Evolution, Biogeography, and Ecological Strategies of Legume Root Nodule Symbioses and Their Agricultural Significance

The Leguminosae (Fabaceae) is one of the largest and most evolutionary diverse plant families that comprises emblematic examples of symbiotic nitrogen fixation. This striking diversity is reflected across research areas ranging from taxonomy and phylogenomics to symbiosis, microbiology, biogeography, and functional traits. Insights into the genetic and molecular bases of root nodule symbiosis (RNS) are rapidly accumulating, enabling advances in agriculture and the engineering of biological nitrogen fixation (BNF). Nevertheless, substantial gaps remain, particularly regarding non-model species and the ecological and evolutionary drivers of RNS. This review summarises current knowledge on legume RNS and highlights that while the trait has likely contributed to legume diversification, the evolutionary trajectory of nodulation is complex, involving multiple gains, losses, and variations in symbiotic strategies across lineages. Nodule morphology and organogenesis, including determinate and indeterminate types, reveal structural and functional differences that may influence adaptability and BNF efficiency, although direct comparisons under varying environmental conditions remain limited. Ecological traits, such as drought tolerance, seed dormancy, and specialised pollination and defence mechanisms, interact with RNS to facilitate survival across diverse habitats. Case studies on Lebeckia ambigua and soybean wild relatives demonstrate how insights from non-model legumes can contribute to sustainable agriculture by improving stress resilience, expanding symbiotic partnerships, and broadening the genetic base for crop improvement. Future research should expand to non-model species and systematically assess nodulation, symbiotic efficiency, and environmental responsiveness to fully harness the potential of legumes for ecological and agricultural applications.