Submitted to: Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/27/1997
Publication Date: N/A
Citation: Interpretive Summary: A major component in all plants, lignin is a polymer that limits digestion of plants by animals, and is removed from wood to make paper. There has been interest recently in using genetic biotechnologies to knock out specific genes that produce crucial precursors of lignin. Early expectations were that this would reduce the amount of lignin. Now we are finding that the plant continues to produce lignin but from different compounds than normal. Nature has already explored some of these mutant possibilities herself. Workers at North Carolina State University recently identified a pine mutant that was naturally deficient in such an enzyme, cinnamyl alcohol dehydrogenase (CAD). The wood was brown/red, characteristic of a build-up of a compound from which lignin precursors are made. Structural examination did indeed show that the amount of this component had doubled in the mutant. More striking was that the mutant lignin contained a new compound not normally associated with lignins (although it is present in small amounts). Faced with an inability to produce the normal lignin precursor, the plant has apparently found a way to either greatly enhance production of the new precursor, or provide a new route to it. The lignin, while structurally very different, is clearly capable of filling the roles of mechanical support and water transport in the plant -- the plant is perfectly viable! These results illustrate the unexpected flexibility plant systems have with their lignin. Indications are that it will be possible to manipulate lignin composition well beyond traditional bounds to produce plant materials with higher polysaccharide digestibility, ultimately improving the utilization of plant energy.
Technical Abstract: Dihydroconiferyl alcohol is incorporated as a predominant precursor into the lignin of a loblolly pine mutant. This mutant is severely depleted in the lignin biosynthetic enzyme, cinnamyl alcohol dehydrogenase (CAD; E.C. 188.8.131.52), that converts coniferaldehyde to coniferyl alcohol, the major lignin monomer in conifers. NMR spectra clearly identify the compositional differences between the lignin in the mutant vs. normal pine. As anticipated from the reduction in CAD activity, coniferyl alcohol subunits are reduced in the lignin of the mutant, and coniferaldehyde subunits accumulate. However, most of the aldehydes are present in extractable components and are not extensively polymerized into the lignin; their level in the mutant lignin is approximately double those of normal lignins. More importantly, the lignin in the mutant contains a large amount of a novel subunit derived from dihydroconiferyl alcohol, accounting for ~30% of the lignin subunits. The conventional enzymatic pathway for lignin biosynthesi cannot account for this precursor. These results have important implications for the genetic engineering of lignin, particularly for the modification of lignin by suppression of CAD.