The Role of Arabidopsis Heterotrimeric G-Protein Subunits in MLO2 Function and MAMP-Triggered Immunity

Heterotrimeric G-proteins, composed of G alpha, G beta, and G gamma subunits, regulate many fundamental processes in plants. In animals, ligand binding to seven transmembrane (7TM) cell surface receptors designated G-protein coupled receptors (GPCR) leads to heterotrimeric G-protein activation. Because the plant G-protein complex is constitutively active, the exact role of plant 7TM proteins in this process is unclear. Members of the mildew resistance locus O (MLO) family represent the best-characterized 7TM plant proteins. Although genetic ablation of either MLO2 or G-proteins alters susceptibility to pathogens in Arabidopsis thaliana, it is unknown whether G-proteins directly couple signaling through MLO2. Here, we exploited two well-documented phenotypes of mlo2 mutants, broad-spectrum powdery mildew resistance and spontaneous callose deposition in leaf mesophyll cells, to assess the relationship of MLO2 proteins to the G-protein complex. Although our data reveal modulation of antifungal defense responses by G beta and G gamma subunits and a role for the G gamma 1 subunit in mlo2-conditioned callose deposition, our findings overall are inconsistent with a role of MLO2 as a canonical GPCR. We discovered that mutants lacking the G beta subunit show delayed accumulation of a subset of defense-associated genes following exposure to the microbe-associated molecular pattern flg22. Moreover, G beta mutants were found to be hypersusceptible to spray inoculation with the bacterial pathogen Pseudomonas syringae.