Chapter 16
Cell Wall Cross-Linking in Grasses by Ferulates and Diferulates Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 2, 2015 | http://pubs.acs.org Publication Date: August 13, 1998 | doi: 10.1021/bk-1998-0697.ch016
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John Ralph , Ronald D. Hatfield , John H. Grabber , Hans-Joachim G. Jung , Stéphane Quideau , and Richard F. Helm 1,4
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U.S. Dairy Forage Research Center, 1925 Linden Drive West, Madison, WI 53706 Department of Forestry, University of Wisconsin, Madison, WI 53706 Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55108 2
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Ferulate polysaccharide esters in grasses enter into free-radical coupling reactions in the cell wall. By radical dimerization of ferulates, polysaccharide-polysaccharide cross-linking is effected. A range of diferulate isomers are produced, only one of which had been quantitated in the past. Diferulates have been underestimated by factors of up to 20, belittling their contribution to functions in the cell wall. Both ferulates and diferulates participate in lignification reactions and become intimately bound with lignin. Under-quantitationis significant since it is not possible to release ferulate or diferulates from some of the structures generated. Overall ferulates play a significant role in cell wall development and impact upon polysaccharide utilization in grasses. Cross-linking of plant cell wall components is expected to have a marked influence on various properties of the cell wall (7, 2). For example, in grasses, several authors have postulated the effects of isodityrosine and diferulate cross-links on cell-wall accessibility, extensibility, plasticity, digestibility and adherence (3,4). The topic has been reviewed extensively (7, 5-10). In woody species, the cross-linking mechanism to receive most attention involves the direct attachment of polysaccharides to lignin via nucleophilic addition to intermediate lignin quinone methides during lignification. An American Chemical Society Symposium recently addressed some of these issues (77). The extent of such cross-linking and how the plant is able to exert control on this seemingly 'opportunistic' approach has yet to be fully determined. It is well known that grasses have substantial amounts of hydroxycinnamic acids intimately associated with the cell wall, as has been detailed in a number of reviews (7, 5-10, 12). This chapter will concern itself only with ferulate, which has the major cross-linking role. It was recognized early on that ferulic acid, an ideally structured difunctional molecule, could have a role in cross-linking polysaccharides with lignin (Figures 1 and 2). 4
Current address: Department of Chemistry, Pennsylvania State University, University Park, PA 16802 Current address: Department of Wood Science and Forest Products, Virginia Polytechnic Institute and State University, Blacksburg, V A 24061-0346
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©1998 American Chemical Society
In Lignin and Lignan Biosynthesis; Lewis, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
In Lignin and Lignan Biosynthesis; Lewis, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Figure 1. Ferulates, exported into the cell wall region as esters of polysaccharides, (primarily arabinoxylans in grasses), will cross-link with lignin via two mechanisms to yield a cross-linked lignin-ferulatepolysaccharide (LFP) complex from which the ferulate may or may not be releasable. The 'passive' mechanism (ferulate addition to lignin quinone methides) gives complexes (LFP complex A) from which the ferulate is fully releasable. The 'active' mechanism, where ferulate radicals crosscouple with lignin monomer or oligomer radicals, produces an L F P (Complex B) from which ferulate is only partially releasable.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 2, 2015 | http://pubs.acs.org Publication Date: August 13, 1998 | doi: 10.1021/bk-1998-0697.ch016
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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on November 2, 2015 | http://pubs.acs.org Publication Date: August 13, 1998 | doi: 10.1021/bk-1998-0697.ch016
L = Lignin sidcchain or
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