SALINITY ADAPTATION OF PLASMA-MEMBRANE H-ATPASE IN THE SALT-MARSH PLANT SPARTINA PATENS - ATP HYDROLYSIS AND ENZYME-KINETICS()

Spartina patens, an intertidal C-4 grass, grows in the upper salt mars h and tolerates coastal seawater salinity. The regulation of ion movem ent across the plasma membrane (PM) for plant salt tolerance is though t to be achieved by an electrochemical gradient generated by plasma me mbrane H+-ATPase. in this study, the change of PM H+-ATPase in respons e to NaCl was characterized for S. patens callus. Callus was cultured for 10 weeks under salinity levels of 0 mM, 170 mM, 340 mM, and 510 mM NaCl. Plasma membrane was isolated from a Dextran/PEG aqueous polymer two-phase system and the purity was demonstrated with membrane enzyme markers. There was a significant increase (up to 2-3-fold) of PM H+-A TPase activity when callus was grown on media containing NaCl. The inc remental activation of PM H+-ATPase activity would enable the cell to tolerate higher cytoplasmic NaCl concentrations. PM H+-ATPase appeared to have a higher V-max and a lower substrate concentration (K-m) to r each V-max. When growth medium salinity increased from 0 mM to 170 and 340 mM, the V-max of H+-ATPase increased from 0.64 to 1.00 and 1.73, respectively, while the K-m decreased from 3.58 to 2.07 and 2.44 mM, r espectively. In vitro NaCl inhibition kinetic data revealed a pattern of non-competitive inhibition by NaCl on PM H+-ATPase. The response of PM H+-ATPase in S. patens callus suggests that this species has evolv ed mechanisms that can regulate this important enzyme when cells are e xposed to NaCl.