Plant salt response: Perception, signaling, and tolerance

1Soil salinization is one of the most adverse environmental stressors, severely limiting plant growth and development and threatening agricultural production worldwide. In addition to naturally occurring soil salinization, the situation is even exacerbated by excessive chemical fertilizers and soil amendments, improper irrigation practices, and the effect of seawater exposure. It is estimated that crop production on at least 20% of global irrigated land is impaired. About 2 million ha (~1%) of world agricultural land is under accelerating salinization each year. Salt stress causes plant growth inhibition, abnormal development, and metabolic disturbance. The detrimental effects of elevated salinity on plants include (i) osmotic stress as sodium accumulates in the soil and (ii) ionic stress ( Figure 1 ). Osmotic stress caused by hyperosmotic soil solution disrupts plant cell turgor. In contrast, ionic stress is characterized by disordered sodium (Na+)/potassium (K+) balance inside the cell, disrupting various metabolic and physiological processes. These two processes were reported to be temporally and spatially separated in plants during the salt stress response. However, there exists a significant overlap between osmotic and ionic stress in both early and downstream signaling. In addition, the elevated content of reactive oxygen species (ROS) in plants is also observed upon salt exposure ( Figure 1 ). Electrons leaked from the electron transport chain (ETC) can react with O2 during aerobic metabolism to produce ROS. The toxic level of ROS seriously disrupts normal cellular metabolism through the oxidation of macromolecules like lipids, proteins, and nucleic acids, resulting in severe oxidative damage.

2In general, there are two plant types regarding their distinct salt tolerance. Some plants, termed halophytes, naturally grow in or even adapt to the saline environment with more than 200 mM NaCl. By contrast, most other plant species are salt-sensitive glycophytes, and their growth and development are adversely inhibited by soil salinization. Due to their sessile lifestyle, plants have evolved with sophisticated but effective strategies with considerable plasticity in morphology, physiology and metabolism to deal with multiple environmental stimuli. Therefore, a series of signaling pathways have been established in plants in response to salt stress, including the sensory mechanisms, the networks that mediate osmotic adjustment, redox balance and ionic homeostasis, and other stress response mechanisms ( Figure 1 ). In this review, we briefly summarize the new findings of salt stress responses in plants, focusing on recent advances in salt-induced signal perception and transduction. Phytohormone and light signal are essential for plant development and stress response, and some of their regulatory elements in have been also found to play essential roles in coordinating salt stress response in plants. Here we also summarize the mechanisms of phytohormone and light signal-mediated salt tolerance of plants. Finally, we discuss the possible roles of plant microbiota in plant salt tolerance. Understanding the mechanisms of plant salt tolerance provides new opportunities for engineering salt-tolerant plants even grown in saline regions, making it a promising way to keep agricultural productivity and ecological security worldwide.

3Plants might sense the alteration of salt concentration in the extracellular space and the change of mechanical effects on cellular structures caused by salt stress, triggering a series of signal transduction processes in plants ( Figure 2 ). The initial salt stress signals mainly include excess apoplastic Na+, cytosolic Ca2+ ([Ca2+]cyt) level increase, ROS accumulation, and 3,5-cyclic guanosine monophosphate (cGMP) production. Under saline condition, excess Na+ enters plant roots through nonselective cation channels (NSCCs), which mediate toxic sodium (Na+) influx into the cell across the plasma membrane (PM). Cyclic nucleotide-gated channels (CNGCs) are the main NSCCs, which are implicated in ionic homeostasis during salt response. The initial salt-induced signals might contribute to regulating NSCCs in plants during salt response. NSCCs can be blocked by Ca2+. The apoplastic Ca2+ concentration in root cells is probably in the region of 0.2-0.4 mM, which is enough to reduce NSCCs-mediated flux by 30-50%. The remaining flux can be further diminished by channel blockers like Gd3+ and La3+. Other channels and transporters may also contribute to salt stress perception, but their regulatory role in sodium sensing and import in planta is debated.

