Abstract :

The maintenance of protein homeostasis is one of the cornerstones of cellular functions. It involves the precise regulation of translation, the folding of newly synthesized proteins, their post-translational modifications, their sorting and trafficking within the cell, and their degradation. The endoplasmic reticulum (ER) plays crucial roles in these processes, supporting the synthesis of one third to a quarter of total proteins in eukaryotic cells. Environmental factors intensify the workload on the protein folding machinery. When the ER is unable to meet the cell's demand for protein folding, unfolded or misfolded polypeptides accumulate within the ER lumen, leading to a proteotoxic stress known as ER stress. This situation actuates the Unfolded Protein Response (UPR), a retrograde, ER-to-nucleus, signalling pathway which aim is to increase the ER folding capacity, preventing an excessive build-up of misfolded or aberrant proteins that may occur during stressful situations. In Arabidopsis, the canonical UPR invokes the ribonuclease IRE1 and three bZIP transcription factors that are anchored in ER membranes (bZIP17, bZIP28 and bZIP60) and made soluble, therefore active, through distinct mechanisms when ER stress occurs. Target genes encompasses those coding for chaperones, co-chaperones and additional factors involved in the ER-Quality Control system, increasing ER folding capacity.

We use the model plant Arabidopsis thaliana to question the role of UPR and ER-Quality control system during plant infection by necrotrophic fungal pathogen such as Botrytis cinerea, the causal agent of grey mould. Our data indicate that B. cinerea infection induces and ER-stress. Furthermore, mutations in UPR regulators leads to either a greater susceptibility or an increase tolerance of plants to fungal infection, without affecting the expression of defence genes or cell death markers. It indicates that UPR modulates the Arabidopsis response to B. cinerea by a yet-to-be-identified mechanism. Interestingly, we identified the NAC053/NTL4 transcription factor as a potential actor of this unknown mechanism. UPR pathway is a conserved mechanism across kingdoms but in the green lineage, only few data are available in basal plants and algae. In the chlorophyte model Chlamydomonas reinhardtii, pioneer investigations uncovered the presence of a functional IRE1-dependent UPR branch; but to date there is no evidence reporting the occurrence of a bZIP17/28 branch in these organisms. We carried out an orthology-based approach to identify orthologues of Arabidopsis UPR proteins in twenty different species from glaucophyte to higher land plants. We then focused on the freshwater filamentous microalgae Klebsormidium nitens which is considered as a model organism for the study of plant adaptation to terrestrial life. The ER-stress inducer tunicamycin and heat stress have provoked the activation of UPR in K. nitens. By a functional complementation analysis using Arabidopsis upr mutants, we then identified K. nitens main UPR regulators and confirmed orthology analysis.