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

Nubia B Eloy,José Nicomedes,Ruben Vanholme,Kris Morreel,Wout Boerjan,Rebecca A Smith,Wannes Voorend,Wu Lan,Igor Cesarino,Andreas Pallidis,John Ralph,Marina De Lyra Soriano Saleme,Geert Goeminne

doi:10.1104/pp.16.01108

Abstract

Lignin is a phenolic heteropolymer that is deposited in secondary-thickened cell walls, where it provides mechanical strength. A recent structural characterization of cell walls from monocot species showed that the flavone tricin is part of the native lignin polymer, where it is hypothesized to initiate lignin chains. In this study, we investigated the consequences of altered tricin levels on lignin structure and cell wall recalcitrance by phenolic profiling, nuclear magnetic resonance, and saccharification assays of the naturally silenced maize (Zea mays) C2-Idf (inhibitor diffuse) mutant, defective in the CHALCONE SYNTHASE Colorless2 (C2) gene. We show that the C2-Idf mutant produces highly reduced levels of apigenin- and tricin-related flavonoids, resulting in a strongly reduced incorporation of tricin into the lignin polymer. Moreover, the lignin was enriched in β-β and β-5 units, lending support to the contention that tricin acts to initiate lignin chains and that, in the absence of tricin, more monolignol dimerization reactions occur. In addition, the C2-Idf mutation resulted in strikingly higher Klason lignin levels in the leaves. As a consequence, the leaves of C2-Idf mutants had significantly reduced saccharification efficiencies compared with those of control plants. These findings are instructive for lignin engineering strategies to improve biomass processing and biochemical production.

Highlights

Lignin is a phenolic heteropolymer that is deposited in secondary-thickened cell walls, where it provides mechanical strength
Because tricin units are incorporated into native maize lignin, we further evaluated whether the absence of this monomer, as well as its consequences on lignin structure, affect the saccharification efficiency in stems and leaves of C2-Idf; B; Pl ;R-scm2 W22 (C2-Idf) plants
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, making it a potential target for bioenergy crop improvement

Summary

Introduction

Lignin is a phenolic heteropolymer that is deposited in secondary-thickened cell walls, where it provides mechanical strength. The leaves of C2-Idf mutants had significantly reduced saccharification efficiencies compared with those of control plants. These findings are instructive for lignin engineering strategies to improve biomass processing and biochemical production. The remarkable ability of lignin to incorporate a variety of monomers highlights the inherent malleability of lignification, confirms the original combinatorial chemical coupling theory of lignification (Ralph et al, 2008a), and suggests that genetic engineering of novel lignins is a promising strategy to tailor plants with improved processing properties (Vanholme et al, 2012; Wilkerson et al, 2014; Mottiar et al, 2016)

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Conclusion