Glutathione and glutathione reductase: A boon in disguise for plant abiotic stress defense operations

Reactive oxygen species (ROS) were initially recognized as toxic by-products of aerobic metabolism. In recent years, it has become apparent that ROS plays an important signaling role in plants, controlling processes such as growth, development and especially response to biotic and abiotic environmental stimuli. The major members of the ROS family include free radicals like O2● −, OH● and non-radicals like H2O2 and 1O2. The ROS production in plants is mainly localized in the chloroplast, mitochondria and peroxisomes. There are secondary sites as well like the endoplasmic reticulum, cell membrane, cell wall and the apoplast. The role of the ROS family is that of a double edged sword; while they act as secondary messengers in various key physiological phenomena, they also induce oxidative damages under several environmental stress conditions like salinity, drought, cold, heavy metals, UV irradiation etc., when the delicate balance between ROS production and elimination, necessary for normal cellular homeostasis, is disturbed. The cellular damages are manifested in the form of degradation of biomolecules like pigments, proteins, lipids, carbohydrates and DNA, which ultimately amalgamate in plant cellular death. To ensure survival, plants have developed efficient antioxidant machinery having two arms, (i) enzymatic components like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR); (ii) non-enzymatic antioxidants like ascorbic acid (AA), reduced glutathione (GSH), α-tocopherol, carotenoids, flavonoids and the osmolyte proline. These two components work hand in hand to scavenge ROS. In this review, we emphasize on the different types of ROS, their cellular production sites, their targets, and their scavenging mechanism mediated by both the branches of the antioxidant systems, highlighting the potential role of antioxidant

Plants face many stressful conditions during their lifetimes and because of their sessile nature they have to adapt to these conditions in order to survive. One unfortunate and unavoidable consequence of all major biotic and abiotic stresses is the overproduction of reactive oxygen species (ROS). ROS are highly reactive and toxic chemical entities and can cause serious damage to cellular proteins, lipids, carbohydrates and DNA, leading to irreparable metabolic dysfunction and cell death. Plant cells and their organelles, particularly the chloroplasts, mitochondria and peroxisomes have antioxidant defence systems, composed of enzymatic and non-enzymatic components, to counter the deleterious effects of ROS and/or to perform signalling functions. It is an established fact that the timely induction of antioxidant defences is a key to protection of plant cells from oxidative damage due to stress. Enzymatic antioxidants include superoxide dismutase, catalase, peroxidases and glutathione reductase, while the major non-enzymatic antioxidants are compatible osmolytes (glycinebetaine, GB; and proline), ascorbic acid, reduced glutathione, α-tocopherol, amino acids and polyphenols. Stimulated biosynthesis and accumulation of low molecular weight compatible osmolytes is one of the most effective mechanisms evolved by plants to maintain their cellular integrity and ensure survival when exposed to multiple abiotic stresses. Glycinebetaine, an N-trimethyl derivative of glycine and a quaternary ammonium compound, is one of the most studied and efficient compatible solutes. Due to its unique structural features, it interacts both with the hydrophobic and hydrophilic domains of macromolecules, including enzymes and proteins. GB has been reported to protect plants from the antagonistic effects of a range of abiotic stresses, by maintaining the water balance between plant cells and environment, osmotic adjustment, protecting the thylakoid membrane system, protein stabilization, photosystem and photosynthetic electron transport chain protection and by modulating ROS detoxification. In recent years, GB has attained unprecedented attention due to its multifunctional roles in plants under stressful conditions. In this chapter, we summarize our understanding of ROS formation under abiotic stress and GB biosynthesis and accumulation, as an adaptive mechanism, with particular emphasis on the new insights into the biochemical and molecular mechanisms involved in GB-mediated abiotic oxidative stress tolerance in plants.

