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BIOENERGETICS AND PHOTOSYNTHESIS

Na⁺/H⁺ Exchange Activity in the Plasma Membrane of Arabidopsis¹

Department of Plant Sciences, University of Arizona, Tucson, Arizona 85721 (Q.-S.Q., J.-K.Z., K.S.S.); and Instituto de Biotecnologia UNAM A.P. 510–3 Colonia Miraval, Cuernavaca, Morelos, Mexico 62250 (B.J.B., R.V.-E.)

In plants, Na⁺/H⁺ exchangers in the plasma membrane are critical for growth in high levels of salt, removing toxic Na⁺ from the cytoplasm by transport out of the cell. The molecular identity of a plasma membrane Na⁺/H⁺ exchanger in Arabidopsis (SOS1) has recently been determined. In this study, immunological analysis provided evidence that SOS1 localizes to the plasma membrane of leaves and roots. To characterize the transport activity of this protein, purified plasma membrane vesicles were isolated from leaves of Arabidopsis. Na⁺/H⁺ exchange activity, monitored as the ability of Na to dissipate an established pH gradient, was absent in plants grown without salt. However, exchange activity was induced when plants were grown in 250 mM NaCl and increased with prolonged salt exposure up to 8 d. H⁺-coupled exchange was specific for Na, because chloride salts of other monovalent cations did not dissipate the pH gradient. Na⁺/H⁺ exchange activity was dependent on Na (substrate) concentration, and kinetic analysis indicated that the affinity (apparent K_m) of the transporter for Na⁺ is 22.8 mM. Data from two experimental approaches supports electroneutral exchange (one Na⁺ exchanged for one proton): (a) no change in membrane potential was measured during the exchange reaction, and (b) Na⁺/H⁺ exchange was unaffected by the presence or absence of a membrane potential. Results from this research provide a framework for future studies into the regulation of the plant plasma membrane Na⁺/H⁺ exchanger and its relative contribution to the maintenance of cellular Na⁺ homeostasis during plant growth in salt.

¹ This work was supported by the National Institutes of Health (grant no. R01GM59138 to J.-K.Z.) and by the Southwest Consortium on Plant Genetics and Water Resources (to J.-K.Z. and K.S.S.). B.J.B. and R.V.-E. were supported by Dirección General de Asuntas para el Personal Académico (grant no. IN#230998) and Consejo Nacional de Ciencia y Tecnológia (grant no. 33054–N). Q.-S.Q. was supported in part by the Major State Basic Research and Development Plan of the People's Republic of China (grant no. G1999011705).

^* Corresponding author; e-mail schumake{at}ag.arizona.edu; fax 520–621–7186.

Received June 20, 2002; returned for revision August 12, 2002; accepted January 26, 2003.

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