SOS1, HKT1;5, and NHX1 Synergistically Modulate Na+ Homeostasis in the Halophytic Grass Puccinellia tenuiflora

Increasing Ca2+ accumulation in salt glands under salt stress increases stronger selective secretion of Na+ in Plumbago auriculata tetraploids

Under salt stress, recretohalophyte Plumbago auriculata tetraploids enhance salt tolerance by increasing selective secretion of Na+ compared with that in diploids, although the mechanism is unclear. Using non-invasive micro-test technology, the effect of salt gland Ca2+ content on Na+ and K+ secretion were investigated in diploid and tetraploid P. auriculata under salt stress. Salt gland Ca2+ content and secretion rates of Na+ and K+ were higher in tetraploids than in diploids under salt stress. Addition of exogenous Ca2+ increased the Ca2+ content of the salt gland in diploids and is accompanied by an increase in the rate of Na+ and K+ secretion. With addition of a Ca2+ channel inhibitor, diploid salt glands retained large amounts of Ca2+, leading to higher Ca2+ content and Na+ secretion rate than those of tetraploids. Inhibiting H2O2 generation and H+-ATPase activity altered Na+ and K+ secretion rates in diploids and tetraploids under salt stress, indicating involvement in regulating Na+ and K+ secretion. Our results indicate that the increased Na+ secretion rate of salt gland in tetraploids under salt stress was associated with elevated Ca2+ content in salt gland.

Physiological responses and transcriptomic analysis of StCPD gene overexpression in potato under salt stresses

Introduction

The potato (Solanum tuberosum L.), one of the most vital food crops worldwide, is sensitive to salinity. Brassinosteroids (BRs) are crucial in tolerance to various abiotic stresses. The constitutive photomorphogenesis and dwarf (CPD) gene encodes C-3 oxidase, which is a rate-limiting enzyme that controls the synthesis of BRs.

Methods

In this study, we used StCPD gene overexpression (T) and un-transgenic (NT) plants obtained from our former research to illustrate adaptive resistance to salt stress at levels of phenotype; cell ultrastructure, physiology, and biochemistry; hormone; and transcription.

Results

Results showed the accumulation of 2,4-epibrassionolide (EBL) in T potatoes. We found that under high salt situations, the changed Na+/K+ transporter gene expression was linked with the prevalent ionic responses in T plants, which led to lower concentrations of K+ and higher concentrations of Na+ in leaves. Furthermore, RNA-sequencing (RNA-seq) data elucidated that gene expressions in NT and T plants were significantly changed with 200-mM NaCl treatment for 24 h and 48 h, compared with the 0-h treatment. Functional enrichment analysis suggested that most of the differentially expressed genes (DEGs) were related to the regulation of BR-related gene expression, pigment metabolism process, light and action, and plant hormone signal transduction.

Discussion

These findings suggested that StCPD gene overexpression can alleviate the damage caused by salt stress and enhance the salt resistance of potato plantlets. Our study provides an essential reference for further research on BR regulation of plant molecular mechanisms in potatoes with stress tolerance.

Eugenol improves salt tolerance via enhancing antioxidant capacity and regulating ionic balance in tobacco seedlings

Salt stress inhibits plant growth by disturbing plant intrinsic physiology. The application of exogenous plant growth regulators to improve the plant tolerance against salt stress has become one of the promising approaches to promote plant growth in saline environment. Eugenol (4-allyl-2- methoxyphenol) is the main ingredient in clove oil and it is known for its strong antioxidant and anti-microbial activities. Eugenol also has the ability of inhibiting several plant pathogens, implying the potential use of eugenol as an environmental friendly agrichemical. However, little is known about the possible role of eugenol in the regulation of plant tolerance against abiotic stress. Therefore, here we investigated the effectiveness of phytochemical eugenol in promoting salt tolerance in tobacco seedlings through physiological, histochemical, and biochemical method. The seedling roots were exposed to NaCl solution in the presence or absence of eugenol. Salt stress inhibited seedling growth, but eugenol supplementation effectively attenuated its effects in a dose-dependent manner, with an optimal effect at 20 µM. ROS (reactive oxygen species) accumulation was found in seedlings upon salt stress which was further resulted in the amelioration of lipid peroxidation, loss of membrane integrity, and cell death in salt-treated seedlings. Addition of eugenol highly suppressed ROS accumulation and reduced lipid peroxidation generation. Both enzymatic and non-enzymatic antioxidative systems were activated by eugenol treatment. AsA/DHA and GSH/GSSG were also enhanced upon eugenol treatment, which helped maintain redox homeostasis upon salinity. Eugenol treatment resulted in an increase in the content of osmoprotectants (e.g. proline, soluble sugar and starch) in salt-treated seedlings. Na+ levels decreased significantly in seedlings upon eugenol exposure. This may result from the upregulation of the expression of two ionic transporter genes, SOS1 (salt-hypersensitive 1) and NHX1 (Na+/H+ anti-transporter 1). Hierarchical cluster combined correlation analysis uncovered that eugenol induced salt tolerance was mediated by redox homeostasis and maintaining ionic balance in tobacco seedlings. This work reveals that eugenol plays a crucial role in regulating plant resistant physiology. This may extend its biological function as a novel biostimulant and opens up new possibilities for improving crop productivity in the saline agricultural environment.

Salt Stress Inhibits Photosynthesis and Destroys Chloroplast Structure by Downregulating Chloroplast Development-Related Genes in Robinia pseudoacacia Seedlings

Soil salinization is an important factor limiting food security and ecological stability. As a commonly used greening tree species, Robinia pseudoacacia often suffers from salt stress that can manifest as leaf yellowing, decreased photosynthesis, disintegrated chloroplasts, growth stagnation, and even death. To elucidate how salt stress decreases photosynthesis and damages photosynthetic structures, we treated R. pseudoacacia seedlings with different concentrations of NaCl (0, 50, 100, 150, and 200 mM) for 2 weeks and then measured their biomass, ion content, organic soluble substance content, reactive oxygen species (ROS) content, antioxidant enzyme activity, photosynthetic parameters, chloroplast ultrastructure, and chloroplast development-related gene expression. NaCl treatment significantly decreased biomass and photosynthetic parameters, but increased ion content, organic soluble substances, and ROS content. High NaCl concentrations (100-200 mM) also led to distorted chloroplasts, scattered and deformed grana lamellae, disintegrated thylakoid structures, irregularly swollen starch granules, and larger, more numerous lipid spheres. Compared to control (0 mM NaCl), the 50 mM NaCl treatment significantly increased antioxidant enzyme activity while upregulating the expression of the ion transport-related genes Na⁺/H⁺ exchanger 1(NHX 1) and salt overly sensitive 1 (SOS 1) and the chloroplast development-related genes psaA, psbA, psaB, psbD, psaC, psbC, ndhH, ndhE, rps7, and ropA. Additionally, high concentrations of NaCl (100-200 mM) decreased antioxidant enzyme activity and downregulated the expression of ion transport- and chloroplast development-related genes. These results showed that although R. pseudoacacia can tolerate low concentrations of NaCl, high concentrations (100-200 mM) can damage chloroplast structure and disturb metabolic processes by downregulating gene expression.

