Conversion of Tryptophan, Indole-3-Pyruvic Acid, Indole-3-Lactic Acid and Indole to Indole-3-Acetic Acid by Azospirillum brasilense Sp7

Abstract

The free-living nitrogen-fixing soil bacterium Azospirillum brasilense Sp7 produces indole-3-acetic acid (IAA) from tryptophan (Trp). Ammonia and oxygen are involved in the processes of IAA production and nitrogen fixation. Oxygen is required during the conversion of Trp to IAA via indole-3-acetamide (IAM). One ammonia molecule is released during the conversion of Trp to IAA. Under microaerophilic conditions, the cells were found to preferentially catalyze the enzymatic conversion of the tryptophan deaminated compound indole-3-pyruvic acid (IPyA) to IAA. Addition of ammonia caused a decrease in the conversion of Trp and IPyA but not indole-3-lactic acid (ILA) to IAA. Indole-3-pyruvic acid was found in the growth medium after addition of either Trp or ILA, indicating the function of an aminotransferase pathway for IAA synthesis from Trp. Three tryptophan-aminotransferases were observed on nondenaturing gels. No tryptophan-aminotransferase activity was observed upon addition of IPyA to the bacterial culture. Addition of 0.5 mM indole to the growth medium resulted in accumulation of IAA by an enzymatic process. Holotryptophanase activity was detected in cell-free extracts of A. brasilense Sp7 grown in a medium amended with indole. Addition of Trp to A. brasilense Sp7 culture as the sole N source partially inhibited nitrogenase activity. Study of the factors affecting both IAA production and nitrogen fixation is important for better understanding of the bacterial-plant association.