The Arabidopsis Na+/H+ antiporters NHX1 and NHX2 control vacuolar pH and K+ homeostasis to regulate growth, flower development, and reproduction

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.

A tonoplast-localized TPK-type K+ transporter (TPKa) regulates potassium accumulation in tobacco

Potassium ion (K+) is one of the most essential nutrients for the growth and development of tobacco (Nicotiana tabacum L.), however, the molecular regulation of K+ concentration in tobacco remains unclear. In this study, a two-pore K (TPK) channel gene NtTPKa was cloned from tobacco, and NtTPKa protein contains the unique K+ selection motif GYGD and its transmembrane region primarily locates in the tonoplast membrane. The expression of NtTPKa gene was significantly increased under low-potassium stress conditions. The concentrations of K+ in tobacco were significantly increased in the NtTPKa RNA interference lines and CRISPR/Cas9 knockout mutants. In addition, the transport of K+ by NtTPKa was validated using patch clamp technique, and the results showed that NtTPKa channel protein exclusively transported K+ in a concentration-dependent manner. Together, our results strongly suggested that NtTPKa is a key gene in maintaining K+ homeostasis in tobacco, and it could provide a new genetic resource for increasing the concentration of K+ in tobacco.

The signalling pathways, calcineurin B-like protein 5 (CBL5)-CBL-interacting protein kinase 8 (CIPK8)/CIPK24-salt overly sensitive 1 (SOS1), transduce salt signals in seed germination in Arabidopsis

For plant growth under salt stress, sensing and transducing salt signals are central to cellular Na+ homoeostasis. The calcineurin B-like protein (CBL)-CBL-interacting protein kinase (CIPK) complexes play critical roles in transducing salt signals in plants. Here, we show that CBL5, an ortholog of CBL4 and CBL10 in Arabidopsis, interacts with and recruits CIPK8/CIPK24 to the plasma membrane. Yeast cells coexpressing CBL5, CIPK8/CIPK24 and SOS1 demonstrated lesser Na+ accumulation and a better growth phenotype than the untransformed or SOS1 transgenic yeast cells under salinity. Overexpression of CBL5 improved the growth of the cipk8 or cipk24 single mutant but not the cipk8 cipk24 double mutant under salt stress, suggesting that CIPK8 and CIPK24 were the downstream targets of CBL5. Interestingly, seed germination in cbl5 was severely inhibited by NaCl, which was recovered by the overexpression of CBL5. Furthermore, CBL5 was mainly expressed in the cotyledons and hypocotyls, which are essential to seed germination. Na+ efflux activity in the hypocotyls of cbl5 was reduced relative to the wild-type under salt stress, enhancing Na+ accumulation. These findings indicate that CBL5 functions in seed germination and protects seeds and germinating seedlings from salt stress through the CBL5-CIPK8/CIPK24-SOS1 pathways.

The potassium transporter TaNHX2 interacts with TaGAD1 to promote drought tolerance via modulating stomatal aperture in wheat

Drought is a major global challenge in agriculture that decreases crop production. γ-Aminobutyric acid (GABA) interfaces with drought stress in plants; however, a mechanistic understanding of the interaction between GABA accumulation and drought response remains to be established. Here we showed the potassium/proton exchanger TaNHX2 functions as a positive regulator in drought resistance in wheat by mediating cross-talk between the stomatal aperture and GABA accumulation. TaNHX2 interacted with glutamate decarboxylase TaGAD1, a key enzyme that synthesizes GABA from glutamate. Furthermore, TaNHX2 targeted the C-terminal auto-inhibitory domain of TaGAD1, enhanced its activity, and promoted GABA accumulation under drought stress. Consistent with this, the tanhx2 and tagad1 mutants showed reduced drought tolerance, and transgenic wheat with enhanced TaNHX2 expression had a yield advantage under water deficit without growth penalty. These results shed light on the plant stomatal movement mechanism under drought stress and the TaNHX2-TaGAD1 module may be harnessed for amelioration of negative environmental effects in wheat as well as other crops.

Designing salt stress-resilient crops: Current progress and future challenges

Excess soil salinity affects large regions of land and is a major hindrance to crop production worldwide. Therefore, understanding the molecular mechanisms of plant salt tolerance has scientific importance and practical significance. In recent decades, studies have characterized hundreds of genes associated with plant responses to salt stress in different plant species. These studies have substantially advanced our molecular and genetic understanding of salt tolerance in plants and have introduced an era of molecular design breeding of salt-tolerant crops. This review summarizes our current knowledge of plant salt tolerance, emphasizing advances in elucidating the molecular mechanisms of osmotic stress tolerance, salt-ion transport and compartmentalization, oxidative stress tolerance, alkaline stress tolerance, and the trade-off between growth and salt tolerance. We also examine recent advances in understanding natural variation in the salt tolerance of crops and discuss possible strategies and challenges for designing salt stress-resilient crops. We focus on the model plant Arabidopsis (Arabidopsis thaliana) and the four most-studied crops: rice (Oryza sativa), wheat (Triticum aestivum), maize (Zea mays), and soybean (Glycine max).

Climate-resilient crops: Lessons from xerophytes

Developing climate-resilient crops is critical for future food security and sustainable agriculture under current climate scenarios. Of specific importance are drought and soil salinity. Tolerance traits to these stresses are highly complex, and the progress in improving crop tolerance is too slow to cope with the growing demand in food production unless a major paradigm shift in crop breeding occurs. In this work, we combined bioinformatics and physiological approaches to compare some of the key traits that may differentiate between xerophytes (naturally drought-tolerant plants) and mesophytes (to which the majority of the crops belong). We show that both xerophytes and salt-tolerant mesophytes have a much larger number of copies in key gene families conferring some of the key traits related to plant osmotic adjustment, abscisic acid (ABA) sensing and signalling, and stomata development. We show that drought and salt-tolerant species have (i) higher reliance on Na for osmotic adjustment via more diversified and efficient operation of Na+ /H+ tonoplast exchangers (NHXs) and vacuolar H+ - pyrophosphatase (VPPases); (ii) fewer and faster stomata; (iii) intrinsically lower ABA content; (iv) altered structure of pyrabactin resistance/pyrabactin resistance-like (PYR/PYL) ABA receptors; and (v) higher number of gene copies for protein phosphatase 2C (PP2C) and sucrose non-fermenting 1 (SNF1)-related protein kinase 2/open stomata 1 (SnRK2/OST1) ABA signalling components. We also show that the past trends in crop breeding for Na+ exclusion to improve salinity stress tolerance are counterproductive and compromise their drought tolerance. Incorporating these genetic insights into breeding practices could pave the way for more drought-tolerant and salt-resistant crops, securing agricultural yields in an era of climate unpredictability.

Genome-wide identification of the cation/proton antiporter (CPA) gene family and functional characterization of the key member BnaA05.NHX2 in allotetraploid rapeseed

Rapeseed (Brassica napus L.) is susceptible to nutrient stresses during growth and development; however, the CPA (cation proton antiporter) family genes have not been identified in B. napus and their biological functions remain unclear. This study was aimed to identify the molecular characteristics of rapeseed CPAs and their transcriptional responses to multiple nutrient stresses. Through bioinformatics analysis, 117 BnaCPAs, consisting of three subfamilies: Na+/H+ antiporter (NHX), K+ efflux antiporter (KEA), and cation/H+ antiporter (CHX), were identified in the rapeseed genome. Transcriptomic profiling showed that BnaCPAs, particularly BnaNHXs, were transcriptionally responsive to diverse nutrient stresses, including Cd toxicity, K starvation, salt stress, NH4+ toxicity, and low Pi. We found that the salt tolerance of the transgenic rapeseed lines overexpressing BnaA05.NHX2 was significantly higher than that of wild type. Subcellular localization showed that BnaA05.NHX2 was localized on the tonoplast, and TEM combined with X-ray energy spectrum analysis revealed that the vacuolar Na+ concentrations of the BnaA05.NHX2-overexpressing rapeseed plants were significantly higher than those of wild type. The findings of this study will provide insights into the complexity of the BnaCPA family and a valuable resource to explore the in-depth functions of CPAs in B. napus.

