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

Abstract

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...

The endoplasmic reticulum-associated degradation is necessary for plant salt tolerance

“…Previous studies have shown that UPR is induced upon salt stress [18] and that many stress-related genes are upregulated by tunicamycin (TM) and dithiothreitol (DTT), two ER stress inducers [19,20]. Some proteins that have functions in both ER quality control (ERQC) and stress responses also have been identified [21][22][23]. These studies suggest that ER stress and UPR are involved in plant responses to environmental stresses.…”

Arabidopsis thaliana Pattern Recognition Receptors for Bacterial Elongation Factor Tu and Flagellin Can Be Combined to Form Functional Chimeric Receptors

“…However, further deletions at the N terminus as in EFR⌬5 or further truncation from its C-terminal side resulted in proteins lacking affinity for elf peptides. Recent reports have shown that accumulation of functional EFR strongly depends on quality control steps and glycosylation processes in the secretory pathway of the plant (27,28). The constructs used in our experiments all contained an N-terminal signal sequence, and they led to accumulation of C-terminally tagged proteins with the size of appropriately glycosylated EFR derivatives ( Fig.…”

Emerging role of ER quality control in plant cell signal perception

“…Therefore, ER serves as a site of quality control for proteins, especially those bound for secretion. The ER-QC in plants, monitoring the folding and glycosylation state of the extracellular domain of receptors, is composed of three systems based on studies in animals and yeasts: BiP-mediated protein retention, thiolmediated retention and calnexin/calreticulin (CNX/CRT) cycle on the glycoproteins (Buck et al, 2007;Li et al, 2009). The first one is dependent on BiP-ERdj3-GP96/GRP94 complex, which resembles the cytosolic Hsp70-Hsp40-Hsp90 complex in assisting the folding of the non-native polypeptides by recognizing the exposed hydrophobic patches .…”