Indole-3-acetic acid and fusicoccin cause cytosolic acidification of corn coleoptile cells.

Microelectrodes were used to measure simultaneously the effects of indole-3-acetic acid (IAA) on membrane potential and cytosolic pH of corn coleoptile cells. IAA caused an initial depolarization followed by hyperpolarization, the latter displaying rhythmic oscillations. The extent of the changes in membrane potential was dependent on IAA concentration, and hyperpolarization, but not depolarization, could be detected with concentrations of IAA as low as 10 nM. Membrane hyperpolarization was preceded by a decrease in cytosolic pH. The decrease commenced approximately 5 min after adding IAA and continued for 15-20 min before reaching a new steady state approximately 0.1 pH unit lower than the original pH. The decrease in pH was readily detectable after treatment with 0.1 muM IAA. Fusicoccin and acetate, which, like IAA, induce elongation growth, caused a similar drop in cytosolic pH and subsequent membrane hyperpolarization, the decrease in pH commencing within seconds. The addition of fusicoccin to IAA-treated cells resulted in a further cytosolic acidification and membrane hyperpolarization. The two substances probably change cytosolic pH via different mechanisms. The results imply that one of the primary effects of auxins in coleoptiles is to lower cytosolic pH.