A defined range of guard cell calcium oscillation parameters encodes stomatal movements

Oscillations in cytosolic calcium concentration ([Ca2+](cyt)) are central regulators of signal transduction cascades(1), although the roles of individual [Ca2+](cyt) oscillation parameters in regulating downstream physiological responses remain largely unknown. In plants, guard cells integrate environmental and endogenous signals to regulate the aperture of stomatal pores(2) and [Ca2+](cyt) oscillations are a fundamental component of stomatal closure(3,4). Here we systematically vary [Ca2+](cyt) oscillation parameters in Arabidopsis guard cells using a 'calcium clamp'(3,5-7) and show that [Ca2+](cyt) controls stomatal closure by two mechanisms. Shortterm 'calcium-reactive' closure occurred rapidly when [Ca2+](cyt) was elevated, whereas the degree of long-term steady-state closure was 'calcium programmed' by [Ca2+](cyt) oscillations within a defined range of frequency, transient number, duration and amplitude. Furthermore, in guard cells of the gca2 mutant(8), [Ca2+](cyt) oscillations induced by abscisic acid and extracellular calcium had increased frequencies and reduced transient duration, and steady-state stomatal closure was abolished. Experimentally imposing [Ca2+](cyt) oscillations with parameters that elicited closure in the wild type restored long-term closure in gca2 stomata. These data show that a defined window of guard cell [Ca2+](cyt) oscillation parameters programs changes in steady-state stomatal aperture.