Fusicoccin-induced, K+-stimulated proton secretion and acid-induced growth of apical root segments

1. (1) Fusicoccin (FC) markedly stimulates proton secretion in isolated apical segments from pea, bean and maize roots. This effect of FC is already detectable after 1 min, and reaches a maximal rate approx. 10 min after the addition of the glucoside. 2. (2) FC-induced proton secretion is strongly stimulated by the presence in the medium of K+ (and not by that of Na+ or Li+) at concentrations higher than 2·10-4 M, while it is not significantly influenced by substitution of Cl- with the slowly penetrating SO4= or benzenesulfonate anions. Net K+ uptake is significantly stimulated by FC. 3. (3) Experiments of titration at constant pH in the range between pH 5.9 and 4 show that the rate of proton secretion in the absence of FC strongly depends on external pH. In contrast, FC-induced increase of the rate of proton extrusion appears very little affected by external pH in the same pH range. 4. (4) FC markedly stimulates elongation of the root segments. This effect is enhanced by the presence of K+ (but not of Na+ or Li+) in the medium. In the absence of FC low pH values of the medium strongly promote irreversible elongation of apical segments of maize roots. Maximal elongation rate is observed at pH 3.7. The effect of acid on elongation is consistently lesser than that of FC, and is not additive to it. The presence of K+ (and not of Na+) in the medium strongly increases FC-induced elongation at pH 6, while it has little effect at pH 3.7. These results are interpreted as indicating that FC stimulates in roots the same energy-dependent proton secretion mechanism previously shown to be activated by FC in stem, coleoptiles, leaves, cotyledons and embryos from dormant seeds. In roots, as in stem and coleoptiles, FC-induced stimulation of cell enlargement seems largely, but not completely explained by its capacity to decrease the pH in the cell wall space.