Biology subjects
Publications and source records attributed to Gressent, F..
As an adaptation to flooding, few legume species have the original ability to develop nitrogen-fixing nodules on the stem. By surveying wetlands of Madagascar, we found a large occurrence and diversity of stem nodulation in Aeschynomene and Sesbania legumes. They represent opportunities to investigate different modalities of the nitrogen-fixing symbiosis in legumes.
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.
Among legumes (Fabaceae) capable of nitrogen-fixing nodulation, several Aeschynomene spp. use a unique symbiotic process that is independent of Nod factors and infection threads. They are also distinctive in developing root and stem nodules with photosynthetic bradyrhizobia. Despite the significance of these symbiotic features, their understanding remains limited. To overcome such limitations, we conducted genetic studies of nodulation in Aeschynomene evenia, supported by the development of a genome sequence for A. evenia and transcriptomic resources for 10 additional Aeschynomene spp. Comparative analysis of symbiotic genes substantiated singular mechanisms in the early and late nodulation steps. A forward genetic screen also showed that AeCRK, coding a novel receptor-like kinase, and the symbiotic signaling genes AePOLLUX, AeCCamK, AeCYCLOPS, AeNSP2 and AeNIN, are required to trigger both root and stem nodulation. This work demonstrates the utility of the A. evenia model and provides a cornerstone to unravel new mechanisms underlying the rhizobium-legume symbiosis.