NA+ H+ ANTIPORTER IN TONOPLAST VESICLES FROM RICE ROOTS/

The Na+/H+ antiporter in vacuolar membranes transports Nat from the cy toplasm to vacuoles using a pH gradient generated by proton pumps; it is considered to be related to salinity tolerance, Rice (Oryza sativa L.) is a salt-sensitive crop whose vacuolar antiporter is unknown, The vacuolar pH of rice roots, determined by P-31-nuclear magnetic resona nce (NMR), increased from 5.34 to 5.58 in response to 0.1 M NaCl treat ment, Transport of protons into the tonoplast vesicles from rice roots was fluorometrically measured, Efflux of protons was accelerated by t he addition of Na+, Furthermore, the influx of Na-22(+) into the tonop last vesicles was accelerated by a pH gradient generated by proton-tra nslocating adenosine 5'-triphosphatase (H+-ATPase) and proton-transloc ating inorganic pyrophosphatase (H+-PPase), We concluded that this Na/H+ antiporter functioned as a Na+ transporter in the vacuolar membran es, The antiporter had a K-m of 10 mM for Na+ and was competitively in hibited by amiloride and its analogues, The K-i values for 5-(N-methyl -N-isobutyl)-amiloride (MIA), 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) , and 5-(N,N-hexamethylene)-amiloride (HMA) were 2.2, 5.9, and 2.9 mu M, respectively, Unlike barley, a salt-tolerant crop, NaCl treatment d id not activate the antiporter in rice roots, The amount of antiporter in the vacuolar membranes may be one of the most important factors de termining salt tolerance.