Contrasting biochemical compositions and microbial interactions of English oak and black poplar root mucilage

Root mucilage plays a crucial role in plant-soil interactions, yet its composition and functions for trees remain largely unexplored. We investigated the root mucilage of two tree species with contrasting growth strategies: the slow-growing English oak (Quercus robur L.) and the fast-growing black poplar (Populus nigra L.). Our analyses focused on the polysaccharide composition of mucilage and its microbial interactions. English oak mucilage polysaccharides consisted of 54% galactose, 16% mannose, 11% arabinose, 7% xylose, and 12% glucuronic acid, with no detectable glucose or galacturonic acid. In contrast, black poplar mucilage polysaccharides contained 26% galactose, 14% mannose, 14% glucose, 22% arabinose, 7% xylose, 14% glucuronic acid, and 3% galacturonic acid. Both mucilage types were hexose-rich, resembling the hexose-to-pentose ratio common in microbial sources in soil. Black poplar mucilage had a higher uronic acid-to-neutral monosaccharide ratio and greater K⁺ and Na⁺ concentrations than English oak mucilage. Functionally, black poplar mucilage increased the growth of Pseudomonas fluorescens SBW25, suggesting the provision of readily available carbon sources. Conversely, English oak mucilage suppressed bacterial growth, plausibly due to antimicrobial compounds that may slow microbial decomposition and promote carbon sequestration.