Submitted to: Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2014
Publication Date: 3/28/2014
Citation: Macho, A., Schwessinger, B., Ntoukakis, V., Brutis, A., Segonzac, C., Roy, S., Kadota, Y., Oh, M., Sklenar, J., Huber, S.C., et al. 2014. A bacterial tyrosine phosphatase inhibits plant pattern recognition receptor activation. Science. 343:1509-1512. Interpretive Summary: An important mechanism in plant defense against pathogens involves sensing of pathogen-derived molecules, known as pathogen-associated molecular patterns or PAMPs, by specific receptor kinases in the plasma membrane. These PAMPs trigger the plant’s innate immunity system to initiate defense responses. Certain pathogens attempt to thwart this system by injecting effector proteins into the plant host cells that specifically block key steps in the innate immunity system. The Arabidopsis receptor kinase known as EFR recognizes the bacterial elongation factor Tu, and thereby recognizes the bacterial infection. Recent results indicate that autophosphorylation of EFR on tyrosine residues is essential for activity. Interestingly, one of the Pseudomonas syringae effector proteins is referred to as HopAO1 and is known to be an active protein tyrosine phosphatase. HopAO1 blocks plant immune responses in vivo and appears to do so by binding to EFR and dephosphorylating essential phosphotyrosine residues. These results illustrate a novel virulence mechanism employed by bacterial effectors and may yield insights to approaches to reduce the impact of crop bacterial diseases.
Technical Abstract: Perception of pathogen-associated molecular patterns (PAMPs) by surface-localised pattern-recognition receptors (PRRs) is a key component of plant innate immunity. Most known plant PRRs are receptor kinases and initiation of PAMP-triggered immunity (PTI) signalling requires phosphorylation of the PRR complex. However, the exact phosphorylation events and their biological roles are still unknown. Tyrosine (Tyr) phosphorylation of plant receptor kinases was only demonstrated recently, but its role in innate immunity has never been reported. Here, we reveal that PRR tyrosine phosphorylation is critical for PTI signalling and immunity. We show that the Arabidopsis leucine-rich repeat receptor kinase EFR, which perceives bacterial EF-Tu (or its immunogenic epitope elf18), is phosphorylated on tyrosine residues and that this modification is critical for EFR activation upon ligand binding. A single residue, Y836, located prior the catalytic loop of EFR kinase domain, is required for EFR activation, downstream responses and immunity to the phytopathogenic bacterium Pseudomonas syringae. Successful pathogens must suppress PTI, and many bacteria do so by injecting effector proteins into the plant cell via the type-III secretion system. Consistent with the critical role of PRR tyrosine phosphorylation in initiating PTI signalling, the P. syringae type III-secreted effector HopAO1, which is a tyrosine phosphatase, directly targets EFR to inhibit its activation and subsequent immunity. Our results shed light on a novel regulatory mechanism controlling plant immune signalling that is critical for anti-bacterial immunity.