Adaptive Mechanisms of Halophytes and Their Potential in Improving Salinity Tolerance in Plants

Soil salinization, which is aggravated by climate change and inappropriate anthropogenic activities, has emerged as a serious environmental problem, threatening sustainable agriculture and future food security. Although there has been considerable progress in developing crop varieties by introducing salt tolerance-associated traits, most crop cultivars grown in saline soils still exhibit a decline in yield, necessitating the search for alternatives. Halophytes, with their intrinsic salt tolerance characteristics, are known to have great potential in rehabilitating salt-contaminated soils to support plant growth in saline soils by employing various strategies, including phytoremediation. In addition, the recent identification and characterization of salt tolerance-related genes encoding signaling components from halophytes, which are naturally grown under high salinity, have paved the way for the development of transgenic crops with improved salt tolerance. In this review, we aim to provide a comprehensive update on salinity-induced negative effects on soils and plants, including alterations of physicochemical properties in soils, and changes in physiological and biochemical processes and ion disparities in plants. We also review the physiological and biochemical adaptation strategies that help halophytes grow and survive in salinity-affected areas. Furthermore, we illustrate the halophyte-mediated phytoremediation process in salinity-affected areas, as well as their potential impacts on soil properties. Importantly, based on the recent findings on salt tolerance mechanisms in halophytes, we also comprehensively discuss the potential of improving salt tolerance in crop plants by introducing candidate genes related to antiporters, ion transporters, antioxidants, and defense proteins from halophytes for conserving sustainable agriculture in salinity-prone areas.


Introduction
Land degradation resulting from salinity puts global agriculture in jeopardy, as it engulfs 3600 million hectares (Mha) out of 5200 Mha of world agricultural land, leading to the loss of USD 27.5 billion every year [1,2]. Furthermore, reports anticipate that the world population will increase to 9.7 billion by 2050, which would require 70% greater food production by that time, exerting additional pressure on the shrinking areas of agricultural land [3]. To feed the growing human population, many parts of the world in peril of drought are still using salt-contaminated ground water for irrigation [4]. Inappropriate Plants grown in salinity-affected areas accumulate higher levels of toxic ions responsible for different physiological abnormalities, including ionic imbalance, impaired gas exchange performance, loss of water homeostasis, alterations in the levels of metabo-lites, and reactive oxygen species (ROS)-mediated oxidative damage to the cellular compartments [10,11]. To counteract the detrimental effects of salt stress, plants adopt some fundamental mechanisms, including (i) toxic ion (Na + and Cl − ) exclusions or their compartmentation into vacuoles or old tissues; (ii) compatible solute accumulations; (iii) synthesis of numerous stress adaptation-related endogenous metabolites; and (iv) synthesis and activation of antioxidant enzymes [e.g., catalase, (CAT), superoxide dismutase (SOD), ascorbate peroxidase (APX), glutathione peroxidase (GPX), peroxidase (POD), and glutathione S-transferase (GST)], and non-enzymatic antioxidants [e.g., carotenoids, phenolic compounds, flavonoids, ascorbate (AsA), and glutathione (GSH)] [10,[12][13][14]. Unfortunately, the traditional crops cannot deploy salt tolerance potential to survive in salinity-affected soils, and a wide range of genetic diversity for salt tolerance in conventional crops, even local landraces, remains elusive. Interestingly, some plants, such as halophytes, comprising only <2% of the world's flora, can sustain and thrive under extreme salinity through the adoption of intrinsic mechanisms related to selective ion uptake, Na + compartmentalization, ion homeostasis, production of defense metabolites, salt gland and salt bladder formations, and improved antioxidant properties [14,15]. Given their ability to tolerate high salinity, the cultivation of halophytes on salt-contaminated soils has been introduced as an alternative solution to economical desalination and restoration of highly degraded saline and sodic soils through the process, called phytoremediation [16]. It is worth noting that molecular analyses, such as transcriptome sequencing, have been widely used to investigate the gene expression dynamics of numerous species in response to salt stress [17]. Nevertheless, exploration of salt tolerance mechanisms of halophyte species with the final goal of transferring their salt tolerance-associated traits and/or genes to traditional crops is timely and imperative, which may also contribute to ensuring sustainable agriculture in salinity-affected areas.
Considering the above facts, the present review focuses on the latest update of the adverse effects of salinity both on soils and plants, as well as the intrinsic physiological, biochemical, and molecular mechanisms that halophytes employ to adapt under salinity. In addition, mechanistic aspects of desalinization and phytoremediation potential of halophytes for improving agricultural output from salt-contaminated soils will also be appraised. Finally, a comprehensive update on transcriptomic studies in halophytes and the genetic potential of halophytes for enhancing salt tolerance in traditional crops will be discussed by considering the genes associated with salt tolerance-related traits of halophytes.

Adverse Effects of Salinity on Soil Properties
Salinity causes immense alterations in soil properties. For instance, high salt concentrations in soils may result in high bulk density, soil compaction, poor soil structure, clay dispersion, and surface crusting [18]. In addition, salinity-induced poor hydraulic conductivity can severely impede water infiltration, leading to poor growth performance and yield reduction in plants [19]. Accumulation of salt in soils also has detrimental effects on soil microbial communities and their enzymatic activities [20]. Because of their increased sensitivity to environmental perturbations, soil microbiomes and associated enzyme activities are regarded as early indicators for measuring the degree of soil degradation and changes in soil quality [21]. Microbial activities play critical roles in the decomposition of soil organic matter (OM), mineralization of soil, and stabilization of soil aggregates [22,23]. Salinity-induced osmotic imbalance can lead to water leakage from the microbial cells and, consequently, plasmolysis and death of microorganisms [24]. On the other hand, microorganisms may live in low osmotic potential environments by accumulating osmolytes, which is an energy-intensive process [25]. Soil enzymes, including ureases, dehydrogenases, CATs, alkaline phosphatases, and β-glucosidases, derived from soil microbes, plant root exudates, and plant and animal residues, play central roles in the regulation of all the processes involved in OM decomposition, as well as in the cycling of soil nutrients [26]. Despite their immense influence on the alteration of soil quality, only a few studies were conducted to assess the potentiality of these enzymes in changing soil quality in coastal areas. In addition, comprehensive information on how salinity influences soil quality is still ambiguous due to the intricate nature of soils [25]. Therefore, future research should focus on unraveling their intrinsic processes in a spatio-temporal manner.

