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ENVIRONMENTAL STRESS AND ADAPTATION TO STRESS

Conservation of the Salt Overly Sensitive Pathway in Rice^{1,[C],[W],[OA]}

Instituto de Recursos Naturales y Agrobiología, Consejo Superior de Investigaciones Científicas, Sevilla 41012, Spain (J.M.-A., X.J., I.M., J.M.P., F.J.Q.); Departamento de Biotecnología, Escuela Técnica Superior de Ingenieros Agrónomos, Universidad Politécnica de Madrid, Madrid 28040, Spain (B.G.); and Department of Botany and Plant Sciences, Institute of Integrative Genome Biology, University of California, Riverside, California 92521 (J.-K.Z.)

The salt tolerance of rice (Oryza sativa) correlates with the ability to exclude Na⁺ from the shoot and to maintain a low cellular Na⁺/K⁺ ratio. We have identified a rice plasma membrane Na⁺/H⁺ exchanger that, on the basis of genetic and biochemical criteria, is the functional homolog of the Arabidopsis (Arabidopsis thaliana) salt overly sensitive 1 (SOS1) protein. The rice transporter, denoted by OsSOS1, demonstrated a capacity for Na⁺/H⁺ exchange in plasma membrane vesicles of yeast (Saccharomyces cerevisiae) cells and reduced their net cellular Na⁺ content. The Arabidopsis protein kinase complex SOS2/SOS3, which positively controls the activity of AtSOS1, phosphorylated OsSOS1 and stimulated its activity in vivo and in vitro. Moreover, OsSOS1 suppressed the salt sensitivity of a sos1-1 mutant of Arabidopsis. These results represent the first molecular and biochemical characterization of a Na⁺ efflux protein from monocots. Putative rice homologs of the Arabidopsis protein kinase SOS2 and its Ca²⁺-dependent activator SOS3 were identified also. OsCIPK24 and OsCBL4 acted coordinately to activate OsSOS1 in yeast cells and they could be exchanged with their Arabidopsis counterpart to form heterologous protein kinase modules that activated both OsSOS1 and AtSOS1 and suppressed the salt sensitivity of sos2 and sos3 mutants of Arabidopsis. These results demonstrate that the SOS salt tolerance pathway operates in cereals and evidences a high degree of structural conservation among the SOS proteins from dicots and monocots.

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 (pardo{at}cica.es).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.092635

^* Corresponding author; e-mail pardo{at}cica.es; fax 34–954624002.

Received November 4, 2006; accepted November 20, 2006; published December 1, 2006.

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