Optimization of auxin production by Streptomyces californicus CLV91 for plant growth promotion

Streptomyces spp. are widely recognized for their capacity to produce bioactive secondary metabolites, including plant growth regulators such as indole-3-acetic acid (IAA), which have promising applications in sustainable agriculture. Streptomyces californicus represents a relatively unexplored species in terms of auxin biosynthesis. This study aimed to evaluate and optimize IAA production by S. californicus CLV91, assessing its potential as a biostimulant. The isolate CLV91 was identified via whole-genome sequencing and cultivated in ISP2 medium under standard and optimized conditions. IAA quantification was performed using the Salkowski colorimetric method. The impact of culture conditions on auxin synthesis was assessed, and plant growth promotion assays were conducted using Phaseolus vulgaris seeds. Under standard conditions, S. californicus CLV91 synthesized 550 µg mL⁻¹ of IAA, and supplementation with 0.2 g L⁻¹ L-tryptophan increased production by 55.42%, reaching 816 ± 17.26 µg mL⁻¹. Despite the enhanced production, combining all optimized variables led to a reduction in auxin levels, suggesting metabolic stress under cumulative conditions. In plant assays, CLV91-derived auxin significantly increased root elongation, with a maximum effect of 62.7% at 20 µg mL⁻¹. These findings demonstrate the potential of S. californicus CLV91 as a microbial source of auxins for use in biostimulant formulations, supporting its further investigation for field-scale applications in sustainable crop management.