K+ and pH homeostasis in plant cells is controlled by a synchronized K+ /H+ antiport at the plasma and vacuolar membrane

Stomatal movement involves ion transport across the plasma membrane (PM) and vacuolar membrane (VM) of guard cells. However, the coupling mechanisms of ion transporters in both membranes and their interplay with Ca2+ and pH changes are largely unclear. Here, we investigated transporter networks in tobacco guard cells and mesophyll cells using multiparametric live-cell ion imaging and computational simulations. K+ and anion fluxes at both, PM and VM, affected H+ and Ca2+ , as changes in extracellular KCl or KNO3 concentrations were accompanied by cytosolic and vacuolar pH shifts and changes in [Ca2+ ]cyt and the membrane potential. At both membranes, the K+ transporter networks mediated an antiport of K+ and H+ . By contrast, net transport of anions was accompanied by parallel H+ transport, with differences in transport capacity for chloride and nitrate. Guard and mesophyll cells exhibited similarities in K+ /H+ transport but cell type-specific differences in [H+ ]cyt and pH-dependent [Ca2+ ]cyt signals. Computational cell biology models explained mechanistically the properties of transporter networks and the coupling of transport across the PM and VM. Our integrated approach indicates fundamental principles of coupled ion transport at membrane sandwiches to control H+ /K+ homeostasis and points to transceptor-like Ca2+ /H+ -based ion signaling in plant cells.

Genome-wide association studies for earliness, MYMIV resistance, and other associated traits in mungbean (Vigna radiata L. Wilczek) using genotyping by sequencing approach

Yellow mosaic disease (YMD) remains a major constraint in mungbean (Vigna radiata (L.)) production; while short-duration genotypes offer multiple crop cycles per year and help in escaping terminal heat stress, especially during summer cultivation. A comprehensive genotyping by sequencing (GBS)-based genome-wide association studies (GWAS) analysis was conducted using 132 diverse mungbean genotypes for traits like flowering time, YMD resistance, soil plant analysis development (SPAD) value, trichome density, and leaf area. The frequency distribution revealed a wide range of values for all the traits. GBS studies identified 31,953 high-quality single nucleotide polymorphism (SNPs) across all 11 mungbean chromosomes and were used for GWAS. Structure analysis revealed the presence of two genetically distinct populations based on ΔK. The linkage disequilibrium (LD) varied throughout the chromosomes and at r2 = 0.2, the mean LD decay was estimated as 39.59 kb. Two statistical models, mixed linear model (MLM) and Bayesian-information and Linkage-disequilibrium Iteratively Nested Keyway (BLINK) identified 44 shared SNPs linked with various candidate genes. Notable candidate genes identified include FPA for flowering time (VRADI10G01470; chr. 10), TIR-NBS-LRR for mungbean yellow mosaic India virus (MYMIV) resistance (VRADI09G06940; chr. 9), E3 ubiquitin-protein ligase RIE1 for SPAD value (VRADI07G28100; chr. 11), WRKY family transcription factor for leaf area (VRADI03G06560; chr. 3), and LOB domain-containing protein 21 for trichomes (VRADI06G04290; chr. 6). In-silico validation of candidate genes was done through digital gene expression analysis using Arabidopsis orthologous (compared with Vigna radiata genome). The findings provided valuable insight for marker-assisted breeding aiming for the development of YMD-resistant and early-maturing mungbean varieties.

Impact of salinity stress on cotton and opportunities for improvement through conventional and biotechnological approaches

Excess salinity can affect the growth and development of all plants. Salinization jeopardizes agroecosystems, induces oxidative reactions in most cultivated plants and reduces biomass which affects crop yield. Some plants are affected more than others, depending upon their ability to endure the effects of salt stress. Cotton is moderately tolerant to salt stress among cultivated crops. The fundamental tenet of plant breeding is genetic heterogeneity in available germplasm for acquired characteristics. Variation for salinity tolerance enhancing parameters (morphological, physiological and biochemical) is a pre-requisite for the development of salt tolerant cotton germplasm followed by indirect selection or hybridization programs. There has been a limited success in the development of salt tolerant genotypes because this trait depends on several factors, and these factors as well as their interactions are not completely understood. However, advances in biochemical and molecular techniques have made it possible to explore the complexity of salt tolerance through transcriptomic profiling. The focus of this article is to discuss the issue of salt stress in crop plants, how it alters the physiology and morphology of the cotton crop, and breeding strategies for the development of salinity tolerance in cotton germplasm.

Genome-wide identification and expression analysis of the NHX gene family under salt stress in wheat (Triticum aestivum L)

Salt stress affects plant growth and development, resulting in the loss of crop yield across the world, and sodium-proton antiporters (NHXs) are one of the genes known to promote salt tolerance in transgenic plants. In this study, we conducted a comprehensive genome-wide analysis and expression profile of NHX genes in wheat under salinity stress. We identified 30 TaNHX genes in wheat based on the Na+/H+ exchanger domain, with all genes containing an amiloride motif except one, a known for inhibiting Na+ ions in plants. Phylogenetic analysis classified these genes into three classes with subfamilies: 12 were localized in vacuoles, while 18 were in the endoplasmic reticulum and plasma membrane. Promoter analysis revealed stress-related cis-acting elements, indicating their potential role in abiotic stress tolerance. The non-synonymous (Ka)/synonymous (Ks) ratios highlighted that the majority of TaNHX genes experienced robust purifying selection throughout their evolutionary history. Transcriptomis data analysis and qRT-PCR demonstrated distinct expression patterns for TaNHX genes across various tissues when subjected to salt stress. Additionally, we predicted 20 different miRNA candidates targeting the identified TaNHX genes. Protein-protein interaction prediction revealed NHX6's involvement in the SOS1 pathway, while NHX1 gene exhibit proton antiporter activity. Molecular dynamics (MD) simulations were also conducted to examine the interactions of TaNHX1, TaNHX2, and TaNHX3. These results represent a significant advancement in our understanding of the molecular mechanisms governing Na+ transporters. This may also offer promising avenues for future studies aimed at unraveling the intricate details of their biological roles and applications.

Halophilic Plant-Associated Bacteria with Plant-Growth-Promoting Potential

The salinization of soils is a growing agricultural concern worldwide. Irrigation practices, drought, and climate change are leading to elevated salinity levels in many regions, resulting in reduced crop yields. However, there is potential for a solution in the microbiome of halophytes, which are naturally salt-tolerant plants. These plants harbor a salt-tolerant microbiome in their rhizosphere (around roots) and endosphere (within plant tissue). These bacteria may play a significant role in conferring salt tolerance to the host plants. This leads to the possibility of transferring these beneficial bacteria, known as salt-tolerant plant-growth-promoting bacteria (ST-PGPB), to salt-sensitive plants, enabling them to grow in salt-affected areas to improve crop productivity. In this review, the background of salt-tolerant microbiomes is discussed and their potential use as ST-PGPB inocula is explored. We focus on two Gram-negative bacterial genera, Halomonas and Kushneria, which are commonly found in highly saline environments. These genera have been found to be associated with some halophytes, suggesting their potential for facilitating ST-PGPB activity. The study of salt-tolerant microbiomes and their use as PGPB holds promise for addressing the challenges posed by soil salinity in the context of efforts to improve crop growth in salt-affected areas.

