Tricin, a Flavonoid Monomer in Monocot Lignification  

Abstract

Tricin was recently discovered in lignin preparations from wheat (Triticum aestivum) straw and subsequently in all monocot samples examined. To provide proof that tricin is involved in lignification and establish the mechanism by which it incorporates into the lignin polymer, the 49-Ob-coupling products of tricin with the monolignols (p-coumaryl, coniferyl, and sinapyl alcohols) were synthesized along with the trimer that would result from its 49-O-b-coupling with sinapyl alcohol and then coniferyl alcohol. Tricin was also found to cross couple with monolignols to form tricin-(49-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 (Zea mays) lignin revealed that the tricin moieties are found in even the highest molecular weight fractions, ether linked to lignin units, demonstrating that tricin is indeed incorporated into the lignin polymer. These findings suggest that tricin is fully compatible with lignification reactions, is an authentic lignin monomer, 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 of monolignols 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.Lignin, a complex phenylpropanoid polymer in the plant cell wall, is predominantly deposited in the cell walls of secondary-thickened cells (Vanholme et al., 2010). It is synthesized via oxidative radical coupling reactions from three prototypical monolignols, p-coumaryl, coniferyl, and sinapyl alcohols, differentiated by their degree of methoxylation ortho to the phenolic hydroxyl group. Considered within the context of the entire polymer, the main structural features of lignin can be defined in terms of its p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units, derived respectively from these three monolignols (Ralph, 2010). Several novel monomers, all deriving from the monolignol biosynthetic pathway, have been found to incorporate into lignin in wild-type and transgenic plants. For example, monolignol acetate, p-hydroxybenzoate, and p-coumarate ester conjugates have all been shown to incorporate into lignin polymers and are the source of naturally acylated lignins (Ralph et al., 2004;Lu and Ralph, 2008); lignins derived solely from caffeyl alcohol were found in the seed coats of both monocot and dicot plants (Chen et al., 2012a(Chen et al., , 2012b; lignins derived solely from 5-hydroxyconiferyl alcohol were found in a cactus (for example, in a member of the genera Astrop...

Silencing CHALCONE SYNTHASE in Maize Impedes the Incorporation of Tricin into Lignin and Increases Lignin Content

“…In dicots and gymnosperms, the lignin polymer is initiated by monolignol dimerization, giving rise to b-ether, phenylcoumaran, and resinol dimers via b-O-4, b-5, and b-b coupling, respectively. In monocots, there is an additional dimeric unit, the tetrahydrofuran structure derived from the b-b coupling of sinapyl p-coumarate (Lan et al, 2015(Lan et al, , 2016a. Nevertheless, in monocots, lignin only has a relatively low frequency of b-5 and b-b interunit linkages, which could most easily be explained by the alternative chain initiation, involving either tricin (Lan et al, 2015) or (di)ferulates (Ralph et al, 1995;Grabber et al, 2000;Ralph, 2010) or both.…”

“…Moreover, tricin is already well recognized as a valuable healthpromoting compound due to its antioxidant, antiaging, anticancer, and cardioprotective potential (Zhou and Ibrahim, 2010;Li et al, 2016). Recent findings show that tricin incorporates into the lignin polymer of maize plants via 49-O-b cross-coupling with normal and acylated monolignols, acting as an initiation site for lignin polymerization because it can only start a lignin chain (Lan et al, 2015(Lan et al, , 2016a. Moreover, the initiation role of tricin has helped explain a long-standing enigma of how the lignin chain is initiated in monocots.…”

“…Tricin is the first flavonoid that is recognized to be an authentic monomer involved in lignification (del Río et al, 2012;Lan et al, 2015). Moreover, tricin is already well recognized as a valuable healthpromoting compound due to its antioxidant, antiaging, anticancer, and cardioprotective potential (Zhou and Ibrahim, 2010;Li et al, 2016).…”

“…Recent structural characterization of cell walls from monocot species using NMR showed that the flavone tricin [5,7-dihydroxy-2-(4-hydroxy-3,5-dimethoxyphenyl)-4H-chromen-4-one] is part of the native lignin polymer in wheat (Triticum aestivum; del Río et al, 2012;Zeng et al, 2013), coconut (Cocos nucifera) coir (Rencoret et al, 2013), bamboo (Phyllostachys pubescens; Wen et al, 2013), maize (Zea mays; Lan et al, 2015), and sugarcane (Saccharum officinarum; del . Tricin is the first flavonoid that is recognized to be an authentic monomer involved in lignification (del Río et al, 2012;Lan et al, 2015).…”

Relationships between Biomass Composition and Liquid Products Formed via Pyrolysis

“…Wheat straw, for example, has a G:S:H ratio of 64:30:6 (Bule et al, 2013). Grass lignin possesses high levels of p-coumarate esters (Hatfield et al, 2008) and can also be etherified by tricin and ferulic acid (Ralph et al, 1995;Lan et al, 2015), as discussed further below. Woody gymnosperm lignins are different from angiosperm lignins, being primarily composed of G-units and a lower amount of H-units (Boerjan et al, 2003).…”