Nodule formation in legume plants was assumed to be exclusively initiated by the binding of bacterial host-specific lipochitooligosaccharidic Nod factors to kinase-like receptors of the plant. Recently, sequence analysis of the genomes of two photosynthetic Bradyrhizobium strains (ORS278 and BTAi1), symbiotic of some Aeschynomene species, overturned this dogma because these bacteria lack the canonical nodulation genes (nodABC) involved in the synthesis of Nod factors [1]. This indicates that certain rhizobia use another mechanism to trigger nodule organogenesis in legumes. The most relevant characteristic of the photosynthetic Bradyrhizobium genomes will be presented. To identify the bacterial genes involved during this symbiosis, we individually screened a large Tn5 mutant library (20,000 mutants) of Bradyrhizobium ORS278 strain for their inability to induce nodules on Aeschynomene or to fix nitrogen. Results of this screening strategy will also be discussed. (Texte intégral)