Contents Summary978I.Introduction979II.Phenolic metabolism979III.Lignin biosynthesis and structure983IV.Alternative lignin monomers for biofuel applications985V.Candidate alternative monolignols in biomimetic systems991VI.From phenolic profiling to lignomics992VII.Phenolic pathway engineering towards alternative monolignols993Acknowledgements994References994 Summary Lignin, a phenolic polymer in the secondary wall, is the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second‐generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation into the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites made by plants meet these requirements, and several have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of compounds by phenolic profiling is discussed, as phenolic profiling serves in pathway elucidation and for the detection of incorporation of alternative lignin monomers.

Lignin is the main cause of lignocellulosic biomass recalcitrance to industrial enzymatic hydrolysis. By partially replacing the traditional lignin monomers by alternative ones, lignin extractability can be enhanced. To design a lignin that is easier to degrade under alkaline conditions, curcumin (diferuloylmethane) was produced in the model plant Arabidopsis thaliana via simultaneous expression of the turmeric (Curcuma longa) genes DIKETIDE-CoA SYNTHASE (DCS) and CURCUMIN SYNTHASE

These authors contribute equally to this work. Keywords: Arabidopsis thaliana.Ca-dehydrogenase, lignin biosynthesis, NMR, Sphingobium sp. SYK-6. SummaryBacteria-derived enzymes that can modify specific lignin substructures are potential targets to engineer plants for better biomass processability. The Gram-negative bacterium Sphingobium sp. SYK-6 possesses a Ca-dehydrogenase (LigD) enzyme that has been shown to oxidize the a-hydroxy functionalities in b-O-4-linked dimers into a-keto analogues that are more chemically labile. Here, we show that recombinant LigD can oxidize an even wider range of b-O-4-linked dimers and oligomers, including the genuine dilignols, guaiacylglycerol-b-coniferyl alcohol ether and syringylglycerol-b-sinapyl alcohol ether. We explored the possibility of using LigD for biosynthetically engineering lignin by expressing the codon-optimized ligD gene in Arabidopsis thaliana. The ligD cDNA, with or without a signal peptide for apoplast targeting, has been successfully expressed, and LigD activity could be detected in the extracts of the transgenic plants. UPLC-MS/MS-based metabolite profiling indicated that levels of oxidized guaiacyl (G) b-O-4-coupled dilignols and analogues were significantly elevated in the LigD transgenic plants regardless of the signal peptide attachment to LigD. In parallel, 2D NMR analysis revealed a 2.1-to 2.8-fold increased level of G-type a-keto-b-O-4 linkages in cellulolytic enzyme lignins isolated from the stem cell walls of the LigD transgenic plants, indicating that the transformation was capable of altering lignin structure in the desired manner.

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.