Metabolic engineering of p-hydroxybenzoate in poplar lignin

In planta production of the nylon precursor beta-ketoadipate

Beta-ketoadipate (βKA) is an intermediate of the βKA pathway involved in the degradation of aromatic compounds in several bacteria and fungi. Beta-ketoadipate also represents a promising chemical for the manufacturing of performance-advantaged nylons. We established a strategy for the in planta synthesis of βKA via manipulation of the shikimate pathway and the expression of bacterial enzymes from the βKA pathway. Using Nicotiana benthamiana as a transient expression system, we demonstrated the efficient conversion of protocatechuate (PCA) to βKA when plastid-targeted bacterial-derived PCA 3,4-dioxygenase (PcaHG) and 3-carboxy-cis,cis-muconate cycloisomerase (PcaB) were co-expressed with 3-deoxy-D-arabinoheptulosonate 7-phosphate synthase (AroG) and 3-dehydroshikimate dehydratase (QsuB). This metabolic pathway was reconstituted in Arabidopsis by introducing a construct (pAtβKA) with stacked pcaG, pcaH, and pcaB genes into a PCA-overproducing genetic background that expresses AroG and QsuB (referred as QsuB-2). The resulting QsuB-2 x pAtβKA stable lines displayed βKA titers as high as 0.25 % on a dry weight basis in stems, along with a drastic reduction in lignin content and improvement of biomass saccharification efficiency compared to wild-type controls, and without any significant reduction in biomass yields. Using biomass sorghum as a potential crop for large-scale βKA production, techno-economic analysis indicated that βKA accumulated at titers of 0.25 % and 4 % on a dry weight basis could be competitively priced in the range of $2.04-34.49/kg and $0.47-2.12/kg, respectively, depending on the selling price of the residual biomass recovered after βKA extraction. This study lays the foundation for a more environmentally-friendly synthesis of βKA using plants as production hosts.

Chromatic symphony of fleshy fruits: functions, biosynthesis and metabolic engineering of bioactive compounds

Fleshy fruits are popular among consumers due to their significant nutritional value, which includes essential bioactive compounds such as pigments, vitamins, and minerals. Notably, plant-derived pigments are generally considered safe and reliable, helping to protect humans against various inflammatory diseases. Although the phytochemical diversity and their biological activities have been extensively reviewed and summarized, the status of bioactive nutrients in fleshy fruits, particularly with a focusing on different colors, has received less attention. Therefore, this review introduces five common types of fleshy fruits based on coloration and summarizes their major bioactive compounds. It also provides the latest advancements on the function, biosynthesis, and metabolic engineering of plant-derived pigments. In this review, we emphasize that promoting the consumption of a diverse array of colorful fruits can contribute to a balanced diet; however, optimal intake levels still require further clinical validation. This review may serve as a useful guide for decisions that enhance the understanding of natural pigments and accelerate their application in agriculture and medicine.

Suppression of Chorismate Mutase 1 in Hybrid Poplar to Investigate Potential Redundancy in the Supply of Lignin Precursors

Chorismate is an important branchpoint metabolite in the biosynthesis of lignin and a wide array of metabolites in plants. Chorismate mutase (CM), the enzyme responsible for transforming chorismate into prephenate, is a key regulator of metabolic flux towards the synthesis of aromatic amino acids and onwards to lignin. We examined three CM genes in hybrid poplar (Populus alba × grandidentata; P39, abbreviated as Pa×g) and used RNA interference (RNAi) to suppress the expression of Pa×gCM1, the most highly expressed isoform found in xylem tissue. Although this strategy was successful in disrupting Pa×gCM1 transcripts, there was also an unanticipated increase in lignin content, a shift towards guaiacyl lignin units, and more xylem vessels with smaller lumen areas, at least in the most severely affected transgenic line. This was accompanied by compensatory expression of the other two CM isoforms, Pa×gCM2 and Pa×gCM3, as well as widespread changes in gene expression and metabolism. This study investigates potential redundancy within the CM gene family in the developing xylem of poplar and highlights the pivotal role of chorismate in plant metabolism, development, and physiology.