4Calcium signal functions as an essential secondary messenger, and salt stress leads to rapid and transient [Ca2+]cyt elevations to trigger salt tolerance responses. During initial salt response, calcium signal plays a vital role in osmosensing within seconds of exposure to osmotic stress caused by the saline conditions. REDUCED HYPEROSMOLALITY- INDUCED [Ca2+]cyt INCREASE1 (OSCA1) was initially identified as a hyperosmolality-activated Ca2+-permeable cation channel responsible for [Ca2+]cyt increase, might be a potential osmosensor in plants. A Ca2+-responsive phospholipid-binding BONZAI (BON) protein was recently reported to mediate hyperosmotic stress tolerance by positively regulating osmotic stress-induced [Ca2+]cyt increase, ABA accumulation, and gene expression, indicating a possible role of the membrane-associated Ca2+-responsive BON proteins in osmotic sensing and signaling. Considering that salt stress triggers osmotic stress, it is possible that the aforementioned Ca2+ transporters and Ca2+-responsive BON protein might participate in initial salt sensing and signaling by mediating Ca2+ signal in plants. Recently, the Arabidopsis thaliana (Arabidopsis) PM glycosyl inositol phosphorylceramide (GIPC) sphingolipids was found to function as a sensor to sense Na+ level and regulate salt stress response by gating ionic stress-induced Ca2+ signaling. GIPC sphingolipid biosynthesis is catalyzed by the protein of monocation-induced Ca2+ increases1 (MOCA1), and extracellular Na+ ions can bind GIPC sphingolipids to gate PM Ca2+ influx channels. Consistently, the moca1 mutant is hypersensitive to salt stress and lacks cation-evoked Ca2+ spikes and waves.

5High salinity stress rapidly triggers H2O2 bursts in plant cells, which might function as essential salt stress signal. HYDROGEN-PEROXIDE-INDUCED CA2+ INCREASES1 (HPCA1) functions as a H2O2 sensor that perceives the stress-induced extracellular H2O2 burst and generates increased [Ca2+]cyt under stress stimuli. Mathematical modeling showed that under salt stress, crosstalk between ROS and Ca2+ signaling is necessary to spread the Ca2+ signal between cells. Accordingly, the atrbohd atrbohf double mutant, which is hypersensitive to salt stress, exhibited a reduction in cytosolic free Ca2+ and PM Ca2+ influx. Importantly, Jiang et al., showed that lack of AtrbohF causes hypersensitivity of shoots to soil salinity with high Na+ accumulation in root vasculature cells and xylem sap. AtrbohF-mediated ROS production in root vasculature contributes to Na+ concentration limitation, thus protecting shoot cells from transpiration-dependent excess Na+ delivery. Moreover, the Ca2+ signaling complex CBL1/9-CIPK26 interacts with and phosphorylates AtRbohF. Thus, in the early stages of salt stress, ROS and Ca2+ signals work together to affect ionic homeostasis in plants.

6The rise of cellular cGMP can be detected within seconds after applying salinity and osmotic stress. Furthermore, cGMP inhibits Na+ influx in several plant species, while it can regulate transcription of various genes related to salinity stress and promote K+ uptake. Importantly, cGMP has a negative effect on the inward Na+ flux acrried by NSCCs.

7Peptide ligands have also emerged as essential mediators of cell-to-cell communication during plant growth and stress responses, possibly through the action of their PM-localized receptors, RECEPTOR-LIKE KINASEs (RLKs). For instance, the defense-related peptide PLANT ELICITOR PEPTIDE AtPep3 and its receptor PEP1 RECEPTOR 1 (PEPR1) are implicated in plant salt response, and PEPR1 loss of function nearly abolishes AtPep3-induced salt resistance in Arabidopsis. Recently, Zhou et al., reported that the RECEPTOR-LIKE KINASE 7 (RLK7), another PM receptor kinase in Arabidopsis, can be recognized by a secreted peptide, PAMP-INDUCED SECRETED PEPTIDE 3 (PIP3), and form an active ligand-receptor signaling cascade that modulates plant salt tolerance in Arabidopsis by activating MPK3/MPK6 cascade. Furthermore, LEUCINE-RICH REPEAT EXTENSINs, RAPID ALKALINIZATION FACTOR peptides and FERONIA (FER), a member from the Catharanthus roseus RLK1-like (CrRLK1L) family in Arabidopsis, may form a functional module that connects salt stress-induced cell-wall alterations to salt stress responses. In addition, FER-dependent signaling may elicit a cell-specific Ca2+ signal to maintain cell wall integrity during salt stress and root growth recovery after salt exposure. Although some peptides are implicated in plant salt response, upregulation of their coding genes and the maturation of these peptides during salt stress response might depend on the initial signals such as Ca2+.

