Regulation of Reactive Oxygen Species and Antioxidant Defense in Plants under Salinity

The generation of oxygen radicals and their derivatives, known as reactive oxygen species, (ROS) is a part of the signaling process in higher plants at lower concentrations, but at higher concentrations, those ROS cause oxidative stress. Salinity-induced osmotic stress and ionic stress trigger the overproduction of ROS and, ultimately, result in oxidative damage to cell organelles and membrane components, and at severe levels, they cause cell and plant death. The antioxidant defense system protects the plant from salt-induced oxidative damage by detoxifying the ROS and also by maintaining the balance of ROS generation under salt stress. Different plant hormones and genes are also associated with the signaling and antioxidant defense system to protect plants when they are exposed to salt stress. Salt-induced ROS overgeneration is one of the major reasons for hampering the morpho-physiological and biochemical activities of plants which can be largely restored through enhancing the antioxidant defense system that detoxifies ROS. In this review, we discuss the salt-induced generation of ROS, oxidative stress and antioxidant defense of plants under salinity.


Introduction
Abiotic stresses are closely associated with climate change, and they hamper the growth and development of plants; consequently, the yield and quality of crops are also negatively affected. Therefore, the sustainability of global agricultural production is threatened by abiotic stresses [1,2]. Salt stress is one of the detrimental abiotic stresses which greatly reduces crop growth and productivity [3]. Around the world, about 20-50% of irrigated land areas are affected by salt stress [4]. An increase in the salinity level in plant growing media leads to an increase in endogenous sodium (Na + ) and chloride (Cl − ) contents. Both Na + and Cl − ions can create life-threatening conditions for plants, but between them, Cl − is more dangerous [5]. Salinity-induced osmotic stress, ionic stress and nutrient imbalance as well as their secondary effects altogether lead to the overgeneration of reactive oxygen species (ROS) [5,6].
Oxygen radicals and their derivatives, called ROS, are produced by different cellular metabolisms in various cellular compartments. The major ROS are hydrogen peroxide (H 2 O 2 ), superoxide (O 2 •− ), singlet oxygen ( 1 O 2 ) and the hydroxyl radical ( • OH) [2,7]. Although ROS production is a general phenomenon and a part of cellular metabolism in plants, environmental stresses lead to excess generation of ROS which are not only highly In plant cells, ROS production occurs due to incomplete or partial reduction of oxygen molecules. Therefore, ROS production is a general phenomenon because they are produced as a result of the oxidation-reduction reaction of several metabolic processes in multiple locations and compartments of plant cells. However, abiotic stress including salinity increases ROS generation which exceeds the equilibrium between antioxidant defense and ROS production [2,7]. In plant cells, a prominent source of ROS production is spilling electrons from the ETC of photosynthesis and respiration, transition metal ion decompartmentalization and redox reactions [7]. Several reactions associated with ROS production are depicted in Figure 2.

ROS Generation in Plant Cells
Reactive oxygen species are the byproduct of aerobic metabolism in different cell organelles such as chloroplasts, mitochondria, peroxisomes, plasma membranes and cell wall (Figure 3 [24,25]). A specific ROS generation in a cell is highly localized, and different pathways are intensively involved in this process [26,27]. For a better understanding of ROS scavenging tactics in different subcellular compartments, at first, it is obligatory to study the subcellular compartment-specific ROS generation in cells. •− -superoxide anion; XOD-xanthine oxidase; SODsuperoxide dismutase; NADPH-nicotinamide adenine dinucleotide phosphate; UO-urate oxidase [24].
Rigid cell wall formation is accelerated under stress conditions when ROS along with POD trigger the polymerization of glycoproteins and phenolic compounds [28]. This cell wall-related POD catalyzes H 2 O 2 in the presence of NADH, where the NADH is derived from malate dehydrogenase [28]. In addition, diamine oxidases reduce diamines or polyamines (PAs) to quinine and thus produce ROS in the cell wall [29].
Free radicals are also produced in the plasma membrane under abiotic stress. The higher generation of O 2 •− in the plasma membrane is reconciled by NADPH oxidase and quinine reductase where NADPH oxidase transports the electron from the cytoplasmic NADPH and forms O 2 •− which, later on, is converted to H 2 O 2 [30]. Chloroplasts are considered as the prime site for ROS production that relies on the interactions of chlorophyll (chl) and light [28,31]. Under stress conditions, stomatal conductance (g s ) and the CO 2 assimilation rate are greatly reduced and thus form excited triplet chlorophyll ( 3 Chl*) that impedes in photosynthetic ETC, and promote ROS overgeneration [32,33]. Photosystems I and II (PS I and PS II) are the major sites in chloroplasts where 1

