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...
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...
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