HKT1 and SOS1 are two key Na⁺ transporters that modulate salt tolerance in plants. Although much is known about the respective functions of HKT1 and SOS1 under salt conditions, few studies have examined the effects of HKT1 and SOS1 mutations on the expression of other important Na⁺ and K⁺ transporter genes. This study investigated the physiological parameters and expression profiles of AtHKT1;1, AtSOS1, AtHAK5, AtAKT1, AtSKOR, AtNHX1, and AtAVP1 in wild-type (WT) and athkt1;1 and atsos1 mutants of Arabidopsis thaliana under 25 mM NaCl. We found that AtSOS1 mutation induced a significant decrease in transcripts of AtHKT1;1 (by 56–62% at 6–24 h), AtSKOR (by 36–78% at 6–24 h), and AtAKT1 (by 31–53% at 6–24 h) in the roots compared with WT. This led to an increase in Na⁺ accumulation in the roots, a decrease in K⁺ uptake and transportation, and finally resulted in suppression of plant growth. AtHKT1;1 loss induced a 39–76% (6–24 h) decrease and a 27–32% (6–24 h) increase in transcripts of AtSKOR and AtHAK5, respectively, in the roots compared with WT. At the same time, 25 mM NaCl decreased the net selective transport capacity for K⁺ over Na⁺ by 92% in the athkt1;1 roots compared with the WT roots. Consequently, Na⁺ was loaded into the xylem and delivered to the shoots, whereas K⁺ transport was restricted. The results indicate that AtHKT1;1 and AtSOS1 not only mediate Na⁺ transport but also control ion uptake and the spatial distribution of Na⁺ and K⁺ by cooperatively regulating the expression levels of relevant Na⁺ and K⁺ transporter genes, ultimately regulating plant growth under salt stress.

Chloride (Cl–) is pervasive in saline soils, and research on its influence on plants has mainly focused on its role as an essential nutrient and its toxicity when excessive accumulation occurs. However, the possible functions of Cl– in plants adapting to abiotic stresses have not been well documented. Previous studies have shown that the salt tolerance of the xerophytic species Pugionium cornutum might be related to high Cl– accumulation. In this study, we investigated the Cl–-tolerant characteristics and possible physiological functions of Cl– in the salt tolerance and drought resistance of P. cornutum. We found that P. cornutum can accumulate a large amount of Cl– in its shoots, facilitating osmotic adjustment and turgor generation under saline conditions. Application of DIDS (4,4´-diisothiocyanostilbene-2,2´-disulfonic acid), a blocker of anion channels, significantly inhibited Cl– uptake, and decreased both the Cl– content and its contribution to leaf osmotic adjustment, resulting in the exacerbation of growth inhibition in response to NaCl. Unlike glycophytes, P. cornutum was able to maintain NO3– homeostasis in its shoots when large amounts of Cl– were absorbed and accumulated. The addition of NaCl mitigated the deleterious effects of osmotic stress on P. cornutum because Cl– accumulation elicited a strong osmotic adjustment capacity. These findings suggest that P. cornutum is a Cl–-tolerant species that can absorb and accumulate Cl– to improve growth under salt and drought stresses.

Summary: Uncovering the mechanisms underlying stress‐resistant traits in xerophytes thriving in harsh environments can aid the genetic improvement of crops. The xerophyte Zygophyllum xanthoxylum features high Na⁺ accumulation in leaves, mediated by the vacuolar antiporter ZxNHX1. Co‐expression of ZxNHX1 and vacuolar H⁺‐PPase gene ZxVP1‐1 has been demonstrated to enhance the stress resistance and biomass of alfalfa. However, it remains unknown if ZxNHX1 outperforms its homologues from the Na⁺‐excluding and stress‐sensitive glycophytes such as Arabidopsis in enhancing plant stress resistance and yield. Here, we found that expression of ZxNHX1 conferred superior growth under salt stress in alfalfa, compared to the Arabidopsis homologue AtNHX1. When expressed in yeast, ZxNHX1 displays stronger Na⁺/H⁺ but weaker K⁺/H⁺ exchange activity than AtNHX1. Under both K⁺ sufficient and deficient conditions, an Arabidopsis atnhx1‐1 mutant expressing ZxNHX1 accumulated higher Na⁺ and lower K⁺ concentrations, with more Na⁺ being sequestered into vacuoles and a larger proportion of K⁺ retained in the cytosol. This optimized cellular ion distribution ensures energy‐conserving osmotic adjustment, leading to stronger stress resistance and higher biomass than plants expressing AtNHX1. Moreover, ZxNHX1 governed the root uptake and root‐to‐leaf transport of Na⁺ at the whole‐plant level, whereas AtNHX1 acted mainly in K⁺ transport processes. We also identified a polar residue Thr265 in a membrane‐spanning region of ZxNHX1 that influences its Na⁺ and K⁺ selectivity. These findings reveal a new energy‐conserving, Na⁺‐based osmotic adjustment mechanism that can enhance crop stress resistance without sacrificing yield, providing an effective way for utilizing saline soils to expand crop production into marginal lands.

Revealing the genetic basis for stress-resistant traits in extremophile plants will yield important information for crop improvement. Zygophyllum xanthoxylum, an extant species of the ancient Mediterranean, is a succulent xerophyte that can maintain a favorable water status under desert habitats; however, the genetic basis of this adaptive trait is poorly understood. Furthermore, the phylogenetic position of Zygophyllales, to which Z. xanthoxylum belongs, remains controversial. In this study, we sequenced and assembled the chromosome-level genome of Z. xanthoxylum. Phylogenetic analysis showed that Zygophyllales and Myrtales form a separated taxon as a sister to the clade comprising fabids and malvids, clarifying the phylogenetic position of Zygophyllales at whole-genome scale. Analysis of genomic and transcriptomic data revealed multiple critical mechanisms underlying the efficient osmotic adjustment using Na⁺ and K⁺ as ‘cheap’ osmolytes that Z. xanthoxylum has evolved through the expansion and synchronized expression of genes encoding key transporters/channels and their regulators involved in Na⁺/K⁺ uptake, transport, and compartmentation. It is worth noting that ZxCNGC1; 1 (cyclic nucleotide-gated channels) and ZxCNGC1; 2 constituted a previously undiscovered energy-saving pathway for Na⁺ uptake. Meanwhile, the core genes involved in biosynthesis of cuticular wax also featured an expansion and up-regulated expression, contributing to the water retention capacity of Z. xanthoxylum under desert environments. Overall, these findings boost the understanding of evolutionary relationships of eudicots, illustrate the unique water retention mechanism in the succulent xerophyte that is distinct from glycophyte, and thus provide valuable genetic resources for the improvement of stress tolerance in crops and insights into the remediation of sodic lands.