O 2 and O 2
•− are largely produced [2]. However, the amount of O 2 •− produced in PS I through the Mehler reaction is converted to H 2 O 2 with the help of SOD.
In the non-green parts of plants, mitochondria are the major site for ROS generation [2]. ROS produced in mitochondria reduce both mitochondrial energy transportation and other subcellular functions. Respiratory complexes I and III are the main sources of mitochondrial ROS, especially O 2 •− . However, the produced O 2 •− in both complexes due to electron leakage is eventually catalyzed by Mn-SOD and Cu-Zn-SOD and produces H 2 O 2 [33,34].
Another vital site for ROS production is the peroxisomes, where a number of oxidases catalyze different reactions and generate H 2 O 2 and O 2 •− as byproducts. It is considered that glycolate oxidase (GOX) is the main source of ROS production in peroxisomes [35]. This GOX in peroxisomes causes stomatal closure; as a result, the stomatal gas exchange rate is greatly reduced and thus reduces CO 2 for RuBisCO generation, causing photorespiration and H 2 O 2 production [36]. In addition, xanthine oxidase (XOD) activity can also generate O 2 •− and uric acid in the peroxisomal matrix, which are further dismutased to H 2 O 2 by metalloenzymes SOD and urate oxidase, respectively [37,38].
In the endoplasmic reticulum (ER), both O 2 •− and H 2 O 2 are produced from the fatty acid oxidation by GOX and urate oxidase activities [39]. In addition, a small amount of O 2 •− is generated in the ER as a byproduct of oxidation and hydroxylation processes which involve cytochrome P450 and cytochrome P540 reductase in the presence of reduced NADPH [27].
Compared to other cell compartments, the ROS production rate is comparatively lower in the cytosol where the redox balance is highly maintained by cytoplasmic NADPH as a central component. However, besides ROS production, the cytosol conducts a pivotal role in the redox signaling process in plant cells. In general, the ROS signaling from different cell organelles passes through the cytoplasm to modulate gene expression in the cell nucleus [40].
Different enzymes contribute to the generation of ROS in apoplasts, among which quinine reductase, NADPH oxidase, SOD and POD are the most important [10,12].

Consequence of ROS in Plant Cells
Overproduction of ROS disrupts the equilibrium between ROS accumulation and scavenging that ultimately damages different cellular biomolecules through protein oxidation, lipid peroxidation, enzyme inactivation, chl degradation and destruction of nucleic acids under stressful conditions ( Figure 4; [8,13,41]). However, various factors influence the extent of biomolecular damage which include the concentration of the biomolecule(s), the location of the target biomolecule(s), the site of ROS generation and the reaction rate between the target biomolecule(s) and ROS [13].
The lipid membrane of cells is damaged by oxidative burst, resulting in lipid peroxidation. The extent of peroxidation differs quantitatively between underground and aboveground plant tissues depending on the types of ROS, which is measured by the content of the final product, malondialdehyde (MDA) [29,42]. However, lipid peroxidation not only hampers membrane permeability but also causes electrolyte leakage (EL) and deactivation of enzymes and receptors, as well as accelerating the oxidation process of nucleic acids and proteins [43].
It is well documented that ROS are involved in protein oxidation. However, not all ROS attack all proteins; rather, they cause protein denaturation in a selective manner, among which • OH is the most notorious in nature, causing damage to protein molecules non-selectively [2]. Oxidation of proteins can be both an irreversible and reversible process. Enzymes that contain Fe-S in the center can be damaged irreversibly by the O 2 •− radical, and such type of damage causes cellular dysfunction. On the contrary, glutathionylation and S-nitrosylation are reversible changes that can mediate the redox regulation in plants [44].
Nucleic acids are the structural components of proteins and DNA which are rapidly oxidized by the action of ROS. Both chloroplastic and mitochondrial DNA are largely oxidized, compared to that of nucleus DNA, due to their proximity to the ROS production site. Among free radicals, • OH is the most pernicious damaging radical for DNA that modifies nucleotide bases (purine and pyrimidine) by oxidizing the sugar residues in the DNA strands [45]. The DNA replication or transcription process is permanently ceased if the damage induced by ROS is completely irreparable [46]. Consequently, many biochemical processes such as irregular protein synthesis and damage of photosynthetic proteins are directly arrested by DNA damage. In addition, signal transduction, transcription, replication errors and, as a whole, genetic instability are the common fate of DNA due to oxidative stress [13].
In plants, the glycolysis pathway and the enzymes of the TCA cycle are the first targets of a free radical attack. For instance, glyceraldehyde 3-phosphate dehydrogenase and fructose-1,6-bisphosphate aldolase are two common enzymes of the pentose phosphate pathway which are inhibited while ROS production is increased due to oxidative stress [2,47].

Plant Responses to Salinity
Plants are posed with salt stress through two mechanisms. One is osmotic stress, which is a rapid mechanism (within minutes to days), hinders the water uptake and is responsible for stomatal closure, ultimately reducing cell expansion and division. Additionally, the other mechanism, which is slower (days to weeks), is ionic toxicity, which creates an ionic imbalance, disrupts ionic homeostasis and cellular functions and also causes premature senescence. Salt stress is detrimental to plants during the germination, growth and development stages and results in significant yield reduction ( Figure 5). Excess accumulation of the salt concentration initiates changes in the mineral distribution, vulnerability of the cell membrane, loss of integrity and a reduced turgor pressure as a result of ionic disequilibrium [48], and, in the extreme case, salt stress results in plant death [14]. Salinity imposes both osmotic stress and ionic toxicity on plants by impairing water uptake, stomatal opening and the ionic balance. As a result, the morphological, physiological and anatomical characteristics of plants start to show changes negatively and finally cause yield loss.

