Putative involvement of cytosolic Ca2+ and GTP-binding proteins in cyclic-GMP-mediated induction of stomatal opening by auxin in Commelina communis L.

To investigate whether cyclic GMP (cGMP) would mediate, in an intracellular Ca2+-dependent manner, coupling of auxin to stomatal opening, the stomatal opening responses to the auxin indolyl-3-butyric acid (IBA) and to the cGMP membrane-permeable derivative 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) were compared in epidermal strips of Commelina communis. In this comparison were studied possible effects of intracellular Ca modulators. GTP-binding protein (G-protein) modulators and selective inhibitors of enzymatic reactions which use or generate cGMP. The stomatal response to IBA was almost similarly reversed by the Ca2+ buffer I,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) the intracellular Ca2+-release inhibitors ruthenium red and procaine, the inactive cGMP analog Rp-8-bromoguanosine 3',5'-cyclic monophosphorothioate (Rp-8-Br-cGMPS), the inhibitor of cGMP-producing guanylyl cyclase LY 83583: the G-protein inhibitor mas17 and the G-protein antagonist pGlu-Gln-D-Trp-Phe-D-Trp-D-Trp-Met-NH. Comparison with stomatal opening in response to 8-Br-cGMP, which was almost completely suppressed by either BAPTA, ruthenium red, procaine or Rp-8-Br-cGMPS, strongly suggests that cGMP acts downstream of G-protein activation as a second messenger for IBA signal transduction and that the cGMP pathway likely depends on cytosolic Ca2+-signaling.