Climate change-associated environmental vagaries have amplified the incidence of pests and pathogens on plants, thus imparting the increased quest for management strategies. Plants respond to stresses through intricate signaling networks that regulate diverse cellular mechanisms. Reactive oxygen species (ROS) are cardinal towards the maintenance of normal plant activities as well as improving stress management. Plants that exhibit a fine balance between ROS levels and its management apparently mitigate stresses better. There have been very many compendiums on signaling and management of ROS during several abiotic stresses. However, expansion of knowledge related to ROS induction and homeostasis during biotic stresses is pertinent. Hence, considering its importance, we provide insights in this review on how plants signal and manage ROS upon an oxidative burst during their interaction with pathogens and herbivores. Substantial degree of molecular changes and pivotal roles of ROS have been detected during phyto-pathogen/herbivore interactions, opening novel platforms to understand signaling/management of events under varied biotic stresses. It is interesting to know that, though plants react to biotic stresses through oxidative burst, receptors and elicitors involved in the signal transduction differ across stresses. The review provides explicit details about the specific signaling of ROS production in plants under pathogen and herbivore attack. Furthermore, we also provide an update about tackling the accumulated ROS under biotic stresses as another pivotal step. ROS signaling and homeostasis can be exploited as critical players and a fulcrum to tackle biotic stresses, thus paving the way for futuristic combinatorial stress management strategies. KEY POINTS: •The review is a comprehension of redox signaling and management in plants during herbivory and pathogen infection •Reactive oxygen species (ROS) is an important factor during normal plant activities as well as in their response to stresses. Diverse modes of ROS signaling and management have been observed during both biotic stresses independently •Exploration of plant biology in multi-stress resistant plants like the crop wild relatives could pave the way for combinatorial management of stress for a better tomorrow.

Abiotic stresses (e.g., heavy metals, drought, cold, or combinations) induce oxidative stress with overproduction of reactive oxygen species (ROS). We have investigated the antioxidative responses of Albizia julibrissin Durazz. (silk tree, Fabaceae). Four-month-old plants grown in sand cultures were subjected to various single or sequential treatments involving exposure to cadmium (50–250 μmol L−1 Cd), lead (1000–5000 μmol L−1 Pb), chilling at 4 °C (CH), or drought (DR), for a period of 7–45 days. Leaf extracts were assayed for glutathione peroxidase (GPX), glutathione-disulfide reductase (GR), catalase (CAT), soluble proline (Pro), ascorbate peroxidase (APX), and guaiacol peroxidase (GUAPX). Cd and Pb accumulation in the leaves was also measured. CAT activity decreased strongly with increasing Pb exposure and after CH. It was also found to be reduced after Cd and DR treatments. GR activity increased highly in nearly all treatments, most strongly at high Cd or Pb, after DR + CH, and after CH followed by Cd. GUAPX and GPX showed similar trends of increase. APX activity dropped after CH, but increased after low Cd treatment and in CH + DR sequential stresses. Massive accumulation of soluble Pro occurred after 14–21 days in highly Cd- or Pb-stressed plants. CH or DR acclimation led to some alterations of antioxidative responses, particularly for CAT, GR, and APX. Our data indicate that GSH, GSH-linked redox systems, peroxidases, and Pro are possibly the more important antioxidants under severe stress.

Reactive oxygen species (ROS) are dangerous molecules because they can cause damage to proteins, lipids and nucleic acids. ROS production in plants increases at low temperatures; particularly sub-tropical species are sensitive to ROS when they are exposed to temperatures below 10°C. To prevent damage from ROS and to scavenge them, plants developed an enzymatic and a non-enzymatic antioxidant system. The enzymatic antioxidant system was studied in this work during the post-harvest cold storage of aubergine using the real time PCR technique in order to monitor changes in the transcript level of genes encoding for ROS scavenging enzymes. Changes in Mn Superoxide-dismutase, Catalase, Glutathione reductase, Ascorbate peroxidase gene expression were analyzed during storage at 0°C and at 10°C up to 10 days. Catalase gene expression is stimulated during storage at 10°C and inhibited at 0°C. Equally, Glutathione reductase gene expression is upregulated in this milder refrigeration condition. On the contrary Mn-superoxide dismutase and Ascorbate peroxidase transcript levels decrease with respect to the initial amount for both temperatures. This suggests that oxidative stress can cause damage during cold storage of aubergine. The expression of ROS scavenging enzymes is positively regulated during the 10°C cold storage contrary to the 0°C cold storage, indicating the presence of oxidative stress and a tissue reaction.

