Maize Tricin-Oligolignol Metabolites and Their Implications for Monocot Lignification

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.).