Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/1999
Publication Date: N/A
Citation: Interpretive Summary: A major component in all plants, lignin is a complex component that limits digestion of plants by animals, and is what must be 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 or to up-regulate genes to produce lignins with better properties. Chapple's group at Purdue successfully knocked out a lignin gene that is responsible for converting from one type of lignin to a simpler lignin in a mutant plant. More importantly, they then put this gene back into the mutant plant and produced plants with more of the simpler lignin than any plant that has been studied to date. Studies at the Dairy Forage Research Center show these lignins to be structurally quite normal and illustrate the massive compositional shifts from the more complex to the simpler lignin. Such studies reveal the incredible plasticity of the lignification process. Chapple's group aims to upregulate the gene in hardwood trees where the simpler lignin will allow easier pulping, and perhaps eventually into softwoods; softwoods have the more complex lignin and allowing them to produce the simpler lignin may allow the commercially important softwoods to be pulped more easily than at present. Similar genetic transformations in forage crop plants may also improve digestability by ruminants, potentially reducing animal wastes. Such studies are at the heart of efforts to improve agricultural sustainability and maximize our plant resources.
Technical Abstract: NMR of isolated lignins from an Arabidopsis mutant deficient in ferulate 5- hydroxylase (F5H) and transgenic plants derived from the mutant by overexpressing the F5H gene have provided detailed insight into the compositional and structural differences between these lignins. Wild-type Arabidopsis has a guaiacyl-rich syringyl-guaiacyl lignin typical of other dicots, with prominent beta-aryl ether, phenylcoumaran, resinol, biphenyl/dibenzodioxocin, and cinnamyl alcohol end-group structures. The lignin isolated from the F5H-deficient fah1-2 mutant contained only traces of syringyl units, and consequently enhanced phenylcoumaran and dibenzodioxocin levels. In fah1-2 transgenics in which the F5H gene was overexpressed under the control of the cauliflower mosaic virus 35S promoter, a guaiacyl-rich syringyl/guaiacyl lignin similar to the wild type was produced. In contrast, the isolated lignin from the fah1-2 transgenics in which F5H expression was driven by the cinnamate 4-hydroxylase promoter was almost entirely syringyl in nature. This simple lignin contained predominantly beta-aryl ether units, mainly with erythro-stereochemistry, with some resinol structures. No phenylcoumaran or dibenzodioxocin structures (which require guaiacyl units) were detectable. The overexpression of syringyl units in this transgenic resulted in a lignin with a higher syringyl content than that reported for any plant to date.