Biosynthesis and molecular actions of specialized 1,4-naphthoquinone natural products produced by horticultural plants

Antibacterial phenolic compounds from the flowering plants of Asia and the Pacific: coming to the light

CONTEXT

The emergence of pan-resistant bacteria requires the development of new antibiotics and antibiotic potentiators.

OBJECTIVE

This review identifies antibacterial phenolic compounds that have been identified in Asian and Pacific Angiosperms from 1945 to 2023 and analyzes their strengths and spectra of activity, distributions, molecular masses, solubilities, modes of action, structures-activities, as well as their synergistic effects with antibiotics, toxicities, and clinical potential.

METHODS

All data in this review was compiled from Google Scholar, PubMed, Science Direct, Web of Science, and library search; other sources were excluded. We used the following combination of keywords: 'Phenolic compound', 'Plants', and 'Antibacterial'. This produced 736 results. Each result was examined and articles that did not contain information relevant to the topic or coming from non-peer-reviewed journals were excluded. Each of the remaining 467 selected articles was read critically for the information that it contained.

RESULTS

Out of ∼350 antibacterial phenolic compounds identified, 44 were very strongly active, mainly targeting the cytoplasmic membrane of Gram-positive bacteria, and with a molecular mass between 200 and 400 g/mol. 2-Methoxy-7-methyljuglone, [6]-gingerol, anacardic acid, baicalin, vitexin, and malabaricone A and B have the potential to be developed as antibacterial leads.

CONCLUSIONS

Angiosperms from Asia and the Pacific provide a rich source of natural products with the potential to be developed as leads for treating bacterial infections.

Plant anthraquinones: Classification, distribution, biosynthesis, and regulation

Anthraquinones are polycyclic compounds with an unsaturated diketone structure (quinoid moiety). As important secondary metabolites of plants, anthraquinones play an important role in the response of many biological processes and environmental factors. Anthraquinones are common in the human diet and have a variety of biological activities including anticancer, antibacterial, and antioxidant activities that reduce disease risk. The biological activity of anthraquinones depends on the substitution pattern of their hydroxyl groups on the anthraquinone ring structure. However, there is still a lack of systematic summary on the distribution, classification, and biosynthesis of plant anthraquinones. Therefore, this paper systematically reviews the research progress of the distribution, classification, biosynthesis, and regulation of plant anthraquinones. Additionally, we discuss future opportunities in anthraquinone research, including biotechnology, therapeutic products, and dietary anthraquinones.

Effect of Agitation and Temporary Immersion on Growth and Synthesis of Antibacterial Phenolic Compounds in Genus Drosera

Sundews (Drosera sp.) are the source of biologically active secondary metabolites: phenolic acids, flavonoids, and 1,4-naphtoquinones. Because obtaining them from the natural environment is impossible (rare and endangered species), in this study modifications of traditional tissue cultures grown in solid medium (SM), such as agitated cultures (ACs) (cultures in liquid medium with rotary shaking) and temporary immersion bioreactors PlantformTM (TIB), were used for multiplication of four sundew species: Drosera peltata, Drosera indica, Drosera regia, and Drosera binata, with simultaneously effective synthesis of biologically active phenolic compounds. Each species cultivated on SM, AC, and TIB was tested for biomass accumulation, the content of total phenols and selected phenolic derivative concentrations (DAD-HPLC), the productivity on of phenolic compounds, as well as its antibacterial activity against two human pathogens: Staphylococcus aureus and Escherichia coli. The results showed that the type of culture should be selected for each species separately. Phytochemical analyses showed that the synthesis of secondary metabolites from the groups of phenolic acids, flavonoids, and 1,4-naphthoquinones can be increased by modifying the cultivation conditions. D. regia turned out to be the richest in phenolic compounds, including 1,4-naphtoquinones: plumbagin and ramentaceone. Extracts from D. indica and D. regia tissue showed strong antibacterial activity against both pathogens. It has also been shown that the growth conditions of sundews can modify the level of secondary metabolites, and thus, their biological activity.

Biosynthesis of brevinic acid from lawsone

The menaquinone-pathway (men) is widespread in bacteria and key to the biosynthesis of intriguing small molecules such as the essential vitamin menaquinone and the natural dye lawsone. The violet molecule brevinic acid is another proposed product of men, but its direct biosynthetic precursor has remained doubtful. In this study, we isolated brevinic acid from E. coli and confirmed its non-enzymatic formation from lawsone and homocysteine involving an intermediate acetylation or phosphorylation step. We furthermore compared our proposed substrates in a non-enzymatic assay against the previously hypothesized precursor DHNA and showed that the reaction with activated lawsone derivatives proceeded faster, more selective, and with complete turnover. This supports our proposed biosynthesis of brevinic acid from lawsone and enables a cost effective, larger-scale synthesis of brevinic acid.

MeJA-induced hairy roots in Plumbago auriculata L. by RNA-seq profiling and key synthase provided new insights into the sustainable production of plumbagin and saponins

Naturally synthesized secondary metabolites in plants are considered an important source of drugs, food additives, etc. Among them, research on natural plant medicinal components and their synthesis mechanisms has always been of high concern. We identified a novel medicinal floral crop, Plumbago auriculata L., that can be treated with methyl jasmonate (MeJA) for the rapid or sustainable production of natural bioactives from hairy roots. In the study, we globally analyzed the changes in the accumulation of plumbagin and others in the hairy roots of Plumbago auriculata L. hairy roots (PAHR) 15834 in P. auriculata L. based on 100 μmol/L of MeJA treatment by RNA-seq profiling, and we found that there was a significant increase in the accumulation of plumbagin and saponin before 24 h. To explain the principle of co-accumulation, it showed that MeJA induced JA signaling and the shikimic acid pathway, and the methylvaleric acid (MVA) pathway was activated downstream subsequently by the Mfuzz and weighted gene co-expression analysis. Under the shared metabolic pathway, the high expression of PAL3 and HMGR promoted the activity of the "gateway enzymes" phenylalanine ammonia lyase (PAL) and 3-hydroxy-3-methylglutaryl CoA reductase (HMGR), which respectively induced the high expression of key reaction enzyme genes, including chalcone synthase (CHS), isopentenyl diphosphate (IPP), and farnesyl pyrophosphate synthase (FPS), that led to the synthesis of plumbagin and saponin. We speculated that large amounts of ketones and/or aldehydes were formed under the action of these characteristic enzymes, ultimately achieving their co-accumulation through polyketone and high-level sugar and amino acid metabolism. The study results provided a theoretical basis for carrying out the factory refinement and biosynthesis of plumbagin and saponins and also provided new ideas for fully exploiting multifunctional agricultural crops and plants and developing new agricultural by-products.

Chemically Mediated Plant-Plant Interactions: Allelopathy and Allelobiosis

Plant-plant interactions are a central driver for plant coexistence and community assembly. Chemically mediated plant-plant interactions are represented by allelopathy and allelobiosis. Both allelopathy and allelobiosis are achieved through specialized metabolites (allelochemicals or signaling chemicals) produced and released from neighboring plants. Allelopathy exerts mostly negative effects on the establishment and growth of neighboring plants by allelochemicals, while allelobiosis provides plant neighbor detection and identity recognition mediated by signaling chemicals. Therefore, plants can chemically affect the performance of neighboring plants through the allelopathy and allelobiosis that frequently occur in plant-plant intra-specific and inter-specific interactions. Allelopathy and allelobiosis are two probably inseparable processes that occur together in plant-plant chemical interactions. Here, we comprehensively review allelopathy and allelobiosis in plant-plant interactions, including allelopathy and allelochemicals and their application for sustainable agriculture and forestry, allelobiosis and plant identity recognition, chemically mediated root-soil interactions and plant-soil feedback, and biosynthesis and the molecular mechanisms of allelochemicals and signaling chemicals. Altogether, these efforts provide the recent advancements in the wide field of allelopathy and allelobiosis, and new insights into the chemically mediated plant-plant interactions.

