Nitrogen Fixation in Tropical Legumes: the who with whom and more

We investigated the symbiotic relationships 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 9.8% globally. Several threatened precious-wood trees were also analyzed for the first time. Our analysis revealed root nodules in 78% of the species (67% in Caesalpinioideae and 84% in Papilionoideae). Additionally, we developed a predictive model for nodulation in legumes, with fine-root color and flower symmetry as the sole predictors. Nodulating plants consistently presented smaller average root diameters and higher nitrogen content in both leaves and roots, regardless of the subfamily. By integrating both culture-dependent and culture-independent techniques, we successfully identified the rhizobial symbionts associated with the root nodules of all the legumes examined.