Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 11/23/2007
Publication Date: 8/1/2008
Publication URL: http://www.dfrc.ars.usda.gov/DFRCWebPDFs/2008-Ralph-RecAdvInPolyphenolRes-36-66.pdf
Citation: Ralph, J., Brunow, G., Harris, P., Dixon, R.A., Schatz, P.F., Boerian, W. 2008. Lignification: Are lignins biosynthesized via simple combinatorial chemistry or via proteinaceous control and template replication? In: Daayf, F., El Hadrami, A., Adam, L., Ballance, G.M., editors. Recent Advances in Polyphenols Research. Oxford, United Kingdom: Wiley-Blackwell Publishing. p. 36-66. Interpretive Summary: Lignification is the plant cell wall process by which the lignin polymers that hold the fibers together are produced. Lignins add structural rigidity to the cell wall, aid in water transport up the plant stem, and function in defense against various pathogens. The current theory, based on that developed some 50 years ago, holds that the polymerization process is under simple chemical control. That is, unlike the synthesis of proteins and carbohydrates, for example, enzymes or proteins are not directly involved in preparing the polymer. The theory continues to accommodate all of the currently known facts. A more recent challenge-hypothesis suggests that such a process is unparalleled in nature and proposes that lignification is via strictly controlled biosynthesis on special protein arrays. The lignin polymer is then proposed to replicate itself. Without supporting evidence, the hypothesis has been unilaterally raised to theory status and the existing theory denigrated as an unsupported notion. Clearly, wider debate on the mechanism of lignification is needed. In examining some of the key arguments we find that those raised against the existing theory are invalid. Arguments for the new contender advocating absolute control are found to be convoluted, ambiguous, or reliant on misinterpreted data. In addition to the challenge-hypothesis' failure to accommodate known facts about lignin, the need for complex and convoluted arguments to explain such basic features as the malleability of the lignification process, lignins' lack of optical activity, and its structural complexity violates basic scientific principles. The challenge-hypothesis is incompatible with monomer substitution, in which the building blocks of the polymer may be replaced by other components, yet the evidence is now well established in various transgenic plants. The logical conclusion from examining the facts is that the new hypothesis fails to challenge the current theory.
Technical Abstract: Lignification is the plant cell wall process by which lignin polymers are produced from phenolic monomers. These monomers are primarily the three hydroxycinnamyl (p-coumaryl, coniferyl and sinapyl) alcohols. Structural analysis of cell wall lignins and mechanistic studies of possible biosynthetic pathways have led to a general consensus among workers in the field that the polymer is a product of oxidative coupling of phenols. The theory of combinatorial coupling of phenoxy radicals formed from the monolignols under simple chemical control (based on original work by Freudenberg) continues to accommodate all the currently known facts of lignin chemistry. This state of affairs has been challenged in recent years without any basis in factual evidence. A new challenge-hypothesis has been unilaterally raised to theory status and the existing theory denigrated as an unsupported notion. We have examined the key arguments in the new hypothesis and find that those raised against the existing theory are invalid. Arguments for the new contender, advocating absolute proteinaceous control, are found to be convoluted, ambiguous, or reliant on misinterpreted data. The logical conclusion from examining the facts is that the new hypothesis fails to challenge the current theory.