Ascorbate-Glutathione Pathway and Stress Tolerance in Plants

Naser A. Anjum Shahid Umar Ming-Tsair Chan ●

●

Editors

Editors Naser A. Anjum Centre for Environmental and Marine Studies (CESAM) and Department of Chemistry University of Aveiro 3810–193 Aveiro Portugal [emailprotected]; [emailprotected]

Shahid Umar Department of Botany Faculty of Science Hamdard University New Delhi India [emailprotected]

Ming-Tsair Chan Academia Sinica Biotechnology Center in Southern Taiwan Sinshih Township Tainan County 74146 Taiwan [emailprotected]

ISBN 978-90-481-9403-2 e-ISBN 978-90-481-9404-9 DOI 10.1007/978-90-481-9404-9 Springer Dordrecht Heidelberg London New York Library of Congress Control Number: 2010934293 © Springer Science+Business Media B.V. 2010 No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission from the Publisher, with the exception of any material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

A Great Philanthropist, Thinker, Visionary & The Founder of Jamia Hamdard (Hamdard University), New Delhi, INDIA

Janab Hakeem Abdul Hameed (1908–1999)

Foreword

Anti-scorbutic factor (Vitamin C), ascorbic acid, was isolated from paprika by A. Szent-Gyorgyi, and its anti-scorbutic effect was confirmed in guinea-pigs (Biochem. J. 27: 278-285 (1933). Occurrence of anti-scorbutic factor in fruits, vegetables and adrenal glands of vertebrates had been deduced from their protective effects against scurvy. However, its isolation from either lemon or adrenal glands was a very heavy work in 1920-30, which required several tons (!) of the materials and several years, because of its low contents and disturbing components. In 1933, Szent-Gyorgyi in Szeged, Hungary, established a simple way for preparation of 450 g crystalline ascorbic acid within a month from local paprika (Hungarian red pepper, Capscicum annuum). This isolation was initiated by his finding of high reducing potentials in paprika juice, as deduced by disappearance of blue dibromophenolindophenol. Even in the current data book on food nutrients, the content of vitamin C in paprika is one of the highest ones among vegetables and fruits, indicating a sharp sense of Szent-Gyorgyi focusing to paprika as the starting material for the isolation of anti-scorbutic factor. In those days, a large amount of ascorbate was required for the determination of its structure, which was accomplished by W. Haworth in England. The revealed structure opened a gate to synthesize ascorbate, which was soon done by W. Haworth and T. Reichstein, independently. Thus, ascorbate is the first vitamin which is chemically synthesized. Based on these works and the following many works and surveys on the antioxidants in plants, World Cancer Research Fund (2007) recommended; “Eat at least five servings (total at least 400 g) of a variety of non-starchy vegetables and of fruits every day, to protect from cancer”, as one of the personal recommendations for foods to escape from cancer. This is a very reasonable one considering that plants are always exposed to most stressful environments among organisms; strong sun light, highest oxygen concentration in leaf tissues, and low homeostasis in respects of temperature and other environmental factors. Thus, plants are expected to contain very effective antioxidants at high contents for survival under natural environments, therefore, both vegetables and fruits are very rich in the antioxidants which are able to protect DNA from ROS. Even though ascorbate isolated from plants has contributed so much to understand its nutritional effect for human, the physiological and biochemical functions of ascorbate in plants themselves have remained obscure for many years. In plants, vii

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Foreword

it had been supposed that ascorbate (AsA), with glutathione (GSH), plays a role as general antioxidants as in animals. However, ascorbate-specific reaction or enzyme in plants was not identified, except for ascorbate oxidase, up to 1980. When chloroplasts are exposed to higher photon intensities over that required for CO2-fixation, excess photons induce the reduction of oxygen in place of CO2. Mehler found the photoproduction of H2O2 in chloroplasts and then, in 1970s, this H2O2 is shown to be produced through the SOD-catalyzed disproportionation of superoxide, which is the primary photoreduced product of oxygen in PS I (Asada). Because even very low amounts of H2O2 inactivate the CO2-fixation enzymes, plant chloroplasts should equip the effective scavenging system of H2O2. Chloroplasts do not contain any catalase, then, peroxidase is a possible scavenger of H2O2. In 198081, ascorbate-specific peroxidase (APX) was found in Euglena cells (Shigeoka et al.) which do not contain catalase, and also in plant chloroplasts (Nakano and Asada). Further, the regeneration of AsA from the oxidized ascorbates (monodehydroascorbate, MDHA radical & dehydroascorbate, DHA) by respective reductases has been characterized including glutathione peroxidase (GPX) and glutathione reductase (GR), all of them are required to scavenge ROS in chloroplasts. Thus, ascorbate-glutathione (AsA-GSH) and related enzymes are essential to scavenge effectively ROS generated in chloroplasts under environmental stress. In addition to chloroplasts, APX and other related enzymes for either scavenging or adjustment of ROS levels have been found in other cell organelles, such as mitochondria, peroxisomes, plasma membranes, apoplasts and other cell compartments. Further, the superoxide-generating, plasma membrane-bound NADPH oxidase has been found and plays a role in the generation for physiologically functional ROS. These systems have an intimate relation with AsA-GSH, as in chloroplasts. The present monograph, edited by eminent scientists Drs. Naser A. Anjum, Shahid Umar and Ming-Tsair Chan, covers the current progress on the physiologically functional ROS, in relation to AsA-GSH. I believe that the revealed mechanisms of signaling and other functions of ROS with intimate correlations of AsA-GSH pathway in plants, and its further progress should contribute to understand the ROS-signal systems in responses to under biotic and biotic stresses. Further, they should provide the focusing points to be analyzed further to support higher crop yields. Furthermore, the ROS-related signaling system mediate signaling system mediated or adjusted by AsA-GSH system in plants should give a clue to understand the similar systems for human health, as have been done since AsA was isolated from paprika. July 29th 2010

Kozi Asada Kyoto University (Emeritus) Kyoto, JAPAN e-mail: [emailprotected]

Preface

Since last several decades, increasing agricultural productivity has been a challenging task to fulfill the requirements of enough food to feed rapidly increasing world population in the changing environment. Both biotic and abiotic stress factors are continued to negatively affect various aspects of plant growth and development leading to relative decrease in the potential maximum yields by more than fifty percent. These stress factors have been shown to affect various aspects of plant system including the acceleration in the formation of reactive oxygen species (ROS). Although, reactive oxygen species are important signal molecules that regulate plant responses to environmental stress factors but these must be rapidly processed and/or detoxified if oxidative damage is to be averted in cells. The ascorbate (AsA)-glutathione (GSH) pathway is a key part of the network of reactions involving enzymes and metabolites with redox properties for the detoxification of ROS, and thus to avert the ROS-accrued oxidative damage in plants. Both AsA and GSH are intimately linked in terms of their major physiological functions in AsA-GSH pathway and many of these processes are correlated with endogenous AsA-GSH levels especially under stress conditions. In addition to having major role during vital phases of the plant life cycle, AsA and GSH determine the lifetime of reactive oxygen species within the cellular environment and provide crucial protection against oxidative damage. While research into the responses of individual components of plant antioxidant defense system has benefited greatly from advances in molecular technology, the cross-talks and inter-relationships studies on the physiological, biochemical and molecular aspects of the cumulative response of various components of AsA-GSH pathway to stress factors and their significance in plant stress tolerance have received comparatively very little or no attention. The present book has concentrated more on cumulative responses of the components of AsA-GSH pathway in plant stress tolerance with emphasis on the unique insights and advances gained by molecular exploration than whole plant antioxidant defense system. In fact, these studies/reports based on inter-relationships and/ or cross-talks are expected to lead to understand and improve the mechanisms of stress tolerance in plants. Therefore, the present volume would definitely be an ideal source of scientific information to the advanced students, junior researchers, faculty and scientists involved in agriculture, plant sciences, molecular biology, biochemistry, biotechnology and related areas. ix

Preface

We are thankful to contributors for their interests, significant contributions and cooperation that eventually made the present volume possible. Thanks are also due to all the well-wishers, teachers, seniors, research students and affectionate family members. Without their unending support, motivation and encouragements the present grueling task would have never been accomplished. We would like to offer our sincere thanks to Mr. Jacco Flipsen, Ineke Ravesloot, Purushothaman Saravanan and their team at Springer and SPi for their continuous support which made our efforts successful. Last but not least, the financial supports to our research from Foundation for Science & Technology (FCT), PORTUGAL (SFRH/BPD/64690/2009), Council of Scientific & Industrial Research (CSIR), New Delhi, INDIA [(9/112(0401)2K8-EMR-I (312208/2K7/1)], Agricultural Biotechnology Research Center (ABRC), Academia Sinica, TAIWAN, Potash Research Institute of India (PRII), Gurgaon, INDIA and International Potash Institute (IPI), SWITZERLAND are gratefully acknowledged. August 6th 2010 Aveiro, Portugal New Delhi, India Tainan, Taiwan

Naser A. Anjum Shahid Umar Ming-Tsair Chan

Contents

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway in Plant Stress Tolerance......................................................... Dariusz Latowski, Ewa Surówka, and Kazimierz Strzałka

2 Ascorbate and Glutathione in Organogenesis, Regeneration and Differentiation in Plant In vitro Cultures....................................... Jarosław Tyburski and Andrzej Tretyn

3 Role of Ascorbate Peroxidase and Glutathione Reductase in Ascorbate–Glutathione Cycle and Stress Tolerance in Plants........ Cai-Hong Pang and Bao-Shan Wang

4 The Ascorbate–Gluathione Cycle and Related Redox Signals in Plant–Pathogen Interactions................................................. 115 Elżbieta Kuźniak 5 Regulation of the Ascorbate–Glutathione Cycle in Plants Under Drought Stress.............................................................................. 137 Adriano Sofo, Nunzia Cicco, Margherita Paraggio, and Antonio Scopa 6 Glutathione and Herbicide Resistance in Plants................................... 191 Zornitsa Ivanova Katerova and Lyuba Petar-Emil Miteva 7 Ascorbate and Glutathione: Protectors of Plants in Oxidative Stress................................................................................... 209 Qaisar Mahmood, Raza Ahmad, Sang-Soo Kwak, Audil Rashid, and Naser A. Anjum

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8 Changes in the Glutathione and Ascorbate Redox State Trigger Growth During Embryo Development and Meristem Reactivation at Germination.......................................... 231 Claudio Stasolla 9 A Winning Two Pair: Role of the Redox Pairs AsA/DHA and GSH/GSSG in Signal Transduction................................................ 251 Günce Şahin and Mario C. De Tullio 10 Involvement of AsA/DHA and GSH/GSSG Ratios in Gene and Protein Expression and in the Activation of Defence Mechanisms Under Abiotic Stress Conditions...................................... 265 Vasileios Fotopoulos, Vasileios Ziogas, Georgia Tanou, and Athanassios Molassiotis 11 Ascorbate–Glutathione Cycle: Enzymatic and Non-enzymatic Integrated Mechanisms and Its Biomolecular Regulation............................................................ 303 Juan Pablo Martínez and Héctor Araya 12 Coordinate Role of Ascorbate–Glutathione in Response to Abiotic Stresses.................................................................................... 323 Imran Haider Shamsi, Sisi Jiang, Nazim Hussain, Xianyong Lin, and Lixi Jiang 13 Regulation of Genes Encoding Chloroplast Antioxidant Enzymes in Comparison to Regulation of the Extra-plastidic Antioxidant Defense System.................................................................... 337 Margarete Baier, Nicola T. Pitsch, Marina Mellenthin, and Wei Guo 14 The Peroxisomal Ascorbate–Glutathione Pathway: Molecular Identification and Insights into Its Essential Role Under Environmental Stress Conditions...................................... 387 Sigrun Reumann and Francisco J. Corpas 15 Identification of Potential Gene Targets for the Improvement of Ascorbate Contents of Genetically Modified Plants......................... 405 Adebanjo A. Badejo and Muneharu Esaka Index.................................................................................................................. 429

Contributors

Raza Ahmad Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan Naser A. Anjum Centre for Environmental and Marine Studies (CESAM) & Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal Academia Sinica, Agricultural Biotechnology Research Center (ABRC), Taiwan and Department of Botany, Division of Plant Physiology and Biochemistry, Faculty of Life Sciences, Aligarh Muslim University, Aligarh-202 002, UP, India [emailprotected]; [emailprotected] Héctor Araya Faculty of Medicine, Department of Nutrition, University of Chile, Independencia 1027, Santiago, Chile and Faculty of Pharmacy, Department of Food and Nutrition, University of Valparaíso, Avda Gran Bretaña 1111, Valparaíso, Región de Valparaíso, Chile and Regional Center for the Study of Healthy Foods (CREAS), Blanco 1623, Oficina 1402, Edificio Torres Mar del Sur 2-, Valparaíso Adebanjo A. Badejo Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan [emailprotected] Margarete Baier Plant Science, Heinrich-Heine-University, Universitaetsstrasse 1, 40225 Düsseldorf, Germany and Plant Physiology, Free University Berlin, Königin-Luise-Strasse 12-16, 14195 Berlin, Germany [emailprotected] Nunzia Cicco Istituto di Metodologie per l’Analisi Ambientale, Consiglio Nazionale delle Ricerche, C. da S. Loia, Zona Industriale, 85050 Tito Scalo (PZ), Italy xiii

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Contributors

Francisco J. Corpas Departamento de Bioquímica, Biología Celular y Molecular de Plantas, Estación Experimental del Zaidín (EEZ), CSIC, Granada, Spain Mario C. De Tullio Plant Biology and Pathology, Aldo Moro University, Bari, Italy [emailprotected] Muneharu Esaka Graduate School of Biosphere Sciences, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8528, Japan [emailprotected] Vasileios Fotopoulos Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology, PC 3036 Limassol, Cyprus Wei Guo Plant Science, Heinrich-Heine-University, Universitaetsstrasse 1, 40225 Düsseldorf, Germany Nazim Hussain Institute of Crop Science, College of Agriculture and Biotechnology, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR of China Lixi Jiang Institute of Crop Science, College of Agriculture and Biotechnology, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR of China [emailprotected] Sisi Jiang Institute of Agricultural Chemistry, College of Environmental and Resource Sciences, 288 Kaixuan Roas, Hangzhou 310029, PR of China Zornitsa Ivanova Katerova Acad. M. Popov Institute of Plant Physiology, Bulgarian Academy of Sciences, Acad. G. Bonchev Str., Bl. 21, 1113 Sofia, Bulgaria [emailprotected] Elżbieta Kuźniak Department of Plant Physiology and Biochemistry, University of Łódź, 90-237 Łódź, Banacha 12/16, Poland [emailprotected] Sang-Soo Kwak Environmental Biotechnology Research Center, Korea Research Institute of Bioscience and Biotechnology (KRIBB), Eoeun-dong 52, Yuseong, Daejon, 305-806, South Korea

Contributors

Dariusz Latowski Faculty of Biochemistry, Biophysics and Biotechnology, Department of Plant Physiology and Biochemistry, Jagiellonian University, Gronostajowa 7, 30-387 Krakow, Poland Xianyong Lin Institute of Agricultural Chemistry, College of Environmental and Resource Sciences, 288 Kaixuan Roas, Hangzhou 310029, PR of China Qaisar Mahmood Department of Environmental Sciences, COMSATS Institute of Information Technology, Abbottabad 22060, Pakistan [emailprotected] Juan Pablo Martínez Institute of Agriculture Research (INIA), Chorrillos 86, La Cruz, Región de Valparaíso, Chile and Regional Center for the Study of Healthy Foods (CREAS), Blanco 1623, Oficina 1402, Edificio Torres Mar del Sur 2, Valparaíso [emailprotected]; [emailprotected] Marina Mellenthin Plant Science, Heinrich-Heine-University, Universitaetsstrasse 1, 40225 Düsseldorf, Germany Lyuba Petar-Emil Miteva Resbiomed EOOD, 81 B, Bulgaria Blvd, Sofia, Bulgaria Athanassios Molassiotis Faculty of Agriculture, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece [emailprotected] Cai-Hong Pang Shandong Academy of Forestry, Jinan 250014, Shandong, P.R. China Margherita Paraggio Istituto di Metodologie per l’Analisi Ambientale, Consiglio Nazionale delle Ricerche, C. da S. Loia, Zona Industriale, 85050 Tito Scalo (PZ), Italy Nicola T. Pitsch Plant Science, Heinrich-Heine-University, Universitaetsstrasse 1, 40225 Düsseldorf, Germany Audil Rashid Department of Environmental Sciences, PMAS Arid Agriculture University, Rawalpindi, Pakistan

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Contributors

Sigrun Reumann Faculty of Science and Technology, Centre for Organelle Research (CORE), University of Stavanger, Stavanger, Norway [emailprotected] Günce Şahin Abant Izzet Baysal University, Bolu, Turkey Antonio Scopa Dipartimento di Scienze dei Sistemi Colturali, Forestali e dell’Ambiente, Università degli Studi della Basilicata, Via dell’Ateneo Lucano 10, 85100 Potenza, Italy Imran Haider Shamsi Institute of Crop Science, College of Agriculture and Biotechnology, Zhejiang University, 268 Kaixuan Road, Hangzhou 310029, PR of China Adriano Sofo Dipartimento di Scienze dei Sistemi Colturali, Forestali e dell’Ambiente, Università degli Studi della Basilicata, Via dell’Ateneo Lucano 10, 85100 Potenza, Italy [emailprotected] Kazimierz Strzałka Faculty of Biochemistry, Biophysics and Biotechnology, Department of Plant Physiology and Biochemistry, Jagiellonian University, Gronostajowa 7, 30-387 Krakow, Poland [emailprotected] Claudio Stasolla Department of Plant Science, University of Manitoba, Winnipeg R3T 2N2, Manitoba, Canada [emailprotected] Ewa Surówka Institute of Plant Physiology, Polish Academy of Sciences, Niezapominajek 21, 30-239 Krakow, Poland Georgia Tanou Faculty of Agriculture, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece Andrzej Tretyn Department of Biotechnology, Institute of General and Molecular Biology, Nicolaus Copernicus University, Gagarina 9, 87-100 Toruń, Poland Jarosław Tyburski Department of Biotechnology, Institute of General and Molecular Biology, Nicolaus Copernicus University, Gagarina 9, 87-100 Toruń, Poland [emailprotected]

Contributors

Bao-Shan Wang Key Lab of Plant Stress Research, College of Life Sciences, Shandong Normal University, Jinan 250014, Shandong, P.R. China [emailprotected] Vasileios Ziogas Faculty of Agriculture, Aristotle University of Thessaloniki, 54 124 Thessaloniki, Greece

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Chapter 1

Regulatory Role of Components of Ascorbate–Glutathione Pathway in Plant Stress Tolerance Dariusz Latowski, Ewa Surówka, and Kazimierz Strzałka

Abstract Ascorbate (Asc) and glutathione (GSH) are important molecules functioning in several vital processes in plant cells, including the ascorbate– glutathione cycle. They are involved in basic metabolic reactions normally occurring in plants as well as in those evoked by abiotic or biotic stresses. Asc and GSH are localized in most of cell compartments such as cytoplasm, mitochondria, peroxisomes and chloroplasts, the Asc being additionally also found in the apoplast. These small molecular weight compounds protect cells against oxidative stress and damage by detoxifying reactive oxygen species (ROS) and ROS-generated toxic metabolic products. They do this either directly by scavenging or indirectly through the activation of defense mechanisms. Asc and GSH are engaged in maintaining cellular redox homeostasis, being at the same time involved in redox signaling. They may interact with different molecules and signaling pathways during the resistance responses. Besides the total level of Asc and GSH in the cell the ratio between reduced and oxidized forms of these molecules play an important role in the activation of various defense mechanisms. Furthermore, glutathione is involved in specific functions such as detoxification of heavy metals, transfer and storage of sulfur, regulation of expression of defense-related genes and protein activity, while ascorbate acts as a signal-transducing molecule, cofactor of some enzymes and biosynthetic precursor of oxalic and l-tartaric acids. Moreover all kinds of xanthophylls cycle, considered as the most important photoprotective mechanism in plants, are strongly dependent on the acid form of ascorbate. The biochemical, physiological and genetic aspects of the involvement of ascorbate and glutathione, localized in different cell compartments, in controlling cellular redox state, plant stress tolerance, and defense mechanisms will be discussed.

D. Latowski and K. Strzałka (*) Faculty of Biochemistry, Biophysics and Biotechnology, Department of Plant Physiology and Biochemistry, Jagiellonian University, Gronostajowa 7, 30-387 Krakow, Poland e-mail: [emailprotected] E. Surówka Institute of Plant Physiology, Polish Academy of Sciences, Niezapominajek 21, 30-239 Krakow, Poland N.A. Anjum et al. (eds.), Ascorbate-Glutathione Pathway and Stress Tolerance in Plants, DOI 10.1007/978-90-481-9404-9_1, © Springer Science+Business Media B.V. 2010

D. Latowski et al.

Keywords Ascorbate • Glutathione • Antioxidant compartmentalisation • Cellular redox state • Defence mechanisms • Signal transduction pathways • Stress tolerance • Xanthophyll cycle

1 Introduction Exposure of plants to stresses such as high light stress (HL), ultraviolet radiation, drought stress and desiccation, salt stress, chilling, heat shock, heavy metals, air pollutants, mechanical stress, nutrient deprivation and pathogen attack and can give rise to excess accumulation of reactive oxygen species (ROS) at the cellular level (Stohs and Bagchi 1995; Noctor and Foyer 1998; Alscher etal. 2002; Schützendübel and Polle 2002). In plant cells ROS such as superoxide radical (O2⋅−), hydrogen peroxide (H2O2), hydroxyl radical (·OH), and singlet oxygen (1O2) are mainly produced in chloroplasts, mitochondria, and peroxisomes as well as by the plasma membrane–bound NADPH oxidases and the cell wall–bound NAD(P)H oxidase– peroxidase (Foyer and Noctor 2000; Jiang and Zhang 2001; del Río et al. 2002; Sweetlove et al. 2002; Foyer and Noctor 2003; Apel and Hirt 2004; Kristensen et al. 2004; Mittler et al. 2004; Møller and Kristensen 2004; Papadakis and Roubelakis-Angelakis 2005; Papadakis and Roubelakis-Angelakis 2005; Halliwell 2006; Ślesak etal. 2007). The enhanced production of ROS during stress can pose a threat to cells but it is also thought that ROS act as signals for the activation of stress-response and defense pathways (Karpinski etal. 1999; Desikan etal. 2001; Knight and Knight 2001; Mittler 2002). ROS can be viewed as cellular indicators of stress and as secondary messengers involved in the stress-response signal transduction pathway thus, their level has to be kept under tight control by ROSscavenging mechanisms, including enzymatic and non-enzymatic antioxidants. Under normal growth conditions, the antioxidative defence system provides the adequate protection against reactive oxygen species (Foyer and Halliwell 1976; Fridovich 1986; Asada and Takahashi 1987). However, under abiotic and biotic stresses the balance between ROS production and scavenging them by antioxidants may be changed and then ROS are generated in excess leading to cell death (Fig.1) (Apel and Hirt 2004; Mittler etal. 2004; Dietz 2003; Gechev and Hille 2005). The extent to which ROS accumulate is determined by the antioxidative system, which enables organisms to maintain proteins and other cellular components in an active state for metabolism and provide a dynamic metabolic interface between stress perception by plant cell and physiological responses (Mahalingam and Fedoroff 2003; Foyer and Noctor 2005, 2009; Mittler etal. 2004; Van Breusegem and Dat 2006; Ślesak etal. 2007; Niewiadomska and Borland 2008). Variable contents and concentrations of antioxidant between individual cell compartments enable plants to generate ROS-related signaling involving fundamental processes in plant cells. Moreover rapid, compartment-specific changes in redox state and ROS, related to redox signaling, can be achieved either by repression/activation of the antioxidant defense system or by modifying ROS (especially O2⋅-, H2O2) production or by both

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

Initiators: drought, salinity, high light, UV, chilling, gaseous pollutants, senescence, heavy metals, wounding, xenobiotics, herbivores, pathogenes

Excess excitation energy

ROS generation H2O2, O2.−, .OH, O21, ROOH mainly in: chloroplasts, mitochondria, peroxisomes, apoplast

Enzymatic scavengers: - superoxide dismutase (SOD) - catalase (CAT) - Peroxidases (POD), e.g. ascorbate peroxidase (APX). glutathione peroxidase (GPX) - glutathione reductase (GR)

Non-enzymatic scavengers - carotenoids (e.g. zeaxanthin) - tocopherols - ascorbic acid (Asc) - glutathione (GSH)

Antioxidant system

Death/defense response/the increase of stress tolerance

Fig. 1 The response of plants to different abiotic and biotic stresses. The antioxidant system enables plants to regulate reactive oxygen species (ROS) level and influence on ROS-dependent signal induction O2.− SOD H2O2

Asc

GSSG

NAD(P) APX

MDHAR

NAD(P)H

DHAR GR

NAD(P)H H2O

NAD(P)

MDHA DHA

GSH

Fig.2 The scheme of ascorbate–glutathione cycle. Asc – ascorbate, APX – ascorbate peroxidase, DHA – dehydroascorbate, DHAR – dehydroascorbate reductase, MDHA – monodehydroascorbate, GR – glutathione reductase, GSH – reduced glutathione, GSSG – oxidized glutathione

in individual organellas (Alscher 1997; Foyer and Noctor 2000; Pastori and Foyer 2002; Xiong etal. 2002; Foyer and Noctor 2005; Kuźniak and Skłodowska 2005; Fernandez-García etal. 2009). Superoxide dismutase (SOD) – the enzyme known as

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the first line of defence, catalyzes the dismutation of O2⋅- radicals to molecular oxygen and H2O2 (Fridovich 1986; Alscher etal. 2002). H2O2 scavenging is accomplished by catalase, various peroxidases, and the ascorbate–glutathione pathway, also known as the Halliwell–Asada cycle (Fig.2), which is catalyzed by a set of four enzymes (Willekens et al. 1997; Wojtaszek 1997; Noctor 1998; Kingston-Smith and Foyer 2000; Jung 2003; Asada 2006; Queval etal. 2008; Almagro etal. 2009; Cosio and Durand 2009). First, the hydrogen peroxide is scavenged via the oxidation of ascorbate (2,3-endioll-gulonic acid-g-lactone; l-ascorbic acid; Asc) by ascorbate peroxidase (APX). This enzyme is involved in the oxidation of Asc to monodehydroascorbate (MDHA), which can be converted back to Asc via monodehydroascorbate reductase (MDHAR). MDHA is further rapidly converted to dehydroascorbate (DHA), which is converted back to Asc by the action of dehydroascorbate reductase (DHAR). DHAR utilizes glutathione (g-l-glutamyl-l-cysteinyl glycine; GSH), which is regenerated by glutathione reductase (GR) from its oxidized form, glutathione disulfide (GSSG). The ascorbate–glutathione cycle operates in chloroplasts, mitochondria, peroxisomes, cytosol, and in the apoplast (Smirnoff 2000; Mittler 2002, 2004; Pignocchi and Foyer 2003; Asada 2006). Apart from the implication of ascorbate and glutathione in Halliwell–Asada cycle, these water soluble compounds could act independently. The specificity of functions of Asc and GSH are related to their compartmentalization (Veljovic-Jovanovic etal. 2001).

2 The Importance of Ascorbate–Glutathione Cycle Compounds in Different Cell Comparments Under Stress Conditions Majority of plant cells contain in most compartments very large quantities of ascorbate (vitamin C) in milimolar concentrations (10–300 mM) and glutathione (0.1– 25 mM) (Noctor and Foyer 1998; Smirnoff 2000, 2002; Noctor etal. 2002; Ruiz and Blumwald 2002; Mou etal. 2003; Ball etal. 2004; Freeman etal. 2004; Gomez etal. 2004; Shao etal. 2008). Hence, they have the capacity to deal with very high fluxes of H2O2 production and with the complex network of reactions surrounding ROS and keeping redox homeostasis (Noctor etal. 2002). The important roles of Asc and GSH for tolerance towards environmental stresses were emphasize in investigations involvement mutants and transgenic plants lines with altered levels of these antioxidants (Pastori etal. 2003; Ball etal. 2004). Seven ascorbate-deficient (vtc) mutants for Arabidopsis thaliana have been identified. These mutants represent four different VTC loci (VTC1, VTC2, VTC3, VTC4) involved in the maintenance of the Asc pool in the cell (Table1). Enzymes encoded by genes localized in these loci involved in major pathway of Asc biosynthesis in all chlorophyll-containing plants, in which GDP-d-mannose, formed from d-mannose-1-phosphate, is successively converted to GDP-l-galactose, l-galactose-1-phosphate, l-galactose, l-galactono-1,4-lactone, and finally to

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

Table1 Comaparison of phenotype of ascorbate (vtc) deficient mutant plants Genotype of Arabidopsis Published phenotype thaliana mutants characteristics References Conklin etal. 1996, 2000, vtc1-1 No phenotypic effects from Veljovic-Jovanovic etal. increased oxidative stress 2001, Gatzek etal. 2002, increased sensitivity to Pastori etal. 2003, Barth ozone; the ascorbate levels etal. 2004, Filkowski etal. 25–30% of wild-type levels; 2004, Müller-Moulé etal. slower growth rate; no 2004, Larkindale etal. 2005, effect on photosynthesis Pavet etal. 2005, Colville rate or hydrogen peroxide and Smirnoff 2008; Gao and levels under moderate Zhang 2008 light intensity; sensitive to UV-B and UV-C radiation; reduced basal thermotolerance microarrays analysis: 171 genes differentially expressed when compare to WT; 12.9% genes involved in cell defence – increase of pathogenesis related (PR) proteins; 60% higher concentration of ABA; increased resistance to infection by virulent pathogens; increased activity of peroxidase other than APX Conklin etal. 2000, Larkindale vtc1-2 Increased sensitivity to ozone; the etal. 2005 ascorbate levels 25–30% of wild-type levels; reduced basal thermotolerance Conklin etal. 2000, Veljovicvtc2-1 Increased sensitivity to ozone; the Jovanovic etal. 2001, Havaux ascorbate levels 25–30% of 2003, Pastori etal. 2003, Barth wild-type levels; slower growth etal. 2004, Müller-Moulé etal. rate; no effect on photosynthesis 2004, Larkindale etal. 2005, rate or hydrogen peroxide levels Pavet etal. 2005 under moderate light intensity; reduced ability to zeaxanthin synthesis under high light; reduced basal thermotolerance; greater thermoinduced photon emission; increased resistance to infection by virulent pathogens reduced acclimation to high light demonstrated as the increase of lipid peroxidation and bleaching; in spite of 30% higher levels of GSH than the wild type (continued)

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Table1 (continued) Genotype of Arabidopsis Published phenotype thaliana mutants characteristics vtc2-2

vtc2-3

vtc3

vtc4

References

Conklin etal. 2000, Müller-Moulé The ascorbate levels 25–30% of etal. 2004, Larkindale etal. wild-type levels; slightly sensitive 2005 to ozone than wild-type plants; reduced ability to synthesise zeaxanthin under high ligh reduced basal thermotolerance Conklin etal. 2000, Müller-Moulé The ascorbate levels 50% of wildetal. 2004 Larkindale etal. type levels in vtc2-3; slightly 2005 sensitive to ozone than wildtype plants; reduced ability to synthesise zeaxanthin under high light; reduced basal thermotolerance Conklin etal. 2000 The ascorbate levels 50% of wildtype levels; slightly sensitive to ozone than wild-type plants Conklin etal. 2000 The ascorbate levels 50% of wildtype levels; slightly sensitive to ozone than wild-type plants

l-ascorbate (Wheeler et al. 1998; Conklin et al. 1999, 2000, 2006; Dowdle et al. 2007; Smirnoff etal. 2004; Ishikawa etal. 2006; Colville and Smirnoff 2008; Linster etal. 2007; Linster and Clarke 2008). Plants possess also alternative Asc biosynthetic pathway, which is suggested to play an important role under certain conditions, at particular developmental stages, and it seems to be engaged in salvage mechanism for carbon resulting from, i.e., the breakdown of cell walls (Davey etal. 1999; Agius etal. 2003). This Asc biosynthetic pathway includes the conversion of the uronic acids, d-glucuronic acid (d-GlcUA) and a precursor of l-galactono-1,4-lactone (l-GalL) such as d-galacturonic acid, to l-ascorbic acid (Davey etal. 1999; Agius etal. 2003; Lorence etal. 2004). In both biosynthetic pathway, the synthesis of l-galactono-1,4-lactone is rate-limiting step for Asc biosynthesis (Bartoli etal. 2000; Millar etal. 2002; Foyer and Noctor 2005; Bartoli etal. 2006). The regulation of the expression genes related to Asc metabolism in leaves of Arabidopsis plants was shown to be closely related to the photosynthetic electron transport (Yabuta et al. 2007, 2008) as well as jasmonic acid (Turner etal. 2002; Wolucka etal. 2005; Bartoli etal. 2006; Dowdle etal. 2007; Maruta et al. 2008). Plants make Asc via de novo synthesis pathways (Wheeler etal. 1998) and carbon skeleton re-cycling networks (Smirnoff etal. 2004). The four glutathione-deficient mutants for A. thaliana have been characterized: cad2-1 (Cobbett etal. 1998), rax1 (Ball etal. 2004) and pad2 (Parisy etal. 2007) and rml1 (Vernoux etal. 2000) and applied in the studies of GSH importance in plant stress response (Table2).

Table2 Comaparison of genotype and phenotype of glutathione deficient mutant plants Genotype of Arabidopsis thaliana mutants Gene locus and enzyme of mutation Published phenotype characteristics 75–80% Reduction in GSH content Mutations in the gene At4g23100 which cad2-1 when compared to wild type; encodes g-glutamylcysteine synthetase two amino acid deletion (P238, (g-ECS, GSH1) K239); no phenotype in the absence of stress; the reduced ability to produce sufficient amounts of heavy metal binding phytochelatins from the precursor GSH; cadium hypersensitive rax1 Mutations in the gene At4g23100 R229K amino acid change; 50–80% which encodes g-ECS decrease in the GSH level; lost of ability to properly control expression of cytosolic APX2 pad2 Mutations in the gene At4g23100 80% Reduction in GSH content; which encodes g-ECS, GSH1 pathogen-sensitive mutant; increased susceptibility to Phytophthora brassicae infection; phytoalexin deficient rml1 Mutations in the gene At4g23100, Shows a D259N amino acid change; which encodes g-ECS; GSH1 not capable of maintaining cell division in the root meristem; arrested plant development even under optimal growth conditions GSH1 knockout mutant Embryonic lethal phenotype; embryos produce no GSH and develop to their full size but just after greening, mutant embryos bleach GSH2 knockout mutants Exhibit a seedlings; lethal phenotype Pasternak etal. 2008

Cairns etal. 2006

Vernoux etal. 2000, Cairns etal. 2006

Glazebrook and Ausubel 1994, Glazebrook et al. 1996, 1997; Parisy etal. 2007

Ball etal. 2004

References Cobbett etal. 1998

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway 7

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These mutants contain the mutation in the gene At4g23100, which encodes g-glutamylcysteine synthetase (g-ECS, GSH1) (Meyer 2008). The knockout of GSH1 through T-DNA insertions causes an embryonic lethal phenotype (Cairns etal. 2006). Similarly, knockout of glutathione synthetase (GSH2) through T-DNA insertions also causes lethal phenotype. So far, it is the state of the art in that the only isolated mutants of GSH2 are T-DNA insertion mutants (Kopriva and Rennenberg 2004; Mullineaux and Rausch 2005; Meyer 2008; Pasternak et al. 2008; Szalai et al. 2009). These two enzymes: GSH1 and GSH2 catalyse ATP-dependent reactions in the glutathione synthesis pathway. First enzyme catalyses the formation of g-glutamylcysteine (g-EC) from l-glutamate and cysteine and the second one bound a glycine to the dipeptide (g-glutamylcysteine). Some authors point out that g-glutamylcysteine synthetase is the controlling enzyme for overall flux through the pathway and synthesis of g-EC is important step in the pathway (Jez etal. 2004; Hothorn etal. 2006; Hicks etal. 2007; Rennenberg etal. 2007; Meyer etal. 2008). GSH1 is exclusively located in the plastids, whereas GSH2, is located in both plastids and cytosol (Wachter 2004; Wachter and Rausch 2005; Mullineaux and Rausch 2005; Wachter etal. 2005; Hothorn etal. 2006; Hicks etal. 2007; Pasternak etal. 2008). In A. thaliana, GSH1 and GSH2 are present as single genes (May and Leaver 1994; Ullmann etal. 1996; The Arabidopsis Genome Initiative 2000; Mullineaux and Rausch 2005).

2.1 Apoplast and Symplast The apoplast is the extraprotoplastic matrix of plant cells, consisting of all compartments from the external face of the plasmalemma to the cell wall (Dietz 1996). It represents the first line of contact between the plant and its external environment (Wojtaszek 1997; Bolwell etal. 1999). The apoplast plays crucial role in the perception of abiotic and biotic stresses as well as in plant defense and survival. Apoplast functions also as a highly sensitive and flexible compartment in signaltransducing processes throughout the plasmalemma. Through the cross-talk between apoplast and cytoplasm (symplast) the level as well as redox status of Asc in the apoplast are regulated (Fig. 3) (Diaz-Vivancos et al. 2006 ; Pignocchi and Foyer 2003 ; Pignocchi etal. 2003, 2006). Asc is regenerated though Asc–GSH cycle or several mechanisms that include MDHAR in the symplast (Fig. 3) (Horemans et al. 2000; Pignocchi et al. 2003; Foyer 2004). Several types of Asc and DHA specific carriers localised in plasmalemma such as, i.e., Asc-dependent cytochrome b561 transporters are engaged in these processes (Foyer 2004; Preger et al. 2005). Asc-dependent cytochrom b561 was shown to be reduced by Asc on one face of plasmalemma and oxidized on the other by MDHA or other substrates for MDHA reductase (MDHAR, Preger etal. 2005). However, the preferably transported from the apoplastic to the cytoplasmic side of the membrane is DHA and this process is dependent upon plasma membrane proton gradient (Kollist etal. 2008).

Apoplast Cell Wall

Initiator e.g. O3.

Plasmalemma

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

Symplast (Cytosol)

Chloroplast NADPH

2nd detoxifying response

ROS

ROS Asc

Asc

GSSG

NAD(P)H

GSH

NADP

NAD(P)H

O3 ROS DHA

DHA

MDHA AO

Asc Cyt b

Asc

MDHA

1st existing detoxifying barrier

Fig.3 Scavenging of ROS by ascorbate in apoplastic space of the cell. ROS – reactive oxygen species, Asc – ascorbate, DHA – dehydroascorbate, MDHA – monodehydroascorbate, GSH – reduced glutathione, GSSG – oxidized glutathione, AO – ascorbate oxidase

The capacity of redox buffering in the apoplast is much weaker than inside the cell (Horemans etal. 2000; Pignocchi etal. 2003), and the homeostasis in the apoplast is easily perturbed and this is why environmental factors can be perceived by the cell throughout the components of the apoplastic space (Lyons etal. 1999; Sattelmacher 2001; Bolwell etal. 2002; Pignocchi and Foyer 2003). The apoplast matrix does not contain NAD(P)H as well as GSH and its derivatives (Noctor 2006; Zechmann etal. 2006, 2008) and it is equiped with antioxidative enzymes such as SOD, MDHAR as well as Asc. Asc is considered as its major antioxidant (Hernández etal. 2001; Foyer and Noctor 2005, 2009; Diaz-Vivancos etal. 2006) and it is present in the apoplast at millimolar concentrations (up to about 10% of the of the whole cell Asc content). These all apoplastic antioxidants are inter-linked with oxidants like (O2⋅−) and H2O2, which are also present in the apoplast especially under stress conditions (Hernández etal. 2001; Pignocchi and Foyer 2003). It is well known that abiotic and biotic stress factors like: pathogens, drought, salinity, air pollutants or strong light cause oxidation of Asc directly or indirectly (Mittler 2002, 2004; Foyer and Noctor 2003, 2005, 2009). The oxidation of Asc occurring in the apoplast and the related alteration in its redox state are the primary symptoms of oxidative burst. However, with the other hand these alterations are related to defense responses through the regulation of signal transduction cascades and gene expression patterns. The ascorbate redox state in the apoplast is largely independent of that in the symplasm which is relatively constant throughout

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the life of a cell. Moreover, the sensors located in the apoplast may have different properties compared with those operating in the symplast (Delauney etal. 2002). The redox state of the Asc pool in the apoplast is also regulated by ascorbate oxidase (AO), which is a cell wall-localized enzyme that uses oxygen to catalyse the oxidation of Asc to the unstable radical MDHA that rapidly disproportionates to yield DHA and Asc (Fotopoulos etal. 2006). AO activity, through modulation of the redox state of the apoplastic AA pool, strongly influences the responses of plant cells to external and internal stimuli. As it was presented on transgenic tobacco (Nicotiana tabacum) plants with modified cell wall-localized AO, oxidation of apoplastic Asc was associated with the loss of the auxin response, higher mitogenactivated protein kinase activities, and susceptibility to a virulent strain of the pathogen Pseudomonas syringae (Pignocchi and Foyer 2003; Foyer and Noctor 2006; Pignocchi etal. 2006). The control of AO by light and other factors such as gibberellic acid (GA), salicylic acid (SA) and auxin may modulate defense processes (Pignocchi and Foyer 2003; Pignocchi etal. 2006). In addition to Asc, plants also accumulate GSH in compartments such as the chloroplast, cytosol and mitochondria. Plants maintain most cytoplasmic thiols in the reduced (2SH) state because of the low thioldisulfide redox potential imposed by millimolar amounts of the thiol buffer, glutathione. Although the amounts of GSH which is present inside the cell are probably small (Gukasyan etal. 2002), the intracellular ratio GSHred/GSHox, which determines the redox potential, could have been high and sufficient to reduce ROS.

