Roles of Transport and Binding in the Specific ΔpH-Dependent Accumulation of Auxin by Zucchini Membrane Vesicles

Abstract

The growth of plants is coordinated in part by the polar transport of the endogenous auxin indoleacetic acid (IAA). Studies on the pH-dependent accumulation of auxin by cells and tissues suspended in buffered auxin solutions [4, 13, 15–17, 19, 21, 22] have led to the realization that this process involves some of the same elements as polar transport in isolated tissues and intact plants [2, 5, 6]. The available evidence suggests a model for the ΔpH-dependent uptake and transport of IAA (Fig. 1) that includes: (1) permeation of the neutral lipophilic form of the weak acid (IAAH) through the lipid bilayer; (2) a saturable symport of the auxin anion (IAA⁻) with protons; and (3) a passive efflux of LAA⁻ via a specific saturable anion channel (carrier) sensitive to napthylphthalamic (NPA) [20] and 2,3,5-triiodobenzoic acids (TIBA), herbicides known to stimulate cellular auxin accumulation at the expense of polar auxin transport [3, 5, 19, 21, 22]. Localization of the IAA“ efflux sites at the base of cells is believed to be the physical basis for polar transport [8].