Mechanisms of Salinity-Mediated Adverse Effects on Plants
Elevated soil salinity can hinder plant performance in several ways. Salinity-induced water paucity severely affects seed germination by lowering the osmotic potential of the soil water, which interrupts imbibition in plant seeds [27,28]. Any disruption in seed imbibition can alter enzyme activities and protein metabolism, perturb hormonal balance, retard seed reserve utilization, and very often distort the ultrastructure of cells, tissues, and organs [29][30][31]. Reduction of plant growth and biomass after germination is the foremost negative impact of soil salinity, which depends on salinity levels, duration of salt exposure and type of plant species [27]. Plant cells can lose water after plant exposure to salinity within a few seconds, and as a result, cell shrinkage occurs [32]. Cells regain their original volume over hours, but cell elongation rates are reduced [33]. Over days, cell division rates are also severely reduced, leading to decreased plant growth [34]. Salinity can also cause visible injuries in the oldest leaves, the demise of older and younger leaves, which may result in death of the plants before seed maturation [32,33]. Nevertheless, the reduced growth rate of plants under salinity is predominantly regulated by root responses, as roots are directly exposed to saline soils, and act as a key conduit for mineral translocation to aerial parts of the plant [34]. As mentioned above, salt stress lowers the water potential of soil solution, which also interrupts water conductivity of roots, ultimately leading to a reduction in water and mineral influx to the plant body [35].
High concentrations of Na + and Cl − in soil solution restrict uptake and accumulation of macronutrients like Ca 2+ and Mg 2+ , which perturbs signal transduction, membrane stability, chlorophyll synthesis, and stomatal opening [34,36]. Furthermore, salinity's adverse effects on photosynthesis are intricately associated to the destruction of photosynthetic pigments, and the underlying facts are still not adequately elucidated. Exposure of plants to salt stress causes a decrease in stomatal density, stomatal conductance, photosynthetic activity, and carbon assimilation, but an increase in mesophyll resistance, thereby reducing light absorption efficiency of photosystem (PS)-I and PS-II [37]. Salt stress also suppresses the activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), a key enzyme involved in carbon dioxide (CO 2 ) assimilation during the dark phase of photosynthesis [38]. In addition, salinity also induces the production of ROS, including hydroxyl radical (OH • ), superoxide (O 2 •− ), hydrogen peroxide (H 2 O 2 ) and singlet oxygen ( 1 O 2 ), in cellular organelles like chloroplasts, mitochondria, peroxisomes, and endoplasmic reticulum [12,13]. The elevated levels of ROS can negatively affect plant growth and developmental processes by causing protein synthesis reduction, cell membrane destruction, genomic instability, and damage to photosynthetic apparatus [13,39]. Although Cl − is the most abundant anion in saline soils, its effects on plant performance, as well as its uptake and transport mechanisms within plants have still received less attention than those of Na + [40]. As an essential micronutrient, Cl − plays a putative role in enzyme activation in the cytoplasm, oxygen evolution during photosynthesis, stabilization of membrane potential, and maintenance of turgor pressure and pH of the plant cells [40]. However, once excessively accumulated, Cl − has even more detrimental effects than Na + in destabilizing the normal functions of the cells [40]. All above-mentioned salinity-induced adverse effects on plants trigger diminution of crop yield, although the intensity and damage vary depending on plant species.

Halophytes and Their Classification
Halophytes are plants that have the ability to survive and complete their life cycles in environments with high salinity, without suffering major negative impacts on their growth or development [15]. Based on their levels of tolerance to different types of saline soils, halophytes can broadly be categorized into two groups: obligate halophytes and facultative halophytes. Obligate halophytes can sustain their sufficient growth and development in high saline habitats containing salinity levels similar to seawater [41]. On the other hand, facultative halophytes are generally found in low saline habitats, although they can thrive in salt-devoid environments as well [42]. Additionally, Von Sengbusch [43] used the eco-physiological characteristics to distinguish obligate and facultative halophytes from habitat-independent halophytes. Habitat-independent halophytes are not necessarily native to saline habitats; they usually prefer to grow in salt-free soils but can cope with the saline conditions [43].
Nevertheless, the comprehensive classification of halophytes is still ambiguous because scientists defined halophytes by considering different aspects [44], including integration of anatomical observations, intensity of salinity and mechanisms of salt tolerance [45,46]. More recently, Santos et al. [47] developed a database of halophytes, namely eHALOPH (http://www.sussex.ac.uk/affiliates/halophytes/; accessed on 20 July 2021), which accumulates the information of plant species that can survive under 80 mM or higher salt concentrations. The authors classified the halophytes into seven categories: hydrohalophytes, chasmophytes, phreatophytes, psammophiles, xerophytes, weedy halophytes, and xero-halophytes. Hydro-halophytes typically grow in aquatic conditions or on wet soil; chasmophytes are found on cliff-tops, rocky and sandy seashores, and saltmarshes; phreatophytes are deep-rooted plants obtaining water from a deep underground source that may or may not be saline; psammophiles are usually found in sandy soils; xerophytes are adapted to extreme drought-prone areas; weedy halophyte species are predominantly invading and colonize highly disturbed sites or regions; and xero-halophytes are adapted to inland salt desert and saline habitats [47].
In addition, based on the physiological basis of salt tolerance as well as the accumulation and transportation of ions, Breckle et al. [48] classified halophytes into three major categories, including recretohalophytes, euhalophytes, and salt-exclusion halophytes (also referred as salt excluders). Recretohalophytes have unique salt-secreting structures, such as salt glands and salt bladders, whereas euhalophytes have the ability to dilute absorbed salts in their succulent leaves or stems. Salt excluders, on the other hand, can prevent the uptake of salt ions from soils or shed leaves containing toxic levels of salt [49,50]. With these complex classifications, it is important to acquire knowledge on their extent of salt tolerance levels, and what the intrinsic mechanisms they adopt to combat salinity effects.

