The ins and outs of intracellular ion homeostasis: NHX-type cation/H(+) transporters

SOS2-AFP2 module regulates seed germination by inducing ABI5 degradation in response to salt stress in Arabidopsis

Soil salinity pose a significant challenge to global agriculture, threatening crop yields and food security. Understanding the salt tolerance mechanisms of plants is crucial for improving their survival under salt stress. AFP2, a negative regulator of ABA signaling, has been shown to play a crucial role in salt stress tolerance during seed germination. Mutations in AFP2 gene lead to increased sensitivity to salt stress. However, the underline mechanisms by which AFP2 regulates seed germination under salt stress remain elusive. In this study, we identified a protein interaction between AFP2 and SOS2, a Ser/Thr protein kinase known to play a critical role in salt stress response. Using a combination of genetic, biochemical, and physiological approaches, we investigated the role of the SOS2-AFP2 module in regulating seed germination under salt stress. Our findings reveal that SOS2 physically interacts with AFP2 and stabilizes it, leading to the degradation of the ABI5 protein, a negative transcription factor in seed germination under salt stress. This study sheds light on previously unknown connections within salt stress and ABA signaling, paving the way for novel strategies to enhance plant resilience against environmental challenges.

Genome-Wide Identification of the Cation/Proton Antiporter (CPA) Gene Family and Functional Analysis of AtrNHX8 under Salt Stress

Amaranthus tricolor is an important vegetable, and its quality is affected by salt stress. Cation/proton antiporters (CPA) contribute to plant development and tolerance to salt stress. In this study, 35 CPA genes were identified from a genome database for A. tricolor, including 9 NHX, 5 KEA, and 21 CPA2 genes. Furthermore, in A. tricolor, the expression levels of most AtrNHX genes were higher at a low salinity level (50 or 100 mM NaCl) than in the control or 200 mM NaCl treatment. Levels of most AtrNHX genes were elevated in the stem. Moreover, AtrNHX8 was homologous to AtNHX4, which is involved in the regulation of sodium homeostasis and salt stress response. After AtrNHX8 overexpression in Arabidopsis thaliana, seed germination was better, and the flowering time was earlier than that of wild-type plants. Additionally, the overexpression of AtrNHX8 in A. thaliana improved salt tolerance. These results reveal the roles of AtrNHX genes under salt stress and provide valuable information on this gene family in amaranth.

Weighted gene co-expression network analysis reveals hub genes regulating response to salt stress in peanut

Peanut (Arachis hypogaea L.) is an important oilseed crop worldwide. However, soil salinization becomes one of the main limiting factors of peanut production. Therefore, developing salt-tolerant varieties and understanding the molecular mechanisms of salt tolerance is important to protect peanut yield in saline areas. In this study, we selected four peanut varieties with contrasting response to salt challenges with T1 and T2 being tolerance and S1 and S2 being susceptible. High-throughput RNA sequencing resulted in more than 314.63 Gb of clean data from 48 samples. We identified 12,057 new genes, 7,971of which have functional annotations. KEGG pathway enrichment analysis of uniquely expressed genes in salt-tolerant peanut revealed that upregulated genes in the root are involved in the MAPK signaling pathway, fatty acid degradation, glycolysis/gluconeogenesis, and upregulated genes in the shoot were involved in plant hormone signal transduction and the MAPK signaling pathway. Na+ content, K+ content, K+/ Na+, and dry mass were measured in root and shoot tissues, and two gene co-expression networks were constructed based on weighted gene co-expression network analysis (WGCNA) in root and shoot. In this study, four key modules that are highly related to peanut salt tolerance in root and shoot were identified, plant hormone signal transduction, phenylpropanoid biosynthesis, starch and sucrose metabolism, flavonoid biosynthesis, carbon metabolism were identified as the key biological processes and metabolic pathways for improving peanut salt tolerance. The hub genes include genes encoding ion transport (such as HAK8, CNGCs, NHX, NCL1) protein, aquaporin protein, CIPK11 (CBL-interacting serine/threonine-protein kinase 11), LEA5 (late embryogenesis abundant protein), POD3 (peroxidase 3), transcription factor, and MAPKKK3. There were some new salt-tolerant genes identified in peanut, including cytochrome P450, vinorine synthase, sugar transport protein 13, NPF 4.5, IAA14, zinc finger CCCH domain-containing protein 62, beta-amylase, fatty acyl-CoA reductase 3, MLO-like protein 6, G-type lectin S-receptor-like serine/threonine-protein kinase, and kinesin-like protein KIN-7B. The identification of key modules, biological pathways, and hub genes in this study enhances our understanding of the molecular mechanisms underlying salt tolerance in peanuts. This knowledge lays a theoretical foundation for improving and innovating salt-tolerant peanut germplasm.

The PbbHLH62/PbVHA-B1 module confers salt tolerance through modulating intracellular Na+/K+ homeostasis and reactive oxygen species removal in pear

The vacuolar H+-ATPase (V-ATPase) is a multi-subunit membrane protein complex, which plays pivotal roles in building up an electrochemical H+-gradient across tonoplast, energizing Na+ sequestration into the central vacuole, and enhancing salt stress tolerance in plants. In this study, a B subunit of V-ATPase gene, PbVHA-B1 was discovered and isolated from stress-induced P. betulaefolia combining with RT-PCR method. The RT-qPCR analysis revealed that the expression level of PbVHA-B1 was upregulated by salt, drought, cold, and exogenous ABA treatment. Subcellular localization analyses showed that PbVHA-B1 was located in the cytoplasm and nucleus. Moreover, overexpression of PbVHA-B1 gene noticeably increased the ATPase activity and the tolerance to salt in transgenic Arabidopsis plants. In contrast, knockdown of PbVHA-B1 gene in P.betulaefolia by virus-induced gene silencing had reduced resistance to salt stress. In addition, using yeast one-hybride (Y1H) and yeast two-hybride (Y2H) screens, PbbHLH62, a bHLH transcription factor, was identified as a partner of the PbVHA-B1 promoter and protein. Then, we also found that PbbHLH62 positively regulate the expression of PbVHA-B1 and the ATPase activity after salt stress treatment. These findings provide evidence that PbbHLH62 played a critical role in the salt response. Collectively, our results demonstrate that a PbbHLH62/PbVHA-B1 module plays a positive role in salt tolerance by maintain intracellular ion and ROS homeostasis in pear.

A charged existence: A century of transmembrane ion transport in plants

If the past century marked the birth of membrane transport as a focus for research in plants, the past 50 years has seen the field mature from arcane interest to a central pillar of plant physiology. Ion transport across plant membranes accounts for roughly 30% of the metabolic energy consumed by a plant cell, and it underpins virtually every aspect of plant biology, from mineral nutrition, cell expansion, and development to auxin polarity, fertilization, plant pathogen defense, and senescence. The means to quantify ion flux through individual transporters, even single channel proteins, became widely available as voltage clamp methods expanded from giant algal cells to the fungus Neurospora crassa in the 1970s and the cells of angiosperms in the 1980s. Here, I touch briefly on some key aspects of the development of modern electrophysiology with a focus on the guard cells of stomata, now without dispute the premier plant cell model for ion transport and its regulation. Guard cells have proven to be a crucible for many technical and conceptual developments that have since emerged into the mainstream of plant science. Their study continues to provide fundamental insights and carries much importance for the global challenges that face us today.

The crossing of two unwound transmembrane regions that is the hallmark of the NhaA structural fold is critical for antiporter activity

Cell pH and Na+ homeostasis requires Na+/H+ antiporters. The crystal structure of NhaA, the main Escherichia coli Na+/H+ antiporter, revealed a unique NhaA structural fold shared by prokaryotic and eukaryotic membrane proteins. Out of the 12 NhaA transmembrane segments (TMs), TMs III-V and X-XII are topologically inverted repeats with unwound TMs IV and XI forming the X shape characterizing the NhaA fold. We show that intramolecular cross-linking under oxidizing conditions of a NhaA mutant with two Cys replacements across the crossing (D133C-T340C) inhibits antiporter activity and impairs NhaA-dependent cell growth in high-salts. The affinity purified D133C-T340C protein binds Li+ (the Na+ surrogate substrate of NhaA) under reducing conditions. The cross-linking traps the antiporter in an outward-facing conformation, blocking the antiport cycle. As many secondary transporters are found to share the NhaA fold, including some involved in human diseases, our data have importance for both basic and clinical research.

