The Arabidopsis intracellular Na+/H+ antiporters NHX5 and NHX6 are endosome associated and necessary for plant growth and development

Screening and functional characterization of salt-tolerant NAC gene family members in Medicago sativa L

Introduction

Alfalfa is the most widely cultivated high-quality perennial leguminous forage crop in the world. In China, saline-alkali land represents an important yet underutilized land resource. Cultivating salt-tolerant alfalfa varieties is crucial for the effective development and utilization of saline-alkali soils and for promoting the sustainable growth of grassland-livestock farming in these regions. The NAC (NAM, ATAF, and CUC) family of transcription factors plays a key role in regulating gene expression in response to various abiotic stresses, such as drought, salinity and extreme temperatures, thereby enhancing plant stress tolerance.

Methods

This study evaluated the structure and evolutionary relationship of the members of the NAC-like transcription factor family in alfalfa using bioinformatics. We identified 114 members of the NAC gene family in the Zhongmu No.1 genome and classified them into 13 subclasses ranging from I to XIII. The bioinformatics analysis showed that subfamily V might be related to the response to salt stress. Gene expression analysis was conducted using RNA-seq and qRT-PCR, and MsNAC40 from subfamily V was chosen for further investigation into salt tolerance.

Results

MsNAC40 gene had an open reading frame of 990 bp and encoded a protein containing 329 amino acids, with a molecular weight of 3.70 KDa and a conserved NAM structural domain. The protein was hydrophilic with no transmembrane structure.After treating both the MsNAC40 overexpressing plants and the control group with 150 mmol/L NaCl for 15 days, physiological and biochemical measurements revealed that these plants had significantly greater height, net photosynthetic rate, stomatal conductance, and transpiration rate compared to the control group, while their conductivity was significantly lower. Additionally, the levels of abscisic acid in the roots and leaves, along with the activities of peroxidase, superoxide dismutase, and catalase in the leaves, were significantly higher in the overexpressing plants, whereas the malondialdehyde content was significantly lower. Moreover, the Na+ content in the overexpressing plants was significantly reduced, while the K+/Na+ ratio was significantly increased compared to the control group.

Discussion

These results indicated that the MsNAC40 gene improved the salt tolerance of Pioneer Alfalfa SY4D, but its potential mechanism of action still needs to be further explored.

CLCf is an endosomal resident proton/chloride antiporter during salt stress

CHLORIDE CHANNEL f (CLCf) has a role in maintaining trans-Golgi Network/Early Endosome function under all conditions and is unlikely to mediate plasma membrane chloride transport during salt stress.

Salinity survival: molecular mechanisms and adaptive strategies in plants

Soil salinity is a significant environmental challenge that threatens plant growth and development, adversely affecting global food crop production. This underscores the critical need to elucidate the molecular mechanisms underlying plant salt tolerance, which has profound implications for agricultural advancement. Recent progress in plant salt tolerance has greatly improved our understanding of the molecular mechanisms of plant responses to salt stress and precision design breeding as an effective strategy for developing new salt-tolerant crop varieties. This review focuses on the model plant species Arabidopsis thaliana and important crops, namely, wheat (Triticum aestivum), maize (Zea mays), and rice (Oryza sativa). It summarizes current knowledge on plant salt tolerance, emphasizing key aspects such as the perception and response to salt stress, Na+ transport, Na+ compartmentalization and clearance, changes in reactive oxygen species induced by salt stress, and regulation of plant stem cell development under salt stress conditions. The review might provide new and valuable information for understanding the molecular mechanisms of plant response and adaptation to salt stress.

The SlWRKY42-SlMYC2 module synergistically enhances tomato saline-alkali tolerance by activating the jasmonic acid signaling and spermidine biosynthesis pathway

Tomato (Solanum lycopersicum) is an important crop but frequently experiences saline-alkali stress. Our previous studies have shown that exogenous spermidine (Spd) could significantly enhance the saline-alkali resistance of tomato seedlings, in which a high concentration of Spd and jasmonic acid (JA) exerted important roles. However, the mechanism of Spd and JA accumulation remains unclear. Herein, SlWRKY42, a Group II WRKY transcription factor, was identified in response to saline-alkali stress. Overexpression of SlWRKY42 improved tomato saline-alkali tolerance. Meanwhile, SlWRKY42 knockout mutants, exhibited an opposite phenotype. RNA-sequencing data also indicated that SlWRKY42 regulated the expression of genes involved in JA signaling and Spd synthesis under saline-alkali stress. SlWRKY42 is directly bound to the promoters of SlSPDS2 and SlNHX4 to promote Spd accumulation and ionic balance, respectively. SlWRKY42 interacted with SlMYC2. Importantly, SlMYC2 is also bound to the promoter of SlSPDS2 to promote Spd accumulation and positively regulated saline-alkali tolerance. Furthermore, the interaction of SlMYC2 with SlWRKY42 boosted SlWRKY42's transcriptional activity on SlSPDS2, ultimately enhancing the tomato's saline-alkali tolerance. Overall, our findings indicated that SlWRKY42 and SlMYC2 promoted saline-alkali tolerance by the Spd biosynthesis pathway. Thus, this provides new insight into the mechanisms of plant saline-alkali tolerance responses triggered by polyamines (PAs).

Ion homeostasis and coordinated salt tolerance mechanisms in a barley (Hordeum vulgare L.)doubled haploid line

Salinization poses a significant challenge in agriculture. Identifying salt-tolerant plant germplasm resources and understanding their mechanisms of salt tolerance are crucial for breeding new salt-tolerant plant varieties. However, one of the primary obstacles to achieving this goal in crops is the physiological complexity of the salt-tolerance trait. In a previous study, we developed a salt-tolerant barley doubled haploid (DH) line, designated as DH20, through mutagenesis combined with microspore culture, establishing it as an idea model for elucidating the mechanisms of salt tolerance. In this study, ion homeostasis, key osmotic agents, antioxidant enzyme activities and gene expression were compared between Hua30 (the original material used as a control) and DH20. The results indicated that under salt treatment, DH20 exhibited significantly higher shoot fresh and dry weight, relative plant height, shoot K+/Na+ ratio, improved stomatal guard cell function, and better retention of chloroplast ultrastructure compared to Hua30. Notably, the K+ efflux in DH20 was significantly lower while the Na+ and H+ efflux was significantly higher than those in Hua30 under salt stress in mesophyll cells. Furthermore, the activities of ascorbate peroxidase, superoxide dismutase, and peroxidase, along with the levels of proline, betaine, malondialdehyde, and soluble protein, were correlated with ion efflux and played a vital role in the response of DH20 to salt stress. Compared to Hua30, the relative expression levels of the HvSOS1, HvSOS2, HvSOS3, HvHKT1;3, HvNHX1, HvNHX2, and HvNHX3 genes, which showed a strong correlation with Na+, K+, and H+ efflux, exhibited significant differences at 24 h under salt stress in DH20. These findings suggest that ion homeostasis, key osmolytes, antioxidant enzyme activities, and associated gene expression are coordinated in the salt tolerance of DH20, with K+ retention and Na+ and H+ efflux serving as important mechanisms for coping with salt stress. These findings present new opportunities for enhancing salinity tolerance, not only in barley but in other cereals as well, including wheat and rice, by integrating this trait with other traditional mechanisms. Furthermore, MIFE measurements of NaCl-induced ion fluxes from leaf mesophyll provide plant breeders with an efficient method to screen germplasm for salinity stress tolerance in barley and potentially other crops. Clinical trial number: Not applicable.

