Membrane-Localized Extra-Large G Proteins and Gβγof the Heterotrimeric G Proteins Form Functional Complexes Engaged in Plant Immunity in Arabidopsis

In animals, heterotrimeric G proteins, comprising Gα, Gβ,and Gγsubunits, are molecular switches whose function tightly depends on Gαand Gβγinteraction. Intriguingly, in Arabidopsis (Arabidopsis thaliana), multiple defense responses involve Gβγ, but notGα. We report here that the Gβγdimer directly partners with extra-large G proteins (XLGs) to mediate plant immunity. Arabidopsis mutants deficient in XLGs, Gβ, and Gγare similarly compromised in several pathogen defense responses, including disease development and production of reactive oxygen species. Genetic analysis of double, triple, and quadruple mutants confirmed that XLGs and Gβγfunctionally interact in the same defense signaling pathways. In addition, mutations in XLG2 suppressed the seedling lethal and cell death phenotypes ofBRASSINOSTEROID INSENSITIVE1-associated receptor kinase1-interacting receptor-like kinase1mutants in an identical way as reported for Arabidopsis Gβ-deficient mutants. Yeast (Saccharomyces cerevisiae) three-hybrid and bimolecular fluorescent complementation assays revealed that XLG2 physically interacts with all three possible Gβγdimers at the plasma membrane. Phylogenetic analysis indicated a close relationship between XLGs and plant Gαsubunits, placing the divergence point at the dawn of land plant evolution. Based on these findings, we conclude that XLGs form functional complexes with Gβγdimers, although the mechanism of action of these complexes, including activation/deactivation, must be radically different form the one used by the canonical Gαsubunit and are not likely to share the same receptors. Accordingly, XLGs expand the repertoire of heterotrimeric G proteins in plants and reveal a higher level of diversity in heterotrimeric G protein signaling.