2.2 Chloroplasts Chloroplasts are frequently exposed to ROS in the light, due to the presence of high redox potential components, excited states of pigments, and generation of free electrons during photosynthetic electron transport in the thylakoid membrane. The energy sensing and signalling pathways initiated in chloroplasts in relation to the photochemical and biochemical dysfunctions of photosynthesis extend beyond these organelles such as mitochondria and affect also processes other than photosynthesis (Mullineaux and Karpinski 2002; Rossel et al. 2002; Ball et al. 2004; Mateo etal. 2004; Mittler etal. 2004; Kuźniak etal. 2009). Therefore ROS generated in excess have to be effectively scavenged, and this can be done by several systems/mechanisms like the xanthophyll cycle, photorespiration and other changes in metabolic activity (Demmig-Adams and Adams 1996; Kozaki and Takeba 1996; Eskling etal. 1997; Osmond etal. 1997), and a number of other nonenzymatic and enzymatic antioxidants such as i.e. carotenoids and a-tocopherol (vitamin E) (Smirnoff 1993; Niyogi 2000; Mullineaux and Karpinski 2002; Apel and Hirt 2004; Mittler etal. 2004; Foyer and Noctor 2005, 2009; Hofius etal. 2004; Munné-Bosch 2004; Munné-Bosch and Falk 2004; Plaxton and Podestá 2006). Within the chloroplastic mechanisms of protection against the potentially deleterious effects of ROS, a pivotal role play Asc and GSH. The reduced forms of Asc and

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

GSH, together with antioxidant enzymes: APX, MDHAR, DHAR, glutathione peroxidise (GPx) and glutathione reductase (GR) make up the ascorbate–glutathione cycle (Fig.2), and they are involved in ROS scavenging, similarly to water–water cycle that enables the electron flow through the photosynthetic electron transport carriers at limited availability of electron acceptors, such as NADP+ and/or CO2 (Asada etal. 1999; Asada 2006). Asc and GSH are water-soluble antioxidants and they are able to act directly. They scavenge lipid hydroperoxide (LOOH) and remove H2O2 generated during photosynthetic processes in the light. Asc and GSH serve both as cofactors for peroxidases and as reductants. Asc is the main substrate for H2O2 reduction by thylakoid-bound and stromal ascorbate peroxidases (Melhorn etal. 1993). Ascorbate peroxidase reduces H2O2 to water producing two molecules of oxidised ascorbate, monodehydroascorbate (MDHA), which rapidly disproportionates to Asc and DHA (Asada 2006). The MDHAR and DHAR are responsible for regenerating Asc, the later using GSH as reducing substrate. Glutathione reductases regenerate GSH in a NADPH-dependent reaction (Foyer and Halliwell 1976). The low antioxidant buffering could allow oxidative signals to accumulate within the lumen, and this may be important in redox signal transduction leading to programmed cell death (PCD) (Van Breusegem and Dat 2006). Asc and GSH help to maintain the integrity of the photosynthetic membranes under oxidative stress (Havaux 2003; Noctor and Foyer 1998; Asada 2006; Smirnoff and Wheeler 2000; Munné-Bosch and Alegre 2002). Asc may indirectly protect a-tocopherol by scavenging ROS, and it may participate in the recycling of a-tocopheroxyl radicals to a-tocopherol (Veljovic-Jovanovic et al. 2001; Pastori and Foyer 2002), thus resulting in DHA accumulation under stress (Munné-Bosch and Alegre 2002, 2003, 2007; Packer etal. 1979; Smirnoff and Wheeler 2000). In the lumen Asc is the electron donor in the enzymatic conversion of violaxanthin to zeaxanthin by the lumenal violaxanthin deepoxidase. In experiments with Ascdeficient Arabidopsis thaliana mutants it was demonstrated that the expose of vtc2-2 mutant to high light was accompanied with reduced nonphotochemical quenching (NPQ) related to the alteration in xanthophyl cycle pigments pattern (Conklin etal. 2000; Veljovic-Jovanovic etal. 2001; Müller-Moulé etal. 2002, 2003, 2004), while in other ascorbate deficient mutants follow oxidative stress responses (Kiddle etal. 2003; Pastori etal. 2003; Rossel etal. 2002; Davletova etal. 2005; Mahalingam et al. 2005). Some authors (Rautenkranz etal. 1994; Horemans et al. 2000) presented data supporting the increased transport of Asc from the cytosol to chloroplasts in stressed plants, and thylakoid membranes are able to transport Asc by diffusion (Foyer and Lelandais 1996; Horemans etal. 2000). The unique status of chloroplasts is also related to the relationship of C, N and S assimilation pathways in these organelles that are influenced by a broad spectrum of environmental conditions (Paul and Foyer 2001). The complexity of these chloroplast regulations may be emphasize by GSH and salicylic acid (SA) biosynthesis (Kuźniak et al. 2009). The high concentrations of GSH found in the chloroplast could be to some extent dependent on import from the cytosol, although the contributions of the two compartments to GSH formation may vary between different

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cells and these processes also influence physiological status (Foyer etal. 2001). The wheat chloroplasts are able to import glutathione and this activity is not affected by either light or ATP (Noctor etal. 2002). Transcripts for one of the transporters with high specificity to GSH, cloned in Arabidopsis have been shown to be induced by xenobiotic exposure, and not by H2O2 or cadmium (Cagnac etal. 2004). Another key player in glutathione content is Cys availability, which concentration is a limiting factor for g-ECS activity (Harms etal. 2000; Droux 2004). Bick etal. ( 2001) shown that ozone exposure leads to up-regulation of sulphur assimilation in response to decrease in GSH:GSSG, via activation of adenosine 5¢-phosphosulphate reductase and a total tissue glutathione level is typically 5–20 times more abundant than free Cys and is highly mobile throughout the plant (Herschbach and Rennenberg 1995). Moreover, glycine supply for biosynthesis of glutathione from g-glutamylcysteine (g-EC) is to some extent light dependent, and has been suggested to come from the photorespiratory cycle (Kopriva and Rennenberg 2004). Together with metabolites such as O-acetylserine (Hirai etal. 2003), GSH may act as an indicator of sulphur status. A simple regulatory mechanism has been proposed with regard to the regulation of demand driven sulphur assimilation by positive signals such as O-acetylserine and negative signals such as GSH (Kopriva and Rennenberg 2004).

2.3 Mitochondria and Peroxisomes Besides chloroplasts, mitochondria are also an important cellular site for production of ROS and are themselves a target of oxidative damage. Thus, these organelles are equipped with various antioxidant defences that help to inhibit any enhancement of reactive oxygen species production. The constituents of the Asc–GSH cycle have also been localized in these organelles and GSH and GPx represent the most important antioxidant compounds in mitochondria (Jiménez et al. 1997). The Asc synthesis is related to the level of the mitochondrial electron transport chain through cytochrome c pathway and non-phosphorylating pathway related to alternative oxidase (AOX). Since these both mitochondrial pathways are involved in the regulation of photosynthesis it could be supposed that the Asc synthesis is to some extend the respond of cells to stress factors. Jiménez etal. (1998) demonstrated on pea leaves that the ascorbate–glutathione cycle compounds are also present in peroxisomes. In peroxisomes the increase of glutathione pools is a result of senescence when the strong oxidative damage takes place. The differential response to senescence of the mitochondrial and peroxisomal ascorbate–glutathione cycle compounds suggests that mitochondria could be affected by oxidative damage earlier than peroxisomes. Willekens etal. (1997) have demonstrated the specific oxidation of the glutathione pool in catalase-deficient barley suggested that the increase of GSH content may be a compensatory mechanism. Kopriva and Rennenberg (2004) pointed out that photorespiration pathway in peroxisomes may supply glycine for glutathione synthesis in normal metabolism or under stress conditions. Changes in the redox state of GSH in mitochondria correlate with changes in oxidative damage

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

of mitochondrial DNA. Thus, the GSH redox status plays an important role in controlling critical functions for mitochondrial and cell survival. In lines with altered aconitase and malate dehydrogenase (MDH) activities revealed a specific increase of ascorbate in leaves (Carrari etal. 2003; Nunes-Nesi etal. 2005), what is coupled to a general up-regulation of levels of transcripts encoding genes associated with photosynthesis (Urbanczyk-Wochniak etal. 2006). Sweetlove etal. (2007) suggest that redox signals emanating from the mitochondria are important in setting the cellular machinery to maintain the overall redox balance, and experiments on transgenic/mutant plants hint at important role for ascorbate in the co-ordination of major pathways of energy metabolism in the leaf (Nunes-Nesi etal. 2007). Chew et al. (2003) stated that the key components of the ascorbate–glutathione cycle in Arabidopsis cell organelles are encoded in nuclear genome by single organellar targeted isoforms that are dual localized in the chloroplast stroma and the mitochondria. The defined set of proteins has dual activities in the intermembrane space and matrix and many of these proteins are dual-targeted to chloroplasts and Moreover, changes in transcript level of APX, MDHAR, and GR genes were induced by oxidative stresses imposed on chloroplasts and/or mitochondria and elevated transcript levels were maintained during photosynthetic operation in the light.

3 The Action of Ascorbate and Glutathione as Important Components of the Antioxidative System The effects of oxidative stress depend on the type of cell, the level of oxidative stress experienced, and the protective mechanisms functioning in that cell type. The sensitivity of cells to oxidative stress is determined by their intrinsic antioxidant systems, in particular the levels of Asc and GSH (Banki etal. 1996). The level of antioxidants is related to the potential extent of antioxidative protection and the balance between their synthesis, oxidation, and regeneration (Boo etal. 2000; Robinson and Bunce 2000; Mittler etal. 2001; Tausz etal. 2001; Herbinger etal. 2002). Positive correlations between high antioxidant capacity and environmental stress tolerance have frequently been found. The relationship between the dehydroascorbate/ascorbate redox couples and the glutathione disulfide/glutathione (GSSG/2GSH) ratio are frequently used as markers of plant stress (Noctor and Foyer 1998). The most prominent and best established functions of Asc and GSH are those of crucial antioxidants in the Asc–GSH cycle (Arrigoni 1994; Cordoba and GonzalesReyes 1994; Noctor and Foyer 1998; Asada 1999). The Asc–GSH cycle serves the removal of H2O2, which are inevitably formed as by-products of the normal metabolism or as a consequence of environmental stresses. This cycle comprises three interdependent redox couples: Asc/DHA, GSH/GSSG, and NADPH/NADP+ (May etal. 1998).

D. Latowski et al. Protein protection

Biosynthetic pathways for defence-related phytohormones ethylene, gibberellins

Resistance to ozon

Cofactor in different reactions Regeneration of carotenes and xanthophylls

Growth regulation

Electron donor in biosynthetic reactions

Electron acceptor

Ascorbate level and/or Asc redox status (Asc/MDHA/DHA)

Regeneration of α−tocopherol

Gluthatione level and/or GSH redox status (GSSG +GSH/GSSH)

Antioxidative defence

Asc regeneration

Acclimatory processes

Protection proteins from denaturation (oxidation of protein thiol groups)

Control of defence gene expression Redox control and information transduction

Regulation of transthylakoid ∆ pH (photosynthetic electron transport stoichiometry) Thiol buffer Detoxyfication of organic compounds xenobiotics, anthocyanins

Pathogen response

Heavy metal tolerance (phytochelatin synthesis)

Fig.4 The metabolic processes influenced by ascorbate and glutathione

Plant cells synthesize Asc and use it as a hydrophilic redox buffer to provide protection against oxidative challenge (Fig.4). Asc is known as a major low molecular antioxidant in plants protecting them by directly reaction with ROS such as O2⋅−, ⋅OH, 1O2 and organic radicals. Indirectly Asc protect cells against ROS by participation in: • Ascorbate photonic energy dissipation mechanisms, such as the water–water cycle (Asada 1999), ascorbate–glutathione and xanthophyll cycles • Recycling of lipid soluble a-tocopherol by reduction of its oxidised form (Munné-Bosch and Alegre 2002) • Protection of enzymes (Padh 1990) Arabidopsis thaliana plants grown in high light (HL) accumulate more Asc and have higher ascorbate peroxidase activity than plants grown in low light (LL) (Tabata etal. 2002; Müller-Moulé etal. 2004; Bartoli etal. 2006; Dowdle etal. 2007). The Asc levels in leaves show a diurnal rhythm (Dutilleul etal. 2003; Pignocchi et al. 2003; Tamaoki et al. 2003), vary with the daylight conditions associated with long-term seasonal variations (Grace and Logan 1996) and the maintain of a constant Asc pool level is dependent on the rate of its biosynthesis (Pallanca and Smirnoff 2000; Chen etal. 2003). It was shown that leaves of tropical plants with lacking DHAR activity under HL turn yellow and accumulated high level of flavonoid pigments (Yamasaki etal. 1999). The overexpression of DHAR increases total tissue ascorbate pools (Chen etal. 2003; Chen and Gallie 2004, 2005). An increased content of Asc protects proteins and lipids against oxidative damage in plants subjected to water and salt stress (Tambussi etal. 2000; Shalata and

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

Neumann 2001). The protective effects of Asc was documented also in plants exposed to ozone (Sanmartin etal. 2003). Retsky etal. (1993) demonstrated that plants accumulate also DHA during stress. DHA is able to act as an antioxidant. The effectivity of DHA was even higher than Asc when low-density lipoprotein from copper ion induced oxidation was investigated. Parallel to Asc, glutathione, plays an important functions in cellular defence and protection. Its reduced form, GSH, exists interchangeably with the oxidized form, GSSG. Schafer and Buettner (2001) pointed out that the GSSG/2GSH couple is the major cellular redox buffer and GR may be a central determinant of antioxidant capacity by controlling the GSH/GSSG ratio. Due to the stability of the disulfide bridge in GSSG, it is more stable than Asc and tocopherol, particularly in dry biological systems (Kranner etal. 2006). In most tissues, glutathione is maintained in GSH form. However, under stressful conditions its redox status: [GSH/(GSSG + GSH)] changes and the relative amount of GSSG may be enhanced considerably (Noctor and Foyer 1998; Wingsle etal. 1999). The efficiency with which GSSG can be re-converted to GSH during the reductive inactivation of peroxides contributes to the centrality of GSH in antioxidant defences. The significant decrease in GSH:GSSG ratio, followed by glutathione pool accumulation after exposure to ozone (Gupta et al. 1991; Pasqualini et al. 2001; Bick et al. 2001). A similar response precedes leaf bleaching in maize subjected to chilling (Gomez et al. 2004). The involvement of GR in the maintenance of a high GSH/GSSG ratio in plants during stress conditions was observed in tomato under salinity, desiccated Myrothamnus flabellifolia and in wheat under drought stress (Shalata etal. 2001; Kranner etal. 2002; Kocsy etal. 2004). May etal. (1998) have also suggested that the pool size of GSH in plants enable to percept changes from the environment and it is able to up-regulate defense mechanisms involved other redox active components such as i.e., Asc, NADPH pools during environmental stress. Thus, the size of the reduced glutathione pool shows marked alterations in response to a number of environmental conditions and the activation of GSH synthesis and accumulation are a general feature of enhanced oxidation processes (Foyer and Noctor 2003, 2005, 2009). The response of the glutathione pool is also highly dependent on stress location. The concentration and/or redox state of glutathione correlates also with the adaptation of plants to abiotic and biotic stresses and tolerance of xenobiotics. The considerable increases in the pool of GSH have been measured in response to chilling, heat shock, pathogen attack, air pollution, drought, extreme temperatures and heavy metal stress (Dietz 2003; Edwards etal. 1991; Foyer and Noctor 1998; Gupta et al. 1991; Kocsy et al. 1996, 2000, 2004a,b; May et al. 1998; Madamanchi and Alscher 1991; Nieto-Sotelo and Ho 1986; Saito 2004; Parisy etal. 2007). It can react directly with a range of ROS, and indirectly throughout enzyme-catalysed reactions link GSH to the detoxification of H2O2 in the ascorbate– glutathione cycle. GSH, is the reducing co-factor for several enzymes involved in ROS detoxification, in particular peroxidases, as well as for enzymes such as formaldehyde dehydrogenase that may be important in processing products of lipid peroxidation. Additionally, in reactions catalyzed by glutathione S-transferases

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(GSTs) GSH is involved in removing lipid peroxides, methylglyoxal, and herbicides (Moons 2005; Rausch etal. 2007; Yadav etal. 2008; Szalai etal. 2009). Different types of GSTs catalyse conjugation of GSH to xenobiotics or endogenous metabolites, while others have peroxidase or isomerase activities (Edwards etal. 2000; Noctor 2006). Exposure of Arabidopsis cell cultures to oxidative stress (by the addition of aminotriazole, menadione or fenchlorazole) lead to the increase of cellular glutathione content and g-ECS at the activity but not mRNA level in the bundle sheath exposed to chilling. This phenomenom, however was not observed in mesophyll cells, what suggested different GSH content in the two types of cells (Gómez etal. 2004). The increase of g-ECS activity and GSH level was related to cadmium tolerance in tomato and chilling tolerance in maize (Chen and Goldsbrough 1994; Kocsy etal. 1996). Transgenic poplars overexpressing g-ECS has the higher, than wild-type, possibility to accumulate Cd and parallel presented the increase tolerance to chloracetanilide herbicides (Gullner et al. 2001; Koprivova et al. 2002). The enhanced synthesis of GSH together with phytochelatins in various plant species exposed to cadmium stress was also described (Mendoza-Cózatl etal. 2005). In Arabidopsis the abundance of GSH1 transcripts coded g-ECS was increased after treatment with heavy metals such as cadmium and copper (Xiang and Oliver 1998), while in maize as a response to chilling stress (Gomez et al. 2004). In Brassica juncea the abundance of GSH1 and GSH2 transcripts was increased by cadmium (Schäfer etal. 1998). Ozone and catalase deficiency are both known to trigger production of jasmonic acid (JA), which has been shown to increase GSH1 and GSH2 transcripts (Xiang and Oliver 1998; Harada etal. 2000). Moreover, the expression of the gene coding g-ECS in Arabidopsis thaliana plants exposed to chilling stress and in maize fumigated with ozone were also described (Gómez et al. 2004; Sasaki-Sekimoto et al. 2005). The expression of the GR, GST, and GPX genes and resulting in increases in the activities of the corresponding enzymes was stated in Euphorbia esula plants exposed to xenobiotic (diclofop-methyl) (Anderson and Davis 2004). However, in pea plants affected by heat stress increased GR gene expression was not accompanied by the higher GR activity (Kurganova etal. 1999; Escaler etal. 2000). Kocsy etal. (2000, 2004) have demonstrated in S-labeling experiments that osmotic stress, high temperature, and cold treatments 35 induced a greater increase in GSH synthesis in tolerant wheat genotypes than in sensitive ones. Furthermore, some authors (Durner etal. 1999; Díaz etal. 2003) presented that glutathione may be required as a relatively stable carrier of nitric oxide (NO), in the form of S-nitrosoglutathione (GSNO). Barroso etal. (2006) presented that GSNO participate in response of plants to Cd stress. Barker etal. (1996) demonstrated that the oxidative damage (i.e., by reactive nitrogen species – RNS) only occurs in the presence of a marked GSH deficiency. GSH, together with Asc are generally considered to constitute an antioxidant buffer that is involved in control of ROS level. Mittova etal. (2003) suggested that the enzymes of GSH synthesis and metabolism are induced together in response to stress which, in turn affects the level of ascorbate and glutathione.

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

4 The Role of Ascorbate–Glutathione Cycle in Regulation of the Xanthophyll Cycles Activity In almost all species of plants exists a photoprotection mechanism, strongly dependent on ascorbate, called common name “xanthophyll cycle”. Term “xanthophyll cycle” means cyclic enzymatic removal of epoxy groups from xanthophylls under strong light condition (de-epoxidation) and reverse reaction, during which the de-epoxidised xanthophylls are epoxidised. First reaction is catalyzed by de-epoxidases and the second one by epoxidases. The xanthophyll cycles take place in the thylakoid membranes. De-epoxidases are located in the thylakoid lumen and bind to membrane when the lumen becomes acidified, while epoxidases are postulated as stromal enzymes, connected with thylakoid membrane and they operate at rather neutral pH. Asc is necessary for all known types of de-epoxidases. Three xanthophylls which are de-epoxidated in nature are violaxanthin (Vx), diadinoxanthin (Ddx) and lutein-epoxide (Lx). Depending on what kind of xanthophyll is de-epoxidased three different xanthophyll cycles are distinguished (Fig.5) (Jahns etal. 2009). When Vx, undergoing de-epoxidation, the cycle is called violaxanthin cycle (Fig.5a). Since it is widely distributed among all higher plants, ferns, mosses, some green algae like Phaeophyta, Chlorophyta or some Rhodophyta (Siefermann-Harms 1985) and it was the first xanthophyll cycle which has been described in the literature (Sapozhnikov etal. (1957) it is called xanthophyll cycle. Pioneering work by the groups of Yamamoto and Hager in the 1960s and 1970s established the basic biochemical characteristics of the Vx cycle (Hager 1969) in which Vx is reversibly converted to Zx via the intermediate antheraxanthin (Ax) (Yamamoto 1979). These two-step reactions, during which two epoxy groups are stepwise removed (in the de-epoxidation reactions) or inserted (in the epoxidation reactions), are catalyzed by the Vx de-epoxidase (VDE) and the Zx epoxidase (ZE), respectively (Fig.5a). In one of marine algae species, called Mantoniella squamata, a modification of the Vx cycle was found (Goss etal. 1998). The most intriguing feature of this type of the cycle is that the second step of the de-epoxidation i.e. conversion of Ax to Zx is very slow and, as a consequence, and contrary to typical Vx cycle, Ax is accumulated in place of Zx. These observations can lead to the conclusion, that VDE from Mantoniella has less affinity to Ax than VDEs from other plants. Up till now this is the only known modification of the xanthophylls cycle involved Vx. In diatoms and some other groups of algae, the xanthophyll cycle is called diadinoxanthin cycle (Fig. 5b) because it comprises the reversible conversion of monoepoxide, diadinoxanthin (Ddx) and epoxide-free, diatoxanthin (Dtx) (Stransky and Hager 1970; Olaizola etal. 1994; Coesel etal. 2008). Analysis of genome of one of diatoms, Phaeodactylum tricornutum, indicates one VDE gene and two VDE-like genes, designated as PtVDE and PtVDL1 and 2, respectively. The first was proposed to be involved in the Vx cycle, which is also present in these organisms, while the latter were suggested to be more specialized in the chromist-specific Ddx-cycle. Proteins coded by VDL genes contain larger lipocalin domain than typical VDE and they were found only in chlorophyll c-containing chromist algae. This

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OH O O HO

VDE

Violaxanthin

VDE

Antheraxanthin

O HO

zeaxanthin Violaxanthin cycle

O HO

VDE

Diadinoxanthin

Diatoxanthin Diadinoxanthin cycle

OH O

VDE

Lutein epoxide OH

Luteina Lutein epoxide cycle

Fig.5 Three different types of xanthophyll cycles: (a) violaxanthin cycle, called also commonly xanthophyll cycle, (b) diadinoxanthin cycle, (c) epoxy lutein cycle

is why these enzymes were suggested to be able to more efficiently bind, and eventually de-epoxidise, brown algal-specific molecules such as diadinoxanthin. They were called diadinoxanthin de-epoxidases (DDE) (Coesel etal. 2008). The third type of xanthophyll cycle is lutein epoxide cycle which has been identified in green tomato fruits (Rabinowitch et al. 1975) and recently in holoparasitic Cuscuta reflexa (Bungard etal. 1999), hemiparasitic plants (Matsubara etal. 2001, 2003), Quercus species (Garcia-Plazaola et al. 2002), a tropical tree Inga sapindoides, and others (Garcia-Plazaola et al. 2007). This cycle comprises reversible light-induced changes at the level of lutein epoxide (Lx) and lutein (L) (Fig.5c). These reactions are probably catalysed by the same enzymes like Vx-cycle (Yamamoto and Higashi 1978; Goss 2003; Garcia-Plazaola etal. 2003, 2007).

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

The main postulated function of the xanthophyll cycle, connected with production of de-epoxidised xanthophylls, is to allow harmless energy dissipation by the NPQ process. NPQ is recognised as a central regulatory mechanism for protecting plants from photodamage (Arsalane et al. 1994; Olaizola and Yamamoto 1994; Frank etal. 1996; Grouneva etal. 2008, 2009; Matsubara etal. 2001, 2005; Müller etal., 2001; Garcìa-Plazaola etal. 2003). Additional postulated functions of the xanthophyll cycles are: • Protection against oxidative stress of lipids (Havaux et al. 1991; Sarry et al. 1994) • Photoreception of blue light (Quinones and Zeiger 1994; Srivastava and Zeiger 1995) • Modulation of the physical properties of the thylakoid membrane (Gruszecki and Strzałka 1991; Tardy and Havaux 1997) • Part of abscisic acid synthesis pathway (Marin etal. 1996) All known enzymes of the xanthophyll cycles have almost the same properties and requirements. Factors that influence the de-epoxidases activity include the availability of substrates, redox potential, lipid composition, temperature and pH. At low light intensities (lumenal pH around 7) the enzymes are water soluble. Upon shift to high light conditions the pH of the lumen drops, with the overall effect that de-epoxidation commences and shows an optimum around pH 5 for VDE or 5,5 for DDE. Vx de-epoxidation starts at pH-values below 6.5 while conversion of Ddx to Dtx starts at pH 7 and the de-epoxidation state increases rapidly with increasing acidification (Jakob etal. 2001). The drop in pH is necessary for binding of enzymes to the membrane but also for the protonation of ascorbate (pKa 4.1) to create acid form of reduced ascorbate (AscH) (Fig. 6) (Bratt etal. 1995). All known types of de-epoxidases require ascorbate (Asc) as reductant to carry out de-epoxidation (Hager 1969; Yamamoto 1979). The possible role of Asc availability in regulation of de-epoxidases activity was discovered when inhibition of VDE by competitive consumption of Asc by ascorbate peroxidase (APX) upon addition of hydrogen peroxide was noticed (Neubauer and Yamamoto, 1994). Bratt et al. (1995) showed that the optimum Asc concentration for VDE activity is strongly pH-dependent. At pH 4.5–5.5 the enzyme becomes saturated at 10–20 mM Asc (Table3), whereas at pH 6.0 it is not saturated even at 100 mM. The Asc concentration in chloroplasts is supposed to be in the range of 10–50 mM (Gillham and Dodge 1986; Schöner and Krause 1990; Foyer 1993) although it may increase in response to stress factors such as high light irradiances and chilling (Gillham and Dodge 1987; Schöner and Krause 1990). The Asc-dependent pH optimum is similar for both VDE located in the thylakoids and partially purified enzyme (Yamamoto 1979), albeit the partially purified VDE shows a broader pH optimum and a shift to higher pH in comparison with VDE in thylakoids (Table3). These differences may be caused by the limitation of Asc passage across the thylakoid membrane at higher pH, due to the negative charge of the base form of Asc (Asc−) (Fig.6). These results suggest that VDE has a pH-dependent KM for Asc

D. Latowski et al. AscH

CH2OH HO

DHA

pK = 4.1 - 4.2 Asc-

CH2OH HO

H O-

pK = 11.8 CH2OH HO CH

Asc2O

H O

O-

Fig.6 Different forms of ascorbate. Three reduced forms (AscH, Asc-, Asc2−) and dehydroascorbate (DHA) as oxidised form of ascorbate (Asc) Table3 The pH-optimum of VDE or DDE activity in dependence on the Asc concentration a Optimum pH of Asc concentration VDE activity in partially purified Optimum pH of partially [mM] intact thylakoids VDE activity purified DDEb 0.5 4.7 4.88 5.00 3.0 4.8–4.9 5.15 5.30 30.0 5.0 5.25 5.58 Bratt etal. 1995 Grouneva etal. 2006

(Table4) and that Asc is not simply a cofactor but a co-substrate for VDE (Bratt etal. 1995; Eskling etal. 1997). Interestingly, all determined values of the KM for Asc fell within the range of 0.10 ± 0.02 mM when Asc concentrations are expressed for the reduced acid form of Asc (AscH) and assuming a pKa of 4.1. This suggests that not the negatively charged Asc but rather the acidic form AscH is the substrate for VDE or DDE (Fig6). AscH as a protonated form of Asc is an endogenous electron and proton donor for de-epoxidation and activates VDE and DDE (Yamamoto 1979; Bratt etal. 1995; Sokolove and Marsho 1976; Neubauer and Yamamoto 1994; Eskling etal. 1997). These de-epoxidases catalyze electron and proton transfer from AscH to one or two

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway Table4 Apparent KM values (mM) of violaxanthin de-epoxidase (VDE) and diadinoxanthin de-epoxidase (DDE) for Asc at different pH values. ND - not determined

KM VDE (mM) pH 4.5 5.0 5.5 6.0 6.5

0.3 1.0 2.5 10 –

ND 2.3 ± 0.09 5.0 ± 0.56 13.8 ± 0.91 ND

KM DDE** 0.6 ± 0.05 0.7 ± 0.04 1.2 ± 0.05 6.6 ± 0.34 8.7 ± 0.79

Bratt etal. 1995 Grouneva etal. 2006

light

Lumen H

Thylakoid membrane

Stroma

CH2OH HO CH

H AscH HO

HO DE

epoxy xanthophyll

CH2OH HO

H DHA O

de-epoxy xanthophyll O

DHA

H2O

GSSG NADP

Hypothetical transporter

CH2OH HO

GSH

NADPH

H HO

O Asc-

Fig.7 Role of ascorbate in de-epoxidation of epoxy xanthophylls in xanthophyll cycle. Asc− – ascobate, AscH – reduced form of ascorbate (ascorbic acid), DE – de-epoxidase, DHA – dehydroascorbate, GSH – reduced form of glutathione, GSSG – oxidised form of glutathione

epoxide groups of epoxy-xanthophylls by that creating completely de-epoxidised xanthophylls like Zx or Dtx, water and dehydroascorbate (DHA) (Fig.7) (Hager 1969). AscH is oxidised during de-epoxidation to DHA, but no mechanism engaged in rereduction of DHA into Asc in thylakoid lumen is known and therefore a transport system for the DHA in the thylakoid membrane has been postulated (Bratt etal. 1995) although such system has never been shown. Foyer and Lelandais (1996) suggested presence of a carrier for Asc in the plasma membrane and chloroplast envelope but no transport system for Asc across the

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thylakoid membrane. Because the negatively charged form of Asc may not penetrate the membrane by simple diffusion, the transport of Asc into thylakoid lumen might be facilitated by a postulated DHA transporter in exchange for DHA (Eskling etal. 1997). This putative system may allow the rereduction of DHA to Asc in the stroma via the GSH cycle consuming NADPH and GSH and protonation of Asc to create the AscH required for de-epoxidation would finally be possible in lumen at low pH as given in the light (Fig.7). The concentration of the reduced acid form of ascorbate would be determined by the light-driven acidification of the thylakoid lumen. It was reported that plants grown at high light intensities contained higher levels of Asc and oxygen radical scavenging enzymes than plants grown under low light (Gillham and Dodge 1987; Schöner and Krause 1990). Both the Asc concetration and the amount of V, A, Z increase when plants are grown at high light intensities (Demmig-Adams etal. 1995; Logan etal. 1996). The requirement of an Asc transporter in the thylakoid membrane and the possible limitation of VDE activity by Asc consuming reactions in the stroma have further been supported by studies with isolated chloroplasts (Neubauer and Yamamoto 1994). Grouneva et al. (2006) found that partially purified DDE has a three to four times higher affinity for the AscH than VDE. Similarly to VDE (Bratt etal. 1995) the ascorbate affinity of DDE strongly depends on the pH value but the KM value of DDE for Asc at pH 5 was determined to be 0.7 mM while the KM for VDE was 2.3 mM (Table4) (Grouneva etal. 2006). At high Asc concentrations a strong shift of the pH optimum towards higher pH values was observed and DDE was found to be still active at almost neutral pH values, showing very fast and strong response in activity to small pH changes in the thylakoid lumen. It was also shown that the ascorbate affinity is an intrinsic feature of the respective enzyme and does not depend on the nature of the substrate being de-epoxidized. The high affinity of DDE for ascorbate indicates that, even at a limited availability of reduced ascorbate, high enzyme activity is possible at low pH values. On the other hand, at high ascorbate concentrations, DDE activity can be shifted towards neutral pH values, thereby facilitating a very fast and strong response to small pH changes in the thylakoid lumen (Grouneva etal. 2006). The different Asc affinities of VDE and DDE may result from differences in their amino acid sequences, where four highly conserved histidine residues have been shown to be important for the Asc binding by VDE. Substitution of the histidines in positions H121 and H124, as well as in positions H 167 and H 173, resulted both in changes of enzyme activities and Asc affinities of the modified spinach de-epoxidases (Emanuelsson etal. 2003). It should be emphasized that the two features do not correlate, meaning that a lower de-epoxidase activity does not necessarily result in a lower affinity for Asc. This indicates that the respective histidine residues might be responsible for the binding of Asc as well as for enzymatic activity. It is suggested that a modification of the histidineenriched protein region might have taken place in the diatom de-epoxidase, leading to the enhanced affinity of DDE for its co-substrate Asc. The higher Asc affinity of DDE may also reflect physiological differences between diatoms and higher plants. At a limited availability of reduced form of ascorbic acid (AscH), high enzyme activity at low pH values is possible. This situation

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

can occur invivo under high light illumination, where high Asc consumption by APX and a strongly stimulated Mehler reaction are taking place (Claquin et al. 2004). The high Asc affinity of DDE would ensure efficient Ddx de-epoxidation even if large amounts of Asc were used for other photoprotective processes. Besides, a first series of measurements of the Asc content of diatom cells suggests that diatoms, in contrast to higher plants and green algae, contain a highly oxidized ascorbate pool under high light illumination (Grouneva etal. 2006). At high Asc concentrations, DDE activity can be moreover shifted towards neutral pH values, thereby facilitating a very fast and strong response to small pH changes in the thylakoid lumen. This situation is also observed in intact diatom cells that are dark incubated for longer time periods. In these dark-adapted cells, slight pH changes in the thylakoid lumen, because of chlororespiratory electron flow, have been reported to induce significant de-epoxidation of Ddx to Dtx (Jakob etal. 2001). As shown above, availability of AscH is the main mechanism by which the ascorbate–glutathione pathway controls all types of the xanthophyll cycles, hence also all protective functions regulated by these cycles.

5 The Role of Compounds of Ascorbate–Glutathione Cycle in the Controlling Cellular Redox State and Their Involvement in Signal Transduction Pathways The redox homeostasis in plant cells is defined by the presence of pools of antioxidant compounds that have possibility to absorb and buffer reductants and oxidants. The coordinated action of different components of the antioxidative systems maintain the cellular redox homeostasis and redox metabolism as well as it is related to signal generation and signal transmission. Venis and Napier (1997) defined signal transduction as a sequence of events that is initiated by a stimulus and culminates in a characteristic physiological or biochemical response. Noctor (2005) distinguished, at least two types of stress signaling defined as “dynamic” and “static” act at a physiological level linked to redox metabolites. It is suggested that both types of signalling through redox couples may coexist. A ‘dynamic’ signaling is postulated to involved a change in redox status or concentration of redox active compounds, while ‘static’ signalling could simply draw on existing pools of redox active compounds or it is related to sensing of a threshold concentration or redox potential. Dietz (2003) grouped redox signals on three types based on their origin and mode of action in signal transmission. Type-I signals derive specifically from single pathways, type-II signals integrate redox information from various pathways and type-III redox signals indicate more extreme redox imbalances and their transmission depends on cross-talk with other signaling cascades. Similarly, Pfannschmidt etal. (2001, 2003) defined three classes of redox signals based on perturbation of the photosynthetic electron transfer depending on the light intensity. Class 1 signals originate from specific redox pairs in the photosynthetic electron transport chain, e.g. the reduced and oxidized PQ. Class 2 signals depend on the

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redox state of stromal thioredoxins, NAD(P)H as well as GSH and Asc, whereas Class 3 signals are mediated by ROS and other signaling molecules. Each of these signals has the potential to function as developmental and environmental stimulus and to change the metabolic state of a given plant cell by short term metabolic control within seconds to minutes or long term genetic mechanisms on the time scale of hours. Baier and Dietz (2005) pointed out that signals transmitted by redox shifts in cellular redox components or by ROS are distinguished from signals transmitted by second messengers synthesized inside chloroplasts. The potent signalling effects of ROS require their concentrations to be controlled by a battery of antioxidants that determine the lifetime and intensity of the signal (Mittler 2002; Mittler etal. 2004). Thus, this flexible and complex regulatory system is involved not only in maintaining the prooxidant/antioxidant equilibrium but also in a specific redox-based stress sensing and signalling (Bräutigam etal. 2009). The individual plant cell compartments like chloroplasts, mitochondria, peroxisomes, cytosol and apoplast posses different antioxidant buffering capacities determined by differences in synthesis, transport, and/or degradation that allow to control redox-sensitive signals independently. On the other hand, the interactions between these cell compartments as well as the complex network of reactions surrounding ROS generation and consumption, are crucial for redox homeostasis in metabolism (Scheibe etal. 2005), and ROS signaling (Noctor 2006; Foyer and Noctor 2005, 2009; Joo etal. 2005). Local perturbation of redox buffering system is likely an important process in the transmission of ROS signals, particularly those related to stress conditions. Specific compartment-based signaling can be achieved, among others, via differential changes in the amounts and relative reduction states of the ascorbate and glutathione pools, which are part of the cellular information-rich redox buffers (Schafer and Buettner 2001; Dietz 2003; Noctor 2006). The interactions between the redox couples such as NAD(P)/NAD(P)H, GSSG/GSH or additionally in plants DHA/AscH are involved in maintaining of cellular redox homeostasis (Fig.8). The regeneration of oxidised glutathione and the reduction of monodehydroascorbate to ascorbate by monodehydroascorbate reductase are NAD(P)H-dependent, demonstrating a metabolic link between the NAD(P)-dependent redox system and the low molecular weight antioxidants. The Asc and GSH, in addition to pyridine nucleotides such as NADH or NADPH, are the key mediators of redox transfer in the soluble phase of plant cells. This function requires that their reduced forms react slowly with oxygen and its more reactive derivatives so that oxidation is enzyme dependent (Noctor 2006; Wormuth et al. 2007). In photosynthetically active cells which are not strongly affected by stress factors, balance between the amounts of NADPH, ROS, Asc and GSH is kept. However, under stress conditions leading to ROS production, the redox balance at a cellular level may be affected. Stress signaling linked to the three redox metabolites: NAD(P), GSH and Asc have been found to be crucial to set the appropriate defence responses and to achieve a successful acclimation by plant. The redox active elements, i.e. ferredoxin, NADPH and GSH, are dependent to some extent, on the photosynthetic electron transport chain or other metabolic processes and feed electrons into the network (Dietz 2003; Kuźniak etal. 2009). The NAD(P)+/NAD(P)H, GSSG/GSH,

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

Thioredoxin Glutaredoxin Peroxiredoxins Signalling

Other metabolic and cell components

Gluthathine (GSSG +GSH/GSSH)

NAD(P)H ROS

Ascorbate (Asc/MDHA/DHA) Tocopherols

Fig.8 Potential plant cells for influence on cellular redox level and signal induction. The relationship between some components of the cell redox buffers

and DHA/AscH redox couples, integrated with the redox regulatory network, could undergo a distinct physiological functions. Redox systems involved in sensing, transferring and multiplying signals are responsible for coordination of respiration, photorespiration and alternative respiration to keep photosynthesis working at high level. These events can be viewed in terms of the interacting pathways contributing to the adjustment of metabolic processes to a new state of homeostasis, usually referred to as acclimation. Both oxidants and antioxidants fulfill signaling roles to provide information on plant defense, using kinase-dependent and independent pathways that are initiated by redox-sensitive receptors modulated by thiol status (Kuźniak et al. 2009). Thiol-based regulation may be important in plant acclimation to environmental change, particularly where redox interactions play a key role in the orchestration of the abiotic stress response (Desikan etal. 2005). In plants, thiol containing of proteins are oxidized by ROS, either directly or indirectly, to give relatively stable oxidation products with modified physical conformations or biochemical activities. The major low molecular weight antioxidants such as Asc and GSH determine the specificity of the signal. Low molecular antioxidant are a part of network associated with the mechanisms by which plant cells sense the environment and make appropriate adjustments to gene expression, metabolism and physiology (Foyer and Noctor 2005). Asc can act as a signal transducing molecule in plants (Pastori et al. 2003; Fotopoulos et al. 2008) participating in the interaction with the environment, for instance to ozone (Sanmartin et al. 2003), pathogens and oxidizing agents (Fotopoulos etal. 2006), and water stress (Fotopoulos etal. 2008). Pastori etal. (2003)

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demonstrated that the leaves of the vtc1 mutant (Table1), which have only 30% of the Asc content of the wild type, show extensive re-adjustments in gene expression that can be reversed by Asc feeding. This finding suggests that Asc is involved in metabolic cross-talk between redox-regulated pathways. However, when Asc concentrations are not supplemented by feeding, ROS do not accumulate in vtc1 mutants (Veljovic-Jovanovic et al. 2001). Moreover, it could be noted that also DHA may act as a potential factor in the signaling pathway. The chemical structure of DHA, with its vicinal carbonyl group, gives the molecule its peculiar reactivity (Deutsch 2000) with, amongst other compounds, thiols, whereby it induces oxidative protein folding (Banhegyi et al. 2003). Reversible modification of specific proteins by DHA could be important in cell signaling. It seems that Asc oxidation to DHA constitutes a vital signal transduction module governing plant reaction to stressful environmental conditions. A large fraction of total leaf DHA is probably located in the apoplast (Veljovic-Jovanovic et al. 2001; De Pinto and De Gara 2004). The Asc/DHA ratio in this compartment is postulated to be crucial in plant signalling (Pignocchi and Foyer 2003). Redox reactions within the apoplast can influence signal transduction cascades. The apoplast is also a major site of NO synthesis as ascorbate reductant (Bethke etal. 2004). It was also demonstrated that the interaction of GSH with NO the S-nitrosoglutathione (GSNO) is formed and it may interconnect the ROS- and reactive nitrogen-based signaling pathways (Neill etal. 2002) as well as it is thought to function in plants as a mobile reservoir of NO bioactivity (Durner etal. 1999; Díaz etal. 2003). Glutathione, the next component of ascorbate–glutathione cycle is an abundant metabolite in plants that functions as modulator or signal transducer (Noctor etal. 2002; Gomez etal. 2004; Noctor and Foyer 1998). GSH and GSSH may also function as signal molecules in many cellular processes. It shows strong interplay between concentration and the redox state. Glutathione pool level might be regulated, at least partially, by hydrogen peroxide and on the redox state of the plastochinon pool (Karpinski etal. 1997, 1999). Conditions that trigger accumulation of GSSG often also lead to the subsequent increase in total glutathione and are related, among others, to the stress-induced changes in the H2O2 content (Neill etal. 2002; Foyer and Noctor 2003, 2005, 2009; Noctor 2006; Dietz 2008; Quan etal. 2008). This phenomenon is observed in catalase-deficient barley (Smith etal. 1985), during exposure of poplar to ozone (Gupta et al. 1991), following chilling of maize (Gomez etal. 2004), during incompatible interactions between barley and powdery mildew (Vanacker etal. 2000) and during the developing of photoperiod-dependent strategies in the acclimation of Arabidopsis thaliana plants (Queval etal. 2007). Many observations suggest that changes in glutathione status may be as important as enhanced ROS pools in redox signaling (Creissen et al. 1999; Vanacker et al. 2000; Mou etal. 2003; Ball et al. 2004; Gomez etal. 2004; Evans et al. 2005). When GSH levels are low the cell environment will be oxidising and the functioning of enzymes, particularly those with thiol groups, will be altered. Moreover, the GSH/GSSG ratio as well as enzymes such as GR involved in controlling the GSH/ GSSG ratio are able to determined of antioxidant capacity and thus may be involved in redox signaling. The GSH/GSSG couple is able to modify the activity of various

1 Regulatory Role of Components of Ascorbate–Glutathione Pathway

compounds (enzymes, regulatory proteins) directly through the reduction/oxidation of their disulfide bridges/sulfhydryl groups and through the (de)glutathionylation of sulfhydryl groups. With the other hand GSH/GSSG couple is an indicator of the general cellular thiol-disulphide redox balance (Fig.9). The regulation of proteins by the GSH/GSSG couple may occur also due to cross-talk between GSH/GSSG and other redox systems like glutathionylation or thiol-disulfide transition, which may have a role in signaling and plant responses to stress conditions (Rausch etal. 2007). Protein glutathionylation (i.e., the formation of a reversible mixed-disulfide bond between a small-molecular-weight thiol such as glutathione and specific cysteine residues of various proteins) seems to play an important role in perception of glutathione status by the cell. Therefore, accumulation of GSSG similar like ROS-catalyzed generation of protein thiol radicals may be sufficient to trigger protein glutathionylation. Glutathionylation may also occur independently of enhanced ROS production or redox perturbation of the glutathione pool. As a target for glutathionylation two Calvin cycle enzymes such as aldolase

HOOC

H N

N H

Oxidative stress (oxidants)

NH2

COOH

Glutathlone homeostasis in the cell: synthesis, sink, partitioning, transport

GSH

NADP

NADPH

O HO

N H

GSSH O HO

NH2

N H

OH O

NH2

H N

O OH

Fig.9 The glutathione in the reduced and oxidized form. The glutathione pool avability and the redox state of glutathione pool are related to ROS level and metabolic processes

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and triose phosphate isomerase have been found (Ito etal. 2003). In Arabidopsis, oxidative stress-induced glutathionylation was described for a number of other proteins, including several GSTs and glutaredoxins (Grx) (Dixon et al. 2005). Glutaredoxins take part in transfer electrons reversibly between GSH and target proteins and they are involved in redox-regulated processes participating in stress responses (Hisabori et al. 2007; Vieira et al. 2006; Meyer 2008; Kuźniak et al. 2009). Glutathionylation could be an important redox signaling mechanism allowing cells to sense and signal harmful stress conditions and trigger appropriate responses and adaptation, and has been shown to be involved in the regulation of several signal transduction pathways (Ghezzi 2005; Hurd et al. 2005; Michelet etal. 2006; Dalle-Donne etal. 2007, 2008; Gallogly and Mieyal 2007; Ghezzi and Di Simplicio 2007; Townsend 2007; Rouhier et al. 2008, Gao et al. 2008). This posttranslational modification can protect cysteine thiols against irreversible oxidation to sulfinic or sulfonic acid, modulate enzyme activity by modification of catalytic site Cys residues or affect biological activity by competing with other thiol modifications (Foyer and Noctor 2005, 2009; Noctor 2006; Gao etal. 2008). This modification apparently occurs mainly under oxidative/nitrosative stress conditions but may also be important under normal conditions, especially for regeneration of several thiol peroxidases (Rouhier etal. 2008). Glutathione together with H2O2 and/or O2⋅− can act both as messenger molecules in cellular signal transduction pathways in several organelles and as factors in plant defense responses (del Río etal. 1996; Karpinski etal. 1997).