Elemental cryo-imaging reveals SOS1-dependent vacuolar sodium accumulation

Increasing soil salinity causes significant crop losses globally; therefore, understanding plant responses to salt (sodium) stress is of high importance. Plants avoid sodium toxicity through subcellular compartmentation by intricate processes involving a high level of elemental interdependence. Current technologies to visualize sodium, in particular, together with other elements, are either indirect or lack in resolution. Here we used the newly developed cryo nanoscale secondary ion mass spectrometry ion microprobe1, which allows high-resolution elemental imaging of cryo-preserved samples and reveals the subcellular distributions of key macronutrients and micronutrients in root meristem cells of Arabidopsis and rice. We found an unexpected, concentration-dependent change in sodium distribution, switching from sodium accumulation in the cell walls at low external sodium concentrations to vacuolar accumulation at stressful concentrations. We conclude that, in root meristems, a key function of the NHX family sodium/proton antiporter SALT OVERLY SENSITIVE 1 (also known as Na+/H+ exchanger 7; SOS1/NHX7) is to sequester sodium into vacuoles, rather than extrusion of sodium into the extracellular space. This is corroborated by the use of new genomic, complementing fluorescently tagged SOS1 variants. We show that, in addition to the plasma membrane, SOS1 strongly accumulates at late endosome/prevacuoles as well as vacuoles, supporting a role of SOS1 in vacuolar sodium sequestration.

NHX5/NHX6/SPY22 complex regulates BRI1 and brassinosteroid signaling in Arabidopsis

NHX5 and NHX6, Arabidopsis endosomal antiporters, play a vital role in facilitating ion and pH homeostasis in endosomal compartments. Studies have found that NHX5 and NHX6 are essential for protein trafficking, auxin homeostasis, and plant growth and development. Here, we report the role of NHX5 and NHX6 in brassinosteroid (BR) signaling. We found that hypocotyl growth was enhanced in nhx5 nhx6 under epibrassinolide (eBR) treatment. nhx5 nhx6 bri1 was insensitive to eBR treatment, indicating that NHX5 and NHX6 are downstream of the BRI1 receptor in BR signaling. Moreover, confocal observation with both hypocotyls and root tips showed that BRI1-YFP localization in the plasma membrane (PM) was reduced in nhx5 nhx6. Interestingly, brefeldin A (BFA) treatment showed that formation of the BFA bodies containing BRI1 and their disassembling were disrupted in nhx5 nhx6. Further genetic analysis showed that NHX5/NHX6 and SYP22 may act coordinately in BR signaling. NHX5 and NHX6 may regulate SYP22 function by modulating cellular K+ and pH homeostasis. Importantly, NHX5 and NHX6 colocalize and interact with SYP22, but do not interact with BRI1. In summary, our findings indicate that NHX5/NHX6/SYP22 complex is essential for the regulation of BRI1 recycling and PM localization. The H+-leak facilitated by NHX5 and NHX6 offers a means of controlling BR signaling in plants.

Mechanical wounding improves salt tolerance by maintaining root ion homeostasis in a desert shrub

Soil salinization, especially in arid environments, is a leading cause of land degradation and desertification. Excessive salt in the soil is detrimental to plants. Plants have developed various sophisticated regulatory mechanisms that allow them to withstand adverse environments. Through cross-adaptation, plants improve their resistance to an adverse condition after experiencing a different kind of adversity. Our analysis of Ammopiptanthus nanus, a desert shrub, showed that mechanical wounding activates the biosynthesis of jasmonic acid (JA) and abscisic acid (ABA), enhancing plasma membrane H+-ATPase activity to establish an electrochemical gradient that promotes Na+ extrusion via Na+/H+ antiporters. Mechanical wounding reduces K+ loss under salt stress, improving the K/Na and maintaining root ion balance. Meanwhile, mechanical damage enhances the activity of antioxidant enzymes and the content of osmotic substances, working together with cellular ions to alleviate water loss and growth inhibition under salt stress. This study provides new insights and approaches for enhancing salt tolerance and stress adaptation in plants by elucidating the signaling mechanisms of cross-adaptation.

Genome-wide identification and drought stress-induced expression analysis of the NHX gene family in potato

NHX proteins are transmembrane antiporters belonging to the cation/proton antiporter gene family, with a conserved Na+ (K+)/H+ exchange (PF00999) protein domain. NHXs play a prominent role in plant growth, development, and defense. However, the role of NHX gene family in potato (Solanum tuberosum L.) is yet to be known. In this study, we conducted a genome-wide analysis of the potato NHX gene family. A total of 25 StNHX family members were identified to be unevenly distributed on 10 chromosomes. The proteins ranged in length from 252 to 1,153 amino acids, with molecular masses ranging from 27516.32 to 127860.87 kD, and isoelectric points (pI) ranging from 4.96 to 9.3. Analyses of gene structures and conserved motifs indicated that StNHX genes in the same phylogenetic cluster are conserved. Phylogenetic analysis divided the StNHX genes into three subfamilies (Classes I, II, and III). Synteny analysis indicated that StNHX gene family Class III of NHX and all Arabidopsis thaliana NHXs shared a close evolutionary relationship. Analysis of cis-acting elements in the upstream 1,500 bp promoter region of potato NHX genes showed that these genes could be regulated by light, stress, and hormones such as abscisic acid and gibberellic acid. Protein-protein interaction network analysis indicated that StNHX proteins may participate in the regulation of potato growth and stress response. Besides, To determine a potential role of these genes in tissue development and drought response, we analyzed the RNA-seq data of different DM potato tissues. The results showed that NHX genes exhibited distinct tissue-specific expression patterns. We further examined the expression patterns of StNHX in different tissues (leaves, roots, shoots, tubers, stolons, and flowers) during the flowering stage in 'Jizhangshu NO.8.' potato. The qRT-PCR results further confirmed the importance of StNHX genes in potato plant growth and development. We further analyzed the RNA-seq data (DM potato) under different abiotic stresses (salt, drought, and heat), and found that the expression of StNHX genes was induced under abiotic stress. qRT-PCR analysis of shoots and roots of 'Jizhangshu NO.8.' potato treated for 0, 6, 12, and 24 h with 15% PEG6000 confirmed that the 25 StNHX genes are involved in the response to drought stress in potato. The results of this study may be useful for selecting appropriate candidate genes for the breeding of new drought-tolerant potato varieties. Furthermore, this study lays a foundation for prospective analysis of StNHX gene functions.

