Reactive Oxygen Species in Plants E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch

Seitenzahl: 893

Veröffentlichungsjahr: 2017

Beschreibung

Describes the basics of ROS metabolism in plants and examines the broad range of ROS signaling mechanisms New discoveries about the effects of reactive oxygen species (ROS) on plants have turned ROS from being considered a bane into a boon, because their roles have been discovered in many plant developmental processes as signaling molecules. This comprehensive book teaches about the role of ROS metabolism in plants and how they affect various developmental processes. It also discusses in detail the advancements made in understanding the ROS signaling. Reactive Oxygen Species in Plants: Boon Or Bane - Revisiting the Role of ROS begins by presenting the basic introduction to ROS and deciphers the detailed knowledge in ROS research. It then examines the broad range of ROS signaling mechanisms as well as how they may be beneficial for plants and human beings. This book also describes both the bane and boon aspects of ROS with their impact on plants, and how the recent revelations have compelled us to rethink ROS turning from stressors to plant regulators. * Compiles, for the first time, the wholesome knowledge in ROS research and their cellular signaling * Includes new discoveries and in-depth discussions about the advancements made in the field * Discusses reactive oxygen species which are involved in a broad range of biological processes Reactive Oxygen Species in Plants: Boon Or Bane - Revisiting the Role of ROS will help scientists to utilize the functions of ROS signaling for plants and also enable readers to gain a deeper knowledge of ROS research and signaling. It is highly recommended for researchers, scientists, and academicians in plant science as well for advanced undergraduate and postgraduate students.

Leseprobe

Table of Contents

Cover

Title Page

List of Contributors

1 Generation Mechanisms of Reactive Oxygen Species in the Plant Cell: An Overview

Introduction

ROS biochemistry and their effects

Production sites of ROS

General mechanisms to ameliorate the Toxic Effects of ROS

Conclusions and Future Perspectives

References

2 Abiotic Stress, Generation of Reactive Oxygen Species, and Their Consequences: An Overview

Introduction

Types of ROS

Major players in ROS signaling

Consequences of ROS generation During Abiotic Stresses

Plant Cellular Defenses Against ROS activities

Regulation of the Antioxidants Governs Abiotic Stress Tolerance in Plants

Conclusion

Acknowledgments

References

3 Balancing Roles of Reactive Oxygen Species in Plants’ Response to Metalloid Exposure

Introduction

ROS: Oxidizing Agents During Metalloid Exposure

ROS: Signaling Roles During Metalloid Exposure

Challenges and Future Prospects

References

4 Role of Reactive Oxygen Species in Magnetoprimed Induced Acceleration of Germination and Early Growth Characteristics of seeds

Introduction

Germination and Seedling Vigor After Magnetopriming

Activity of Enzymes Related to Germinating Magnetoprimed Seeds

Reactive Oxygen Species (ROS) and Seed Germination After Magnetopriming

Antioxidant Enzyme Activities During Germination After Magnetopriming

Conclusions and Future Prospects

References

5 Reactive Oxygen Species: Generation, Damage, and Quenching in Plants During Stress

Introduction

Types of ROS

ROS Production in plants

Various Stresses Generate ROS in Plants

Quenching of ROS in plants

ROS Induce Signaling in Plants

ROS Induce Programmed Cell Death (PCD) in plants

Conclusions

Acknowledgments

References

6 Effects of Reactive Oxygen Species on Crop Productivity: an Overview

Introduction

Causes of ROS Overproduction and their Limitation on Productivity

Targeted Approaches and Outcomes

Approaches to Overcome Crop Yield Loss

Conclusion

Acknowledgments

References

Web References

7 Reactive Oxygen Species and Photosynthetic Functioning: past and present

Introduction

Activation of Oxygen and Free Radicals

Oxidative Stress and Damage to Membranes

Electron Paramagnetic Resonance (EPR) as a Tool to Study ROS from Photosynthesis

Oxidative Stress and Photosynthetic Functioning

Sites of ROS Production: Past and Present

Maintaining Function: The Ascorbate/Glutathione Cycle and its Interrelation with Lipoic Acid in Chloroplast

References

8 Reactive Oxygen Species and Response of the Calvin–Benson Cycle: an Overview

Introduction

ROS Generation in Plants: a Consequence of Limited CO

2

Fixation/Calvin–Benson Cycle

ROS‐Mediated Inhibition of the Calvin–Benson Cycle

Mechanism of Enzyme Inhibition by ROS: Protein Oxidation

Effect of ROS on Calvin–Benson Cycle Intermediates

Conclusion

References

9 Role of Reactive Oxygen Species in Photophosphorylation and Damage to D1 Protein: Past and Present

Introduction

Stress

ROS Production Site

Effects of ROS

Photosystem II (PSII): Structure, Function, Damage, and Repair

Mechanism of PSII Repair

Irreversible Damage to D1 Protein

Plant Protection Against ROS‐Mediated Damage

Conclusion

Acknowledgments

References

10 Reactive Oxygen Species and Antioxidants: A Continuous Scuffle within the Cell

Introduction

Antioxidant Defense Machinery

Reactive Oxygen Species

ROS Generation and Antioxidant Scavenging

Conclusion

References

11 Quenching of Reactive Oxygen Species Inside the Cell: Physiological, Biochemical, and Molecular Mechanisms

Introduction

ROS Quenching: Physiological and Biochemical Aspects

Molecular Mechanism of ROS Quenching

Conclusion

References

12 Adjustment of Plant Metabolism Against Reactive Oxygen Species: Past and Present

Introduction

ROS Homeostasis in Plants

Regulation of the Antioxidant System in Plants

ROS‐Scavenging Enzymatic Antioxidants

Non‐enzymatic Antioxidants

Transcription Factors

Conclusion and Future Perspectives

References

13 The Regulation of Plant Development: Cross‐talk of Reactive Oxygen Species and Plant Hormones

Introduction

ROS Producers and ROS Scavengers

Cross‐talk of ROS With Phytohormones is Required for Plant Development

Conclusion

References

14 Cross‐talk of Reactive Oxygen Species and Nitric Oxide in Various Processes of Plant Development: Past and Present

Nitric oxide (NO) and Related Intermediates: Reactive Nitrogen Species (RNS)

Physico‐chemical properties of NO: A Comparison With ROS

Common sites of ROS and NO Production Within Plant Cells

Transfer of NO action: NO‐dependent Posttranslational Modifications

ROS–NO Interaction During Plant Growth and Development

Conclusion

Acknowledgements

References

15 Reactive Oxygen Species Signaling and Seed Germination: An Overview

Introduction

Influence of ROS on male‐female cross‐talk During Fertilization

From Embryogenesis to Endosperm Cleavage

Conclusions

Future prospects

Acknowledgments

References

16 Reactive Oxygen Species Signaling and Root Hair Development

Reactive Oxygen Species (ROS)

Root Hair

ROS and Root Hairs

ROS and Ca

2+

in Root Hair Growth

Influence of ROS and Mg Availability on Root Hair Development

Effect of ROS and Other Substances on Root Hairs

Future Perspectives

Acknowledgments

References

17 Role of Reactive Oxygen Species Signaling in Cell Proliferation and Differentiation: An Overview

Introduction

Basics of Reactive Oxygen Species

ROS in the Proliferation of Cells

Conclusions

References

Index

End User License Agreement

List of Tables

Chapter 01

Table 1.1 Production of reactive oxygen species (ROS) and membrane damage under different stress conditions and their effect on plants.

Chapter 08

Table 8.1 Calvin–Benson cycle enzymes inhibited by reactive oxygen species (ROS).

Chapter 10

Table 10.1 Non enzymatic antioxidants of the cell.

Table 10.2 Enzymatic antioxidants of the cell.