Distribution of Acetogenic Naphthoquinones in Droseraceae and Their Chemotaxonomic Utility

Chemotaxonomy is the link between the state of the art in analytical chemistry and the systematic classification and phylogenetic analysis of biota. Although the characteristic secondary metabolites from diverse biotic sources have been used in pharmacology and biological systematics since the dawn of mankind, only comparatively recently established reproducible methods have allowed the precise identification and distinction of structurally similar compounds. Reliable, rapid screening methods like TLC (Thin Layer Chromatography) can be used to investigate sufficiently large numbers of samples for chemotaxonomic purposes. Using distribution patterns of mutually exclusive naphthoquinones, it is demonstrated in this review how a simple set of chemical data from a representative sample of closely related species in the sundew family (Droseraceae, Nepenthales) provides taxonomically and phylogenetically informative signal within the investigated group and beyond.

Juglone, a plant-derived 1,4-naphthoquinone, binds to hydroxylamine oxidoreductase and inhibits the electron transfer to cytochrome c554

IMPORTANCE

Nitrification, the microbial conversion of ammonia to nitrate via nitrite, plays a pivotal role in the global nitrogen cycle. However, the excessive use of ammonium-based fertilizers in agriculture has disrupted this cycle, leading to groundwater pollution and greenhouse gas emissions. In this study, we have demonstrated the inhibitory effects of plant-derived juglone and related 1,4-naphthoquinones on the nitrification process in Nitrosomonas europaea. Notably, the inhibition mechanism is elucidated in which 1,4-naphthoquinones interact with hydroxylamine oxidoreductase, disrupting the electron transfer to cytochrome c554, a physiological electron acceptor. These findings support the notion that phytochemicals can impede nitrification by interfering with the essential electron transfer process in ammonia oxidation. The findings presented in this article offer valuable insights for the development of strategies aimed at the management of nitrification, reduction of fertilizer utilization, and mitigation of greenhouse gas emissions.

Lactiplantibacillus plantarum uses ecologically relevant, exogenous quinones for extracellular electron transfer

IMPORTANCE

While quinones are essential for respiratory microorganisms, their importance for microbes that rely on fermentation metabolism is not understood. This gap in knowledge hinders our understanding of anaerobic microbial habitats, such in mammalian digestive tracts and fermented foods. We show that Lactiplantibacillus plantarum, a model fermentative lactic acid bacteria species abundant in human, animal, and insect microbiomes and fermented foods, uses multiple exogenous, environmental quinones as electron shuttles for a hybrid metabolism involving EET. Interestingly, quinones both stimulate this metabolism as well as cause oxidative stress when extracellular electron acceptors are absent. We also found that quinone-producing, lactic acid bacteria species commonly enriched together with L. plantarum in food fermentations accelerate L. plantarum growth and medium acidification through a mainly quinone- and EET-dependent mechanism. Thus, our work provides evidence of quinone cross-feeding as a key ecological feature of anaerobic microbial habitats.

Metal-Free C-H Sulfamidation of 1,4-Naphthoquinone in Water

Direct sulfamidation of 1,4-naphthoquinones using N-methoxy sulfonamides under metal-free conditions in water was developed. Base-mediated nucleophilic addition of N-methoxy sulfonamides, followed by N-O bond cleavage allowed the formation of enesulfonamides. Further, the synthesis of pyrrolonaphthoquinones proved the practicability of the current approach.

Hybrids of 1,4-Naphthoquinone with Thymidine Derivatives: Synthesis, Anticancer Activity, and Molecular Docking Study

One of the most essential health problems is cancer, the first or second cause of death worldwide. Head and neck cancers are hard to detect due to non-specific symptoms. The treatment often relies on a combination of radio and chemotherapy. For this reason, the research of new anticancer compounds is fundamental. The natural and synthetic compounds with 1,4-naphthoquinone scaffold is characterized by high anticancer activity. The study aimed to evaluate the synthesis and anticancer activity of hybrids 1,4-naphthoquinone with thymidine derivatives. The series of compounds allows us to check the influence of the substituent in the C3' position of the thymidine moiety on the cytotoxicity against squamous cancer cell lines (SCC-9 and SCC-25) and submandibular gland cancer (A-253). An annexin V/propidium iodide (PI) co-staining assay shows that derivatives cause the apoptotic in SCC-25 and A-253 cell lines. The molecular docking study examined the interaction between the active site of the BCL-2 protein and the hybrids.

Structural and pharmacological diversity of 1,4-naphthoquinone glycosides in recent 20 years

1,4-naphthoquinones are the most widespread naphthoquinone compounds. Recently, many 1,4-naphthoquinone glycosides with different structural features have been obtained from both nature and synthesis, which has led to an increasing variety of naphthoquinone glycosides. In this paper, the structure variety and biological activity in recent 20 years are reviewed, and classified them according to the source and structure characteristics. Meanwhile the synthetic methods of O-, S-, C- and N-naphthoquinone glycosides and their structure activity relationships are also described. It was referred that the presence of polar groups of C2 and C5 and non-polar groups attached to C3 on the naphthoquinone ring are beneficial for their biological activities. It will provide more comprehensive literature resources for the future research of 1, 4-naphthoquinone glycosides and lay a theoretical foundation.

Chimaphila umbellata; a biotechnological perspective on the coming-of-age prince's pine

Chimaphila umbellata has been studied for almost two centuries now, with the first paper exploring the phytochemistry of the plant published in 1860. Almost all contemporary studies focus on the biotechnological advances of C. umbellata including its utilization as a natural alternative in the cosmetic, food, biofuel, and healthcare industry, with a special focus on its therapeutic uses. This literature review critically investigates the significance and applications of secondary metabolites extracted from the plant and presses on the biotechnological approaches to improve its utilization. C. umbellata is home to many industrially and medicinally important phytochemicals, the majority of which belong to phenolics, sterols, and triterpenoids. Other important compounds include 5-hydroxymethylfurfural, isohomoarbutin, and methyl salicylate (the only essential oil of the plant). Chimaphilin is the characteristic phytochemical of the plant. This review focuses on the phytochemistry of C. umbellata and digs into their chemical structures and attributes. It further discusses the challenges of working with C. umbellata including its alarming conservation status, problems with in-vitro cultivation, and research and development issues. This review concludes with recommendations based on biotechnology, bioinformatics, and their crucial interface.

An overview on the antibacterial properties of juglone, naphthazarin, plumbagin and lawsone derivatives and their metal complexes

Bacterial resistance development represents a serious threat to human health across the globe and has become a very serious clinical problem for many classes of antibiotics. Hence, there is a constant and urgent need for the discovery and development of new effective antibacterial agents to stem the emergence of resistant bacteria. 1,4-naphthoquinones are an important class of natural products and have been known for decades as a privileged scaffold in medicinal chemistry regarding their many biological properties. The significant biological properties of specific 1,4-naphthoquinones hydroxyderivatives have drawn the attention of researchers in order to find new derivatives with an optimized activity, mainly as antibacterial agents. Based on juglone, naphthazarin, plumbagin and lawsone moieties, structural optimization was realized with the purpose of improving the antibacterial activity. Thereupon, relevant antibacterial activities have been observed on different panels of bacterial strains including resistant ones. In this review, we highlight the interest of developing new 1,4-naphthoquinones hydroxyderivatives and some metal complexes as promising antibacterial agents alternatives. Here, we thoroughly report for the first time both the antibacterial activity and the chemical synthesis of four different 1,4-naphthoquinones (juglone, naphthazarin, plumbagin and lawsone) from 2002 to 2022 with an emphasis on the structure-activity relationship, when applicable.

