Submitted to: Phytochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/1995
Publication Date: N/A
Interpretive Summary: Plants cannot be fully digested by animals that eat them. The sugar components (polysaccharides), which alone would be fully digestible, are tied to other components in the plant fiber that make them inaccessible. One mechanism the plant uses to tie fibers together is with a connecting agent called ferulic acid. Ferulic acid, attached to a fiber, can attach to another fiber-bound ferulic acid linking the two fibers together by a ferulic acid-ferulic acid bridge. We found that, even though ferulic acid accounts for only a small portion of the plant, 50% would join together to tie up fibers. Also, ferulic acids and their bridges link to another indigestible component of the plant called lignin. Ninety percent of the ferulic acid on fibers became attached to lignin. We feel that a great deal of the indigestibility of plants can be attributed to ferulic acid bridging and are currently studying the dependence of the amount of bridging on plant digestibility using a system where we have extensive control over the bridging process. The information gained will form a basis for selectively breeding or genetically modifying plants for improved digestibility.
Technical Abstract: Peroxidase-mediated dimerization of ferulate monomers and incorporation of ferulates into lignin were investigated with primary walls from suspension cultured maize (Zea mays cv Black Mexican). Growing cultures with 0 to 50 mM 2-aminoindan-2-phosphonic acid (AIP) reduced total ferulate concentrations in cell walls from 17.2 mg g-1 to 4.3 mg g-1 and increased the proportion of dehydrodimers to total ferulate from 15 to 24%, indicating that maize cells compensated for reduced ferulate deposition by increasing the formation of dehydrodimers. In a separate study, dimerization of ferulate monomers by wall-bound peroxidase was increased 260% by adding dilute hydrogen peroxide to nonlignified cell walls, suggesting that dehydrodimer formation was limited by the availability of hydrogen peroxide. About 45% of the dehydrodimers were coupled by 8-5 linkages, with 8-8, 8-O-4 and 5-5 coupled dehydrodimers each comprising 15 to 20% of the total. The quantity of ferulic and dehydrodiferulic acids released by saponification was reduced by 90% when exogenously supplied hydroxycinnamyl alcohols were polymerized into nonlignified walls by wall- bound peroxidase and in situ generated hydrogen peroxide. Only 40% of the ferulate incorporated into lignin was recovered following hydrolysis of a- and b-aryl ether linkages, suggesting that most of the ferulate was coupled to lignin by stable C-C or enol-ether linkages formed by oxidative coupling mechanisms. These results indicate that primary cell walls become extensively cross-linked by ferulic and dehydrodiferulic acids during lignification, and that only a portion of ferulates in lignified tissues are measurable by current solvolytic methods.