The IRE1-bZIP60 branch of the unfolded protein response is required for the Arabidopsis immune response to Botrytis cinerea

The unfolded protein response (UPR) is a signalling pathway activated when endoplasmic reticulum (ER) proteostasis is disturbed. We investigated the contribution of the UPR in the Arabidopsis thaliana response to two necrotrophic fungi, Botrytis cinerea and Alternaria brassicicola. The IRE1-bZIP60 branch of the UPR was specifically activated upon infection with both pathogenic fungi, as evidenced by the production of active bZIP60 transcription factor forms and the increased expression of UPR-responsive genes. We also demonstrated using reverse genetics that the IRE1-bzIP60 axis was necessary to restrict foliar necrotic symptoms induced by both fungi. Furthermore, mutants deficient in two ER quality control components were more susceptible to infection by B. cinerea. By contrast, investigating the involvement of CDC48, an AAA+ ATPAse that assists the ER-associated degradation 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 the Arabidopsis immune response to B. cinerea, we quantified defence gene and cell death marker expression in bzip60 single and ire1 double mutants. None of these genes were misregulated in mutant backgrounds, indicating that the IRE1-bZIP60 branch of the 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.