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.

Stress salinity in plants: New strategies to cope with in the foreseeable scenario

The excess of salts in soils causes stress in most plants, except for some halophytes that can tolerate higher levels of salinity. The excess of Na+ generates an ionic imbalance, reducing the K+ content and altering cellular metabolism, thus impacting in plant growth and development. Additionally, salinity in soil induces water stress due to osmotic effects and increments the production of reactive oxygen species (ROS) that affect the cellular structure, damaging membranes and proteins, and altering the electrochemical potential of H+, which directly affects nutrient absorption by membrane transporters. However, plants possess mechanisms to overcome the toxicity of the sodium ions, such as internalization into the vacuole or exclusion from the cell, synthesis of enzymes or protective compounds against ROS, and the synthesis of metabolites that help to regulate the osmotic potential of plants. Physiologic and molecular mechanisms of salinity tolerance in plants will be addressed in this review. Furthermore, a revision of strategies taken by researchers to confer salt stress tolerance on agriculturally important species are discussed. These strategies include conventional breeding and genetic engineering as transgenesis and genome editing by CRISPR/Cas9.

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

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

Plastid KEA-type cation/H+ antiporters are required for vacuolar protein trafficking in Arabidopsis

Arabidopsis plastid antiporters KEA1 and KEA2 are critical for plastid development, photosynthetic efficiency, and plant development. Here, we show that KEA1 and KEA2 are involved in vacuolar protein trafficking. Genetic analyses found that the kea1 kea2 mutants had short siliques, small seeds, and short seedlings. Molecular and biochemical assays showed that seed storage proteins were missorted out of the cell and the precursor proteins were accumulated in kea1 kea2. Protein storage vacuoles (PSVs) were smaller in kea1 kea2. Further analyses showed that endosomal trafficking in kea1 kea2 was compromised. Vacuolar sorting receptor 1 (VSR1) subcellular localizations, VSR-cargo interactions, and p24 distribution on the endoplasmic reticulum (ER) and Golgi apparatus were affected in kea1 kea2. Moreover, plastid stromule growth was reduced and plastid association with the endomembrane compartments was disrupted in kea1 kea2. Stromule growth was regulated by the cellular pH and K+ homeostasis maintained by KEA1 and KEA2. The organellar pH along the trafficking pathway was altered in kea1 kea2. Overall, KEA1 and KEA2 regulate vacuolar trafficking by controlling the function of plastid stromules via adjusting pH and K+ homeostasis.

Salt stress responses and alleviation strategies in legumes: a review of the current knowledge

Salinity is one of the most significant environmental factors limiting legumes development and productivity. Salt stress disturbs all developmental stages of legumes and affects their hormonal regulation, photosynthesis and biological nitrogen fixation, causing nutritional imbalance, plant growth inhibition and yield losses. At the molecular level, salt stress exposure involves large number of factors that are implicated in stress perception, transduction, and regulation of salt responsive genes' expression through the intervention of transcription factors. Along with the complex gene network, epigenetic regulation mediated by non-coding RNAs, and DNA methylation events are also involved in legumes' response to salinity. Different alleviation strategies can increase salt tolerance in legume plants. The most promising ones are Plant Growth Promoting Rhizobia, Arbuscular Mycorrhizal Fungi, seed and plant's priming. Genetic manipulation offers an effective approach for improving salt tolerance. In this review, we present a detailed overview of the adverse effect of salt stress on legumes and their molecular responses. We also provide an overview of various ameliorative strategies that have been implemented to mitigate/overcome the harmful effects of salt stress on legumes.

LeGRXS14 Reduces Salt Stress Tolerance in Arabidopsis thaliana

Salt stress represents a significant abiotic stressor for plants and poses a severe threat to agricultural productivity. Glutaredoxins (GRXs) are small disulfide reductases that can scavenge cellular reactive oxygen species and are crucial for plant growth and development, particularly under stressful circumstances. Although CGFS-type GRXs were found to be involved in various abiotic stresses, the intrinsic mechanism mediated by LeGRXS14, a tomato (Lycopersicon esculentum Mill.) CGFS-type GRX, is not yet fully understood. We discovered that LeGRXS14 is relatively conserved at the N-terminus and exhibits an increase in expression level under salt and osmotic stress conditions in tomatoes. The expression levels of LeGRXS14 in response to osmotic stress peaked relatively rapidly at 30 min, while the response to salt stress only peaked at 6 h. We constructed LeGRXS14 overexpression Arabidopsis thaliana (OE) lines and confirmed that LeGRXS14 is located on the plasma membrane, nucleus, and chloroplasts. In comparison to the wild-type Col-0 (WT), the OE lines displayed greater sensitivity to salt stress, resulting in a profound inhibition of root growth under the same conditions. Analysis of the mRNA levels of the WT and OE lines revealed that salt stress-related factors, such as ZAT12, SOS3, and NHX6, were downregulated. Based on our research, it can be concluded that LeGRXS14 plays a significant role in plant tolerance to salt. However, our findings also suggest that LeGRXS14 may act as a negative regulator in this process by exacerbating Na+ toxicity and the resulting oxidative stress.

Comparative transcriptome analysis reveals molecular regulation of salt tolerance in two contrasting chickpea genotypes

Salinity is a major abiotic stress that causes substantial agricultural losses worldwide. Chickpea (Cicer arietinum L.) is an important legume crop but is salt-sensitive. Previous physiological and genetic studies revealed the contrasting response of two desi chickpea varieties, salt-sensitive Rupali and salt-tolerant Genesis836, to salt stress. To understand the complex molecular regulation of salt tolerance mechanisms in these two chickpea genotypes, we examined the leaf transcriptome repertoire of Rupali and Genesis836 in control and salt-stressed conditions. Using linear models, we identified categories of differentially expressed genes (DEGs) describing the genotypic differences: salt-responsive DEGs in Rupali (1,604) and Genesis836 (1,751) with 907 and 1,054 DEGs unique to Rupali and Genesis836, respectively, salt responsive DEGs (3,376), genotype-dependent DEGs (4,170), and genotype-dependent salt-responsive DEGs (122). Functional DEG annotation revealed that the salt treatment affected genes involved in ion transport, osmotic adjustment, photosynthesis, energy generation, stress and hormone signalling, and regulatory pathways. Our results showed that while Genesis836 and Rupali have similar primary salt response mechanisms (common salt-responsive DEGs), their contrasting salt response is attributed to the differential expression of genes primarily involved in ion transport and photosynthesis. Interestingly, variant calling between the two genotypes identified SNPs/InDels in 768 Genesis836 and 701 Rupali salt-responsive DEGs with 1,741 variants identified in Genesis836 and 1,449 variants identified in Rupali. In addition, the presence of premature stop codons was detected in 35 genes in Rupali. This study provides valuable insights into the molecular regulation underpinning the physiological basis of salt tolerance in two chickpea genotypes and offers potential candidate genes for the improvement of salt tolerance in chickpeas.

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.

Melatonin-mediated endogenous nitric oxide coordinately boosts stability through proline and nitrogen metabolism, antioxidant capacity, and Na+/K+ transporters in tomato under NaCl stress

The interactions between nitric oxide (NO) and melatonin in alleviating sodium chloride (NaCl) toxicity in plants are poorly comprehended. Here, the associations between the exogenous application of melatonin and endogenous NO levels in inducing tomato seedlings' defense response during NaCl toxicity were investigated. The results indicated that the application of melatonin (150 μM) increased height (23.7%) and biomass (32.2%), improved chlorophyll (a (137%) and b (92.8%)), and proline metabolisms, and reduced the contents of superoxide anion radicals (49.6%), hydrogen peroxide (31.4%), malondialdehyde (38%), and electrolyte leakage (32.6%) in 40-day-old tomato seedlings grown under NaCl (150 mM) treatment. Melatonin increased the antioxidant defense system in NaCl-stressed seedlings by increasing the activity of the antioxidant enzymes. Melatonin also improved N metabolism and endogenous NO content in NaCl-stressed seedlings by upregulating the activity of enzymes implicated in N assimilation. Furthermore, melatonin improved ionic balance and reduced Na content in NaCl-exposed seedlings by upregulating the expression of genes involved in K/Na ratio homeostasis (NHX1-4) and increasing the accumulation of mineral nutrients (P, N, Ca, and Mg). However, the addition of cPTIO (100 μM; an NO scavenger) reversed the beneficial impacts of melatonin, indicating the effective function of NO in melatonin-induced defense mechanisms in NaCl-stressed tomato seedlings. Therefore, our results revealed that melatonin improves the tolerance of tomato plants during NaCl toxicity by mediating internal NO.

