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 K_m for Mg²⁺-ATP. The predominant effect of salt-stress was reduce V_max of Mg²⁺-ATPase activity from 69 μmol P_i (mg protein)^-1 h^-1 in control roots to 39 μmol P_i (mg protein) ^-1 h^-1 in salt-treated roots.