Polyphenolic Composition, Antioxidant Activity, and Cytotoxic Effect of Male Floral Buds from Three Populus Species Growing in the South of Romania

Three poplar species widely distributed in southern Romania were investigated for their chemical composition and bioactivity. Male buds from black poplar (Populus nigra L.), white poplar (Populus alba L.), and Euroamerican hybrid poplar (Populus × euramericana (Dode) Guinier.) were analyzed using HPTLC, HPLC, GC-MS, and spectrophotometric assays. The analysis revealed predominantly polyphenolic compounds, including phenolic acids and flavonoids, secondary metabolites recognized for their antioxidant properties, particularly valuable in alleviating oxidative stress disorders. Heavy metal content was measured using atomic absorption spectroscopy, and antioxidant capacity was assessed through DPPH and FRAP assays alongside a cytotoxicity evaluation. Polyphenolic content ranged from 19.26 to 33.37 mg GAE/g DW and flavonoid content from 2.15 to 4.45 mg RE/g DW. All three species demonstrated notable antioxidant capacity and cytotoxic activity. Hydroethanolic extracts of P. nigra and P. euramericana showed higher antioxidant activity than aqueous extracts, with P. nigra achieving the lowest IC50 value overall, highlighting the influence of solvent choice on antioxidant efficacy. Furthermore, poplar hydroethanolic extracts exhibited concentration-dependent cytotoxicity against fibroblast-like human osteosarcoma MG63 cell lines, with IC50 values of 42.55 µg/mL for P. nigra, 40.87 µg/mL for P. × euramericana, and 132.49 µg/mL for P. alba, underscoring significant interspecies variability in cytotoxic potency. These findings suggest that male floral buds from Romanian poplar species may serve as valuable sources of bioactive compounds with therapeutic potential.

Decarboxylation of Hydroxybenzoic Acids to Phenol Via Deep Eutectic Solvents

The development of greener and more sustainable synthesis processes for manufacturing commodity chemicals is of great importance. The majority of current phenol production methods involve harsh reaction conditions with high energy consumption, causing severe environmental pollution. In this study, we present a novel approach for the decarboxylation of hydroxybenzoic acids (HBAs) to phenol using a choline chloride-urea (ChCl-urea) deep eutectic solvent (DES). Our study reveals the remarkable dual performance of ChCl-urea both as a catalyst and solvent for the decarboxylation of HBA, resulting in a high phenol yield (94 mol %) under mild reaction conditions. The proposed reaction pathway, established through a combination of experiments and computational simulations, enhances our understanding of this process. The recyclability of the DES system during decarboxylation was also assessed. Our findings demonstrate that the integration of DES into conventional chemical processes can pave the way for sustainable manufacturing, exemplifying a novel approach for producing phenol from abundant natural resources using designer solvents.

The origin and metabolic fate of 4-hydroxybenzoate in Arabidopsis

MAIN CONCLUSION

The contribution of p-coumarate β-oxidation and kaempferol cleavage to the pools of glycosylated, free and cell wall-bound 4-hydroxybenzoate is organ-dependent in Arabidopsis. 4-Hydroxybenzoate (4-HB) is a vital precursor for a number of plant primary and specialized metabolites, as well as for the assembly of the plant cell wall. In Arabidopsis, it is known that 4-HB is derived independently from phenylalanine and tyrosine, and that the metabolism of phenylalanine into 4-HB proceeds via at least two biosynthetic routes: the β-oxidation of p-coumarate and the peroxidative cleavage of kaempferol. The precise contribution of these precursors and branches to 4-HB production, however, is not known. Here, we combined isotopic feeding assays, reverse genetics, and quantification of soluble (i.e., free and glycosylated) and cell wall-bound 4-HB to determine the respective contributions of phenylalanine, tyrosine, β-oxidation of p-coumarate, and peroxidative cleavage of kaempferol to 4-HB biosynthesis in Arabidopsis tissues. Over 90% of 4-HB was found to originate from phenylalanine in both leaves and roots. Soluble 4-HB level varied significantly between organs, while the proportion of cell wall-bound 4-HB was relatively constant. In leaves and flowers, glycosylated and cell wall-bound 4-HB were the most and least abundant forms, respectively. Flowers displayed the highest specific content of 4-HB, while free 4-HB was not detected in roots. Although p-coumarate β-oxidation and kaempferol catabolism were found to both contribute to the supply of 4-HB in all tissues, the proportion of kaempferol-derived 4-HB was higher in roots than in leaves and flowers. Within the β-oxidative branch, p-coumaroyl-CoA ligase 4-CL8 (At5g38120) bore a preponderant role in the production of soluble and cell wall-bound 4-HB in leaves, while p-coumaroyl-CoA ligase At4g19010 appeared to control the biosynthesis of soluble 4-HB in flowers. Furthermore, analysis of a series of Arabidopsis T-DNA mutants corresponding to the three major UDP-glucosyltransferases known to act on 4-HB in vitro (UGT75B1, UGT89B1, and UGT71B1) showed that none of these enzymes appeared in fact to have a significant role in the glycosylation of 4-HB in vivo.

