A Receptor-like Cytoplasmic Kinase Phosphorylates the Host Target RIN4, Leading to the Activation of a Plant Innate Immune Receptor

Abstract

SUMMARY Plants have evolved sophisticated surveillance systems to recognize pathogen effectors delivered into host cells. RPM1 is an NB-LRR immune receptor that recognizes the Pseudomonas syringae effectors AvrB and AvrRpm1. Both effectors associate with and affect the phosphorylation of RIN4, an immune regulator. Although the kinase and the specific mechanisms involved are unclear, it has been hypothesized that RPM1 recognizes phosphorylated RIN4. Here we identify RIPK as a RIN4-interacting receptor-like protein kinase that phosphorylates RIN4. In response to bacterial effectors, RIPK phosphorylates RIN4 at amino acid residues T21, S160, and T166. RIN4 phosphomimetic mutants display constitutive activation of RPM1-mediated defense responses and RIN4 phosphorylation is induced by AvrB and AvrRpm1 during P. syringae infection. RIPK knockout lines exhibit reduced RIN4 phosphorylation and blunted RPM1-mediated defense responses. Taken together, our results demonstrate that the RIPK kinase associates with and modifies an effector-targeted protein complex to initiate host immunity.

Two redundant receptor-like cytoplasmic kinases function downstream of pattern recognition receptors to regulate activation of SA biosynthesis in Arabidopsis

“…In addition to Pto and PBS1, RLCKs such as BIK1, PBL1, RIPK, and OsRLCK185 are also targeted by pathogen effector proteins (J. Liu et al, 2011;Feng et al, 2012;Yamaguchi et al, 2013), further supporting the importance of RLCKs in plant immunity.…”

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

“…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). RIPK interacts with the P. syringae effector AvrB and either directly phosphorylates the immune regulator RPM1-INTERACTING PROTEIN4 (RIN4) or promotes AvrB-mediated RIN4 phosphorylation; phosphorylated RIN4 then activates RPM1 (Chung et al, 2011;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).…”

“…During effector-triggered immunity, another RLCK, RPM1-INDUCED PROTEIN KINASE (RIPK), is involved in activating the NB-LRR protein RPM1 (Liu et al, 2011). RIPK interacts with the P. syringae effector AvrB and either directly phosphorylates the immune regulator RPM1-INTERACTING PROTEIN4 (RIN4) or promotes AvrB-mediated RIN4 phosphorylation; phosphorylated RIN4 then activates RPM1 (Chung et al, 2011;Liu et al, 2011).…”

PBL13 is a serine/threonine protein kinase that negatively regulates Arabidopsis immune responses.

“…Another Arabidopsis RLCK VII, RESISTANCE TO PSEUDOMONAS SYRINGAE PV. MACULICOLA1 (RPM1)-INDUCED PROTEIN KI-NASE (RIPK), interacts with the plasma membraneassociated protein RPM1 INTERACTING4 (RIN4; Liu et al, 2011). RIN4 is targeted by several P. syringae effectors and monitored by the NLR proteins RPM1 and RPS2 (Mackey et al, 2002(Mackey et al, , 2003Axtell and Staskawicz, 2003).…”