Shoot Na+ exclusion and increased salinity tolerance engineered by cell type-specific alteration of Na+ transport in Arabidopsis

Metabolic adaptations leading to an enhanced lignification in wheat roots under salinity stress

Analysis of salinity tolerance processes in wheat has focused on salt exclusion from shoots while root phenotypes have received limited attention. Here, we consider the varying phenotypic response of four bread wheat varieties that differ in their type and degree of salt tolerance and assess their molecular responses to salinity and changes in root cell wall lignification. These varieties were Westonia introgressed with Nax1 and Nax2 root sodium transporters (HKT1;4-A and HKT1;5-A) that reduce Na+ accumulation in leaves, as well as the 'tissue tolerant' Portuguese landrace Mocho de Espiga Branca that has a mutation in the homologous gene HKT1;5-D and has high Na+ concentration in leaves. These three varieties were compared with the relatively more salt-sensitive cultivar Gladius. Through the use of root histochemical analysis, ion concentrations, as well as differential proteomics and targeted metabolomics, we provide an integrated view of the wheat root response to salinity. We show different metabolic re-arrangements in energy conversion, primary metabolic machinery and phenylpropanoid pathway leading to monolignol production in a genotype and genotype by treatment-dependent manner that alters the extent and localisation of root lignification which correlated with an improved capacity of wheat roots to cope better under salinity stress.

Variation in TaSPL6-D confers salinity tolerance in bread wheat by activating TaHKT1;5-D while preserving yield-related traits

Na+ exclusion from above-ground tissues via the Na+-selective transporter HKT1;5 is a major salt-tolerance mechanism in crops. Using the expression genome-wide association study and yeast-one-hybrid screening, we identified TaSPL6-D, a transcriptional suppressor of TaHKT1;5-D in bread wheat. SPL6 also targeted HKT1;5 in rice and Brachypodium. A 47-bp insertion in the first exon of TaSPL6-D resulted in a truncated peptide, TaSPL6-DIn, disrupting TaHKT1;5-D repression exhibited by TaSPL6-DDel. Overexpressing TaSPL6-DDel, but not TaSPL6-DIn, led to inhibited TaHKT1;5-D expression and increased salt sensitivity. Knockout of TaSPL6-DDel in two wheat genotypes enhanced salinity tolerance, which was attenuated by a further TaHKT1;5-D knockdown. Spike development was preserved in Taspl6-dd mutants but not in Taspl6-aabbdd mutants. TaSPL6-DIn was mainly present in landraces, and molecular-assisted introduction of TaSPL6-DIn from a landrace into a leading wheat cultivar successfully improved yield on saline soils. The SPL6-HKT1;5 module offers a target for the molecular breeding of salt-tolerant crops.

SOS1 tonoplast neo-localization and the RGG protein SALTY are important in the extreme salinity tolerance of Salicornia bigelovii

The identification of genes involved in salinity tolerance has primarily focused on model plants and crops. However, plants naturally adapted to highly saline environments offer valuable insights into tolerance to extreme salinity. Salicornia plants grow in coastal salt marshes, stimulated by NaCl. To understand this tolerance, we generated genome sequences of two Salicornia species and analyzed the transcriptomic and proteomic responses of Salicornia bigelovii to NaCl. Subcellular membrane proteomes reveal that SbiSOS1, a homolog of the well-known SALT-OVERLY-SENSITIVE 1 (SOS1) protein, appears to localize to the tonoplast, consistent with subcellular localization assays in tobacco. This neo-localized protein can pump Na+ into the vacuole, preventing toxicity in the cytosol. We further identify 11 proteins of interest, of which SbiSALTY, substantially improves yeast growth on saline media. Structural characterization using NMR identified it as an intrinsically disordered protein, localizing to the endoplasmic reticulum in planta, where it can interact with ribosomes and RNA, stabilizing or protecting them during salt stress.

Salt tolerance in mungbean is associated with controlling Na and Cl transport across roots, regulating Na and Cl accumulation in chloroplasts and maintaining high K in root and leaf mesophyll cells

Salinity tolerance requires coordinated responses encompassing salt exclusion in roots and tissue/cellular compartmentation of salt in leaves. We investigated the possible control points for salt ions transport in roots and tissue tolerance to Na+ and Cl- in leaves of two contrasting mungbean genotypes, salt-tolerant Jade AU and salt-sensitive BARI Mung-6, grown in nonsaline and saline (75 mM NaCl) soil. Cryo-SEM X-ray microanalysis was used to determine concentrations of Na, Cl, K, Ca, Mg, P, and S in various cell types in roots related to the development of apoplastic barriers, and in leaves related to photosynthetic performance. Jade AU exhibited superior salt exclusion by accumulating higher [Na] in the inner cortex, endodermis, and pericycle with reduced [Na] in xylem vessels and accumulating [Cl] in cortical cell vacuoles compared to BARI Mung-6. Jade AU maintained higher [K] in root cells than BARI Mung-6. In leaves, Jade AU maintained lower [Na] and [Cl] in chloroplasts and preferentially accumulated [K] in mesophyll cells than BARI Mung-6, resulting in higher photosynthetic efficiency. Salinity tolerance in Jade AU was associated with shoot Na and Cl exclusion, effective regulation of Na and Cl accumulation in chloroplasts, and maintenance of high K in root and leaf mesophyll cells.

A high-affinity potassium transporter (MeHKT1) from cassava (Manihot esculenta) negatively regulates the response of transgenic Arabidopsis to salt stress

BACKGROUND

High-affinity potassium transporters (HKTs) are crucial in facilitating potassium uptake by plants. Many types of HKTs confer salt tolerance to plants through regulating K+ and Na+ homeostasis under salinity stress. However, their specific functions in cassava (Manihot esculenta) remain unclear.

RESULTS

Herein, an HKT gene (MeHKT1) was cloned from cassava, and its expression is triggered by exposure to salt stress. The expression of a plasma membrane-bound protein functions as transporter to rescue a low potassium (K+) sensitivity of yeast mutant strain, but the complementation of MeHKT1 is inhibited by NaCl treatment. Under low K+ stress, transgenic Arabidopsis with MeHKT1 exhibits improved growth due to increasing shoot K+ content. In contrast, transgenic Arabidopsis accumulates more Na+ under salt stress than wild-type (WT) plants. Nevertheless, the differences in K+ content between transgenic and WT plants are not significant. Additionally, Arabidopsis expressing MeHKT1 displayed a stronger salt-sensitive phenotype.

CONCLUSION

These results suggest that under low K+ condition, MeHKT1 functions as a potassium transporter. In contrast, MeHKT1 mainly transports Na+ into cells under salt stress condition and negatively regulates the response of transgenic Arabidopsis to salt stress. Our results provide a reference for further research on the function of MeHKT1, and provide a basis for further application of MeHKT1 in cassava by molecular biological means.

Structural insights into the Oryza sativa cation transporters HKTs in salt tolerance

The high-affinity potassium transporters (HKTs), selectively permeable to either Na+ alone or Na+/K+, play pivotal roles in maintaining plant Na+/K+ homeostasis. Although their involvement in salt tolerance is widely reported, the molecular underpinnings of Oryza sativa HKTs remain elusive. In this study, we elucidate the structures of OsHKT1;1 and OsHKT2;1, representing two distinct classes of rice HKTs. The dimeric assembled OsHKTs can be structurally divided into four domains. At the dimer interface, a half-helix or a loop in the third domain is coordinated by the C-terminal region of the opposite subunit. Additionally, we present the structures of OsHKT1;5 salt-tolerant and salt-sensitive variants, a key quantitative trait locus associated with salt tolerance. The salt-tolerant variant of OsHKT1;5 exhibits enhanced Na+ transport capability and displays a more flexible conformation. These findings shed light on the molecular basis of rice HKTs and provide insights into their role in salt tolerance.