Germination
Salinity reduces the germination of seeds by its osmotic properties and toxicity mechanism at the time of germination. Moreover, seedling establishment is also greatly hampered in salinity as it impedes imbibition, disturbs active metabolism and hinders embryonic tissue development [49]. In Oryza sativa, salt stress can cause an adverse effect on the germination stage by inhibiting gibberellic acid (GA) activity in the seed, which, in turn, can curtail seed germination by up to 71% [50]. A similar trend of decline was reported by Shu et al. [51], where it was explained that salt stress favors ABA synthesis while inhibiting the GA synthesis pathway. This phenomenon creates an imbalance in the GA/ABA ratio and in turn, the GA content is decreased, which is beneficial for the seed germination process [52]. During germination, a higher concentration of salt causes osmotic stress in water-deficient conditions that can reduce the Triticum aestivum germination rate [53]. Moreover, high salinity has the potential to increase the mean germination time while decreasing the germination rate and, subsequently, lower the germination percentage of Helianthus annuus L. [54].

Growth
Salt stress hampers cell prolongation in growing tissue, which, in turn, reduces the leaf area and dry matter assimilation in the plant [5]. In the case of long-term salinity, the plant exhibits a lower photosynthesis rate, lower nutrient storage and less growth hormones [55]. Growth reduction upon salt exposure of seedlings can be found in the reduced root and shoot weights of Solanum lycopersicum. Additionally, the damage is more prominent in the root fresh weight (FW) and dry weight (DW) by 40 and 35%, respectively, compared to the shoot FW and DW by 19 and 29%, respectively [56]. Accumulation of Na + and Cl − in leaf tissue modified plant growth hormones, enzyme activity, stomatal closure and photosynthetic activity that resulted in lower assimilation of CO 2 and, finally, a reduced plant height and DW under salinity [57].

Photosynthesis
Salinity generates an unfavorable condition for plant photosynthesis by affecting g s , sap flow and the transpiration rate (T r ) through accumulating higher concentrations of NaCl within plant cells [58]. Shortage of the net photosynthesis rate (P n ), T r and g s resulted in a reduction in the chl a, chl b and carotenoid contents in S. lycopersicum, and, finally, photosynthesis was hampered to a great extent [59]. A decline in the chl a, chl b and carotenoid contents has also been recorded in T. aestivum [60] and S. lycopersicum [61] along with a reduced chl a+b content. In the salt stress condition, the plant develops an increased chl a/b ratio compared to the stress-free condition, and this imbalanced condition triggers reduced photosynthesis in plants [62]. Importantly, salinity can develop a physiological drought condition in the plant, which acts on stomatal closure and lessens the photosynthetic CO 2 assimilation as well [63]. Plants exhibit chl degradation even at short exposure to salinity and can reach a high severity level with the prolongation of stress [64].

Ionic Imbalance
Ionic imbalance or toxicity is known as a secondary effect of salt stress that causes disturbance in the plant life cycle. Upon salt exposure, imbalance of nutrition is presumed as a primary phenomenon as it can hamper the supply, uptake and translocation of nutrients in plants [65]. Salinity has the capacity to depolarize the root plasma membrane while inducing the guard cell outward-rectifying K channels, which, in turn, results in a higher Na + content with a lower K + content in the plant [66]. Excess Na + increased the Na + /K + ratio while decreasing calcium (Ca 2+ ) and Mg 2+ in both the roots and shoots of S. lycopersicum seedlings [56]. A similar trend was recorded in Lens culinaris [64] and Vigna radiata [67]. An excess Na + concentration causes disturbance in plant metabolic activities by replacing the K + content from the enzymatic components of the cell [68]. Moreover, lower uptake of K + , Ca 2+ and Mg 2+ , caused by excess Na + accumulation, subsequently induces an imbalance in mineral homeostasis and ionic stress in the plant [61,69]. Rahman et al. [70] found that under salt stress, ion homeostasis was hampered in O. sativa seedlings by the increment in Na + and the Na + /K + ratio, compared to the stress-free condition. In this study, the shoots exhibited a higher concentration of Na + than the roots. Along with the reduction in K + , Ca 2+ and Mg 2+ , salt stress also negatively affected the zinc (Zn) content, both in the roots and shoots of the plant.

Water Relation
At a low level of salinity, the plant may not show a difference in water uptake, but at high levels of salt stress, the plant, especially its shoot, is found to be injured greatly with a reduced relative water content (RWC) and a dehydrated condition at the cellular level [71]. S. lycopersicum was recorded to be affected by this osmotic stress under salinity, expressed through a reduced RWC, which was also found to be restored after the recovery period [61]. In addition, nutrient imbalance is a common phenomenon in salt stress conditions, caused by reduced water uptake and translocation, as a consequence of accumulated Na + and Cl − ions in the cytoplasm [72]. A reduction in the RWC was also reported in T. aestivum [60], O. sativa [70,73] and Sorghum bicolor [74].