Differentiation of advanced generation mutant wheat lines: Conventional techniques versus Raman spectroscopy

This research aimed to assess the feasibility of utilizing Raman spectroscopy in plant breeding programs. For this purpose, the evaluation of the mutant populations set up the application of 4 mM NaN₃ to the somatic embryos obtained from mature wheat (Triticum aestivum L. Adana-99 cv.) embryos. Advanced wheat mutant lines, which were brought up to the seventh generation with salt stress tolerance by following in vitro and in vivo environments constructed by mutated populations, were evaluated using conventional techniques [measurement of antioxidant enzyme activities (SOD, CAT, and POX), total chlorophyll, TBARS, and proline contents; measurement of the concentration of Na⁺ and K⁺ ions; and evaluation of gene expression by qPCR (TaHKT2;1, TaHKT1;5, TaSOS1, TaNa⁺/H⁺ vacuolar antiporter, TaV-PPase, TaV-ATPase, and TaP5CS)] and Raman spectroscopy. In this research, no significant difference was found in the increase of SOD, CAT, and POX antioxidant enzyme activities between the salt-treated and untreated experimental groups of the commercial cultivar, while there was a statistically significant increase in salt-treated advanced generation mutant lines as compared to control and the salt-treated commercial cultivar. Proline showed a statistically significant increase in all experimental groups compared to the untreated commercial cultivar. The degradation in the amount of chlorophyll was lower in the salt-treated advanced generation mutant lines than in the salt-treated commercial cultivar. According to gene expression studies, there were statistical differences at various levels in terms of Na⁺ and/or K⁺ uptake from soil to plant (TaHKT2;1, TaHKT1;5, and TaSOS1), and Na⁺ compartmentalizes into the cell vacuole (TaNa⁺/H⁺ vacuolar antiporter, Ta vacuolar pyrophosphatase, and Ta vacuolar H⁺-ATPase). The expression activity of TaP5CS, which is responsible for the transcription of proline, is similar to the content of proline in the current study. As a result of Raman spectroscopy, the differences in peaks represent the protein-related bands in mutant lines having a general decreasing trend in intensity when compared to the commercial cultivar. Amide-I (1,630 and 1,668 cm^-1), Histidine, Lysine, Arginine, and Leucine bands (823, 849, 1,241, 1,443, and 1,582 cm^-1) showed decreasing wavenumbers. Beta-carotene peaks at 1,153 and 1,519 cm^-1 showed increasing trends when the normalized Raman intensities of the mutant lines were compared.

Enhanced Na+ and Cl- sequestration and secretion selectivity contribute to high salt tolerance in the tetraploid recretohalophyte Plumbago auriculata Lam

Enhanced secretion of Na⁺ and Cl^- in leaf glands and leaf vacuolar sequestration of Na⁺ or root retention of Cl^-, combined with K⁺ retention, contribute to the improved salt tolerance of tetraploid recretohalophyte P. auriculata. Salt stress is one of the major abiotic factors threatening plant growth and development, and polyploids generally exhibit higher salt stress resistance than diploids. In recretohalophytes, which secrete ions from the salt gland in leaf epidermal cells, the effects of polyploidization on ion homeostasis and secretion remain unknown. In this study, we compared the morphology, physiology, and ion homeostasis regulation of diploid and autotetraploid accessions of the recretohalophyte Plumbago auriculata Lam. after treatment with 300 mM NaCl for 0, 2, 4, 6, and 8 days. The results showed that salt stress altered the morphology, photosynthetic efficiency, and chloroplast structure of diploid P. auriculata to a greater extent than those of its tetraploid counterpart. Moreover, the contents of organic osmoregulatory substances (proline and soluble sugars) were significantly higher in the tetraploid than in the diploid, while those of H₂O₂ and malondialdehyde (MDA) were significantly lower. Analysis of ion homeostasis revealed that the tetraploid cytotype accumulated more Na⁺ in stems and leaves and more Cl^- in roots but less K⁺ loss in roots compared with diploid P. auriculata. Additionally, the rate of Na⁺ and Cl^- secretion from the leaf surface was higher, while that of K⁺, Mg²⁺, and Ca²⁺ secretion was lower in tetraploid plants. X-ray microanalysis of mesophyll cells revealed that Na⁺ mainly accumulated in different cellular compartments in the tetraploid (vacuole) and diploid (cytoplasm) plants. Our results suggest that polyploid recretohalophytes require the ability to sequester Na⁺ and Cl^-(via accumulation in leaf cell vacuoles or unloading by roots) and selectively secrete these ions (through salt glands) together with the ability to prevent K⁺ loss (by roots). This mechanism required to maintain K⁺/Na⁺ homeostasis in polyploid recretohalophytes under high salinity provides new insights in the improved maintenance of ion homeostasis in polyploids under salt stress.

Isolation, characterization, and functional verification of salt stress response genes of NAC transcription factors in Ipomoea pes-caprae

Adverse environmental stress is a major environmental factor threatening food security, which is why improving plant stress resistance is essential for agricultural productivity and environmental sustainability. The NAC (NAM, ATAF, and CUC) transcription factors (TFs) play a dominant role in plant responses to abiotic and biotic stresses, but they have been poorly studied in Ipomoea pes-caprae. In this research, 12 NAC TFs, named IpNAC1-IpNAC12, were selected from transcriptome data. The homologous evolution tree divided IpNACs into four major categories, and six IpNACs were linearly associated with Arabidopsis ANAC genes. From the gene structures, protein domains, and promoter upstream regulatory elements, IpNACs were shown to contain complete NAC-specific subdomains (A-E) and cis-acting elements corresponding to different stress stimuli. We measured the expression levels of the 12 IpNACs under abiotic stress (salt, heat, and drought) and hormone treatment (abscisic acid, methyl jasmonate, and salicylic acid), and their transcription levels differed. IpNAC5/8/10/12 were located in the nucleus through subcellular localization, and the overexpressing transgenic Arabidopsis plants showed high tolerance to salt stress. The cellular Na⁺ homeostasis content in the mature and elongation zones of the four IpNAC transgenic sweetpotato roots showed an obvious efflux phenomenon. These conclusions demonstrate that IpNAC5/8/10/12 actively respond to abiotic stress, have significant roles in improving plant salt tolerance, and are important salt tolerance candidate genes in I. pes-caprae and sweetpotato. This study laid the foundation for further studies on the function of IpNACs in response to abiotic stress. It provides options for improving the stress resistance of sweetpotato using gene introgression from I. pes-caprae.

