Biosynthesis of indole-3-acetic acid via the indole-3-acetamide pathway in Streptomyces spp.

Various Streptomyces spp. including S. violaceus, S. scabies, S. griseus, S. exfoliatus, S. coelicolor and S. lividans secrete indole-3-acetic acid (IAA) when fed with L-tryptophan (Trp). Production of IAA was detected in Streptomyces strains causing potato scab as well as in non-pathogenic strains. The pathways for IAA synthesis from Trp were investigated in S. violaceus and S. exfoliatus. Indole-3-acetamide (IAM), indole-3-lactic acid (ILA), indole-3-ethanol (IEt) and IAA were identified by HPLC and GC-MS. Streptomyces cells were capable of catabolizing IAM, ILA, IEt and indole-3-acetaldehyde (IAAld) into IAA. Incorporation of radioactivity into IAM, IAA and IAL but not IEt was detected when cells were fed with L-[3-¹⁴C]tryptophan. Results indicate the presence of the IAM pathway (Trp → IAM → IAA) and the possible presence of additional pathways for IAA biosynthesis in Streptomyces.