Signalling in guard cells and regulation of ion channel activity

A review is presented of the properties of ion channels in plasmalemma and tonoplast of stomatal guard cells, their regulation, with particular reference to Ca2+ and protein phosphorylation/dephosphorylation, and of the evidence for ABA-induced changes in specific ion channels, with an attempt to identify the signalling chains involved in each such change. A key question is whether a local increase in Ca2+, close to cell membranes and capable of triggering Ca2+-dependent changes in a variety of ion channels, is a universal feature of the ABA-reponse. If this is so, then there exist Ca2+-coupled mechanisms for most of the observed changes, including inhibition of the inward K+ channel and activation of the slow anion channel in the plasmalemma, and activation of two channels in the tonoplast, the K+-selective (VK) channel and the slow vacuolar (SV) channel, initiating efflux of both anions and cations from the vacuole. The detailed signalling chains are not complete, and the role of protein phosphorylation/dephosphorylation is not clearly defined, nor linked to ABA. Control of the outward K+ channel is Ca2(+)-independent; its activation by ABA may be mediated by cytoplasmic alkalinization, but the role of protein dephosphorylation in the signalling chain has still to be clarified. If Ca2+ is not available as second messenger, then the signalling chains involved have hardly begun to be understood. Detailed comparison of the efflux transients in different conditions provides evidence that ABA changes the 'set-point' of a stretch-activated channel, initiating loss of vacuolar K+. The inclusion of Ba2+ in the bathing solution has effects similar to those of reduced ABA concentration, a delay in initiating the vacuolar transient, and a slower rise to a reduced peak height. It is suggested that this could be the result of inhibition of the process of Ca2+ release from internal stores, by blocking a charge-balancing K+ flux.