Salt Stress Induced Plant Physio-biochemical and Molecular Responses: A Review

A broad spectrum of abiotic stresses like acidity alkalinity, salinity, and microbial infections are harmful to the plants. Salinity stress is among the most crucial environmental stresses to the productivity of crops worldwide ( lowers 2004; Munns and Tester 2008). Soil salinity shows a high concentration of soluble salts in the soil moisture of the rhizosphere. According to the United Nations Environment Programme, nearly 50% of the world's cultivated land is saline ( lowers and Yeo 1995). Salt stress in plants can occur in two ways; first, high soil salt concentration makes it difficult for roots to absorb water, and second, the high content of salt within the plant may prove fatal as it is toxic (Munns and Tester 2008). Saline stress in plants may hamper the various processes such as flowering, fruit quality, seed germination, survival percentage, which leads to decreased crop productivity (Sairam and Tyagi 2004). Severe consequences of salinity on plant growth may be because of ion toxicity in cell and osmotic stress (Hussain et al., 2007). Lipids are major constituents in the cell membrane, which help to maintain cell tolerance to saline stress. Increased saline levels disturb the arrangement of lipids and proteins in the cell membrane, which leads to defective lipid metabolism (Guo et al., 2019). The most challenging circumstance for salt stress is the degradation of chlorophyll content, which terminates photosynthesis level in plants, and it also hampers the stomatal conductance, respiration rate (Doganlar et al., 2010).

The initial reaction shown by salt-stressed plants is water potential decrease, which hampers water use efficiency, resulting in significant toxic damage and reduced yield (James et al., 2011). Water relations and osmotic adjustment regulate salt-stressed plants' growth responses (Munns et al., 1983). Salinity stress inhibits water content in many crop yielding plants such as pea tomato, Mentha, balm, showing a deleterious reduction in crop yield (Ozturk and Unlukara 2004). Metabolic adjustments in response to salinity stress are dynamic and multifaceted, e.g., under salt stress, the most dramatic change occurs in the ice plant (Guan et al., 2020). Salt stress can bring out a shift from C 3 to CAM pathway in this succulent plant by the induction of some enzymatic machinery, e.g., Phosphoenolpyruvate

(Romano et al., 2020) carboxylase, within a few hours, and the transition occurs within 6 to 8 days. The striking feature of CAM plants is scotoactive stomata, by which they can use water efficiently by showing minimum transpiration rate (Guan et al., 2020).

Some metabolic changes are common to most plants, whereas others are specific, e.g., organic solutes of low molecular weight show salt-stress induced accumulation. These solutes comprise amino acids (proline or glutamate), betaines (glycine betaine and alanine betaine), and polyols. During nitrogen deficiency, plants usually accumulate sulfonium compounds, like dimethyl-sulfonium propionate, which are physiologically similar to nitrogen-containing betaines. These organic solutes act antagonistically to inorganic solutes like Na and Cl ions, even at high concentrations, and are not detrimental to the enzymes and cell organelles. That is why these organic compounds preferentially function as compatible osmolytes (Rhodes and Hanson 1993; Ashraf and Harris 2004). Studies revealed that higher plants could tolerate high salinity by exclusion or accumulation of salt. Salt excluders have the property to remove salts from the whole plant or particular organs. In such situations membrane permeability supports the intake of K⁺ over Na+. Thus, the crops which exclude salts are distinguished by the low concentration of Na⁺ and Cl⁺ ions (Chen et al., 2018). Salt accumulators possess the ability to uptake high salt content through the following two mechanisms. In the first mechanism, plants can withstand high concentrations of intercellular salts by tolerant cell membranes, a common feature of salt-stressed plants, e.g., Halophytes. In this way, the ratio of Na⁺ to K⁺ in tissues is apparent. The second mechanism involves eliminating excess salt that enters the plant from which roots can uptake ions to prevent their detrimental effects (Yensen 2006).

Salt tolerance through physiological and biochemical mechanisms

Salt stress negatively affects plant growth through various constraints like osmotic stress, wherein water uptake by plants is affected, triggering significant events like the arrest of the shoot and root (Munns et al., 2000; ricke et al., 2004). Therefore, the osmotic adjustment becomes the requirement for plants in such conditions. Though reduced, the root growth rate resumes within an hour by regaining cell turgor by inorganic ions uptake and changing cell wall composition like Na+ binding affecting Na+ passage and other ion bindings (Shabala and Lew 2002; Byrt et al., 2018). The assimilation of CO2 is also affected by osmotic stress through the rapid closing of stomata, resulting from reduced xylem pressure. The roots sense and send the hydraulic signals at fast speed while shoots transduce these signals immediately to alter its metabolism, and ion channels in guard cells decode the altered xylem pressure to alter stomatal conductance ( uruichi et al., 2008; Christmann et al., 2013; Shabala et al., 2016).

Salt stress-induced ion toxicity, mostly related to increased Na⁺ and Cl^- accumulation, also restricts plant growth. Na⁺ toxicity lethal effect on plants though unknown can occur through enzyme inhibition; for example, many enzymes functioning in primary metabolism, regulated by K^+, get inhibited through the replacement of K⁺ by Na⁺ (Wu et al., 2018). Salt sensitive plants exhibit increasing physiological dysfunctions with increasing shoot Cl^-levels; thus, salt tolerance demands the exclusion of Cl^- from shoots (Geilfus 2018; Teakle and Tyerman 2010). Halophytes can withstand Cl^- concentration more than 1M without any adverse effect, and Cl^- affects the plant by causing less availability of macronutrients like S and N, as Cl^-share the anion transporters with SO 4 ^2- and NO 3 ^-(Bazihizina et al., 2019). Salt stress, mostly Na⁺ specific, cause PCD (programmed cell death) in plant roots, and events like cytochrome c release and DNA fragmentation occur rapidly (Jiang et al., 2008). Mutants of Arabidopsis lacking GORK, a K⁺ channel, show reduced or no such PCD events, suggesting entry of Na⁺ into cytosol causes membrane depolarisation, K⁺ efflux, activation of PCD executing endonucleases, and caspase-like proteases (Demidchik et al., 2010; Demidchik 2014).

