Four glycosyltransferase genes are responsible for synthesis and accumulation of different flavonol glycosides in apple tissues

Flavonols are widely synthesized throughout the plant kingdom, playing essential roles in plant physiology and providing unique health benefits for humans. Their glycosylation plays significant role in improving their stability and solubility, thus their accumulation and function. However, the genes encoding the enzymes catalyze this glycosylation remain largely unknown in apple. This study utilized a combination of methods to identify genes encoding such enzymes. Initially, candidate genes were selected based on their potential to encode UDP-dependent glycosyltransferases (UGTs) and their expression patterns in response to light induction. Subsequently, through testing the in vitro enzyme activity of the proteins produced in Escherichia coli cells, four candidates were confirmed to encode a flavonol 3-O-galactosyltransferase (UGT78T6), flavonol 3-O-glucosyltransferase (UGT78S1), flavonol 3-O-xylosyltransferase/arabinosyltransferase (UGT78T5), and flavonol 3-O-rhamnosyltransferase (UGT76AE22), respectively. Further validation of these genes' functions was conducted by modulating their expression levels in stably transformed apple plants. As anticipated, a positive correlation was observed between the expression levels of these genes and the content of specific flavonol glycosides corresponding to each gene. Moreover, overexpression of a flavonol synthase gene, MdFLS, resulted in increased flavonol glycoside content in apple roots and leaves. These findings provide valuable insights for breeding programs aimed at enriching apple flesh with flavonols and for identifying flavonol 3-O-glycosyltransferases of other plant species.

Scutellarin Alleviates Ischemic Brain Injury in the Acute Phase by Affecting the Activity of Neurotransmitters in Neurons

Cerebral ischemic stroke is a common neuron loss disease that is caused by the interruption of the blood supply to the brain. In order to enhance the CIS outcome, both identifying the treatment target of ischemic brain damage in the acute phase and developing effective therapies are urgently needed. Scutellarin had been found to be beneficial to ischemic injuries and has been shown to have potent effects in clinical application on both stroke and myocardial infarction. However, whether scutellarin improves ischemic brain damage in the acute phase remains unknown. In this study, the protective effects of scutellarin on ischemic brain damage in the acute phase (within 12 h) were illustrated. In middle cerebral artery occlusion and reperfusion (MCAO/R) modeling rats, the Z-Longa score was significantly down-regulated by 25% and 23.1%, and the brain infarct size was reduced by 26.95 ± 0.03% and 25.63 ± 0.02% when responding to high-dose and low-dose scutellarin treatments, respectively. H&E and TUNEL staining results indicated that the neuron loss of the ischemic region was improved under scutellarin treatment. In order to investigate the mechanism of scutellarin's effects on ischemic brain damage in the acute phase, changes in proteins and metabolites were analyzed. The suppression of scutellarin on the glutamate-inducing excitatory amino acid toxicity was strongly indicated in the study of both proteomics and metabolomics. A molecular docking experiment presented strong interactions between scutellarin and glutamate receptors, which score much higher than those of memantine. Further, by performing a parallel reaction monitoring-mass spectrometry (PRM-MS) study on both the cortex and hippocampus tissue of the ischemic region, we screened the scutellarin-regulating molecules that are involved in both the release and transportation of neurotransmitters. It was found that the aberrant levels of glutamate receptors, including EAAT2, GRIN1, GRIN2B, and GRM1, as well as of other glutamatergic pathway-involving proteins, including CAMKK2, PSD95, and nNOS, were significantly regulated in the ischemic cortex. In the hippocampus, EAAT2, GRIN1, nNOS, and CAM were significantly regulated. Taken together, scutellarin exerts potent effects on ischemic brain damage in the acute phase by regulating the activity of neurotransmitters and reducing the toxicity of excitatory amino acids in in neurons.

Functional characterization of a Flavonol 3-O-rhamnosyltransferase and two UDP-rhamnose synthases from Hypericum monogynum

