SIGNAL REDUNDANCY, GATES AND INTEGRATION IN THE CONTROL OF ION CHANNELS FOR STOMATAL MOVEMENT

As many as three distinct signalling pathways and their messengers-ent ailing changes in cytoplasmic-free Ca2+ ([Ca2+](i)), cytoplasmic pH (p H(i)) and protein phosphorylation-may underpin K+ and anion channel co ntrol during stomatal movements. Such a degree of redundancy is probab ly not unique among plant cells, and is wholly consistent with the abi lity of the guard cells to integrate the wide range of environmental a nd hormonal stimuli that affect stomatal aperture. In principle, signa l convergence enables a spectrum of graded responses extending beyond simple interference ('crosstalk'): it allows one pathway to gate trans mission via the next, so boosting or muting the final 'integrated sign al' that reaches the effector. Current evidence supports such a role f or the ABI1 protein phosphatase and, by inference, protein kinase elem ents in gating K+ channel sensitivity to pH(i) and ABA. In turn, gatin g 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 c ell plasma membrane, their coupling may be seen to add a further layer of control necessary for co-ordinating the ensemble of channel respon se during stomatal movements.