Molecular cloning and characterization of a flavonoid-O-methyltransferase with broad substrate specificity and regioselectivity from Citrus depressa

O-methylation modifications in the biosynthetic pathway of dibenzocyclooctadiene lignans

Schisandra chinensis, a well-known traditional Chinese herb used for hepatitis treatment, contains dibenzocyclooctadiene lignans as its primary active compounds, which undergo extensive multi-site O-methylation. However, O-methyltransferases (OMT) involved in this process have not been previously reported. This study employed transcriptomic analysis of S. chinensis treated with methyl jasmonate, alongside expression profiling, phylogenetic analysis, and heterologous expression to characterize the functional roles of OMTs. The study identified 4 OMTs: SchiOMT4, SchiOMT12, SchiOMT16, and SchiOMT22, which catalyze C-3 O-methylation of caffeic acid and Caffeyl aldehyde to form ferulic acid and coniferyl aldehyde. Additionally, SchiOMT12 and SchiOMT16 methylated gomisin L2 at C-3, while SchiOMT16 also O-methylation schisanhenol at C-14 and performed sequential O-methylation at C-3 and C-12 of gomisin J. Molecular docking further clarified the regioselectivity of SchiOMT16 and SchiOMT12, elucidating the differences in their catalytic activities. This study is the first to identify methyltransferases involved in the subsequent modifications of dibenzocyclooctadiene lignans, underscoring the broad substrate range, selective O-methylation, and regulatory importance of OMTs in their biosynthesis.

Tailoring the Regioselectivity of Lentinula edodes O-Methyltransferases for Precise O-Methylation of Flavonoids

A novel O-methyltransferase, LeOMT4, from Lentinula edodes was identified, expressed, and characterized. Although its catalytic activity was lower than that of the previously reported LeOMT2, LeOMT4 displayed strong regioselectivity for the meta-hydroxy group across different catecholic compounds, producing e.g., ferulic acid with an almost exclusive regioisomeric ratio of 98:2 and homoeriodictyol with a ratio of 82:18 (3'-product:4'-product). Leveraging the high sequence and predicted structural similarity between LeOMT2 and LeOMT4, key sites for the tailoring of LeOMT2 were identified through site-directed mutagenesis. This approach aimed for robust mutants retaining the high specific activity of LeOMT2, while enhancing regioselectivity. A single amino acid substitution, F182Y, enabled a regioisomeric ratio of 91:9 for the production of homoeriodictyol. Notably, another single amino acid substitution, I53M reversed the regioselectivity to 2:98 in favor of hesperetin. This strategy enables the selective production of sought-after pharmacologically active flavonoids (butein) and flavor-active flavonoids (homoeriodictyol, hesperetin, hesperetin dihydrochalcone).

Enzymatic Synthesis and Evaluation of Eight Methylated Quercetin Products: In Vitro Chemical Properties and Adipogenesis Regulation

The methoxylated modification of flavonoids has been reported to enhance stability and permeability; however, its effect on the improvement of activity is not clear. In this study, Citrus depressa flavonoid O-methyltransferase 5 and Sorghum vulgare 7-O-methyltransferase were recombinantly expressed and successfully converted quercetin (QUE) into eight methoxylated products, which were isolated and identified with a purity exceeding 95%. All products except rhamnetin (RHA) showed improved stability, while only 5,7,3',4'-EMQ, 7,3',4'-TMQ, and 3,7,3',4'-EMQ had higher uptake ratios. Compared to QUE, 5,7,3',4'-EMQ and RHA significantly reduced the intracellular triglyceride level, while 3,5,7,3',4'-PMQ, 3,3',4'-TMQ and 3,7,3',4'-EMQ increased it. 5,7,3',4'-EMQ and RHA also significantly downregulated both the mRNA and protein levels of peroxisome proliferator-activated receptor γ, while 3,5,7,3',4'-PMQ and 3,7,3',4'-EMQ upregulated PPARγ at the transcriptional level to about ten times higher than that of QUE. The structure-activity relationship analysis highlighted the importance of C3-OH retention and dual methoxylation of the A-ring. In summary, this study efficiently produced eight structurally well-defined QUE methoxylation products via biotransformation, established an in vitro initial structure-activity relationship for regulating adipogenesis, and provided a potential structure for PPARγ regulation, a central target of lipid metabolism.

Two Caffeoyl-CoA O-methyltransferase-like enzyme are involved in the biosynthesis of polymethoxyflavones in Citrus reticulata 'Chachiensis'

Polymethoxyflavones (PMFs) are abundant in citrus plants and exhibit remarkable biological activities in the inhibition of the occurrence and development of various tumours. O-methyltransferase (OMTs)-mediated methylation plays a crucial role in the biosynthesis of PMFs. However, the OMTs catalyzing the methylation of hydroxyflavonoids in citrus have not been fully identified. In this study, we identified two novel CCoAOMT-like enzymes in Citrus reticulata 'Chachiensis', designated CrcCCoAOMT7-1 and CrcCCoAOMT7-2. The transient overexpression and virus-induced gene silencing experiments further confirmed the essential role of CrcCCoAOMT7-1/7-2 in the regulation of PMF synthesis in citrus. Furthermore, in vitro functional analysis showed that CrcCCoAOMT7-1 preferentially catalyzes flavones, while CrcCCoAOMT7-2 exhibits a preference for flavanones and quercetin (flavonol). The two enzymes methylate the 3'-, 4'-, 3-, 6-, 8-OH sites and the 3'-, 4'-, 6-, 7-, 8-OH sites with hydroxyflavonoids, respectively, covering nearly all methylation sites during the biosynthesis of PMFs. Functional diversification of CCoAOMTs expended our understanding of PMFs biosynthesis modes in citrus. This study enriches the PMF biosynthetic pathway in citrus and lays the foundation for the selection and breeding of high-quality Citrus reticulata 'Chachiensis' mandarins, as well as the synthesis of natural anticancer PMFs through genetic engineering.

Cytotoxic Effects of Citrus Peels on Breast Tumor: Opportunities for Waste to Raw Material Conversion

Citrus species have long been known for their rich nutritional value. Recent research has shed light on their therapeutic potential, particularly in cancer treatment. Citrus peels, on the other hand, often discarded as waste, contain a wealth of bioactive compounds, such as flavonoids, coumarins, and essential oil components, which have proven medicinal properties. Converting Citrus peels from waste products to medicinal raw materials is a crucial approach in both healthcare and sustainability. Therefore, the present study aims to investigate the cytotoxic potential of the peels of Citrus aurantium L., Citrus maxima (Burm.) Merr. (syn. Citrus grandis), Citrus medica L. and Citrus paradisi Macfad. cultivated in Türkiye, and to find out the compounds responsible for the cytotoxic activity. The cytotoxic effects of the peel extracts were evaluated on MCF-7 cell lines according to bioactivity-guided fractionation and isolation assay procedures. The compounds CAS-5 (isomeranzin), CAS-10 (3-methoxy nobiletin), CAS-11 (nobiletin), and CAS-12 (tangeretin) were isolated. In silico analyses conducted on the isolated compounds provided supporting information for the results obtained from in vitro experiments regarding their anticancer activity. Indeed, one of the key components of Citrus fruits is polymethoxy flavonoids (PMFs), a group of bioactive constituents recognized for their anti-inflammatory, antioxidant, and anticancer activities. As a valuable byproduct of Citrus waste, PMFs offer a dual benefit by reducing waste while providing a natural source of bioactive compounds and making them an exciting research area in cancer management. The therapeutic promise of PMFs lies not only in their ability to combat cancer but also in their potential to contribute to sustainable practices.

