Recalcitrance barriers in plant cell walls: From hierarchical organization to targeted depolymerization strategies

The plant cell wall represents the most abundant renewable reservoir of organic carbon on Earth, and its efficient conversion is therefore critical for sustainable development. However, the intrinsic resistance of the cell wall to depolymerization remains a major bottleneck that limits biomass refining efficiency. This review systematically examines the chemical origins and structural foundations of anti-depolymerization barriers, establishing a multiscale theoretical framework for their analysis. At the molecular scale, the crystalline hydrogen-bonding network of cellulose, the high bond dissociation energies of lignin C–C linkages, and hemicellulose side-chain modifications collectively confer strong chemical resistance. At the intermolecular scale, lignin–carbohydrate complex (LCC) architectures, hydrogen-bond networks, π–π stacking, and van der Waals interactions synergistically form dense interfacial layers. At the microscopic scale, the cell wall’s layered architecture, pore size distribution, and microfibril orientation further restrict reactant transport and molecular accessibility. Based on these insights, a range of chemical, physical, biological, and interfacial strategies has been developed to selectively weaken structural barriers across multiple scales, thereby enabling efficient depolymerization and controlled conversion. This review emphasizes the critical roles of in situ characterization and multiscale simulations in elucidating interfacial dynamics and guiding targeted depolymerization. Looking ahead, the advancement of multiscale synergistic regulation and green, low-energy depolymerization technologies is expected to provide a robust theoretical and methodological foundation for overcoming biomass conversion bottlenecks and achieving efficient, high-value utilization.