Functional verification and screening of protein interacting with the slPHB3

slPHB3 was cloned from Salix linearistipularis, the amino acid sequence blast and phylogenetic tree analysis showed that slPHB3 has the most similarity with PHB3 from Populus trichocarpa using DNAMAN software and MEGA7 software. RT-qPCR results confirmed that the expression of slPHB3 was induced obviously under stress treatments. The growth of recombinant yeast cells was better than that of the control group under the stress treatment, indicating that slPHB3 may be involved in the stress response of yeast cells. The transgenic tobacco was treated with different concentrations of NaCl, NaHCO3 and H2O2, fresh weigh of overexpression tobacco were heavier than wild-types. The results showed that transgenic tobacco was more tolerant to salt and oxidation than wild-type tobacco. Expression of important genes including NHX1 and SOS1 in salt stress response pathways are steadily higher in overexpression tobacco than that in wild-types. We identified 17 proteins interacting with slPHB3 by yeast two-hybrid technique, most of these proteins were relation to the stresses. The salt tolerance of slPHB3 expressing yeast and slPHB3 overexpressing plants were better than that of the control. Ten stress-related proteins may interact with slPHB3, which preliminarily indicated that slPHB3 had a certain response relationship with salt stress. The study of slPHB3 under abiotic stress can improve our understanding of PHB3 gene function.

Regulation of Na+/H+ exchangers, Na+/K+ transporters, and lignin biosynthesis genes, along with lignin accumulation, sodium extrusion, and antioxidant defense, confers salt tolerance in alfalfa

Accumulation of high sodium (Na⁺) leads to disruption of metabolic processes and decline in plant growth and productivity. Therefore, this study was undertaken to clarify how Na⁺/H⁺ exchangers and Na⁺/K⁺ transporter genes contribute to Na⁺ homeostasis and the substantial involvement of lignin biosynthesis genes in salt tolerance in alfalfa (Medicago sativa L.), which is poorly understood. In this study, high Na⁺ exhibited a substantial reduction of morphophysiological indices and induced oxidative stress indicators in Xingjiang Daye (XJD; sensitive genotype), while Zhongmu (ZM; tolerant genotype) remained unaffected. The higher accumulation of Na⁺ and the lower accumulation of K⁺ and K⁺/(Na⁺ + K⁺) ratio were found in roots and shoots of XJD compared with ZM under salt stress. The ZM genotype showed a high expression of SOS1 (salt overly sensitive 1), NHX1 (sodium/hydrogen exchanger 1), and HKT1 (high-affinity potassium transporter 1), which were involved in K⁺ accumulation and excess Na⁺ extrusion from the cells compared with XJD. The lignin accumulation was higher in the salt-adapted ZM genotype than the sensitive XJD genotype. Consequently, several lignin biosynthesis-related genes including 4CL2, CCoAOMT, COMT, CCR, C4H, PAL1, and PRX1 exhibited higher mRNA expression in salt-tolerant ZM compared with XJD. Moreover, antioxidant enzyme (catalase, superoxide dismutase, ascorbate peroxidase, and glutathione reductase) activity was higher in ZM relative to XJD. This result suggests that high antioxidant provided the defense against oxidative damages in ZM, whereas low enzyme activity with high Na⁺ triggered the oxidative damage in XJD. These findings together illustrate the ion exchanger, antiporter, and lignin biosysthetic genes involving mechanistic insights into differential salt tolerance in alfalfa.

Foliar spray of commercial seaweed and amino acid-derived biostimulants promoted phytoremediation potential and salinity stress tolerance in halophytic grass, Puccinellia distans

Plants pretreatment with various chemicals has often been used to diminish salinity stress impact on plants. An experiment was carried out to determine the effect of foliar spray of two commercially available biostimulants (Algabon® [0.5 g/l] and Bonamid® [2 g/l]) on the growth and tolerance of halophytic grass, Pucccinellia distans under non-salinity condition (NSC) and salinity condition (SC). The greenhouse experiment was set up in a completely randomized design with three treatments repeated three times. Our results showed that biomass, leaf relative water content, chlorophyll content, K+ content, K+/Na+ ratio, and protein and N contents were all negatively affected by 300 mM NaCl. The results obtained in the present study showed the beneficial effects of the pretreatments of two biostimulants on P. distans seedlings under non-salinity stress conditions with respect to increasing plant biomass, photosynthetic pigments, K+ content, the content of proteins, and nitrogen percentage. The results suggested that foliar spray of Bonamid® could partly diminish NaCl-caused stress on P. distans seedlings, probably due to higher accumulation of shoot biomass, photosynthetic pigments, K+/Na+ ratio, protein and N contents, phytoremediation potential, as well as upregulation of Na+/H+ antiporters located in plasma membranes and vacuoles. The highest phytoremediation potential (PP) of shoots and total biomass was detected in the plants sprayed with Bonamid® by 50.8 and 42.7% respectively, relative to that in salinity-stressed control plants. Interestingly, foliar spray with two biostimulants decreased osmoprotectants and antioxidant compounds content of shoots under salinity stress conditions. Collectively, it could be concluded that a noticeable feature of pretreatment of P. distans seedlings with Algabon® and Bonamid® is the increase in growth under NSC, whereas under SC only pretreated plants with amino acid-derived biostimulant (Bonamid®) can (partly) diminish the NaCl-induced deleterious effects in P. distans seedlings through the compartmentalization of salts in vacuoles (by upregulation of Na+/H+ antiporters).

Lb1G04202, an Uncharacterized Protein from Recretohalophyte Limonium bicolor, Is Important in Salt Tolerance

With global increases in saline soil, it has become increasingly important to decipher salt-tolerance mechanisms and identify strategies to improve salt tolerance in crops. Halophytes complete their life cycles in environments containing ≥200 mM NaCl; these remarkable plants provide a potential source of genes for improving crop salt tolerance. Recretohalophytes such as Limonium bicolor have salt glands that secrete Na⁺ on their leaf epidermis. Here, we identified Lb1G04202, an uncharacterized gene with no conserved domains, from L. bicolor, which was highly expressed after NaCl treatment. We confirmed its expression in the salt gland by in situ hybridization, and then heterologously expressed Lb1G04202 in Arabidopsis thaliana. The transgenic lines had a higher germination rate, greater cotyledon growth percentage, and longer roots than the wild type (WT) under NaCl treatments (50, 100 and 150 mM). At the seedling stage, the transgenic lines grew better than the WT and had lower Na⁺ and malonyldialdehyde accumulation, and higher K⁺ and proline contents. This corresponded with the high expression of the key proline biosynthesis genes AtP5CS1 and AtP5CS2 under NaCl treatment. Isotonic mannitol treatment showed that Lb1G04202 overexpression significantly relieved osmotic stress. Therefore, this novel gene provides a potential target for improving salt tolerance.

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.

