|Jung, Hans Joachim|
Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/1998
Publication Date: N/A
Interpretive Summary: The feeding value of forage crops to cattle is reduced by a chemical compound known as lignin. Lignin is in the fiber of forages and limits digestion of the polysaccharides present in fiber. Also, lignin must be removed from wood in order to produce paper, and most of the cost and pollution related to paper production is associated with lignin removal. As a result, much effort is being targeted at the genetic modification through biotechnology of lignin addition to forages and trees. We reviewed the progress made in manipulating the lignin content of plants through biotechnology. Published research shows that when the genetic modifications actually reduce the amount of lignin present in plants, then the plants grow abnormally and cannot tolerate stress. However, other experiments indicate that when the genetic modification causes plants to alter the type of lignin deposited in the plant fiber, but not the amount of lignin, then these modified plants grow normally. The digestibility of forage fiber and the amount of chemical treatment required to produce paper have been improved by some of these modifications of lignin type. While further research will be necessary to refine the genetic modifications of plants to improve their feeding value and ease of paper production, it is evident that manipulation of lignin through biotechnology will provide improved forage and tree crops.
Technical Abstract: Because of the sizable economic benefits that might be achieved, considerable research effort has been targeted toward reducing the amount of lignin or modifying lignin structure to facilitate pulping of trees and improving digestibility of forage crops. As a result, genes for many of the steps in the lignin biosynthesis pathway have been used in transgenic experiments. Results of these experiments have added to our understanding of the process of lignification and point toward the critical aspects of lignin needed for normal plant growth and development. Down-regulation of cinnamoyl-CoA reductase (CCR) results in reduced lignin concentration in transgenic tobacco, and these plants exhibit stunted growth and partially collapsed xylem structures. In contrast, down-regulation of cinnamyl alcohol dehydrogenase (CAD) does not impact lignin concentration, and the transgenic plants appear normal in their growth and development. Both CCR and CAD transgenics incorporate unusual building blocks into their lignins The down-regulated CCR transgenics have high concentrations of tyramine ferulates, apparently because feruloyl-CoA accumulates due to down- regulation of CCR and is then available for linkage to tyramine rather than being converted to cinnamaldehydes. In contrast, down-regulated CAD transgenics directly incorporate the precursor cinnamaldehydes into lignin. It is apparent that plants have a requirement for adequate concentrations of lignin in their cell walls, but the composition of this lignin may be of relatively little importance.