Melatonin enhances KCl salinity tolerance by maintaining K+ homeostasis in Malus hupehensis

Large amounts of potash fertilizer are often applied to apple (Malus domestica) orchards to enhance fruit quality and yields, but this treatment aggravates KCl-based salinity stress. Melatonin (MT) is involved in a variety of abiotic stress responses in plants. However, its role in KCl stress tolerance is still unknown. In the present study, we determined that an appropriate concentration (100 μm) of MT significantly alleviated KCl stress in Malus hupehensis by enhancing K+ efflux out of cells and compartmentalizing K+ in vacuoles. Transcriptome deep-sequencing analysis identified the core transcription factor gene MdWRKY53, whose expression responded to both KCl and MT treatment. Overexpressing MdWRKY53 enhanced KCl tolerance in transgenic apple plants by increasing K+ efflux and K+ compartmentalization. Subsequently, we characterized the transporter genes MdGORK1 and MdNHX2 as downstream targets of MdWRKY53 by ChIP-seq. MdGORK1 localized to the plasma membrane and enhanced K+ efflux to increase KCl tolerance in transgenic apple plants. Moreover, overexpressing MdNHX2 enhanced the KCl tolerance of transgenic apple plants/callus by compartmentalizing K+ into the vacuole. RT-qPCR and LUC activity analyses indicated that MdWRKY53 binds to the promoters of MdGORK1 and MdNHX2 and induces their transcription. Taken together, our findings reveal that the MT-WRKY53-GORK1/NHX2-K+ module regulates K+ homeostasis to enhance KCl stress tolerance in apple. These findings shed light on the molecular mechanism of apple response to KCl-based salinity stress and lay the foundation for the practical application of MT in salt stress.

Analysis on the salt tolerance of Nitraria sibirica Pall. based on Pacbio full-length transcriptome sequencing

KEY MESSAGE

Nitraria sibirica Pall. regulates its tolerance to salt stress mainly by adjusting ion balance, modifying cell wall structure, and activating signal transduction pathways. N. sibirica, as a typical halophyte, can not only effectively restore saline-alkali land, but also has high economic value. However, studies on its salt tolerance at combining molecular and physiological levels were limited. In this study, the salt tolerance of N. sibirica was analyzed based on Pacbio full-length transcriptome sequencing, and the salt tolerance in the physiological level was verified by key genes. The results showed that 89,017 full-length transcripts were obtained, of which 84,632 sequences were annotated. A total of 86,482 coding sequences (CDS) were predicted and 6561 differentially expressed genes (DEGs) were identified. DEGs were significantly enriched in "sodium ion homeostasis", "response to osmotic stress", "reactive oxygen species metabolic process", "defense response by cell wall thickening", "signal transduction", etc. The expression levels for most of these DEGs increased under salt stress. A total of 69 key genes were screened based on weighted gene co-expression network analysis (WGCNA), of which 33 were first reported on salt tolerance. Moreover, NsRabE1c gene with the highest expression level was selected to verify its salt tolerance. Over-expression of NsRabE1c gene enhanced the germination potential and root length of transgenic Arabidopsis thaliana plants without salt treatment as compared to those of Col-0 and AtRabE1c mutant. The expression levels of NsRabE1c decreased in the growth stagnation phase, while significantly increased in the growth recovery phase under salt stress. We predicted that NsRabE1c gene help N. sibirica resist salt stress through the regulation of plant growth. The results of this study deepen the understanding of salinity resistance in N. sibirica.

Melatonin promotes Al3+ compartmentalization via H+ transport and ion gradients in Malus hupehensis

Soil acidification in apple (Malus domestica) orchards results in the release of rhizotoxic aluminum ions (Al3+) into soil. Melatonin (MT) participates in plant responses to abiotic stress; however, its role in AlCl3 stress in apple remains unknown. In this study, root application of MT (1 μM) substantially alleviated AlCl3 stress (300 μM) in Pingyi Tiancha (Malus hupehensis), which was reflected by higher fresh and dry weight, increased photosynthetic capacity, and longer and more roots compared with plants that did not receive MT treatment. MT functioned mainly by regulating vacuolar H+/Al3+ exchange and maintaining H+ homeostasis in the cytoplasm under AlCl3 stress. Transcriptome deep sequencing analysis identified the transcription factor gene SENSITIVE TO PROTON RHIZOTOXICITY 1 (MdSTOP1) was induced by both AlCl3 and MT treatments. Overexpressing MdSTOP1 in apple increased AlCl3 tolerance by enhancing vacuolar H+/Al3+ exchange and H+ efflux to the apoplast. We identified 2 transporter genes, ALUMINUM SENSITIVE 3 (MdALS3) and SODIUM HYDROGEN EXCHANGER 2 (MdNHX2), as downstream targets of MdSTOP1. MdSTOP1 interacted with the transcription factor NAM ATAF and CUC 2 (MdNAC2) to induce MdALS3 expression, which reduced Al toxicity by transferring Al3+ from the cytoplasm to the vacuole. Furthermore, MdSTOP1 and MdNAC2 coregulated MdNHX2 expression to increase H+ efflux from the vacuole to the cytoplasm to promote Al3+ compartmentalization and maintain cation balance in the vacuole. Taken together, our findings reveal an MT-STOP1 + NAC2-NHX2/ALS3-vacuolar H+/Al3+ exchange model for the alleviation of AlCl3 stress in apple, laying a foundation for practical applications of MT in agriculture.

Structure-Guided Identification of Critical Residues in the Vacuolar Cation/Proton Antiporter NHX1 from Arabidopsis thaliana

Cation/Proton Antiporters (CPA) acting in all biological membranes regulate the volume and pH of cells and of intracellular organelles. A key issue with these proteins is their structure-function relationships since they present intrinsic regulatory features that rely on structural determinants, including pH sensitivity and the stoichiometry of ion exchange. Crystal structures are only available for prokaryotic CPA, whereas the eukaryotic ones have been modeled using the former as templates. Here, we present an updated and improved structural model of the tonoplast-localized K+, Na+/H+ antiporter NHX1 of Arabidopsis as a representative of the vacuolar NHX family that is essential for the accumulation of K+ into plant vacuoles. Conserved residues that were judged as functionally important were mutated, and the resulting protein variants were tested for activity in the yeast Saccharomyces cerevisiae. The results indicate that residue N184 in the ND-motif characteristic of CPA1 could be replaced by the DD-motif of CPA2 family members with minimal consequences for their activity. Attempts to alter the electroneutrality of AtNHX1 by different combinations of amino acid replacements at N184, R353 and R390 residues resulted in inactive or partly active proteins with a differential ability to control the vacuolar pH of the yeast.

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.