Sodium hyperaccumulators in the Caryophyllales are characterized by both abnormally large shoot sodium concentrations and [Na]shoot/[Na]root quotients greater than unity

BACKGROUND AND AIMS

Some Caryophyllales species accumulate abnormally large shoot sodium (Na) concentrations in non-saline environments. It is not known whether this is a consequence of altered Na partitioning between roots and shoots. This paper tests the hypotheses (1) that Na concentrations in shoots ([Na]shoot) and in roots ([Na]root) are positively correlated among Caryophyllales, and (2) that shoot Na hyperaccumulation is correlated with [Na]shoot/[Na]root quotients.

METHODS

Fifty two genotypes, representing 45 Caryophyllales species and 4 species from other angiosperm orders, were grown hydroponically in a non-saline, complete nutrient solution. Concentrations of Na in shoots and in roots were determined using inductively coupled plasma mass spectrometry (ICP-MS).

KEY RESULTS

Sodium concentrations in shoots and roots were not correlated among Caryophyllales species with normal [Na]shoot, but were positively correlated among Caryophyllales species with abnormally large [Na]shoot. In addition, Caryophyllales species with abnormally large [Na]shoot had greater [Na]shoot/[Na]root than Caryophyllales species with normal [Na]shoot.

CONCLUSIONS

Sodium hyperaccumulators in the Caryophyllales are characterized by abnormally large [Na]shoot, a positive correlation between [Na]shoot and [Na]root, and [Na]shoot/[Na]root quotients greater than unity.

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.

Sex-specific photosynthetic capacity and Na+ homeostasis in Populus euphratica exposed to NaCl stress and AMF inoculation

Soil salinity and associated land degradation are major ecological problems. Excess Na⁺ ions in soil impede the plant photosynthetic process and Na⁺ homeostasis status. Arbuscular mycorrhizal fungi (AMF) can alleviate salt stress in host plants. Although a number of studies have demonstrated that Na⁺ accumulation is decreased by mycorrhizae, the molecular mechanisms involved have received little attention from researchers. Populus euphratica is a typical natural woody tree with excellent salt tolerance. Due to its symbiosis forming capability with AMF, we explored the influence of Funneliformis mosseae on the growth, photosynthesis, and expression of three genes involved in Na⁺ homeostasis within dioecious P. euphratica under salt stress. The results indicated that salt stress significantly increases Na⁺ contents and inhibits growth status and photosynthetic capacity, especially in females. However, AMF had positive effects on the growth status, photosynthetic capacity and Na⁺ homeostasis, especially in males. The expression levels of NHX1 in shoots and HKT1 and SOS1 in roots, all of which are involved in Na⁺ homeostasis, were upregulated by F. mosseae under salt stress. For males, the beneficial effect of AMF centered on extruding, sequestering and long-distance transporting of Na⁺ ions . For females, the beneficial effect of AMF centered on extruding excessive Na⁺.

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.

Salicylic acid modulates ACS, NHX1, sos1 and HKT1;2 expression to regulate ethylene overproduction and Na+ ions toxicity that leads to improved physiological status and enhanced salinity stress tolerance in tomato plants cv. Pusa Ruby

Tomato is an important crop for its high nutritional and medicinal properties. The role of salicylic acid (SA) in 1-aminocyclopropane-1-carboxylate synthase (ACS), sodium-hydrogen exchanger (NHX1), salt overly sensitive 1 (sos1) and high-affinity K⁺ transporter (HKT1;2) transcripts, and ACS enzyme activity and ethylene (ET) production, and growth and physiological attributes was evaluated in tomato cv. Pusa Ruby under salinity stress. Thirty days-old seedlings treated with 0 mM NaCl, 250 mM NaCl, 250 mM NaCl plus 100 µM SA were assessed for different growth and physiological parameters at 45 DAS. Results showed ACS, NHX1, sos1 and HKT1;2 transcripts were significantly changed in SA treated plants. The ACS enzyme activity and ET content were considerably decreased in SA treated plants. Shoot length (SL), root length (RL), number of leaves (NL), leaf area per plant (LA), shoot fresh weight (SFW) and root fresh weight (RFW) were also improved under SA treatment. Conversely, the electrolyte leakage and sodium ion (Na⁺) content were significantly reduced in SA treated plants. In addition, the endogenous proline and potassium ion (K⁺) content, and K⁺/Na⁺ ratio were considerably increased under SA treatment. Likewise, antioxidant enzymes (SOD, CAT, APX and GR) profile were better in SA treated plant. The present findings suggest that SA reverse the negative effects of salinity stress and stress induced ET production by modulating ACS, NHX, sos1 and HKT1;2 transcript level, and improving various growth and physiological parameters, and antioxidants enzymes profile. This will contribute to a better understanding of salinity stress tolerance mechanisms of tomato plants involving SA and ET cross talk and ions homeostasis to develop more tolerant plant.

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

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

Genome-wide identification and function analysis of HMAD gene family in cotton (Gossypium spp.)

BACKGROUND

The abiotic stress such as soil salinization and heavy metal toxicity has posed a major threat to sustainable crop production worldwide. Previous studies revealed that halophytes were supposed to tolerate other stress including heavy metal toxicity. Though HMAD (heavy-metal-associated domain) was reported to play various important functions in Arabidopsis, little is known in Gossypium.

RESULTS

A total of 169 G. hirsutum genes were identified belonging to the HMAD gene family with the number of amino acids ranged from 56 to 1011. Additionally, 84, 76 and 159 HMAD genes were identified in each G. arboreum, G. raimondii and G. barbadense, respectively. The phylogenetic tree analysis showed that the HMAD gene family were divided into five classes, and 87 orthologs of HMAD genes were identified in four Gossypium species, such as genes Gh_D08G1950 and Gh_A08G2387 of G. hirsutum are orthologs of the Gorai.004G210800.1 and Cotton_A_25987 gene in G. raimondii and G. arboreum, respectively. In addition, 15 genes were lost during evolution. Furthermore, conserved sequence analysis found the conserved catalytic center containing an anion binding (CXXC) box. The HMAD gene family showed a differential expression levels among different tissues and developmental stages in G. hirsutum with the different cis-elements for abiotic stress.

CONCLUSIONS

Current study provided important information about HMAD family genes under salt-stress in Gossypium genome, which would be useful to understand its putative functions in different species of cotton.

