Stimulation of Weak Acid Uptake and Increase in Cell Sap pH as Evidence for Fusicoccin- and K+-Induced Cytosol Alkalinization

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Abstract
In maize root segments fusicoccin induced a consistent increase in cell sap pH (taken as representative of vacuolar pH). This effect was markedly enhanced by the presence of K+ in the medium, whereas in the absence of fusicoccin K+ did not significantly influence cell sap pH. Treatment with a weak acid at 2 mm concentration inhibited the uptake of a different (14C-labeled) weak acid fed at a lower concentration, thus suggesting that acidification of the cytoplasm inhibits weak acid uptake. Fusicoccin and K+ increased the rate of uptake of 5,5-dimethyloxazolidine-2,4-dione, butyric acid, or isobutyric acid slightly when fed separately, strongly when fed in combination. The synergism between fusicoccin and K+ in stimulating weak acid uptake was parallel to that observed for the stimulation of H+ extrusion. Application of the weak acid distribution method to a condition of `quasi-equilibrium' indicated that fusicoccin induces a cytosolic pH increase of about 0.14 unit. These results are interpreted as providing circumstantial evidence that fusicoccin- and K+- induced stimulation of H+ extrusion led to an alkalinization of the cytosol, and that other early metabolic responses, such as an increase in malate level, are a consequence of the increase in cytosolic pH.