Anatomical Modifications
Besides morphological and physiological changes, salinity may also alter the anatomical characteristics of a plant through modifying the cell wall and nucleus components and leaf structure, especially the ultrastructure of leaf chloroplasts. Due to the close proximity to the saline-affected soil, salt-induced roots were seen to be hampered by the disrupted nuclei and nuclear membrane of T. aestivum [75]. Similar results were recorded in S. lycopersicum [76], where salinity changed the root structure through decreasing different layers of the columella, cortex cells and cell sizes and enlarging cell nucleoli, ultimately leading to transforming the cell shape and vacuoles. Interestingly, salt-induced Brassica napus roots were seen to produce an apoplastic barrier near the root apex to moderate the over-accumulated toxic ions [77]. Salinity causes an alteration in leaf structure by reducing the thickness of the epidermis and mesophyll and, consequently, causes disturbance in water uptake and turgidity [78]. The stomatal frequency was found to be reduced by 10.5 and 22% in B. juncea L. [79] and T. aestivum [80], respectively, at the salt-induced condition, with a partially closed stomatal aperture, compared to the stress-free condition. Upon salt exposure, the chloroplast ultrastructure was recorded to change by distorting thylakoids and compressing granular and stroma lamellae with massive plastoglobuli, subsequently resulting in a reduced photosynthetic pigment content of the plant [81].

Crop Yield
Salt stress causes alterations in the morphological and physiological characteristics of plants and, subsequently, reduces the yield to a great extent. For example, upon exposure to salt, the yield-contributing characteristics of H. annuus, e.g., head diameter, 100-seed weight and oil percentage, were found to be reduced 24, 28 and 5%, respectively, with a severe (26%) seed yield reduction [82]. A similar trend was found in Hordeum vulgare, where the tiller number, spike length and 100-grain weight were reduced by 53, 40 and 41%, respectively, in saline conditions [83]. Apart from this, the salt-induced condition interrupts the fertilization process at the time of grain filling through reducing pollen viability and stigma receptivity along with providing minimized photoassimilates. This incident ultimately leads to a yield reduction in the grains [84].

Crop Quality
Salinity causes variation in the qualitative characteristics of plants such as the sugar, citric acid, cellulose and oil contents according to the level of concentration. Upon salt exposure, the cellulose content and sucrose movement are greatly hampered [85]; as cellulose deposition is known as the prime component of fiber quality, this reduction affects the fiber quality in a dose-dependent manner. Salt-induced Mentha piperita (a beneficial medicinal and aromatic plant) was seen to lose quality in respect to essential oil and menthol contents compared to the control condition as salinity hampers its growth, photosynthesis and nutrient content to a great extent [86]. Salt-induced S. lycopersicum performed positively in respect to electrical conductivity (EC), total soluble solids (TSS), titratable acidity, citric acid content and oxide reduction potential (ORP) [62,87]. As plants produce more soluble sugars and organic acids to compete with the ionic toxicity, generated from Na + and Cl − ions, this can be the reason behind the increment in EC, TSS and titratable acidity in S. lycopersicum [88]. Moreover, in saline conditions, plants exhibit early maturation, which promotes sugar accumulation within the fruit and, subsequently, increases TSS [89].

Salinity-Induced Oxidative Stress in Plants
When plants are exposed to salinity, the concentration of Na + and Cl − ions increases abundantly in the soil, which are accumulated at a higher rate, thus reducing the essential ions in plants. Hereby, it disrupts the plant-water relation by creating drought-like conditions, resulting in osmotic stress which is liable for the reduction in g s and photosynthetic enzyme activities, leading to ROS generation in plants [90]. Besides this, it also triggers ionic stress by inhibiting K + accumulation and interrupting the nutrient balance in plants; thus, by altering redox homeostasis, salinity hampers the flow of electrons from central transport chains to oxygen reduction pathways in different organelles, which leads to the overgeneration of ROS in plants [14]. Other activities such as carbon metabolism, ion redis-tribution, ABA accumulation and alkalization are also interfered with upon exposure to salinity and foster ROS generation in plants ( Figure 6; [91]). These overproduced ROS are accountable for the damage of nucleic acids and oxidation of different biomolecules, i.e., proteins, lipids, carbohydrates and DNA. Thus, fluctuations occur in their functions and properties, leading to physiological and biochemical alterations and, ultimately, creating oxidative stress in plants, Table 1 [6,10]. Mohsin et al. [60] reported that when T. aestivum seedlings are exposed to salt stress at the rate of 150 mM NaCl (mild) and 250 mM NaCl (severe) for 5 d, MDA and H 2 O 2 contents were increased by 63 and 116%, and 78 and 108%, respectively, compared to the unstressed seedlings. Similarly, Siddiqui et al. [92] showed increased MDA (by 116%), H 2 O 2 (by 198%) and O 2 •− (by 263%) under salt-stressed (100 mM NaCl) seedlings of S. lycopersicum, while increased H 2 O 2 by 58 and 97% and MDA by 74 and 113% were found under 100 mM and 160 mM NaCl stress, respectively, compared to the controls [56]. Recently, a contrasting response was observed by Ali et al. [93], where the MDA content was decreased but H 2 O 2 was increased upon exposure to salt stress (150 mM NaCl) in S. lycopersicum plants. The extent of damage caused by salinity-induced oxidative stress could be varied among different portions of plants. In most cases, the roots suffer more compared to other tissues as they are directly in contact with the saline conditions, followed by the shoots and leaves [94]. Upon exposure to 100 mM NaCl, a greater increase in the MDA content (by 116%) was observed in the roots than the leaves (by 106%), whereas the H 2 O 2 content was greater in the leaves (by 149%) than the roots (by 34%), compared to untreated S. bicolor plants [95]. Similarly, Jabeen et al. [96] observed a higher amount of MDA content in the roots (44%) than the leaves (38%) compared to the control when Glycine max plants were exposed to 100 mM NaCl. Derbali et al. [97] evaluated four genotypes of Chenopodium quinoa (cvs. Tumeko, Red Faro, Kcoito and UDEC-5) under different doses of salt (100, 300 and 500 mM NaCl) and reported that both the MDA and H 2 O 2 contents increased in a dose-dependent manner in these four genotypes. However, the lowest accumulation of MDA and H 2 O 2 was observed in the salt-resistant genotype (cv. UDEC-5) compared to the salt-sensitive genotypes (cvs. Tumeko, Red Faro, Kcoito) at different NaCl levels. Derbali et al. [98] further reported that the post-stress recovery capacity was higher in the salt-resistant genotype of C. quinoa (cv. UDEC-5) compared to the salt-sensitive genotype (cv. Kcoito). From the above-mentioned examples, it can be stated that plant responses against salinity vary among species, and also different genotypes/varieties of the same species.

