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The phenylpropanoid pathway serves as a rich source of metabolites in plants and provides precursors for lignin biosynthesis. Lignin first appeared in tracheophytes and has been hypothesized to have played pivotal roles in land plant colonization. In this review, we summarize recent progress in defining the lignin biosynthetic pathway in lycophytes, monilophytes, gymnosperms, and angiosperms. In particular, we review the key structural genes involved in p -hydroxyphenyl-, guaiacyl-, and syringyl-lignin biosynthesis across plant taxa and consider and integrate new insights on major transcription factors, such as NACs and MYBs. We also review insight regarding a new transcriptional regulator, 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase, canonically identified as a key enzyme in the shikimate pathway. We use several case studies, including EPSP synthase, to illustrate the evolution processes of gene duplication and neo-functionalization in lignin biosynthesis. This review provides new insights into the genetic engineering of the lignin biosynthetic pathway to overcome biomass recalcitrance in bioenergy crops.

Lignocellulose polysaccharides are encrusted by lignin, which has long been considered an obstacle for efficient use of polysaccharides during processes such as pulping and bioethanol fermentation. Hence, numerous transgenic plant lines with reduced lignin contents have been generated, leading to more efficient enzymatic saccharification and forage digestion. However, lignin is also a potential feedstock for aromatic products and an important direct-combustion fuel, or a by-product fuel in polysaccharide utilization such as pulping and bioethanol production. For aromatic feedstock production, the complicated structure of lignin along with its occlusion within polysaccharide matrices makes lignin utilization intractable. To alleviate these difficulties, simplification of the lignin structure is an important breeding objective for future high-value utilization of lignin. In addition, higher lignin contents are beneficial for increasing heating values of lignocellulose, because lignin has much larger heating values than polysaccharides, cellulose and hemicelluloses. Structural modification of lignin may also be effective in increasing heating values of lignocellulose biomass, because the heating value of p -hydroxyphenyl lignin is highest, followed by those of guaiacyl lignin and of syringyl lignin in this order. Herein, recent developments for augmenting lignin contents and for lignin structural modifications, to improve its utilization by metabolic engineering, are outlined.

Lignin, an energy-rich and adaptable polymer comprising phenylpropanoid monomers utilized by plants for structural reinforcement, water conveyance, and defense mechanisms, ranks as the planet’s second most prevalent biopolymer, after cellulose. Despite its prevalence, lignin is frequently underused in the process of converting biomass into fuels and chemicals. Instead, it is commonly incinerated for industrial heat due to its intricate composition and resistance to decomposition, presenting obstacles for targeted valorization. In contrast to chemical catalysts, biological enzymes show promise not only in selectively converting lignin components but also in seamlessly integrating into cellular structures, offering biocatalysis as a potentially efficient pathway for lignin enhancement. This review comprehensively summarizes cutting-edge biostrategies, ligninolytic enzymes, metabolic pathways, and lignin-degrading strains or consortia involved in lignin degradation, while critically evaluating the underlying mechanisms. Metabolic and genetic engineering play crucial roles in redirecting lignin and its derivatives towards metabolic pathways like the tricarboxylic acid cycle, opening up novel avenues for its valorization. Recent advancements in lignin valorization are scrutinized, highlighting key challenges and promising solutions. Furthermore, the review underscores the importance of innovative approaches, such as leveraging digital systems and synthetic biology, to unlock the commercial potential of lignin-derived raw materials as sustainable feedstocks. Artificial intelligence-driven technologies offer promise in overcoming current challenges and driving widespread adoption of lignin valorization, presenting an alternative to sugar-based feedstocks for bio-based manufacturing in the future. The utilization of available lignin residue for synthesis of high-value chemicals or energy, even alternative food, addresses various crises looming in the food-energy-water nexus.

Doc number: 12 Abstract Background: Lignin is one of the three major components in plant cell walls, and it can be isolated (dissolved) from the cell wall in pretreatment or chemical pulping. However, there is a lack of high-value applications for lignin, and the commonest proposal for lignin is power and steam generation through combustion. Organosolv ethanol process is one of the effective pretreatment methods for woody biomass for cellulosic ethanol production, and kraft process is a dominant chemical pulping method in paper industry. In the present research, the lignins from organosolv pretreatment and kraft pulping were evaluated to replace polyol for producing rigid polyurethane foams (RPFs). Results: Petroleum-based polyol was replaced with hardwood ethanol organosolv lignin (HEL) or hardwood kraft lignin (HKL) from 25% to 70% (molar percentage) in preparing rigid polyurethane foam. The prepared foams contained 12-36% (w/w) HEL or 9-28% (w/w) HKL. The density, compressive strength, and cellular structure of the prepared foams were investigated and compared. Chain extenders were used to improve the properties of the RPFs. Conclusions: It was found that lignin was chemically crosslinked not just physically trapped in the rigid polyurethane foams. The lignin-containing foams had comparable structure and strength up to 25-30% (w/w) HEL or 19-23% (w/w) HKL addition. The results indicated that HEL performed much better in RPFs and could replace more polyol at the same strength than HKL because the former had a better miscibility with the polyol than the latter. Chain extender such as butanediol could improve the strength of lignin-containing RPFs.

Aromatic compounds have broad applications and have been the target of biosynthetic processes for several decades. New biomolecular engineering strategies have been applied to improve production of aromatic compounds in recent years, some of which are expected to set the stage for the next wave of innovations. Here, we will briefly complement existing reviews on microbial production of aromatic compounds by focusing on a few recent trends where considerable work has been performed in the last 5 years. The trends we highlight are pathway modularization and compartmentalization, microbial co-culturing, non-traditional host engineering, aromatic polymer feedstock utilization, engineered ring cleavage, aldehyde stabilization, and biosynthesis of non-standard amino acids. Throughout this review article, we will also touch on unmet opportunities that future research could address.

