Tryptophan conversion to indole-3-acetic acid via indole-3-acetamide in Azospirillum brasilense Sp7

The phytohormone indole-3-acetic acid is involved in several types of microorganism-plant interactions. In the most widely studied pathway, tryptophan-2-monooxygenase converts tryptophan to the intermediate indole-3-acetamide, and indole-3-acetamide hydrolase catalyzes the conversion of indole-3-acetamide to indole-3-acetic acid. The genetic determinants for these enzymatic conversions are iaaM and iaaH, respectively. This pathway has been observed in many pathogenic and symbiotic soil bacteria. The associative soil bacteria of the genus Azospirillum are known to promote plant growth, probably via the secretion of phytohormones, including indole-3-acetic acid. The following evidence is presented for the existence of the above-described indole-3-acetic acid pathway in Azospirillum brasilense Sp7: the high toxicity of α-methyltryptophan as compared with that of 5-methyltryptophan; indole-3-acetic acid formation in vivo from indole-3-acetamide; the existence of two tryptophan-induced proteins, one of which has a molecular weight similar to that of tryptophan-2-monooxygenase; tryptophan-2-monooxygenase activity observed on nondenaturing gel; the existence of a protein with high tryptophan-2-monooxygenase activity with a molecular weight similar to that of one of the tryptophan-induced proteins on a two-dimensional gel; and the partial homology between the iaaM gene, which encodes tryptophan-2-monooxygenase in Pseudomonas savastanoi, and A. brasilense Sp7 total DNA.