Antioxidant Defense System in Plants under Salinity
Plants have antioxidants that are employed in scavenging ROS by working in coordination with non-enzymatic antioxidants (ascorbate (AsA), GSH, phenolic compounds, flavonoids, alkaloids, α-tocopherol, non-protein amino acids, etc.) and enzymatic antioxidants (SOD, APX, CAT, DHAR, MDHAR, GPX, GR, GST, POD/POX polyphenol oxidase, PPO, PRX, thioredoxin (TRX), etc.), and that protect them from oxidative damage (Figure 7; [13,24]). In plants, SOD is the major antioxidant that activates the first line of defense against ROS-induced damage by converting O 2 •− into H 2 O 2 ; this is further accumulated by CAT, APX, GPX, PPO, PRX and TRX enzymes or catalyzed in the Asada−Halliwell pathway (AsA-GSH cycle) [124,125]. To detoxify ROS in the AsA-GSH cycle, plants have nonenzymatic antioxidants (AsA and GSH) and four enzymatic antioxidants (APX, DHAR, MDHAR and GR); thus, by minimizing ROS, they can maintain redox homeostasis in plants, Table 2 [2,8,24]. Furthermore, H 2 O 2 and other xenobiotics are also detoxified with the help of the GST and GPX enzymes [126,127]. Many researchers have proposed that the activities of these antioxidants depend on the salinity threshold, duration of salinity exposure and growth stages of plants [128]. For instance, Ali et al. [129] found altered antioxidant enzyme activities in scavenging ROS under two concentrations of NaCl (80 mM and 160 mM), with a maximum reduction in SOD, POD, APX and CAT of 49, 43, 39 and 52% in cv. P1574, and 67, 46, 47 and 61% in cv. Hycorn-11, respectively, which are two Z. mays cultivars, at 160 mM NaCl stress and concluded that the P1574 cultivar is more salt-tolerant than the Hycorn-11 cultivar. Similarly, upon exposure to 100 mM NaCl, total phenols (by 60%) and AsA (by 55%) were increased in S. lycopersicum plants together with enhanced SOD, CAT, POD and PPO activities [130]. Jiang et al. [131] highlighted the TaSOS1 gene in response to salt stress in two spring T. aestivum genotypes, Seri M82 (salt-sensitive) and CIGM90.863 (salt-tolerant), and observed a higher expression of most of the 18 TaSOS1 genes in the roots of salt-tolerant seedlings than the salt-sensitive seedlings. Recently, several exogenous protectants (i.e., GA, salicylic acid (SA), melatonin (MT), silicon, selenium) have been used to upregulate the antioxidant machinery in plants under salinity [132][133][134][135]. For instance, Ahmad et al. [133] found increased activities of SOD (9%), APX (13%), CAT (26%), GR (40%) and POD (98%) when seed priming was conducted with 0.5 mM GA. Foliar spraying of SA (0.5 mM) enhanced AsA, GSH, total phenols and anthocyanin biosynthesis and increased SOD and CAT activities, thus reducing the H 2 O 2 and O 2 •− content in salt-stressed seedlings of Vicia faba [134]. Combined application of silicon and selenium has been reported to increase CAT, GR, SOD and APX activities and synthesis of AsA and GSH [135]. Pretreatment of seeds of Avena nuda with MT (50 or 100 µM) upregulates CAT, APX, POD and SOD activities, thus reducing H 2 O 2 (by 34%), O 2 •− (by 26%) and MDA (by 51%), compared to salt-treated plants [132]. Chen et al. [136] also reported that seed priming with MT reduces the accumulation of H2O2 and MDA and the percentage of EL and enhances the germination rate under salt stress conditions. Furthermore, Zhang et al. [137] observed that both foliar and root application of MT (100 µM) increased SOD, POD, CAT and APX activities under salt stress and decreased H2O2, O2•− and MDA accumulation in Beta vulgaris. Table 2. Activities of antioxidant defense system against salinity.

