Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 12/12/2002
Publication Date: 12/30/2002
Citation: Interpretive Summary:
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.