The electrical response of Avena coleoptile cortex to auxins: evidence in vivo for activation of a Cl- conductance

The electrical response to auxin perfusion by cortex tissue of peeled Avena sativa L. coleoptiles was examined using microelectrode measurements of the membrane potential. Applied at 10 micromolars, the known auxins indole-3-acetic acid (IAA), alpha-naphthalene acetic acid (alpha NAA), 2,4-dichlorophenoxyacetic acid, and indole-3-butyric acid, the two analogues beta-naphthalene acetic acid and 2,3-dichlorophenoxyacetic acid, and acetic acid transiently (1-30 min) depolarized the membrane potential; but a subsequent hyperpolarization, possibly resulting from activation of the plasma-membrane H+-ATPase, occurred only with active auxins. The magnitude and duration of the depolarizations induced by the different compounds varied. The alpha NAA-induced depolarization consisted of two depolarization events. The first reached a maximum within 2-3 min and the second between 6 and 10 min. The magnitude of the first alpha NAA-induced depolarization varied with external concentration of Cl-suggesting that it represents an efflux of Cl-. A known anion-channel blocker, anthracene-9-carboxylic acid, at 1 micromolar prevented the Cl--sensitive depolarization induced by alpha NAA. The same concentration failed to block alpha NAA-induced elongation of peeled coleoptile segments, though higher concentrations (20 micromolars and 100 micromolars) increasingly inhibited growth. Applied at 10 micromolars, the naturally occurring auxin, IAA, induced a single depolarization (maximal between 6 and 8 min) which was insensitive to external Cl- concentration. The depolarization induced by 10 micromolars IAA, as well as that induced by acetic acid and the second depolarization induced by 10 micromolars IAA, increased in magnitude when the pH of the perfusate was lowered from 6.0 to 4.5. When IAA was applied at a higher concentration (100 micromolars) the magnitude of the depolarization increased, lengthened, and also contained an early, Cl--sensitive, depolarization (maximal between 2 and 4 min). The Cl--sensitive depolarization induced by alpha NAA also increased when alpha NAA was increased from 10 to 100 micromolars, but, while 10 and 100 micromolars IAA and 10 micromolars alpha NAA induced elongation of peeled Avena coleoptile segments, 100 micromolars alpha NAA did not. Our results suggest that: (i) the putative Cl- currents induced by 10 micromolars alpha NAA and 100 micromolars IAA are the result of the opening of a population of Cl--permeable anion channels, (ii) activation of these channels is not required for the auxin-induced growth of Avena coleoptiles, (iii) the pH-sensitive depolarization induced by alpha NAA and IAA is not an auxin-specific response and might result from a weak acid protonophore effect, and (iv) the delayed hyperpolarization is a specific response to auxins in Avena coleoptiles.