Signalling gates in abscisic acid-mediated control of guard cell ion channels

Multiple signalling pathways and their messengers - entailing changes in cytosolic-free Ca2+ ([Ca2+](i)), pH (pH(i)) and protein phosphorylation - underpin K+ and anion channel control during stomatal movements. This redundancy is wholly consistent with the ability of the guard cells to integrate the wide range of environmental and hormonal stimuli that affect stomatal aperture. Signal redundancy effects a spectrum of graded responses by linking pathways to gate signal transmission, and so boosts or mutes the final 'integrated signal' that reaches each ion channel. All evidence supports a role for the ABI1 protein phosphatase and protein kinase elements in gating K+ channel sensitivity to pH(i) and ABA. Changes in [Ca2+](i), in turn, are demonstrably sensitive to pH(i). Because each of these signal elements modulate and, in turn, are influenced by the activity of different sets of ion channels, the additional couplings engender a remarkably complex network, layering positive and negative controls with the ion channels that facilitate ion fluxes for stomatal movement.