Involvement of Cyclic AMP in ABA- and Ca2--Mediated Signal Transduction of Stomatal Regulation in Vicia faba

Abstract

The focus of this study is to investigate possible involvement of cyclic AMP in regulation of Vicia stomatal movements. The presence of 0.1 mM 8-Br-cAMP, a membrane-permeable analogue of cAMP, alone in the incubation medium did not affect stomatal opening in the light in leaf epidermal peel experiments. However, addition of 0.1 mM 8-Br-cAMP completely reversed exogenous ABA- and Ca2+-induced inhibition of stomatal opening. Consistent with these results, patch-clamping experiments showed that intracellular addition of 0.5 mM or 1 mM cAMP significantly reversed the inhibition of whole-cell inward K+ currents by internally supplied 13 μM Ca2+ or 10 μM ABA in stomatal guard cell protoplasts, respectively. Furthermore, intracellular addition of either 10 μM prostaglandin E1 (PGE1, an adenylate cyclase activator) or 1 mM 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor) mimicked the effect of exogenous cAMP on the removal of ABA- or Ca2+ inhibition of inward K+-current. These results suggest that a cAMP signaling pathway is involved in signal transduction in stomatal regulation by interacting with ABA and Ca2+ signaling cascades. A hypothetical mechanism by which cAMP may regulate K+ in stomatal guard cells is also discussed.