Changes in Root Exudate Composition Drive the Evolution of Their Ecological Functions Towards Improved Chemical Defense and Plant-Microbe Interactions.
Alterations in root exudates and their ecological significance across plant evolutionary divergence time remain elusive due to methodological challenges. Herein, we measured root exudate composition of 20 woody species and assessed its associations with root traits and rhizosphere microbiota to investigate the ecological functions of root exudates throughout plant evolution. The root exudates showed increased richness, polarity, unsaturation and aromaticity with divergence time. Furthermore, fungal richness markedly increased with divergence time, accompanied by the rhizosphere enrichment of copiotrophic bacteria (e.g., Bacteroidota and Actinobacteriota), ectomycorrhizal fungi and several functional microbes. Additionally, competitive root traits (e.g., specific root length and root nitrogen concentration) were positively correlated with exudate richness and chemical complexity in ectomycorrhizal (EcM) trees, whereas they were related to the exudation rates in the arbuscular mycorrhizal (AM) trees. Microbial richness and strategies were closely linked to exudate chemistry, and the exudate-microbe interactions were more tightly coupled in the rhizosphere of EcM trees than in that of the AM trees. Shifts in exudate composition and their associated microbial communities may reflect enhanced chemical defense and rhizosphere communication as plants evolve. These results provide evidence that root strategies extend beyond the root itself to include exudates and the rhizosphere, thereby advancing our understanding of root traits-exudate-microbiota associations across plant evolution.