ORM-mediated regulation of sphingolipid biosynthesis is essential for nodule formation in Aeschynomene evenia

Legumes are able to establish symbiotic interactions with nitrogen-fixing rhizobia that are accomodated in root-derived organs, the nodules. Rhizobia recognition triggers a plant symbiotic signalling pathway activating two coordinated processes: infection and nodule organogenesis. How these are orchestrated in legumes species utilizing intercellular infection and lateral root-base nodulation remain elusive. Here, we show that Aeschynomene evenia OROSOMUCOID PROTEIN 1 (AeORM1), a key regulator of sphingolipid biosynthesis, is required for nodule formation in this legume species. Using A. evenia orm1 mutants, we demonstrate that alterations in AeORM1 function result in numerous early aborted nodules, exhibiting defense-like reactions, and shortened lateral roots. Consistantly, AeORM1 was expressed during lateral root initiation and elongation, including at lateral root bases where nodule primordia form in the presence of symbiotic bradyrhizobia. Sphingolipidomics revealed that mutations in AeORM1 leaded to sphingolipid overaccumulation in roots, in particular the very-long-chain-fatty-acid (VLCFA)-containing ceramids relative to the wild-type plants. Taken together, our findings reveal that AeORM1-regulated sphingolipid homeostasis is essential for rhizobial infection and nodule organogenesis, and indicate a shared requirement for nodule formation and lateral root development in A. evenia.