Fusicoccin- and light-induced activation and in vivo phosphorylation of phosphoenolpyruvate carboxylase in vicia guard cell protoplasts

The in vivo regulation of phosphoenolpyruvate carboxylase (PEPCase; EC 4.1. 1.31), was studied in purified guard cell protoplasts (GCPs) of Vicia faba L. Incubation of GCPs with fusicoccin (FC) led to the rapid activation of P EPCase and reduced its sensitivity towards the feedback-inhibitor malate. T his was accompanied by an increase in the phosphorylation state of the enzy me. Additionally, PEPCase could be transiently activated by white light. Ac tivation and phosphorylation of PEPCase upon illumination were dependent on the presence of potassium in the incubation medium. Treatment of GCPs with inhibitors of H+-ATPases, and with abscisic acid (ABA) suppressed the acti vation of PEPCase in a concentration-dependent manner. Treatment of protopl asts with butyrate also led to PEPCase activation, suggesting a role for th e cytosolic pH (pH,,) in the signal transduction process. The presented dat a indicate that guard cell PEPCase is regulated by reversible phosphorylati on of at least one isoform and elucidate first components of the signaling pathway. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.