Submitted to: Plant Physiology Supplement
Publication Type: Abstract Only
Publication Acceptance Date: August 2, 1997
Publication Date: N/A
Technical Abstract: Many grass lignins are unique in containing relatively large amounts of p- coumaric acid (pCA) attached via an ester linkage to C-9 position of lignin monomers. In the case of corn, pCA constitutes 15-18% of a dioxane soluble enzyme lignin. In addition, the major portion of the pCA appears to be specifically attached to sinapyl alcohol residues. A perplexing problem is the function of pCA and why or how it remains unincorporated into the growing lignin molecule. Studies with isolated corn wall peroxidases show that pCA undergoes rapid radical coupling in the presence of hydrogen peroxide yet sinapyl alcohol undergoes very slow radical coupling. Analysis of corn wall peroxidases revealed no specific enzymes that would lead to the preferred incorporation of sinapyl alcohol as seen in other plants. The addition of sinapyl p-coumarate conjugate to a reaction mixture containing peroxidase, sinapyl alcohol, and hydrogen peroxide stimulated the rate of sinapyl alcohol radical coupling by 10-20 fold. Based on spectral analysis it appears that pCA radicals form rapidly but prefer to transfer these radicals to sinapyl alcohol. The newly formed sinapyl alcohol radicals form cross-coupled products. Radicals of sinapyl alcohol and pCA do not form cross-coupled products. As long as hydrogen peroxide is limiting pCA remains uncoupled. The role of pCA may be as an attached radical transfer mechanism for optimizing radical coupling of sinapyl alcohol into the growing lignin polymer. Attachment of some pCA to sinapyl alcohol insures localization of the radical transfer mechanism in areas where sinapyl alcohol is being incorporated into lignin.