Plant innate immunity depends in part on recognition of pathogenassociated 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), UDPglucose 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.endoplasmic reticulum ͉ innate immunity ͉ receptor kinase P lant innate immunity involves three main processes: Recognition of conserved pathogen-associated molecular patterns (PAMPs) leading to PAMP-triggered immunity (PTI), suppression of defense by pathogen effectors, and recognition of specific effectors by cytoplasmic host proteins resulting in effector-triggered immunity (ETI) (1-4). Three pattern recognition receptors (PRRs) that can initiate PTI are known in the plant model Arabidopsis thaliana. Bacterial flagellin, and its peptide surrogate flg22 are recognized by the leucine-rich repeat receptor kinase (LRR-RK) FLS2 (5), bacterial elongation factor (EF)-Tu, and its surrogate peptide elf18 are recognized by the related LRR-RK EFR (6), while recognition of fungal chitin and unknown bacterial PAMP(s) depend on CERK1, a LysM domain RK (7-9).EFR and FLS2 are glycosylated transmembrane proteins (6, 10) and therefore need to enter the secretory pathway to mature and to reach their final plasma membrane destination. The endoplasmic reticulum (ER) is the first organelle of the secretory pathway and is responsible for the proper folding and assembly of polypeptides that are then directed to the Golgi. After translocation in the ER, newly synthesized polypeptides interact with different chaperones that will assist them to fold properly and to avoid aggregation in a process called ER quality control (ER-QC) (11). Misfolded proteins are directed to ER-associated degradation, leading to their clearance by the ubiquitin-proteasome in the cytosol (12). Most of our k...