Complete biosynthetic pathway of furochromones in Saposhnikovia divaricata and its evolutionary mechanism in Apiaceae plants

Furochromones are specific bioactive secondary metabolites of many Apiaceae plants. Their biosynthesis remains largely unexplored. In this work, we dissect the complete biosynthetic pathway of major furochromones in the medicinal plant Saposhnikovia divaricata by characterizing prenyltransferase, peucenin cyclase, methyltransferase, hydroxylase, and glycosyltransferases. De novo biosynthesis of prim-O-glucosylcimifugin and 5-O-methylvisamminoside is realized in Nicotiana benthamiana leaves. Through comparative genomic and transcriptomic analyses, we further find that proximal duplication and high expression of a pentaketide chromone synthase gene SdPCS, together with the presence of a lineage-specific peucenin cyclase gene SdPC, lead to the predominant accumulation of furochromones in the roots of S. divaricata among surveyed Apiaceae plants. This study paves the way for metabolic engineering production of furochromones, and sheds light into evolutionary mechanisms of furochromone biosynthesis among Apiaceae plants.

Proteomics: An Essential Tool to Study Plant-Specialized Metabolism

Plants are a valuable source of specialized metabolites that provide a plethora of therapeutic applications. They are natural defenses that plants use to adapt and respond to their changing environment. Decoding their biosynthetic pathways and understanding how specialized plant metabolites (SPMs) respond to biotic or abiotic stress will provide vital knowledge for plant biology research and its application for the future sustainable production of many SPMs of interest. Here, we focus on the proteomic approaches and strategies that help with the study of plant-specialized metabolism, including the: (i) discovery of key enzymes and the clarification of their biosynthetic pathways; (ii) study of the interconnection of both primary (providers of carbon and energy for SPM production) and specialized (secondary) metabolism; (iii) study of plant responses to biotic and abiotic stress; (iv) study of the regulatory mechanisms that direct their biosynthetic pathways. Proteomics, as exemplified in this review by the many studies performed to date, is a powerful tool that forms part of omics-driven research. The proteomes analysis provides an additional unique level of information, which is absent from any other omics studies. Thus, an integrative analysis, considered versus a single omics analysis, moves us more closely toward a closer interpretation of real cellular processes. Finally, this work highlights advanced proteomic technologies with immediate applications in the field.

Genome-Wide Characterization of Solanum tuberosum CCoAOMT Gene Family and Identification of StCCoAOMT Genes Involved in Anthocyanin Biosynthesis

BACKGROUND

The caffeoyl-CoA-O methyltransferase (CCoAOMT) family plays essential roles in the methylation of various secondary metabolites, including anthocyanins. Despite the wide identification of the CCoAOMT family in plants, the characterization and function of CCoAOMT protein members in Solanum tuberosum remain poorly understood.

METHODS AND RESULTS

In this study, a total of 12 StCCoAOMT members were identified in the genome of S. tuberosum using the Blastp and HMM search and were unevenly located on eight chromosomes. Collinearity analysis revealed that four tandem duplicated gene pairs and two segmental duplicated gene pairs existed in the S. tuberosum genome, demonstrating that duplication events play a key role in the expansion of the CCoAOMT family. All StCCoAOMTs were clustered into group I and group II based on phylogenetic analysis, which was further verified by the conserved motifs and gene structures analysis. The cis-acting elements analysis illustrated that StCCoAOMTs might be important for photosynthesis, hormone responses, and abiotic stress. Expression analysis demonstrated that StCCoAOMT genes have diverse transcript levels in various tissues and that StCCoAOMT10 was significantly expressed in purple potatoes with abundant anthocyanin content according to RNA-seq data and qRT-PCR assays. In addition, the subcellular localization assay validated that the StCCoAOMT10 protein was mainly localized in the cytoplasm and nucleus.

CONCLUSIONS

These results will be of great importance for a better understanding of the features of CCoAOMT family members, especially of the candidate genes involved in the methylation of anthocyanins in S. tuberosum, and also for improving the nutritional quality of S. tuberosum.

Kinetic and in silico structural characterization of norbelladine O-methyltransferase of Amaryllidaceae alkaloids biosynthesis

Amaryllidaceae alkaloids are a diverse group of alkaloids exclusively reported from the Amaryllidaceae plant family. In planta, their biosynthesis is still not fully characterized; however, a labeling study established 4'-O-methylnorbelladine as the key intermediate compound of the pathway. Previous reports have characterized O-methyltransferases from several Amaryllidaceae species. Nevertheless, the formation of the different O-methylnorbelladine derivatives (3'-O-methylnorbelladine, 4'-O-methylnorbelladine, and 3'4'-O-dimethylnorbelladine), the role, and the preferred substrates of O-methyltransferases are not clearly understood. In this study, we performed the biochemical characterization of an O-methyltransferase candidate from Narcissus papyraceus (NpOMT) in vitro and in vivo, following biotransformation of norbelladine in Nicotiana benthamiana having transient expression of NpOMT. Docking analysis was further used to investigate substrate preferences, as well as key interacting residues of NpOMT. Our study shows that NpOMT methylates norbelladine preferentially at the 4'-OH position in vitro and in planta. Interestingly, NpOMT also catalyzed the synthesis of 3',4'-O-dimethylnorbelladine from norbelladine and 4'-O-methylnorbelladine during in vitro enzymatic assay. Furthermore, we show that NpOMT methylates 3,4-dihydroxybenzylaldehyde and caffeic acid in a nonregiospecific manner to produce meta/para monomethylated products. This study reveals a novel catalytic potential of an Amaryllidaceae O-methyltransferase and its ability to regioselectively methylate norbelladine in the heterologous host N. benthamiana.

Diversity and regioselectivity of O-methyltransferases catalyzing the formation of O-methylated flavonoids

Flavonoids and their methylated derivatives have immense market potential in the food and biomedical industries due to their multiple beneficial effects, such as antimicrobial, anti-inflammatory, and anticancer activities. The biological synthesis of flavonoids and their derivatives is often accomplished via the use of genetically modified microorganisms to ensure large-scale production. Therefore, it is pivotal to understand the properties of O-methyltransferases (OMTs) that mediate the methylation of flavonoids. However, the properties of these OMTs are governed by their: sources, substrate specificity, amino acid residues in the active sites, and the intricate mechanism. In order to obtain a clue for the selection of suitable OMTs for the biosynthesis of a target methylated flavonoid, we made a comprehensive review of the currently reported results, with a particular focus on their comparative regioselectivity for different flavonoid substrates. Additionally, the possible mechanisms for the diversity of this class of enzymes were explored using molecular simulation technology. Finally, major gaps in our understanding and areas for future studies were discussed. The findings of this study may be useful in selecting genes that encode OMTs and designing enzyme-based processes for synthesizing O-methylated flavonoids.

Beyond pathways: Accelerated flavonoids candidate identification and novel exploration of enzymatic properties using combined mapping populations of wheat

Although forward-genetics-metabolomics methods such as mGWAS and mQTL have proven effective in providing myriad loci affecting metabolite contents, they are somehow constrained by their respective constitutional flaws such as the hidden population structure for GWAS and insufficient recombinant rate for QTL. Here, the combination of mGWAS and mQTL was performed, conveying an improved statistical power to investigate the flavonoid pathways in common wheat. A total of 941 and 289 loci were, respectively, generated from mGWAS and mQTL, within which 13 of them were co-mapped using both approaches. Subsequently, the mGWAS or mQTL outputs alone and their combination were, respectively, utilized to delineate the metabolic routes. Using this approach, we identified two MYB transcription factor encoding genes and five structural genes, and the flavonoid pathway in wheat was accordingly updated. Moreover, we have discovered some rare-activity-exhibiting flavonoid glycosyl- and methyl-transferases, which may possess unique biological significance, and harnessing these novel catalytic capabilities provides potentially new breeding directions. Collectively, we propose our survey illustrates that the forward-genetics-metabolomics approaches including multiple populations with high density markers could be more frequently applied for delineating metabolic pathways in common wheat, which will ultimately contribute to metabolomics-assisted wheat crop improvement.

