PR10 RNase activity drives immunity-mediated nodule cell death in Medicago littoralis

Establishment and maintenance of the legume-rhizobium symbiosis require a precise balance between host immune responses and symbiotic accommodation. In Medicago littoralis R108, mutation of NAD1 causes necrotic nodules accompanied by an overactivated immune response, indicating that tight immune regulation is essential for successful rhizobial colonization. Here, we identify members of the pathogenesis-related 10 (PR10) family as critical determinants of nodule function in M. littoralis. MltPR10 genes are transcriptionally upregulated in nad1-1 nodules. Strikingly, nad1-1 pr10c pr10d triple mutants produce nodules with almost normal morphology and restored nitrogenase activity compared with the necrotic nodules of nad1-1. MlPR10d exhibits robust ribonuclease activity against both plant and bacterial RNA, whereas the MlPR10d^Y150F variant abolishes its RNase activity and fails to restore the necrotic phenotype when introduced into the nad1-1 pr10c pr10d-mutant plants. Together, these findings indicate that PR10 RNase activity directly influences nodule cell fate and may function as a terminal executioner of RNA degradation during immunity-overactivated nodule cell death. Our results reveal a mechanistic link between immune regulation and nodule functionality and suggest molecular targets for improving biological nitrogen fixation in legumes.