Ca2+ signalling and control of guard-cell volume in stomatal movements

Stomatal guard cells are unique as a plant cell model and, because of the d epth of knowledge now to hand on ion transport and its regulation, serve as an excellent model for the analysis of stimulus-response coupling in highe r plants. Parallel controls - mediated by Ca2+, H+ protein kinases and phos phatases - regulate the gating of the K+ and Cl- channels that facilitate s olute flux for stomatal movements. A growing body of evidence now indicates that oscillations in the cytosolic free concentration of Ca2+ contribute t o a 'signalling cassette: which is integrated within these events through a n unusual coupling with membrane voltage. Additional developments during th e past two years point to events in membrane traffic that play complementar y roles in stomatal control. Research in these areas, especially, is now ad ding entirely new dimensions to our understanding of guard cell signalling.