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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.