Involvement of cyclic AMP in ABA- and Ca2+-mediated signal transduction ofstomatal regulation in Vicia faba

The focus of this study is to investigate possible involvement of cyclic AM P in regulation of Vicia stomatal movements. The presence of 0.1 mM 8-Br-cA MP, a membrane-permeable analogue of cAMP, alone in the incubation medium d id not affect stomatal opening in the light in leaf epidermal peel experime nts. However, addition of 0.1 mM 8-Br-cAMP completely reversed exogenous AB A- and Ca2+-induced inhibition of stomatal opening. Consistent with these r esults, patch-clamping experiments showed that intracellular addition of 0. 5 mM or 1 mM cAMP significantly reversed the inhibition of whole-cell inwar d K+ currents by internally supplied 13 mu M Ca2+ or 10 mu M ABA in stomata l guard cell protoplasts, respectively. Furthermore, intracellular addition of either 10 mu M prostaglandin E-1 (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+-induce d inhibition of inward K+-current. These results suggest that a cAMP signal ing pathway is involved in signal transduction in stomatal regulation by in teracting with ABA and Ca2+ signaling cascades. A hypothetical mechanism by which cAMP may regulate K+-channels in stomatal guard cells is also discus sed.