Role of malate synthesis mediated by phosphoenolpyruvate carboxylase in guard cells in the regulation of stomatal movement

To clarify the pathway and role of malate synthesis in guard cells, epiderm al strips isolated from Vicia faba L. leaflets were treated with 3,3-dichlo ro-2-dihydroxyphosphinoylmethyl-2-propenoate (DCDP), a specific inhibitor o f phosphoenolpyruvate carboxylase (PEPC), When dark-closed stomata were ill uminated, malate accumulated in guard cells and stomata opened; these were inhibited by 60% and 30%, respectively, by 5 mM DCDP treatment, When light- opened stomata were treated with DCDP, both malate level in guard cells and stomatal aperture decreased. Treatment with 5 mM DCDP partially inhibited CO2 incorporation into malate in guard cells. Treatment with mannitol at 0. 4 M (osmotic stress) in the light increased malate level in guard cells and closed stomata. DCDP treatment decreased both malate level and stomatal ap erture under stressed condition, These results show that malate synthesis i n the light under both non-stressed and stressed conditions is dependent on PEPC activity. The extent of the decrease in malate level by DCDP treatmen t was larger under stressed condition than under nonstressed condition, sug gesting that osmotic stress may enhance the activity of this pathway of mal ate synthesis which is induced by light. Role of malate synthesis in guard cells is discussed.