Genetic Diversity and Synergistic Modulation of Salinity Tolerance Genes in Aegilops tauschii Coss

Aegilops tauschii Coss. (2n = 2x = 14, DD) is a problematic weed and a rich source of genetic material for wheat crop improvement programs. We used physiological traits (plant height, dry weight biomass, Na⁺ and K⁺ concentration) and 14 microsatellite markers to evaluate the genetic diversity and salinity tolerance in 40 Ae. tauschii populations. The molecular marker allied with salinity stress showed polymorphisms, and a cluster analysis divided the populations into different groups, which indicated diversity among populations. Results showed that the expression level of AeHKT1;4 and AeNHX1 were significantly induced during salinity stress treatments (50 and 200 mM), while AeHKT1;4 showed relative expression in roots, and AeNHX1 was expressed in leaves under the control conditions. Compared with the control conditions, the expression level of AeHKT1;4 significantly increased 1.7-fold under 50 mM salinity stress and 4.7-fold under 200 mM salinity stress in the roots of Ae. tauschii. AeNHX1 showed a relative expression level of 1.6-fold under 50 mM salinity stress and 4.6-fold under 200 mM salinity stress compared with the control conditions. The results provide strong evidence that, under salinity stress conditions, AeHKT1;4 and AeNHX1 synergistically regulate the Na⁺ homeostasis through regulating Na⁺ transport in Ae. tauschii. AeNHX1 sequestrated the Na⁺ into vacuoles, which control the regulation of Na⁺ transport from roots to leaves under salinity stress conditions in Ae. tauschii.

Genome-wide DNA methylation analysis and biochemical responses provide insights into the initial domestication of halophyte Puccinellia tenuiflora

Puccinellia tenuiflora was domesticated for two years by growing it under non-saline conditions, providing epigenetic and biochemical insights into the initial domestication of extreme halophytes. Some halophytes have economic value as crop species. The domestication of halophytes may offer hope in solving the problem of soil salinization. We domesticated a wild halophyte, Puccinellia tenuiflora, for two years by growing it under non-saline conditions in a greenhouse and used re-sequencing, genome-wide DNA methylation, biochemical, and transcriptome analyses to uncover the mechanisms underlying alterations in the halophyte's tolerance to saline following domestication. Our results showed that non-saline domestication altered the methylation status for a number of genes and transposable elements, resulting in a much higher frequency of hypomethylation than hypermethylation. These modifications to DNA methylation were observed in many critical salinity-tolerance genes, particularly their promoter regions or transcriptional start sites. Twenty-nine potassium channel genes were hypomethylated and three were hypermethylated, suggesting that the DNA methylation status of potassium channel genes was influenced by domestication. The accelerated uptake of potassium is a major salinity tolerance characteristic of P. tenuiflora. We propose that modifications to the DNA methylation of potassium channel genes may be associated with the development of salinity tolerance in this species. By assessing whether non-saline domestication could change the salinity tolerance of P. tenuiflora, we demonstrated that non-saline domesticated plants are less tolerant to saline, which may be attributable to altered sucrose metabolism. DNA methylation and transposable elements may, therefore, be integrated into an environment-sensitive molecular engine that promotes the rapid domestication of P. tenuiflora to enable its use as a crop plant.

Comparative Genomics and Transcriptomics of the Extreme Halophyte Puccinellia tenuiflora Provides Insights Into Salinity Tolerance Differentiation Between Halophytes and Glycophytes

Halophytes and glycophytes exhibit clear differences in their tolerance to high levels of salinity. The genetic mechanisms underlying this differentiation, however, remain unclear. To unveil these mechanisms, we surveyed the evolution of salinity-tolerant gene families through comparative genomic analyses between the model halophyte Puccinellia tenuiflora and glycophytic Gramineae plants, and compared their transcriptional and physiological responses to salinity stress. Under salinity stress, the K⁺ concentration in the root was slightly enhanced in P. tenuiflora, but it was greatly reduced in the glycophytic Gramineae plants, which provided a physiological explanation for differences in salinity tolerance between P. tenuiflora and these glycophytes. Interestingly, several K⁺ uptake gene families from P. tenuiflora experienced family expansion and positive selection during evolutionary history. This gene family expansion and the elevated expression of K⁺ uptake genes accelerated K⁺ accumulation and decreased Na⁺ toxicity in P. tenuiflora roots under salinity stress. Positively selected P. tenuiflora K⁺ uptake genes may have evolved new functions that contributed to development of P. tenuiflora salinity tolerance. In addition, the expansion of the gene families involved in pentose phosphate pathway, sucrose biosynthesis, and flavonoid biosynthesis assisted the adaptation of P. tenuiflora to survival under high salinity conditions.

Transcriptome analysis of upland cotton revealed novel pathways to scavenge reactive oxygen species (ROS) responding to Na2SO4 tolerance

Salinity is an extensive and adverse environmental stress to crop plants across the globe, and a major abiotic constraint responsible for limited crop production threatening the crop security. Soil salinization is a widespread problem across the globe, threatening the crop production and food security. Salinity impairs plant growth and development via reduction in osmotic potential, cytotoxicity due to excessive uptake of ions such as sodium (Na⁺) and chloride (Cl^-), and nutritional imbalance. Cotton, being the most cultivated crop on saline-alkaline soils, it is of great importance to elucidate the mechanisms involved in Na₂SO₄ tolerance which is still lacking in upland cotton. Zhong 9835, a Na₂SO₄ resistant cultivar was screened for transcriptomic studies through various levels of Na₂SO₄ treatments, which results into identification of 3329 differentially expressed genes (DEGs) in roots, stems and leave at 300 mM Na₂SO₄ stress till 12 h in compared to control. According to gene functional annotation analysis, genes involved in reactive oxygen species (ROS) scavenging system including osmotic stress and ion toxicity were significantly up-regulated, especially GST (glutathione transferase). In addition, analysis for sulfur metabolism, results in to identification of two rate limiting enzymes [APR (Gh_D05G1637) and OASTL (Gh_A13G0863)] during synthesis of GSH from SO₄^2-. Furthermore, we also observed a crosstalk of the hormones and TFs (transcription factors) enriched in hormone signal transduction pathway. Genes related to IAA exceeds the rest of hormones followed by ubiquitin related genes which are greater than TFs. The analysis of the expression profiles of diverse tissues under Na₂SO₄ stress and identification of relevant key hub genes in a network crosstalk will provide a strong foundation and valuable clues for genetic improvements of cotton in response to various salt stresses.

5-Aminolevulinic Acid Improves Morphogenesis and Na+ Subcellular Distribution in the Apical Cells of Cucumis sativus L. Under Salinity Stress

Soil salinity causes damage to plants and a reduction in output. A natural plant growth regulator, 5-aminolevulinic acid (ALA), has been shown to promote plant growth under abiotic stress conditions. In the present study, we assessed the effects of exogenously applied ALA (25 mg L^-1) on the root architecture and Na⁺ distribution of cucumber (Cucumis sativus L.) seedlings under moderate NaCl stress (50 mmol L^-1). The results showed that exogenous ALA improved root length, root volume, root surface area, and cell activity in the root tips, which were inhibited under salt stress. In addition, although salinity stress increased the subcellular Na⁺ contents, such as those of the cell wall, nucleus, plastid, and mitochondria, ALA treatment reduced these Na⁺ contents, except the soluble fraction. Molecular biological analysis revealed that ALA application upregulated both the SOS1 and HA3 transcriptional and translational levels, which suggested that the excretion of Na⁺ into the cytoplasm cloud was promoted by exogenous ALA. Meanwhile, exogenously applied ALA also upregulated the gene and protein expression of NHX1 and VHA-A under salinity stress, which suggested that the compartmentalization of Na⁺ to the vacuole was enhanced. Overall, exogenous ALA mitigated the damage caused by NaCl in cucumber by enhancing Na⁺ redistribution and increasing the cytoactivity of root cells.