Plant Species Level(s) of Salt Stress Antioxidant Defense References
G. max cv. A3935 50 (low), 100 (medium) and 150 (high) mM NaCl; 30 d High salinity reduces SOD activity in roots (28%) and leaves (38%). APX activity decreased in leaves by 20% (low) and 57% (high), but slightly increased in roots at low salinity (10%) and decreased by 29% under high salinity. GR and CAT activities decreased in both roots and leaves under high salinity. [138] T. aestivum cv. Pradip 150 and 300 mM NaCl; 4 d AsA content sharply decreased. GSH content and GSH/GSSG ratio increased.
Slight increase in APX and GST activities, whereas activities of CAT, DHAR, MDHAR, GR and GPX decreased at 300 mM NaCl stress. [99] B. napus cv. BINA Sharisha 3 100 and 200 mM NaCl; 2d AsA and GSH content decreased, but GSSG content increased at 200 mM NaCl. GPX, GST and GR activities increased at 100 mM NaCl. Increase in APX activity, and decrease in CAT, DHAR, MDHAR, GR, GST and GPX activities at 200 mM NaCl stress. O. sativa cv. KDML105 60, 120 and 160 mM NaCl; 3 d SOD, APX and GR activities increase with increasing NaCl concentrations. CAT activity reduced by 1.6-fold at 160 mM NaCl. [103] G. max cv. PK9305 100 mM NaCl; 7 d Activities of CAT, SOD and PPO observed more in leaves than roots. Higher activity of POX in roots than shoots.

Signaling of ROS in the Regulation of Salinity
Salt stress directly induces primary stresses such as osmotic and ionic stresses as well as imposing secondary stresses such as oxidative stress caused by ROS. These ROS play a signaling function at a certain low concentration which is variable depending upon many factors [139]. In recent decades, research on salt stress signals and adaptation proposed different pathways which still remain hazy because of the complex interaction between and among biomolecules (Figure 8; [139][140][141]). Under salt stress, ROS production induces mitogen-activated protein kinase (MAP) cascades which mediates ionic, osmotic and ROS homeostasis [139,142]. The salt overly sensitive signaling (SOS) pathway functions in maintaining ionic homeostasis, through extruding Na + into the apoplast. Under salinity, excess intracellular or extracellular Na + triggers, and ROS-induced MAPs stimulate, a Ca 2+ signal and production. Ca 2+ together with the SOS pathway excludes Na + to maintain ionic homeostasis [141,143,144]. ROS signals stimulate antioxidant defense and ROS scavenging which confer biomembrane protection and restore membrane function for maintaining ionic and nutrient homeostasis [141,144].
The ROS-induced ABA production and ROS-induced activation of MAPs and the Ca 2+ signal regulate stomatal opening and closing. The osmoregulation and plant water status are maintained properly as a result [140,142]. Nitric oxide is also considered as one of the regulators of stomatal opening and closing [145]. ROS signal sensors/receptors can induce SA which also has a role in osmoregulation. Different signaling molecules or hormones can be stimulated by ROS which have diversified physiological, growth and developmental functions in the salt adaptation of plants. The ROS signal induces JA to modulate lignin biosynthesis, and GA to affect germination, and it induces auxin to modulate growth [146]. Ethylene-ROS interaction has a differential function. Ethylene enhances ROS generation that activates Na + and K + transport, where expression of different genes is involved. The modulation of ETH signaling controls AsA biosynthesis under salinity. The synchronizing action of ABA and ETH modulates the AsA biosynthesis during salt exposure. The Ca 2+ signaling persuaded by ROS signaling participates in AsA biosynthesis [3]. Several research reports demonstrated that exogenous application of hormones and signaling molecules regulated or enhanced the antioxidant defense system, conferred osmoprotection and improved physiology, which defended against oxidative damage under salt stress conditions [147][148][149]. There might be other hormones/hormonelike molecules or signaling molecules in this complex and hazy salt stress adaptation pathway. Knowledge on the signaling functions of ROS is misty as it is connected to the signaling function and biological function of different biomolecules. The dual role of ROS-creating oxidative stress and having a signaling function-is well known, but the mechanisms, interplay and cross-talk with other molecules and components are complex and yet to be discovered.

