Signal Transduction in Sinapis alba Root Hairs: Auxins as External Messengers

Abstract

In developing root hairs of Sinapis alba the effects of externally applied indole-3-acetic acid (IAA) and other auxins have been investigated with respect to membrane potential, membrane conductance, cytosolic Ca2 + and pH. Following a delay of roughly 30s, 10- 12 to lO- IO M IAA slowly hyperpolarize, 10- 7 M IAA rapidly depolarize the root hairs, while 10- 9 M has hardly an effect. We show that these voltage responses are not the result of a change in membrane conductance or permeability, but are presumably caused by a change in H+ ATPase activity. The other tested auxins and analogues yielded comparable effects, but with much lower effectivity (IAA > 1-NAA > 2,4-D ~ 2-NAA > 2,3-D). Cytosolic Ca2+ and pH were decreased during depolarization by 0.2 and 0.4 units, respectively. No such changes were observed during hyperpolarization or about 1 h after the first encounter of the root hairs with IAA. We propose that IAA is a natural external signal for roots while competing with neighboring organisms for nutrients and salts, and suggest a signal chain with the plasma membrane H+ ATPase as a target protein. The delay in response to IAA, the time dependency, and the extremely low effective IAA concentrations point to the existence of a IAA receptor. Since the IAA-induced shifts in cytosolic pH and Ca2+ occur simultaneously with the depolarization, the question whether these ions are cellular messengers and part of an IAA-triggered signal chain is critically discussed.