Chapter 13

Table 13.1 Major sites for reactive oxygen species (ROS) production and scavenging in the cell.

Chapter 14

Table 14.1 Some relevant physico‐chemical properties of reactive oxygen and nitrogen species.

Table 14.2 List of antioxidant enzyme proteins modulated by

S

‐nitrosylation or tyrosine nitration.

List of Illustrations

Chapter 01

Figure 1.1 Schematic representation of major sites involved in reactive oxygen species (ROS) production and different scavenging mechanisms in plant cells. Abiotic and biotic stresses cause generation of toxic reactive oxygen species (ROS) such as the superoxide radical (), singlet oxygen (

1

O

2

), hydrogen peroxide (H

2

O

2

), and hydroxyl radical (·OH). These interact with several essential macromolecules and metabolites causing cellular damage. Moreover, the process of formation of ROS due to the spilling of electrons (e

‐

) from chloroplasts (ETS or Mehler’s reaction), mitochondria (ETS involved in respiratory chain), peroxisomes (ETS involved in photorespiration) and plasma membranes (ETS); these electrons are taken up by molecular O

2

and quickly converted into superoxide radical (SOR). SOR produced during stress conditions is detoxified by superoxide dismutase (SOD) activity, leading to formation of H

2

O

2

. Detoxification mechanisms involve enzymatic as well non‐enzymatic antioxidants to mitigate ROS‐induced damage in plants. The ascorbate–glutathione (AsA–GSH) cycle plays an important role in H

2

O

2

breakdown. H

2

O

2

is reduced to H

2

O with the help of ascorbate peroxidase (APX) using ascorbate (AsA) as the specific electron donor. APX is present in different organelles such as chloroplast (chlAPX), mitochondria (mitAPX), peroxisome (mAPX), and cytosol (cAPX). It protects plants from oxidative damage by delivering the electrons as well as minimizing excess production of ROS at these subcellular compartments. APX, ascorbate peroxidase; AsA, ascorbic acid; Car, carotenoids; CAT, catalase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; ER, endoplasmic reticulum; ETS, electron transport system; Fenton, breakdown of H

2

O

2

to highly reactive ·OH in the presence of iron; GSH, reduced glutathione; GSSG, oxidized glutathione; GR, glutathione reductase; H

2

O

2

, hydrogen peroxide; MDHA, monodehydroascorbate; MDHAR, monodehydroascorbate reductase; POD, peroxidase; Pro, proline content; SOD, superoxide dismutase.

Chapter 02

Figure 2.1 The ascorbate‐glutathione (AsA‐GSH) cycle is often regarded as the redox‐regulating hub of the cell. Here, hydrogen peroxide produced on detoxification of superoxide radical by superoxide dismutase (SOD) is immediately scavenged by ascorbate peroxidase (APX) to produce a water molecule. During this reduction, AsA acts as the reducing equivalent and itself gets oxidized into monodehydroascorbate (MDHA). MDHA can be recycled back to AsA by the action of MDHA reductase (MDHAR). The reducing equivalent in this reaction is NADPH, which gets oxidized by MDHAR into NADP (not shown in figure). Usually, MDHA produces dehydroascorbate (DHA) which also gets recycled to form ascorbate catalyzed by DHA reductase (DHAR). For recycling of DHA to AsA, DHAR utilizes the cellular reducing equivalent, that is, reduced glutathione (GSH), and oxidizes it to oxidized glutathione (GSSG). GSH is regenerated from GSSG by the catalytic reaction of glutathione reductase (GR) via the oxidation of NADPH to NADP. Thus, the AsA‐GSH cycle regulates the GSH:GSSG ratio in the cell to scavenge the ROS and minimize widespread oxidative damage.

Figure 2.2 Under control conditions, cells maintain a very high ratio of reduced glutathione to oxidized glutathione (GSH:GSSG), which rapidly reduces during extreme oxidative stress due to higher utilization of GSH for scavenging ROS. Transgenic plants or better adapted plants capable of tolerating such oxidative stresses can maintain a high GSH:GSSG ratio even during suboptimal conditions due to a more efficient ascorbate‐glutathione (AsA‐GSH)cycle.

Chapter 03

Figure 3.1 Cascades of cross‐talk among reactive oxygen species (ROS), Ca

2+

, and nitric oxide (NO) involving the Embden–Meyerhof–Parnas (EMP) pathway and Ca‐dependent kinase cascade during metalloid exposure of plants. CDPK, calcium‐dependent protein kinase; GR, glutathione reductase; GRX, glutaredoxin; GST, glutathione

S

‐transferase; PRX, peroxiredoxin; SOD, superoxide dismutase.

Figure 3.2 A simplified representation of the ethylene production cascade under cadmium exposure. ACS, 1‐aminocyclopropane‐1‐carboxylate synthase; MAPK, mitogen‐activated protein kinase; MPK, mitogen‐activated kinase; OXI1, serine/threonine protein kinase oxidative signal‐inducible 1.

Chapter 04

Figure 4.1 Effect of magnetopriming (static magnetic field treatment 200 mT for 1 h) on the percentage germination, speed of germination, and growth of root and shoot of soybean (left panels) and maize seedlings (right panels). UT, untreated; MT, magnetic field treated.

Figure 4.2 Proposed mechanism for seed germination by magnetopriming. Magnetoprimed seeds after imbibition accumulate reactive oxygen species (ROS) and nitric oxide (NO). ROS upregulate catabolism of abscisic acid (ABA) through NO, and also gibberellic acid (GA) biosynthesis. A high concentration of ABA also inhibits GA biosynthesis, but a balance of these two hormones jointly controls seed dormancy and hasten the germination of seeds through NO and ROS.

Chapter 05

Figure 5.1 An overview of reactive oxygen species (ROS) production and its quenching in plants. APX, ascorbate peroxide; CAT, catalase; DHAR, dehydroascorbate reductase; GR, glutathione reductase; MDHAR, monodehydroascorbate reductase; POD, peroxidase; SOD, superoxide dismutase.

Figure 5.2 Role of reactive oxygen species (ROS) and the biological consequences with and without stress.

Figure 5.3 Reactive oxygen species (ROS) influence gene expression.

Figure 5.4 Signaling mechanisms through the mitogen‐activated protein kinase (MAPK) pathway.

Chapter 06

Figure 6.1 Production (million metric tons) of major crops worldwide from 2014/15 to 2016/17.

Figure 6.2 World cereal yield, annual yield increase, and population growth, 1961–2013. R square of the fitted trend line of cereal yield is 0.99.

Figure 6.3 Projected and past undernourished people globally and by region from 1990–92 to 2030.

Figure 6.4 Cereal production targets.

Top:

Global cereal production has risen from 877 million metric tons in 1961 to 2351 million metric tons in 2007 (light gray). However, to meet predicted demands production will need to rise to over 4000 million metric tons by 2050 (dark gray). The rate of yield increase must move from the blue trend line (32 million metric tons per year) to the red dotted line (44 million metric tons per year) to meet this demand, an increase of 37%. The inset table shows the 2007 data for the three major cereals.

Bottom:

The greatest demand for yield increases will be from countries in the developing world.

Chapter 07

Figure 7.1 Univalent reductive chain reactions of oxygen. , superoxide radical; H

2

O

2

, hydrogen peroxide;

.

OH, hydroxyl radical.

Figure 7.2 Electron paramagnetic resonance (EPR) spectrum of Tiron semiquinone.

Figure 7.3 Experimental (a) and simulated (b) electron paramagnetic resonance (EPR) spectra of 5‐(diethoxyphosphoryl)‐5‐methyl‐1‐pyrroline‐

N

‐oxide (DEPMPO) obtained after 5 min of continuous illumination of thylakoid membranes from wheat. Arrows show the pure lines corresponding to the superoxide adduct (DEPMPO‐OOH); asterisks show the pure lines corresponding to the hydroxyl radical adduct (DEPMPO‐OH).