Plant Plasma Membrane Proton Pump: One Protein with Multiple Functions

In plants, the plasma membrane proton pump (PM H⁺-ATPase) regulates numerous transport-dependent processes such as growth, development, basic physiology, and adaptation to environmental conditions. This review explores the multifunctionality of this enzyme in plant cells. The abundance of several PM H⁺-ATPase isogenes and their pivotal role in energizing transport in plants have been connected to the phenomena of pleiotropy. The multifunctionality of PM H⁺-ATPase is a focal point of numerous studies unraveling the molecular mechanisms of plant adaptation to adverse environmental conditions. Furthermore, PM H⁺-ATPase is a key element in plant defense mechanisms against pathogen attack; however, it also functions as a target for pathogens that enable plant tissue invasion. Here, we provide an extensive review of the PM H⁺-ATPase as a multitasking protein in plants. We focus on the results of recent studies concerning PM H⁺-ATPase and its role in plant growth, physiology, and pathogenesis.

New insights into the influence of NHX-type Cation/H+ antiporter on flower color in Phalaenopsis orchids

Vacuolar sodium/proton Na⁺(K⁺)/H⁺ exchanger (NHX) influence color formation because of their effects on cellular pH and Na⁺/K⁺ homeostasis. Research regarding NHXs has mainly focused on the vacuolar NHX family members. However, the NHX functions related to Phalaenopsis flower coloration remain relatively uncharacterized. In this study, we cloned and characterized PeNHX1, a vacuolar cation/H⁺ antiporter-encoding gene that is highly expressed in the Phalaenopsis equestris (orchid) flower lip. Phylogenetic and sequence analyses showed that PeNHX1 is a vacuolar NHX protein family member that is similar to other known vacuolar antiporters. The PeNHX1-GFP fusion protein was clearly localized to the vacuolar membrane in a transient transfection assay. A quantitative real-time PCR analysis revealed the increased expression of PeNHX1 in different flower developmental stages. Moreover, it was more highly expressed in the lip than in the other flower organs. On the basis of virus-induced gene silencing, we determined that decreased PeNHX1 expression significantly reduces P. equestris petal coloration. Furthermore, the overexpression of PeNHX1 in Phalaenopsis Big Chili caused the pH to increase and the petal color to change from red to blue. The results indicate that NHX1 may mediates the Na ⁺ or K⁺/H⁺ exchange, thereby regulating the vacuolar pH to promote blue coloration. This research provides a theoretical basis for the development of orchid varieties with blue flowers.

Saline-alkali stress tolerance is enhanced by MhPR1 in Malus halliana leaves as shown by transcriptomic analyses

qRT-PCR analysis showed that MhPR1 was strongly induced by saline-alkali stress. Overexpression of MhPR1 enhanced tolerance to saline-alkali stress in transgenic tobacco (Nicotiana tabacum L.) and apple calli. Abstract: Soil salinization seriously threaten apple growth in Northwest loess plateau of China. Malus halliana has developed special system to adapt to saline-alkali environmental stress. To obtain a more detailed understanding of the adaptation mechanisms involved in M. halliana, a transcriptomic approach was used to analyze the leaves' pathways in the stress and its regulatory mechanisms. RNA-Seq showed that among the 16,246 investigated unigenes under saline-alkali stress, 7268 genes were up-regulated and 8978 genes were down-regulated. KEGG analysis indicated that most of the enriched saline-alkali-responsive genes were mainly involved in plant hormone, calcium signal transduction, amino acids, carotenoid and flavonoids biosynthesis, carbon and phenylalanine metabolism, and other secondary metabolites. Expression profile analysis by quantitative real-time PCR confirmed that the maximum up-regulation of MhPR1 under saline-alkali stress was 7.1 folds in leaves. Overexpression of MhPR1 enhanced tolerance to saline-alkali stress in transgenic tobacco (Nicotiana tabacum L.) and apple calli. Taken together, our results demonstrate that MhPR1 encodes a saline-alkali-responsive transcriptional activator and provide valuable information for further study of PR1 functions in apple.

The Role of Serendipita indica (Piriformospora indica) in Improving Plant Resistance to Drought and Salinity Stresses

Simple Summary

Environmental stresses are one of the biggest threats to modern agriculture, and climate change has heightened the risks of these stresses in different parts of the world. Among all the environmental stresses, salinity and drought are a severe threat to arid and semi-arid regions of the world, and for a long time, scientists have been searching for ways to reduce the risk of these stresses. In recent decades, solutions have been developed to reduce the risk of environmental stress on plants by identifying beneficial soil microorganisms. This study was conducted to identify morphophysiological and molecular changes of plants in coexistence with Serendipita indica and their impact on drought and salinity stress reduction. The study also has investigated the stressors’ impact on plants and the plants’ mechanisms to cope with them; Furthermore, sharing results with researchers provides a clear path for future research.

Abstract

Plant stress is one of the biggest threats to crops, causing irreparable damage to farmers’ incomes; Therefore, finding suitable, affordable, and practical solutions will help the agricultural economy and prevent the loss of millions of tons of agricultural products. Scientists have taken significant steps toward improving farm productivity in the last few decades by discovering how beneficial soil microorganisms enhance plant resistance to environmental stresses. Among these microorganisms is Serendipita indica, which the benefits of coexisting this fungus with plant roots have been extensively explored in recent years. By investigating fungus specification and its effects on plants’ morphological, physiological, and molecular traits, the present study seeks to understand how Serendipita indica affects plant resistance to salinity and drought conditions. Furthermore, this study attempts to identify the unknown mechanisms of action of the coexistence of Serendipita indica with plants in the face of stress using information from previous studies. Thus, it provides a way for future research to assess the impact of this fungus on tackling environmental stresses and enhancing agricultural productivity.

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.

Metabolites Reprogramming and Na+/K+ Transportation Associated With Putrescine-Regulated White Clover Seed Germination and Seedling Tolerance to Salt Toxicity

Soil salinization is a serious challenge to many countries worldwide. Putrescine (Put) is related to the improvement of seed germination under salt stress, but molecular and metabolic mechanisms are still not fully understood. The objectives of this study were to determine the effect of seed soaking with Put on germination characteristics under salt stress induced by 100 mm sodium chloride (NaCl) and to further analyze subsequent stress tolerance associated with amylolysis, oxidative damage, sodium (Na+)/ potassium (K+) accumulation and transportation, and metabolic homeostasis in white clover (Trifolium repens cv. Haifa) seedlings. The results showed that seed soaking with Put significantly alleviated salt-induced decreases in the endogenous Put content, germination rate, germination vigor, germination index, Rl/SL, and fresh/dry weight of seedlings. Put application also significantly promoted starch metabolism through activating α-amylase and β-amylase activities under salt stress. The metabolomic analysis showed that seed soaking with Put significantly increased the accumulation of polyamines (Put and spermidine), amino acids (γ-aminobutyric acid, glutamate, alanine, proline, citrulline, etc.), organic acids (ketopentanic acid, malonic acid, malic acid, ketopentanic acid, cis-sinapinic acid, etc.), lipids and fatty acids (glycerol, stearic acid, linoleic acid, palmitic acid, etc.), sugars (levoglucosan, fucose, and anhydro-D-galactose), alcohols (myo-inositol, allo-inositol, hexadecanol, and threitol), and other metabolites (thymine, xanthine, adenine, guanine, and glycerol 1-phosphate, etc.) associated with enhanced tricarboxylic acid (TCA) cycle and γ-aminobutyric acid (GABA) shunt contributing to better osmotic adjustment, cell membrane stability, energy supply, and metabolic homeostasis when seeds germinated under salt stress. In addition, Put significantly up-regulated the AsSOS1, NHX6, SKOR, HKT1, and HKT8 expression levels which played critical roles in Na+ rejection and K+ retention resulting in higher K+/Na+ ratio during seed germination under salt stress. The Put-induced up-regulation of HAL2 transcription level could reduce the toxicity of 3'-phosphoadenosine-5'-phosphosulfate (PAPS) in cells. Current findings will provide an integrative understanding of Put-induced salt tolerance associated with amylolysis, metabolic regulation, and ionic homeostasis during seed germination.

