The Arabidopsis heterotrimeric G-protein β subunit, AGB1, is required for guard cell calcium sensing and calcium-induced calcium release.

Cytosolic calcium concentration ([Ca ] ) and heterotrimeric G-proteins are universal eukaryotic signaling elements. In plant guard cells, extracellular calcium (Ca ) 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, AGB1, is required for four guard cell Ca responses: induction of stomatal closure; inhibition of stomatal opening; [Ca ] oscillation; and inositol 1,4,5-trisphosphate (InsP3) production. Stomata in wild-type Arabidopsis (Col) and in mutants of the canonical Gα subunit, GPA1, showed inhibition of stomatal opening and promotion of stomatal closure by Ca . 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 . These behaviors contrast with ABA-regulated stomatal movements, which involve GPA1 and AGB1/AGG3 dimers, illustrating differential partitioning of G-protein subunits among stimuli with similar ultimate impacts, which may facilitate stimulus-specific encoding. AGB1 knockouts retained reactive oxygen species and NO production, but lost YC3.6-detected [Ca ] oscillations in response to Ca , initiating only a single [Ca ] spike. Experimentally imposed [Ca ] oscillations restored stomatal closure in agb1. Yeast two-hybrid and bimolecular complementation fluorescence experiments revealed that AGB1 interacts with phospholipase Cs (PLCs), and Ca induced InsP3 production in Col but not in agb1. In sum, G-protein signaling via AGB1/AGG1/AGG2 is essential for Ca -regulation of stomatal apertures, and stomatal movements in response to Ca apparently require Ca -induced Ca release that is likely dependent on Gβγ interaction with PLCs leading to InsP3 production.