ELEVATION OF CYTOPLASMIC CALCIUM BY CAGED CALCIUM OR CAGED INOSITOL TRISPHOSPHATE INITIATES STOMATAL CLOSURE

STOMATAL pores at the leaf surface regulate water loss during transpiration and CO2 uptake for photosynthesis through changes in the turgor of the surrounding guard cells. Such regulation occurs in response to a wide range of environmental stimuli (such as light, CO2 level and abscisic acid). These are thought to be transduced through changes in cytosolic calcium levels1-3, and several treatments causing stomatal closure are accompanied by increases in cytosolic Ca2+ (ref. 4), probably from mobilization of intracellular Ca2+ stores. We report here the use of the fluorescent Ca2+ indicator Fluo-3 (refs 5,6) to follow changes in stomatal aperture and cytoplasmic Ca2+ concentration as Ca2+ or inositol 1,4,5-trisphosphate (InsP3) are released into the cell from their caged (photoactivatable) forms6-9. Increasing Ca2+ concentration to greater than ̃600 nM induced stomatal closure. Similarly, release of InsP3 initiated influx of Ca2+ into the cytosol which was followed by stomatal closure. These results suggest that Ca2+ and InsP3 may act as second messengers leading to guard cell closure. © 1990 Nature Publishing Group.