6 Influence of Ascorbate and Glutathione on Gene Expression Associated with Biotic and Abiotic Stress Response Apart from the well documented role of antioxidative enzymes and antioxidants in removing H2O2 and other ROS, there is also a strong interaction between oxidants and antioxidants, at the level of gene expression and translation (May etal. 1998; Karpinski etal. 1999). Asc and GSH as well as Asc/DHA or GSH/GSSG redox couples have been demonstrated as a transcriptional regulators and being regulated by metabolic processes (Baier etal. 2000; Noctor etal. 2000; Veljovic-Jovanovic etal. 2001) (Fig.10). The changes in the cellular redox balance constitute an early event in H2O2 signal transduction as reduction of the cellular redox buffer and thus the cell’s ability to maintain a high GSH/GSSG ratio potentiated the plant’s antioxidant response. The perturbs cellular redox balance may serve as an inducer for the set of defenserelated genes such as: H2O2-induced expression of glutathione-S-trasferase1 (GST1) gene (Rentel and Knight 2004) or genes of pathogenesis-related (PR) proteins (Foyer etal. 1997; Durner etal. 1998; Foyer and Noctor 2003, 2005, 2009). PR proteins are crucial components of defense mechanism and they may contribute to the innate immunity of plants. They are induced in response to pathogen infection and/or related to stress situations and are used widely as marker genes/proteins

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ROS

Disruption of cell homeostasis

Plastoquinone pool redox status (chloroplasts) NADPH/NADP (chloroplasts, cytoplasm) redox buffers: Asc/DHA(hydrophylic, chloroplasts, cytosol, mitochondria, peroxisomes, apoplast) GSH/GSSG (hydrophylic; chloroplats, cytosol, mitochondria, peroxisomes) Tocopherol (lipophylic, chloroplast membranes)

Redox balance sensors/transmiters and transcription factor activation (thioredoxins, glutaredoxins, peroxiredoxins, protein didisfide isomerases, redox-active target proteins, TFs, NPR1)

Changes in gene expression (e.i. for CuZnSOD, GR, chalcone synthase, phenylalarine ammonia lyase, pathogen related protein (PR1))

Cellular redox status

Fig.10 The role of the redox active components in the regulation of gene expression

to study the plant self-defense mechanism(s). The PR proteins are classified into 14 groups (PR-1 to PR-14) based on genes sequence or predicted sequence of amino acids, serological relationships and biological activity of their structural hom*ologues within the groups (van Loon and van Strien 1999). The low levels of Asc, can act as an elicitor of pathogen resistance responses and leads to the induction of PR genes (PR-1, PR-2 and PR-5), as shown by the analysis of the vtc1 and vtc2 mutants (Pastori etal. 2003; Conklin and Barth 2004; Barth etal. 2004; Pavet etal. 2005). Feeding Asc to leaves of Asc deficient vtc1 mutant represses the accumulation of PR-1 transcripts, whereas the levels of PR-5 and PR-2 transcripts were not significantly changed (Pastori et al. 2003) and these effects are accompanied by enhanced glutathione level (Mou etal. 2003; Gomez etal. 2004). Horling etal. (2003) demonstrated that the treatment of Arabidopsis plants with ascorbate declined in shoots of these plants the level of transcript of the other proteins engaged in stress response such as the chloroplast-targeted peroxiredoxins namely the two-cysteine (2-Cys) peroxiredoxins (prx Q and prx II E). These enzymes are involved in protecting photosynthesis. Baier etal. (2000) point out that the decrease of 2-Cys peroxiredoxins in transgenic A. thaliana plants caused up-regulation of genes related to ascorbate metabolism such as stromal ascorbate peroxidase and particularly monodehydroascorbate reductase, while gene products like glutathione

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reductase involved in glutathione metabolism were unaltered similarly to catalase, and superoxide dismutase. On the hand Lamkemeyer etal. (2006) observed alter transcripts of glutathione-related genes in Prx Q knockout plants. The expression of defense-related gene is also related to the absolute amounts of glutathione as well as the ratio of GSH to GSSG. The oxidation of even a small part of the GSH in a cell can shift significantly the GSH:GSSG ratio, with consequent implications for patterns of gene expression (Hwang etal. 1992; Sen 2000). Such changes allow the organism to respond to oxidative stress caused by altered cellular or environmental conditions. As it was presented by some authors changes in the redox state of GSH in plants exposed to HL stress are associated with the increased activity of heat shock transcription factors (HSFs) and raised abscisic acid (ABA) level (Jabs et al. 1996; Karpinski etal. 1997, 1999; Fryer etal. 2003; Ball etal. 2004; Chang etal. 2004; Mateo etal. 2006; Kuźniak etal. 2009). Moreover, both GSH and GSSG elicit phenylalanine ammonia-lyase (Phe ammonia lyase) enzyme activity and phytoalexin accumulation (Edwards etal. 1991). It has been demonstrated that the promoters of genes involved in the synthesis of phytoalexins include GSH-responsive elements (Levine et al. 1994). In carrot, inhibition of glutathione synthesis induced phytoalexin accumulation (Guo etal. 1993). Exogenous application of GSH increased level of Phe ammonia lyase and chalcone synthase transcripts in bean cell suspensions (Wingate etal. 1988). Chalcone synthase (CHS) is pivotal for the biosynthesis of flavonoid antimicrobial phytoalexins and anthocyanin pigments, catalyzes the first and key regulatory step in the branch of phenylpropanoid biosynthesis pathway specific for synthesis of flavonoid pigments and UV protectants (Ferrer etal. 1999; Ryder etal. 1987). Parisy etal. (2007) demonstrated that the low GSH concentration in the pad2-1 mutant (Table2) did not affect the transcript abundance of g-ECS and GSHS. Hovewer, after inoculation of this mutant with Phytophthora brassicae gene expression for these enzymes was much more strongly induced than in the wild type. Ball etal. (2004) determined the transcriptional response of the allelic glutathione biosynthetic mutants rax1-1 and cad2-1 (Table2). Typical target genes with altered transcript accumulation under stress are those for PR proteins, e.g. PR1, chitinase, and PAL (encoding phenylalanine ammonium lyase), and antioxidant enzymes as peroxidases, dehydroascorbate reductase and CuZn-superoxide dismutase. The sets of target genes widely overlap with the transcripts induced by pathogens and wounding (Cheong etal. 2002; Mahalingam etal. 2003). It is also observed that GSH as a one of thiols similarly to thioredoxin and Cys enhance nuclear transcription factor kappa (NF-k) in mesophyll cells of maize leaves and this suggests that redox control of translation is a key determinant of protein abundance in this plant (Pastori etal. 2000). The demonstration of thiol modulation of nonexpressor of pathogen related genes (NPR1) and its interacting transcription factors (Després etal. 2003) potentially resolves some of the issues surrounding pathogen-induced changes in glutathione status (Edwards et al. 1991, Vanacker etal. 2000) as well as induction of PR genes in plants in which glutathione pools are enhanced by g-ECS overexpression or in response to the decrease of antioxidative enzyme capacity (Creissen etal. 1999; Rizhsky etal. 2002). In Medicago trunculata

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roots GSH synthesis pathway is regulated by NO-donors (sodium nitroprusside and nitrosoglutathione) treatment and g-ESC and GSHS genes for gamma- glutamylcysteine synthetase (g-ECS), glutathione synthetase (GSHS), respectively are up regulated (Innocenti etal. 2007). Asc and GSH and Asc/DHA or GSH/GSSG redox couples may regulate the expression of genes involved in plants stress response with the one side, but synthesis of these molecules as well as metabolism is affected by the abiotic and biotic stress factors. Enzymes involved in Asc and GSH metabolism (Szalai etal. 2009) are regulated at different levels in various plant species and the control of the corresponding enzymes may depend on the organ and cell type and on the developmental stage.

7 Relationship Between Different Components of Ascorbate–Glutathione Cycle and Metabolic Processes Involved in Plant Defense Response The induction of defence pathways is a key feature of response of all organisms to stress. The dynamic interactions between cell compartments are important for effective stress signal relay and the induction of defense mechanisms. Under stress conditions plants regulate their resource allocation in a way that increases stress tolerance (Mateo etal. 2004; Feys etal. 2005). Plant defense may be modulated simultaneously in response to changes in Asc availability via changes in the ABA and ABA/giberelin acid (GA) signaling pathways. The abundance of key transcripts is modulated by Asc availability in leaves. Thus, plants sense not only changes in the redox state of their Asc pool but also the absolute amount of this major redox buffer (Foyer 2004). Asc plays a crucial role in protection and regulation of photosynthesis, acting among others in the Mehler peroxidase reaction with ascorbate peroxidase to regulate the redox state of photosynthetic electron carriers (Foyer and Allen 2003). Futhermore, higher Asc levels may support the growth of plants under conditions of high salinity (Shalata and Neumann 2001). In opposite to these data, it was demonstrated that the deficient in Asc mutant vtc1 (Table1) is more resistant to bacterial and fungal pathogens (Barth et al. 2004). Millar et al. (2003) showed that Asc synthesis and hence stress responses in plants is controlled by respiration. Asc and its metabolic precursors give rise to oxalic acid (OxA) found in calcium oxalate crystals in specialized crystal idioblast cells in plants as well as for soluble oxalate accumulated in leaf vacuoles (Wagner 1981; Kostman et al. 2001). The accumulation of OA crystals in plant tissues is suggested to be involved in regulation of cellular calcium levels and sequestration of toxic metals, and to confer resistance to herbivory (Deutsch 2000; Horner etal. 2000; Keates etal. 2000; Franceschi and Nakata 2005). Asc participates also in synthesis of ethylene, gibberellins and anthocyanins (Smirnoff and Wheeler 2000; Smirnoff 2000). Asc, functions also as a biosynthetic precursor of l-tartaric acids (TA). TA plays a critical role in determining the suitability of

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grapes for use in winemaking – berry. TA is largely responsible for controlling juice pH and through TA addition during vinification, the winemaker can minimize oxidative and microbial spoilage, thereby promoting both organoleptic and ageing potentials of the finished wine (DeBolt etal. 2006). Moreover, Asc, GSH and H2O2 function as upstream/downstream components of hormone-mediated signal transduction. Some authors (Chen and Gallie 2006; Fotopoulos etal. 2008) described altered stomatal behaviour in ascorbate oxidase (AO) over-expressing tobacco plants, and they suggested that DHA may act as an early activator of stomatal closure thus improving the control over transpiration. Under stressful conditions Asc oxidation to DHA take place and, in turn, this molecule can modulate plant responses to stress by regulating ABA synthesis and increasing hydrogen peroxide production (López-Carbonell etal. 2006). Involvement of apoplastic Asc, in stress responses enable plants to cope with unfavorable conditions by modulating plant growth. Asc and AO in the apoplast are key players in both cell division and cell elongation. Many environmental and metabolic factors influence the rate and direction of cell elongation (Gonzalez-Reyes etal. 1994). DHA is believed to signal the redox state of the apoplastic environment, and hence to allow the cell to perceive stress in the environment. DHA accumulation in the apoplast may trigger the arrest of cell division (Potters etal. 2000). Glutathione is abundant and ubiquitous thiol with proposed roles in the storage and transport of reduced sulphur, the synthesis of proteins and nucleic acids and as a modulator of enzyme activity. Stable protein disulphide bonds are relatively rare except in quiescent tissues such as seeds, where GSSG is allowed to accumulate. Moreover, GSH is a substrate for several reductive enzymes, including enzymes that reduce peroxides (Foyer and Noctor 2005a,b). Glutathione is used also in detoxification of reactive ketoaldehydes such as methylglyoxal what is a potential target for engineering tolerance to stresses such as high salinity (Singla-Pareek etal. 2003; Noctor 2006, Noctor and Foyer 2005). Other indirect roles of glutathione are in heavy metal detoxification as a precursor to phytochelatins (Cobbett and Goldsborough 2002) and as a transport form of Cys (Kopriva and Rennenberg 2004). Cold treatments and the certain metals such cadmium or copper that favour ROS production can induce transcripts for the enzymes of glutathione synthesis (Rodríguez-Serrano et al. 2009). The redox changes in the glutathione pool as well as changes in hydrogen peroxide levels are genetically and functionally interconnected with the SA signaling pathway regulating light acclimatory processes (Karpinska et al. 2000, 2003; Mateo et al. 2004, 2006). Glutathione is able to induce PR transcript induction (Gómez etal. 2004a,b), whereas localized cell death occurs in Asc deficient plants (Pastori etal. 2003). These effects point to opposing functions for GSH and Asc in redox signal transduction. A striking relationship between glutathione oxidation and mitochondrial DNA damage during aging has also been reported (Yen et al. 1994; Esteve et al. 1999). In addition, accumulation of GSSG is often associated with tissue death or quiescence (Wachter etal. 2005). Kranner etal. (2006) demonstrated that the alteration in the half-cell reduction potential of the GSH/GSSG couple (EGSSG/2GSH), a major cellular antioxidant and redox buffer is part of the

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signaling cascade that initiates programmed cell death (PCD), finally causing internucleosomal DNA fragmentation in the final, or execution phase, of PCD (Kranner et al. 2006; Schafer and Buettner 2001). Changes of the redox state of glutathione was demonstrated, among others, as the response to excess light stress (Karpinski etal. 1997, 1999; Panchuk etal. 2002; Fryer etal. 2003; Ball etal. 2004; Chang etal. 2004). Acknowledgement This work was supported by the Polish Ministry of Science and Higher Education (project No. 50/N-DFG/2007/0).

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Chapter 2

Ascorbate and Glutathione in Organogenesis, Regeneration and Differentiation in Plant In vitro Cultures Jarosław Tyburski and Andrzej Tretyn

Abstract The prerequisite for shoot, root or somatic embryo formation in plant in vitro culture is the development of meristem from dedifferentiated cells of the explant tissue. Auxin and cytokinin levels and their relative ratios play a decisive role in inducing the morphogenetic pathways leading to shoot, root or somatic embryo formation in plant invitro cultures. Exogenous auxin is required to maintain the high rate of an unorganised growth in plant cell suspension cultures. On the other hand, the proliferation of hairy root cultures is usually dependent on endogenous hormonal factors. Auxin and cytokinin execute their regulatory role by being involved in a cross-talk with numerous endogenous factors affecting cell division and differentiation. Among them, ascorbate/dehydroascorbate (ASC/DHA), glutathione/glutathione disulphide (GSH/GSSG) redox pair, H2O2 and other components of cellular redox systems play an important role in triggering developmental responses in plant in vitro culture. Ascorbate, glutathione and related enzymes participate in the responses to auxin/ cytokinin treatments. In addition, they can even directly affect hormone metabolism in tissue. Ascorbate and glutathione have important regulatory roles in the process of cell-cycle progression within the meristems, where they participate in redox-dependent determination of proliferation and quiescence patterns. The mechanism underlying the regulatory effects of ascorbate and glutathione in cell divisions is not fully elucidated; however, it seems to be related to the regulation of nucleotide synthesis. Ascorbate levels in apoplast modulate the rate of organ elongation by increasing cell wall extensibility. Besides the effects on cell proliferation and growth, ascorbate and glutathione concentrations as well as the enzymes of their metabolism protect the invitro cultured tissues against oxidative stress. This function is of particular importance during root regeneration and the elicitation of metabolite production by hairy root cultures, where increased levels of oxidising agents are often required to stimulate both processes. In this review, we report recent studies on the involvement of ascorbate and glutathione in the processes of regeneration and proliferation in plant tissue culture.

J. Tyburski (*) and A. Tretyn Department of Biotechnology, Institute of General and Molecular Biology, Nicolaus Copernicus University, Gagarina 9, 87-100 Toruń, Poland e-mail: [emailprotected]

N.A. Anjum et al. (eds.), Ascorbate-Glutathione Pathway and Stress Tolerance in Plants, DOI 10.1007/978-90-481-9404-9_2, © Springer Science+Business Media B.V. 2010

J. Tyburski and A. Tretyn

Keywords Ascorbate • Glutathione • In vitro • Rooting • Shoot regeneration • Somatic embryogenesis

1 Regenerating Plants from In vitro Cultured Cells, Tissues and Organs The capacity of plant cells to undergo inducible morphogenetic pathways is essential for various biotechnological applications of plant cell culture, such as clonal propagation and genetic transformation. Morphogenesis in plant tissue culture may occur in two ways. It may result in the formation of bipolar structures called somatic embryos, equipped with shoot and root meristem. Alternatively, organogenesis may occur leading to the formation of unipolar structures, i.e. shoots or roots. Whereas, a somatic embryo directly develops into a complete plantlet, shoot organogenesis must be followed by root organogenesis at the base of a shoot before the newly regenerated plant is transferred to ex vitro conditions (Warren 1993) (Fig.1). The process of somatic embryogenesis may be divided into two phases: induction and expression. During the induction phase, differentiated somatic cells of an explant, undergo de-differentiation, acquire embryogenic competence and proliferate as embryogenic cells. In the expression phase, the embryogenic cells differentiate to form somatic embryos (Namasivayam 2007). In few experimental models, somatic embryos follow a sequence of development comprising the stages of embryo formation similar to those, which are observed during zygotic embryogenesis, i.e. globular, heart shape and torpedo. Such developmental pattern occurs during embryogenesis of carrot callus cells cultured as a suspension in liquid medium. This system is one of the most controllable somatic embryogenesis processes, and is often used as a model system in studies on cell differentiation. However, usually, developmental events that give rise to a somatic embryo in culture show far more variation than the equivalent process in the ovule. Somatic embryos exhibit a much greater range of sizes and shapes than the zygotic embryos (Warren 1993).

Fig.1 Most important developmental pathways in plant tissue culture. See text for details

2 Ascorbate and Glutathione in Organogenesis

Somatic embryogenesis can proceed as a direct or indirect process. In direct embryogenesis, which is a relatively rare event, embryos develop directly on the surface of organised tissue such as leaf, stem, zygotic embryo, etc. Alternatively, the much more common indirect somatic embryogenesis requires an intermediary step of meristematic cluster formation preceding the embryo development. This kind of embryogenesis often occurs in cell suspensions and callus culture (Namasivayam 2007). Typically in meristematic clusters, only a few surface cells give rise to embryos; however the presence of cells that do not undergo embryogenesis is necessary within the cluster to complement cells, which are directly involved in embryo formation (Kreuger and van Holst 1993). Induction of plant explants for somatic embryo formation generally requires a pretreatment on auxin-supplemented medium. Among auxins, the most frequently used was 2,4-diphenoxyacetic acid (2,4-D). However, other auxins like b-naphtoxyacetic (NAA) and indole butyric acid (IBA) were also used for this purpose (Razdan 2003). The formation of meristematic clusters containing embryogenically competent cells occurs during the period of auxin treatment. On the other hand, the following phase, when competent cells develop into embryos, requires the reduction or removal of auxin from the medium. This is achieved by transferring cells to a new medium with a very low level of auxin or no auxin at all (Razdan 2003). Nevertheless, auxin treatment is most frequently used to induce somatic embryogenesis. However, the effect of other plant growth regulators should not be overlooked (Jimenez 2001). For example, in the case of nucellus cultures of Vitis, simultaneous presence of NAA and benzylaminopurine (BAP) were inductive for embryogenesis. In some systems, somatic embryogenesis was induced by abscisic acid (ABA) (Nishiwaki etal. 2000) or even occurred on hormonefree medium (Choi etal. 1998). Plant organogenesis invitro is a more controllable process than embryogenesis (Warren 1993). Different morphogenetic pathways, i.e. shoot or root formation, may be induced in culture by application of the appropriate hormones in the medium. Although the exact nature of these hormonal signals may vary between species, the balance of auxin to cytokinin has been found to have a consistent effect on the type of regenerated organs. A relatively high ratio between auxin and cytokinin promotes the regeneration of roots. Whereas, shoot organogenesis is a preferred type of differentiation on culture media supplemented with high cytokinin and low auxin concentration (Skoog and Miller 1957; Christianson and Warnick 1983). The explants cells proliferate to form callus when the same concentrations of auxin and cytokinin are added to the medium. Callus cells are usually grown on solid media. Alternatively, long term cultures of friable callus may be grown on liquid media as cell suspension cultures (Zhao etal. 2008) (Fig.1). Root cultures may be derived without the use of exogenous growth regulators by infecting plant explants with Agrobacterium rhizogenes. This gram-negative soil bacterium transfers a DNA segment (T-DNA) from its large root-inducing (Ri) plasmid into the genome of the infected plant. This T-DNA carries a set of genes that encode enzymes which control auxin and cytokinin biosynthesis. The new hormonal balance induces the formation of proliferating roots, called hairy roots, that emerge at the wounding site. The hairy root phenotype is characterized by fast

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hormone-independent growth, increased lateral root branching and genetic stability (Guillon etal. 2006) (Fig.1). Temporal requirements for a specific balance of phytohormones for the organogenesis process indicate that organ regeneration is accomplished in three phases (Christianson and Warnick 1983). In the first phase, cells of the explants acquire competence, which is defined as the ability to respond to hormonal signals. The competence acquisition involves dedifferentiation of explants cells, which re-enter cell cycle. Competent cells are then canalized and determined for specific organ formation under the influence of phytohormone balance through the second phase, referred to as induction phase. Organ primordia differentiate from induced explants cells and their further development and outgrowth occurs during the third phase. These processes usually proceed independently of the exogenously supplied phytohormones (Christianson and Warnick 1983; Sugiyama 1999). Studies using Arabidopsis temperature sensitive mutants (srd1, srd2, srd3) defective for shoot and/or root redifferentiation revealed further complexity of the process of competence acquisition (Ozawa et al. 1998). It has been found that hypocotyl explants grown on callusinducing medium first become competent in root organogenesis and then gain competence in shoot organogenesis. Therefore, dedifferentiation stage is divided into two sub-phases occurring sequentially one after another. Finally, explants cells are competent in regeneration of both roots and shoots. The transition from an incompetent state to competence in root organogenesis and from competence in root organogenesis to competence in root and shoot organogenesis requires the functions of SRD2 and SRD3 gene, respectively (Ozawa etal. 1998). In spite of more than 50 years of studies, molecular and biochemical processes underlying morphogenesis in tissue culture are not fully understood. However, the growing list of genes which are known to be specifically involved in organogenesis and/or somatic embryogenesis mark significant advances in the field of elucidating the mechanism of plant regeneration in vitro (Philips 2004). It was found that the SRD2 gene, involved in control of proliferation competence and dedifferentiation, encodes for a nuclear protein responsible for activation of snRNA transcription. At present, it remains unclear which molecular event, subsequent to the activation of snRNA transcription, is responsible for the elevation of cell proliferation competence (Ohtani and Sugiyama 2005). Re-entering the cell cycle by quiescent cells during dedifferentiation is correlated with expression of cell cycle-related genes such as cyclins and cyclin-dependent kinases (CDK). Among them, gene cdc2At coding PSTAIRE domain-containing CDK and CYCD3 coding a D-type cyclin are possibly involved in committing the dedifferentiating cells to the cell cycle (Sugiyama 1999). A developmental pathway leading to shoot organogenesis was found to be related to genes involved in cytokinin perception and signalling (Sugiyama 1999, 2000). The establishment and maintenance of shoot meristem is dependent on meristem identity genes such as SHOOT MERISTEMLESS (STM), WUSHEL (WUS) and CLAVATA (CLV). These genes function for the establishment of shoot apical meristems only after dedifferentiated cells are determined for shoot organogenesis (Philips 2004). In accordance with the essential role of auxin in root regeneration, an important role of genes engaged in auxin perception and signalling was also

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identified. The ROOTING AUXIN CASCADE (RAC) gene coding for Rac/Rop GTPase (Tao etal. 2002) mediating an auxin-signalling pathway is involved in an early stage of auxin perception specific to the formation of adventitious roots (Sugiyama 1999). Similar to organogenesis, the process of somatic embryogenesis involves reprogramming of gene expression patterns. Vegetative-embryonic transition is marked by changes in expression of gene coding for somatic embryogenesis receptor-like kinase (SERK), which was identified as a specific marker distinguishing individual embryo-forming cells from non-embryogenic cells in carrot suspension culture. The transcription factors LEAFY COTYLEDON 1 (LEC1) and WUSHEL (WUS) are possibly involved in inducing and maintaining embryogenesis in culture (Namasivayam 2007).

2 Interaction of Ascorbate and Glutathione with Auxin and Cytokinin A decisive role in regulating the morphogenetic pathways in plant tissue culture is attributed to auxin and cytokinin. Below, we describe possible links between these growth regulators and several aspects of ascorbate and glutathione metabolism. These interactions may possibly be important for understanding the roles of these antioxidants in differentiation and growth in plant invitro culture.

2.1 Auxin An apoplastic enzyme ascorbate oxidase (AOX) provides a clear link between regulatory action of auxin and ASC. Highest levels of AOX activity were detected in young and growing parts of the plant. For example, AOX activity in rapidly growing immature pumpkin fruits was 15–20 times higher than that in mature fruits, which no longer increase in size (Esaka etal. 1992). In tobacco plants, high levels of AOX transcript were detected in young and growing parts like upper leaf, upper stem and root but little or none in old tissues such as lower leaf and lower stem (Kato and Esaka 1996). Similarly, in tomato seedling roots, AOX activity was stimulated by auxin treatment in the apical part of the organ, whereas no stimulation was observed in the proximal part of root (Tyburski etal. 2008). The relation between auxin and AOX expression and activity in pumpkin fruits was demonstrated by Esaka et al. (1992). AOX activity in pumpkin fruit tissue, cultured invitro, strongly increased when culture medium was supplemented with synthetic auxin, 2,4-D. It was shown that the enzyme’s activity is regulated by auxin at the level of gene expression. Gene expression analysis showed that AOX mRNA level also increased after transfer onto the culture medium in the presence of 2,4-D. The highest steady-state level of AOX mRNA appeared 2 days after transfer, and decreased thereafter. In the absence of 2,4-D, no increase in AOX transcript

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level was detected in fruit tissue (Esaka et al. 1992). AOX expression was also induced by auxin in tobacco leaves and this induction was associated with stimulation of plant growth (Pignocchi etal. 2003). AOX is able to react with indole-3-acetic acid (IAA) as a substrate at least invitro. It was demonstrated that AOX may effectively decrease IAA concentration in radish roots via oxidative decarboxylation forming, as a main product, oxindole-3-methanol (Kerk etal. 2000). While the principal mechanism of auxin turnover in shoots engages non-decarboxylative pathway (Woodward and Bartel 2005), the analysis of IAA metabolite in various parts of root tissue shows that decarboxylation is a main IAA-degradation pathway in roots. Oxidative decarboxylation occurred almost exclusively in the root tip with possible participation of AOX. This finding implicates an existence of a regulatory loop involving AOX, IAA and ASC in the root tip cells (Kerk etal. 2000). Further insight into the relation between AOX and auxin was provided by studies on AOX-over-expressing tobacco plants. It was demonstrated that overexpression of AOX results in oxidation of the redox state of apoplast, which is followed by the reduction in sensitivity to exogenous auxin. While wild-type seedlings reacted to spraying with 0.5 mM NAA with enhancement of shoot growth (i.e. advanced development of cotyledon and leaves) and increase in fresh weight, AOX–over-expressing seedlings, submitted to the same treatment, accumulated 25–30% less biomass. Although lateral root proliferation following auxin treatment was observed in both, control line and AOX–over-expressing line, this effect was much more evident in wild-type seedlings. Desensitisation to auxin in AOX-over-producing plants was related to constitutive induction of auxin signalling pathway in these plants (Pignocchi etal. 2006). Auxin stimulates ROS production in apoplast and employs them as mediators in the regulation of plant growth reactions (Joo etal. 2001; Schopfer etal. 2002; Liszkay etal. 2004). Because apoplast is already oxidised in AOX-over-expressing plants, a further stimulation of ROS production by auxin has minimal effects on oxidative signalling processes leading to auxin insensitivity. Moreover, the activity of auxin-dependent MAP kinase pathway was doubled in leaves of AOX-over-expressing seedlings when compared to control wild-type seedlings. These data emphasise the role of apoplast redox-mediated changes in developmental reactions of plants to auxin treatments (Pignocchi etal. 2006). Besides AOX, the effects of auxin treatment on the activities of other ASC metabolising enzymes were reported. Changes in the activities of AOX and Halliwel–Asada cycle enzyme activity were analysed in the proximal and distal parts of tomato seedling roots grown invitro (Tyburski etal. 2008). Roots respond to auxin treatment with an increase in lateral root formation which occurs primarily in the proximal part of root and concurrent inhibition of root elongation, which affects root elongation zone localised in proximity of the root apex (Cleland 2004; Tanimoto 2005). It was demonstrated that a 3-day-long treatment with 1 mM IAA decreased the activities of ASC-regenerating enzymes, i.e. monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR) within both, distal (elongating) and proximal (lateral root producing) part of the root. ASC-oxidising

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enzymes were stimulated by IAA. However, auxin’s stimulatory effect on ascorbate peroxidise (APX) activity was restricted to proximal part of the root, whereas, AOX activity increased exclusively within apical part of the root. These changes were accompanied with an increase in the participation of DHA in total pool of ascorbate. These findings show that the effects of exogenous auxin on ascorbate metabolism depend on the zone of root which is exposed to an auxin stimulus (Tyburski etal. 2008). Generally, auxin application is usually followed by increased oxidation of the ascorbate pool due to stimulatory effects on AOX or ascorbate peroxidase (APX) activity and/or expression. This finding is consistent with an overall mode of auxin action, which triggers auxin-specific signalling pathways, oxidising the cellular environment (Joo etal. 2001; Schopfer etal. 2002).

2.2 Cytokinin A role of endogenous cytokinin in regulation of ASC and GSH enzyme activity was studied using transgenic tobacco plants expressing supplementary ipt – gene under a control of the constitutive promoter for small subunit of RUBISCO (Pssu-ipt). Over-expression of bacterial ipt gene encoding isopentenyl transferase, a key enzyme in cytokinin synthesis, in plant cells led to an increase in endogenous cytokinin content in transgenic plants. When compared to non-transformed control, active forms of cytokinins increased seven times in transgenic shoots grafted onto nontransgenic rootstock and 12 times in transgenic shoots propagated invitro. Tobacco plants over-expressing ipt, exhibited typical traits of cytokinin over-production syndrome, including reduced root growth, reduced apical dominance, and retarded plant and leaf senescence. Transgenic plants were also characterised by alterations in water regime, disturbance in photosynthesis and over-expression of PATOGENESIS RELATED PROTEIN 1 (PR1). These findings were interpreted as symptoms of permanent stress, which is caused directly and indirectly by cytokinin over-production (Synková etal. 2004. Transgenic plants produced more H2O2 than control plants; however, higher activities of ascorbate/glutathione enzymes were detected in cytokinin over-producing plants as well. APX activity was two times higher in transgenic plants grown either invitro or invivo conditions when compared to the control. Glutathione reductase (GR) activity was also doubled in ipt transformants, however, this increase was observed only in invitro cultured plants, whereas there was no difference in GR activity between control and transgenic plants grown in ex vitro conditions. In addition, catalase (CAT) activity was stimulated in transgenic plants; on the other hand, superoxide dismutase activity was halved. These findings suggest that cytokinins may regulate H2O2 levels by changing the activities of principal enzymes of H2O2 turnover (Synková etal. 2004, 2006). Cytokinin over-expression had important consequences for redox metabolism during senescence. This process is, to a large extent, dependent on ROS accumulation in chloroplasts, which causes chlorophyll bleaching, lipid peroxidation and eventual loss of chloroplast integrity (Navabpour etal. 2003; Bhattacharjee 2005).

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Control plants exhibited high activities of antioxidant enzymes in early stages of plant development, i.e. until the onset of flowering, and a decline in later stages. Contrary to the control, transgenic plants showed increase in CAT, GR and APX in later stages of development, i.e. during flowering and forming seeds. These data suggest that cytokinin control of senescence involves the regulation of the activities of aforementioned H 2O2-processing enzymes (Synková etal. 2006). Similar effects of exogenously applied cytokinin were demonstrated. Wheat leaf segments treated with 10−4 M BAP exhibit delayed senescence, which manifests itself in the retention of chlorophylls and chloroplast proteins (Zavaleta-Mancera et al. 2007). Consequently, during incubation period following exposure to BAP, chloroplasts in the cells of treated leaf segments remained intact significantly longer, whereas those of control segments exhibited various symptoms of degradation. BAP-treated leaves accumulated significantly lower levels of H2O2. This finding together with increasing the content of xanthophylls, at least partly, explains delayed senescence of leaves pre-treated with BAP. Decrease in H2O2 was due to enhanced activity in CAT and APX enzymes. During 6-day-long incubation period, in control leaves CAT activity decreased from the fourth day on and APX activity decreased on the sixth day. BAP treatment prior to incubation period efficiently prevented the decrease in the activities of both enzymes. It was concluded that the mechanism of cytokinin-dependent delay in leaf senescence involves the reduction in H2O2 levels due to the hormone’s stimulatory effect on CAT and APX activities (Zavaleta-Mancera etal. 2007).