Figure 7.4 Examples of spin labels: (a) 3‐maleimido proxyl; (b) 5‐doxyl‐stearic acid; (c) 16‐doxyl‐stearic acid.

Figure 7.5 Kinetics of superoxide production by thylakoids from wheat upon illumination. Arrows indicate the switch on (ON) and the switch off (OFF) of the lamp.

Figure 7.6 Kinetics of superoxide radical production upon continuous illumination of thylakoids from wheat.

Figure 7.7 Experimental electron paramagnetic resonance (EPR) spectra of 5‐(diethoxyphosphoryl)‐5‐methyl‐1‐pyrroline‐

N

‐oxide (DEPMPO) obtained after 5 min of continuous illumination of (a) PSII membrane fragments and (b) thylakoid membranes from wheat. (c) EPR spectrum of the superoxide adduct DEPMPO‐OOH obtained in the presence of xanthine/xanthine oxidase. Arrows show the pure lines corresponding to the superoxide adduct (DEPMPO‐OOH); asterisks show the pure lines corresponding to the hydroxyl radical adduct (DEPMPO‐OH).

Chapter 08

Figure 8.1 Generation of reactive oxygen species (ROS) and overall impact of ROS on the Calvin–Benson cycle as represented in the chloroplast. Cyt, cytochrome; GAPDH, glyceraldehyde‐3‐phosphate dehydrogenase; FBPase, fructose 1,6‐bisphosphatase; Fd, ferredoxin; G3P, glyceraldehyde 3‐phosphate; OEC, oxygen‐evolving complex; PC, Plastocyanin; PSI, photosystem I; PSII, photosystem II; PRK, phosphoribulokinase; RuBP, ribulose 1,5‐bisphosphate; SBPase, sedoheptulose bisphosphatase.

Chapter 09

Figure 9.1 Sites of production of reactive oxygen species (ROS) in plants. ROS are produced at several locations in the cell, including chloroplasts, mitochondria, plasma membrane, peroxisomes, apoplast, endoplasmic reticulum, and cell wall.

Figure 9.2 Proposed model of reactive oxygen species (ROS) production and amplification under conditions of low O

2

in plant cells (orange arrows indicate hypoxia‐dependent mechanisms). GDP, guanosine diphosphate; GTP, guanosine triphosphate; HRU1, hypoxia responsive universal stress protein 1; MAPK, mitogen‐activated protein kinase; RBOHD, respiratory burst oxidase homolog D; Rho‐like small G‐protein of plants (ROP) – GTPase activating protein (GAP);

Figure 9.3 Assembly of the dimeric photosystem II (PSII) complex in

Synechocystis

sp. PCC 6803. The low molecular mass (LMM) PsbE, PsbF, PsbH, PsbI, and PsbK subunits and the extrinsic PsbO, PsbU, and PsbV subunits are designated by the appropriate upper case letters. For clarity, assembly factors and many of the LMM subunits are not included. Cytochrome

b

559 is composed of a heterodimer of the PsbE and PsbF subunits.

Chapter 10

Figure 10.1 Effects of cellular concentration of reactive oxygen species (ROS) in plants.

Figure 10.2 Metabolic pathways involved in the reduction of molecular oxygen (O

2

) leading to generation of various intermediate reactive oxygen species (ROS) in plants. APX, ascorbate peroxidase; CAT, catalase; GPX, glutathione peroxidase; SOD, superoxide dismutase.

Figure 10.3 Reactive oxygen species (ROS) generation sites in mitochondria and antioxidants involved in scavenging of generated ROS. GPX, glutathione peroxidase; GR, glutathione reductase; GRX, glutaredoxin; GSH, reduced glutathione; GSSG, oxidized glutathione; SOD, superoxide dismutase; TRX, thioredoxin.

Figure 10.4 Reactive oxygen species (ROS) generation sites in chloroplasts and antioxidants involved in scavenging the generated ROS. AsA‐GSH cycle, ascorbate‐glutathione cycle; SOD, superoxide dismutase; tAPX, thylakoid bound ascorbate peroxidase.

Figure 10.5 Reactive oxygen species (ROS) generation sites in peroxisomes and antioxidants involved in scavenging the generated ROS. CAT, catalase; ETC, electron transport chain; GPX, glutathione peroxidase; NOS, nitric oxide synthase; SOD, superoxide dismutase; XOS, Xanthine oxidase.

Figure 10.6 Reactive oxygen species (ROS) generation sites in apoplast and antioxidants involved in scavenging the generated ROS. APX, ascorbate peroxidase; ASH‐GSH cycle, ascorbate‐glutathione cycle; SOD, superoxide dismutase.

Chapter 11

Figure 11.1 The major signaling pathways activated in response to reactive oxygen species (ROS). MAPK, mitogen‐activated protein kinase; MAPKK, MAPK kinase; MAPKKK, MAPKK kinase; TFs, transcription factors.

Chapter 12

Figure 12.1 Reactive oxygen species (ROS) interplay during biotic and abiotic stress in plants. Stress stimulus received by plant sensors is mediated by various stress signal transduction pathways, such as the mitogen‐activated protein kinase (MAPK) cascade, which activates defense‐related proteins and modulates plant metabolism by reducing ROS generation. Increased ROS generation by NADPH oxidase under stress activates the plant’s antioxidant defense system, which is involved in ROS scavenging.

Figure 12.2 Enzymatic and non‐enzymatic antioxidants in plants. Plants have an efficient antioxidant system that includes both enzymatic and non‐enzymatic antioxidants. Enzymatic antioxidants include SOD (superoxide dismutase), dehydroascorbate reductase (DHAR), monodehydroascorbate reductase (MDHAR), catalase (CAT), and ascorbate peroxidase (APX). Non‐enzymatic antioxidants include glutathione (GSH), glutathione

S

‐transferases (GST), and ascorbic acid (AsA). DHA, dehydroascorbate; GPX, glutathione peroxidase; GR, glutathione reductase; GSSG, oxidized glutathione; PSI, photosystem I.

Chapter 13

Figure 13.1 Reactive oxygen species (ROS) and phytohormones are required for different growth and development processes in plant life.

Figure 13.2 The impact of cellular reactive oxygen species (ROS) homeostasis and interplay with plant hormones. ABA, abscisic acid; BR, brassinosteroids; CK, cytokinin; ET, ethylene; SL, strigolactones.

Chapter 14

Figure 14.1 Schematic overview of the most relevant subcellular sites and mechanisms of reactive oxygen species (ROS) and nitric oxide (NO) generation in plant cells.

1.

Chloroplast: NO may originate from different sources such as nitrite or

L

‐arginine, is generated in the electron transport chain (ETC), H

2

O

2

and other forms are produced by enzymatic activities (e.g., superoxide dismutase, SOD).

2.

Mitochondrion: both NO and are produced in the ETC. SODs convert to H

2

O

2

.

3.

Peroxisome: the major source of NO is xanthine oxidoreductase (XOR), which produces also . Another putative NO source in peroxisomes is the mammalian‐like nitric oxide synthase (NOS) enzyme.

4.

Plasma membrane: NO is synthesized by nitrite‐nitric oxide reductase (Ni‐NOR) in the root cells, while NADPH oxidase enzyme complex (NOX) catalyzes the formation of . Other ROS forms are produced enzymatically (e.g., SOD).

5.

Cell wall: NO is produced in a non‐enzymatic reduction reaction at low pH in the presence of reductants such as ascorbic acid (AsA). ROS can be formed,

inter alia

, by the activity of cell wall‐associated peroxidases.

