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

Abstract

Heterotrimeric G-Proteins are signal transduction complexes comprised of three subunits, Gα, Gβand 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 cerk1-4 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.

G-Protein Phosphorylation: Aspects of Binding Specificity and Function in the Plant Kingdom

“…In addition, XLG2 and XLG3 interact with BIK1, FLS2, and RbohD (NADPH/respiratory burst oxidase protein D), and the complementation of knockout plants with AtXLG2 S141/148/150/151A expression abolishes flg22-induced phosphorylation and lowers ROS response compared to that in wild-type plants [ 47 ]. In contrast, XLG2 signaling with CERK1 (CHITIN ELICITOR RECEPTOR KINASE 1) under chitin elicitation is not affected by the same N-terminal mutations [ 77 ].…”

Studies on plant immunity: dissection of the Cf-4/SOBIR1-containing complex in Nicotiana benthamiana

“…Agroinfiltration of none of these tandem constructs resulted in a reduction of the Avr4-triggered apoplastic ROS burst (Figure S6B). Since the Arabidopsis XLGs have been shown to be redundantly required for the ROS burst triggered upon challenge with flg22 239,257 , we also tested the flg22-triggered ROS burst in the Gα-and Gβ-agroinfiltrated N. benthamiana:Cf-4 plants (Figure S6C).…”

“…We included all the genes encoding Gα subunits of N. benthamiana in our silencing experiments, because of the evidence of redundancy in their function. For instance, Arabidopsis XLG1, XLG2 and XLG3 are redundantly required for regulating root morphogenesis , in addition to the flg22-induced ROS burst 257 . However, the role of XLG1 in modulating the flg22-induced ROS burst is unclear, as there is evidence suggesting that only XLG2 and XLG3 are part of the XLG-Gβγ complexes that regulate signalling by the receptor FLS2 239,240 .…”