3 Ascorbate and Glutathione as Regulators of Cell Division in the Root Apical Meristem There is a growing body of evidence indicating that the role of ascorbate and glutathione in plants extends beyond their intensively explored antioxidant function (De Pinto and De Gara 2004). It has been demonstrated that high ascorbate and glutathione levels are required for normal progression of the cell cycle in meristematic tissues (Liso etal. 1984, 1988; Potters etal. 2000; Vernoux etal. 2000; Jiang etal. 2003). Ascorbate is directly involved in the regulation of two processes that mediate morphogenic responses in plants: cell division and elongation. The reduced ascorbate (ASC) as well as oxidised form of this compound (dehydroascorbate, DHA) play an important role in the regulation of mitotic activity in the meristems (De Tullio etal. 1999; De Pinto etal. 2000; Potters etal. 2002). It was observed that ASC promotes cell-cycle progression in the root apical meristem by stimulating G1-S transition. If ASC is added to the cells of the root quiescent centre (QC), it induces these normally non-dividing cells to pass from G1 into the S phase (Liso etal. 1988). A direct link between ASC/DHA and GSH/GSSG redox states and auxindependent regulation of mitotic activity in discrete parts of the apical root meristem

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has been demonstrated. The apical root meristem comprises of proximal meristem (PM), which produces the tissues of the root proper and the distal meristem (DM) that produces root cap. Between the actively dividing cells of PM and DM, the population of slowly dividing cells, called quiescent centre (QC) is localised. QC cells spend extended periods of time in G1 phase, dividing, on average, every 200 h (Jiang etal. 2006). It was demonstrated that laser ablation of QC cells leads to differentiation of the initial cells previously attached to it. Therefore, it was concluded that the function of QC is maintaining the dedifferentiated state of initial cells in adjacent meristems in a cell-contact-dependent manner (Bonke etal. 2005). Differences in the cell division rate between the meristems and QC depend on the polar transport of auxin from shoot apex, where the hormone synthesis takes place, to the root tip. Directions of hormone transport in the root are determined by the patterns of PIN family auxin – efflux carrier protein expression (Friml 2003). Auxin is transported acropetally towards the root tip through the root stele tissue employing cells that express PIN1 auxin efflux carrier (Friml 2003). The hormone is transported to QC and columella initials where high levels of auxin accumulate (Kerk and Feldman 1995). Part of auxin is further transported from QC to columella by PIN4 protein. An auxin efflux protein PIN3, specifically expressed by columella cells is localised in lateral sides of plasma membrane that enables basipetally directed lateral auxin transport through lateral parts of the root cap and cortex/ endoderm cells expressing PIN2 auxin carrier (Benkova etal. 2003; Friml 2003; Blilou etal. 2005). It was demonstrated that auxin accumulation in QC switches the redox balance in the cells towards a more oxidised state (Jiang etal. 2003). When compared to rapidly dividing PM initials, cells of the QC are characterised by the elevated levels of DHA (×1,000), GSSG (×10), O2·− (×15.6) and H2O2 (×34). On the other hand, rapidly dividing cells of PM contain reduced forms of ASC and GSH and do not accumulate ROS. Auxin-induced oxidised intracellular environment plays a decisive role in maintaining the low cell division rate in QC (Kerk and Feldman 1995; Jiang etal. 2003). Treating roots with an inhibitor of auxin polar transport, such as NPA, results in a relocation of auxin maximum from QC to cortical and procambial region of proximal meristem. This alteration in auxin distribution was followed by mitotic activation of QC. This correlated with the development of less oxidised status in the QC and more oxidised status in the adjacent PM, to which the auxin maximum was shifted (Jiang etal. 2003). An increase in ASC oxidation in QC is attributed to high AOX activity. Both, AOX transcript and protein accumulate very distinctly in QC, whereas proximal and distal meristem cells were characterised by much lower AOX activity. Because AOX expression is stimulated by auxin (Kato and Esaka 1999; Kerk and Feldman 1995; Liso et al. 2004), high levels of enzyme activity are maintained in QC by IAA accumulating within this structure. Consequently, high rate of ASC oxidation keeps the oxidised ascorbate redox balance (Kerk and Feldman 1995). QC cells exhibit lower abilities for regenerating ASC from DHA because of the absence of DHAR activity. However, the activity of MDHAR was similar in QC

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and adjacent meristem cells, which possibly prevents the ascorbate pool in QC from total oxidation. QC cells were equipped with several times lower levels of GR activities, which explains decreased abilities for recycling GSH from GSSG (Jiang etal. 2003). Highly oxidised intracellular environment of QC seems to slow down cell division rate by reducing mitochondrial activity. QC cells are characterised by lowered mitochondrial membrane potential, which indicates the decrease in the production of ATP and NADH. Critical levels of these compounds are necessary to satisfy the G1-S checkpoint energy requirement. Therefore, reduced ATP/NADH levels in QC cells result in a decrease in the energy supply, which may cause the decrease in the cell division rate in QC (Jiang etal. 2006). Alteration in mitochondrial activity in QC may be related to inhibition of expression of some nuclear-encoded mitochondrial proteins. Among them, the reductions in tricarboxylic acid cycle enzymes have been demonstrated (Jiang et al. 2006). On the other hand, transcripts encoded by the mitochondrial genome were not decreased in QC cells when compared to adjacent meristem cells. It was also demonstrated that, in spite of constitutive oxidative stress conditions, which often result in a loss of mitochondrial membrane integrity, followed by the induction of apoptosis pathway, the overall cellular ultra-structure of the QC, including that of the mitochondria is typical of that found in unstressed plant cells. The intactness of the mitochondria, and other cell organelles of QC cells is possibly due to protective functions of residual levels of reduced ascorbate and glutathione still present within these cells (Jiang etal. 2006). The mechanism of ASC-dependent stimulation of cell divisions is still not sufficiently explained. Some data suggest that the stimulatory effect of ASC on cell divisions in root apical meristem results from the involvement of ASC in hydroxyproline synthesis. Hydroxyproline-containing proteins play a decisive role in cell cycle regulation and ASC is a co-factor of peptidyl-prolyl hydroxylase – a hydroxyproline-synthesising enzyme (De Tullio etal. 1999). Another hypothesis indicates the role of ASC in inducing the activity of ribonucleotide reductase (RNR). The enzyme reduces ribonucleotides to deoxyribonucleotids and therefore is an essential enzyme for DNA synthesis during the S phase. ASC is supposed to be required for effective reconstitution of an iron centre of RNR by increasing the release of Fe2+ from intracellular stock such as ferritin (Potters etal. 2002). A complementary explanation stresses the role of AOX in oxygen management and the regulation of metabolic rate in plant cells. According to this idea, ASC oxidation catalysed by AOX as an oxygen-consuming reaction decreases the oxygen availability in the cell. This results in slowing down of the metabolism and is followed, as observed in QC cells, by a decrease in the rate of cell divisions (De Tullio etal. 2007). Finally, ascorbate affects plant development being engaged in the synthesis of several growth regulators, such as ethylene, abscisic acids and gibberellins. It functions in these processes as a co-factor of dioxygenases – enzymes playing a decisive role in the synthesis of aforementioned hormones (Dong etal. 1992; Liu etal. 1999; Arrigoni and De Tullio, 2000; Lopéz-Carbonell etal. 2006).

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4 Glutathione as a Regulatory Factor in Plant Development It has been previously demonstrated that GSH has many functions in plants including an important role in the antioxidant system, sulphur metabolism and detoxification of xenobiotics (Noctor and Foyer 1998). In addition, similar to ASC, a regulatory role of GSH in some aspects of plant development has been reported. It was demonstrated that the timing of the development of inflorescence and flowering in the rosette plants: Arabidopsis thaliana (Ogawa etal. 2001) and Eustoma grandiflorum (Yanagida et al. 2004) is regulated by changes in the rate of GSH biosynthesis. Ogawa et al. (2001) have shown that the effects of endogenous GSH levels on flowering in Arabidopsis are dependent on the stage of plant development. Depleting GSH by treating plants with a specific inhibitor of GSH biosynthesis, buthionine-l- suphoxymine (BSO), promoted flowering when it was applied at the onset of transition to flowering. On the other hand, when plants were exposed to BSO from the beginning of culture, flowering was delayed. The GSH-deficient Arabidopsis mutant cad2 also exhibited delayed flowering when compared to wild type plants (Ogawa etal. 2001). Later it was demonstrated that levels of endogenous GSH, decisive for the proper timing of flowering, are regulated by the availability of ATP synthesised during photosynthesis and therefore are dependent on the intensity of photosynthesis (Ogawa etal. 2004). GSH is also required for vernalization-induced flowering of rosette plant Eustoma grandiflorum. It was shown that vernalization was efficiently replaced by feeding with GSH, or its precursor cysteine (however not by other thiols) in promoting flower induction. On the other hand, the inductive effect of vernalizaton on bolting was suppressed by BSO, without decreasing the plant growth rate (Yanagida etal. 2004). GSH is also involved in the regulation of the phytohormone-induced differentiation of tracheary elements (TE) in Zinnia and Arabidopsis (Henmi et al. 2001). This process was promoted by GSSG, if the substance was applied during the early period of culture, whereas the effect was completely reversed when GSSG was applied at a later period of culture. The expression of glutathione reductase (GR) was down-regulated during TE development and exogenous GSH suppressed TE formation. Over-expressing GR in Arabidopsis had the same effect. Because GSSG stimulated TE differentiation only in the presence of appropriate growth regulators, the authors conclude that GSSG-dependent regulation cooperates with phytohormones to induce TE differentiation (Henmi etal. 2001). Presence of the reduced form of glutathione (GSH) in tissue was found to be necessary to maintain cell divisions in Arabidopsis root meristem (SánchezFernández etal. 1997; Vernoux etal. 2000). GSH was localised in actively dividing initial cells while it was absent in slowly dividing cells of the quiescent centre, that suggests that growing tissues have a requirement for glutathione. Cell divisions of meristem initials were stimulated by exogenous GSH and inhibited by treatment with the inhibitor of GSH biosynthesis BSO (Sánchez-Fernández et al. 1997). Moreover, Arabidopsis plants hom*ozygous for a mutation in the ROOT MERISTEMLESS 1 (RML1) gene, coding for g-glutamylcysteine synthetase

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(enzyme of GSH biosynthesis), accumulating only 3% of the wild type GSH level, were unable to maintain cell divisions in the root apical meristem. On the other hand, root cells of the mutant plants divided normally when fed with exogenous GSH. Cells in the root meristem of the mutant plant were arrested at the G1-S transition of the cell cycle, which resulted in inhibition of root growth. However, while post-embryonic root development was blocked due to cell division arrest, the overall organisation of cell types in the root apex was not changed when compared to wild type plants. The rml1 phenotype can be induced by treating wild-type seedlings by lowering GSH levels with BSO. Similar to rlm1 plants, cells in the root apical meristem of BSO-treated seedlings are arrested in G1 phase. It should be noted that the GSH requirement of cell division process is a root-specific phenomenon. The shoot meristem of rlm1 seedlings is able to produce all the above-ground organs with timing similar to that of wild type plants (Vernoux etal. 2000).

5 Plant Cell Suspension Cultures as Model Systems in Studies on the Mechanism of Ascorbate and Glutathione Role in Cell Proliferation The long-term cultures of plant cells and small cell aggregates named cell suspension cultures, are characterised by the highest structural and metabolic hom*ogeneity in plant in vitro culture. Cell suspension cultures are usually derived from callus cultures and grown in liquid media under constant shaking and aeration. As quickly dividing and easily synchronizable cell populations, which follow specific growth kinetics, cell suspensions are frequently used in studies on the mechanism of cell cycle regulation (Menges and Murray 2002). Cell suspension cultures of Arabidopsis and tobacco cell line BY2 were used as model systems in studies on ascorbate and glutathione involvement in the regulation of cell divisions. Timing of phases during cell suspension growth kinetics was found to be synchronized with changes in cellular ASC and GSH levels. A three- to fourfold increase in the endogenous ASC content was observed during the exponential growth phase, and a peak in ASC coincided with a peak in the mitotic index in BY2 culture (De Pinto et al. 1999). Ascorbate was also abundant in the exponential phase of Arabidopsis cell culture growth cycle. When the growth rate in culture declined, ascorbate levels decreased to about half its original value (Pellny etal. 2009). In addition, an increase in GSH levels was observed during a proliferation phase of the growth cycle (De Pinto etal. 1999). Cell proliferation in tobacco cell culture was strongly stimulated by exogenous GSH, however, was not affected by GSSG (De Pinto etal. 1999, 2000). Stimulation of ASC biosynthesis in BY-2 by adding ascorbate precursor galactono-g-lactone (GalL) to the culture medium accelerated culture growth by promoting cell division. In contrast to the reduced form of ascorbate, exogenous DHA strongly blocked cell divisions in BY2 culture (De Pinto etal. 1999). Studies using synchronized BY-2 cells revealed that addition of 1 mM DHA to cells in G1 phase induced a delay in

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cell cycle progression. DHA-treated cells reached the same value of mitotic index as untreated cells but several hours later. DHA added to the medium was quickly reduced after the uptake by the cells, leading to a strong increase in ASC intracellular level. Because oxidative stress induced by various environmental factors favours oxidation of apoplastic ASC to DHA, it was concluded that slowing down the cell cycle progression in the presence of high DHA levels may function as an adaptation strategy to surveillance under stress condition. (Potters etal. 2000). Studies on BY2 cell culture revealed that the DHA effect on cell proliferation could not be reproduced by GSSG treatment (De Pinto etal. 1999). However, total depletion of cellular GSH with BSO resulted in a cell cycle delay similar to that caused by DHA treatment. Simultaneous application of both compounds completely blocked mitotic activity. On the other hand, combined addition of GSH and DHA resulted in delay similar to DHA alone. This finding suggests that the ASC/ DHA pair has a specific regulatory role on cell division rather than merely acting as a general redox pair and that ascorbate and glutathione control cell cycle using independent pathways (Potters etal. 2004). It is noteworthy that DHA is only capable of slowing down the cell cycle when added during G1 phase. Addition of DHA during G2 phase did not affect the cell cycle progression. This suggests that DHA influences cell cycle through processes that are specific for G1 or S phase. However, the precise mechanism of DHAdependent cell cycle regulation still remains to be deciphered (Potters etal. 2004). Changes in the activities in ASC and GSH metabolising enzymes were synchronized with ASC, GSH levels and rate of cell division in BY2 cell culture. Similar to ASC and GSH content the activities of the Halliwel–Asada cycle enzymes rise during exponential phase of the cell culture growth cycle. These finding suggests that cell division process is marked by the intensification in ascorbate and glutathione turnover. Apart from the ASC-dependent H2O2 scavenging that is particularly high in dividing cells, other ascorbate-consuming processes like hydroxyproline-rich protein or nucleotide synthesis are responsible for an increased ascorbate recycling (De Pinto etal. 2000). A regulatory role for glutathione with respect to cell proliferation in Arabidopsis cell culture was demonstrated to be linked to pyridine nucleotide metabolism and activity of poly (ADP-ribose) polymerase (PARP) activity (Pellny et al. 2009). PARP is a nuclear enzyme which transfers ADP-ribose units from NAD+ residues on target proteins, resulting in branched chains of ADP-ribose polymers. ADP-ribosylation is important in regulating processes such as DNA repair, modification of histone structure and chromatin remodelling (Kraus and Lis 2003). PARP activity level correlated with the rate of cell division in Arabidopsis cell culture; lower activity was observed on the first day after inoculation but values increased rapidly during the exponential growth phase. Subsequently, when the growth rate in culture declined, PARP activity decreased (Pellny etal. 2009). Expression patterns of the major Arabidopsis PARP-coding genes (PARP1 and PARP2) increased progressively during the exponential phase, giving a peak when growth was the highest and declined thereafter. High PARP1 and PARP2 expression levels were associated with increases in the NAD + NADH pool and oxidised state

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of NAD/NADH redox pair. Because PARP is a NAD-cleaving enzyme, this reflects the requirement for the pyridine nucleotide substrate. Intracellular glutathione (but not ascorbate) levels increased in parallel with PARP activity and expression. The putative reason for glutathione increase during exponential phase is the necessity to buffer cellular oxidation caused by enhanced oxidation of NAD pool. At this phase of the growth cycle, glutathione is present in the cytoplasm. However, almost total cellular glutathione is recruited to the nucleus at the end of the phase of exponential growth. GSH movement between cytoplasm and nucleus may possibly have important consequences for both, redox buffering in the cytoplasm and direct regulation of gene expression. A close correlation between glutathione levels and PARP activity suggests that there is a relationship between these parameters. PARP is a redoxsensitive enzyme whose activity could be regulated by the GSH in the nucleus, via thiol-disulphide exchange mechanism or glutathionylation (Pellny et al. 2009). However, these ideas should be supported by further experiments.

6 Regulation of Cell Growth by Ascorbate Besides the effects on cell division rate, ascorbate affects organ growth by participating in the process of cell wall stiffening, which regulates the cell expansion within the organ elongation zones (Green and Fry 2005a). An increase in activity of apoplastic AOX, detected in rapidly elongating tissues, results in the production of monodehydroascorbate (MDHA) in the plant cell wall. This compound activates plasma membrane H+-ATPase (Gonzales-Reyes etal. 1992; Asard etal. 1995). An increase in the activity of this enzyme is followed by an acidification of apoplast that according to the acid growth theory leads to the loosening of cell wall structure and facilitates cell growth (Cosgrove 2001). Moreover, MDHA in apoplast functions as an acceptor in transmembrane electron transport. The plasma membrane redox system, involving cytochrome B, transports electrons from NADH across the plasma membrane onto MDHA, which is reduced to ASC. The intensification of this process results in plasma membrane hyperpolarisation which is followed by activation of plasma membrane H+-ATPase that promotes cell growth as mentioned above. Simultaneously, local cytoplasm acidification resulting from NADH oxidation activates vacuolar H+-ATPase. Consequently, an increase in cell vacuolization occurs which further enhances cell expansion (Gonzales-Reyes et al. 1992; Horemans etal. 2000). MDHA generated by AOX activity in the apoplastic spaces may undergo further spontaneous oxidation to DHA (Noctor and Foyer 1998). This compound as well as a product of its degradation – oxalate – may directly cause cell wall loosening and/or enhance cell vacuolization (Lin and Varner 1991). Another mechanism linking apoplastic ASC with loosening of cell walls in growing segments of plant organs involves the inhibitory effect of this antioxidant on peroxidase-dependent processes of cross-linking cell wall polymers. ASC prevents oxidative formation of diferulate bridges between cell wall polysaccharides in elongating onion roots (Córdoba-Pedregosa etal. 1996; Takahama and Oniki 1992)

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and pine hypocotyls (Sánchez et al. 1997). ASC inhibited the polymerisation of monomers of cell wall structural protein extension by blocking isodityrosine crosslinking (Córdoba and Gonzales-Reyes 1994) and delayed cell wall lignification by reducing phenoxyl radicals, which serve as precursors in peroxidase-dependent lignin synthesis (Takahama and Oniki 1991; Padu 1999). Ascorbate may also stimulate a pro-oxidative process within the plant cell wall, which results in local production of hydroxyl radical (˙OH) (Fry 1992; Green and Fry 2005a, b). This highly reactive compound is able to cause an oxidative scission of cell wall polysaccharide chains, which is followed by loosening the cell wall structure that again promotes cell growth (Schopfer et al. 2001; Schopfer 2002). The mechanism of ASC-dependent ˙OH was proposed to have a non-enzymatic nature and to require O2 and a transition metal such as Cu2+. It was demonstrated that in the presence of traces of Cu2+, ASC reduces O2 to H2O2, ASC also reduces Cu2+ to Cu+. These two products: H2O2 and Cu+, can participate in a Fenton reaction. In the course of this reaction, Cu2+ is regenerated with concurrent ˙OH formation (Fry 1992). Besides ascorbate itself, the components of its breakdown pathway may also act as pro-oxidative agents that stimulate the generation hydroxyl radical (˙OH) in the cell wall. DHA generated by ASC oxidation may undergo an apoplast-specific degradation pathway via oxidase-dependent or non-enzymatic, conversion of DHA to 4-O-oxalyl-l-threonate. The latter compound is further converted to l-threonate and oxalate as final degradation products. This reaction is catalysed by apoplastic oxalyl esterases but may also occur non-enzymatically. During these steps additional H2O2 molecules are formed, which can take part in a Fenton reaction. Moreover, in many plants l-threonate is oxidated to l-threarate in a reaction, which uses two O2 and generates two H2O2 molecules. In tissues possessing apoplastic oxalate oxidase, the oxalate can also yield H2O2 when oxidised (Green and Fry 2005a, b).

7 A Role of Ascorbate in Somatic Embryogenesis Studies on the roles of ASC in the formation of somatic embryos were conducted using white spruce (Picea glauca) embryogenesis as a model system. An embryogenic tissue for the induction of somatic embryogenesis was generated from zygotic embryos (Stasolla and Yeung 1999). A well-characterised process of somatic embryogenesis in this plant generally consists of proliferation and maturation stages. These stages are characterised by different hormonal requirements. The first stage is characterised by proliferating tissue on the auxin- and cytokinin-containing medium. At this stage, the formation of filamentous embryos occurs. These structures are composed of a suspensor region subtending clusters of small, non-vacuolated cells of the embryo proper. Then, the tissue is transferred onto the ABA-supplemented maturation medium. During the culture in the presence of ABA, cell divisions occurs which leads to an increase in the size of the embryo proper. Subsequently,

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a well-developed shoot and root pole become visible. Finally, the embryos develop a ring of cotyledons at the shoot apical region. Mature embryos are transferred onto the germination medium where, root and shoot emergence occurs (Stasolla etal. 2002). White spruce somatic embryo maturation and germination were marked by important changes in ASC metabolism. The differences in ASC content, ASC/DHA redox state, and the activities of ascorbate-metabolising enzymes were analysed using both embryogenic and non-embryogenic cell lines. While no differences in ASC metabolism were observed between embryogenic and non-embryogenic cell lines in the proliferation medium, after the transfer onto the maturation medium, the rate of ASC synthesis sharply increased in embryogenic lines, whereas it remained constant in the non-embryogenic line. Gradual decrease in the APX activity followed by a shift within the total ascorbate pool towards the reduced form was observed in embryogenic lines. On the other hand, the non-embryogenic line was characterised with significantly higher APX activity and oxidised state of ascorbate pool. Switching of the ASC/DHA ratio towards the reduced state and increase in ASC levels in embryogenic lines are possibly required for the progression of cell divisions within developing embryos (Stasolla and Yeung 2001). In contrast to zygotic embryos, which undergo desiccation before subsequent germination preceded by imbibition, mature somatic embryos usually directly develop to plantlets. However, the germination frequency of somatic embryos of coniferous species, including white spruce, was found to be greatly increased by partial drying treatment (PDT) preceding transfer onto germination medium. Stimulatory effect of PDT results from an alteration in storage product deposition, decreased synthesis of ethylene and ABA, and changes in the pattern of nucleotide synthesis and utilization (Stasolla et al. 2002). PDT applied to the mature embryos was characterised by several changes in ASC metabolism. Firstly, ASC levels in the embryos as well as the activities of its redox enzymes significantly declined. Secondly, ASC/DHA ratio shifted to a more oxidised state, with equal participation of ASC and DHA in total ascorbate pool observed at the end of PDT. After PDT, white spruce embryos are transferred onto a hormone-free germination medium, where the induction of root and shoot meristem activity occurs. Upon the transfer onto the germination medium a restoration of ASC synthesis and ASC metabolism were observed. Besides the onset of ASC synthesis, a reduction of DHA accumulated during PDT occurs which contributes to an increase in the ASC level. Elevated ASC levels are required for activation of meristematic activity in germinating embryo. Moreover, an increase in ASC levels are followed by rise in APX activity necessary for detoxification of H2O2 generated after the recovery of oxidative metabolism (Stasolla and Yeung 2001). The process of shoot emergence was found to be significantly affected by ASC added to the germination medium (Stasolla and Yeung 1999). An optimised concentration of ASC significantly enhanced shoot development. Moreover, ASC-treated embryos were larger and produced dark green leaves at the shoot pole. Control embryos, germinated on ascorbate-free medium were smaller than and not as green

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as those cultured in the presence of ASC. In contrast to shoot meristem, the root pole of the embryo did not react with an increase of root growth to ASC treatments (Stasolla and Yeung 1999). An importance of ASC for embryo meristem reactivation at germination was confirmed by a complementary approach using lycorine – an inhibitor of ASC biosynthesis. Lycorine inhibits the last step of ASC synthesis, i.e. conversion of GalL to ascorbate (Arrigoni 1994). It was demonstrated that ASC depletion following the lycorine administration to the germination medium prevents cellular divisions in the shoot apical meristem (Stasolla and Yeung 2007). The mechanism of ASC stimulatory effect on the shoot apex development involves prevention of the peroxidase-dependent cell wall stiffening within shoot apical meristem. Experimental manipulations resulting in an increase in ASC content, i.e. supplementing germination medium with ASC or GalL, decrease the activities of ferulic acid peroxidase (FPOX) and guaiacol peroxidase (GPOX). On the other hand, activities of the aforementioned enzymes increase in lycorine-treated embryos. FPOX and GPOX are responsible for cross-linkage of cell wall compounds, which leads to cell wall stiffening and prevents cell division and cell growth. Therefore, the critical levels of endogenous ASC in the apical meristem are necessary to maintain cell wall plasticity prerequisite for both cell growth and proliferation (Stasolla and Yeung 2007).

8 A Role of Glutathione in Somatic Embryogenesis A series of studies revealed an importance of glutathione concentration and redox state for the process of somatic embryo formation in the white spruce tissue culture. In contrast to ASC treatments, which stimulated mainly postembryonic developmental events within the shoot meristem, altering glutathione level affected all stages of somatic embryo formation and further development: cell proliferation on maintenance medium, embryo maturation and germination. GSH supplementation to the maintenance medium had a stimulatory effect on cell divisions in the embryogenic tissue resulting in higher fresh weigh increase when compared to untreated control. On the other hand, the rate of growth of GSSG-treated tissue was comparable to that of control (Belmonte etal. 2003). The addition of exogenously supplied GSH or GSSG to the ABA-containing maturation medium affected the number of embryos formed in culture as well as their quality. An optimised GSH concentration in the medium resulted in an increase in the number of embryos formed, this increased number was, however, mostly due to the production of low quality embryos, unable to regenerate viable plants. In contrast to GSH-treatment, supplementing medium with GSSG had a marginal effect on total embryo population. However, supplementing with GSSG strongly increased embryo quality, which manifested in higher number of embryos equipped with four or more cotyledons and possessing high germination potential. It should be noted that the aforementioned effect of GSH or GSSG applications were limited to the narrow ranges of concentration. Higher level of

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both glutathione redox forms inhibited overall embryo-forming capacity in cultured tissue and impeded embryo development (Belmonte and Yeung 2004; Belmonte etal. 2005a). GSSG treatment caused an increased accumulation of starch granules, lipids and protein bodies in cortical and procambial cells of GSSG-treated embryos. Storage product deposition in cells of treated embryos may be required for the acquisition of desiccation tolerance, thus enhancing the viability of embryos. Another striking feature of embryos exposed to GSSG was the proper structure of meristems, whereas meristems of untreated embryos were frequently disrupted by intercellular spaces, which led to a failure in meristem reactivation during postembryonic development (Belmonte etal. 2005a). The improved architecture of shoot apical meristems in GSSG treated embryos may be due to the reduction of ethylene production. Disorganisation of cell division patterns in meristem resulting in intercellular spaces formation was related to the accumulation of this hormone in the culture vessel. In embryos grown on GSSG-supplemented medium endogenous ethylene level did not vary throughout the culture period. On the other hand, in control cuttings ethylene production increased markedly after 10 days of culture and remained high until the end of culture period (Belmonte etal. 2005a). Ethylene synthesis in white spruce seems to be under control of GHS/GSSG redox status. It was demonstrated that the gene encoding aminocyclopropane-1carboxylate (ACC) oxidase, an enzyme responsible for the final step of ethylene biosynthesis is induced by GSH treatment at all the stages of somatic embryo development. A switch of the glutathione pool towards oxidised form by GSSG administration is believed to have inhibitory effect on ACC oxidase expression that consequently decreases ethylene levels (Stasolla etal. 2004). However, the down regulation of ACC oxidase transcript level imposed by GSSG application still needs to be demonstrated to prove this hypothesis. Given that supplementing GSH to the medium favours cell proliferation during early stages of somatic embryogenesis and increases total embryo yield, whereas effect of GSSG treatment manifests itself at embryo maturation period, a protocol of sequential treatments with GSH and GSSG was developed to profit the beneficial effects of both glutathione redox forms. An optimised procedure that resulted in the highest embryo yield, containing the highest percentage of high quality embryos consisted of 7 days of GSH treatment followed by culture in the presence of GSSG during the remaining part of 40-days-long culture period (Belmonte etal. 2005a). Similar stimulation of somatic embryogenesis was achieved by replacing sequential GSH/GSSG treatment with supplementing maturation medium with optimised concentration of BSO. It was observed that the glutathione redox state turned towards a more oxidised one with 0.01 mM or higher concentration of BSO. Compared to control embryos, the GSH/GSH + GSSG ratio declined quickly in BSO-treated embryos over the course of culture period, reaching its minimal value in fully mature embryos. In contrast to BSO-treated embryos, control ones were characterised by reduced glutathione redox status throughout the culture period (Belmonte and Stasolla 2007; Stasolla etal. 2008).

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Oxidation of glutathione pool in tissues grown in the presence of BSO was beneficial for the efficiency of embryogenesis and the quality of embryos formed. In contrast to control embryos, those formed in the presence of 0.01 mM BSO were characterised by proper organisation of shoot apical meristem with its sub-apical domain composed by tightly packed cells. Moreover, BSO-treated embryos contained more initial cells in root apical meristem when compared to the untreated control (Belmonte and Stasolla 2007; Stasolla etal. 2008). This finding is consistent with the present knowledge which emphasises the importance of an oxidised environment for the establishment of quiescence and cell division patterns in the apical root meristem (Jiang etal. 2003). Since glutathione levels and redox state are coupled to ascorbate through ASC–GSH cycle, BSO treatment affected both ascorbate levels and the activities of several ascorbate enzymes. Generally, BSO treatment decreased total ascorbate levels in embryos and transiently stimulated the expression of genes coding for APX and MDHAR, which resulted in an increase in the activities of these enzymes. A stimulatory effect of BSO on the APX activity during the early phase of embryo development may have particular importance for somatic embryo development due to decreasing toxic levels of H2O2 generated during active embryonic growth (Stasolla etal. 2008).

9 Glutathione-Dependent Changes in Nucleotide Metabolism During Somatic Embryogenesis Studies on nucleotide metabolism in cultures grown on GSH-supplemented media revealed that stimulatory effect of GSH treatment on cell proliferation in white spruce culture grown on maintenance medium is possibly related to alterations in purine and pyrimidine metabolism (Belmonte etal. 2003, 2005b). Both purine and pyrimidine nucleotides serve as building blocks for nucleic acid synthesis. Moreover, purine nucleotides participate in bio-energetic processes. In white spruce cells, both purine and pyrimidine nucleotides can be synthesised de novo from precursor molecules, or through the salvage mechanism, that utilises bases and nucleosides as substrates (Stasolla etal. 2003). The effects of GSH or GSSG supplementation to the maintenance medium on pyrimidine metabolism was studied by following the metabolic fate of radiolabelled orotic acid, precursor of the de novo synthesis pathway and uridine and uracil, respective intermediates of the salvage and degradation pathway. Inclusion of either GSH or GSSG to the medium increased the levels of pyrimidine nucleoside triphosphates and nucleic acids in cultured cells. It was demonstrated that elevated GSH levels result in activation of the denovo synthesis and reduction of the degradation pathway, whereas GSSG increases the activity of salvage pathway. Compared to the control tissue, grown on GSH-free medium, tissues cultured in the presence of 0.2 mM GSH were characterised by increased production of UMP from orotic acid leading to high cellular levels of UTP and CTP. The enlargement of

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pyrimidine nucleotide pool observed in GSH-treated tissue was due to an increase in the activity of orotate phosphoribosyl transferase, an enzyme that converts orotic acid to orotidine-5¢-monophosphate during the first step of pyrimidyne nucleotide synthesis pathway (Bellmonte etal. 2005b). Induction of the de novo synthesis of pyrimidine nucleotides by GSH was accompanied by a reduction of the degradation pathway. Pyrimidine nucleotides undergoing degradation are converted to uracil, which in further steps is degraded to b-ureidopropionate and CO2 (Stasolla etal. 2003). If the tissue was grown in the presence of GSH, substantially less radiolabelled uracil was converted to these degradation products. Instead, uracil was recovered into nucleotides and nucleic acids (Belmonte etal. 2005b). In contrast to GSH, in the GSSG-treated tissue, the activities of synthesis and degaradation pathways of pyrimidine nucleotides were at control level. On the other hand, GSSG increased the efficiency of the salvage pathway by stimulating the phosphorylation of uridine to uridine monophosphate (UMP). Following further phosphorylation UMP converts to UTP, which may directly be incorporated to RNA synthesis or serve as a substrate in CTP- or UDP-glucose synthesis. The reaction of UMP synthesis from uridine may be catalysed by uridine kinase or nucleoside phosphotransferase. Given that both enzymes respond to GSSG treatment with a simultaneous rise in their activities, indicates the possible mechanism of GSSG-dependent increase in the rate of uridine salvage (Belmonte etal. 2005b). Similar approach was applied to test the effects of GSH or GSSG supplementation on purine nucleotide metabolism in white spruce embryogenic tissue grown on maintenance medium. The activity of synthesis pathway was studied determining the rate of incorporation of purine synthesis precursor AICAR to AMP, ATP + ADP and nucleic acids. It was observed that GSH or GSSG treatments did not significantly increase the efficiency of AICAR conversion into aforementioned compounds. This finding suggests that GSH and GSSG do not affect purine synthesis pathway (Belmonte et al. 2003). On the other hand, the differential salvage activity was observed between control and GSH-treated tissue manifested in an increased incorporation of the salvage pathway intermediates, adenine and adenosine, during ATP synthesis in GSH-treated tissue. High levels of ATP during exponential growth phase of the embryogenic tissue may be required not only as a building block for nucleic acid synthesis but also as an intermediate involved in bio-energetic processes related to cell proliferation. Finding that only low levels of labelled adenine and adenosine was recovered in nucleic acids, suggests that increased ATP synthesis is the principal outcome of GSH-dependent stimulation of purine salvage pathway (Belmonte etal. 2003). The rate of the degradation of purine nucleotides was not affected by GSH or GSSG treatments. However, in GSSG-treated tissue, a substantial amount of degradation pathway intermediate – inosine – was recruited for the ATP production compared to untreated control and GSH-treated tissue (Belmonte etal. 2003). The data reported above show that the possible mechanism underlying the stimulation of the cell proliferation in white spruce embryogenic tissue by glutathione is related to an enhancement of the production of nucleotides serving as “building

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blocks” for nucleic acid synthesis of substrates in bio-energetic reactions (Belmonte etal. 2003, 2005b). It was demonstrated that GSH availability regulates the activities of enzymes of nucleotide metabolism at the level of gene expression. The level of transcript coding for adenosine kinase was markedly higher in GSH-treated embryos during embryo maturation phase and germination. GSH supplementation to the medium reduced the expression of genes encoding for uracil phosphotransferase and nucleoside diphosphate kinase during the maturation phase. On the other hand, GSH stimulated the expression of uridinylate kinase in mature embryos of white spruce (Stasolla etal. 2004).

10 Glutathione-Induced Changes in Patterns of Gene Expression During Somatic Embryo Formation The beneficial effect of GSH on embryo conversion is due to profound changes in gene expression patterns observed upon GSH administration. Microarray studies revealed that genes involved in large number of metabolic and regulatory pathways are differentially expressed between control and GSH-treated tissue. Compared with early stages of somatic embryo development, the total number of differentially expressed genes increased during embryo maturation (Stasolla etal. 2004). Many genes encoding for proteins involved in protein synthesis like ribosomal proteins, initiation and translation factors as well as late embryogenic abundant proteins were down-regulated in the presence of GSH during embryo maturation phase. The lower transcript level of genes related to protein synthesis, together with the reduced accumulation of protein bodies in embryos grown on GSH-supplemented medium is interpreted to be directly related to the switch to germination mode without PDT (Stasolla etal. 2004). Some genes involved in carbohydrate metabolism, such as several glucanases, acetyl-CoA synthase, phosphoenolpyruvate carboxylase were repressed by the presence of GSH. On the other hand, expression levels of other group of genes involved in carbohydrate processing, represented by transketolase, fructokinase, aldolase and aconitase hydratase were elevated in GSH-treated embryos (Stasolla et al. 2004). Changes in the expression of aforementioned genes may account for an increased starch deposition in the cell of GSH-treated embryos (Stasolla etal. 2004). An establishment of functional shoot and root apical meristem during somatic embryo maturation is dependent on expression of genes involved in meristem identity and organisation (Golz 2005). It was observed that several genes falling into that category, including the hom*ologues of CLAVATA 1 (CLV 1), NO APICAL MERISTEM (NAM) and ARGONAUTE (AGO) were substantially stimulated by GSH treatment during the late phases of embryo development (Stasolla etal. 2004). The aforementioned genes regulate division patterns within shoot apical meristem (Golz 2005). Among genes that regulate differentiation within root apical meristem, a SCARECROW (SCR) gene was found be up-regulated in mature embryos grown on GSH-supplemented medium (Stasolla etal. 2004).

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Relation between expression patterns of genes involved in meristem patterning and glutathione levels was studied with regard to spruce KNOTTED-like homeobox (KNOX) gene HBK 1. The HBK 1 gene is preferentially expressed in the shoot apical meristem of spruce where, similar to its Arabidopsis hom*ologue SHOOT MERISTEMELESS (STM), plays decisive role in meristematic cell specification (Sundås-Larsson etal. 1998; Hjortswang etal. 2002). In situ hybridisation experiments using probes specific to HBK1 mRNA revealed that long-term culture in the presence of GSSG extended the HBK 1 expressing zone within an apical shoot pole. Localisation pattern of HBK 1 was similar for both control and GSSG-treated embryos during the first 10 days of culture. HBK 1 transcript was restricted to the apical pole of embryos. Differences in the expression pattern were visible at day 20 when HBK 1 expressing zone was still restricted to the apical cells in control embryos, whereas was extended to the sub-apical cells in treated embryos. Moreover, less cells expressed HBK 1 gene if meristem was disrupted by intercellular spaces. These findings raise a possibility that the improvement of the meristem organisation by GSSG is possibly related to conferring stem cell identity to larger population of cells through stimulation the HBK 1 expression (Belmonte etal. 2005b). A transformation approach using white spruce cells transformed with HBK 3 gene in sense or antisense orientation revealed that representatives of HBK gene family control both early and late stages of somatic embryogenesis in spruce. Overexpression of HBK 3 gene in cells of HBK 3 – sense line promoted the initiation of embryo formation in embryogenic tissue. On the other hand, embryo formation was strongly reduced in transgenic lines where down-regulation of HBK 3 expression was imposed due to antisense HBK 3 transformation. Compared to the control embryos, those over-expressing HBK 3 formed larger shoot apical meristems and exhibited higher competence for generating viable plants with no phenotypic aberrations (Belmonte etal. 2007). Further studies on HBK 3 over- or under-expressing lines of embryogenic cells revealed an intimate relationship between HBK 3 expression level and ASC and GSH metabolism. Both, GSH and ASC levels were significantly higher in cells over-expressing HBK 3 especially after the onset of somatic embryo formation from embryogenic tissue induced by transfer from plant growth regulator-supplemented medium onto plant growth regulator-free medium. This increase was due to the activities of ASC and GSH regenerating enzymes. The activities of DHAR, MDHAR and GR were elevated in cells of HBK 3 over-expressing line. These findings suggest that HBK 3 expression might regulate the transition from cell proliferation to embryo formation in embryogenic tissue through alterations in ascorbate and glutathione metabolism (Belmonte and Stasolla 2009). Similar to GSH/GSSG treatments, an application of BSO, resulting in a shift in the glutathione pool towards its oxidised form, induced the expression of genes involved in shoot meristem formation. Treatment of Brassica napus somatic embryos with 0.1 mM BSO resulted in an increase in the expression levels of ARGONAUTE 1 (AGO 1) transcript during the globular and early torpedo stages of development (Stasolla etal. 2008). AGO 1 is a protein required for both shoot meristem formation and identity, characterised by conserved PAZ and PIWI domains

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that are engaged in protein-protein interactions. This protein is closely related to CLAVATA 3 – a well recognised factor engaged in meristem patterning and maintenance (Lynn et al. 1999). During the same stage of embryo development, BSO treatment increased the expression of gene coding for CLE 27 protein (Stasolla etal. 2008), engaged in meristem size regulation (Fiers etal. 2007). During next stages of embryo formation, i.e. when embryos complete their histodifferentiation programme and switch to post-embryonic development, besides AGO 1 and CLE 27, the level of ZWILLE transcript increased in BSO-treated embryos (Stasolla etal. 2008). ZWILLE gene codes for protein specifying the expression pattern of STM, which is also involved in the maintenance of apical shoot meristem (Moussian et al. 1998). Finally, in embryos cultured on BSO-supplemented medium, the expression of B. napus STM hom*ologue increased in a late stage of embryo development (Stasolla et al. 2008). The data mentioned above, demonstrate that the imposition of an oxidised environment, effected by BSO application results in a sequential induction of a set of genes responsible for proper organisation of shoot apical meristem. This finding accounts for a beneficial effect of BSO treatment on the structure of the apical meristem.

