Submitted to: Organic and Biomolecular Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/7/2002
Publication Date: N/A
Citation: Interpretive Summary: Monolignols are the building blocks from which the plant cell wall polymer lignin is produced. It is lignin that holds fibers together in woody and forage plants; it is essential for the plant, but limits plant cell wall digestibility by ruminant animals and impedes industrial pulping to make paper. Consequently there is a lot of effort directed toward selection and genetic methods for altering lignins. The last step of the monolignol biosynthetic pathway is catalyzed by the enzyme CAD (cinnamyl alcohol dehydrogenase). When CAD is deficient, a monolignol precursor coniferaldehyde builds up and becomes incorporated into lignins. We conducted a series of experiments by incorporating coniferaldehyde into synthetic lignins. After detailed analysis by NMR spectroscopy, we have a better database to understand how these components are incorporated into lignins in plants. The incorporation obviously alters the lignin structure and properties (including its color). Some alterations can be exploited in various industrial processes, but such modifications of lignin also impact the plant, since the properties required for water transport and structural support are also altered. These studies are aimed at understanding the limitations to cell wall utilization in various natural and industrial processes.
Technical Abstract: Peroxidase/hydrogen-peroxide-mediated radical coupling of 4-hydroxycinnamaldehydes produces 8-O-4-, 8-5-, and 8-8-coupled dehydrodimers as had been documented earlier, as well as the 5-5-coupled dehydrodimer. The 8-5-dehydrodimer is however produced kinetically in its cyclic phenylcoumaran form at neutral pH. Synthetic polymers produced from mixtures of hydroxycinnamaldehydes and normal monolignols provide the next level of complexity. Spectral data from dimers, oligomers, and synthetic polymers have allowed a more substantive assignment of aldehyde components in lignins isolated from a CAD-deficient pine mutant and an antisense-CAD-downregulated transgenic tobacco. The CAD-deficient pine lignin shows enhanced levels of the typical benzaldehyde and cinnamaldehyde end-groups, along with evidence for two types of 8-O-4-coupled coniferaldehyde units. The CAD-downregulated tobacco also has higher levels of hydroxycinnamaldehyde and hydroxybenzaldehyde (mainly syringaldehyde) incorporation, but the analogous two types of 8-O-4-coupled products are the dominant features. 8-8-Coupled units are also clearly evident. There is clear evidence for coupling of hydroxycinnamaldehydes with each other and then incorporating into the lignin, as well as for the incorporation of hydroxycinnamaldehyde monomers onto the growing lignin polymer. Coniferaldehyde and sinapaldehyde (as well as vanillin and syringaldehyde) co-polymerize with the traditional monolignols into lignins and do so at enhanced levels when the normal monolignol production is impacted by CAD-deficiency. The implication is that, particularly in angiosperms, the aldehydes behave like the traditional monolignols and should probably be regarded as authentic lignin monomers in normal and CAD-deficient plants.