Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/23/2012
Publication Date: 5/23/2012
Citation: Grabber, J.H., Ress, D., Ralph, J. 2012. Identifying new lignin bioengineering targets: impact of epicatechin, quercetin glycoside, and gallate derivatives on the lignification and fermentation of maize cell walls. Journal of Agricultural and Food Chemistry. 60:5152-5160. Interpretive Summary: Plant cell walls are the world’s most abundant source of polysaccharides (complex carbohydrates) for fermentation into biofuels and chemicals, but harsh and costly chemical pretreatments must currently be used to liberate these polysaccharides from another cell wall polymer called lignin. Lignin is also a major barrier limiting the digestion of many feeds by livestock. Therefore, we are conducting a series of studies aimed at identifying new ways to modify lignin in plants to enhance the conversion of cell wall polysaccharides into useful products. In these studies, we artificially lignified cell walls from corn withcnormal lignin precursors plus various flavonoid and gallate ester derivatives, which are secondary plant metabolites not normally associated with lignification. Many of these derivatives readily copolymerized into lignin and their incorporation into lignin in some cases dramatically enhanced fermentability of cell walls even without chemical pretreatments. These results provide compelling evidence that bioengineering of plants to incorporate flavonoid and/or gallate derivatives such as epigallocatechin gallate into lignin should substantially enhance the availability of cell wall polysaccharides for biofuel, chemical, or livestock production.
Technical Abstract: Apoplastic targeting of secondary metabolites compatible with monolignol polymerization may provide new avenues for designing lignins that are less inhibitory toward fiber fermentation. To identify suitable monolignol substitutes, we artificially lignified maize cell walls with normal monolignols plus various epicatechin, quercetin glycoside, and gallate ester derivatives added as 0 or 45% by weight of the precursor mixture. Monolignol substitutes had variable effects on peroxidase activity, but all considerably dropped lignification pH. Epigallocatechin gallate, epicatechin gallate, epicatechin vanillate, epigallocatechin, galloylhyperin, and pentagalloylglucose formed wall-bound lignins at moderate to high concentrations with monolignols and their incorporation increased 48 h in vitro ruminal fiber fermentability by 20 to 33% relative to lignified controls. By contrast, ethyl gallate and corilagin severely depressed lignification and increased 48 h fermentability by about 50%. The results suggest several flavonoid and gallate ester derivatives are promising lignin bioengineering targets for improving the fermentability of non-pretreated cell walls.