Ion Homeostasis

NaCl is the primary soil salinity stress determinant, so the mechanisms underlying Na+ transport and its sequestration have remained the researcher's primary focus. The accumulation of a high concentration of Na+ in the root cell cytoplasm is prevented by excluding Na+ from uptake and vacuolar compartmentalization of Na+ (Theerawitaya et al., 2020) . Na+ uptake by roots is carried by various ion transporters in which two essential types include non-selectively operating cation channels, HKT2 (high-affinity K+ transporters), and CNGCs (cyclic nucleotide-gated) or GLRs (glutamate receptor-like) (Demidchik 2018; Mian et al., 2011). Other suggested transporters involved in Na+ uptake include HAK5, AKT1 LCT1 (low-affinity cation transporter1), and PIP2-1 (aquaporins) (Schachtman et al., 1997; Mian et al., 2011; Isayenkov and Maathuis 2019; Kronzucker and Britto 2011; Byrt et al., 2017). The counterbalance of Na+ uptake occurs through Na+ expulsion through SOS1 Na+/H+ exchanger and vesicles, and about 95% Na+ returns to the rhizosphere (Shi et al., 2002b; Shabala et al., 2020). Na+ vacuolar sequestration is done by Na+/H+ tonoplast antiporters of the NHX family, which show elevated transcript levels and activity in glycophytes upon salt exposure and constitutive activity in halophytes (Bassil and Blumwald 2014; Shabala and Mackay 2011). Two types of leak channels in tonoplasts SV and V (fast), need to be tightly regulated to block the leakage of Na+ back into the cytosol (Shabala et al., 2020). urther, Na+/H+ antiporter's higher affinity for K+ than Na+ suggests the operation of other mechanisms like vacuolar trafficking in delivering Na+ to the vacuole (Bassil et al., 2019; Baral et al., 2015). The loading of Na+ into xylem occurs both actively and passively, and the importance of mode of uptake depends on the time after salt exposure (Ishikawa et al., 2018b). NSCC mediate the passive Na+ xylem loading while SOS1 Na+/ H+ antiporter and HKT2 (K+/Na+ symporter) carry the active loading (Guo et al., 2010; El Mahi et al., 2019; Jabnoune et al., 2009). SOS1 Na+/H+ antiporter occurs abundantly in xylem parenchyma and HKT2 in stellar tissues of the root. Saline conditions favour passive K+ outward movement from parenchyma cells into the xylem due to depolarization, which creates a driving force for Na+ xylem loading (Ishikawa et al., 2018b). The CCC (cation-chloride cotransporters), which mediate K+ Cl-, and Na+ symport, create a driving force by transporting Cl- into the xylem passively, for active Na+ xylem loading (Ishikawa et al., 2018a). HKT1 (Class I)

transporters remove Na⁺ from xylem sap, and these transporters are more selective for Na⁺ than K⁺ (Munns et al., 2012). Also, Na⁺ is recirculated back to the roots from the shoots through the phloem involving mainly HKT1, and this Na⁺ gets stored in stellar parenchyma cells to prevent phototoxicity and damage to shoot meristematic tissues and growing leaves (Kobayashi et al., 2017; Shabala 2017).

Osmolytes and Osmoprotection

Osmolytes, compatible solutes or osmoprotectants are polar, soluble, uncharged, and chemically varied organic compounds, not interfering with cellular processes even if accumulated at high concentrations. These include glycine betaine, proline , polyols, sugar hydroxyproline, β-alanine, polyamines, and LEA (late embryogenesis abundant) proteins (Tahir et al., 2012; Wang and Nii 2000; Kerepesi and Galiba 2000; Saxena et al., 2013). Different plant species show variations for amounts of organic osmolytes synthesized and accumulated, like proline accumulated in varied plant groups, whereas β-alanine is accumulated only in some members of plumbaginaceae (Saxena et al., 2013; Hanson et al., 1994). The external osmolarity determines the osmolyte accumulation, and thus through water influx, osmolytes function in the protection of structures and maintenance of cell osmolarity (Hasegawa et al., 2000).

The quaternary ammonium compounds that physiologically act as potent compatible plant osmolytes under salt stress are glycine betaine, (β)-alanine betaine, proline betaine, choline-O-sulphate, hydroxyproline betaine, and pipecolate betaine (Ashraf and Harris 2004). GB shows ubiquitous occurrence in organisms like cyanobacteria, bacteria, fungi, algae, animals, and several families of plants like families Chenopodiaceae and Gramineae over an extensive range of abiotic stress conditions (Turkan and Demiral 2009; Lokhande and Penna 2012). This osmolyte is mostly found in chloroplasts and plays a significant role in stromal adjustment and thylakoid membrane protection, and hence and photosynthetic activity is maintained (Jagendorf and Takabe 2001). PS-II (photosystem II) complex is protected by GB in saline conditions (Annunziata et al., 2019). Against osmotic stress, GB prevents the membrane and enzyme (like Rubisco) destabilization induced by heat (Giri 2011). In angiosperms, synthesis of GB within the cell occurs from choline via ethanolamine, by two sequential oxidation reactions catalyzed by enzymes, namely CMO (choline monooxygenase) and BADH (betaine aldehyde dehydrogenase) respectively (Luo et al., 2012a; Missihoun et al., 2015). In some plants, another biosynthetic pathway involving two N-methyl transferase enzymes shows GB synthesis from glycine (Ahmad et al., 2012). Studies revealed that GB improves salt tolerance in maize, tobacco, potato, rice, barley, tomato belonging to different agronomical crops. The exploitation of these plants in biotechnology for GB synthesis has provided tolerance against various abiotic stresses (Sairam and Tyagi 2004; Turkan and Demiral 2009). GB's unique structure enables it to interact with macromolecules like enzymes through hydrophilic and hydrophobic domains. It functions in stabilization of proteins, osmotic adjustment, prevention of damage to photosynthetic apparatus, and lowering ROS (Reactive Oxygen Species) levels (Ashraf and oolad 2007; Saxena et al., 2013; Cha-Um and Kirdmanee 2010). Pre-treatment of rice seedlings with GB can prevent damages like grana disintegration, thylakoid swelling, mitochondrial swelling, and foliar spray of GB, leading to increased photosynthetic rate, pigment stabilization, and enhanced growth (Rahman et al., 2015; Ahmad et al., 2012).