Impact of cytotoxic plant naphthoquinones, juglone, plumbagin, lawsone and 2-methoxy-1,4-naphthoquinone, on Chlamydomonas reinhardtii reveals the biochemical mechanism of juglone toxicity by rapid depletion of plastoquinol

Hydrophilic, untethered 1,4-naphthoquinones (1,4-NQs) are plant secondary metabolites that are often excreted into the environment and play a role in various plant-microbial, plant-fungal, plant-insect and plant-plant interactions. The biological activity of 1,4-NQs is mainly related to their redox properties, i.e. the ability to undergo redox cycling in cells. These compounds may also undergo electrophilic addition to thiol-containing compounds. The aim of this study was to compare the impact of juglone, plumbagin, lawsone and 2-methoxy-1,4-naphthoquinone (2-met-NQ) on the antioxidant response of the green microalga Chlamydomonas reinhardtii. The algae were incubated with the examined compounds under low light for 6 h and the content of photosynthetic pigments, prenyllipid antioxidants, ascorbate, soluble thiols, proline, and superoxide dismutase activity was assessed. To examine the interaction between photosynthetic activity and naphthoquinone toxicity, we carried out the second experiment, in which C. reinhardtii was incubated with 1,4-NQs for 1 h under high light or in darkness. The pro-oxidant action of the examined 1,4-NQs depended on their reduction potentials, which decrease in order: juglone > plumbagin > 2-met-NQ > lawsone. Lawsone did not display pro-oxidant properties. Exposure to high light strongly enhanced the pro-oxidant effect of juglone, plumbagin, and 2-met-NQ, which is thought to result from the interception of the electrons from photosynthetic electron transfer chain. Only juglone was able to cause a fast depletion of plastoquinol, which may be an important mode of action of this allelochemical, responsible for its high toxicity to plants.

Quinones as Promising Compounds against Respiratory Viruses: A Review

Respiratory viruses represent a world public health problem, giving rise to annual seasonal epidemics and several pandemics caused by some of these viruses, including the COVID-19 pandemic caused by the novel SARS-CoV-2, which continues to date. Some antiviral drugs have been licensed for the treatment of influenza, but they cause side effects and lead to resistant viral strains. Likewise, aerosolized ribavirin is the only drug approved for the therapy of infections by the respiratory syncytial virus, but it possesses various limitations. On the other hand, no specific drugs are licensed to treat other viral respiratory diseases. In this sense, natural products and their derivatives have appeared as promising alternatives in searching for new compounds with antiviral activity. Besides their chemical properties, quinones have demonstrated interesting biological activities, including activity against respiratory viruses. This review summarizes the activity against respiratory viruses and their molecular targets by the different types of quinones (both natural and synthetic). Thus, the present work offers a general overview of the importance of quinones as an option for the future pharmacological treatment of viral respiratory infections, subject to additional studies that support their effectiveness and safety.

Strategies to study the metabolic origins of specialized plant metabolites: The specialized 1,4-naphthoquinones

One of the hallmarks of specialized plant metabolites is that they are produced using precursors from central metabolism. Therefore, in addition to identifying and characterizing the pathway genes and enzymes involved in synthesizing a specialized compound, it is critical to study its metabolic origins. Identifying what primary metabolic pathways supply precursors to specialized metabolism and how primary metabolism has diversified to sustain fluxes to specialized metabolite pathways is imperative to optimizing synthetic biology strategies for producing high-value plant natural products in crops and microbial systems. Improved understanding of the metabolic origins of specialized plant metabolites has also revealed instances of recurrent evolution of the same compound, or nearly identical compounds, with similar ecological functions, thereby expanding knowledge about the factors driving the chemical diversity in the plant kingdom. In this chapter, we describe detailed methods for performing tracer studies, chemical inhibitor experiments, and reverse genetics. We use examples from investigations of the metabolic origins of specialized plant 1,4-naphthoquinones (1,4-NQs). The plant 1,4-NQs provide an excellent case study for illustrating the importance of investigating the metabolic origins of specialized metabolites. Over half a century of research by many groups has revealed that the pathways to synthesize plant 1,4-NQs are the result of multiple events of convergent evolution across several disparate plant lineages and that plant 1,4-NQ pathways are supported by extraordinary events of metabolic innovation and by various primary metabolic sources.

How Structure-Function Relationships of 1,4-Naphthoquinones Combat Antimicrobial Resistance in Multidrug-Resistant (MDR) Pathogens

Antimicrobial resistance (AMR) is one of the top ten health-related threats worldwide. Among several antimicrobial agents, naphthoquinones (NQs) of plant/chemical origin possess enormous structural and functional diversity and are effective against multidrug-resistant (MDR) pathogens. 1,4-NQs possess alkyl, hydroxyl, halide, and metal groups as side chains on their double-ring structure, predominantly at the C-2, C-3, C-5, and C-8 positions. Among 1,4-NQs, hydroxyl groups at either C-2 or C-5 exhibit significant antibacterial activity against Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, Enterobacter spp. (ESKAPE) and MDR categories. 1,4-NQs exhibit antibacterial activities like plasmids curing, reactive oxygen species generation, efflux pumps inhibition, anti-DNA gyrase activity, membrane permeabilization, and biofilm inhibition. This review emphasizes the structure-function relationships of 1,4-NQs against ESKAPE and MDR pathogens based on a literature review of studies published in the last 15 years. Overall, 1,4-NQs have great potential for counteracting the antimicrobial resistance of MDR pathogens.

Interactions of Antibacterial Naphthoquinones with Mesoporous Silica Surfaces: A Physicochemical and Theoretical Approach

1,4-naftoquinone (NQ) molecules have been extensively evaluated as potent antibacterial compounds; however, their use is limited, since they have low water solubility and exhibit toxicities in healthy eukaryotic cells. A possible path to overcoming these challenges is the use of particulate vehicles, such as SBA-15, which is a biocompatible and biodegradable mesoporous silica material, that may enhance drug delivery and decrease dosages. In this work, an isotherm model-based adsorption of three NQs into SBA-15 microparticles was evaluated. Interactions between NQs and SBA-15 microparticles were modeled at the B3LYP/6-31+G(d,p) level of theory to understand the nature of such interactions. The results demonstrated that the adsorption of NQ, 2NQ, and 5NQ into SBA-15 fit the Freundlich adsorption model. According to theorical studies, physisorption is mediated by hydrogen bonds, while the most stable interactions occur between the carbonyl group of NQ and silica surfaces. Both experimental and theoretical results contribute to a deeper understanding of the use of SBA-15 or similar particles as nanovehicles in such a way that NQs can be modified in carbonyl or C3 to enhance adsorptions. The theoretical and experimental results were in accordance and contribute to a deeper understanding of how interactions between NQ-type molecules and SiO₂ materials occur.

