Exploring symbiotic legume-rhizobia relationships across tropical species

The nitrogen-fixing symbiosis with rhizobia significantly contributes to the successful establishment of legumes in tropical environments. Here, we explored the symbiotic interactions between legumes and rhizobia across 109 tropical species. We evaluated the presence of root nodules in 72 genera spanning four Fabaceae subfamilies, encompassing 53% of the genera in Costa Rica and approximately 9.4% globally. Our analysis revealed root nodules in 78% of the species belonging to Caesalpinioideae (67%) and Papilionoideae (84%). Also, we formulated a predictive model for nodulation in legumes, with fine-root color and flower symmetry as the main predictor criteria. Nodulating taxa consistently presented higher N levels in tissues, thinner roots, and a pronounced phylogenetic influence on nodulation. We also identified the bacterial genera associated with all nodulating legumes. Caesalpinioideae formed symbiotic relationships with ten rhizobial genera, whereas Papilionoideae were associated with only four. Bradyrhizobium emerged as the predominant symbiont in nodules, while Rhizobium, Mesorhizobium, Agrobacterium, Paraburkholderia, and Cupriavidus exhibited a more restricted host range. Furthermore, genomic analysis suggests that 15 out of the 22 sequenced rhizobial strains may represent novel species. Our study enhances the understanding of the different dimensions that comprise legume-rhizobia interactions, incorporating a diverse array of previously unexamined tropical forest legume taxa.