Cell wall fermentation kinetics are impacted more by lignin content and ferulate cross-linking than by lignin composition

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 by 37%, rate decreased by 37%, and the extent declined by 18% as cell wall lignin content increased from 0.5 to 124 mg g(-1). 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 alcohols 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. (C) 2008 Society of Chemical Industry