Physiological Mechanisms Associated with Halophyte Adaptation to Soil Salinity
Comprehensive research on halophytic plants is crucial for a better understanding of their salt tolerance mechanisms and their exploration for breeding purposes. To withstand and continue to be grown well in a saline environment, halophytic plants employ different morphophysiological strategies, such as salt exclusion, salt excretion through specialized organs, i.e., salt glands and/or salt bladders, and dilution of salt ions by succulence. We will review these morphophysiological mechanisms in light of their functions in halophytes' resistance to salt stress (Figures 2 and 3; Table 1).

Salt Exclusion
Halophytic plants, particularly hydro-halophytes and some phreatophytes, were reported to exclude excessive salts by root system-induced ultrafiltration mechanisms. More specifically, installing apoplastic barriers at the roots improves bypass flow resistance, resulting in effective exclusion of salts from the roots and, as a result, a reduced accumulation of toxic ions in the aboveground shoots through the transpiration stream [51] (Figure 2; Table 1). The root endodermis and exodermis are two distinct cell barrier layers with highly specialized roles ascribed to two particular cell wall features: (a) the Casparian band (CB) and (b) the suberin lamella (SL) (Figure 2). The CB, a paracellular lignin deposition in the endodermis and exodermis cell walls, forces all apoplastic transport into the strictly regulated symplastic system [52,53]. Furthermore, SL, primarily made of long-chain fatty acids, permeates the whole exodermis/endodermis cell wall, providing a hydrophobic barrier that aids in ion and water uptake regulation [54,55]. SL is thought to play an essential role in preventing the direct movement of water and ions from the apoplast into the endodermal protoplasts [56,57]. It was recently discovered that SL acts as an apoplastic barrier at the site of lateral root emergence where CBs have not yet developed [57]. A limited number of studies have been performed on halophytes and shown that apoplastic barriers are crucial for salt adaptation of Suaeda maritima [58], Avicennia officinalis [59], S. salsa [60], and A. marina [61]. However, in comparison with CBs, there is less evidence for the role of SL in plant salt tolerance; and hence, more research is needed to assess the role of SL in ion exclusion from plants under salinity.
Importantly, salt overly sensitive 1 (SOS1) in the cortical cells plays an essential role in exclusion of Na + from roots ( Figure 2). Recently, Liu et al. [62] reported that S. salsa exhibited greater expression of S. salsa SOS1 (SsSOS1) in roots of intertidal S. salsa population than in those of inland S. salsa population, which was correlated with the increased exclusion of Na + by roots; and thus, it could thrive in a high saline environment (Table 1). Similarly, greater expression of Karelinia caspia SOS1 (KcSOS1) gene in the roots of the recretohalophyte K. caspia was also observed [63] (Table 1). However, the mechanisms underlying root system-induced ultrafiltration under saline conditions remain elusive and require further in-depth investigations. Many reports have recommended that salt exclusion mechanism as a cure to reduce accumulations of toxic levels of Na + and Cl − in plant bodies [64]. A strong argument could, however, be raised against this mechanism since it may elevate soil salinity levels by restriction of salt uptake by roots [65].

