Conserved DEAD-box RNA helicases regulate RNA silencing and plant immunity and are targeted by a powdery mildew effector.

RNA silencing is a conserved defense mechanism that protects plants against viruses, bacteria, fungi, and oomycetes. Although diverse pathogens have evolved effectors that suppress host RNA silencing, whether powdery mildew fungi exploit this strategy and the underlying molecular mechanisms remain unknown. Here, we identify CSEP0004, a secreted effector from the barley powdery mildew fungus Blumeria hordei, as an RNA silencing suppressor that promotes fungal virulence. CSEP0004 physically interacts with the evolutionarily conserved DEAD-box RNA helicases Hordeum vulgare RNA helicase 37 (HvRH37) and HvRH52. Genetic analyses demonstrate that HvRH37 and HvRH52 positively regulate chitin-triggered reactive oxygen species production, resistance to B. hordei, and small RNA accumulation in barley, while their Arabidopsis homologs AtRH11/37/52 perform conserved functions in small RNA accumulation. Mechanistically, HvRH37 and HvRH52 associate with HvAGO1, a core component of the RNA-induced silencing complex, and function as biochemically active RNA helicases capable of binding and unwinding double-stranded RNA. CSEP0004 disrupts the HvRH37/52-AGO1 association and inhibits HvRH37/52 helicase activity in a dose-dependent manner. Together, our findings uncover a previously unrecognized mechanism by which a fungal effector suppresses host RNA silencing through targeting conserved DDX3-like DEAD-box RNA helicases, establishing these helicases as key regulators of RNAi-mediated immunity and identifying RNA helicase-dependent RNA remodeling as a critical regulatory node exploited by fungal pathogens.