In contrast to other amino acids like methionine, arginine, and cysteine, which show a decreased concentration, proline shows an increase in concentration under salt stress (El-Shintinawy and El-Shourbagy 2001). The accumulation of proline (Liu et al., 2006) provides a measure of improvement of salinity stress, and its intracellular accumulation serves as a nitrogen reserve for after recovery utilization (Saxena et al., 2013; Ben Ahmed et al., 2010). The Pro concentration is metabolically regulated, and synthesis of this imino acid occurs in plastids and cytoplasm, while it is broken down to L-glutamate (Kadioglu et al., 2012) in mitochondria. In plants, Pro has two precursors viz; glutamate (Kadioglu et al., 2012) and ornithine (Orn). Pro synthesis occurs from Glu via glutamic – ϒ- semialdehyde and pyrroline-5-carboxylate (P5C). The conversion of Glu to P5C is catalyzed by P5C synthase (P5CS) succeeded by P5C reductase (P5CR), which reduces P5C to Pro (Ashraf and oolad 2007). Probiosynthesis also takes place from another precursor Orn which is transaminated to P5C with the involvement of an enzyme mitochondrial Orn –ϒ-aminotransferase (Ong et al., 2016; Verbruggen and Hermans 2008). Pro biosynthesis is negatively regulated by a bHLH T MYC2, by limiting the expression of P5CS (Verma et al., 2020). Pro can quench 1O2, act as a free radical scavenger, buffer cellular redox potential, and mitigate cytoplasmic acidosis under stress (Babiychuk et al., 1995; Matysik et al., 2002; Lee et al., 2008). In Olea europaea, proline supplements improve salt tolerance by enhancing enzymatic antioxidants, photosynthesis osmotic balance, and plant growth, while in tobacco, it enhances salt tolerance through the acceleration of antioxidant defence pathway-specific enzymatic ac under stress (Babiychuk et al., 1995; Matysik et al., 2002; Lee et al., 2008). Inlular structures like proteins and membranes are stabilized by Pro through cluster formation with water, thereby preventing their denaturation, and membranes are protected through the maintenance of cellular ion homeostasis and osmotic balance (Ashraf and Harris 2004; Lee et al., 2008; Gleeson et al., 2005). Silicon supplementation increases the Pro content 3 to 6 days and decreases it 9 to 12 days after salt exposure and negative correlation between Pro and cytokinin exist after 3 days of stress, suggesting communication between cytokinin and Pro metabolism. Silicon induced Pro increase that can regulate cytokinin to confer salt tolerance (Zhu et al., 2020c)

Sugar and Sugar Alcohols:

The correlation between salt tolerance and fluctuations in soluble carbohydrate concentrations shows that the accumulation of carbohydrates like sugars (fructose, sucrose, glucose) and starch occurs due to salt stress (Parida and Das 2005). At higher salinity (400-500 mM NaCl), plants like Cakile maritime and Aster tripolium accumulate more elevated amounts of total soluble carbohydrates and proline (Megdiche et al., 2008; Geissler et al., 2010). Sugar alcohol and sugars function in storing carbon, scavenging radicals, osmoregulation, and osmoprotection (Ahmad and Sharma 2008; Lee et al., 2008; Adams et al., 2005). urthermore, there is deliberation about the sugar and sugar alcohols as molecular chaperones (Liu et al., 2006; Hasegawa et al., 2000). In Setaria sphacelata, starch and sugar show reduction in amounts under water stress for the shortterm while soluble sugars increase and starch decrease in long-term stress, suggesting a metabolic shift towards sucrose because starch turnover is more affected than sucrose (da Silva and Arrabaca 2004).

Trehalose, a non-reducing sugar, helps angiosperms protect from abiotic stress and alleviates salt-induced oxidative stress by preventing ROS accumulation (Mostofa et al., 2015). Salt tolerance increases in transgenic rice overexpressing OsTRE1, a trehalase gene (Islam et al., 2019). While functioning as osmoprotectant and osmolyte, it prevents membranes and proteins from undergoing denaturation (Ashraf and Harris 2004). Transgenic plants exhibit a reduction in photo-oxidative damage and increased photosynthetic rate with increased trehalose levels under salt stress. It protects biological molecules from injuries induced by desiccation through its potential for water absorption and in maize seedling growth improved via regulation of the glyoxalase system and antioxidant by trehalose through a reduction in ROS, methylglyoxal, malondialdehyde, and Na+/K+ (Rohman et al., 2019; Penna 2003). Sugar alcohols (Polyols) can be acyclic like mannitol or cyclic polyols like pinitol, having multiple functional OH-groups, and act as osmolytes, ROS scavengers, and chaperones (Ashraf and oolad 2007). These can work as osmolytes in osmotic adjustment and osmoprotectants through two indistinguishable means and osmotic adjustment occurs through the facilitation of cytoplasmic water retention and apoplast or vacuolar Na⁺ sequestration (Parida and Das 2005). Accumulation of polyols in grapes under water deficit conditions provides a means for refining water use efficiency and grapevine practices (Conde et al., 2015).

Polyamines

Polyamines or organic amines, including Spd (spermidine), Put (putrescine), and Spm (spermine)

increase salt tolerance when applied exogenously, or their levels increased in transgenic plants (Bueno and Cordovilla 2019; Rathinapriya et al., 2020; Ji et al., 2019). Different stresses induce polyamines that regulate the functioning of specific ion channels to control ionic influxes and play a protective role, as evidenced by the outcome of exogenous application (Zhao et al., 2007; Pottosin and Shabala 2014). Transgenic overexpression and loss of function mutants support polyamine's protective role (Ji et al., 2019; Zarza et al., 2017). The endogenous level of plant polyamines increase after salt exposure, and polyamine catabolism regulates the endogenous level and the oxidative catabolism catalyzing enzymes of polyamines, amine oxidases, are essential in imparting salt tolerance (Takahashi and Kakehi 2010). Polyamines maintain membrane integrity, regulate gene expression governing osmolytes' metabolism, reduce ROS production and control Na⁺ and Cl^- accumulation in various plant organs (Ji et al., 2019; Rathinapriya et al., 2020; Seo et al., 2019). Spermidine and spermine, when exogenously applied to like in soybean seedlings, enhance photosynthesis and ROS metabolism, thereby improve growth and enhance salt tolerance (Wang and Yin 2014; Baniasadi et al., 2018). SAMDC (S-adenosylmethionine decarboxylase) upregulation and ODC (ornithine decarboxylase) and ADC (arginine decarboxylase) downregulation led to decreased Put and increased Spm and Spd accumulation, alleviating the salt-induced inhibition of growth (Takahashi et al., 2017). Spd and Spm affect many metabolic pathways, and in grape seedlings, polyamines with ABA alleviate salt stress (Paul and Roychoudhury 2017; Sun et al., 2018). ADC2 deletion increases salt sensitivity, whereas ADC expression leads to enhanced osmotic adjustment and better plant growth (Naka et al., 2010; Espasandin et al., 2018).

LEA Proteins

Late embryogenesis abundant proteins occur in plants and animals that prevent desiccation, protein aggregation, or osmotic stresses ( uruki et al., 2012; Hundertmark et al., 2012; Hand et al., 2011). Named after their finding in maturing seeds, constitutive expression, or salinity stress-induced expression of LEA proteins are reported (Cumming 1999; Liu et al., 2013). LEA proteins protect various plant molecular or cellular structures from damaging effects of H2O2 through hydration, buffering, ion sequestration, direct protection of membranes or other proteins, or unfolded protein renaturation and phosphorylation is supposedly to have an enormous impact on the functioning of LEA proteins under stress conditions (Zhang et al., 2000; Goyal et al., 2005; Liu et al., 2017). IpLEA, gene product from Ipomoea pes-caprae, belonging to LEA group 2 functions in drought and salt tolerance through the facilitation of water homeostasis or ROS scavenging (Zheng et al., 2019). LEA group 1 proteins (XsLEA1-8) in Arabidopsis thaliana, while avoiding damage to subcellular structures, enhance the drought and salt tolerance (Artur et al., 2019). LsEm1 (LEA group1), from lettuce, when overexpressed in rice, alters the expression of other genes like OsCDPK25, OsCDPK15, OsCDPK13, OsCDPK9, and rab21 positively leading to increased salt and drought tolerance (Xiang et al., 2018). CaDHN5 (Luo et al., 2019) overexpression upregulated several salt-responsive genes, increasing osmotic and salt tolerance (Luo et al., 2019).

