Bak1-Mediated Phosphorylation Of Canonical G Protein Alpha During Flagellin Signaling In Arabidopsis

Heterotrimeric G proteins consisting of G alpha, G beta and G gamma are conserved signaling hubs in eukaryotes. Without analogs to canonical animal G protein-coupled receptors, plant cells are thought to use RGS1 and a yet unknown mechanism to regulate the activity of G alpha. Meanwhile, the exact role of canonical G alpha in plant innate immunity remains controversial. Here, we report multiple immune deficiencies in the null allele of Arabidopsis G alpha (GPA1) in response to bacterial flg22 elicitor, clarifying a positive regulatory role of GPA1 in flg22 signaling. We also detect overall increased phosphorylation of GPA1 but reduced phosphorylation at Thr(19) upon flg22 elicitation. Interestingly, flg22 could not induce phosphorylation of GPA1(T19A) and GPA1(T19D), suggesting that the dynamic Thr(19) phosphorylation is required for GPA1 to respond to flg22. Moreover, flg22-induced GPA1 phosphorylation is largely abolished in the absence of BAK1 in vivo, and BAK1 could phosphorylate GPA1 but not GPA1(T19A) in vitro at the phosphorylation sites identified in vivo, suggesting BAK1 is likely the kinase for GPA1 phosphorylation in response to flg22. Furthermore, the T19A mutation could promote flg22-induced association, rather than dissociation, between GPA1 and RGS1. Taken together, our findings shed new insights into the function and regulation of GPA1 in Arabidopsis defense signaling.