Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 7, 2008
Publication Date: November 1, 2008
Citation: Hatfield, R.D., Ralph, J., Grabber, J.H. 2008. A potential role for sinapyl p-coumarate as a radical transfer mechanism in grass lignin formation. Planta. 228(6):919-928. Interpretive Summary: Corn silage is an important component of dairy rations, particularly in the more humid regions of the U.S. Fiber (cell walls) is a major component of corn silage and is mainly made up of carbohydrates, protein, and an indigestible component called lignin. Fiber provides a source of nutritional energy for dairy cows and is critical for maintaining good animal health. However, only about 40% of the fiber is digested by the animal due to the binding of lignin to the fiber carbohydrates. We have found that corn lignin is unique in that it contains high amounts of p-coumaric acid (pCA). Our studies indicate that pCA does not have a structural role but is important in helping corn plants form lignin within the cell wall. If rapid lignin formation is dependent upon pCA availability, we may be able to help control lignin formation by altering the amount of pCA available during fiber development in corn. In order to obtain the maximum nutritional energy from corn silage it is important to limit the influence of lignin on fiber digestibility. This information may provide geneticists and/or molecular biologists a way to genetically alter corn to improve cell wall digestibility. Improving corn silage digestibility would lead to better feed utilization in dairy rations and decrease manure solid wastes.
Technical Abstract: Lignins from maize and other grasses differentiate themselves from other plants in uniquely containing up to 20% p-coumaric acid (pCA) acylated to the C-9 position of syringyl units. The function of pCA acylation of lignin and its aversion to undergo oxidative coupling during lignification is not understood. UV monitoring of single- substrate reactions of extractable and cell-wall-bound maize peroxidases with lignin precursors and p-hydroxycinnamate cell-wall models revealed that radical coupling of coniferyl alcohol, ethyl ferulate and methyl pCA proceeded 5 to 50 times faster than sinapyl alcohol. Poor oxidation of sinapyl alcohol by maize peroxidases was unexpected since this monolignol is the chief component of lignin in most maize tissues. Adding catalytic amounts of ethyl ferulate, methyl pCA or sinapyl p-coumarate (the presumed precursor of p-coumarolyated lignin) enhanced oxidative coupling of sinapyl alcohol by maize peroxidases by 10-20 fold. Subsequent UV, HPLC, and NMR studies with maize peroxidases suggested that rapidly oxidized p-hydroxycinnnamate esters preferentially transferred radicals to stimulate oxidative coupling of sinapyl alcohol. Oxidative coupling of p-hydroxcinnamate esters commenced only after sinapyl alcohol coupling was complete. Since cell wall ferulates are largely consumed by extensive cross-coupling with monolignols at the early stages of lignification, our results indicate that direct acylation of sinapyl alcohol with pCA could insure efficient oxidation of this monolignol during the later stages of lignification when syringyl-rich lignins are formed in maize. Their propensity to forgo oxidative coupling in favor of radical transfer may also partially explain why most pCA esters exist as uncoupled terminal units in grass lignins.