8The immediate problem plants face is dehydration due to osmotic stress caused by high salinity, and plants can initiate osmotic adjustment to maintain cell volume and turgor under salt stress ( Figures 1, 2 ). The osmotic adjustment is a process by which plants enhance their water availability by synthesizing compatible solutes, known as osmolytes, in their cells. The osmolytes mainly include organic substances and inorganic ions, and proline, glycine betaine, and soluble carbohydrates, have been proven effectively regulate osmotic pressure by lowering the osmotic potential in the cytosolic compartment, thus preventing cellular dehydration during salt stress. Due to its strong hydration ability, proline accumulation is an effective strategy to prevent protein dehydration and denaturation under osmotic stress in plants. Data showed that treatment with proline to two contrasting cultivars of Brassica juncea could alleviate the adverse effects of salinity on photosynthesis and seed yield. Furthermore, proline can function as an O2 quencher, thus removing excessive ROS produced under stress. Glycine betaine, a water-soluble amphoteric quaternary ammonium compound, is also an essential osmotic regulator in higher plants, playing a vital role in stress alleviation. Exogenous application of glycine betaine can mitigate salt-induced damage in maize seedlings. Non-structural carbohydrates, such as glucose, sucrose, fructan, and starch, are also found to accumulate in plants under salt stress and function in osmotic adjustment, therefore enhancing the salt tolerance of plants. Significantly, carbohydrates also act as sugar signaling in plant response to an environmental stimulus, making it another layer of regulation in plant salt tolerance. Apart from organic solutes, inorganic ions have been demonstrated as significant contributors to osmotic adjustment during saline conditions. Sodium and chloride are toxic to plants, but salt-tolerant plants might take them as essential osmolytes to maintain the external osmolarity to avoid osmotic imbalance and sustain growth. Plants enhance their environmental adaptability by accumulating inorganic ions like K+ under salt stress. It has been revealed that K+ is an influential contributor to osmotic regulation, making it a crucial element for plant growth and stress response.

9ROS in plant cells mainly includes oxygen radicals, like superoxide (*O2 -), hydroxyl radical (*OH), and some non-radicals, such as hydrogen peroxide (H2O2), singlet oxygen (1O2), and ozone (O3). The generation sites of ROS in salt-stressed plants mainly include chloroplasts, mitochondria, apoplast, and peroxisomes. ROS might damage cells at excess levels while act as crucial signaling molecules essential for stress signaling at lower concentrations ( Figure 2 ). The interaction between ROS and ethylene (ET) has been identified as the primary signal that mediates salinity stress in rice. Excessive accumulation of ROS induced by salt stress is toxic to plants and causes oxidative damage to cellular constituents, leading to cell death. A high level of ROS (especially H2O2) leads to DNA damage and distorts genomic stability ( Figure 1 ). To detoxify ROS generated by salt stress, plants have evolved a set of antioxidant strategies, mainly including enzymatic and non-enzymatic systems to protect cells from oxidative damage ( Figure 1 ). Enzymatic systems include a set of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). Numerous studies have shown that salt-tolerant species improve their salt tolerance by enhancing the antioxidant defense system under salt stress. Non-enzymatic antioxidants, such as glutathione (GSH), ascorbate (AsA), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), can effectively scavenge some highly toxic ROS in plants. Data showed that GSH and AsA levels increase with an elevated level of *O2 - and H2O2 in Camellia sinensis (L.) exposed to 300 mM NaCl stress. Enhanced activities of MDHAR and DHAR enzymes were reported to be induced by salt stress, thus decreasing cell membrane damage in sugar beet M14. Consequently, plant antioxidant defense is proficient in maintaining ROS balance for salt tolerance. Importantly, as the interaction between O2 and reduced ETC components leads to ROS production, one effective strategy to reduce toxic ROS level is to prevent ETC over-reduction. The mitochondrial alternative oxidases (AOXs) participate in electron overflow when the cytochrome ETC chain is saturated with electrons due to impaired electron transport under challenging conditions, preventing further reduction of ubiquinone and stabilizing the whole ETC. Our recent work demonstrates that SIZ1-mediated SUMOylation of R2R3-MYB transcription factor MYB30 modulates plant salt tolerance through the action of AOX1a. MYB30 binds the promoter of AOX1a and upregulates its expression in response to salt stress to maintain the cellular redox homeostasis through enhanced alternative respiration pathway.

