Plasma membrane H+-ATPase activity is involved in adaptation of tomato calli to NaCl

A tomato (Lycopersicon esculentum Mill. cv, Pera) callus culture tolerant to NaCl was obtained by successive subcultures of NaCl-sensitive calli in medium supplemented with 50 mM NaCl, NaCl-tolerant calli grew better than NaCl-sensitive calli in media supplemented with 50 and 100 mM NaCl, Analysis of callus ion content showed a strong increase in Na+ and Cl- both in NaCl-tolerant and -sensitive calli grown in media containing NaCl for one subculture. Cells from NaCl-tolerant calli showed a higher H+ extrusion activity than those from NaCl-sensitive calli grown for one subculture in the presence of NaCl, The inhibition of H+ extrusion by NaCl-sensitive cells was correlated with an inhibition of microsomal vanadate-sensitive H+-ATPase (EC 3.6.1.35) and ATP-dependent H+ transport, while the stimulation of H+ extrusion by cells tolerant to 50 mM NaCl was correlated with an increase in plasma membrane ATP-dependent H+ transport. The increase of ATP-dependent H+ extrusion in plasma membranes isolated from 50 m;M NaCl-tolerant calli was not a result of stimulation of a vanadate-sensitive ATP hydrolytic activity or an increase in passive permeability to H+, Relative to NaCl-sensitive calli, plasma membrane H+-ATPase from calli tolerant to 50 mM NaCl showed a lower g, for Mg2+-ATP. Our results indicate that tolerance of tomato calli to 50 mM NaCl increases the affinity of plasma membrane Hf-ATPase for the substrate ATP and stimulates the H+-pumping activity of this enzyme without modifying its phosphohydrolytic activity.