REDUCTION IN PLASMA-MEMBRANE ATPASE ACTIVITY OF TOMATO ROOTS BY SALT STRESS

ATPase activity was characterized in a plasma membrane-enriched fraction isolated from tomato (Lycopersicon exculentum Mill., cv. Heinz 1350) roots grown in the absence or presence of salinity stress (-4 bars, 60 mM NaCl plus 12 mM CaCl2). The enzyme exhibited the following properties in both non-stressed and salt-stressed roots: (1) activated by divalent cations (Mg2+ ≻ Mn2+ ≻ Co2+ ≻ Ni2+ ≻ Ca2+ ≻ Zn2+) and further stimulated by monovalent cations (Na+ = K+ ≻ Rb+ ≻ Li+); (2) pH optima for Mg2+ activation and KCl-stimulation of 7.0 and 6.5, respectively; (3) selective for Mg2+-ATP as substrate; (4) sensitive to N,N′-dicyclohexylcarbodiimide and vanadate but insensitive to azide and oligomycin; (5) not stimulated synergistically by Na+ plus K+. Exposing roots to salt-stress altered the kinetics of Mg2+-ATPase activity. Simple Michaelis-Menten kinetics were observed when Mg2+-ATP was used as substrate for both control and salt-treated roots. Salt-stress had little effect on the apparent Km for Mg2+-ATP. The predominant effect of salt-stress was reduce Vmax of Mg2+-ATPase activity from 69 μmol Pi (mg protein)-1 h-1 in control roots to 39 μmol Pi (mg protein) -1 h-1 in salt-treated roots. © 1990.