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 cascades¹, 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 pores² 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. Short-term '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 mutant⁸, [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.