Detection of the IAA Biosynthetic Pathway from Tryptophan via Indole-3-Acetamide in Bradyrhizobium spp.

The IAA biosynthetic pathway of tryptophan to IAA via IAM was detected in Bradyrhizobium spp. (slow-growing Rhizobium) but not in Rhizobium spp. (fast-growing Rhizobium). A simple method using rapid HPLC analysis to measure the conversion from NAM to NAA was developed to detect indole-3-acetamide hydrolase activity in cultures of bacteria. Most of the Bradyrhizobium strains produce large amounts of NAA converted from NAM under our assay conditions. In addition, GC/MS analysis of purified extracts from cultures of B.japonicum wild-type strain J1063, grown in a tryptophan-supplemented liquid medium, demonstrated the presence of IAM and IAA. The results strongly suggest that biosynthesis of IAA in Bradyrhizobium spp. involves the same pathway as that operating in Pseudomonas savastanoi and Agrobacterium tumefaciens.