Salt Excretion
Halophytes, particularly recretohalophytes, directly secrete remarkably higher contents of salt ions onto the leaf surface, for dealing with maintenance of cellular ion homeostasis, through a unique epidermal structure called salt glands, making them superior compared with other classes of halophytes and non-halophytes [50] (Figure 3). Several species of the Limonium genus, including L. bicolor, L. aureum, L. gmelinii, L. otolepis, and L. sinuatum, are good examples of recretohalophytes, which boost salt tolerance by enhancing salt secretion via the salt glands [66][67][68] (Table 1). Species of another recretohalophyte genus, Tamarix, are salt-tolerant by increasing the density of salt glands and the rate of salt secretion per salt gland [69] (Table 1). However, salt glands in different halophytes species possess diverse structural characteristics, and the number of cells that make up salt glands are different [50]. Still, their common features include the followings: (i) salt glands are surrounded by a thickened cuticle; (ii) having many plasmodesmata between the cells, a large number of highly developed mitochondria and many small vesicles in the cytoplasm; and (iii) having no chloroplasts in the cells [50,70].
Multi-cellular salt glands varying from 4 to 40 cells are found in dicotyledonous recretohalophytes, including L. bicolor [66], whereas bi-cellular salt glands are prominent in monocotyledonous recretohalophytes of the Poaceae family [71]. Ions secreted through salt glands include various cations (Na + , K + , Ca 2+ , Mg 2+ , Fe 2+ , Mn 2+ , and Zn 2+ ) and anions (Cl − , Br − , I − , SO 4 2− , PO 4 3− , and NO 3 − ), depending largely on the environment [50]. The actual pathways of salt secretion from salt glands remain to be determined, although several possible pathways based on recent studies have been suggested by Yuan et al. [70] and Lu et al. [50]. Nevertheless, it was also demonstrated that salt glands might impose detrimental threats to plants by causing leaf dehydration because the glands produce salt crystals on the leaf surface [65]. This problem can be minimized through de novo synthesis of compatible solutes, but it requires high energy, i.e., 30 to 109 molecules of ATP to produce one molecule of any compatible solute, which ultimately leads to a penalty in crop yield [65]. Putative salt uptake and accumulation mechanisms in an ideal halophyte plant (modified from Zhao et al. [11], Arif et al. [34], and Arbelet-Bonnin et al. [72]. Na + is transported from soils to/within plant root cells via two major pathways: symplastic and apoplastic. The major transporters/channels embedded in the root epidermis for up-taking Na + are GLRs, CNGCs, LCT1, PIP2, HKT2, HAK5, and AKT1. In the presence of Na + , K + is competitively uptaken via HKT2, HAK5, and AKT1. The entrance of Cl − into the root cells is mediated by separate transporters, including NRT. NSCC and GORK channels, on the other hand, are involved in salinity-induced K + exclusion from the root epidermis. Apoplastic barriers, consisting of Casparian bands and suberin lamellae in the epidermis and endodermis of the roots, can block and/or limit the bypass flow of Na + and other solutes moving to the aboveground parts. Passive Na + entry into the xylem is facilitated by NSCCs, while cotransporters like SOS1, CCC, and HKT2 are required for active loading of Na + into the xylem. Na + can return to xylem parenchyma using HKT1. The xylem-loaded Na + , K + and Cl − can be transported up through the transpiration stream, and subsequently enter into the aboveground organs via CCC and HKT1. Abbreviations: AKT1, Arabidopsis K + transporter 1 (a shaker-type K + channel); CCC, cation-chloride cotransporter; CNGC, cyclic nucleotide-gated channel; GORK, gated outwardly rectifying K + channel; GLR, glutamate receptor-like channel; HKT, high-affinity potassium transporter; HAK, high-affinity potassium uptake transporter; LCT, low-affinity cation transporter; NRT, nitrate transporter; NSCC, non-selective cation channel; PIP2, plasma membrane intrinsic protein; SOS1, salt overly sensitive 1.
Epidermal bladder cells (EBCs), also known as salt bladders, can be found in more than half of the halophytes [65,73]. The EBC have a modified non-glandular trichome structure that is divided into an apical and basal cell, and the apical cell gains a globular shape, typically with a cell volume of 500 nL and an average diameter of 1 mm [65,73,74] ( Figure 3). The diameter of EBCs often stretches~10× larger in comparison with normal epidermal cells to sequestrate excessive salts, by as much as 1000× more than the traditional leaf cell vacuoles can do [65].  [11], Arif et al. [34], Arbelet-Bonnin et al. [72], Tran et al. [75], and Himabindu et al. [76]. The absorption of Na + in plant cells is counterbalanced by active Na + extrusion through SOS1 Na + /H + exchanger (I). The RBOH-mediated ROS formation in the plasma membrane can activate ANN1induced Ca 2+ -signaling pathway (I). The Na + bound GIPCs can trigger Ca 2+ influx through an unknown Ca 2+ channel (I). Ca 2+ influx mediated by either ANN1 or GIPCs is essential for activating the SOS-signaling pathway (I). The SOS3 relays salt-triggered Ca 2+ signals to SOS2 kinase, forming the SOS3-SOS2 protein complex that phosphorylates SOS1 to induce Na + efflux (I). To maintain ion homeostasis, halophytes can also sequestrate excess Na + into the vacuoles and/or excrete Na + via two unique structures, namely salt glands and salt bladders (II and III). It is also possible to avoid excessive Na + accumulation in the cytosol by sequestrating it into the vacuoles (IV). Predominantly, Na + -sequestration in the vacuoles is conferred by tonoplast-based Na + /H + exchangers of the NHX family (e.g., NHX1 or NHX2), fueled by either H + -PPase or H + -ATPase pumps (IV). Another equally important mechanism of vacuolar Na + -sequestration is the efficient control of tonoplast slow vacuolar (SV) and fast vacuolar (FV) activating ion channels, which may allow vacuolar Na + to leak back into the cytosol (IV). Once, if both channels are downregulated or blocked by choline, Na + leakage is inhibited, resulting in an increased level of salt tolerance (IV). On the other hand, Cl − influx in the vacuoles is mediated by CLC (IV). To maintain cytosolic Ca 2+ homeostasis under saline conditions, the influx of Ca 2+ into the vacuole is mediated by CAX1 (IV). In response to salt stress, halophytes often accumulate various osmoprotectants, including proline, total free amino acids, sugars, polyols, glycine betaine, and GABA, which play vital roles in conferring an efficient osmotic adjustment (V). In addition, changes in the levels of different phytohormones may also help in stress adaptation; for instance, increased ABA level aids in controlling stomatal closure to maintain better water status at the tissue level (VI). It is worth mentioning that changes in the levels of phytohormones (e.g., JA, SA, and GA) in halophytes under salt stress varies dependently on the type of halophytes (VI).  More fundamentally, EBCs have long been considered imperative for maintaining low Na + levels in leaves, especially in young leaves [77]. Because younger leaves have small and underdeveloped vacuoles in the mesophyll cells, they rely solely on EBCs for internal salt sequestration [77]. Besides serving as a potential hotspot for storage of excessive Na + and Cl − , EBCs may also store different metabolic compounds, ROS-scavenging metabolites and organic osmoprotectants [78]. A positive correlation between the presence and/or the number of EBCs and salt tolerance was reported in Chenopodium quinoa [74,78]. Some candidate genes related to EBC formation were recently identified using transcriptome analysis in C. quinoa, Mesembryanthemum crystallinum, and Atriplex centralasiatica species [79][80][81] ( Table 1). More fundamentally, Imamura et al. [82] first identified a gene, named reduced epidermal bladder cells (REBC), involved in the EBC development in C. quinoa, laying the foundation for elucidating the EBC-associated stress tolerance mechanism in halophytes.

Succulence
Halophytes also adopt succulence mechanisms for ion homeostasis and accumulation of osmoprotectants to maintain cell turgor pressure under saline conditions [83,84] (Table 1). Succulent plants, popularly referred to as halo-succulents, have been characterized by thick leaves and stems, increased size of mesophyll cells, smaller intercellular space, enhanced tissue water content and high turgor pressure [83]. Succulent leaves possess more and stretched-sized mitochondria, which presumes to provide the excess energy required for salt compartmentalization and sequestration [84]. A large number of halophytes, including Salsola drummondii, Achras sapota, and Sarcocornia fruticosa, were reported to have efficient succulent mechanisms to confer salt tolerance via accumulation of elevated salts in their leaves and stems while maintaining improved photosynthetic efficiency [85][86][87] (Table 1). However, there is still much to learn about the 'nuts and bolts' of succulence. Therefore, current research should focus on (i) how the complex structural-functional coordination regulates the physiological processes in succulent plants; (ii) transcriptomics and geneediting techniques to provide insights into the functional biology of succulent plants; and (iii) genetic engineering to introduce succulence traits into non-succulent plants to enhance their resilience to salinity-caused environmental perturbations. • Enhanced levels of proline and soluble sugars for osmotic adjustment • Improved ascorbic acid and glutathione levels, and the activity of SOD, GPX, APX, CAT, and GR for protection against oxidative stress [89]

