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

The <i>Ustilaginoidea virens</i> 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 <i>Oryza sativa</i> 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 <i>UvSfk1</i> essential for <i>U. virens</i> virulence. Thus, OsRNS4 might act as a carrier to transport miRNAs from rice into fungi, which target the virulence genes in <i>U. virens</i> 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.