Submitted to: Phytochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 9/9/2002
Publication Date: N/A
Citation: Interpretive Summary: Genetically modified plants provide insights into a plant's biochemical processes. It has been found that plants will change the makeup and structure of their lignin (a polymer that holds fibers together in woody and forage plants) if they are not able to produce lignins normally. When the lignin genes, COMT and CCoAOMT, are deficient, the resulting plants are more digestible by ruminant animals. Both genes have been altered and their effects characterized in other plants (such as aspen or poplar or tobacco) Here we have examined the affects of COMT and CCoAOMT deficiencies specifically on alfalfa cell wall components (which lignin is a part of). The typical signatures of COMT-deficiency are observed in the COMT transgenic plant; COMT-deficient plants cannot make sinapyl alcohol, one of the normal components of lignin, but instead supplement their lignin production by using the immediate precursor to the normal component, so-called 5-hydroxyconiferyl alcohol. In addition, a new type of end-unit (an unique dead-end on the lignin polymer where no more units can be attached) was identified in the lignin of COMT-deficient alfalfa, 5-hydroxyguaiacyl glycerol units. The CCoAOMT-deficiency results in a general decrease in the normal components of alfalfa lignin which include both coniferyl and sinapyl alcohol. Both gene deficiencies show the flexibility of the alfalfa plant to still produce lignin albeit modified. By regulating the expression of these genes in alfalfa, a more digestible plant is produced for ruminant animals. Studies such as this are at the heart of efforts to improve the utilization of valuable plant resources.
Technical Abstract: Isolated lignins from alfalfa deficient in caffeic acid 3-O-methyltransferase contained benzodioxanes resulting from the incorporation of the novel monomer, 5-hydroxylconiferyl alcohol. Due to the high level incorporated into the soluble lignin fraction and the use of sensitive NMR instrumentation, unique structural features were revealed. A new type of end-unit, the 5-hydroxyguaiacyl glycerol unit, was identified. It was possible to establish that coniferyl alcohol, sinapyl alcohol, and the novel 5-hydroxyconiferyl alcohol can cross-couple with the 5-hydroxyguaiacyl units that are formed in the lignin, the latter giving rise to extended chains of benzodioxane units. There is also evidence that 5-hydroxyconiferyl alcohol couples with normal (guaiacyl or syringyl) lignin units. Lignin in the alfalfa deficient in caffeoyl CoA 3-O-methyltransferase was structurally similar to the control lignin but the transgenic exhibited a dramatic decrease in lignin content (~20%) and modest increase in cellulose (~10%) reflecting a 30% increase in cellulose:lignin ratio. The compositional changes in both transgenics potentially allow enhanced utilization of alfalfa as a major forage crop by increasing the digestibility of its stem fraction.