In Arabidopsis thaliana, the SOS1 (Salt Overly Sensitive 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 SOS3͞SOS2 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 ϩ 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-␤ -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 wildtype 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. INTRODUCTIONPlant growth depends on mineral nutrients absorbed from the soil by roots. Although Na ϩ is a major cation present in soil solutions, Na ϩ is not considered an essential mineral for most plants. In saline soils, high concentrations of Na ϩ disrupt K ϩ and other mineral nutrition, create hyperosmotic stress, and cause secondary problems such as oxidative stress (Zhu, 2001). These adverse effects contribute to plant growth inhibition and even plant death.Many cytosolic enzymes are activated by K ϩ and inhibited by Na ϩ (Flowers et al., 1977). Three mechanisms are available to plant cells to prevent excessive accumulation of Na ϩ in the cytosol (Niu et al., 1995;Blumwald et al., 2000; Zhu, 2001). First, Na ϩ entry to plant cells may be restricted by selective ion uptake. Nonselective cation channels have been proposed to mediate substantial Na ϩ entry into plant roots, but genes encoding these channels have yet to be identified (Amtmann and Sanders, 1999; Tyerman and Skerrett, 1999). The cloned transporters HKT1 and LCT1 have Na ϩ permeability when expressed in yeast or oocytes, suggesting that they also may be candidate Na ϩ influx transporters (Rubio et al., 1995;Schachtman et al., 1997). Recently, studies in yeast demonstrated that the magnitude of the membrane potential affects net Na ϩ influx into cells. Mutations in the yeast PMP3 gene lead to membrane hyperpolarization, increased Na ϩ influx, and salt sensitivity (Navarre and Goffeau, 2000).Second, internalized Na ϩ can be stored in vacuoles. Vacuolar compartmentation is an efficient strategy for plant cells to deal with salt s...

High concentrations of Na+ in saline soils inhibit plant growth and reduce agricultural productivity. We report here that CaMV 35S promoter driven overexpression of the Arabidopsis thaliana SOS1 gene, which encodes a plasma membrane Na+/H+ antiporter, improves plant salt tolerance in A. thaliana. Transgenic plants showed substantial upregulation of SOS1 transcript levels upon NaCl treatment, suggesting post-transcriptional control of SOS1 transcript accumulation. In response to NaCl treatment, transgenic plants overexpressing SOS1 accumulated less Na+ in the xylem transpirational stream and in the shoot. Undifferentiated callus cultures regenerated from the transgenic plants were also more tolerant of salt stress, which was correlated with reduced Na+ content in the transgenic cells. These results show that improved salt tolerance could be achieved by limiting Na+ accumulation in plant cells.

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 SOS2͞SOS3 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 SOS2͞SOS3 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 SOS2͞SOS3-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.S oil salinity is a prevalent abiotic stress for crop plants. Excess salts in the soil solution interfere with mineral nutrition and water uptake, and lead to the undue accumulation of toxic ions (1). Maladies associated to salt stress are membrane disorganization, impaired nutrient and water acquisition, metabolic toxicity, inhibition of photosynthesis, and production of reactive oxygen species. In most instances, ion toxicity results from immoderate Na ϩ uptake caused by its steep inward electrochemical gradient. Plant growth under salt stress depends, among other concomitant processes, on the re-establishment of proper cellular ion homeostasis. Low cytosolic Na ϩ content is preserved by the concerted interplay of regulated ion uptake, vacuolar compartmentation, and active extrusion to the extracellular milieu (2). Vacuolar partitioning of Na ϩ and other ions also contributes to the maintenance of cellular water relations in a hypertonic medium. Energy-dependent exclusion of Na ϩ from the cytosol is coupled to downhill reverse transport of H ϩ by Na ϩ ͞H ϩ antiporters located in both the plasma membrane and tonoplast.The Arabidopsis thaliana SOS1 protein is the first putative plasma membrane Na ϩ ͞H ϩ antiporter to be described in plants (3,4). Arabidopsis sos1 mutants were isolated in a genetic screen for plants hypersensitive to NaCl, together with sos2 and sos3 mutants (5). SOS2 is a Ser͞Thr protein kinase in which t...

Salt cress (Thellungiella halophila) is a small winter annual crucifer with a short life cycle. It has a small genome (about 2 3 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 215°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 stressrelevant 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.Salinity is a severe and increasing constraint on the productivity of agricultural crops. High concentrations of salts in the soil have a strong inhibitory effect on the growth and harvestable yield of all crop species. Secondary salinization significantly impairs crop production on at least 20% of irrigated land worldwide (Ghassemi et al., 1995), and irrigated agriculture contributes more than 30% of global agricultural production (Hillel, 2000). Salinization of arable land arising from poor water management has led to the decline of past civilizations, and it threatens the long-term sustainability of many current large-scale irrigation systems, especially those in Asia (Sharma and Goyal, 2003). Soil salinity almost always originates from previous exposure to seawater (Flowers et al., 1986). Although it is believed that, for most of the Earth's history, the salt level of the oceans was much lower than at present (Serrano et al., 1997), all plant spec...

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Cell surface proteoglycans have been implicated in many aspects of plant growth and development, but genetic evidence supporting their function has been lacking. Here, we report that the Salt Overly Sensitive5 ( SOS5 ) gene encodes a putative cell surface adhesion protein and is required for normal cell expansion. The sos5 mutant was isolated in a screen for Arabidopsis salt-hypersensitive mutants. Under salt stress, the root tips of sos5 mutant plants swell and root growth is arrested. The root-swelling phenotype is caused by abnormal expansion of epidermal, cortical, and endodermal cells. The SOS5 gene was isolated through map-based cloning. The predicted SOS5 protein contains an N-terminal signal sequence for plasma membrane localization, two arabinogalactan protein-like domains, two fasciclin-like domains, and a C-terminal glycosylphosphatidylinositol lipid anchor signal sequence. The presence of fasciclinlike domains, which typically are found in animal cell adhesion proteins, suggests a role for SOS5 in cell-to-cell adhesion in plants. The SOS5 protein was present at the outer surface of the plasma membrane. The cell walls are thinner in the sos5 mutant, and those between neighboring epidermal and cortical cells in sos5 roots appear less organized. SOS5 is expressed ubiquitously in all plant organs and tissues, including guard cells in the leaf.

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.