Evidence supporting a model of voltage-dependent uptake of auxin intoCucurbitavesicles

摘要

The accumulation of 14Cindole-3-acetic acid (IAA), of 3Htetra-phenyl phosphonium ion as a membrane potential probe, and of 14Cbutyric acid as probe for pH gradients was studied with membrane vesicles from etiolated hypocotyls ofCucurbita pepo. Ion gradients (K+, H+) were applied in the presence and absence of specific ionophores e.g. valinomycin or carbonylcyanidem-chlorophenylhydrazone. In all cases tested, the accumulation of 14CIAA 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.