BackgroundIn the biorefinery utilizing lignocellulosic biomasses, lignin decomposition to value-added phenolic derivatives is a key issue, and recently biocatalytic delignification is emerging owing to its superior selectivity, low energy consumption, and unparalleled sustainability. However, besides heme-containing peroxidases and laccases, information about lignolytic biocatalysts is still limited till date.ResultsHerein, we report a promiscuous activity of tyrosinase which is closely associated with delignification requiring high redox potentials (>1.4 V vs. normal hydrogen electrode [NHE]). The promiscuous activity of tyrosinase not only oxidizes veratryl alcohol, a commonly used nonphenolic substrate for assaying ligninolytic activity, to veratraldehyde but also cleaves the 4-O-5 and Cα–Cβ bonds in 4-phenoxyphenol and guaiacyl glycerol-β-guaiacyl ether (GGE) that are dimeric lignin model compounds. Cyclic voltammograms additionally verified that the promiscuous activity oxidizes lignin-related high redox potential substrates.ConclusionThese results might be applicable for extending the versatility of tyrosinase toward biocatalytic delignification as well as suggesting a new perspective for sustainable lignin utilization. Furthermore, the results provide insight for exploring the previously unknown promiscuous activities of biocatalysts much more diverse than ever thought before, thereby innovatively expanding the applicable area of biocatalysis.

The emergence of self-organized industrial symbiosis (IS) is based on the expectations of industrial actors regarding financial and/or environmental benefits through symbiotic inter-company linkages. One such linkage is the exchange of by-products as substitutes for primary raw materials. However, the company generating the by-product may even not be aware of potential application fields in other industries. In cases where the by-product triggers an innovation, the very early phase of the innovation process (“early front-end”—EFE) is extremely important, as it is here that a first rough picture of future application fields must be defined. In contrast to traditional market innovations of industries, the EFE of IS innovations is triggered by the existence of a certain by-product. As conventional innovation models are not very helpful in supporting the EFE decisions in IS innovations, our paper aims to establish a link between self-organized IS and innovation by creating a specific theoretical framework for the support of EFE decisions. We thus introduce the “stage-gate model of self-organized IS innovations” and place a particular emphasis on the early phases within this model. Subsequently, we illustrate the application of the early phases of the model in a case study on lignin utilization in the Austrian paper and pulp industry (P&P industry). In this way, the study contributes to a better understanding of the peculiarities and conditions of EFE decisions in IS innovations and their significance in the emergence of self-organized IS networks.

Lignocellulose is the most abundant renewable resource in nature, providing a large supply of lignin. The efficient separation and utilization of lignin from lignocellulose can help alleviate the current shortage of fossil fuels. Ionic liquids, as green solvents, have been widely applied in the field of biorefining. However, most research has focused on the extraction and purification of cellulose, while lignin is often treated as a by-product. The high-value utilization of lignin has currently emerged as a hot topic. This review summarizes recent advances in the extraction of lignin from lignocellulose using ionic liquids and the mechanisms of lignin extraction. Additionally, it briefly discusses the applications of ionic liquids in the high-value utilization of lignin, including lignin depolymerization, modification, the preparation of lignin-based functional materials, and biofuels. This review aims to provide ideas for the extraction and high-value utilization of lignin through ionic liquids.

The oxidation of four lignins obtained by organosolv pulping of eucalyptus wood (Acetosolv-eucalyptus Acetosolv lignin [EAL]), sugarcane bagasse (Acetosolv-bagasse Acetosolv lignin [BAL] and in acetone/water/FeCl3-bagasse acetone/water lignin [BAWL]), and a softwood mixture (Organocell, Munich, Germany) was performed to obtain vanillin, vanillic acid, and oxidized lignin. Experiments were carried out in acetic acid under oxygen flow using HBr, cobalt(II), and manganese(II) acetates as catalysts. After 10 h the total vanillin and vanillic acid yields were BAL 0.05 mmol, EAL 0.38 mmol, BAWL 0.45 mmol, and Organocell 0.84 mmol. Acetosolv lignins are cross-linked, which explains the lower yields in mononuclear products. The reaction volume (deltaV) of this reaction is -817 cm3/mol, obtained in experiments performed under oxygen pressure, showing the high influence of pressure on the oxidation. The major part of the lignin stays in solution (oxidized lignin), which was analyzed by infrared spectroscopy, showing an increase in carbonyl and hydroxyl groups in comparison with the original lignin. The oxidized lignin can be used as chelating agent in the treatment of effluents containing heavy metals.

Residual lignin affects the physical and chemical performance of lignin-containing cellulose nanofibers (LCNFs). In this work, LCNFs were prepared from sugarcane bagasse powder (SBP) through p -toluenesulfonic acid ( p -TsOH) hydrolysis and the subsequent homogenization treatment. By adjusting the concentration of p -TsOH and hydrolysis temperature, LCNFs with lignin content of 4.69–17.53% were obtained, and the effects of lignin content on the chemical structure, crystallinity, size, hydrophobicity and thermal stability of LCNFs were systematically studied. With the increase of lignin content, the diameters and average water contact angles of LCNFs were increased (from 169.65 to 781.56 nm and 39.74–86.16°, respectively), while the crystallinities were decreased. The thermal stabilities of LCNFs were decided both by lignin content and the crystallinity. The by-product lignin nanoparticles (LNPs) with an average diameter of 50–500 nm were generated with LCNFs, further improving the resource utilization value of SBP. This study provides theoretical and experimental basis for the subsequent processing of films with different hydrophobic properties and materials with higher mechanical properties and thermal stability. Graphical abstract