The Na+/H+ antiporter GbSOS1 interacts with SIP5 and regulates salt tolerance in Gossypium barbadense

Cotton is a globally cultivated economic crop and is a major source of natural fiber and edible oil. However, cotton production is severely affected by salt stress. Although Salt Overly Sensitive 1 (SOS1) is a well-studied Na⁺/H⁺ antiporter in multiple plant species, little is known about its function and regulatory mechanism in cotton. Here, we cloned a salt-induced SOS1 from sea-island cotton. Real-time quantitative PCR analysis revealed that GbSOS1 was induced by multiple stresses and phytohormones. Silencing GbSOS1 through virus-induced gene silencing significantly reduced cotton resistance to high Na⁺ but mildly affected Li⁺ tolerance. On the other hand, overexpression of GbSOS1 enhanced salt tolerance in yeast, Arabidopsis, and cotton largely due to the ability to maintain Na⁺ homeostasis in protoplasts. Yeast-two-hybrid assays and bimolecular fluorescence complementation identified a novel protein interacting with GbSOS1 on the plasma membrane, which we named SOS Interaction Protein 5 (SIP5). We found that the SIP5 gene encoded an unknown protein localized on the cell membrane. Silencing SIP5 significantly increased cotton tolerance to salt, exhibited by less wilting and plant death under salt stress. Our results revealed that GbSOS1 is crucial for cotton survival in saline soil, and SIP5 is a potentially negative regulator of SOS1-mediated salt tolerance in cotton. Overall, this study provides a theoretical basis for elucidating the molecular mechanism of SOS1, and a candidate gene for breeding salt-tolerant crops.

Indian Himalayan natural Arabidopsis thaliana accessions with abolished miR158 levels exhibit robust miR173-initiated trans-acting cascade silencing

Small RNAs (sRNAs) such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) are short 20-24-nucleotide non-coding RNAs. They are key regulators of gene expression in plants and other organisms. Several 22-nucleotide miRNAs trigger biogenesis cascades of trans-acting secondary siRNAs, which are involved in various developmental and stress responses. Here we show that Himalayan Arabidopsis thaliana accessions having natural mutations in the miR158 locus exhibit robust cascade silencing of the pentatricopeptide repeat (PPR)-like locus. Furthermore, we show that these cascade sRNAs trigger tertiary silencing of a gene involved in transpiration and stomatal opening. The natural deletions or insertions in MIR158 led to improper processing of miR158 precursors, thereby blocking synthesis of mature miR158. Reduced miR158 levels led to increased levels of its target, a pseudo-PPR gene that is targeted by tasiRNAs generated by the miR173 cascade in other accessions. Using sRNA datasets derived from Indian Himalayan accessions, as well as overexpression and knockout lines of miR158, we show that absence of miR158 led to buildup of pseudo-PPR-derived tertiary sRNAs. These tertiary sRNAs mediated robust silencing of a gene involved in stomatal closure in Himalayan accessions lacking miR158 expression. We functionally validated the tertiary phasiRNA that targets NHX2, which encodes a Na⁺ -K⁺ /H⁺ antiporter protein, thereby regulating transpiration and stomatal conductance. Overall, we report the role of the miRNA-TAS-siRNA-pseudogene-tertiary phasiRNA-NHX2 pathway in plant adaptation.

Effects of Arbuscular Mycorrhizal Fungus on Sodium and Chloride Ion Channels of Casuarina glauca under Salt Stress

Casuarina glauca is an important coastal protection forest species, which is exposed to high salt stress all year round. Arbuscular mycorrhizal fungi (AMF) can promote the growth and salt tolerance of C. glauca under salt stress. However, the effects of AMF on the distribution of Na⁺ and Cl^- and the expression of related genes in C. glauca under salt stress need to be further explored. This study explored the effects of Rhizophagus irregularis on plant biomass, the distribution of Na⁺ and Cl^-, and the expression of related genes in C. glauca under NaCl stress through pot simulation experiments. The results revealed that the mechanisms of Na⁺ and Cl^- transport of C. glauca under NaCl stress were different. C. glauca took a salt accumulation approach to Na⁺, transferring Na⁺ from roots to shoots. Salt accumulation of Na⁺ promoted by AMF was associated with CgNHX7. The transport mechanism of C. glauca to Cl^- might involve salt exclusion rather than salt accumulation, and Cl^- was no longer transferred to shoots in large quantities but started to accumulate in roots. However, AMF alleviated Na⁺ and Cl^- stress by similar mechanisms. AMF could promote salt dilution of C. glauca by increasing biomass and the content of K⁺, compartmentalizing Na⁺ and Cl^- in vacuoles. These processes were associated with the expression of CgNHX1, CgNHX2-1, CgCLCD, CgCLCF, and CgCLCG. Our study will provide a theoretical basis for the application of AMF to improve salt tolerance in plants.

Salinity-Induced Cytosolic Alkaline Shifts in Arabidopsis Roots Require the SOS Pathway

Plants have evolved elaborate mechanisms to sense, respond to and overcome the detrimental effects of high soil salinity. The role of calcium transients in salinity stress signaling is well established, but the physiological significance of concurrent salinity-induced changes in cytosolic pH remains largely undefined. Here, we analyzed the response of Arabidopsis roots expressing the genetically encoded ratiometric pH-sensor pHGFP fused to marker proteins for the recruitment of the sensor to the cytosolic side of the tonoplast (pHGFP-VTI11) and the plasma membrane (pHGFP-LTI6b). Salinity elicited a rapid alkalinization of cytosolic pH (pHcyt) in the meristematic and elongation zone of wild-type roots. The pH-shift near the plasma membrane preceded that at the tonoplast. In pH-maps transversal to the root axis, the epidermis and cortex had cells with a more alkaline pHcyt relative to cells in the stele in control conditions. Conversely, seedlings treated with 100 mM NaCl exhibited an increased pHcyt in cells of the vasculature relative to the external layers of the root, and this response occurred in both reporter lines. These pHcyt changes were substantially reduced in mutant roots lacking a functional SOS3/CBL4 protein, suggesting that the operation of the SOS pathway mediated the dynamics of pHcyt in response to salinity.

Mechanisms of plant saline-alkaline tolerance

Saline-alkaline soil affects crop growth and development, thereby suppressing the yields. Human activities and climate changes are putting arable land under the threat of saline-alkalization. To feed a growing global population in limited arable land, it is of great urgence to breed saline-alkaline tolerant crops to cope with food security. Plant salt-tolerance mechanisms have already been explored for decades. However, to date, the molecular mechanisms underlying plants responses to saline-alkaline stress have remained largely elusive. Here, we summarize recent advances in plant response to saline-alkaline stress and propose some points deserving of further exploration.

Potassium nutrient status drives posttranslational regulation of a low-K response network in Arabidopsis

Under low-potassium (K+) stress, a Ca2+ signaling network consisting of calcineurin B-like proteins (CBLs) and CBL-interacting kinases (CIPKs) play essential roles. Specifically, the plasma membrane CBL1/9-CIPK pathway and the tonoplast CBL2/3-CIPK pathway promotes K+ uptake and remobilization, respectively, by activating a series of K+ channels. While the dual CBL-CIPK pathways enable plants to cope with low-K+ stress, little is known about the early events that link external K+ levels to the CBL-CIPK proteins. Here we show that K+ status regulates the protein abundance and phosphorylation of the CBL-CIPK-channel modules. Further analysis revealed low K+-induced activation of VM-CBL2/3 happened earlier and was required for full activation of PM-CBL1/9 pathway. Moreover, we identified CIPK9/23 kinases to be responsible for phosphorylation of CBL1/9/2/3 in plant response to low-K+ stress and the HAB1/ABI1/ABI2/PP2CA phosphatases to be responsible for CBL2/3-CIPK9 dephosphorylation upon K+-repletion. Further genetic analysis showed that HAB1/ABI1/ABI2/PP2CA phosphatases are negative regulators for plant growth under low-K+, countering the CBL-CIPK network in plant response and adaptation to low-K+ stress.