HORMONAL REGULATION OF SALINITY STRESS

Many phytohormones like ethylene, GA, ABA jasmonic acid (JA), and salicylic acid coordinate and integrate, enabling plants to respond and adapt to salt stress while ABA among these acts in different abiotic stresses. Water deficit and osmotic stress induced by salt stress result in rapid ABA production in shoots and roots (Shi et al., 2002b). ABA binding to receptors like RCAR (REGULATORY COMPONENTS O ABA RECEPTORS), PYL (PRY1-LIKE), and PRY1 (PYRABACTIN RESISTANCE1) leads to inhibition of PP2C phosphatases and promotion of SnRK2s activity that can activate many anion efflux channels resulting in turgor pressure loss and stomatal closure (Duarte et al., 2019; Zhang et al., 2017). Previously ABA was thought to be produced in roots exposed to osmotic stress and regulate stomatal closure; however, evidence like upregulation of gene NCED3 encoding enzyme in leaf parenchyma upon osmotic stress, catalyzing the first step of ABA biosynthesis, suggest ABA delivery to shoot from roots is not necessary (Endo et al., 2008; Buckley

2019). Besides, synthesizing ABA in roots would require precursor transport from leaves (Zhang et al., 2018). The role of ABA produced in roots under salt stress is still unclear even though ABA produced in roots has a higher concentration than leaves (Shi et al., 2002a). ABA in osmotically stressed roots stimulates NADPH oxidase that induces H2O2 production and accumulation, triggering stomatal closure and thus provides a clue of its role in salt stress response (Nath et al., 2019). OsABAR1 encoding GRAM (glucosyltransferases-like GTPase activators and Myotubularin) protein positively regulates the ABA pathway and enhances tolerance to salt and drought through ABA (Zheng et al., 2020).

Several JA biosynthesis genes upregulate by salt stress, suggesting the role of JA signaling in salt stress-induced plant responses (Kazan 2015; Geng et al., 2013). Salt stress inhibits root elongation through JA signaling. Physical interaction of RSS3 (RICE SALT SENSITIVE3) protein with class C bHLH (basic-helixloop-helix) T s (transcription factors) and JAZ (JASMONATE ZIM_DOMAIN) proteins induce cell elongation whereas RSS3 mutants show enhanced expression of JA-responsive genes, suggesting JA functions in root growth inhibition under salt stress (Toda et al., 2013). GaJAZ1 affects salt tolerance in cotton through downregulating genes containing G-box ciselement and reprogramming of defence associated gene expression (Zhao et al., 2020). OsCYP94C2b gene, encoding an enzyme catalyzing the conversion of jasmonoyl isoleucine (active form) to the inactive one, when overexpressed, increases the chances of survival in rice, suggesting JA regulates salt tolerance negatively. Though, TaORP1, the gene for JA biosynthesis, overexpression, or JA exogenous application, result in enhanced salt tolerance, suggesting positive regulation of salt tolerance by JA (Kang et al., 2005; Dong et al., 2013). GA (Gibberellic acid) plays a critical role in plant growth regulation under stress conditions, and endogenous GAs reduce while DELLA proteins increase in seedlings on exposure to salt stress (Colebrook et al., 2014; Magome et al., 2008). DELLA proteins mediate salt-induced growth restriction and help plants survive (Achard et al., 2006).

GA deficiency in vivo enhances salt tolerance through Na⁺ sequestration in vacuoles and ROS homeostasis (Zhang et al., 2020f). Ethylene is vital in imparting salt tolerance as evidenced by increased salt tolerance by applying ACC (1-aminocyclopropane-1-carboxylic acid) the precursor of ethylene, and salt hypersensitivity due to mutations in genes involved in ethylene signaling (Cao et al., 2007; Peng et al., 2014). Ethylene positively regulates salt tolerance by modulating ROS scavenging and generation pathways (Peng et al., 2014).

SA enhances the osmoprotectant (glycine betaine, polyamines, proline) accumulation and enzymatic antioxidant activities under salt stress, suggesting its positive role in conferring salt tolerance (Misra and Misra 2012). SA induces salt tolerance by the restoration of membrane integrity and increased carotenoid and chlorophyll content, resulting in enhanced accumulation of soluble sugar and K⁺ in the root (El-Tayeb 2005). Increased antioxidant metabolism and enhanced sulfur and nitrogen assimilation promoted by SA lead to an increased photosynthetic reduction (Nazar et al., 2011). SA treatment enables plants to adapt under saline conditions by reducing NaCl promoted H⁺ influx and K⁺ efflux (Jayakannan et al., 2013). BR (Brassinosteroid) increase antioxidant enzyme (GPX, APX, POX, and SOD) activity and lead to the accumulation of tocopherol, reduced glutathione, and ascorbate (non-enzymatic antioxidants), thereby alleviating adverse salinity effects (El-Mashad and Mohamed 2012).

Antioxidant responses of salinity tolerance

ROS, mostly hydroxyl radical (OH-), hydrogen peroxide (H2O2), singlet oxygen (1O2), and singlet anion (O2.-), show an increased production under different environmental stresses, including the salt stress and mitochondria, chloroplasts, peroxisomes, and apoplast are their main sites of generation (Miller et al., 2010). NADPH oxidases AtRboh and AtRbohD, polyamine oxidase, diamine oxidase, and peroxidase mediate the apoplastic ROS production, and salt stress leads to the upregulation of AtRboh and AtRbohD genes, whereas plant mutant atrboh /artboarD exhibit salt hypersensitivity (Ma et al., 2012). Salt induced ROS production by AtRboh /AtRbohD leads to cytosolic K+ influx resulting in decreased Na+/K+ ratios. AtRboh restricts xylem Na+ distribution, thereby hampers Na+ transport from root to shoot, whereas AtRbohD helps in the propagation of environmental stimuli induced longdistance signal (Miller et al., 2009; Jiang et al., 2012). Salt induced antioxidative response against H2O2 production mediated by AtRboh /D helps decrease cellular oxidative damage (Ben Rejeb et al., 2015). The chloroplast is the primary singlet oxygen and hydrogen peroxide producer compared to other organelles (Davletova et al., 2005). During photosynthetic electron transport, the photoreduction of O2 to O2- takes place and is termed as Mehler Reaction. In the plastoquinone pool, the formation of superoxides is due to the reduction of molecular oxygen. In PS-I during ETC, this reduction is executed by plastosemiquinone, ferredoxin, or by sulfur redox centres (Dat et al., 2000). The conversion of superoxides to H2O2 occurs either naturally or by enzyme SOD action, and the hydroxyl radicals are produced from hydrogen peroxide (Pospisil et al., 2019). In plant cells, peroxisomes are the leading producers of intracellular hydrogen peroxide, and H2O2 production induced by high oxygen and low CO2 levels under salt stress enhance photorespiration (Wingler et al., 2000; Del Rio and Lopez-Huertas 2016). The sites for O2- production in peroxisomes are peroxisomal matrix and peroxisomal membrane. In the peroxisomal matrix the generation of O2- radicals occurs through xanthine and hypoxanthine oxidation to uric acid by enzyme xanthine oxidase (Gutteridge and Halliwell 2000). Salinity induces ROS production (O2--) in mitochondria through ubiquinone over-reduction leading to the release of electrons from the electron transport chain to oxygen (Miller et al., 2010).

