Mechanism of blue-light-induced plasma-membrane depolarization in etiolated cucumber hypocotyls

A large, transient depolarization of the plasma membrane precedes the rapid blue-light (BL)-induced growth suppression in etiolated seedlings of Cucumis sativus L. The mechanism of this voltage transient was investigated by applying inhibitors of ion channels and the plasma-membrane H+-ATPase, by manipulating extracellular ion concentrations, and by measuring cell input resistance and ATP levels. The depolarizing phase was not affected by Ca2+-channel blockers (verapamil, La3+) or by reducing extracellular free Ca2+ by treatment with ethylene glycol-bis(β-aminoethyl ether)-N,N,N′,N′-tetraacetic acid (EGTA). However, these treatments did reduce the rate of repolarization, indicating an inward movement of Ca2+ is involved. No effects of the K+-channel blocker tetraethylammonium (TEA+) were detected. Vanadate and KCN, used to inhibit the H+-ATPase, reduced or completely inhibited the BL-induced depolarization. Levels of ATP increased by 11—26% after 1—2 min of BL. Input resistance of trichome cells, measured with double-barreled microelectrodes, remained constant during the onset of the depolarization but decreased as the membrane voltage became more positive than — 90 mV. The results indicate that the depolarization mechanism initially involves inactivation of the H+-ATPase with subsequent transient activation of one or more types of ion channels.