Engineering Curcumin Biosynthesis in Poplar Affects Lignification and Biomass Yield.

Lignocellulosic biomass is recalcitrant toward deconstruction into simple sugars mainly due to the presence of lignin. By engineering plants to partially replace traditional lignin monomers with alternative ones, lignin degradability and extractability can be enhanced. Previously, the alternative monomer curcumin has been successfully produced and incorporated into lignified cell walls of Arabidopsis by the heterologous expression of DIKETIDE-CoA SYNTHASE (DCS) and CURCUMIN SYNTHASE2 (CURS2). The resulting transgenic plants did not suffer from yield penalties and had an increased saccharification yield after alkaline pretreatment. Here, we translated this strategy into the bio-energy crop poplar. Via the heterologous expression of DCS and CURS2 under the control of the secondary cell wall CELLULOSE SYNTHASE A8-B promoter (ProCesA8-B), curcumin was also produced and incorporated into the lignified cell walls of poplar. ProCesA8-B:DCS_CURS2 transgenic poplars, however, suffered from shoot-tip necrosis and yield penalties. Compared to that of the wild-type (WT), the wood of transgenic poplars had 21% less cellulose, 28% more matrix polysaccharides, 23% more lignin and a significantly altered lignin composition. More specifically, ProCesA8-B:DCS_CURS2 lignin had a reduced syringyl/guaiacyl unit (S/G) ratio, an increased frequency of p-hydroxyphenyl (H) units, a decreased frequency of p-hydroxybenzoates and a higher fraction of phenylcoumaran units. Without, or with alkaline or hot water pretreatment, the saccharification efficiency of the transgenic lines was equal to that of the WT. These differences in (growth) phenotype illustrate that translational research in crops is essential to assess the value of an engineering strategy for applications. Further fine-tuning of this research strategy (e.g., by using more specific promoters or by translating this strategy to other crops such as maize) might lead to transgenic bio-energy crops with cell walls more amenable to deconstruction without settling in yield.

Low molecular weight and highly functional RCF lignin products as a full bisphenol a replacer in bio-based epoxy resins.

Herein, we present a full lignocellulose-to-chemicals valorization chain, wherein low molecular weight and highly functional lignin oligomers, obtained from reductive catalytic fractionation (RCF) of pine wood, were used to fully replace bisphenol A (BPA) for synthesizing bio-based epoxy resins.

Reductive catalytic fractionation: state of the art of the lignin-first biorefinery.

Reductive catalytic fractionation (RCF) of lignocellulose is an emerging biorefinery scheme that combines biomass fractionation with lignin depolymerisation. Central to this scheme is the integration of heterogeneous catalysis, which overcomes the tendency of lignin to repolymerise. Ultimately, this leads to a low-Mw lignin oil comprising a handful of lignin-derived monophenolics in close-to-theoretical yield, as well as a carbohydrate pulp. Both product streams are considered to be valuable resources for the bio-based chemical industry. This Opinion article sheds light on recently achieved milestones and consequent research opportunities. More specifically, mechanistic studies have established a general understanding of the elementary RCF steps, which include (i) lignin extraction, (ii) solvolytic and catalytic depolymerisation and (iii) stabilisation. This insight forms the foundation for recently developed flow-through RCF. Compared to traditional batch, flow-through RCF has the advantage of (i) separating the solvolytic steps from the catalytic steps and (ii) being a semi-continuous process; both of which are beneficial for research purposes and for industrial operation. Although RCF has originally been developed for 'virgin' biomass, researchers have just begun to explore alternative feedstocks. Low-value biomass sources such as agricultural residues, waste wood and bark, are cheap and abundant but are also often more complex. On the other side of the feedstock spectrum are high-value bio-engineered crops, specifically tailored for biorefinery purposes. Advantageous for RCF are feedstocks designed to (i) increase the total monomer yield, (ii) extract lignin more easily, and/or (iii) yield unconventional, high-value products (e.g. alkylated catechols derived from C-lignin). Taking a look at the bigger picture, this Opinion article highlights the multidisciplinary nature of RCF. Collaborative efforts involving chemists, reactor engineers, bioengineers and biologists working closer together are, therefore, strongly encouraged.

Perspective on Lignin Oxidation: Advances, Challenges, and Future Directions.

Lignin valorization has gained increasing attention over the past decade. Being the world's largest source of renewable aromatics, its valorization could pave the way towards more profitable and more sustainable lignocellulose biorefineries. Many lignin valorization strategies focus on the disassembly of lignin into aromatic monomers, which can serve as platform molecules for the chemical industry. Within this framework, the oxidative conversion of lignin is of great interest because it enables the formation of highly functionalized, valuable compounds. This work provides a brief overview and critical discussion of lignin oxidation research. In the first part, oxidative conversion of lignin models and isolated lignin streams is reviewed. The second part highlights a number of challenges with respect to the substrate, catalyst, and operating conditions, and proposes some future directions regarding the oxidative conversion of lignin.