EXTRA LARGE G-PROTEIN 2 (XLG2) mediates cell death and hyperimmunity via a novel, apoplastic ROS-independent pathway in Arabidopsis thaliana

ABSTRACT Heterotrimeric G-Proteins are signal transduction complexes comprised of three subunits, Gα, Gβ and Gγ, and are involved in many aspects of plant life. The non-canonical Gα subunit XLG2 mediates PAMP-induced ROS generation and immunity downstream of PRRs. A mutant of the chitin receptor component CERK1, cerk1 -4, maintains normal chitin signalling capacity, but shows excessive cell death upon infection with powdery mildews. We identified XLG2 mutants as suppressors of the cerk1 -4 phenotype. We generated stably transformed Arabidopsis lines expressing Venus-XLG2 and numerous mutated variants. These were analysed by confocal microscopy, Western blotting and pathogen infection. We also crossed cerk1 -4 with several mutants involved in immunity and analysed their phenotype. Phosphorylation of XLG2 was investigated by quantitative proteomics. Mutations in XLG2 complex partners AGB1 and AGG1 have a partial cerk1 -4 suppressor effect. The cerk1 -4 phenotype is independent of NADPH oxidase-generated ROS, BAK1 and SOBIR1, but requires PUB2. XLG2 mediates cerk1 -4 cell death at the cell periphery. Integrity of the XLG2 N-terminal domain, but not its phosphorylation, is essential for correct XLG2 localisation and cerk 1-4 signalling. Our results suggest that XLG2 transduces signals from an unknown cell surface receptor that activates an apoplastic ROS-independent cell death pathway in Arabidopsis .