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

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

Overexpression of stress granule protein TZF1 enhances salt stress tolerance by targeting ACA11 mRNA for degradation in Arabidopsis

Tandem CCCH zinc finger (TZF) proteins play diverse roles in plant growth and stress response. Although as many as 11 TZF proteins have been identified in Arabidopsis, little is known about the mechanism by which TZF proteins select and regulate the target mRNAs. Here, we report that Arabidopsis TZF1 is a bona-fide stress granule protein. Ectopic expression of TZF1 (TZF1 OE), but not an mRNA binding-defective mutant (TZF1H186Y OE), enhances salt stress tolerance in Arabidopsis. RNA-seq analyses of NaCl-treated plants revealed that the down-regulated genes in TZF1 OE plants are enriched for functions in salt and oxidative stress responses. Because many of these down-regulated mRNAs contain AU- and/or U-rich elements (AREs and/or UREs) in their 3'-UTRs, we hypothesized that TZF1-ARE/URE interaction might contribute to the observed gene expression changes. Results from RNA immunoprecipitation-quantitative PCR analysis, gel-shift, and mRNA half-life assays indicate that TZF1 binds and triggers degradation of the autoinhibited Ca2+-ATPase 11 (ACA11) mRNA, which encodes a tonoplast-localized calcium pump that extrudes calcium and dampens signal transduction pathways necessary for salt stress tolerance. Furthermore, this salt stress-tolerance phenotype was recapitulated in aca11 null mutants. Collectively, our findings demonstrate that TZF1 binds and initiates degradation of specific mRNAs to enhance salt stress tolerance.

Genomes of multicellular algal sisters to land plants illuminate signaling network evolution

Zygnematophyceae are the algal sisters of land plants. Here we sequenced four genomes of filamentous Zygnematophyceae, including chromosome-scale assemblies for three strains of Zygnema circumcarinatum. We inferred traits in the ancestor of Zygnematophyceae and land plants that might have ushered in the conquest of land by plants: expanded genes for signaling cascades, environmental response, and multicellular growth. Zygnematophyceae and land plants share all the major enzymes for cell wall synthesis and remodifications, and gene gains shaped this toolkit. Co-expression network analyses uncover gene cohorts that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution.

Structural, developmental and functional analyses of leaf salt glands of mangrove recretohalophyte Aegiceras corniculatum

Salt secretion is an important strategy used by the mangrove plant Aegiceras corniculatum to adapt to the coastal intertidal environment. However, the structural, developmental and functional analyses on the leaf salt glands, particularly the salt secretion mechanism, are not well documented. In this study, we investigated the structural, developmental and degenerative characteristics and the salt secretion mechanisms of salt glands to further elucidate the mechanisms of salt tolerance of A. corniculatum. The results showed that the salt gland cells have a large number of mitochondria and vesicles, and plenty of plasmodesmata as well, while chloroplasts were found in the collecting cells. The salt glands developed early and began to differentiate at the leaf primordium stage. We observed and defined three stages of salt gland degradation for the first time in A. corniculatum, where the secretory cells gradually twisted and wrinkled inward and collapsed downward as the salt gland degeneration increased and the intensity of salt gland autofluorescence gradually diminished. In addition, we found that the salt secretion rate of the salt glands increased when the treated concentration of NaCl increased, reaching the maximum at 400 mM NaCl. The salt-secreting capacity of the salt glands of the adaxial epidermis is significantly greater than that of the abaxial epidermis. The real-time quantitative PCR results indicate that SAD2, TTG1, GL2 and RBR1 may be involved in regulating the development of the salt glands of A. corniculatum. Moreover, Na+/H+ antiporter, H+-ATPase, K+ channel and Cl- channel may play important roles in the salt secretion of salt glands. In sum mary, this study strengthens the understanding of the structural, developmental and degenerative patterns of salt glands and salt secretion mechanisms in mangrove recretohalophyte A. corniculatum, providing an important reference for further studies at the molecular level.

The RNA landscape of Dunaliella salina in response to short-term salt stress

Using the halotolerant green microalgae Dunaliella salina as a model organism has special merits, such as a wide range of salt tolerance, unicellular organism, and simple life cycle and growth conditions. These unique characteristics make it suitable for salt stress study. In order to provide an overview of the response of Dunaliella salina to salt stress and hopefully to reveal evolutionarily conserved mechanisms of photosynthetic organisms in response to salt stress, the transcriptomes and the genome of the algae were sequenced by the second and the third-generation sequencing technologies, then the transcriptomes under salt stress were compared to the transcriptomes under non-salt stress with the newly sequenced genome as the reference genome. The major cellular biological processes that being regulated in response to salt stress, include transcription, protein synthesis, protein degradation, protein folding, protein modification, protein transport, cellular component organization, cell redox homeostasis, DNA repair, glycerol synthesis, energy metabolism, lipid metabolism, and ion homeostasis. This study gives a comprehensive overview of how Dunaliella salina responses to salt stress at transcriptomic level, especially characterized by the nearly ubiquitous up-regulation of the genes involving in protein folding, DNA repair, and cell redox homeostasis, which may confer the algae important mechanisms to survive under salt stress. The three fundamental biological processes, which face huge challenges under salt stress, are ignored by most scientists and are worth further deep study to provide useful information for breeding economic important plants competent in tolerating salt stress, other than only depending on the commonly acknowledged osmotic balance and ion homeostasis.

The Molecular Mechanism of Potassium Absorption, Transport, and Utilization in Rice

Potassium is essential for plant growth and development and stress adaptation. The maintenance of potassium homeostasis involves a series of potassium channels and transporters, which promote the movement of potassium ions (K+) across cell membranes and exhibit complex expression patterns and regulatory mechanisms. Rice is a major food crop in China. The low utilization rate of potassium fertilizer limits the yield and quality of rice. Elucidating the molecular mechanisms of potassium absorption, transport, and utilization is critical in improving potassium utilization efficiency in rice. Although some K+ transporter genes have been identified from rice, research on the regulatory network is still in its infancy. Therefore, this review summarizes the relevant information on K+ channels and transporters in rice, covering the absorption of K+ in the roots, transport to the shoots, the regulation pathways, the relationship between K+ and the salt tolerance of rice, and the synergistic regulation of potassium, nitrogen, and phosphorus signals. The related research on rice potassium nutrition has been comprehensively reviewed, the existing research foundation and the bottleneck problems to be solved in this field have been clarified, and the follow-up key research directions have been pointed out to provide a theoretical framework for the cultivation of potassium-efficient rice.

Genome-wide identification and expression analysis of Na+/H+antiporter (NHX) genes in tomato under salt stress

Plant Na +/H + antiporter (NHX) genes enhance salt tolerance by preventing excessive Na+ accumulation in the cytosol through partitioning of Na+ ions into vacuoles or extracellular transport across the plasma membrane. However, there is limited detailed information regarding the salt stress responsive SlNHXs in the most recent tomato genome. We investigated the role of this gene family's expression patterns in the open flower tissues under salt shock in Solanum lycopersicum using a genome-wide approach. A total of seven putative SlNHX genes located on chromosomes 1, 4, 6, and 10 were identified, but no ortholog of the NHX5 gene was identified in the tomato genome. Phylogenetic analysis revealed that these genes are divided into three different groups. SlNHX proteins with 10-12 transmembrane domains were hypothetically localized in vacuoles or cell membranes. Promoter analysis revealed that SlNHX6 and SlNHX8 are involved with the stress-related MeJA hormone in response to salt stress signaling. The structural motif analysis of SlNHX1, -2, -3, -4, and -6 proteins showed that they have highly conserved amiloride binding sites. The protein-protein network revealed that SlNHX7 and SlNHX8 interact physically with Salt Overly Sensitive (SOS) pathway proteins. Transcriptome analysis demonstrated that the SlNHX2 and SlNHX6 genes were substantially expressed in the open flower tissues. Moreover, quantitative PCR analysis indicated that all SlNHX genes, particularly SlNHX6 and SlNHX8, are significantly upregulated by salt shock in the open flower tissues. Our results provide an updated framework for future genetic research and development of breeding strategies against salt stress in the tomato.

