Oomycete pathogens Produce RNA Silencing Repressors to Promote Infection

Effectors are essential virulence proteins produced by a broad range of parasites including viruses, bacteria, fungi, oomycetes, protozoa, insects and nematodes. Upon entry into host cells, pathogen effectors manipulate specific physiological processes or signaling pathways to subvert host immunity. Most effectors, especially those of eukaryotic pathogens, remain functionally uncharacterized. Here we show that two effectors from the oomycete plant pathogen Phytophthora sojae suppress RNA silencing in plants by inhibiting the biogenesis of small RNAs. Ectopic expression of these Phytophthora suppressors of RNA silencing enhances plant susceptibility to both a virus and Phytophthora, demonstrating that some eukaryotic pathogens have evolved virulence proteins that target host RNA silencing processes to promote infection. These findings reveal RNA silencing suppression as a common strategy used by pathogens across kingdoms to cause disease, and are consistent with RNA silencing having important roles in host defense.