Abstract
Plants offer a simpler and cheaper alternative to mammalian animal models for the study of endoplasmic reticulum glycoprotein folding quality control (ERQC). In particular, the Arabidopsis thaliana (At) innate immune response to bacterial peptides provides an easy means of assaying ERQC function in vivo. A number of mutants that are useful to study ERQC in planta have been described in the literature, but only for a subset of these mutants the innate immune response to bacterial elicitors has been measured beyond monitoring plant weight and some physio-pathological parameters related to the plant immune response. In order to probe deeper into the role of ERQC in the plant immune response, we monitored expression levels of the Phosphate-induced 1 (PHI-1) and reticulin-oxidase homologue (RET-OX) genes in the At ER α-Glu II rsw3 and the At UGGT uggt1-1 mutant plants, in response to bacterial peptides elf18 and flg22. The elf18 response was impaired in the rsw3 but not completely abrogated in the uggt1-1 mutant plants, raising the possibility that the latter enzyme is partly dispensable for EF-Tu receptor (EFR) signaling. In the rsw3 mutant, seedling growth was impaired only by concomitant application of the At ER α-Glu II NB-DNJ inhibitor at concentrations above 500 nM, compatibly with residual activity in this mutant. The study highlights the need for extending plant innate immune response studies to assays sampling EFR signaling at the molecular level.
Highlights
Glycoproteins traversing the secretory pathway of eukaryotic cells reach their cellular or extracellular destinations after folding in the endoplasmic reticulum (ER) [1]
Two missense mutants have been reported in Arabidopsis for the ER α–glucosidase II (Glu II) α catalytic subunit: the Arabidopsis thaliana (At) Glu II S517F known as psl5-1 mutant and the At ER α–Glu II S599F known as rsw3 mutant (Table S2)
We showed that germinated embryos of At psl5-1 were unable to growth in the presence of 70 μM NB-DNJ iminosugar [20], suggesting that At psl5-1 may possess residual activity, the latter only completely abrogated by the iminosugar treatment
Summary
Glycoproteins traversing the secretory pathway of eukaryotic cells reach their cellular or extracellular destinations after folding in the endoplasmic reticulum (ER) [1]. N-linked glycan, yielding a mono-glucosylated N-linked glycan [3]: thanks to the action of Glu. II on the native glycan, all glycoproteins have at least one chance to associate with CNX/CRT and profit from chaperone/foldase assisted folding. The same ERQC usher allowing glycoprotein entry into the calnexin cycle, enables escape from it: after having mediated the exposure of a mono-glucosylated glycan, Glu II catalyzes a second Glc cleavage, removing the inner glucose from the glycan, making it no longer a substrate for ER lectins CNX/CRT. Thanks to the mono-glucosylated N-linked glycans generated by Glu II, easy/quick-to-fold glycoproteins bind ERQC lectins and chaperones/foldases long enough to form successfully, and progress towards the Golgi down the secretory pathway
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