Submitted to: Journal of the French Academy of Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2004
Publication Date: 5/1/2004
Citation: Grabber, J.H., Ralph, J., Lapierre, C., Barriere, Y. 2004. Genetic and molecular basis of grass cell wall biosynthesis and degradability i. lignin-cell wall matrix interactions. Journal of the French Academy of Science. v. 327. p. 455-465. Interpretive Summary: Forage grasses are a major feedstuffs for livestock and grass cereal crops like maize, rice, wheat, oats and rye supply most of the dietary energy needs of people and many types of livestock. Fiber makes up 25 to 80% of the dry weight of grasses and it is composed primarily of complex carbohydrates. These carbohydrates are potentially an important source of digestible energy for livestock and they may also be converted into various chemicals for use in automobile fuels, plastics, and other products. Unfortunately, the enzymatic breakdown of complex carbohydrates into sugars is limited by an indigestible component in fiber known as 'lignin'. In the 1st part of this paper, we review how the formation and chemical makeup of lignins and their attachment to structural polysaccharides influences the enzymatic degradability of fiber. This review will help scientists to develop improved grasses and processing methods so that lignin is less of a barrier to fiber digestion. Ultimately these studies will lower the cost and environmental impact of converting grasses and cereal crop residues into food, fuel, and industrial products.
Technical Abstract: Lignification of cell walls is a major factor limiting the degradability of grasses in the rumen or large intestine of herbivores. Monolignols transported from the cytosol to the apoplast undergo oxidase-mediated polymerization to form lignins composed mainly of guaiacyl (G) and syringyl (S) units with lesser amounts of p-hydroxyphenyl (H) units. Lignification is a spatially and temporally regulated phenomenon, and lignin composition and structure differs between primary and secondary cell walls and among tissues types. Grass lignins are cross-linked to the cell wall matrix by oxidative coupling of monolignols to ferulate and diferulate xylan esters and possibly by nucelophilic addition of structural sugars and amino acids to lignin quinone-methide intermediates. In some cases, cross-linking between lignin polymers is probably mediated by photocatalyzed cyclodimers of p-coumarate esters on syringyl units. Variations in lignin deposition, cross-linking and, in some cases, composition influence the enzymatic degradability of grass cell walls.