The cell biology of primary cell walls during salt stress

Salt stress simultaneously causes ionic toxicity, osmotic stress, and oxidative stress, which directly impact plant growth and development. Plants have developed numerous strategies to adapt to saline environments. Whereas some of these strategies have been investigated and exploited for crop improvement, much remains to be understood, including how salt stress is perceived by plants and how plants coordinate effective responses to the stress. It is, however, clear that the plant cell wall is the first contact point between external salt and the plant. In this context, significant advances in our understanding of halotropism, cell wall synthesis, and integrity surveillance, as well as salt-related cytoskeletal rearrangements, have been achieved. Indeed, molecular mechanisms underpinning some of these processes have recently been elucidated. In this review, we aim to provide insights into how plants respond and adapt to salt stress, with a special focus on primary cell wall biology in the model plant Arabidopsis thaliana.

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.

Functional and biotechnological cues of potassium homeostasis for stress tolerance and plant development

Potassium (K+) is indispensable for the regulation of a plethora of functions like plant metabolism, growth, development, and abiotic stress responses. K+ is associated with protein synthesis and entangled in the activation of scores of enzymes, stomatal regulation, and photosynthesis. It has multiple transporters and channels that assist in the uptake, efflux, transport within the cell as well as from soil to different tissues, and the grain filling sites. While it is implicated in ion homeostasis during salt stress, it acts as a modulator of stomatal movements during water deficit conditions. K+ is reported to abate the effects of chilling and photooxidative stresses. K+ has been found to ameliorate effectively the co-occurrence of drought and high-temperature stresses. Nutrient deficiency of K+ makes leaves necrotic, leads to diminished photosynthesis, and decreased assimilate utilization highlighting the role it plays in photosynthesis. Notably, K+ is associated with the detoxification of reactive oxygen species (ROS) when plants are exposed to diverse abiotic stress conditions. It is irrefutable now that K+ reduces the activity of NADPH oxidases and at the same time maintains electron transport activity, which helps in mitigating the oxidative stress. K+ as a macronutrient in plant growth, the role of K+ during abiotic stress and the protein phosphatases involved in K+ transport have been reviewed. This review presents a holistic view of the biological functions of K+, its uptake, translocation, signaling, and the critical roles it plays under abiotic stress conditions, plant growth, and development that are being unraveled in recent times.

Phytoremediation of DEHP and heavy metals co-contaminated soil by rice assisted with a PGPR consortium: Insights into the regulation of ion homeostasis, improvement of photosynthesis and enrichment of beneficial bacteria in rhizosphere soil

The coexistence of di (2-ethylhexyl) phthalate (DEHP), Cd, and Zn poses a serious challenge to soil ecosystems. This study aimed to evaluate the phytoremediation potential of rice assisted with a plant growth promoting rhizobacteria (PGPR) consortium for the remediation of DEHP, Cd, and Zn co-contaminated soil. The consortium consisted of four bacterial strains, all of which exhibited Cd-Zn resistance and DEHP degradability. The results showed that the rice assisted by the bacterial consortium dissipated 86.1% DEHP while removing 76.0% Cd²⁺ and 92.2% Zn²⁺ from soil within 30 d. The presence of the PGPR consortium promoted plant growth and improved soil enzymatic activity, which may have helped enhance the removal of DEHP and heavy metals from the soil. Moreover, the application of the consortium modified the bacterial community and increased the relative abundance of bacteria related to DEHP degradation (Sphingomonas, Xanthobacteraceae), heavy metal immobilization (Massilia), and soil nutrient cycling (Nitrospira, Vicinamibacterales), which promoted plant growth and the removal of DEHP and heavy metals from soil. Notably, the DEHP and heavy metal contents in rice decreased substantially during the phytoremediation process. Therefore, the PGPR consortium could be beneficial for enhancing the removal of DEHP and heavy metals from the soil, without inducing the accumulation of these pollutants in rice. In general, this study confirmed that the combined use of rice and the PGPR consortium could remedy DEHP and heavy metal co-contaminated soil economically and ecologically without simultaneously posing risks for rice consumption.

ROS scavenging and ion homeostasis is required for the adaptation of halophyte Karelinia caspia to high salinity

The halophyte Karelinia caspia has not only fodder and medical value but also can remediate saline-alkali soils. Our previous study showed that salt-secreting by salt glands is one of main adaptive strategies of K. caspia under high salinity. However, ROS scavenging, ion homeostasis, and photosynthetic characteristics responses to high salinity remain unclear in K. caspia. Here, physio-biochemical responses and gene expression associated with ROS scavenging and ions transport were tested in K. caspia subjected to 100-400 mM NaCl for 7 days. Results showed that both antioxidant enzymes (SOD, APX) activities and non-enzymatic antioxidants (chlorogenic acid, α-tocopherol, flavonoids, polyamines) contents were significantly enhanced, accompanied by up-regulating the related enzyme and non-enzymatic antioxidant synthesis gene (KcCu/Zn-SOD, KcAPX6, KcHCT, KcHPT1, Kcγ-TMT, KcF3H, KcSAMS and KcSMS) expression with increasing concentrations of NaCl. These responses are beneficial for removing excess ROS to maintain a stable level of H2O2 and O2 - without lipid peroxidation in the K. caspia response to high salt. Meanwhile, up-regulating expression of KcSOS1/2/3, KcNHX1, and KcAVP was linked to Na+ compartmentalization into vacuoles or excretion through salt glands in K. caspia. Notably, salt can improve the function of PSII that facilitate net photosynthetic rates, which is helpful to growing normally in high saline. Overall, the findings suggested that ROS scavenging systems and Na+/K+ transport synergistically contributed to redox equilibrium, ion homeostasis, and the enhancement of PSII function, thereby conferring high salt tolerance.

Genome-wide identification of cysteine-rich receptor-like kinases in sweet cherry reveals that PaCRK1 enhances sweet cherry resistance to salt stress

Forty PaCRKs have been identified from sweet cherry and overexpression PaCRK1 in sweet cherry enhances its resistance to salt stress. Cysteine-rich receptor-like kinases (CRKs), a large subgroup of the receptor-like kinases, play an important role in plant development and stress response. However, knowledge about CRKs and its function against adverse environmental stresses in sweet cherry were lacking. In this study, 40 PaCRKs were identified from sweet cherry (Prunus avium) genome database. Phylogenetic analysis indicated that PaCRKs could be classified into six subgroups. Transcriptome analysis showed that the expression levels of most PaCRKs were changed under external environmental stresses. Functional study showed that PaCRK1 overexpression could enhance Arabidopsis and sweet cherry tolerance to salt stress. Moreover, biochemical analysis showed that PaCRK1 increased salt tolerance of sweet cherry by regulating the expression of antioxidation-related genes and their enzyme activities. This study provides a comprehensive understanding of PaCRKs in sweet cherry and elucidates the potential role of PaCRKs in response to various environmental stimuli.

Regulation of pollen tube growth by cellular pH and ions

Tip growth of the pollen tube is a model system for the study of cell polarity establishment in flowering plants. The tip growth of the pollen tube displays an oscillating pattern corresponding to cellular ion and pH dynamics. Therefore, cellular pH and ions play an important role in pollen growth and development. In this review, we summarized the current advances in understanding the function of cellular pH and ions in regulating pollen tube growth. We analyzed the physiological roles and underlying mechanisms of cellular pH and ions, including Ca²⁺, K⁺, and Cl^-, in regulating pollen tube growth. We further examined the function of Ca²⁺ in regulating cytoskeletons, small G proteins, and cell wall development in relation to pollen tube growth. We also examined the regulatory roles of cellular pH in pollen tube growth as well as pH regulation of ion flow, cell wall development, auxin signaling, and cytoskeleton function in pollen. In addition, we assessed the regulation of pollen tube growth by proton pumps and the maintenance of pH homeostasis in the trans-Golgi network by ion transporters. The interplay of ion homeostasis and pH dynamics was also assessed. We discussed the unanswered questions regarding pollen tube growth that need to be addressed in the future.