Rhamnosyltransferase (RT) and rhamnose synthase (Rhs) are the key enzymes that are responsible for the biosynthesis of rhamnosides and UDP-l-rhamnose (UDP-Rha) in plants, respectively. How to discover such enzymes efficiently for use is still a problem to be solved. Here, we identified HmF3RT, HmRhs1, and HmRhs2 from Hypericum monogynum, which is abundant in flavonol rhamnosides, with the help of a full-length and high throughput transcriptome sequencing platform. HmF3RT could regiospecifically transfer the rhamnose moiety of UDP-Rha onto the 3-OH position of flavonols and has weakly catalytic for UDP-xylose (UDP-Xyl) and UDP-glucose (UDP-Glc). HmF3RT showed well quercetin substrate affinity and high catalytic efficiency with K_m of 5.14 μM and k_cat/K_m of 2.21 × 10⁵ S^-1 M^-1, respectively. Docking, dynamic simulation, and mutagenesis studies revealed that V129, D372, and N373 are critical residues for the activity and sugar donor recognition of HmF3RT, mutant V129A, and V129T greatly enhance the conversion rate of catalytic flavonol glucosides. HmRhs1 and HmRhs2 convert UDP-Glc to UDP-Rha, which could be further used by HmF3RT. The HmF3RT and HmRhs1 co-expressed strain RTS1 could produce quercetin 3-O-rhamnoside (quercitrin), kaempferol 3-O-rhamnoside (afzelin), and myricetin 3-O-rhamnoside (myricitrin) at yields of 85.1, 110.7, and 77.6 mg L^-1, respectively. It would provide a valuable reference for establishing a better and more efficient biocatalyst for preparing bioactive flavonol rhamnosides by identifying HmF3RT and HmRhs.

Glycosylation of Aromatic Glycosides by a Promiscuous Glycosyltransferase UGT71BD1 from Cistanche tubulosa

Multiple-glycosylated glycosides are a major source of bioactive leads. However, most of the currently reported glycosyltransferases (GTases) mainly catalyze glycosylation of aglycones without sugar group substitution. GTases accepting diverse glycosides as substrates are rarely reported. In this article, a new GTase UGT71BD1 was identified from Cistanche tubulosa, a desert herb plant abundant with various phenylethanoid glycosides (PhGs). Interestingly, UGT71BD1 showed no activity toward the aglycone of PhGs. Instead, it could catalyze the further glycosylation of PhG compounds to produce new phenylethanoid multiglycosylated glycosides, including the natural rarely separated tetraglycoside PhGs. Extensive assays found the unprecedented substrate promiscuity of UGT71BD1 toward diverse glycosides including flavonoid glycosides, stilbene glycosides, and coumarin glycosides, performing further mono- or diglycosylation with efficient conversion rates. Using UGT71BD1, six multiglycosylated glycosides were prepared and structurally identified by NMR spectroscopy. These products showed enhanced pharmacological activities compared with the substrates. Docking, dynamic simulation, and mutagenesis studies identified key residues for UGT71BD1's activity and revealed that the sugar modules in glycosides play crucial roles in substrate recognition, thus partly illuminating the unusual substrate preference of UGT71BD1 toward diverse glycosides. UGT71BD1 could be a potential enzyme tool for glycosylation of diverse glycosides.

Specific decorations of 17-hydroxygeranyllinalool diterpene glycosides solve the autotoxicity problem of chemical defense in Nicotiana attenuata

The native diploid tobacco Nicotiana attenuata produces abundant, potent anti-herbivore defense metabolites known as 17-hydroxygeranyllinalool diterpene glycosides (HGL-DTGs) whose glycosylation and malonylation biosynthetic steps are regulated by jasmonate signaling. To characterize the biosynthetic pathway of HGL-DTGs, we conducted a genome-wide analysis of uridine diphosphate glycosyltransferases (UGTs) and identified 107 family-1 UGT members. The transcript levels of three UGTs were highly correlated with the transcript levels two key HGL-DTG biosynthetic genes: geranylgeranyl diphosphate synthase (NaGGPPS) and geranyllinalool synthase (NaGLS). NaGLS's role in HGL-DTG biosynthesis was confirmed by virus-induced gene silencing. Silencing the Uridine diphosphate (UDP)-rhamnosyltransferase gene UGT91T1 demonstrated its role in the rhamnosylation of HGL-DTGs. In vitro enzyme assays revealed that UGT74P3 and UGT74P4 use UDP-glucose for the glucosylation of 17-hydroxygeranyllinalool (17-HGL) to lyciumoside I. Plants with stable silencing of UGT74P3 and UGT74P5 were severely developmentally deformed, pointing to a phytotoxic effect of the aglycone. The application of synthetic 17-HGL and silencing of the UGTs in HGL-DTG-free plants confirmed this phytotoxic effect. Feeding assays with tobacco hornworm (Manduca sexta) larvae revealed the defensive functions of the glucosylation and rhamnosylation steps in HGL-DTG biosynthesis. Glucosylation of 17-HGL is therefore a critical step that contributes to the resulting metabolites' defensive function and solves the autotoxicity problem of this potent chemical defense.

