OsRNS4, with salinity-induced response, functions as a positive regulator of leaf senescence in rice

S-like RNases, a subclass of the RNase T2 family, are conserved ribonucleases in plants. However, their roles in leaf senescence remain poorly characterized, especially in monocot species. OsRNS4, previously identified as a salinity stress-induced S-like RNase in rice (Oryza sativa), lacks most of the conserved residues within the two catalytic active sites (CAS), suggesting a non-canonical mechanism of action. Expression analyses revealed that OsRNS4 is strongly induced during dark-induced senescence and by abscisic acid (ABA), a key regulator of stress and aging. Functional characterization using loss- and gain-of-function mutants demonstrated that OsRNS4 promotes leaf senescence: knockout mutants exhibited delayed senescence, whereas overexpression lines showed accelerated senescence under dark-induced conditions. Altered OsRNS4 expression also affected the transcript levels of senescence-associated genes and ABA signaling-related genes. Furthermore, we identified OsPHR2, a transcription factor previously implicated in phosphate-starvation signaling, positively controls OsRNS4 expression as well as senescence- and ABA-related genes. Both osrns4 and osphr2 mutants displayed reduced sensitivity to exogenous ABA-induced senescence. Together, these findings uncover a previously unrecognized role for OsRNS4 as a positive regulator of leaf senescence through ABA-mediated signaling pathways.