11 Ascorbate and Glutathione Involvement in Adventitious Root Formation In vitro Adventitious rooting consists of two stages: formation of root primordia and its subsequent outgrowth. The phase of primordia formation can be subdivided on the induction phase, when molecular and biochemical events related to dedifferentiation and competence acquirement occur, and initiation phase, characterised by organised cell divisions in the developing root primordium (Gaspard et al. 1997; Li et al. 2009a). Rooting is affected by numerous endogenous and exogenous factors, with the principal role of auxin as a chief regulator of adventitious root formation. Blocking the transport of endogenous auxin to seedling rooting zone inhibits rooting (De Klerk etal. 1999). In the regulation of adventitious rooting process, ascorbate and glutathione seem to be involved in a complex interplay between auxin and other components of cellular redox systems. It was demonstrated that auxin stimulatory effect on rooting is mediated by NO (Pagnussat etal. 2002) and the NO-dependent signalling pathway, leading to root formation, is mediated by H2O2 (Liao etal. 2009). The signalling role of H2O2 in root formation was explored in studies on rooting of cucumber (Li etal. 2007) and Mung bean seedling cuttings (Li etal. 2009a). It was found that the culture in the presence of 20–40 mM H2O2 significantly increased the number of adventitious roots per explant formed by cucumber cuttings, when compared to the untreated control (Li etal. 2007). Mung bean cuttings reacted with increased rooting to H2O2 concentrations ranging from 1–100 mM. Cuttings underwent 8–24-h-long pulse H2O2 treatment followed by culture on the H2O2-free medium. Stimulatory effect of H2O2 treatment was observed both in the absence and in the presence of exogenous

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IBA as an auxin source. Promoting effect of H2O2 on rooting decreased if a relatively higher H2O2 concentration and longer treatment times were applied (Li et al. 2009a). Decreasing H2O2 content, with diphenyloiodonium (DPI, NADPH oxidase inhibitor), catalase or ASC prevented the stimulation of rooting. Application of 4 mM ASC significantly decreased the number of regenerated roots and eliminated the stimulatory effect of H2O2 if these two substances were applied simultaneously (Li et al. 2009a). In contrast to H2O2, IBA still stimulated rooting in the presence of 4 mM ASC in Mung bean, and IAA stimulated rooting in the presence of 2 mM ASC in cucumber (Li etal. 2007, 2009a). On the other hand, ASC in concentration of 6 mM completely prevented rooting in the absence or presence of either H2O2 or IBA. These results suggested that H2O2 or IBA promotion of adventitious root formation was blocked by certain concentrations of ASC (Li etal. 2009a). Measurements of H2O2 concentration in cucumber seedling explants during subsequent phases of rooting on auxin-free medium revealed that H2O2 peaks during the early induction phase. It may suggest that elevated H2O2 levels may possibly be required in early events of rooting, i.e. dedifferentiation and acquiring competence (Li et al. 2007). In Mung bean cuttings cultured on basal medium an induction phase of rooting was marked by a progressive increase in POX and APX activities. At the switch from induction to initiation phase, activities of both enzymes strongly decrease. It was demonstrated that IBA treatment transiently decreased POX and APX activities during induction phase of rooting, which may be correlated with an increase in endogenous H2O2 level, required for the induction of adventitious roots. Therefore, APX activity may be involved in an auxin-dependent mechanism of regulation of H2O2 level during adventitious rooting and the early decrease in its activity may be one mechanism by which IBA and H2O2 promote adventitious rooting (Li etal. 2009b). It should be noted that although the moderate oxidation stimulates rooting, H2O2-overproduction, resulting in severe oxidative stress inhibited root formation (Pal Singh etal. 2009). Imin and co-workers (2007) show that both reduced (GSH) and oxidised (GSSG) form of glutathione markedly enhance the number of roots formed by callus derived from leaf explants of Medicago truncatula cultured on auxin-supplemented medium. In their experiments leaf segments were grown for 3 weeks on proliferation medium supplemented with NAA or NAA in combination with either GSSG or GSH. Significantly, more roots were produced by explants grown on NAA-containing media supplemented with GSH or GSSG than on media supplemented with NAA alone, which suggests that glutathione may act synergistically with exogenous auxin in the stimulation of root regeneration (Imin et al. 2007). Because in the absence of NAA, root formation was negligible (Imin etal. 2007), it would be interesting to find out if GSH or GSSG may, to some extent, replace the hormone in the stimulation of root formation. However, the authors do not include any data on effects of GSH or GSSG on root formation on NAA-free media (Imin etal. 2007). During early stages of rooting in tomato seedling cuttings, when the root primordia are formed, total glutathione pool is characterised by higher participation of GSSG in the total glutathione pool in comparison to the later stages when the outgrowth

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and elongation of newly formed roots occurs (Tyburski and Tretyn 2010). Higher oxidation of glutathione pool during the formation of root primordia by tomato seedling cuttings may result from an increase in the rate of ascorbate turnover in Halliwell–Asada cycle and a strong rise in the activity of dehydroascorbate reductase at this stage of root formation. Subsequent days of rooting are characterised by a decline in the activity of dehydroascorbate reductase and other enzymes of ascorbate metabolism (Tyburski etal. 2006). Consequently, higher amounts of reduced glutathione can be accumulated in the rooting zone (Tyburski etal. 2006). Supplementing the rooting medium with GSH increased the number of roots formed by tomato seedling cuttings grown on an auxin-free medium. However, the stimulatory effect of GSH was restricted to a narrow range of concentrations spanning from 1 or 2.5 mM GSH. Strongest stimulation of root formation occurred when plants were simultaneously treated with auxin and GSH. Treatments with GSSG did not affect root formation if cuttings were grown on the basal medium, however, an optimised GSSG concentration strongly enhanced the rooting-stimulatory effect of auxin treatment. BSO did not affect the number of roots formed by cuttings grown on BM and only slightly decreased the efficiency of rooting in the presence of IAA, which shows that depletion of GSH from rooting zones does not inhibit rooting. This finding suggests that root formation may occur in the absence of GSH but the process is stimulated by its presence (Tyburski and Tretyn 2010). Root regeneration by tomato seedling cuttings proceeds on a highly synchronized manner with a 3-day-long period of primordia formation encompassing initiation and induction phase and subsequent phase of primordia elongation, which manifests in root emergence on the fourth day of culture. Induction and initiation phases of rooting in tomato seedling cuttings are characterised by rapid increase in the content of ASC in the explant rooting zone. The accumulation of ASC in the tissue correlates with the biosynthetic capability from GalL and with the activities of enzymes regenerating ASC from its oxidised forms (MDHAR and DHAR). Simultaneously, the sharp increase in H2O2 content and in the activities of APX and AOX were observed. ASC peaks on the third day of rooting, i.e. at the switch from the phase of primordia organisation to the phase of primordia elongation. With the beginning of elongation phase a dramatic decrease in ASC content was observed. In contrast to ASC, DHA levels remained constant during root formation. Similar to ASC, the H2O2 level, as well as, the activities of ASC-metabolising enzymes dropped at the onset of the root elongation phase. These finding suggest that the increase in the endogenous level of ASC observed at the beginning of rooting may be explained by the necessity of the regulation of H2O2 level in the rooting zone during the formation of root primordia. This point of view is supported by the finding that the activities of H2O2 scavenging enzymes: APX, POX and CAT rise simultaneously with the ASC (Tyburski etal. 2006). Functions of ascorbate in adventitious rooting seem to extend beyond the regulation of H2O2 levels (Tyburski etal. 2006). It has been observed that the addition of exogenous GalL, ASC or DHA to the rooting medium affect the root formation by tomato seedling cuttings. The ascorbate precursor (GalL) as well as ASC and DHA modified the rooting response in a similar way, i.e. stimulated the formation

J. Tyburski and A. Tretyn

Fig.2 The effect of ASC, DHA and IAA on rooting of tomato seedling cuttings. The cuttings were cultured 7 days on basal medium (a), or on the same medium supplemented with 2 mM ASC (b), 2 mM DHA (c) or 1 mM IAA 9 (d). Similar to exogenous auxin, ASC and DHA stimulate the formation of roots but inhibit their elongation

of roots but inhibited their elongation. Therefore, their effect was similar to that of exogenous auxin, which also increased the number of regenerated roots, simultaneously reducing their length (Tyburski etal. 2006) (Fig.2). It is noteworthy that DHA was more effective in inducing abundant root formation than ASC and GalL, having higher stimulatory effect than auxin. Moreover, because ASC added to the medium, being an unstable molecule, is partly oxidised to DHA and GalL after being converted to ASC is subsequently oxidised to DHA. It has beenfound that treatments with DHA and GalL that induced more roots than ASC, resulted in a significant increase in DHA content in the rooting zones in comparison with ASC-treated cuttings. Therefore, the oxidised form of ascorbate is supposed to have a decisive role in the stimulation of rooting (Tyburski etal. 2006). Rooting of cuttings does not occur, or is severely reduced, if auxin transport to the rooting zone is blocked by application of auxin transport inhibitors like NPA or TIBA. These treatments result in a suboptimal auxin level in explant rooting zone that block root formation (Visser etal. 1995; Ludwig-Müller etal. 2005). Several redox agents were able to restore root development in NPA or TIBA treated explants. NO and H2O2, which are believed to mediate in the auxin signalling pathway, were able to trigger adventitious root formation in Tagetes erecta seedling cuttings cultured on medium supplemented with 10 mM NPA (Liao etal. 2009). On the other hand, an inhibitory effect of 1 mM TIBA on rooting of tomato seedling cuttings was reversed by 4 mM ASC. It has been found that only ASC (but neither GalL nor DHA) was able to reverse the stimulatory effect of TIBA on root formation, although the stimulatory effect of GalL and DHA on rooting of cuttings grown on medium without TIBA was comparable to the one of ASC or even stronger. The question of why ASC added to rooting the medium is able to reverse the inhibition of rooting by TIBA while the GalL administration, which actually efficiently increases the ASC content in the root-forming tissue remains unresolved (Tyburski etal. 2006).

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12 The Roles of Ascorbate, Glutathione and Related Enzymes in the Elicitation of Metabolite Synthesis in Root Cultures Isolated root systems grown in bioreactors as suspensions on liquid media serve as important source of metabolites for commercial purposes (Flores etal. 1999; Guillon etal. 2006). It was demonstrated that changes in activities of the enzymes of ASC and GSH metabolism may influence both root development in culture as well as the rate of metabolite production, which often needs elicitation by oxidative stressinducing factors. The production of ginsenoside by adventitious cultures of Panax ginseng was enhanced by increasing O2 concentration from 20% to 40%. The same effect was achieved when roots were treated with optimised concentrations of H2O2. An increase in metabolite production was observed between 15 and 45 days after the exposition to O2 started. On the other hand, treatments with 25 and 50 mM H2O2 resulted in increased ginsenoside levels after only 7 days after treatment. As inferred from malonyl dialdehyde (MDA) content and H2O2 levels in O2-treated roots, this treatment imposed a mild oxidative stress, which seems to have a stimulatory effect on the induction of ginsenoside production. To counter the oxidative damage, antioxidant enzymes of ascorbate–glutathione cycle were induced by O2 application. An increase in APX activity was detected after 15 days of treatment and from that day on high enzyme activity was observed during the rest of culture period with maximum after 45 days. Other enzymes, i.e. MDHAR, DHAR and GR behaved in a similar manner. Increase in their activities was coordinated with increases in the activities of other constituents of antioxidant system such as superoxide dismutase (SOD), CAT, glutathione peroxidase (GPX) and guaiacol peroxidase (POX). These finding suggest that during O2/H2O2-elicited ginsenoside production, the intensity of oxidative stress is regulated in a coordinated manner by synchronized changes in the activities of antioxidant enzymes (Ali etal. 2005a). Redox agents played a decisive role also in the regulation of root proliferation in culture. The proliferation of Panax isolated adventitious roots was stimulated by NO. It was demonstrated that supplementing media with NO producing producers like sodium nitroprusside (SNP) increased the number of rootlets per explant. On the other hand, decreasing NO levels with NO-scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl3-oxide (PTIO), prevented root proliferation (Tewari etal. 2007). NO stimulatory effect was blocked if roots were treated with DPI – an inhibitor of O2·−-producing enzyme: NADPH-oxidase. This observation together with the finding that NO stimulates NADPH oxidase in ginseng roots suggest that NO stimulatory effect on root proliferation is mediated by O2·−. However, along with increasing the pro-oxidant activity of NADPH oxidase, NO stimulated the activity of the set of antioxidant enzymes including SOD, CAT, POX, APX, DHAR and GR. Treatment with SNP also increased the levels of total ascorbate and the participation of ASC in the pool of total ascorbate. These effects were prevented by the application of NO scavenger. Therefore, a coordinated stimulation of pro-oxidant and antioxidant systems by NO impose a moderate oxidative stress conditions which promote root proliferation in culture (Tewari etal. 2008).

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The onset of secondary metabolite production in hairy root culture often requires the treatments with elicitors; the signalling molecules of plant defence responses (Guillon etal. 2006). Elicitor treatments frequently induce oxidative stress, which sometimes is a direct factor triggering metabolite synthesis (Huerta-Heredia etal. 2009). When biotransformation process in a hairy root culture is dependent of ROS production, antioxidant activity of ASC may have an inhibitory effect, as it was shown for the tetracycline phytoremediation by hairy roots of sunflower (Gujarathi et al. 2005). However, in other experimental systems, ASC, GSH and associated enzymes play an important role in protecting cultured roots from oxidative damage during elicitation (Ali etal. 2005b). The stimulation of saponin production by hairy root cultures of Panax ginseng and Panax quinqefolium by methyl jasmonate (MeJA) was accompanied by a gradual increase in H2O2 content. In addition, P. quinqefolium root culture showed a marked increase in lipooxygenase activity upon MeJA treatment, which usually results in increased lipid peroxidation. However, the markers of lipid peroxidation declined upon elicitation in roots of both species of ginseng, which suggests that in spite of increased oxidation no severe oxidative damage occurred in treated roots. It was shown that MeJA-induced oxidative stress was mitigated by an increase in the activities of various components of the antioxidant system. Upon elicitation, ASC content and ASC/DHA ratio as well as SOD and POX activities increased in both species. On the other hand, a decrease in CAT activity was observed. APX activity increased after MeJA treatment in P. ginseng but not in P. quinqefolium. In contrast, P. quinqefolium was characterised by increased DHAR activity but the activity of this enzyme decreased in P. ginseng. These findings indicate that, the enzymes of ASC–GSH cycle play a protective role against oxidative stress during the elicitation of metabolite production in hairy root culture (Ali etal. 2005b).

13 Ascorbate and Glutathione Roles in Shoot Organogenesis In vitro Relatively less attention, when compared with somatic embryogenesis and root formation, was paid on the role of ascorbate and glutathione in shoot differentiation in plant tissue culture. Early reports suggest that ASC may enhance shoot organogenesis in tobacco callus culture (Joy etal. 1988). Addition of 0.8 mM ASC to the shoot-forming medium increased shoot formation by 45% over the control in young callus, having a high organogenetic potential. Different concentrations of exogenous ASC (0.4–0.8 mM) were able to restore regenerative capacities of old callus tissue, characterised by reduced ability to regenerate shoots. Treatment with ASC also speeded up the shoot-forming process with the appearance of primordia as early as day 10 in culture, compared to more than 12 days in control tissue. It has also been shown that ASC treatment reversed the inhibitory effect of exogenous gibberelic acid on root formation. The stimulatory effect of ASC on shoot differentiation in tobacco callus was linked to the content of soluble sugars, which increased in tissue

2 Ascorbate and Glutathione in Organogenesis

during ASC treatment. However, the mechanism of ascorbate involvement in shoot organogenesis in tobacco has not been explained (Joy etal. 1988). Treatments with reduced glutathione improved the development of isolated shoot tips of apple (Nomura etal. 1998). Isolated shoot meristems were grown on basal medium supplemented with BAP for shoot development and then transferred onto the medium containing IBA for rooting the differentiated shoots. Efficiency of shoot tip culture was strongly reduced by explant browning observed within 1 day after the start of culture. Explant browning was prevented by dipping the shoot tips in 0.1 mM GSH, prior to inoculation on a solid medium. Besides inhibiting browning, dip treatment increased the number of shoot tips that developed into normal shoots (Nomura etal. 1998). Treated shoots developed into normal shoots with an efficiency of 100%. In the untreated control, rates of shoot development from untreated shoots did not exceed 60%. Promotion of normal shoot development by the dip treatment was also expressed in terms of the length of first leaves from cultured shoot tips. The dip-treated shoot tips produced longer first leaves when compared to untreated controls. Interestingly, the promotion of shoot development was observed only if pulse GSH treatment was applied before the start of culture and after that, shoot tips were grown on basal medium. If instead of GSH pulse treatment, shoot tips were cultured on medium supplemented with GSH, shoot development was not stimulated. On the other hand, cultures on medium containing GSH promoted callus proliferation at the base of shoot tips (Nomura etal. 1998). Reducing conditions imposed by adding antioxidants (ASC, GSH and a-tocopherol) stimulated shoot organogenesis from leaf segment of gladiolus (Dutta Gupta and Datta 2003). Both, frequency of shoot organogenesis as well as the number of number of shoots per explant were increased with the addition of antioxidants at 0.5 mM concentration. However, higher concentrations were found to be inhibitory. Compared to a-tocopherol and ASC, GSH was noted to be most effective. On the other hand, H2O2 added to the culture medium in concentrations ranging from 0.005 to 0.15 mM strongly inhibited shoot organogenesis. Moreover, the process of shoot formation was marked by a gradual increase in the CAT and POX activities with a concurrent decrease in the SOD activity. Given that, CAT and POX are H2O2-consuming enzymes, whereas high SOD activity results in elevated H2O2 levels, it was concluded that shoot organogenesis requires a reducing environment (Dutta Gupta and Datta 2003). In contrast to shoot organogenesis, somatic embryo formation on the gladiolus leaf explants was promoted by oxidising factors. The frequency of somatic embryogenesis and the number of embryos per responding culture were decreased by ASC, GSH and a- tocopherol in a dose-dependent manner. On the other hand, H2O2 at 100 mM concentration increased the frequency of somatic embryogenesis by about 18%. The process of somatic embryo formation was accompanied, in its initial phase, by an increase in SOD activity and progressive reduction in CAT and POX activities, which creates conditions favourable for H2O2 accumulation (Dutta Gupta and Datta 2003). Similar to gladiolus, initial phase of somatic embryo formation in the callus of Lycium barbarum was characterised by increase in SOD activity accompanied by decrease in POX and CAT activities, which resulted in a marked increase in intracellular H2O2 levels. Moreover, the treatments that increase H2O2 levels, such as inhibiting

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CAT activity with aminotriazole or adding H2O2 to the culture medium promoted somatic embryogenesis. On the other hand, inhibiting SOD activity with N,N¢diethyldithiocarbonate caused a decrease in the frequency of somatic embryogenesis (Kairong etal. 1999). The aforementioned data suggest that shoot organogenesis and somatic embryogenesis differ in their redox requirements, being promoted by reducing or oxidising environment, respectively. However, it should be kept in mind that in some shoot regeneration systems, similar to somatic embryogenesis, oxidising processes may also have a stimulatory effect on organogenesis. This was the case for shoot organogenesis in strawberry callus, where H2O2 levels and SOD activity increased during early phases of shoot regeneration when meristemoid formation and vascular tissue occurs in calli grown on regeneration medium. Simultaneously, a decline in H2O2 scavenging enzymes; CAT and POX was observed (Tian et al. 2003). Similar to somatic embryogenesis systems (Kairong etal. 1999; Dutta Gupta and Datta 2003), shoot organogenesis percentage in strawberry was stimulated by exogenous H2O2 and decreased by SOD inhibitor – N,N¢-diethyldithiocarbonate. Moreover, it was shown that types of strawberry calli, which showed high regeneration capacity, had several times higher H2O2 and O2− levels compared to calli with low-regeneration capacity (Tian etal. 2004).

14 Conclusion The studies reported in this review demonstrate that ascorbate- and glutathionedependent biochemical systems play important roles in many aspects of plant tissue culture, affecting growth, differentiation and metabolism. Ascorbate and glutathione participate in plant regeneration, being involved in the mechanisms regulating cell divisions in newly formed meristems and participating in hormone metabolism and signalling. Moreover, an antioxidant activity of these molecules manifested in Halliwell–Asada cycle protects in vitro cultured tissues against oxidative stress. Diverse functions of these antioxidants open vast possibilities of using them for the improvement of tissue culture and plant regeneration methods. However, further studies are required to fully exploit the properties of ascorbate and glutathione for manipulating developmental processes in plant tissue culture.

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Chapter 3

Role of Ascorbate Peroxidase and Glutathione Reductase in Ascorbate–Glutathione Cycle and Stress Tolerance in Plants Cai-Hong Pang and Bao-Shan Wang

Abstract Ascorbate–glutathione (AsA–GSH) cycle is an important component of the scavenging system for reactive oxygen compounds in plants. The member of this pathway involves the antioxidants: AsA, GSH and the antioxidatnt enzymes such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAsAR), dehydroascorbate reductase (DAsAR) and glutathione reductase (GR). APX and GR are the key enzymes of the AsA–GSH cycle. APX utilizes AsA as the electron donor reducing H2O2 to water, and prevents the accumulation of a toxic level of H2O2 in photosynthetic organisms under stress conditions. GR converts oxidized glutathione (GSSG) to GSH using NAD(P)H as an electron donor. And thus a highly ratio of GSH/GSSG and AsA/DAsA is maintained at the intracellular level by this reaction during oxidative stress. In general, APX and GR activity have been shown to increase in various plant species under different stress conditions. Transgenic plants showed that APX and GR play an important role in providing resistance to oxidative stress caused by diferent stressors such as paraquat, methyl viologen, ozone, drought, heavy metals, high light, salinity, and chilling. In this review, recent progress in research on APX, GR and stress tolerance will be discussed.

Keywords Ascorbate peroxidase · Glutathione reductase · AsA-GSH cycle · Stress tolerance

C.-H. Pang Shandong Academy of Forestry, Jinan 250014, Shandong, P.R. China B.-S. Wang (*) Key Lab of Plant Stress Research, College of Life Sciences, Shandong Normal University, Jinan 250014, Shandong, P.R. China e-mail: [emailprotected] N.A. Anjum et al. (eds.), Ascorbate-Glutathione Pathway and Stress Tolerance in Plants, DOI 10.1007/978-90-481-9404-9_3, © Springer Science+Business Media B.V. 2010

C.-H. Pang and B.-S. Wang

1 Properties and Functions of Ascorbate Peroxidase and Glutathione Reductase of the AsA–GSH cycle in plants In plants, the glutathione–ascorbate cycle operates in the cytosol, mitochondria, plastids and peroxisomes (Jimenez etal. 1998; Meyer 2009). In the first step of the cycle, H2O2 is reduced to water by APX using AsA as the specific electron donor. At the same time AsA was oxidized to monodehydroascorbate (MDAsA). MDAsA is directly reduced to AsA by monodehydroascorbate reductase (MDAsAR) or spontaneously disproportionated to dehydroascorbate (DAsA). DAsA is reduced to AsA by dehydroascorbate reductase (DAsAR) at the expense of GSH, yielding GSSG. Finally, GSH must be regenerated using GSSG which is reduced by GR using NADPH as electron donor. Thus AsA and GSH are not consumed; the net electron flow is from NADPH to H2O2. We simplified the ascorbate–glutathione cycle in a plant cell in Fig.1. The ascorbate–glutathione cycle provides a mechanism for the dissipation of the excess of reducing power in plant cell, in particular in chloroplasts under stress. This cycle play an immportant role in keeping the equilibrium between the ROS production and scavenging (Pang and Wang 2007). High levels of both ascorbate and glutathione pools are a requisite for this cycle (Anderson etal. 1983).

apoplast cytoso

plasma membrane H2O2 chloroplas

mitochondri

APX

AsA

H2O MDAsA

MDAsAR NAD (P)+ NAD (P)H NAD (P)H NAD (P)+ GR

microbody

GSH

GSSG

DAsA

DAsAR

Fig.1 Simplified scheme of AsA–GSH cycle in plant cellsThis cycle is mainly localized in the cytosol, chloroplasts, mitochondria and microbody. APX and GR play a critical role in AsA–GSH cycle. In the AsA–GSH cycle, solid arrows represent enzyme-mediated reactions and dotted arrows indicate spontaneous reactions. Abbreviations: AsA, Ascorbate; MDAsA, monodehydroascorbate; DAsA, dehydroascorbate; MDAsAR, monodehydroascorbate reductase; DAsAR, dehydroascorbate reductase; GSH, reduced glutathione; GSSG, oxidized glutathione; APX, ascorbate peroxidase; GR, glutathione reductase

3 Role of Ascorbate Peroxidase and Glutathione Reductase

2 Characteristics of Ascorbate Peroxidase APXs (EC1.11.1.11) are enzymes that detoxify H2O2 using AsA as a substrate. The reaction they catalyzed is the transfer of electrons from AsA to a H2O2, producing dehydroascorbate and water (Raven 2000). The reaction is as follows: Ascorbate + Hydrogenperoxide → Dehydroascorbate + Water APX isoenzymes have high affinity for AsA as electron donors. One of the most characteristic properties of APX, which distinguishes it from guaiacol peroxidase, Cyt c peroxidase and glutathione peroxidase, is its instability in the absence of AsA. When the concentration of AsA is lower than 20 mM, APX was inactivated rapidly and lost their stability (Shigeoka etal. 2002). Jespersen etal. (1997) suggested that tryptophan-175 of chloroplast APX increased their substrate specificity. So the chlAPX is more labile than cAPX in the absence of AsA. The half-life of chlAPX in ascorbate-depleted media is about 15 s, while those of cAPX isoforms are about 60 min (Miyake and Asada 1996; Yoshimura et al. 1998). The instability of chlAPX may be one reason that APX was not found in photosynthetic orgnisms for a long time. Ascorbate peroxidase is a haem-containing enzyme whose prosthetic group is protoporphyrin. Iron plays an important role in the catalytic site. This was clearly shown by applying iron deficiency conditions in sugar beet (Zaharieva and Abadia 2003). Cyanide and azide are the inhibitors of APX (Shigeoka etal. 1980). APX is also inhibited by thiolmodifying reagents (r-chloromercuribenzoate), thiol and suicide inhibitors such as hydroxyurea and r-aminophenol (Chen and Asada 1990). Some functional amino acid residues have been identified in the APX catalytic domain by analysis site-directed mutagenesis. Arg172 was changed to lysine, glutamine and asparagine, the variants of APX are incapable of oxidizing AsA to form Compound II. It is suggested that Arg172 of pea cAPX plays a key role in AsA utilization (Bursey and Poulos 2000). Arg172, Lys30 are the location of ascorbate binding site and the hydrogen-bonding interactions (Sharp etal. 2003). Arg-38 has a possible functional role in the control of substrate binding and orientation (Raven et al. 2001). The active site structures, including the hydrogen-bonding interactions between the proximal His-163 ligand, a buried Asp-208 residue, and a Trp-179 residue, are same to cytochrome c peroxidase (CCP). The difference between the APX and CCP is the presence of a cation binding site in APX (Patterson and Poulos 1995). Glu 112 forms a salt bridge with Lys20 and Arg24 of the opposing subunit near the axis of dyad symmetry between the subunits and is related to the dimer interface including an alteration in solvent structure (Mandelman etal. 1998a). AsA oxidation does not occur at the exposed heme edge but at an alternate binding site in the vicinity of Cys32 near Arg172 and the heme propionates (Mandelman etal. 1998b).

C.-H. Pang and B.-S. Wang

2.1 Isoforms and Subcelluar Localization of Acorbate Peroxidase As shown in Fig.1, APX isoenzymes are distributed in at least four distinct cellular compartments: cytosolic APX (cAPX), chloroplast APX (chlAPX) including stromal APX (sAPX) and thylakoid membrane-bound APX (tAPX), microbody membranebound APX (mAPX, including glyoxysome and peroxisome), and mitochondrial APX (mitAPX, a membrane-bound form). APX has also been found in the protozoan Trypanosoma cruzi (Boveris etal. 1980) and the bovine eye (Wada etal. 1998). The subcellular localization of APX isoforms is determined by the signal peptides and transmembrane domains in the N- and C- terminal regions (Shigeoka etal. 2002; Teixeira etal. 2004). A novel pumpkin APX isoenzyme was found to be localized on membranes of microbodies (Yamaguchi etal. 1995) and spinach glyoxysomes APX was found to be located on the external side of the organelles (Ishikawa etal. 1998). The targeting signal of peroxisomal APX comprises a C-terminal transmembrane domain (TMD, rich in valine and alanine) and the followed positively charged domain containing five amino acid rsidues (Mullen and Trelease 2000). These isoforms are indirectly sorted to the peroxisomes via a subdomain of the rough endoplasmic reticulum (Lisenbee etal. 2003). chlAPX isoforms possess a transit peptide consisting of approximately 70 residues in the N-terminus rich in Ser and Thr (Shigeoka et al. 2002). At C-terminal, tAPX isoforms present a transmembrane hydrophobic domain which is responsible for spanning to the stroma-exposed thylskoid membranes in chloroplasts (Ishikawa etal. 1996). Chew etal. (2003) showed that the Arabidopsis stromal APX is dual-targeted to chloroplasts stroma and mitochondria. An additional methionine residue 25 residues from the initiator methionine resulted in an additional band upon translation.This is maybe the reason that the APX isoform produces an ambiguity targeting peptide in the N-terminus.

2.2 Ascorbate Peroxidase Family and its Evolution in Plants On the basis of the amino acid sequences plant peroxidases were classified into three classes (Welinder 1992). Class I contains the intracellular peroxidase of prokaryotic origin including yeast cytochrome c peroxidase (CCP) (Dabrowska etal. 2007; Shigeoka etal. 2002), class II consists of secretory fungal peroxidases which include ligninases, or lignin peroxidases (LiPs), and manganesedependent peroxidases (MnPs). And class III consists of the secretory plant peroxidases. APXs belong to the class I family (Dabrowska etal. 2007; Shigeoka etal. 2002). They are found in chloroplastic organisms and putatively in cyanobacterial (Miyake etal. 1991). The phylogenetic tree was constructed using ClustalW software based on the deduced amino acid sequences of the APX isoenzymes. It was found that the APX isoforms can be divided into four groups (two kinds of cAPX, chlAPX and mAPX) (Shigeoka etal. 2002). Diversification of cAPX and chlAPX was the first step of

3 Role of Ascorbate Peroxidase and Glutathione Reductase

the APX evolution. They have the same originand and were resulted from duplication events in the basal Virdiplantae (Shigeoka et al. 2002; Passardi et al. 2007; Dabrowska et al. 2007). ChlAPX can be divided into two subclasses: tAPX and sAPX. Peroxisomal APX isoforms are only present in land plants. They probably originated from cytosolic isoforms that gained an exon encoding the membranebound domain and the targeting peptide (Teixeira et al. 2004). APX isoforms in higher plants show high hom*ology (70–90%) within each group and 50–70% hom*ology with each other (Shigeoka etal. 2002).

2.3 Ascorbate Peroxidase-Encoding Genes in Higher Plants The genes encoding APXs in higher plants have been isolated and characterized. Seven APX genes were identified in the tomato genome including three cytosolic isoforms, two peroxisomal isoforms and two chloroplastic ones (Najami etal. 2008). Rice APX gene famiy is comprised of genes encoding two cytosolic, two putative peroxisomal, and four putative chloroplastic APXs (Teixeira etal. 2006). Nine genes were suggested as APX genes in Arabidopsis thaliana which include three cytosolic, three peroxisomal and two chloroplastic (one thylakoid-bound and one whose product is targeted to both chloroplast stroma and mitochondria) (Chew etal. 2003). There are two or more putative cAPXs in higher plants.The pea cAPX gene (APX1) is the one whose structure is best known (Mittler and Zilinskas 1992). APX1 is encoded by a single copy gene and contains nine introns which have a high AT nucleotides content. The first intron is in the 5¢UTR region. Steady state APX1 transcript level was increased by drought, heat, and application of ethephon, abscisic acid, and the superoxide-generating agent paraquat. This phenomenon is possibly in relation to the regulatory elements in the promoter of APX1. These include two repeats of a putative plant heat-shock element (HSE) and a reversed antiperoxidative element (ARE). In addition, two repeats of a putative xenobiotic responsive element (CACGCA) were also found at the positions: −282 and −302 and a putative GPEI enhancer was found at position 97. Recent analysis of knockout plants deficient in cytosolic APX1 (KO-Apx1) revealed that cAPX plays a key role in protecting chloroplasts under light stress conditions (Pnueli etal. 2003; Davletova etal. 2005). The cAPX isoforms are hom*odimers. Subunit molecular weight of rice cytosolic APX1 and APX2 is 28.5 and 34 kDa respectively (Sharma and Dubey 2004). The molecular weight of pea cAPX is 57.5 kDa and its subunit is 29.5 kDa (Mittler and Zilinskas 1991). Ascorbate peroxidase gene, ecoding spinach chlAPX isoenzymes, consists of 13 exons and 12 introns. Exons 1 ~ 11 encode the common amino acid sequence of sAPX and tAPX. There are two splice acceptor sites in the exon 12 and 13 seperated by 14 bp nucleotides. Exon 12 contains one condon for Asp-365 before the termination codon and the entire 3¢-UTR including potential polyadenylation signal (AATAA) of sAPX mRNA. The final exon consists of the sequence coding hydrophobic C-terninal region,TGA terminal codon and the entire 3¢-UTR including potential polyadenylation signal (AATATA) of tAPX mRNA. By alternative splicing

C.-H. Pang and B.-S. Wang

of the intron 11 and 12 in the 3¢-termina, two kinds of mRNA are generated by one chlAPX gene. The N-termianl amino acids of sAPX are completely identical with those of tAPX, while c-terminal amino acids differ. A similar finding was also made in spinach (Ishikawa etal. 1997), pumpkin (Mano etal. 1997) tabacco and Suaeda salsa (data unpublished). While in Arabidopsis thaliana sAPX and tAPX are encoded by different genes. The nucleotide sequences of their cDNAs share only 66.1% identity (Jespersen etal. 1997). APX isoforms located in organelle are monomer with different molecular weight. tAPX with a molecular weight of 40 kDa is about 10 kDa higher than that of the sAPX from spinach chloroplasts (Miyake etal. 1993). The reason for difference in molecular mass between tAPX and sAPX is related to the transmembrane domain in the C-terminal. The spatial expression patterns of the APX genes were studied in tomato (Najami et al. 2008). Among the cytosolic APX genes, SlAPX1 was mainly expressed in the root, SlAPX2 in the root, stem, and fruit while SlAPX3 in the stem and fruit. Interestingly, the relative abundance of the three cytosolic APX transcripts, and especially that of SlAPX3, was lower in the leaves than in the root, stem, and fruit. Transcripts of SlAPX4 were found to be accumulated mostly in the leaves and stem, and SlAPX5 in the leaves, stem, and fruit. Of the two chloroplastic genes, SlAPX6 was highly expressed in leaves, less in fruit, stem and root and SlAPX7 was contrary. It suggested that the expression pattern of APX genes is different at tissue level.

3 Characteristics of Glutathione Reductase Glutathione reductase (GR, EC1.6.4.2), which converts oxidized glutathione (GSSG) to reduced glutathione (GSH) using NADPH, is ubiquitous in living organisms. It is necessary for maintaining the high ratio of GSH/GSSG in the plant cells and accelerating the H2O2 scavenging pathway in plants particularly under stress conditions (Smith etal. 1989). GR catalyses following reaction: GSSG + NADPH + H + = 2 GSH + NADP + GR has high specificities for its substrates, although some glutathione conjugates and mixed glutathione disulphides can be reduced by GR (Gaullier et al. 1994). Most GRs from plants have a high affinity for NADPH ( Glutathione reductase GR1 isoform 1

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Reumann etal. 2009 Kataya and Reumann 2010

Fig.2 Peroxisomal immunogold electron microscopy localizations of several enzymes of the ascorbate–glutathione cycle. Panel (a): electron micrograph showing the immunocytochemical localization of APX in peroxisomes of cucumber cotyledons (Reproduced, with permission, from Corpas etal. 1994). Panel (b): electron micrograph showing the immunocytochemical localization of GR in pea leaves peroxisomes (Reproduced, with permission, from Romero-Puertas etal. 2006). Panel (c): electron micrograph showing the immunocytochemical localization of MDAR in pea leaves peroxisomes (Reproduced, with permission, from Leterrier etal. 2005). CW, cell wall; P, peroxisome; M, mitochondrion; CH, chloroplast. Bar = 0.5 µm

did not affect Arabidopsis growth and development suggesting that its peroxidase function could be compensated for by other antioxidant enzymes (Narendra etal. 2006). It currently remains unknown whether a soluble APX isoform is also part of the peroxisomal AsA–GSH cycle, similar to MDAR (see below). Arabidopsis is predicted to have eight to nine APXs (Jespersen etal. 1997; Panchuk etal. 2002;

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Chew et al. 2003; Narendra et al. 2006). Based on similarities in sequence and secondary structure, and the presence of predicted peroxisome targeting signals type 1 or type 2 (PTS1/2), APX5 and APX4 (LKL>) have been proposed to be integral to the peroxisome membrane and localized in the matrix, respectively (Panchuk etal. 2002). However, experimental evidence for their association with peroxisomes is not yet reported. The expression and activity of catalase and APX (and possibly MDAR, DHAR, and GR) must be coordinated to control intraperoxisomal H2O2 levels generated under physiological or adverse environmental stress conditions, as well as the release of H2O2 release from peroxisomes. However, data yielding insight into the regulation of the Asc–glutathione cycle at the transcriptional or post-translational level are extremely scarce. The five genes involved in the peroxisomal AsA–GSH cycle are constitutively expressed at significant levels at all developmental stages, indicating a constitutive detoxification function under standard growth conditions (Fig.3). In addition, several genes are reported to be further induced by abiotic and biotic stress (see Section3). In addition to gene induction, the peroxisomal AsA–GSH cycle might be regulated at the post-translational level. An isoform of the 14-3-3 protein family, G-box regulating factor 6 (GRF6), also annotated as GF14l, has been detected in a yeast two-hybrid screen and characterized to interact with APX3 (Yan et al. 2002). 14-3-3 proteins generally bind to phosphorylated proteins and regulate signal transduction and protein activity (DeLille etal. 2001), suggesting that APX3 activity or protein–protein interactions might be regulated by reversible phosphorylation. The same 14-3-3 protein (GF14l) has been identified, along with APX3 itself, in the proteome of Arabidopsis leaf peroxisomes. Together with the absence of

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Fig.3 Gene expression analysis of the five Arabidopsis enzymes of the peroxisomal AsA–GSH cycle at different developmental plant stages. The expression data derive from microarray experiments and were retrieved using Genevestigator (www.genevestigator.com; Zimmermann et al. 2004, 2005). High and low expression levels are reflected semi-quantitatively by dark and light blue coloring, respectively. The numbers indicate the number of underlying microarrays

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predicted matrix targeting signals, the data strongly suggest that GF14l associates with leaf peroxisomes by its physical interaction with the cytosolic domain of APX3 (Reumann etal. 2009). In vivo validation of such an indirect protein targeting mechanism to peroxisomes might not be trivial and remains to be reported. Monodehydroascorbate generated by APX is converted back into ascorbate by MDAR using NAD(P)H as the reductant (Hossain and Asada 1984; Asada and Takahashi 1987). Arabidopsis peroxisomes possess two MDARs; MDAR1 is a PTS1containing matrix enzyme (AKI>), whereas MDAR4 is an integral membrane protein with its catalytic domain facing the matrix side (Lisenbee etal. 2005) (Fig.2). The differential sub-compartmental localization of the two enzymes indicates slightly divergent roles in AsA recycling, i.e., detoxification of H2O2 escaping from peroxisomes (MDAR4, see above) and clearance of intra-peroxisomal H2O2 (MDAR1). By application of a forward genetic screen, Eastmond (2007) demonstrated that MDAR4 is essential to prevent oil bodies from incurring oxidative damage, when in close proximity to peroxisomes, and triacylglycerol lipase from inactivation. The loss-offunction mutant is conditionally seedling-lethal because its seeds are unable to break down storage oil to provide carbon skeletons and energy for early seedling growth. Dehydroascorbate, produced nonenzymatically from AsA or monodehydroascorbate, is reduced to ascorbate by DHAR using reduced glutathione as the reducing co-factor (Hossain and Asada 1984). Arabidopsis encodes five DHAR isoforms that are either shown to be or predicted to be cytosolic or mitochondrial/plastidic (Chew etal. 2003). DHAR1 (At1g19570) was detected in Arabidopsis leaf peroxisomes by proteomics and confirmed to be peroxisome-localized by invivo subcellular targeting analysis (Reumann etal. 2009). Similar to a number of newly identified matrix proteins, DHAR1 does not carry a predicted PTS neither type 1 nor type 2, and is thus predicted to be imported by a novel import pathway. Glutathione reductase (GR) is a major enzyme of the antioxidative defense system and plays an important physiological role in maintaining and regenerating reduced glutathione in response to biotic and abiotic stresses in plants (RomeroPuertas etal. 2006). GR mediates the reduction of GSSG to GSH by using NADPH as an electron donor. In contrast to the large gene families of APX, MDAR, and DHAR in Arabidopsis, only two GR isoforms are encoded in the Arabi dopsis genome. Proteomic experiments identified GR1 (At3g24170) in Arabidopsis peroxisomes from both mature leaves and suspension-cultured cells (Eubel etal. 2008; Reumann etal. 2007, 2009). GR1 orthologs were previously reported to be cytosolic. However, expression of GR1 as an N-terminal fusion protein with a fluorescent reporter protein, GR1 has validated its peroxisomal localization (Kataya and Reumann 2010). The efficiency of peroxisome targeting, however, was weak upon expression from a strong promoter, consistent with the idea that the enzyme is dually targeted to peroxisomes and the cytosol invivo. The C-terminal tripeptide of GR1, TNL>, was shown to represent a novel, albeit weak, PTS1 (Kataya and Reumann 2010). Arabidopsis GR1 is the first plant protein that is dually targeted to peroxisomes and the cytosol. Strikingly, Arabidopsis GR2 is likewise dually targeted, namely to chloroplasts and mitochondria (Creissen etal. 1995). Nonetheless, the underlying

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regulatory mechanism of dual targeting to chloroplast and mitochondria remains unknown. Dual targeting of GR1 orthologs to both peroxisomes and the cytosol appears to be conserved across Viridiplantae and might partly be determined by the weak peroxisome targeting efficiency of TNL>. Additionally, in light of the importance of the peroxisomal AsA–GSH cycle under sudden conditions of catalase inactivation (Feierabend and Engel 1986), the ratio of GR1 distribution between peroxisomes and the cytosol may be dynamic and adjustable depending on H2O2 overproduction in the matrix by post-transcriptional and/or post-translational mechanisms (Kataya and Reumann 2010). Since GR1 provides reduced glutathione not only for DHAR but also for several glutathione-dependent enzymes that have recently been discovered in plant peroxisomes (see below), the physiological function of peroxisomal GR can hardly be overestimated.

3 Peroxisomal AsA–Glutathione Enzymes Under Environmental Stress Conditions Plants are exposed to continuous environmental changes. In many cases, these changes can induce oxidative stress that provokes cellular damage. Therefore, plants have developed a battery of antioxidative enzymatic and non-enzymatic mechanisms to attenuate or overcome the negative effects of ROS. In this context, the AsA–GSH cycle is, together with catalase, superoxide dismutase (SOD) and other peroxidases, a first line of defense against the deleterious effect of ROS. Usually, the antioxidant systems are located close to the place of ROS generation, including cytosol, chloroplasts, mitochondria and peroxisomes. Under environmental stress conditions, H2O2 may be overproduced in the peroxisomal matrix, causing oxidation of proteins, lipids, metabolites, and co-factors in the matrix, the peroxisome membrane and likely, upon diffusion across the membrane, other cell compartments. Thus, the presence of all components (enzymatic and non-enzymatic) of the AsA–GSH cycle in plant peroxisomes is an auxiliary strategy for plant cells to maintain the H2O2 level in this cell compartment, which could have a relevant significance under adverse stress conditions.

