Structural basis for the activity regulation of Salt Overly Sensitive 1 in Arabidopsis salt tolerance

A putative Na+/H+ antiporter BpSOS1 contributes to salt tolerance in birch

White birch (Betula platyphylla Suk.) is an important pioneer tree which plays a critical role in maintaining ecosystem stability and forest regeneration. The growth of birch is dramatically inhibited by salt stress, especially the root inhibition. Salt Overly Sensitive 1 (SOS1) is the only extensively characterized Na+ efflux transporter in multiple plant species. The salt-hypersensitive mutant, sos1, display significant inhibition of root growth by NaCl. However, the role of SOS1 in birch responses to salt stress remains unclear. Here, we characterized a putative Na+/H+ antiporter BpSOS1 in birch and generated the loss-of-function mutants of the birch BpSOS1 by CRISPR/Cas9 approach. The bpsos1 mutant exhibit exceptional increased salt sensitivity which links to excessive Na+ accumulation in root, stem and old leaves. We observed a dramatic reduction of K+ contents in leaves of the bpsos1 mutant plants under salt stress. Furthermore, the Na+/K+ ratio of roots and leaves is significant higher in the bpsos1 mutants than the wild-type plants under salt stress. The ability of Na+ efflux in the root meristem zone is found to be impaired which might result the imbalance of Na+ and K+ in the bpsos1 mutants. Our findings indicate that the Na+/H+ exchanger BpSOS1 plays a critical role in birch salt tolerance by maintaining Na+ homeostasis and provide evidence for molecular breeding to improve salt tolerance in birch and other trees.

SOS1 gene family in mangrove (Kandelia obovata): Genome-wide identification, characterization, and expression analyses under salt and copper stress

BACKGROUND

Salt Overly Sensitive 1 (SOS1), a plasma membrane Na+/H+ exchanger, is essential for plant salt tolerance. Salt damage is a significant abiotic stress that impacts plant species globally. All living organisms require copper (Cu), a necessary micronutrient and a protein cofactor for many biological and physiological processes. High Cu concentrations, however, may result in pollution that inhibits the growth and development of plants. The function and production of mangrove ecosystems are significantly impacted by rising salinity and copper contamination.

RESULTS

A genome-wide analysis and bioinformatics techniques were used in this study to identify 20 SOS1 genes in the genome of Kandelia obovata. Most of the SOS1 genes were found on the plasma membrane and dispersed over 11 of the 18 chromosomes. Based on phylogenetic analysis, KoSOS1s can be categorized into four groups, similar to Solanum tuberosum. Kandelia obovata's SOS1 gene family expanded due to tandem and segmental duplication. These SOS1 homologs shared similar protein structures, according to the results of the conserved motif analysis. The coding regions of 20 KoSOS1 genes consist of amino acids ranging from 466 to 1221, while the exons include amino acids ranging from 3 to 23. In addition, we found that the 2.0 kb upstream promoter region of the KoSOS1s gene contains several cis-elements associated with phytohormones and stress responses. According to the expression experiments, seven randomly chosen genes experienced up- and down-regulation of their expression levels in response to copper (CuCl2) and salt stressors.

CONCLUSIONS

For the first time, this work systematically identified SOS1 genes in Kandelia obovata. Our investigations also encompassed physicochemical properties, evolution, and expression patterns, thereby furnishing a theoretical framework for subsequent research endeavours aimed at functionally characterizing the Kandelia obovata SOS1 genes throughout the life cycle of plants.

Structure and mechanism of the osmoregulated choline transporter BetT

The choline-glycine betaine pathway plays an important role in bacterial survival in hyperosmotic environments. Osmotic activation of the choline transporter BetT promotes the uptake of external choline for synthesizing the osmoprotective glycine betaine. Here, we report the cryo-electron microscopy structures of Pseudomonas syringae BetT in the apo and choline-bound states. Our structure shows that BetT forms a domain-swapped trimer with the C-terminal domain (CTD) of one protomer interacting with the transmembrane domain (TMD) of a neighboring protomer. The substrate choline is bound within a tryptophan prism at the central part of TMD. Together with functional characterization, our results suggest that in Pseudomonas species, including the plant pathogen P. syringae and the human pathogen Pseudomonas aeruginosa, BetT is locked at a low-activity state through CTD-mediated autoinhibition in the absence of osmotic stress, and its hyperosmotic activation involves the release of this autoinhibition.

