The IRE1-bZIP60 branch of Unfolded Protein Response is required for Arabidopsis immune response to Botrytis cinerea.

The Unfolded Protein Response (UPR) is a signalling pathway activated when endoplasmic reticulum (ER) proteostasis is disturbed. We have investigated the contribution of UPR in Arabidopsis thaliana response to two necrotrophic fungi Botrytis cinerea and Alternaria brassicicola. We found out that the IRE1-bZIP60 branch of UPR was specifically activated upon infection with both pathogenic fungi, as evidenced by the production of the active bZIP60 transcription factor forms and the increased expression of UPR-responsive genes. We also demonstrated using reverse genetics that IRE1-bzIP60 axis was necessary to restrict foliar necrotic symptoms induced by both fungi. Furthermore, mutants deficient for two ER-QC components were also more susceptible to infection by B. cinerea. By contrast, investigating the involvement of CDC48, an AAA+-ATPAse that assist ER-Associated Degradation (ERAD) pathway, we showed that a series of mutants and transgenics are more resistant to B. cinerea. To gain molecular insights into how the ER shapes Arabidopsis immune response to B. cinerea, we quantified defence gene and cell death marker expression in bzip60 single and ire1 double mutants. None of those genes were misregulated in mutant backgrounds, indicating that IRE1-bZIP60 branch of UPR modulates the Arabidopsis response to B. cinerea by a yet-to-be-identified mechanism. Interestingly, we identified NAC053 as a potential actor of this unknown mechanism.