Broadly Conserved Fungal Effector BEC1019 Enhances Virulence and Suppresses Host Cell Death

The interaction of barley, Hordeum vulgare L., with the biotrophic powdery mildew fungus, Blumeria graminis f. sp. hordei, is an ideal model to address fundamental questions in host resistance and susceptibility.  Effector proteins secreted by B. graminis act to inhibit, induce, or accelerate host processes to allow colonization and nutrient acquisition by the pathogen.  Previous research identified a collection of novel effectors secreted by B. graminis that contribute to pathogen virulence.  Blumeria effector candidate (BEC) 1019, a predicted metalloprotease, is expressed in haustorial feeding structures, and Host-Induced-Gene-Silencing (HIGS) of BEC1019 restricts haustorial development in compatible interactions.  Here we show that Barley stripe mosaic virus-Induced Gene Silencing (BSMV-VIGS) of BEC1019 significantly reduces the accumulation of fungal biomass in infected barley leaves.  Delivery of BEC1019 to the host cell cytoplasm via Xanthomonas type III secretion suppresses cultivar non-specific hypersensitive reaction (HR) induced by Xanthomonas oryzae pv. oryzicola, as well as cultivar specific HR induced by the AvrPphB effector from Pseudomonas syringae pv. phaseolicola.  Orthologs of BEC1019 are present in 97 of 241 sequenced fungal genomes, including plant pathogens Magnaporthe oryzae (rice blast) and Fusarium graminearum (Fusarium head blight of barley and wheat), as well as the human pathogens Candida albicans and Aspergillus fumigatus.  The C. albicans ortholog phenocopies BEC1019 in the Xanthomonas type III secretion assay.  Site-directed mutagenesis of BEC1019 implicates a conserved ETVIC motif as required for suppression of HR.  Taken together, BEC1019 represents a new class of biotrophic fungal effector notable for its broad conservation.  Supported by NSF-PGRP grant 09-22746 to RW, AB, DN, and JAD and NIH grant R01 GM046451 to RWI.