Submitted to: Phytochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/16/2000
Publication Date: N/A
Citation: Interpretive Summary: Mutant and transgenic plants provide useful insights into the biochemical processes occurring in normal plants. It has recently been found that plants will change the makeup and structure of their lignin (the polymer that holds fibers together in woody and forage plants) if they are not able to produce lignins normally. When enzymes in the lignin pathway (including COMT [caffeic acid O-methyl transferase, now more aptly termed 5-hydroxyconiferyl aldehyde O-methyl transferase]and CAD [cinnamyl alcohol dehydrogenase] described here) are deficient, the plants use components that they can make by normal processes as novel precursors for lignin formation. This study extends the preliminary findings reported in previous papers, detailing the structural changes that result from COMT- and CAD-deficiency. It is becoming increasingly evident that plants are able to incorporate substantial amounts of components that are not normally considered to be precursors of lignins. This means that breeding or genetic engineering can be used to enhance these components in plants if they are found to be of value. Such studies are at the heart of efforts to improve the utilization of valuable plant resources in processes ranging from digestion of forages by ruminants to industrial paper production by chemical pulping.
Technical Abstract: Studying lignin-biosynthetic-pathway mutants and transgenics provides insights into plant responses to perturbations to the lignification system, and enhances our understanding of normal lignification. When enzymes late in the pathway are downregulated, significant changes in the composition and structure of lignin may result. NMR spectroscopy provides powerful diagnostic tools for elucidating structures in the lignin polymer, hinting at the chemical and biochemical changes that have occurred. COMT (caffeic acid O-methyl transferase, now more aptly termed 5-hydroxyconiferyl aldehyde O-methyl transferase) downregulation in poplar results in production and apparent export to the cell wall of 5-hydroxyconiferyl alcohol which incorporates into lignins via typical radical coupling reactions, to produce novel benzodioxane units in the lignin. CAD (cinnamyl alcohol dehydrogenase) downregulation results in the incorporation of the hydroxycinnamyl aldehyde monolignol precursors into lignin. Sinapyl aldehyde cross-couples with both guaiacyl and syringyl units in the growing polymer, whereas coniferyl aldehyde cross-couples only with syringyl units, reflecting simple chemical cross-coupling propensities. The clear incorporation of these monomers implies that the plant is sending the products of incomplete monolignol biosynthesis out to the cell wall for incorporation. The recognition that novel units can incorporate into lignin also provides significantly expanded opportunities for engineering the composition and consequent properties of lignin for improved utilization of valuable plant resources.