Discovery of a novel flavonol O-methyltransferase possessing sequential 4'- and 7-O-methyltransferase activity from Camptotheca acuminata Decne

The biosynthetic route for flavonol in Camptotheca acuminata has been recently elucidated from a chemical point of view. However, the genes involved in flavonol methylation remain unclear. It is a critical step for fully uncovering the flavonol metabolism in this ancient plant. In this study, the multi-omics resource of this plant was utilized to perform flavonol O-methyltransferase-oriented mining and screening. Two genes, CaFOMT1 and CaFOMT2 are identified, and their recombinant CaFOMT proteins are purified to homogeneity. CaFOMT1 exhibits strict substrate and catalytic position specificity for quercetin, and selectively methylates only the 4'-OH group. CaFOMT2 possesses sequential O-methyltransferase activity for the 4'-OH and 7-OH of quercetin. These CaFOMT genes are enriched in the leaf and root tissues. The catalytic dyad and critical substrate-binding sites of the CaFOMTs are determined by molecular docking and further verified through site-mutation experiments. PHE181 and MET185 are designated as the critical sites for flavonol substrate selectivity. Genomic environment analysis indicates that CaFOMTs evolved independently and that their ancestral genes are different from that of the known Ca10OMT. This study provides molecular insights into the substrate-binding pockets of two new CaFOMTs responsible for flavonol metabolism in C. acuminata.

Neofunctionalization of an OMT cluster dominates polymethoxyflavone biosynthesis associated with the domestication of citrus

Polymethoxyflavones (PMFs) are a class of abundant specialized metabolites with remarkable anticancer properties in citrus. Multiple methoxy groups in PMFs are derived from methylation modification catalyzed by a series of hydroxylases and O-methyltransferases (OMTs). However, the specific OMTs that catalyze the systematic O-methylation of hydroxyflavones remain largely unknown. Here, we report that PMFs are highly accumulated in wild mandarins and mandarin-derived accessions, while undetectable in early-diverging citrus species and related species. Our results demonstrated that three homologous genes, CreOMT3, CreOMT4, and CreOMT5, are crucial for PMF biosynthesis in citrus, and their encoded methyltransferases exhibit multisite O-methylation activities for hydroxyflavones, producing seven PMFs in vitro and in vivo. Comparative genomic and syntenic analyses indicated that the tandem CreOMT3, CreOMT4, and CreOMT5 may be duplicated from CreOMT6 and contributes to the genetic basis of PMF biosynthesis in the mandarin group through neofunctionalization. We also demonstrated that N17 in CreOMT4 is an essential amino acid residue for C3-, C5-, C6-, and C3'-O-methylation activity and provided a rationale for the functional deficiency of OMT6 to produce PMFs in early-diverging citrus and some domesticated citrus species. A 1,041-bp deletion in the CreOMT4 promoter, which is found in most modern cultivated mandarins, has reduced the PMF content relative to that in wild and early-admixture mandarins. This study provides a framework for reconstructing PMF biosynthetic pathways, which may facilitate the breeding of citrus fruits with enhanced health benefits.

Biosynthetic Pathway and Bioactivity of Vanillin, a Highly Abundant Metabolite Distributed in the Root Cortex of Tea Plants (Camellia sinensis)

Volatiles are important for plant root stress resistance. The diseases in tea root are serious, causing major losses. The volatile composition in tea root and whether it can resist diseases remain unclear. In this study, the volatile composition in different tea tissues was revealed. The vanillin content was higher in the root (mainly in root cortex) than in aerial parts. The antifungal effects of vanillin on pathogenic fungi in tea root were equal to or greater than those of other metabolites. O-methyltransferase (CsOMT), a key enzyme in one of two biosynthetic pathways of vanillin, converted protocatechualdehyde to vanillin in vitro. Furthermore, its characteristics and kinetic parameters were studied. In Arabidopsis thaliana protoplasts, the transiently expressed CsOMT was localized in the cytoplasm and nucleus. These findings have clarified the formation and bioactivities of volatiles in tea roots and provided a theoretical basis for understanding how tea plants resist root diseases.

Genome-wide characterization of COMT family and regulatory role of CsCOMT19 in melatonin synthesis in Camellia sinensis

BACKGROUND

Caffeic acid O-methyltransferase (COMT) is a key enzyme that regulates melatonin synthesis and is involved in regulating the growth, development, and response to abiotic stress in plants. Tea plant is a popular beverage consumed worldwide, has been used for centuries for its medicinal properties, including its ability to reduce inflammation, improve digestion, and boost immune function. By analyzing genetic variation within the COMT family, while helping tea plants resist adversity, it is also possible to gain a deeper understanding of how different tea varieties produce and metabolize catechins, then be used to develop new tea cultivars with desired flavor profiles and health benefits.

RESULTS

In this study, a total of 25 CsCOMT genes were identified based on the high-quality tea (Camellia sinensis) plant genome database. Phylogenetic tree analysis of CsCOMTs with COMTs from other species showed that COMTs divided into four subfamilies (Class I, II, III, IV), and CsCOMTs was distributed in Class I, Class II, Class III. CsCOMTs not only undergoes large-scale gene recombination in pairs internally in tea plant, but also shares 2 and 7 collinear genes with Arabidopsis thaliana and poplar (Populus trichocarpa), respectively. The promoter region of CsCOMTs was found to be rich in cis-acting elements associated with plant growth and stress response. By analyzing the previously transcriptome data, it was found that some members of CsCOMT family exhibited significant tissue-specific expression and differential expression under different stress treatments. Subsequently, we selected six CsCOMTs to further validated their expression levels in different tissues organ using qRT-PCR. In addition, we silenced the CsCOMT19 through virus-induced gene silencing (VIGS) method and found that CsCOMT19 positively regulates the synthesis of melatonin in tea plant.

CONCLUSION

These results will contribute to the understanding the functions of CsCOMT gene family and provide valuable information for further research on the role of CsCOMT genes in regulating tea plant growth, development, and response to abiotic stress.

Strategies to fortify the nutritional values of polished rice by implanting selective traits from brown rice: A nutrigenomics-based approach

Whole-grain cereals are important components of a healthy diet. It reduces the risk of many deadly diseases like cardiovascular diseases, diabetes, cancer, etc. Brown rice is an example of whole grain food, which is highly nutritious due to the presence of various bioactive compounds (flavonoids, phenolics, vitamins, phytosterols, oils, etc.) associated with the rice bran layer of brown rice. White rice is devoid of the nutritious rice bran layer and thus lacks the bioactive compounds which are the major attractants of brown rice. Therefore, to confer health benefits to the public at large, the nutrigenomic potential of white rice may be improved by integrating the phytochemicals associated with the rice bran layer of brown rice into it via biofortification processes like conventional breeding, agronomic practices, metabolic engineering, CRISPR/Cas9 technology, and RNAi techniques. Thus, this review article focuses on improving the nutritional qualities of white/polished rice through biofortification processes, utilizing new breeding technologies (NBTs).