3.1 Abiotic Stress Conditions The term abiotic stress includes many environmental factors that can cause an increased formation of ROS, including high light, extreme temperature, heavy metals, salt, and drought. Knowledge of the specific contribution of peroxisomes in the mechanism of response against these stresses is very limited. Even less is known about the AsA–GSH cycle, considering that the description of this cycle in peroxisomes is relatively new. Table 2 summarizes the available data of some of the components of the AsA–GSH cycle from isolated peroxisomes from different

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Table2 Peroxisomal Asc–glutathione enzyme activities and gene expression under environmental stress conditions Abiotic or biotic stress Plant species condition Peroxisomal enzyme Reference Dark induced senescent Decrease in APX and Jiménez etal. Pea (Pisum leaves MDAR activities 1998 sativum) Cadmium (50 µM) Increase in APX and GR Romero-Puertas activities etal. 1999, 2006 Leterrier etal. Wounding, low temperature Increase in MDAR expression 2005 (8°C), 2,4-dichlorophenoxyacetic acid (2,4-D) Palma etal. 2006 Decrease in APX and Nitrate-fed and nodulated MDAR activities, plants at two different increase in DHAR growth stages (9 and 12 and GR activities weeks) Heat, salt, and abscisic acid Increase in the transcript Shi etal. 2001 Barley (Hordeum vulgare) treatments level of APX Light stress (500 mE m−2 s−1), Increase in the transcript Kavitha etal. Grey mangrove (Avicennia level of APX 2008 NaCl (500mM), H2O2 marina) (90mM), Fe(III) citrate (1.0 mM) Tobacco (Nicotiana Oxidative stress damage Overexpression of Wang etal. 1999 tobacum) caused by aminotriazole Arabidopsis APX3 Decrease in GR activity Mateos etal. Pepper (Capsicum Fruit ripening during the 2003 annuum) transition from green to red fruit Mittova etal. NaCl (100 mM) Increased APX and Tomato 2003, 2004 MDAR activities and (Lycopersicon reduced GR activity esculentum) Kuzniak and Botrytis Decrease in APX, GR, Skłodowska MDAR, 2005a and DHAR activities Powdery mildew Chen etal. 2006 Wheat (Triticum Enhanced APX aestivum) expression APX, ascorbate peroxidase; MDAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase

organs and plant species. Peroxisomal isoenzymes of the AsA–GSH cycle must be investigated in isolated peroxisomes to avoid an overlap with the non-peroxisomal isoenzymes, which points to some of the difficulties in analyzing peroxisomal enzymes and their activities.

3.2 Biotic Stress Conditions During pathogen infection, it has been proposed that peroxisomes are involved mechanistically in systemic defense responses (Kimura etal. 2001). Peroxisomes

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are involved in the production of lipid-based signaling molecules, such as jasmonic acid, as well as in the generation of ROS and the modulation of antioxidative enzymes (Koo etal. 2006; del Río etal. 2006). For example, it has been reported that pathogens can induce rapid changes in H2O2 leading to a variety of physiological responses in plants (Bolwell 1999; Shetty etal. 2007). It remains to be demonstrated to what extent peroxisomes contribute to the overall oxidative burst. Catalase has been reported to be activated by calcium/calmodulin (Yang and Poovaiah 2002). This regulatory mechanism might play a role in pathogen-induced ROS production. However, to our knowledge, little information is available on how the AsA–GSH pathway is affected by pathogen attack. For example, tomato peroxisomes exhibit a decrease in the activities of APX, GR, MDAR, and DHAR when infected by the fungus Botrytis cinerea, which was also accompanied by a significant decrease in AsA and GSH concentrations (Kuzniak and Skłodowska 2005a).

4 Peroxisomal Redox Status of Ascorbate, Glutathione and NADPH Under Environmental Stress Conditions For the function of the AsA–GSH cycle, there is a close interplay among AsA, GSH, and NADPH. These multifunctional redox metabolites play essential roles in both the AsA–GSH cycle and in mediating signaling processes. Moreover, the concentrations of these metabolites can be greatly modified in response to a variety of environmental factors, mainly those that cause increased oxidative stress. Several of these molecules have been clearly detected in plant peroxisomes by different approaches, but the available information remains incomplete (Donaldson 1982; Jiménez etal. 1997). Glutathione (g-Glu-Cys-Gly) is one of the major, soluble, low molecular weight antioxidants, as well as the major non-protein thiol in plant cells (0.2–10 mM, Foyer and Noctor 2003). Glutathione not only fulfils multiple metabolic functions, such as detoxification of xenobiotics, ROS, and heavy metals (Tausz etal. 2004), but also acts as an independent redox-signaling molecule (Foyer and Noctor 2005). In this sense, GSH can also react with nitric oxide (NO) to form S-nitrosoglutathione (GSNO), which may function as an intracellular NO reservoir. GSNO may also act as a vehicle for NO throughout the cell, as a connection between ROS and RNS (Corpas etal. 2008). Glutathione has a wide distribution in the different subcellular compartments including mitochondria, nuclei, chloroplasts, cytosol and also in peroxisomes (Müller et al. 2004; Fernández-García etal. 2009). The peroxisomal localization of GSH has been demonstrated by either HPLC in isolated peroxisomes or by electron-microscopic immunogold cytochemistry in different plant species under normal and adverse conditions (Jiménez etal. 1997; Mateos etal. 2003; Müller etal. 2004; Zechmann etal. 2008; FernándezGarcía etal. 2009; Kolb etal. 2009). In pumpkin plants infected by zucchini yellow mosaic virus (ZYMV), a general rise in GSH has been described in older leaves (as evaluated by electron microscopic immunogold cytochemistry)

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with GSH levels 1.7-fold higher in peroxisomes, 1.6-fold in the cytosol, 1.4fold in mitochondria, and 1.2-fold in nuclei when compared to control cells (Zechmann etal. 2007). Even beyond its function in the peroxisomal AsA–GSH cycle, glutathione is now recognized as a major co-factor of metabolic reactions in plant peroxisomes. For instance, several glutathione-S transferases have recently been discovered in peroxisomes (Reumann etal. 2007, 2009; Dixon etal. 2009). Ascorbate is the most abundant non-thiol (10–100 mM) small molecule antioxidant in plants (Foyer and Noctor 2003). Ascorbate has also been detected in plant peroxisomes by HPLC, and its concentrations were shown to be dynamic and affected by different growth conditions. For example, during natural and induced leaf senescence, the peroxisomal Asc content increased (Jiménez etal. 1998; Palma etal. 2006). Similar antioxidant changes have been described in peroxisomes isolated from pepper fruits where the concentration of peroxisomal AsA increased during fruit ripening (conversion of green to red peppers), while the concentration of peroxisomal GSH remained constant (Mateos et al. 2003). In contrast, the infection of tomato plants by Botrytis cinerea caused a significant reduction in the concentration of both AsA and GSH in peroxisomes (Kuzniak and Skłodowska 2005b). Likewise, a reduction in the concentration of Asc in root tomato peroxisomes has been reported under salinity stress (100 mM NaCl, Mittova etal. 2004). NADPH is the specific donor of reducing equivalents to GR. Therefore, the production of this pyridine nucleotide inside of peroxisomes is required for a functional AsA–GSH cycle. The presence of NADP(H) in peroxisomes was reported by Donaldson (1982), where isolated glyoxysomes from castor beans contained 16% of the total NADP(H); although, to our knowledge, there is not any additional data. It has been reported that the recycling of NADPH from NADP can be carried out in plant peroxisomes by at least three dehydrogenases: glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and isocitrate dehydrogenase (ICDH) (Corpas et al. 1998, 1999, 2001). Specific Arabidopsis isoforms of both 6PGDH and ICDH have been localized to peroxisomes invivo (Reumann etal. 2007; Eubel etal. 2008). Consequently, any factors that could affect the activity of these peroxisomal dehydrogenases may also affect the function of the AsA–GSH cycle.

5 Release of Peroxisomal H2O2 and ROS Signaling Molecules Hydrogen peroxide is relatively stable and has been shown to act as a signaling molecule in different physiological and phytopathological processes (Alvarez et al. 1998; Bolwell 1999; López-Huertas et al. 2000; Orozco-Cárdenas et al. 2001; Shetty etal. 2007). A theoretical model, based on the latency of peroxisomal catalase from rat liver, concludes that the peroxisomal membrane represents a barrier to the free diffusion of H2O2. However, it could not be excluded that around 2% of the H2O2 produced in peroxisomes can diffuse out of the organelle

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(Poole 1975). In support of this prediction, Mueller etal. (2002) have demonstrated the release of H 2O 2 using a chemiluminescence method combined with in vitro analysis of isolated rat liver peroxisomes. Interestingly, the diffusion of H2O2 out of peroxisomes appears to be controlled not only by the compartmentalization function of the peroxisomal membrane, but also at second level where the highly packed peroxisomal matrix itself seems to act as an additional diffusion barrier (Fritz et al. 2007). While catalase generally removes matrix H2O2 efficiently, its spatial separation from urate oxidase prevents the removal of H2O2 derived from urate oxidation. It has been demonstrated that urate oxidase can directly release H 2O 2 into the cytoplasm via 5-nm primary tubules in crystalline cores (Fritz etal. 2007). Plant peroxisomes can be considered more efficient in keeping H2O2 under control as compared to mammalian and fungi peroxisomes because they have catalase in the matrix and APX in the membrane (Fig.2). However, under stress conditions, the generation of ROS can overcome the capacity of the peroxisomal antioxidative system. Consequently, it has been hypothesized that some ROS, mainly H2O2, can be released to the cytosol where they may trigger some signal transduction pathways (Corpas et al. 2001). In plants, there is some evidence that support this hypothesis. For example, in catalase-deficient Arabidopsis plants exposed to high light intensity (1,600–1,800 µmol m−2 s−1 for 3–8 h), there is a correlated increase of cellular H2O2 levels with the up-regulation of 349 transcripts and the down-regulation of 88 genes. Under these high light conditions, H2O2 seems to play a key role in the transcriptional up-regulation of small heat shock proteins (Vandenabeele etal. 2004; Vanderauwera etal. 2005). On the other hand, electron microscopic analysis of leaves from cadmiumtreated pea plants showed a significant H2O2 accumulation in the membrane of peroxisomes when in close contact with chloroplasts and the tonoplast (Romero-Puertas etal. 2004).

6 Conclusions Considering that metabolism of peroxisomes is very dynamic because it is adapted to many physiological and adverse conditions, the Asc–glutathione pathway must be studied in great detail at the molecular level and in relation with other peroxisomal components, such as the NADPH-recycling system and RNS. At the enzymatic level, reverse genetics must be applied to all genes to investigate the exact physiological function of all enzymes, including pathway regulation at the transcriptional, posttranscriptional and post-translational level and the regulation of dual targeting. Our understanding of sub-compartmentalization of the peroxisomal AsA–GSH cycle into membrane-bound and matrix-located pathways remains fragmentary and might require the identification of additional isoforms (e.g., soluble APX). A significant gap concerns the transfer of pathway intermediates across the peroxisome membrane. Future research must also focus on interactions among the different

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molecules present in these organelles, such as H2O2, AsA, GSH, nitric oxide (NO) and S-nitrosoglutathione (GSNO), and their metabolic enzymes. All of these molecules are also involved in signaling processes and consequently in the crosstalk among the different cell compartments. Acknowledgements SR and FJC were supported by UiS funding and grants from the Ministry of Education and Science (BIO2009-12003-C02-01), respectively.

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Mateos RM, León AM, Sandalio LM, Gómez M, del Río LA, Palma JM (2003) Peroxisomes from pepper fruits (Capsicum annuum L.): purification, characterisation and antioxidant activity. J Plant Physiol 60:1507–1516 McCartney AW, Greenwood JS, Fabian MR, White KA, Mullen RT (2005) Localization of the tomato bushy stunt virus replication protein p33 reveals a peroxisome-to-endoplasmic reticulum sorting pathway. Plant Cell 17:3513–3531 Mittler R, Zilinskas BA (1991) Purification and characterization of pea cytosolic ascorbate peroxidase. Plant Physiol 97:962–968 Mittova V, Guy M, Tal M, Volokita M (2004) Salinity up-regulates the antioxidative system in root mitochondria and peroxisomes of the wild salt-tolerant tomato species Lycopersicon pennellii. J Exp Bot 55:1105–1113 Mittova V, Tal M, Volokita M, Guy M (2003) Up-regulation of the leaf mitochondrial and peroxisomal antioxidative systems in response to salt-induced oxidative stress in the wild salt-tolerant tomato species Lycopersicon pennellii. Plant Cell Environ 26:845–856 Moschou PN, Sanmartin M, Andriopoulou AH, Rojo E, Sanchez-Serrano JJ, RoubelakisAngelakis KA (2008) Bridging the gap between plant and mammalian polyamine catabolism: a novel peroxisomal polyamine oxidase responsible for a full back-conversion pathway in Arabidopsis. Plant Physiol 147:1845–1857 Mueller S, Weber A, Fritz R, Mütze S, Rost D, Walczak H, Völkl A, Stremmel W (2002) Sensitive and real-time determination of H2O2 release from intact peroxisomes. Biochem J 363:483–491 Mullen RT, Trelease RN (1996) Biogenesis and membrane properties of peroxisomes: does the boundary membrane serve and protect? Trends Plant Sci 1:389–394 Mullen RT, Lisenbee CS, Miernyk JA, Trelease RN (1999) Peroxisomal membrane ascorbate peroxidase is sorted to a membranous network that resembles a subdomain of the endoplasmic reticulum. Plant Cell 11:2167–2185 Müller M, Zechmann B, Zellnig G (2004) Ultrastructural localization of glutathione in Cucurbita pepo plants. Protoplasma 223:213–219 Narendra S, Venkataramani S, Shen G, Wang J, Pasapula V, Lin Y, Kornyeyev D, Holaday AS, Zhang H (2006) The Arabidopsis ascorbate peroxidase 3 is a peroxisomal membrane-bound antioxidant enzyme and is dispensable for Arabidopsis growth and development. J Exp Bot 57:3033–3042 Orozco-Cárdenas ML, Narváez-Vásquez J, Ryan CA (2001) Hydrogen peroxide acts as a second messenger for the induction of defense genes in tomato plants in response to wounding, systemin, and methyl jasmonate. Plant Cell 13:179–191 Palma JM, Corpas FJ, del Río LA (2009) Proteome of plant peroxisomes: new perspectives on the role of these organelles in cell biology. Proteomics 9:2301–2312 Palma JM, Jiménez A, Sandalio LM, Corpas FJ, Lundqvist M, Gómez M, Sevilla F, del Río LA (2006) Antioxidative enzymes from chloroplasts, mitochondria, and peroxisomes during leaf senescence of nodulated pea plants. J Exp Bot 57:1747–1758 Panchuk II, Volkov RA, Schoffl F (2002) Heat stress- and heat shock transcription factor-dependent expression and activity of ascorbate peroxidase in Arabidopsis. Plant Physiol 129:838–853 Poole B (1975) Diffusion effects in the metabolism of hydrogen peroxide by rat liver peroxisomes. J Theor Biol 51:149–167 Pracharoenwattana I, Smith SM (2008) When is a peroxisome not a peroxisome? Trends Plant Sci 13:522–525 Reumann S, Babujee L, Ma C, Wienkoop S, Siemsen T, Antonicelli GE, Rasche N, Luder F, Weckwerth W, Jahn O (2007) Proteome analysis of Arabidopsis leaf peroxisomes reveals novel targeting peptides, metabolic pathways, and defense mechanisms. Plant Cell 19:3170–3193 Reumann S, Quan S, Aung K, Yang P, Manandhar-Shrestha K, Holbrook D, Linka N, Switzenberg R, Wilkerson CG, Weber AP, Olsen LJ, Hu J (2009) In-depth proteome analysis of Arabidopsis leaf peroxisomes combined with in vivo subcellular targeting verification indicates novel metabolic and regulatory functions of peroxisomes. Plant Physiol 150:125–143

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Chapter 15

Identification of Potential Gene Targets for the Improvement of Ascorbate Contents of Genetically Modified Plants Adebanjo A. Badejo and Muneharu Esaka

Abstract Up to half of the ~6.8 billion people living on planet earth live on less than $3 a day and also suffer from at least one micronutrient deficiency especially in Africa and Southeast Asia. Human lacks the capacity to synthesize vitamin C (ascorbic acid) and its deficiency causes scurvy. As excess vitamin C cannot be stored in the human’s body, there is the need to regularly consume fruits and vegetables to supply this essential compound. In plants, it is multifunctional and indispensable. Overexpression of heterogonous genes to introduce novel traits into plants offers an effective way to increase the vitamin content of crops. Although many alternative biosynthesis routes for vitamin C have been proposed, the Smirnoff–Wheeler (l-galactose) pathway has been proven to be the functional pathway in Arabidopsis and many other fruit-bearing plants. Identifying limiting genes in the biosynthesis pathways and overexpression of such genes severally and collectively as well as in combination with genes from ascorbate recycling pathway may lead to the generation of transgenic plants with ‘substantial’ amount of vitamin C for both human nutrition leading to reduced ‘hidden hunger’ and agronomic purposes. Proper dissemination of scientifically proven safety information about such transgenic plants will also increase public confidence in selecting and consuming such nutritionally enhanced genetically modified food crops. Keywords Biosynthesis pathway • Gene expression • Genetic engineering • Genetically modified foods

A.A. Badejo Faculty of Life and Environmental Science, Shimane University, Matsue, Shimane 690-8504, Japan e-mail: [emailprotected] M. Esaka (*) Graduate School of Biosphere Sciences, Hiroshima University, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8528, Japan e-mail: [emailprotected]

N.A. Anjum et al. (eds.), Ascorbate-Glutathione Pathway and Stress Tolerance in Plants, DOI 10.1007/978-90-481-9404-9_15, © Springer Science+Business Media B.V. 2010

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1 Ascorbic Acid in Living Organisms According to the Food and Agriculture Organization of the United Nations, close to 50% of the world’s population are suffering from diseases caused by lack of essential minerals and vitamins (FAO 2006). The developed societies have taken measures to reduce such diseases by ensuring that fruits and vegetables are included in their diet. These fruits and vegetables are rich sources of antioxidants that have also been found to reduce the risk of certain cancers and cardiovascular diseases (Hanco*ck and Viola 2005a). Unfortunately, malnutrition is widespread in the developing world where the populace depends upon monotonous cereal-rich diets that lacks the essential vitamins and minerals (Timmer 2003). Many vertebrates synthesize ascorbic acid. Fish, amphibians and reptiles synthesize ascorbic acid in the kidney; mammals on the other hand produce it in the liver (Chatterjee 1973; Moreau and Dabrowski 1998). African trypanosome, Trypanosoma brucei, and the American trypanosome, Trypanosoma cruzi have been shown to synthesis ascorbic acid in a single-membrane organelle, the glycosome (Wilkinson et al. 2005). Although the biosynthesis capacity of most eukaryotic organisms appeared early in the evolutionary history especially the fish (Moreau and Dabrowski 1998); many including human, have lost the capacity to synthesize the compound. The mutation and non-functional l-gulono-1,4-lactone oxidase, the enzyme catalyzing the last step in the biosynthesis of ascorbic acid, incapacitated human’s ability to synthesize the compound (Chatterjee 1973). Most plant species studied so far have been found to synthesize ascorbic acid (Davey etal. 2000; Smirnoff etal. 2001) including the photosynthesizing algae Euglena gracilis (Ishikawa et al. 2008). Yeasts are unusual in that they predominantly synthesize erythroascorbate, a fivecarbon analogue of ascorbic acid that possesses all of its biochemical properties (Huh etal. 1998). It took more than a century from the time the Scottish doctor James Lind found that administration of fresh citrus fruits could cure scurvy to the time it was first isolated by Svent Gyorgy (Davey etal. 2000). The studies on the biosynthesis of ascorbic acid in plants have been very interesting as it generates many direct and indirect connecting routes to the final product and has been reviewed by many authors (Smirnoff and Wheeler 2000; Valpuesta and Botella 2004; Hanco*ck and Viola 2005b). It was towards the close of the last century that Wheeler etal. (1998) proposed a functional biosynthesis pathway for ascorbic acid (Fig.1). The pathway has since been supported by genetic, molecular and biochemical proves (Conklin etal. 2006; Dowdle etal. 2007; Linster etal. 2007). Ascorbic acid is present in all subcellular compartments such as mitochondria, apoplast (cell wall), cytosol, vacuoles, chloroplasts (Rautenkranz et al. 1994; Smirnoff and Wheeler 2000), with concentration ranging from 20 to 300 mM depending on the location within the plant. Except the last enzyme in the biosynthetic pathway that is localized in the inner mitochondria, the whole pathway is cytosolic (Smirnoff 2000). Under normal condition, the ascorbic acid pool in plant is maintained at close to 90% in the reduced state (Foyer 1993). Ascorbate oxidase and Ascorbate peroxidase are the two enzymes that catalyze the oxidation of ascorbic

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B D-Glucose 6-P

Smirnoff-Wheeler (L-galactose) Pathway

D-Galacturonate Pathway

D-fructose 6-P

D-mannose 1-P

D-galacturonate NADPH NADP

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5 (VTC1)

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GDP-L-galactose 7 (VTC2)

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D-glucuronate

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3’,5’- epimerization

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GDP-D-mannose 6

L-Gulose Pathway

GDP-L-gulose

D-mannose 6-P

Uronic acid (Animal) Pathway

Methylgalacturonate

NADH

NADPH NADP

L-gulonate

L-galactono-1,4-lactone

Cyt Cox

L-gulono-1,4-lactone

Cyt Cred

Ascorbic Acid (Vitamin C)

Fig.1 Biosynthetic pathway of ascorbic acid. The pathway D represent the animal pathway. The flow in B represents the Smirnoff-Wheeler (l-galactose) pathway and the network of other alternative pathways are shown in A, C and E. The reactions are catalyzed by: 1. phosphoglucose mutase; 2. phosphoglucose isomerase (EC 5.3.9.1); 3. phosphomannose isomerase (EC 5.3.1.8); 4. phosphomannose mutase (EC 5.4.2.8); 5. GDP-mannose pyrophosphorylase (VTC1) (EC 2.7.7.22); 6. GDP-mannose-3′,5′-epimerase (EC5.1.3.18); 7. GDP-l-galactose phosphorylase; 8. l-galactose 1-phosphate phosphatase (VTC4); 9. l-galactose dehydrogenase; 10. l-galactono1,4-lactone dehydrogenase (EC 1.3.2.3); 11. nucleotide pyrophosphatase or sugar-1-P guanylyltransferase; 12. sugar phosphatase; 13. sugar dehydrogenase; 14. UDP-glucose pyrophosphorylase; 15. UDP-glucose dehydrogenase; 16. UDP-glucuronate pyrophosphorylase; 17. Glucuronate 1-kinase; 18. d-glucuronate reductase; 19. Aldonolactonase (EC 3.1.1.18); 20. l-gulonolactone oxidase or dehydrogenase; 21. myo-inositol 1-P synthase; 22. myo-inositol 1-P phosphatase; 23. myo-inositol oxygenase; 24. (pectin) methylesterase (EC 3.1.1.11); 25. d-galacturonate reductase (EC 1.1.1.203)

acid and formation of monodehydroascorbate (MDHA) radical (Fig. 2). MDHA reductase uses NAD(P)H as a reductant to reduce MDHA back to ascorbic acid in the cytosol or chloroplast. In the chloroplast, thylakoid-associated ferredoxin has been found to reduce MDHA more effectively than MDHAR (Asada 1999). However MDHA emanating from the thylakoid lumen as a result of the action of violaxanthin deepoxidase is not always available as a substrate for both reductants; thus a drop in pH of the lumen causes MDHA to just disproportionate into dehydroascorbate (DHA) and ascorbic acid (Asada 1999). DHA reductase uses glutathione as a reductant to reduce DHA back to ascorbate leading to the ascorbate– glutathione cycle (Fig.2). In cases where there are not enough DHA reductase

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NADPH

GSSH

AO APX

DHAR

NADP

MDHAR

H2 O H2 O 2

NADPH

H2O

Monodehydroascorbic acid (MDHA) Disproportionation at low pH

Dehydroascorbic acid (DHA) NADP +

GSH

Hydrolysis at >pH 7

2,3 Diketogluconic acid (DKG)

Fig. 2 The ascorbate–glutathione (Halliwell–Foyer–Asada) cycle. The enzymes catalyzing the reactions are as follows; AO: Ascorbate oxidase (EC 1.10.3.3); GR: Glutathione reductase (EC 1.8.1.7); DHAR: Dehydroascorbate reductase (EC 1.8.5.1); MDHAR: Monodehydroascorbate reductase (EC 1.6.5.4); APX: Ascorbate peroxidase (EC 1.11.1.11). GSH: Glutathione; GSSH: Oxidized Glutathione. The reaction mechanism is discussed in the text. The disproportionation of MDHA at low pH is shown with broken arrows

(as the apoplast does not have much DHA reductase and glutathione) to reduce DHA to ascorbic acid, the excess DHA which is very unstable above pH7 undergoes irreversible hydrolysis to 2,3-diketogulonic acid. The degradation of ascorbic acid has been extensively discussed (Hanco*ck and Viola 2005b). Since micronutrients are highly invaluable in human diets and more so billions of people in the developing countries suffer from at least one micronutrient deficiency, till date many multinational companies like Roche, BASF, and Cognis Health and Nutrition are spending millions of euros to increase their vitamin production facilities (Herbers 2003).

2 Functions of Ascorbic Acid 2.1 Antioxidant Adverse environmental conditions and normal aerobic metabolic processes such as photosynthesis or respiration leads to the generation of reactive oxygen species (ROS) in the form of superoxide, singlet oxygen, ozone, and hydrogen peroxide. ROS are very toxic to the plants and if the amount generated are not controlled it

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can lead to oxidation of protein resulting in adverse effect on plants (Smirnoff 1996). Ascorbic acid acts as an antioxidant wherein it protects plant by detoxifying the ROS through decomposition (Conklin 2001).

2.2 Enzyme Co-factor Ascorbic acid is highly involved in the regeneration of a-tocopherol commonly called vitamin E from the tocopheroxyl radical thus providing membrane protection (Thomas et al. 1992). About 10–20% of the ascorbate is found in the thylakoid lumen of plant where it functions greatly as a co-factor for the enzymes violaxanthin de-epoxidase (the enzyme catalyzing the formation of the pigment zeaxanthin from violaxanthin and antheroxanthin in the xanthophyll cycle where it is necessary for dissipation of excess excitation energy), and ethylene forming enzyme (necessary for the ripening of various fruits), 2-oxoacid-dependent dioxygenases (important in the synthesis of abscisic acid and gibberellic acid) (Eskling etal. 1997; Davey etal. 2000; Smirnoff 2000; Arrigoni and De Tullio 2002).

2.3 Cell Division and Growth Large concentration of ascorbate is found in the meristematic tissues that are the major sites for cell division (Conklin 2001) and it has been shown to be involved in cell division and expansion (Kato and Esaka 1999, 2000, see review by Davey etal. 2000). Tobacco Bright Yellow-2 cells with antisense l-galactono-1,4-lactone dehydrogenase grew with abnormal shapes as well as a reduction in cell growth because the ascorbate content was only 70% of the wild type (Tabata etal. 2001). On the other hand when the same gene was overexpressed in tobacco BY2 cells the mitotic index was higher in the transgenic cells than the wild type (Tokunaga etal. 2005). Keller et al. (1999) also reported that transgenic potatoes with antisense GDP-mannose pyrophosphorylase are smaller in size, had lower ascorbate content and the leaves undergo senescence faster compared to the wild type. All these strongly support the involvement of ascorbic acid in growth and cell division.

3 One Product (Ascorbate) with Multiple Pathways 3.1 Inversion and Non-inversion Routes The synthesis of ascorbate in plant was thought to be similar to that of animal where the compound was synthesized from UDP-d-Glucose through the “inversion” pathway in which the C1 of glucose becomes C6 of ascorbate and C6 becomes C1 (Smirnoff

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and Wheeler 2000). Although plants were capable of converting d-galacturonic acid derivatives (Mapson and Isherwood 1956) and l-galactono-g-lactone to ascorbate (Mapson etal. 1954) just as the animals could convert d-glucuronate into l-gulono1,4-lactone and finally to ascorbate; when labeled hexoses were fed to plants it disproves the synthesis of ascorbate through “inversion” of carbon (Loewus etal. 1958; Loewus 1963). An alternative pathway involving “non-inversion” of carbon was later proposed in which ascorbate was formed directly from glucose via d-glucosone (d-arabino-hexos-2-ulose) and l-sorbosone (l-xylo-hexo-2-ulose) (Loewus et al. 1990). Although the reaction takes place at a very slow rate, the enzyme catalyzing the last step of the reaction involving NADP-dependent oxidation of l-sorbosone to ascorbic acid was partially purified (Loewus etal. 1990) to justify the existence of the pathway. Unfortunately, there were no strong evidences to support the first two reactions which involve the conversion of d-glucose to d-sorbosone and epimerization of d-glucosone to l-sorbosone, and so the pathway was not regarded as a major route to ascorbate biosynthesis. Available evidences support l-glactono-1,4-lactone as an effective precursor of ascorbate (Ostergaard etal. 1997; Davey etal. 1999) but there were uncertainties as to whether it is present in plant and also the possibility to synthesizing it without “inversion” of the hexose carbon. Over a decade ago Wheeler etal. (1998) were able to demonstrate the presence of l-galactose dehydrogenase in pea and also the NAD+ dependent oxidation of l-galactose at C1 to l-galactono-1,4-lactone thus proposing the Smirnoff–Wheeler (l-galactose) pathway. In this newly proposed biosynthetic pathway, GDP-galactose formed from GDP-mannose undergoes two sequential oxidation reaction leading to l-galactono-1,4-lactone and ascorbic acid without the inversion of carbon chain. Recent works on fruit bearing plants such as blackcurrant, peach, kiwifruit, and acerola show correlation between ascorbic acid content of these fruits and gene expression of the enzymes involved in the Smirnoff–Wheeler pathway (Hanco*ck et al. 2007; Imai et al. 2009; Bulley et al. 2009; Badejo etal. 2009a). In some of the early radiolabel experiments with detached strawberry, it was shown that ascorbic acid could be synthesized in vivo from d-glucuronic acid, d-glucuronolactone, and d-galacturonic acid methyl ester with little redistribution of radiolabel and also without the inversion of carbon chain (Loewus etal. 1958). In linking this to the proposed pathway by the Wheeler group, the d-galacturonic acid methyl ester in plant could possibly be reduced by a non-specific aldoketo reductase activity, to form l-galactono-1,4-lactone which is the substrate for l-galactono-1,4-lactone dehydrogenase (Davey et al. 2000). Unfortunately, the conversion of d-glucuronolactone, d-glucuronic acid methyl ester and l-gulonolactone to ascorbic acid did not surface in the Smirnoff–Wheeler pathway (Wheeler etal. 1998) thus the suggestion for the possibility of a separate uronic acid pathway (Smirnoff etal. 2001). It was not long before the d-galacturonic acid reductase was cloned and characterized from strawberry fruits thus providing molecular evidence for the uronic acid pathway in which pectin-derived d-galacturonic acid is reduced to l-galactonic acid and later to the terminal substrate in the synthesis of ascorbic acid (Agius et al. 2003). Another key enzyme in this uronic acid pathway is

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Aldonolactonase which is highly vital to the synthesis of ascorbate in photosynthesizing algae Euglena gracilis (Ishikawa etal. 2008). Till date Aldonolactonase and/or its orthologs have not been cloned from plants neither has it been found in among higher plants data base (Ishikawa etal. 2008).

3.2 Other Alternative Routes The isolation of Arabidopsis vitamin C (vtc1) mutant that encodes GDP-mannose pyrophosphorylase (GMP) lends credence to the Smirnoff–Wheeler pathway (Conklin et al. 1999; Conklin 2001) at the same time opened up yet another possible link to the synthesis of ascorbic acid in higher plant. GMP catalyzes the conversion of d-mannose-1-P to GDP-d-mannose. GDP-mannose-3¢,5¢ epimerase has been known to catalyze the conversion of GDP-d-mannose to GDP-l-galactose (Wolucka etal. 2001). The same group also found that another product, GDP-lgulose, is formed alongside GDP-l-galactose in the epimerization reaction (Wolucka and Van Montagu 2003). While GDP-l-galactose is formed through the 3¢,5¢-epimerization of GDP-d-mannose, GDP-l-gulose is formed through the 5¢-epimerization. This reaction product established a new branch of ascorbic acid biosynthesis pathway in plants with a link to the biosynthesis pathway in animals (Wolucka and Van Montagu 2003; Valpuesta and Botella 2004). Lorence etal. (2004) showed molecular and biochemical evidence for a possible biosynthetic route using myo-inositol (MI) as the initial substrate. In the MI pathway, the conversion of d-glucose-6-phosphate to myo-inositol 1-phosphate by myo-inositol 1-phosphate synthase (IPS) precedes the dephosphorylation of myo-inositol-1phosphate by the enzyme myo-inositol-1-phosphate phosphatase to form MI. Myo-inositol oxygenase (MIOX) then catalyzes the oxidation of MI to d-glucuronate (Lorence etal. 2004) before undergoing additional steps in which l-gulonate and l-gulono lactone are formed prior to ascorbic acid synthesis. It is worth noting that myo-inositol was not considered as a major precursor to ascorbic acid. Furthermore no correlation could be found between the gene expression of MI pathway genes (IPS and MIOX) and high ascorbate accumulation in kiwifruit (Bulley etal. 2009). All these alternative pathways reveal a complex picture of ascorbic acid biosynthesis in planta than had been expected (Valpuesta and Botella 2004).

4 Key Ascorbate Biosynthesis Enzymes of Smirnoff–Wheeler Pathway 4.1 Phosphomannose Isomerase (PMI) On the basis of sequence similarity, three classes of PMI (EC 5.3.1.8) have been defined. Bacteria and human PMIs belong to the type1 (Coulin etal. 1993). PMI

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activity has been confirmed from ManA (Rv3255c), an enzyme that has been shown to be essential for mycobacterial growth in vitro (Patterson et al. 2003). Unfortunately, PMI is often not expressed in plants (Gao et al. 2005; Zhu et al. 2005; Badejo etal. 2009b) which makes the origin of the d-mannose 6-phosphate unclear. Furthermore, PMI has been used as an antibiotic-free positive selection marker in plant biotechnology (Wenck and Hansen 2005). Recently, two putative PMI genes were identified in Arabidopsis thaliana (PMI1, At3g02570; and PMI2, At1g67070), and have been used to show that under normal growth conditions, PMI1 but not PMI2 is essential for the biosynthesis of AsA in A. thaliana leaves (Maruta etal. 2008).

4.2 Phosphomannomutase Phosphomannosemutases (PMM; EC 5.4.2.8) are phosphotransferases belonging to the superfamily of haloalkanoic acid dehalogenases. In the functional AsA biosynthetic pathway, PMM catalyzes the interconversion of mannose 6-phosphate and mannose 1-phosphate, and is required for the synthesis of GDP-d-mannose (Wheeler et al. 1998; Dowdle et al. 2007). It has also been found that PMM is constitutively expressed in both the vegetative and reproductive organs of plants (Qian etal. 2007; Badejo etal. 2009b). PMM is required in the synthesis of d-mannose which is vital not only for AsA biosynthesis but also in post-translational modifications, such as protein glycosylation and glycosylphosphatidylinositol (Hoeberichts etal. 2008). In human a mutation in the PMM gene was reported to result in a fatal clinical disorder that provokes impaired neurological development and increased childhood mortality (Van Schaftingen and Jaeken 1995). Most plants contain a single PMM gene (Qian etal. 2007; Hoeberichts etal. 2008; Badejo etal. 2009b) and till date no PMM knockout mutants have been reported. Efforts to generate knockdown mutants were unsuccessful (Hoeberichts etal. 2008). Two point mutations were located on the first exon of the PMM gene outside the conserved DXDX(T/V) motif characteristic of the phosphotransferases. Sequence comparison with the wild type shows that the first mutation resulted in a GGA to AGA codon change (codon 7), leading to a substitution of a neutral glycine residue with a basic arginine; while in the second a CGA to CAA codon change (codon 37) occurred thereby replacing an arginine with a neutral glutamine (Hoeberichts etal. 2008).

4.3 GDP-d-Mannose Pyrophosphorylase GDP-d-mannose pyrophosphorylase (GMP; EC 2.7.7.22), localized to the cytosol, is the enzyme that catalyzes the synthesis of GDP-mannose (the activated form of mannose), an important step in the formation of all guanosine-containing sugar nucleotides in plants. Prokaryotes and eukaryotes use this GDP-mannose in the

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synthesis of complex structural proteins. It is highly essential for the synthesis of three different structural carbohydrates in the plant cell wall – mannans, l-fucose and l-galactose (Bewley etal. 1997; Bonin etal. 1997; Wheeler etal. 1998). The vitamin C deficient Arabidopsis mutant VTC1 locus encodes GMP (Conklin etal. 2000). The vtc1-1 mutant which is ozone-sensitive was isolated from ethylmethanesulfonate-mutagenized pools and is incapable of properly converting glucose and mannose to ascorbate thus possessing only about a quarter of the wild-type ascorbate content (Conklin etal. 1999; Conklin 2001).

4.4 GDP-d-Mannose-3¢,5¢-Epimerase GDP-d-mannose-3¢,5¢-epimerase (GME; EC 5.1.3.18) represent the intersection between the pathway that leads to AsA synthesis and that of the cell wall polysaccharides and glycoprotein in plants. Upon the formation of GDP-d-Mannose by GMP, GME catalyzes the formation of GDP-l-galactose from GDP-d-mannose. It is one of the most highly conserved proteins involved in the biosynthesis of AsA (Wolucka and Van Montagu 2007). In reporting an alternative pathway for the biosynthesis of AsA, Wolucka and Van Montagu (2003) identified another epimerization product, GDP-l-gulose, released by the catalytic action of GME in addition to the already established GDP-l-galactose. GME is unique as it performs three chemical reactions-oxidation, epimerization and reduction- at the same active site (Major et al. 2005). The conserved NAD binding domain in rice GME compared to that of Arabidopsis reflected on the reaction it catalyzed; as GME was inhibited by GDP and strongly assisted by NAD+ (Watanabe etal. 2006).

4.5 GDP-l-Galactose Phosphorylase l-galactose-1-P has been described as the first metabolite in the Smirnoff–Wheeler pathway that is dedicated wholly to ascorbate biosynthesis. The enzyme GDP-lgalactose phosphorylase (GGP) catalyzes the conversion of GDP-l-galactose to l-galactose-1-phosphate (Laing et al. 2007). Vitamin C deficient Arabidopsis mutants were first identified as ozone sensitive, with characteristically low vitamin C contents and later mapped to a locus on chromosome 4 that was designated VTC2 (vitamin C 2) (Conklin etal. 2000; Conklin 2001). The mutated gene identified as At4g26850 was found to encode a novel protein. At4g26850 shows high similarity to another Arabidopsis gene At5g55120 both of which were uncharacterized (Laing et al. 2007). In attempt to characterize the protein sequence predicted for At4g26850, it was found by BLAST (Basic Logic Alignment Search Tool) that it shared similarities with HIT (histidine triad) protein family with the characteristic HXHXH (where X is a hydrophobic amino acid) motif (Laing etal. 2007). The HIT enzymes share a lot in common with the VTC gene. All the known HIT enzymes

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use the second His residue (His238 in Arabidopsis VTC2) of the HIT motif in a covalent nucleotidylylation reaction where the intermediate enzyme can then be broken down by simple hydrolysis, phosphrolysis or a specific phosphorylated compound (Linster and Clarke 2008). VTC2 and kiwifruit hom*olog were found to specifically convert GDP-l-galactose to l-galactose-1-P and to require the presence of a guanylyl acceptor other than water to catalyze the reaction (Linster etal. 2007; Dowdle etal. 2007; Laing etal. 2007). Linster etal. (2008) went on to provide evidence for covalent guanylylation of the second His residue of the HIT motif of Arabidopsis VTC2 thus proving that VTC2 is a member of the phosphorylasetransferase (but not hydrolase) branch of the HIT protein superfamily. Laing et al. (2007) named the Arabidopsis At4g26850 gene as GDP-lgalactose-d-mannose-1-phosphate guanyltransferase proving with an indirect enzyme coupled assay that the gene is able to use a range of hexose-1-P sugars. The inability of the enzyme to effectively transfer guanosine monophosphate from GDP-l-galactose-1-phosphate to either pyrophosphate or phosphate led to the conclusion that it is neither a pyrophosphorylase nor a phosphorylase (Laing et al. 2007). One the other hand two other independent groups (Dowdle et al. 2007, Linster et al. 2007) referred to the same gene as GDP-l-galactose pyrophosphorylase because they were able to find high VTC2 activities in the presence of inorganic phosphate. This little but highly controversial enzyme activity led to the proposed VTC2 cycle (Wolucka and Van Montagu 2007). Further works by the Linster group was able to clarify that VTC2 acts as a phosphorylase forming GDP, and not a transferase in which GDP-hexose is formed at the end of the reaction (Fig.3) because the VTC2 has phosphorylase activity that was 100-fold higher than the transferase activity (Linster etal. 2008; Linster and Clarke 2008). At5g55120 was also found to be GGP and was designated VTC5 (Dowdle etal. 2007). The kinetic data and sequence identity of VTC5 is highly similar to those of VTC2 but the gene expression of VTC2 in reproductive and vegetative tissues of Arabidopsis was far greater than those of VTC5 (Dowdle etal. 2007).

