A novel fungal effector from Puccinia graminis suppressing RNA silencing and plant defense responses.

Fungal plant pathogens, like rust-causing biotrophic fungi, secrete hundreds of effectors into plant cells to subvert host immunity and promote pathogenicity on their host plants by manipulating specific physiological processes or signal pathways, but the actual function has been demonstrated for very few of these proteins. Here, we show that the PgtSR1 effector proteins, encoded by two allelic genes (PgtSR1-a and PgtSR1-b), from the wheat stem rust pathogen Puccinia graminis f. sp. tritici (Pgt), suppress RNA silencing in plants and impede plant defenses by altering the abundance of small RNAs that serve as defense regulators. Expression of the PgtSR1s in plants revealed that the PgtSR1s promote susceptibility to multiple pathogens and partially suppress cell death triggered by multiple R proteins. Overall, our study provides the first evidence that the filamentous fungus P. graminis has evolved to produce fungal suppressors of RNA silencing and indicates that PgtSR1s suppress both basal defenses and effector triggered immunity.