In Vitro Antimicrobial Effects and Inactivation Mechanisms of 5,8-Dihydroxy-1,4-Napthoquinone

Naphthoquinones are an important class of natural organic compounds that have antimicrobial effects. However, the mechanisms of their action remain to be elucidated. Therefore, the antimicrobial activity of the chemically synthesized naphthoquinone derivative, 5,8-dihydroxy-1,4-naphthoquinone, was investigated in this study against 10 different microorganisms. Its inhibitory activity was evident against Bacillus cereus, Proteus vulgaris, Salmonella enteritidis, Staphylococcus epidermidis, S. aureus, and Candida albicans, and its MIC50 values were determined to be 14, 10, 6, 2, 4, 1.2, and <0.6 µg/mL, respectively. Moreover, the crystal violet uptake, TTC dehydrogenase activity, protein/DNA leakage, and DNA damage of the compound in these microorganisms were also investigated to reveal the antimicrobial mechanisms. In addition, scanning electron microscopy was used to detect physiological damage to the cell membrane of S. epidermidis, S. aureus, and C. albicans, which was most severe in the crystal violet uptake assay. The overall results showed that 5,8-dihydroxy-1,4-naphthoquinone exhibited its effects on S. aureus, S. epidermidis, and C. albicans by various mechanisms, especially membrane damage and membrane integrity disruption. It also caused DNA leakage and damage along with respiratory chain disruption (78%) in C. albicans. Similarly, it caused varying degrees of reduction in the respiratory activity of S. aureus (47%), S. epidermidis (16%), B. cereus (12%), S. enteritidis (9%), and P. vulgaris (8%). Therefore, 5,8-dihydroxy-1,4-naphthoquinone proved to be a very effective antifungal and antibacterial agent and could be considered a new potential drug candidate, inspiring further discoveries in these microorganisms.

Functional genomics-enabled characterization of CYP81B140 and CYP81B141 from Plumbago zeylanica L. substantiates their involvement in plumbagin biosynthesis

Novel cytochrome P450s, CYP81B140 and CYP81B141 from Plumbago zeylanica were functionally characterized to understand their involvement in polyketide plumbagin biosynthesis. Further, we propose 3-methyl-1-8-naphthalenediol and isoshinanolone as intermediates for plumbagin biosynthesis. Plumbago zeylanica L. (P. zeylanica) is a medicinally important plant belonging to the family Plumbaginaceae. It comprises the most abundant naphthoquinone plumbagin having anti-cancer activity. Only the polyketide synthase (PKS) enzyme has been identified from the biosynthetic pathway which catalyzes iterative condensation of acetyl-CoA and malonyl-CoA molecules. The plumbagin biosynthesis involves hydroxylation, oxidation, hydration and dehydration of intermediate compounds which are expected to be catalyzed by cytochrome P450s (CYPs). To identify the CYPs, co-expression analysis was carried out using PKS as a candidate gene. Out of the eight identified CYPs, CYP81B140 and CYP81B141 have similar expression with PKS and belong to the CYP81 family. Phylogenetic analysis suggested that CYP81B140 and CYP81B141 cluster with CYPs from CYP81B, CYP81D, CYP81E and CYP81AA subfamilies which are known to be involved in the hydroxylation and oxidation reactions. Moreover, artificial microRNA-mediated transient individual silencing and co-silencing of CYP81B140 and CYP81B141 significantly reduced plumbagin and increased the 3-methyl-1-8-naphthalenediol and isoshinanolone content. Based on metabolite analysis, we proposed that 3-methyl-1-8-naphthalenediol and isoshinanolone function as intermediates for plumbagin biosynthesis. Transient silencing, over-expression and docking analysis revealed that CYP81B140 is involved in C-1 oxidation, C-4 hydroxylation and [C2-C3] hydration of 3-methyl-1-8-naphthalenediol to form isoshinanolone, whereas CYP81B141 is catalyzing [C2-C3] dehydration and C-4 oxidation of isoshinanolone to form plumbagin. Our results indicated that both CYP81B140 and CYP81B141 are promiscuous and necessary for plumbagin biosynthesis. This is the first report of identification and functional characterization of P. zeylanica-specific CYPs involved in plumbagin biosynthetic pathway and in general hexaketide synthesis in plants.

Dynamics of alkannin/shikonin biosynthesis in response to jasmonate and salicylic acid in Lithospermum officinale

Alkannin/shikonin and their derivatives are specialised metabolites of high pharmaceutical and ecological importance exclusively produced in the periderm of members of the plant family Boraginaceae. Previous studies have shown that their biosynthesis is induced in response to methyl jasmonate but not salicylic acid, two phytohormones that play important roles in plant defence. However, mechanistic understanding of induction and non-induction remains largely unknown. In the present study, we generated the first comprehensive transcriptomic dataset and metabolite profiles of Lithospermum officinale plants treated with methyl jasmonate and salicylic acid to shed light on the underlying mechanisms. Our results highlight the diverse biological processes activated by both phytohormones and reveal the important regulatory role of the mevalonate pathway in alkannin/shikonin biosynthesis in L. officinale. Furthermore, by modelling a coexpression network, we uncovered structural and novel regulatory candidate genes connected to alkannin/shikonin biosynthesis. Besides providing new mechanistic insights into alkannin/shikonin biosynthesis, the generated methyl jasmonate and salicylic acid elicited expression profiles together with the coexpression networks serve as important functional genomic resources for the scientific community aiming at deepening the understanding of alkannin/shikonin biosynthesis.

Plumbagin resurrect colistin susceptible against colistin-resistant Pseudomonas aeruginosa in vitro and in vivo

The global emergence and spread of multi-drug resistant (MDR) strains is becoming increasingly worrisome due to the overuse of broad-spectrum antibiotics. Colistin, the last resort for treating MDR strains infections, has once again returned to the clinician’s choice. However, with the widespread use of colistin, colistin-resistant gram-negative bacteria (GNB) have subsequently emerged, including colistin-resistant Pseudomonas aeruginosa (COL-R PA). Therefore, available solutions are urgently needed to respond to this situation. Here, we inspiringly found that the combination of plumbagin and colistin had an efficiently inhibitory effect for colistin-resistant P. aeruginosa in vitro through checkerboard assay and time-kill assay. The combinatorial inhibition of biofilm formation was clearly demonstrated by crystal violet staining and scanning electron microscopy (SEM), and this combination can not only inhibited biofilm formation but also eradicated the mature biofilm. Erythrocytes hemolysis test showed that plumbagin has negligible hemolysis ability. In addition, the increased survival rate of Galleria mellonella (G. mellonella) larva confirmed this combination as same as effective in vivo. As for the mechanism of this combination, propidium iodide (PI) staining showed colistin combined with plumbagin could significantly change the membrane permeability, thus exerting synergistic antibacterial activity. In conclusion, the combination of plumbagin and colistin shows a prominently synergistic antibacterial effect in vitro and in vivo, providing a promising option for the therapy of COL-R PA infection.

Quinoid Pigments of Sea Urchins Scaphechinus mirabilis and Strongylocentrotus intermedius: Biological Activity and Potential Applications

This review presents literature data: the history of the discovery of quinoid compounds, their biosynthesis and biological activity. Special attention is paid to the description of the quinoid pigments of the sea urchins Scaphechinus mirabilis (from the family Scutellidae) and Strongylocentrotus intermedius (from the family Strongylocentrotidae). The marine environment is considered one of the most important sources of natural bioactive compounds with extremely rich biodiversity. Primary- and some secondary-mouthed animals contain very high concentrations of new biologically active substances, many of which are of significant potential interest for medical purposes. The quinone pigments are products of the secondary metabolism of marine animals, can have complex structures and become the basis for the development of new natural products in echinoids that are modulators of chemical interactions and possible active ingredients in medicinal preparations. More than 5000 chemical compounds with high pharmacological potential have been isolated and described from marine organisms. There are three well known ways of naphthoquinone biosynthesis-polyketide, shikimate and mevalonate. The polyketide pathway is the biosynthesis pathway of various quinones. The shikimate pathway is the main pathway in the biosynthesis of naphthoquinones. It should be noted that all quinoid compounds in plants and animals can be synthesized by various ways of biosynthesis.

