Spencer C. Brown,2
Clémence Chaintreuil,1 and
1IRD, Laboratoire des Symbioses Tropicales et Méditerranéennes, UMR IRD/SupAgro/INRA/UM2/CIRAD, Campus International de Baillarguet, TA A-82/J, 34398 Montpellier Cedex 5, France; 2CNRS, IBiSA imagerie Gif, Institut des Sciences du Végétal, UPR 2355, 23/24, avenue de la Terrasse, 91198 Gif-sur-Yvette, France; 3CIRAD, Laboratoire de Cytogénétique Moléculaire, UMR AGAP, TA-A 108/03, 34398 Montpellier Cedex 5, France
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Accepted 22 March 2012.
Research on the nitrogen-fixing symbiosis has been focused, thus far, 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 spp. are nodulated by photosynthetic Bradyrhizobium spp. 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 mechanisms of this Nod-independent process, we propose Aeschynomene evenia as a model legume because it presents all the characteristics required for genetic and molecular analysis. It is a short-perennial and autogamous species, with a diploid and relatively small genome (2n = 20; 460 Mb/1C). A. evenia ‘IRFL6945’ is nodulated by the well-characterized photosynthetic Bradyrhizobium sp. strain ORS278 and is efficiently transformed by Agrobacterium rhizogenes. Aeschynomene evenia is genetically homozygous but polymorphic accessions were found. A manual hybridization procedure has been set up, allowing directed crosses. Therefore, it should be relatively straightforward to unravel the molecular determinants of the Nod-independent process in A. evenia. This should shed new light on the evolution of rhizobium–legume symbiosis and could have important agronomic implications.
© 2012 The American Phytopathological Society