Signal redundancy, gates and integration in the control of ion channels for stomatal movement

As many as three distinct signalling pathways and their messengers-entailing changes in cytoplasmic-free Ca2+ ([Ca2+](i)), cytoplasmic pH (pH(i)) and protein phosphorylation-may underpin K+ and anion channel control during stomatal movements. Such a degree of redundancy is probably not unique among plant cells, and is wholly consistent with the ability of the guard cells to integrate the wide range of environmental and hormonal stimuli that affect stomatal aperture. In principle, signal convergence enables a spectrum of graded responses extending beyond simple interference ('crosstalk'): it allows one pathway to gate transmission via the next, so boosting or muting the final 'integrated signal' that reaches the effector. Current evidence supports such a role for the ABI1 protein phosphatase and, by inference, protein kinase elements in gating K+ channel sensitivity to pH(i) and ABA. In turn, gating of changes in [Ca2+](i) may also be subject to pH(i). Because these signal pathways affect discrete subsets of ion channels at the guard cell plasma membrane, their coupling may be seen to add a further layer of control necessary for co-ordinating the ensemble of channel response during stomatal movements.