Similar Characteristics of siRNAs of Plant Viruses Which Replicate in Plant and Fungal Hosts.

RNA silencing is a host innate antiviral mechanism which acts via the synthesis of viral-derived small interfering RNAs (vsiRNAs). We have previously reported the infection of phytopathogenic fungi by plant viruses such as cucumber mosaic virus (CMV) and tobacco mosaic virus (TMV). Furthermore, fungal RNA silencing was shown to suppress plant virus accumulation, but the characteristics of plant vsiRNAs associated with the antiviral response in this nonconventional host remain unknown. Using high-throughput sequencing, we characterized vsiRNA profiles in two plant RNA virus-fungal host pathosystems: CMV infection in phytopathogenic fungus and TMV infection in phytopathogenic fungus . The relative abundances of CMV and TMV siRNAs in the respective fungal hosts were much lower than those in the respective experimental plant hosts, and . However, CMV and TMV siRNAs in fungi had similar characteristics to those in plants, particularly in their size distributions, proportion of plus and minus senses, and nucleotide preference for the 5' termini of vsiRNAs. The abundance of TMV siRNAs largely decreased in mutants with a deletion in either () or genes which encode key proteins for the production of siRNAs and antiviral responses. However, deletion of both and restored TMV siRNA accumulation in , indicating the production of -independent siRNAs with no antiviral function in the absence of the and genes. Our results suggest that fungal RNA silencing recognizes and processes the invading plant RNA virus genome in a similar way as in plants.