Mechanistic insight in the selective delignification of wheat straw by three white-rot fungal species through quantitative C-IS py-GC-MS and whole cell wall HSQC NMR.

Background: The white-rot fungi (), (), and () have been shown to be high-potential species for selective delignification of plant biomass. This delignification improves polysaccharide degradability, which currently limits the efficient lignocellulose conversion into biochemicals, biofuels, and animal feed. Since selectivity and time efficiency of fungal delignification still need optimization, detailed understanding of the underlying mechanisms at molecular level is required. The recently developed methodologies for lignin quantification and characterization now allow for the in-depth mapping of fungal modification and degradation of lignin and, thereby, enable resolving underlying mechanisms.

Results: Wheat straw treated by two strains of ( and , ( and and ( and was characterized using semi-quantitative py-GC-MS during fungal growth (1, 3, and 7 weeks). The remaining lignin after 7 weeks was quantified and characterized using C lignin internal standard based py-GC-MS and whole cell wall HSQC NMR. Strains of the same species showed similar patterns of lignin removal and degradation. and outperformed in terms of extent and selectivity of delignification (≥>>. The highest lignin removal [66% (w/w); 1] was obtained after 7 weeks, without extensive carbohydrate degradation (factor 3 increased carbohydrate-to-lignin ratio). Furthermore, though after treatment with and comparable amounts of lignin remained, the structure of the residual lignin vastly differed. For example, C-oxidized substructures accumulated in treated lignin up to 24% of the total aromatic lignin, a factor two higher than in -treated lignin. Contrarily, ferulic acid substructures were preferentially targeted by (and ). Interestingly, -spent lignin was specifically depleted of tricin (40% reduction). The overall subunit composition (H:G:S) was not affected by fungal treatment.

Conclusions: and are both able to effectively and selectively delignify wheat straw, though the underlying mechanisms are fundamentally different. We are the first to identify that degrades the major --4 ether linkage in grass lignin mainly via C--aryl cleavage, while C-C cleavage of inter-unit linkages predominated for Our research provides a new insight on how fungi degrade lignin, which contributes to further optimizing the biological upgrading of lignocellulose.