Broadly Conserved Fungal Effector BEC1019 Suppresses Host Cell Death and Enhances Virulence of Powdery Mildew in Barley (Hordeum vulgare L.)

The interaction of barley, Hordeum vulgare L., with the biotrophic powdery mildew fungus, Blumeria graminis f. sp. hordei (Bgh), is an ideal model to address fundamental questions in host resistance and susceptibility.  Effector proteins secreted by Bgh suppress or induce host processes to promote nutrient acquisition and colonization.  The 130-Mb Blumeria genome harbors ca. 540 predicted secreted effectors, designated BECs (Blumeria Effector Candidates) or CSEPs (Candidate Secreted Effector Proteins).  Large-scale RNA-Seq of Bgh-infected resistance-signaling mutants indicates that distinct subsets of BECs/CSEPs are differentially regulated at penetration, or in haustoria.  BEC1019, a predicted single-copy metalloprotease, is differentially expressed in haustoria among all barley loss-of-function mutants.  BSMV-mediated silencing of BEC1019 significantly reduces fungal colonization of barley epidermal cells, demonstrating that BEC1019 plays a central role in virulence.  In addition, 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.  BEC1019 homologs are present in 96 of 241 sequenced fungal genomes, including plant pathogens, human pathogens, and free-living non-pathogens.  Comparative analysis revealed variation at several amino acid positions that correlate with fungal lifestyle, and several highly conserved, non-correlated motifs.  Site-directed mutagenesis of one of these, ETVIC, compromises the HR suppressing activity of BEC1019.  We postulate that BEC1019 represents an ancient, broadly important fungal protein family, members of which have evolved to function as effectors in plant and animal hosts.

Supported by NSF-PGRP grant 09-22746 to RW, AB, DN, and JAD and NIH grant R01 GM046451 to RWI.