Biomanufacturing of value-added chemicals from lignin

Lignin valorization faces persistent biomanufacturing challenges due to the heterogeneous and toxic carbon substrates derived from lignin depolymerization. To address the heterogeneous nature of aromatic feedstocks, plant cell wall engineering and 'lignin first' pretreatment methods have recently emerged. Next, to convert the resulting aromatic substrates into value-added chemicals, diverse microbial host systems also continue to be developed. This includes microbes that (1) lack aromatic metabolism, (2) metabolize aromatics but not sugars, and (3) co-metabolize both aromatics and sugars, each system presenting unique pros and cons. Considering the intrinsic complexity of lignin-derived substrate mixtures, emerging and non-model microbes with native metabolism for aromatics appear poised to provide the greatest impacts on lignin valorization via biomanufacturing.

Synthetic-biology approach for plant lignocellulose engineering

Plant biomass is an abundant, renewable resource that offers multiple advantages for the production of green chemicals and recombinant proteins. However, the adoption of plant-based systems by industry is hindered because mammalian and other cell cultures are well-established and better characterized in an industrial setting, and thus it is difficult for plant-based processes to gain a foothold in the marketplace. Therefore, additional benefits of plant-based systems may be essential to tip the balance in favor of sustainable plant-derived products. A crucial factor in biomass valorization is to design mid- to high-value co-products that can be derived cost-effectively from the residual lignocellulose (LC). However, the utility of LC remains limited because LCs are, in general, too recalcitrant for industries to utilize their components (lignin, cellulose, and hemicelluloses). To overcome this issue, in planta engineering to reduce LC recalcitrance has been ongoing in recent decades, with essential input from synthetic biology owing to the complexity of LC pathways and the massive number of genes involved. In this review, we describe recent advances in LC manipulation and eight strategies for redesigning the pathways for lignin and structural glycans to reduce LC recalcitrance while mitigating against the growth penalty associated with yield loss.

Enabling Lignin Valorization Through Integrated Advances in Plant Biology and Biorefining

Despite lignin having long been viewed as an impediment to the processing of biomass for the production of paper, biofuels, and high-value chemicals, the valorization of lignin to fuels, chemicals, and materials is now clearly recognized as a critical element for the lignocellulosic bioeconomy. However, the intended application for lignin will likely require a preferred lignin composition and form. To that end, effective lignin valorization will require the integration of plant biology, providing optimal feedstocks, with chemical process engineering, providing efficient lignin transformations. Recent advances in our understanding of lignin biosynthesis have shown that lignin structure is extremely diverse and potentially tunable, while simultaneous developments in lignin refining have resulted in the development of several processes that are more agnostic to lignin composition. Here, we review the interface between in planta lignin design and lignin processing and discuss the advances necessary for lignin valorization to become a feature of advanced biorefining.

Functional Complexity on a Cellular Scale: Why In Situ Analyses Are Indispensable for Our Understanding of Lignified Tissues

Lignins are a key adaptation that enables vascular plants to thrive in terrestrial habitats. Lignin is heterogeneous, containing upward of 30 different monomers, and its function is multifarious: It provides structural support, predetermined breaking points, ultraviolet protection, diffusion barriers, pathogen resistance, and drought resilience. Recent studies, carefully characterizing lignin in situ, have started to identify specific lignin compositions and ultrastructures with distinct cellular functions, but our understanding remains fractional. We summarize recent works and highlight where further in situ lignin analysis could provide valuable insights into plant growth and adaptation. We also summarize strengths and weaknesses of lignin in situ analysis methods.