A multifunctional true caffeoyl coenzyme A O-methyltransferase enzyme participates in the biosynthesis of polymethoxylated flavones in citrus

Polymethoxylated flavones (PMFs) have received extensive attention due to their abundant bioactivities. Citrus peels specifically accumulate abundant PMFs, and methylation modification is a key step in PMF biosynthesis; however, the function of reported O-methyltransferase (OMT) in citrus is insufficient to elucidate the complete methylation process of PMFs. In this study, we analyzed the accumulation pattern of PMFs in the flavedo of the sweet orange (Citrus sinensis) cultivar "Bingtangcheng" at different developmental stages. We found that accumulation of PMFs was completed at the early stage of fruit development (60-d after flowering). Furthermore, we characterized a true caffeoyl-CoA O-methyltransferase (named CsCCoAOMT1) from C. sinensis. Functional analysis in vitro showed that CsCCoAOMT1 preferred flavonoids to caffeoyl-CoA and esculetin. This enzyme efficiently methylated the 6-, 7- 8-, and 3'-OH of a wide array of flavonoids with vicinal hydroxyl groups with a strong preference for quercetin (flavonol) and flavones. The transient overexpression and virus-induced gene silencing experiments verified that CsCCoAOMT1 could promote the accumulation of PMFs in citrus. These results reveal the function of true CCoAOMTs and indicate that CsCCoAOMT1 is a highly efficient multifunctional O-methyltransferase involved in the biosynthesis of PMFs in citrus.

Functional Analysis of a Methyltransferase Involved in Anthocyanin Biosynthesis from Blueberries (Vaccinium corymbosum)

Anthocyanins are natural water-soluble pigments that widely exist in plants, with various biological activities, including antioxidant, anti-obesity, and anti-diabetic activities. Currently, monomeric anthocyanins are mainly obtained through natural sources, which limits their availability. In the biosynthesis of anthocyanins, anthocyanin methyltransferases are recognized to play important roles in the water solubility and structural stability of anthocyanins. Blueberries are a rich source of anthocyanins with more than 30 chemical structures. However, the enzymes that were responsible for the methylation of anthocyanidin cores in blueberries had not been reported. Here, blueberries (Vaccinium corymbosum) have been selected as the candidate for characterization of the key enzyme. Phylogenic analysis, enzymatic activity assay, homology modeling, molecular simulation, protein expression and purification assay, site-directed mutation, isothermal titration calorimetry assay, and enzyme kinetic assay were used to identify the enzymatic function and molecular mechanism of VcOMT, which was responsible for the methylation of anthocyanidin cores. VcOMT could use delphinidin as a substrate but not cyanidin, petunidin, anthocyanins, flavonols, and flavonol glycosides. Ile191 and Glu198 were both identified as important amino acid residues for the binding interactions of anthocyanidins with VcOMT.

Functional Diversification and Structural Origins of Plant Natural Product Methyltransferases

In plants, methylation is a common step in specialized metabolic pathways, leading to a vast diversity of natural products. The methylation of these small molecules is catalyzed by S-adenosyl-l-methionine (SAM)-dependent methyltransferases, which are categorized based on the methyl-accepting atom (O, N, C, S, or Se). These methyltransferases are responsible for the transformation of metabolites involved in plant defense response, pigments, and cell signaling. Plant natural product methyltransferases are part of the Class I methyltransferase-superfamily containing the canonical Rossmann fold. Recent advances in genomics have accelerated the functional characterization of plant natural product methyltransferases, allowing for the determination of substrate specificities and regioselectivity and further realizing the potential for enzyme engineering. This review compiles known biochemically characterized plant natural product methyltransferases that have contributed to our knowledge in the diversification of small molecules mediated by methylation steps.

The draft genome and multi-omics analyses reveal new insights into geo-herbalism properties of Citrus grandis 'Tomentosa'

Citrus grandis 'Tomentosa' (CGT) (Huajuhong, HJH) is a widely used medicinal plant, which is mainly produced in Guangdong and Guangxi provinces of South China. Particularly, HJH from Huazhou (HZ) county of Guangdong province has been well-regarded as the best national product for geo-herbalism. But the reasons for geo-herbalism property in HJH from HZ county remains a mystery. Therefore, a multi-omics approach was applied to identify the nature of the geo-herbalism in CGT from three different regions. The comprehensive screening of differential metabolites revealed that the Nobiletin content was significantly different in HZ region compared to other regions, and could be employed as a key indicator to determine the geo-herbalism. Furthermore, the high-quality genome (N50 of 9.12 Mb), coupled with genomics and transcriptomics analyses indicated that CGT and Citrus grandis are closely related, with a predicted divergence time of 19.1 million years ago (MYA), and no recent WGD occurred in the CGT, and the bioactive ingredients of CGT were more abundant than that of Citrus grandis. Interestingly, Nobiletin (Polymethoxyflavones) content was identified as a potential indicator of geo-herbalism, and O-methyltransferase (OMT) genes are involved in the synthesis of Polymethoxyflavones. Further multi-omics analysis led to the identification of a novel OMT gene (CtgOMT1) whose transient overexpression displayed significantly higher Nobiletin content, suggesting that CtgOMT1 was involved in the synthesis of Nobiletin. Overall, our findings provide new data resources for geo-herbalism evaluation, germplasm conservation and insights into Nobiletin biosynthesis pathways for the medicinal plant C. grandis 'Tomentosa'.

Heterologous production of Cannabis sativa-derived specialised metabolites of medicinal significance - Insights into engineering strategies

Cannabis sativa L. has been known for at least 2000 years as a source of important, medically significant specialised metabolites and several bio-active molecules have been enriched from multiple chemotypes. However, due to the many levels of complexity in both the commercial cultivation of cannabis and extraction of its specialised metabolites, several heterologous production approaches are being pursued in parallel. In this review, we outline the recent achievements in engineering strategies used for heterologous production of cannabinoids, terpenes and flavonoids along with their strength and weakness. We provide an overview of the specialised metabolism pathway in C. sativa and a comprehensive list of the specialised metabolites produced along with their medicinal significance. We highlight cannabinoid-like molecules produced by other species. We discuss the key biosynthetic enzymes and their heterologous production using various hosts such as microbial and eukaryotic systems. A brief discussion on complementary production strategies using co-culturing and cell-free systems is described. Various approaches to optimise specialised metabolite production through co-expression, enzyme engineering and pathway engineering are discussed. We derive insights from recent advances in metabolic engineering of hosts with improved precursor supply and suggest their application for the production of C. sativa speciality metabolites. We present a collation of non-conventional hosts with speciality traits that can improve the feasibility of commercial heterologous production of cannabis-based specialised metabolites. We provide a perspective of emerging research in synthetic biology, allied analytical techniques and plant heterologous platforms as focus areas for heterologous production of cannabis specialised metabolites in the future.

Identification and characterization of two Isatis indigotica O-methyltransferases methylating C-glycosylflavonoids

Isatis indigotica accumulates several active substances, including C-glycosylflavonoids, which have important pharmacological activities and health benefits. However, enzymes catalyzing the methylation step of C-glycosylflavonoids in I. indigotica remain unknown. In this study, three O-methyltransferases (OMTs) were identified from I. indigotica that have the capacity for O-methylation of the C-glycosylflavonoid isoorientin. The Type II OMTs IiOMT1 and IiOMT2 efficiently catalyze isoorientin to form isoscoparin, and decorate one of the aromatic vicinal hydroxyl groups on flavones and methylate the C6, C8, and 3'-hydroxyl positions to form oroxylin A, wogonin, and chrysoeriol, respectively. However, the Type I OMT IiOMT3 exhibited broader substrate promiscuity and methylated the C7 and 3'-hydroxyl positions of flavonoids. Further site-directed mutagenesis studies demonstrated that five amino acids of IiOMT1/IiOMT2 (D121/D100, D173/D149, A174/A150R, N200/N176, and D248/D233) were critical residues for their catalytic activity. Additionally, only transient overexpression of Type II OMTs IiOMT1 and IiOMT2 in Nicotiana benthamiana significantly increased isoscoparin accumulation, indicating that the Type II OMTs IiOMT1 and IiOMT2 could catalyze the methylation step of C-glycosylflavonoid, isoorientin at the 3'-hydroxyl position. This study provides insights into the biosynthesis of methylated C-glycosylflavonoids, and IiOMTs could be promising catalysts in the synthesis of bioactive compounds.