Comparative Analysis of Root Na+ Relation under Salinity between Oryza sativa and Oryza coarctata

Na⁺ toxicity is one of the major physiological constraints imposed by salinity on plant performance. At the same time, Na⁺ uptake may be beneficial under some circumstances as an easily accessible inorganic ion that can be used for increasing solute concentrations and maintaining cell turgor. Two rice species, Oryza sativa (cultivated rice, salt-sensitive) and Oryza coarctata (wild rice, salt-tolerant), demonstrated different strategies in controlling Na⁺ uptake. Glasshouse experiments and gene expression analysis suggested that salt-treated wild rice quickly increased xylem Na⁺ loading for osmotic adjustment but maintained a non-toxic level of stable shoot Na⁺ concentration by increased activity of a high affinity K⁺ transporter HKT1;5 (essential for xylem Na⁺ unloading) and a Na⁺/H⁺ exchanger NHX (for sequestering Na⁺ and K⁺ into root vacuoles). Cultivated rice prevented Na⁺ uptake and transport to the shoot at the beginning of salt treatment but failed to maintain it in the long term. While electrophysiological assays revealed greater net Na⁺ uptake upon salt application in cultivated rice, O. sativa plants showed much stronger activation of the root plasma membrane Na⁺/H⁺ Salt Overly Sensitive 1 (SOS1) exchanger. Thus, it appears that wild rice limits passive Na⁺ entry into root cells while cultivated rice relies heavily on SOS1-mediating Na⁺ exclusion, with major penalties imposed by the existence of the "futile cycle" at the plasma membrane.

Evolutionary Significance of NHX Family and NHX1 in Salinity Stress Adaptation in the Genus Oryza

Rice (Oryza sativa), a staple crop for a substantial part of the world’s population, is highly sensitive to soil salinity; however, some wild Oryza relatives can survive in highly saline environments. Sodium/hydrogen antiporter (NHX) family members contribute to Na⁺ homeostasis in plants and play a major role in conferring salinity tolerance. In this study, we analyzed the evolution of NHX family members using phylogeny, conserved domains, tertiary structures, expression patterns, and physiology of cultivated and wild Oryza species to decipher the role of NHXs in salt tolerance in Oryza. Phylogenetic analysis showed that the NHX family can be classified into three subfamilies directly related to their subcellular localization: endomembrane, plasma membrane, and tonoplast (vacuolar subfamily, vNHX1). Phylogenetic and structural analysis showed that vNHX1s have evolved from streptophyte algae (e.g., Klebsormidium nitens) and are abundant and highly conserved in all major land plant lineages, including Oryza. Moreover, we showed that tissue tolerance is a crucial trait conferring tolerance to salinity in wild rice species. Higher Na⁺ accumulation and reduced Na⁺ effluxes in leaf mesophyll were observed in the salt-tolerant wild rice species O. alta, O. latifolia, and O. coarctata. Among the key genes affecting tissue tolerance, expression of NHX1 and SOS1/NHX7 exhibited significant correlation with salt tolerance among the rice species and cultivars. This study provides insights into the evolutionary origin of plant NHXs and their role in tissue tolerance of Oryza species and facilitates the inclusion of this trait during the development of salinity-tolerant rice cultivars.

Genome-Wide Identification, Primary Functional Characterization of the NHX Gene Family in Canavalia rosea, and Their Possible Roles for Adaptation to Tropical Coral Reefs

Canavalia rosea, distributed in the coastal areas of tropical and subtropical regions, is an extremophile halophyte with good adaptability to high salinity/alkaline and drought tolerance. Plant sodium/hydrogen (Na⁺/H⁺) exchanger (NHX) genes encode membrane transporters involved in sodium ion (Na⁺), potassium ion (K⁺), and lithium ion (Li⁺) transport and pH homeostasis, thereby playing key roles in salinity tolerance. However, the NHX family has not been reported in this leguminous halophyte. In the present study, a genome-wide comprehensive analysis was conducted and finally eight CrNHXs were identified in C. rosea genome. Based on the bioinformatics analysis about the chromosomal location, protein domain, motif organization, and phylogenetic relationships of CrNHXs and their coding proteins, as well as the comparison with plant NHXs from other species, the CrNHXs were grouped into three major subfamilies (Vac-, Endo-, and PM-NHX). Promoter analyses of cis-regulatory elements indicated that the expression of different CrNHXs was affected by a series of stress challenges. Six CrNHXs showed high expression levels in five tested tissues of C. rosea in different levels, while CrNHX1 and CrNHX3 were expressed at extremely low levels, indicating that CrNHXs might be involved in regulating the development of C. rosea plant. The expression analysis based on RNA-seq showed that the transcripts of most CrNHXs were obviously decreased in mature leaves of C. rosea plant growing on tropical coral reefs, which suggested their involvement in this species' adaptation to reefs and specialized islands habitats. Furthermore, in the single-factor stress treatments mimicking the extreme environments of tropical coral reefs, the RNA-seq data also implied CrNHXs holding possible gene-specific regulatory roles in the environmental adaptation. The qRT-PCR based expression profiling exhibited that CrNHXs responded to different stresses to varying degrees, which further confirmed the specificity of CrNHXs' in responding to abiotic stresses. Moreover, the yeast functional complementation test proved that some CrNHXs could partially restore the salt tolerance of the salt-sensitive yeast mutant AXT3. This study provides comprehensive bio-information and primary functional identification of NHXs in C. rosea, which could help improve the salt/alkaline tolerance of genetically modified plants for further studies. This research also contributes to our understanding of the possible molecular mechanism whereby NHXs maintain the ion balance in the natural ecological adaptability of C. rosea to tropical coral islands and reefs.

NHX Gene Family in Camellia sinensis: In-silico Genome-Wide Identification, Expression Profiles, and Regulatory Network Analysis

Salt stress affects the plant growth and productivity worldwide and NHX is one of those genes that are well known to improve salt tolerance in transgenic plants. It is well characterized in several plants, such as Arabidopsis thaliana and cotton; however, not much is known about NHXs in tea plant. In the present study, NHX genes of tea were obtained through a genome-wide search using A. thaliana as reference genome. Out of the 9 NHX genes in tea, 7 genes were localized in vacuole while the remaining 2 genes were localized in the endoplasmic reticulum (ER; CsNHX8) and plasma membrane (PM; CsNHX9), respectively. Furthermore, phylogenetic relationships along with structural analysis which includes gene structure, location, and protein-conserved motifs and domains were systematically examined and further, predictions were validated by the expression analysis. The dN/dS values show that the majority of tea NHX genes is subjected to strong purifying selection under the course of evolution. Also, functional interaction was carried out in Camellia sinensis based on the orthologous genes in A. thaliana. The expression profiles linked to various stress treatments revealed wide involvement of NHX genes from tea in response to various abiotic factors. This study provides the targets for further comprehensive identification, functional study, and also contributed for a better understanding of the NHX regulatory network in C. sinensis.

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

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

Understanding the potential of root microbiome influencing salt-tolerance in plants and mechanisms involved at the transcriptional and translational level

Soil salinity severely affects plant growth and development and imparts inevitable losses to crop productivity. Increasing the concentration of salts in the vicinity of plant roots has severe consequences at the morphological, biochemical, and molecular levels. These include loss of chlorophyll, decrease in photosynthetic rate, reduction in cell division, ROS generation, inactivation of antioxidative enzymes, alterations in phytohormone biosynthesis and signaling, and so forth. The association of microorganisms, viz. plant growth-promoting rhizobacteria, endophytes, and mycorrhiza, with plant roots constituting the root microbiome can confer a greater degree of salinity tolerance in addition to their inherent ability to promote growth and induce defense mechanisms. The mechanisms involved in induced stress tolerance bestowed by these microorganisms involve the modulation of phytohormone biosynthesis and signaling pathways (including indole acetic acid, gibberellic acid, brassinosteroids, abscisic acid, and jasmonic acid), accumulation of osmoprotectants (proline, glycine betaine, and sugar alcohols), and regulation of ion transporters (SOS1, NHX, HKT1). Apart from this, salt-tolerant microorganisms are known to induce the expression of salt-responsive genes via the action of several transcription factors, as well as by posttranscriptional and posttranslational modifications. Moreover, the potential of these salt-tolerant microflora can be employed for sustainably improving crop performance in saline environments. Therefore, this review will briefly focus on the key responses of plants under salinity stress and elucidate the mechanisms employed by the salt-tolerant microorganisms in improving plant tolerance under saline environments.

