OsRNS4-mediated cross-kingdom RNA interference is inhibited by <i>Ustilaginoidea virens</i> effector SCRE2 to promote infection in rice

The Ustilaginoidea virens effector SCRE2 has been previously identified to be an essential virulence factor. To elucidate molecular mechanisms underlying SCRE2 virulence function, we identify that SCRE2 interacts with Oryza sativa S-like ribonuclease family member OsRNS4. As a catalytically inactive RNase, OsRNS4 positively regulates rice immunity, depending on its RNA binding ability. OsRNS4 is unconventionally secreted through tetraspanin-positive extracellular vesicles (EVs). OsRNS4 binds to certain groups of rice miRNAs, whereas SCRE2 prevents the binding of OsRNS4 to miRNAs. Especially, OsRNS4-bound miR528-5p in rice is detected in false smut balls and silences the gene UvSfk1 essential for U. virens virulence. Thus, OsRNS4 might act as a carrier to transport miRNAs from rice into fungi, which target the virulence genes in U. virens and thereby contributes to rice resistance against fungal pathogens. As an unidentified counter-defense mechanism, SCRE2 suppresses cross-kingdom RNA interference to promote infection through inhibiting OsRNS4 binding to miRNAs.