Receptor-like Cytoplasmic Kinases Integrate Signaling from Multiple Plant Immune Receptors and Are Targeted by a Pseudomonas syringae Effector

Abstract

Cell-surface-localized plant immune receptors, such as FLS2, detect pathogen-associated molecular patterns (PAMPs) and initiate PAMP-triggered immunity (PTI) through poorly understood signal-transduction pathways. The pathogenic Pseudomonas syringae effector AvrPphB, a cysteine protease, cleaves the Arabidopsis receptor-like cytoplasmic kinase PBS1 to trigger cytoplasmic immune receptor RPS5-specified effector-triggered immunity (ETI). Analyzing the function of AvrPphB in plants lacking RPS5, we find that AvrPphB can inhibit PTI by cleaving additional PBS1-like (PBL) kinases, including BIK1, PBL1, and PBL2. In unstimulated plants, BIK1 and PBL1 interact with FLS2 and are rapidly phosphorylated upon FLS2 activation by its ligand flg22. Genetic and molecular analyses indicate that BIK1, and possibly PBL1, PBL2, and PBS1, integrate immune signaling from multiple immune receptors. Whereas AvrPphB-mediated degradation of one of these kinases, PBS1, is monitored by RPS5 to initiate ETI, this pathogenic effector targets other PBL kinases for PTI inhibition.

Pathogen-Associated Molecular Pattern-Triggered Immunity: Veni, Vidi…?

“…Despite the role of the MAPKs 3, 4, and 6, the involvement of the upstream MAPKK and MAPKKK is less clear and a link to the upstream receptor complexes is missing. This could involve cytoplasmic kinases such as BIK1, which is phosphorylated upon flg22 elicitation and transphosphorylates FLS2 and BAK1 (Lu et al, 2010;Zhang et al, 2010), but interaction with the MAPK cascade has not been demonstrated. The MAPKs 3, 4, and 6 are prominent signaling kinases in many stress-related pathways mediating specific transcriptional changes.…”

“…BAK1 is a member of a small family of LRRRKs with partly redundant functions, and it is possible that PRRs are also regulated by other SERKs. Besides some evidence for protein phosphatases and the cytoplasmic kinase BIK1, which interacts with FLS2 and possibly BAK1 (Lu et al, 2010;Zhang et al, 2010), the dynamics, composition, stoichiometry of, and interactions within unstimulated and activated PRR complexes remain elusive.…”

CAST AWAY, a Membrane-Associated Receptor-Like Kinase, Inhibits Organ Abscission in Arabidopsis

“…Palmitoylation of N-terminal Cys residues is also predicted to allow reversible membrane association for CST and many other class VII RLCKs ( Fig. 4C; Sorek et al, 2009;Zhang et al, 2010). To visualize CST protein within Arabidopsis cells, we generated a CST-GFP fusion construct driven by the constitutive viral 35S promoter that could be transfected into mesophyll protoplasts.…”

“…A recent study has revealed that, like PBS1, BIK1 and several PBS1-like (PBL) RLCKs are also substrates of the bacterial AvrPphB protease effector . This discovery and other work suggests that a bacterial effector can suppress PAMP-triggered immunity in plants by cleaving RLCKs known (BIK1) or proposed (PBL1, PBL2) to positively interact with RLKs that bind PAMPs, including FLS2, CHITIN ELICITOR RECEPTOR KINASE1, and the EF-Tu RE-CEPTOR (Zipfel et al, 2006;Miya et al, 2007;Zhang et al, 2010). During effector-triggered immunity, another RLCK, RPM1-INDUCED PROTEIN KINASE (RIPK), is involved in activating the NB-LRR protein RPM1 (Liu et al, 2011).…”

“…Although receptor-like cytoplasmic kinases (RLCKs) account for at least 125 of the 610 annotated RLKs in Arabidopsis (Arabidopsis thaliana), much remains to be learned about their functions within cell signaling complexes (Shiu and Bleecker, 2001;Goring and Walker, 2004;Shiu et al, 2004;Jurca et al, 2008). Several of the 46 RLCKs assigned to the class VII subfamily have been found to function in pathogen response and developmental signaling pathways (Swiderski and Innes, 2001;Shao et al, 2003;Murase et al, 2004;Muto et al, 2004;Veronese et al, 2006;Ade et al, 2007;Lu et al, 2010;Zhang et al, 2010;Liu et al, 2011).…”