Assessing the chemical composition and ecological relevance of root exudates in legume species

Global food security and environmental sustainability are closely linked to positive plant-soil feedback, for example through reduced dependence on synthetic agrochemicals. Beyond their nutritional value as protein- and lipid-rich crops, legumes have an essential ecological function by enhancing soil fertility and consequently reducing the use of synthetic fertilizers. Legume root systems exude a diverse array of secondary metabolites (SM) into the soil. These exudates affect the rhizosphere microbiome (structure and functions) as well as nutrient availability to the plant and stimulate microbial activity in the soil. Thereby they modulate ecological interactions, which may result in positive plant-soil feedback that can contribute to improve soil health and resilience of plant production systems. Considering the importance of legume root exudate and SMs to food production and environment protection, this review fills a gap in the literature summarizing the state-of-the-art regarding the elucidation of their chemodiversity and functional significance. For that, we have compiled the literature on legume root exudates of the last 25 years (from 1999 to 2024) resulting in a comprehensive list of secondary metabolites. We also discussed how they are accessed and their biological relevance in root symbiosis. We found that root exudates were characterized in 22 legume species, which led to the identification of 92 SM. The most reported metabolites are shikimate and phenylpropanoid derivatives, especially flavonoids. Based on the information organized in this review, we observed that the rapid progress of metabolomics and high-resolution analytical techniques provided invaluable data on legume exudate chemical diversity, although other chemical classes remain underrepresented. Besides, this knowledge remains fragmented across few legume taxa, with most research concentrated on a limited number of model and crop legume species. We argue that expanding the research beyond cultivated species, and combining metabolomics strategies to access their chemical diversity with ecological functions will strengthen the knowledge on their regulatory mechanisms and their influence on rhizosphere microbiome. Ultimately, by addressing legume root exudate chemistry and ecological functions, this comprehensive review expands our current understanding and brings new insights on how such metabolites contribute to plant performance, enhance soil health and promote sustainable agricultural practices.