Epigallocatechin Gallate Incorporation Into Lignin Enhances the Inherent Fermentability, Alkaline Degradability, and Enzymatic Saccharifiability of Artificially Lignified Maize Cell Walls.

Polyphenolic flavonoids and gallate esters are potentially attractive targets for lignin bioengineering because their copolymerization with normal monolignols could reduce lignin hydrophobicity and cross-linking to polysaccharides, or facilitate delignification by biomass pretreatments. To test this hypothesis, we biomimetically lignified maize cell walls with normal monolignols (coniferyl and sinapyl alcohols) plus a series of epicatechin, quercetin glycoside, and gallate derivatives, each added as 45% of the precursor mixture. Epigallocatechin gallate (EGCG) was among the most promising alternate monomers examined because it readily formed wall-bound lignin with normal monolignols and increased the inherent ruminal fermentability of non-pretreated cell walls by 33% relative to lignified controls. Subsequent in vitro peroxidase-catalyzed polymerization experiments revealed that both gallate ester and pyrogallol moieties in EGCG underwent radical cross-coupling with monolignols forming mainly benzodioxane units in the polymer. Incorporation of EGCG into lignin permitted extensive alkaline delignification of cell walls (73 to 90%) that far exceeded lignified controls (44 to 62%). Improved delignification may be attributed to cleavage of ester intra-unit linkages within EGCG and trapping of quinone methide intermediates by EGCG to block benzyl ether cross-linking of lignin to structural polysaccharides. Alkaline-insoluble residues from EGCG-lignified walls yielded 30% more total sugars than lignified controls during enzymatic saccharification. Overall, our results suggest that apoplastic deposition of EGCG for incorporation into lignin could be a promising plant genetic engineering target for improving the fermentation, delignification and saccharification of biomass crops.