Melatonin as a regulator of plant ionic homeostasis: implications for abiotic stress tolerance

Melatonin is a highly conserved and ubiquitous molecule that operates upstream of a broad array of receptors in animal systems. Since melatonin was discovered in plants in 1995, hundreds of papers have been published revealing its role in plant growth, development, and adaptive responses to the environment. This paper summarizes the current state of knowledge of melatonin's involvement in regulating plant ion homeostasis and abiotic stress tolerance. The major topics covered here are: (i) melatonin's control of H+-ATPase activity and its implication for plant adaptive responses to various abiotic stresses; (ii) regulation of the reactive oxygen species (ROS)-Ca2+ hub by melatonin and its role in stress signaling; and (iii) melatonin's regulation of ionic homeostasis via hormonal cross-talk. We also show that the properties of the melatonin molecule allow its direct scavenging of ROS, thus preventing negative effects of ROS-induced activation of ion channels. The above 'desensitization' may play a critical role in preventing stress-induced K+ loss from the cytosol as well as maintaining basic levels of cytosolic Ca2+ required for optimal cell operation. Future studies should focus on revealing the molecular identity of transporters that could be directly regulated by melatonin and providing a bioinformatic analysis of evolutionary aspects of melatonin sensing and signaling.

A soybean sodium/hydrogen exchanger GmNHX6 confers plant alkaline salt tolerance by regulating Na+/K+ homeostasis

Alkaline soil has a high pH due to carbonate salts and usually causes more detrimental effects on crop growth than saline soil. Sodium hydrogen exchangers (NHXs) are pivotal regulators of cellular Na⁺/K⁺ and pH homeostasis, which is essential for salt tolerance; however, their role in alkaline salt tolerance is largely unknown. Therefore, in this study, we investigated the function of a soybean NHX gene, GmNHX6, in plant response to alkaline salt stress. GmNHX6 encodes a Golgi-localized sodium/hydrogen exchanger, and its transcript abundance is more upregulated in alkaline salt tolerant soybean variety in response to NaHCO₃ stress. Ectopic expression of GmNHX6 in Arabidopsis enhanced alkaline salt tolerance by maintaining high K⁺ content and low Na⁺/K⁺ ratio. Overexpression of GmNHX6 also improved soybean tolerance to alkaline salt stress. A single nucleotide polymorphism in the promoter region of NHX6 is associated with the alkaline salt tolerance in soybean germplasm. A superior promoter of GmNHX6 was isolated from an alkaline salt tolerant soybean variety, which showed stronger activity than the promoter from an alkaline salt sensitive soybean variety in response to alkali stress, by luciferase transient expression assays. Our results suggested soybean NHX6 gene plays an important role in plant tolerance to alkaline salt stress.

Sodium Accumulation in Infected Cells and Ion Transporters Mistargeting in Nodules of Medicago truncatula: Two Ugly Items That Hinder Coping with Salt Stress Effects

The maintenance of intracellular nitrogen-fixing bacteria causes changes in proteins’ location and in gene expression that may be detrimental to the host cell fitness. We hypothesized that the nodule’s high vulnerability toward salt stress might be due to alterations in mechanisms involved in the exclusion of Na+ from the host cytoplasm. Confocal and electron microscopy immunolocalization analyses of Na+/K+ exchangers in the root nodule showed the plasma membrane (MtNHX7) and endosome/tonoplast (MtNHX6) signal in non-infected cells; however, in mature infected cells the proteins were depleted from their target membranes and expelled to vacuoles. This mistargeting suggests partial loss of the exchanger’s functionality in these cells. In the mature part of the nodule 7 of the 20 genes encoding ion transporters, channels, and Na+/K+ exchangers were either not expressed or substantially downregulated. In nodules from plants subjected to salt treatments, low temperature-scanning electron microscopy and X-ray microanalysis revealed the accumulation of 5–6 times more Na+ per infected cell versus non-infected one. Hence, the infected cells’ inability to withstand the salt may be the integral result of preexisting defects in the localization of proteins involved in Na+ exclusion and the reduced expression of key genes of ion homeostasis, resulting in premature senescence and termination of symbiosis.

Over-Expression of an R2R3 MYB Gene, MdMYB108L, Enhances Tolerance to Salt Stress in Transgenic Plants

Plants are affected by various abiotic stresses during their growth and development. In plants, MYB transcription factors are involved in various physiological and biochemical processes, including biotic and abiotic stress responses. In this study, we functionally analyzed MdMYB108L. We examined the transcriptional activity of MdMYB108L under salt stress and determined that the N-terminal domain of MdMYB108L, which was significantly induced under salt stress, has transcriptional activity. MdMYB108L overexpression increased the germination rate, main root length, and the antioxidant activity of catalase and peroxidase in transgenic Arabidopsisthaliana seeds, while reducing reactive oxygen species (ROS) accumulation. MdMYB108L overexpression also increased the photosynthetic capacity of hairy root tissue (leaves) under salt stress. In addition, the MdMYB108L transcription factor bound to the MdNHX1 promoter positively regulated the transcription of the salt tolerance gene MdNHX1 in apples, improving the salt stress tolerance of transgenic plants. These findings have implications for improving the agricultural yields of apple trees under salt stress.

Epigenomic Features and Potential Functions of K+ and Na+ Favorable DNA G-Quadruplexes in Rice

DNA G-quadruplexes (G4s) are non-canonical four-stranded DNA structures involved in various biological processes in eukaryotes. Molecularly crowded solutions and monovalent cations have been reported to stabilize in vitro and in vivo G4 formation. However, how K⁺ and Na⁺ affect G4 formation genome-wide is still unclear in plants. Here, we conducted BG4-DNA-IP-seq, DNA immunoprecipitation with anti-BG4 antibody coupled with sequencing, under K⁺ and Na⁺ + PEG conditions in vitro. We found that K⁺-specific IP-G4s had a longer peak size, more GC and PQS content, and distinct AT and GC skews compared to Na⁺-specific IP-G4s. Moreover, K⁺- and Na⁺-specific IP-G4s exhibited differential subgenomic enrichment and distinct putative functional motifs for the binding of certain trans-factors. More importantly, we found that K⁺-specific IP-G4s were more associated with active marks, such as active histone marks, and low DNA methylation levels, as compared to Na⁺-specific IP-G4s; thus, K⁺-specific IP-G4s in combination with active chromatin features facilitate the expression of overlapping genes. In addition, K⁺- and Na⁺-specific IP-G4 overlapping genes exhibited differential GO (gene ontology) terms, suggesting they may have distinct biological relevance in rice. Thus, our study, for the first time, explores the effects of K⁺ and Na⁺ on global G4 formation in vitro, thereby providing valuable resources for functional G4 studies in rice. It will provide certain G4 loci for the biotechnological engineering of rice in the future.

Overdominant expression of related genes of ion homeostasis improves K+ content advantage in hybrid tobacco leaves

BACKGROUND

Potassium(K+) plays a vital role in improving the quality of tobacco leaves. However, how to improve the potassium content of tobacco leaves has always been a difficult problem in tobacco planting. K+ content in tobacco hybrid is characterized by heterosis, which can improve the quality of tobacco leaves, but its underlying molecular genetic mechanisms remain unclear.