10Under salinity stress, excess toxic sodium ions (Na+) enter and accumulate in plant cells, disrupting ion homeostasis, especially Na+/K+ balance ( Figures 1, 2 ). Keeping ionic homeostasis is a prerequisite for plant growth during salt stress, since disordered ionic homeostasis leads to the disruption of cellular metabolism. Plants have developed sophisticated and effective mechanisms to keep optimal levels of Na+ by removing or vacuolar compartmentalizing Na+ from the cytoplasm, and a variety of carrier and channel proteins, symporters, and antiporters participate in this process ( Figures 1, 2 ). On the other hand, K+ retention in the cytosol is esstisal for Na+/K+ balance during salt response. The Na+/H+ antiporters that transport Na+ in exchange for H+ achieve the regulation of Na+ levels in the cytoplasm in plant cells. The PM-localized Na+/H+ antiporters can transport Na+ to the apoplast, and the vacuole-localized Na+/H+ antiporters are responsible for vacuolar sequestration of Na+. The increased accumulation of Na+ in vacuoles might also act as an osmoticum, enhancing salt tolerance ability. NHXs are putative Na+/H+ exchangers that transport Na+ from the cytoplasm to the vacuole, holding plant resistance to salt stress. Several reports showed that overexpression of NHX confers salinity tolerance in many plant species. For example, constitutive overexpression of AtNHX1 significantly increases salt tolerance in rice, wheat, tomato, and cotton. Taken together, Na+ exclusion, vacuolar Na+ sequestration, and K+ retention in the cytosol are essential for plant salt tolerance ( Figure 2 ).

11The Salt Overly Sensitive (SOS) regulatory pathway plays a pivotal role in regulating ionic homeostasis through modulating the activity of Na+/H+ antiporters under salt stress ( Figure 2 ). The SOS pathway effectively maintains the Na+ homeostasis by transporting excess Na+ from the cytosol to the apoplast, thus preventing the accumulation of Na+ to toxic levels. The SOS signaling pathway includes Salt Overly Sensitive-1 (SOS1), a PM Na+/H+ antiporter, the serine/threonine protein kinase SOS2, and two calcium sensors, SOS3 and SCaBP8/CBL10 (SOS3-like calcium-binding protein 8). When grown in normal condition, SOS pathway is “off” via the action of 14-3-3 proteins and GIGANTEA (GI) which interact with SOS2 and repress its kinase activity. High salinity initiates a calcium signal that activates the SOS pathway ( Figure 2 ). Under salt stress, 14-3-3 proteins are released from SOS2 and degraded through proteasomal pathways; SOS3/SCaBP8 protein perceives the increased [Ca2+]cyt, recruits SOS2 to the PM, and activates its activity. Subsequently, the activated SOS2 phosphorylates SOS1, thus enhancing the transport activity of SOS1 and transporting Na+ from cytosol to apoplast. In addition, SOS3/SCaBP8-SOS2 module might also positively regulate the activities of other transporters involved in ionic homeostasis like vacuolar Na+/H+ exchanger (NHX). Remarkably, SOS1-mediated Na+ exclusion in plants during salt response is regulated by MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) signaling pathways (Figure 2). For instance, MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6 physically interact with and phosphorylates SOS1 and salt stress-induced PA, and thus mediate salt response and suppresse Na+ accumulation via in shoots. Phosphatase MAP KINASE PHOSPHATASE1 (MKP1) can be phosphorylated by MPK6, and MKP1 exhibits a negative effect on MPK3/6 activity. A recent study revealed that the mkp1 mutation improves salt tolerance by restraining Na+ accumulation in shoots. The salt tolerance in mkp1 might be attributed to the activation of SOS1 via the elevation of MPK6.

12Besides Arabidopsis thaliana, SOS genes have been identified in many other plants, such as Triticum aestivum L. and Oryza sativa. Overexpression of SOS genes could improve salt tolerance by regulating ionic homeostasis. Importantly, it is worth noting that the entry of Na+ across the tonoplast membrane or PM is driven by the proton motive force established by proton pumps in the tonoplast or PM. Activation of H+-ATPase and H+ pyrophosphatases generates such proton motive force across the PM, thus activating most of the ion and metabolite transport. Vacuolar H+-ATPase (V-ATPase) is the most prevailing H+ pump in the plant cells. Studies have revealed that enhancing the expression level of V-ATPase could improve salt tolerance. Interestingly, both V-ATPases and PPase are also thought to be regulated by the SOS components.