A novel maize F-bZIP member, ZmbZIP76, functions as a positive regulator in ABA-mediated abiotic stress tolerance by binding to ACGT-containing elements

The group F-bZIP transcription factors (TFs) in Arabidopsis are involved in nutrient deficiency or salt stress responses. Nevertheless, our learning about the functions of group F-bZIP genes in maize remains limited. Here, we cloned a new F-bZIP gene (ZmbZIP76) from maize inbred line He344. The expression of ZmbZIP76 in maize was dramatically induced by high salt, osmotic stress and abscisic acid. Accordingly, overexpression of ZmbZIP76 increased tolerance of transgenic plants to salt and osmotic stress. In addition, ZmbZIP76 functions as a nuclear transcription factor and upregulates the expression of a range of abiotic stress-responsive genes by binding to the ACGT-containing elements, leading to enhanced reactive oxygen species (ROS) scavenging capability, increased abscisic acid level, proline content, and ratio of K+/Na+, reduced water loss rate, and membrane damage. These physiological changes caused by ZmbZIP76 ultimately enhanced tolerance of transgenic plants to salt and osmotic stress.

Identification and transcriptomic profiling of salinity stress response genes in colored wheat mutant

Background

Salinity is a major abiotic stress that prevents normal plant growth and development, ultimately reducing crop productivity. This study investigated the effects of salinity stress on two wheat lines: PL1 (wild type) and PL6 (mutant line generated through gamma irradiation of PL1).

Results

The salinity treatment was carried out with a solution consisting of a total volume of 200 mL containing 150 mM NaCl. Salinity stress negatively impacted germination and plant growth in both lines, but PL6 exhibited higher tolerance. PL6 showed lower Na+ accumulation and higher K+ levels, indicating better ion homeostasis. Genome-wide transcriptomic analysis revealed distinct gene expression patterns between PL1 and PL6 under salt stress, resulting in notable phenotypic differences. Gene ontology analysis revealed positive correlations between salt stress and defense response, glutathione metabolism, peroxidase activity, and reactive oxygen species metabolic processes, highlighting the importance of antioxidant activities in salt tolerance. Additionally, hormone-related genes, transcription factors, and protein kinases showed differential expression, suggesting their roles in the differential salt stress response. Enrichment of pathways related to flavonoid biosynthesis and secondary metabolite biosynthesis in PL6 may contribute to its enhanced antioxidant activities. Furthermore, differentially expressed genes associated with the circadian clock system, cytoskeleton organization, and cell wall organization shed light on the plant's response to salt stress.

Conclusions

Understanding these mechanisms is crucial for developing stress-tolerant crop varieties, improving agricultural practices, and breeding salt-resistant crops to enhance global food production and address food security challenges.

Structures and ion transport mechanisms of plant high-affinity potassium transporters

Plant high-affinity K+ transporters (HKTs) mediate Na+ and K+ uptake, maintain Na+/K+ homeostasis, and therefore play crucial roles in plant salt tolerance. In this study, we present cryoelectron microscopy structures of HKTs from two classes, class I HKT1;1 from Arabidopsis thaliana (AtHKT1;1) and class II HKT2;1 from Triticum aestivum (TaHKT2;1), in both Na+- and K+-bound states at 2.6- to 3.0-Å resolutions. Both AtHKT1;1 and TaHKT2;1 function as homodimers. Each HKT subunit consists of four tandem domain units (D1-D4) with a repeated K+-channel-like M-P-M topology. In each subunit, D1-D4 assemble into an ion conduction pore with a pseudo-four-fold symmetry. Although both TaHKT2;1 and AtHKT1;1 have only one putative Na+ ion bound in the selectivity filter with a similar coordination pattern, the two HKTs display different K+ binding modes in the filter. TaHKT2;1 has three K+ ions bound in the selectivity filter, but AtHKT1;1 has only two K+ ions bound in the filter, which has a narrowed external entrance due to the presence of a Ser residue in the first filter motif. These structures, along with computational, mutational, and electrophysiological analyses, enable us to pinpoint key residues that are critical for the ion selectivity of HKTs. The findings provide new insights into the ion selectivity and ion transport mechanisms of plant HKTs and improve our understanding about how HKTs mediate plant salt tolerance and enhance crop growth.

Calcium regulation of the Arabidopsis Na+/K+ transporter HKT1;1 improves seed germination under salt stress

Calcium is known to improve seed-germination rates under salt stress. We investigated the involvement of calcium ions (Ca2+) in regulating HIGH-AFFINITY K+ TRANSPORTER 1 (HKT1; 1), which encodes a Na+/K+ transporter, and its post-translational regulator TYPE 2C PROTEIN PHOSPHATASE 49 (PP2C49), in germinating Arabidopsis (Arabidopsis thaliana) seedlings. Germination rates of hkt1 mutant seeds under salt stress remained unchanged by CaCl2 treatment in wild-type Arabidopsis, whereas pp2c49 mutant seeds displayed improved salt-stress tolerance in the absence of CaCl2 supplementation. Analysis of HKT1;1 and PP2C49 promoter activity revealed that CaCl2 treatment results in radicle-focused expression of HKT1;1 and reduction of the native radicle-exclusive expression of PP2C49. Ion-content analysis indicated that CaCl2 treatment improves K+ retention in germinating wild-type seedlings under salt stress, but not in hkt1 seedlings. Transgenic seedlings designed to exclusively express HKT1;1 in the radicle during germination displayed higher germination rates under salt stress than the wild type in the absence of CaCl2 treatment. Transcriptome analysis of germinating seedlings treated with CaCl2, NaCl, or both revealed 118 upregulated and 94 downregulated genes as responsive to the combined treatment. Bioinformatics analysis of the upstream sequences of CaCl2-NaCl-treatment-responsive upregulated genes revealed the abscisic acid response element CACGTGTC, a potential CaM-binding transcription activator-binding motif, as most prominent. Our findings suggest a key role for Ca2+ in mediating salt-stress responses during germination by regulating genes that function to maintain Na+ and K+ homeostasis, which is vital for seed germination under salt stress.

Na+-preferential ion transporter HKT1;1 mediates salt tolerance in blueberry

Soil salinity is a major environmental factor constraining growth and productivity of highbush blueberry (Vaccinium corymbosum). Leaf Na+ content is associated with variation in salt tolerance among blueberry cultivars; however, the determinants and mechanisms conferring leaf Na+ exclusion are unknown. Here, we observed that the blueberry cultivar 'Duke' was more tolerant than 'Sweetheart' and accumulated less Na+ in leaves under salt stress conditions. Through transcript profiling, we identified a member of the high-affinity K+ transporter (HKT) family in blueberry, VcHKT1;1, as a candidate gene involved in leaf Na+ exclusion and salt tolerance. VcHKT1;1 encodes a Na+-preferential transporter localized to the plasma membrane and is preferentially expressed in the root stele. Heterologous expression of VcHKT1;1 in Arabidopsis (Arabidopsis thaliana) rescued the salt hypersensitivity phenotype of the athkt1 mutant. Decreased VcHKT1;1 transcript levels in blueberry plants expressing antisense-VcHKT1;1 led to increased Na+ concentrations in xylem sap and higher leaf Na+ contents compared with wild-type plants, indicating that VcHKT1;1 promotes leaf Na+ exclusion by retrieving Na+ from xylem sap. A naturally occurring 8-bp insertion in the promoter increased the transcription level of VcHKT1;1, thus promoting leaf Na+ exclusion and blueberry salt tolerance. Collectively, we provide evidence that VcHKT1;1 promotes leaf Na+ exclusion and propose natural variation in VcHKT1;1 will be valuable for breeding Na+-tolerant blueberry cultivars in the future.

Comparison of the Morpho-Physiological and Molecular Responses to Salinity and Alkalinity Stresses in Rice

Rice is a major food crop that has a critical role in ensuring food security for the global population. However, major abiotic stresses such as salinity and alkalinity pose a major threat to rice farming worldwide. Compared with salinity stress, there is limited progress in elucidating the molecular mechanisms associated with alkalinity tolerance in rice. Since both stresses coexist in coastal and arid regions, unraveling of the underlying molecular mechanisms will help the breeding of high-yielding stress-tolerant rice varieties for these areas. This study examined the morpho-physiological and molecular response of four rice genotypes to both salinity and alkalinity stresses. Geumgangbyeo was highly tolerant and Mermentau was the least tolerant to both stresses, while Pokkali and Bengal were tolerant to only salinity and alkalinity stress, respectively. A set of salinity and alkalinity stress-responsive genes showed differential expression in the above rice genotypes under both stress conditions. The expression patterns were consistent with the observed morphological responses in these rice genotypes, suggesting the potential role of these genes in regulating tolerance to these abiotic stresses. Overall, this study suggested that divergence in response to alkalinity and salinity stresses among rice genotypes could be due to different molecular mechanisms conferring tolerance to each stress. In addition to providing a basis for further investigations into differentiating the molecular bases underlying tolerance, this study also emphasizes the possibilities of developing climate-resilient rice varieties using donors that are tolerant to both abiotic stresses.