The Adjustment Strategy of Venus Flytrap Photosynthetic Apparatus to UV-A Radiation

The objective of this study was to investigate the response of the photosynthetic apparatus of the Venus flytrap (Dionaea muscipula J. Ellis) to UV-A radiation stress as well as the role of selected secondary metabolites in this process. Plants were subjected to 24 h UV-A treatment. Subsequently, chl a fluorescence and gas exchange were measured in living plants. On the collected material, analyses of the photosynthetic pigments and photosynthetic apparatus proteins content, as well as the contents and activity of selected antioxidants, were performed. Measurements and analyses were carried out immediately after the stress treatment (UV plants) and another 24 h after the termination of UV-A exposure (recovery plants). UV plants showed no changes in the structure and function of their photosynthetic apparatus and increased contents and activities of some antioxidants, which led to efficient CO₂ carboxylation, while, in recovery plants, a disruption of electron flow was observed, resulting in lower photosynthesis efficiency. Our results revealed that D. muscipula plants underwent two phases of adjustment to UV-A radiation. The first was a regulatory phase related to the exploitation of available mechanisms to prevent the over-reduction of PSII RC. In addition, UV plants increased the accumulation of plumbagin as a potential component of a protective mechanism against the disruption of redox homeostasis. The second was an acclimatization phase initiated after the running down of the regulatory process and decrease in photosynthesis efficiency.

Review of Shikonin and Derivatives: Isolation, Chemistry, Biosynthesis, Pharmacology and Toxicology

Shikonin and its derivatives, isolated from traditional medicinal plant species of the genus Lithospermum, Alkanna, Arnebia, Anchusa, Onosma, and Echium belonging to the Boraginaceae family, have numerous applications in foods, cosmetics, and textiles. Shikonin, a potent bioactive red pigment, has been used in traditional medicinal systems to cure various ailments and is well known for its diverse pharmacological potential such as anticancer, antithrombotic, neuroprotective, antidiabetic, antiviral, anti-inflammatory, anti-gonadotropic, antioxidants, antimicrobial and insecticidal. Herein, updated research on the natural sources, pharmacology, toxicity studies, and various patents filed worldwide related to shikonin and approaches to shikonin’s biogenic and chemical synthesis are reviewed. Furthermore, recent studies to establish reliable production systems to meet market demand, functional identification, and future clinical development of shikonin and its derivatives against various diseases are presented.

Lack of antagonism between salicylic acid and jasmonate signalling pathways in poplar

Salicylic acid (SA) and jasmonic acid (JA) often play distinct roles in plant defence against pathogens. Research from Arabidopsis thaliana has established that SA- and JA-mediated defences are more effective against biotrophs and necrotrophs, respectively. These two hormones often interact antagonistically in response to particular attackers, with the induction of one leading to suppression of the other. Here, we report a contrasting pattern in the woody perennial Populus: positive SA-JA interplay. Using genetically engineered high SA lines of black poplar and wild-type lines after exogenous hormone application, we quantified SA and JA metabolites, signalling gene transcripts, antifungal flavonoids and resistance to rust (Melampsora larici-populina). Salicylic acid and JA metabolites were induced concurrently upon rust infection in poplar genotypes with varying resistance levels. Analysis of SA-hyperaccumulating transgenic poplar lines showed increased jasmonate levels, elevated flavonoid content and enhanced rust resistance, but no discernible reduction in growth. Exogenous application of either SA or JA triggered the accumulation of the other hormone. Expression of pathogenesis-related (PR) genes, frequently used as markers for SA signalling, was not correlated with SA content, but rather activated in proportion to pathogen infection. We conclude that SA and JA pathways interact positively in poplar resulting in the accumulation of flavonoid phytoalexins.

Investigation on Metabolites in Structural Diversity from The Deep-Sea Sediment-Derived Bacterium Agrococcus sp. SCSIO 52902 and Their Biosynthesis

Deep-sea sediment-derived bacterium may make full use of self-genes to produce more bioactive metabolites to adapt to extreme environment, resulting in the discovery of novel metabolites with unique structures and metabolic mechanisms. In the paper, we systematically investigated the metabolites in structurally diversity and their biosynthesis from the deep-sea sediment-derived bacterium Agrococcus sp. SCSIO 52902 based on OSMAC strategy, Molecular Networking tool, in combination with bioinformatic analysis. As a result, three new compounds and one new natural product, including 3R-OH-1,6-diene-cyclohexylacetic acid (1), linear tetradepsipeptide (2), N¹,N⁵-di-p-(EE)-coumaroyl-N¹⁰-acetylspermidine (3) and furan fatty acid (4), together with nineteen known compounds (5–23) were isolated from the ethyl acetate extract of SCSIO 52902. Their structures were elucidated by comprehensive spectroscopic analysis, single-crystal X-ray diffraction, Marfey’s method and chiral-phase HPLC analysis. Bioinformatic analysis revealed that compounds 1, 3, 9 and 13–22 were closely related to the shikimate pathway, and compound 5 was putatively produced by the OSB pathway instead of the PKS pathway. In addition, the result of cytotoxicity assay showed that compound 5 exhibited weak cytotoxic activity against the HL-60 cell line.

The Manchurian Walnut Genome: Insights into Juglone and Lipid Biosynthesis

BACKGROUND

Manchurian walnut (Juglans mandshurica Maxim.) is a tree with multiple industrial uses and medicinal properties in the Juglandaceae family (walnuts and hickories). J. mandshurica produces juglone, which is a toxic allelopathic agent and has potential utilization value. Furthermore, the seed of J. mandshurica is rich in various unsaturated fatty acids and has high nutritive value.

FINDINGS

Here, we present a high-quality chromosome-scale reference genome assembly and annotation for J. mandshurica (n = 16) with a contig N50 of 21.4 Mb by combining PacBio high-fidelity reads with high-throughput chromosome conformation capture data. The assembled genome has an estimated sequence size of 548.7 Mb and consists of 657 contigs, 623 scaffolds, and 40,453 protein-coding genes. In total, 60.99% of the assembled genome consists of repetitive sequences. Sixteen super-scaffolds corresponding to the 16 chromosomes were assembled, with a scaffold N50 length of 33.7 Mb and a BUSCO complete gene percentage of 98.3%. J. mandshurica displays a close sequence relationship with Juglans cathayensis, with a divergence time of 13.8 million years ago. Combining the high-quality genome, transcriptome, and metabolomics data, we constructed a gene-to-metabolite network and identified 566 core and conserved differentially expressed genes, which may be involved in juglone biosynthesis. Five CYP450 genes were found that may contribute to juglone accumulation. NAC, bZip, NF-YA, and NF-YC are positively correlated with the juglone content. Some candidate regulators (e.g., FUS3, ABI3, LEC2, and WRI1 transcription factors) involved in the regulation of lipid biosynthesis were also identified.

CONCLUSIONS

Our genomic data provide new insights into the evolution of the walnut genome and create a new platform for accelerating molecular breeding and improving the comprehensive utilization of these economically important tree species.

