Abstract
The permeability properties of gibberellin A(1) (GA(1)) were examined in membrane vesicles isolated from cowpea hypocotyls. The rate of GA(1) uptake was progressively greater as pH decreased, indicating that the neutral molecule is more permeable than anionic GA(1). Membrane vesicles used in this study possessed a tonoplast-type H(+)-translocating ATPase as assayed by MgATP-dependent quenching of acridine orange fluorescence and methylamine uptake. However, GA(1) uptake was not stimulated by MgATP. At concentrations in excess of 1 micromolar, GA(1), GA(5), and GA, collapsed both MgATP-generated and artifically imposed pH gradients, apparently by shuttling H(+) across the membrane as neutral GA. The relatively high permeability of neutral GA and the potentially detrimental effects of GA in uncoupling pH gradients across intracellular membranes supports the view that GA(1) accumulation and compartmentation must occur by conversion of GA(1) to more polar metabolites.