Expression of S-adenosylmethionine Hydrolase in Tissues Synthesizing Secondary Cell Walls Alters Specific Methylated Cell Wall Fractions and Improves Biomass Digestibility.

Aymerick Eudes,George Wang,Dominique Loqué,Nanxia Zhao,Blake A Simmons,Noppadon Sathitsuksanoh,Taek Soon Lee,Seema Singh,Jeemeng Lao,Edward E K Baidoo,Sasha Yogiswara,Jenny C Mortimer,Jay D Keasling

doi:10.3389/fbioe.2016.00058

Abstract

Plant biomass is a large source of fermentable sugars for the synthesis of bioproducts using engineered microbes. These sugars are stored as cell wall polymers, mainly cellulose and hemicellulose, and are embedded with lignin, which makes their enzymatic hydrolysis challenging. One of the strategies to reduce cell wall recalcitrance is the modification of lignin content and composition. Lignin is a phenolic polymer of methylated aromatic alcohols and its synthesis in tissues developing secondary cell walls is a significant sink for the consumption of the methyl donor S-adenosylmethionine (AdoMet). In this study, we demonstrate in Arabidopsis stems that targeted expression of AdoMet hydrolase (AdoMetase, E.C. 3.3.1.2) in secondary cell wall synthesizing tissues reduces the AdoMet pool and impacts lignin content and composition. In particular, both NMR analysis and pyrolysis gas chromatography mass spectrometry of lignin in engineered biomass showed relative enrichment of non-methylated p-hydroxycinnamyl (H) units and a reduction of dimethylated syringyl (S) units. This indicates a lower degree of methylation compared to that in wild-type lignin. Quantification of cell wall-bound hydroxycinnamates revealed a reduction of ferulate in AdoMetase transgenic lines. Biomass from transgenic lines, in contrast to that in control plants, exhibits an enrichment of glucose content and a reduction in the degree of hemicellulose glucuronoxylan methylation. We also show that these modifications resulted in a reduction of cell wall recalcitrance, because sugar yield generated by enzymatic biomass saccharification was greater than that of wild-type plants. Considering that transgenic plants show no important diminution of biomass yields, and that heterologous expression of AdoMetase protein can be spatiotemporally optimized, this novel approach provides a valuable option for the improvement of lignocellulosic biomass feedstock.

Highlights

Lignin is a phenolic polymer produced by oxidative polymerization of methylated hydroxycinnamyl alcohols synthesized from phenylalanine (Figure 1A)
We evaluated in Arabidopsis the impact of expressing S-adenosylmethionine hydrolase (AdoMetase) in tissues producing secondary cell walls (SCWs)
G and S lignin units, which represent more than 95% of lignin units, consist of 10 and 11 carbon skeletons, respectively, and harbor one or two methyl groups added by AdoMet-dependent methyltransferases (CCoAOMT and caffeic acid O-methyltransferase (COMT))

Summary

Introduction

Lignin is a phenolic polymer produced by oxidative polymerization of methylated hydroxycinnamyl alcohols (or monolignols) synthesized from phenylalanine (Figure 1A). Guaiacyl (G) units are derived from coniferyl alcohol, which contains one methyl group, and syringyl (S) units are derived from dimethylated sinapyl alcohol (Figure 1A). G and S units are the most common lignin monomers in angiosperms, whereas p-hydroxyphenyl (H) units derived from the polymerization of non-methylated p-coumaryl alcohol are typically less abundant (Boerjan et al, 2003). S-adenosylmethionine (AdoMet) is a key intermediate in onecarbon metabolism that serves as a universal methyl-group donor for the methylation of a large number of metabolites (Hanson and Roje, 2001). In addition to being used as a substrate by AdoMet-utilizing methyltransferases, it acts as a precursor in the synthesis of polyamines, nicotianamine, phytosiderophores, 5′-deoxyadenosyl radicals, and ethylene (Roje, 2006). As part of the methionine salvage cycle (or Yang cycle), AdoMet is synthesized from methionine (Met) by AdoMet synthetase (Figure 1B) (Albers, 2009)

Methods

Results

Conclusion