RESULTS

Through a two-year field experiment, G70×GDH11 with strong heterosis and K326×GDH11 with weak heterosis were screened out. Transcriptome analyses revealed that 80.89% and 57.28% of the differentially expressed genes (DEGs) in the strong and weak heterosis combinations exhibited an overdominant expression pattern, respectively. The genes that up-regulated the overdominant expression in the strong heterosis hybrids were significantly enriched in the ion homeostasis. Genes involved in K+ transport (KAT1/2, GORK, AKT2, and KEA3), activity regulation complex (CBL-CIPK5/6), and vacuole (TPKs) genes were overdominant expressed in strong heterosis hybrids, which contributed to K+ homeostasis and heterosis in tobacco leaves.

CONCLUSIONS

K+ homeostasis and accumulation in tobacco hybrids were collectively improved. The overdominant expression of K+ transport and homeostasis-related genes conducted a crucial role in the heterosis of K+ content in tobacco leaves.

Salt stress mitigation in Lathyrus cicera by combining different microbial inocula

Arid and semi-arid areas are considered vulnerable to various environmental constraints which are further fortified by climate change. Salinity is one of the most serious abiotic factors affecting crop yield and soil fertility. Till now, no information is available on the effect of salinity on development and symbiotic nitrogen (N₂) fixation in the legume species Lathyrus cicera. Here, we evaluated the effect of different microbial inocula including nitrogen-fixing Rhizobium laguerreae, arbuscular mycorrhizal fungus (AMF) Rhizophagus irregularis, a complex mixed inoculum of AMF isolated from rhizospheric soil in "Al Aitha", and various plant growth-promoting bacteria (PGPB) including Bacillus subtilus, Bacillus simplex and Bacillus megaterium combined with Rhizobium, the AMF consortium, or R. irregularis on alleviating salt stress in this legume. A pot trial was conducted to evaluate the ability of different microbial inocula to mitigate adverse effects of salinity on L. cicera plants. The results showed that salinity (100 mM NaCl) significantly reduced L. cicera plant growth. However, inoculation with different inocula enhanced plant growth and markedly promoted various biochemical traits. Moreover, the combined use of PGPB and AMF was found to be the most effective treatment in mitigating deleterious effects of salinity stress on L. cicera. In addition, this co-inoculation upregulated the expression of two marker genes (LcHKT1 and LcNHX7) related to salinity tolerance. Our findings suggest that the AMF/PGPB formulation has a great potential to be used as a biofertilizer to improve L. cicera plant growth and productivity under saline conditions.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01205-4.

Genomic & structural diversity and functional role of potassium (K+) transport proteins in plants

Potassium (K⁺) is an essential macronutrient for plant growth and productivity. It is the most abundant cation in plants and is involved in various cellular processes. Variable K⁺ availability is sensed by plant roots, consequently K⁺ transport proteins are activated to optimize K⁺ uptake. In addition to K⁺ uptake and translocation these proteins are involved in other important physiological processes like transmembrane voltage regulation, polar auxin transport, maintenance of Na⁺/K⁺ ratio and stomata movement during abiotic stress responses. K⁺ transport proteins display tremendous genomic and structural diversity in plants. Their key structural features, such as transmembrane domains, N-terminal domains, C-terminal domains and loops determine their ability of K⁺ uptake and transport and thus, provide functional diversity. Most K⁺ transporters are regulated at transcriptional and post-translational levels. Genetic manipulation of key K⁺ transporters/channels could be a prominent strategy for improving K⁺ utilization efficiency (KUE) in plants. This review discusses the genomic and structural diversity of various K⁺ transport proteins in plants. Also, an update on the function of K⁺ transport proteins and their regulatory mechanism in response to variable K⁺ availability, in improving KUE, biotic and abiotic stresses is provided.

Genome-wide identification, molecular characterization, and gene expression analyses of honeysuckle NHX antiporters suggest their involvement in salt stress adaptation

Background

Ion homeostasis is an essential process for the survival of plants under salt stress. Na⁺/H⁺ antiporters (NHXs) are secondary ion transporters that regulate Na⁺ compartmentalization or efflux reduce Na⁺ toxicity and play a critical role during plant development and stress responses.

Methods and Results

To gain insight into the functional divergence of NHX genes in honeysuckle, a total of seven LjNHX genes were identified on the whole genome level and were renamed according to their chromosomal positions. All LjNHXs possessed the Na⁺/H⁺ exchanger domain and the amiloride-binding site was presented in all NHX proteins except LjNHX4. The phylogenetic analysis divided the seven NHX genes into Vac-clade (LjNHX1/2/3/4/5/7) and PM-clade (LjNHX6) based on their subcellular localization and validated by the distribution of conserved protein motifs and exon/intron organization analysis. The protein-protein interaction network showed that LjNHX4/5/6/7 shared the same putatively interactive proteins, including SOS2, SOS3, HKT1, and AVP1. Cis-acting elements and gene ontology (GO) analysis suggested that most LjNHXs involve in the response to salt stress through ion transmembrane transport. The expression profile analysis revealed that the expression levels of LjNHX3/7 were remarkably affected by salinity. These results suggested that LjNHXs play significant roles in honeysuckle development and response to salt stresses.

Conclusions

The theoretical foundation was established in the present study for the further functional characterization of the NHX gene family in honeysuckle.

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.

The phosphoinositide PI(3,5)P2 inhibits the activity of plant NHX proton/potassium antiporters: Advantages of a novel electrophysiological approach

In the present work, we discuss the way in which the parallel application of the patch-clamp technique and the 2′,7′-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) fluorescence detection for recording luminal proton changes allows the functional characterization of nonelectrogenic potassium/proton vacuolar antiporters of the NHX (Na+/H+ exchanger) family. Moreover, we review the functional role of the tonoplast-specific phosphoinositide PI(3,5)P2, able to simultaneously inhibit the activity of NHXs and CLC-a transporters, whose coordinated action can play an important role in the water balance of plant cells.

The Halophyte Species Solanum chilense Dun. Maintains Its Reproduction despite Sodium Accumulation in Its Floral Organs

Salinity is a growing global concern that affects the yield of crop species, including tomato (Solanum lycopersicum). Its wild relative Solanum chilense was reported to have halophyte properties. We compared salt resistance of both species during the reproductive phase, with a special focus on sodium localization in the flowers. Plants were exposed to NaCl from the seedling stage. Salinity decreased the number of inflorescences in both species but the number of flowers per inflorescence and sepal length only in S. lycopersicum. External salt supply decreased the stamen length in S. chilense, and it was associated with a decrease in pollen production and an increase in pollen viability. Although the fruit set was not affected by salinity, fruit weight and size decreased in S. lycopersicum. Concentrations and localization of Na, K, Mg, and Ca differed in reproductive structures of both species. Inflorescences and fruits of S. chilense accumulated more Na than S. lycopersicum. Sodium was mainly located in male floral organs of S. chilense but in non-reproductive floral organs in S. lycopersicum. The expression of Na transporter genes differed in flowers of both species. Overall, our results indicated that S. chilense was more salt-resistant than S. lycopersicum during the reproductive phase and that differences could be partly related to dissimilarities in element distribution and transport in flowers.

