The characterization of differential calcium signalling in tobacco guard cells

Summary Two novel approaches for the study of Ca2+‐mediated signal transduction in stomatal guard cells are described. Stimulus‐induced changes in guard‐cell cytosolic Ca2+ ([Ca2+]cyt) were monitored using viable stomata in epidermal strips of a transgenic line of Nicotiana plumbaginifolia expressing aequorin (the proteinous luminescent reporter of Ca2+) and in a new transgenic line in which aequorin expression was targeted specifically to the guard cells. The results indicated that abscisic acid (ABA)‐induced stomatal closure was accompanied by increases in [Ca2+]cyt in epidermal strips. In addition to ABA, mechanical and low‐temperature signals directly affected stomatal behaviour, promoting rapid closure. Elevations of guard‐cell [Ca2+]cyt play a key role in the transduction of all three stimuli. However, there were striking differences in the magnitude and kinetics of the three responses. Studies using Ca2+ channel blockers and the Ca2+ chelator EGTA further suggested that mechanical and ABA signals primarily mobilize Ca2+ from intracellular store(s), whereas the influx of extracellular Ca2+ is a key component in the transduction of low‐temperature signals. These results illustrate an aspect of Ca2+ signalling whereby the specificity of the response is encoded by different spatial or kinetic Ca2+ elevations.