The association at 0° C of 3Hindole-3-acetic acid (IAA) with membrane vesicles prepared fromCucurbita pepoL. hypocotyls at pH 7.9 and resuspended at pH 6.0 was greatly diminished by osmotic shrinkage. Nonradioactive IAA inhibited a large proportion of this association thus revealing a saturable component, also decreased by osmotic shrinkage, which mainly represents operation of an auxin uptake carrier, with saturable binding having only a minor role. The contribution of this carrier to the steady state of IAA transport changed in the same direction as the proton motive force (transmembrane pH and electrical potential difference) which was manipulated using the K+ionophore valinomycin with differing K+concentration gradients over a range of assay pH values. We conclude that the uptake carrier is electroimpelled with IAA-/2H+(or IAAH/H+) symport as a possible mechanism. The same procedures were used to examine the behaviour of the IAA efflux carrier, whose inhibition by N-1-naphthylphthalamic acid (NPA) causes an increase in IAA accumulation by the vesicles. The extent of NPA-stimulation was linked to the magnitude of the electrical potential difference (negative inside) across the vesicle membranes. We conclude that IAA transport by the efflux carrier is electroimpelled, with IAA-a likely substrate
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