Submitted to: Review Article
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/7/2011
Publication Date: 2/22/2012
Publication URL: http://handle.nal.usda.gov/10113/56461
Citation: Tobimatsu, Y., Elumalai, S., Grabber, J.H., Davidson, C.L., Pan, X., Ralph, J. 2012. Hydroxycinnamate conjugates as potential monolignol replacements: In vitro lignification and cell wall studies with rosmarinic acid. Review Article. 5:676-686. 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 hindering 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 this study, we artificially lignified cell walls from corn with normal precursors (i.e. monolignols) plus rosmarinic acid, which is a natural antioxidant found in many culinary herbs but not normally a component of lignin. We found that rosmarinic acid readily formed polymers with normal monolignols and it rendered lignin easier to remove with chemical pretreatments. Incorporation of rosmarinic acid into lignin also improved the production and fermentation of sugar from cell walls, both prior to and following chemical pretreatment of cell walls. These results provide compelling evidence that bioengineering of plants to incorporate rosmarinic acid into lignin should substantially enhance the availability of cell wall polysaccharides for biofuel, chemical, or livestock production.
Technical Abstract: The plasticity of lignin biosynthesis should permit the inclusion of new compatible phenolic monomers such as rosmarinic acid (RA) and analogous catechol derivatives to create cell wall lignins that are less recalcitrant to biomass processing. In vitro lignin polymerization experiments revealed that RA readily underwent peroxidase-catalyzed copolymerization with monolignols and lignin oligomers to form polymers with new benzodioxane inter-unit linkages. Incorporation of RA permitted extensive depolymerization of synthetic lignins by mild alkaline hydrolysis, presumably by cleavage of ester intraunit linkages within RA. Copolymerization of RA with monolignols into maize cell walls by in-situ peroxidases strikingly enhanced alkaline lignin extractability and promoted subsequent cell wall saccharification by fungal enzymes. Interestingly, incorporating RA also improved cell wall saccharification by fungal enzymes and by rumen microflora even without alkaline pretreatments, possibly by modulating lignin hydrophobicity and/or limiting cell wall cross-linking. Consequently, we anticipate that bioengineering approaches for partial monolignol substitution with RA and analogous plant cinnamates would permit more efficient utilization of plant fiber for biofuel or livestock production.