In Arabidopsis thaliana , the SOS1 ( S alt O verly S ensitive 1 ) locus is essential for Na ⁺ and K ⁺ homeostasis, and sos1 mutations render plants more sensitive to growth inhibition by high Na ⁺ and low K ⁺ environments. SOS1 is cloned and predicted to encode a 127-kDa protein with 12 transmembrane domains in the N-terminal part and a long hydrophilic cytoplasmic tail in the C-terminal part. The transmembrane region of SOS1 has significant sequence similarities to plasma membrane Na ⁺ /H ⁺ antiporters from bacteria and fungi. Sequence analysis of various sos1 mutant alleles reveals several residues and regions in the transmembrane as well as the tail parts that are critical for SOS1 function in plant salt tolerance. SOS1 gene expression in plants is up-regulated in response to NaCl stress. This up-regulation is abated in sos3 or sos2 mutant plants, suggesting that it is controlled by the SOS3/SOS2 regulatory pathway.

The salt tolerance locus SOS1 from Arabidopsis has been shown to encode a putative plasma membrane Na⁺/H⁺ antiporter. In this study, we examined the tissue-specific pattern of gene expression as well as the Na⁺ transport activity and subcellular localization of SOS1. When expressed in a yeast mutant deficient in endogenous Na⁺ transporters, SOS1 was able to reduce Na⁺ accumulation and improve salt tolerance of the mutant cells. Confocal imaging of a SOS1–green fluorescent protein fusion protein in transgenic Arabidopsis plants indicated that SOS1 is localized in the plasma membrane. Analysis of SOS1 promoter–β-glucuronidase transgenic Arabidopsis plants revealed preferential expression of SOS1 in epidermal cells at the root tip and in parenchyma cells at the xylem/symplast boundary of roots, stems, and leaves. Under mild salt stress (25 mM NaCl), sos1 mutant shoot accumulated less Na⁺ than did the wild-type shoot. However, under severe salt stress (100 mM NaCl), sos1 mutant plants accumulated more Na⁺ than did the wild type. There also was greater Na⁺ content in the xylem sap of sos1 mutant plants exposed to 100 mM NaCl. These results suggest that SOS1 is critical for controlling long-distance Na⁺ transport from root to shoot. We present a model in which SOS1 functions in retrieving Na⁺ from the xylem stream under severe salt stress, whereas under mild salt stress it may function in loading Na⁺ into the xylem.

Elevated sodium (Na⁺) decreases plant growth and, thereby, agricultural productivity. The ion transporter high-affinity K⁺ transporter (HKT)1 controls Na⁺ import in roots, yet dysfunction or overexpression of HKT1 fails to increase salt tolerance, raising questions as to HKT1's role in regulating Na⁺ homeostasis. Here, we report that tissue-specific regulation of HKT1 by the soil bacterium Bacillus subtilis GB03 confers salt tolerance in Arabidopsis thaliana. Under salt stress (100 mM NaCl), GB03 concurrently down- and upregulates HKT1 expression in roots and shoots, respectively, resulting in lower Na⁺ accumulation throughout the plant compared with controls. Consistent with HKT1 participation in GB03-induced salt tolerance, GB03 fails to rescue salt-stressed athkt1 mutants from stunted foliar growth and elevated total Na⁺ whereas salt-stressed Na⁺ export mutants sos3 show GB03-induced salt tolerance with enhanced shoot and root growth as well as reduced total Na⁺. These results demonstrate that tissue-specific regulation of HKT1 is critical for managing Na⁺ homeostasis in salt-stressed plants, as well as underscore the breadth and sophistication of plant–microbe interactions.

The Arabidopsis thaliana SOS1 protein is a putative Na ⁺ /H ⁺ antiporter that functions in Na ⁺ extrusion and is essential for the NaCl tolerance of plants. sos1 mutant plants share phenotypic similarities with mutants lacking the protein kinase SOS2 and the Ca ²⁺ sensor SOS3. To investigate whether the three SOS proteins function in the same response pathway, we have reconstituted the SOS system in yeast cells. Expression of SOS1 improved the Na ⁺ tolerance of yeast mutants lacking endogenous Na ⁺ transporters. Coexpression of SOS2 and SOS3 dramatically increased SOS1-dependent Na ⁺ tolerance, whereas SOS2 or SOS3 individually had no effect. The SOS2/SOS3 kinase complex promoted the phosphorylation of SOS1. A constitutively active form of SOS2 phosphorylated SOS1 in vitro independently of SOS3, but could not fully substitute for the SOS2/SOS3 kinase complex for activation of SOS1 in vivo . Further, we show that SOS3 recruits SOS2 to the plasma membrane. Although sos1 mutant plants display defective K ⁺ uptake at low external concentrations, neither the unmodified nor the SOS2/SOS3-activated SOS1 protein showed K ⁺ transport capacity in vivo , suggesting that the role of SOS1 on K ⁺ uptake is indirect. Our results provide an example of functional reconstitution of a plant response pathway in a heterologous system and demonstrate that the SOS1 ion transporter, the SOS2 protein kinase, and its associated Ca ²⁺ sensor SOS3 constitute a functional module. We propose a model in which SOS3 activates and directs SOS2 to the plasma membrane for the stimulatory phosphorylation of the Na ⁺ transporter SOS1.

Salt cress (Thellungiella halophila) is a small winter annual crucifer with a short life cycle. It has a small genome (about 2 × Arabidopsis) with high sequence identity (average 92%) with Arabidopsis, and can be genetically transformed by the simple floral dip procedure. It is capable of copious seed production. Salt cress is an extremophile native to harsh environments and can reproduce after exposure to extreme salinity (500 mm NaCl) or cold to −15°C. It is a typical halophyte that accumulates NaCl at controlled rates and also dramatic levels of Pro (>150 mm) during exposure to high salinity. Stomata of salt cress are distributed on the leaf surface at higher density, but are less open than the stomata of Arabidopsis and respond to salt stress by closing more tightly. Leaves of salt cress are more succulent-like, have a second layer of palisade mesophyll cells, and are frequently shed during extreme salt stress. Roots of salt cress develop both an extra endodermis and cortex cell layer compared to Arabidopsis. Salt cress, although salt and cold tolerant, is not exceptionally tolerant of soil desiccation. We have isolated several ethyl methanesulfonate mutants of salt cress that have reduced salinity tolerance, which provide evidence that salt tolerance in this halophyte can be significantly affected by individual genetic loci. Analysis of salt cress expressed sequence tags provides evidence for the presence of paralogs, missing in the Arabidopsis genome, and for genes with abiotic stress-relevant functions. Hybridizations of salt cress RNA targets to an Arabidopsis whole-genome oligonucleotide array indicate that commonly stress-associated transcripts are expressed at a noticeably higher level in unstressed salt cress plants and are induced rapidly under stress. Efficient transformation of salt cress allows for simple gene exchange between Arabidopsis and salt cress. In addition, the generation of T-DNA-tagged mutant collections of salt cress, already in progress, will open the door to a new era of forward and reverse genetic studies of extremophile plant biology.