Regulation of Cytosolic pH: The Contributions of Plant Plasma Membrane H+-ATPases and Multiple Transporters

Cytosolic pH homeostasis is a precondition for the normal growth and stress responses in plants, and H+ flux across the plasma membrane is essential for cytoplasmic pH control. Hence, this review focuses on seven types of proteins that possess direct H+ transport activity, namely, H+-ATPase, NHX, CHX, AMT, NRT, PHT, and KT/HAK/KUP, to summarize their plasma-membrane-located family members, the effect of corresponding gene knockout and/or overexpression on cytosolic pH, the H+ transport pathway, and their functional regulation by the extracellular/cytosolic pH. In general, H+-ATPases mediate H+ extrusion, whereas most members of other six proteins mediate H+ influx, thus contributing to cytosolic pH homeostasis by directly modulating H+ flux across the plasma membrane. The fact that some AMTs/NRTs mediate H+-coupled substrate influx, whereas other intra-family members facilitate H+-uncoupled substrate transport, demonstrates that not all plasma membrane transporters possess H+-coupled substrate transport mechanisms, and using the transport mechanism of a protein to represent the case of the entire family is not suitable. The transport activity of these proteins is regulated by extracellular and/or cytosolic pH, with different structural bases for H+ transfer among these seven types of proteins. Notably, intra-family members possess distinct pH regulatory characterization and underlying residues for H+ transfer. This review is anticipated to facilitate the understanding of the molecular basis for cytosolic pH homeostasis. Despite this progress, the strategy of their cooperation for cytosolic pH homeostasis needs further investigation.

Integrative transcriptomic and metabolomic analysis of D-leaf of seven pineapple varieties differing in N-P-K% contents

BACKGROUND

Pineapple (Ananas comosus L. Merr.) is the third most important tropical fruit in China. In other crops, farmers can easily judge the nutritional requirements from leaf color. However, concerning pineapple, it is difficult due to the variation in leaf color of the cultivated pineapple varieties. A detailed understanding of the mechanisms of nutrient transport, accumulation, and assimilation was targeted in this study. We explored the D-leaf nitrogen (N), phosphorus (P), and potassium (K) contents, transcriptome, and metabolome of seven pineapple varieties.

RESULTS

Significantly higher N, P, and K% contents were observed in Bali, Caine, and Golden pineapple. The transcriptome sequencing of 21 libraries resulted in the identification of 14,310 differentially expressed genes in the D-leaves of seven pineapple varieties. Genes associated with N transport and assimilation in D-leaves of pineapple was possibly regulated by nitrate and ammonium transporters, and glutamate dehydrogenases play roles in N assimilation in arginine biosynthesis pathways. Photosynthesis and photosynthesis-antenna proteins pathways were also significantly regulated between the studied genotypes. Phosphate transporters and mitochondrial phosphate transporters were differentially regulated regarding inorganic P transport. WRKY, MYB, and bHLH transcription factors were possibly regulating the phosphate transporters. The observed varying contents of K% in the D-leaves was associated to the regulation of K⁺ transporters and channels under the influence of Ca²⁺ signaling. The UPLC-MS/MS analysis detected 873 metabolites which were mainly classified as flavonoids, lipids, and phenolic acids.

CONCLUSIONS

These findings provide a detailed insight into the N, P, K% contents in pineapple D-leaf and their transcriptomic and metabolomic signatures.

Post-translational regulation of the membrane transporters contributing to salt tolerance in plants

This review article summarises the role of membrane transporters and their regulatory kinases in minimising the toxicity of Na+ in the plant under salt stress. The salt-tolerant plants keep their cytosolic level of Na+ up to 10-50mM. The first line of action in this context is the generation of proton motive force by the plasma membrane H+-ATPase. The generated proton motive force repolarises the membrane that gets depolarised due to passive uptake of Na+ under salt stress. The proton motive force generated also drives the plasma membrane Na+/H+ antiporter, SOS1 that effluxes the cytosolic Na+ back into the environment. At the intracellular level, Na+ is sequestered by the vacuole. Vacuolar Na+ uptake is mediated by Na+/H+ antiporter, NHX, driven by the electrochemical gradient for H+, generated by tonoplast H+ pumps, both H+ATPase and PPase. However, it is the expression of the regulatory kinases that make these transporters active through post-translational modification enabling them to effectively manage the cytosolic level of Na+, which is essential for tolerance to salinity in plants. Yet our knowledge of the expression and functioning of the regulatory kinases in plant species differing in tolerance to salinity is scant. Bioinformatics-based identification of the kinases like OsCIPK24 in crop plants, which are mostly salt-sensitive, may enable biotechnological intervention in making the crop cultivar more salt-tolerant, and effectively increasing its annual yield.

Arabidopsis antiporter CHX23 and auxin transporter PIN8 coordinately regulate pollen growth

Both the antiporter CHX23 (Cation/Proton Exchangers 23) and auxin transporter PIN8 (PIN-FORMED 8) are localized in the ER and regulate pollen growth in Arabidopsis. But how these two proteins regulate pollen growth remains to be studied. Here, we report that CHX23 and PIN8 act coordinately in regulating pollen growth. The chx23 mutant was reduced in pollen growth and normally shaped pollen grains, and complementation with CHX23 restored both pollen growth and normal pollen morphology. NAA treatments showed that CHX23 was crucial for pollen auxin homeostasis. The pin8 chx23 double mutant was decreased in pollen growth and normal pollen grains, indicating the joint effort of CHX23 and PIN8 in pollen growth. In vivo germination assay showed that CHX23 and PIN8 were involved in the early stage of pollen growth. CHX23 and PIN8 also function collaboratively in maintaining pollen auxin homeostasis. PIN8 depends on CHX23 in regulating pollen morphology and response to NAA treatments. CHX23 co-localized with PIN8, but there was no physical interaction. KCl and NaCl treatments showed that pollen growth of chx23 was reduced less than Col-0; pin8 chx23 was reduced less than chx23 and pin8. Together, CHX23 may regulate PIN8 function and hence pollen growth through controlling K⁺ and Na⁺ homeostasis mediated by its transport activity.

Mechanisms and regulation of organic acid accumulation in plant vacuoles

In fleshy fruits, organic acids are the main source of fruit acidity and play an important role in regulating osmotic pressure, pH homeostasis, stress resistance, and fruit quality. The transport of organic acids from the cytosol to the vacuole and their storage are complex processes. A large number of transporters carry organic acids from the cytosol to the vacuole with the assistance of various proton pumps and enzymes. However, much remains to be explored regarding the vacuolar transport mechanism of organic acids as well as the substances involved and their association. In this review, recent advances in the vacuolar transport mechanism of organic acids in plants are summarized from the perspectives of transporters, channels, proton pumps, and upstream regulators to better understand the complex regulatory networks involved in fruit acid formation.

Towards Molecular Understanding of the pH Dependence Characterizing NhaA of Which Structural Fold is Shared by Other Transporters

Na⁺/H⁺ antiporters comprise a super-family (CPA) of membrane proteins that are found in all kingdoms of life and are essential in cellular homeostasis of pH, Na⁺ and volume. Their activity is strictly dependent on pH, a property that underpins their role in pH homeostasis. While several human homologues have long been drug targets, NhaA of Escherichia coli has become the paradigm for this class of secondary active transporters as NhaA crystal structure provided insight into the architecture of this molecular machine. However, the mechanism of the strict pH dependence of NhaA is missing. Here, as a follow up of a recent evolutionary analysis that identified a 'CPA motif', we rationally designed three E. coli NhaA mutants: D133S, I134T, and the double mutant D133S-I134T. Exploring growth phenotype, transport activity and Li⁺-binding of the mutants, we revealed that Asp133 does not participate directly in proton binding, nor does it directly dictate the pH-dependent transport of NhaA. Strikingly, the variant I134T lost some of the pH control, and the D133S-Il134T double mutant retained Li⁺ binding in a pH independent fashion. Concurrent to loss of pH control, these mutants bound Li⁺ more strongly than the WT. Both positions are in close vicinity to the ion-binding site of the antiporter, attributing the results to electrostatic interaction between these residues and Asp164 of the ion-binding site. This is consistent with pH sensing resulting from direct coupling between cation binding and deprotonation in Asp164, which applies also to other CPA antiporters that are involved in human diseases.

