Determinants of Host Specificity in the Bradyrhizobium Japonicum-Soybean Symbiosis

One of the most fascinating, and still largely unexplained, aspects of Rhizobium/Bradyrhizobium- legume interaction is the host specificity exhibited. Investigations of Rhizobium meliloti and R. leguminosarum bv. viciae and trifolii have shown that host specificity is determined by the respective nodD gene as well as unique host specificity genes (reviewed in Long 1989). Against this background of work with various Rhizobium species, comparatively little is known about the determinants of host specificity in Bradyrhizobium species. Clearly, the specificity of the nodD gene product for specific isoflavone inducers from the plant plays an important role in legume infection by Bradyrhizobium (Kosslak et a1 1988; Banfalvi et al 1988). In addition, a few loci have been identified by mutagenesis in Bradyrhizobium that appear to affect nodulation on only particular host species (Bender et al 1987; Nieuwkoop et al 1987; Hahn and Hennecke 1988).