Fruit Peels: Food Waste as a Valuable Source of Bioactive Natural Products for Drug Discovery

Fruits along with vegetables are crucial for a balanced diet. These not only have delicious flavors but are also reported to decrease the risk of contracting various chronic diseases. Fruit by-products are produced in huge quantity during industrial processing and constitute a serious issue because they may pose a harmful risk to the environment. The proposal of employing fruit by-products, particularly fruit peels, has gradually attained popularity because scientists found that in many instances peels displayed better biological and pharmacological applications than other sections of the fruit. The aim of this review is to highlight the importance of fruit peel extracts and natural products obtained in food industries along with their other potential biological applications.

Comparative transcriptomics of wild and commercial Citrus during early ripening reveals how domestication shaped fruit gene expression

BACKGROUND

Interspecific hybridizations and admixtures were key in Citrus domestication, but very little is known about their impact at the transcriptomic level. To determine the effects of genome introgressions on gene expression, the transcriptomes of the pulp and flavedo of three pure species (citron, pure mandarin and pummelo) and four derived domesticated genetic admixtures (sour orange, sweet orange, lemon and domesticated mandarin) have been analyzed at color break.

RESULTS

Many genes involved in relevant physiological processes for domestication, such sugar/acid metabolism and carotenoid/flavonoid synthesis, were differentially expressed among samples. In the low-sugar, highly acidic species lemon and citron, many genes involved in sugar metabolism, the TCA cycle and GABA shunt displayed a reduced expression, while the P-type ATPase CitPH5 and most subunits of the vacuolar ATPase were overexpressed. The red-colored species and admixtures were generally characterized by the overexpression in the flavedo of specific pivotal genes involved in the carotenoid biosynthesis, including phytoene synthase, ζ-carotene desaturase, β-lycopene cyclase and CCD4b, a carotenoid cleavage dioxygenase. The expression patterns of many genes involved in flavonoid modifications, especially the flavonoid and phenylpropanoid O-methyltransferases showed extreme diversity. However, the most noticeable differential expression was shown by a chalcone synthase gene, which catalyzes a key step in the biosynthesis of flavonoids. This chalcone synthase was exclusively expressed in mandarins and their admixed species, which only expressed the mandarin allele. In addition, comparisons between wild and domesticated mandarins revealed that the major differences between their transcriptomes concentrate in the admixed regions.

CONCLUSION

In this work we present a first study providing broad evidence that the genome introgressions that took place during citrus domestication largely shaped gene expression in their fruits.

Molecular Characterisation of Flavanone O-methylation in Eucalyptus

Flavonoids are ubiquitous polyphenolic compounds in plants, long recognised for their health-promoting properties in humans. Methylated flavonoids have received increasing attention due to the potential of methylation to enhance medicinal efficacy. Recently, Eucalyptus species with high levels of the O-methylated flavanone pinostrobin have been identified. Pinostrobin has potential commercial value due to its numerous pharmacological and functional food benefits. Little is known about the identity or mode of action of the enzymes involved in methylating flavanones. This study aimed to identify and characterise the methyltransferase(s) involved in the regiospecific methylation of pinostrobin in Eucalyptus and thereby add to our limited understanding of flavanone biosynthesis in plants. RNA-seq analysis of leaf tips enabled the isolation of a gene encoding a flavanone 7-O-methyltransferase (EnOMT1) in Eucalyptus. Biochemical characterisation of its in vitro activity revealed a range of substrates upon which EnOMT1 acts in a regiospecific manner. Comparison to a homologous sequence from a Eucalyptus species lacking O-methylated flavonoids identified critical catalytic amino acid residues within EnOMT1 responsible for its activity. This detailed molecular characterisation identified a methyltransferase responsible for chemical ornamentation of the core flavanone structure of pinocembrin and helps shed light on the mechanism of flavanone biosynthesis in Eucalyptus.

Diversification of Chemical Structures of Methoxylated Flavonoids and Genes Encoding Flavonoid-O-Methyltransferases

The O-methylation of specialized metabolites in plants is a unique decoration that provides structural and functional diversity of the metabolites with changes in chemical properties and intracellular localizations. The O-methylation of flavonoids, which is a class of plant specialized metabolites, promotes their antimicrobial activities and liposolubility. Flavonoid O-methyltransferases (FOMTs), which are responsible for the O-methylation process of the flavonoid aglycone, generally accept a broad range of substrates across flavones, flavonols and lignin precursors, with different substrate preferences. Therefore, the characterization of FOMTs with the physiology roles of methoxylated flavonoids is useful for crop improvement and metabolic engineering. In this review, we summarized the chemodiversity and physiology roles of methoxylated flavonoids, which were already reported, and we performed a cross-species comparison to illustrate an overview of diversification and conserved catalytic sites of the flavonoid O-methyltransferases.

One-Pot Biocatalytic In Vivo Methylation-Hydroamination of Bioderived Lignin Monomers to Generate a Key Precursor to L-DOPA

Electron-rich phenolic substrates can be derived from the depolymerisation of lignin feedstocks. Direct biotransformations of the hydroxycinnamic acid monomers obtained can be exploited to produce high-value chemicals, such as α-amino acids, however the reaction is often hampered by the chemical autooxidation in alkaline or harsh reaction media. Regioselective O-methyltransferases (OMTs) are ubiquitous enzymes in natural secondary metabolic pathways utilising an expensive co-substrate S-adenosyl-l-methionine (SAM) as the methylating reagent altering the physicochemical properties of the hydroxycinnamic acids. In this study, we engineered an OMT to accept a variety of electron-rich phenolic substrates, modified a commercial E. coli strain BL21 (DE3) to regenerate SAM in vivo, and combined it with an engineered ammonia lyase to partake in a one-pot, two whole cell enzyme cascade to produce the l-DOPA precursor l-veratrylglycine from lignin-derived ferulic acid.

Evolution of DIMBOA-Glc O-Methyltransferases from Flavonoid O-Methyltransferases in the Grasses

O-Methylated benzoxazinoids (BXs) and flavonoids are widespread defenses against herbivores and pathogens in the grasses (Poaceae). Recently, two flavonoid O-methyltransferases (FOMTs), ZmFOMT2 and ZmFOMT3, have been reported to produce phytoalexins in maize (Zea mays). ZmFOMT2 and ZmFOMT3 are closely related to the BX O-methyltransferases (OMTs) ZmBX10-12 and ZmBX14, suggesting a common evolutionary origin in the Poaceae. Here, we studied the evolution and enzymatic requirements of flavonoid and BX O-methylation activities in more detail. Using BLAST searches and phylogenetic analyses, we identified enzymes homologous to ZmFOMT2 and ZmFOMT3, ZmBX10-12, and ZmBX14 in several grasses, with the most closely related candidates found almost exclusively in species of the Panicoideae subfamily. Biochemical characterization of candidate enzymes from sorghum (Sorghum bicolor), sugar cane (Saccharum spp.), and teosinte (Zea nicaraguensis) revealed either flavonoid 5-O-methylation activity or DIMBOA-Glc 4-O-methylation activity. However, DIMBOA-Glc 4-OMTs from maize and teosinte also accepted flavonols as substrates and converted them to 3-O-methylated derivatives, suggesting an evolutionary relationship between these two activities. Homology modeling, sequence comparisons, and site-directed mutagenesis led to the identification of active site residues crucial for FOMT and BX OMT activity. However, the full conversion of ZmFOMT2 activity into BX OMT activity by switching these residues was not successful. Only trace O-methylation of BXs was observed, indicating that amino acids outside the active site cavity are also involved in determining the different substrate specificities. Altogether, the results of our study suggest that BX OMTs have evolved from the ubiquitous FOMTs in the PACMAD clade of the grasses through a complex series of amino acid changes.

