Co-ordination of signalling elements in guard cell ion channel control

Fine regulation of solutes transport across the guard cell plasma membrane for osmotic modulation is essential for the maintenance of the proper stomatal aperture in response to environmental stimuli. The major osmotica, K+, and malate are transported through selective ion channels in the plasma membrane and tonoplast of guard cells. To date, a number of ion channels have been shown to operate in the guard cell plasma membrane: outwardly- and inwardly-rectifying K+ channels (1K,in and 1K,out), slowly- and rapid-activating anion channels, and stretch-activated non-selective channels. Slow and fast vacuolar channels (SV and FV) and voltage-independent K+-selective (VK) channels have been found at the guard cell tonoplast. On the molecular level, the regulation of the stomatal aperture is achieved by precise spatial and temporal co-ordination of channel activities through a network of signalling cascades that can be triggered, among others, by plant hormones. ABA and auxin as regulators of stomatal aperture have received most attention to date. It is clear now that the effect of these hormones on ion channels is mediated by second messengers pHi and [Ca2+]i. The effect of ABA is generally associated with increases in pHi, while auxin acidifies the cytosol. Both of these hormones may induce elevation in [Ca2+]i. Phosphorylation is another important factor in cellular signalling. The ABI1 gene encoding a 2C-type protein phosphatase has been shown to be a key element of ABA-dependent cascades. Plasma membrane voltage is also an important component of signalling and channel control, and has recently been shown to influence cytosolic-free [Ca2+]. Thus, transduction of these, and associated Cytoplasmic signals is clearly non-linear, and is probably important for providing a plasticity of cellular response to external and environmental stimuli. Understanding the interdependence and hierarchy of signalling elements now presents a major challenge for research in plant biology.