Cyclic adenosine monophosphate regulates calcium channels in the plasma membrane of Arabidopsis leaf guard and mesophyll cells

The effect of cAMP on Ca²⁺-permeable channels from Arabidopsis thaliana leaf guard cell and mesophyll cell protoplasts was studied using the patch clamp technique. In the whole cell configuration, dibutyryl cAMP was found to increase a hyperpolarization-activated Ba²⁺ conductance (I _Ba). The increase of I_Ba was blocked by the addition of GdCl₃. In excised outside-out patches, the addition of dibutyryl cAMP consistently activated a channel with particularly fast gating kinetics. Current/voltage analyses indicated a single channel conductance of ∼13 picosiemens. In patches where we measured some channel activity prior to cAMP application, the data suggest that cAMP enhances channel activity without affecting the single channel conductance. The cAMP activation of these channels was reversible upon washout. The results obtained with excised patches indicate that the cAMP-activated I_Ba seen in the whole cell configuration could be explained by a direct effect of cAMP on the Ca²⁺ channel itself or a close entity to the channel. This work represents the first demonstration using patch clamp analysis of the presence in plant cell membranes of an ion channel directly activated by cAMP.