Nobiletin as a chemopreventive natural product against cancer, a comprehensive review

As a leading cause of death, second only to heart disease, cancer has always been one of the burning topics in medical research. When targeting multiple signal pathways in tumorigenesis chemoprevention, using natural or synthetic anti-cancer drugs is a vital strategy to reduce cancer damage. However, toxic effects, multidrug resistance (MDR) as well as cancer stem cells (CSCs) all prominently limited the clinical application of conventional anticancer drugs. With low side effects, strong biological activity, unique mechanism, and wide range of targets, natural products derived from plants are considered significant sources for new drug development. Nobiletin is one of the most attractive compounds, a unique flavonoid primarily isolated from the peel of citrus fruits. Numerous studies in vitro and in vivo have suggested that nobiletin and its derivatives possess the eminent potential to become effective cancer chemoprevention agents through various cellular and molecular levels. This article aims to comprehensively review the anticancer efficacy and specific mechanisms of nobiletin, enhancing our understanding of its chemoprevention properties and providing the latest research findings. At the end of this review, we also give some discussion and future perspectives regarding the challenges and opportunities in nobiletin efficient exploitation.

Characterization of a Caffeic Acid 8-O-Methyltransferase from Citrus and Its Function in Nobiletin Biosynthesis

Nobiletin (3',4',5,6,7,8-hexamethoxyflavone) is a polymethoxylated flavonoid specifically accumulated in citrus fruit with numerous beneficial effects to human health. In this study, a novel O-methyltransferase (CitOMT2) was isolated from three citrus varieties, Ponkan mandarin (Citrus reticulata Blanco), Nou 6 ("King mandarin" × "Mukaku-kishu"), and Satsuma mandarin (Citrus unshiu Marc.), and its functions were characterized in vitro. The gene expression results showed that CitOMT2 was highly expressed in the two nobiletin abundant varieties of Ponkan mandarin and Nou 6. However, the expression level of CitOMT2 was low in the flavedo of Satsuma mandarin, in which only a small amount of nobiletin was accumulated. Functional analysis suggested that CitOMT2 was a caffeic acid 8-O-methyltransferase, and it catalyzed the O-methylation of 7,8-dihydroxyflavone at 8-OH. As the methylation of flavone at 8-OH was required for nobiletin biosynthesis, the results presented in this study suggested that CitOMT2 was a key gene regulating nobiletin accumulation in citrus fruit.

One-Pot Biocatalytic In Vivo Methylation-Hydroamination of Bioderived Lignin Monomers to Generate a Key Precursor to L-DOPA

Electron‐rich phenolic substrates can be derived from the depolymerisation of lignin feedstocks. Direct biotransformations of the hydroxycinnamic acid monomers obtained can be exploited to produce high‐value chemicals, such as α‐amino acids, however the reaction is often hampered by the chemical autooxidation in alkaline or harsh reaction media. Regioselective O‐methyltransferases (OMTs) are ubiquitous enzymes in natural secondary metabolic pathways utilising an expensive co‐substrate S‐adenosyl‐l‐methionine (SAM) as the methylating reagent altering the physicochemical properties of the hydroxycinnamic acids. In this study, we engineered an OMT to accept a variety of electron‐rich phenolic substrates, modified a commercial E. coli strain BL21 (DE3) to regenerate SAM in vivo, and combined it with an engineered ammonia lyase to partake in a one‐pot, two whole cell enzyme cascade to produce the l‐DOPA precursor l‐veratrylglycine from lignin‐derived ferulic acid.

Tangeretin as an adjuvant and chemotherapeutic sensitizer against various types of cancers: a comparative overview

OBJECTIVES

Cancer is a leading cause of disabling morbidities and death worldwide. Although there are various strategies for the management of cancer, the severe adverse effects negatively impact the patient's quality of life. In addition, the development of resistance limits the efficacy of many chemotherapeutics. Many natural agents are capable of reducing the adverse effects associated with chemotherapy and improving the therapeutic outcome. Tangeretin, a polymethoxy flavone, is one of the promising natural anticancer agents.

KEY FINDINGS

Tangeretin not only targets various malignancies but also synergizes chemotherapeutic agents and reverses cancer resistance. Hence, the application of tangeretin as an adjuvant in cancer chemotherapy would be a promising strategy.

SUMMARY

This work critically highlighted the proposed anticancer activity of tangeretin and discussed its potential combination with various chemotherapeutic agents. Additionally, it shed light on tangeretin chemical derivatives with improved pharmacokinetic and pharmacodynamic activity. Finally, this review described flavonoid biosynthetic pathways and how bioengineering can be employed to enhance the production yield of tangeretin. Thus, this work paves the way for the rational clinical utilization of tangeretin as a safe and effective adjuvant in chemotherapeutic protocols.

Genome-wide identification and characterization of COMT gene family during the development of blueberry fruit

BACKGROUND

Caffeic acid O-methyltransferases (COMTs) play an important role in the diversification of natural products, especially in the phenylalanine metabolic pathway of plant. The content of COMT genes in blueberry and relationship between their expression patterns and the lignin content during fruit development have not clearly investigated by now.

RESULTS

Ninety-two VcCOMTs were identified in Vaccinium corymbosum. According to phylogenetic analyses, the 92 VcCOMTs were divided into 2 groups. The gene structure and conserved motifs within groups were similar which supported the reliability of the phylogenetic structure groupings. Dispersed duplication (DSD) and whole-genome duplication (WGD) were determined to be the major forces in VcCOMTs evolution. The results showed that the results of qRT-PCR and lignin content for 22 VcCOMTs, VcCOMT40 and VcCOMT92 were related to lignin content at different stages of fruit development of blueberry.

CONCLUSION

We identified COMT gene family in blueberry, and performed comparative analyses of the phylogenetic relationships in the 15 species of land plant, and gene duplication patterns of COMT genes in 5 of the 15 species. We found 2 VcCOMTs were highly expressed and their relative contents were similar to the variation trend of lignin content during the development of blueberry fruit. These results provide a clue for further study on the roles of VcCOMTs in the development of blueberry fruit and could promisingly be foundations for breeding blueberry clutivals with higher fruit firmness and longer shelf life.