Overexpression of a plasmalemma Na+/H+ antiporter from the halophyte Nitraria sibirica enhances the salt tolerance of transgenic poplar

The plasmalemma Na+/H+ antiporter Salt Overly Sensitive 1 (SOS1) is responsible for the efflux of Na+ from the cytoplasm, an important determinant of salt resistance in plants. In this study, an ortholog of SOS1, referred to as NsSOS1, was cloned from Nitraria sibirica, a typical halophyte that grows in deserts and saline-alkaline land, and its expression and function in regulating the salt tolerance of forest trees were evaluated. The expression level of NsSOS1 was higher in leaves than in roots and stems of N. sibirica, and its expression was upregulated under salt stress. Histochemical staining showed that β-glucuronidase (GUS) driven by the NsSOS1 promoter was strongly induced by abiotic stresses and phytohormones including salt, drought, low temperature, gibberellin, and methyl jasmonate, suggesting that NsSOS1 is involved in the regulation of multiple signaling pathways. Transgenic 84 K poplar (Populus alba × P. glandulosa) overexpressing NsSOS1 showed improvements in survival rate, root biomass, plant height, relative water levels, chlorophyll and proline levels, and antioxidant enzyme activities versus non-transgenic poplar (NT) under salt stress. Transgenic poplars accumulated less Na+ and more K+ in roots, stems, and leaves, which had a lower Na+/K+ ratio compared to NT under salt stress. These results indicate that NsSOS1-mediated Na+ efflux confers salt tolerance to transgenic poplars, which show more efficient photosynthesis, better scavenging of reactive oxygen species, and improved osmotic adjustment under salt stress. Transcriptome analysis of transgenic poplars confirmed that NsSOS1 not only mediates Na+ efflux but is also involved in the regulation of multiple metabolic pathways. The results provide insight into the regulatory mechanisms of NsSOS1 and suggest that it could be used to improve the salt tolerance of forest trees.

GhCLCc-1, a Chloride Channel Gene from Upland Cotton, Positively Regulates Salt Tolerance by Modulating the Accumulation of Chloride Ions

The ionic toxicity induced by salinization has adverse effects on the growth and development of crops. However, researches on ionic toxicity and salt tolerance in plants have focused primarily on cations such as sodium ions (Na+), with very limited studies on chloride ions (Cl-). Here, we cloned the homologous genes of Arabidopsis thaliana AtCLCc, GhCLCc-1A/D, from upland cotton (Gossypium hirsutum), which were significantly induced by NaCl or KCl treatments. Subcellular localization showed that GhCLCc-1A/D were both localized to the tonoplast. Complementation of Arabidopsis atclcc mutant with GhCLCc-1 rescued its salt-sensitive phenotype. In addition, the silencing of the GhCLCc-1 gene led to an increased accumulation of Cl- in the roots, stems, and leaves of cotton seedlings under salt treatments, resulting in compromised salt tolerance. And ectopic expression of the GhCLCc-1 gene in Arabidopsis reduced the accumulation of Cl- in transgenic lines under salt treatments, thereby enhancing salt tolerance. These findings elucidate that GhCLCc-1 positively regulates salt tolerance by modulating Cl- accumulation and could be a potential target gene for improving salt tolerance in plants.

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

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

Inverse regulation of SOS1 and HKT1 protein localization and stability by SOS3/CBL4 in Arabidopsis thaliana

To control net sodium (Na+) uptake, Arabidopsis plants utilize the plasma membrane (PM) Na+/H+ antiporter SOS1 to achieve Na+ efflux at the root and Na+ loading into the xylem, and the channel-like HKT1;1 protein that mediates the reverse flux of Na+ unloading off the xylem. Together, these opposing transport systems govern the partition of Na+ within the plant yet they must be finely co-regulated to prevent a futile cycle of xylem loading and unloading. Here, we show that the Arabidopsis SOS3 protein acts as the molecular switch governing these Na+ fluxes by favoring the recruitment of SOS1 to the PM and its subsequent activation by the SOS2/SOS3 kinase complex under salt stress, while commanding HKT1;1 protein degradation upon acute sodic stress. SOS3 achieves this role by direct and SOS2-independent binding to previously unrecognized functional domains of SOS1 and HKT1;1. These results indicate that roots first retain moderate amounts of salts to facilitate osmoregulation, yet when sodicity exceeds a set point, SOS3-dependent HKT1;1 degradation switches the balance toward Na+ export out of the root. Thus, SOS3 functionally links and co-regulates the two major Na+ transport systems operating in vascular plants controlling plant tolerance to salinity.