Tricin, a flavonoid monomer in monocot lignification

Tricinwas recentlydiscoveredinligninpreparations fromwheat (Triticum aestivum) strawandsubsequentlyinallmonocotsamplesexamined. To provide proof that tricin is involved in lignification and establish the mechanism bywhich it incorporates into the lignin polymer, the 4¢-O-b coupling products of tricinwith themonolignols (p-coumaryl, coniferyl, and sinapyl alcohols) were synthesized alongwith the trimer that would result fromits 4¢-O-b-couplingwith sinapyl alcohol and then coniferyl alcohol. Tricinwas also found to cross couplewithmonolignols to form tricin-(4¢-O-b)-linked dimers in biomimetic oxidations using peroxidase/hydrogen peroxide or silver (I) oxide. Nuclear magnetic resonance characterization of gel permeation chromatography-fractionated acetylated maize (Zeamays) lignin revealed that the tricinmoieties are found in even the highestmolecularweight fractions, ether linked to lignin units, demonstrating that tricin is indeed incorporated into the lignin polymer.Thesefindingssuggest that tricin is fullycompatiblewithlignification reactions, isanauthentic ligninmonomer, and,because it can only start a lignin chain, functions as a nucleation site for lignification in monocots. This initiation role helps resolve a long-standing dilemma that monocot lignin chains do not appear to be initiated by monolignol homodehydrodimerization as they are in dicots that have similar syringyl-guaiacyl compositions. The term flavonolignin is recommended for the racemic oligomers and polymers ofmonolignols that start from tricin (or incorporate other flavonoids) in the cell wall, in analogy with the existing term flavonolignan that is used for the lowmolecular mass compounds composed of flavonoid and lignan moieties.