The role of ion-transporting proteins in the evolution of salt tolerance in charophyte algae

Species within the genus Chara have variable salinity tolerance. Their close evolutionary relationship with embryophytes makes their study crucial to understanding the evolution of salt tolerance and key evolutionary processes shared among the phyla. We examined salt-tolerant Chara longifolia and salt-sensitive Chara australis for mechanisms of salt tolerance and their potential role in adaptation to salt. We hypothesize that there are shared mechanisms similar to those in embryophytes, which assist in conferring salt tolerance in Chara, including a cation transporter (HKT), a Na⁺ /H⁺ antiport (NHX), a H⁺ -ATPase (AHA), and a Na⁺ -ATPase (ENA). Illumina transcriptomes were created using cultures grown in freshwater and exposed to salt stress. The presence of these candidate genes, identified by comparing with genes known from embryophytes, has been confirmed in both species of Chara, with the exception of ENA, present only in salt-tolerant C. longifolia. These transcriptomes provide evidence for the contribution of these mechanisms to differences in salt tolerance in the two species and for the independent evolution of the Na⁺ -ATPase. We also examined genes that may have played a role in important evolutionary processes, suggested by previous work on the Chara braunii genome. Among the genes examined, cellulose synthase protein (GT43) and response regulator (RRB) were confirmed in both species. Genes absent from all three Chara species were members of the GRAS family, microtubule-binding protein (TANGLED1), and auxin synthesizers (YUCCA, TAA). Results from this study shed light on the evolutionary relationship between Chara and embryophytes through confirmation of shared salt tolerance mechanisms, as well as unique mechanisms that do not occur in angiosperms.

Regulation of salt-stressed sunflower (Helianthus annuus) seedling's water status by the coordinated action of Na+/K+ accumulation, nitric oxide, and aquaporin expression

Among abiotic stresses, salt stress is a major threat to crop production all over the world. Present work demonstrates the profuse accumulation of Na+ in 2-day-old, dark-grown sunflower (Helianthus annuus L.) seedlings roots in response to salt stress (NaCl). The pattern of K+ accumulation in response to salt stress is similar to that of Na+ but on relatively lower scale. Application of nitric oxide (NO) donor (DETA) scales down Na+ accumulation in salt-stressed seedlings. The impact of NO donor on K+ accumulation is, however, different in control and salt-stressed seedling roots. In control seedlings, it enhances K+ accumulation, whereas, it gets reduced in salt-stressed seedlings. Specialised channels called 'aquaporins' (AQPs) play a major role maintaining the water status and transport across plant parts under salt-stress. Thus, accumulation of plasma-membrane intrinsic proteins (PIPs) and tonoplast-intrinsic proteins (TIPs), localised on plasma-membrane and vacuolar-membrane, respectively was undertaken in 2-day-old, dark-grown seedling roots. Salt stress increased the abundance of these isoforms, whereas, NO application resulted in decreased accumulation of PIP2 and TIP1. PIP1 and TIP2 isoforms remained undetectable. Present work thus, puts forward a correlation between AQP expression and ions (Na+ and K+) homeostasis in response to salt stress and NO.

Membrane voltage as a dynamic platform for spatiotemporal signaling, physiological, and developmental regulation

Membrane voltage arises from the transport of ions through ion-translocating ATPases, ion-coupled transport of solutes, and ion channels, and is an integral part of the bioenergetic "currency" of the membrane. The dynamics of membrane voltage-so-called action, systemic, and variation potentials-have also led to a recognition of their contributions to signal transduction, both within cells and across tissues. Here, we review the origins of our understanding of membrane voltage and its place as a central element in regulating transport and signal transmission. We stress the importance of understanding voltage as a common intermediate that acts both as a driving force for transport-an electrical "substrate"-and as a product of charge flux across the membrane, thereby interconnecting all charge-carrying transport across the membrane. The voltage interconnection is vital to signaling via second messengers that rely on ion flux, including cytosolic free Ca2+, H+, and the synthesis of reactive oxygen species generated by integral membrane, respiratory burst oxidases. These characteristics inform on the ways in which long-distance voltage signals and voltage oscillations give rise to unique gene expression patterns and influence physiological, developmental, and adaptive responses such as systemic acquired resistance to pathogens and to insect herbivory.

Diurnal variation of transitory starch metabolism is regulated by plastid proteins WXR1/WXR3 in Arabidopsis young seedlings

Transitory starch is the portion of starch that is synthesized during the day in the chloroplast and usually used for plant growth overnight. Here, we report altered metabolism of transitory starch in the wxr1/wxr3 (weak auxin response 1/3) mutants of Arabidopsis. WXR1/WXR3 were previously reported to regulate root growth of young seedlings and affect the auxin response mediated by auxin polar transport in Arabidopsis. In this study the wxr1/wxr3 mutants accumulated transitory starch in cotyledon, young leaf, and hypocotyl at the end of night. WXR1/WXR3 expression showed diurnal variation. Grafting experiments indicated that the WXRs in root were necessary for proper starch metabolism and plant growth. We also found that photosynthesis was inhibited and the transcription level of DIN1/DIN6 (Dark-Inducible 1/6) was reduced in wxr1/wxr3. The mutants also showed a defect in the ionic equilibrium of Na+ and K+, consistent with our bioinformatics data that genes related to ionic equilibrium were misregulated in wxr1. Loss of function of WXR1 also resulted in abnormal trafficking of membrane lipids and proteins. This study reveals that the plastid proteins WXR1/WXR3 play important roles in promoting transitory starch degradation for plant growth over night, possibly through regulating ionic equilibrium in the root.

The soybean plasma membrane-localized cation/H+ exchanger GmCHX20a plays a negative role under salt stress

Cation/H+ -exchanger (CHX) perform diverse functions in plants, including being a part of the protective mechanisms to cope with salt stress. GmCHX1 has been previously identified as the causal gene in a major salt-tolerance quantitative trait locus (QTL) in soybean, but little is known about another close paralog, GmCHX20a, found in the same QTL. In this study, GmCHX20a was characterized along with GmCHX1. The expression patterns of the two genes and the direction of Na+ flux directed by overexpression of these two transporters are different, suggesting that they are functionally distinct. The ectopic expression of GmCHX20a led to an increase in salt sensitivity and osmotic tolerance, which was consistent with its role in increasing Na+ uptake into the root. Although this seems counter-intuitive, it may in fact be part of the mechanism by which soybean could counter act the effects of osmotic stress, which is commonly manifested in the initial stage of salinity stress. On the other hand, GmCHX1 from salt-tolerant soybean was shown to protect plants via Na+ exclusion under salt stress. Taken together these results suggest that GmCHX20a and GmCHX1 might work complementally through a concerted effort to address both osmotic stress and ionic stress as a result of elevated salinity.

Insight into the direct interaction of Na+ with NhaA and mechanistic implications

Na⁺/H⁺ antiporters comprise a family of membrane proteins evolutionarily conserved in all kingdoms of life that are essential in cellular ion homeostasis. While several human homologues have long been drug targets, NhaA of Escherichia coli has become the paradigm for this class of secondary active transporters as NhaA crystals provided insight in the structure of this molecular machine. However, structural data revealing the composition of the binding site for Na⁺ (or its surrogate Li⁺) is missing, representing a bottleneck in our understanding of the correlation between the structure and function of NhaA. Here, by adapting the scintillation proximity assay (SPA) for direct determination of Na⁺ binding to NhaA, we revealed that (i) NhaA is well adapted as the main antiporter for Na⁺ homeostasis in Escherichia coli and possibly in other bacteria as the cytoplasmic Na⁺ concentration is similar to the Na⁺ binding affinity of NhaA, (ii) experimental conditions affect NhaA-mediated cation binding, (iii) in addition to Na⁺ and Li⁺, the halide Tl⁺ interacts with NhaA, (iv) whereas acidic pH inhibits maximum binding of Na⁺ to NhaA, partial Na⁺ binding by NhaA is independent of the pH, an important novel insight into the effect of pH on NhaA cation binding.

