The Putative Plasma Membrane Na⁺/H⁺ Antiporter SOS1 Controls Long-Distance Na⁺ Transport in Plants

Abstract

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...

Efeito do estresse salino em genótipos tetraplóides de bananeira

“…O controle da distribuição do Na + nos órgãos e tecidos da planta, bem como no interior da célula, se dá em função da atividade dos transportadores de Na + /H + que, dependendo do nível de salinização, garantem a manutenção da homeostase iônica (Shi et al, 2002). A exclusão do Na + através da membrana plasmática resulta na extrusão do Na + para outras células ou tecidos ou para a solução do solo.…”

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress

“…The same notion is applicable to other techniques used to reveal the tissuespecific patterning of transporter expression. For example, using a GFP fusion technique, the preferential expression of the SALT OVERLY SENSITIVE1 (SOS1) Na + /H + exchanger was reported for the epidermal cells of the root tip and xylem/symplast boundary (Shi et al, 2000(Shi et al, , 2002. Yet, the functional analysis of Arabidopsis (Arabidopsis thaliana) sos1 transport mutants has revealed significant differences in root K + retention ability between sos1 and wild-type plants in the mature root epidermis , where no GFP signals were detected (Shi et al, 2000).…”

Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance

“…The plants were covered to maintain humidity. The first two drops of xylem sap were discarded, and subsequent drops of xylem sap were collected from each plant with micropipettes (Drummond) (Shi et al, 2002;Sunarpi et al, 2005). Samples were dried overnight at 958C and dissolved in 5% HNO 3 solution.…”