Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/25/2008
Publication Date: 10/15/2008
Citation: Grabber, J.H., Mertens, D.R., Kim, H., Funk, C., Lu, F., Ralph, J. 2008. Cell wall fermentation kinetics are impacted more by lignin content and ferulate cross-linking than by lignin composition. Journal of the Science of Food and Agriculture. (89):122-129. Interpretive Summary: Cell walls make up 40 to 80% of the dry weight of grasses and they are composed primarily of polysaccharides (polymers of sugar molecules) cemented together with a polymer known as lignin. Unfortunately, lignin hinders the enzymatic breakdown of polysaccharides into sugars and this impairs the value of forage grasses as feedstuffs for livestock and as renewable feedstocks for the industrial production of ethanol fuels and other products. In this study, we artificially lignified cell walls from corn (Zea mays L.) with various building blocks of lignin found in normal, mutant, and genetically engineered plants. We also manipulated the cell wall deposition of ferulate, a molecule responsible for attaching (cross-linking) polysaccharides to lignin. When the cell walls were incubated with rumen bacteria, we found that reducing both the amount of lignin formed in cell walls and its cross-linking to polysaccarides by ferulate enhanced the rate and extent of cell wall digestion. In contrast, shifts in lignin composition had no impact on cell wall degradability. Thus genetic engineering or selection of grasses for lower lignin content or cross-linking should be more effective for improving cell wall degradability than altering lignin composition.
Technical Abstract: BACKGROUND: We used a biomimetic model system to ascertain how reductions in ferulate-lignin cross-linking and shifts in lignin composition influence ruminal cell wall fermentation. Primary walls from maize cell suspensions with normal or reduced feruloylation were artificially lignified with various monolignols previously identified in normal, mutant, and transgenic plants. Cell wall fermentability was determined from gas production during in vitro incubation with rumen microflora and by analysis of non-fermented polysaccharides. RESULTS: Hemicellulose fermentation lag time increased 37%, rate decreased by 37%, and extent declined by 18% as cell wall lignin content increased from 0.5 to 124 mg/g. Lignification increased lag time for cellulose fermentation by 12-fold. Ferulate-lignin cross-linking accounted for at least one-half of the inhibitory effect of lignin on cell wall fermentation. Incorporating sinapyl p-coumarate, a precursor of p-coumaroylated grass lignin, increased the extent of hemicellulose fermentation by 5%. Polymerizing varying ratios of coniferyl and sinapyl alcohol or incorporating 5-hydroxyconiferyl alcohol, coniferaldehyde, sinapyl acetate, or dihydroconiferyl alcohol into lignin did not alter the kinetics of cell wall fermentation. CONCLUSION: The results indicate that selection or engineering of plants for reduced lignification or ferulate-lignin cross-linking will improve fiber fermentability more than current approaches for shifting lignin composition.