Hormonal Regulation and ROS under Salinity
Plant hormones or phytohormones are chemicals that are present in plants in small amounts but have a great impact on plant growth and development. Plant hormones play different biological roles in the presence of environmental stress and regulate plant growth positively or negatively. Under salinity, diverse signaling pathways coordinately work to mitigate salt stress [150]. Flexible regulation of phytohormone signaling helps plants to adapt under salinity (Figure 9). In plants, ABA is an important stress-responsive hormone that plays a crucial role in osmotic stress and especially in ROS-mediated signaling pathways. Salt-induced osmotic stress in plants causes a higher accumulation of ABA [151,152]. This higher accumulation of ABA helps to mitigate the negative effect of salinity by improving photosynthesis and the osmolyte content and reducing ROS-mediated toxicity [153]. It also triggers the accumulation of K + and Ca 2+ which inhibit the uptake of Na + and Cl − [154]. ABA also regulates Na + /K + homeostasis and the H 2 O 2 content under salt stress [155]. Abscisic acid is known as a vital salinity-mediated signal which regulates salinity-responsive genes. Overexpression of the MAX2 gene increased the resistance to salt stress, which was associated with redox homeostasis and ABA [156]. Osmotic homeostasis is regulated by ABA-activated SnRK2s which are responsible for the breakdown of starch into sugar and derivate osmolytes [157]. To scavenge ROS, ABA-generated H 2 O 2 triggered the accumulation of NO and activated protein kinase (MAPK) [139]. The MAPK cascade signaling upregulates the antioxidant genes and acts against salinity [158]. Salt stress reduced the lateral root length in plants which mediated ABA signaling [159]. Similar to ABA, auxin also alters the root apical meristem under salt stress. In Arabidopsis thaliana, receptor for activated C kinase 1 (RACK1) is involved in the biosynthesis of the miR393 gene which mediates both ABA and auxin regulation to induce redox homeostasis and alter lateral root growth under salinity [160]. During salt exposure, accumulation of excess ROS altered the auxin distribution, induced oxidative damage and caused a reduction in the root apical meristem [161]. Seed germination modulated membrane-bound transcription factor (NTM2), altered auxin signaling in plants which caused overexpression of the IAA30 gene and induced salt tolerance in the Arabidopsis plant [162]. Plant growth and development depend on seed germination which is closely related to GA. DELLA proteins act as a growth suppressor of plants under salt stress. GA binds with the GIBBERELLIN INSENSITIVE DWARF1 (GID1) receptor which recruits DELLA proteins and forms the GA-GID1-DELLA complex [163]. Thus, GA reduces DELLAs and salt tolerance in plants. Some GA-regulated genes, viz., AtGA2ox7 and OsGA2ox5, also help to increase salinity tolerance in A. thaliana and O. sativa plants, respectively [164,165].
Under the saline condition, plants regulate CK production along with ROS homeostasis to induce salt tolerance in plants. CK is a master phytohormone which acts as a free ROS scavenger and opposite to ABA and induces salt tolerance in plants [166]. CK receptor genes, viz., AHK2, AHK3 and AHK4, help to modulate the stress signal in plants which regulates the osmolyte content and membrane integrity [167]. Histidine kinase (HK) of CK receptors plays an important role in CK regulation and stress responses [168]. In the presence of CK, receptor gene CRE1 changes to its HK form and acts as a negative regulator of stress signaling. However, in the absence of CK, it converts into its phosphatase form and does not show any negative reactions [169]. Due to the higher CK content, receptor genes AHK2 and AHK3 antagonistically regulate ABA synthesis in Arabidopsis and consequently inhibit the ABA action in germinating seeds and seedling growth through the CK receptor HKs [170]. However, ROS production is reduced by the overexpression of CK biosynthetic gene IPT8 which helps to upregulate antioxidant enzyme activity under salinity in Arabidopsis [171]. Likewise, JA also works against salt stress and helps to mediate ROS. A high concentration of JA was noticed in salt-treated plants, which helps to reduce oxidative damage and shows a positive relationship with salt tolerance [172]. Qiu et al. [173] also observed an increase in the salt tolerance of T. aestivum by exogenous application of JA through the reduction in the Na + content, ROS and lipid peroxidation. The salt tolerance mechanism of JA is modulated by jasmonate ZIM domain (JAZ) transcription factors which repress MYC2 transcription factors. These MYC2 transcription factors are used by both JA and ABA to regulate salt responses where the functioning of JA activates ABA-regulated genes RD22 and AtADH1 under salt stress to induce salt tolerance [173]. In Arabidopsis, the PnJAZ1 gene regulates the JA pathway to induce salt tolerance and cause upregulation of ABA-regulated genes, viz., AtABI3 and AtRD22, to regulate seed germination and seedling growth under salinity [174].
In plants, salinity tolerance is induced by SA via activating the GORK channel which helps to maintain the membrane potential and K + loss in plant cells [175]. It also upregulates the activity of the H + -ATPase enzyme which helps in K + retention in the saline condition. However, SA did not retard the accumulation of Na + in plant roots but, instead, helped to reduce the Na + concentration in the roots [176]. In Arabidopsis, expression of the nudt7 gene reduced the shoot Na + concentration during prolonged salt stress [177]. In barley, the application of SA helps to increase P n , the carotenoid content and membrane integrity and to reduce ROS toxicity which ultimately helps to increase the K + and osmolyte concentration in plant roots [178]. It also caused overexpression of the P5CS gene which causes Pro accumulation and thus induces salt tolerance in Dianthus superbus [179]. ETH homeostasis and signaling have a direct relation with salt stress. Salt stress regulates key components of ETH signaling and also causes overexpression of the EIN3/EIL1 gene which is known as a defense-related gene. This gene helps to reduce the H 2 O 2 content and upregulate antioxidant enzymes to induce salt tolerance in Arabidopsis [180]. Similarly, brassinosteroid (BR) also helps to mediate salt stress in S. lycopersicum by the overexpression of BRI1 or BSK5 [181]. In saline conditions, BRASSINOSTEROID INSENSITIVE1 KINASE INHIBITOR1 (BKI1) translocated BRASSINAZOLE RESISTANT 1 (BZR1) and BRI1-EMSSUPPRESSOR 1 (BES1) and caused overexpression of the BZR1 gene in the Arabidopsis plant which controlled plant growth and development [182]. In Arabidopsis, BR-mediated salt tolerance signaling is linked with ABA, ETH and SA pathways which indicates the cross-talk of phytohormones in mitigating salt stress [183].

