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Tricin [5,7-dihydroxy-2-(4-hydroxy-3,5-dimethoxyphenyl)-4H-chromen-4-one] is a flavone that has been found to be incorporated in grass lignin polymers via 4′–O–β coupling. Herein, we investigated the tricin-lignin structure using nuclear magnetic resonance (NMR) methods by comparing the 1H–13C heteronuclear correlation (HSQC) NMR spectra of the isolated lignin with a series of dimeric and trimeric tricin-4′–O–β-ether model compounds. Results showed that the tricin moiety significantly affects the chemical shift of the Cβ/Hβ of 4′–O–β unit, producing peaks at around δC/δH 82.5–83.5/4.15–4.45, that differ from the Cβ/Hβ correlations from normal 4–O–β units formed solely by monolignols, and that have to date been unassigned., The authors thank the China Scholarship Council, State Education Department, for supporting living expenses for Wu Lan’s Program in the Department of Biological System Engineering, University of Wisconsin, Madison. W.L., F.Y., F.L., and J.R. (John Ralph) were funded by the DOE Great Lakes Bioenergy Research Center (DOE BER Office of Science DE-FC02-07ER64494 and DE-SC0018409). J.R. (Jorge Rencoret) and J.C.d.R. were funded by the Spanish Projects CTQ2014-60764-JIN and AGL2017-83036-R (co-financed by FEDER funds).

22 páginas.-- 5 figuras.-- 2 tablas.-- 58 referencias.-- Supplemental Data inhttp://www.plantphysiol.org/content/167/4/1284/suppl/DC1, Open access.-- Artículo en acceso abierto, © 2014 American Society of Plant Biologists. All Rights Reserved. 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., This work was supported by the Department of Energy Great Lakes Bioenergy Research Center (grant no. DE–FC02–07ER64494) and the Multidisciplinary Research Partnership Biotechnology for a Sustainable Economy (grant no. 01MRB510W) of Ghent University.

11 páginas.-- 5 figuras.-- 1 tabla.-- 47 referencias.-- The following supplemental materials are available in the https://doi.org/10.1104/pp.16.02012, Lignin is an abundant aromatic plant cell wall polymer consisting of phenylpropanoid units in which the aromatic rings display various degrees of methoxylation. Tricin [5,7-dihydroxy-2-(4-hydroxy-3,5-dimethoxyphenyl)-4H-chromen-4-one], a flavone, was recently established as a true monomer in grass lignins. To elucidate the incorporation pathways of tricin into grass lignin, the metabolites of maize (Zea mays) were extracted from lignifying tissues and profiled using the recently developed ‘candidate substrate product pair’ algorithm applied to ultra-high-performance liquid chromatography and Fourier transform-ion cyclotron resonance-mass spectrometry. Twelve tricin-containing products (each with up to eight isomers), including those derived from the various monolignol acetate and p-coumarate conjugates, were observed and authenticated by comparisons with a set of synthetic tricin-oligolignol dimeric and trimeric compounds. The identification of such compounds helps establish that tricin is an important monomer in the lignification of monocots, acting as a nucleation site for starting lignin chains. The array of tricin-containing products provides further evidence for the combinatorial coupling model of general lignification and supports evolving paradigms for the unique nature of lignification in monocots. Lignin is one of the major components in plant cell walls and is deposited predominantly in the walls of secondarily thickened cells. It is a complex phenylpropanoid polymer composed primarily of p-hydroxyphenyl (H), guaiacyl (G), and syringyl (S) units derived from the monolignols p-coumaryl 2h, coniferyl 2g, and sinapyl 2s alcohols, respectively (Fig. 1; Freudenberg and Neish, 1968). These monolignols are biosynthesized in the cytoplasm and translocated to the cell wall, where they are oxidized by laccases and peroxidases to monolignol radicals (Boerjan et al., 2003; Dixon and Reddy, 2003; Ralph et al., 2004b; Vanholme et al., 2008, 2010; Bonawitz and Chapple, 2010; Mottiar et al., 2016). The polymer can be started by radical coupling between two monolignol radicals to form a dehydrodimer from which the chain extends by endwise polymerization with additional monolignols, producing β-O-4-, β-5-, β-1-, and β-β-linked units in the lignin. Two growing oligomers also may radically couple to increase the polymer size, producing 4-O-5- and 5-5-linked units. During such radical coupling reactions, therefore, the monomer-derived units are linked together via various C-C and C-O bonds with different frequencies depending primarily on the monomer distribution and supply (Ralph et al., 2004b)., This work was supported by the Department of Energy Great Lakes Bioenergy Research Center (grant no. DE–FC02–07ER64494 to W.L., F.L., and J.R.), by Stanford University’s Global Climate and Energy Program (to J.R., K.M., and W.B.), by the Institute for the Promotion of Innovation through Science and Technology in Flanders via the SBO project Bioleum (to W.B. and K.M.), by the China Scholarship Council's State Education Department for his Ph.D. work in the Department of Biological System Engineering, University of Wisconsin, Madison (to W.L.).