Plants use low ROS levels as signals to regulate growth and development in response to different stresses, whereas higher ROS concentration under stress conditions like salt stress has harmful effects. ROS disrupts cell integrity and leads to membrane lipid peroxidation, protein denaturation, pigment breakdown damage to DNA and enzymes, and carbohydrate oxidation (Groß et al., 2013). ROS activates various ion channels like Ca+ channels sensitive to H2O2 in guard cells and root epidermal cells, Na+ cation channels, K+ efflux channels, Ca2+ pumps sensitive to OH-, disturbing ionic homeostasis (Demidchik 2018; Zepeda-Jazo et al., 2011). Plant ROS scavenging machinery detoxify the oxidative stress triggered by high salinity induced ROS accumulation, and ROS homeostasis maintenance plays a critical role in salt tolerance (Bose et al., 2014). ROS scavenging induced salt tolerance includes both the activity of enzymatic antioxidants like SOD (superoxide dismutase), (Groß et al., 2013) glutathione reductase CAT (catalases), and APX (ascorbate peroxidase) and non-enzymatic antioxidant (glutathione, ascorbate tocopherols) accumulation (Hanin et al., 2016). Enhanced glutathione levels in chloroplasts peroxisomes, and mitochondria increase, whereas reduced ascorbate levels, as in vtc2-1mutant, decrease salt tolerance (Bose et al., 2014; Koffler et al., 2015). Salt stress impairs the anthocyanin production, as seen in air1 (anthocyanin-impaired-response1) mutant evidencing the role of anthocyanin accumulation in salt tolerance (Van Oosten et al., 2013). Glutathione acts as a ROS scavenger through its reaction with H2O2 superoxide, and hydroxyl radicals, and while acting in the ascorbate-glutathione cycle, it helps in the regeneration of ascorbate ( oyer et al., 1997). In Allium cepa, exogenous application of glutathione restores the cell viability and plasma-membrane permeability under salt stress, and ascorbate plus glutathione application resulted in increased plant height and branch number and increased content of phenols, carbohydrates mineral ions, and xanthophylls (Aly-Salama and Al-Mutawa 2010) (Rawia et al., 2011). Ascorbate, an important antioxidant, mitigates the harmful impacts of salt stress and helps in plant recovery after salt stress exposure (Agarwal and Shaheen 2007; Munir and Aftab 2011). Plants can synthesize a lipophilic antioxidant known as alpha-tocopherol or Vit. E. In combination with other antioxidants, alpha-tocopherol scavenges free radicals (Munne-Bosch and Alegre 2003; Massacci et al., 2008). It plays a crucial role in protecting the structure and function of PS-II as it chemically combines with oxygen in the chloroplast (Lopez-Huertas et al., 2000). Alpha-tocopherol assists in membrane stabilization and mitigates tolerance of plants during oxidative stress (Munne-Bosch and Alegre 2003). Overexpression of OsVTE (encoding tocopherol cyclase in rice) IbTC (encoding tocopherol cyclase in sweet potato) enhanced the salt tolerance, and such a plant showed less accumulation of H2O2 (Ouyang et al., 2011; Kim et al., 2019). Superoxide anion is converted to H2O2 by SOD and H2O2 to water by APX in chloroplasts, and APX gene overexpression leads to enhancement in salt tolerance (Asada 2006; Badawi et al., 2004). Under salinity stress, the activity of SOD increases in Catharanthus roseus and Morus alba (Jaleel et al., 2008; Ahmad et al., 2010). In mitochondria, Mn-SOD (manganese SOD) and alternative oxidase (AOX) increase salt tolerance through ROS detoxification (Giraud et al., 2008). In rice, two isoforms of chaperone protein, NCA1a, and NCA1b interact mutually exclusive with CAT to control its activity and confer salt tolerance (Liu et al., 2019a).

MAPK signalling cascade and salt stress

Over the period, plants have developed various mechanisms to counter and overcome the stress conditions for their survival, and among these mechanisms, signaling pathways, production, and shifting of signal molecules are crucial. Signal molecules are the by-products of biochemical reactions received by plant receptors, usually found in the cellular membrane resulting in different gene expression, including stress genes, and help the plants tolerate and survive under stress conditions (Hamel et al., 2006; Colcombet and Hirt 2008). Various signaling pathways get triggered during stress conditions, including MAPK (mitogen-activated protein kinase) signaling cascade. MAPK components are a group of enzymes that enable plants to respond to various stimuli by different stresses and activate plant responses like the activation of antioxidant enzymes. MAPK components get triggered by the activation loop's phosphorylation or dephosphorylation by upstream kinases, and phosphatases can suppress such activation (Lee et al., 2009). Arabidopsis genome exhibits the ability to activate 5 MKP molecules, which include IBR5, PHS1, DSPTP1, AtMKP1, and AtMKP2. Among these molecules, AtMKP2 and DSPTP1 can dephosphorylate Arabidopsis MAPK molecules (MPK3,4, and 6) in-vitro, and MAPK component activation, and the expression of stress genes may increase plant response to stress (Lee and Ellis 2007).