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Cakile maritima 400 • Enhanced levels of amino acids like asparagine, alanine, lysine, proline, leucine, glycine, and threonine for amino acid biosynthesis • Improved levels of sugars, such as glycerol, xylulose, and myo-inositol for osmoprotection • Enhanced levels of gamma amino butyric acid (GABA) and glutamic acid for osmotic adjustment and photorespiration process [97]

Biochemical Mechanisms Associated with Halophyte Adaptation to Salinity
Plants, including halophytes, adopt several biochemical strategies to adapt against salinity-caused adverse effects. For instance, ion homeostasis, osmotic adjustment, and ROS-detoxification are among the important biochemical mechanisms used by plants to protect their cellular constituents from the detrimental effects of excessive salt [76] (Figures 2 and 3; Table 1). Here, we summarize these three biochemical mechanisms associated with halophyte responses and tolerance to salt stress.

Ion Homeostasis
Various channels and transporters are actively involved in accommodating Na + , Cl − , and K + in the tissues of halophytic plants. It is well known that the uptake of Na + from soils to roots occurs mainly via the apoplastic and symplastic pathways [34]. To transport salt ions via the symplastic pathway, transporters and channels are needed. Generally, nonselective cation channels (NSCCs), including cyclic nucleotide-gated channels (CNGCs) and glutamate receptor-like channels (GLRs), high-affinity K + transporters (e.g., HKT2 and HAK5) and aquaporins, such as plasma membrane intrinsic protein (PIP) isoforms, are mainly responsible for root-Na + uptake ( Figure 2). The influx of Na + can also be aided by low-affinity cation transporter (LCT1) and Arabidopsis K + transporter 1 (AKT1) (Figure 2) [34]. The inward movement of Cl − in root cells is mediated by H + /Cl − symporters, Cl − /H + co-transporters, and nitrate transporters (NRTs). During high salinity, several anion channels, including slow anion channels (SLAC channels), are involved in the passive transport of Cl − [114]. Cation chloride cotransporters (CCCs) help in Cl − influx in the xylem and shoot tissues [115]. The chloride channels (CLCs) found on tonoplast are responsible for Cl − sequestration [75] (Figure 3). In the presence of Na + , K + is competitively uptaken via HKT2, HAK5, and AKT1 [11].
To prevent the accumulation of excess salt in their cytosol, halophytes mainly adopt two strategies: (i) prevention of salt entrance to the roots and (ii) vacuolar sequestration of salt ions [76] (Figure 3). However, these mechanisms are not mutually exclusive, and a particular plant may employ multiple strategies depending on the circumstances. For example, ion homeostasis in the cytosol is a unique strategy that plants follow to limit salt ion accumulation by compartmentalizing them effectively, which is essential for plant survival under salinity-triggered hostile conditions. The SOS system is crucial for perceiving salt-mediated Ca 2+ signals, as well as for the exclusion of excess Na + by roots, loading of Na + into the xylem and sequestration of excess Na + into the vacuoles using Na + /H + exchangers (NHXs) [11,116] (Figure 3). The SOS system is predominantly comprised of SOS3/SOS3-like calcium-binding proteins (SCaBPs), SOS2 (a serine/threonine-protein kinase) and SOS1 (a plasma membrane Na + /H + antiporter) [11,76] (Figure 3). Importantly, SOS4 (pyridoxal kinase) and SOS5 have recently been discovered, with SOS4 being involved in the biosynthesis of pyridoxal-5-phosphate (vitamin B6) that plays a key role in Na + /K + homeostasis in plants, and SOS5 being a putative cell surface adhesion protein that is required for normal cell expansion [116]. Generally, SOS3 relays salt-triggered Ca 2+ signals to SOS2 kinase, forming an SOS3-SOS2 protein complex that phosphorylates SOS1 to induce Na + efflux [11,84] (Figure 3). The elevated expression of the SOS1 gene and its correlation with the improvement of salt tolerance have been reported in the obligate halophyte Cakile maritima [104], ornamental halophyte Lobularia maritima [94], and recretohalophyte Karelinia caspia [63] (Table 1). Likewise, Marriboina et al. [107] and Arbelet-Bonnin et al. [104] reported that the simultaneous expression of SOS1, SOS2, and SOS3 contributed to high salinity tolerance in the phreatophyte Pongamia pinnata and obligate halophyte C. maritima species, respectively (Table 1).

