Ca(2+) signalling and control of guard-cell volume in stomatal movements.

Stomatal guard cells are unique as a plant cell model and, because of the depth of knowledge now to hand on ion transport and its regulation, serve as an excellent model for the analysis of stimulus-response coupling in higher plants. Parallel controls - mediated by Ca(2+), H(+) protein kinases and phosphatases - regulate the gating of the K(+) and Cl(-) channels that facilitate solute flux for stomatal movements. A growing body of evidence now indicates that oscillations in the cytosolic free concentration of Ca(2+) contribute to a 'signalling cassette', which is integrated within these events through an unusual coupling with membrane voltage. Additional developments during the past two years point to events in membrane traffic that play complementary roles in stomatal control. Research in these areas, especially, is now adding entirely new dimensions to our understanding of guard cell signalling.