Juglone and 1,4-Naphthoquinone-Promising Nematicides for Sustainable Control of the Root Knot Nematode Meloidogyne luci

The scarce availability of efficient and eco-friendly nematicides to control root-knot nematodes (RKN), Meloidogyne spp., has encouraged research toward the development of bionematicides. Naphthoquinones, juglone (JUG) and 1,4-naphthoquinone (1,4-NTQ), are being explored as alternatives to synthetic nematicides to control RKN. This study expands the knowledge on the effects of these natural compounds toward M. luci life cycle (mortality, hatching, penetration, reproduction). M. luci second-stage juveniles (J2)/eggs were exposed to each compound (250, 150, 100, 50, and 20 ppm) to monitor nematode mortality and hatching during 72 h and 15 days, respectively. Tomato seedlings were then inoculated with 200 J2, which had been exposed to JUG/1,4-NTQ for 3 days. The number of nematodes inside the roots was determined at 3 days after inoculation, and the final population density was assessed at 45 days after inoculation. Moreover, the potential mode of action of JUG/1,4-NTQ was investigated for the first time on RKN, through the assessment of reactive oxygen species (ROS) generation, acetylcholinesterase (AChE) in vitro inhibitory activity and expression analysis of ache and glutathione-S-transferase (gst) genes. 1,4-NTQ was the most active compound, causing ≥50% J2 mortality at 250 ppm, within 24 h. At 20 and 50 ppm, hatching was reduced by ≈50% for both compounds. JUG showed a greater effect on M. luci penetration and reproduction, decreasing infection by ≈80% (50 ppm) on tomato plants. However, 1,4-NTQ-induced generation of ROS and nematode vacuolization was observed. Our study confirms that JUG/1,4-NTQ are promising nematicidal compounds, and new knowledge on their physiological impacts on Meloidogyne was provided to open new avenues for the development of innovative sustainable nematicides.

Deadenylase-dependent mRNA decay of GDF15 and FGF21 orchestrates food intake and energy expenditure

Hepatokines, secretory proteins from the liver, mediate inter-organ communication to maintain a metabolic balance between food intake and energy expenditure. However, molecular mechanisms by which hepatokine levels are rapidly adjusted following stimuli are largely unknown. Here, we unravel how CNOT6L deadenylase switches off hepatokine expression after responding to stimuli (e.g., exercise and food) to orchestrate energy intake and expenditure. Mechanistically, CNOT6L inhibition stabilizes hepatic Gdf15 and Fgf21 mRNAs, increasing corresponding serum protein levels. The resulting upregulation of GDF15 stimulates the hindbrain to suppress appetite, while increased FGF21 affects the liver and adipose tissues to induce energy expenditure and lipid consumption. Despite the potential of hepatokines to treat metabolic disorders, their administration therapies have been challenging. Using small-molecule screening, we identified a CNOT6L inhibitor enhancing GDF15 and FGF21 hepatokine levels, which dramatically improves diet-induced metabolic syndrome. Our discovery, therefore, lays the foundation for an unprecedented strategy to treat metabolic syndrome.

Designing a Redox Noninnocent Phenalenyl-Based Copper(II) Complex: An Autotandem Catalyst for the Selective Oxidation of Polycyclic Aromatic Hydrocarbons (PAHs)

A square-planar [CuIIL] complex 1, based on the redox-active phenalenyl unit LH2 = 9,9'-(ethane-1,2-diylbis(azanediyl))bis(1H-phenalen-1-one), is prepared and structurally characterized by single-crystal X-ray diffraction analysis. Complex 1 crystallizes at room temperature with the P1 space group. The molecular structure of 1 reveals the presence of intriguing C-H···Cu intermolecular anagostic interactions of the order ∼2.7715 Å. Utilizing the presence of anagostic interactions and the free nonbonding molecular orbitals (NBMOs) of the closed-shell phenalenyl unit in 1, the oxidation reactions of some industrially important polycyclic aromatic hydrocarbons (PAHs) in the presence of the [CuIIL] complex under very mild conditions have been reported. The direct conversion of anthracene-9-carbaldehyde to 9,10-anthraquinone in one step concludes that the catalyst shows dual activity in the chemical transformations. This also includes the first report of a "single-step" catalytic transformation of pyrene-1-carbaldehyde to the synthetically difficult pyren-4-ol, a precursor for the synthesis of several novel fluorescent probes for cell imaging.

Allelopathy as an evolutionary game

In plants, most competition is resource competition, where one plant simply preempts the resources away from its neighbors. Interference competition, as the name implies, is a form of direct interference to prevent resource access. Interference competition is common among animals that can physically fight, but in plants, one of the main mechanisms of interference competition is allelopathy. Allelopathic plants release cytotoxic chemicals into the environment which can increase their ability to compete with surrounding organisms for limited resources. The circumstances and conditions favoring the development and maintenance of allelochemicals, however, are not well understood. Particularly, despite the obvious benefits of allelopathy, current data suggest it seems to have only rarely evolved. To gain insight into the cost and benefit of allelopathy, we have developed a 2 × 2 matrix game to model the interaction between plants that produce allelochemicals and plants that do not. Production of an allelochemical introduces novel cost associated with both synthesis and detoxifying a toxic chemical but may also convey a competitive advantage. A plant that does not produce an allelochemical will suffer the cost of encountering one. Our model predicts three cases in which the evolutionarily stable strategies are different. In the first, the nonallelopathic plant is a stronger competitor, and not producing allelochemicals is the evolutionarily stable strategy. In the second, the allelopathic plant is the better competitor, and production of allelochemicals is the more beneficial strategy. In the last case, neither is the evolutionarily stable strategy. Instead, there are alternating stable states, depending on whether the allelopathic or nonallelopathic plant arrived first. The generated model reveals circumstances leading to the evolution of allelochemicals and sheds light on utilizing allelochemicals as part of weed management strategies. In particular, the wide region of alternative stable states in most parameterizations, combined with the fact that the absence of allelopathy is likely the ancestral state, provides an elegant answer to the question of why allelopathy seems to rarely evolve despite its obvious benefits. Allelopathic plants can indeed outcompete nonallelopathic plants, but this benefit is simply not great enough to allow them to go to fixation and spread through the population. Thus, most populations would remain purely nonallelopathic.

Integrative analysis of the shikonin metabolic network identifies new gene connections and reveals evolutionary insight into shikonin biosynthesis

Plant specialized 1,4-naphthoquinones present a remarkable case of convergent evolution. Species across multiple discrete orders of vascular plants produce diverse 1,4-naphthoquinones via one of several pathways using different metabolic precursors. Evolution of these pathways was preceded by events of metabolic innovation and many appear to share connections with biosynthesis of photosynthetic or respiratory quinones. Here, we sought to shed light on the metabolic connections linking shikonin biosynthesis with its precursor pathways and on the origins of shiknoin metabolic genes. Downregulation of Lithospermum erythrorhizon geranyl diphosphate synthase (LeGPPS), recently shown to have been recruited from a cytoplasmic farnesyl diphosphate synthase (FPPS), resulted in reduced shikonin production and a decrease in expression of mevalonic acid and phenylpropanoid pathway genes. Next, we used LeGPPS and other known shikonin pathway genes to build a coexpression network model for identifying new gene connections to shikonin metabolism. Integrative in silico analyses of network genes revealed candidates for biochemical steps in the shikonin pathway arising from Boraginales-specific gene family expansion. Multiple genes in the shikonin coexpression network were also discovered to have originated from duplication of ubiquinone pathway genes. Taken together, our study provides evidence for transcriptional crosstalk between shikonin biosynthesis and its precursor pathways, identifies several shikonin pathway gene candidates and their evolutionary histories, and establishes additional evolutionary links between shikonin and ubiquinone metabolism. Moreover, we demonstrate that global coexpression analysis using limited transcriptomic data obtained from targeted experiments is effective for identifying gene connections within a defined metabolic network.