The main MAPK molecules which get activated during stress are MPK6, MPK4, and MPK3, and under salt stress, M2K2 activates MPK4 (Colcombet and Hirt 2008). The upregulation of M2K2 and downregulation of MPK4 and MPK6 through MAPK cascade is related to Me2K (Teige et al., 2004). In Arabidopsis , AtMPK6 AtMPK4, and AtMPK3 are regulated by salt stress whereas salt stress results in MAPK's (ZmMPK5, ZmMAPK3, and ZmSIMK1) expression in corn ( Zea mays ) (Droillard et al., 2002; Droillard et al., 2004; Ding et al., 2009; Wang et al., 2010). Accumulation of ZmMPK3 RNA occurs in plants by treating it with ethylene, salicylic acid, hydrogen peroxide, ABA, or under salinity stress (Wang et al., 2010). In Arabidopsis ZmSIMK expression enhances plant salt tolerance (Kong et al., 2011). StMAPK3 regulates salt and osmotic tolerance through affecting the activities of enzymes like CAT, SOD, peroxidases, and concentration of Pro, H 2 O 2, and malondialdehyde (Zhu et al., 2020b). In Arabidopsis , overexpression of the AtM2K3 gene increases plant tolerance to salinity and its sensitivity to ABA, implicating the signalling role of AtM2K3 in ABA activation and plant tolerance under salinity (Hwa and Yang 2007). In salt-tolerant peppermint, MAPK signaling regulates essential oil biosynthesis under salt conditions (Li et al., 2016). Wheat MAPK phosphatase, TMKP1 when overexpressed, increases germination rates, enhanced antioxidant activities like SOD, CAT, and peroxidases, leading to salt tolerance enhancement (Zaidi et al., 2016). In Populus trichocarpa overexpression of MAPK kinase (PtMAPKK4) results in improved germination, growth, and tolerance.

Salt responses at the molecular level: gene expression and transcriptional regulation

Gene expression in plants is regulated through varied mechanisms resulting in specific gene expression under specific conditions. Salt stress-induced differential expression of thousands of genes is determined by salt exposure duration or strength (Zeller et al., 2009). Transcriptome analyses of plants after salt stress alone or combined with other stresses propose widespread crosstalk between signaling pathways of salt stress and other stresses (Rasmussen et al., 2013). T s (transcription factors) constitute important regulators controlling gene expression, and T s from prominent T families like ER /AP2, NAC, bZIP, WRKY, MYB act in salt response. SlMYB102, R2R3-type MYB T overexpression confers salt tolerance through lowering ROS generation, K+/Na+ ratio maintenance, decreasing electrolyte leakage, enhancing the activity of SOD, CAT, APX, and peroxidase, increase in Pro, glutathione, and ascorbate concentrations, and upregulating several stress-associated genes like SICPK3, SICPK1, SIHAK5, SINHX4, SINHX3, SISOS2, and SISOS1 (Zhang et al., 2020e). AtMYB49 (R2R3-type MYB T ) increases salt tolerance through changes in cutin deposition, elevating Ca2+ level and through upregulating genes for LEA and peroxidases, improving antioxidant capacity (Zhang et al., 2020c). Another MYB T , VcMYB4a in Vaccinium corymbosum, is suggested an essential abiotic stress repressor as its overexpression leads to enhanced salt sensitivity (Zhang et al., 2020a).

Upregulation and downregulation of bZIP genes occur in salt-sensitive and salt-tolerant wheat cultivars, respectively, when exposed to an extended period of salinity, whereas in rice and wheat, NAC overexpression leads to salt tolerance (Johnson et al., 2002; Nakashima et al., 2007). LpNAC13 acting oppositely regulates drought tolerance negatively and salt tolerance positively, and its overexpression results in increased chlorophyll and proline content, increased enzymatic antioxidants and decreased malondialdehyde content under salt stress, whereas opposite results are found under drought conditions (Wang et al., 2020b). CB (C-repeat binding factor) from the AP2/ER family plays a positive role in the salt response, as evidenced by an increase in salt tolerance by overexpressing CB 3/DREB1A (dehydration responsive element binding protein 1A) and salt hypersensitivity in mutants of CB genes (Zhao and Zhu 2016; Kasuga et al., 1999). DREB2, DREB1/CB , and AREB/AB T s regulate abiotic stress responses transcriptionally, and T s, Z P179, and OsNAC5, are suggested to regulate the accumulation of LEA proteins, sugar, and proline under salinity stress, thereby conferring salt tolerance (Mizoi et al., 2012; ujita et al., 2013; Song et al., 2011). NnDREB2C overexpression in salt-stressed Arabidopsis increased germination, chlorophyll content, survival rates, lowered conductivity and resulted in increased tolerance to salt and drought through upregulation of PIP (Plasma membrane intrinsic proteins) genes (Ziyuan et al., 2020). OsSTAP1 encoding AP2/ER T overexpression enhances salt tolerance through increasing activities of CAT, POD, and SOD, lowering Na+/K+ ratios and upregulating stress associated genes like peroxidase and other ER encoding genes, suggesting its positive regulation of salt tolerance (Wang et al., 2020c). AtWRKY8 in Arabidopsis shows increased expression and binds directly to the target gene RD29A promoter under salt stress (Hu et al., 2013). IbWRKY2 enhances salt and drought tolerance in sweet potato through content increase for Pro and ABA, activity increase for SOD, and lowering H2O2 and malondialdehyde content (Zhu et al., 2020a). SlWRKY28 in Salix linearistipularis is found to improve tolerance to alkaline salt stress by regulating genes involved in the pathway of ROS scavenging (Wang et al., 2020a).

Regulation of gene expression by ABA mostly occurs through the bZIP T subfamily, AREB/AB , like AB 2 overexpression, enhances plant tolerance to multiple stresses (Choi et al., 2000; Kim et al., 2004). High salt and ABA hypersensitivity are exhibited by plants overexpressing DIG (Dynamic Influencer of Gene expression)/DIL (DIG-like), ABA-responsive T s (Song et al., 2016). MYC2, T involved in jasmonate signaling positively regulates salt tolerance, and EIN3 T involved in ethylene signaling enhances salt tolerance via DELLA proteins (Zhao et al., 2014; Peng et al., 2014). EIN3 act through ESE1 (Ethylene and Salt Inducible1) and ER 1 (Ethylene Response actor1) and activate salt responsive gene expression (Cheng et al., 2013; Zhang et al., 2011a). SER 1 (SALT-RESPONSIVE ER 1) directly binds to MAPK6, MAPK5, Z P179 (ZINC

INGER PROTEIN179), and DREB2A promoters and confers salt tolerance (Schmidt et al., 2013). Some T s can be controlled by different kinases that function in salinity tolerance like OsRMC encoding a receptor kinase that negatively regulates salt tolerance, is downregulated through binding of its promoter by two T s, OsEREBP1 and OsEREBP2 (Serra et al., 2013). Also, OsEREBP1 expression is not affected by ABA, salt, or severe cold but is regulated by moderate cold and drought slightly while ABA, cold, salinity enhance OsEREBP2 expression and drought, evidencing the role of OsEREBP2 in salt tolerance (Serra et al., 2013). OSBZ8, a bZIP T , in rice activated by phosphorylation through the SN -1 group of serine/threonine kinase, is expressed highly in salt-tolerant than the salt insensitive cultivars (Gupta et al., 2012). CaSBP12 (SBP-box T ) negatively regulates the salt tolerance through modulation of ROS signaling (Zhang et al., 2020b).

