Dynamic G protein alpha signaling in Arabidopsis innate immunity

Heterotrimeric G proteins composed of Gα, Gβ and Gγ subunits are evolutionarily conserved signaling modules involved in diverse biological processes in plants and animals. The role and action of Gα remain largely enigmatic in plant innate immunity. We have recently demonstrated that Arabidopsis Gα (GPA1) is a key component of a new immune signaling pathway activated by bacteria-secreted proteases. Here we show that GPA1 is also involved in the signaling network of Arabidopsis in response to the bacterial flagellin epitope flg22. Specifically, GPA1 plays a pivotal role in an immune pathway involving the flg22 receptor FLS2, co-receptor BAK1, Regulator of G Signaling 1 (RGS1), and Arabidopsis Gβ (AGB1), in which flg22 elicits GPA1/AGB1 dissociation from the FLS2/BAK1/RGS1 receptor complex. Consequently, we observed flg22-induced degradation of FLS2, BAK1 and RGS1 but not GPA1 or AGB1. We also found that GPA1 constitutively interacts with the NADPH oxidase RbohD to potentiate flg22-induced ROS burst independently of the central cytoplasmic kinase BIK1. Taken together, our work sheds multiple novel insights into the functions and regulatory mechanisms of GPA1 in Arabidopsis innate immunity.