Abstract
Research on the nitrogen-fixing symbiosis has been so far 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 semi-aquatic Aeschynomene species present the distinctive feature to form nitrogen-fixing nodules on both roots and stems following elicitation by photosynthetic bradyrhizobia that do not produce Nod factors. This interaction is believed to represent a living testimony of the ancestral state of the rhizobium-legume symbiosis. To decipher the molecular mechanisms of this unique Nod-independent nitrogen-fixing symbiosis, we previously identified A. evenia C. Wright as an appropriate model legume, because it displays all the requisites for molecular and genetic approaches. To advance the use of this new model legume species, here we characterized the intraspecific diversity found in A. evenia. For this, the accessions available in germplasm banks were collected and subjected to morphological investigations, genotyping with RAPD and SSR markers, molecular phylogenies using ITS and single nuclear gene sequences, and cross-compatibility tests. These combined analyses revealed an important intraspecific differentiation that led us to propose a new taxonomic classification for A. evenia comprising two subspecies and four varieties. The A. evenia ssp. evenia contains var. evenia and var. pauciciliata whereas A. evenia ssp. serrulata comprises var. serrulata and var. major. This study provides information to exploit efficiently the diversity encountered in A. evenia and proposes subsp. evenia as the most appropriate subspecies for future projects aimed at identifying plant determinants of the Nod-independent symbiotic process.
Highlights
The pantropical genus Aeschynomene belongs to the Dalbergieae tribe, an important group within the papilionoid legumes that is represented by peanut (Arachis hypogaea), the second most important crop legume
To investigate the intraspecific diversity in A. evenia, we thoroughly characterized a set of twenty seven accessions procured from USDA (USA), CIAT (Colombia), AusPGRIS (Australia) and the Senegal herbarium (University Cheikh Anta Diop, Dakar) (Table 1)
In the light of the data obtained with this study, we propose a revision of the taxonomic classification for A. evenia with new genotype delimitations
Summary
The pantropical genus Aeschynomene belongs to the Dalbergieae tribe, an important group within the papilionoid legumes that is represented by peanut (Arachis hypogaea), the second most important crop legume. This genus includes approximately 150 species, one half of them from the new world, mainly South and Central America from where the genus originates, the other half found across the tropical regions of Africa, South-East Asia, Australia and the Pacific Islands [1]. Half of the Aeschynomene species are rather xeric and are found in savannas or dry forests. The other half is composed of hydrophyte species growing in marshes, rice fields, waterlogged meadows, along stream and river banks [1]
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