Degradation of lignin β-aryl ether units in Arabidopsis thaliana expressing LigD, LigF and LigG from Sphingomonas paucimobilis SYK-6.

Ewelina Mnich,John Ralph,Sarah Liu,Peter Ulvskov,Jesper Harholt,Geert Goeminne,Nanna Bjarnholt,Fachuang Lu,Mohammed S Motawie,Paula Oyarce,Bodil Jørgensen,Wout Boerjan,Ruben Vanholme,Birger L Møller

doi:10.1111/pbi.12655

Abstract

SummaryLignin is a major polymer in the secondary plant cell wall and composed of hydrophobic interlinked hydroxyphenylpropanoid units. The presence of lignin hampers conversion of plant biomass into biofuels; plants with modified lignin are therefore being investigated for increased digestibility. The bacterium Sphingomonas paucimobilis produces lignin‐degrading enzymes including LigD, LigF and LigG involved in cleaving the most abundant lignin interunit linkage, the β‐aryl ether bond. In this study, we expressed the LigD, LigF and LigG (LigDFG) genes in Arabidopsis thaliana to introduce postlignification modifications into the lignin structure. The three enzymes were targeted to the secretory pathway. Phenolic metabolite profiling and 2D HSQC NMR of the transgenic lines showed an increase in oxidized guaiacyl and syringyl units without concomitant increase in oxidized β‐aryl ether units, showing lignin bond cleavage. Saccharification yield increased significantly in transgenic lines expressing LigDFG, showing the applicability of our approach. Additional new information on substrate specificity of the LigDFG enzymes is also provided.

Highlights

Lignin is one of the most abundant biopolymers in the world
The S. paucimobilis LigD, LigF, LigG gene sequences were codonoptimized for expression in Arabidopsis
For expression of the entire pathway, LigD, LigF and LigG were expressed in tri-cistronic constructs in which the three coding sequences were linked by the 2A gene sequence coding for a self-processing polypeptide similar to the one found in the Foot-and-mouth disease virus (El Amrani et al, 2004)

Summary

Introduction

Lignin is one of the most abundant biopolymers in the world. Together with cellulose, hemicelluloses, pectin and additional minor components that constitute the plant cell wall, lignin offers mechanical strength and affords protection against pathogens. Lignin fills up spaces between polysaccharides in the secondary plant cell wall and may be covalently cross-linked to some of these. The properties of lignin impede enzymatic lignocellulose deconstruction and constitute a major bottleneck in biofuel production (Pauly and Keegstra, 2010; Van Acker et al, 2013; Vanholme et al, 2013a). Altering lignin composition and structure might be a better strategy to improve the efficiency of biomass processing (Bonawitz and Chapple, 2010; Li et al, 2008; Ralph, 2007; Sederoff et al, 1999; Vanholme et al, 2008, 2012)

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Discussion

Conclusion