Ion Changes and Signaling under Salt Stress in Wheat and Other Important Crops

High concentrations of sodium (Na+), chloride (Cl-), calcium (Ca2+), and sulphate (SO42-) are frequently found in saline soils. Crop plants cannot successfully develop and produce because salt stress impairs the uptake of Ca2+, potassium (K+), and water into plant cells. Different intracellular and extracellular ionic concentrations change with salinity, including those of Ca2+, K+, and protons. These cations serve as stress signaling molecules in addition to being essential for ionic homeostasis and nutrition. Maintaining an appropriate K+:Na+ ratio is one crucial plant mechanism for salt tolerance, which is a complicated trait. Another important mechanism is the ability for fast extrusion of Na+ from the cytosol. Ca2+ is established as a ubiquitous secondary messenger, which transmits various stress signals into metabolic alterations that cause adaptive responses. When plants are under stress, the cytosolic-free Ca2+ concentration can rise to 10 times or more from its resting level of 50-100 nanomolar. Reactive oxygen species (ROS) are linked to the Ca2+ alterations and are produced by stress. Depending on the type, frequency, and intensity of the stress, the cytosolic Ca2+ signals oscillate, are transient, or persist for a longer period and exhibit specific "signatures". Both the influx and efflux of Ca2+ affect the length and amplitude of the signal. According to several reports, under stress Ca2+ alterations can occur not only in the cytoplasm of the cell but also in the cell walls, nucleus, and other cell organelles and the Ca2+ waves propagate through the whole plant. Here, we will focus on how wheat and other important crops absorb Na+, K+, and Cl- when plants are under salt stress, as well as how Ca2+, K+, and pH cause intracellular signaling and homeostasis. Similar mechanisms in the model plant Arabidopsis will also be considered. Knowledge of these processes is important for understanding how plants react to salinity stress and for the development of tolerant crops.

Genome-wide identification of a novel Na+ transporter from Bienertia sinuspersici and overexpression of BsHKT1;2 improved salt tolerance in Brassica rapa

Salt stress is an ever-increasing stressor that affects both plants and humans. Therefore, developing strategies to limit the undesirable effects of salt stress is essential. Sodium ion exclusion is well known for its efficient salt-tolerance mechanism. The High-affinity K+ Transporter (HKT) excludes excess Na+ from the transpiration stream. This study identified and characterized the HKT protein family in Bienertia sinuspersici, a single-cell C4 plant. The HKT and Salt Overly Sensitive 1 (SOS1) expression levels were examined in B. sinuspersici and Arabidopsis thaliana leaves under four different salt stress conditions: 0, 100, 200, and 300 mM NaCl. Furthermore, BsHKT1;2 was cloned, thereby producing stable transgenic Brassica rapa. Our results showed that, compared to A. thaliana as a glycophyte, the HKT family is expanded in B. sinuspersici as a halophyte with three paralogs. The phylogenetic analysis revealed three paralogs belonging to the HKT subfamily I. Out of three copies, the expression of BsHKT1;2 was higher in Bienertia under control and salt stress conditions than in A. thaliana. Stable transgenic plants overexpressing 35S::BsHKT1;2 showed higher salt tolerance than non-transgenic plants. Higher biomass and longer roots were observed in the transgenic plants under salt stress than in non-transgenic plants. This study demonstrates the evolutionary and functional differences in HKT proteins between glycophytes and halophytes and associates the role of BsHKT1;2 in imparting salt tolerance and productivity.

How do plants maintain pH and ion homeostasis under saline-alkali stress?

Salt and alkaline stresses often occur together, severely threatening plant growth and crop yields. Salt stress induces osmotic stress, ionic stress, and secondary stresses, such as oxidative stress. Plants under saline-alkali stress must develop suitable mechanisms for adapting to the combined stress. Sustained plant growth requires maintenance of ion and pH homeostasis. In this review, we focus on the mechanisms of ion and pH homeostasis in plant cells under saline-alkali stress, including regulation of ion sensing, ion uptake, ion exclusion, ion sequestration, and ion redistribution among organs by long-distance transport. We also discuss outstanding questions in this field.

Plant salinity stress, sensing, and its mitigation through WRKY

Salinity or salt stress has deleterious effects on plant growth and development. It imposes osmotic, ionic, and secondary stresses, including oxidative stress on the plants and is responsible for the reduction of overall crop productivity and therefore challenges global food security. Plants respond to salinity, by triggering homoeostatic mechanisms that counter salt-triggered disturbances in the physiology and biochemistry of plants. This involves the activation of many signaling components such as SOS pathway, ABA pathway, and ROS and osmotic stress signaling. These biochemical responses are accompanied by transcriptional modulation of stress-responsive genes, which is mostly mediated by salt-induced transcription factor (TF) activity. Among the TFs, the multifaceted significance of WRKY proteins has been realized in many diverse avenues of plants' life including regulation of plant stress response. Therefore, in this review, we aimed to highlight the significance of salinity in a global perspective, the mechanism of salt sensing in plants, and the contribution of WRKYs in the modulation of plants' response to salinity stress. This review will be a substantial tool to investigate this problem in different perspectives, targeting WRKY and offering directions to better manage salinity stress in the field to ensure food security.

The HKT1 Na+ transporter protects plant fertility by decreasing Na+ content in stamen filaments

Salinity stress can greatly reduce seed production because plants are especially sensitive to salt during their reproductive stage. Here, we show that the sodium ion transporter AtHKT1;1 is specifically expressed around the phloem and xylem of the stamen in Arabidopsis thaliana to prevent a marked decrease in seed production caused by salt stress. The stamens of AtHKT1;1 mutant under salt stress overaccumulate Na+, limiting their elongation and resulting in male sterility. Specifically restricting AtHKT1;1 expression to the phloem leads to a 1.5-fold increase in the seed yield upon sodium ion stress. Expanding phloem expression of AtHKT1;1 throughout the entire plant is a promising strategy for increasing plant productivity under salinity stress.

Leaf sodium homeostasis controlled by salt gland is associated with salt tolerance in mangrove plant Avicennia marina

Avicennia marina, a mangrove plant growing in coastal wetland habitats, is frequently affected by tidal salinity. To understand its salinity tolerance, the seedlings of A. marina were treated with 0, 200, 400 and 600 mM NaCl. We found the whole-plant dry weight and photosynthetic parameters increased at 200 mM NaCl but decreased over 400 mM NaCl. The maximum quantum yield of primary photochemistry (Fv/Fm) significantly decreased at 600 mM NaCl. Transmission electron microscopy observations showed high salinity caused the reduction in starch grain size, swelling of the thylakoids and separation of the granal stacks, and even destruction of the envelope. In addition, the dense protoplasm and abundant mitochondria in the secretory and stalk cells, and abundant plasmodesmata between salt gland cells were observed in the salt glands of the adaxial epidermis. At all salinities, Na+ content was higher in leaves than in stems and roots; however, Na+ content increased in the roots while it remained at a constant level in the leaves over 400 mM NaCl treatment, due to salt secretion from the salt glands. As a result, salt crystals on the leaf adaxial surface increased with salinity. On the other hand, salt treatment increased Na+ and K+ efflux and decreased H+ efflux from the salt glands by the non-invasive micro-test technology, although Na+ efflux reached the maximum at 400 mM NaCl. Further real-time quantitative PCR analysis indicated that the expression of Na+/H+ antiporter (SOS1 and NHX1), H+-ATPase (AHA1 and VHA-c1) and K+ channel (AKT1, HAK5 and GORK) were up-regulated, and only the only Na+ inward transporter (HKT1) was down-regulated in the salt glands enriched adaxial epidermis of the leaves under 400 mM NaCl treatment. In conclusion, salinity below 200 mM NaCl was beneficial to the growth of A. marina, and below 400 mM, the salt glands could excrete Na+ effectively, thus improving its salt tolerance.