Osmotic Adjustment
Plants can attain osmotic adjustment by employing two strategies: (i) increased uptake of inorganic ions, and (ii) de novo synthesis of organic osmolytes to maintain turgor pressure and organellar volume within the growing plant cells [11]. Fundamentally, Na + and Cl − in vacuoles, as well as K + and compatible organic solutes like proline and sugars in the cytoplasm, are crucial to assure osmotic adjustment in the cells in a salt-caused osmotic environment (Figure 3) [117]. Generally, salinity-induced K + efflux from the cell takes place via two major types of channels, including guard cell outward rectifying K + (GORK) channels and/or ROS-activated NSCCs [11] (Figure 2). Although GORK channels play a central role in stress-induced K + loss from the cytosol, comprehensive research into the molecular background of GORK channel-mediated osmotic balance in halophytes is still elusive. Nonetheless, by studying two contrasting C. quina accessions, Kiani-Pouya et al. [95] have recently revealed that the transcript levels of CqGORK in roots remained unchanged or decreased in salt-tolerant accession but increased in salt-sensitive accession, implying GORK's role in K + retention for better osmotic adjustment in the cells of salt-sensitive C. quina. Furthermore, the ion selectivity approach of HAKs/HKTs has been reported to confer greater uptake of K + in Eutrema parvula [105] and Hordeum brevisubulatum [99] and, thus, contributed to improved K + /Na + homeostasis (Table 1). Nonetheless, future studies should be taken to decipher how the movement of K + between intra-cellular and inter-cellular membranes by K + transporters contribute to the adjustment of the osmotic environment in salt-exposed halophytic plants.
Halophytes often produce a multitude of osmoprotectant molecules, including amino acids, soluble sugars, sugar alcohols and glycine betaine to maintain osmotic balance under salinity [118] (Figure 3; Table 1). The accumulation of proline and the elevated expression of delta 1-pyrroline-5-carboxylate synthase (P5CS) gene involved in proline biosynthesis, were associated with improved salt tolerance in M. crystallinum [75] and Urochondra setulosa [96] ( Table 1). In addition, salt tolerance by glycine betaine has also been recorded in Atriplex halimus [108] and Sesuvium portulacastrum [109] (Table 1). It is worth mentioning that metabolite profiling in halophytes has recently gained momentum as a means of gaining a comprehensive understanding of the levels of accumulated osmolytes in relation to salt tolerance. For instance, Panda et al. [88] and Kumari and Parida [106] uncovered the saltresponsive metabolites and metabolic pathways in xero-halophyte Haloxylon salicornicum and facultative halophyte Salvadora persica plant, respectively (Table 1). They found that salt-exposed H. salicornicum and S. persica had greater levels of sugars, sugar alcohols, and amino acids, indicating that these osmolytes might contribute to the osmotic adjustment, resulting in salt tolerance in these plants (Table 1). Furthermore, metabolite profiling of C. maritima exposed to 400 mM sodium chloride (NaCl) showed that proline, glycine betaine and gamma amino butyric acid (GABA) significantly accumulated, while sugar levels surprisingly decreased, indicating that amino acids and amino acids-derived molecules played the primary roles in osmoprotection to cope with salt stress in this halophyte [97] ( Table 1). Nonetheless, further exploration of metabolite changes in other halophytes should be undertaken to deepen our existing knowledge on putative salt tolerance mechanisms in halophytes.

ROS-Detoxification
Plants rely on efficient antioxidant defense mechanisms to protect them from oxidative stress, induced by the overaccumulation of ROS under elevated salinity [119]. To alleviate ROS-induced damage, plants possess a robust antioxidant defense system, which comprises of non-enzymatic antioxidants like flavonoids, AsA, GSH, and tocopherols, and enzymatic antioxidants, such as SOD, CAT, APX, glutathione reductase (GR), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), GPX, and GST [39] (Figure 3). The improved salt tolerance of various halophytes is associated with their ability to maintain ROS homeostasis by a proper activation of their antioxidant system under salt stress [89,90,94,100,101,120]. For example, in H. salicornicum plant, the higher activity of SOD, higher level of GSH and higher ratios of AsA/dehydroascorbate (DHA) and GSH/oxidized glutathione (GSSG) play a pivotal role in scavenging of ROS to withstand oxidative stress and improve salt tolerance [101] (Table 1).
In response to salt stress, halophytic plants modulate the levels of endogenous phytohormones in order to regulate various defense mechanisms, including antioxidant defense [91,121]. Detailed roles of phytohormones in halophyte responses to abiotic stresses have been reviewed in-depth elsewhere [121]. In halophytes, phytohormone accumulation varies depending on the types of halophytes exposed to salt stress. For instance, jasmonic acid (JA) content increased in S. persica [106] but dropped in Aster tripolium [91] and H. salicornicum [88] under salt stress ( Figure 3, Table 1). The levels of salicylic acid did not alter significantly in S. persica [106] and H. salicornicum [88], while being remarkably dropped in A. tripolium [91] (Figure 3). The content of gibberellic acid (GA) increased noticeably in S. salsa [98], while it decreased in H. salicornicum [88] (Figure 3, Table 1). Because JA can be produced via linolenic acid oxidation [122], the JA level in plants is linked to the amount of ROS and the rate of ROS-induced lipid peroxidation [123]. Indeed, a lower level of JA was positively correlated with the reduced level of ROS under salt stress [88]. On the other hand, the level of abscisic acid (ABA), which acts as a critical hormone in stomatal closure to reduce transpiration, significantly increased in Lycium humile, S. persica, H. salicornicum and A. tripolium in response to salt stress [88,91,92,106] (Table 1). Panda et al. [88] reported that enhanced ABA accumulation helped in reduction of oxidative stress in salt-exposed H. salicornicum. The generation of ROS and subsequent redox processing are an integral part of hormone regulation and have a role in controlling plant development and stress tolerance [124]. Therefore, future studies should focus on spatial-temporal regulation of ROS generation and identification of proteins that could detect changes in ROS. Such information may help us further understand the crosstalk among multiple hormones signaling pathways in halophytes, and their hormone-modulated stress-tolerant phenotype.

