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United States Department of Agriculture

Agricultural Research Service

Title: Lignins: Natural Polymers from Oxidative Coupling of 4-Hydroxyphenylpropanoids

Authors
item Ralph, John
item Lundquist, Knut - U CHALMERS, SWEDEN
item Brunow, Gosta - U HELSINKI, FINLAND
item Lu, Fachuang - UW MADISON
item Kim, Hoon - UW MADISON
item Schatz, Paul
item Marita, Jane
item Hatfield, Ronald
item Boerjan, Wout - U GENT, BELGIUM

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 6, 2003
Publication Date: January 15, 2004
Citation: Ralph, J., Lundquist, K., Brunow, G., Lu, F., Kim, H., Schatz, P.F., Marita, J.M., Hatfield, R.D., Ralph, S.A., Christensen, J.H., Boerjan, W. 2004. Lignins: natural polymers from oxidative coupling of 4-hydroxyphenylpropanoids. Phytochemistry Reviews 3(1):29-60.

Interpretive Summary: A major component in all plants, lignin is a complex component that limits digestion of plants by animals, and must be removed from wood to make fine paper. An understanding of the nature of lignins is evolving as a result of detailed structural studies, recently aided by the availability of mutant and transgenic plants. The currently accepted theory by which the lignin polymer is synthesized is by combinatorial-like coupling reactions of lignin monomers (monolignols) and the growing polymer, simply under controls normal for chemical reactions. As a result, the actual structure of the lignin macromolecule is not absolutely defined or determined. The 'randomness' of linkage generation and the astronomical number of possible combinations of even a simple polymer structure, suggest that there is little likelihood that there are two molecules of lignin over a certain (regrettably unspecified) size on this earth that are identical! A recent challenge to the currently accepted theory, attempting to bring lignin into line with other carefully synthesized polymers, can be summarily dismissed for its inability to explain the most basic details of lignin structure. The amazing plasticity of the combinatorial polymerization reactions may allow monomer substitution and significant variations in final structure which, in many cases, the plant appears to tolerate. As such, lignification is seen as a marvelously engineered solution to allow plants considerable flexibility in dealing with various environmental stresses, and conferring on plants a striking ability to remain viable even when humans or nature impact 'required' lignin-biosynthetic-pathway genes/enzymes. Studies on lignins and the process of lignification are at the heart of efforts to improve agricultural sustainability and maximize the utilization of our plant resources.

Technical Abstract: An understanding of the nature of lignins is evolving as a result of detailed structural studies, recently aided by the availability of lignin-biosynthetic-pathway mutants and transgenics. The currently accepted theory by which the lignin polymer is (bio)synthesized is via combinatorial-like coupling reactions of monolignols and the growing polymer, under simple chemical control. As a result, the actual structure of the lignin macromolecule is not absolutely defined or determined. The 'randomness' of linkage generation (which is not truly statistically random but governed, as is any chemical reaction, by the supply of reactants, the matrix, etc.) and the astronomical number of possible isomers of even a simple polymer structure, suggest that there is little likelihood that there are two molecules of lignin over a certain (regrettably unspecified) size on this earth that are identical! A recent challenge to the currently accepted theory, attempting to bring lignin into line with other carefully synthesized polymers, can be summarily dismissed for its inability to explain the most basic details of lignin structure. The amazing plasticity of the combinatorial polymerization reactions may allow monomer substitution and significant variations in final structure which, in many cases, the plant appears to tolerate. As such, lignification is seen as a marvelously engineered solution to allow plants considerable flexibility in dealing with various environmental stresses, and conferring on plants a striking ability to remain viable even when humans or nature impact 'required' lignin-biosynthetic-pathway genes/enzymes.

Last Modified: 4/24/2014
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