A Fungal siRNA Is Involved in Genetic Robustness of <i>VmRDR</i> ‐Mediated Virulence in <i>Valsa mali</i> During Apple Infection

Genetic robustness refers to the ability of organisms to maintain normal phenotypes in the face of genetic variation, such as gene deletion. In plant pathogenic fungi, RNA-dependent RNA polymerases (RdRPs) play a crucial role in RNA interference (RNAi) signalling amplification in plant-pathogen interactions. However, the genetic robustness of RdRP-mediated fungal virulence and the molecular mechanisms regulating robustness remain elusive. In this study, we characterized the virulence genetic robustness following VmRDR3 knockout in Valsa mali during apple infection, and revealed an siRNA-mediated regulatory mechanism for genetic robustness. It was demonstrated that VmRDR3 knockout could induce compensatory upregulation of paralogous gene VmRDR2, which resulted in the stable abundance of sRNAs and the induction of new sRNAs generation, such as Vm-siR43. This siRNA specifically degrades MdWRKY3 (a disease resistance-related WRKY transcription factor gene in apple) in a sequence-specific manner, thereby suppressing host resistance. Concurrently, Vm-siR43 silences the fungal hypothetical protein gene VmHy5, impairing mycelial growth of V. mali. Our findings reveal a novel epigenetic regulation mechanism underlying RdRP-mediated fungal virulence robustness, operating through posttranscriptional gene silencing by a fungal siRNA. The results advance the understanding of the functional complexity of fungal RNAi components in coordinating pathogen adaptation and infection of the host.