Figure 14.2 Signal interactions between reactive oxygen species (ROS) and nitric oxide (NO) during reproductive developmental processes such as flowering, pollen tube growth, and fruit ripening and during seed developmental events like embryogenesis and germination. Solid lines indicate experimentally verified links, while dashed lines represent putative interactions. ABI, ABA insensitive (transcription factor); APX, ascorbate peroxidase; AUX, auxin; CAT, catalase; CO,

CONSTANS

; ET, ethylene; FLC,

FLOWERING LOCUS

C; GC, guanylate cyclase; Glb1, class 1 hemoglobin gene; GSH, glutathione; GSNOR,

S

‐nitrosoglutathione reductase; LFY,

LEAFY

; LOX, lipoxygenase; NOX, NADPH oxidase; PA, polyamine; POX, peroxidase; SA, salicylic acid; SnRK, sucrose nonfermenting 1‐related protein kinase; SOD, superoxide dismutase.

Figure 14.3 Summary of signal relationships between reactive oxygen species (ROS) and nitric oxide (NO) during different processes of vegetative plant development. Solid lines indicate experimentally verified links, while dashed lines represent hypothesized interactions. AR, adventitious root; Asa, ascorbic acid; AUX, auxin; CAT, catalase; CDK, cyclin‐dependent kinase; CYC, cyclin; GC, guanylate cyclase; LR, lateral root; MAPK, mitogen‐activated protein kinase; NOX, NADPH oxidase; NR, nitrate reductase; PIN, PIN‐FORMED protein; POX, peroxidase; PR, primary root; RH, root hair; SOD, superoxide dismutase; TIR, TRANSPORT INHIBITOR RESPONSE protein.

Chapter 15

Figure 15.1 An heuristic overview of the pollen–pistil interaction based on the hydration status mediated by reactive oxygen species (ROS) and EXO70A1.

Figure 15.2 A model for the involvement of ROS‐ and Ca

2+

‐mediated mechanisms in the guidance of sperm cells toward the ovule. Signaling patterns have been conceived and adapted from various references cited in the text. However, the main concept has been adapted specially from the following: Boisson‐Dernier

et al

. (2009), Capron

et al

. (2008), Duan

et al

. (2010, 2013, 2014), Dresselhaus and Franklin‐Tong (2013), Iwano

et al

. (2009), Lausser and Dresselhaus (2010), Müller

et al

. (2013), Palanivelu

et al

. (2003), Palanivelu and Tsukamoto (2012), Rodriguez‐Serrano

et al

. (2014), Serrano and Olmedilla (2012); Serano

et al

. (2015), Van Doorn

et al

. (2011), and Zhang

et al

. (2014). The model shows the general role of proteins involved in the signaling mediated by ROS and Ca

2+

, during the fusion of male and female gametes. Detailed mechanism(s) of individual genes/proteins cannot be interpreted from this figure. ROS, reactive oxygen species; PCD, programmed cell death; Ca

2+

, calcium.

Figure 15.3 A simplified representation of the involvement of reactive oxygen species (ROS) and antioxidant enzymes in the germination of dormant and non‐dormant seeds. HR, hypersensitive response; PCD, programmed cell death; SAR, systemic acquired resistance.

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Reactive Oxygen Species in Plants

Boon Or Bane ‐ Revisiting the Role of ROS

Edited By

Vijay Pratap Singh

Govt. Ramanuj Pratap Singhdev Post Graduate CollegeChhattisgarh, India

Samiksha Singh

Ranjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of Allahabad, Allahabad, India

Durgesh Kumar Tripathi

Centre of Advanced Study in BotanyBanaras Hindu UniversityVaranasi, India

Sheo Mohan Prasad

Ranjan Plant Physiology and Biochemistry LaboratoryDepartment of BotanyUniversity of AllahabadAllahabad, India

Devendra Kumar Chauhan

D.D. Pant Interdisciplinary Research LaboratoryDepartment of BotanyUniversity of Allahabad, Allahabad, India

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List of Contributors

Madhoolika AgrawalLaboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

S.B. AgrawalLaboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Neelam AtriMMV, Banaras Hindu University, Varanasi, India

Aditya BanerjeePost Graduate Department of Biotechnology, St Xavier’s College (Autonomous), Kolkata, India

Nivedita ChaudharyLaboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Krishna Kumar ChoudharyLaboratory of Air Pollution and Global Climate Change, Department of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Institute of Soil, Water andEnvironmental Sciences, Volcani Center, Agricultural Research Organization, Bet Dagan, Israel

Farah DeebaPlant Ecology & Environmental Science, CSIR‐National Botanical Research Institute, Rana Pratap Marg, Lucknow, India

Gábor FeiglDepartment of Plant Biology, University of Szeged, Szeged, Hungary

Sunil K. GuptaPlant Ecology & Environmental Science; and Academy of Scientific and Innovative Research (AcSIR), CSIR‐National Botanical Research Institute, Rana Pratap Marg, Lucknow, India

Gajendra S. JeenaCSIR‐Central Institute of Medicinal and Aromatic Plants, Lucknow, India

Byoung Ryong JeongHorticulture Major, Division of Applied Life Science (BK21 Plus), Graduate School; Institute of Agriculture and Life Science; and Research Institute of Life Science, Gyeongsang National University, Jinju, South Korea

Sunita KatariaSchool of Life Sciences, Devi Ahilya Vishwavidyalaya, Khandwa Road Campus, Indore, India

Zsuzsanna KolbertDepartment of Plant Biology, University of Szeged, Szeged, Hungary

Chun‐Yan LiuInstitute of Root Biology, Yangtze University, Jingzhou, Hubei, China

Abinaya ManivannanInstitute of Agriculture and Life Science, Gyeongsang National University, Jinju, South Korea

Shakti MehrotraCSIR‐Central Institute of Medicinal and Aromatic Plants, Lucknow, India

Sonal MishraUniversity of Jammu, Jammu, India

Vivek PandeyPlant Ecology & Environmental Science; and Academy of Scientific and Innovative Research (AcSIR), CSIR‐National Botanical Research Institute, Rana Pratap Marg, Lucknow, India

Ujjal J. PhukanCSIR‐Central Institute of Medicinal and Aromatic Plants, Lucknow, India

Calogero PinzinoResearch National Council (CNR), Istituto di Chimica dei Composti Organo Metallici (ICCOM), Area della Ricerca del CNR di PISA, Pisa, Italy

Sheo Mohan PrasadRanjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Syed Naved QuadriCenter for Transgenic Plant Development, Department of Biotechnology, Jamia Hamdard University, New Delhi, India

Mike Frank QuartacciDepartment of Agriculture, Food and Environment, University of Pisa, Pisa, Italy

Aryadeep RoychoudhuryPost Graduate Department of Biotechnology, St Xavier’s College (Autonomous), Kolkata, India

Cristina SgherriDepartment of Agriculture, Food and Environment, University of Pisa, Pisa, Italy

Marisha SharmaPlant Ecology & Environmental Science, CSIR‐National Botanical Research Institute, Rana Pratap Marg, Lucknow, India

Rakesh K. ShuklaCSIR‐Central Institute of Medicinal and Aromatic Plants, Lucknow, India

Anita SinghRanjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Madhulika SinghRanjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Prabhakaran SoundararajanDivision of Applied Life Science (BK21 Plus), Graduate School, Gyeongsang National University, Jinju, South Korea

Vikas SrivastavaDepartment of Plant Sciences, Central University of Jammu, Jammu, India

Dibyendu TalukdarDepartment of Botany, R.P.M. College, Hooghly, India

Sanjesh TiwariRanjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Santwana TiwariRanjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Supriya TiwariDepartment of Botany, Institute of Science, Banaras Hindu University, Varanasi, India

