Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/20/2003
Publication Date: 7/17/2003
Citation: Grabber, J.H., Hatfield, R.D., Ralph, J. 2003. Apoplastic ph and monolignol addition rate effects on lignin formation and cell wall degradability in maize. Journal of Agricultural and Food Chemistry. 51:4984-4989. Interpretive Summary: Fiber makes up 15 to 80% of the dry weight of plants. Fiber is composed mainly of complex carbohydrates. These carbohydrates are potentially an important source of digestible energy for livestock and they are also a renewable source of sugars that may 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. The three-dimensional shape of lignin and its attachment to carbohydrates by benzyl-ether and benzyl-ester cross-links are thought to influence the breakdown of complex carbohydrates by enzymes. To test this hypothesis, we artificially formed lignins of varying shapes and degrees of cross-linking in fiber isolated from corn (Zea mays L.). We found that the three-dimensional shape of lignin did not affect the breakdown of complex carbohydrates by digestive enzymes. In contrast, cross-linking of carbohydrates to lignin reduced their breakdown by about 25%. Therefore, reducing benzyl-ether and benzyl-ester cross-linking between lignin and carbohydrates should lower the cost and environmental impact of converting fibrous crops into food, fuel, and industrial products.
Technical Abstract: The influence of lignin structure and pH dependent lignin cross-linking on cell wall degradability were investigated by gradual end-wise or rapid bulk polymerization of coniferyl alcohol into primary maize walls suspended in pH 4 or 5.5 buffers. Based on thioacidolysis, the linear end-wise lignins had up to 270% more ether inter-unit linkages and 70% fewer end-groups than the branched bulk lignins but these differences in lignin structure did not influence cell wall degradability. By contrast, lowering the pH from 5.5 to 4.0 during lignification reduced the enzymatic degradability of cell walls by about 25%. This pH dependent depression in degradability was only noted when lignins were formed under conditions that enhance the formation alpha-ester and alpha-ether cross-links via quinone methide intermediates. Consequently, the abundance of these cross-links and not differences in lignin structure probably contributes to the poor degradability of primary walls relative to secondary walls in lignified grass tissues.