Incorporation of flavonoid derivatives or pentagalloyl glucose into lignin enhances cell wall saccharification following mild alkaline or acidic pretreatments

Authors: Grabber, John; SANTORO, NICHOLAS - University Of Michigan; FOSTER, CLIFF - University Of Michigan; ELUMALAI, SASIKUMAR - University Of Wisconsin; RALPH, JOHN - University Of Wisconsin; PAN, XUEJUN - University Of Wisconsin

Submitted to: BioEnergy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2015
Publication Date: 3/17/2015
Publication URL: http://handle.nal.usda.gov/10113/62857
Citation: Grabber, J.H., Santoro, N., Foster, C.E., Elumalai, S., Ralph, J., Pan, X. 2015. Incorporation of flavonoid derivatives or pentagalloyl glucose into lignin enhances cell wall saccharification following mild alkaline or acidic pretreatments. BioEnergy Research. doi 10.1007/s12155-015-9605-2.

Interpretive Summary: Plant cell walls are the world’s most abundant source of sugars for fermentation into biofuels. Prior to their fermentation into biofuels, however, these sugars must be released from cell walls by enzymes following harsh and costly chemical pretreatments that are required to break down lignin. Therefore, we are testing ways to modify lignin formation in cell walls so that sugars can more readily be produced for fermentation into biofuels. In this study, we artificially lignified cell walls from corn with normal precursors (i.e. monolignols) plus various epicatechin, quercetin galactoside, and gallate ester derivatives, which are not normally components of lignin but are natural antioxidants found in many plants. The yield of glucose from sequential chemical pretreatment and enzymatic hydrolysis of cell walls was used to gauge how lignin alterations affected the availability of fermentable sugars. We found that most alternate precursors readily formed lignin-like polymers with normal precursors (monolignols), and many improved the production of fermentable sugars following mild acid and especially mild alkaline pretreatment. Our results provide compelling evidence that several of these alternate lignin precursors would be a promising plant genetic engineering target for improving the production of biofuels from biomass crops.

Technical Abstract: Partial substitution of normal monolignols with phenolic precursors from other metabolic pathways may improve the susceptibility of lignified biomass to chemical pretreatment and enzymatic saccharification for biofuel production. Flavonoids and gallate esters readily undergo oxidative coupling reactions, suggesting they could serve as alternate monomers for forming lignin in plants. To test this premise, primary cell walls of Zea mays (L.) were artificially lignified with normal monolignols plus various flavan-3-ol/phenolic ester derivatives, flavonol glycoside/gallate ester derivatives, or pentagalloyl glucose added as 0 or 45% of the precursor mixture. Most alternate monomers readily copolymerized with normal monolignols, but wall-bound lignin was most efficiently formed with epicatechin, epicatechin gallate, or epigallocatechin gallate. Yields of glucose from a high-throughput digestibility platform were used to examine how lignin modifications affected the susceptibility of cell walls to enzymatic hydrolysis following alkaline or acidic pretreatments of different severities. With the exception of hyperoside, incorporation of alternate monomers into lignin improved yields of enzymatically released glucose by 18-60% after mild alkaline pretreatment and by 6-34% after mild acid pretreatment. Responses due to lignin modification diminished as pretreatment severity increased. Overall, our results suggest that apoplastic deposition of pentagalloyl glucose or gallated flavan-3-ols such as epicatechin gallate or epigallocatechin gallate for incorporation into lignin could be promising plant genetic engineering targets for improving sugar yields from grass biomass crops that are subjected to low temperature alkaline pretreatments.