Differential gene expression patterns and physiological responses improve adaptation to high salinity concentration in pepper accessions

High salinity decreases the productivity of crops worldwide. Pepper is particularly sensitive to high salt concentrations. Herein, we subjected three tolerant pepper accessions (C12, B14 and A25) to high sodium chloride concentration (70 mM NaCl). The aerial and root biomass, leaf and root osmotic potential (Ψπ ), Na+ , Cl- , K+ and proline concentrations and the relative expression of the putative genes CaSOS1, CaHKT1, three CaNHXs and CaP5CS were measured. Different salinity tolerance strategies depending on the pepper accession were identified. In C12, tolerance was attributed to the accumulation of Na+ in vacuoles and endosomes by the activation of vacuolar CaNHXs genes and the reduction in Ψπ ; additionally, the activation of CaHKT1 and CaSOS1 in leaves and roots moved and accumulated Na+ ions in the xylem and xylem parenchyma cells (XPC) as well as expulsed it out of the root cells. A25 accession, on the contrary, was specialized in compartmentalizing Na+ ions in root and leaf vacuoles and root XPC by the up-regulation of CaNHXs and CaHKT1, respectively, avoiding a toxic accumulation in leaves. Finally, B14 accession moved and accumulated Na+ in xylem and XPC, reducing its concentration in roots by the activation of CaSOS1 and CaHKT1. This study shade light on different tolerance mechanisms of pepper plants to overcome salt stress.

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.

A SNARE-like protein from Solanum lycopersicum increases salt tolerance by modulating vesicular trafficking in tomato

Intracellular vesicular trafficking ensures the exchange of lipids and proteins between endomembrane compartments. This is relevant under high salinity conditions, since both the removal of transporters and ion channels from the plasma membrane and the compartmentalization of toxic ions require the formation of vesicles, which can be maintained as multivesicular bodies or be fused to the central vacuole. SNARE proteins (Soluble N-ethylmaleimide-sensitive factor attachment receptor) participate in the vesicle fusion process and give specificity to their destination. Plant genome studies have revealed a superfamily of genes that encode for proteins called SNARE-like. These proteins appear to be participating in vesicular trafficking with similar functions to those of SNARE proteins. A SNARE-like, named SlSLSP6, in Solanum lycopersicum plants has been shown to be induced under high salinity conditions. A phylogenetic relationship of SlSLSP6 with SNARE-like proteins of salinity-tolerant plants, including Salicornia brachiata, Zostera marina and Solanum pennelli, was determined. Considering its amino acid sequence, a putative clathrin adapter complex domain and palmitoylation site was predicted. Subcellular localization analysis evidenced that SlSLSP6 is mostly localized in the plasma membrane. Using transgenic tomato plants, we identified that overexpression of SlSLSP6 increased tolerance to salt stress. This tolerance was evident when we quantified an improvement in physiological and biochemical parameters, such as higher chlorophyll content, performance index, efficiency of photosystem II and relative water content, and lower malondialdehyde content, compared to control plants. At the subcellular level, the overexpression of SlSLSP6 reduced the presence of H2O2 in roots and increased the compartmentalization of sodium in vacuoles during salt stress. These effects appear to be associated with the higher endocytic rate of FM4-64, determined in the plant root cells. Taken together, these results indicate that SlSLSP6 increases tolerance to salt stress by modulating vesicular trafficking through over-induction of the endocytic pathway. This work contributes to understanding the role of this type of SNARE-like protein during salt stress and could be a potential candidate in breeding programs for tolerance to salt stress in tomato plants.

Genome-wide identification of NHX (Na+/H+ antiporter) gene family in Cucurbita L. and functional analysis of CmoNHX1 under salt stress

Soil salinization, which is the accumulation of salt in soil, can have a negative impact on crop growth and development by creating an osmotic stress that can reduce water uptake and cause ion toxicity. The NHX gene family plays an important role in plant response to salt stress by encoding for Na⁺/H⁺ antiporters that help regulate the transport of sodium ions across cellular membranes. In this study, we identified 26 NHX genes in three cultivars of Cucurbita L., including 9 Cucurbita moschata NHXs (CmoNHX1-CmoNHX9), 9 Cucurbita maxima NHXs (CmaNHX1-CmaNHX9) and 8 Cucurbita pepo NHXs (CpNHX1-CpNHX8). The evolutionary tree splits the 21 NHX genes into three subfamilies: the endosome (Endo) subfamily, the plasma membrane (PM) subfamily, and the vacuole (Vac) subfamily. All the NHX genes were irregularly distributed throughout the 21 chromosomes. 26 NHXs were examined for conserved motifs and intron-exon organization. These findings suggested that the genes in the same subfamily may have similar functions while genes in other subfamilies may have functional diversity. The circular phylogenetic tree and collinearity analysis of multi-species revealed that Cucurbita L. had a substantially greater homology relationship than Populus trichocarpa and Arabidopsis thaliana in terms of NHX gene homology. We initially examined the cis-acting elements of the 26 NHXs in order to investigate how they responded to salt stress. We discovered that the CmoNHX1, CmaNHX1, CpNHX1, CmoNHX5, CmaNHX5, and CpNHX5 all had numerous ABRE and G-box cis-acting elements that were important to salt stress. Previous transcriptome data showed that in the mesophyll and veins of leaves, many CmoNHXs and CmaNHXs, such as CmoNHX1, responded significantly to salt stress. In addition, we heterologously expressed in A. thaliana plants in order to further confirm the response of CmoNHX1 to salt stress. The findings demonstrated that during salt stress, A. thaliana that had CmoNHX1 heterologously expression was found to have decreased salt tolerance. This study offers important details that will aid in further elucidating the molecular mechanism of NHX under salt stress.

Identification and Functional Characterization of WRKY, PHD and MYB Three Salt Stress Responsive Gene Families in Mungbean (Vigna radiata L.)

WRKY-, PHD-, and MYB-like proteins are three important types of transcription factors in mungbeans, and play an important role in development and stress resistance. The genes' structures and characteristics were clearly reported and were shown to contain the conservative WRKYGQK heptapeptide sequence, Cys4-His-cys3 zinc binding motif, and HTH (helix) tryptophan cluster W structure, respectively. Knowledge on the response of these genes to salt stress is largely unknown. To address this issue, 83 VrWRKYs, 47 VrPHDs, and 149 VrMYBs were identified by using comparative genomics, transcriptomics, and molecular biology methods in mungbeans. An intraspecific synteny analysis revealed that the three gene families had strong co-linearity and an interspecies synteny analysis showed that mungbean and Arabidopsis were relatively close in genetic relationship. Moreover, 20, 10, and 20 genes showed significantly different expression levels after 15 days of salt treatment (p < 0.05; Log2 FC > 0.5), respectively. Additionally, in the qRT-PCR analysis, VrPHD14 had varying degrees of response to NaCl and PEG treatments after 12 h. VrWRKY49 was upregulated by ABA treatment, especially in the beginning (within 24 h). VrMYB96 was significantly upregulated in the early stages of ABA, NaCl, and PEG stress treatments (during the first 4 h). VrWRKY38 was significantly upregulated by ABA and NaCl treatments, but downregulated by PEG treatment. We also constructed a gene network centered on the seven DEGs under NaCl treatment; the results showed that VrWRKY38 was in the center of the PPI network and most of the homologous Arabidopsis genes of the interacted genes were reported to have response to biological stress. Candidate genes identified in this study provide abundant gene resources for the study of salt tolerance in mungbeans.