Qiang‐Sheng WuInstitute of Root Biology, Yangtze University, Jingzhou, Hubei, China; Department of Chemistry, Faculty of Science, University of Hradec Kralove, Hradec Kralove, Czech Republic

Shivam YadavMolecular Biology Section, Center of Advanced Study in Botany, Banaras Hindu University, Varanasi, India

Yu‐Jie YangInstitute of Root Biology, Yangtze University, Jingzhou, Hubei, China

De‐Jian ZhangInstitute of Root Biology, Yangtze University, Jingzhou, Hubei, China

1Generation Mechanisms of Reactive Oxygen Species in the Plant Cell: An Overview

Santwana Tiwari, Sanjesh Tiwari, Madhulika Singh, Anita Singh, and Sheo Mohan Prasad

Ranjan Plant Physiology and Biochemistry Lab., Department of Botany, University of Allahabad, Allahabad, India

Introduction

During the course of evolution, life on the Earth started in a reducing environment and about 3.2 billion years ago, the reducing environment changed to an oxidizing one due to the appearance and proliferation of the first oxygen‐evolving photosynthetic organisms, that is, cyanobacteria (Schopf et al., 2007). In other words, the cyanobacteria are considered to be the first organisms to release oxygen in the environment by means of an oxygen evolving complex (OEC) (Bekker et al., 2004). The outermost orbital of the dioxygen (O2) molecule has two unpaired electrons having same spin quantum number, and this enables O2 to accept electrons one at a time efficiently, and generate the reactive oxygen species (ROS). Out of the total O2 utilized by plants, 1% is diverted to produce ROS in various cell organelles (del Rio et al., 2002). Reactive oxygen species are essential by‐products of all aerobic organisms that are produced during normal metabolic processes as well as under stress conditions. The ROS‐producing subcellular organelles are mainly mitochondria, chloroplasts, peroxisomes, cytosol, and plasma membrane (Corpas et al., 2015; Hasan et al., 2016) (Figure 1.1). The roles of ROS are contradictory, they may have negative as well as positive roles depending upon their concentrations in the particular cell organelles. At higher concentrations, ROS cause damaging effects on proteins, DNA/RNA, and lipids by oxidative modification in plant cells (Gill and Tuteja, 2010; Hasan et al., 2016). On the other hand, previous evidence clearly showed that at lower concentrations ROS act as signaling molecules in plants for regulating developmental pathways and control of redox homeostasis and defense responses against pathogens and environmental stress (Wood et al., 2003; Apel and Hirt, 2004). There are well‐described mechanisms in prokaryotes where the concentration of ROS directly activates transcription factors that overexpress the genes to combat oxidative stress (Kiley and Storz, 2004). There are several ROS, such as superoxide radical (), hydroxyl radical (·OH), hydroperoxyl radical (HO2·), hydrogen peroxide (H2O2), alkoxy radical (RO·), peroxy radical (ROO·), singlet oxygen (1O2), and excited carbonyl (RO*), all of which are cytotoxic to plants at elevated concentrations (Dismukes et al., 2001; Karuppanapandian et al., 2011). In the cell organelles accumulation of superoxide enhances oxidative stress rather than playing a role in redox signaling. However, in some cases it damages certain proteins that activate specific signaling pathways and consequently leads to death of the particular cell (Chen et al., 2009). Hydrogen peroxide (H2O2) acts as a signaling molecule that diffuses across membranes and triggers specific signal transduction pathways (Veal and Day, 2011). The balance between production and elimination of ROS is dependent upon various biotic and abiotic factors such as temperature, heavy metal concentration, drought, salinity, UV radiation, light, nutrient deficiency, and excessive use of pesticides and/or herbicides as well as pathogen attacks. This disturbance ultimately leads to increased concentration of ROS in particular cell organelles. The damaging effects of ROS are ameliorated by different antioxidative defense systems. The antioxidant system consists of enzymatic antioxidants, namely superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione S‐transferase (GST), ascorbate peroxidase (APX) and non‐enzymatic antioxidants, including non‐protein thiols (NP‐SH) ascorbate, tocopherol, caretenoid and cysteine. These act together as a machine to detoxify ROS (Kumar et al., 2015; Singh et al., 2016; Tripathi et al., 2012a,b, 2016a,b) (see Figure 1.1). Among these, SOD is considered as first line of defense; it dismutates the superoxide and subsequently H2O2 is generated. Further, H2O2 detoxification involves different enzyme systems in distinct cellular compartments. The peroxidase family includes ascorbate peroxidase (APX), glutathione peroxidase (GPX), and peroxidase (POD) for the elimination of H2O2. Another enzyme, CAT, is important in the removal of H2O2 generated in peroxisomes. GPX also reduces H2O2 as well as organic and lipid hydroperoxides by using glutathione (GSH) as substrate. Among the various H2O2‐detoxifying enzymes, APX plays the most essential role in scavenging ROS. APX is present in thylakoid, glyoxisome, chloroplast stroma, and cytosol, and is involved in the scavenging of H2O2 through water‐water and ascorbate‐glutathione (AsA‐GSH) cycles, utilizing AsA as the electron donor. One of the antioxidant enzymes, GST, participates in herbicide detoxification, hormone homeostasis, and regulation of apoptosis and also is involved in plant responses to biotic and abiotic stresses. Non‐enzymatic antioxidants include the major cellular redox buffers ascorbate and glutathione, as well as tocopherol, flavonoids, alkaloids, and carotenoids.

Figure 1.1 Schematic representation of major sites involved in reactive oxygen species (ROS) production and different scavenging mechanisms in plant cells. Abiotic and biotic stresses cause generation of toxic reactive oxygen species (ROS) such as the superoxide radical (), singlet oxygen (1O2), hydrogen peroxide (H2O2), and hydroxyl radical (·OH). These interact with several essential macromolecules and metabolites causing cellular damage. Moreover, the process of formation of ROS due to the spilling of electrons (e‐) from chloroplasts (ETS or Mehler’s reaction), mitochondria (ETS involved in respiratory chain), peroxisomes (ETS involved in photorespiration) and plasma membranes (ETS); these electrons are taken up by molecular O2 and quickly converted into superoxide radical (SOR). SOR produced during stress conditions is detoxified by superoxide dismutase (SOD) activity, leading to formation of H2O2. Detoxification mechanisms involve enzymatic as well non‐enzymatic antioxidants to mitigate ROS‐induced damage in plants. The ascorbate–glutathione (AsA–GSH) cycle plays an important role in H2O2 breakdown. H2O2 is reduced to H2O with the help of ascorbate peroxidase (APX) using ascorbate (AsA) as the specific electron donor. APX is present in different organelles such as chloroplast (chlAPX), mitochondria (mitAPX), peroxisome (mAPX), and cytosol (cAPX). It protects plants from oxidative damage by delivering the electrons as well as minimizing excess production of ROS at these subcellular compartments. APX, ascorbate peroxidase; AsA, ascorbic acid; Car, carotenoids; CAT, catalase; DHA, dehydroascorbate; DHAR, dehydroascorbate reductase; ER, endoplasmic reticulum; ETS, electron transport system; Fenton, breakdown of H2O2 to highly reactive ·OH in the presence of iron; GSH, reduced glutathione; GSSG, oxidized glutathione; GR, glutathione reductase; H2O2, hydrogen peroxide; MDHA, monodehydroascorbate; MDHAR, monodehydroascorbate reductase; POD, peroxidase; Pro, proline content; SOD, superoxide dismutase.

Source: Adapted from Jajic et al., 2015.

With the above context, the present chapter gives an overview of reactive oxygen species, their production sites, and biochemistry as well as the mechanism for their amelioration, particularly in the plant system.

