Regulation of ROS Metabolism in Plants under Environmental Stress: A Review of Recent Experimental Evidence

1Ensuring food security is one of the most defined concerns and high-priority areas among plant scientists. Meanwhile, agricultural productivity is under immense pressure in the scenario of stringent climate change and an ever-increasing affluent population. Under such a scenario, it is imperative to understand the coordinated processes of plant metabolism to improve crop productivity. Due to changing climate, abiotic stresses become one of the most critical factors that severely reduce crop productivity. For instance, environmental stresses lead to altered nutrient acquisition and biosynthetic capacity that can inhibit plant growth. Furthermore, under abiotic stresses, modification of signaling pathways, expression of genes, proteins, and post-translation may occur, which activates numerous stress-responsive transcription factors to adapt the downstream responses needed to support an effective defense to specific abiotic stress challenges.

2The existence of aerobic organisms, such as plants, depends on oxygen for energy generation systems. Moreover, plants also produce O2 during the photosynthesis. Molecular oxygen could be excited, forming reactive oxygen species (ROS). This ROS includes singlet oxygen (1O2), superoxide radical (O2•−), hydroxyl radical (•OH), and hydrogen peroxide (H2O2). The generation of toxic ROS as a by-product occurs in various cellular sites, such as mitochondria, chloroplast, peroxisome, and apoplast. Under normal conditions, a balance exists between the production and scavenging of ROS by the coordinated action of antioxidant defense system components within the plants. The generation of ROS at a lower level during normal growing conditions takes part in crucial intra- and inter-cellular signaling processes for inducing a positive response in antioxidant defense systems and the biological processes such as cellular proliferation, differentiation, or stress adaptation mechanisms.

3On the other hand, disproportionation between ROS generation and scavenging leads to oxidative damage under stress conditions hampering normal cellular function, damaging biomolecules such as carbohydrates, lipids, proteins, and DNA, and finally causes cell death. Thus, a stringent regulation between ROS production and scavenging that aids a plant in utilizing ROS as a defense molecule under stress conditions is crucial. To regulate cell redox homeostasis under a stressful situation, plants utilize a multifaceted and strong antioxidant defense system where non-enzymatic and enzymatic components perform their function in sensing and elimination/detoxification of excess ROS. Therefore, different plants have varied capacities to tolerate oxidative stress that depends on the ability of their antioxidant machinery. Moreover, ROS induced redox signals regulate various vital processes of the cellular environment. For example, ROS-induced redox signaling in plants induces programs of gene expression and aids in maintaining cellular redox homeostasis.

4Moreover, the progression of different basic biological processes related to cellular differentiation, proliferation, and cell death are also attributed to ROS-induced redox signaling. Previously, it was thought that these biological processes are attributed to ROS-induced oxidative damage; but recent reports suggest that these processes are triggered by the ROS activating programmed cell death pathway. Henceforth, it is evident that ROS-induced redox signaling plays a vital function in the various physiological responses.

5Therefore, this review provides an in-depth look at recent findings related to ROS metabolism domains in plants such as ROS production sites, antioxidant networks, and their synergistic and counter effects under environmental stresses with a particular focus on oxidative stress.

6Partially reduced or activated molecular O2 or the primary product or by-product of O2-containing molecules showing higher reactivity than atmospheric O2 are ROS. The transfer of energy or electrons produces ROS from O2 molecules; the most common cellular ROS are H2O2, O2•−, •OH, and 1O2 in plants.

7Both free radical and non-radical ROS are produced in cells (Figure 1). Among them, O2•−, OH•, alkoxyl radical (RO•), and peroxyl radical (ROO•) are the free radicals, while H2O2 and 1O2 are the non-radicals. Some other non-radical ROS, such as hypochlorous acid (HOCl), and excited carbonyl (RO*), are also found in plants. Moreover, some acidic compounds (hypochlorous acid, HOCl; hypoiodous acid, HOI; and hypobromous acid, HOBr) and some radical compounds like carbonate (CO3•−) are also incorporated into ROS.

8Besides, peroxyl radical (LOO•), alkoxyl radical (LO•), hydroperoxyl radical (HO2•), peroxynitrite (HNO3−), ozone (O3), and trichloromethyl peroxyl radical (Cl3COO•) could also be found in biological systems.

9Under stress conditions, the elevation of ROS relies on subcellular ROS metabolism and their transformation from one to another, which varies for different ROS types, cellular compartments, and even cell types (Figure 2). The diffusion distance of different ROS is also different; but it is to be noted that H2O2 may be one that is capable of moving from one cell compartment (or even cell) to another. Therefore, cellular ROS accumulation also depends on the compartmental specific signaling effect and ROS detoxification.

10To better understand ROS scavenging tactics, it is necessary to know the subcellular compartment-specific ROS generation. Different cell compartments have their specific ROS generation and detoxification capacity, being of vital importance for regulating ROS scavenging systems and keeping the ROS below the threshold level for protecting cellular components or initiating signaling cascade.

11The chloroplast is one of the leading ROS production sites in plants, where ROS generation, directly and indirectly, depends on the interaction of chlorophyll (chl) and light. Here, triplet chl and electron transport chain (ETC), specifically PSI and PSII, are the primary sources for ROS production. In PSI, O2•− is produced by Mehler reaction and then superoxide dismutase (SOD) converts them into H2O2. In the presence of metal ions such as Fe2+, more highly reactive short-lived HO• are formed from O2•− and H2O2. Environmental stresses cause stomatal closure, and consequent lowering of CO2 level provokes the chloroplastic ROS production.

12Abiotic stresses induce higher photorespiration in the peroxisome. Here, glycolate oxidase (GOX) is the crucial player for elevated ROS production. In the peroxisome, O2•− can be produced at the organelle matrix by xanthine oxidase (XOD) and peroxisomal membrane NADPH oxidase. Metalloenzymes, SODs, later dismutate O2•− into H2O2. Different types of SODs, including Cu-Zn-SOD and Mn-SOD, have been discovered in peroxisomes of various plant species. Peroxisome is considered one of the major sites for H2O2 generation. Besides, photorespiration GOX reaction, O2•− disproportionation, β-oxidation of fatty acids, flavin oxidase polyamine oxidase, sulfite oxidase, copper amine oxidase, and sarcosine oxidase activity are the prime metabolic processes through which H2O2 are produced in peroxisomes.

13In the non-green plant organs, particularly in the root, mitochondria are the major ROS generation sites. Mitochondrial ETC holds electrons with sufficient free energy, from which 0.2–2% of electrons transferred interact with O2 to produce ROS, and hence, apparently 1–2% of O2 becomes partially reduced. In such a case, complex I and III in the ETC of mitochondria are the two major sites for ROS production. On abiotic stresses, excessive amounts of ROS are accumulated through electron leakage from both complexes I and III, and produce O2•−, which is later catalyzed by Mn-SOD and Cu-Zn-SOD and produce H2O2.

14Complex structures made by poly-saccharides, phenolics, and proteins in the plant cell walls are the active site for ROS production. Stressed-plants become prone to growth reduction by differential cell wall growth, where ROS, along with peroxidase, triggers polymerization of glycoproteins and phenolic compounds to make cell walls rigid. These cell wall-associated peroxidases catalyze H2O2 generation in the presence of NADH, where the NADH is solely provided from malate dehydrogenase. Additionally, diamine oxidases cause ROS generation in the cell wall by reducing diamines or polyamines to quinine. Under stress conditions, lipoxygenase (LOX)–induced polyunsaturated fatty acid (PUFA) hydroperoxidation is another potential source of ROS.

15In the plasma membrane, O2•− generation is mediated by NADPH oxidase and quinine reductase, where electron transportation from cytoplasmic NADPH is catalyzed by NADPH oxidase to form O2•−, which is again converted into H2O2.

16Reactive oxygen species are also produced in the endoplasmic reticulum, glyoxysome, and cytosol. In the endoplasmic reticulum, Cyt P450 produces O2•− as a by-product during reaction with an organic substrate to form a free radical intermediate (Cyt P450-ROO−), where NADPH is the electron donor. In glyoxysome, both O2•− and H2O2 are produced from the fatty acid oxidation by GOX and urate oxidase activities. Moreover, both XOD and aldehyde oxidase (AO) are actively engaged in ROS production in the cytosol.