13Under salinity stress, excessive Na+ leads to K+ loss in plant cells. The transporters HAK/KT/KUP play an essential role in maintaining Na+/K+ homeostasis during salt stress, which is involved in enhancing K+ absorption and reducing Na+ accumulation inside the cells. In rice, OsHAK1 dominates the Na+-sensitive high affinity K+ uptake system. Constitutive expression of OsHAK5 in BY2 cells enhances K+ accumulation under saline condition and confers salt tolerance in these cells. OsHAK5 might mainly function in shoot tissues and its overexpression leads 43-115% increase in K+/Na+ ratio compared to WT plants in shoot but not root. OsHAK21 is also reported to mediate K+ absorption across the PM and play an essential role in maintaining the Na+/K+ homeostasis in rice under salt stress. The mutant of oshak21 accumulates less K+ and considerably more Na+ in both shoots and roots compared with the wild type. Research also suggests that Arabidopsis NADPH oxidases ARABIDOPSIS THALIANA RESPIRATORY BURST OXIDASE HOMOLOG D (AtrbohD) and AtrbohF function in ROS-dependent regulation of Na+/K+ homeostasis under salinity stress possibly regulating inward K+ currents under both normal and salt stress conditions.

14Phytohormones are small chemicals that play an essential role in plant growth and development. Evidence indicates that phytohormones mediate various stress resistance, such as salt, osmotic, drought, cold, and pathogen stress. Numerous studies have shown that plant hormone signaling plays integrated and sophisticated roles at different vegetative stages, in different tissues, or under various environmental stimuli. How plant hormones, including abscisic acid (ABA), brassinosteroid (BR), ethylene (ET), gibberellin (GA), salicylic acid (SA), and jasmonic acid (JA), mediate salinity signals to regulate plant salt stress tolerance is briefly summarized here ( Figure 3 ).

15ABA functions as an essential central integrator to activate adaptive signaling cascades during the salt stress response in plants. Under abiotic stresses, including salinity and water deficit, endogenous ABA levels increase rapidly, and enhanced ABA signaling activates sucrose nonfermenting 1-related protein kinases (SnRK2s). SnRK2s are the central components in ABA signaling networks and play critical roles in ion transport, osmoregulation, ROS production, gene transcription, and the closing of stomata. Stomata are the primary place for plant transpiration, and ABA-regulated stomatal opening and closing are critical for plants to respond to salt stress. OST1/SnRK2.6 interacts with and phosphorylates specific ion channels, such as the potassium channel KAT1 and the slow anion channel SLAC1, to mediate K+ efflux and anion currents in guard cells, thus enhancing stomatal closure during salt and osmotic stress. SnRK2.2/2.3/2.6 phosphorylate and positively control various ABA-responsive element (ABRE)-binding protein/ABRE-binding factor (AREB/ABF) transcription factors, further regulating osmotic stress response in plants. ABA-activated SnRK2s also regulate osmotic stress tolerance by controlling the BAM1- and AMY3-dependent degradation of starch into sugar and sugar-derived osmolytes. It is deserved to determine whether similar mechanisms are involved in the osmotic regulation during salt stress in plants.

16Salt stress leads to the increase of [Ca2+]cyt. Ca2+ signaling plays a real-time and influential role in response to salinity stress. ABA effectively helps plants survive salt stress by integrating with the versatile second messenger Ca2+ via provoking PM-bound channels or releasing Ca2+ from intracellular Ca2+ pools. The damage to the cell wall caused by Na+ can be perceived by the kinase FER, which mediates salt stress signaling by increasing [Ca2+]cyt. In contrast, ABA flexibly controls FER activity through the dephosphorylation of ABA INSENSITIVE 2 (ABI2). In addition, ABA-activated SnRKs can phosphorylate the membrane-bound NADPH oxidase AtrbohF, modulating ROS homeostasis in plant response to high salinity. A recent study has demonstrated that salt-induced ABA and Ca2+ signaling can fine-tune AtrbohF activity by activating SnRK2.6 and CIPK11/26 signaling modules. Together, ABA, Ca2+ and ROS exhibit complicated signaling crosstalk to control plant resistance to salt stress.