Improving ATPase and PPase activities, nutrient uptake and growth of salt stressed ajowan plants by salicylic acid and iron-oxide nanoparticles

Salicylic acid and iron-oxide nanoparticles alleviated salt toxicity and improved plant growth by stimulating the activities of H⁺-ATPase and H⁺-PPase and preventing nutrient imbalance. Two factorial experiments were undertaken in a greenhouse during 2018 and 2019, to evaluate the impacts of SA (1 mM) and nano-Fe₂O₃ (3 mM) sprays at 7 leaves and flowering stages on vacuolar H⁺-pumps, growth and essential oil of salt-subjected (0, 4, 8 and 12 dS m^-1 NaCl) ajowan plants. Measurements of plant traits were started at about 12 days after the last foliar spray and continued up to maturity. The H⁺-ATPase and H⁺-PPase activities and root ATP content were enhanced under low salinity, but higher salinities reduced these parameters. Rising salinity enhanced Na uptake and translocation, endogenous SA and DPPH activity, while reduced K⁺/Na⁺ ratio and nutrients uptake, leading to a reduction in plant biomass. Treatment with SA, nano-Fe₂O₃ and their combination improved H⁺-pumps activities and ATP content in roots and leaves. The SA-related treatments caused the highest activities of H⁺-pumps in roots, but Fe-related treatments resulted in the highest activities of these pumps in leaves. Increasing H⁺-pumps activities reduced sodium uptake and translocation and enhanced nutrients uptake. Foliar treatments, especially SA + nano-Fe₂O₃ augmented endogenous SA, DPPH activity, and plant growth in salt-stressed plants. Essential oil contents of vegetative and inflorescence organs under severe salinity and seeds under moderate and severe salinities were enhanced. Maximum essential oil was obtained from seeds of SA + nano-Fe₂O₃-treated plants, which was strongly correlated with endogenous SA and DPPH. Nevertheless, the SA + nano-Fe₂O₃ was the best treatment for diminishing salt toxicity and improving ajowan plant growth and essential oil production.

Current Understanding of Role of Vesicular Transport in Salt Secretion by Salt Glands in Recretohalophytes

Soil salinization is a serious and growing problem around the world. Some plants, recognized as the recretohalophytes, can normally grow on saline-alkali soil without adverse effects by secreting excessive salt out of the body. The elucidation of the salt secretion process is of great significance for understanding the salt tolerance mechanism adopted by the recretohalophytes. Between the 1950s and the 1970s, three hypotheses, including the osmotic potential hypothesis, the transfer system similar to liquid flow in animals, and vesicle-mediated exocytosis, were proposed to explain the salt secretion process of plant salt glands. More recently, increasing evidence has indicated that vesicular transport plays vital roles in salt secretion of recretohalophytes. Here, we summarize recent findings, especially regarding the molecular evidence on the functional roles of vesicular trafficking in the salt secretion process of plant salt glands. A model of salt secretion in salt gland is also proposed.

The C-terminal tail of the plant endosomal-type NHXs plays a key role in its function and stability

Typically, Na⁺/H⁺ antiporters (NHXs) possess a conserved N-terminus for cation binding and exchange and a hydrophilic C-terminus for regulating the antiporter activity. Plant endosomal-type NHXs play important roles in protein trafficking, as well as K⁺ and vesicle pH homeostasis, however the role of the C-terminal tail remains unclear. Here, the function of MnNHX6, an endosomal-type NHX in mulberry, was investigated using heterologous expression in yeast. Functional and localization analyses of C-terminal truncation and mutations in MnNHX6 revealed that the C-terminal conserved region was responsible for the function and stability of the protein and its hydrophobicity, which is a key domain requirement. Nuclear magnetic resonance spectroscopy provided direct structural evidence and yeast two-hybrid screening indicated that this functional domain was also necessary for interaction with sorting nexin 1. Our findings demonstrate that although the C-terminal tail of MnNHX6 is intrinsically disordered, the C-terminal conserved region may be an important part of the external mouth of this transporter, which controls protein function and stability by serving as an inter-molecular cork with a chain mechanism. These findings improve our understanding of the roles of the C-terminal tail of endosomal-type NHXs in plants and the ion transport mechanism of NHX-like antiporters.

Genome-wide identification, characterization and transcriptional profiling of NHX-type (Na+/H+) antiporters under salinity stress in soybean

This study was aimed at the genome-wide identification, a comprehensive in silico characterization of NHX genes from soybean (Glycine max L.) and their tissue-specific expression under varied levels (0-200 mM NaCl) of salinity stress. A total of nine putative NHX genes were identified from soybean. The phylogenetic analysis confirmed a total of five sub-groups and GmNHXs were distributed in three of them. Bioinformatics analyses confirmed all GmNHXs as ion transporters in nature, and all were localized on the vacuolar membrane. Several cis-acting regulatory elements involved in hormonal signal-responsiveness and abiotic stress including salinity responses were identified in the promoter regions of GmNHXs. Amiloride, which is a known Na⁺/H⁺ exchanger activity inhibitor, binding motifs were observed in all the GmNHXs. Furthermore, the identified GmNHXs were predicted-targets of 75 different miRNA candidates. To gain an insight into the functional divergence of GmNHX transporters, qRT-PCR based gene expression analysis was done in control and salt-treated root, stem and leaf tissues of two contrasting Indian soybean varieties MAUS-47 (tolerant) and Gujosoya-2 (sensitive). The gene up-regulation was tissue-specific and varied amongst the soybean varieties, with higher induction in tolerant variety. Maximum induction was observed in GmNHX2 in root tissues of MAUS-47 at 200 mM NaCl stress. Overall, identified GmNHXs may be explored further as potential gene candidates for soybean improvement.

Regulatory Network of Cotton Genes in Response to Salt, Drought and Wilt Diseases (Verticillium and Fusarium): Progress and Perspective

In environmental conditions, crop plants are extremely affected by multiple abiotic stresses including salinity, drought, heat, and cold, as well as several biotic stresses such as pests and pathogens. However, salinity, drought, and wilt diseases (e.g., Fusarium and Verticillium) are considered the most destructive environmental stresses to cotton plants. These cause severe growth interruption and yield loss of cotton. Since cotton crops are central contributors to total worldwide fiber production, and also important for oilseed crops, it is essential to improve stress tolerant cultivars to secure future sustainable crop production under adverse environments. Plants have evolved complex mechanisms to respond and acclimate to adverse stress conditions at both physiological and molecular levels. Recent progresses in molecular genetics have delivered new insights into the regulatory network system of plant genes, which generally includes defense of cell membranes and proteins, signaling cascades and transcriptional control, and ion uptake and transport and their relevant biochemical pathways and signal factors. In this review, we mainly summarize recent progress concerning several resistance-related genes of cotton plants in response to abiotic (salt and drought) and biotic (Fusarium and Verticillium wilt) stresses and classify them according to their molecular functions to better understand the genetic network. Moreover, this review proposes that studies of stress related genes will advance the security of cotton yield and production under a changing climate and that these genes should be incorporated in the development of cotton tolerant to salt, drought, and fungal wilt diseases (Verticillium and Fusarium).

Cu and Na contents regulate N uptake of Leymus chinensis growing in soda saline-alkali soil

Leymus chinensis (L. chinensis) is the dominant plant in the eastern margins of the Eurasian temperate grasslands. It is a very robust species, exhibiting good saline-alkali resistance and stabilizing soil. In this study, 67 soil samples and L. chinensis were collected in western Jilin province, China. The contents of N, P, K, S, Mn, Fe, Zn, Cu and Na were measured, revealing that the growth of L. chinensis was mainly restricted by N based on the stoichiometric N: P ratios of plant. Furthermore, path analysis indicated that N was significantly correlated with K, S, Cu, and Zn. Imbalances in the homeostasis of these four elements may thus constrain N. The homeostasis index of Cu (H_Cu) in sites with 100%-70% of vegetation cover was only 0.79, it was classified as a sensitive element. However, K, S and Zn, whose concentrations in L. chinensis were significantly related to those of N, exhibited no homeostatic characteristics. These results suggest that when seeking to treat saline-alkali stress, it is important to add fertilizers containing K, S, and Zn to avoid growth limitation. Na⁺, an ion associated with high soil alkalinity, exhibited weak homeostasis in L. chinensis even in sites with only 40%-10% of vegetation cover. When soil Na exceeded 16000 mg/kg, the homeostasis mechanism of L. chinensis appeared to be overwhelmed, resulting in rapid and probably harmful accumulation of Na. Proper control of N content can alleviate the toxicity of Na stress in L. chinensis and enhance its Na tolerance. Together, these results suggest that combined fertilization with N, K, S, Zn and Cu should be applied to improve grasslands growth. The results of this study can provide a reference basis for sustainable grassland management.

