XVI International Botanical Congess

Rhizobia are bacteria that fix nitrogen in nodules that they induce on the roots of legumes. They provide an instructive paradigm for plant-microbe interactions. All known rhizobia are alpha proteobacteria, but they do not form an exclusive clade. What they all share is a set of nodulation (nod) genes, which direct the synthesis of substituted lipooligochitins (Nod factors). These induce the plant responses that lead to nodule formation. The biochemical functions of the nod gene products are known, but in most cases the genes are only distantly related to homologs in other organisms or elsewhere in the genome. Hence the evolutionary origin of the nod genes is still obscure, but it seems clear that the core genes, nodDABCIJ, had a single origin but have since been shared among all the rhizobial lineages by lateral transfer. Their phylogeny does not match that of housekeeping genes in the bacteria. Indeed, in some respects nod gene relationships resemble those of the legume host plants. This is only to be expected since variation in the nod genes is a major determinant of the host range specificity in this symbiosis.