Enzymatic synthesis of myricetin 3-O-galactoside through a whole-cell biocatalyst

Objective

Myricetin 3-O-galactoside is an active compound with pharmaceutical potential. The insufficient supply of this compound becomes a bottleneck in the druggability study of myricetin 3-O-galactoside. Thus, it is necessary to develop a biosynthetic process for myricetin 3-O-galactoside through metabolic engineering.

Methods

Two genes OcSUS1 and OcUGE1 encoding sucrose synthase and UDP-glucose 4-epimerase were introduced into BL21(DE3) to reconstruct a UDP-D-galactose (UDP-Gal) biosynthetic pathway in Escherichia coli. The resultant chassis strain was able to produce UDP-Gal. Subsequently, a flavonol 3-O-galactosyltransferase DkFGT gene was transformed into the chassis strain producing UDP-Gal. An artificial pathway for myricetin 3-O-galactoside biosynthesis was thus constructed in E. coli.

Results

The obtained engineered strain was demonstrated to be capable of producing myricetin 3-O-galactoside, reaching 29.7 mg/L.

Conclusion

Biosynthesis of myricetin 3-O-galactoside through engineered E. coli could be achieved. This result lays the foundation for the large-scale preparation of myricetin 3-O-galactoside.

Metabolic Profiling and Transcriptome Analysis of Mulberry Leaves Provide Insights into Flavonoid Biosynthesis

Flavonoids are widely distributed in mulberry leaves and have been recognized for their beneficial physiological effects on the human health. Here, we analyzed variations in 44 flavonoid compounds among 91 mulberry resources. Metabolic profiling revealed that O-rhamnosylated flavonols and malonylated flavonol glycosides, including rutin and quercetin 3-O-(6″-O-malonylglucoside) (Q3MG), were absent from Morus notabilis and multiple mulberry (Morus alba L.) resources. Transcriptome and phylogenetic analyses of flavonoid-related UDP-glycosyltransferases (UGTs) suggested that the flavonol 3-O-glucoside-O-rhamnosyltransferase (FGRT) KT324624 is a key enzyme involved in rutin synthesis. A recombinant FGRT protein was able to convert kaempferol/quercetin 3-O-glucoside to kaempferol 3-O-rutinoside (K3G6″Rha) and rutin. The recombinant FGRT was able to use 3-O-glucosylated flavonols but not flavonoid aglycones or 7-O-glycosylated flavonoids as substrates. The enzyme preferentially used UDP-rhamnose as the sugar donor, indicating that it was a flavonol 3-O-glucoside: 6″-O-rhamnosyltransferase. This study provided insights into the biosynthesis of rutin in mulberry.

Family-1 UDP glycosyltransferases in pear (Pyrus bretschneideri): Molecular identification, phylogenomic characterization and expression profiling during stone cell formation

Stone cells are a characteristic trait of pear fruits, and excessive stone cell formation has a significant negative impact on the texture and flavour of the pulp. Lignin is one of the main components of stone cells. Family-1 uridine diphosphate-glycosyltransferases (UGTs) are responsible for the glycosylation modification of monolignols. However, information remains limited regarding the relationship between UGTs and stone cell formation. To address this problem, we identified 139 UGTs from the pear genome, which were distributed in 15 phylogenetic groups (A-M, O, and P). We also performed a collinearity analysis of UGTs among four Rosaceae plants (pear, peach, mei, and strawberry). Phylogenetic analysis suggested that 13 PbUGTs might be related to the glycosylation of monolignols. Analysis of expression patterns demonstrated that most putative monolignol glycosylation-related PbUGTs not only showed high expression levels in flowers and buds but were also induced by exogenous ABA, SA, and MeJA. In addition, the transcript level of Pbr005014.1 (named PbUGT72AJ2) was consistent with the changing trend of lignin content in pear fruit, and the transcript level was also higher in 'Dangshan Su' pear with higher lignin and stone cell contents. Subcellular localization results showed that PbUGT72AJ2 was located mainly in the cytomembrane and cytoplasm. Based on our study, PbUGT72AJ2 is considered to be a monolignol glycosylation-related UGT. Our results provide an important source for the identification of UGTs and a foundation for the future understanding and manipulation of lignin metabolism and stone cell formation in pear fruit.

Deep eutectic solvents used as extraction solvent for the determination of flavonoids from Camellia oleifera flowers by high-performance liquid chromatography

INTRODUCTION

Camellia oleifera flowers are rich in flavonoids, but there has been little attention on their application. A simple and reliable method for determining the content of flavonoids in C. oleifera flowers would be very helpful for the utilisation of agriculture resources.

