Potential of indigenous rhizobia for enhancing plant growth and improving faba bean (Vicia faba L.) yield in Mekelle Ethiopia

Native rhizobia are a sustainable alternative to exotic inoculants and excessive agrochemicals in agricultural practices, as they avoid adaptability issues and undesirable effects. This study aimed to identify and characterize native rhizobia from faba bean roots, evaluate their plant growth-promoting properties, and assess plant inoculation effects. Rhizobia were isolated using Yeast Mannitol agar medium and confirmed through biochemical and presumptive tests. Plant growth-promoting features were assessed in vitro, while nodulation and nitrogen fixation were evaluated at molecular level. The effects of the isolates were tested on faba bean seedlings using pot experiment. Totally, 61 faba bean nodule isolates were obtained, categorized into 8 based on morpho-cultural and biochemical features, and 16 isolates selected for further experiments.. All 16 rhizobia isolates (100%) showed positive results in the production of indole-3-acetic acid, ammonia, and exo-polysaccharide, despite varying production capacity among them. Of 16 isolates, 11 (68.75%) formed clear halo zones, with solubilization indices ranging from 0.60 to 2.75. Isolate MR24 showed an expected nifH replicon of approximately 360 bp, while MR1, MR2, and MR48 showed approximately 781 bp unpredicted nifH gene fragment. In addition, MR2, MR6, MR24, and MR26 produced approximately 850 bp unpredicted nodA gene replicon. Inoculation with these isolates significantly (P ≤ 0.05) increased the shoot /root length, shoot fresh/dry weight, and number of nodules compared to the control. Whereas, the exotic inoculant showed better root/ shoot length and shoot dry weight. The isolates can be used as bioinoculants for faba bean growing.