Calcium-calmodulin signalling is involved in light-induced acidification by epidermal leaf cells of pea, Pisum sativum L

Pathways of signal transduction of red and blue lightdependent acidification by leaf epidermal cells were studied using epidermal strips of the Argenteum mutant of Pisum sativum. In these preparations the contribution of guard cells to the acidification is minimal. The hydroxypyridine nifedipine, a Ca -channel blocker, partly inhibited the response to both blue and red light, while the phenylalkylamine, verapamil, a Ca -channel blocker that has been shown in plant cells also to block K-channels, caused nearly complete inhibition. The Ca + -channel activator S(-)Bay K 8644 induced acidification when added in the dark and diminished the light-induced lowering of the extracellular pH. The Ca + -ionophores, ionomycin and A23187, also reduced the light response. Furthermore, the light-induced acidification was inhibited by the calmodulin antagonists W-7 and trifluoperazine, but not by W-5. These calmodulin inhibitors completely inhibited the red light-induced acidification, but inhibited the response to blue light by only 60-70%. In general, inhibition by compounds affecting Ca-calmodulin signalling was always stronger on the red light response than that on the blue light response (with the exception of verapamil that blocked both the red and blue light responses equally well). This differential effect on red and blue light-induced responses indicates a role for Ca + CaM signalling in both the red and blue light responses, while a second process, independent of Ca is activated by blue light.