Science doi:10.1126/science.1248849

Credit: ALBERTO MACHO

PAMP-triggered immunity (PTI) is initiated in plants when pathogen-associated molecular patterns (PAMPs)—typically fragments of pathogen biomolecules—interact with cell-surface pattern-recognition receptors (PRRs). For instance, the EF-TU RECEPTOR (EFR) in Arabidopsis thaliana is involved in antibacterial responses initiated by elf18, a peptide fragment of bacterial elongation factor Tu. Pathogenic bacteria have also evolved countermeasures to suppress these PTI responses. Macho et al. now highlight the importance of tyrosine phosphorylation of EFR (and other non-arginine-aspartate kinases) in PTI and in the evasive actions of pathogenic bacteria. Treatment of A. thaliana seedlings with different tyrosine kinase inhibitors diminishes the plant's response to PAMPs. Probing or immunoprecipitating EFR with anti-phosphotyrosine antibodies revealed that EFR is activated by tyrosine phosphorylation in vitro and in planta in an elf18-dependent way. Systematic mutagenesis of EFR tyrosines and in planta analysis revealed Tyr836 as a key phosphorylation site and regulator of EFR signaling activity. A targeted yeast two-hybrid screen revealed that HopA01—a virulence factor from Pseudomonas syringae that has protein tyrosine phosphatase (PTP) activity—binds EFR and reduces its tyrosine phosphorylation levels in plants. Taken together, the study suggests that the phosphorylation states of key tyrosine residues in PRRs are central to PTI signaling and thus may be susceptible to counterattack by effectors that are secreted by pathogens to blunt the host immune response.