IAA Biosynthetic Pathway from Tryptophan via Indole-3-pyruvic Acid in Enterobacter cloacae

Indole-3-acetic acid (IAA) was identified as a tryptophan (Trp) catabolite in the cultures of Enterobacter cloacae isolated from the rhizosphere of well-grown cucumbers. This strain produced IAA in a tryptophan-supplemented liquid medium at levels of up to 1 mg/ml, and indole-3-lactic acid (ILA) and tryptophol (Tol) were also produced. Under aerobic conditions, IAA was produced in higher concentrations than ILA and Tol, but under less aerobic conditions, ILA and Tol were produced in higher concentrations than IAA. In metabolic studies with eight kinds of indole substrates, E. cloacae converted Trp, indole-3-pyruvic acid (IPyA), and indole-3-acetaldehyde (IAAld) to IAA. These results strongly suggest that the biosynthesis of IAA in E. cloacae involves the pathway of Trp to IAA via IPyA and IAAld.