The Arabidopsis heterotrimeric G‐protein β subunit, AGB 1, is required for guard cell calcium sensing and calcium‐induced calcium release

Summary Cytosolic calcium concentration ([Ca 2+ ] cyt ) and heterotrimeric G‐proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca o ) is as strong a stimulus for stomatal closure as the phytohormone abscisic acid ( ABA ), but underlying mechanisms remain elusive. Here, we report that the sole Arabidopsis heterotrimeric Gβ subunit, AGB 1, is required for four guard cell Ca o responses: induction of stomatal closure; inhibition of stomatal opening; [Ca 2+ ] cyt oscillation; and inositol 1,4,5‐trisphosphate (InsP3) production. Stomata in wild‐type Arabidopsis (Col) and in mutants of the canonical Gα subunit, GPA 1 , showed inhibition of stomatal opening and promotion of stomatal closure by Ca o . By contrast, stomatal movements of agb1 mutants and agb1 / gpa1 double‐mutants, as well as those of the agg1agg2  Gγ double‐mutant, were insensitive to Ca o . These behaviors contrast with ABA ‐regulated stomatal movements, which involve GPA 1 and AGB 1/ AGG 3 dimers, illustrating differential partitioning of G‐protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus‐specific encoding. AGB 1 knockouts retained reactive oxygen species and NO production, but lost YC 3.6‐detected [Ca 2+ ] cyt oscillations in response to Ca o , initiating only a single [Ca 2+ ] cyt spike. Experimentally imposed [Ca 2+ ] cyt oscillations restored stomatal closure in agb1 . Yeast two‐hybrid and bimolecular complementation fluorescence experiments revealed that AGB 1 interacts with phospholipase Cs (PLCs), and Ca o induced InsP3 production in Col but not in agb1 . In sum, G‐protein signaling via AGB 1/ AGG 1/ AGG 2 is essential for Ca o ‐regulation of stomatal apertures, and stomatal movements in response to Ca o apparently require Ca 2+ ‐induced Ca 2+ release that is likely dependent on Gβγ interaction with PLC s leading to InsP3 production.