Specific ER quality control components required for biogenesis of the plant innate immune receptor EFR

Plant innate immunity depends in part on recognition of pathogen-associated molecular patterns (PAMPs), such as bacterial flagellin, EF-Tu, and fungal chitin. Recognition is mediated by pattern-recogntition receptors (PRRs) and results in PAMP-triggered immunity. EF-Tu and flagellin, and the derived peptides elf18 and flg22, are recognized in Arabidopsis by the leucine-rich repeat receptor kinases (LRR-RK), EFR and FLS2, respectively. To gain insights into the molecular mechanisms underlying PTI, we investigated EFR-mediated PTI using genetics. A forward-genetic screen for Arabidopsis elf18-insensitive (elfin) mutants revealed multiple alleles of calreticulin3 (CRT3), UDP-glucose glycoprotein glucosyl transferase (UGGT), and an HDEL receptor family member (ERD2b), potentially involved in endoplasmic reticulum quality control (ER-QC). Strikingly, FLS2-mediated responses were not impaired in crt3, uggt, and erd2b null mutants, revealing that the identified mutations are specific to EFR. A crt3 null mutant did not accumulate EFR protein, suggesting that EFR is a substrate for CRT3. Interestingly, Erd2b did not accumulate CRT3 protein, although they accumulate wild-type levels of other ER proteins. ERD2B seems therefore to be a specific HDEL receptor for CRT3 that allows its retro-translocation from the Golgi to the ER. These data reveal a previously unsuspected role of a specific subset of ER-QC machinery components for PRR accumulation in plant innate immunity.