4.6 l-Galactose-1-Phosphate Phosphatase l-Galactose-1-phosphate phosphatase (GPP) was previously classified as myoinositol-1-phosphate phosphatase but phylogenetic analyses as well biochemical characterizations have been used to clarify the differences (Laing et al. 2004; Conklin etal. 2006). GGP purified from kiwifruit is highly specific in its hydrolysis of l-galactose-1-phosphate in a reaction that is highly dependent on Mg2+ with concentration less than 2 mM to form l-galactose (Laing et al. 2004). Ascorbate deficient Arabidopsis mutant VTC4 encodes the gene GPP. Alignment shows high identity between Arabidopsis and kiwifruit GPP and both are highly active against l-galactose-1-phosphate compared to myo-inositol-1-phosphate (Laing etal. 2004).

15 Identification of Potential Gene Targets for the Improvement of Ascorbate Contents GDP-L-galactose

VTC2

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L-galactose-1-P

Phosphorylase Reaction

GDP L-galactose-1-P VTC2-His238

GDP-L-galactose

Guanylated Enzyme Intermediate

+ VTC2-His

238

GDP-D-mannose Transferase Reaction

Fig.3 The proposed reactions catalyzed by VTC2 adapted from Linster and Clarke (2008). The reaction undergoes two sequential steps in which l-galactose-1-P is formed when guanylated enzyme (guanylylated active site His) intermediate emerges upon the use of a-phosphate of GDPl-galactose. In the latter reaction the dominant phosphorylase reaction (depicted with thick arrow) generates GDP while the less dominant transferase reaction (depicted with the thin) forms GDPd-mannose. The broken arrows on the left and right represent reactions from d-glucose to GDPl-galactose and from l-galactose 1P to ascorbic acid respectively

4.7 l-Galactose Dehydrogenase (GDH) l-Galactose dehydrogenase (GDH; EC 1.1.1.117) catalyzes the penultimate step involving the conversion of l-galactose to l-galactono-1,4-lactone in an irreversible reaction by oxidizing the C1 of l-galactose in the Smirnoff–Wheeler pathway (Wheeler etal. 1998). GDH purified from spinach has high substrate specificity toward l-galactose but none for l-fructose, l-mannose, l-xylose, l-arabinose, d-galactose, d-glucose, l-fucose, d-mannose and d-arabinose (Mieda etal. 2004). Among the characteristics of this cytosol localized gene is that it has a Km of 0.43 and 3.7 mM for l-galactose and l-gulose respectively in pea and can be inhibited up to 65% and 90% by 5 mM oxidized glutathione and 1 mM N-ethylmaleimide respectively (Gatzek et al. 2002). The spinach GDH activity was enhanced by more than 15% in 1 mM DTT and 1 mM glutathione (Mieda etal. 2004). Antisense suppression of GDH activity in A. thaliana reduces foliar ascorbate concentration but was not thought to have pleiotropic effect on other enzymes required for regenerating ascorbate from its oxidized state; however overexpression of the gene in plant was only able to increase GDH activity but not the ascorbate content (Gatzek etal. 2002).

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4.8 l-Galactono 1,4 Lactone Dehydrogenase (GalLDH) l-Galactono 1,4 lactone dehydrogenase (GalLDH; EC 1.3.2.3) catalyzes the last step in the biosynthesis pathway for ascorbate in plants. It is localized to the inner membrane of the mitochondria (Siendone etal. 1999) and uses cytochrome c as an electron acceptor (Ostergaard etal. 1997; Imai etal. 1998). GalLDH is incapable of catalyzing the synthesis of ascorbate in the presence of potassium cyanide, an inhibitor of cytochrome c oxidase (Bartoli et al. 2000). It is one of the enzymes in the Smirnoff– Wheeler pathway (Wheeler etal. 1998) whose activity in plant has been known for a long time (Mapson etal. 1954). The enzyme has been purified from a number of plant species including potato and cauliflower (Oba etal. 1995; Ostergaard etal. 1997). Unlike the animal l-gulono-1,4-lactone oxidase that can utilize l-gulono-1,4-lactone and l-galactono-1,4-lactone, GalLDH uses l-galactono-1,4-lactone as a specific substrate (Oba etal. 1995; Ostergaard etal. 1997).

5 Factors Affecting Ascorbate Biosynthesis and Accumulation Researches have shown that many factors control the ascorbic acid contents in plants. It has been shown earlier that ascorbate content of plant changes with the light intensity available to the plant (Tabata etal. 2002). The AsA content is diurnally regulated in a number of plant species where the peak of accumulation is recorded when daylight intensity is maximal and this correlates with the photosynthetic activity (Tamaoki etal. 2003; Chen and Gallie 2004). Total foliar ascorbate of potato leaves was found to be dependent on the age of the leaves in the plant (Bartoli etal. 2000). The ascorbate content of the young leaves was ten times more than those of leaves undergoing senescence (Bartoli etal. 2000). Ascorbate is also found in different concentrations in the vegetative and reproductive organs of plants. The highest concentrations was recorded in the fruits of acerola (Badejo etal. 2007) while the flowers and roots of Arabidopsis has the highest and lowest concentrations respectively (Lorence et al. 2004). Various cell compartments in plants also accumulate ascorbate to different degrees. The chloroplast and cytoplasm have relatively higher concentration compared to the vacuole of plant cell (Foyer et al. 1983; Davey et al. 2000). Two major substrates in the biosynthesis pathway, namely GDP-d-mannose and l-galactose, are of great importance in the control of ascorbate concentration (Wheeler etal. 1998). GDP-d-mannose and the major epimerization product, GDP-l-galactose, are channeled not only to ascorbate biosynthesis but also to cell-wall polysaccharide (xyloglucan and rhamnogalacturonans) and/or glycoprotein biosynthesis (Roberts 1971; Rayon et al. 1999). The diversion to cell-wall polysaccharide and protein glycosylation will put pressure on the denovo GDP-d-mannose available for ascorbate biosynthesis. The steps from l-galactose-1-P are fully committed to ascorbate biosynthesis as l-galactose is only a minor component of cell wall polysaccharide (Roberts 1971).

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6 Targeted Genes for Ascorbate Improvement 6.1 Genes from Smirnoff–Wheeler Pathway The overexpression and virus-induced gene silencing knockdown experiments performed on Arabidopsis and tobacco (Nicotiana benthamiana), respectively, indicated that PMM is playing an active role in ascorbic acid biosynthesis (Hoeberichts et al. 2008). With viral-vector mediated ectopic expression in N. benthamiana, PMM expression increased thereby resulting in an increase in the ascorbate content of the transgenic plant by 20~50% compared to the wild type (Qian etal. 2007). In the same research the Qian team also performed the transgenic expression of Arabidopsis thaliana PMM–GFP fusion protein in Arabidopsis which also increased ascorbate content by 25–33%. Transgenic tobacco overexpressing acerola (Malpighia glabra) PMM was found to have ascorbate level that was threefold that of the wild type plant (Badejo etal. 2009b). It has been shown that biosynthesis in situ rather than translocation contributed to the accumulation of ascorbate in blackcurrant fruits with metabolic and genetic proofs that ascorbate is synthesized through the Smirnoff–Wheeler pathway (Hanco*ck et al. 2007). The gene expression studies on kiwifruit (Actinidia spp.) showed that there is a correlation between the rise in ascorbate concentration and the expression of the Smirnoff–Wheeler pathway biosynthetic genes especially GDP-l-galactose guanyltransferase (GDP-l-galactose phosphorylase (GGP)) (Bulley etal. 2009). In the fruits of Actinidia eriantha, not only GGP was found to correlate with the increase in ascorbate concentration, the expression of GDPmannose-3,5-epimerase (GME) was also found to be vital in regulating the ascorbate concentration (Bulley et al. 2009). When GGP gene was over expressed in Arabidopsis and transiently in tobacco it was found to elevate ascorbate level by more than threefold (Laing et al. 2007; Bulley et al. 2009). Transgenic tobacco transformed with GME and GGP, showed an increase in the ascorbate content that was twice the amonut observed with either genes over expressed alone (Bulley etal. 2009). The ascorbate content of tomatoes in which GME gene was silenced by RNAi were found to be reduced by 20–40% in the fruits 20 days post anthesis, and by 40–60% in the young fully expanded leaves (Gilbert etal. 2009). Analysis of the fruit pericarp of the RNAi transgenic tomatoes showed a significant reduction in both the cell number and cell size (Gilbert etal. 2009). With acerola fruits having a high ascorbate content, it was found that the ascorbate concentration of the fruit is positively correlated with the genes in the Smirnoff–Wheeler pathway especially GDP-d-mannose pyrophosphorylase (GMP), GME and GDP-l-galactose phosphorylase (GGP) (Badejo etal. 2008, 2009a). In attempt to understand why acerola has such high ascorbate content, the transcript expressions of the biosynthesis genes were compared to that of the model plant A. thaliana and it found that transcript expression of acerola was 5–700-fold higher (Badejo etal. 2009a). Antisense inhibition of GMP activity in potato resulted in a decrease in the ascorbate content of the transgenic potato as well as the mannose

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content of the cell wall polysaccharide (Keller etal. 1999), but when acerola GMP gene was overexpressed in tobacco it resulted in two- to threefold increase in the ascorbate content of the transgenic tobacco (Badejo etal. 2008). The Bulley team found it difficult to clone the last gene in the ascorbate biosynthesis pathway, l-galactono-1,4-lactone dehydrogenase (GalLDH) with degenerate primers. The expression of GalLDH could not be measured because there were no kiwifruit orthologues in the HortResearch Actinidia EST database. In acerola however GalLDH was cloned and its transcript level in the leaves was found to be the lowest among the five genes involved in the Smirnoff–Wheeler pathway that were analyzed (Badejo et al. 2009a). Although GalLDH expression have been found to correlate with ascorbate content (Badejo etal. 2009a; Pateraki etal. 2004; Tamaoki et al. 2003), it does not seem to exert strong control on ascorbate flux neither did the penultimate gene GDH (Gatzek et al. 2002). When GalLDH was knocked out in tomato, no change was recorded in the total ascorbate content of the young leaves and fruits (Alhagdow etal. 2007).

6.2 Genes from d-Galacturonate Pathway Agius et al. (2003) have been able to show that ascorbate could be synthesized through d-galacturonic acid derived from the pectin of ripening strawberry fruits. The d-galacturonic acid which is also an abundant component in the cell wall of all plants is reduced to l-galactonic acid by the enzyme d-galacturonate reductase (Smirnoff 2003). This gene has expression correlating with ascorbate content in ripening strawberry fruits (Agius et al. 2003). Overexpression of the gene in Arabidopsis thaliana resulted in two- to threefold increase in the ascorbate content of the transgenic plants (Table1). Although Aldonolactonase has not been reported in higher plants, RNAi silencing of Aladonolactonase in photosynthetic algae Euglena gracilis halted the cell growth, only to be restored on supplementation with one of l-galactono-1,4-lactone, d-galacturonate or d-glucuronate (Ishikawa etal. 2008).

6.3 Genes from Animal Pathway and myo-Inositol Route At the beginning of the millennium, Jain and Nessler (2000) showed that ascorbate content of plants could be increased with overexpression of the rat gene l-gulonog-lactone oxidase (GLO) (Table1). Interestingly the same rat gene was also able to rescue the ascorbate content of the Arabidopsis vitamin C mutants including vtc1 when overexpressed in them (Radzio etal. 2003). However, plants overexpressing rat GLO still uses and prefers l-galactono-g-lactone of the Smirnoff–Wheeler pathway as a precursor over l-gulono-g-lactone (from the animal pathway) when fed artificially to the tobacco leaves (Jain and Nessler 2000). In the animal pathway

Effect on AsA pool 1.5–2.0-Fold increase 4.0–7.0-Fold increase 2.0–3.0-Fold increase 0.25–0.33-Fold increase ~2.0-Fold increase 2.0–3.0-Fold increase 2.0–3.0-Fold increase 3.0-Fold increase 4.0-Fold increase 2.0–3.0-Fold increase ~1.9-Fold increase 7.0-Fold increase

Reference Tokunaga etal.(2005) Jain and Nessler (2000) Lorence etal. (2004) Qian etal. (2007) Badejo etal. (2009b) Badejo etal. (2008) Badejo etal. (2009c) Laing etal. (2007) Bulley etal. (2009) Agius etal. (2003) Chen etal. (2003) Bulley etal. (2009)

Genetically modified plants with increased ascorbate content. The genes engineered are: GalLDH, l-galactono-1,4-lactone dehydrogenase; GLO, l-gulonog-lactone oxidase; MIOX, myo-inositol oxygenase; PMM, phosphomannomutase; GMP, GDP-d-mannose pyrophosphorylase; GGP, GDP-l-galactose phosphorylase; GalUAR, d-galacturonic acid reductase; DHAR, Dehydroascorbate reductase; GME, GDP-d-mannose-3¢,5¢-epimerase

Table1 Changes in the ascorbate content of some transgenic plants Engineered gene Gene source Plant engineered GalLDH Tobacco Tobacco BY2 cells GLO Rat Lettuce MIOX Arabidopsis Arabidopsis PMM Arabidopsis Arabidopsis PMM Acerola Tobacco GMP Acerola Tobacco GGP Acerola Tobacco GGP Kiwifruit Tobacco GGP Kiwifruit Arabidopsis GalUAR Strawberry Arabidopsis DHAR Wheat Maize GME/GGP Kiwifruit Tobacco

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glucuronate reductase catalyzes the formation of l-gulonic acid from l-glucuronic acid and the latter is acted upon by aldono lactonase to form the penultimate substrate to ascorbate synthesis (Fig. 1). The d-glucuronic acid can be formed through the action of myo-inositol oxygenase (Fig.1). Lorence etal. (2004) showed that ectopic expression of myo-inositol oxygenase doubled the ascorbate content of transgenic Arabidopsis thaliana (Table 1). Also overexpression of purple acid phosphatase gene AtPAP15 which is involved in myo-inositol metabolism was also found to double the ascorbate content of transgenic A. thaliana (Zhang etal. 2008).

6.4 Genes from Ascorbate Recycling Pathway Poplar tree hybrids expressing ascorbate recycling gene glutathione reductase showed slight increase in ascorbate concentration and were able to withstand oxidative stress (Foyer etal. 1995). When A. thaliana MDHA reductase that is essential for maintaining reduced ascorbate in plant was overexpressed in tobacco, it resulted in about twofold higher ascorbate contents in the transgenic plant compared to the wild type (Eltayeb et al. 2007). DHAR, which regenerates the reduced form of ascorbate, elevated the ascorbic acid content of transgenic tobacco in which it was overexpressed (Chen etal. 2003).

7 Factors for and Against Genetic Engineering Since most staple foods are low in ascorbate content, consumption of the recommended dietary allowance is being achieved by adding chemically synthesized vitamin C to foods (fortification), or by taking vitamin C pills (supplementation). Supplementation and fortification have been plagued with many demerits such as the increasing resistance among consumers to the use of additives in foods and a tightening of the legislation on foods and drinks especially in the developed world (Hanco*ck and Viola 2005a). The recurrent cost, inadequate distribution system and lack of industrial food system makes implementation very difficult in developing countries (Bouis 2002). All these pose a great challenge to supplementation and fortification of foods. Biofortification, which is the process of enriching the nutrient content of crops as they grow by employing genetic engineering of genes in the plants, is a promising and viable option to nutrient enhancement in plants (Naqvi etal. 2008). Genetic modifications in the past decade have been used to elucidate the biosynthetic pathways for important health components of crops. Research progress made in the area of genomics of food science and nutrition has improved the understanding of how common dietary chemicals affect human and animal health by altering the expression and/or structure of the genome (van Ommen 2004). Genetic engineering certainly increases the nutritional contents of plants. One important factor that has deterred the progress of genetic engineering technology when considered in the

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past decade is insufficient support from the upstream and downstream sectors and the importance of genetically engineered crops/foods have been watered down in many literatures lacking scientific facts (Vain 2006). Bioavailability of the nutrient in the transgenic plants is very vital to its acceptability, as increased levels of nutrients are not necessarily correlated with enhanced bioavailability (Jeong and Guerinot 2008). Further scientific research into the bioavailability of nutritionally enhanced genetically modified crops/foods will either debunk or strengthen the non-science based speculations and fears about the acceptance of transgenic crops. Also, there has been the need to cope with meager funding, the hostile market conditions combined with the tight regulatory frame work surrounding transgenic crops.

8 Genetic Engineering-Solution to Food Security and Malnutrition ‘Hidden hunger’, described as the micronutrient malnutrition inherent in human diets that are adequate in calories but lack vitamins and/or mineral elements (White and Broadley 2009), is ravaging a significant portion of the world population. Genetic engineering is one area of science that offers the opportunity to exponentially increase the rate of food production by avoiding losses due to diseases and pests and by increasing tolerance under adverse conditions (Bhalla 2006) as in the case of ascorbic acid that play roles in resistance to environmental stresses such as ozone, high temperature and UV radiation (Conklin etal. 1996; Sanmartin etal. 2003; Chen and Gallie 2005). Genetic engineering is also laden with potentials to breed crops with novel desirable characteristics, such as reduced allergenicity (Herman etal. 2003), improved nutritional qualities (Paine etal. 2005; Diaz de la Garza et al. 2007; Naqvi et al. 2008; Badejo et al. 2008, 2009b, c), hybrid seed production (Bhalla and Smith 1998) and improved plant productivity (Sakamoto etal. 2005). Many of the crops to which genetic engineering is being applied are the same as those being targeted for many years by plant breeding (Dale 1993). Genetically engineered foods have the potential to reach relatively remote rural populations that conventional interventions are not now reaching and with increased agricultural productivity (Bouis 2002). Most diets rich in fruits and vegetables are associated with decreased risk of certain cancers and cardiovascular diseases, and this has been attributed to the high concentrations of antioxidants such as ascorbic acid in such foods. Production of these fruits and vegetables could also be enhanced through the application of mineral fertilizers. There is an increasing use of nitrogen fertilizers on agricultural soils worldwide resulting in the accumulation of ammonium in those soils (Britto etal. 2001). It was reported recently that mutation in GMP, one of the vital genes in the biosynthesis of vitamin C and essential for the synthesizing of d-mannose, confer hypersensitivity to ammonium (Qin etal. 2008). Defective protein glycosylation in the root rather than decreased vitamin C content in the mutant was reported to

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account for the hypersensitivity to ammonium (Qin etal. 2008). Genetic manipulation of GMP through over expression in plant has resulted in an increase in vitamin C content (Badejo etal. 2008). d-mannose formed through the activity of GMP is not channeled to vitamin C biosynthesis only but also to protein glycosylation and many other cellular processes. GDP-l-fucose is a constituent of glycoprotein and it is also synthesized from GDP-mannose (Bonin etal. 1997). The deficiency and/or a null mutant of GMP gene may be lethal (Conklin et al. 1999). Any genetic manipulation that increases the de novo synthesis of d-mannose or its activated form, GDP-mannose, may possibly increase ascorbate content as well as improve on protein glycosylation. Theoretically the same manipulation should doubtless reduce the sensitivity to ammonium in the natural and industrial fertilizers thereby enhancing the growth of plants. Also biofortification of crops through the application of mineral fertilizers, combined with breeding varieties with an increased ability to acquire mineral elements, has been advocated for incorporating essential mineral elements into the staple foods commonly consumed worldwide (White and Broadley 2009).

9 Concluding Remarks The first decade of this millennium has witnessed an unprecedented progress in the study relating to improving ascorbate concentration in plants. Most of the overexpression of ascorbate biosynthesis genes have been performed on the leafy ‘vegetables’ and ‘unfortunately’ it is only the rat gene l-gulono-g-lactone oxidase overexpressed in lettuce that yielded sevenfold increase (Jain and Nessler 2000). Albeit attractive, incorporation of a rat gene in readily consumable vegetable may not be a great choice for a willing consumer. Overexpression of genes from a high ascorbic acid containing plant, acerola, did not increase ascorbic acid content of plants beyond threefold (Badejo etal. 2008, 2009b, c) neither has the genes from Arabidopsis raised ascorbate content exponentially (Ishikawa et al. 2006). Thus, integrating increased biosynthesis with increased recycling capacity by overexpressing not only single but a combination of plant genes from ‘all’ ascorbate biosynthesis pathways and the ascorbate–glutathione recycling pathway may aid in tailoring plants with improved ascorbate contents at will and this is going to be of great benefit to many people worldwide. It will also ensure the generation of plants that is highly resistance to pathogens and environmental stress. While it is worth noting that genetic engineering only is not a panacea for all nutritional deficiencies, it can be a promising and cost-effective additional tool, complementing existing micronutrient interventions. It is still very difficult to discuss genetically modified foods without dividing people into different schools of thought as the expected benefits are given less credence than the feared risks, however a ban on genetically modified crops, as Conner etal (2003) described, could limit the options of farmers and be imprudent rather than precautionary especially to the developing world where the technology will have a lot to offer.

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Index

A ABA and ABA/giberelin acid (GA) signaling pathways, 31 ABA metabolism, 241 Abiotic and biotic stresses, 2, 3, 8, 9, 15, 28–31, 103, 142, 165, 178, 182, 196, 199, 232, 265–290, 304–306, 315–317, 323–330, 354, 392–396 Abiotic stress conditions, 265–290, 394–395 Abiotic stress-driven oxidative stress, 268 Abiotic stresses, 25, 28–31, 103, 130, 155, 160, 165, 178, 182, 196, 199, 232, 265–290, 304–306, 311, 315, 316, 323–330, 354, 393, 394 Abnormal meristems, 239, 245 Abscisic acid synthesis pathway, 19 Abutilon theophrasti, 201 ACC. See 1-Aminocyclopropane-1-carboxylase Acclimation, 5, 24–26, 102, 138, 176, 177, 179, 273, 276–278, 283, 311, 314 ACCO. See 1-Aminocyclopropane-1carboxylate oxidase Acer saccharinum, 143, 171 Action of ascorbate and glutathione, 13–16 Activation of defence mechanisms, 265–290 Adaptation, 15, 28, 67, 142, 160, 169, 210, 215, 221–223, 304, 329, 341 ADP-ribose polymers, 67 Adventitious root formation, 59, 77–80 Adventitious rooting, 77–79 Adverse abiotic stress conditions, 265 Agricultural productivity, 272, 290, 421 Agrobacterium rhizogenes, 57 Air pollutants/strong light, 2, 9, 17, 284, 287 Air pollution, 15 Aldolase, 27, 75 Allium, 166 Allium schoenoprasum, 143, 166 Allocasuarina luehmannii, 169

Alternative splicing, 341, 344–345 Al-tolerant, 281 Aluminium (Al), 281–282, 358, 359 AM. See Arbuscular mycorrhizal 1-Aminocyclopropane-1-carboxylase (ACC), 72, 242, 255 1-Aminocyclopropane-1-carboxylate oxidase (ACCO), 255 Aminotriazole, 16, 84, 103, 395 Anaerobic environment, 194 Androgenesis, 233, 241 Angiosperm, 233, 241, 246, 345 Animal pathway, 407, 418–420 Anoda, 166 Antheraxanthin (Ax), 17 Antioxidant compounds, pools of, 23 Antioxidant defense system, 2, 170, 210–211, 222, 269, 275, 279, 337–372 Antioxidant enzymes, 11, 30, 62, 81, 99, 101, 129, 140, 142, 156, 158–160, 162, 164–168, 171, 173–177, 179–183, 220, 223, 269, 273, 275–277, 282, 284, 289, 290, 304, 309, 313, 337–372, 391 Antioxidant enzymes in higher plants, cellular localization, 340 Antioxidants, 2, 59, 99, 115, 138, 197, 210, 240, 251, 266, 304, 324, 339, 391, 406 Antioxidative enzymes, 9, 28, 30, 123, 127, 182, 210, 222, 306, 307, 310, 325, 394, 396 Antioxidative protection, 13, 156, 222, 267, 290, 307, 342, 359, 361, 369 Antioxidative system, components, 2, 13–16, 23 Antiperoxidative element (ARE), 95 Antisense constructs, 354, 358 AO. See Ascorbate oxidase Apoplasm, 340

429

430 Apoplast, 4, 8–10, 24, 26, 32, 60, 68, 69, 116–122, 124, 125, 127, 128, 138, 142, 196, 218, 219, 267, 271, 287–289, 306, 308, 309, 311, 312, 338, 339, 406, 408 oxidative metabolism, 117–122 Apoplastic AA pool, 10 Apoplastic ascorbate/phenolic/peroxidase system, 119 Apoplastic ascorbate pool, 117, 118, 120, 219 Apoptosis, 64, 215, 338 signaling, 215 APX. See Ascorbate peroxidase APX1-deficient mutant, 314 Arabidopsis, 6, 8, 12, 13, 16, 28, 29, 58, 65–67, 76, 94, 98, 104, 124, 127, 168, 175, 179, 194, 196, 211–213, 215–217, 235, 239, 240, 255–259, 273, 274, 278, 281–284, 286, 288, 289, 306, 311, 313, 315, 330, 342, 344, 345, 347, 352–360, 362–368, 391–393, 395, 397, 398, 411, 413, 414, 416–419, 422 Arabidopsis mutant, 65, 194, 216, 255, 257, 278, 288, 315, 413, 414 Arabidopsis thaliana (A. thaliana), 4–8, 11, 14, 16, 29, 65, 95, 97–99, 102–104, 118, 123–125, 129, 143, 166, 173, 196, 198, 211, 216, 217, 223, 271–273, 279, 281, 282, 288, 289, 313, 314, 330, 341–343, 350, 354, 356, 358, 360, 363–366, 412, 415, 417, 418, 420 Arbuscular mycorrhizal (AM), 159–161 Arbutus unedo, 143, 170 ARE. See Antiperoxidative element ARGONAUTE (AGO), 75, 239 ARGONAUTE 1 (AGO 1), 76, 239 AsA-and GSH-mediated functions, 130 AsA/DHA and GSH/GSSG ratios, 15, 26, 28, 126, 127, 164, 172, 198–199, 235–239, 245, 265–290, 309, 310 AsA–GSH cycle-related mechanism, 122 AsA-requiring enzymes, 255 Asc and GSH and Asc/DHA or GSH/GSSG redox couples, 28, 31 Asc/DHA and GSH/GSSG redox states, 62 ratio, 26, 70, 82 redox state, 70 Asc–GSH cycle, 8, 12, 13, 73, 82 Ascorbate (AsA/Asc) biosynthesis, 66, 71, 254 biosynthetic pathway, 6 consumption, 22, 23 content, 66, 70, 71, 79, 80, 82, 405–422 deficient mutants, 4, 11, 118, 284, 414

Index developmental stages, 6 forms, 10, 20 metabolism, 70, 84 oxidised form of, 20 pool, 4, 10, 14, 23, 31, 61, 64, 70, 117, 118, 120, 125, 172, 219, 233, 235, 244, 269, 273 recycling, 268, 271, 288, 393 recycling pathway, 270, 288, 420 redox state, 9, 119, 120, 157, 231–246, 259, 268, 271, 275, 277, 284, 288 signalling module, 253–256 synthesis, 245, 254, 255, 273, 280, 284, 325, 413, 420 transporter, 22 Ascorbate-and glutathione-dependent biochemical systems, 84 Ascorbate-deficient (vtc), 4, 11, 118, 284, 414 Ascorbate–glutathione (AsA–GSH) coordinated role of, 218–223, 323–330 cycle, 4–14, 17–28, 31–33, 81, 91–106, 116, 118, 120–126, 130, 137–184, 210, 220, 252–254, 258–260, 267, 268, 270, 273, 275, 276, 279, 282, 286, 303–318, 324, 326–327, 389–392, 394–398 interaction of, 59–62 pathway components, 1–33, 100, 130, 171, 222, 223, 254, 258, 275, 387–399 regulation, 246, 290 Ascorbate-metabolising enzymes, 70 Ascorbate oxidase (AO), 9, 10, 32, 118–120, 219, 242, 268, 271, 274, 289, 406, 408 Ascorbate peroxidase (APX) characteristics of, 92–96 encoding genes, 95–96 family, 94 functions, 91–92 isoforms, 93–95, 155, 267, 391 protein content, 101 Ascorbic acid, 21, 22, 183, 210, 218, 219, 221, 222, 243, 251–253, 255, 256, 305, 308, 324, 325, 328, 406–411, 415–417, 420–422 in living organisms, 406–408 functions of, 324, 408–409 synthesis, 325, 406, 410, 411 Ascorbic acid free radical (AFR), 243–245 a-tochopherol, 267 ATP-dependent reactions, 8 ATP-sulfurylase activity, 222 Atrazine, 192, 198, 200–202 Auxin, 10, 57–63, 69, 77–80, 192, 217, 234, 236, 311 Ax. See Antheraxanthin

Index B Benzylaminopurine (BAP), 57, 62, 83 Betula pendula, 288 Biochemical dysfunctions, 10 Biochemical machinery, 364, 388 Bioinformatic analysis, 344 Biomolecular regulation, 303–318 Bioreactors, 81 Biosynthesis, 4, 6, 11, 12, 14, 30, 65, 66, 71, 72, 118, 169, 179, 180, 192, 195–196, 202, 213, 216, 217, 219, 221, 223, 235, 236, 241–243, 253–255, 258, 279–282, 308, 324, 325, 340, 357, 359, 360, 388, 406, 410–417, 421, 422 Biosynthetic pathway of ascorbic acid, 243, 253, 284, 407 Biotic and abiotic stress, 2, 3, 8, 9, 15, 31, 103, 142, 165, 178, 182, 196, 199, 232, 265–290, 304–306, 315–317, 323–330, 354, 393–396 Biotic and abiotic stress response, 28–31 Biotic stress conditions, 125, 395–396 Botrytis cinerea (B. cinerea), 118, 119, 121–124, 127, 396, 397 Brassica B.campestris, 98, 222, 281 B.juncea, 16, 202, 222, 279 B.napus, 76, 77, 233–234, 237, 239–242, 246 BSO-treated embryos, 72, 73, 77 Buffering, 9, 24, 68, 117, 118, 120, 272, 359 Bupleurum, 166 Bupleurum chinense, 143, 166 C Cadmium (Cd), 12, 16, 32, 101, 102, 104, 221, 222, 278–281, 395, 398 stress, 16, 102, 103, 221, 279–281 tolerance, 221, 279 tolerance, tomato, 16 Calendula officinalis, 273 Callus, 56–58, 66, 78, 82–84 Calmodulin, 311, 369, 370, 396 Calvin cycle enzymes, 27, 129, 258 C3 and C4 plant, 142, 163, 172 Capparis ovata (C. ovata), 143, 161 Capsicum annuum, 219, 395 cAPXs. See Cytosolic ascorbate peroxidases Ca2+ signals, 104, 105, 351, 369 Casuarina, 99 Catharanthus, 166 Catharanthus roseus, 144, 167, 181 CCP. See Cytochrome c peroxidase Cd-hyperaccumulating ecotype, 221

431 CDK. See Cyclin-dependent kinases CDNB. See 1-Chloro-2,4-dinitrobenzene Cell compartments, 2, 24, 155, 211, 258, 259, 308, 389, 390, 394, 399, 416 Cell cycle, 58, 62, 64, 66, 67, 236, 238, 244, 257, 325, 350 Cell death, 2, 11, 32, 33, 102, 120, 123, 124, 142, 222, 237, 238, 266, 273, 287, 357 Cell division, 7, 32, 62–73, 77, 84, 217, 235, 236, 240, 241, 243, 244, 409 Cell division and growth, 409 Cell growth, 68–69, 71, 325, 350, 409, 418 Cell proliferation, 58, 66–68, 71–74, 76, 235, 237, 244, 255 Cell redox buffers, 25 Cell suspension cultures, 57, 66 Cellular antioxidant, 32, 222, 346, 362 Cellular antioxidant defense system, component of, 222 Cellular homeostasis, 240, 266, 267, 280 Cellular information-rich redox buffers, 24 Cellular proliferation, 235 Cellular receptors, 312 Cellular redox balance, 28, 212, 215, 267, 271, 309 Cellular redox environment, 126–128, 232, 245, 310 Cellular redox homeostasis, 23, 24, 126, 215, 280, 325–326 Cellular redox level, 25 Cellular redox regulation, 290, 325, 371 Cellular redox state, 23–28, 116, 126, 127, 215, 220, 232, 233, 240, 245, 259, 268, 274, 281, 282 Cellular redox systems, 77 Cereals, 162–165, 199, 286, 406 Chaperones, 342, 363 Chelation of heavy metals, 278 Chilling, 2, 15, 16, 19, 26, 99, 100, 102, 146, 161, 164, 165, 216, 266, 275, 276, 306 Chilling-low temperature stress, 275–277 Chilling tolerance, maize, 16, 165 Chitinase, 30 Chlamydomonas reinhardtii, 96, 97, 341, 342, 371 Chloracetanilide herbicides, 16 1-Chloro-2,4-dinitrobenzene (CDNB), 202 Chlorophyll fluorescence, 161 Chlorophyta, 17 Chloroplastic DHAR (chlDHAR), 270, 315 Chloroplastic/mitochondrial GR genes, 98, 213 Chloroplasts antioxidants, 337–372 redox signals, 339, 369

432 Chrysopogon zizanioides, 201 Cicer arietinum, 329 9-cis-epoxycarotenoid dioxygenase gene (SgNCED1), 180 Cistus clusii, 144, 170 CLAVATA (CLV), 58 CLAVATA 1 (CLV 1), 75 CLAVATA 3, 77, 239 Cleavage polyembryony, 238 C, N and S, 11 Coffea canephora, 144, 157 Cold treatments, 16, 32, 346 Compartmentalization, 125, 195, 201, 287, 367, 388, 398 Compartmentalization of metabolic reactions, 388 Compartments, 2, 4, 8, 10, 11, 24, 26, 31, 93, 103, 105, 116–118, 120, 122–126, 130, 155, 183, 199, 210, 211, 213, 235, 256–259, 267, 269, 308, 309, 339, 340, 343, 368, 389, 390, 394, 396, 399, 406, 416 Component of plant antioxidant networks, 215 Conifer, 233, 246 Controlling cellular redox state, 23–28 Copper (Cu), 15, 16, 32, 139, 277, 281, 283, 284, 309 availability, 364–366 Co-regulation, 124, 362, 364, 366, 367 Crop productivity, 269, 329 Cross-talk, 8, 23, 26, 27, 123, 129, 355, 366, 399 Cucumis sativus, 145, 180 Cucurbita pepo, 281–282 CUPSHAPED COTYLEDONS, 239 Cuscuta reflexa, 18 CuZn-superoxide dismutase, 30, 139 Cyclin-dependent kinases (CDK), 58 Cysteines, 8, 27, 28, 65, 97, 104, 128, 129, 199, 210, 214–216, 235, 256–258, 275–277, 324, 339, 353–355, 359, 369, 370 Cytochrome c peroxidase (CCP), 93, 94, 183 Cytokinin, 57, 58, 61–62, 69, 181, 234 levels, 61 Cytosol, 4, 8, 10, 11, 24, 91, 92, 97, 98, 116, 118–120, 122, 124, 127–129, 139, 140, 155, 168, 179, 184, 195, 196, 211–213, 258, 259, 267, 279, 287, 288, 308, 309, 314, 324, 326, 340, 342, 343, 355, 368, 371, 389, 390, 393, 394, 396–398, 406, 407, 412, 415 Cytosolic antioxidant enzymes, 346, 348–359, 362 Cytosolic ascorbate peroxidases (cAPXs), 93–95, 101–103, 154, 278, 283, 285, 341, 349–350

Index D DDE. See Diadinoxanthin de-epoxidases Ddx. See Diadinoxanthin Dedifferentiation, 55–84, 218, 236, 238–240, 243, 308, 368 De-epoxidases, 17, 19–22, 139, 219, 267, 308, 409 De-epoxidation, 19–23, 157, 164 of epoxy xanthophylls, 17, 21 Defense mechanisms, 15, 28, 31, 175, 182, 198, 232, 306–309, 361 Defense pathways, 2 Defense-related genes, 28, 30, 256, 354 Degradation of glutathione, 195–196 Dehydrascorbate reductase, 310 Dehydroascorbate (DHA) AsA redox potential, 259 ascorbate redox couples, 13 Dehydroascorbate reductase (DHAR), 3, 4, 11, 14, 30, 60, 63, 76, 79, 81, 82, 92, 116, 119, 122–127, 129, 138–142, 144–146, 149, 151–154, 157, 163, 164, 168, 171, 172, 175, 176, 180, 220, 222, 268, 270, 271, 273, 274, 277, 278, 280, 282–286, 288, 289, 307, 310, 315, 326, 340, 343, 358, 389–396, 408, 419, 420 Depletion of GSH pool, 280 Desertification, 305 Desert plants, 100, 176 Detoxification heavy metals, 32, 215, 219 H2O2, 15, 70, 92, 138, 267, 280, 286, 339, 388, 393 xenobiotics, 65, 126, 139, 197, 198, 200–201, 396 Developmental pathways, 56, 58 DHA. See Dehydroascorbate DHAR. See Dehydroascorbate reductase Diadinoxanthin (Ddx), 17–19, 21, 23 Diadinoxanthin de-epoxidases (DDE), 18–23 Diatoms, 17, 22, 23 Diatoxanthin (Dtx), 17, 19, 21, 23 Differentiation, 55–84, 218, 236, 238–240, 243, 308, 368 2,4-Diphenoxyacetic acid (2,4-D), 57, 59, 192, 193, 200, 236, 395 Diphenyloiodonium (DPI), 78, 81 Dismutation, 4, 210 Distal meristem (DM), 63, 153 Dithiol disulfide exchange, 210 D-mannose/L-galactose pathway, 308 D-mannose-1-phosphate, 4, 414 DNA-coded molecule, 195 DNA segment (T-DNA), 57

Index DPI. See Diphenyloiodonium Drought stress, 2, 15, 99–101, 137–184, 219, 269–271, 314, 328–329, 348 stress and desiccation, 2 stressed, 159, 160, 167, 168, 175 stressed plants, 142–170, 173–177, 179–183 tolerance, 100, 142, 155, 157–162, 165, 173, 177–183, 269–271, 315 Drought-mediated oxidative stress, 141, 155, 156, 183 Dry mass, 170, 222, 223 Dtx. See Diatoxanthin Dual-targeting, 97, 123, 313, 344–345, 394, 398 E EBR. See 2,4-Epibrassinosteroid Electrolyte leakage, 156, 157, 163, 165, 171 Electron donor, 11, 91–93, 119, 140, 141, 218, 308, 324, 326, 327, 342, 393 Elicitation, 81–82 Embryo conversion, 75, 243–245 development, 57, 72, 73, 75, 77, 231–246 formation, 56, 57, 71, 75–77, 83, 232, 233 Embryogenesis, 56–59, 69–76, 82–84, 213, 233, 234, 236–238, 240–242, 244–246, 257, 327, 388 Endogenous electron, 20 Endogenous metabolites, 16 Enhanced affinity, 22 Environment conditions, 11, 15, 26, 30, 121, 123, 290, 305, 338, 389, 408 factors, 9, 67, 138, 232, 394, 396 fluctuations, 116 irradiance, 158, 172 stimuli, 24, 338 Environment stress conditions, 105, 176, 341, 387–399 factors, 312, 314 Enzymatic and non-enzymatic antioxidant pathways, 266 Enzymatic and non-enzymatic integrated mechanisms, 303–318 Enzymatic and non-enzymatic molecules, 266 Enzymatic antioxidants, 10, 121, 123, 138, 140, 156, 158, 160, 163, 168, 172, 179, 304, 310 Enzymatic conjugation, 193 Enzymatic mechanisms, 117, 259 Enzymatic reactions, 222

433 Enzyme ascorbate–glutathione cycle, 81, 139, 140, 143, 155, 164, 171–173, 183, 184, 210, 270, 273, 282, 286, 303–318, 391 catalysed reactions, 15 co-factor, 409 Smirnoff–Wheeler pathway, 410–416 2,4-Epibrassinosteroid (EBR), 220 Escherichia coli, 98, 99, 177, 194, 213, 215, 342 Ethylene, 31, 64, 70, 72, 118, 241, 242, 254, 255, 353, 409 Eucalyptus globulus, 145, 156 Eukaryotes, 98, 211, 212, 253, 257, 406, 412 Eupatorium adenophorum, 100, 171 Eupatorium odoratum, 100 Euphorbia, 165 Euphorbia esula, 16, 166, 270 Eustoma grandiflorum, 65 Evolution metabolic network, 259–260 metabolism, 195 Expressional regulation of antioxidant enyzmes, 346–363 External applications, 180–183 External environment, 8, 126 Extra-plastidic antioxidant defenses, 337–372 Extra-plastidic antioxidant systems, 339–345 Extraprotoplastic matrix, 8 Extreme abiotic stress conditions, 266 Extreme temperatures, 15, 176, 394 F Fabaceae, 194 Factors affecting ascorbate biosynthesis and accumulation, 416 Factors for and against genetic engineering, 420–421 fa*gus sylvatica, 145, 170 FAO. See Food and Agriculture Organization Fence model, 254 Fenchlorazole, 16 Ferredoxin-thioredoxin system, 311 Ferulic acid peroxidase (FPOX), 71 First line of defence, 4, 287, 363, 394 Fluxes of H2O2 production, 4 Food and Agriculture Organization (FAO), 406 Food security and malnutrition, 421–422 Foyer–Halliwell-cycle, 343 FPOX. See Ferulic acid peroxidase Functional proteins, 174 G GA. See Gibberellic acid Galactono-g-lactone (GalL), 66, 71, 79, 80, 280

