Unveiling the Potential of Non-Rhizobial Microorganisms to Improve Faba Bean Growth

The growing demand for sustainable agricultural practices has renewed interest in legumes such as faba bean ( Vicia faba L.) due to their nutritional importance and capacity to enhance soil fertility through symbiotic nitrogen fixation. While nodulation is classically attributed to rhizobial genera, legume nodules are now recognized as complex microbial niches that also harbor non-rhizobial endophytes (NREs). Although NREs cannot induce nodulation independently, they can promote plant growth through nutrient acquisition, phytohormone production, and stress tolerance. In this study, bacterial communities associated with faba bean nodules collected from Aït Ourir were isolated and characterized, and their plant growth–promoting traits and co-inoculation effects were evaluated. A total of 80 isolates were obtained, comprising 9 rhizobia and 71 NREs. All rhizobial strains formed nodules, with strain F4.10 showing the highest symbiotic efficiency. Among the NREs, 65% exhibited nitrogen-fixation potential, while selected strains demonstrated phosphate solubilization (up to 17.76 mg/L) and indole-3-acetic acid production (up to 44.21 µg/mL). Co-inoculation of F4.10 with selected NREs significantly enhanced shoot length (29%), root length (47%), and root dry weight (152%) compared to single inoculation or non-inoculated controls. These results highlight the functional diversity of faba bean nodule microbiota and support the development of multifunctional biofertilizers for sustainable legume production.