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Tonoplast vesicles were isolated from leaf mesophyll tissue of the inducible Crassulacean acid metabolism plant Mesembryanthemum crystallinum to investigate the mechanism of vacuolar Na + accumulation in this halophilic species. In 8-week-old plants exposed to 200 HIM NaCI for 2 weeks, tonoplast H + -ATPaSe activity was approximately doubled compared with control plants of the same age, as determined by rates of both ATP hydrolysis and ATP-dependent H + transport. Evidence was also obtained for the presence of an electroneutral Na + /H + antiporter at the tonoplast that is constitutively expressed, since extravesicular Na + was able to dissipate a pre-existing transmembrane pH gradient. Initial rates of H + efflux showed saturation kinetics with respect to extravesicular Na + concentration and were 2.1-fold higher from vesicles of salt-treated plants compared with the controls. Na + -dependent H + efflux also showed a high selectivity for Na + over K + , was insensitive to the transmembrane electrical potential difference, and was more than 50% inhibited by 200 μM N-amidino-3,5-diamino-6-chloropyrazine-carboxamide hydrochloride. The close correlation between increased Na + /H + antiport and H + -ATPase activities in response to salt treatment suggests that accumulation of the very high concentrations of vacuolar Na + found in M. crystallinum is energized by the H + electrochemical gradient across the tonoplast.

Type

Journal article

Journal

Plant Physiology

Publication Date

01/10/1995

Volume

109

Pages

549 - 556