17As the by-products of aerobic metabolism, ROS are found to generate naturally in plants. However, when exposed to a stressful environment, the over-generation of ROS disrupts the equilibrium between ROS accumulation and scavenging, causing oxidative damage of biomolecules that cause cellular damage and death. Under oxidative stress, biomolecules like lipids, proteins, DNA, carbohydrates, polynucleic acids, et cetera, become reversibly or irreversibly modified (Figure 3).

18Under oxidative stress, over-accumulated ROS causes lipid peroxidation (LPO), causing chain reactions and creates free radicals, resulting in lipid denaturation. In the membrane phospholipids, the most susceptible sites for ROS attack are the carbon (C) atoms and the ester linkage between fatty acids and glycerol. Moreover, 1O2 and OH• attack the plasma membrane polyunsaturated fatty acids (linoleic and linolenic acids). Lipid peroxidation is divided into three definite consecutive stages, including (i) initiation, (ii) propagation/progression, and (iii) termination. Of all three steps, initiation is the rate-limiting step. In this process, H+ is subtracted from the methylene group (-CH2), giving rise to a carbon-centered radical -•CH- or lipid-free radical (L•). Due to H+ subtraction, the double bond in the fatty acid weakens, facilitating the C-H bond. Thus, the lipids remain vulnerable to free radicals and peroxidation. In the propagation stage, L• activates O2, generating the lipid peroxyl radical (LOO•), and the abstracting of a second H+ from nearby fatty acid produces the lipid hydroperoxide (LOOH) and causes another L•. Subsequently, the LOOH is cleaved by the reduced transition metals like Fe2+ or Cu+, forming a highly reactive radical called the lipid alkoxyl (LO•) and resulting in the formation of different aldehydes, alkanes, lipid epoxides, and so on. The termination of the LPO occurs through the formation of various lipid dimmers from lipids-derived radicals.

19Singlet oxygen evolved in the PSII reaction center, further reacts with the lipid double bonds forming LOOH. Moreover, 1O2 can be produced from the reaction of two LOO• damaging membranes’ fluidity, collapsing membrane function, and causing the oxidation and dysfunctioning of membrane proteins, receptors, and localized enzymes, respectively. Furthermore, by damaging membranes, severe LPO collapses their barrier function; thus, the cellular organelles disintegrate. Besides, LPO causes oxidative malfunctions of proteins, DNA, and RNA together with forming some disrupted and altered aldehyde substances like malondialdehyde (MDA), acrolein, and 4-hydroxy-2-nonenal (HNE), which are the oxidative stress markers in plants.

20Reactive oxygen species or other by-products of oxidative stress alter the covalent bond and induce protein oxidation. Any proteinogenic amino acid can be oxidized by ROS, altering the metabolic, structural, transport, or regulatory activities of proteins. As a secondary effect, some lipid peroxidation products like hydroxynonenal and MDA can react with proteins that modify arginine (Arg), lysine (Lys), proline (Pro), threonine (Thr), and tryptophan (Trp) and result in proteolytic degradation. These proteinogenic amino acids give rise to free carbonyl groups and become more susceptible to proteolytic activity. Essential plant protein complexes including PSI protein complex (PSI-A to PSI-O and LHCI (Lhca1, Lhca2, Lhca3, Lhca4)), D1 protein of PSII, ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCo), and SOD require the presence of •OH and 1O2 for oxidative damages, where the study revealed that 1O2 is specifically found to affect D1 protein; whereas, increased activity of catalytically-active transition metal (•OH) is required for the damages of the others. Collectively, ROS (•OH, 1O2, and H2O2) result in the oxidation of side chain residues of amino acids, protein-protein cross-linkages formation, protein backbone oxidation, as well as protein fragmentation. Reactive oxygen species (•OH, 1O2) mainly target the proteins having sulfur-containing amino acids and thiol groups. The cysteine (Cys) and methionine (Met) modified residues containing thiol and sulfur groups are inevitably attacked by •OH and 1O2. Oxidative damages of proteins are mostly irreversible except those with S-containing amino acids (Met and Cys). The •OH and 1O2 form oxidized derivatives of amino acids sequentially (cysteine → cystine → cysteine sulfenic acid → cysteine sulfinic acid → cysteic acid). Therefore, disulfide bond formation, Cys oxidation, nitrosylation, glutathionylation, and sulfhydration result from redox modification of proteins. Furthermore, Kale et al. proposed that, the formation of HO• at Mn4O5Ca cluster and the nonheme iron of PSII resulted in the oxidation of specific amino acid residues of the proteins (D1, D2). Besides, oxidative modification of D1:130E and D2:246M residues were also evident by the formation of O2•− (by reducing O2). Again, several native amino acid residues were found to be oxidized by ROS; for example, D1, D2, and CP43 subunits in the locality of the cluster Mn4O5Ca, a peptide of the D1 protein (130E–136R), and some other oxidized amino residues in the location of PheoD1 and QA were demonstrated in several studies.

21The main damaging factor for polynucleic acids are the •OH, which alters nucleotide bases (purine and pyrimidine) by abstracting H+ from the C-H bonds of 2-deoxyribose and methyl group, causing deoxyribose radical, hydroxyl methyl urea, thymine glycol, et cetera, and breaks the double-stranded DNA into single-strands. Damage to DNA due to oxidative stress has been reported by several researchers. Furthermore, ROS also damages DNA nucleotide by oxidizing deoxyribose sugar, modifying nucleotide bases, abstracting nucleotides, and DNA protein cross-linking. As a consequence of base oxidation, some damaging products (8-hydroxyquinine, dehydro-2′-deoxyguanosine, etc.) are formed, which cause irreparable cross-links and those are very lethal to plant cells. If these damages are not repaired before the next replication or transcription, DNA denaturation and unfolding are the obvious results. Additionally, incorrect protein sequences are also found as important consequences. Damage of DNA affects plant growth and development, directly affecting various physiological processes; for example, abnormal synthesis of protein and damage of photosynthetic proteins, et cetera. It can also arrest transcription, signal transduction, replication errors, and whole genomic instability. Besides, DNA bases are not always damaged by direct oxidation but also through the reactive intermediates (generated from ROS attack) reacting with the macromolecules. For instance, polyunsaturated fatty acid residues of membrane phospholipids are often attacked by oxygen radicals and are considered vital indirect oxidative damage. Malondialdehyde, acrolein, and crotonaldehyde are some reactive by-products of membrane LPO.

22In plants, under oxidative stress, redox modification of glycolysis and TCA cycle enzymes are among the primary damaging responses. With an increased ROS level, pentose phosphate pathway enzymes, for example, glyceraldehyde 3-phosphate dehydrogenase and fructose-1,6-bisphosphate aldolase, are inhibited. To cope with the high ROS level, these inhibited enzymes in the pentose phosphate pathway increase the carbon flux (by increasing cycle metabolites, ribose 5-P, and ribulose 5-P intermediates) to produce necessary NADPH. Besides, enzymes of the TCA cycle are severely affected during oxidative stresses, and also inhibit enzyme aconitase and accelerate citrate biosynthesis. Contrarily, citrate accumulation plays vital roles as an introducer of alternative oxidase ROS detoxification under stress. Lehman et al. also demonstrated that under oxidative stress, reduced glycolysis and metabolism of the TCA cycle and amino acid have resulted in the plant. In this study, C flow in Arabidopsis root was investigated, kinetic analysis of 13C-Glc showed a decreased labeling for citrate, isocitrate, fumarate, malate, succinate, and 2-oxoglutarate, which suggested an inhibited C flux throughout the TCA cycle.

