Jean-François ARRIGHI - Genetics of the tropical legume Aeschynomene evenia to shed light on the Nod-independent symbiosis

Abstract

Research on the nitrogen-fixing symbiosis has been mainly focused on two model legumes, Medicago truncatula and Lotus japonicus, which use a sophisticated infection process involving infection thread formation. However, in 25% of the legumes, the bacterial entry occurs more simply in an intercellular fashion. Among them, some Aeschynomene species are nodulated by photosynthetic Bradyrhizobium strains that do not produce Nod factors.

To advance in our knowledge of this unique Nod-independent symbiotic process, we performed a molecular phylogeny of the genus Aeschynomene that evidenced the Nod-independent infection process arose in a single clade. Within this lineage, A. evenia appears to be the most appropriate species to constitute a bona fide model legume since it presents all the characteristics required for functional analysis.

Taking advantage of the intraspecific diversity in A. evenia, we have recently developed a genetic map that provides the first insights on the genome structure of this species and represents a significant tool for the mapping of symbiotic loci. This genetic map will support the “AeschyNod” project which aims at performing a “mutagenesis” strategy to generate and screen plant mutants defective for nodulation. To make successful this strategy, we are also developing a reference genome sequence for A. evenia, which will accelerate the map-based cloning of symbiotic genes.

The acquired knowledge should bring important highlights on the specific determinants of the Nod-independent process and the evolution of the nitrogen-fixing symbiosis in the legume family.

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