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ARS Home » Midwest Area » Madison, Wisconsin » U.S. Dairy Forage Research Center » Research » Publications at this Location » Publication #163638
Title: PROFILING OF OLIGOLOGNOLS REVEALS MONOLIGNOL COUPLING CONDITIONS IN LIGNIFYING POPLAR XYLEM

Author
item MORREEL, KRIS - GENT U BELGIUM
item Ralph, John
item KIM, HOON - UW MADISON
item GOEMINNE, GEERT - GENT U BELGIUM
item LU, FACHUANG - UW MADISON
item RALPH, SALLY - US FOREST PRODUCTS
item MESSENS, ERIC - GENT U BELGIUM
item BOERJAN, WOUT - GENT U BELGIUM

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/25/2004
Publication Date: 11/1/2004
Citation: Morreel, K., Ralph, J., Kim, H., Goeminne, G., Lu, F., Ralph, S., Messens, E., Boerjan, W. 2004. Profiling of oligolognols reveals monolignol coupling conditions in lignifying poplar xylem. Plant Physiology. 136:3537-3549.

Interpretive Summary: Lignin is a polymer that is mainly present in plant cell walls, where it provides strength and impermeability, allowing transport of water and solutes through the vascular system. There is wide interest in understanding the process of lignin biosynthesis and deposition because of its economic relevance: during chemical pulping, lignin needs to be extracted from the wood chips, a process that is expensive and environmentally hazardous. In addition, lignin limits the digestibility of forages. Hence, plant varieties with altered lignin contents may have improved performance as fodder crops or in the production of paper and pulp. The lignin polymer is made essentially from building block called monolignols. In addition, a variety of other phenolic compounds may be co-polymerized into the lignin polymer. Soluble low molecular weight components that are destined for lignin were characterized here. Attempts are made to discriminate them from products of alternate biochemical pathways. This fraction was severely depleted in transgenic poplars with reduced lignin content. We have identified the structures of 35 phenolic compounds, most of which are derived from two, three- or four monolignols. In addition, the structures of two of the compounds demonstrate the presence of a novel monomer that must now be considered as an authentic lignin precursor in poplar. All products are consistent with a current theory for lignification involving 'combinatorial' chemical coupling reactions. Such studies are ultimately aimed at improving the utilization of valuable plant resources.

Technical Abstract: Lignin is an aromatic heteropolymer, abundantly present in the walls of secondary thickened cells. Although much research has been devoted to the structure and composition of the polymer to obtain insight into lignin polymerization, the low molecular weight oligolignol fraction has escaped a detailed characterization. This fraction, however, can serve as a simple and accessible model to reveal details about the chemical coupling of monolignols, in contrast to the rather inaccessible polymer. We have profiled the methanol-soluble oligolignol fraction of poplar xylem, a tissue with extensive lignification. Using LC-MS/MS, chemical synthesis, and NMR, we have elucidated the structures of 35 compounds, most of which were dimers, trimers and tetramers derived from coniferyl alcohol, sinapyl alcohol, their aldehyde analogs, or vanillin. All structures are in agreement with the random coupling hypothesis for lignin polymerization. The structures of two oligomers each containing a gamma-p-hydroxybenzoylated syringyl unit, strongly suggest that sinapyl p-hydroxybenzoate has to be considered as an authentic precursor for lignin polymerization in poplar. This is the first paper cataloging the oligolignol fraction in the lignifying tissue of poplar.