Rhizobium meliloti nodulation genes and their regulation

Nodulation of legume roots by rhizobia leads to the establishment of nitrogen fixing symbiosis. Genes determining nodule induction have been identified in several Rhizobium, Bradyrhizobium and Azorhizobium species. One set of nodulation genes, the common nod genes (nodABC) are essential and conserved in all rhizobia both functionally and at the DNA sequence level, while other sets of genes determine the host-specificity of nodulation (hsn) and/or contribute to a more efficient nodulation ability (19, for review). These nodulation genes are barely expressed in free-living cells. When rhizobia are exposed to exudates of the plant hosts, the nodulation genes are transcriptionally activated which requires also the regulatory NodD protein. The plant signals activating nod gene expression are various products of the phenylpropanoid pathway, primarily flavonoids or isoflavones (22, for review). The two classes of induced nodulation genes provide functions leading to root hair curling and induction of meristematic cell division. These plant responses are evoked by a family of signal molecules synthetized by different sets of nod gene products (2, 3, 8, 28). A host-specific signal has been recently identified by Lerouge et al. (21) as sulphated and acylated glucosamine oligosaccharide.