23Drought stress induces stomatal closure and reduces CO2 fixation in plant leaves. Consequently, this stress creates disequilibrium between light capturing and utilization; as a result, the photosynthetic rate is reduced. During drought, photochemistry of chloroplasts become altered, and an imbalance between the electron release and acceptance results in the increased generation of ROS from the excess light energy in the photosystems. In fact, the absorbed light energy that cannot go to CO2 fixation, produce ROS. Drought stress-induced H2O2 generation directly results from photorespiration. Additionally, under drought stress, if chloroplast are exposed to excess light energy, ferredoxin becomes highly reduced; the regeneration of NADP+ is hindered, which interferes with the acceptance of electrons causing reduced ETC as well as greater electron leakage, which contribute in overproducing ROS. Drought-induced major and obvious oxidative stress markers are LPO (MDA) and H2O2 accumulation causing the dysfunctioning of various cellular and physiological processes, including stomatal conductance, membrane functions, water-use efficiency, carboxylation efficiency, respiration, photosynthesis, transpiration, and so on. Reports indicate increased MDA and H2O2 contents under drought stress in many plant species, for example, rapeseed, maize, soybean, alfalfa, chili, et cetera, which together with the other toxic ROS gives rise to oxidative damages (Table 1).

24Performance of Vigna radiata L. plants were studied under drought stress (5% polyethylene glycol, PEG; 48 h), singly or together with high temperature (HT) stress and then compared with the control grown plants. Drought and HT either singly or in combination caused higher generation of ROS including free radicals and non-radicals (H2O2 and O2•−) along with the enhanced activity of the oxidative enzyme (LOX) resulting in increased LPO indicated by higher MDA levels in V. radiata L., compared to the control. Hasanuzzaman et al. also documented increased MDA and H2O2 content in Brassica napus L. cv. Bina Sharisha-3 under drought stresses (10 and 20% PEG), where the addition of PEG-6000 in the growth medium resulted in osmotic stress. Nahar et al. found overgeneration of toxic free radicals O2•− with increased LPO and thiobarbituric acid reactive substances (TBARS) in Oryza sativa L. plants when subjected to drought (15 and 20% PEG) for seven days. Sarker and Oba revealed higher electrolyte leakage (EL) resulting from increasing the drought stress severity in Amaranthus tricolor plants. In another study, declined EL level by 11, 26, and 47% under mild to severe levels of drought stresses induced by 5, 10, and 15% PEG (3 w), respectively, were reported in Glycine max. Triticum aestivum L. subjected to severe drought stress (70% FC, field capacity) showed maximum TBARS, EL, and H2O2 contents of 31, 25, and 38%, respectively, compared to controls. Hussain et al. also demonstrated similar oxidative stress in drought-affected Zea mays L. when investigating the effects of drought stress (50% FC, 15 d) on two-hybrid maize varieties, and found an overproduction of toxic ROS (O2•−, H2O2, and •OH) and enhanced accumulation of MDA leading to oxidative stress condition. While working with O. sativa L. subsp. japonica. cv. Nipponbare; drought stress (20% PEG) increased O2•− by 23%, enhanced H2O2 content by 1.21-fold, and increased MDA content by 16%, compared to controls, which were liable for creating oxidative damage. Interestingly, drought-induced higher oxidative stress intensities varied among the cultivars and tolerant lentil cultivar (PDL-2) accumulated less ROS and reduced oxidative damage and showed better performance, compared to the sensitive cultivar (JL-3) under drought stress. From these reports, it is obvious that drought stress induces oxidative stress in plants via enhancing toxic ROS levels that are deleterious for vital processes in plants.

25Salinity restricts crop productivity, particularly in the arid and semi-arid regions as well as in coastal soils. Salinity poses osmotic stress, ion toxicity, genotoxicity, nutritional deficiency, as well as initiates overproduction of ROS, leading to oxidative stress. Under salinity stress, the root tissues suffer the most, followed by mature and young leaves, which are the least affected. Both mild (75 mM NaCl) and severe (150 mM NaCl) doses of salinity caused the increase in MDA, H2O2, and EL of roots and mature leaf pairs of maize plants, including higher damage severity found under severe stress. Using comet assay, Saha et al. studied the oxidative DNA damage of mung bean under salt stress. They observed salinity-induced enhanced damage of DNA in seven day old mung bean seedlings, which was correlated to oxidative stress. They also found accelerated ROS accumulation in a dose-dependent manner causing larger DNA damage. Salt tolerant T. aestivum cv. BARI Gom-28 showed higher H2O2 and MDA content by 230 and 61% where 41 and 90%, respectively, were found in sensitive cultivars. Although higher H2O2 was observed in tolerant plants along with lower cellular damage than sensitive plants and these higher H2O2 might be performing signaling roles. It can be suggested that salt-sensitive cultivars suffered more from oxidative stress. Two-fold higher ROS generation (O2•− and H2O2) along with higher MDA and EL were observed in mung beans under 100 mM NaCl and thus depicted increased oxidative damage. Salt stress caused two times higher ROS generation with elevated LPO and EL in rice roots. Similarly, salinity-mediated elevation of oxidative stress markers including O2•−, H2O2, EL, and MDA by 157, 176, 158, and 94%, respectively, observed in tomatoes. From these reports, it is obvious that salinity causes oxidative stress which is lethal for vital processes in plants.

26Metals/metalloids toxicity interrupts not only morpho-physiological traits but also causes enhanced oxidative stress resulting from lack of balance between antioxidant defense system and ROS production. El-Amier et al. reported increased levels of LPO and H2O2 accumulation in Ni (100 µM) stressed Pisum sativum L. seedlings. Such increments of oxidative stress indicators was even higher in the same crop with the same concentration of Cd as well. Meanwhile, Cd stress raised the MDA and H2O2 production in different crops including V. radiata L., B. napus L., B. juncea L., A. thaliana, and Cucumis sativus under different levels of stress. Apart from MDA and H2O2, the rate of O2•− production was also higher in V. radiata L.. Another study demonstrated the oxidative damages under Pb stress conditions in wheat plants. Nahar et al. reported higher levels of H2O2, O2•−, and MDA contents, and LOX activity by 83, 110, 97 and 72%, respectively, in V. radiata L. cv. BARI Mung-2 when exposed to Al stress (0.5 mM) for 48 h. B. juncea seedlings exposed to Cr stress (0.15 and 0.3 mM, 5 d) exhibited higher TBARS and H2O2 contents as well as LOX than that of the control plants. Thus, it is clear that metals/metalloids toxicity increased oxidative stress as depicted by oxidative stress markers such as MDA and H2O2.

27When plants are exposed to HT, heat inactivation occurs on both sides of the electron acceptor and donor in PSII. On the PSII electron donor side, heat inactivation is linked with the inhibition of oxygen-evolving complex (OEC) through removing the extrinsic proteins from their binding sites via the release of Ca2+ and Mn2+, which is required for H2O splitting. On the PSII electron acceptor side, heat inactivation impaired the electron flow from QA to QB due to the increased redox potential of QA/QA•−. This impairs electron flow, which can exert damaging effects on the PSII reaction centers, including D1 and D2 proteins; causing further damage to D1 protein generating 1O2 on the PSII electron acceptor side. On the electron donor side of PSII, due to incomplete oxidation of H2O, H2O2 forms, after that it has been converted into •OH radicals by Fenton reaction. Heat inactivation also occurred due to the moderate temperature in spinach thylakoids causing LPO and damage to D1 and LHCII as well. Moreover, it has been suggested that sink capacity declines due to the slowing down of carbon fixation, resulting in an increased excitation pressure in the chloroplasts. In PSII, this high excitation pressure causes photoinhibition. High temperature also affects the reaction center of PSI resulting in photoinhibition, which is a rare case in nature due to the P700 oxidation system of the plant that can suppress the excessive excitation pressure of P700. Several studies reported that unlike PSII, PSI is not a limiting factor in the overall photosynthetic activity of plants. High temperature (35/32 °C day/night) stress slowed down the electron flow to the PSII reaction center and reduced quantum efficiency (Fv/Fm) and down-regulated photochemistry of PSII of rice (cv. IR64 and Huanghuazhan). However, short-lived and unstable 1O2 has a great impact on photosynthesis once it is formed. Moreover, due to instability and higher oxidation-reduction activity, O2•− is a vital precursor for various ROS products. In a study, 21-d-old purslane (Portulaca oleracea L.) seedlings were treated with HT (42 °C, 7 d) showed higher levels of O2•−, EL, and MDA content by 2.4, 3.84-fold, and 23%, respectively, than that of controls.