Chromosome-level genomes of multicellular algal sisters to land plants illuminate signaling network evolution

The filamentous and unicellular algae of the class Zygnematophyceae are the closest algal relatives of land plants. Inferring the properties of the last common ancestor shared by these algae and land plants allows us to identify decisive traits that enabled the conquest of land by plants. We sequenced four genomes of filamentous Zygnematophyceae (three strains of Zygnema circumcarinatum and one strain of Z. cylindricum) and generated chromosome-scale assemblies for all strains of the emerging model system Z. circumcarinatum. Comparative genomic analyses reveal expanded genes for signaling cascades, environmental response, and intracellular trafficking that we associate with multicellularity. Gene family analyses suggest that Zygnematophyceae share all the major enzymes with land plants for cell wall polysaccharide synthesis, degradation, and modifications; most of the enzymes for cell wall innovations, especially for polysaccharide backbone synthesis, were gained more than 700 million years ago. In Zygnematophyceae, these enzyme families expanded, forming co-expressed modules. Transcriptomic profiling of over 19 growth conditions combined with co-expression network analyses uncover cohorts of genes that unite environmental signaling with multicellular developmental programs. Our data shed light on a molecular chassis that balances environmental response and growth modulation across more than 600 million years of streptophyte evolution.

RAP2.6 enhanced salt stress tolerance by reducing Na+ accumulation and stabilizing the electron transport in Arabidopsis thaliana

The transcription factors of the AP2/ERF family are involved in plant growth and development and responses to biotic and abiotic stresses. Here, we found RAP2.6, a transcription factor which belongs to the ERF subfamily, was responsive to salt stress in Arabidopsis. Under salt stress conditions, rap2.6 mutant seedlings were the sensitivity deficiency to salt stress which was reflected in higher germination rate and longer root length compared to the wild type. Also, the expressions of salt-related gene including SOS1, SOS2, SOS3, NHX1, NHX3, NHX5 and HKT1 in rap2.6 mutant seedlings were lower than the wild type under salt stress. rap2.6 mutant adult lacked salt stress tolerance based on the results of the phenotype, survival rates and ion leakage. Compared to wild type, rap2.6 mutant adult accumulated more Na⁺ in leaves and roots while the salt-related gene expressions were lower. In addition, the photosynthetic electron transport and PSII energy distribution in rap2.6 mutant plant leaves had been more seriously affected under salt stress conditions compared to the wild type. In summary, this study identified essential roles of RAP2.6 in regulating salt stress tolerance in Arabidopsis.

Transcription Factor SiDi19-3 Enhances Salt Tolerance of Foxtail Millet and Arabidopsis

Salt stress is an important limiting factor of crop production. Foxtail millet (Setaria italica L.) is an important model crop for studying tolerance to various abiotic stressors. Therefore, examining the response of foxtail millet to salt stress at the molecular level is critical. Herein, we discovered that SiDi19-3 interacts with SiPLATZ12 to control salt tolerance in transgenic Arabidopsis and foxtail millet seedlings. SiDi19-3 overexpression increased the transcript levels of most Na+/H+ antiporter (NHX), salt overly sensitive (SOS), and calcineurin B-like protein (CBL) genes and improved the salt tolerance of foxtail millet and Arabidopsis. Six SiDi19 genes were isolated from foxtail millet. Compared with roots, stems, and leaves, panicles and seeds had higher transcript levels of SiDi19 genes. All of them responded to salt, alkaline, polyethylene glycol, and/or abscisic acid treatments with enhanced expression levels. These findings indicate that SiDi19-3 and other SiDi19 members regulate salt tolerance and other abiotic stress response in foxtail millet.

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

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

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

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

Function of Rice High-Affinity Potassium Transporters in Pollen Development and Fertility

Plant High-affinity K+ transporters/K+ uptake permeases/K+ transporters (HAK/KUP/KT) transporters have been predicted as membrane H+-K+ symporters in facilitating K+ uptake and distribution, while their role in seed production remains to be elucidated. In this study, we report that OsHAK26 is preferentially expressed in anthers and seed husks and located in the Golgi apparatus. Knockout of either OsHAK26 or plasma membrane located H+-K+ symporter gene OsHAK1 or OsHAK5 in both Nipponbare and Dongjin cultivars caused distorted anthers, reduced number and germination rate of pollen grains. Seed-setting rate assay by reciprocal cross-pollination between the mutants of oshak26, oshak1, oshak5 and their wild types confirmed that each HAK transporter is foremost for pollen viability, seed-setting and grain yield. Intriguingly, the pollens of oshak26 showed much thinner wall and were more vulnerable to desiccation than those of oshak1 or oshak5. In vitro assay revealed that the pollen germination rate of oshak5 was dramatically affected by external K+ concentration. The results suggest that the role of OsHAK26 in maintaining pollen development and fertility may relate to its proper cargo sorting for construction of pollen walls, while the role of OsHAK1 and OsHAK5 in maintaining seed production likely relates to their transcellular K+ transport activity.

Plant endosomes as protein sorting hubs

Endocytosis, secretion, and endosomal trafficking are key cellular processes that control the composition of the plasma membrane. Through the coordination of these trafficking pathways, cells can adjust the composition, localization, and turnover of proteins and lipids in response to developmental or environmental cues. Upon being incorporated into vesicles and internalized through endocytosis, plant plasma membrane proteins are delivered to the trans-Golgi network (TGN). At the TGN, plasma membrane proteins are recycled back to the plasma membrane or transferred to multivesicular endosomes (MVEs), where they are further sorted into intralumenal vesicles for degradation in the vacuole. Both types of plant endosomes, TGN and MVEs, act as sorting organelles for multiple endocytic, recycling, and secretory pathways. Molecular assemblies such as retromer, ESCRT (endosomal sorting complex required for transport) machinery, small GTPases, adaptor proteins, and SNAREs associate with specific domains of endosomal membranes to mediate different sorting and membrane-budding events. In this review, we discuss the mechanisms underlying the recognition and sorting of proteins at endosomes, membrane remodeling and budding, and their implications for cellular trafficking and physiological responses in plants.

The genome of a mangrove plant, Avicennia marina, provides insights into adaptation to coastal intertidal habitats

Whole-genome duplication, gene family and lineage-specific genes analysis based on high-quality genome reveal the adaptation mechanisms of Avicennia marina to coastal intertidal habitats. Mangrove plants grow in a complex habitat of coastal intertidal zones with high salinity, hypoxia, etc. Therefore, it is an interesting question how mangroves adapt to the unique intertidal environment. Here, we present a chromosome-level genome of the Avicennia marina, a typical true mangrove with a size of 480.43 Mb, contig N50 of 11.33 Mb and 30,956 annotated protein-coding genes. We identified 621 Avicennia-specific genes that are mainly related to flavonoid and lignin biosynthesis, auxin homeostasis and response to abiotic stimulus. We found that A. marina underwent a novel specific whole-genome duplication, which is in line with a brief era of global warming that occurred during the paleocene-eocene maximum. Comparative genomic and transcriptomic analyses outline the distinct evolution and sophisticated regulations of A. marina adaptation to the intertidal environments, including expansion of photosynthesis and oxidative phosphorylation gene families, unique genes and pathways for antibacterial, detoxifying antioxidant and reactive oxygen species scavenging. In addition, we also analyzed salt gland secretion-related genes, and those involved in the red bark-related flavonoid biosynthesis, while significant expansions of key genes such as NHX, 4CL, CHS and CHI. High-quality genomes in future investigations will facilitate the understand of evolution of mangrove and improve breeding.

