High concentration of adenosine or kinetin riboside induces stomatal closure in Vicia faba, probably through inhibition of adenylate cyclase

A high concentration (0.1 mM) of adenosine (Ade) or kinetin riboside (KR) provoked stomatal closure in Vicia faba, in light. One mM guanosine 5' -triphosphate (GTP) also elicited stomatal closure and had a marked closing effect when applied together with 0.1 mM Ade or KR. Guanosine 5' -triphosphate potentiated the closing effect of 0.1 mM Ade or KR on opened stomata, in light. This potentiating effect was very fast. A treatment with cholera toxin or Bordetella pertussis toxin stimulated stomatal aperture and both toxins were highly effective in attenuating, or even in cancelling; the Ade or KR closing effect. B. pertussis toxin was able to cancel methyladenosine-induced stomatal closure but not that induced by 2'-deoxiadenosine. The results support the proposition that the stomatal closing effect of 0.1 mM Ade or KR could be accounted for by inhibition of the adenylate cyclase present in plasmalemma of guard cells of V. faba, with the involvement of GTP, probably through a regulatory component of the enzyme. Moreover, the results obtained from Ade-analogues and B. pertussis toxin assays suggest that the inhibition of the enzyme may occur via specific plasma membrane-receptor.