17BRs are a class of steroid phytohormones in plants and play pivotal roles in plant growth, development, and response to adverse stresses. BRs have been widely reported to improve salt stress tolerance in a range of plants, including Arabidopsis, rice (Oryza sativa), tomato (Lycopersicon esculentum), and mustard (Brassica napus). BR can significantly inhibit ROS generation by enhancing antioxidant capacity under saline conditions. Exogenous treatment with 24-epibrassinolide (EBL), an active by-product from brassinolide biosynthesis, effectively improves salt tolerance in soybean through regulating enzymatic antioxidants and osmolyte accumulation. The exogenous application of BR has been reported to improve photosynthetic efficiency in different plant species. In a recent study, EBL application could alleviate the detrimental effects of salt stress on chloroplasts and photosynthesis in Robinia pseudoacacia L. seedlings. Moreover, exogenous BR application could also relieve salt toxicity by regulating the activity of Na+/H+ antiporters and NHX. These results highlight the potential role of BR in plant salt resistance.

18BR-induced enhanced tolerance to salinity is closely associated with BR signaling. When extracellular BR hormones directly bind to one of its membrane-localized receptors, BRASSINOSTEROID-INSENSITIVE 1 (BRI1), BRI1-LIKE 1 (BRL1) or BRL3, and the coreceptor BAK1(SERK3) in Arabidopsis, an efficient phosphorylation cascade, to relay BR signals to BRI1-EMS-SUPPRESSOR1 (BES1) and BRASSINAZOLE-RESISTANT1 (BZR1) family TFs, therefore controlling BR-regulated gene expression. Salt stress leads to root growth inhibition in plants due to a reduced level of BZR1 in the nucleus and the repression of BR signaling. However, exogenous BR application can even partially enhance salt-induced growth inhibition. Studies indicate that overexpression of vascular BR receptor BRL3 promotes the accumulation of osmoprotectant metabolites, including proline and sugars, which play essential roles in osmoregulation under salt stress. Overexpression of SERK2, an interacting partner of BR receptor in rice, significantly enhances grain size and salt stress resistance. The accumulation of SERK2 induced by salt stress confers early BR signaling on the PM to enhance the salt stress response. SlBZR1, a BZR/BES TF in tomatoes, positively regulates BR signaling and salt stress tolerance in tomatoes and Arabidopsis. BIN2 is a critical negative component of BR signaling, which also acts as an essential molecular switch to balance plant growth recovery and salt stress response in Arabidopsis; however, BR signaling might not be implicated in BIN2-SOS2 module during salt response and growth recovery regulation. Importantly, multilayer crosstalks between BR and ABA have been observed. BIN2 activates ABA signaling through the phosphorylation of SnRK2.2 and SnRK2.3. In contrast, phosphatases ABI1 and ABI2, two major negative players in ABA signaling, can mediate the dephosphorylation of BIN2, abolishing the activity of BIN2 and enhancing the transduction of the BR signaling pathway. It is possible that BR and ABA together control plant growth and salt stress response

19ET and GA have been also reported to be involved in plant salt stress response. Plants rapidly generate gaseous ET under salt stress. Increased endogenous ET or treatment with ACC, an ET precursor, both can enhance plant salt tolerance. ET promotes plant salt tolerance by maintaining the homeostasis of Na+/K+ and reducing ROS by inducing antioxidant defense. ET signaling is essential in plant salt tolerance. Loss of function of ET receptor ETHYLENE RESPONSE 1(ETR1) and ETHYLENE INSENSITIVE 4 (EIN4) or CONSTITUTIVE TRIPLE RESPONSE 1 (CTR1), a negative regulator of ET signaling, confers enhancement of salt tolerance, while loss-of-function mutants of ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-LIKE 1 (EIL1), two ET-activated TFs, exhibited higher salt sensitivity in contrast to wild-type plants. As a well-known regulator of seed germination, GA positively regulates plant growth. By contrast, reduced bioactive GA levels or signaling after germination is required for plant tolerance to salt stress. Consistently, the GA-deficient mutant ga1-3 exhibits remarkable tolerance to salt stress; the growth of seedlings lacking GA signaling repressors GAI, RGA, RGL1, and RGL2 is less inhibited by salt stress compared with the corresponding wild-type plants. Moreover, overexpression of the GA catabolic gene CYP71D8L improves rice tolerance to salinity stress by affecting GA homeostasis. The phytohormone SA is also repeatedly reported to take part in salt tolerance. For example, the exogenous application of SA together with nitric oxide (NO) significantly alleviates the NaCl-mediated oxidative damage in Vigna angularis by enhancing the synthesis of osmotic substances and improving photosynthesis. A recent study showed that priming the seed germination of Leymus chinensis in SA solution relieves salt-induced osmotic damage by accumulating K+. It might be due to the improved ATP content and H+-ATPase activity in the membrane of root cells. In addition, SA might also crosstalk with other hormones, such as ABA, ET, and GA, which are closely correlated with the activation of osmotic adjustment and maintenance of ionic homeostasis.