Networks of Physiological Adjustments and Defenses, and Their Synergy With Sodium (Na+) Homeostasis Explain the Hidden Variation for Salinity Tolerance Across the Cultivated Gossypium hirsutum Germplasm

The abilities to mobilize and/or sequester excess ions within and outside the plant cell are important components of salt-tolerance mechanisms. Mobilization and sequestration of Na+ involves three transport systems facilitated by the plasma membrane H+/Na+ antiporter (SOS1), vacuolar H+/Na+ antiporter (NHX1), and Na+/K+ transporter in vascular tissues (HKT1). Many of these mechanisms are conserved across the plant kingdom. While Gossypium hirsutum (upland cotton) is significantly more salt-tolerant relative to other crops, the critical factors contributing to the phenotypic variation hidden across the germplasm have not been fully unraveled. In this study, the spatio-temporal patterns of Na+ accumulation along with other physiological and biochemical interactions were investigated at different severities of salinity across a meaningful genetic diversity panel across cultivated upland Gossypium. The aim was to define the importance of holistic or integrated effects relative to the direct effects of Na+ homeostasis mechanisms mediated by GhHKT1, GhSOS1, and GhNHX1. Multi-dimensional physio-morphometric attributes were investigated in a systems-level context using univariate and multivariate statistics, randomForest, and path analysis. Results showed that mobilized or sequestered Na+ contributes significantly to the baseline tolerance mechanisms. However, the observed variance in overall tolerance potential across a meaningful diversity panel were more significantly attributed to antioxidant capacity, maintenance of stomatal conductance, chlorophyll content, and divalent cation (Mg2+) contents other than Ca2+ through a complex interaction with Na+ homeostasis. The multi-tier macro-physiological, biochemical and molecular data generated in this study, and the networks of interactions uncovered strongly suggest that a complex physiological and biochemical synergy beyond the first-line-of defense (Na+ sequestration and mobilization) accounts for the total phenotypic variance across the primary germplasm of Gossypium hirsutum. These findings are consistent with the recently proposed Omnigenic Theory for quantitative traits and should contribute to a modern look at phenotypic selection for salt tolerance in cotton breeding.

Genome-Wide Characterization and Expression Analysis of NHX Gene Family under Salinity Stress in Gossypium barbadense and Its Comparison with Gossypium hirsutum

Cotton is an important economic crop affected by different abiotic stresses at different developmental stages. Salinity limits the growth and productivity of crops worldwide. Na⁺/H⁺ antiporters play a key role during the plant development and in its tolerance to salt stress. The aim of the present study was a genome-wide characterization and expression pattern analysis under the salinity stress of the sodium-proton antiporter (NHX) of Gossypium barbadense in comparison with Gossypium hirsutum. In G. barbadense, 25 NHX genes were identified on the basis of the Na⁺_H⁺ exchanger domain. All except one of the G. barbadense NHX transporters have an Amiloride motif that is a known inhibitor of Na⁺ ions in plants. A phylogenetic analysis inferred three classes of GbNHX genes-viz., Vac (GbNHX1, 2 and 4), Endo (GbNHX6), and PM (GbNHX7). A high number of the stress-related cis-acting elements observed in promoters show their role in tolerance against abiotic stresses. The Ka/Ks values show that the majority of GbNHX genes are subjected to strong purifying selection under the course of evolution. To study the functional divergence of G. barbadense NHX transporters, the real-time gene expression was analyzed under salt stress in the root, stem, and leaf tissues. In G. barbadense, the expression was higher in the stem, while in G. hirsutum the leaf and root showed a high expression. Moreover, our results revealed that NHX2 homologues in both species have a high expression under salinity stress at higher time intervals, followed by NHX7. The protein-protein prediction study revealed that GbNHX7 is involved in the CBL-CIPK protein interaction pathway. Our study also provided valuable information explaining the molecular mechanism of Na⁺ transport for the further functional study of Gossypium NHX genes.

Progress in the Study and Use of Seawater Vegetables

As global soil salinization increases, halophytes that can grow in saline soils are the primary choice for improving soil quality. Some halophytes can even be irrigated with seawater and used as vegetables. These so-called seawater vegetables include those that can be planted on saline and alkali soils and some edible halophytes and ordinary vegetables that are salt-tolerant. The cultivation of seawater vegetables on saline soil has become a matter of increasing interest. In this review, we focus on the salt-tolerance mechanisms and potential applications of some seawater vegetables. We also summarize their value to health, medicine, industry, and the economy as a whole. Further improvement and development to support the use of seawater vegetables will require in-depth research at the cellular and molecular levels.

Genome of extreme halophyte Puccinellia tenuiflora

Background

Puccinellia tenuiflora, a forage grass, is considered a model halophyte given its strong tolerance for multiple stress conditions and its close genetic relationship with cereals. This halophyte has enormous values for improving our understanding of salinity tolerance mechanisms. The genetic information of P. tenuiflora also is a potential resource that can be used for improving the salinity tolerance of cereals.

Results

Here, we sequenced and assembled the P. tenuiflora genome (2n = 14) through the combined strategy of Illumina, PacBio, and 10× genomic technique. We generated 43.2× PacBio long reads, 123.87× 10× genomic reads, and 312.6× Illumina reads. Finally, we assembled 2638 scaffolds with a total size of 1.107 Gb, contig N50 of 117 kb, and scaffold N50 of 950 kb. We predicted 39,725 protein-coding genes, and identified 692 tRNAs, 68 rRNAs, 702 snRNAs, 1376 microRNAs, and 691 Mb transposable elements.

Conclusions

We deposited the genome sequence in NCBI and the Genome Warehouse in National Genomics Data Center. Our work may improve current understanding of plant salinity tolerance, and provides extensive genetic resources necessary for improving the salinity and drought tolerance of cereals.

