Production of indole-3-acetic acid by bradyrhizobium japonicum: A correlation with genotype grouping and rhizobitoxine production

Bioassays show that rhizobitoxine-producing strains of Bradyrhizobium japonicum excreted another phytotoxic compound into their culture fluid. This compound was purified and identified by HPLC and mass spectrometry as indole-3-acetic acid (IAA). The levels of IAA produced by the different strains of B. japonicum, for which the genotype groups have been determined with respect to the degree of base substitution in and around nifDKE, were quantified using gas chromatography/mass spectrometry and a deuterated internal standard. Genotype II strains, which produce rhizobitoxine, excreted more than 20μof IAA into their culture fluid. However, no IAA was detected in the culture supernatants of genotype I strains, which do not produce rhizobitoxine. This was true even when tryptophan was added to the medium. Moreover, cells of genotypes I and II strains, which were grown under our 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 genotype II strains that produce rhizobitoxine.