Medicago root nodule microbiomes: insights into a complex ecosystem with potential candidates for plant growth promotion

Purpose: Studying the legume nodule microbiome is important for understanding the development and nutrition of the plants inhabited by the various microbes within and upon them. We analyzed the microbiomes of these underground organs from both an important crop plant (Medicago sativa) and a related legume (M. polymorpha) using metagenomic and culture-based techniques to identify the main cultivatable contributors to plant growth enhancement. Methods: Using high-throughput sequencing, culturing, and in planta techniques, we identified and analyzed a broad population of the bacterial taxa within Medicago nodules and the surrounding soil. Results: Fifty-one distinct bacterial strains were isolated and characterized from nodules of both Medicago species and their growth-promoting activities were studied. Sequencing of 16S rRNA gene amplicons showed that in addition to Ensifer, the dominant genus, a large number of Gram-positive bacteria belonging to the Firmicutes and Actinobacteria were also present. After performing ecological and plant growth-promoting trait analyses, selecting the most promising strains, and then performing in planta assays, we found that strains of Bacillus and Micromonospora among others could play important roles in supporting the growth, health, and productivity of the host plant. Conclusion: To our knowledge, the comparison of the biodiversity of the microbiota of undomesticated vs. cultivated Medicago roots and nodules is novel and shows the range of potential Plant Growth-Promoting Bacteria that could be used for plants of agricultural interest. These and other nodule-isolated microbes could also serve as inoculants with rhizobia with the goal of replacing synthetic fertilizers and pesticides for sustainable agriculture.