28Awasthi et al. tested HT stress (32/20 °C day/night, 7 d) in heat-tolerant and heat-sensitive chickpea plants and observed 6.5-fold increased H2O2 content in leaves of sensitive genotypes than in tolerant ones (5.7-fold). In contrast, MDA content and EL increased by 2.9–6.2-fold and 1.2–1.8-fold in heat-tolerant and sensitive plants, respectively. Furthermore, HT (45/30 ± 2 °C) at three reproductive phases of the cotton plant caused 0.78 times increased MDA content than controls, which affected the cell organelles. Contrarily, Liu et al. found no substantial change in the content of O2•− and MDA in rice seeds but H2O2 increased by 1.27-fold under HT stress (38 °C, 5 d) compared to controls. From these reports, it is clear that HT stress causes oxidative stress as depicted by the oxidative stress markers.

29In several plant species, low temperature (LT) also increases ROS and induces oxidative stress. For example, Guo et al. exposed two sweet sorghum inbred lines (Sorghum bicolor L. cv. M81-E and Roma) to LT (10 °C; 0, 12, 24, 36, and 48 h) and observed that MDA content increased to a maximum in both M81-E and Roma by 266% after 48 h of LT stress. Zhang et al. studied two rice cultivars, japonica (Nipponbare) and indica (93-11), under LT stress (2 ± 1 °C; 10, 33, 57 h), where a higher level of H2O2 at 33 h of stress imposition was reported in cv. Nipponbare, compared to cv. 93-11. A similar trend for H2O2 overgeneration was also reported by Diao et al. and Ghanbari and Sayyari in tomatoes under LT stress (4 °C, 24 h; and 3 °C, 6 h and 6 d). In another study, LT stressed (12 °C, 6 d) rice seedlings showed a 180% increased MDA and 49% EL, compared to controls. However, it was interestingly seen by Jan et al. that MDA content increased by 16.79% after 24 h, whereas it decreased by 12.21% after 48 h in tomato plants; where EL also showed a similar trend. To evaluate the LT tolerance correlation with the presence of oxidative stress markers in rice shoots and roots, Hsu and Hsu experimented with eight Taiwan rice cultivars exposed to LT (15 °C for 4 d) in a programmable incubator. They observed that the higher growth rate of the LT treated cultivars had a higher level of H2O2 in their shoots than the roots. In comparison, the LT treated slow-growing cultivars had higher levels of H2O2 along with higher MDA and EL in their roots than the shoots that caused the interruption of the nutrient uptake from root to shoot. It suggested the presence of higher H2O2 content correlated positive growth in the shoots but negative in the roots during LT stress (Table 1).

30In the natural ecosystem, sudden extreme climate change events like a flood can hamper the natural distribution of plants or even cause extinction. Besides causing hypoxia; flooding or waterlogging (WL) may also result in anoxia, which hampers respiration and generates toxic compounds causing impaired metabolic processes. Ceased growth and biomass production, disturbance in the light interception and root hydraulic conductivity, limitation of stomatal conductance and CO2 assimilation, reduced photosynthesis and respiration, and altered accumulation of the secondary metabolites are the main reasons for yield reduction. Such impairments of metabolic processes result in ROS generation and oxidative damages under WL condition.

31Studies have revealed flooding-induced increments of LPO, ROS overgeneration, and other oxidative damages in different crops. Some cereal crops, for example, Z. mays L., S. bicolor L., Hordeum vulgare L., et cetera, showed a remarkable increase in LPO and accumulation of H2O2, O2•−, and •OH under different durations of WL condition. Li et al. chose eighteen maize genotypes to evaluate the WL (2 d)-induced changes in LPO. They observed that MDA content increased in only in four genotypes, compared to control plants, but declined in other genotypes, which might be due to the scavenging of ROS by antioxidant enzymes. Two barley (WL sensitive cv. TF57 and WL tolerant cv. TF58) cultivars exposed to WL condition for 21 days resulted in a higher increase of both O2•− and MDA contents in susceptible TF57 cultivar, compared to the tolerant TF58 (Table 1;).

32The primary energy source for plants is light. Still, high light (HL) impairs the photosynthetic ETC in PSII, which may lead to the production of 1O2, resulting in a delayed recovery period of D1 in the PSII core. Furthermore, during electron transfer to O2, O2•− can be produced at PSI by Mehler reaction or at PSII through QA to QB pathway, and H2O2 can be formed by the univalent reduction of O2 at ETC in the plant cell (Table 1;).

33According to Awad et al., A. thaliana double mutants deficient in two plastids 2-Cys PRXs (2-Cys PRX A and B, 2cpa 2cpb) and triple mutants deficient in 2-Cys PRXs and tAPX (2cpa 2cpb tapx) leads to accumulation of O2•− and H2O2 and causes photo-bleaching of leaf tissue in HL stress. Iris pumila grown in full sunlight had 20% higher MDA content than those grown in shade. Shengxin et al. also noted increased MDA, O2•−, and H2O2 values in rapeseed (B. napus L. cv. Zhongshuang11) seedlings under HL (550 ± 20 µmol photons m−2 s−1, 16 h). In contrast, Lima et al. found no significant change in MDA and H2O2 in 55-d-old cashew plants (Anacardium occidentale L.) treated with water deficit followed by HL (850 µmol photons m−2 s−1, 5 d) showing resistance against water deficit and HT stresses. Recently, Zha et al. divided lettuce plants (Lactuca sativa L. cv. ”Yidali”) into three groups of light intensity such as low light (LL; 100 µmol m−2 s−1), medium light (ML; 200 µmol m−2 s−1), and high light (HL; 300 µmol m−2 s−1). They observed that the H2O2 content increased in lettuce leaves at ML and HL, whereas O2•− content increased only in the HL. These increased levels of H2O2 and O2•− contents were seen during the first 6 days of treatment but decreased at 9 days with increased trends found again at 12 days of treatment. The MDA content showed a similar trend to O2•− content. In contrast, it was also reported that the H2O2 and O2•− contents remained relatively constant, respectively, at LL stress on the first 9 days of treatment and again increased at 12 days as seen in HL lettuce leaves. All these reports show that HL stress enhanced oxidative stress as depicted by oxidative stress markers. At the same time, the resistant plant genotypes are less susceptible to oxidative damage, compared to sensitive genotypes.

34The UV radiation (200–400 nm) is detrimental to nucleotides and proteins; consequently, exposure to this radiation causes excess ROS production in plants. Considerable damage to proteins and membranes exerts inhibiting effects on the functioning of mitochondria and chloroplasts, resulting in ROS production. Apart from that, reduction in other plant metabolic functions like CO2 assimilation, stomatal conductance, electron transport, and net photosynthesis may also account for the production of ROS in plants exposed to UV radiation. Different experiments have been conducted to understand the UV-radiation-induced oxidative damages, including higher H2O2 and O2•− generation, MDA content, and EL. Tripathi et al. observed increased contents of O2•−, H2O2, MDA, and higher EL in T. aestivum L. seedlings exposed to two levels of UV-B radiation: ambient (8.6 kJ m−2 d−1) and enhanced (ambient + 2.8 kJ m−2 d−1). G. max L. plants grown under UV-C light with 0.284 mW cm−2 intensity (20 min d−1) resulted in remarkably higher O2•−, H2O2, and MDA contents. Four hours of UV-B radiation causes higher levels of EL, MDA, and O2•− contents in Morus alba seedlings under dark conditions. Enhanced ROS and oxidative stress biomarkers reveal that UV-radiation poses an oxidative stress condition.

35Stomata, a crucial interface for gas exchange between plants and the atmosphere, are reported to be affected by the O3 concentration. O3 imposes phytotoxic impacts on plants via entering through stomata. Exposure to elevated levels of ozone induces oxidative stresses in plants via dissolving entered O3 in the aqueous phase of substomatal cavity producing excessive ROS beyond the scavenging capacity of a plant’s intrinsic defense machinery. Depending on the concentration and environmental situations, O3 affects the plants to different degrees by causing specific biochemical and molecular responses. For plants acclimated to O3 stress, O3 signals can initiate PCD during biotic and abiotic stress conditions.

