Chemotaxis of Bradyrhizobium japonicum to soybean exudates.

The chemotactic response of Bradyrhizobium japonicum toward soybean seed and root exudates was examined. Assays using various isoflavones and fractionated exudate indicated that isoflavones are not the principal attractants in exudates. Likewise, induction of nod genes with isoflavones or seed exudate before assay did not enhance chemotaxis. Screening of numerous compounds revealed that only dicarboxylic acids and the amino acids glutamate and aspartate were strong attractants. The presence of glutamate, aspartate, and dicarboxylic acids in appreciable concentrations in soybean seed and root exudates indicates that these compounds likely represent natural chemoattractants for B. japonicum.