Biomimetic Cell Wall Model Studies to Identify New Lignin Bioengineering Targets for Improving Biomass Susceptibility to Pretreatment and Enzymatic Saccharification.

Increasingly, bioengineering of lignin to contain atypical building blocks from other metabolic pathways is being pursued to custom-design lignin that is easier to remove by chemical pretreatments and less inhibitory toward polysaccharide saccharification. Because plants produce such a diverse array of phenolics that could serve as building blocks (monomers) for lignin formation, cell wall model studies are invaluable as a screening tool for identifying the most promising genetic engineering targets for biomass crops. Therefore, over a decade ago, we began using a well-characterized biomimetic model based on maize cell walls to investigate the copolymerization of normal monolignols with coniferyl ferulate – a secondary metabolite containing two phenolic moieties connected by an alkali- and acid-labile ester linkage. As expected, cell walls lignified with coniferyl ferulate were more susceptible to pretreatment and more extensively saccharified by polysaccharidases. This finding prompted the cloning of a monolignol ferulate acyltransferase gene with the aim of expressing it in plants to enhance the production of biofuels, chemicals, or paper. Other model studies have recently demonstrated the utility of another biphenolic ester conjugate (rosmarinic acid) and of several flavonoid derivatives (e.g. epigallocatechin gallate) for enhancing cell wall delignification, saccharification and fermentation. Ongoing studies are examining the utility of other types of phenolics that may reduce lignin hydrophobicity, limit its cross-linking to structural polysaccharides, or render it easier to remove by chemical pretreatments. Overall, our findings should have broad application in plant bioengineering programs aimed at improving the conversion cellulosic biomass into biofuels or other products.