36The sudden reaction of O3 with membrane fatty acids motivates peroxidative processes. In pomegranate, O3 exposure caused a 10% increase in O2•−, 225% in H2O2, and MDA by 2-fold, compared to controls. Dolker and Agarwal demonstrated a significant elevation in MDA content in Ischaemum rugosum Salisb and Malvastrum coromandelianum L. under elevated O3 exposure of nine months. In wheat, exposure to O3 showed an increase in H2O2, O2•−, OH• and MDA levels. Lee et al. reported an increase in MDA under O3 (86 and 56%) stress compared to controls at 7 and 14 days after exposure, respectively. A similar trend was shown for O3 exposure regarding H2O2, O2•−, and •OH. Ozone-induced oxidative stress is also associated with the alteration in gas exchange, photosynthetic efficiency, and water relations.

37Under extreme pH conditions, the plasma membrane proton pumps try to combat the stress by influx and efflux of H+. Still, pH alteration and the excitation pressure lead to the toxic free radical generation together with the severe disruption of cellular and enzymatic activity failures. For instance, Bhuyan et al. investigated the effects of extreme acidic pH (3.5) on wheat (T. aestivum L. cvs. BARI Gom-21, 24, 25, 26, and 30) genotypes and increased H2O2 content together with increased LPO and LOX activity in all cultivars was found. Similarly, Liu et al. found increased cell membrane injury and MDA content in Medicago sativa L. cv. Gongnong No. 1, under alkaline stress (pH 11.2). Later on, Bhuyan et al. studied T. aestivum L. cv. BARI Gom-25 with both extremely acidic (pH 4.0) and alkaline (pH 8.5) pH and found increased levels of MDA and H2O2 and LOX activity pointed out oxidative stress under extreme pH. Therefore, the reviewed research findings presented in this section indicate that soil acidity and alkalinity stresses increase oxidative damage in plants.

38In addition, acidity stress accelerated toxic metals/metalloids (Fe, Cu, Mn, Zn, and Al) toxicity, together with essential nutrients (P, Mg, Ca, K, and Na due to substantial replacement of cations for H+) deficiency and considered as a major limiting factor for plant growth in acid soils. Similarly, alkalinity stress creates P, Fe, Zn, Mn, Cu, Mo, and B deficiencies. Contrarily, alkaline soils are characterized by B, Na, and Cl toxicities. Nutrient deficiency and metals/metalloids toxicity induced oxidative stress is well studied. Acidity induced N, P, K, Ca, Mg, and S deficiency as well as Fe, Mn, B, Zn, and Cu excess was found, with increased ROS level and oxidative stress as well as disrupted redox balance and antioxidant defense. Therefore, it could be assumed that acidity or alkalinity induced nutrient deficiency and the metals/metalloids toxicity might be one of the causes for oxidative stress in plants.

39On the other hand, plant cells require cytoplasmic pH 7.0–7.5 to maintain the normal physiological activities. It was reported that a single unit decrease of external growing media pH reduces 0.1 units of the cytoplasmic pH. Similarly, increases in external growing media pH causes precipitation of P and other metal ions, consequently increases the absorption of inorganic anions, and disrupts the ion balance. Therefore, both acidic and alkaline pH of growing media alter pH homeostasis, inactivate enzymes, and overgenerate ROS and creates oxidative stress and are considered as the major limiting factors for plant growth.

40Herbicides are often used in cultivated crop plants to easily control weeds. Still, the unconscious use of herbicides may cause oxidative stress in plants. Herbicides increase oxidative stress by overproducing ROS, which destroys plant cell membranes, lipids, photosynthetic pigments, and enzyme activities; therefore, they affect plant growth and productivity (Table 1). Herbicide glyphosate caused oxidative stress in plants by restricting the shikimate pathway, which leads to overproduction of ROS, which disrupted redox homeostasis. The application of glyphosate significantly inhibited the growth of H. vulgare L. in response to the higher accumulation of H2O2 (82% in leaves and 123% in roots) and O2•−, which increased the LPO (MDA; 45% in leaves and 104% in roots). Glyphosate application in tomato also increased H2O2 and O2•− by 40 and 100%, respectively. Liu et al. observed the increased MDA content with increasing the concentration of picloram in Eupatorium adenophorum. Oxidative stress is also observed in various plants after the application of paraquat. Oxidative stress indicators such as MDA, H2O2, and O2•− considerably increased by the application of paraquat in mustard. Besides, the application of the herbicide 2,4-D and its formulation imposed oxidative stress by increasing XOD and LOX activity in pea plants and concomitant increase in MDA content in M. aquaticum plants.

41Alves et al. applied different doses of fomesafen and sulfentrazone in Raphanus sativus, Avena sativa, Lupinus albus, and V. sativa, and, where a higher generation of TBARS and altered activity of CAT, APX, and GPX indicate the oxidative stress. Among different species, V. sativa, R. sativus, and L. albus showed a higher damaging effect from sulfentrazone (1.2 kg ha−1). Effect of different herbicide applications such as 2,4-D, metsulfuron, metribuzin, iodosulfuron, clodinafop, and bentazon, were studied in the wheat plant. Photosynthesis, transpiration rate, and stomatal conductance were reduced under herbicide toxicity. Lipid peroxidation, CAT activity, and phenols contents were higher, while chl and carotenoids were lower in herbicide stressed wheat plants. Due to the toxic effect of different herbicides, including oxyfluorfen, oxyfluorfen, and pendimethalin, rice plants showed phytotoxicity with reduced height, altered metabolism, accumulation of ROS, and alteration of non-enzymatic and enzymatic components of antioxidant defense machinery. Reduction in photosynthetic pigments, Pro accumulation, protein content, photosynthesis rate, and efficiency of carboxylation, as well as excessive generation of LPO was noticeable as an outcome of the phytotoxic effect of herbicides. A substantial increase in MDA and activity of GR and SOD indicated the paraquat-induced oxidative damage in soybean plants.

42Antioxidants are considered vital components for scavenging ROS, which play a critical role in abiotic stresses. Plants have developed a multifaceted antioxidant defense network to reduce ROS overgeneration under different abiotic stresses (Figure 4;). The antioxidant defense system consists of several antioxidants of low molecular weight (AsA, ascorbate; GSH, glutathione; non-protein amino acids; phenolic compounds; α-tocopherol; and some alkaloids) and antioxidant enzymes (SOD; CAT, catalase; POX, peroxidases; APX, ascorbate peroxidase; MDHAR, monodehydroascorbate reductase; DHAR, dehydroascorbate reductase; GR, glutathione reductase; GPX, glutathione peroxidase; GST, glutathione S-transferase).

43Ascorbic acid (vitamin C) is a strong water-soluble antioxidant, abundant in the active growing parts such as meristems, photosynthetic cells, root tips, flowers, and young fruits. Having the potential to donate electrons as a co-enzyme, the AsA participates significantly in scavenging ROS upon stresses. Ascorbate also participates in regenerating α-tocopherol from tocopherol radical scavenging O2•− and •OH. The growth and development of plants are considerably influenced by AsA production under abiotic stresses. By regulating cellular water status, AsA improved enzymatic detoxification of ROS (H2O2) to protect the cells. In plants, many phytohormone biosynthesis pathways are regulated by AsA. Therefore, the exogenous application of AsA increased plant growth by regulating hormonal balance and ion homeostasis. Reports suggested that exogenous AsA application maintained the growth and biochemical processes of cauliflower, wheat, cucumber, canola, soybean, rapeseed, and grapes.

44As a low molecular weight antioxidant and a non-protein thiol, GSH plays a critical function in regulating intracellular defense by scavenging ROS. Besides, GSH maintains redox homeostasis as a component of the AsA-GSH cycle. It also plays vital roles in detoxification of xenobiotics, signal transduction, transportation of sulfate, and metabolites conjugation. Glutathione also detoxifies atmospheric pollutants such as O3 and NO2. It also actively participates in recycling the AsA and α-tocopherol. Upon stress, the frequent stimulation of GSH indicated the possible role of regulating defense mechanisms.