Modification of the Pseudomonas aeruginosa toxin 2-heptyl-1-hydroxyquinolin-4(1H)-one and other secondary metabolites by methyltransferases from mycobacteria

The opportunistic pathogen Pseudomonas aeruginosa, one of the most prevalent species in infections of the cystic fibrosis lung, produces a range of secondary metabolites, among them the respiratory toxin 2-heptyl-1-hydroxyquinolin-4(1H)-one (2-heptyl-4-hydroxyquinoline N-oxide, HQNO). Cultures of the emerging cystic fibrosis pathogen Mycobacteroides abscessus detoxify HQNO by methylating the N-hydroxy moiety. In this study, the class I methyltransferase MAB_2834c and its orthologue from Mycobacterium tuberculosis, Rv0560c, were identified as HQNO O-methyltransferases. The P. aeruginosa exoproducts 4-hydroxyquinolin-2(1H)-one (DHQ), 2-heptylquinolin-4(1H)-one (HHQ), and 2-heptyl-3-hydroxyquinolin-4(1H)-one (the 'Pseudomonas quinolone signal', PQS), some structurally related (iso)quinolones, and the flavonol quercetin were also methylated; however, HQNO was by far the preferred substrate. Both enzymes converted a benzimidazole[1,2-a]pyridine-4-carbonitrile-based compound, representing the scaffold of antimycobacterial substances, to an N-methylated derivative. We suggest that these promiscuous methyltransferases, newly termed as heterocyclic toxin methyltransferases (Htm), are involved in cellular response to chemical stress and possibly contribute to resistance of mycobacteria toward antimicrobial natural compounds as well as drugs. Thus, synthetic antimycobacterial agents may be designed to be unamenable to methyl transfer. ENZYMES: S-adenosyl-l-methionine:2-heptyl-1-hydroxyquinolin-4(1H)-one O-methyl-transferase, EC 2.1.1.

Expression and functional analysis of the nobiletin biosynthesis-related gene CitOMT in citrus fruit

Nobiletin, a polymethoxy flavone (PMF), is specific to citrus and has been reported to exhibit important health-supporting properties. Nobiletin has six methoxy groups at the 3′,4′,5,6,7,8-positions, which are catalyzed by O-methyltransferases (OMTs). To date, researches on OMTs in citrus fruit are still limited. In the present study, a novel OMT gene (CitOMT) was isolated from two citrus varieties Satsuma mandarin (Citrus unshiu Marc.) and Ponkan mandarin (Citrus reticulata Blanco), and its function was characterized in vitro. The results showed that the expression of CitOMT in the flavedo of Ponkan mandarin was much higher than that of Satsuma mandarin during maturation, which was consistent with the higher accumulation of nobiletin in Ponkan mandarin. In addition, functional analysis showed that the recombinant protein of CitOMT had methylation activity to transfer a methyl group to 3′-hydroxy group of flavones in vitro. Because methylation at the 3′-position of flavones is vital for the nobiletin biosynthesis, CitOMT may be a key gene responsible for nobiletin biosynthesis in citrus fruit. The results presented in this study will provide new strategies to enhance nobiletin accumulation and improve the nutritional qualities of citrus fruit.

Microbial lipases and their industrial applications: a comprehensive review

Lipases are very versatile enzymes, and produced the attention of the several industrial processes. Lipase can be achieved from several sources, animal, vegetable, and microbiological. The uses of microbial lipase market is estimated to be USD 425.0 Million in 2018 and it is projected to reach USD 590.2 Million by 2023, growing at a CAGR of 6.8% from 2018. Microbial lipases (EC 3.1.1.3) catalyze the hydrolysis of long chain triglycerides. The microbial origins of lipase enzymes are logically dynamic and proficient also have an extensive range of industrial uses with the manufacturing of altered molecules. The unique lipase (triacylglycerol acyl hydrolase) enzymes catalyzed the hydrolysis, esterification and alcoholysis reactions. Immobilization has made the use of microbial lipases accomplish its best performance and hence suitable for several reactions and need to enhance aroma to the immobilization processes. Immobilized enzymes depend on the immobilization technique and the carrier type. The choice of the carrier concerns usually the biocompatibility, chemical and thermal stability, and insolubility under reaction conditions, capability of easy rejuvenation and reusability, as well as cost proficiency. Bacillus spp., Achromobacter spp., Alcaligenes spp., Arthrobacter spp., Pseudomonos spp., of bacteria and Penicillium spp., Fusarium spp., Aspergillus spp., of fungi are screened large scale for lipase production. Lipases as multipurpose biological catalyst has given a favorable vision in meeting the needs for several industries such as biodiesel, foods and drinks, leather, textile, detergents, pharmaceuticals and medicals. This review represents a discussion on microbial sources of lipases, immobilization methods increased productivity at market profitability and reduce logistical liability on the environment and user.

Application of Enzymes in Regioselective and Stereoselective Organic Reactions

Nowadays, biocatalysts have received much more attention in chemistry regarding their potential to enable high efficiency, high yield, and eco-friendly processes for a myriad of applications. Nature’s vast repository of catalysts has inspired synthetic chemists. Furthermore, the revolutionary technologies in bioengineering have provided the fast discovery and evolution of enzymes that empower chemical synthesis. This article attempts to deliver a comprehensive overview of the last two decades of investigation into enzymatic reactions and highlights the effective performance progress of bio-enzymes exploited in organic synthesis. Based on the types of enzymatic reactions and enzyme commission (E.C.) numbers, the enzymes discussed in the article are classified into oxidoreductases, transferases, hydrolases, and lyases. These applications should provide us with some insight into enzyme design strategies and molecular mechanisms.

Characterization of a caffeoyl-CoA O-methyltransferase-like enzyme involved in biosynthesis of polymethoxylated flavones in Citrus reticulata

Polymethoxylated flavones (PMFs), which accumulate exclusively in fruit peel of citrus, play important physiological and pharmacological roles but the genetic basis for the methylation of flavonoids has not been fully elucidated in citrus. Here we characterize a caffeoyl-CoA O-methyltransferase-like enzyme, designated CrOMT1. The expression pattern of CrOMT1 was highly correlated with the concentration of the three major PMFs in two different citrus fruit tissues during fruit maturation. Exposure of fruit to UV-B radiation sharply increased the level of CrOMT1 transcripts and also led to the accumulation of three PMFs. The potential role of CrOMT1 was studied by testing the catalytic activity of recombinant CrOMT1 with numerous possible substrates in vitro. The enzyme could most efficiently methylate flavones with neighboring hydroxy moieties, with high catalytic efficiencies found with 6-OH- and 8-OH-containing compounds, preferences that correspond precisely with the essential methylation sites involved in the synthesis of the three naturally occurring PMFs in Citrus reticulata. This indicates that CrOMT1 is capable of in vitro methylation reactions required to synthesize PMFs in vivo. Furthermore, transient overexpression of CrOMT1 increased levels of the three major PMFs in fruit, indicating that CrOMT1 is likely to play an essential role in the biosynthesis of PMFs in citrus.

Nobiletin in Cancer Therapy: How This Plant Derived-Natural Compound Targets Various Oncogene and Onco-Suppressor Pathways

Cancer therapy is a growing field, and annually, a high number of research is performed to develop novel antitumor drugs. Attempts to find new antitumor drugs continue, since cancer cells are able to acquire resistance to conventional drugs. Natural chemicals can be considered as promising candidates in the field of cancer therapy due to their multiple-targeting capability. The nobiletin (NOB) is a ubiquitous flavone isolated from Citrus fruits. The NOB has a variety of pharmacological activities, such as antidiabetes, antioxidant, anti-inflammatory, hepatoprotective, and neuroprotective. Among them, the antitumor activity of NOB has been under attention over recent years. In this review, we comprehensively describe the efficacy of NOB in cancer therapy. NOB induces apoptosis and cell cycle arrest in cancer cells. It can suppress migration and invasion of cancer cells via the inhibition of epithelial-to-mesenchymal transition (EMT) and EMT-related factors such as TGF-β, ZEB, Slug, and Snail. Besides, NOB inhibits oncogene factors such as STAT3, NF-κB, Akt, PI3K, Wnt, and so on. Noteworthy, onco-suppressor factors such as microRNA-7 and -200b undergo upregulation by NOB in cancer therapy. These onco-suppressor and oncogene pathways and mechanisms are discussed in this review.

