Maize Tricin-Oligolignol Metabolites and their Implications for Monocot Lignification

Abstract

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 tricincontaining 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

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

“…We recently reported all these flavonolignols (which were previously termed tricin oligolignols) to be present in maize internodes (Lan et al, 2016a). Also, when specifically searching for oligolignols without tricin, we found trilignols and tetralignols coupled via all three types of bonds (b-O-4, b-b, and b-5), as expected for combinatorial coupling Ralph, 2010).…”

“…The flavone tricin was recently reported to be part of the lignin polymer in the cell walls of grasses, and it was suggested to act as an initiation site for lignification (del Río et al, 2012;Lan et al, 2015Lan et al, , 2016a, making it a potential target for bioenergy crop improvement. The biosynthesis of tricin has been described to be accomplished via CHS and CHI activity (Shih et al, 2008;Dixon and Pasinetti, 2010).…”

“…This term is in analogy with the existing term flavonolignan that is used for the lowmolecular-mass compounds composed of flavonoid and lignan moieties (Begum et al, 2010) and the term oligolignol that is used for oligomers of canonical monolignols (Morreel et al, 2004(Morreel et al, , 2010b. The fact that some flavonolignols were found without further decoration with sugars and their lack of optical activity suggest that these might be considered as lignin oligomers (Lan et al, 2015(Lan et al, , 2016a. Alternatively, they were in the process of being further decorated to become part of the hexosylated flavonolignol pool.…”

“…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.…”