Effect of Surface and Membrane Potentials on IAA Uptake and Binding by Zucchini Membrane Vesicles

The polar transport of the endogenous hormone controlling extension growth of plant cells, indoleacetic acid (IAA), is thought to depend on transmembrane pH and electrical gradients resulting in part from the action of proton ATPases in the plasma membrane. According to a recent hypothesis (see Goldsmith, 1977 for review), elements of this transport process are: (1) permeation of the membrane by the undissociated lipophilic indoleacetic acid (IAAH) from the acidic apoplast, followed by dissociation of the weak acid and accumulation of the IAA anion (IAA-) in the alkaline cytoplasm; (2) a saturable symport of IAA- with one or more protons; (3) a carrier-mediated efflux of IAA- down a considerable electrochemical gradient. The efflux is greater from the basal than the apical end of cells and is thought to be responsible for the overall polarity of the process. This step is also the site of action of napthylphthalamic acid (NPA) and herbicides that inhibit polar transport but stimulate net accumulation of auxin by tissues and cells (Sussman & Goldsmith, 1981ab Thomson et al, 1973).