Transport of Gibberellin A1 in Cowpea Membrane Vesicles

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Abstract
The permeability properties of gibberellin A1 (GA1) were examined in membrane vesicles isolated from cowpea hypocotyls. The rate of GA1 uptake was progressively greater as pH decreased, indicating that the neutral molecule is more permeable than anionic GA1. 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, GA1 uptake was not stimulated by MgATP. At concentrations in excess of 1 micromolar, GA1, GA5, and GA9 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 GA1 accumulation and compartmentation must occur by conversion of GA1 to more polar metabolites.