Salt Stress-Regulation of Root Water Uptake in a Whole-Plant and Diurnal Context

This review focuses on the regulation of root water uptake in plants which are exposed to salt stress. Root water uptake is not considered in isolation but is viewed in the context of other potential tolerance mechanisms of plants-tolerance mechanisms which relate to water relations and gas exchange. Plants spend between one third and half of their lives in the dark, and salt stress does not stop with sunset, nor does it start with sunrise. Surprisingly, how plants deal with salt stress during the dark has received hardly any attention, yet any growth response to salt stress over days, weeks, months and years is the integrative result of how plants perform during numerous, consecutive day/night cycles. As we will show, dealing with salt stress during the night is a prerequisite to coping with salt stress during the day. We hope to highlight with this review not so much what we know, but what we do not know; and this relates often to some rather basic questions.

Transcriptome Revealed GhPP2C43-A Negatively Regulates Salinity Tolerance in an Introgression Line from a Semi-wild Upland Cotton

Salt damage is one of the major threats to sustainable cotton production owing to the limited arable land in China mainly occupied by the production of staple food crops. Salt-stress tolerant cotton varieties are lacking in production and, the mechanisms underpinning salt-stress tolerance in cotton remain enigmatic. Here, DM37, an intraspecific introgression line from G. hirsutum race yucatanense acc TX-1046 into the G. hirsutum acc TM-1 background, was found to be highly tolerant to salt stress. Its seed germination rate and germination potential were significantly higher than the recipient TM-1 under salt stress. Physiological analysis showed DM37 had higher proline content and Peroxidase activity, as well as lower Na+/K+ ratios at the seedling stage, consistent with higher seedling survival rate after durable salt stress. Furthermore, comparative transcriptome analysis revealed that responsive patterns to salt stress in DM37 were different from TM-1. Weighted Correlation Network Analysis (WGCNA) demonstrated that co-expression modules associated with salt stress in DM37 also differed from TM-1. Out of them, GhPP2C43-A, a phosphatase gene, exhibited negative regulation of salt-stress tolerance verified by VIGS and transgenic Arabidopsis. Gene expression showed GhPP2C43-A in TM-1 was induced by durable salt stress but not in DM37 probably attributing to the variation of cis-element in its promoter, thereby being conferred different salt-stress tolerance. Our result would provide new genes/germplasms from semi-wild cotton in salt-stress tolerant cotton breeding. This study would give us new insights into the mechanisms underpinning the salt-stress tolerance in cotton.

Transcriptome, Biochemical and Phenotypic Analysis of the Effects of a Precision Engineered Biostimulant for Inducing Salinity Stress Tolerance in Tomato

Salinity stress is a major problem affecting plant growth and crop productivity. While plant biostimulants have been reported to be an effective solution to tackle salinity stress in different crops, the key genes and metabolic pathways involved in these tolerance processes remain unclear. This study focused on integrating phenotypic, physiological, biochemical and transcriptome data obtained from different tissues of Solanum lycopersicum L. plants (cv. Micro-Tom) subjected to a saline irrigation water program for 61 days (EC: 5.8 dS/m) and treated with a combination of protein hydrolysate and Ascophyllum nodosum-derived biostimulant, namely PSI-475. The biostimulant application was associated with the maintenance of higher K⁺/Na⁺ ratios in both young leaf and root tissue and the overexpression of transporter genes related to ion homeostasis (e.g., NHX4, HKT1;2). A more efficient osmotic adjustment was characterized by a significant increase in relative water content (RWC), which most likely was associated with osmolyte accumulation and upregulation of genes related to aquaporins (e.g., PIP2.1, TIP2.1). A higher content of photosynthetic pigments (+19.8% to +27.5%), increased expression of genes involved in photosynthetic efficiency and chlorophyll biosynthesis (e.g., LHC, PORC) and enhanced primary carbon and nitrogen metabolic mechanisms were observed, leading to a higher fruit yield and fruit number (47.5% and 32.5%, respectively). Overall, it can be concluded that the precision engineered PSI-475 biostimulant can provide long-term protective effects on salinity stressed tomato plants through a well-defined mode of action in different plant tissues.

Molecular membrane separation: plants inspire new technologies

Plants draw up their surrounding soil solution to gain water and nutrients required for growth, development and reproduction. Obtaining adequate water and nutrients involves taking up both desired and undesired elements from the soil solution and separating resources from waste. Desirable and undesirable elements in the soil solution can share similar chemical properties, such as size and charge. Plants use membrane separation mechanisms to distinguish between different molecules that have similar chemical properties. Membrane separation enables distribution or retention of resources and efflux or compartmentation of waste. Plants use specialised membrane separation mechanisms to adapt to challenging soil solution compositions and distinguish between resources and waste. Coordination and regulation of these mechanisms between different tissues, cell types and subcellular membranes supports plant nutrition, environmental stress tolerance and energy management. This review considers membrane separation mechanisms in plants that contribute to specialised separation processes and highlights mechanisms of interest for engineering plants with enhanced performance in challenging conditions and for inspiring the development of novel industrial membrane separation technologies. Knowledge gained from studying plant membrane separation mechanisms can be applied to developing precision separation technologies. Separation technologies are needed for harvesting resources from industrial wastes and transitioning to a circular green economy.

Emerging Roles of Salicylic Acid in Plant Saline Stress Tolerance

One of the most important phytohormones is salicylic acid (SA), which is essential for the regulation of plant growth, development, ripening, and defense responses. The role of SA in plant-pathogen interactions has attracted a lot of attention. Aside from defense responses, SA is also important in responding to abiotic stimuli. It has been proposed to have great potential for improving the stress resistance of major agricultural crops. On the other hand, SA utilization is dependent on the dosage of the applied SA, the technique of application, and the status of the plants (e.g., developmental stage and acclimation). Here, we reviewed the impact of SA on saline stress responses and the associated molecular pathways, as well as recent studies toward understanding the hubs and crosstalk between SA-induced tolerances to biotic and saline stress. We propose that elucidating the mechanism of the SA-specific response to various stresses, as well as SA-induced rhizosphere-specific microbiome modeling, may provide more insights and support in coping with plant saline stress.

Root epidermis-specific expression of a phosphate transporter TaPT2 enhances the growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions

Although attempts to improve the phosphate (Pi) uptake and use efficiency by constitutively overexpressing phosphate transporters have resulted in enhanced Pi or total phosphorous contents, growth promotion by Pi acquisition was observed in only a few cases. This study examined the effect of the tissue-specific overexpression of phosphate transporter on Pi acquisition and plant growth. We cloned cDNA for a wheat phosphate transporter, TaPT2, using PCR and confirmed its Pi transport activity in Arabidopsis suspension cells. The overexpression of TaPT2 by the Arabidopsis Shaker family inward rectifying potassium channel 1 (AKT1) promoter, dominantly expressed in root epidermal cells, resulted in increased root and shoot growth of transgenic Arabidopsis under Pi-replete and Pi-depleted conditions. However, their Pi and total P contents did not increase. The overexpression of TaPT2 by the constitutive promoter, actin8 (ACT8), increased shoot total P contents in transgenic plants, but did not promote their growth. These results suggested that enhanced Pi uptake in root epidermal cells is suitable as a driving force for Pi transport from roots to shoots, improving subsequent Pi use in shoots. Thus, the root epidermal cell-specific expression of TaPT2 may be a simple and promising strategy for enhancing plant Pi uptake and efficiency.

