Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2000
Publication Date: N/A
Citation: Interpretive Summary: Mutant and transgenic plants provide useful insights into biochemical processes occurring in normal plants. It has recently been found that plants will change the makeup and structure of their lignins (the polymers that hold fibers together in woody and forage plants) if they are not able to produce lignins normally. In this case, the final step in the biosynthetic pathway to one of lignin's two major components (sinapyl alcohol) was blocked. The lignins incorporated significant quantities of the precursor right before the blocked step, 5-hydroxyconiferyl alcohol. The structures that are produced in the lignins were studied so that the mechanism of their incorporation can be elucidated and their utilization can be addressed. In fact this component incorporates fully into the lignin and produces a new type of structure that was identified by NMR in this study. 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 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: Benzodioxane structures have been found to result from incorporation of 5-hydroxyconiferyl alcohol monomers into lignins isolated from poplars that are deficient in COMT, an O-methyl transferase operating late on the lignin biosynthetic pathway. The enzyme, which methylates 5-hydroxyconiferyl aldehyde, is one of two enzymes required to produce lignin syringyl (3,5-dimethoxy-4-hydroxyphenyl) units. The benzodioxanes are formed by coupling of a monolignol (at its beta-position) with a 5-hydroxyguaiacyl unit (at its 4-O-position) followed by internal trapping of the resultant quinone methide (at its alpha-position) by the phenolic OH at C-5. Such reactions can be mimicked in vitro to provide model compounds for authentication of the structures in the lignin. Short-range 13C-1H NMR correlation experiments (HMQC or HSQC) on the lignins reveal the unique alpha- and beta-C/H pairs, which can be further characterized by 2D and 3D HMQC-TOCSY experiments. The clear incorporation of 5-hydroxyconiferyl alcohol units into lignins in COMT-deficient plants provides further evidence of the metabolic plasticity of lignification -- plants are capable of integrating phenols other than the three traditional monolignols into their lignins when faced with reduced capacity to produce the traditional monolignols.