Improving crop salt tolerance using transgenic approaches: An update and physiological analysis

Salinization of land is likely to increase due to climate change with impact on agricultural production. Since most species used as crops are sensitive to salinity, improvement of salt tolerance is needed to maintain global food production. This review summarises successes and failures of transgenic approaches in improving salt tolerance in crop species. A conceptual model of coordinated physiological mechanisms in roots and shoots required for salt tolerance is presented. Transgenic plants overexpressing genes of key proteins contributing to Na⁺ 'exclusion' (PM-ATPases with SOS1 antiporter, and HKT1 transporter) and Na⁺ compartmentation in vacuoles (V-H⁺ ATPase and V-H⁺ PPase with NHX antiporter), as well as two proteins potentially involved in alleviating water deficit during salt stress (aquaporins and dehydrins), were evaluated. Of the 51 transformations, with gene(s) involved in Na⁺ 'exclusion' or Na⁺ vacuolar compartmentation that contained quantitative data on growth and include a non-saline control, 48 showed improvements in salt tolerance (less impact on plant mass) of transgenic plants, but with only two tested in field conditions. Of these 51 transformations, 26 involved crop species. Tissue ion concentrations were altered, but not always in the same way. Although glasshouse data are promising, field studies are required to assess crop salinity tolerance.

Plant NHX Antiporters: From Function to Biotechnological Application, with Case Study

Salt stress is one of the major abiotic stresses that negatively affect crops worldwide. Plants have evolved a series of mechanisms to cope with the limitations imposed by salinity. Molecular mechanisms, including the upregulation of cation transporters such as the Na+/H+ antiporters, are one of the processes adopted by plants to survive in saline environments. NHX antiporters are involved in salt tolerance, development, cell expansion, growth performance and disease resistance of plants. They are integral membrane proteins belonging to the widely distributed CPA1 sub-group of monovalent cation/H+ antiporters and provide an important strategy for ionic homeostasis in plants under saline conditions. These antiporters are known to regulate the exchange of sodium and hydrogen ions across the membrane and are ubiquitous to all eukaryotic organisms. With the genomic approach, previous studies reported that a large number of proteins encoding Na+/H+ antiporter genes have been identified in many plant species and successfully introduced into desired species to create transgenic crops with enhanced tolerance to multiple stresses. In this review, we focus on plant antiporters and all the aspects from their structure, classification, function to their in silico analysis. On the other hand, we performed a genome-wide search to identify the predicted NHX genes in Argania spinosa L. We highlighted for the first time the presence of four putative NHX (AsNHX1-4) from the Argan tree genome, whose phylogenetic analysis revealed their classification in one distinct vacuolar cluster. The essential information of the four putative NHXs, such as gene structure, subcellular localization and transmembrane domains was analyzed.

Chitosan regulates metabolic balance, polyamine accumulation, and Na+ transport contributing to salt tolerance in creeping bentgrass

BACKGROUND

Chitosan (CTS), a natural polysaccharide, exhibits multiple functions of stress adaptation regulation in plants. However, effects and mechanism of CTS on alleviating salt stress damage are still not fully understood. Objectives of this study were to investigate the function of CTS on improving salt tolerance associated with metabolic balance, polyamine (PAs) accumulation, and Na+ transport in creeping bentgrass (Agrostis stolonifera).

RESULTS

CTS pretreatment significantly alleviated declines in relative water content, photosynthesis, photochemical efficiency, and water use efficiency in leaves under salt stress. Exogenous CTS increased endogenous PAs accumulation, antioxidant enzyme (SOD, POD, and CAT) activities, and sucrose accumulation and metabolism through the activation of sucrose synthase and pyruvate kinase activities, and inhibition of invertase activity. The CTS also improved total amino acids, glutamic acid, and γ-aminobutyric acid (GABA) accumulation. In addition, CTS-pretreated plants exhibited significantly higher Na+ content in roots and lower Na+ accumulation in leaves then untreated plants in response to salt stress. However, CTS had no significant effects on K+/Na+ ratio. Importantly, CTS enhanced salt overly sensitive (SOS) pathways and also up-regulated the expression of AsHKT1 and genes (AsNHX4, AsNHX5, and AsNHX6) encoding Na+/H+ exchangers under salt stress.

CONCLUSIONS

The application of CTS increased antioxidant enzyme activities, thereby reducing oxidative damage to roots and leaves. CTS-induced increases in sucrose and GABA accumulation and metabolism played important roles in osmotic adjustment and energy metabolism during salt stress. The CTS also enhanced SOS pathway associated with Na+ excretion from cytosol into rhizosphere, increased AsHKT1 expression inhibiting Na+ transport to the photosynthetic tissues, and also up-regulated the expression of AsNHX4, AsNHX5, and AsNHX6 promoting the capacity of Na+ compartmentalization in roots and leaves under salt stress. In addition, CTS-induced PAs accumulation could be an important regulatory mechanism contributing to enhanced salt tolerance. These findings reveal new functions of CTS on regulating Na+ transport, enhancing sugars and amino acids metabolism for osmotic adjustment and energy supply, and increasing PAs accumulation when creeping bentgrass responds to salt stress.

Sexual Differences in Physiological and Transcriptional Responses to Salinity Stress of Salix linearistipularis

Willow (Salix), a dioecious plant, is an important ornamental tree species in the world. Salix linearistipularis, a perennial woody plant species naturally distributed on the Songnen Plain saline-alkali land in northeast China, has a high saline condition. To study the sexual differences of S. linearistipularis in salinity tolerance, the physiological and transcriptional responses to salinity were compared between female and male cuttings. Under salinity stress, the female leaves exhibited higher superoxide dismutase and peroxidase activities and photosynthetic capacity, and lower H2O2 contents than those of male leaves. Under salinity stress, sodium (Na+) accumulation in female leaves was lower than that in the male leaves. The non-invasive micro-test showed that the net Na+ efflux in the salt-treated female roots was higher than that in male roots. Physiological responses revealed that female cuttings were more tolerant than males, which may be mainly due to females having lower leaf Na+ accumulation and higher root Na+ efflux capacity than males. Transcriptional analyses showed that 108 differentially expressed salt-responsive genes were identified in both female and male roots; most of these showed sexual differences in expression patterns under salinity stress. RNA-seq combined with qPCR analysis showed that the salt-induced expression of four Na+/H+ antiporter (NHX) genes (SlNHX3, 5, 6, 7) in female roots was higher than that in male roots. Transcriptional analyses revealed that the higher Na+ efflux capacity in female roots than in male roots may be closely related to the differential expression of salt-responsive genes, especially NHX genes.

Identification and characterization of multiple abiotic stress tolerance genes in wheat

Wheat is produced worldwide over six continents with the European Union, China, India, Russia, and the United States as major producer countries. The productivity was recorded 749 million tons by harvesting from 220-million-hectare land. It is the need of the hour to develop stress-tolerant wheat varieties to enhance the productivity by 60% to provide food security to 9.6 billion-world population by 2050. Although the genotypes have been identified for heat, drought and salt tolerance, their underlying mechanism for tolerance is poorly understood. The detailed understanding of the mechanism and identification of critical factors participating in multiple abiotic stress tolerance is essential. In the present study, the contrasting wheat genotypes were intensely characterized and assessed for the expression of different stress responsive genes under lab conditions. The expression analysis revealed that SHN1, DREB6, NHX2 and AVP1 were found to be highly induced under heat, salt and drought stresses in wheat. Thus, these genes can be used as signature genes to identify the multiple stress-tolerant varieties in the breeding program. The novel variants of these genes can be targeted through breeding or genetic engineering or genome editing strategies to develop multiple abiotic stress tolerant wheat varieties.