Na^+/H^+ Antiporter in Tonoplast Vesicles from Rice Roots

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The Na^+/H^+ antiporter in vacuolar membranes transports Na^+ from the cytoplasm 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 saltsensitive crop whose vacuolar antiporter is unknown. The vacuolar pH of rice roots, determined by ^<31>P-nuclear magnetic resonance (NMR), increased from 5.34 to 5.58 in response to 0.1 M NaCl treatment. Transport of protons into the tonoplast vesicles from rice roots was fluorometrically measured. Efflux of protons was accelerated by the addition of Na^+. Furthermore, the influx of ^<22>Na^+ into the tonoplast vesicles was accelerated by a pH gradient generated by proton-translocating adenosine 5'-triphosphatase (H^+-ATPase) and proton-translocating inorganic pyrophosphatase (H^+-PPase). We concluded that this Na^+/H^+ antiporter functioned as a Na^+ transporter in the vacuolar membranes. The antiporter had a K_m of 10mM for Na^+ and was competitively inhibited 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, and2.9 μM, respectively. Unlike barleya, a salt-tolerant crop, NaCl treatment did not activate the antiporter in rice roots. The amount of antiporter in the vacuolar membranes may be one of the most important factors determining salt tolerance.
日本植物生理学会の論文