The plant cell wall-dynamic, strong, and adaptable-is a natural shapeshifter

Mythology is replete with good and evil shapeshifters, who, by definition, display great adaptability and assume many different forms-with several even turning themselves into trees. Cell walls certainly fit this definition as they can undergo subtle or dramatic changes in structure, assume many shapes, and perform many functions. In this review, we cover the evolution of knowledge of the structures, biosynthesis, and functions of the 5 major cell wall polymer types that range from deceptively simple to fiendishly complex. Along the way, we recognize some of the colorful historical figures who shaped cell wall research over the past 100 years. The shapeshifter analogy emerges more clearly as we examine the evolving proposals for how cell walls are constructed to allow growth while remaining strong, the complex signaling involved in maintaining cell wall integrity and defense against disease, and the ways cell walls adapt as they progress from birth, through growth to maturation, and in the end, often function long after cell death. We predict the next century of progress will include deciphering cell type-specific wall polymers; regulation at all levels of polymer production, crosslinks, and architecture; and how walls respond to developmental and environmental signals to drive plant success in diverse environments.

Wood of trees: Cellular structure, molecular formation, and genetic engineering

Wood is an invaluable asset to human society due to its renewable nature, making it suitable for both sustainable energy production and material manufacturing. Additionally, wood derived from forest trees plays a crucial role in sequestering a significant portion of the carbon dioxide fixed during photosynthesis by terrestrial plants. Nevertheless, with the expansion of the global population and ongoing industrialization, forest coverage has been substantially decreased, resulting in significant challenges for wood production and supply. Wood production practices have changed away from natural forests toward plantation forests. Thus, understanding the underlying genetic mechanisms of wood formation is the foundation for developing high-quality, fast-growing plantation trees. Breeding ideal forest trees for wood production using genetic technologies has attracted the interest of many. Tremendous studies have been carried out in recent years on the molecular, genetic, and cell-biological mechanisms of wood formation, and considerable progress and findings have been achieved. These studies and findings indicate enormous possibilities and prospects for tree improvement. This review will outline and assess the cellular and molecular mechanisms of wood formation, as well as studies on genetically improving forest trees, and address future development prospects.

The effect of nitrogen source and levels on hybrid aspen tree physiology and wood formation

Nitrogen can be taken up by trees in the form of nitrate, ammonium and amino acids, but the influence of the different forms on tree growth and development is poorly understood in angiosperm species like Populus. We studied the effects of both organic and inorganic forms of nitrogen on growth and wood formation of hybrid aspen trees in experimental conditions that allowed growth under four distinct steady-state nitrogen levels. Increased nitrogen availability had a positive influence on biomass accumulation and the radial dimensions of both xylem vessels and fibers, and a negative influence on wood density. An optimal level of nitrogen availability was identified where increases in biomass accumulation outweighed decreases in wood density. None of these responses depended on the source of nitrogen except for shoot biomass accumulation, which was stimulated more by treatments complemented with nitrate than by ammonium alone or the organic source arginine. The most striking difference between the nitrogen sources was the effect on lignin composition, whereby the abundance of H-type lignin increased only in the presence of nitrate. The differential effect of nitrate is possibly related to the well-known role of nitrate as a signaling compound. RNA-sequencing revealed that while the lignin-biosynthetic genes did not significantly (FDR <0.01) respond to added NO3 - , the expression of several laccases, catalysing lignin polymerization, was dependent on N-availability. These results reveal a unique role of nitrate in wood formation and contribute to the knowledge basis for decision-making in utilizing hybrid aspen as a bioresource.

Lignin Hydrogenolysis: Phenolic Monomers from Lignin and Associated Phenolates across Plant Clades

The chemical complexity of lignin remains a major challenge for lignin valorization into commodity and fine chemicals. A knowledge of the lignin features that favor its valorization and which plants produce such lignins can be used in plant selection or to engineer them to produce lignins that are more ideally suited for conversion. Sixteen biomass samples were compositionally surveyed by NMR and analytical degradative methods, and the yields of phenolic monomers following hydrogenolytic depolymerization were assessed to elucidate the key determinants controlling the depolymerization. Hardwoods, including those incorporating monolignol p-hydroxybenzoates into their syringyl/guaiacyl copolymeric lignins, produced high monomer yields by hydrogenolysis, whereas grasses incorporating monolignol p-coumarates and ferulates gave lower yields, on a lignin basis. Softwoods, with their more condensed guaiacyl lignins, gave the lowest yields. Lignins with a high syringyl unit content released elevated monomer levels, with a high-syringyl polar transgenic being particularly striking. Herein, we distinguish phenolic monomers resulting from the core lignin vs those from pendent phenolate esters associated with the biomass cell wall, acylating either polysaccharides or lignins. The basis for these observations is rationalized as a means to select or engineer biomass for optimal conversion to worthy phenolic monomers.

