Stimulus-Induced Oscillations in Guard Cell Cytosolic Free Calcium

Ca2+ is implicated as a second messenger in the response of stomata to a range of stimuli. However, the mechanism by which stimulus-induced increases in guard cell cytosolic free Ca2+ (Ca2+]i) are transduced into different physiological responses remains to be explained. Oscillations in Ca2+]i may provide one way in which this can occur. We used photometric and imaging techniques to examine this hypothesis in guard cells of Commelina communis. External Ca2+ (Ca2+]e), which causes an increase in Ca2+]i, was used as a closing stimulus. The total increase in Ca2+]i was directly related to the concentration of Ca2+]e, both of which correlated closely with the degree of stomatal closure. Increases were oscillatory in nature, with the pattern of the oscillations dependent on the concentration of Ca2+]e. At 0.1 mM, Ca2+]e induced symmetrical oscillations. In contrast, 1.0 mM Ca2+]e induced asymmetric oscillations. Oscillations were stimulus dependent and modulated by changing Ca2+]e. Experiments using Ca2+ channel blockers and Mn2+-quenching studies suggested a role for Ca2+ influx during the oscillatory behavior without excluding the possible involvement of Ca2+ release from intracellular stores. These data suggest a mechanism for encoding the information required to distinguish between a number of different Ca2+-mobilizing stimuli in guard cells, using stimulus-specific patterns of oscillations in Ca2+]i.