The differential modulation of secondary metabolism induced by a protein hydrolysate and a seaweed extract in tomato plants under salinity

Climate change and abiotic stress challenges in crops are threatening world food production. Among others, salinity affects the agricultural sector by significantly impacting yield losses. Plant biostimulants have received increasing attention in the agricultural industry due to their ability to improve health and resilience in crops. The main driving force of these products lies in their ability to modulate plant metabolic processes involved in the stress response. This study's purpose was to investigate the effect of two biostimulant products, including a protein hydrolysate (Clever HX^®) and a seaweed extract with high amino acids content (Ascovip^®), and their combination, on the metabolomics profile of tomato crops grown under salt stress (150 mM NaCl). Several stress indicators (leaf relative water content, membrane stability index, and photosynthesis activity) and leaf mineral composition after salinity stress exposure were assessed to evaluate stress mitigation, together with growth parameters (shoot and root biomasses). After that, an untargeted metabolomics approach was used to investigate the mechanism of action of the biostimulants and their link with the increased resilience to stress. The application of the biostimulants used reduced the detrimental effect of salinity. In saline conditions, protein hydrolysate improved shoot dry weight while seaweed extracts improved root dry weight. Regarding stress indicators, the application of the protein hydrolysate was found to alleviate the membrane damage caused by salinity stress compared to untreated plants. Surprisingly, photosynthetic activity significantly improved after treatment with seaweed extracts, suggesting a close correlation between root development, root water assimilation capacity and photosynthetic activity. Considering the metabolic reprogramming after plant biostimulants application, protein hydrolysates and their combination with seaweed extracts reported a distinctive metabolic profile modulation, mainly in secondary metabolite, lipids and fatty acids, and phytohormones biosynthetic pathways. However, treatment with seaweed extract reported a similar metabolic reprogramming trend compared to salinity stress. Our findings indicate a different mechanism of action modulated by protein hydrolysate and seaweed extract, suggesting stronger activity as a stress mitigator of protein hydrolysate in tomato crops under salinity stress.

Potential abiotic stress targets for modern genetic manipulation

Research into crop yield and resilience has underpinned global food security, evident in yields tripling in the past 5 decades. The challenges that global agriculture now faces are not just to feed 10+ billion people within a generation, but to do so under a harsher, more variable, and less predictable climate, and in many cases with less water, more expensive inputs, and declining soil quality. The challenges of climate change are not simply to breed for a "hotter drier climate," but to enable resilience to floods and droughts and frosts and heat waves, possibly even within a single growing season. How well we prepare for the coming decades of climate variability will depend on our ability to modify current practices, innovate with novel breeding methods, and communicate and work with farming communities to ensure viability and profitability. Here we define how future climates will impact farming systems and growing seasons, thereby identifying the traits and practices needed and including exemplars being implemented and developed. Critically, this review will also consider societal perspectives and public engagement about emerging technologies for climate resilience, with participatory approaches presented as the best approach.

A genome-wide association study identifies novel players in Na and Fe homeostasis in Arabidopsis thaliana under alkaline-salinity stress

In nature, multiple stress factors occur simultaneously. The screening of natural diversity panels and subsequent Genome-Wide Association Studies (GWAS) is a powerful approach to identify genetic components of various stress responses. Here, the nutritional status variation of a set of 270 natural accessions of Arabidopsis thaliana grown on a natural saline-carbonated soil is evaluated. We report significant natural variation on leaf Na (LNa) and Fe (LFe) concentrations in the studied accessions. Allelic variation in the NINJA and YUC8 genes is associated with LNa diversity, and variation in the ALA3 is associated with LFe diversity. The allelic variation detected in these three genes leads to changes in their mRNA expression and correlates with plant differential growth performance when plants are exposed to alkaline salinity treatment under hydroponic conditions. We propose that YUC8 and NINJA expression patters regulate auxin and jasmonic signaling pathways affecting plant tolerance to alkaline salinity. Finally, we describe an impairment in growth and leaf Fe acquisition associated with differences in root expression of ALA3, encoding a phospholipid translocase active in plasma membrane and the trans Golgi network which directly interacts with proteins essential for the trafficking of PIN auxin transporters, reinforcing the role of phytohormonal processes in regulating ion homeostasis under alkaline salinity.

New Insight into Plant Saline-Alkali Tolerance Mechanisms and Application to Breeding

Saline-alkali stress is a widespread adversity that severely affects plant growth and productivity. Saline-alkaline soils are characterized by high salt content and high pH values, which simultaneously cause combined damage from osmotic stress, ionic toxicity, high pH and HCO₃^-/CO₃^2- stress. In recent years, many determinants of salt tolerance have been identified and their regulatory mechanisms are fairly well understood. However, the mechanism by which plants respond to comprehensive saline-alkali stress remains largely unknown. This review summarizes recent advances in the physiological, biochemical and molecular mechanisms of plants tolerance to salinity or salt- alkali stress. Focused on the progress made in elucidating the regulation mechanisms adopted by plants in response to saline-alkali stress and present some new views on the understanding of plants in the face of comprehensive stress. Plants generally promote saline-alkali tolerance by maintaining pH and Na⁺ homeostasis, while the plants responding to HCO₃^-/CO₃^2- stress are not exactly the same as high pH stress. We proposed that pH-tolerant or sensitive plants have evolved distinct mechanisms to adapt to saline-alkaline stress. Finally, we highlight the areas that require further research to reveal the new components of saline-alkali tolerance in plants and present the current and potential application of key determinants in breed improvement and molecular breeding.

Salinity-Induced Physiological Changes in Pea (Pisum sativum L.): Germination Rate, Biomass Accumulation, Relative Water Content, Seedling Vigor and Salt Tolerance Index

Salinity affects and limits the yield potential of pulse crops. Therefore, an experiment was conducted to evaluate the salinity-induced physiological response of field peas by estimating the germination rate (%), accumulation of biomass, relative water content, and seedling vigor and salt tolerance index. The treatments included four salinity levels (NaCl) (i.e., 0 (control), 8, 12, and 16 dS m^-1, respectively) and eight field pea genotypes (i.e., BD4175, BD4182, BD4225, BD6944, BD4176, BD4193, BD4493, and BD4496). All treatments were arranged in a factorial completely randomized design and repeated four times. Results indicated that the percentage and rate of germination, percentage reduction of fresh and dry weight, relative water content, seedling vigor index, and salt tolerant index of all genotypes of field peas were influenced significantly by the different levels of salinity. The radicle and plumule of all field pea genotypes were damaged by applying 12 and 16 dS m^-1 salt stress. However, among these eight pea genotypes, two genotypes, namely BD4175 and BD4225, performed better under the 8 dS m^-1 level of salinity and these two genotypes may be recommended for cultivation in field conditions of saline coastal areas of Bangladesh, and can also be used in future breeding programs for the development of salt-tolerant pea cultivars.

Improving Wheat Salt Tolerance for Saline Agriculture

Salinity is a major abiotic stress that threatens crop yield and food supply in saline soil areas. Crops have evolved various strategies to facilitate survival and production of harvestable yield under salinity stress. Wheat (Triticum aestivum L.) is the main crop in arid and semiarid land areas, which are often affected by soil salinity. In this review, we summarize the conventional approaches to enhance wheat salt tolerance, including cross-breeding, exogenous application of chemical compounds, beneficial soil microorganisms, and transgenic engineering. We also propose several new breeding techniques for increasing salt tolerance in wheat, such as identifying new quantitative trait loci or genes related to salt tolerance, gene stacking and multiple genome editing, and wheat wild relatives and orphan crops domestication. The challenges and possible countermeasures in enhancing wheat salinity tolerance are also discussed.

Natural variation in growth and leaf ion homeostasis in response to salinity stress in Panicum hallii

Soil salinity can negatively impact plants growth, development and fitness. Natural plant populations restricted to coastal environments may evolve in response to saline habitats and therefore provide insights into the process of salinity adaptation. We investigated the growth and physiological responses of coastal and inland populations of Panicum hallii to experimental salinity treatments. Coastal genotypes demonstrated less growth reduction and superior ion homeostasis compared to the inland genotypes in response to saline conditions, supporting a hypothesis of local adaptation. We identified several QTL associated with the plasticity of belowground biomass, leaf sodium and potassium content, and their ratio which underscores the genetic variation present in this species for salinity responses. Genome-wide transcriptome analysis in leaf and root tissue revealed tissue specific overexpression of genes including several cation transporters in the coastal genotype. These transporters mediate sodium ion compartmentalization and potassium ion retention and thus suggests that maintenance of ionic homeostasis of the coastal genotypes might be due to the regulation of these ion transporters. These findings contribute to our understanding of the genetics and molecular mechanisms of salinity adaptation in natural populations, and widens the scope for genetic manipulation of these candidate genes to design plants more resilient to climate change.

