Allen et al. studied how calcium signaling in Arabidopsis guard cells regulates stomatal closure. Calcium oscillations were imposed by changing the buffer from depolarizing (high KCl) to hyperpolarizing (low KCl with calcium) and calcium transients were measured by using a calcium indicator based on green fluorescent protein. Calcium transients induced closure of the stomatal pore, and cellular responsiveness was sensitive to the length of time between oscillations (maximal closure at a period of 10 min), as well as the duration of the oscillations (maximal closure at a transient duration of 5 min). Steady-state stomatal closure (that persisted after the oscillations ceased) was only detected when the transients were 10 min apart and not when the transients were 20 min apart or in response to a persistent 20-min calcium plateau. The last two mentioned conditions produced short-term decreases in aperture, but did not result in steady-state decreases in aperture. This suggests that the information for long-term changes in stomatal closure is encoded by the calcium oscillation pattern. Allen et al. analyzed calcium transients and stomatal closure in wild-type guard cells and guard cells from the growth controlled by abscisic acid (gca2) mutant in response to either calcium-induced calcium oscillations or abscisic acid (ABA). Cells from the gca2 plants had a much shorter period and duration for calcium oscillations produced by either treatment (calcium or ABA). They did not respond with steady-state stomatal closure, although the cells did exhibit short-term decreases in aperture. Steady-state stomatal closure could be restored if calcium oscillations similar to wild type were imposed; thus, the cells have an intact downstream signaling pathway, but cannot initiate the proper calcium oscillation code to trigger the pathway.
G. J. Allen, S. P. Chu, C. L. Harrington, K. Schumacher, T. Hoffman, Y. Y. Tang, E. Grill, J. I. Schroeder, A defined range of guard cell calcium oscillation parameters encodes stomatal movements. Nature 411, 1053-1057 (2001). [Online Journal]