Engineering salinity tolerance in plants: progress and prospects

There is a need to integrate conceptual framework based on the current understanding of salt stress responses with different approaches for manipulating and improving salt tolerance in crop plants. Soil salinity exerts significant constraints on global crop production, posing a serious challenge for plant breeders and biotechnologists. The classical transgenic approach for enhancing salinity tolerance in plants revolves by boosting endogenous defence mechanisms, often via a single-gene approach, and usually involves the enhanced synthesis of compatible osmolytes, antioxidants, polyamines, maintenance of hormone homeostasis, modification of transporters and/or regulatory proteins, including transcription factors and alternative splicing events. Occasionally, genetic manipulation of regulatory proteins or phytohormone levels confers salinity tolerance, but all these may cause undesired reduction in plant growth and/or yields. In this review, we present and evaluate novel and cutting-edge approaches for engineering salt tolerance in crop plants. First, we cover recent findings regarding the importance of regulatory proteins and transporters, and how they can be used to enhance salt tolerance in crop plants. We also evaluate the importance of halobiomes as a reservoir of genes that can be used for engineering salt tolerance in glycophytic crops. Additionally, the role of microRNAs as critical post-transcriptional regulators in plant adaptive responses to salt stress is reviewed and their use for engineering salt-tolerant crop plants is critically assessed. The potentials of alternative splicing mechanisms and targeted gene-editing technologies in understanding plant salt stress responses and developing salt-tolerant crop plants are also discussed.

Calcium signaling and salt tolerance are diversely entwined in plants

In plants dehydration imposed by salinity can invoke physical changes at the interface of the plasma membrane and cell wall. Changes in hydrostatic pressure activate ion channels and cause depolarization of the plasma membrane due to disturbance in ion transport. During the initial phases of salinity stress, the relatively high osmotic potential of the rhizosphere enforces the plant to use a diverse spectrum of strategies to optimize water and nutrient uptake. Signals of salt stress are recognized by specific root receptors that activate an osmosensing network. Plant response to hyperosmotic tension is closely linked to the calcium (Ca2+) channels and interacting proteins such as calmodulin. A rapid rise in cytosolic Ca2+ levels occurs within seconds of exposure to salt stress. Plants employ multiple sensors and signaling components to sense and respond to salinity stress, of which most are closely related to Ca2+ sensing and signaling. Several tolerance strategies such as osmoprotectant accumulation, antioxidant boosting, polyaminses and nitric oxide (NO) machineries are also coordinated by Ca2+ signaling. Substantial research has been done to discover the salt stress pathway and tolerance mechanism in plants, resulting in new insights into the perception of salt stress and the downstream signaling that happens in response. Nevertheless, the role of multifunctional components such as Ca2+ has not been sufficiently addressed in the context of salt stress. In this review, we elaborate that the salt tolerance signaling pathway converges with Ca2+ signaling in diverse pathways. We summarize knowledge related to different dimensions of salt stress signaling pathways in the cell by emphasizing the administrative role of Ca2+ signaling on salt perception, signaling, gene expression, ion homeostasis and adaptive responses.

Combined Boron Toxicity and Salinity Stress-An Insight into Its Interaction in Plants

The continuously changing environment has intensified the occurrence of abiotic stress conditions. Individually, boron (B) toxicity and salinity stress are well recognized as severe stress conditions for plants. However, their coexistence in arid and semi-arid agricultural regions has shown ambiguous effects on plant growth and development. Few studies have reported that combined boron toxicity and high salinity stress have more damaging effects on plant growth than individual B and salt stress, while other studies have highlighted less damaging effects of the combined stress. Hence, it is interesting to understand the positive interaction of this combined stress so that it can be effectively employed for the improvement of crops that generally show the negative effects of this combined stress. In this review, we discussed the possible processes that occur in plants in response to this combined stress condition. We highly suggest that the combined B and salinity stress condition should be considered as a novel stress condition by researchers; hence, we recommend the name "BorSal" for this combined boron toxicity and high salinity state in the soil. Membrane-bound activities, mobility of ions, water transport, pH changes, transpiration, photosynthesis, antioxidant activities, and different molecular transporters are involved in the effects of BorSal interaction in plants. The discussed mechanisms indicate that the BorSal stress state should be studied in light of the involved physiological and molecular processes that occur after B and salt interaction in plants.

The Role of Soil Fungi in K+ Plant Nutrition

K⁺ is an essential cation and the most abundant in plant cells. After N, its corresponding element, K, is the nutrient required in the largest amounts by plants. Despite the numerous roles of K in crop production, improvements in the uptake and efficiency of use of K have not been major focuses in conventional or transgenic breeding studies in the past. In research on the mineral nutrition of plants in general, and K in particular, this nutrient has been shown to be essential to soil-dwelling-microorganisms (fungi, bacteria, protozoa, nematodes, etc.) that form mutualistic associations and that can influence the availability of mineral nutrients for plants. Therefore, this article aims to provide an overview of the role of soil microorganisms in supplying K⁺ to plants, considering both the potassium-solubilizing microorganisms and the potassium-facilitating microorganisms that are in close contact with the roots of plants. These microorganisms can influence the active transporter-mediated transfer of K⁺. Regarding the latter group of microorganisms, special focus is placed on the role of endophytic fungus. This review also includes a discussion on productivity through sustainable agriculture.

Genome-Wide Identification of Na+/H+ Antiporter (NHX) Genes in Sugar Beet (Beta vulgaris L.) and Their Regulated Expression under Salt Stress

Salinity is one of the major environment factors that limits the growth of plants and the productivity of crops worldwide. It has been shown that Na+ transporters play a central role in salt tolerance and development of plants. The objective of this study was to identify Na+/H+ antiporter (NHX) genes and investigate their expression patterns in sugar beet (Beta vulgaris L.) subjected to various concentrations of NaCl. A total of five putative NHX genes were identified and distributed on four chromosomes in sugar beet. Phylogenetic analysis revealed that these BvNHX genes are grouped into three major classes, viz Vac- (BvNHX1, -2 and -3), Endo- (BvNHX4), and PM-class NHX (BvNHX5/BvSOS1), and within each class the exon/intron structures are conserved. The amiloride-binding site is found in TM3 at N-terminus of Vac-class NHX proteins. Protein-protein interaction (PPI) prediction suggested that only BvNHX5 putatively interacts with calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK), implying it might be the primary NHX involved in CBL-CIPK pathway under saline condition. It was also found that BvNHX5 contains one abscisic acid (ABA)-responsive element (ABRE), suggesting that BvNHX5 might be involved in ABA signal responsiveness. Additionally, the qRT-PCR analysis showed that all the BvNHX genes in both roots and leaves are significantly up-regulated by salt, and the transcription levels under high salinity are significantly higher than those under either low or moderate salinity. Taken together, this work gives a detailed overview of the BvNHX genes and their expression patterns under salt stress. Our findings also provide useful information for elucidating the molecular mechanisms of Na+ homeostasis and further functional identification of the BvNHX genes in sugar beet.