Transcriptomic Analysis Reveals LncRNAs Associated with Flowering of Angelica sinensis during Vernalization

Angelica sinensis is a “low-temperature and long-day” perennial plant that produces bioactive compounds such as phthalides, organic acids, and polysaccharides for various types of clinical agents, including those with cardio-cerebrovascular, hepatoprotective, and immunomodulatory effects. To date, the regulatory mechanism of flowering under the photoperiod has been revealed, while the regulatory network of flowering genes during vernalization, especially in the role of lncRNAs, has yet to be identified. Here, lncRNAs associated with flowering were identified based on the full-length transcriptomic analysis of A. sinensis at vernalization and freezing temperatures, and the coexpressed mRNAs of lncRNAs were validated by qRT-PCR. We obtained a total of 2327 lncRNAs after assessing the protein-coding potential of coexpressed mRNAs, with 607 lncRNAs aligned against the TAIR database of model plant Arabidopsis, 345 lncRNAs identified, and 272 lncRNAs characterized on the SwissProt database. Based on the biological functions of coexpressed mRNAs, the 272 lncRNAs were divided into six categories: (1) chromatin, DNA/RNA and protein modification; (2) flowering; (3) stress response; (4) metabolism; (5) bio-signaling; and (6) energy and transport. The differential expression levels of representatively coexpressed mRNAs were almost consistent with the flowering of A. sinensis. It can be concluded that the flowering of A. sinensis is positively or negatively regulated by lncRNAs, which provides new insights into the regulation mechanism of the flowering of A. sinensis.

Transcriptomic Analysis Elaborates the Resistance Mechanism of Grapevine Rootstocks against Salt Stress

Grapes are subject to a wide range of climatic conditions during their life cycle, but the use of rootstocks can effectively ameliorate the effects of abiotic stress. However, the tolerance mechanism of different grape rootstock varieties varies under various stresses, and systematic research on this aspect is limited. On the basis of previous research, transcriptome sequencing was performed on three tolerant grape rootstock varieties (3309C, 520A, 1103P) and three intolerant grape rootstock varieties (5BB, 101-14, Beta). In total, 56,478,468 clean reads were obtained. One hundred and ten genes only existed in all combinations during P1 with a downregulated trend, and 178 genes existed only in P1 of tolerant grape rootstock varieties. Salt treatment firstly affected the photosynthesis of leaves, and tolerant varieties weakened or even eliminated this effect through their own mechanisms in the later stage. Tolerant varieties mobilized a large number of MFs during the P2 stage, such as hydrolase activity, carboxypeptidase activity, and dioxygenase activity. Carbon metabolism was significantly enriched in P1, while circadian rhythm and flavonoid biosynthesis were only enriched in tolerant varieties. In the intolerant varieties, photosynthesis-related pathways were always the most significantly enriched. There were large differences in the gene expression of the main signal pathways related to salt stress in different varieties. Salt stress affected the expression of genes related to plant abiotic stress, biotic stress, transcription factors, hormones, and secondary metabolism. Tolerant varieties mobilized more bHLH, WRKY, and MYB transcription factors to respond to salt stress than intolerant varieties. In the tolerant rootstocks, SOS was co-expressed. Among these, SOS1 and SOS2 were upregulated, and the SOS3 and SOS5 components were downregulated. The genes of heat shock proteins and the phenylalanine pathway were upregulated in the tolerant varieties. These findings outline a tolerance mechanism model for rootstocks for coping with osmotic stress, providing important information for improving the resistance of grapes under global climate change.

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.

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

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

The DROL1 subunit of U5 snRNP in the spliceosome is specifically required to splice AT-AC-type introns in Arabidopsis

An Arabidopsis mutant named defective repression of OLE3::LUC 1 (drol1) was originally isolated as a mutant with defects in the repression of OLEOSIN3 (OLE3) after seed germination. In this study, we show that DROL1 is an Arabidopsis homolog of yeast DIB1, a subunit of the U5 small nuclear ribonucleoprotein particle (snRNP) in the spliceosome. It is also part of a new subfamily that is specific to a certain class of eukaryotes. Comprehensive analysis of the intron splicing using RNA sequencing analysis of the drol1 mutants revealed that most of the minor introns with AT-AC dinucleotide termini had reduced levels of splicing. Only two nucleotide substitutions from AT-AC to GT-AG enabled AT-AC-type introns to be spliced in drol1 mutants. Forty-eight genes, including those having important roles in abiotic stress responses and cell proliferation, exhibited reduced splicing of AT-AC-type introns in the drol1 mutants. Additionally, drol1 mutant seedlings showed retarded growth, similar to that caused by the activation of abscisic acid signaling, possibly as a result of reduced AT-AC-type intron splicing in the endosomal Na⁺ /H⁺ antiporters and plant-specific histone deacetylases. These results indicate that DROL1 is specifically involved in the splicing of minor introns with AT-AC termini and that this plays an important role in plant growth and development.

Plant Trans-Golgi Network/Early Endosome pH regulation requires Cation Chloride Cotransporter (CCC1)

Plant cells maintain a low luminal pH in the trans-Golgi-network/early endosome (TGN/EE), the organelle in which the secretory and endocytic pathways intersect. Impaired TGN/EE pH regulation translates into severe plant growth defects. The identity of the proton pump and proton/ion antiporters that regulate TGN/EE pH have been determined, but an essential component required to complete the TGN/EE membrane transport circuit remains unidentified − a pathway for cation and anion efflux. Here, we have used complementation, genetically encoded fluorescent sensors, and pharmacological treatments to demonstrate that Arabidopsis cation chloride cotransporter (CCC1) is this missing component necessary for regulating TGN/EE pH and function. Loss of CCC1 function leads to alterations in TGN/EE-mediated processes including endocytic trafficking, exocytosis, and response to abiotic stress, consistent with the multitude of phenotypic defects observed in ccc1 knockout plants. This discovery places CCC1 as a central component of plant cellular function.

Integrative transcriptome and proteome analyses provide deep insights into the molecular mechanism of salt tolerance in Limonium bicolor

Integrative transcriptome and proteome analyses revealed many candidate members that may involve in salt secretion from salt glands in Limonium bicolor. Limonium bicolor, a typical recretohalophyte, protects itself from salt damage by excreting excess salt out of its cells through salt glands. Here, to provide an overview of the salt-tolerance mechanism of L. bicolor, we conducted integrative transcriptome and proteome analyses of this species under salt treatment. We identified numerous differentially expressed transcripts and proteins that may be related to the salt-tolerance mechanism of L. bicolor. By measuring the Na⁺ secretion rate, were found that this cation secretion rate of a single salt gland was significantly increased after high salinity treatment compared with that in control and then reached the maximum in a short time. Interestingly, transcripts and proteins involved in transmembrane transport of ions were differentially expressed in response to high salinity treatment, suggesting a number of genes and proteins they may play important roles in the salt-stress response. Correlation between differentially expressed transcript and protein profiles revealed several transcripts and proteins that may be responsible for salt tolerance, such as cellulose synthases and annexins. Our findings uncovered many candidate transcripts and proteins in response to the salt tolerance of L. bicolor, providing deep insights into the molecular mechanisms of this important process in recretohalophytes.

