Streptomyces as microbial biofactories: optimization of auxin production and growth-promoting activity of cell-free supernatants in soybean (Glycine max (L.) Merr.).

Auxins are plant growth regulators, and Actinobacteria, particularly Streptomyces, can synthesize indole-3-acetic acid (IAA), the most prevalent auxin, contributing to plant growth promotion. In this study, three auxin-producing Streptomyces isolates (CLV45, CLV104, and CLV194) were characterized, and auxin biosynthesis was evaluated under different cultivation conditions. Isolates were cultivated in ISP2 broth, and auxin-related compounds were quantified using the Salkowski assay and high-performance liquid chromatography (HPLC). Isolates were assigned to the genus Streptomyces based on morphological characteristics and 16S rRNA gene sequence analysis. All isolates produced auxin, albeit with distinct metabolic responses. A One Factor at a Time (OFAT) approach was used to identify favorable culture conditions for each isolate. CLV45 achieved maximal production (day 7, 182.3 µg mL⁻1) with 0.05 g L⁻1 L-tryptophan, 1% dextrose, and 0.5% peptone under 6 h of oxygen restriction; CLV104 showed the highest synthesis (day 7, 958.9 µg mL⁻1) with 0.1 g L⁻1 L-tryptophan, 0.5% glycerol, and 1% ammonium sulfate. For CLV194, no individual factor enhanced auxin production; its highest concentration (577 µg mL⁻1) coincided with its natural production peak at day 5, rather than with any specific treatment. All auxin-containing supernatants promoted soybean root growth, enhancing root elongation by up to 136.3% relative to untreated control following treatment with CLV194 supernatant at 10 µg mL⁻1, with no significant effect on shoot elongation. These findings demonstrate the capacity of Streptomyces isolates to produce high concentrations of auxin-like compounds under controlled cultivation and show that cell-free supernatants (CFS) can promote soybean seedling growth.