nod Genes and Nod signals and the evolution of the rhizobium legume symbiosis

The establishment of the nitrogen-fixing symbiosis between rhizobia and leg umes requires an exchange of signals between the two partners. In response to flavonoids excreted by the host plant, rhizobia synthesize Nod factors ( NFs) which elicit, at very low concentrations and in a specific manner, var ious symbiotic responses on the roots of the legume hosts. NFs from several rhizobial species have been characterized. They all are lipo-chitooligosac charides, consisting of a backbone of generally four or five glucosamine re sidues N-acylated at the non-reducing end, and carrying various O-substitue nts. The N-acyl chain and the other substituents are important determinants of the rhizobial host specificity. A number of nodulation genes which spec ify the synthesis of NFs have been identified. All rhizobia, in spite of th eir diversity, possess conserved nodABC genes responsible for the synthesis of the N-acylated oligosaccharide core of NFs, which suggests that these g enes are of a monophyletic origin. Other genes, the host specific nod genes , specify the substitutions of NFs. The central role of NFs and nod genes i n the Rhizobium-legume symbiosis suggests that these factors could be used as molecular markers to study the evolution of this symbiosis. We have studied a number of NFs which are N-acylated by alpha,beta -unsatur ated fatty acids. We found that the ability to synthesize such NFs does not correlate with taxonomic position of the rhizobia. However, all rhizobia t hat produce NFs such nodulate plants belonging to related tribes of legumes , the Trifolieae, Vicieae, and Galegeae, all of them being members of the s o-called galegoid group. This suggests that the ability to recognize the NF s with alpha,beta -unsaturated fatty acids is limited to this group of legu mes, and thus might have appeared only once in the course of legume evoluti on, in the galegoid phylum.