Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/30/1998
Publication Date: N/A
Citation: Interpretive Summary: A major component in all terrestrial plants, lignin is a polymer that limits digestion of plants by animals, and must be removed from wood to make paper. An approach to reducing the problems associated with lignin has been to try to decrease lignin production in plants by down-regulating some of the crucial enzymes in the lignin biosynthetic pathway. This is done by genetic manipulation of various kinds. Tobacco is often used since its genetics are well defined and it is a good, rapidly growing, model plant system. One interesting observation is that, with some down-regulation attempts, the amount of lignin is not actually reduced. What we are finding is that plants appear to have the flexibility to make lignin from other components when it is not possible to produce the normal lignin building blocks. The effects on lignin amount and lignin structure of down- regulating two important lignin genes, the last two on the pathway, were studied. In one case, CAD, the amount of lignin is similar, but it incorporates building blocks from earlier in the biosynthetic pathway. In the other, CCR, the lignin is reduced to about half the normal level, but the plants cell walls are thinner and often collapsed and the plant is not vigorous; again, the lignin that is formed incorporates components from earlier in the pathway. These studies are aimed at understanding lignification to eventually improve the utilization of plant crops.
Technical Abstract: Homologous antisense constructs were used to downregulate tobacco CAD (cinnamyl alcohol dehydrogenase) and CCR (cinnamoyl-CoA reductase) activities in the lignin monomer biosynthetic pathway. CCR converts activated cinnamic acids (hydroxycinnamoyl-SCoA's) to cinnamaldehydes; cinnamaldehydes are then reduced to cinnamyl alcohols by CAD. The transformations caused the incorporation of non-traditional components int the extractable tobacco lignins, as evidenced by NMR. Isolated lignin of antisense-CAD tobacco contained fewer coniferyl and sinapyl alcohol-derived units that were compensated for by elevated levels of benzaldehydes and cinnamaldehydes. Products from radical coupling of cinnamaldehydes, particularly sinapaldehyde, which were barely discernible in normal tobacco, were major components of the antisense-CAD tobacco lignin. Lignin content was reduced in antisense-CCR tobacco which displayed a markedly reduced vigor. That lignin contained fewer coniferyl alcohol-derived units and significant levels of tyramine ferulate. Tyramine ferulate is a sink for the anticipated build-up of feruloyl-SCoA, and may be up-regulated in response to a deficit of coniferyl alcohol. Although it is not yet clear whether the modified lignins are true structural components of the cell wall, the findings provide further indications of the metabolic plasticity of plant lignification. An ability to produce lignin from alternative monomers would open new avenues for manipulation of lignin by genetic biotechnologies.