OBJECTIVE

To develop an efficient analytical method for the determination of flavonoids in C. oleifera flowers by high-performance liquid chromatography-ultraviolet (HPLC-UV) detection.

METHODOLOGY

Preparing an environmentally-friendly and effective solvent - deep eutectic solvents (DESs) - for compound extraction. Then investigating the influential factors of ultrasound-assisted extraction with DESs by the Box-Behnken design combined with response surface methodology.

RESULTS

DES-5 synthesised with choline chloride and lactic acid (1:2) acquired excellent extractability for four flavonoids (quercetin 3-O-rhamnoside, kaempferol 3-O-rhamnoside and their aglycones) with different polarity. The proposed method, which could simultaneously determine four flavonoids with HPLC-UV detection for the first time, displays satisfactory recovery yields and high precision with inter-day relative standard deviation lower than 5.80%.

CONCLUSION

DESs could be promising solvents for efficiently and selectively extracting bioactive compounds from plant materials, and the analytical method for flavonoids of C. oleifera flowers could provide reference value for its application and be used in other plant resources.

Synthesis of Scutellarein Derivatives with a Long Aliphatic Chain and Their Biological Evaluation against Human Cancer Cells

Scutellarin is the major active flavonoid extracted from the traditional Chinese herbal medicine Erigeron breviscapus (Vant.) Hand-Mazz., which is widely used in China. Recently, accumulating evidence has highlighted the potential role of scutellarin and its main metabolite scutellarein in the treatment of cancer. To explore novel anticancer agents with high efficiency, a series of new scutellarein derivatives with a long aliphatic chain were synthesized, and the antiproliferative activities against Jurkat, HCT-116 and MDA-MB-231 cancer cell lines were assessed. Among them, compound 6a exhibited the strongest antiproliferative effects on Jurkat (IC₅₀ = 1.80 μM), HCT-116 (IC₅₀ = 11.50 μM) and MDA-MB-231 (IC₅₀ = 53.91 μM). In particular, 6a even showed stronger antiproliferative effects than the positive control NaAsO₂ on Jurkat and HCT-116 cell lines. The results showed that a proper long aliphatic chain enhanced the antiproliferative activity of scutellarein.

High throughput mass spectrometry-based characterisation of Arabidopsis thaliana group H glycosyltransferases

In this report, we cloned and characterised four members of group H glycosyltransferases (GTs) by studying their substrate specificities and kinetics. The formation of products and possible glycosylation position was confirmed using MS/MS. The results revealed that 76E1 and 76E5 have broader donor specificity, including UDP-glucose (UDPGlc), UDP-galactose (UDPGal) and UDP-N-acetylglucosamine (UDPGlcNAc) with various flavonoids as acceptor substrates. Pseudo-single substrate kinetics data showed a relatively low K_M, indicating a high affinity for substrate UDPGlc and also supported that 76E5 is more of a galactosyl and N-acetylglucosamine transferase. Sequence alignment and site-directed mutagenesis studies indeed suggested that serine is a crucial residue in the UDPGlcNAc and UDPGal activity.

We cloned and characterised four group H glycosyltransferases by studying their substrate specificities and kinetics. Sequence alignment and site-directed mutagenesis studies showed that serine is a crucial residue for UDPGlcNAc and UDPGal activity.

Functional Characterization of a New Tea (Camellia sinensis) Flavonoid Glycosyltransferase

Tea (Camellia sinensis) is an important commercial crop, in which the high content of flavonoids provides health benefits. A flavonoid glycosyltransferase (CsUGT73A20), belonging to cluster IIIa, was isolated from tea plant. The recombinant CsUGT73A20 in Escherichia coli exhibited a broad substrate tolerance toward multiple flavonoids. Among them, kaempferol was the optimal substrate compared to quercetin, myricetin, naringenin, apigenin, and kaempferide. However, no product was detected when UDP-galactose was used as the sugar donor. The reaction assay indicated that rCsUGT73A20 performed multisite glycosidation toward flavonol compounds, mainly forming 3-O-glucoside and 7-O-glucoside in vitro. The biochemical characterization analysis of CsUGT73A20 showed more K7G product accumulated at pH 8.0, but K3G was the main product at pH 9.0. Kinetic analysis demonstrated that high pH repressed the glycosylation reaction at the 7-OH site in vitro. Besides, the content of five flavonol-glucosides was increased in CsUGT73A20-overexpressing tobaccos (Nicotiana tabacum).