Identification of Bradyrhizobium nod genes involved in host-specific nodulation

Three loci important for soybean nodulation by Bradyrhizobium japonicum were delimited by Tn5 mutagenesis on a 5.3-kilobase EcoRI fragment adjacent to the nodABC genes. Results of hybridization studies suggested that this region is conserved in Bradyrhizobium species but absent in all Rhizobium species. lacZ translational fusions of two of the loci contained in this region were found to be inducible by host-produced flavonoid chemicals via a mechanism requiring a functional nodD gene product. A mutation in one of the loci was found to result in an alteration of the host range of B. japonicum. This mutation appears to block nodulation at the step at which plant root cortical cell division is induced.