Salinity induces discontinuous protoxylem via a DELLA-dependent mechanism promoting salt tolerance in Arabidopsis seedlings

Salinity is detrimental to plants and developmental adjustments limiting salt uptake and transport is therefore important for acclimation to high salt. These parameters may be influenced by xylem morphology, however how plant root xylem development is affected by salt stress remains unclear. Using molecular and genetic techniques and detailed phenotypic analyses, we demonstrate that salt causes distinct effects on Arabidopsis seedling root xylem and reveal underlying molecular mechanisms. Salinity causes intermittent inhibition of protoxylem cell differentiation, generating protoxylem gaps, in Arabidopsis and several other eudicot seedlings. The extent of protoxylem gaps in seedlings positively correlates with salt tolerance. Reduced gibberellin signalling is required for protoxylem gap formation. Mutant analyses reveal that the xylem differentiation regulator VASCULAR RELATED NAC DOMAIN 6 (VND6), along with secondary cell wall producing and cell wall modifying enzymes, including EXPANSIN A1 (EXP1), are involved in protoxylem gap formation, in a DELLA-dependent manner. Salt stress is likely to reduce levels of bioactive gibberellins, stabilising DELLAs, which in turn activates multiple factors modifying protoxylem differentiation. Salt stress impacts seedling survival and formation of protoxylem gaps may be a measure to enhance salt tolerance.

Synthetic bacterial community derived from a desert rhizosphere confers salt stress resilience to tomato in the presence of a soil microbiome

The root bacterial microbiome is important for the general health of the plant. Additionally, it can enhance tolerance to abiotic stresses, exemplified by plant species found in extreme ecological niches like deserts. These complex microbe-plant interactions can be simplified by constructing synthetic bacterial communities or SynComs from the root microbiome. Furthermore, SynComs can be applied as biocontrol agents to protect crops against abiotic stresses such as high salinity. However, there is little knowledge on the design of a SynCom that offers a consistent protection against salt stress for plants growing in a natural and, therefore, non-sterile soil which is more realistic to an agricultural setting. Here we show that a SynCom of five bacterial strains, originating from the root of the desert plant Indigofera argentea, protected tomato plants growing in a non-sterile substrate against a high salt stress. This phenotype correlated with the differential expression of salt stress related genes and ion accumulation in tomato. Quantification of the SynCom strains indicated a low penetrance into the natural soil used as the non-sterile substrate. Our results demonstrate how a desert microbiome could be engineered into a simplified SynCom that protected tomato plants growing in a natural soil against an abiotic stress.

Molecular Mechanisms of Plant Responses to Salt Stress

Saline-alkali soils pose an increasingly serious global threat to plant growth and productivity. Much progress has been made in elucidating how plants adapt to salt stress by modulating ion homeostasis. Understanding the molecular mechanisms that affect salt tolerance and devising strategies to develop/breed salt-resilient crops have been the primary goals of plant salt stress signaling research over the past few decades. In this review, we reflect on recent major advances in our understanding of the cellular and physiological mechanisms underlying plant responses to salt stress, especially those involving temporally and spatially defined changes in signal perception, decoding, and transduction in specific organelles or cells.

Genetic analysis and identification of VrFRO8, a salt tolerance-related gene in mungbean

Mungbean (Vigna radiata (L.) R. Wilczek) is an important legume crop of Asia. Salt concentrations typically causes major yield reductions in mungbean. Although the biochemical and genetic basis of salt tolerance-related gene are well studied in Arabidopsis and soybean, limited information concerning the salt tolerance-related genes in mungbean. To address this issue, we mined salt tolerance related genes using the survival rate trait and 160,1405 SNPs in 112 mungbean accessions. As a result, VrFRO8 significantly associated with salt-stress were identified in the GWAS analysis. The candidate gene VrFRO8 was evidenced by comparative genomics, transcriptome and RT-qPCR analysis. The expression level of VrFRO8 was significantly up-regulated (P-value = 0.001) after salt treatment compared with the control group. Moreover, 188 genes and 158 transcription factors related to salt-stress signal transduction pathway were mined, and 18 genes (18/188) had higher expression level in the salt-tolerant varieties than salt-sensitive varieties. And, the function of VrFRO8 was predicted in mungbean, the protein interaction between VrFRO8 and seven related-genes were found by molecular structure analysis. VrFRO8 might reduce SOD contents by influence Fe²⁺/Fe³⁺ ratio under the damage of salt stress. This study used multi-omics data to mine a key genes significantly associated with salt tolerance, and constructed a VrFRO8-related PPI network for salt tolerance, which would lay a solid foundation for further molecular biology research of VrFRO8 and mungbean breeding.

CRISPR/Cas9 Mediated Knockout of the OsbHLH024 Transcription Factor Improves Salt Stress Resistance in Rice (Oryza sativa L.)

Salinity stress is one of the most prominent abiotic stresses that negatively affect crop production. Transcription factors (TFs) are involved in the absorption, transport, or compartmentation of sodium (Na⁺) or potassium (K⁺) to resist salt stress. The basic helix-loop-helix (bHLH) is a TF gene family critical for plant growth and stress responses, including salinity. Herein, we used the CRISPR/Cas9 strategy to generate the gene editing mutant to investigate the role of OsbHLH024 in rice under salt stress. The A nucleotide base deletion was identified in the osbhlh024 mutant (A91). Exposure of the A91 under salt stress resulted in a significant increase in the shoot weight, the total chlorophyll content, and the chlorophyll fluorescence. Moreover, high antioxidant activities coincided with less reactive oxygen species (ROS) and stabilized levels of MDA in the A91. This better control of oxidative stress was accompanied by fewer Na⁺ but more K⁺, and a balanced level of Ca²⁺, Zn²⁺, and Mg²⁺ in the shoot and root of the A91, allowing it to withstand salt stress. Furthermore, the A91 also presented a significantly up-regulated expression of the ion transporter genes (OsHKT1;3, OsHAK7, and OsSOS1) in the shoot when exposed to salt stress. These findings imply that the OsbHLH024 might play the role of a negative regulator of salt stress, which will help to understand better the molecular basis of rice production improvement under salt stress.

An Insight into Abiotic Stress and Influx Tolerance Mechanisms in Plants to Cope in Saline Environments

Simple Summary

This review focuses on plant growth and development harmed by abiotic stress, primarily salt stress. Salt stress raises the intracellular osmotic pressure, leading to hazardous sodium buildup. Plants react to salt stress signals by regulating ion homeostasis, activating the osmotic stress pathway, modulating plant hormone signaling, and altering cytoskeleton dynamics and cell wall composition. Understanding the processes underlying these physiological and biochemical responses to salt stress could lead to more effective agricultural crop yield measures. In this review, researchers outline recent advances in plant salt stress control. The study of plant salt tolerance processes is essential, both theoretically and practically, to improve agricultural output, produce novel salt-tolerant cultivars, and make full use of saline soil. Based on past research, this paper discusses the adverse effects of salt stress on plants, including photosynthesis suppression, ion homeostasis disturbance, and membrane peroxidation. The authors have also covered the physiological mechanisms of salt tolerance, such as the scavenging of reactive oxygen species and osmotic adjustment. This study further identifies specific salt stress-responsive mechanisms linked to physiological systems. Based on previous studies, this article reviews the current methodologies and techniques for improving plant salt tolerance. Overall, it is hoped that the above-mentioned points will impart helpful background information for future agricultural and crop plant production.

Abstract

Salinity is significant abiotic stress that affects the majority of agricultural, irrigated, and cultivated land. It is an issue of global importance, causing many socio-economic problems. Salt stress mainly occurs due to two factors: (1) soil type and (2) irrigation water. It is a major environmental constraint, limiting crop growth, plant productivity, and agricultural yield. Soil salinity is a major problem that considerably distorts ecological habitats in arid and semi-arid regions. Excess salts in the soil affect plant nutrient uptake and osmotic balance, leading to osmotic and ionic stress. Plant adaptation or tolerance to salinity stress involves complex physiological traits, metabolic pathways, the production of enzymes, compatible solutes, metabolites, and molecular or genetic networks. Different plant species have different salt overly sensitive pathways and high-affinity K⁺ channel transporters that maintain ion homeostasis. However, little progress has been made in developing salt-tolerant crop varieties using different breeding approaches. This review highlights the interlinking of plant morpho-physiological, molecular, biochemical, and genetic approaches to produce salt-tolerant plant species. Most of the research emphasizes the significance of plant growth-promoting rhizobacteria in protecting plants from biotic and abiotic stressors. Plant growth, survival, and yield can be stabilized by utilizing this knowledge using different breeding and agronomical techniques. This information marks existing research areas and future gaps that require more attention to reveal new salt tolerance determinants in plants—in the future, creating genetically modified plants could help increase crop growth and the toleration of saline environments.