Halophyte-Mediated Phytoremediation of Salinity-Affected Soils
Extensively used methods to reclaim salt-affected areas are leaching and adding chemicals and organic amendments [125]. Leaching facilitates the movement of salt particles from the surface horizon to the deeper horizon. However, lack of water during leaching and its effects on reduced soil stability, contents of total nitrogen and total organic carbon, microbial functions and overall soil fertility limit its practice in salt-contaminated areas [126]. Chemical approach involves an ion exchange process through existing and/or adding calcium carbonate (CaCO 3 ) or other chemical substitutes to the soils; however, poor solubility and quality and high prices restrain further expansion of chemical uses in reclaiming of saline soils [126,127]. Interestingly, optimal rates of the different organic amendment (not more than 50 T ha −1 ) can efficiently increase dissolution of CaCO 3 [128], as well as improve soil-physical (e.g., structure, permeability, water holding capacity, etc.) and chemical properties (e.g., pH, cation exchange capacity, etc.) that favors plant growth and microbial activities [129]. However, benefits are restrained due to the risk of releasing various chemicals, including heavy metals, phenolic compounds, ethylene and ammonia, excess salts, and organic acids from uncategorized organic wastage [129][130][131][132]. All of these constraints have forced the farmers to look for alternative options that are environment-friendly, cost-effective, and aesthetically pleasing.
Phytoremediation, popularly known as vegetative bioremediation or biological reclamation, is a promising option over other remediation techniques, which involves cultivating certain salt-tolerant species to remove salt ions from the soil to restore salinity-degraded soils [20,126,133]. Its acceptance is increasing progressively due to numerous benefits. For example, phytoremediation (i) is economically viable, as it does not require the purchase of chemicals; (ii) enhances nutrient availability for plants in soil, which helps boost crop yield; (iii) removes excess salts more uniformly, even from higher depth when compared with gypsum; and (iv) is environment-friendly as it sequestrates carbon in both aboveand below-ground biomasses [47]. Unfortunately, information regarding mechanistic insights into phytoremediation of saline soils using halophytes is still elusive. Because of complicated interactions, it is challenging to visualize plants' complete and holistic mode-of-action in the saline soil remediation process, and their beneficial effects on soil properties. According to our current knowledge, we have illustrated a well-established and recognized underlying phytoremediation mechanism of halophytes in Figure 4.
First, halophytic plants can help remove salts by taking salt ions through root system and store them in aboveground plant parts (Figure 4). S. salsa has recently been successfully used to reclaim saline soil in the northwest of China due to its salt extraction potential, which ranges from 3.75 to 3.91 T ha −1 year −1 [133]. It is important to note that S. salsa can theoretically remove 3.0-3.8 T Na + ha −1 with a population density of 15 plants m −2 [134]. Liang and Shi [134] also reported that the aboveground biomass of S. salsa plants accumulated approximately 27 mg of salt g −1 dry weight. Shaygan et al. [135] demonstrated that the shoots of Tecticornia pergranulata could accumulate 98 g of salt Kg −1 dry weight.
Sodium chloride is often used to lower the freezing point of water on North American highways to prevent the risk of vehicle crashes and injuries [136]. The same study [136] showed that A. patula would take six years to remove all the salt from an area contaminated with 1540 g Cl − per m 2 , if salt application was halted. On the other hand, since salt is applied every winter, A. patula would be a potential candidate for the management of salt amount accumulated over time in the roadside soil and groundwater.
Second, some studies suggest that planting halophytes could reduce salt ion accumulation in soils by improving soil physical properties. Briefly, the deep-rooted halophytic plants have the potential to act as tillage tools. This is popularly referred to as biological drilling and has been reported to stabilize soil structure by reducing soil bulk density, thereby increasing soil porosity and hydraulic properties that augment Na + leaching and replace it with other cations, besides inducing higher nutrient use efficiency [137,138] ( Figure 4). In line with this finding, planting S. salsa with cotton (Gossypium hirsutum) as an intercropping system significantly decreased soil salinity and bulk density, while increasing soil porosity, soil organic carbon, root growth, total aboveground biomass, and cotton yield, when compared with the traditional monocropping system [134]. In addition, A. nummularia showed improvement of hydraulic conductivity and reduction in bulk density, which led to enhanced macropores and water infiltration rate under 40.86 dS m −1 salinity level [138]. Ashraf et al. [139] also reported that planting halophytes reduced bulk density, while it increased soil porosity and hydraulic conductivity, which eventually improved salt ion leaching from saline-alkali soil.
Third, root respiration and decomposition of organic matter play a pivotal role in reducing Na + content in the saline-sodic soils because they can increase CO 2 concentration in the soil atmosphere [140]. In brief, irrigation water and soil moisture-induced availability of soil water react with CO 2 to form carbonic acid (H 2 CO 3 ) (Figure 4). The H 2 CO 3 dissociation and N 2 -fixation in halophyte roots lead to the release of the proton (H + ), which causes dissolution of calcite (CaCO 3 ) to produce Ca 2+ , H 2 O and CO 2 [140] (Figure 4). The released Ca 2+ in the soil could facilitate the removal of Na + from the cation exchange sites of soil colloid [140].The exchanged Na + content in soils is then eliminated via two routes: (i) uptake through halophytic plant roots, and (ii) leaching as a result of flooding irrigation prior to sowing, which ultimately contributes to the reduction in soil sodicity and salinity ( Figure 4).
Nonetheless, in non-calcareous soils, augmentation of CO 2 results in the lowering of soil pH that enhances dissolution of CaCO 3 to make Ca 2+ available to interchange with Na + [140]. Planting two facultative halophytes, S. verrucosum and Bacopa monnieri, for 240 days reduced the electrical conductivity (EC) value from 11.13 to 7.97 dS m −1 , pH from 7.84 to 7.42, while improving soil porosity from 54.71 to 57.23%. Notably, the combined plantation of these two halophytes had a phytodesalination capacity of 1.21 T Na + ha −1 , which helped prepare the soil for Zea mays to thrive and boosted yields from 7.2 to 8.5 T ha −1 [141]. S. monoica, a salt marsh halophyte, has been shown to lower the soil EC value from 4.75 to 2.10 dS m −1 and pH from 8.40 to 6.73 [142].
However, the use of halophytes as a desalinization tool cannot be guaranteed in all situations, since this approach is affected by environmental inconsistencies, such as rainfall and temperature, or cultivation modes, such as irrigation and non-irrigation, as well as the risk of recycling back of the salt particles through plant parts [143]. Therefore, robust assessments of the potential of various halophytes by analyzing their mode-ofaction are needed prior to their application in the desalinization of salt-affected areas. In addition, further research will be required to testify the suitability of the plant-uptakemediated phytoremediation process, and if found significant, it will open a new avenue for restoring salinity-affected areas, notably non-calcareous soils and soils under non-leaching circumstances.  [140] and Jesus et al. [126]. Deep-rooted plants help reduce soil bulk density and increase soil porosity, enabling storage of excessive salt ions in deeper soil layers through leaching. On the other hand, the reclamation of saline soils needs a source of Ca 2+ that replaces excessive Na + from the cation exchange sites of soil colloid. Decay of plant parts enhances soil organic matter (SOM) content, leading to the improvement of soil fertility. The microbes-mediated SOM decomposition and root respiration increase the concentration of carbon dioxide (CO 2 ) in the soil atmosphere. Water (H 2 O) from irrigation and soil moisture sources reacts with CO 2 to generate carbonic acid (H 2 CO 3 ). In halophyte roots, H 2 CO 3 dissociation and N 2 -fixation release proton (H + ) that enhances dissolution of calcium carbonate (CaCO 3 ) to release Ca 2+ , H 2 O and CO 2 . The released Ca 2+ could help remove Na + from cation exchange sites of soil colloid. Finally, the exchanged Na + can be uptaken by roots, and compartmentalized in aboveground shoots or leached into the deeper layer of soils via irrigation. In non-calcareous soils, an increase in CO 2 induces the release of H + , which results in a decrease in the pH of the soil. The acidic nature of soil enhances the dissolution of CaCO 3 , which helps reduce the amount of salt ions in soils via Ca 2+ exchange with Na + .

