Conservation of the Salt Overly Sensitive Pathway in Rice^{<a href="#fn1" rid="fn1" class=" fn">1</a>,}^{<a href="#fn2" rid="fn2" class=" fn">[C]</a>}^{<a href="#fn3" rid="fn3" class=" fn">[W]</a>}^{<a href="#fn4" rid="fn4" class=" fn">[OA]</a>}

Abstract

Rice (Oryza sativa) is one of the most important cereal crops in tropical and temperate regions of the world. Among all common environmental stresses, salinity is a major factor decreasing the yield in rice cultivation in coastal areas and in irrigated farmlands. Problems associated with salinity are water deficit imposed by the greater osmolarity of the soil solution and the cellular damage inflicted by excessive ion accumulation in plant tissues. Comparison of rice subspecies and varieties differing in tolerance to salinity has shown that greater tolerance correlates with the ability to exclude Na ^{from the shoot and maintain a low Na/K ratio (}Golldack et al., 2003; Lee et al., 2003; Ren et al., 2005). For instance, the salt-sensitive variety IR29 accumulated Na ^{in leaves at 5- to 10-fold greater concentrations than the salt-tolerant lines BK or Pokkali (}Golldack et al., 2003). In contrast, shoot K ^{concentration per se showed no relation to salinity tolerance in} japonica spp. and only weak correlation in indica spp. varieties (Golldack et al., 2003; Lee et al., 2003). Because steady accumulation of Na ^{is what injures the cells of leaves at moderate salinity levels (}Flowers et al., 1991; Munns, 1993), restricting the translocation of Na ^{is a mechanism for salt tolerance that plays a major role in rice (}Lee et al., 2003; Ren et al., 2005). The gene SKC1/HKT8, responsible for a major quantitative trait locus imparting a high K^{/Na balance in shoots and salt tolerance, encodes an Na-selective transporter of the HKT family that regulates long-distance transport of Na (}Ren et al., 2005). SKC1/HKT8 participates in reabsorption of Na ^{at the xylem parenchyma, thereby restricting the buildup of toxic concentrations of Na in photosynthetic tissues (}Ren et al., 2005). The related rice gene HKT1 is preferentially expressed in root xylem parenchyma and in cells adjacent to phloem vessels in leaves, suggesting that it could also be involved in the regulation of long-distance transport of Na ⁽Golldack et al., 2002). The mechanism by which rice roots take up Na ^{is uncertain. Anatomical discontinuities in the root endodermis may lead to uncontrolled apoplastic bypass flow of ions and their subsequent discharge into the vascular bundle (}Yeo et al., 1987; Yadav et al., 1996). In this process, ion transporters would play a minor role, or none at all, and the natural variability of salt tolerance among cultivars would be determined by genes controlling developmental traits (Koyama et al., 2001). However, the kinetics of Na ^{uptake by rice roots is consistent with enzymatic processes driven by ion transporters (}Garciadeblas et al., 2003). Although the molecular identities of these transporters remain to be established, the kinetic properties of OsHKT1 in heterologous systems recapitulate those of whole roots (Garciadeblas et al., 2003). In wheat (Triticum aestivum), TaHKT1 is primarily expressed in the root cortex and down-regulation of TaHKT1 by RNAi reduced Na ^{uptake and enhanced salt tolerance, indicating that TaHKT1 mediated Na uptake (}Laurie et al., 2002).