20As a stress-related hormone, JA has been also found to be involved in salt-induced growth inhibition. Salt stress induces the expression of JA biosynthesis-related genes in leaves and roots, leading to increased JA production. Exogenous application of JA significantly alleviates salt-induced damage by increasing the antioxidative enzyme activities and maintaining Na+/K+ balance. JA signaling plays an essential role in plant salt tolerance. The crucial component activating JA signaling, MYC2, is implicated in salt-mediated JA-dependent inhibition of cell elongation in the elongation zone of Arabidopsis primary roots. Additionally, MYC2 contributes to salt tolerance by regulating the proline biosynthesis gene in Arabidopsis. Jasmonate ZIM-domain (JAZ) proteins are the core components of the JA signaling pathway, and their roles in plant salt stress response have been characterized in many species. A recent study has shown that GaJAZ1 interacts with GaMYC2 to inhibit the expression of downstream genes, increasing salt tolerance in Gossypium hirsutum. On the other hand, overexpression of CYP94C2b, a cytochrome P450 family protein involving JA catabolism, enhanced viability under salt conditions and delayed the salt stress-induced leaf senescence in rice.

21The light signaling networks in plants begin with the perception of light signals, ultimately leading to changes in plant development and stress response. Emerging evidence shows that light signaling is vital in shaping plant salt stress response ( Figure 3 ). For example, light signaling can affect salt stress-induced transcriptional memory response of P5CS1-mediated proline accumulation in Arabidopsis. In addition, PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), a negative regulator of the phytochrome signaling pathway, negatively regulates plant salt tolerance by downregulating the expression of stress tolerance genes. CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1), a master of the light signaling pathway, also regulates salt stress tolerance. Salt treatment can promote the translocation of COP1 to the cytosol; the cop1 mutants exhibited a significantly impaired resistance to salt stress than the wild-type plants at the germination and seedling stages. A recent study further confirmed that COP1 controls plant salt stress tolerance by modulating sucrose content. Moreover, constitutive nuclear-localized ELONGATED HYPOCOTYL 5 (HY5), a bZIP family TF acting as a critical regulator in light signaling and seedling development, can promote proline biosynthesis by connecting light and salt stress signals. Recently, a study on tobacco suggests that NtHY5 enhances salt stress tolerance by positively regulating light-mediated flavonoid biosynthesis.

22Circadian clock regulates many physiological and developmental processes in plants, and its phase and period are adjusted by light, temperature, and nutrient input. It has been found that salt tolerance is also regulated by the circadian clock via modulating the expression of salt-responsive genes like RD29A and SOS1. The protein abundance of PM Na+/H+ antiporter SOS1 appears to occur in a diurnal cycle. Interestingly, a recent work revealed that SOS1 specifically functions as a salt-specific circadian clock regulator via GI in Arabidopsis. SOS1 directly interacts with GI in a salt-dependent manner and stabilizes this protein to sustain a proper clock period under saline conditions for the homeostasis of the salt response under high or daily fluctuating salt levels ( Figure 3 ). The regulatory role of light signals in plant salt tolerance needs to be clarified next.

23Plants host a diverse community of microorganisms on and inside organs such as roots and leaves, collectively termed the plant microbiota. Accumulating evidence indicates that plant microbiota plays a vital role in plant adaptation and resistance to saline soil. Notably, multiple groups of root-associated microbes, including plant-growth-promoting rhizobacteria (PGPR) and endophytic bacteria, are essential to improve plant tolerance to high salinity ( Figure 3 ). PGPR alleviates the toxicity of salt stress on plants mainly by regulating ionic homeostasis, accumulating osmolytes, activating antioxidant capacity, and enhancing essential nutrient uptake. For instance, a recent study suggested that the bacterial strain E. cloacae PM23 mediated salt tolerance in maize by modulating plant physiology, antioxidant defense, and compatible solute accumulation.