Proto Kranz-like leaf traits and cellular ionic regulation are associated with salinity tolerance in a halophytic wild rice

Species of wild rice (Oryza spp.) possess a wide range of stress tolerance traits that can be potentially utilized in breeding climate-resilient cultivated rice cultivars (Oryza sativa) thereby aiding global food security. In this study, we conducted a greenhouse trial to evaluate the salinity tolerance of six wild rice species, one cultivated rice cultivar (IR64) and one landrace (Pokkali) using a range of electrophysiological, imaging, and whole-plant physiological techniques. Three wild species (O. latifolia, O. officinalis and O. coarctata) were found to possess superior salinity stress tolerance. The underlying mechanisms, however, were strikingly different. Na+ accumulation in leaves of O. latifolia, O. officinalis and O. coarctata were significantly higher than the tolerant landrace, Pokkali. Na+ accumulation in mesophyll cells was only observed in O. coarctata, suggesting that O. officinalis and O. latifolia avoid Na+ accumulation in mesophyll by allocating Na+ to other parts of the leaf. The finding also suggests that O. coarctata might be able to employ Na+ as osmolyte without affecting its growth. Further study of Na+ allocation in leaves will be helpful to understand the mechanisms of Na+ accumulation in these species. In addition, O. coarctata showed Proto Kranz-like leaf anatomy (enlarged bundle sheath cells and lower numbers of mesophyll cells), and higher expression of C4-related genes (e.g., NADPME, PPDK) and was a clear outlier with respect to salinity tolerance among the studied wild and cultivated Oryza species. The unique phylogenetic relationship of O. coarctata with C4 grasses suggests the potential of this species for breeding rice with high photosynthetic rate under salinity stress in the future.

Reduced apoplastic barriers in tissues of shoot-proximal rhizomes of Oryza coarctata are associated with Na+ sequestration

Oryza coarctata is the only wild rice species with significant salinity tolerance. The present work examines the role of the substantial rhizomatous tissues of O. coarctata in conferring salinity tolerance. Transition to an erect phenotype (shoot emergence) from prostrate growth of rhizome tissues is characterized by marked lignification and suberization of supporting sclerenchymatous tissue, epidermis, and bundle sheath cells in aerial shoot-proximal nodes and internodes in O. coarctata. With salinity, however, aerial shoot-proximal internodal tissues show reductions in lignification and suberization, most probably related to re-direction of carbon flux towards synthesis of the osmporotectant proline. Concurrent with hypolignification and reduced suberization, the aerial rhizomatous biomass of O. coarctata appears to have evolved mechanisms to store Na+ in these specific tissues under salinity. This was confirmed by histochemical staining, quantitative real-time reverse transcription-PCR expression patterns of genes involved in lignification/suberization, Na+ and K+ contents of internodal tissues, as well as non-invasive microelectrode ion flux measurements of NaCl-induced net Na+, K+, and H+ flux profiles of aerial nodes were determined. In O. coarctata, aerial proximal internodes appear to act as 'traffic controllers', sending required amounts of Na+ and K+ into developing leaves for osmotic adjustment and turgor-driven growth, while more deeply positioned internodes assume a Na+ buffering/storage role.

An efficient root transformation system for CRISPR/Cas9-based analyses of shoot-root communication in cucurbit crops

Cucurbit crops are suitable models for studying long-distance signaling in horticultural plants. Although thousands of substances are graft transmissible in cucurbits, functional studies have been hampered by the lack of efficient genetic transformation systems. Here, we report a convenient and efficient root transformation method for several cucurbit crops that will facilitate studies of functional genes and shoot-root crosstalk. We obtained healthy plants with completely transformed roots and non-transgenic shoots within 6 weeks. Furthermore, we combined this root transformation method with grafting, which allowed for gene manipulation in the rootstock. We validated our system by exploring salt tolerance mechanisms using a cucumber (Cucumis sativus)/pumpkin (Cucurbita moschata Duch.) (scion/rootstock) graft in which the sodium transporter gene High-affinity K+ transporter1 (CmoHKT1;1) was edited in the pumpkin rootstock, and by overexpressing the pumpkin tonoplast Na+/H+ antiporter gene Sodium hydrogen exchanger4 (CmoNHX4) in cucumber roots.

ZRT-IRT-Like PROTEIN 6 expression perturbs local ion homeostasis in flowers and leads to anther indehiscence and male sterility

Metallic micronutrients are essential throughout the plant life cycle. Maintaining metal homeostasis in plant tissues requires a highly complex and finely tuned network controlling metal uptake, transport, distribution and storage. Zinc and cadmium hyperaccumulation, such as observed in the model plant Arabidopsis halleri, represents an extreme evolution of this network. Here, non-ectopic overexpression of the A. halleri ZIP6 (AhZIP6) gene, encoding a zinc and cadmium influx transporter, in Arabidopsis thaliana enabled examining the importance of zinc for flower development and reproduction. We show that AhZIP6 expression in flowers leads to male sterility resulting from anther indehiscence in a dose-dependent manner. The sterility phenotype is associated to delayed tapetum degradation and endothecium collapse, as well as increased magnesium and potassium accumulation and higher expression of the MHX gene in stamens. It is rescued by the co-expression of the zinc efflux transporter AhHMA4, linking the sterility phenotype to zinc homeostasis. Altogether, our results confirm that AhZIP6 is able to transport zinc in planta and highlight the importance of fine-tuning zinc homeostasis in reproductive organs. The study illustrates how the characterization of metal hyperaccumulation mechanisms can reveal key nodes and processes in the metal homeostasis network.

Light regulation of potassium in plants

Essential macronutrient potassium (K) and environmental signal light regulate a number of vital plant biological processes related to growth, development, and stress response. Recent research has shown connections between the perception of light and the regulation of K in plants. Photoreceptors-mediated wavelength-specific light perception activates signaling cascades which mediate stomatal movement by altering K⁺influx/efflux via K⁺ channels in the guard cells. The quality, intensity, and duration of light affect the regulation of K nutrition and crop quality. Blue/red illumination or red combined blue light treatment increases the expression levels of K transporter genes, K uptake and accumulation, leading to increased lycopene synthesis and improved fruit color in tomato. Despite the commonalities of light and K in multiple functions, our understanding of light regulation of K and associated physiological and molecular processes is fragmentary. In this review, we take a look at the light-controlled K uptake and utilization in plants and propose working models to show potential mechanisms. We discuss major light signaling components, their possible involvement in K nutrition, stomatal movement and crop quality by linking the perception of light signal and subsequent regulation of K. We also pose some outstanding questions to guide future research. Our analysis suggests that the enhancement of K utilization efficiency by manipulation of light quality and light signaling components can be a promising strategy for K management in crop production.

Salt stress proteins in plants: An overview

Salinity stress is considered the most devastating abiotic stress for crop productivity. Accumulating different types of soluble proteins has evolved as a vital strategy that plays a central regulatory role in the growth and development of plants subjected to salt stress. In the last two decades, efforts have been undertaken to critically examine the genome structure and functions of the transcriptome in plants subjected to salinity stress. Although genomics and transcriptomics studies indicate physiological and biochemical alterations in plants, it do not reflect changes in the amount and type of proteins corresponding to gene expression at the transcriptome level. In addition, proteins are a more reliable determinant of salt tolerance than simple gene expression as they play major roles in shaping physiological traits in salt-tolerant phenotypes. However, little information is available on salt stress-responsive proteins and their possible modes of action in conferring salinity stress tolerance. In addition, a complete proteome profile under normal or stress conditions has not been established yet for any model plant species. Similarly, a complete set of low abundant and key stress regulatory proteins in plants has not been identified. Furthermore, insufficient information on post-translational modifications in salt stress regulatory proteins is available. Therefore, in recent past, studies focused on exploring changes in protein expression under salt stress, which will complement genomic, transcriptomic, and physiological studies in understanding mechanism of salt tolerance in plants. This review focused on recent studies on proteome profiling in plants subjected to salinity stress, and provide synthesis of updated literature about how salinity regulates various salt stress proteins involved in the plant salt tolerance mechanism. This review also highlights the recent reports on regulation of salt stress proteins using transgenic approaches with enhanced salt stress tolerance in crops.