434 Gamma-glutamylcysteine synthetase (g-ECS), 7, 8, 31, 65, 195, 196, 217, 219, 236 GDP-D-mannose, 415 GDP-D-mannose-3¢,5¢-epimerase, 413 GDP-D-mannose pyrophosphorylase, 412–413, 417 GDP-L-galactose, 415 GDP-L-galactose phosphorylase, 407 Gene expression analysis, 59, 361, 364, 367, 392 coordination of, 368 influence, 28–31, 355 patterns of, 30, 59, 75–77, 175 and protein expression, 266 General stress response, 327–328 Genes from D-galacturonate pathway, 418 from Smirnoff–Wheeler pathway, 417–418 Genes encoding, enzymes of arcobate glutathione cycle, 313 Genetically modified foods, 421, 422 Genetically modified plants, 405–422 Genetic engineering, 193, 218, 284, 288, 315, 317, 420–422 Genetic manipulation, 270, 275, 276, 316, 422 Genetic transformation, 56, 276 Genotype and phenotype, 7 Germination, 70, 71, 75, 147, 168, 171, 213, 217, 231–246, 257, 275, 388 GHS/GSSG redox status, 72 Gibberellic acid (GA), 10, 181, 409 Glomus versiforme, 159 Glucose-6-phosphate dehydrogenase, 124, 180, 397 Glutaredoxins (GRXs), 28, 128–130, 210, 211, 214–217, 259, 342 Glutathione (GSH) administration, 75 and ascorbate redox state, 231–246 biosynthesis, 65, 66, 213, 216, 217, 221, 258 deficiency, 16, 257 deficient mutant, 6, 7 dependent changes, 73–75 functions of, 197 homeostasis, 120 metabolism, 12, 30, 31, 59, 76, 81, 195, 212, 220, 235, 236, 243, 257, 267, 285–287 oxidation and transport, 199 oxidation ratio, 169 peroxidase expression, 362–363 pool, 12, 15, 24, 26, 27, 30, 32, 72, 73, 76, 78, 79, 92, 125–127, 141, 142, 172,

Index 173, 194, 223, 235, 240–242, 245, 269, 270, 273, 285–287, 355 pool size of, 15 redox state, 72, 126, 235–236, 241, 269, 275, 278, 285 redox status, 13, 15 reductase-encoding genes, 98–99 reductase family, 98 signalling module, 257–258 synthesis, 8, 12, 15, 16, 30–32, 196, 199, 201, 211–213, 217, 221, 235, 237, 257, 272, 275, 277, 279, 287, 324 synthesizing capacity, 277 Glutathione (g-L-glutamyl-L-cysteinyl glycine; GSH), 4 Glutathione-and ascorbate-mediated redox regulation, 126, 128 Glutathione-and ascorbate-related redox regulations, 126–130 Glutathione disulfide (GSSG), 3, 4, 9, 12, 13, 15, 21, 26, 27, 30, 32, 63–67, 71–74, 76, 78, 79, 92, 96, 97, 119, 120, 123–125, 128, 139, 141, 143, 145, 146, 149, 151, 152, 155, 163, 164, 169, 172, 182, 197, 199, 200, 210, 211, 213, 221, 232, 240, 242, 245, 309, 314, 324, 327, 370, 390, 393 Glutathione disulfide-glutathione (GSH/ GSSG) couple, 26, 27, 32, 128, 199, 232, 245, 267, 269 Glutathione disulfide/glutathione (GSSG/2GSH) ratio, 13 Glutathione-induced changes, 75–77 Glutathione peroxidase (GPX), 3, 11, 12, 16, 81, 93, 130, 139, 163, 179, 197, 210, 221, 222, 267, 277, 290, 307, 329, 330, 339, 340, 342 Glutathione reductase (GR), 3, 11, 61, 65, 91–106, 116, 119, 139, 140, 195, 197, 210, 211, 220–222, 235, 268, 306, 310, 324, 326, 328, 329, 343, 344, 359, 389–391, 393, 408, 420 characteristics of, 96–99 Glutathione–short historical survey and evolution, 194–195 Glutathione-S-transferase (GST), 163, 307, 358 significance of, 201–202 Glutathione synthetase (GSHS), 8, 30, 31, 195, 196, 202, 324, 330 Glutathionylation, 27, 28, 68, 129, 214, 253, 258 Glutharedoxins, 232, 342 Glycine, 8, 12, 196, 257, 412 Glycine max, 146, 161, 194, 365 Gossypium hirsutum, 146, 166

Index G1 phase, 63, 66, 67, 236 GPOX. See Guaiacol peroxidase GPX. See Glutathione peroxidase GR. See Glutathione reductase GR isoforms, 96, 155, 179, 393 GRXs. See Glutaredoxins GSH. See Glutathione GSH/GSH + GSSG ratio, 72 GSHS. See Glutathione synthetase GSH1 transcripts, 16 G1-S phase, 244 GSSG. See Glutathione disulfide GSSG-treated embryos, 72, 76, 242 GST. See Glutathione-S-transferase GST-mediated herbicide resistance, 201 Guaiacol peroxidase (GPOX), 71, 81, 93, 197, 210, 221, 222 H Haem-containing enzyme, 93 Hairy root culture, 57, 82 Half-life, 93, 192 Halliwell–Asada cycle, 4, 79, 84, 198, 232, 243 Halliwell–Foyer–Asada cycle, 408 H+-ATPase, 68 Heat shock, 2, 15, 95, 233, 266, 277, 314, 352 Heat shock factors, (HSFs) 105, 350, 353 Heatshock proteins (HSPs), 278, 312, 327, 368, 398 Heat shock transcription factors (HSFs), 30, 283 Heat stress, 16, 102, 277–278, 313, 314, 348, 352, 356 Heavy metals, 2, 7, 16, 32, 101, 139, 192, 211, 215, 219, 223, 235, 258, 266, 278, 279, 305, 308, 326, 394, 396 Heavy metal stress, 15, 223, 278–283 Helianthus annuus, 100, 146, 172, 181, 272, 329 Hemiparasitic plants, 18 Herbaceous species, 165–168 Herbicide Resistance Action Committee, 191 Herbicides, 16, 192–194, 197–202, 258, 304 classifications, 192–193 conjugation, 193, 197, 199–200, 258 defense system, 197–198 resistance, 191–202 stress, 192, 199–200, 202 Heterocomplex formation, 363–367 Hexuronic acid, 252 High light (HL), 5, 6, 11, 14, 19, 22, 23, 99, 102, 103, 156, 170, 173, 179, 266, 278, 283–285, 304, 311, 313, 316, 338, 353, 354, 362, 363, 389, 394, 398 High light stress, 2, 173, 284, 285, 350

435 Histodifferentiation, 77, 241, 245 Holoparasitic, 18 hom*o-glutathiones, 194, 195, 202, 211, 324 Hordeum vulgare, 101, 395 Horizontal connection, 253, 254, 258–259 HRGPs. See Hydroxyproline rich glycoproteins HSFs. See Heat shock factors; Heat shock transcription factors HSPs. See Heatshock proteins Hydrilla verticillata, 221 Hydrohymethylglutathione, 194 Hydrophilic redox buffer, 14, 116 Hydroxyproline, 64, 67, 120, 244, 255 Hydroxyproline-reach proteins, 244 Hydroxyproline rich glycoproteins (HRGPs), 67, 120, 255, 256 Hydroxyproline synthesis, 64 Hypoxia, 286 I IBA. See Indole butyric acid Imbalance between ROS and antioxidant defense, 115, 222, 223, 304 Immunogold electron microscopy, 391 Indole alkaloid content, 167 Indole butyric acid (IBA), 57, 78, 83 Influence, 3, 10–12, 14, 19, 25, 26, 28–32, 58, 67, 81, 119, 166, 167, 169, 180, 181, 192, 196, 218, 223, 284, 308, 329, 340, 355, 364, 370, 371 Inga sapindoides, 18 Inversion and non-inversion routes, 409–411 In vitro cultures, 55–84, 194 cells, 56–59, 61, 66, 84 organs, 56–59 tissues, 56–60 In vitro embryogenesis, 233, 236–238 Ionic toxicity, 272, 329 Ipomoea batatas, 362 Irradiance, 19, 158–160, 166, 172, 176, 177, 304 Isoenzyme patterns, 289, 341–344 Isoforms and subcelluar localization, 93–94 of glutathione reductase, 97 J Jasmonic acid (JA), 6, 16, 128, 223, 396 K Ketoconazole (KCZ), 181 Knockout, 8, 30, 95, 212, 341, 352–354, 359, 361, 366, 367, 372

436 Knockout mutant, 7, 196, 412 KNOTTED-like homeobox (KNOX), 76 L Late embryogenesis abundant (LEA) proteins, 75, 242, 327 Laurus azorica, 146, 156, 157 Lavandula spica, 161 L-buthione sulfoximine, 65–67, 72, 73, 76, 77, 79, 235, 237, 239–242, 359, 360 Leaf water potential, 157, 222 LEAFY COTYLEDON 1 (LEC1), 59 L-galactono 1,4 lactone dehydrogenase, 169, 253, 254, 284, 407, 409, 410, 416, 418, 419 L-galactose dehydrogenase, 284, 407, 410, 415, 418 L-galactose-1-phosphate, 4, 413, 414 L-galactose-1-phosphate phosphatase, 407, 414 Light stress, 33, 95, 102, 283–285, 352, 395 Lignin peroxidases (LiPs), 94 Ligustrum vulgare, 146, 160, 161 Lipid peroxidation, 5, 15, 61, 82, 123, 139, 157, 158, 161, 162, 164, 168, 182, 183, 197, 223, 266, 275, 281, 282, 289, 329 Lipid peroxides, 16, 223, 305, 342 Lower affinity, 22 Low light (LL), 14, 19, 22, 169, 179, 283, 284 Low oxygen stress, 285–287 Lumen, 11, 17, 19, 21–23, 138, 139, 259, 342, 407, 409 Lutein epoxide (Lx), 17, 18 Lycium barbarum, 83 Lycopersicon esculentum, 146, 219, 395 Lycopersicon peruvianum, 100 Lycorine, 71, 243, 245, 254 M Maize, 15, 16, 26, 30, 96, 100, 163–165, 171, 192, 197, 199, 202, 217, 219, 220, 256, 270, 275–277, 283, 306, 314, 328, 343, 356, 363, 419 Malondialdehyde (MDA), 81, 156, 160, 162, 163, 165, 172, 174, 175, 220, 222, 223, 390 Malpighia glabra, 417 Malus domestica, 146, 169 Manganesedependent peroxidases (MnPs), 94 Mantoniella squamata, 17 MAPK. See Mitogen-activated protein kinase Markers of plant stress, 13, 28 Mass spectrometry, 176

Index MDA. See Malondialdehyde MDEs. See Microspore-derived embryos MDHA. See Monodehydroascorbate MDHAR. See Monodehydroascorbate reductase Mechanical stress, 2, 103 Mehler reaction, 23, 31, 283 Membrane-bound antioxidant compounds, 139 Membrane lipid peroxidation, 266 Membrane stability, 158, 162, 182 Menadione, 16 Meristems, 7, 56–58, 62–66, 70–77, 83, 84, 213, 216–218, 239–242, 244, 245, 256, 257 reactivation, 71, 72, 231–246 Mesophyll cells, 16, 30, 171, 180, 236, 288, 338 Metabolic activity, 10, 138, 259 Metabolic electron consumption, 338 Metabolic hom*ogeneity, 66 Metabolic processes, 14, 24, 25, 27, 28, 31–33, 117, 218, 232, 235, 245, 304, 308, 310, 408 Metabolic reactions, 324, 338, 388, 397 Metabolic regulation, 286, 310–315 Metabolism, 2, 6, 12, 13, 16, 23–25, 29–31, 59, 61, 64, 65, 67, 70, 73–76, 79, 84, 115–122, 125, 138, 141, 142, 159, 166, 168, 177, 180, 192, 193, 195–197, 200–202, 210, 212, 214, 215, 218, 220, 221, 223, 233, 235–236, 238, 240, 241, 243–245, 252, 253, 257, 266, 268–271, 275, 278, 283, 306, 308, 311, 315, 326, 330, 371, 388–389, 398, 420 Metabolism and modulation of growth and development, 235–236 Metabolite synthesis, 81–82 Metallo-Chaperone, 365–366 Metal pollution, 278 Metal stress, 220, 222, 223 Metaphase, 244 Methionine sulfoxide reductases, 210 Methylglyoxal (MG), 16, 32, 275 Methyl viologen, 102, 103, 215, 314, 346, 367 Microarray experiments, 240, 256, 392 Microarray studies, 75 Microbodies, 92–94, 101, 102, 138, 340, 341 MicroRNAs, 364 Microspore-derived and somatic embryos, 234 Microspore-derived embryogenesis system, 234, 237 Microspore-derived embryos (MDEs), 233, 234, 237, 239–242 Mineral ion toxicity, 304 Mineral nutrients deficiency, 304 miRNA mechanism, 366

Index Mitochondria, 2, 4, 10, 12–13, 24, 32, 64, 91–95, 97, 98, 116, 122–125, 138, 164, 173, 211, 213, 214, 218, 219, 253, 257, 259, 266, 267, 282, 289, 306–310, 313, 314, 324–326, 340–345, 363, 389, 393, 394, 396, 397, 406, 416 Mitochondrial and peroxisomal ascorbate– glutathione cycle, 12 Mitogen-activated protein kinase (MAPK), 10, 350 cascade, 104, 105, 311, 350–356, 361, 371 pathway, 60, 311 Mitosis, 142, 325 Mitotic activity, 62, 63, 67, 237, 243, 244 Mitotic index, 66, 67, 409 Modulation of ascorbate–glutathione cycle, 311–313 of redox state, 10, 219, 232, 235–236, 243–245 of ROS-antioxidant interaction, 310 Molecular identification, 387–399 Molecular oxygen, 4, 283, 330 Monodehydroascorbate (MDHA), 3, 4, 8–11, 24, 68, 92, 119, 139–141, 145, 154, 172, 218, 219, 241, 243, 256, 268, 271, 273, 308, 309, 326, 340, 389, 390, 393, 407, 408, 420 Monodehydroascorbate reductase (MDHAR), 4, 8, 9, 11, 13, 24, 29, 60, 63, 73, 76, 79, 81, 92, 116, 118, 119, 122–124, 138–142, 145, 147, 149, 151, 153, 154, 157, 164, 168, 171, 172, 175, 180, 218, 270, 273, 280, 282–286, 288, 289, 307–310, 313, 314, 326, 340, 344, 345, 357–359, 389–391, 395, 407, 408 Monoepoxide, 17 Morphogenesis, 56, 58, 233, 236, 241, 245 Morphogenetic pathways, 56, 57, 59 mRNA level, 16, 59, 76, 95, 99, 101–103, 140, 178, 255, 256, 270, 279, 284, 286, 288, 289, 314, 315, 357, 364, 366, 368 Multifunctional compounds, 116 Mutants, 4–8, 11, 13, 26, 29–31, 58, 65, 66, 118, 123, 125, 173, 194, 196, 212, 213, 216, 217, 236, 240, 255–257, 278, 284, 288, 314, 315, 346, 360, 361, 366, 388, 393, 411–414, 418, 421, 422 Myo-inositol route, 418–420 Myrica faya, 156 N b-Naphtoxyacetic (NAA), 57, 60, 78 NAD(P), 24

437 NADH oxidation, 24, 64, 67, 68, 389 NAD/NADH redox pair, 67–68 NADPH-dependent reaction, 11 NAD pool, 38 NAM. See NO APICAL MERISTEM Natural and anthropogenic phenomena, 192 Natural metabolism, 192 Near-isogenic lines (NILs), 277 Necrotrophic pathogens, 120, 123, 129 Nernst equation, 126, 199 Net CO2 assimilation rate, 169, 183, 285 Net photosynthesis rates, 164, 314 Nickel (Ni), 282–283 Nicotiana tabacum, 10, 98, 147, 179, 271, 289, 314 NILs. See Near-isogenic lines Nitric oxide, 16, 26, 31, 77, 80, 81, 180, 182, 272–274, 306, 396, 399 N, N¢-diethyldithiocarbonate, 84 NO APICAL MERISTEM (NAM), 75 Non-enzymatic and enzymatic integration, 303–318 Non-enzymatic antioxidant pools, 142–170 Non-enzymatic antioxidants, 2, 10, 123, 128, 138, 142–170, 172, 180, 183, 210, 214, 222, 223, 266, 305, 307, 309, 310 Non-enzymatic antioxidant system, 138, 222, 223, 304, 339 Non-enzymatic compounds, 307–309, 330 Nonexpressor of pathogen related genes (NPR1), 30, 104, 105, 127–129, 355, 363 Non-heme peroxidases, 341 Non-photochemical quenching, 11, 157, 283 Non-protein thiol glutathione, 222 Non-protein thiols, 211, 308, 396 NPR1. See Nonexpressor of pathogen related genes Nucleotide metabolism, 67, 73–75, 244 Nucleus, 68, 104, 124, 127, 128, 340, 355, 361, 367 O O-acetylserine, 12 2-ODDs. See 2-Oxoacid-dependent dioxygenases Olea europaea, 158 OPP. See Oxidative pentose phosphate pathway Organic sulphur-hydrogenating substance, 252 Organogenesis, 55–84, 244 Organ primordia, 58 Oryza sativa, 98, 99, 165, 314, 364 Osmolarity, 166 Osmotic balance, 272

438 Oxalic acid (OxA), 31 Oxalic and l-tartaric acids, 1 Oxidation, 4, 9, 10, 12, 13, 15, 24–28, 30, 32, 60, 61, 63, 64, 67–69, 73, 78, 79, 82, 93, 104, 118–121, 123, 124, 126, 169, 170, 199, 200, 214, 215, 220, 223, 243, 256, 266–270, 272, 273, 275, 278, 280, 282, 285, 286, 289, 305, 307, 311, 315, 324, 326, 338, 339, 388–390, 394, 398, 406, 409–411, 413 Oxidation and reduction of protein thiols, 214 Oxidation of glutathione pool, 73, 79 Oxidative burst, 9, 117, 120, 124, 125, 306, 311, 396 Oxidative damages, 12, 14, 16, 81, 82, 116, 138, 159–161, 163, 164, 168, 171, 179–183, 198, 199, 210, 214–216, 270, 272, 274, 278, 286, 304, 305, 308, 315, 324, 329, 390, 393 Oxidative pentose phosphate pathway (OPP), 124 Oxidative stress, 11, 64, 101, 115, 138, 196, 211, 258, 266, 304, 326, 338, 388, 420 Oxidative stress-promoting chemical agents, 271 Oxidising agents, 55 OxA. See Oxalic acid 2-Oxoacid-dependent dioxygenases (2-ODDs), 255, 409 Oxygen scavengers and chelators, 305 Ozone, 12, 15, 16, 25, 26, 99, 101, 102, 167, 170, 235, 255, 266, 287–290, 304, 313–315, 338, 359, 408, 413, 421 Ozone-sensitive mutants, 255 Ozone stress, 266, 287–290 P Paclobutrazol (PBZ), 181 PAL. See Phenylalanine ammonium lyase Panax ginseng, 81, 82 Panax quinqefolium, 82 PAPS. See Phosphoadenylyl sulfate PAR. See Photosynthetically active radiation Paraquat, 95, 99, 102, 103, 180, 197, 198, 284 PARP-coding genes, 67 Partial drying treatment (PDT), 70, 75 Pathogen attack, 2, 15, 117, 124, 125, 192, 304, 306, 311, 338, 355, 357, 396 Pathogenesis-related (PR) genes, 28–30, 118, 349, 355, 359 proteins, 28–30 Pathogens, 9, 25, 30, 31, 117–129, 142, 157, 213, 216, 338, 396, 422 Pathogen-triggered cell death, 123

Index PATOGENESIS RELATED PROTEIN 1 (PR1), 61 PBZ. See Paclobutrazol PC. See Phytochelatins PCD. See Programmed cell death PDT. See Partial drying treatment Peptide ligands, 278 Peroxiredoxin regulation, 356–357 Peroxiredoxins, 29, 130, 210, 339–342, 356, 357, 359 Peroxisomal ascorbate–glutathione pathway, 12, 387–399 Peroxisomal reactive oxygen species metabolism, 388–389 Peroxisomal redox status of ascorbate, glutathione and NADPH, 396–397 Peroxisomes, 2, 4, 12–13, 91, 94, 97, 116, 122, 124, 125, 266, 306, 307, 309, 326, 338, 388–398 Persea indica, 156, 157 Pesticide, 180, 192 Phaeodactylum tricornutum, 17 Phaeophyta, 17 Phaseolus vulgaris, 123, 178, 279, 287 Phenological stage, 166, 275 Phenotype of ascorbate (vtc) comaparison, 5 Phenylalanine ammonium lyase (PAL), 30, 120, 285 Phloem glutathione, 170 Phosphoadenylyl sulfate (PAPS), 211 Phospholipid hydroperoxide glutathione peroxidase, 139, 307 Phosphomannomutase (PMM), 412, 417 Phosphomannose isomerase (PMI), 411–412 Phosphorylation, 74, 311, 350, 392 Photochemical, 10, 275 Photoinhibition, 141, 157, 284, 285 Photo-oxidative stress, 101, 276, 283, 284, 330, 334, 361, 367, 371 Photoprotection mechanism, 17 Photoprotective pigments, 169 Photoprotective processes, 23 Photorespiration, 10, 12, 25, 266, 306, 307, 330, 388 Photorespiration pathway, 12 Photosynthesis, 10, 12, 13, 25, 29, 31, 61, 65, 139, 158, 161, 164, 167, 172, 177, 179, 182, 192, 214, 216, 223, 266, 271, 287, 288, 313–315, 325, 338, 360, 366, 368, 369, 388, 408 Photosynthetic activity, 275, 288, 371 Photosynthetically active radiation (PAR), 156, 158 Photosynthetic apparatus, 141

Index Photosynthetic cells, 171 Photosynthetic components, 275 Photosynthetic electron transport, 10, 23, 24, 283, 285, 338, 339, 360, 366 Photosynthetic electron transport chain, 23, 24, 285, 338, 339, 366 Photosynthetic operation, 13, 326 Photosynthetic organisms, 308 Photosynthetic performance, 164 Photosynthetic pigments, 166, 182, 183 Photosynthetic potential, 222, 223 Photosystem II (PSII), 157, 177, 192, 193, 216, 283, 284, 314, 330 Physcomitrella patens, 341 Physiological functions, 25, 195, 330, 388, 394, 398 Physiological responses, 2, 396 Physiological status, 12 Phytochelatins (PCs), 16, 32, 139, 196, 213, 217, 220–223, 257, 278, 279, 281, 282 biosynthesis, 217, 279, 281 Phytohormones, 58, 65, 241 Phytophthora brassicae, 30 Phytoremediation, 82, 202, 217 Picea P. abies, 237 P. asperata, 156 P. glauca, 69, 233–234, 237, 246 Pinus canariensis, 169 Piriformospora indica, 175, 315, 358 Pisum sativum, 98, 102, 173, 182 Plant defense response, metabolic processes, 31–33 Plant–pathogen interactions, 115–130, 388 Plants defense mechanisms, 306–309 development, 62, 64–66, 179, 212–216, 235, 236, 257, 325 peroxisomes, 388, 394, 397, 398 regeneration, 58, 84, 236–238 resistance to herbicide stress, 199–200 stress tolerance, 1–33, 105 Plasma membrane, 2, 8, 21, 63, 68, 119, 120, 266, 267, 306, 312, 338, 339 Plastid and extra-plastid antioxidants, 346–349 Plastids, 8, 196, 212, 213, 259, 285, 305, 307, 316, 324, 326, 330, 346–349, 361, 362, 368, 371, 372 Plastid transcription kinase (PTK), 285 Plastochinon pool, 26 Plastocyanin, 366 Plastocyanin expression, 366 PM. See Proximal meristem PMI. See Phosphomannose isomerase (PMI)

439 PMM. See Phosphomannomutase (PMM) Polyacrylamide gel electrophoresis, 176 Poly (ADP-ribose) polymerase (PARP) activity, 67, 68 Poncirus trifoliata, 159 Populus accutifolius, 270 Populus euramericana, 100 Populus kangdingensis, 155 Populus trichocarpa, 98, 314 Post-transcriptional regulation, 357, 363–368 Potential gene targets, 405–422 Precursors, 6, 31, 32, 65, 66, 69, 73, 74, 79, 139, 194–196, 213, 218, 223, 238, 243, 245, 257, 279, 280, 308, 324, 325, 350, 359, 364, 368, 371, 410, 411, 418 Priformospora indica, 306 Primary symptoms of oxidative burst, 9 Production of reactive oxygen species, 115, 304–305 Programmed cell death (PCD), 11, 33, 102, 118, 121, 123, 124, 237, 238, 306 Properties, 10, 19, 84, 91–93, 99, 101, 122, 165, 181, 210, 275, 304, 305, 316, 344, 406 Protection, 2, 10, 13–15, 17, 19, 31, 120, 125, 138, 156–158, 199, 210, 219, 222, 223, 258, 267, 269, 278, 289, 290, 305, 307, 308, 316, 341–344, 354, 359, 361, 366, 369, 390, 409 Protectors, 209–223 Protein folding, 26, 199, 256 glutathonylation, 258 import, 313, 367–371 induced by drought stress, 174 labeling studies, 344 oxidation, 214, 266 protein interaction, 77, 239, 355, 361, 392 synthesis, 75, 99, 102, 105, 197, 314 Proteomic analysis, 176, 281 Proteomic approaches, 344 Proteomic studies, 176–177 Proton donor, 20 Proton-electrochemical gradient, 218, 308 Proximal meristem (PM), 63 Pryrimidine precursors, 245 PSII. See Photosystem II Pseudomonas aeruginosa, 98, 99 Pseudomonas syringae, 10, 125 Pteris vittata, 216 PTK. See Plastid transcription kinase Pyrimidine metabolism, 73 nucleotides, 73, 74, 238, 244

440 Q Quercus species, 18 Quiescent center (QC), 62–64, 216, 217, 235, 244 R RAC. See ROOTING AUXIN CASCADE (RAC) Ratio GSHred/GSHox, 10 Reactions catalyzed by VTC2, 415 Reactive nitrogen species (RNS), 16, 214, 388, 396, 398 Reactive oxygen and nitrogen species, 388 Reactive oxygen species (ROS) detoxification, 15, 116, 286, 339, 357 generation, 24, 117, 339, 366, 394 production, 2, 24, 27, 32, 60, 82, 92, 104, 125, 274, 304, 306, 312, 352, 396 scavenging, 2, 9, 11, 92, 103–105, 116, 124, 130, 155, 177, 182, 215, 222, 223, 258, 267, 273, 279, 280, 310, 312, 314, 315, 328 signaling molecules, 397–398 signal transduction cascade, 104–105 signal transduction pathway, 105 Real time-PCR, 173 Reaumuria soongorica, 100 Redox active components, 29 active elements, ferredoxin, NADPH and GSH, 24 buffers, 24, 25, 116, 130, 139, 219 flux, 116, 210, 269 homeostasis, 4, 23, 24, 117, 125, 126, 215, 325–326 imbalances, 23, 339, 351, 360, 361 metabolites, 23, 24, 396 metabolites, NAD(P), GSH and Asc, 24 pairs, 23, 123, 251–260, 326 potentials, 10, 19, 23, 99, 116, 123, 126, 199, 210, 215, 258, 259, 311, 338 reactions, 26, 307 sensitive proteins, 268, 269 signaling, 2, 26, 28, 117, 120, 126, 127, 130, 198, 212, 214, 266, 286, 289, 309, 342, 367, 396 signals, 13, 23, 115–130, 312, 326, 339, 350, 361, 366–367, 369 state, 2, 9, 10, 12, 15, 23–28, 30–33, 60, 62, 70–73, 116, 118–124, 126–128, 157, 169, 170, 173, 197, 199, 213, 217, 219, 220, 231–246, 257–259, 268, 269, 271, 272, 274, 275, 277, 278, 281, 282,

Index 284, 285, 287–289, 315, 339, 343, 355, 360, 369, 371, 389 state of glutathione, 15, 27, 33, 123, 126, 213, 285, 343 systems, 24, 25, 27, 68, 77, 119 Redox active compounds, 23, 117, 126, 369 pools of, 23 Redox regulation central role of, 304, 310 2-Cys Peroxiredoxin-A, 359–362 protein import, 367–371 Redox couples: Asc/DHA, GSH/GSSG, and NADPH/NADP+, 13 Redox-regulated pathways, 26 Redox-sensitive cellular processes, 216 Redox-sensitive receptors, 25, 104, 105 Reduced ascorbate environment, 245 Reduced ascorbate redox state, 245 Reduced glutathione environment, 237, 238 Reduced glutathione pool, 15, 223, 285 Reduced photosystem I, 338 Reductase, 3, 60, 91, 116, 139, 197, 210, 235, 253, 268, 306, 324, 340, 389 Reduction, 11, 14, 24, 28, 32, 57, 60, 62, 70, 72–74, 83, 99, 118, 119, 126–129, 139, 141, 169, 179, 181, 182, 198–200, 210–217, 219, 238, 240, 241, 243, 244, 258, 266–271, 273–275, 281, 283, 285–287, 304, 307, 309, 310, 315, 317, 318, 324, 326, 327, 340, 343, 355, 361, 371, 389, 390, 393, 397, 409, 413, 417 Regenerating, 11, 56–60, 63, 76, 79, 124, 169, 218, 219, 280, 285, 328, 393, 415 Regeneration, 13, 24, 28, 55–84, 118, 120, 124, 127, 129, 141, 215, 236–239, 242, 267, 273, 283, 308, 314, 339, 340, 342, 343, 409 Regulation of catalase genes, 356 of cytosolic MDHAR, 357–359 of defense genes, 355 of expression genes, 6 of extra-plastidic antioxidant defense system, 337–372 of Fsd expression, 366–367 of genes encoding chloroplast antioxidant enzymes, 337–372 of nucleotide synthesis, 55 of transcript abundance, 348, 358 of transcript abundance of genes, 348 via HSFs, 352–353 Regulators, 28, 57, 59, 62–65, 76, 77, 116, 127, 139, 173, 181, 197, 233, 241, 254, 258, 266, 271, 352, 354, 355, 360, 361, 365

Index Regulatory amplitudes of genes, comparison of 348 Regulatory factor, 65–66 Regulatory proteins, 27, 267, 356 Relative leaf water content, 170, 179 Relative water content (RWC), 156–158, 160–162, 166, 169, 170, 172, 182 Repression/activation, 2 Retama raetam, 176, 177 Rhodophyta, 17 Ribonucleotide reductase (RNR), 64, 210, 211, 244 Ricinus communis, 314, 315 RNA blot analyses, 271 RNAi construct, 314 RNR. See Ribonucleotide reductase RNS. See Reactive nitrogen species Roles of Asc and GSH, 82 Root apical meristem, 62–64, 66, 73, 75, 256 cultures, 57, 81–82, 286 glutathione, 170 primordium, 77 regeneration, 13, 24, 28, 55–84, 118, 120, 124, 127, 129, 141, 215, 236–238, 242, 267, 273, 283, 308, 314, 339, 340, 342, 343, 409 Root-colonizing endophytic fungus, 175 Root-inducing (Ri), 57 ROOTING AUXIN CASCADE (RAC), 59 ROOT MERISTEMLESS 1 (RML1) gene, 65 ROS. See Reactive oxygen species Rosa hybrida, 174 ROS-scavenging mechanisms, 2 RUBISCO (Pssu-ipt), 61, 177, 345 RWC. See Relative water content (RWC) S S-adenosylmethionine, 242 Salicylic acid (SA), 10, 32, 127–129, 180, 182, 216, 274, 306, 355 Salicylic acid (SA) biosynthesis, 11 Salinity, 9, 15, 31, 32, 99, 101, 103, 192, 222, 266, 272–275, 304, 305, 307, 315–318, 327, 329 Salinity stress, 99, 103, 192, 222, 223, 272–275, 305–307, 315, 316, 329, 354, 397 Salinization, 305, 317, 318 Salt dependent-oxidative cell death, 273 Salt-sensitive genotype, 100 Salt stress, 2, 14, 99–101, 162, 178, 180, 272–275, 305, 306, 314, 329–330, 346, 348, 361

441 Salt-tolerant maize, 100 SAM. See Shoot apical meristem SCARECROW (SCR) gene, 75 Scavenging, 2, 4, 22, 67, 79, 84, 92, 96, 116, 124, 138–140, 155, 177, 180–182, 218, 240, 258, 271, 274, 280, 308, 390 Scheme of ascorbate–glutathione (AsA-GSH) cycle, 3, 92 Sclerotinia sclerotiorum, 121 Secondary messengers, 2, 24, 142, 266, 369 Secondary metabolite production, 82 Secondary redox-reactions, 338 Sedum alfredii, 221, 222 Selaginella moellendorffii, 341 Senescence, 12, 61, 62, 221, 281, 287, 338, 356, 358, 397, 409, 416 Sensors of reactive oxygen species-perception, 104 Sequestration of toxic metals, 31 SERK. See Somatic embryogenesis receptor-like kinase Setaria italica, 99, 272, 329 Shoot apical meristem (SAM), 58, 71–73, 75–77, 240, 244, 245 architecture, 239, 242 SHOOT MERISTEMELESS (STM), 58, 76, 77, 239, 240 Shoot organogenesis, 56–58, 82–84, 244 Shoot regeneration, 84 Shoot–root translocation of glutathione, 170 Shrub species, 160–161 Signal molecules of oxidative stress, 103–104 regulation, 312 transduction, 9, 11, 23, 26, 28, 32, 101, 104–106, 142, 174, 184, 219, 251–260, 267, 311, 342, 349, 350, 352, 356, 360, 372, 392 transduction pathways, 2, 23–28, 104–106, 120, 311, 346, 367, 398 Signaling/Signalling cascades, 23, 33, 327, 361, 362, 371, 372 molecules, 82, 116 pathways, 10, 26, 31, 32, 59–61, 77, 80, 103, 117, 118, 121, 127, 128, 130, 212, 276, 290, 309, 311, 313, 325, 326, 350, 359, 361, 368, 371–372 Signalling-related implications, 253 Signal-transducing processes, 8 siRNAs, 357, 358, 364 Small RNAs, 363–367 Smirnoff–Wheeler pathway, 253, 410–418 Smirnoff–Wheeler (l-galactose) pathway, 407, 410, 415

442 S-nitrosoglutathione, 16, 26, 396, 399 SOD. See Superoxide dismutase Sodium nitroprusside (SNP), 31, 81, 180, 182, 183, 272 Solanum lycopersicum, 272 Somatic embryo formation, 56, 57, 69, 75–77, 83 Somatic embryogenesis, 56–59, 69–76, 82–84, 233, 236, 240, 242, 244 Somatic embryogenesis receptor-like kinase (SERK), 59 Somatic embryos, 56, 57, 69–73, 76, 233, 234, 237, 238, 241, 242, 245 Sorghum bicolor, 151, 172 STM. See SHOOT MERISTEMELESS Stomatal conductance, 160, 167, 169, 271, 275 Strategies to improve the antioxidant levels in crops, 315–318 Stress conditions, 4–13, 15, 24, 27, 28, 31, 64, 67, 81, 95, 96, 99–101, 103, 105, 106, 125, 164, 169, 176–179, 192, 213, 235, 257, 265–290, 304, 316, 318, 326, 341, 344, 362, 387–399 factors, 9, 12, 19, 24, 31, 103, 117, 183, 192–194, 197, 266, 312, 314 tolerance, 1–33, 91–106, 162, 163, 168, 174, 182, 219, 275, 278, 284, 287, 306, 314, 316, 329, 330, 357 Stress-response signal transduction pathway, 2 Stromal proteins, 342 Structural, physiological and molecular changes, 238–243 Suaeda salsa, 95, 100, 101 Subcellular localization, 94, 97, 340–344 of antioxidant system, 339–340 Subcellular targeting, 393 Sub-compartmental localization, 390, 393 Superoxide dismutase (SOD), 3, 9, 30, 61, 81–84, 140, 156, 157, 161, 163–168, 171, 172, 174, 175, 180, 181, 183, 210, 220–222, 270, 279, 282, 283, 286, 306, 307, 313, 328, 340, 343–345, 356, 363–367, 372, 388, 394 Superoxide radical, 2, 220, 221, 266, 304, 306, 339, 363 Symplast, 8–10, 100, 275, 289 Synergistic antioxidants, 305 Systemic defence, 121, 130 T Tagetes erecta, 80 Targeted genes for ascorbate improvement, 417–420

Index TDM. See Triadimefon T-DNA insertions, 8, 175, 358 TE. See Tracheary elements T. goesingense, 282 Thiol buffer, 10, 325 concentrations, 194 Thiol/disulfide-containing proteins, 232 Thiol-disulfide interconversions, 257 Thiol-disulphide, 27, 68, 120, 129, 257 Thiol-group, 26, 104, 129, 199, 211, 267, 308, 311, 324, 326, 357 Thiol (–SH) group, 324, 326 Thiol-mediated signalling, 129 Thioredoxins (TRX), 24, 30, 123, 128–130, 210, 211, 214–217, 232, 256, 311, 339, 342, 355, 360, 362, 369, 370 pathway, 311 Thylakoid-associated ferredoxin, 407 Thylakoid lumen, 17, 21–23, 342, 407, 409 Thylakoid membrane, 10, 11, 17, 19, 21, 22, 93, 138, 139, 283, 338, 344, 367 Thymidine, 245 Tobacco, 10, 32, 59–61, 66, 83, 97, 102, 119, 124, 177–180, 202, 212, 243, 259, 268, 271, 273–275, 279, 282–284, 288, 289, 309, 329, 341, 343, 344, 352, 395, 409, 417–420 callus culture, 82 Tolerance, 1–33, 72, 91–106, 142, 155–163, 165, 168, 173, 174, 178–183, 192, 199–202, 217–223, 241, 269, 271–279, 281–284, 287, 288, 304–306, 314–317, 327, 329, 330, 354, 357, 421 Tomato, 15, 16, 18, 95, 96, 99, 100, 104, 118, 122, 124–126, 182, 219, 272, 275, 276, 289, 305–307, 316, 317, 329, 395–397, 418 Tomato seedling, 59, 60, 78–80, 277 Tortula ruralis, 100 Toxic compounds, 192, 193, 197 Toxic metabolic products, 1 Trace metals, 278 Tracheary elements (TE), 65, 236 Transcription chloroplast isoforms, 345 control, 327, 344, 353 factors, 30, 59, 104, 105, 127–130, 178, 210, 215, 256, 258, 267, 269, 311, 312, 350, 351, 354–357, 360–362 regulation, 173–175, 242, 315, 346, 359 Transcripts abundance, 30, 102, 286, 346, 356–360, 362–367

Index abundance regulation of genes, 346–348, 356, 357, 359, 363, 371 factors, 104, 105 levels, 13, 72, 75, 95, 101–104, 118, 173–175, 178, 270, 285, 289, 346, 347, 349, 352–354, 356–359, 362–367, 395, 418 Transgenic plants, 4, 61, 62, 121, 125, 177–180, 183, 202, 215, 271–275, 279, 282, 284, 288–290, 307, 314, 417–421 Transgenic plants lines, 4 Transgenic poplars, 16 Transgenic tobacco, 10, 61, 124, 178–180, 202, 268, 271, 273–275, 279, 282–284, 288, 289, 329, 417, 418, 420 Transmembrane domain, 94, 96, 283, 370 Tree species, 142, 155–160 Triadimefon (TDM), 181 Trifolium repens, 153, 167 Triose phosphate isomerase, 28 Triticum aestivum, 152, 153, 162–164, 169, 182, 194, 286, 288, 315, 395 TRX. See ThioredoxinsTrypanosoma cruzi, 93, 406 Two-dimensional polyacrylamide gel electrophoresis, 176 U Ubiquitous thiol, 32, 235 Ultraviolet radiation, 2 UMP. See Uridine monophosphate United Nations, 406 Up-regulation of sulphur assimilation, 12, 287 Uridine, 73, 74, 245 Uridine monophosphate (UMP), 73, 74, 238, 245 UV-B radiation, 155, 180, 183, 285 V Vaccinium, 165 Vaccinium myrtillus, 153, 166

443 Vernalization-induced, 65 Vicinal cysteine residues, 256 Vigna radiata, 281 Violaxanthin (Vx), 11, 17, 18, 138, 409 Vitis vinifera, 177, 315 W Water deficit, 100, 142, 156, 157, 164, 166, 170, 172, 173, 177, 179, 181, 350 Water-deficit stress, 167, 174, 175, 177, 182, 270 Water-soluble antioxidants, 11, 219 Water–water cycle, 11, 14, 218, 258, 283, 308 Whole ascorbate–glutathione cycle, 171–173 Winning two pair, 251–260 Wolffia arrhiza, 223 WUSHEL (WUS), 58, 59 X Xanthophyll cycle, photorespiration, 10 Xanthophyll cycles activity, regulation, 17–23 Xenobiotics, 12, 15, 16, 65, 95, 139, 192, 196, 200, 201, 258 Xenobitic detoxification, 65, 126, 139, 197, 198, 200–201, 396 Z ZAT10-APx2-pathway, 362 ZAT factors, 353–355 Zea mays, 154, 163–165, 182, 220, 270, 283, 315 Zeaxanthin, 5, 6, 11, 138, 157, 163, 170, 218, 267, 308, 409 Zinc hyperaccumulation, 221 Zinnia, 65 Zinnia elegans, 236 ZWILLE, 77, 240 Zx epoxidase (ZE), 17

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