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Tonoplast Na+/H+ antiport activity and its energization by the vaculoar H+-ATPase in the halophytic plant Mesembryanthemum crystallinum

Barkla, B.J., Zingarelli, L., Blumwald, E., Smith, J.A.C.
Plant physiology 1995 v.109 no.2 pp. 549-556
Mesembryanthemum crystallinum, ion transport, tonoplast, vacuoles, sodium, pH, proton pump, salt tolerance, leaves, mesophyll, membrane permeability, metabolic inhibitors, kinetics, cation exchange
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 mM NaCl 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 micromolar N-amidino-3,5-diamino-6-chloropyrazine carboxamide hydrochloride. The close correlation between increased Na+/H+ antiportand 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.