Exploration and characterization of chemical stimulators to maximize the wax ester production by Euglena gracilis

Euglena gracilis, a phototrophic protist, is a valuable biomass producer that is often employed in sustainable development efforts. E. gracilis accumulates wax esters as byproducts during anaerobic ATP production via the reductive tricarboxylic acid cycle, utilizing the storage carbohydrate β-1,3-glucan paramylon as the carbon source. Here, we report a library screening for chemical stimulators that accelerate both wax ester production and paramylon consumption. Among the 115 compounds tested, we identified nine compounds that increased wax ester production by more than 2.0-fold relative to the solvent control. In the presence of these nine compounds, the paramylon content decreased compared with the control experiment, and the residual paramylon content varied between 7% and 26% of the initial level. The most active compound, 1,4-diaminoanthracene-9,10-dione (OATQ008), stimulated wax ester production up to 2.7-fold within 24 h, and 93% of the cellular paramylon was consumed. In terms of the structural features of the chemical stimulators, we discuss the potential target sites to stimulate wax ester production in mitochondria under anaerobic conditions.

Light emitting diodes optimisation for secondary metabolites production by Droseraceae plants

The most abundant active compound in Droseraceae is plumbagin, a naphthoquinone widely used for medical purposes due to its antimicrobial, antitussive, antimalarial and anticancer properties. In this work, we created a light-emitting diode (LED) based culture illumination setup as an alternative to fluorescent lamps traditionally used as a light source in plant in vitro cultures. The plants of Drosera binata and Drosera peltata cultured under LED illumination grew equally well and produced similar amounts of biologically active compounds as plants grown under fluorescent lamps. The plants were cultured on two media differing in mineral composition, sucrose content and pH. Secondary metabolites were extracted with ethanol from the plants after harvesting. The extracts were subjected to HPLC and microbiological analyses. We observed differences in morphology and secondary metabolism between plants of the same species grown on different media. However, we did not note significant changes in secondary metabolite yield under assessed lighting conditions. We propose LEDs as a more efficient, eco-friendly and economically reasonable source of light for big scale in vitro production of plumbagin in Drosera species than fluorescent lamps.

Tomato transcriptomic response to Tuta absoluta infestation

BACKGROUND

The South America pinworm, Tuta absoluta, is a destructive pest of tomato that causes important losses worldwide. Breeding of resistant/tolerant tomato cultivars could be an effective strategy for T. absoluta management but, despite the economic importance of tomato, very limited information is available about its response to this treat. To elucidate the defense mechanisms to herbivore feeding a comparative analysis was performed between a tolerant and susceptible cultivated tomato at both morphological and transcriptome level to highlight constitutive leaf barriers, molecular and biochemical mechanisms to counter the effect of T. absoluta attack.

RESULTS

The tolerant genotype showed an enhanced constitutive barrier possibly as result of the higher density of trichomes and increased inducible reactions upon mild infestation thanks to the activation/repression of key transcription factors regulating genes involved in cuticle formation and cell wall strength as well as of antinutritive enzymes, and genes involved in the production of chemical toxins and bioactive secondary metabolites.

CONCLUSIONS

Overall, our findings suggest that tomato resilience to the South America pinworm is achieved by a combined strategy between constitutive and induced defense system. A well-orchestrated modulation of plant transcription regulation could ensure a trade-off between defense needs and fitness costs. Our finding can be further exploited for developing T. absoluta tolerant cultivars, acting as important component of integrated pest management strategy for more sustainable production.

Accessing Chemo- and Regioselective Benzylic and Aromatic Oxidations by Protein Engineering of an Unspecific Peroxygenase

Unspecific peroxygenases (UPOs) enable oxyfunctionalizations of a broad substrate range with unparalleled activities. Tailoring these enzymes for chemo- and regioselective transformations represents a grand challenge due to the difficulties in their heterologous productions. Herein, we performed protein engineering in Saccharomyces cerevisiae using the MthUPO from Myceliophthora thermophila. More than 5300 transformants were screened. This protein engineering led to a significant reshaping of the active site as elucidated by computational modelling. The reshaping was responsible for the increased oxyfunctionalization activity, with improved k_cat/K_m values of up to 16.5-fold for the model substrate 5-nitro-1,3-benzodioxole. Moreover, variants were identified with high chemo- and regioselectivities in the oxyfunctionalization of aromatic and benzylic carbons, respectively. The benzylic hydroxylation was demonstrated to perform with enantioselectivities of up to 95% ee. The proposed evolutionary protocol and rationalization of the enhanced activities and selectivities acquired by MthUPO variants represent a step forward toward the use and implementation of UPOs in biocatalytic synthetic pathways of industrial interest.

UPLC-TOF-MS/MS-Based Metabolomics Analysis Reveals Species-Specific Metabolite Compositions in Pitchers of Nepenthes ampullaria, Nepenthes rafflesiana, and Their Hybrid Nepenthes × hookeriana

Hybridization is key to the evolution and diversity of plants in nature. Nepenthaceae comprises a family of diverse tropical carnivorous pitcher plant species with extensive hybridization. However, there is no study to date on the metabolite expression of hybrids in this family. We performed a non-targeted metabolomics analysis of the pitchers of two Nepenthes species with different dietary habits, namely, the semi-detritivorous N. ampullaria and carnivorous N. rafflesiana with their hybrid (N. × hookeriana) for a comparative study. The whole-pitcher samples were extracted in methanol:chloroform:water (3:1:1) via sonication-assisted extraction and analyzed using ultra-performance liquid chromatography time-of-flight mass spectrometry (UPLC-TOF-MS) followed by data analysis to profile chemical compositions. A total of 1,441 metabolite features were profiled from the three species in which 43.3% of features in the hybrid samples were not found in either of its parents. The partial least squares discriminant analysis (PLS-DA) found 324 metabolite features with variable in projection (VIP) values greater than one in which 55 features were statistically significant. This showed that the hybrid is closer to N. rafflesiana, which is consistent to the previous study on gene and protein expressions. A total of 105 metabolites were putatively identified with manual searches using public metabolite databases. Phenols were detected to be the most abundant secondary metabolites due to a high flavonoid content, especially in N. rafflesiana. The most abundant feature 476.3s:449.102 was found to be the most significant VIP for distinguishing between the three species as a chemical marker. This is the first study comparing metabolites in the carnivory organs of different Nepenthes species with comprehensive profiling and putative identification. The differential metabolite compositions in the pitchers of different species might have ecological implications with the hybrid showing intermediate phenotype between the parents as well as manifesting unique metabolites. However, there is no clear evidence of metabolites related to the differences in dietary habits between the hybrid and the two parent species.

Naphthoquinones and Their Derivatives: Emerging Trends in Combating Microbial Pathogens

In the current era, an ever-emerging threat of multidrug-resistant (MDR) pathogens pose serious health challenges to mankind. Researchers are uninterruptedly putting their efforts to design and develop alternative, innovative strategies to tackle the antibiotic resistance displayed by varied pathogens. Among several naturally derived and chemically synthesized compounds, quinones have achieved a distinct position to defeat microbial pathogens. This review unleashes the structural diversity and promising biological activities of naphthoquinones (NQs) and their derivatives documented in the past two decades. Further, realizing their functional potentialities, researchers were encouraged to approach NQs as lead molecules. We have retrieved information that is dedicated on biological applications (antibacterial, antifungal, antiparasitic) of NQs. The multiple roles of NQs offer them a promising armory to combat microbial pathogens including MDR and the ESKAPE (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) group. In bacteria, NQs may exhibit their function in the following ways (1) plasmid curing, (2) inhibiting efflux pumps (EPs), (3) generating reactive oxygen species (ROS), (4) the inhibition of topoisomerase activity. Sparse but meticulous literature suggests the mechanistic roles of NQs. We have highlighted the possible mechanisms of NQs and how the targeted drug synthesis can be achieved via molecular docking analysis. This bioinformatics-oriented approach will explicitly lead to the development of effective and most potent drugs against targeted pathogens. The mechanistic approaches of emerging molecules like NQs might prove a milestone to defeat the battle against microbial pathogens.

