Evidence supporting a model of voltage-dependent uptake of auxin into Cucurbita vesicles.

The accumulation of [(14)C]indole-3-acetic acid (IAA), of [(3)H]tetra-phenyl phosphonium ion as a membrane potential probe, and of [(14)C]butyric acid as probe for pH gradients was studied with membrane vesicles from etiolated hypocotyls of Cucurbita pepo. Ion gradients (K(+), H(+)) were applied in the presence and absence of specific ionophores e.g. valinomycin or carbonylcyanide m-chlorophenylhydrazone. In all cases tested, the accumulation of [(14)C]IAA equals neither potential probe nor pH-probe accumulation, but represents. an intermediate between the two. Auxin molecules seem to be taken up as positively charged ions and a pH gradient is required for accumulation. The uptake mechanism thus appears to be a specific, carrier-mediated cotransport of the anion of IAA and no less than two protons. The initial rates of auxin uptake by the saturable influx carrier, of permeation through the membrane, and of efflux by the phytotropin-affected efflux carrier were analysed.