Structure, Function, and Regulation of the Plasma Membrane Na+/H+ Antiporter Salt Overly Sensitive 1 in Plants

Physiological studies have confirmed that export of Na⁺ to improve salt tolerance in plants is regulated by the combined activities of a complex transport system. In the Na⁺ transport system, the Na⁺/H⁺ antiporter salt overly sensitive 1 (SOS1) is the main protein that functions to excrete Na⁺ out of plant cells. In this paper, we review the structure and function of the Na⁺/H⁺ antiporter and the physiological process of Na⁺ transport in SOS signaling pathway, and discuss the regulation of SOS1 during phosphorylation activation by protein kinase and the balance mechanism of inhibiting SOS1 antiporter at molecular and protein levels. In addition, we carried out phylogenetic tree analysis of SOS1 proteins reported so far in plants, which implied the specificity of salt tolerance mechanism from model plants to higher crops under salt stress. Finally, the high complexity of the regulatory network of adaptation to salt tolerance, and the feasibility of coping strategies in the process of genetic improvement of salt tolerance quality of higher crops were reviewed.

Approaches Involved in the Vegetable Crops Salt Stress Tolerance Improvement: Present Status and Way Ahead

Salt stress is one of the most important abiotic stresses as it persists throughout the plant life cycle. The productivity of crops is prominently affected by soil salinization due to faulty agricultural practices, increasing human activities, and natural processes. Approximately 10% of the total land area (950 Mha) and 50% of the total irrigated area (230 Mha) in the world are under salt stress. As a consequence, an annual loss of 12 billion US$ is estimated because of reduction in agriculture production inflicted by salt stress. The severity of salt stress will increase in the upcoming years with the increasing world population, and hence the forced use of poor-quality soil and irrigation water. Unfortunately, majority of the vegetable crops, such as bean, carrot, celery, eggplant, lettuce, muskmelon, okra, pea, pepper, potato, spinach, and tomato, have very low salinity threshold (ECt, which ranged from 1 to 2.5 dS m-1 in saturated soil). These crops used almost every part of the world and lakes' novel salt tolerance gene within their gene pool. Salt stress severely affects the yield and quality of these crops. To resolve this issue, novel genes governing salt tolerance under extreme salt stress were identified and transferred to the vegetable crops. The vegetable improvement for salt tolerance will require not only the yield influencing trait but also target those characters or traits that directly influence the salt stress to the crop developmental stage. Genetic engineering and grafting is the potential tool which can improve salt tolerance in vegetable crop regardless of species barriers. In the present review, an updated detail of the various physio-biochemical and molecular aspects involved in salt stress have been explored.

Mechanism of high affinity potassium transporter (HKT) towards improved crop productivity in saline agricultural lands

Glycophytic plants are susceptible to salinity and their growth is hampered in more than 40 mM of salt. Salinity not only affects crop yield but also limits available land for farming by decreasing its fertility. Presence of distinct traits in response to environmental conditions might result in evolutionary adaptations. A better understanding of salinity tolerance through a comprehensive study of how Na⁺ is transported will help in the development of plants with improved salinity tolerance and might lead to increased yield of crops growing in strenuous environment. Ion transporters play pivotal role in salt homeostasis and maintain low cytotoxic effect in the cell. High-affinity potassium transporters are the critical class of integral membrane proteins found in plants. It mainly functions to remove excess Na⁺ from the transpiration stream to prevent sodium toxicity in the salt-sensitive shoot and leaf tissues. However, there are large number of HKT proteins expressed in plants, and it is possible that these members perform in a wide range of functions. Understanding their mechanism and functions will aid in further manipulation and genetic transformation of different crops. This review focuses on current knowledge of ion selectivity and molecular mechanisms controlling HKT gene expression. The current review highlights the mechanism of different HKT transporters from different plant sources and how this knowledge could prove as a valuable tool to improve crop productivity.

Maize Golden2-like transcription factors boost rice chloroplast development, photosynthesis, and grain yield

Chloroplasts are the sites for photosynthesis, and two Golden2-like factors act as transcriptional activators of chloroplast development in rice (Oryza sativa L.) and maize (Zea mays L.). Rice OsGLK1 and OsGLK2 are orthologous to maize ZmGLK1 (ZmG1) and ZmGLK2 (ZmG2), respectively. However, while rice OsGLK1 and OsGLK2 act redundantly to regulate chloroplast development in mesophyll cells, maize ZmG1 and ZmG2 are functionally specialized and expressed in different cell-specific manners. To boost rice chloroplast development and photosynthesis, we generated transgenic rice plants overexpressing ZmG1 and ZmG2, individually or simultaneously, with constitutive promoters (pZmUbi::ZmG1 and p35S::ZmG2) or maize promoters (pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2). Both ZmG1 and ZmG2 genes were highly expressed in transgenic rice leaves. Moreover, ZmG1 and ZmG2 showed coordinated expression in pZmG1::ZmG1/pZmG2::ZmG2 plants. All Golden2-like (GLK) transgenic plants had higher chlorophyll and protein contents, Rubisco activities and photosynthetic rates per unit leaf area in flag leaves. However, the highest grain yields occurred when maize promoters were used; pZmG1::ZmG1, pZmG2::ZmG2, and pZmG1::ZmG1/pZmG2::ZmG2 transgenic plants showed increases in grain yield by 51%, 47%, and 70%, respectively. In contrast, the pZmUbi::ZmG1 plant produced smaller seeds without yield increases. Transcriptome analysis indicated that maize GLKs act as master regulators promoting the expression of both photosynthesis-related and stress-responsive regulatory genes in both rice shoot and root. Thus, by promoting these important functions under the control of their own promoters, maize GLK1 and GLK2 genes together dramatically improved rice photosynthetic performance and productivity. A similar approach can potentially improve the productivity of many other crops.

Ammonium and nitrate affect sexually different responses to salt stress in Populus cathayana

Nitrogen (N) fertilization is a promising approach to improve salt tolerance. However, it is poorly known how plant sex and inorganic N alter salt stress-induced Na⁺ uptake, distribution and tolerance. This study employed Populus cathayana Rehder females and males to examine sex-related mechanisms of salt tolerance under nitrate (NO₃ ^- ) and ammonium (NH₄ ⁺ ) nutrition. Males had a higher root Na⁺ efflux, lower root-to-shoot translocation of Na⁺ , and higher K⁺ /Na⁺ , which enhanced salt tolerance under both N forms compared to females. On the other hand, decreased root Na⁺ efflux and K⁺ retention, and an increased ratio of Na⁺ in leaves relative to shoots in females caused greater salt sensitivity. Females receiving NH₄ ⁺ rather than NO₃ ^- had greater net root Na⁺ uptake, K⁺ efflux, and translocation to the shoots, especially in leaves. In contrast, males receiving NO₃ ^- rather than NH₄ ⁺ had increased Na⁺ translocation to the shoots, especially in the bark, which may narrow the difference in leaf damage by salt stress between N forms despite a higher shoot Na⁺ accumulation and lower root Na⁺ efflux. Genes related to cell wall synthesis, K⁺ and Na⁺ transporters, and denaturized protein scavenging in the barks showed differential expression between females and males in response to salt stress under both N forms. These results suggested that the regulation of N forms in salt stress tolerance was sex-dependent, which was related to the maintenance of the K⁺ /Na⁺ ratio in tissues, the ability of Na⁺ translocation to the shoots, and the transcriptional regulation of bark cell wall and proteolysis profiles.

