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Poster Presentation 1-27
The Updated Metabolic Stream Roadmap for Lignin Genetic Engineering
Laigeng Li1, Xiaofei Cheng1, Jacqueline Leshkevich1, Toshiaki Umezava2 and Vincent Chiang1
1Plant Biotechnology Research Center, School of Forestry and Wood Products, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931
2 Wood Research Institute, Kyoto University
Telephone: (906) 487-2293; Fax: (906) 487-2915; E-mail: lli@mtu.edu
Lignin, the second most abundant polymer in plants, has attracted much research interest to understand how it is biosynthesized and how it can be genetically modified in order to use the biomass more efficiently. Although much progress has been achieved for illustration of lignin biosynthetic metabolism roadmap, there is still some uncertainty in a number of the pathway steps.
Following the clarification of the monolignol biosynthesis pathway for the steps of 5-hydroxylation and 5-O-methylation of the intermediates (Osakabe et al.,1999, PNAS, 96:8955-8960; Li et al., 2000, JBC, 275:6537-6545), we recently cloned two lignin-related alcohol dehydrogenase cDNAs in aspen developing xylem tissue (Li et al., 2001, the Plant Cell, 13:1567-1586). The characterization of their recombinant proteins indicated that they are two distinct enzymes and catalyze preferential reduction of coniferyl or sinapyl aldehyde, respectively. Accordingly they are named as coniferyl alcohol dehydrogenases (CAD) and sinapyl alcohol dehydrogenases(SAD). The detailed kinetics data suggested that the reduction of coniferyl and sinapyl aldehydes in monolignol biosynthesis is catalyzed correspondingly by CAD and SAD, instead of only one, that is, cinnamyl alcohol dehydrogenase which has long been credited with converting both coniferyl and sinapyl aldehyde to their corresponding alcohol. The sequence analysis of CAD and SAD cDNAs indicated that they belong to the divergent groups of alcohol dehydrogenases in association with G and S lignin appearance in higher plants. All molecular, biochemical, histochemical and immunolocalizational evidence clearly indicates that CAD gene function is associated with G lignin deposition while SAD is involved in S lignin formation in aspen. Thus, three steps dictated by Cal5H, AldOMT and SAD constitute a consecutive route diverting the metabolic stream to syringyl monolignol, which is evolutionally developed in the line of angiosperm appearance.
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