Co-cultivation of diazotrophic bacteria enhances auxin-mediated root development in maize: implications for microbial inoculants.

This study assessed the co-cultivation of Gluconacetobacter diazotrophicus and Paraburkholderia tropica, two diazotrophic species previously used in sugarcane, to evaluate their compatibility, phytohormone profiles, and effects on maize (Zea mays) root system architecture. Both strains were grown separately and together, followed by metabolomic analyses and plant bioassays under controlled and greenhouse conditions. The co-cultivation maintained stable populations and produced a wider range of phytohormones than individual cultures, including indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), 4-Cl-IAA, gibberellins, cytokinins, and abscisic acid, with many of these compounds released into the culture medium. Short-term bioassays showed that auxins, particularly IAA, were the only phytohormones produced at concentrations high enough to cause quick and consistent changes in root length and surface area, with IAA levels increasing from 14.07 to 15.30 ng mL⁻¹ in monocultures to 30.68 ng mL⁻¹ under co-cultivation, indicating that auxin-driven signaling is the main mechanism behind early root responses. These findings suggest that co-cultivating compatible diazotrophic bacteria is a viable and practical strategy for developing advanced microbial inoculants to enhance maize root architecture and support sustainable crop production.