EXTRA LARGE G-PROTEIN2 mediates cell death and hyperimmunity in the chitin elicitor receptor kinase 1-4 mutant

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 EXTRA LARGE G-PROTEIN2 (XLG2) mediates pathogen-associated molecular pattern (PAMP)-induced reactive oxygen species (ROS) generation and immunity downstream of pattern recognition receptors. A mutant of the chitin receptor component CHITIN ELICITOR RECEPTOR KINASE1 (CERK1), cerk1-4, maintains normal chitin signaling capacity but shows excessive cell death upon infection with powdery mildew fungi. We identified XLG2 mutants as suppressors of the cerk1-4 phenotype. Mutations in XLG2 complex partners ARABIDOPSIS Gβ1 (AGB1) and Gγ1 (AGG1) have a partial cerk1-4 suppressor effect. Contrary to its role in PAMP-induced immunity, XLG2-mediated control of ROS production by RESPIRATORY BURST OXIDASE HOMOLOGUE D (RBOHD) is not critical for cerk1-4–associated cell death and hyperimmunity. The cerk1-4 phenotype is also independent of the co-receptor/adapter kinases BRI1-ASSOCIATED RECEPTOR KINASE 1 (BAK1) and SUPPRESSOR OF BIR1 1 (SOBIR1), but requires the E3 ubiquitin ligase PLANT U-BOX 2 (PUB2). XLG2 localizes to both the cell periphery and nucleus, and the cerk1-4 cell death phenotype is mediated by the cell periphery pool of XLG2. Integrity of the XLG2 N-terminal domain, but not its phosphorylation, is essential for correct XLG2 localization and formation of the cerk1-4 phenotype. Our results support a model in which XLG2 acts downstream of an unknown cell surface receptor that activates an NADPH oxidase–independent cell death pathway in Arabidopsis (Arabidopsis thaliana).

Arabidopsis EXTRA‐LARGE G PROTEIN 1 (XLG1) functions together with XLG2 and XLG3 in PAMP‐triggered MAPK activation and immunity

“…XLG2 and XLG3 have been reported to confer resistance to fungal pathogens (Humphry et al, 2010; Maruta et al, 2015; Petutschnig et al, 2022). To test if XLG1 also contributes to resistance to fungal pathogens, we inoculated WT, xlg2/3 , and xlg triple mutants with Fusarium oxysporum .…”

“…The ROS burst and MAPK activation are critical defense responses activated upon pathogen recognition (Yuan et al, 2021). Some components of the heterotrimeric G proteins have been reported to play essential roles in ROS production during PTI (Liu et al, 2013; Maruta et al, 2015; Liang et al, 2016; Petutschnig et al, 2022). In addition, XLG2, XLG3, and AGB1 interact with FLS2, BIK1, and RbohD and are required for BIK1‐ and RbohD‐mediated ROS production after PAMP recognition (Liang et al, 2016).…”

“…XLGs and Gβγ have essential roles in PTI. The agb1 , agg1/2 , xlg2 , and xlg2/3 mutants are compromised in PAMP‐triggered ROS burst (Lee and Assmann, 1999; Liu et al, 2013; Maruta et al, 2015; Liang et al, 2016; Petutschnig et al, 2022). Here, we demonstrated that XLG1 functions redundantly with XLG2/3 in flg22‐induced ROS production.…”

“…However, whether G proteins function in other PTI responses is not known. XLG2 functions in a RbohD‐generated ROS‐independent pathway in CERK1‐mediated immunity (Petutschnig et al, 2022). Here, we demonstrated that XLGs are not only essential for the PAMP‐induced ROS burst but also interact with MAPK cascade proteins to regulate PRR‐mediated MAPK activation.…”

Conserved Role of Heterotrimeric G Proteins in Plant Defense and Cell Death Progression

“…Several studies have established a positive connection between G proteins and PCD in plants. Specifically, XLG2 and AGGs are known to interact directly with transmembrane receptor-like kinases (RLKs), including BAK1-interacting RLK 1 (BIR1) and flagellinsensitive 2 (FLS2) [10,33,37,38]. Arabidopsis mutants deficient in G proteins, such as agb1, agg1 agg2, and xlg2 xlg3, demonstrate markedly reduced reactive oxygen species (ROS) production induced by flagellin peptide flg22 [10,33,39,40].…”