45The antioxidant tocopherol is found as alpha (α), beta (β), gamma (γ), and delta (δ) forms; mostly synthesized in photosynthetic organs. They protect the photosynthetic membrane by scavenging ROS, mainly 1O2 and •OH.

46The carotenoids mainly accomplish three important activities: (i) absorbing light spectra (between 400 and 550 nm wavelengths), (ii) scavenging harmful ROS during photosynthesis, and (iii) protecting the complex light-harvesting proteins as well as stabilizing thylakoid membranes. Carotenoids have a polyene backbone in their structure, which consists of a series of C=C bonds. This particular characteristic is mainly responsible for pigmentation and ROS quenching ability.

47Flavonoids are low molecular weight and contain hydroxylation patterns in their molecular structure, indicating the antioxidant capacity. Flavonoids decrease cell damage in plants by scavenging free radicals and protecting cell membranes from LPO. The genes related to flavonoid biosynthesis are highly expressed under stress conditions; therefore, activating defense mechanisms by increasing flavonoid levels. Apart from the antioxidant activity, flavonoids can regulate auxin transport in vivo and give photoprotection. Moreover, flavonoids protect plants against UV light damage by absorbing UV radiation and act as sunscreens. Although light is essential for flavonoid biosynthesis, exposure to UV radiation induces higher levels of flavonoids in plants, which further act in the ROS removal mechanism in plants.

48Superoxide dismutase is a metalloenzyme that shows the frontline defense under excessive ROS generation. In most plant cells, the available SOD concentration is ~10−5 M. Based on metal co-factor at active sites, three main SOD types are described—Cu/Zn-SOD, Mn-SOD, and Fe-SOD. In the antioxidant defense network, O2•− is dismutased by SOD into H2O2, this also reduces the option of •OH generation via Haber–Weiss reaction (Figure 4;).

49The tetrameric haem-containing CAT enzyme rapidly decomposes H2O2, producing H2O and O2. All aerobic organisms contain CAT, a unique enzyme for ROS detoxification without any reducing equivalent. Among the antioxidant enzymes CAT possesses, the maximum turnover rate and 26 million H2O2 molecules can be converted by one CAT molecule in one minute. CAT activity is found in peroxisomes, mitochondria, and cytosol.

50Peroxidases are glycoproteins containing a polypeptide chain with 300–350 amino acid residues. The POX contains three domains—among which a proximal heme-binding domain and a distal heme-binding domain is identified, but the other one is still unknown. Peroxidase mainly oxidizes phenolic compounds (PhOH) and produces phenoxyl radical (PhO•), where H2O2 contributes to this reaction as an electron acceptor, and it is converted to 2H2O.

51Another class I heme-peroxidase is APX occurring in several isoforms (cAPX, cytosolic APX; mitAPX, mitochondrial APX; chAPX, chloroplastic APX; and microbody (including peroxisomal and glyoxysomal) APX), mAPX;. All the isoforms function to scavenge H2O2, but the activity stops without the presence of AsA. Within the AsA-GSH cycle, APX participates in detoxifying H2O2 and oxidizes AsA to produce monodehydroascorbate (MDHA) and subsequent dehydroascorbate (DHA) (Figure 4).

52In the AsA-GSH cycle, MDHAR is an NADH or NADPH-dependent flavin adenine dinucleotide enzyme containing a thiol group involved in the phenoxyl radical reduction and AsA regeneration from MDHA. MDRAH has several isoforms based on localization. Therefore, MDHAR genes are present in different cell components like mitochondria, chloroplasts, glyoxysomes, peroxisomes, and cytosol. Respective genes and locations are essential to knowing the role of each isoform. One MDHAR gene can produce two isoforms, and MDHAR genes can vary in different plant species (Figure 4; Table 2;).

53The enzyme DHAR, is monomeric and is included in the GST super-family, plays a vital role to regenerate AsA by an oxidative reaction, where DHA is recycled. In this process, GSSG is from the oxidation of GSH. As a result, the enzyme is also called GSH dehydrogenase or GSH:DHA oxidoreductase.

54In the AsA-GSH cycle, GR is another vital enzyme for regulating the redox homeostasis, which reduces GSSG to GSH (Figure 4; Table 2;).The reaction catalyzed by GR involves two steps—in the first step, NADPH reduces the flavin moiety, which is oxidized and reduced forming a disulfide bridge, which is redox-active and generates a thiolate anion and cysteine. In the next step, one GSSG moiety binds with cysteine forming a disulfide bond, finally releasing GSH.

55The enzyme GPX, is the non-heme peroxidase family member, utilizes GSH and thioredoxin, reduces H2O2, and protects cells from oxidative damage. Eight GPX proteins were identified in Arabidopsis, mainly found in the chloroplast, mitochondria, endoplasmic reticulum, and cytosol. The active site of GPX contains a thiol group (Cys residue) that can bind both GSH and thioredoxin, therefore, considered the redox regulating enzyme (Figure 4;).

56Glutathione S-transferases are a ubiquitous large enzyme family regulating versatile functions within plants. The enzyme has three superfamilies based on their localization, viz. cytosolic, mitochondrial, and microsomal. The enzyme is classified into various types, for example, phi, tau, lambda, and DHAR are found in plants; where phi and tau are highly responsible for environmental stresses mitigation. Moreover, it accelerates the activity of GPX and significantly reduces the reactive electrophile species generation (Figure 4;).

57Under drought stress, an adaptive strategy is activating the antioxidant defense system to fight against the oxidative stresses and to develop tolerance against drought in plants. The AsA and GSH are the strongest among the non-enzymatic antioxidants, which provide significant protection against drought-induced oxidative stress. In O. sativa, upregulation of AsA and GSH under drought stress (PEG, 15 and 20%) reduced oxidative damages. In B. napus cv. Bina Sharisha-3, higher AsA content was found under moderate stress level (10% PEG), but not in severe stress (20% PEG) levels. Moreover, in moderate and severe drought-stressed rapeseed seedlings, GSH content increased by 31 and 26%, respectively, compared to controls, whereas 83 and 225% increases in GSSG contents were also documented. Nahar et al. demonstrated reduced AsA and enhanced DHA contents, which ultimately reduced the overall AsA/DHA ratio by 54% in V. radiata L. under drought stress. The GSH/GSSG ratio was also reduced, compared to controls in drought exposed V. radiata L. seedlings. Elevated antioxidant enzyme activities also contributed to scavenging the toxic free radicals and protected plants from oxidative damage. Hasanuzzaman et al. revealed enzymatic antioxidants; DHAR, MDHAR, GR, and APX, played essential roles along with the non-enzymatic antioxidants (AsA and GSH) to alleviate drought stress-induced oxidative damage in B. napus L. cv. Bina Sharisha-3, which decreased the MDA and H2O2 contents. Upon drought (20% PEG) stress, toxic ROS overgeneration decreased by activated antioxidants in B. rapa L., where endogenous AsA and GSH levels increased together with a notable (23%) enhancement in APX activity, as well as increased CAT, GPX, and GR, activity by 29, 26, and 81%, respectively, compared to unstressed seedlings. Liu et al. investigated the drought stress (20% PEG-6000, 5 d) effects on O. sativa L. subsp. japonica cv. Nipponbare which demonstrated significantly decreased CAT, APX, and SOD activities and remarkably increased peroxidase (POD) activity by 59%. Rezayian et al. studied the effect of mild to severe drought stress (5, 10, and 15% PEG) on G. max L. and found increased SOD and POX activities at a high intensity of stress (15% PEG). Still, most uplifted activities of APX, CAT, and POX were found at low intensity of stress (5% PEG). Meanwhile, uplifted tocopherol (by 26, 26, and 21%) and total phenol (51, 32, and 44%) contents were also demonstrated at three levels of PEG-stresses, respectively. In T. aestivum L. cv. Sakha-94 enhanced GPX activity but significantly inhibited CAT activity under drought stress, compared to controls. When exposed to drought stress (irrigation at 60% of soil FC) for 20 days, an upregulated antioxidant defense system was also found in S. lycopersicum L. In this case, enzymatic antioxidants like APX, CAT, and SOD activities enhanced by 77, 110, and 66% in contrast, and non-enzymatic antioxidants viz. α-tocopherol, GSH, and AsA increased by 103, 93, and 81%, respectively, compared to controls (Table 2;).

