Biosynthesis of indole-3-acetic acid via the indole-3-acetamide pathway in Streptomyces spp -- Manulis et al. 140 (5): 1045 -- Microbiology

This study demonstrates that multiple Streptomyces species, including both pathogenic potato scab-causing strains and non-pathogenic strains, produce the plant hormone indole-3-acetic acid (IAA) when supplied with the amino acid L-tryptophan. Researchers investigated IAA biosynthetic pathways in S. violaceus and S. exfoliatus using HPLC, GC-MS, and radioactive labeling techniques. They identified several indole intermediates including indole-3-acetamide (IAM), indole-3-lactic acid (ILA), and indole-3-ethanol (IEt), and demonstrated that bacterial cells can convert these intermediates into IAA. Radioactive tryptophan incorporation studies confirmed the presence of the IAM pathway (tryptophan → IAM → IAA), which appears to be the primary route. Evidence also suggests additional alternative pathways for IAA synthesis may exist. The widespread capacity for IAA production across diverse Streptomyces species suggests this trait may provide competitive advantages in plant tissues through effects on cell wall loosening, nutrient exudation, and modulation of plant defense responses.

Key findings

• Six different Streptomyces species produce IAA from tryptophan, including both potato scab pathogens and non-pathogenic strains
• The indole-3-acetamide (IAM) pathway is confirmed as a major route for IAA biosynthesis via chemical identification, cell metabolism studies, and radioactive carbon incorporation
• Multiple indole intermediates (IAM, ILA, IEt, IAA) are produced and can be metabolized into IAA by Streptomyces cells
• IAA production may confer selective advantages by inducing plant cell wall loosening, nutrient exudation, and suppressing plant defense responses