Résumé
Two types of interactions have been described during the Aeschynomene-photosynthetic Bradyrhizobium symbioses-a classical one which is Nod factors (NF) dependent and an atypical one which is NF independent. It is the Aeschynomene sp. that determines the signaling modus. Some photosynthetic strains such as ORS285 that do contain the canonical nodABC genes are able to use bath strategies depending on the hast plant. Whereas the nod-Iacking strain such as BTAi 1 and ÛRS278 nodulate only some Aeschynomene species according a Nod-independent process. In this study, we investigate the role of lipopolysaccharides (LPS) in the establishment of the Aeschynomene-Bradyrhizobium symbiosis. Structural analysis of the LPS structure of photosynthetic Bradyrhizobium strains (BTAi1 and ORS278) reveals two original features : i) The only component of the 0- O-antigen region consists of a unique monosaccharide that had never been described before in nature. This new sugar was named Bradyrhizose (Alba S et al. 20 Il), il) the lipid A region displays the covalent attachment of an hopanoid molecule to a very long chain fatty acid. Screening of a mutant library of ûRS285 s train for clones showing an aberrant colony morphology has permitted to identify a mutant affected in a putative 0-antigen ligase that lacks the O-antigen part. Analysis of the symbiotic properties of this mutant shows that this region is important for the establishment of the Nod-dependent process but not for the Nod-independent one. To investigate the biological significance of the Hopanoid-lipid A, we cOl1structed a mutant in the squalene hopene cyclase (shc) gene that catalyzes a key step in hopanoid biosynthesis. This mutant was perfectly able to engage in an efficient symbiotic relationship with the plant but very rapidly the nodules lost their ability to fix nitrogen and senesce prematurely. Altogether these data indicate that Photosynthetic bradyrhizobia display a unique LPS structure that plays an important role in the establishment of the Nod-dependent symbiosis and in the chronic infection of the plant cells. (Résumé d'auteur)