Receptor quality control in the endoplasmic reticulum for plant innate immunity

Abstract

Patternrecognition receptors in eukaryotes initiate defence responses on detection of microbe-associated molecular patterns shared by many microbe species. The Leu-rich repeat receptor-like kinases FLS2 and EFR recognize the bacterial epitopes flg22 and elf18, derived from flagellin and elongation factor-Tu, respectively. We describe Arabidopsis 'priority in sweet life' (psl) mutants that show de-repressed anthocyanin accumulation in the presence of elf18. EFR accumulation and signalling, but not of FLS2, are impaired in psl1, psl2, and stt3a plants. PSL1 and PSL2, respectively, encode calreticulin3 (CRT3) and UDP-glucose:glycoprotein glycosyltransferase that act in concert with STT3A-containing oligosaccharyltransferase complex in an N-glycosylation pathway in the endoplasmic reticulum. However, EFR-signalling function is impaired in weak psl1 alleles despite its normal accumulation, thereby uncoupling EFR abundance control from quality control. Furthermore, salicylic acid-induced, but EFR-independent defence is weakened in psl2 and stt3a plants, indicating the existence of another client protein than EFR for this immune response. Our findings suggest a critical and selective function of N-glycosylation for different layers of plant immunity, likely through quality control of membrane-localized regulators.

Requirement of Vesicle-Associated Membrane Protein 721 and 722 for Sustained Growth during Immune Responses in Arabidopsis

“…Similarly, dysfunction in ER N-linked glycosylation by mutations in genes of UDP-glucose:glyco-protein glycosyltransferase (UGGT) and STT3A, a component of oligosaccharyltransferase complex, also reduces SA-induced defence against P. syringae pv. maculicula (Saijo et al, 2009). These reports indicate that a secretory pathway that traverses the ER and Golgi is critical for successful defense responses in plants.…”

The function of calreticulin in plant immunity

“…Particularly, recent studies suggest that both CRT isoforms are important regulators of plant immunity. 11,12,15 As shown in Figure 2, different atcrt mutants display distinct disease symptoms when infected with Pseudomonas syringae pv tomato DC3000 (Pst DC3000). While atcrt1 and atcrt2 mutant plants are more resistant to the Pst DC3000 infection than wild-type plants, atcrt3 mutants are more sensitive (Fig.…”

“…AtCRT3 functions as a potential component of ER quality control (ER-QC) and is indispensible for the abundance of functional leucine-rich repeat receptor kinase EFR. 11,12 EFR is a pattern-recognition receptor that can specifically recognize bacterial elongation factor (EF)-Tu and its surrogate peptide elf18. This recognition further initiates pathogen-associated molecular pattern-triggered immunity (PTI).…”

“…4 Further studies revealed that CRT not only plays a key role in the intracellular Ca 2+ storage and endoplasmic reticulum (ER) Ca 2+ homeostasis, [5][6][7] but also functions as a crucial molecular chaperone. [8][9][10][11][12] …”

Emerging role of ER quality control in plant cell signal perception

“…Surprisingly, FLS2 accumulation is not affected, nor its mediated microbe response or affected to a less extent. These mutants are loss-of-function mutants of CRT3, UGGT, GIIα, GIIβ and STT3A that act in the CNX/CRT cycle Lu et al, 2009;Nekrasov et al, 2009;Saijo et al, 2009). The HDEL receptor family member (ERD2b) is also required for EFR accumulation through regulating the ER retention of CRT3 .…”