Potential of Halophytes as Genetic Resources for the Engineering of Crops with Improved Salt Tolerance
Halophytes can survive better under ambient salinity than glycophytes, which might be attributable to proper modulation of genes that play crucial roles in triggering their salt tolerance mechanisms [76,144]. High-throughput transcriptome sequencing is increasingly being used to understand the responses of plants to abiotic stresses at gene expression level [145]. Recently, Guo et al. [146] conducted a comparative transcriptome analysis of the model halophyte Puccinellia tenuiflora and several glycophytic Gramineae plants to uncover their underlying molecular responses to excessive salt stress. They found that the greater expression of the gene families involved in K + uptake, sucrose biosynthesis, pentose phosphate pathway and flavonoid biosynthesis might be associated with better growth of P. tenuiflora under high salinity conditions. In Salicornia persica, Aliakbari et al. [147] identified 1595 differentially expressed genes (DEGs) between the control and salt-treated plant shoots, including those encoding transcription factors, transporters, and protein kinases. Functional annotation analyses indicated that improved energy homeostasis and the synthesis of primary metabolites might play pivotal roles in salt tolerance of S. persica. Gene network analysis revealed that Ca + signaling, ABA signaling, and Na + compartmentalization are essential components in conferring salt tolerance in S. persica. Han et al. [148] identified 399 upregulated and 327 downregulated genes in S. rigida plants in responses to different regimes of soil salinity (100, 300, and 500 mM NaCl). Gene ontology (GO) enrichment analysis suggested that the downregulated genes were related to organic substance transport, nitrogen compound transport and intracellular protein transport. In contrast, the upregulated genes were involved in cell wall biogenesis, such as cell wall assembly, extracellular matrix organization, and cell wall formation [148].
Moreover, RNA-seq identified 9144 DEGs in A. centralasiatica leaves under salt stress, with 3819 upregulated and 5325 downregulated genes [81]. GO enrichment analysis showed that salt stress dramatically increased the expression of genes associated with ion transport, ABA-dependent signaling pathway, ROS-scavenging and transcriptional regulation (HB-7 and MYB78) pathway [81]. A de novo transcriptome profiling in the halophyte U. setulosa revealed the upregulation of genes encoding photosynthetic enzymes, transcription factors, mitogen-activated protein kinases (MAPKs), transporter proteins, cell membrane proteins, antioxidant enzymes, and enzymes for synthesis of compatible solutes by salinity [96]. Wang et al. [100] revealed many DEGs, specifically 4903 upregulated and 3800 downregulated genes in Z. macrostachya, in response to 300 mM NaCl. Such expression change in a large number of genes may contribute to improved plant growth performance by modulating plant hormone signal transduction, ion homeostasis via salt secretion, and osmoregulatory substance accumulation and prevention of oxidative damage through the activation of an efficient ROS-scavenging system.
Halophytes with intrinsic and efficient salt tolerance abilities can be used as models for improving crop and/or plant responses to salt stress (Table 2). Furthermore, because crops are predominantly glycophytes, it is critical to highlight the molecular differences between these two plant groups. Such findings could be the key to guiding and optimizing glycophyte transformation using halophytic candidate genes, thereby providing a new avenue for study aimed at preparing crops to cope with elevated soil salinity. However, despite their impressive implication, information regarding identification, metabolic functions, and isolation of crucial genes from halophytes that are associated with salt tolerance is still limited. Therefore, identification and characterization of the novel salt toleranceassociated genes from halophytes, and their subsequent utilization in genetic engineering are important areas of plant research, which deserves more attention to sustainably increase crop production for achieving global food security under the era of climate change.

Conclusions and Future Perspectives
Salt-induced land degradation restrains crop production and, thus, endangers sustainable food production and food security in the world. Climate-driven changes and anthropogenic activities have aggravated salinity problems, pressing the need to cultivate plant species that can withstand ambient salinity. Halophytes are a diverse group of plants with excellent adaptabilities to high salinity at both cell and whole-plant levels, and thus, have significant potential to meet this such cultivation challenge and be successfully integrated into future farming systems. Several halophytes are able to grow naturally in ambient salinity through the adoption of intrinsic salt tolerance mechanisms. These halophytes could be used as potent commercial alternatives to conventional crops, although commercialization of halophytic products has just started. At the same time, the cultivation of halophytic species in salinity-affected areas can reduce salt contamination of soils. Thus, this phytoremediation technique could be the most potent and viable strategy to remediate salinity-affected lands and facilitate crop production in salt-prone areas.
Despite its tremendous potential in soil salinity management, phytoremediation has limitations since it is a time-consuming process and requires screening out of suitable species. Therefore, future research should focus on sorting out halophytic plants with multiple salt tolerance mechanisms, which can produce higher biomass for removing the maximum quantity of salts from contaminated soils. Furthermore, despite tremendous research advances on halophytes (for more than 100 years), some basic physiological, biochemical and molecular mechanisms are still unclear. Hence, in-depth research is eminently needed to unravel halophytes' underlying salt tolerance mechanisms for better soil conservation. Furthermore, recent genetic and omic studies showed that salt tolerance is a complex trait that depends on whole plant responses rather than the action of a single gene or protein. Therefore, exploration of the salt tolerance potential of various halophytes and the development of transgenic plants with superior remediation features are crucial for sustainable agriculture in salinity-affected areas to reduce the curse of global climate change.