Electrostatic Changes in Lycopersicon esculentum Root Plasma Membrane Resulting from Salt Stress

Abstract

Salinity-induced alterations in tomato (Lypersicon esculentum Mill. cv Heinz 1350) root plasma membrane properties were studied and characterized using a membrane vesicle system. Equivalent rates of MgATP-dependent H+-transport activity were measured by quinacrine fluorescence (.DELTA.pH) in plasma membrane vesicles isolated from control or salt-stressed (75 millimolar salt) tomato roots. However, when bis-[3-phenyl-5-oxoisoxazol-4-yl]pentanethine was used to measure MgATP-dependent membrane potential (.DELTA..psi.) formation, salt-stressed vesicles displayed a 50% greater initial quench rate and a 30% greater steady state quench than control vesicles. This differential probe response suggested a difference in surface properties between control and salt-stressed membranes. Fluorescence titration of vesicles with the surface potential probe, 8-anilino-1-napthalenesulphonic acid (ANS) provided dissociation constants (Kd) of 120 and 76 micromolar for dye binding to control and salt-stressed vesicles, respectively. Membrane surface potentials (.psi.o) of -26.0 and -13.7 millivolts were calculated for control and salt-stressed membrane vesicles from the measured Kd values and the calculated intrinsic affinity constant, Ki. The concentration of cations and anions at the surface of control and salt-stressed membranes was estmated using .psi.o values and the Boltzmann equation. The observed difference in membrane surface electrostatic properties was consistent with the measured differences in K+-stimulated kinetics of ATPase activity between control and salt-stressed vesicles and by the differential ability of Cl- ions to stimulate H+-transport activity. Salinity-induced changes in plasma membrane electrostatic properties may influence ion transport across the plasma membrane.