Altering Poplar Cell-Wall Recalcitrance and Accessibility by Regulating the Expression of Caffeoyl Shikimate Esterase for Biorefinery Applications

Wood is the most abundant terrestrial biomass and an important renewable feedstock for biobased materials and chemicals. However, biomass recalcitrance, closely associated with the complex lignin-carbohydrate network, restricts efficient deconstruction and utilization. Here, transgenic poplars with caffeoyl shikimate esterase (CSE) downregulation and overexpression were systematically analyzed to elucidate how CSE expression modulates lignin structure, lignin-carbohydrate complex (LCC) connectivity, and downstream conversion performance. CSE downregulation decreased lignin content, reduced the syringyl/guaiacyl (S/G) ratio, and lowered LCC abundance, which was associated with enhanced cellulose accessibility and reduced biomass recalcitrance without altering polysaccharide composition. In contrast, CSE overexpression promoted lignification and the formation of complex LCC linkages, corresponding to increased biomass recalcitrance. When integrated with deep eutectic solvent (DES) pretreatment, CSE downregulated biomass exhibited improved delignification and enzymatic saccharification. These results reveal a structural role of CSE in coordinating lignin-carbohydrate architecture and provide insights into genetically optimized woody feedstocks for sustainable biorefineries.