HIF-1 Prevents Hematopoietic Cells from Cell Damage by Overproduction of Mitochondrial ROS after Cytokine Stimulation through Induction of PDK-1

Reactive oxygen species and antioxidant machinery in abiotic stress tolerance in crop plants

Halophytes are diverse group of plants with tolerance to high salinity due to specific mechanisms of salt uptake and tolerance. Saline areas are being affected by heavy metal pollution also due to many reasons like industrialization, etc. in the recent years. Use of halophytic species for heavy metal remediation is of significant importance as these plants are naturally present in soils characterized by excess of salts. They possess specific mechanisms for uptake, detoxification and extrusion of salts. These mechanisms could help in heavy metal remediation also. Halophytes are constitutively better equipped to cope with oxidative stress as reactive oxygen species (ROS) are overproduced when plants are exposed to high salt concentrations. Halophytes have higher antioxidant metabolism compared with glycophytes. Heavy metal stress also results in overproduction of ROS. ROS scavenging proteins are particularly critical for plants under salt and heavy metal stress to maintain redox homeostasis. Sulphur metabolism also plays a role in heavy metal detoxification. Heavy metal accumulators have increased cysteine biosynthesis induced by heavy metals. Highly reactive heavy metals such as Cr, Cu and Fe are directly involved in redox reactions and generate ROS, while the redox inactive heavy metals can induce indirect ROS through decrease in antioxidant activity. The oxidative damage caused by overproduction of ROS in heavy metal-contaminated soils of halophytes will be discussed. The antioxidant metabolism of halophytes in terms of heavy metal tolerance also will be discussed as some adaptations to these stresses are common. The role of osmolytes, phytochelatins and metallothionein in heavy metal tolerance will also be discussed.

What can reactive oxygen species (ROS) tell us about the action mechanism of herbicides and other phytotoxins?

Abiotic stresses like high temperature, cold, freezing, drought, salinity, flooding or oxidizing agents cause significant loss in the crop yield and quality. Abiotic stresses cause reactive oxygen species (ROS) production such as singlet oxygen (1O2), hydrogen peroxide (H2O2), superoxide radical (O2•−), hydroxyl radical (OH-), etc., that leads to a significant reduction of crop yield. A major source of ROS production in plants through aerobic metabolism is chloroplast, mitochondria, and peroxisome. The tripartite interactions involving Trichoderma- Phytopathogen-Host have received less attention in contrast to the plant–antagonist, plant–pathogen or pathogen–antagonist interactions. This article explores the possibilities of employing thermotolerant strains of agriculturally important microorganisms (AIMs) for alleviating the oxidative stress induced due heat stress in crops by modulating oxidative and defense network of the host.

Due to their stationary nature, plants are exposed to a diverse range of biotic and abiotic stresses, of which heavy metal (HM) stress poses one of the most detrimental abiotic stresses, targeting diverse plant processes. HMs instigate the overproduction of reactive oxygen species (ROS), and to mitigate the adverse effects of ROS, plants induce multiple defence mechanisms. Besides the negative implications of overproduction of ROS, these molecules play a multitude of signalling roles in plants, acting as a central player in the complex signalling network of cells. One of the ROS-associated signalling mechanisms is the mitogen-activated protein kinase (MAPK) cascade, a signalling pathway which transduces extracellular stimuli into intracellular responses. Plant MAPKs have been implicated in signalling involved in stress response, phytohormone regulation, and cell cycle cues. However, the influence of various HMs on MAPK activation has not been well documented. In this review, we address and summarise several aspects related to various HM-induced ROS signalling. Additionally, we touch on how these signals activate the MAPK cascade and the downstream transcription factors that influence plant responses to HMs. Moreover, we propose a workflow that could characterise genes associated with MAPKs and their roles during plant HM stress responses.