58Variations in antioxidant activities impart differential salt stress tolerance to plants. These variations are present even at the plant organ level. For instance, AbdElgawad et al. studied antioxidant enzyme activity responses in different organs (roots, young leaves, and mature leaves) of salt-stressed maize plants. More specifically, they observed that DHAR and CAT activity increased in all studied organs while APX, SOD, GR, and GST increased in roots following NaCl application. Antioxidant activities of tolerant (BRS Bojuru) and sensitive (BRS Pampa) rice cultivars were significantly different under salt stress, where increased SOD and CAT activities were observed only in tolerant cultivars. Vighi et al. reported that OsGR2, OsGR3, OsAPX3, and OsSOD3-Cu/Zn genes were the markers to differentiate between sensitive and tolerant cultivars upon salinity stress. Chung et al. observed that the antioxidant-related genes GmCATs and GmAPX1 responds under salt stress conditions. Compared to the control plants GmCAT1, GmCAT2, and GmAPX1 expression was upregulated by 2- to 3-fold, 3- to 4-fold, and 8- to 9-fold, respectively, in soybean plants under salt stress. Such higher salt tolerances of tolerant cultivars are linked to higher antioxidant levels than salt-sensitive cultivars.

59Various protectants are reported to increase the salinity-mediated oxidative stress tolerance in plants via modulating antioxidant machinery. For example, salicylic acid and Si supplementation improved salt tolerance in wheat and mung bean, respectively, by increasing APX, CAT, and SOD activities that lowered H2O2 levels, EL, and MDA content. Rady et al. reported that pretreatment with Moringa oleifera leaf extract (MLE; 6%) better regulated both the non-enzymatic antioxidants (GSH, AsA, and AsA redox) and enzymatic antioxidants (APX, GPX, POD, SOD, and CAT) than Pro (12 mM) pretreatment for tackling salt stress. From the above reports, it is evident that alterations in the antioxidant system impart salinity tolerance in plants.

60The tolerance mechanism of plants to metals/metalloids-induced oxidative stress depends on the antioxidant defense capacity. It is positively correlated with the enhancement of the antioxidant enzyme activities and the improved synthesis of non-enzymatic antioxidants. Apart from the role of ROS scavenging, these components, both enzymatic and non-enzymatic, also help in metal chelation to some extent.

61El-Amier et al. observed that P. sativum L. seedlings when exposed to Ni (100 µM NiCl2, 72 h), GSH content was increased linearly up to 48 h and then declined, whereas GSSG content increased up to 36 h and then decreased. The later reduction of GSH and GSSG contents might disrupt the antioxidative system by Ni-toxicity stress. In rice seedlings, 0.25 and 0.5 mM NiSO4.7H2O enhanced both the GSH and GSSG contents, of which GSSG was further declined. Moreover, decreased AsA and increased DHA contents and ultimately reduced AsA/DHA ratio under Ni-stress was reversed by the exogenous application of Si (0.05 mM Na2SiO3) that signifies the ability of Si to upregulate the AsA-GSH cycle. Nickel stress also enhanced APX, MDHAR, DHAR, GR, GPX, and SOD activities, which were further upregulated by the supplementation of Si. The LOX activity was also increased under Ni stress, but Si application reduced LOX actions. Almost 3-fold higher AsA, 2-fold higher GSH, and 4.5-fold higher GSSG contents along with increased SOD and CAT activity were observed in Cajanas cajan seedlings when exposed to 10 µM of As (as Na3AsO4) for 5 days. With higher levels of As stress (150 and 300 µM Na3AsO4, 35 d) increased activities of SOD, CAT and POD were reported in two different cultivars of Chenopodium quinoa, and these enzyme activities were further upregulated with dimethylthiourea (5 mM) treatment.

62The plants typically use antioxidant protection systems to deal with HT stress, which varies among genotypes. Mansoor and Naqvi observed the elevated activity of SOD, POD, and APX in all genotypes of mung bean (V. radiata L.) at 40 °C except CAT. Sarkar et al. observed an increased activity of CAT, GSH, APX, and POX in all tested wheat cultivars at 30 °C. However, all these enzyme activities decreased in heat-sensitive cultivars, compared to tolerant ones at 35 °C. A similar result for enzymatic antioxidants was reported by Ding et al. in cucumber (C. sativus L.) seedlings, in which the activity of SOD, CAT, APX, GR, and POD increased by 16.6, 13, 25.2, 14.4, and 35.4%, respectively, under HT stress (35 °C), while GSH down-regulated as a non-enzymatic antioxidant. In contrast, Awasthi et al. found a higher content of GSH in chickpea (Cicer arietinum L.) plants at HT stress conditions. Djanaguiraman et al. reported that due to the lower activity of CAT and POX during HT stress, SOD activity decreased, and ROS toxicity increased in both pistils and pollen grains of both pearl millet and sorghum plants. However, in cabbage and kale plants, Soengas et al. found a higher activity of SOD, CAT, and GR, compared to controls under HT stress (32 °C), while compared with cabbage, the activity of SOD recorded was higher in kale plants. In a greenhouse experiment, Sarwar et al. exposed cotton plants to HT stress and treated them with growth regulators (water and H2O2). After the treatment duration, they observed that the H2O2 treated plants showed higher SOD (0.75, 0.33, and 0.36-fold) and CAT (2.26, 1.19, and 1.31-fold) activity than plants with water under high, medium, and optimum thermal regimes, respectively.

63Numerous studies have revealed that the elevated SOD activity in plants mediates LT tolerance by scavenging H2O2. Unlike SOD, CAT and APX also catalyze the H2O2. Wang et al. found the higher activity of CAT in indica 93-11 after 24 h of LT stress, while activity declined with increased stress duration. Shi et al., Wani et al., and Mohammadrezakhani et al. observed that due to the LT stress, CAT activity increased significantly in Cynodon dactylon, Capsella bursa-pastoris, and Citrus reticulata. Whereas, higher APX activity in Jatropha macrocarpa even under high H2O2 levels improved LT stress resistance while J. curcas could not withstand LT condition due to the reduced (>6-fold) activity of APX. Cheng et al. kept watermelon (Citrullus lanatus) plants at 10/5 °C for 7 days and observed that GSH/GSSG and AsA/DHA ratios were increased by 30.6 and 214.3%, respectively, compared to controls, which is related to antioxidant defense system activation. Han et al. treated 14-day-old rice seedlings at 12 °C for 6 days and reported that the GSH and GSSG contents as well as GSH/GSSG ratio were increased significantly along with higher activities of SOD, POD, and CAT. Chen et al. also reported that at 4 °C, GR, DHAR, and MDHAR activities increased by 20.26, 7.64, and 16.60%, respectively, and AsA, DHA, and GSH also increased by 12.13, 7.89, and 56.09%, respectively, compared to controls, resulting in direct scavenging of ROS. From the above examples, it is obvious that modulation in antioxidant machinery exerts a positive impact in LT tolerance ability.

64Several crop species exhibit survival abilities under waterlogged or flooded conditions for short or even longer durations sometimes; rice can be a good example. In addition to formation of aerenchyma and adventitious roots, plants may also exhibit the ability of modulating antioxidant defense systems to combat the waterlogging-induced oxidative damages. Some cereal crops like Z. mays L., S. bicolor L., and H. vulgare L. have been shown to be damaged at the cellular level by higher LPO and ROS and further antioxidative defense response. Upregulation or downregulation of these antioxidants has also been reported in G. max L., L. esculentum, Sesamum indicum L., and Spinacia oleracea L.. da-Silva and do Amarante studied WL modulated upregulation of antioxidant defense system in G. max L. and observed higher activities of APX, CAT, and SOD in stressed plants along with higher ROS and MDA accumulation. Moreover, reduced AsA with a higher DHA level indicated ROS scavenging by spending AsA. Waterlogging stress also showed significant variation in SOD, CAT, and POD in B. napus L., regardless of its growing condition (pot or field), along with higher MDA. Some sensitive and tolerant species of Prunus were subjected to WL, and upregulation of antioxidants (AsA and GSH contents; CAT and POD activities) were recorded, which indicated the existence of ROS scavenging systems in tolerant cultivars (Prunus pseudocerasus and P. cerasus × P. canescens).

