A novel Azospirillum vistecanum strain from methane digestate promotes plant growth via indole-3-acetic acid biosynthesis

The urgent need for sustainable agricultural inputs has accelerated the search for microbial alternatives to synthetic agrochemicals. In this study, we report the isolation and comprehensive characterization of a novel strain, Azospirillum vistecanum VT-I1, obtained from a methane-enriched digestate system. This strain demonstrated exceptional plant growth-promoting potential through the biosynthesis of indole-3-acetic acid (IAA), a key phytohormone. Genome sequencing and annotation revealed the presence of genes associated with the indole-3-pyruvate (IPyA) pathway, aro9, ipdC, and aldA, with no detectable IAA-degrading gene clusters, supporting a high net auxin yield. Under optimized culture conditions, VT-I1 produced up to 1.206 mM of IAA, significantly surpassing the levels observed in A. brasilense. Functional assays confirmed the bioactivity of this microbial IAA, which enhanced root development in Exacum affine and improved seed germination in Andrographis paniculata. Collectively, these results establish A. vistecanum VT-I1 as a promising candidate for next-generation biofertilizers, offering a scalable, eco-friendly alternative to chemically synthesized auxins. This work expands our understanding of auxin biosynthesis in rhizobacteria and provides a strong foundation for future field applications and microbiome-based crop enhancement strategies. KEY POINTS: • A novel Azospirillum vistecanum was isolated from methane digestate. • The strain produces high levels of indole-3 acetic acid via the IPyA pathway. • Microbial IAA enhances root growth and seed germination in model plants.