Submitted to: Biomed Central (BMC) Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 17, 2010
Publication Date: March 17, 2010
Repository URL: http://handle.nal.usda.gov/10113/56917
Citation: Grabber, J.H., Schatz, P.F., Kim, H., Lu, F., Ralph, J. 2010. Identifying New Lignin Bioengineering Targets: 1. Monolignol Substitute Impacts on Lignin Formation and Cell Wall Fermentability. Biomed Central (BMC) Plant Biology. 10:114. Interpretive Summary: Cell wall polysaccharides in plant biomass are poorly fermented into ethanol or other industrial products unless they are liberated by harsh and costly chemical treatments from another cell-wall polymer called lignin. Lignin is also a major barrier limiting the digestion of many feeds by ruminant livestock such as cattle or sheep. Current plant engineering efforts are mainly aimed at manipulating the normal pathway of lignin biosynthesis. In the future, plants might be induced to form new types of lignin from molecules produced by other metabolic pathways and this might make lignin less inhibitory toward polysaccharide digestion or easier to remove by biological or chemical treatments. To identify promising new avenues for lignin bioengineering, we artificially lignified cell walls from corn with various combinations of normal "monolignols" (coniferyl and sinapyl alcohols) plus a wide variety of monolignol substitutes. In some cases we found that monolignol substitutes greatly enhanced the digestibility of cell walls by rumen bacteria, the organisms primarily responsible for fiber digestion in cattle and sheep. In ongoing work, we are characterizing the enzymatic breakdown of intact and chemically pretreated cell walls lignified by these and other monolignol substitutes. In subsequent studies, we will examine these and other monolignol substitutes in greater detail to identify promising bioengineering targets for improving plant fiber utilization.
Technical Abstract: Background: Recent discoveries highlighting the metabolic malleability of plant lignification indicate that lignin can be engineered to dramatically alter its composition and properties. Current plant engineering efforts are primarily aimed at manipulating the biosynthesis of normal monolignols but, in the future, apoplastic targeting of phenolics from other metabolic pathways may provide new approaches for designing lignins that are less inhibitory toward polysaccharide fermentation, both with and without biomass pretreatment. To identify promising new avenues for lignin bioengineering, we artificially lignified cell walls from maize cell suspensions with various combinations of normal monolignols (coniferyl and sinapyl alcohols) plus a variety of phenolic monolignol substitutes. Results: Inclusion of feruloylquinic or caffeoylquinic acids with monolignols considerably depressed lignin formation and strikingly improved cell wall fermentability by anaerobic rumen microflora. In contrast, various phenylpropanoids, catechins, mono- and diferuloyl polyol esters readily formed copolymer-lignins with normal monolignols; cell wall fermentability was often moderately enhanced by greater hydroxylation or 1,2-diol functionality of monolignol substitutes. Conclusion: Overall, monolignol substitutes improved the fermentability of non-pretreated cell walls by depressing lignin formation or possibly by reducing lignin hydrophobicity or cross-linking to structural polysaccharides. In ongoing work, we are characterizing the enzymatic saccharification of intact and chemically pretreated cell walls lignified by these and other monolignol substitutes.