Structure-Property Relationship in Selected Naphtho- and Anthra-Quinone Derivatives on the Basis of Density Functional Theory and Car–Parrinello Molecular Dynamics

Intra- and inter-molecular interactions were studied in 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone and 1,4-dihydroxy-anthraquinone to shed more light on the molecular assembly phenomena. The electronic ground and excited states features of the compounds were investigated to find structure-property dependencies. The theoretical study was carried out on the basis of Density Functional Theory (DFT), its Time-Dependent (TD-DFT) extension, and using Car–Parrinello Molecular Dynamics (CPMD). In order to show how the environmental effects modulate the physico-chemical properties, the simulations were performed in vacuo, with the solvent reaction field (Polarizable Continuum Model (PCM) and water as a solvent) and crystalline phase. The intramolecular hydrogen bonds and the bridged proton dynamics were analyzed in detail. The aromatic rings and electronic structure changes were estimated using the Harmonic Oscillator Model of Aromaticity (HOMA) and Atoms in Molecules (AIM) theory. The Symmetry-Adapted Perturbation Theory (SAPT) was employed for interaction energy decomposition in the studied dimers and trimers. It was found that the presence of a polar solvent decreased the energy barrier for the bridged proton transfer. However, it did not significantly affect the aromaticity and electronic structure. The SAPT results showed that the mutual polarization of the monomers in the dimer was weak and that the dispersion was responsible for most of the intermolecular attraction. The intermolecular hydrogen bonds seem to be much weaker than the intramolecular bridges. The TD-DFT results confirmed that the electronic excitations do not play any significant role in the intramolecular proton transfer. The CPMD results indicated that the protons are very labile in the hydrogen bridges. Short proton transfer and proton-sharing events were observed, and a correlation between them in the twin bridges was noticed, especially for the first investigated compound.

Bicyclic Chalcones as Mitotic Inhibitors for Overcoming Androgen Receptor-Independent and Multidrug-Resistant Prostate Cancer

To improve the biological effects of the lead compound 5'-chloro-2,2'-dihydroxychalcone (Cl-DHC), bicyclic aromatic chalcones were designed, synthesized, and evaluated against androgen-independent prostate cancer (PCa) DU145 and PC-3 cell proliferation. Newly synthesized bi-naphthyl derivatives 2 and 3 suppressed the proliferation of these two cell lines and also taxane-resistant prostate cancer cell lines at a submicromolar level. The two compounds were 4-18 times more potent than the parent molecule Cl-DHC. A structure-activity relationship analysis revealed that the orientation of the 10π-electron ring-A naphthalene had a significant effect on the activity. Mode-of-action studies in KB-VIN cells demonstrated that 2 and 3 arrested cells in mitosis at prometaphase and metaphase followed by induction of sub-G1 accumulation. Thus, 2 and 3 have good potential as leads for continued development of treatments for cancers especially for not only androgen-independent PCa but also multidrug-resistant tumors.

Plumbagin, a Potent Naphthoquinone from Nepenthes Plants with Growth Inhibiting and Larvicidal Activities

Some plant species are less susceptible to herbivore infestation than others. The reason for this is often unknown in detail but is very likely due to an efficient composition of secondary plant metabolites. Strikingly, carnivorous plants of the genus Nepenthes show extremely less herbivory both in the field and in green house. In order to identify the basis for the efficient defense against herbivorous insects in Nepenthes, we performed bioassays using larvae of the generalist lepidopteran herbivore, Spodoptera littoralis. Larvae fed with different tissues from Nepenthes x ventrata grew significantly less when feeding on a diet containing leaf tissue compared with pitcher-trap tissue. As dominating metabolite in Nepenthes tissues, we identified a naphthoquinone, plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone). When plumbagin was added at different concentrations to the diet of S. littoralis larvae, an EC₅₀ value for larval growth inhibition was determined with 226.5 µg g^-1 diet. To further determine the concentration causing higher larval mortality, sweet potato leaf discs were covered with increasing plumbagin concentrations in no-choice-assays; a higher mortality of the larvae was found beyond 60 µg plumbagin per leaf, corresponding to 750 µg g^-1. Plant-derived insecticides have long been proposed as alternatives for pest management; plumbagin and derivatives might be such promising environmentally friendly candidates.

Influence of domestication on specialized metabolic pathways in fruit crops

During the process of plant domestication, the selection and traditional breeding for desired characters such as flavor, juiciness and nutritional value of fruits, probably have resulted in gain or loss of specialized metabolites contributing to these traits. Their appearance in fruits is likely due to the acquisition of novel and specialized metabolic pathways and their regulation, driven by systematic molecular evolutionary events facilitated by traditional breeding. Plants change their armory of specialized metabolism to adapt and survive in diverse ecosystems. This may occur through molecular evolutionary events, such as single nucleotide polymorphism, gene duplication and transposition, leading to convergent or divergent evolution of biosynthetic pathways producing such specialized metabolites. Breeding and selection for improved specific and desired traits (fruit size, color, taste, flavor, etc.) in fruit crops through conventional breeding approaches may further alter content and profile of specialized metabolites. Biosynthetic routes of these metabolites have been studied in various plants. Here, we explore the influence of plant domestication and breeding processes on the selection of biosynthetic pathways of favorable specialized metabolites in fruit crops. An orderly clustered arrangement of genes associated with their production is observed in many fruit crops. We further analyzed selection-based acquisition of specialized metabolic pathways comparing first the metabolic profiles and genes involved in their biosynthesis, followed by the genomic organization of such genes between wild and domesticated horticultural crops. Domestication of crop plants favored the acquisition and retention of metabolic pathways that enhanced the fruit value while eliminated those which produced toxic or unfavorable metabolites. Interestingly, unintentional reorganization of complex metabolic pathways by selection and traditional breeding processes has endowed us with flavorful, juicy and nutritionally rich fruits.

Convergent evolution of plant specialized 1,4-naphthoquinones: metabolism, trafficking, and resistance to their allelopathic effects

Plant 1,4-naphthoquinones encompass a class of specialized metabolites known to mediate numerous plant-biotic interactions. This class of compounds also presents a remarkable case of convergent evolution. The 1,4-naphthoquinones are synthesized by species belonging to nearly 20 disparate orders spread throughout vascular plants, and their production occurs via one of four known biochemically distinct pathways. Recent developments from large-scale biology and genetic studies corroborate the existence of multiple pathways to synthesize plant 1,4-naphthoquinones and indicate that extraordinary events of metabolic innovation and links to respiratory and photosynthetic quinone metabolism probably contributed to their independent evolution. Moreover, because many 1,4-naphthoquinones are excreted into the rhizosphere and they are highly reactive in biological systems, plants that synthesize these compounds also needed to independently evolve strategies to deploy them and to resist their effects. In this review, we highlight new progress made in understanding specialized 1,4-naphthoquinone biosynthesis and trafficking with a focus on how these discoveries have shed light on the convergent evolution and diversification of this class of compounds in plants. We also discuss how emerging themes in metabolism-based herbicide resistance may provide clues to mechanisms plants employ to tolerate allelopathic 1,4-naphthoquinones.

Silent constraints: the hidden challenges faced in plant metabolic engineering

Metabolic engineering is embraced as a method to sustainably enhance production of valuable phytochemicals with beneficial properties. However, successful production of these compounds in plants is not always predictable even when the pathways are fully known, frequently due to the lack of comprehensive understanding of plant metabolism as a whole, and interconnections between different primary, secondary, and hormone metabolic networks. Here, we highlight critical hidden constraints, including substrate availability, silent metabolism, and metabolic crosstalk, that impair engineering strategies. We explore how these constraints have historically been manifested in engineering attempts and propose how modern advancements will enable future strategies to overcome these impediments.