Submitted to: Phytochemistry Reviews
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/19/2004
Publication Date: 12/1/2004
Citation: Ralph, J., Bunzel, M., Marita, J.M., Hatfield, R.D., Lu, F., Kim, H., Schatz, P.F., Grabber, J.H., Steinhart, H. 2004. Peroxidase-dependent cross-linking reactions of p-hydroxycinnamates in plant cell walls. Phytochemistry Reviews. 3:79-96 Interpretive Summary: Peroxidases are enzymes involved in the biosynthesis of several components of the plant cell wall. Lignins, that are over-quoted as being the second most abundant terrestrial polymers, comprise some 10-30% of plant biomass. Peroxidases ostensibly provide the oxidizing capability for coupling every single lignin (phenolic) precursor into the complex lignin polymer. This paper is concerned with more subtle reactions on a smaller scale that nevertheless have significant impacts on plant cell wall biosynthesis and architecture. Low levels of cell wall cross-linking strengthens the walls but negatively impacts economically important natural processes such as ruminant digestibility. Grasses have so-called hydroxycinnamates intimately associated with their cell walls. Hydroxycinnamates are also apparent in cereal grains, where they may be responsible for some of the health benefits. Oxidation reactions of hydroxycinnamates are crucial to a diverse array of important cell wall cross-linking mechanisms. New cross-linking mechanisms continue to be found and are reported here. More extensive cross-linking action by the most important hydroxycinnamate, ferulate, has just been revealed with the discovery of trimeric compounds in which three ferulates, attached to their polysaccharides, are cross-linked. Significant issues remain for separating and quantifying these important components and in understanding mechanistic details. Ultimately, researchers will learn more about the impact of these diverse products of cell wall cross-linking have on physiological processes, including ruminant and human nutrition.
Technical Abstract: Peroxidases are heavily implicated in plant cell wall cross-linking reactions, altering the properties of the wall and impacting its utilization. Polysaccharide-polysaccharide cross-linking in grasses is achieved by dehydrodimerization of hydroxycinnamate-polysaccharide esters; a whole range of hydroxycinnamic acid dehydrodimers are released by saponification. Ferulates are the major cross-linking agents, but sinapates have now been implicated in a similar role; sinapate dehydrodimers and sinapate-ferulate cross-products have been discovered. New dehydrodimers have been authenticated, expanding our knowledge of the chemistry, role, and extent of cross-linking reactions. The first ferulate dehydrotrimer has been reported and there are suggestions that it may be only one of several higher oligomers; whether these trimers truly cross-link three independent polysaccharide chains or only two remains to be determined. Hydroxycinnamates and their dehydrodimers also undergo radical coupling reactions with lignin monomers and possibly oligomers, resulting in lignin-polysaccharide cross-linking in the wall. Both polysaccharide-polysaccharide and lignin-polysaccharide cross-links inhibit cell wall degradation in ruminants.