Submitted to: Journal of the Science of Food and Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 6, 1998
Publication Date: N/A
Interpretive Summary: Fiber makes up 15 to 80% of the dry weight of plants. Fiber is composed primarily of polysaccharides and lignin. Polysaccharides can be degraded by some types of fungi and bacteria to simple sugars. These sugars are an important source of energy for humans and for livestock. These sugars may also be converted into ethanol for use in automobile fuels or other purposes. Polysaccharides in fiber are poorly degraded into simple sugars because of their association with lignin. Lignin is normally made up of three different kinds of building blocks--p-coumaryl, coniferyl, and sinapyl alcohols. In some genetically modified plants, coniferaldehyde and other aldehydes become major components of lignin. We ran experiments to determine how lignins formed with coniferaldehyde affect the degradability of polysaccharides in fiber. To do this, we isolated fiber from plant cells that were composed of just polysaccharides. We then formed an artificial lignin within this fiber using differing amounts of coniferyl alcohol and coniferaldehyde. We found that coniferaldehyde lignins were much more inhibitory to polysaccharide degradation than coniferyl alcohol lignins. Based on our results, we recommend that plants containing large amounts of coniferaldehyde lignins should not be used as livestock feeds. However, these lignins may increase the pest resistance of nonfood crops because aldehydes have antibiotic properties. These plants may also be valuable for papermaking or for ethanol production since coniferaldehyde lignin is easily removed from fiber. Studies of this kind provide basic information needed for understanding how biotechnology affects the usefulness of plants used as feeds and as raw materials for making industrial products.
Technical Abstract: Although the enzymatic or ruminal degradability of plants deficient in cinnamyl alcohol dehydrogenase (CAD) is often greater than their normal counterparts, factors responsible for these degradability differences have not been identified. Since lignins in CAD deficient plants often contain elevated concentrations of aldehydes, we used a cell-wall model system to evaluate what effect aldehyde-containing lignins have on the fungal- hydrolase degradability of cell walls. Varying ratios of coniferaldehyde and coniferyl alcohol were polymerized into nonlignified walls of maize (Zea mays L) by wall-bound peroxidase and exogenously supplied H2O2. Coniferaldehyde-containing lignins formed fewer cross-linked structures with other wall components, but they were much more inhibitory to cell wall degradation than lignins formed with coniferyl alcohol. Degradability differences were eliminated if coniferaldehyde lignins were made less hydrophobic by ethanolic sodium borohydride reduction of aldehyde groups t alcohols. These results suggest that aldehyde-containing lignins restrict rather than enhance the degradability of CAD deficient plants. Although incorporation of aldehyde-containing lignins into plants is not desirable from a nutritional standpoint, these lignins may enhance the pest resistance and delignification of plants used for papermaking and other industrial purposes.