Production of Indole-3-Acetic Acid by Bradyrhizobium japonicum: A Correlation with Genotype Grouping and Rhizobitoxine Production

Bioassays show that rhizobitoxine-producing strains of Bradyrhizobiumjaponicum excreted another phytotoxic compound into their culturefluid. This compound was purified and identified by HPLC andmass spectrometry as indole-3-acetic acid (IAA). The levelsof IAA produced by the different strains of B. japonicum, forwhich the genotype groups have been determined with respectto the degree of base substitution in and around nifDKE, werequantified using gas chromatography/mass spectrometry and adeuterated internal standard. Genotype II strains, which producerhizobitoxine, excreted more than 20µof IAA into theirculture fluid. However, no IAA was detected in the culture supernatantsof genotype I strains, which do not produce rhizobitoxine. Thiswas true even when tryptophan was added to the medium. Moreover,cells of genotypes I and II strains, which were grown underour culture conditions, did not show IAA degradation activity.These results suggest that, in wild-type B. japonicum strains,complete IAA biosynthesis is confined exclusively to genotypeII strains that produce rhizobitoxine. (Received April 9, 1990; Accepted October 6, 1990)