Populus endo-glucanase 16 localizes to the cell walls of developing tissues

The hemicelluloses comprise a group of matrix glycans that interact with cellulose microfibrils in plant cell walls and play important roles in establishing wall architecture. The structures of hemicelluloses are determined by carbohydrate-active enzymes (CAZymes) that synthesize, integrate, and break down these polymers. Specifically, endo-glucanase 16 (EG16) enzymes, which are related to the well-known xyloglucan endotransglycosylase/hydrolase (XTH) gene products in Glycoside Hydrolase Family 16 (GH16), have been implicated in the degradation of the β(1,4)-linked backbone of mixed-linkage β(1,3);β(1,4)-glucans (MLG) and xyloglucans. EG16 members are single-copy genes found in most plant clades but are absent from many eudicots, including the model plant Arabidopsis thaliana. Until recently, EG16 members had only been characterized in vitro, establishing their substrate specificity, protein structure, and phylogenetic history, but their biological function was unknown. Here we used a hybrid polar, Populus alba × Populus grandidentata (P39), as a model to examine EG16 expression, subcellular localization, and pheno- and chemotypes of EG16-downregulated P39 plants. Populus EG16 expression is strong in young tissues, but RNAi-mediated downregulation did not impact plant growth nor the fine structure of the hemicellulose xyloglucan, suggesting a restricted or currently unknown role in angiosperm physiology.

Evolution of p-coumaroylated lignin in eudicots provides new tools for cell wall engineering

Ester-linked p-coumarate (pCA) is a hallmark feature of the secondary cell walls in commelinid monocot plants. It has been shown that pCA groups arise during lignin polymerisation from the participation of monolignol conjugates assembled by p-coumaroyl-CoA:monolignol transferase (PMT) enzymes, members of the BAHD superfamily of acyltransferases. Herein, we report that a eudicot species, kenaf (Hibiscus cannabinus), naturally contains p-coumaroylated lignin in the core tissues of the stems but not in the bast fibres. Moreover, we identified a novel acyltransferase, HcPMT, that shares <30% amino acid identity with known monocot PMT sequences. Recombinant HcPMT showed a preference in enzyme assays for p-coumaroyl-CoA and benzoyl-CoA as acyl donor substrates and sinapyl alcohol as an acyl acceptor. Heterologous expression of HcPMT in hybrid poplar trees led to the incorporation of pCA in lignin, but no improvement in the saccharification potential of the wood. This work illustrates the value in mining diverse plant taxa for new monolignol acyltransferases. Furthermore, the occurrence of pCA outside monocot lineages may represent another example of convergent evolution in lignin structure. This discovery expands textbook views on cell wall biochemistry and provides a new molecular tool for engineering the lignin of biomass feedstock plants.

Lignin engineering in forest trees: From gene discovery to field trials

Wood is an abundant and renewable feedstock for the production of pulp, fuels, and biobased materials. However, wood is recalcitrant toward deconstruction into cellulose and simple sugars, mainly because of the presence of lignin, an aromatic polymer that shields cell-wall polysaccharides. Hence, numerous research efforts have focused on engineering lignin amount and composition to improve wood processability. Here, we focus on results that have been obtained by engineering the lignin biosynthesis and branching pathways in forest trees to reduce cell-wall recalcitrance, including the introduction of exotic lignin monomers. In addition, we draw general conclusions from over 20 years of field trial research with trees engineered to produce less or altered lignin. We discuss possible causes and solutions for the yield penalty that is often associated with lignin engineering in trees. Finally, we discuss how conventional and new breeding strategies can be combined to develop elite clones with desired lignin properties. We conclude this review with priorities for the development of commercially relevant lignin-engineered trees.