ROS biochemistry and their effects

Oxygen is necessary for every aerobic organism. In normal conditions it is involved in several biochemical reactions. The reduction of O2 to H2O provides the energy that allows the impressive complexity of higher organisms. However, incomplete reduction of O2 leads to the production of ROS, which are extremely reactive and can oxidize almost every biological molecule. All ROS can react with DNA, proteins, and lipids (Gill and Tuteja, 2010; Singh et al., 2016). Under these conditions, firstly 1O2 is produced; then H2O2 is synthesized via the disproportionation of superoxide catalyzed by SOD, or non‐enzymatically in the process of superoxide diffusion with a low yield of the reaction (Quinlan et al., 2013; Singh et al., 2015). H2O2 is reduced to water with the involvement of ascorbate peroxidase and ascorbate. Ascorbate is oxidized and then regenerated by the reduced glutathione at the expense of NADPH. Reduction of molecular O2 proceeds through four steps, thus generating several O2 radical species (Kalyanaraman et al., 2016). The reaction chain requires initiation at the first step whereas subsequent steps are exothermic and can occur spontaneously, either catalyzed or not. The first step in O2 reduction produces relatively short‐lived ROS that are not readily diffusible: hydroperoxyl (H2O2−) and peroxide (O2 ·−). The second O2 reduction generates hydrogen peroxide (H2O2), which can diffuse upto some distance from its site of production (Quinlan et al., 2013).

Singlet Oxygen Species

Environmental stresses that impact CO2 fixation (Gul et al., 2016), such as drought and salt stress, ozone, and high or low temperatures, reduce NADP+ regeneration during C‐3 cycle, so the photosynthetic electron transport chain is over‐reduced, by which singlet oxygen species are produced in the chloroplasts (Wu and Tang, 2004; Bechtold et al., 2005). The chlorophyll pigments associated with the electron transport system (ETS) are the prime source of singlet oxygen (1O2). The latter may also arise as a by‐product of lipoxygenase activity. Like other reactive oxygen species, 1O2 is also highly destructive, and reacts with most biological molecules at near diffusion‐controlled rates. This mainly occurs due to the excitement of chlorophyll molecules; although the lifetime of excited chlorophyll is short within these aggregates, its duration varies according to physiological conditions. The excited singlet state of chlorophyll is used for the transfer of energy or electrons. However, there are two other possible routes of de‐excitation, radioactive decay (fluorescence) and conversion of the singlet chlorophyll state to the triplet chlorophyll state. The latter interacts with oxygen to produce 1O2.

Superoxide Radical

The half‐life for O2 ·− is approximately 2–4 ms (Saxena et al., 2016). It is produced at different sites in the cell but the mechanism of its production is almost similar at all sites. Reduction of dioxygen by light in the chloroplasts was first shown by the production of acetaldehyde in the presence of ethanol and catalase, and the photo‐reduced product was assumed to be hydrogen peroxide. Under most circumstances, the control of electron flow between photosystems II (PSII) and I (PSI) regulates the reduction state of the acceptor side of PSI. The regulated activation of the C‐3 cycle and control of the rate of electron flow are important factors determining the redox state of the ferredoxin pool (Tóth, et al. 2007). This is important because ferredoxin and the electron carriers on the reducing side of PSI have sufficiently negative electrochemical potentials to donate electrons to oxygen resulting in the formation of superoxide radical . The majority of O2 reduction in vivo is thought to proceed via reduced ferredoxin (Fdred), which reduces molecular oxygen to the superoxide radical (Reaction 1). Hydrogen peroxide is then formed through dismutation of O2−(Reaction 2). The latter occurs spontaneously, but the velocity of the reaction is greatly increased by SOD (Reaction 3):

(Reaction 1)

(Reaction 2)

(Reaction 3)

The major site of superoxide formation lies in the electron transfer chain (ETC), of mitochondria especially at the level of Complex I and Complex III. It was shown in animal mitochondria that the flavin mononucleotide (FMN)‐containing subunit and an iron‐sulfur cluster of the nicotinamide adenine dinucleotide (NADH) dehydrogenase of Complex I are the sites of O2 generation (Chen et al., 2009), especially when this complex is glutathionylated after oxidative stress (Taylor et al., 2003). This complex could amplify ROS production and participate in the regulation of ROS concentrations in the whole cell. The over‐reduction of the ubiquinone pool by Complex I can also lead to a reverse functioning of the chain, and to the formation of large amounts of ROS. In Complex III, the over‐reduction state of the ubiquinone pool can lead to a direct electron transfer to molecular oxygen, and to the formation of superoxide anions. Superoxides are known to be produced during NADPH‐dependent microsomal electron transport. Two possible loci of O2 ·− production in microsomes are auto‐oxidation of the oxycytochrome‐P450 complex that forms during microsomal mixed function oxidase (MFO) reactions, and/or auto‐oxidation of cytochrome P450 reductase, a flavoprotein that contains both FAD and FMN.

Hydrogen peroxide

Hydrogen peroxide is produced by the dismutation of superoxide radicals in a reaction mostly catalyzed by superoxide dismutase (Tripathi et al., 2016). In leaf cells, catalase is exclusively localized in peroxisomes and has not been found in chloroplasts. The hydrogen peroxide in chloroplasts is scavenged by a peroxidase reaction using the photo‐reductant produced in the thylakoid as the electron donor. Thus, diffusion of hydrogen peroxide from chloroplasts to peroxisomes and its scavenging by catalase are very unlikely to occur. The electron donor for the peroxidase reaction has been identified as ascorbate.

H2O2 is moderately reactive, has a relatively long half‐life (1 ms), and can diffuse upto some distance from its site of production. H2O2 may inactivate the enzymes by oxidizing their thiol groups. Dismutation and oxidation reactions of superoxide yield hydrogen peroxide. Hydrogen peroxide, although more oxidizing than superoxide, is biologically less toxic: picomolar intracellular levels of superoxide are lethal, whereas micromolar levels of H2O2 can be tolerated. H2O2 is a potent oxidizer (although not always a fast oxidizer), and is much more diffusible than superoxide, because it is less reactive and is membrane permeable: O2 ·− is generally considered membrane impermeable except in its HO2 · form, which is in low abundance at physiological pH. The biological toxicity of H2O2 through oxidation of ‐SH groups has long been known, and it can be enhanced in the presence of metal catalysts through Haber–Weiss or Fenton‐type reactions (see Figure 1.1). Fenton showed that the formation of toxic hydroxyl radicals (·OH) from hydrogen peroxide (H2O2) is catalyzed by iron ions, called the “Fenton reaction.”

The Fenton or Haber–Weiss reactions are:

Iron ions are required to form toxic · OH radicals. Joseph Weiss and Fritz Haber discovered that O2 ·− can be converted into H2O2 and further to · OH, called the Haber–Weiss reaction. The last species generated by this series of reductions is the hydroxyl radical (·OH). It has high reactivity and has half‐life of less than 1 ms. As a result, it has a very high affinity for biological molecules to react at its site of production.

Hydroxyl radical

The generation of · OH from H2O2 and O2 ·− by the Haber–Weiss process is well known. In this process catalysis is necessary in the presence of a metal since the rate of uncatalyzed reaction is negligible. The hydroxyl radical is highly reactive among ROS. It has a single unpaired electron, and thus can easily bind with oxygen in the triplet ground state. Because cells have no enzymatic mechanism to eliminate · OH, its excess production can ultimately lead to cell death (Tripathi et al., 2016). The oxidation of organic substrates by · OH may proceed by two possible reactions: either by addition of · OH to organic molecules or by abstraction of a hydrogen atom. In this context, organic oxygen radicals such as alkoxy, peroxy, semiquinones, reduced hydrogen peroxide, and hydrogen peroxide‐electron donor complexes, as well as metallo‐oxygen complexes, have been proposed as the ultimate active species besides destructive free · OH. These · OH are thought to be largely responsible for mediating oxygen toxicity in vivo. The hydroxyl radical can potentially react with all biological molecules, including DNA, proteins, and lipids, and almost any constituent of cells, and due to the absence of any enzymatic mechanism for the elimination of this highly reactive ROS, excess production of · OH ultimately leads to cell death (Table 1.1; see also Figure 1.1).