Sodium extrusion at the root-soil interface, as well as some level of Na ^{efflux in every other cell type to achieve ion homeostasis, is presumed to be of critical importance for the salt tolerance of glycophytes (}Tester and Davenport, 2003). Indeed, efficient efflux of Na ^{to the soil solution must function in the roots of several species to minimize net uptake because unidirectional influx of Na is rapid and greatly exceeds the rate of accumulation (}Tester and Davenport, 2003). In wheat roots, high rates of Na ^{efflux were inferred because net uptake was very low relative to unidirectional influx (}Davenport et al., 2005). The Na^{/H antiporter salt overly sensitive (SOS)1 is the only Na efflux protein at the plasma membrane of plants characterized so far. Mutants of Arabidopsis (}Arabidopsis thaliana) lacking SOS1 are extremely salt sensitive and have combined defects in Na ^{extrusion and in the long-distance transport of this ion from root to shoot (}Qiu et al., 2002; Shi et al., 2002). SOS1 is primarily expressed at the root tip epidermis and in xylem parenchyma at the xylem-symplast boundary throughout the plant (Shi et al., 2002). At the root-soil interface, SOS1 would act extruding the excess of Na ^{ions from root epidermal cells. In addition, analysis of the Na root-shoot partition in the} sos1 mutant under different saline regimes indicated that SOS1 also participated in the redistribution of Na ^{between roots and shoot in a complex manner (}Shi et al., 2002). Under moderate saline stress (25 mm NaCl) sos1 mutant plants accumulated less Na ^{in their aerial parts than the wild type, indicating that SOS1 functions in loading Na into the xylem for controlled delivery to the shoot. By contrast, at high salinity (100 m}m NaCl), the roots and aerial parts of sos1 mutant plants accumulated more Na ^{than wild-type plants, which could be caused by the breakdown of Na exclusion at the root epidermis and to the large electrochemical gradient of Na across the xylem-symplast boundary (}Shi et al., 2002; Pardo et al., 2006). In addition, it has been suggested that, under severe salinity stress, the difference in Na ^{concentration between xylem sap and xylem parenchyma cells could be of greater magnitude than the corresponding pH gradient, which would result in reversal of SOS1 activity (assuming an electroneutral exchange) and retrieval of Na from the xylem (}Shi et al., 2002; Tester and Davenport, 2003). The activity of the SOS1 exchanger is regulated through protein phosphorylation by the SOS2/SOS3 kinase complex (Qiu et al., 2002; Quintero et al., 2002). SOS2 is a Ser-Thr protein kinase belonging to the SNF1-related kinase (SnRK)3 family (Gong et al., 2004; Kolukisaoglu et al., 2004). SOS3 is a myristoylated Ca ^{sensor belonging to the recoverin-like family of SOS3-like Ca sensor/binding proteins (SCaBPs)/calcineurin B-like (CBL) proteins (}Gong et al., 2004; Kolukisaoglu et al., 2004). Upon Ca ^{binding, SOS3 undergoes dimerization and enhances the protein kinase activity of SOS2 (}Guo et al., 2001; Sanchez-Barrena et al., 2005). Besides activating SOS2, SOS3 was also shown to recruit SOS2 to the plasma membrane to achieve efficient interaction with SOS1 (Quintero et al., 2002). Mutant plants deficient in either SOS2 or SOS3 share the salt-sensitive phenotype of sos1 plants (Zhu, 2000).

We have begun to characterize Na ^{efflux proteins of rice by isolating a SOS1 homolog, which is encoded by a single-copy gene. We show that OsSOS1 functions as a plasma membrane Na/H antiporter in yeast (}Saccharomyces cerevisiae) cells and that, like its Arabidopsis counterpart, it is phosphorylated and activated by the SOS2-SOS3 protein kinase complex. Ectopic expression of OsSOS1 suppressed the growth defects of an Arabidopsis sos1 mutant line. We have also identified the homologs of AtSOS2 and AtSOS3 (OsCIPK24 and OsCBL4, respectively), which coordinately regulate the activity of OsSOS1. These results show that the SOS pathway for salt tolerance operates in cereals.

Acknowledgments

Notes

^{This work was supported by the Ministerio de Educación y Ciencia (grant no. BIO2003–08501–CO2–01 to J.M.P.), Instituto Nacional de Investigacion y Tecnologia Agraria y Alimentaria (grant no. CPE03–006–C6–3 to J.M.P.), Junta de Andalucía (grant no. CVI–148 to F.J.Q. and J.M.P.), and by the National Institutes of Health (grant no. R01GM59138 to J.-K.Z.). J.M.-A. was supported by a Formacion Profesorado Universitario FPU fellowship from the Ministerio de Educación y Ciencia.}

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: José M. Pardo (se.acic@odrap).

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