Superiority of Bacillus velezensis and Glutamicibacter nicotianae as native non- rhizobial nodulating bacteria for enhancing legume production in low-fertility soils

Although numerous studies have examined the effects of legume-nodulating bacteria (LNB) on legume crops in resource-limited agricultural settings, most research has focused almost exclusively on rhizobia, with limited attention to the role of native non-rhizobial endophytes (NREs) that inhabit legume root nodules. The identity and role of these NREs remain poorly understood. We employed a bottom-up approach, combining culture-dependent isolation techniques, full-length 16S rRNA gene sequencing, greenhouse screening, and a robust randomized block field experiment to isolate, identify, and characterize native LNB, and assess their effectiveness in promoting cowpea and soybean production. Through this integrative strategy, two novel and highly promising NRE isolates, Glutamicibacter nicotianae (Nks5) and Bacillus velezensis (Nka11), were identified based on their enhanced ability to nodulate cowpea and soybean plants, as well as their plant-growth-promoting traits, including phosphate solubilization and indole-3-acetic acid (IAA) synthesis. In greenhouse trials, inoculation with these selected isolates significantly improved cowpea growth parameters compared to the uninoculated control. G. nicotianae increased plant biomass by 30% and grain yield by 77.8%, while B. velezensis increased biomass by 114% and yield by 200% compared to canonical rhizobia and the control. Importantly, these benefits also translated to field conditions. Field trials conducted on a non-promiscuous soybean variety showed that inoculation with the NRE consortium, comprising B. velezensis and G. nicotianae , significantly enhanced nodulation and productivity in nutrient-poor soils. Compared to the control, the consortium increased the number of root nodules by 129%, nodule biomass by 54%, plant biomass by 44%, and grain yield by 82%, underscoring its superior performance and the strong standalone potential of the NRE consortium. Overall, these findings challenge the traditional view that rhizobia alone are responsible for effective legume nodulation, highlighting the value of native NREs as promising, sustainable bioinoculants for improving legume production in tropical agroecosystems. We should prioritize bottom-up isolation and characterization of home-based NREs, evaluate their symbiotic performance alongside rhizobia in the field, and develop affordable, context-appropriate nutrient management strategies. By tackling these research gaps and resource constraints, it may be possible to unlock significant yield gains and bolster the sustainability of legume-based cropping systems in resource-limited agricultural settings.