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The family Leguminosae is one of the largest families of flowering plants, including more than 18,000 species (Young, Johnston, 1989). Their symbiotic bacterial partners are very diverse and do not form a discrete clade. Although they are collectively referred to as rhizobia, they are now classified in distinct Rhizobium, Bradyrhizobium, Azorhizobium and Sinorhizobium (Young, Haukka, 1996). Some rhizobia are indeed more closely related to nonsymbiotic bacteria than they are to other rhizobia. However, all rhizobial species possess common nodulation (nod) genes, the nodABC genes, and consequently produce nodulation signals, the Nod factors, that belong to the same chemical family: they are chitin oligomers of 4 or 5 glucosamine residues that are mono-N-acylated at the terminal non-reducing end. The common chitin oligomer core can be substituted at both ends of the molecule, and each rhizobial species or biovar produces a set of major Nod factors with characteristic substitutions (Denarie et al., 1996). These substitutions are under the control of specific nod genes. In the case of Sinorhizobium meliloti, it has been shown that the three substitutions are essential for effective infection of the host plant, alfalfa. The structure of major Nod factors has now been determined for most rhizobial species and their comparison indicates that Nod factor structure is related to rhizobial host range (Denarie et al., 1996). |