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.

Genetic, Epigenetic, Genomic and Microbial Approaches to Enhance Salt Tolerance of Plants: A Comprehensive Review

Simple Summary

Globally, soil salinity, which refers to salt-affected soils, is increasing due to various environmental factors and human activities. Soil salinity poses one of the most serious challenges in the field of agriculture as it significantly reduces the growth and yield of crop plants, both quantitatively and qualitatively. Over the last few decades, several studies have been carried out to understand plant biology in response to soil salinity stress with a major emphasis on genetic and other hereditary components. Based on the outcome of these studies, several approaches are being followed to enhance plants’ ability to tolerate salt stress while still maintaining reasonable levels of crop yields. In this manuscript, we comprehensively list and discuss various biological approaches being followed and, based on the recent advances in the field of molecular biology, we propose some new approaches to improve salinity tolerance of crop plants. The global scientific community can make use of this information for the betterment of crop plants. This review also highlights the importance of maintaining global soil health to prevent several crop plant losses.

Abstract

Globally, soil salinity has been on the rise owing to various factors that are both human and environmental. The abiotic stress caused by soil salinity has become one of the most damaging abiotic stresses faced by crop plants, resulting in significant yield losses. Salt stress induces physiological and morphological modifications in plants as a result of significant changes in gene expression patterns and signal transduction cascades. In this comprehensive review, with a major focus on recent advances in the field of plant molecular biology, we discuss several approaches to enhance salinity tolerance in plants comprising various classical and advanced genetic and genetic engineering approaches, genomics and genome editing technologies, and plant growth-promoting rhizobacteria (PGPR)-based approaches. Furthermore, based on recent advances in the field of epigenetics, we propose novel approaches to create and exploit heritable genome-wide epigenetic variation in crop plants to enhance salinity tolerance. Specifically, we describe the concepts and the underlying principles of epigenetic recombinant inbred lines (epiRILs) and other epigenetic variants and methods to generate them. The proposed epigenetic approaches also have the potential to create additional genetic variation by modulating meiotic crossover frequency.

ClCd and ClCf act redundantly at the trans-Golgi network/early endosome and prevent acidification of the Golgi stack

The trans-Golgi network/early endosome (TGN/EE) serves as the central hub in which exocytic and endocytic trafficking pathways converge and specificity of cargo routing needs to be achieved. Acidification is a hallmark of the TGN/EE and is maintained by the vacuolar H+-ATPase (V-ATPase) with support of proton-coupled antiporters. We show here that ClCd and ClCf, two distantly related members of the Arabidopsis Cl- channel (ClC) family, colocalize in the TGN/EE, where they act redundantly, and are essential for male gametophyte development. Combining an inducible knockdown approach and in vivo pH measurements, we show here that reduced ClC activity does not affect pH in the TGN/EE but causes hyperacidification of trans-Golgi cisternae. Taken together, our results show that ClC-mediated anion transport into the TGN/EE is essential and affects spatiotemporal aspects of TGN/EE maturation as well as its functional separation from the Golgi stack.

Arbuscular mycorrhizal symbioses alleviating salt stress in maize is associated with a decline in root-to-leaf gradient of Na+/K+ ratio

BACKGROUND

Inoculation of arbuscular mycorrhizal (AM) fungi has the potential to alleviate salt stress in host plants through the mitigation of ionic imbalance. However, inoculation effects vary, and the underlying mechanisms remain unclear. Two maize genotypes (JD52, salt-tolerant with large root system, and FSY1, salt-sensitive with small root system) inoculated with or without AM fungus Funneliformis mosseae were grown in pots containing soil amended with 0 or 100 mM NaCl (incrementally added 32 days after sowing, DAS) in a greenhouse. Plants were assessed 59 DAS for plant growth, tissue Na⁺ and K⁺ contents, the expression of plant transporter genes responsible for Na⁺ and/or K⁺ uptake, translocation or compartmentation, and chloroplast ultrastructure alterations.

RESULTS

Under 100 mM NaCl, AM plants of both genotypes grew better with denser root systems than non-AM plants. Relative to non-AM plants, the accumulation of Na⁺ and K⁺ was decreased in AM plant shoots but increased in AM roots with a decrease in the shoot: root Na⁺ ratio particularly in FSY1, accompanied by differential regulation of ion transporter genes (i.e., ZmSOS1, ZmHKT1, and ZmNHX). This induced a relatively higher Na⁺ efflux (recirculating) rate than K⁺ in AM shoots while the converse outcoming (higher Na⁺ influx rate than K⁺) in AM roots. The higher K⁺: Na⁺ ratio in AM shoots contributed to the maintenance of structural and functional integrity of chloroplasts in mesophyll cells.

CONCLUSION

AM symbiosis improved maize salt tolerance by accelerating Na⁺ shoot-to-root translocation rate and mediating Na⁺/K⁺ distribution between shoots and roots.

Identifying the genetic control of salinity tolerance in the bread wheat landrace Mocho de Espiga Branca

Salinity tolerance in bread wheat is frequently reported to be associated with low leaf sodium (Na+) concentrations. However, the Portuguese landrace, Mocho de Espiga Branca, accumulates significantly higher leaf Na+ but has comparable salinity tolerance to commercial bread wheat cultivars. To determine the genetic loci associated with the salinity tolerance of this landrace, an F2 mapping population was developed by crossing Mocho de Espiga Branca with the Australian cultivar Gladius. The population was phenotyped for 19 salinity tolerance subtraits using both non-destructive and destructive techniques. Genotyping was performed using genotyping-by-sequencing (GBS). Genomic regions associated with salinity tolerance were detected on chromosomes 1A, 1D, 4B and 5A for the subtraits of relative and absolute growth rate (RGR, AGR respectively), and on chromosome 2A, 2B, 4D and 5D for Na+, potassium (K+) and chloride (Cl-) accumulation. Candidate genes that encode proteins associated with salinity tolerance were identified within the loci including Na+/H+ antiporters, K+ channels, H+-ATPase, calcineurin B-like proteins (CBLs), CBL-interacting protein kinases (CIPKs), calcium dependent protein kinases (CDPKs) and calcium-transporting ATPase. This study provides a new insight into the genetic control of salinity tolerance in a Na+ accumulating bread wheat to assist with the future development of salt tolerant cultivars.

