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United States Department of Agriculture

Agricultural Research Service

Title: Modifications in Lignin of Transgenic Alfalfa Downregulated in Caffeic Acid 3-0-Methyltransferase and Caffeoyl Coenzyme a 3-0-Methyltransferase

item Marita, Jane
item Ralph, John
item Hatfield, Ronald
item Guo, Dianjing - NOBLE FOUNDATION
item Chen, Fang - NOBLE FOUNDATION
item Dixon, Richard - NOBLE FOUNDATION

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: December 12, 2002
Publication Date: December 30, 2002

Technical Abstract: Alfalfa is a leading forage crop with high nutritive value because it is rich in protein, minerals, and vitamins, and can retain a low fiber and high energy content. The inability of cattle to digest the fiber fraction efficiently reduces its nutritive value, a consequence of the deposition of lignin and polysaccharides in the stem cell wall componenets during maturation. In alfalfa, coniferyl and sinapyl alcohols give rise to guaiacyl and syringyl units in the lignin polymer. The formation of these alcohols requires the activity of O-methyltransferases. Caffeoyl CoA 3-O-methyltransferase (CCOMT) participates in the production of guaiacyl and syringyl precursors via caffeoyl-CoA as an intermediate, whereas caffeic acid 3-O-methyltransferase (COMT) primarily contributes to the formation of syringyl precursors from 5-hydroxyconiferyl aldehyde. Isolated lignins from COMT-deficient alfalfa contained benzodioxanes resulting from the incorporation of the novel monomer, 5-hydroxylconiferyl alcohol (5-HCA). Due to the high level of 5-HCA incorporated into the lignin of the COMT-deficient alfalfa and the use of sensitive NMR instruments, new structural features were revealed. A new type of end-unit (5-hydroxyguaiacyl glycerol)was identified. It was also established that both coniferyl alcohol and the novel 5-HCA can cross-couple with the 5-hydroxyguaiacyl units formed in lignin, the latter giving rise to extended chains of benzodioxane units. Most strikingly evident in the CCOMT-deficient alfalfa was the dramatic decrease in lignin content and modest increase in cellulose reflecting a 30% increase in cellulose:lignin ratio. These compositional changes potentially enhance the utilization of alfalfa as a major forage crop by increasing the digestibility of its stem fraction.

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