Programmed cell death (PCD) is a process that plays a fundamental role in plant development and responses to biotic and abiotic stresses. Knowledge of plant PCD mechanisms is still very scarce and is incomparable to the large number of studies on PCD mechanisms in animals. Quick and accurate assays, e.g., the TUNEL assay, comet assay, and analysis of caspase-like enzyme activity, enable the differentiation of PCD from necrosis. Two main types of plant PCD, developmental (dPCD) regulated by internal factors, and environmental (ePCD) induced by external stimuli, are distinguished based on the differences in the expression of the conserved PCD-inducing genes. Abiotic stress factors, including heavy metals, induce necrosis or ePCD. Heavy metals induce PCD by triggering oxidative stress via reactive oxygen species (ROS) overproduction. ROS that are mainly produced by mitochondria modulate phytotoxicity mechanisms induced by heavy metals. Complex crosstalk between ROS, hormones (ethylene), nitric oxide (NO), and calcium ions evokes PCD, with proteases with caspase-like activity executing PCD in plant cells exposed to heavy metals. This pathway leads to very similar cytological hallmarks of heavy metal induced PCD to PCD induced by other abiotic factors. The forms, hallmarks, mechanisms, and genetic regulation of plant ePCD induced by abiotic stress are reviewed here in detail, with an emphasis on plant cell culture as a suitable model for PCD studies. The similarities and differences between plant and animal PCD are also discussed.

Although excess reactive oxygen species (ROS) are toxic, physiological concentrations of ROS may function as signaling molecules to mediate various responses. However, given that ROS are diffusible and short-lived, localizing the ROS signal at a precise subcellular location is essential for stimulation of specific redox signaling. In animals, recent studies have indicated lipid microdomain platforms or lipid rafts may be importantly implicated in redox signaling of a variety of cells in response to agonists or stimuli (for a review, see Li and Gulbins 2007). The plant plasma membrane (PM) is in charge of sensing the various environmental modifications faced by the plant cell and triggering the appropriate physiological responses. It thus exemplifies this requirement for an extremely fine-tuning of ROS production in plants, which has been evidenced as a mediator in many different biotic or abiotic stresses leading to significantly different responses. The spatial compartmentalization of ROS-producing enzymes in specialized domains of the plant PM could be one key element of such a regulation.

Pre-treatment of soybean plants with calcium stimulates ROS responses and mitigates infection by Sclerotinia sclerotiorum

Heat stress is an environmental factor that causes oxidative stress. We found previously that acute heat stress stimulates the production of reactive oxygen species (ROS) in the skeletal muscle mitochondria of birds, and that this was accompanied by an increase of the mitochondrial membrane potential (ΔΨ) due to increased substrate oxidation by the electron transport chain. We also showed that avian uncoupling protein (avUCP) expression is decreased by the heat exposure. The present study clarifies whether ΔΨ is a major determinant of the overproduction of ROS due to acute heat stress, and if the decrease in avUCP expression is responsible for the elevation in ΔΨ. Control (24°C) and acute heat-stressed (34°C for 12 h) birds exhibited increased succinate-driven mitochondrial ROS production as indicated by an elevation of ΔΨ, with this increase being significantly higher in the heat-stressed group compared with the control group. In glutamate/malate-energized mitochondria, no difference in the ROS production between the groups was observed, though the mitochondrial ΔΨ was significantly higher in the heat-stressed groups compared with the control group. Furthermore, mitochondria energized with either succinate/glutamate or succinate/malate showed increased ROS production and ΔΨ in the heat-stressed group compared with mitochondria from the control group. These results suggest that succinate oxidation could play an important role in the heat stress-induced overproduction of mitochondrial ROS in skeletal muscle. In agreement with the notion of a decrease in avUCP expression in response to heat stress, proton leak, which was likely mediated by UCP (that part which is GDP-inhibited and arachidonic acid-sensitive), was reduced in the heat-exposed group. We suggest that the acute heat stress-induced overproduction of mitochondrial ROS may depend on ΔΨ, which may in turn result not only from increased substrate oxidation but also from a decrease in the mitochondrial avUCP content.