Characteristics of fusicoccin-induced changes of transmembrane potential and ion uptake in maize root segments

Abstract

Listen

Translate article

The effects of fusicoccin (FC) on the transmembrane electric potential difference (PD) of cortical cells of isolated apical root segments was investigated under various conditions of osmotic pressure, temperature, ionic composition and pH of the incubation medium, and compared with the effects of this substance on the uptake of K +, Rb +, Na + and Cs + ions. FC induced a rapid hyper-polarization of the negative PD, ranging from 20 to 50 mV depending on the ionic composition of the medium. With 5·10 −5M FC maximal PD value was reached in ca. 6 min. Changing the osmotic pressure of the medium from 0.01 to 0.25 osmolar (mannitol) progressively inhibited elongation without affecting the PD change thus showing that the effect of FC does not depend on the increase of cell enlargement. Low temperature (6°C) and the phosphorylation uncoupler ( p-trifluoromethoxy)-carbonyl cyanide-phenylhydrazone (FCCP) rapidly and severely decreased both normal and FC-induced PD. FC stimulated the rate of monovalent cation uptake. In the 1–10 mM concentration range this effect was much larger for K + and Rb + than for Na + and Cs +. K + and (to a lesser extent) Rb + at concentrations higher than 5·10 −4M induced a rapid depolarization, larger in the presence than in the absence of FC, while Na +, Cs + and Li + at concentrations up to 10 −2M did not significantly influence PD. No significant PD changes were observed when the anion Cl − was substituted with SO 4 2− or NO 3 2−. Increasing proton concentration in the medium from pH 6 to pH 4 induced a significant hyperpolarization, probably due to the inhibition of K + uptake by low pH. These data are compared with other results showing the remarkable activity of FC in stimulating proton secretion in root segments, and they are interpreted as indicating that FC activates in roots, as in a wide variety of other plant organs, an electrogenic mechanism of proton extrusion deeply involved in cation transport. The hypothesis is proposed that the differential activity of the monovalent cations in inducing depolarization of PD might depend on their different affinity for the proton/cation electrogenic exchange mechanism.