Bradyrhizobium Japonicum Strains Produce Multiple 2–0-Methylfucose-containing Lipo-Oligosaccharide Nodulation Signals

Abstract

Bradyrhizobium japonicum, a Gram negative soil bacterium, has the ability to establish a nitrogen-fixing symbiosis with soybean, siratro, cowpea, and a few other leguminous plants. This process is termed nodulation and the plant is induced to form a new nitrogen-fixing organ, the nodule. B. japonicum is a member of the family Rhizobiaceae which also includes Rhizobium, Azorhizobium, and Agrobacterium species. In Rhizobium, Azorhizobium, and Bradyrhizobium species the bacterial nodulation (nod or nol) genes are necessary for the establishment of the symbiosis with their respective host plant. The expression of the nodulation genes is controlled, in part, by the product of the nodD gene which encodes a positive transcriptional regulator. Induction of the nodulation genes also requires the presence of specific plant flavonoids. In the case of the association between B. japonicum and soybean, these nod gene-inducing flavonoids are isoflavones (e.g., genistein, daidzein, and their corresponding glycosides, Smit et al. 1992).