Plant processes, mainly under various stress conditions, are influenced by alternative splicing. Alternative splicing regulating proteins like Ser/Arg rich proteins can splice alternatively under salt and other abiotic stresses (Staiger and Brown 2013). In Arabidopsis , alternative splicing involves the symmetrical demethylation of Arg side chains by Type II Arg methyltransferase, PRMT5, and prmt5 mutants are salt sensitive (Zhang et al., 2011b). PRMT5 alters H4R3sme2 (H4 arginine3 symmetric demethylation) and LSm4 methylation status and impacts the salt response and plant growth (Zhang et al., 2011b). Salt, ABA, and mannitol upregulate the At-SKIP (Ski-interacting protein) expression, and altered tolerance of At-SKIP antisense lines or At-SKIP overexpressing lines in response to different stresses suggests the role of alternative splicing in such phenotypic expressions (Staiger and Brown 2013; Lim et al., 2010). In Arabidopsis , nuclear SUMO proteases, OTS1 (Conti et al., 2008) and OTS2 redundantly regulate salt stress responses, and the UBC32 (Ubiquitin conjugase) component of ERAD (endoplasmic reticulum-associated protein degradation) affect salt tolerance through BR (Cui et al., 2012; Conti et al., 2008). The existence of a mutual regulation mechanism between various signals, proteins, and genes is suggested to control various processes specific to abiotic stress adaptability of plants such as in-vitro salt tolerance increases when β-carotene hydroxylase downregulation leads the increased level of total carotenoid and β-carotene (Kim et al., 2012).

Regulation of salt tolerance in plants is also influenced through epigenetic modifications like acetylation, methylation, ubiquitination, and phosphorylation. Many HATs (histone acetyltransferases) that catalyze histone acetylation and HDACs (histone deacetylases) that catalyze histone deacetylation function in salinity stress response such as HDA6 interaction with HD2C results in repression of ABI1 and ABI2 (ABA-responsive genes), thereby decrease salt tolerance (Luo et al., 2012b). Salt tolerance is positively regulated by HDA5,14, 15,18 (class II enzymes), and negatively regulated by HDA6 9, 19 (class I enzymes) belonging to the RPD3 family of histone deacetylases (Ueda et al., 2017). HUB2 (E3 ligase from Arabidopsis) enhances salt and drought tolerance through increasing the monoubiquitination of H2B histone (Zhou et al., 2017; Chen et al., 2019). GmMYB84 encoding a T conferring salt tolerance in soybean depends on DNA methylation, and upstream 690nt to 950nt of its initiation codon, DNA methylation level decreases upon salt exposure leading to its higher expression (Zhang et al., 2020d).

Non-coding RNAs like miRNAs (microRNAs), siRNAs (small interfering RNAs), and lncRNAs (long non-coding RNAs) also mediate salt tolerance. miR-393 and miR-169 are induced, whereas miR-398 is inhibited by salt stress (Sunkar et al., 2012), and miR319 induces salt tolerance through the downregulation of essential genes involved in the methionine cycle and upregulating genes for ethylene synthesis (Liu et al., 2019b). A 24-nt nat-siRNA induces salt tolerance by downregulating the expression of enzyme P5CDH (delta1-pyrroline-5-carboxylate dehydrogenase), thereby inhibiting proline degradation (Borsani et al., 2005). DRIR (Drought Induced lncRNA) regulates salt tolerance by modifying the expression of multiple genes (Qin et al., 2017) and lncRNA973 in cotton increases salt tolerance through the regulation of T s, salt-stress responsive genes, and ROS scavenging genes (Zhang et al., 2019).

Stress priming or utilizing previous stress memory to enhance stress response by plants involves epigenetic modifications, inherited through mitosis or meiosis, like histone methylation and DNA methylation (Eichten et al., 2014). Salt and drought tolerance increase through seed priming using hyperosmotic reagents or NaCl (Sani et al., 2013; Cayuela et al., 1996). With mild seed priming using salt, the H3K27me3 (histone H3 lysine 27 trimethylation) islands get shortened and fractioned, which results in an alteration in the transcriptional responsiveness of many genes upon second stress exposure (Sani et al., 2013).

Na+ toxicity

Enzyme Inhibition

Membrane depolarization

[ K+ efflux through GORK" :

__like proteins______________

Salt Stress

• Replacement of K+ by Na+ resulting Na+ toxicity

CAIter metabolism

• •    Cl" leads less availability of macronutrients (S,N), by sharing anion transporters

Alter Ion channels in Guard cells

Plant Response and Tolerance

Excluding Na+ from uptake

Na+ Vacuolar Compartmentmentalization by Na+/H+ tonoplast antiporter

Hydraulic signals from roots

Homeostasis

la+ expulsion through SOS: Na+/H+exchanger

SalLSensing by Roots

Osmotic" Adjustment

• •    Uptake of Inorganic Ions

• •    Cell wall composition alteration

• •    Stomatai Closure

• •    Exclusion of Cl-or accumulation as in Halophytes

Na+ Uptake through channels: HKT2, CNGCs, HAK5,AKT1, LCT1, PIP2-1

J.

Na+ accumulation in Cytoplasm

• Figure 1: igurerepresenting Plant response and tolerance to NaCl

Glycine Betaine Proline

Sugar and Sugar alcohol

Protein Stabilization Osmotic Adjustment Lowering ROS Content

Stromal Adjustment and Thylakoid membrane Protection

Source of Nitrogen source after stress recovery

Quench Singlet Oxygen

Buffer Cellular Redox Potentia

Mitigate Cytoplasmic Acidosis

Increase Enzymatic Antioxidant Activity

Enhance Photosynthesis

Osmotic Adjustment

Protein and Membrane Stabilization

Storing Carbon

Scavenging Radicals

Osmoregulation and Osmoprotection

Molecular Chaperones

Trehalose: Protect membrane and Prevent protein denaturation

Mannitol and Pinnitol: Cytoplasmic Water retention and Apoplastic/Vacuolar Na* sequestration; ROS scavenger;

Chaperones

Polyamines

LEA Proteins

Spermine, Putrescine, Spermine

• •    Regulate ion influxes through specific Ion Channels

• •    Membrane Integrity

• •    Regulate Osmolyte Metabolism Through Gene Expression regulation

• •    Reduce ROS Production

• •    Control Na* and Cl" Accumulation

• •    Enhance Phtosynthesis

• •    Protection from H₂O₂ Damage Through Hydration and Buffering Ion Sequestration

• •    Membrane and Protein Protection

• •    Unfolded Protein Renaturation and Phosphorylation

• •    Gene Expression Alteration of Genes like OsCDPK25, OsCDPKIS, OsCDPKIS, OsCDPK9, and rab21

  Figure 2: Role of osmolytes under salinity stress

  
  