Characterization of a Flavonoid 3'/5'/7-O-Methyltransferase from Citrus reticulata and Evaluation of the In Vitro Cytotoxicity of Its Methylated Products

O-methylation of flavonoids is an important modification reaction that occurs in plants. O-methylation contributes to the structural diversity of flavonoids, which have several biological and pharmacological functions. In this study, an O-methyltransferase gene (CrOMT2) was isolated from the fruit peel of Citrus reticulata, which encoding a multifunctional O-methyltransferase and could effectively catalyze the methylation of 3'-, 5'-, and 7-OH of flavonoids with vicinal hydroxyl substitutions. Substrate preference assays indicated that this recombinant enzyme favored polymethoxylated flavones (PMF)-type substrates in vitro, thereby providing biochemical evidence for the potential role of the enzyme in plants. Additionally, the cytotoxicity of the methylated products from the enzymatic catalytic reaction was evaluated in vitro using human gastric cell lines SGC-7901 and BGC-823. The results showed that the in vitro cytotoxicity of the flavonoids with the unsaturated C2-C3 bond was increased after being methylated at position 3'. These combined results provide biochemical insight regarding CrOMT2 in vitro and indicate the in vitro cytotoxicity of the products methylated by its catalytic reaction.

Differences in flavonoid pathway metabolites and transcripts affect yellow petal colouration in the aquatic plant Nelumbo nucifera

BACKGROUND

The Asia lotus (Nelumbo nucifera Gaertn.) is an ornamental aquatic plant with high economic value. Flower colour is an important ornamental trait, with much of N. nucifera breeding focusing on its yellow flowers. To explore the yellow flower colouration mechanism in N. nucifera, we analysed its pigment constituents and content, as well as gene expression in the flavonoid pathway, in two N. nucifera cultivars.

RESULTS

We performed metabolomic and gene expression analyses in two N. nucifera cultivars with yellow and white flowers, Molinqiuse (MLQS) and Yeguangbei (YGB), respectively, at five stages of flower colouration. Based on phenotypic observation and metabolite analyses, the later stages of flower colouration (S3-S5) were determined to be key periods for differences between MLQS and YGB, with dihydroflavonols and flavonols differing significantly between cultivars. Dihydroquercetin, dihydrokaempferol, and isorhamnetin were significantly higher in MLQS than in YGB, whereas kaempferol was significantly higher in YGB. Most of the key homologous structural genes in the flavonoid pathway were significantly more active in MLQS than in YGB at stages S1-S4.

CONCLUSION

In this study, we performed the first analyses of primary and secondary N. nucifera metabolites during flower colouration, and found that isorhamnetin and kaempferol shunting resulted in petal colour differences between MLQS and YGB. Based on our data integration analyses of key enzyme expression in the putative flavonoid pathways of the two N. nucifera cultivars, NnFLS gene substrate specificity and differential expression of NnOMTs may be related to petal colour differences between MLQS and YGB. These results will contribute to determining the mechanism of yellow flower colouration in N. nucifera, and will improve yellow petal colour breeding in lotus species.

Nobiletin and Derivatives: Functional Compounds from Citrus Fruit Peel for Colon Cancer Chemoprevention

The search for effective methods of cancer treatment and prevention has been a continuous effort since the disease was discovered. Recently, there has been increasing interest in exploring plants and fruits for molecules that may have potential as either adjuvants or as chemopreventive agents against cancer. One of the promising compounds under extensive research is nobiletin (NOB), a polymethoxyflavone (PMF) extracted exclusively from citrus peel. Not only does nobiletin itself exhibit anti-cancer properties, but its derivatives are also promising chemopreventive agents; examples of derivatives with anti-cancer activity include 3'-demethylnobiletin (3'-DMN), 4'-demethylnobiletin (4'-DMN), 3',4'-didemethylnobiletin (3',4'-DMN) and 5-demethylnobiletin (5-DMN). In vitro studies have demonstrated differential efficacies and mechanisms of NOB and its derivatives in inhibiting and killing of colon cancer cells. The chemopreventive potential of NOB has also been well demonstrated in several in vivo colon carcinogenesis animal models. NOB and its derivatives target multiple pathways in cancer progression and inhibit several of the hallmark features of colorectal cancer (CRC) pathophysiology, including arresting the cell cycle, inhibiting cell proliferation, inducing apoptosis, preventing tumour formation, reducing inflammatory effects and limiting angiogenesis. However, these substances have low oral bioavailability that limits their clinical utility, hence there have been numerous efforts exploring better drug delivery strategies for NOB and these are part of this review. We also reviewed data related to patents involving NOB to illustrate the extensiveness of each research area and its direction of commercialisation. Furthermore, this review also provides suggested directions for future research to advance NOB as the next promising candidate in CRC chemoprevention.

Identification of Hydroxypyrazine O-Methyltransferase Genes in Coffea arabica: A Potential Source of Methoxypyrazines That Cause Potato Taste Defect

The goal of this study is to identify Coffea arabica O-methyltransferase (OMT) genes involved in the biosynthesis of methoxypyrazines. High levels of 2-isopropyl-3-methoxypyrazine (IPMP) and 2-isobutyl-3-methoxypyrazine (IBMP) in coffee beans are associated with the potato taste defect (PTD). Among the 34 putative O-methyltransferase genes identified in the published genome of C. canephora, three genes are highly homologous to known hydroxypyrazine OMT genes. Genes of interest were amplified and sequenced from genomic DNA of single C. arabica beans grown in eight different locations, including regions with endemic PTD. Although C. arabica OMT target sequences were almost identical regardless of source location, individual beans shared numerous polymorphisms in each of the target genes. Two of the predicted C. arabica OMT enzymes were successfully expressed in Escherichia coli and purified, and one enzyme shows slow yet measurable turnover of both 3-isobutyl-2-hydroxypyrazine (IBHP) and 3-isopropyl-2- hydroxypyrazine (IPHP), supporting a possible role of the coffee plant in PTD.

Molecular cloning and functional characterization of an O-methyltransferase catalyzing 4'-O-methylation of resveratrol in Acorus calamus

Resveratrol and its methyl ethers, which belong to a class of natural polyphenol stilbenes, play important roles as biologically active compounds in plant defense as well as in human health. Although the biosynthetic pathway of resveratrol has been fully elucidated, the characterization of resveratrol-specific O-methyltransferases remains elusive. In this study, we used RNA-seq analysis to identify a putative aromatic O-methyltransferase gene, AcOMT1, in Acorus calamus. Recombinant AcOMT1 expressed in Escherichia coli showed high 4'-O-methylation activity toward resveratrol and its derivative, isorhapontigenin. We purified a reaction product enzymatically formed from resveratrol by AcOMT1 and confirmed it as 4'-O-methylresveratrol (deoxyrhapontigenin). Resveratrol and isorhapontigenin were the most preferred substrates with apparent K_m values of 1.8 μM and 4.2 μM, respectively. Recombinant AcOMT1 exhibited reduced activity toward other resveratrol derivatives, piceatannol, oxyresveratrol, and pinostilbene. In contrast, recombinant AcOMT1 exhibited no activity toward pterostilbene or pinosylvin. These results indicate that AcOMT1 showed high 4'-O-methylation activity toward stilbenes with non-methylated phloroglucinol rings.