24Furthermore, PGPR can produce ACC deaminase, which reduces the excessive ET production in plants caused by salt stress. Plants with reduced ET level would finally cope with salt-induced growth inhibition by associating with ACC deaminase-producing microbes. Studies also indicate that PGPR can improve plant salt tolerance by producing a wide range of phytohormones as signal molecules in the rhizospheric region. For instance, it has been revealed that an Algerian Sahara PGPR named strain Pp20 confers maize root tolerance to salt stress via producing and secreting plant growth-promoting hormone indole-3-acetic acid (IAA) and ACC deaminase. In addition, the endophytic bacteria penetrating into the plant root cells possess similar functions in improving salt tolerance compared with PGPR. For example, ACC deaminase-containing endophytic bacteria can ameliorate salt stress in Pisum sativum through reduced oxidative damage and induction of antioxidative defense systems. Remarkably, some root-associated fungal endophytes are also shown to improve plant salt tolerance in terms of growth, ion homeostasis, and osmoregulation. Taken together, both rhizospheric and endophytic bacteria can be employed as effective and eco-friendly adjuncts to promote plant tolerance to salinity ( Figure 3 ).

25In recent years, the plant nanobiotechnology approach has shown great potential to modulate plant stress response. Nanotechnology is the application of small-sized materials with a basic structure of 1–100 nm. A variety of nanomaterials (NMs) have been reported to enhance plant salt tolerance for growth under saline condition. For instance, some metal-based nanoparticles, cerium oxide nanoparticles, silica nanoparticles, titanium dioxide nanoparticles, and zinc oxide nanoparticles, can improve salt resistance in multiple plant species. NMs enhance plant salt tolerance mainly by improving plant photosynthesis performance, promoting ROS detoxification, and maintaining ionic homeostasis and resoring osmotic balance. Compared with the non-nanoparticle control, the application of cerium oxide nanoparticles significantly improved cotton salt tolerance by b maintaining cytosolic Na+/K+ ratio. In addition, zinc oxide nanoparticles have been shown to enhanc salt tolerance in seedlings by improving photosynthetic pigments and antioxidative systems. Remarkably, exogenous application of biocompatible poly (acrylic acid)-coated cerium oxide nanoparticles can improve the production of gasious signaling molecules (i.e., NO), therefore maintaining the redox and ionic homeostasis in rice under salt stress. Although the underlying mechanisms need to be furtherly elucidated, nanobiotechnology could be a promising approach to increase crop yield in saline soils by enhancing plant salt tolerance.

26Over the past decades, much progress has been made in understanding how plants respond and adapt to salt stress. Plants have evolved various regulatory mechanisms to cope with the damages caused by excessive saline ions in the soil. Osmotic adjustment, redox, and ionic homeostasis regulation, and metabolic adjustment are the significant factors associated with plant salt tolerance ( Figure 1 ). To cope with salt stress, plants have to rapidly and effectively perceive changes in Na+ levels and osmotic pressure caused by salt stress. Different sensors mediate stress-signaling sensing, which relays stress signals to secondary messengers that activate signaling cascades and downstream regulatory networks via multiple hormone-mediated signaling pathways. The mechanisms of plant salt response involve a variety of signaling components, transcription factors, and functional genes that directly mediate ionic homeostasis, osmoregulation, and antioxidation ( Figures 2, 3 ). The phytohormone and light signals also mediate salt stress response in plants. Plant microbiota might also contribute to plant resistance. Exploring the molecular mechanisms of plant salt tolerance remains a great challenge. Many salt-responsive new genes still need to be annotated via advanced biotechnologies. The current knowledge of the salt-responsive molecular mechanisms in plants, from salt sensing and signaling to the development of adaptive tolerance mechanisms, still requires further studies. To date, the integration of multi-omics techniques and physiological phenotyping has proven to be a fast and effective method for probing the regulatory mechanism of plant salt tolerance. In particular, identification of upstream components regulating salt stress sensing is of paramount importance. Furthermore, the crosstalk between salt stress signaling networks and phytohormones still requires further investigation. Together, these findings provide valuable knowledge for breeding salt-tolerant crops through biotechnological approaches in the future.

27FX and HZ wrote this manuscript and prepared the illustrations. All the authors contributed to the discussion and agreed to the published version of the manuscript. All authors contributed to the article and approved the submitted version.