Gene Regulation for Antioxidant Defense under Salinity
Antioxidant enzymes are considered the first line of defense to eliminate accumulated ROS. Antioxidant enzymes are encoded by different gene families which are located in different parts of the cell. The activities of these enzymes are controlled by the expression of genes encoding antioxidant enzymes which are varied in crops. Mishra et al. [184] observed increased SOD activity in a tolerant cultivar of O. sativa which occurred due to the expressions of the Cu/Zn-SOD genes. In O. sativa (cv. BRS AG), expression of OsCATB increased with the duration (10, 15 and 20 d) of salt stress, which resulted in increased activity of CAT during the exposure to salinity [185]. GPX genes of T. aestivum demonstrated different expression patterns at a stressed condition, where TaGPX genes showed higher expression at the leaf developmental stage [186]. Glutathione peroxidase is considered as an important antioxidant enzyme involved in the reduction of H 2 O 2 with the help of GSH [2]. Salt-tolerant H. vulgare genotypes showed higher GPX activity which was regulated by the higher expression of the GPX gene [187]. In B. juncea, higher expression of ROS detoxification genes, viz., MDHAR and DHAR, was observed in salttolerant genotypes compared to the sensitive ones [188]. MDHAR and DHAR help to detoxify plant cells with the help of the AsA-GSH cycle [2]. Under salt stress, antioxidant genes of stressed plants are upregulated or downregulated with the severity of stress. Filiz et al. [189] worked with 36 isoform genes of 10 natural Arabidopsis ecotypes under salt stress, where 64% upregulation was demonstrated by CAT genes, and 55% downregulation was recorded in SOD and GPX genes.
Overexpressed genes encoding antioxidant enzymes help to mitigate ROS and induce stress tolerance in plants; some examples are cited in Table 3. Overexpression of Cu/ZnSOD in tobacco plants reduced ROS-induced damage in plants and improved the chl content under salinity [190]. Overexpression of PutAPX in A. thaliana reduced lipid peroxidation and improved plant growth under salt stress [191]. In PaSODand RaAPX-overexpressed potato plants, Shafi et al. [192] observed an improved RWC and chl content, and a reduction in the MDA content, under 150 mM NaCl-induced salt stress.

Conclusions
The generation of ROS in plants is a natural phenomenon that occurs as a part of different metabolic processes in multiple cellular and subcellular compartments. Salinityinduced osmotic stress and ionic toxicity disrupt redox homeostasis and trigger the overproduction of ROS and ROS-induced oxidative stress under salt stress. Several studies revealed that ROS have both negative and positive roles in plants. Along with signaling, ROS aid several basic processes and pathways in plant cells. Lower concentrations of ROS were found to be essential for proliferative pathway activation, cell differentiation and stem cell renewal. On the contrary, excessive accumulation of ROS at the cellular level acts as a stressor and causes oxidative damage of lipids, proteins, carbohydrates and RNA and DNA.
As ROS have a dual role in plants, the metabolism of ROS is crucial for growth, development and adaptation under salinity. The antioxidant defense system plays a vital role in ROS metabolism by scavenging and detoxifying ROS. Plant hormones, amino acids and their derivatives, PAs and vitamin supplementation contributed to ROS metabolism and decreased salt-induced oxidative stress under different levels of salinity by upregulating the antioxidant defense system, biological processes, ion homeostasis, osmolytes and gene expression in several studies. In previous investigations, under different levels of salinity, overexpression of genes helped to mitigate ROS-induced oxidative stress. In recent studies, molecular and genetic tools developed transgenic plants with enhanced activities of antioxidant enzymes and ROS detoxification under salinity.
Although the dual role of ROS is known, the metabolism process of ROS under salinity is complex, including the mechanisms, cross-talk with other molecules and components that are yet to be understood. Therefore, further research is required to understand the metabolism process of ROS more clearly. The understanding of ROS metabolism under salinity will be helpful in mitigating salt-induced oxidative stress. The mechanisms, interplay, cross-talk with other molecules and components are complex and warrant further research.
The present review focuses on ROS production and the antioxidant defense system under salt stress where information about cultivated and common plant species was included mostly from recently published research articles. There are salt-tolerant plant species in nature. Halophytes bear unique morphological, anatomical and physiological behavior, and that is why they are adapted to grow in coastal saline soils, mangrove forests and salt-affected lands of arid and semi-arid regions. Understanding the mechanisms of halophytes, finding out the signaling cascades and identifying salt stress-responsive genes can be useful for managing salt stress-affected cultivated and commonly grown plant species through different agronomic, breeding, biotechnological and other approaches.