Comparative analysis of various root active promoters by evaluation of GUS expression in transgenic Arabidopsis

To prepare various root active promoters for expressing transgenes and prevent gene silencing caused by the repeated use of the same promoter, the expression characteristics of various root active promoters were comparatively evaluated using GUS as a reporter gene. The high-affinity potassium transporter (HKT1;1), the Shaker family potassium ion channel (SKOR), the Shaker family inward rectifying potassium channel (AKT1), the major facilitator superfamily protein (MFS1), and the senescence associated gene 14 (SAG14) promoter from Arabidopsis (Arabidopsis thaliana) were used, and for comparison, four additional constitutive or green tissue specific promoters in the expression vectors were also employed. As the Gateway cloning technology provided by Invitrogen can offer high efficiency and cloning reliability, and easy manipulation of fusion constructs in vitro, our expression vectors are based on binary (destination) vectors compatible with this cloning technique. These destination vectors are also advantageous for stable expression of the transgene, as the heat shock protein terminator is utilized. The AtHKT1;1, SKOR, AKT1, MFS1 and SAG14 promoters were all active in roots but showed slightly different tissue specificities: AtHKT1;1, SKOR, and MFS1 were dominantly active in vascular bundle tissue, while AtHKT1;1 and MFS1- but not SKOR, AKT1, and SAG14-were active in root tips. SKOR showed the strongest root-specificity, and SAG14 showed the highest activity among the five root active promoters. The activity of MFS was developmentally regulated. These destination vectors are now available to express multiple transgenes in transgenic plants, especially in roots.

The highly interactive BTB domain targeting other functional domains to diversify the function of BTB proteins in rice growth and development

The BTB (broad-complex, tram track, and bric-abrac) proteins are involved in developmental processes, biotic, and abiotic stress responses in various plants, but the molecular basis of protein interactions is yet to be investiagted in rice. In this study, the identified BTB proteins were divided into BTB-TAZ, MATH-BTB, BTB-NPH, BTB-ANK, BTB-Skp, BTB-DUF, and BTB-TPR subfamilies based on the additional functional domains found together with the BTB domain at N- and C-terminal as well. This suggesting that the extension region at both terminal sites could play a vital role in the BTB gene family expansion in plants. The yeast two-hybrid system, firefly luciferase complementation imaging (LCI) assay and bimolecular fluorescence complementation (BiFC) assay further confirmed that BTB proteins interact with several other proteins to perform a certain developmental process in plants. The overexpression of BTB genes of each subfamily in Arabidopsis revealed that BTB genes including OsBTB4, OsBTB8, OsBTB64, OsBTB62, OsBTB138, and OsBTB147, containing certain additional functional domains, could play a potential role in the early flowering, branching, leaf, and silique development. Thus we concluded that the presence of other functional domains such as TAZ, SKP, DUF, ANK, NPH, BACK, PQQ, and MATH could be the factor driving the diverse functions of BTB proteins in plant biology.

Gene regulation at transcriptional and post-transcriptional levels to combat salt stress in plants

Soil salinity is a major challenge that will be faced more and more by human population in the near future. Higher salt concentrations in the soil limit the growth and production of crops, which poses serious threats to global food production. Various plant breeding approaches have been followed in the past which are reported to reduce the effect of salt stress by inducing the level of protective metabolites like osmolytes and antioxidants. Conventional breeding approaches are time-consuming and not cost-effective. In recent times, genetic engineering has been largely followed to confer salt tolerance through introgressions of single transgenes or stacking multiple transgenes. However, most of such works are limited only at the laboratory level and field trials are still awaited to prove the long-term efficacy of such transgenics. In this review, we attempt to present a broad overview of the current strategies undertaken to develop halophytic and salt-tolerant crops. The salt-induced damages in the plants are highlighted, followed by representing the novel traits, associated with salt stress, which can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of transcriptional and epigenetic regulation in plants for amelioration of salt-induced damages has been reviewed. The role of post-transcriptional mechanisms such as microRNA regulation, genome editing and alternative splicing, during salt stress, and their implications in the development of salt-tolerant crops are also discussed. Finally, we present a short overview about the role of ion transporters and rhizobacteria in the engineering of salt tolerance in crop species.

To exclude or to accumulate? Revealing the role of the sodium HKT1;5 transporter in plant adaptive responses to varying soil salinity

Arid/semi-arid and coastal agricultural areas of the world are especially vulnerable to climate change-driven soil salinity. Salinity tolerance in plants is a complex trait, with salinity negatively affecting crop yield. Plants adopt a range of mechanisms to combat salinity, with many transporter genes being implicated in Na⁺-partitioning processes. Within these, the high-affinity K⁺ (HKT) family of transporters play a critical role in K⁺ and Na⁺ homeostasis in plants. Among HKT transporters, Type I transporters are Na⁺-specific. While Arabidopsis has only one Na ⁺ -specific HKT (AtHKT1;1), cereal crops have a multiplicity of Type I and II HKT transporters. AtHKT1; 1 (Arabidopsis thaliana) and HKT1; 5 (cereal crops) 'exclude' Na⁺ from the xylem into xylem parenchyma in the root, reducing shoot Na⁺ and hence, confer sodium tolerance. However, more recent data from Arabidopsis and crop species show that AtHKT1;1/HKT1;5 alleles have a strong genetic association with 'shoot sodium accumulation' and concomitant salt tolerance. The review tries to resolve these two seemingly contradictory effects of AtHKT1;1/HKT1;5 operation (shoot exclusion vs shoot accumulation), both conferring salinity tolerance and suggests that contrasting phenotypes are attributable to either hyper-functional or weak AtHKT1;1/HKT1;5 alleles/haplotypes and are under strong selection by soil salinity levels. It also suggests that opposite balancing mechanisms involving xylem ion loading in these contrasting phenotypes exist that require transporters such as SOS1 and CCC. While HKT1; 5 is a crucial but not sole determinant of salinity tolerance, investigation of the adaptive benefit(s) conferred by naturally occurring intermediate HKT1;5 alleles will be important under a climate change scenario.

Arabidopsis MADS-box factor AGL16 is a negative regulator of plant response to salt stress by downregulating salt-responsive genes

Sessile plants constantly experience environmental stresses in nature. They must have evolved effective mechanisms to balance growth with stress response. Here we report the MADS-box transcription factor AGL16 acting as a negative regulator in stress response in Arabidopsis. Loss-of-AGL16 confers resistance to salt stress in seed germination, root elongation and soil-grown plants, while elevated AGL16 expression confers the opposite phenotypes compared with wild-type. However, the sensitivity to abscisic acid (ABA) in seed germination is inversely correlated with AGL16 expression levels. Transcriptomic comparison revealed that the improved salt resistance of agl16 mutants was largely attributed to enhanced expression of stress-responsive transcriptional factors and the genes involved in ABA signalling and ion homeostasis. We further demonstrated that AGL16 directly binds to the CArG motifs in the promoter of HKT1;1, HsfA6a and MYB102 and represses their expression. Genetic analyses with double mutants also support that HsfA6a and MYB102 are target genes of AGL16. Taken together, our results show that AGL16 acts as a negative regulator transcriptionally suppressing key components in the stress response and may play a role in balancing stress response with growth.

Gain-of-function mutations of AtNHX1 suppress sos1 salt sensitivity and improve salt tolerance in Arabidopsis

Soil salinity severely hampers agricultural productivity. Under salt stress, excess Na+ accumulation causes cellular damage and plant growth retardation, and membrane Na+ transporters play central roles in Na+ uptake and exclusion to mitigate these adverse effects. In this study, we performed sos1 suppressor mutant (named sup) screening to uncover potential genetic interactors of SOS1 and additional salt tolerance mechanisms. Map-based cloning and sequencing identified a group of mutants harboring dominant gain-of-function mutations in the vacuolar Na+/H+ antiporter gene AtNHX1. The gain-of-function variants of AtNHX1 showed enhanced transporter activities in yeast cells and increased salt tolerance in Arabidopsis wild type plants. Ion content measurements indicated that at the cellular level, these gain-of-function mutations resulted in increased cellular Na+ accumulation likely due to enhanced vacuolar Na+ sequestration. However, the gain-of-function suppressor mutants showed reduced shoot Na+ but increased root Na+ accumulation under salt stress, indicating a role of AtNHX1 in limiting Na+ translocation from root to shoot. We also identified another group of sos1 suppressors with loss-of-function mutations in the Na+ transporter gene AtHKT1. Loss-of-function mutations in AtHKT1 and gain-of-function mutations in AtNHX1 additively suppressed sos1 salt sensitivity, which indicates that the three transporters, SOS1, AtNHX1 and AtHKT1 function independently but coordinately in controlling Na+ homeostasis and salt tolerance in Arabidopsis. Our findings provide valuable information about the target amino acids in NHX1 for gene editing to improve salt tolerance in crops.