Table 1.1 Production of reactive oxygen species (ROS) and membrane damage under different stress conditions and their effect on plants.

Serial number

Reactive oxygen species & membrane damage

Stress

Plant

Damaging effect

Antioxidants and other defense systems

Reference

1.

, H

2

O

2

, MDA equivalents contents and RNS

Salt and drought

Ailanthus altissima

Reduce chlorophyll fluorescence, stomatal conductance, and NR activity

SOD, CAT activity, and Pro content increased for detoxification

Filippou

et al

., 2014

2.

, H

2

O

2

, and MDA equivalents contents

Temperature

Cucumis sativus

and

Cucurbita ficifolia

Loss of root cell viability; low root zone changes the mitochondrial electron distribution between the COX and AOX pathway in cucumber root

APX, GPOD, and CAT activity; temperature‐mediated production of ROS in cucurbit species may act as signaling molecules, which activate MRR, and subsequently induce expression of genes encoding AOX protein of mitochondria and maintain ROS levels and redox homeostasis

Zhang

et al

., 2012

3.

Electrolyte leakage (EL)

Salt and zinc

Vigna radiata

Reduced plant growth, gas exchange parameters, carbonic anhydrase and nitrate reductase activity

Exogenous epibrassinolide and spermidine application enhances SOD, POD, and CAT activity, which reduces/detoxifies the damaging effect of stress It increases uptake of nutrients, stabilizes the photosynthetic enzyme and pH and enhances carboxylase activity that consequently increases growth of

Vigna

plant

Mir

et al

., 2015

4.

H

2

O

2

and TBARS

Salt

Brassica juncea

Reduced photosynthetic rate, stomatal conductance, intercellular CO

2

concentration, quantum yield efficiency of PSII, rubisco activity, and total nitrogen content, and enhanced accumulation of Na

+

and Cl

−

ions

Provide salinity tolerance by decreasing Na

+

and Cl

−

accumulation and also regulate Pro and ethylene production

Iqbal

et al

., 2015

5.

, H

2

O

2

, MDA equivalents contents, and EL

Arsenic stress

Solanum melongena

Reduced growth, photosynthetic pigment, and chlorophyll fluorescence; seedlings accumulate more arsenic (As) content

Reduce the damaging effect of As by upregulating the synthesis of SOD, POD, CAT, GST activity, and Pro as well as Pro synthetic enzyme

Singh

et al

., 2015

6.

, H

2

O

2

, MDA equivalents contents, and EL

Salt

Solanum melongena

Reduced growth, K

+

content, photosynthetic pigment, and chlorophyll fluorescence; enhanced accumulation of Na

+

The damaging effect of NaCl is reduced by increased synthesis of enzymatic and non‐enzymatic antioxidants

Singh

et al

., 2016

7.

H

2

O

2

and LPO

UV‐B radiation

Ginkgo biloba

Reduced growth, photosynthetic pigment, and total protein content observed in

Ginkgo biloba

Increased PAL activity as well as NO enhance accumulation of UV‐B filters such as flavonoids

Hao

et al

., 2009

8.

LPO and MDA equivalent content

UV‐B radiation

Phaseolus vulgaris

Reduced biomass accumulation, biomass allocation pattern, and physiological and biochemical responses of

P. vulgaris

UV‐B induced enhancement in enzymatic and non‐enzymatic antioxidants, ascorbic acid, Pro and TPC, which provide tolerance against UV‐B damage. UV‐B radiation induces two enzymes, namely chalcone synthase and PAL of the phenylpropanoid pathway, that are essential for synthesis of flavonoids, which act as selective UV‐B filters

Raghuvanshi and Sharma, 2016

AOX, alternative oxidase; APX, ascorbate peroxidase; As, arsenic; CAT, catalase; COX, cytochrome c oxidase; EL, electrolyte leakage; ER, endoplasmic reticulum; GPOD, guaiacol peroxidase; GSH, reduced glutathione; GST, glutathione S‐transferase; H2O2 hydrogen peroxide; LPO, lipid peroxidation; MDA equivalents contents, malondialdehyde; MRR, mitochondrial retrograde regulation; NR, nitrate reductase activity; , superoxide radical; PAL, phenylalanine ammonium‐lyase activity; POD, peroxidase; Pro, proline content; PSII, photosystem II; RNS, reactive nitrogen species; SOD, superoxide dismutase; TBARS, thiobarbituric acid reactive substances; TPC, total phenolic content; UV‐B, ultraviolet B.

In response to all these ROS, peroxidation of lipids is considered as the most harmful process known to occur in every living organism. Membrane injury is sometimes taken as a single parameter to determine the level of lipid demolition under various stresses. ROS are very reactive and damage membranes and various cell components; this results in mobilization of various defense systems to reduce ROS generation and enhance ROS scavenging. This response entails de novo synthesis of antioxidant enzymes (i.e., superoxide dismutase, catalase, ascorbate peroxidase, glutathione reductase) and/or activation of their low molecular weight antioxidant precursors (i.e., ascorbate, glutathione, tocopherols, flavonoids) (Minibaeva and Gordon, 2003; Foyer and Noctor, 2005; Hung et al., 2005). Production and removal of ROS must be strictly controlled in order to avoid oxidative stress. When the level of ROS overwhelms the defense mechanisms, a cell is said to be in a state of “oxidative stress.” However, the balance between production and scavenging of ROS is disturbed under a number of stressful conditions such as salinity, drought, high light levels, toxicity due to metals, pathogens, and so forth (Table 1.1). The damage caused to biomolecules by enhanced levels of ROS can alter core membrane properties like fluidity and ion transport, lead to loss of enzyme activity, affect protein cross‐linking, inhibit protein synthesis, damage DNA, and so forth, ultimately resulting in cell death. When ROS levels exceed a certain threshold, enhanced lipid peroxidation takes place in both the cellular as well as organellar membranes, which, in turn, affects normal cellular functioning. Lipid peroxidation intensifies the oxidative stress through production of lipid‐derived radicals that themselves can react with and damage proteins and DNA. The level of lipid peroxidation has been widely used as an indicator of ROS‐mediated damage to cell membranes under stressful conditions. Two common sites of ROS attack on the phospholipid molecules are the unsaturated (double) bond between two carbon atoms and the ester linkage between glycerol and the fatty acid. The polyunsaturated fatty acids (PUFAs) present in membrane phospholipids are particularly sensitive to attack by ROS. A single · OH can result in peroxidation of many polyunsaturated fatty acids because the reactions involved in this process are part of a cyclic reaction.

Attack by ROS can modify proteins in a variety of ways, some direct and others indirect. Direct modification involves modulation of a protein’s activity through nitrosylation, carbonylation, disulfide bond formation, and glutathionylation. Proteins can be modified indirectly by conjugation with breakdown products of fatty acid peroxidation (Yamauchi et al., 2005). Tissues injured by oxidative stress generally contain increased concentrations of carbonylated proteins, the degree of which is widely used as a marker of protein oxidation (Møller and Kristensen, 2004). Greater alteration of proteins has been reported in plants under various stresses (Romero‐Puertas et al., 2002; Sharma and Dubey, 2005; Maheshwari and Dubey, 2009; Tanou et al., 2009).