65While studying the impact of HL, Shengxin et al. disclosed that the activity of SODs, CAT, and POD decreased in rapeseed seedlings under HL conditions. Similarly, Lu et al. observed decreased POD and SOD activity due to HL stress in tomatoes. In contrast, SOD, CAT, APX, and POD activities upregulated in the moderate light treated Chl-8 rice genotype leaves than Zhefu 802. In HL stress, higher SOD and CAT activity was observed in pgl rice genotype compared to Zhefu 802. Lima et al. demonstrated decreased CAT activity along with upregulated APX and SOD activity in HL exposed (12 h) cashew plants. Furthermore, as total AsA content decreased by 25%, the total GSH content increased by 63% in HL exposed (12 h) cashew plants. Similarly, HL stress down-regulated APX2, DHAR1, CDS1, CDS2, and FDS2 while upregulated APX1, CAT2, and FDS1 in Arabidopsis mutant (pgr5), compared to WT (Table 2;). Flavonoid biosynthesis is light-dependent. Higher light intensities stimulate the flavonoids synthesis in Passiflora suberosa L., which protects the plants from HL damage, as documented by Ni et al.. Moreover, Stewart et al. narrated about water-insoluble antioxidant carotenoids (the xanthophylls zeaxanthin and lutein as well as β-carotene), which maintain the higher growth rate in Lemna gibba L. even under very HL (700 µmol photons m−2 s−1, 7 d). It resulted from a combination of the decline in the chl synthesis and higher zeaxanthin accumulation, which restricted the accumulation of excessive energy from excitation.

66Tripathi et al. exposed T. aestivum L. seedlings to two levels of UV-B radiation: ambient (8.6 kJ m−2 d−1) and enhanced (ambient + 2.8 kJ m−2 d−1) resulted in upregulated CAT and GPX activity along with reduced SOD and APX activities. Such a reduction in SOD activity was also observed in A. thaliana L. (genotype rsr4-1). In contrast, genotype C24 showed increased POD, APX, and GPX activities exposed to UV-B radiation (3.9 kJ m−2, 4 d). UV-C radiation-induced enhancement of SOD and POD activities was also reported in G. max L.. Seedlings of G. max L. grown under UV-C light with 0.284 mW cm−2 intensity (20 min d−1) resulted in enhanced activity of CAT, SOD, and POD that was further upregulated with exogenous application of AsA (10 mg L−1). In Olea europaea L. cv. Galega Vulgar plants, 23.5, and 21.7% higher free radical scavenging capacity (ABTS) was recorded in two levels of UV-B (6.5 and 12.4 kJ m−2 d−1) treatments, respectively. As a result, the levels of H2O2 were almost similar to control plants. However, in addition to such antioxidant defense responses, some terrestrial plants have also been documented to synthesize some protective phytochemicals (e.g., AsA, hydroxycinnamates, flavonoids, vitamins, etc.) when exposed to UV-B radiation.

67Ozone causes various restrictions in plants such as visible foliage injury, early senescence of leaves, stomatal closure, and inhibition of carbon transport, et cetera. Of these, stomatal closure is considered a direct cause of the yield reduction of plants as it reduces carbon uptake/carbon fixation and ultimately induces reduction in photosynthesis. Hence, decreased transportation of fixed carbon towards edible plant parts, for example, grains result in yield reduction due to low supply of source towards sink. Furthermore, O3 directly disturbs the carbon translocation in phloem from leaves to other parts such as roots or edible portions and thus has more influence on yield reduction. Under such scenario, a plant’s ability to withstand O3 stress can stabilize yield reductions. In this regard, antioxidants can play a vital role in regulation and detoxification of toxic ROS. The influence of O3 in regulating antioxidant activities in two rice cultivars was studied by Tammam et al.. Both rice cultivars showed higher SOD and APX activity in shoots, but GR and GST activities were reduced in both shoots and roots. In a two-year study on rice cultivars with contrasting ability towards O3 stress tolerance, Wang et al. demonstrated that CAT, POD, and GPX activities were decreased in O3 sensitive cultivars. The authors concluded that O3 tolerance improved at different growth stages via regulating antioxidant activities in tolerant cultivars. In another study, O3 exposure-induced ROS production was scavenged via regulation of antioxidant activities in wheat leaves.

68Both acidity and alkalinity stress altered the antioxidant defense system of plants. Uplifted APX, CAT, SOD, and POD activities under acidic pH were found in rice and tomato, but Bhuyan et al. found down-regulated SOD and DHAR activities while GR activity remained unchanged in wheat seedlings subjected to acidity stress. Similarly, a higher pH level (alkalinity stress) resulted in lower antioxidant enzyme activities in rice. In contrast, elevated APX, CAT, SOD, and POD activities were increased in Z. mays L. under alkaline stress. Moreover, the redox balance of AsA/DHA severely declined. Although GSH content increased, GSSG content decreased, while GPX, GST, and GR activities were also reduced. Later on, Bhuyan et al. observed the affect of acidity (pH 4.0) and alkalinity (pH 8.5)-stresses on T. aestivum L. and found decreased AsA and GSH contents, with upregulated APX, CAT, GPX, and GST activities under both acidic and alkaline-stress.

69The plants can mitigate herbicide-induced toxicity by two resistance mechanisms. The first one is target-site resistance, in other words, structural modification or activity enhancement of herbicide targeted proteins. The second one is non-target site resistance, in other words, developing the herbicide detoxification system. The herbicide detoxification system contained three enzymatic phases: (i) in phase I, CytP450 monooxygenases; (ii) in phase II, GSH transferases and glycosyl transferases; and (iii) in phase III, tonoplast localized ATP-binding cassette transporters, while GST showed a major role in this system. Several reports proved the antioxidant defense mechanisms in plants to reduce herbicide toxicity. Glyphosate toxicity alleviated in H. vulgare by improving the activity of antioxidant enzyme CAT (80% in leaves and 46% in roots), APX (106% in leaves and 97% in roots), and GST (61% in leaves and 95% in roots). The activity of SOD, POD, CAT, and APX improved in Salvinia natans, which reduced glyphosate-induced oxidative stress. In T. aestivum, Hasanuzzaman et al. reported increased enzymatic antioxidant activities; and AsA and GSH contents to mitigate paraquat toxicity. Higher GSH content and high CAT, APX, GST, and GR activities were also observed to improve antioxidant defense in imazapic-induced Nicotiana tabacum.

70It is well established that ROS play a vital role in regulating various responses under both biotic and abiotic stresses in plants. The domain of plant stress is quite vast, and the number of studies grew dramatically over the last 20 years. The interesting phenomenon among all these studies is the dual role of ROS in plants. Firstly, they are unavoidable toxic metabolic by-products; and secondly, they play signaling roles during stress conditions. Although it seems that ROS are damage causing agents in plants, their importance for stimulating the stress signaling component to stop further losses is also notable. Even with the continuous increase of stress-related publications, there exists quite little novelty in the information. Most reports just confirmed the previous results rather than bringing new insights into this domain. Therefore, there should be attempts at new approaches for obtaining the novel aspects regarding ROS metabolism. It is still an open question of how plants feel stressed and get ready for upcoming stress threats. Due to their highly reactive nature and short half-life, further research is needed on ROS chemistry and metabolism. For instance, different aspects related to ROS metabolism regulation, particularly under multiple environmental stresses, which remain unanswered. Therefore, future studies should focus on more polished techniques such as advanced imaging techniques, fluorescence in situ hybridization (FISH), advanced functional genomics, metabolomics, and proteomics to better understand the ROS metabolism in crop plants. Exogenous ROS application at proper dose and duration to improve signaling cascades and subsequent stress tolerance also warrants further research.