Powdery mildew effectors AVR_A1and BEC1016 target the ER J-domain proteinHvERdj3B required for immunity in barley

Plant disease resistance often occurs upon direct or indirect recognition of pathogen effectors by host nucleotide-binding leucine-rich-repeat (NLR) receptors. The barley powdery mildew fungus,Blumeria graminisf. sp.hordei(Bgh), secretes hundreds of candidate secreted effector proteins (CSEPs) to facilitate pathogen infection and colonization. One of these, CSEP0008, is directly recognized by the barley NLR, MLA1, and therefore designated AVRA1. Here we show that AVRA1and the sequence-unrelatedBgheffector BEC1016 (CSEP0491) suppress immunity in barley cells. We then used yeast 2-hybrid next-generation-interaction screens (Y2H-NGIS), followed by binary Y2H and bimolecular fluorescence complementation, to identify a common barley target of AVRA1and BEC1016, the endoplasmic reticulum (ER)-localized J-domain protein,HvERdj3B. This is an ER protein quality control (ERQC) protein and silencing it increased theBghpenetration rate in barley.HvERdj3B is localized to the ER lumen, and AVRA1and BEC106 translocation into the ER was confirmed using a split GFP system. Together, these results suggest that the barley innate immunity, preventingBghentry into epidermal cells, is dependent on ERQC, which in turn is regulated by J-domain proteinHvERdj3B and the two effectors. Previous work has shown that AVRA1is directly recognized in the cytosol by the immune receptor, MLA1. We speculate whether the AVRA1J-domain target being inside the ER, where it is inapproachable by NLRs, has forced the plant to evolve this challenging direct recognition.