Identification and characterisation of monovalent cation/proton antiporters (CPAs) in Phyllostachys edulis and the functional analysis of PheNHX2 in Arabidopsis thaliana

Plant monovalent cation/proton antiporters (CPAs), types of transmembrane transporters, play important roles in resistance to salt stress. In this study, 37 CPA genes from moso bamboo (Phyllostachys edulis) were identified and characterised. The expression profiles of 10 CPA1 genes (PheNHXs) of moso bamboo were detected by qRT-PCR, which showed that they were specifically expressed in six tissues. In addition, the expression of 10 PheNHXs in leaves and roots changed significantly under 150/200 mM NaCl and 100 μM ABA treatments. In particular, the expression of PheNHX2 in leaves and roots was significantly upregulated under NaCl treatment, thus, we cloned PheNHX2 and analysed its function. Subcellular localisation analysis showed that PheNHX2 was located on the vacuolar membrane. Overexpression of PheNHX2 reduced seed germination and root growth of Arabidopsis thaliana under salt stress, as well as severely affecting cellular Na⁺ and K⁺ content, which in turn reduced the salt tolerance of transgenic Arabidopsis. Measurements of physiological indicators, including chlorophyll content, malondialdehyde content, peroxidase and catalase enzyme activities and relative electrical conductivity, all supported this conclusion. Under salt stress, PheNHX2 also inhibited the expression of some stress-related and ion transport-related genes in transgenic Arabidopsis. Overall, these results indicate that overexpression of PheNHX2 reduces the salt tolerance of transgenic Arabidopsis. This investigation establishes a foundation for subsequent functional studies of moso bamboo CPA genes, and it provides a deeper understanding of PheNHX2 regulation in relation to the salt tolerance of moso bamboo.

Physiological and Biochemical Traits of Two Major Arabidopsis Accessions, Col-0 and Ws, Under Salinity

Salinity affects plant growth and development as shown with the glycophyte model plant, Arabidopsis thaliana (Arabidopsis). Two Arabidopsis accessions, Wassilewskija (Ws) and Columbia (Col-0), are widely used to generate mutants available from various Arabidopsis seed resources. However, these two ecotypes are known to be salt-sensitive with different degrees of tolerance. In our study, 3-week-old Col-0 and Ws plants were treated with and without 150 mM NaCl for 48, 72, or 96 h, and several physiological and biochemical traits were characterized on shoots to identify any specific traits in their tolerance to salinity. Before salt treatment was carried out, a different phenotype was observed between Col-0 and Ws, whose main inflorescence stem became elongated in contrast to Col-0, which only displayed rosette leaves. Our results showed that Col-0 and Ws were both affected by salt stress with limited growth associated with a reduction in nutrient uptake, a degradation of photosynthetic pigments, an increase in protein degradation, as well as showing changes in carbohydrate metabolism and cell wall composition. These traits were often more pronounced in Col-0 and occurred usually earlier than in Ws. Tandem Mass Tags quantitative proteomics data correlated well with the physiological and biochemical results. The Col-0 response to salt stress was specifically characterized by a greater accumulation of osmoprotectants such as anthocyanin, galactinol, and raffinose; a lower reactive oxygen detoxification capacity; and a transient reduction in galacturonic acid content. Pectin degradation was associated with an overaccumulation of the wall-associated kinase 1, WAK1, which plays a role in cell wall integrity (CWI) upon salt stress exposure. Under control conditions, Ws produced more antioxidant enzymes than Col-0. Fewer specific changes occurred in Ws in response to salt stress apart from a higher number of different fascilin-like arabinogalactan proteins and a greater abundance of expansin-like proteins, which could participate in CWI. Altogether, these data indicate that Col-0 and Ws trigger similar mechanisms to cope with salt stress, and specific changes are more likely related to the developmental stage than to their respective genetic background.

Potential Networks of Nitrogen-Phosphorus-Potassium Channels and Transporters in Arabidopsis Roots at a Single Cell Resolution

Nitrogen (N), phosphorus (P), and potassium (K) are three major macronutrients essential for plant life. These nutrients are acquired and transported by several large families of transporters expressed in plant roots. However, it remains largely unknown how these transporters are distributed in different cell-types that work together to transfer the nutrients from the soil to different layers of root cells and eventually reach vasculature for massive flow. Using the single cell transcriptomics data from Arabidopsis roots, we profiled the transcriptional patterns of putative nutrient transporters in different root cell-types. Such analyses identified a number of uncharacterized NPK transporters expressed in the root epidermis to mediate NPK uptake and distribution to the adjacent cells. Some transport genes showed cortex- and endodermis-specific expression to direct the nutrient flow toward the vasculature. For long-distance transport, a variety of transporters were shown to express and potentially function in the xylem and phloem. In the context of subcellular distribution of mineral nutrients, the NPK transporters at subcellular compartments were often found to show ubiquitous expression patterns, which suggests function in house-keeping processes. Overall, these single cell transcriptomic analyses provide working models of nutrient transport from the epidermis across the cortex to the vasculature, which can be further tested experimentally in the future.

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.

The apple palmitoyltransferase MdPAT16 influences sugar content and salt tolerance via an MdCBL1-MdCIPK13-MdSUT2.2 pathway

Protein S-acyltransferases (PATs) are a category of eukaryotic transmembrane proteins that mediate the S-acylation of their target proteins. S-acylation, commonly known as palmitoylation, is a reversible protein modification that regulates the membrane association and function of target proteins. However, the functions and mechanisms of PATs in apple (Malus domestica) remain poorly understood. In this study, an MdPAT family member, MdPAT16, was identified and shown to have palmitoyltransferase activity. We demonstrated that this gene responds to salt stress and that its expression improves plant salt stress resistance. In addition, its overexpression significantly promotes the accumulation of soluble sugars. The same phenotypes were observed in transgenic tissue culture seedlings, transgenic roots, and Arabidopsis thaliana that ectopically expressed MdPAT16. MdPAT16 was shown to interact with MdCBL1 and stabilize MdCBL1 protein levels through palmitoylation. The N-terminal sequence of MdCBL1 contains a palmitoylation site, and its N-terminal deletion led to changes in MdCBL1 protein stability and subcellular localization. The phenotypes of MdCBL1 transgenic roots and transiently injected apple fruits were fully consistent with the sugar accumulation phenotype of MdPAT16. Mutation of the palmitoylation site interfered with this phenotype. These findings suggest that MdPAT16 palmitoylates its downstream target proteins, improving their stability. This may be a missing link in the plant salt stress response pathway and have an important impact on fruit quality.

Genome-wide identification, characterization, and expression analysis of the monovalent cation-proton antiporter superfamily in maize, and functional analysis of its role in salt tolerance

Na⁺, K⁺ and pH homeostasis are important for plant life and they are controlled by the monovalent cation proton antiporter (CPA) superfamily. The roles of ZmCPAs in salt tolerance are not fully elucidated. In this study, we identified 35 ZmCPAs comprising 13 Na⁺/H⁺ exchangers (ZmNHXs), 16 cation/H⁺ exchanger (ZmCHXs), and 6 K⁺ efflux antiporters (ZmKEAs). All ZmCPAs have transmembrane domains and most of them were localized to plasma membrane or tonoplast. ZmCHXs were specifically highly expressed in anthers, while ZmNHXs and ZmKEAs showed high expression in various tissues. ZmNHX5 and ZmKEA2 were up-regulated in maize seedlings under both NaCl and KCl stresses. Yeast complementation experiments revealed the roles of ZmNHX5, ZmKEA2 in NaCl tolerance. Analysis of the maize mutants further validated the salt tolerance functions of ZmNHX5 and ZmKEA2. Our study highlights comprehensive information of ZmCPAs and provides new gene targets for salt tolerance maize breeding.