ARS | Publication request: WHAT MAKES A GOOD MONOLIGNOL SUBSTITUTE

Submitted to: Book Chapter
Publication Type: Book/Chapter
Publication Acceptance Date: January 29, 2005
Publication Date: August 1, 2006
Publisher's URL: http://www.brownwalker.com/book.php?method=ISBN&book=1581124457
Citation: Ralph, J. 2006. What makes a good monolignol substitute? In: Hayashi, T., editor. The Science and Lore of the Plant Cell Wall Biosynthesis, Structure and Function. Boca Raton, FL: Universal Publishers (BrownWalker Press). p. 285-293.

Technical Abstract: A small range of monomers are now known to substitute for the conventional monolignols in various natural and transgenic plants. Monolignol substitution appears to be most successful when the novel monomer behaves, in its chemical radical coupling and cross-coupling reactions, like a normal monolignol. Most important is the beta-O-4-coupling reaction with the phenolic end of the growing polymer to extend the polymer chain. The post-coupling reactions that may be altered by the different functionality on the monomer seem to have less effect. Thus massive changes in the lignin structure occur when 5-hydroxyconiferyl alcohol substitutes for sinapyl alcohol, for example - the coupling reactions are analogous, but post-coupling steps produce novel benzodioxane structures that drastically change the lignin. Observations that plants with monolignol substitution and profoundly altered lignin structure can fare well supports the heretical tongue-in-cheek idea expressed at a conference in 1997 that the exact structure of lignins is not that important to the functioning of the plant. The plant requires certain properties and functionality of its lignins, but does not expend resources dictating those properties by exactly stipulating lignin primary structure. Such biosynthetic malleability functions well for the plant, and also provides significant opportunities for engineering the polymer. Already it has been demonstrated that natural and industrial processes ranging from ruminant digestibility to chemical pulping can be both positively and negatively impacted by alterations to lignin composition and structure. It is also apparent that phenolic components from beyond the monolignol pathway itself (such as the acylated monolignols) may be incorporated into lignins if they have compatible reaction chemistry and are transportable to the wall. Future work should reveal opportunities beyond the interesting deviations achieved by up- and down-regulating genes on the monolignol pathway to date.