Overexpression of PeHKT1;1 Improves Salt Tolerance in Populus

Soil salinization is an increasingly serious threat that limits plant growth and development. Class I transporters of the high-affinity K⁺ transporter (HKT) family have been demonstrated to be involved in salt tolerance by contributing to Na⁺ exclusion from roots and shoots. Here, we isolated the PeHKT1;1 gene from hybrid poplar based on the sequences of the Populus trichocarpa genome. The full-length PeHKT1;1 gene was 2173 bp, including a 1608 bp open reading frame (ORF) encoding 535 amino acids and containing eight distinct transmembrane domains. Multiple sequence alignment and phylogenetic analysis suggested that the PeHKT1;1 protein had a typical S–G–G–G signature for the P-loop domains and belonged to class I of HKT transporters. PeHKT1;1 transcripts were mainly detected in stem and root, and were remarkably induced by salt stress treatment. In further characterization of its functions, overexpression of PeHKT1;1 in Populus davidiana × Populus bolleana resulted in a better relative growth rate in phenotypic analysis, including root and plant height, and exhibited higher catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD) activities than non-transgenic poplar under salt stress conditions. These observations indicated that PeHKT1;1 may enhance salt tolerance by improving the efficiency of antioxidant systems. Together, these data suggest that PeHKT1;1 plays an important role in response to salt stress in Populus.

Mechanisms of Sodium Transport in Plants-Progresses and Challenges

Understanding the mechanisms of sodium (Na⁺) influx, effective compartmentalization, and efflux in higher plants is crucial to manipulate Na⁺ accumulation and assure the maintenance of low Na⁺ concentration in the cytosol and, hence, plant tolerance to salt stress. Na⁺ influx across the plasma membrane in the roots occur mainly via nonselective cation channels (NSCCs). Na⁺ is compartmentalized into vacuoles by Na⁺/H⁺ exchangers (NHXs). Na⁺ efflux from the plant roots is mediated by the activity of Na⁺/H⁺ antiporters catalyzed by the salt overly sensitive 1 (SOS1) protein. In animals, ouabain (OU)-sensitive Na⁺, K⁺-ATPase (a P-type ATPase) mediates sodium efflux. The evolution of P-type ATPases in higher plants does not exclude the possibility of sodium efflux mechanisms similar to the Na⁺, K⁺-ATPase-dependent mechanisms characteristic of animal cells. Using novel fluorescence imaging and spectrofluorometric methodologies, an OU-sensitive sodium efflux system has recently been reported to be physiologically active in roots. This review summarizes and analyzes the current knowledge on Na⁺ influx, compartmentalization, and efflux in higher plants in response to salt stress.

Expression pattern of salt tolerance-related genes in Aegilops cylindrica

Aegilops cylindrica, a salt-tolerant gene pool of wheat, is a useful plant model for understanding mechanism of salt tolerance. A salt-tolerant USL26 and a salt-sensitive K44 genotypes of A. cylindrica, originating from Uremia Salt Lake shores in Northwest Iran and a non-saline Kurdestan province in West Iran, respectively, were identified based on screening evaluation and used for this work. The objective of the current study was to investigate the expression patterns of four genes related to ion homeostasis in this species. Under treatment of 400 mM NaCl, USL26 showed significantly higher root and shoot dry matter levels and K+ concentrations, together with lower Na+ concentrations than K44 genotype. A. cylindrica HKT1;5 (AecHKT1;5), SOS1 (AecSOS1), NHX1 (AecNHX1) and VP1 (AecVP1) were partially sequenced to design each gene specific primer. Quantitative real-time PCR showed a differential expression pattern of these genes between the two genotypes and between the root and shoot tissues. Expressions of AecHKT1;5 and AecSOS1 was greater in the roots than in the shoots of USL26 while AecNHX1 and AecVP1 were equally expressed in both tissues of USL26 and K44. The higher transcripts of AecHKT1;5 in the roots versus the shoots could explain both the lower Na+ in the shoots and the much lower Na+ and higher K+ concentrations in the roots/shoots of USL26 compared to K44. Therefore, the involvement of AecHKT1;5 in shoot-to-root handover of Na+ in possible combination with the exclusion of excessive Na+ from the root in the salt-tolerant genotype are suggested.

Isoform Sequencing and State-of-Art Applications for Unravelling Complexity of Plant Transcriptomes

Single-molecule real-time (SMRT) sequencing developed by PacBio, also called third-generation sequencing (TGS), offers longer reads than the second-generation sequencing (SGS). Given its ability to obtain full-length transcripts without assembly, isoform sequencing (Iso-Seq) of transcriptomes by PacBio is advantageous for genome annotation, identification of novel genes and isoforms, as well as the discovery of long non-coding RNA (lncRNA). In addition, Iso-Seq gives access to the direct detection of alternative splicing, alternative polyadenylation (APA), gene fusion, and DNA modifications. Such applications of Iso-Seq facilitate the understanding of gene structure, post-transcriptional regulatory networks, and subsequently proteomic diversity. In this review, we summarize its applications in plant transcriptome study, specifically pointing out challenges associated with each step in the experimental design and highlight the development of bioinformatic pipelines. We aim to provide the community with an integrative overview and a comprehensive guidance to Iso-Seq, and thus to promote its applications in plant research.

Newly Identified Wild Rice Accessions Conferring High Salt Tolerance Might Use a Tissue Tolerance Mechanism in Leaf

Cultivated rice (Oryza sativa L.) is very sensitive to salt stress. So far a few rice landraces have been identified as a source of salt tolerance and utilized in rice improvement. These tolerant lines primarily use Na⁺ exclusion mechanism in root which removes Na⁺ from the xylem stream by membrane Na⁺ and K⁺ transporters, and resulted in low Na⁺ accumulation in shoot. Identification of a new donor source conferring high salt tolerance is imperative. Wild relatives of rice having wide genetic diversity are regarded as a potential source for crop improvement. However, they have been less exploited against salt stress. Here, we simultaneously evaluated all 22 wild Oryza species along with the cultivated tolerant lines including Pokkali, Nona Bokra, and FL478, and sensitive check varieties under high salinity (240 mM NaCl). Based on the visual salt injury score, three species (O. alta, O. latifolia, and O. coarctata) and four species (O. rhizomatis, O. eichingeri, O. minuta, and O. grandiglumis) showed higher and similar level of tolerance compared to the tolerant checks, respectively. All three CCDD genome species exhibited salt tolerance, suggesting that the CCDD genome might possess the common genetic factors for salt tolerance. Physiological and biochemical experiments were conducted using the newly isolated tolerant species together with checks under 180 mM NaCl. Interestingly, all wild species showed high Na⁺ concentration in shoot and low concentration in root unlike the tolerant checks. In addition, the wild-tolerant accessions showed a tendency of a high tissue tolerance in leaf, low malondialdehyde level in shoot, and high retention of chlorophyll in the young leaves. These results suggest that the wild species employ tissue tolerance mechanism to manage salt stress. Gene expression analyses of the key salt tolerance-related genes suggested that high Na⁺ in leaf of wild species might be affected by OsHKT1;4-mediated Na⁺ exclusion in leaf and the following Na⁺ sequestration in leaf might be occurring independent of tonoplast-localized OsNHX1. The newly isolated wild rice accessions will be valuable materials for both rice improvement to salinity stress and the study of salt tolerance mechanism in plants.