Exploring symbiotic legume-rhizobia relationships across tropical species

Abstract

This study investigates the diversity and ecological patterns of symbiotic relationships between tropical legumes and rhizobia across 109 species representing four subfamilies of Fabaceae in Costa Rica. Root nodules were present in 78% of the examined species, with Caesalpinioideae and Papilionoideae showing the highest nodulation frequencies, while Cercidoideae and Detarioideae exhibited none. Using culture-dependent and metabarcoding approaches, the authors identified all rhizobial symbionts associated with nodulating species, revealing that Caesalpinioideae interacted with ten rhizobial genera, whereas Papilionoideae associated with only four. Bradyrhizobium emerged as the dominant symbiont across taxa. A decision-tree predictive model based on fine-root color and flower symmetry achieved 96% accuracy in forecasting nodulation capacity. Functional analyses showed that nodulating species had thinner roots and significantly higher nitrogen content in leaves and roots. Whole-genome sequencing of selected strains further indicated that 15 of the 22 rhizobia isolates likely represent novel species. Overall, the study provides the most comprehensive characterization to date of legume–rhizobia symbioses in tropical forests and highlights their phylogenetic, morphological, and microbial dimensions.