Production sites of ROS

Oxidative stress occurs when there is a serious imbalance in any cell compartment between production of ROS and antioxidant defense, which leads to cellular damage (Halliwell and Gutteridge, 1999). ROS are a group of free radicals, reactive molecules, and ions that are derived from O2. They are produced in unstressed and stressed cells in several cell organelles, chiefly chloroplasts, mitochondria, and peroxisomes (see Figure 1.1). There is little contribution from the apoplast, cell wall, or endoplasmic reticulum. ROS are continuously formed by the leakage of electrons to O2 from the electron transport in different cell organelles. They are also formed in various metabolic pathways as by‐products, which are localized in different cellular organelles. Photosynthesizing plants are under threat of oxidative damage, because of their oxygenic conditions and the abundance of the photosensitizer in the chloroplast envelope. It has been reported that two primary processes are mainly involved in the formation of ROS during photosynthesis: (i) direct photoreduction of O2 to the superoxide radical by reduced electron transport components associated with PSI; and (ii) reactions linked to the photorespiratory cycle, including rubisco (in chloroplasts) and glycolate‐oxidase and CAT‐peroxidase reactions (in peroxisomes). Of the three major cell organelles, chloroplasts and peroxisomes produce ROS in the presence of light (Foyer and Noctor, 2003) while mitochondria generate ROS in the absence of light. Because ROS can cause damage to proteins, lipids, and DNA, their generation and scavenging must be strictly controlled. To manage this problem, the cell has a survival strategy including mechanisms for scavenging ROS and repairing damage caused by ROS. Excessive ROS reduce the rate of electron transport in the photosynthetic ETC; this leads to activation of alternative pseudocyclic electron transport and photorespiration. Under these conditions, first 1O2 is produced; then H2O2 is synthesized in the reaction of disproportionation of superoxide catalyzed by SOD or non‐enzymatically in the process of superoxide diffusion with a low yield of the reaction.

Chloroplast

Chloroplasts are considered the most powerful source of ROS generation in plants (Foyer et al., 1994). It is the cellular site of photosynthesis, which proceeds through successive redox reactions during which light energy is transferred to different reaction centers of the two photosystems with the help of the light‐harvesting complexes. Oxygen is continuously produced during light‐driven photosynthetic electron transport and simultaneously removed from chloroplasts by reduction and assimilation. There are three types of oxygen‐consuming processes closely associated with photosynthesis: (i) direct reduction of molecular oxygen by photosystem I (PSI) electron transport; (ii) the oxygenase reaction of ribulose 1,5‐bisphosphate carboxylase‐oxygenase (RuBisCO); and (iii) chlororespiration within thylakoid membrane (Alric et al., 2010). The electrons released during the process are transferred to an ultimate acceptor via a path called the photosynthetic electron transport chain. The cyclic electron transport chain includes a number of enzymes on the reducing (acceptor) side of PSI: Fe‐S centers, reduced thioredoxin, and ferredoxin (Noctor and Foyer, 1998). These electron transport components are auto‐oxidizable (Biehler and Fock, 1996). Further generation of ROS is elaborated by the Mehler peroxidase reaction, which explains the transfer of electrons from H2O to O2, resulting in production of O2− at PSII, O2 ·−at PSI, and the trans‐thylakoid proton gradient necessary to drive phosphorylation and photochemistry of PSII. It has been suggested that photoreduction of O2 to water by the Mehler peroxidase pathway in intense light may involve up to 30% of the total electron transport (Oukarroum, 2016). Oxygen reduction sustains significant levels of photosynthetic electron flux, not only through its role in photorespiration but also by its direct reduction through PSI (Asada, 1999). This would suggest that O2 plays an important role as an alternative electron acceptor in photo‐protection. Producing large amounts of ROS is an unavoidable consequence of the photosynthetic reduction of oxygen, and plants have to evolve efficient strategies to deal with the accumulation of these potentially toxic compounds that are integral components of oxygenic photosynthesis.

Mitochondria

Mitochondria, which are considered the cell’s “energy hub,” are believed to be the foremost sites of ROS production. It has been shown that ROS generated in mitochondria and protein oxidation are contributing factors to the “oxidative stress” syndrome in plants (Sweetlove et al., 2002; Kristensen et al., 2004; Møller and Kristensen 2004). It has also been proven that in the dark or in non‐green tissues of plants, mitochondria are a major source of ROS. Firstly, in 1966 it was reported that the respiratory electron transport system produces ROS, and their production can be enhanced in response to various biotic and abiotic stresses. Subsequent innovative work of (Belt et al., 2017) showed that isolated mitochondria produce H2O2. The mitochondrial inner membrane is where respiratory electron transport occurs. This mitochondrial ETS harbors electrons with sufficient free energy to directly reduce O2, which is considered a primary source of ROS generation. It was confirmed later that H2O2 arose from the dismutation of superoxide (O2 ·−) generated within mitochondria. There are two pathways of O2 consumption, namely: (i) O2 consumption via cytochrome oxidase to produce H2O, a process that accounts for more than 95% of O2 consumption under normal conditions; and (ii) direct reduction of O2 to O2 ·− in the flavoprotein region of the NADH dehydrogenase segment of the respiratory chain (Jezek and Hlavata, 2005). During mitochondrial electron transport, the oxygen radical is markedly enhanced in the presence of antimycin A, which blocks electron flow after ubiquinone. This results in the accumulation of reduced ubiquinone, which may undergo auto‐oxidation, resulting in the production of O2 ·− (Li et al., 2016). Several observations reveal ubiquinone as a major H2O2‐generating location of the mitochondrial electron transport chain in vitro, and it would appear that O2 ·− is a major precursor of H2O2 (Winston, 1990). The mitochondrial electron transport chain is comprised of several dehydrogenase complexes that reduce a common pool of ubiquinone (Møller, 1997). The ubiquinone pool is then oxidized by either the cytochrome or the alternative pathway. In general, the main O2 ·− generators in the mitochondria are the ubiquinone radical and NADH dehydrogenases (Richter and Schweizer, 1997). Because the ETC harbors electrons with sufficient free energy to directly reduce molecular oxygen, it is considered the unavoidable primary source of mitochondrial ROS production, a necessary accompaniment to aerobic respiration. Production of ROS will increase if the rate of electrons leaving the ETC through the terminal oxidases is slowed and/or the rate of electron input increases in excess of the ability of the two respiratory pathways to process the electrons, leading to an over‐reduced ubiquinone pool.

Peroxisomes

Peroxisomes are single membrane‐bounded subcellular organelles with an essentially oxidative type of metabolism and a simple morphology that does not reflect the complexity of their enzymatic composition. At the beginning of the 1960s, when peroxisomes were first isolated and characterized from mammalian tissues, their main function was perceived to be the removal of toxic H2O2 by catalase. H2O2 is typically generated in the peroxisomal respiratory pathway by different flavin oxidases (see Figure 1.1). However, it has become increasingly clear that peroxisomes are involved in a range of important cellular functions in almost all eukaryotic cells. An important property of peroxisomes is their metabolic plasticity, because their enzymatic content can vary depending on the organism, cell or tissue type, and environmental conditions (Jezek and Hlavata, 2005). ROS are also generated by major metabolic pathways, especially those in the peroxisomes, and are used as a weapon against invading pathogens in the oxidative burst. There is another route by which H2O2 can be produced during photosynthesis (Elstner, 1982). During carbon assimilation, ribulose 1,5‐bisphosphate carboxylase uses CO2 to carboxylate ribulose 1,5‐bisphosphate. However, ribulose 1,5‐bisphosphate carboxylase can also use O2 to oxygenate ribulose 1,5‐bisphosphate. Oxygenation yields two glycolates, which are then transported from the chloroplasts to the peroxisomes. Therefore, glycolate oxidation is catalyzed by glycolate oxidase yielding H2O2. In addition, the microbodies contain fatty acid beta‐oxidase and xanthine oxidase as H2O2− and O2 ·−