Effect of Fusicoccin on Dark CO(2) Fixation by Vicia faba Guard Cell Protoplasts.

When Vicia faba guard cell protoplasts were treated with fusicoccin, dark (14)CO(2) fixation rates increased by as much as 8-fold. Rate increase was saturated with less than 1 micromolar fusicoccin. Even after 6 minutes of dark (14)CO(2) fixation, more than 95% of the incorporated radioactivity was in stable products derived from carboxylation of phosphoenolpyruvate (about 50% and 30% in malate and aspartate, respectively). The relative distribution of (14)C among products and in the C-4 position of malate (initially more than 90% of [(14)C]malate) was independent of fusicoccin concentration. After incubation in the dark, malate content was higher in protoplasts treated with fusicoccin. A positive correlation was observed between the amounts of (14)CO(2) fixed and malate content.It was concluded that (a) fusicoccin causes an increase in the rate of dark (14)CO(2) fixation without alteration of the relative fluxes through pathways by which it is metabolized, (b) fusicoccin causes an increase in malate synthesis, and (c) dark (14)CO(2) fixation and malate synthesis are mediated by phosphoenolpyruvate carboxylase.