  

  ABA

  ABA

  Ethylene

  Na+ Squestration in Vacuoles

  OsABAR (GRAM)

  ROS Homeostasis

  SALT TOLERANCE

  • Osmoprotectant Accumulation

  • Enzymatic Antioxidants

  • Membrane Integrity

  • Increased Carotenoid & Chlorophyll

  • SolubleSugar & IC

  • S & N Accumulation

  • Increased Photosynthetic Reduction

  • Reduce NaCI Promoted H* influx & K⁺ efflux

  Receptors RCAR, PYL, PRY1

  Antioxidant Enzymes (SOD, POX, APX, GPX) Tocopherol, Reduced

  Gl utath lone & Ascorbate

  Receptors RCAR, PYL, PRY1

  Survival Growth

  I Restriction

  Reprogramming Defense Associated Gene Expression

  Genes havingG-box Cis element

  Root Elongation

  Modulate ROS Generation & ROS Scavenging

  PP2C Phosphatase

  Turgor Pressor loss Stomatai Closure

  Anion Efflux Channels

  DELLA proteins

  SnRK2S

  Figure 3: Role of phytohormones in imparting salt tolerance [GA (Gibberellic Acid), BR (Brassinosteroid), ABA (Abscisic Acid), SA (Salicylic Acid), JA (Jasmonic Acid), GRAM (Glucosyltransferases-like GTPase Activators and Myotubularin), RCAR (Regulatory Components of ABA Receptors), PYL (PRY1-Like), and PRY1 (Pyrabactin

  
  

  Resistance1)

  
  
  

  AtWRKYS

  LpNAC13

  OsNAC

  OsSTAPl

  lbWRKY2

  NAC

  C3F3/DREB1A

  RD29A

  LEA

  Scavenging

  AREB/ABF

  ZFP179

  SIMYB102

  EIN3

  ABA

  ABF2

  bZIP

  DIG/DIL

  ncreased Chlorophyll, Pro, Enzymatic Antioxidants Decreased Malondialdehyde

  CAT, POD, SOD Increase NaVK* ratio lowered Genes for Peroxidases and other ERF encoding Genes

  • Lowering ROS

  • K*/Na⁺ratio maintenance

  • Prevent Electrolyte Leakage

  • Enzymatic Antioxidants Increase

  • Pro, Glu & Ascorbate ncrease

  • Gene Upregulation Like SICPK3

  • Changes in Cutin deposition

  • Ca^2t level ncrease

  • Upregulating LEA& Peroxidase

  • Antioxidant Enzymes

  TFs

  WRKY

  ERF/AP2

  SIWRKY28

  Genes for ROS

  AtMYB49

  Transcriptiona Regulation

  Salt Tolerance

  • Pro & ABA increase

  • Lowering H₂O₂& Melondialdehyde

  I MYB I

  Figure 4: Role of Transcription actors (T s) in salt tolerance.

  
  

  Many studies have described that salinity stress induces within genome DNA methylation and epigenetic variation, either natural or induced by mutations in DNA methylation mechanisms result from gene expression alteration mediating salt tolerance (Karan et al., 2012;

Wang et al., 2014; Wang et al., 2015; Huang et al., 2013). A correlation between histone methylation alterations and inactivation or activation of genes induced by salt (Sun et al., 2019); however, the role of epigenetic changes induced by salt stress in salt tolerance is yet to be arrived at fully.

CONCLUSIONS AND FUTURE PERSPECTIVE

Plant salt tolerance includes a cascade of responses at physiological, biochemical, and molecular levels. Comprehensive research viz; physiological, biochemical and molecular studies have explained various salinity mechanisms regulating ion intake, exchange and balance, osmotic regulation, hormonal metabolism antioxidant metabolism, and stress signaling. Besides, rapid expression of NtNHX1 increases plants salt tolerance by enhancing vacuolar Na⁺ compartmentalization that declines the toxic accumulation of the ion in the cytoplasm that promotes growth in a saline environment. The determination of net Na⁺ flux across the plasma membrane regulates the expression of SOS 1 (Na⁺ efflux) and HKT1 (Na⁺ influx). Environmental stress, particularly osmotic and ionic stresses, are liable for the decrement in yield, especially in arid and semi-arid regions. Production of ROS in different cell organelles chloroplast, mitochondria, and peroxisomes is due to prolonged environmental stresses. The normal functioning of the cell is disturbed by ROS as it attacks biomolecules like DNA, lipids, proteins, and carbohydrates. Under extreme stress conditions, ROS finally leads to cell death. or overcoming oxidative stress, plants have enzymatic and non – enzymatic antioxidants. Many workers have addressed various benefits of SOD, CAT, APX, GR, MDHAR, AsA in overcoming oxidative damage to the cell. There is an accumulation of osmolytes and osmoprotectants, such as proline and glycine betaine, to overcome salt stress's detrimental effects. These compounds help in osmotic adjustment and protecting subcellular structures. Salt tolerance of plants can be enhanced by increasing CO 2 concentration as it alleviates oxidative stress, which activates the oxidative system and increases the accumulation of compatible substances.

LEA proteins play a significant role in plant stress tolerance, but the elaborated mechanisms of plant stress protection remain undetermined. Under abiotic stress distinct signaling pathways are regulated, including the MAPK cascade. MAPK molecules are a group of proteins that can negotiate various plant functions, like cell cycle, plant growth and development, plant response to stress. The integration of different MAPK pathways can be very beneficial for transgenic plant production which is more resistant to salinity stress. Maintaining plant behaviour, particularly Na+ cellular concentration under salt stress, is an important key issue to make plant salt tolerant. MAPK signaling can significantly affect such pathways by regulating proton pumps' activity, Na+ localization into vacuoles, and regulating the cell cycle. There is a crosstalk between different signaling pathways during the stress and the interactions with phytohormones. At the molecular level-specific T s, non-coding RNAs and epigenetic modifications play an essential role in countering salt stress and imparting salt tolerance.

The limitation of crop production due to salt stress and ensuring food security can be overcome by developing salt-tolerant crop plants by utilizing novel technologies like gene editing and resourceful genetic transformation. The prerequisite for these novel technologies is that candidate genes associated with salinity tolerance must be recognized and exploited. Though multiple studies are carried out, the are many areas of understanding salt-induced responses and plant adaptability which need more focus, including how a plant senses the salt stress at the cellular and whole plant level, what sort of changes occur at cellular levels like cell wall modifications, cell organelles response and signal integration, mechanisms of phytohormone involvement. The control of specific gene expression during salinity stress needs special attention to discover and understand the role of key players like T s, noncoding RNAs, and epigenetic modifications.

ACKNOWLEDGMENTS

The authors are thankful to Head, Department of Botany, University of Kashmir for providing necessary facilities for the completion of this manuscript.