Effect of Fusicoccin on Dark 14 CO 2 Fixation by Vicia faba Guard Cell Protoplasts

Abstract

When Vicia faba guard cell protoplasts were treated with fusicoccin,dark (14)CO(2) fixation rates increased by as much as 8-fold. Rateincrease was saturated with less than 1 micromolar fusicoccin. Evenafter 6 minutes of dark (14)CO(2) fixation, more than 95% of theincorporated radioactivity was in stable products derived from carboxylationof phosphoenolpyruvate (about 50% and 30% in malate and aspartate,respectively). The relative distribution of (14)C among productsand in the C-4 position of malate (initially more than 90% of [(14)C]malate)was independent of fusicoccin concentration. After incubation inthe 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 causesan increase in the rate of dark (14)CO(2) fixation without alterationof 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 phosphoenolpyruvatecarboxylase.