Abstract:

Lignin, a crucial component of plant cell walls, together with cellulose and hemicellulose, constitutes the largest renewable organic carbon reservoir in terrestrial ecosystems. Due to its inherent heterogeneity, complex interunit linkages, and highly branched structure, lignin exhibits strong recalcitrance to degradation. This characteristic not only restricts the high-value utilization of lignin, but also greatly limits the efficient conversion of lignocellulose. As a green and sustainable approach, biodegradation has shown great potential for the valorization of lignin and lignocellulosic materials. Advanced studies of lignin biodegradation not only enhance our understanding of global carbon sequestration but also drive technological innovations in biomass conversion and renewable chemical production, offering promising solutions to climate change and energy crises. Fungi, as the primary decomposers in terrestrial ecosystems, have evolved highly sophisticated and diverse enzymatic systems for lignin degradation. However, critical gaps remain in our understanding of fungal-lignin interactions and their underlying mechanisms. In recent years, notable progress has been made in some studies. For example, the soft-rot fungus Parascedosporium putredinis NO1 secretes a novel lignin-oxidizing enzyme capable of cleaving β-ether bonds in lignin in the absence of cofactors, leading to lignin depolymerization and releasing triazine. In addition, the white-rot fungus Echinodontium taxodii generates manganese peroxidase, laccase, and esterase, which act synergistically to selectively delignify bamboo by cleaving cross-linkages between lignin and xylan, thereby enhancing cellulase accessibility and improving saccharification efficiency. Furthermore, Phanerochaete chrysosporium produces lignin peroxidase PcLiP03, which exhibits high affinity for lignin but minimal binding to cellulose, effectively preventing competitive adsorption. This interaction is primarily driven by electrostatic interactions arising from functional groups and hydrophobic interactions related to its structural features. At the metabolic level, ¹³C isotope-labeling studies have revealed that the basidiomycete Agaricus bisporus depolymerizes native lignin and its derivatives extracellularly and subsequently metabolizes the resulting products intracellularly as carbon and energy sources for anabolism. Additionally, the anaerobic fungus Neocallimastigomycete californiae has been shown to mediate lignin depolymerization through small-molecule-mediated redox reactions, altering lignin monomer composition and cleaving multiple interunit linkages. Collectively, these findings provide new insights into fungal lignin degradation mechanisms and highlight the potential for developing efficient, cost-effective, and sustainable lignin degradation technologies. The next step of research should further clarify the enzymatic mechanism of fungal degradation of lignin and continue to explore the potential of fungal metabolic pathways in the high-value utilization of lignin.

Key words: lignin degradation, fungi, biodegradation, lignin depolymerization, lignin valorization