Multifunctional flavonoid dioxygenases: flavonol and anthocyanin biosynthesis in Arabidopsis thaliana L

Safflower CtFLS1-Induced Drought Tolerance by Stimulating the Accumulation of Flavonols and Anthocyanins in Arabidopsis thaliana

Flavonol synthase gene (FLS) is a member of the 2-oxoglutarate-dependent dioxygenase (2-ODD) superfamily and plays an important role in plant flavonoids biosynthetic pathways. Safflower (Carthamus tinctorius L.), a key source of traditional Chinese medicine, is widely cultivated in China. Although the flavonoid biosynthetic pathway has been studied in several model species, it still remains to be explored in safflower. In this study, we aimed to elucidate the role of CtFLS1 gene in flavonoid biosynthesis and drought stress responses. The bioinformatics analysis on the CtFLS1 gene showed that it contains two FLS-specific motifs (PxxxIRxxxEQP and SxxTxLVP), suggesting its independent evolution. Further, the expression level of CtFLS1 in safflower showed a positive correlation with the accumulation level of total flavonoid content in four different flowering stages. In addition, CtFLS1-overexpression (OE) Arabidopsis plants significantly induced the expression levels of key genes involved in flavonol pathway. On the contrary, the expression of anthocyanin pathway-related genes and MYB transcription factors showed down-regulation. Furthermore, CtFLS1-OE plants promoted seed germination, as well as resistance to osmotic pressure and drought, and reduced sensitivity to ABA compared to mutant and wild-type plants. Moreover, CtFLS1 and CtANS1 were both subcellularly located at the cell membrane and nucleus; the yeast two-hybrid and bimolecular fluorescence complementation (BiFC) assay showed that they interacted with each other at the cell membrane. Altogether, these findings suggest the positive role of CtFLS1 in alleviating drought stress by stimulating flavonols and anthocyanin accumulation in safflower.

Synthesis of Flavonols and Assessment of Their Biological Activity as Anticancer Agents

A series of flavanols were synthesized to assess their biological activity against human non-small cell lung cancer cells (A549). Among the sixteen synthesized compounds, it was observed that compounds 6k (3.14 ± 0.29 µM) and 6l (0.46 ± 0.02 µM) exhibited higher potency compared to 5-fluorouracil (5-Fu, 4.98 ± 0.41 µM), a clinical anticancer drug which was used as a positive control. Moreover, compound 6l (4'-bromoflavonol) markedly induced apoptosis of A549 cells through the mitochondrial- and caspase-3-dependent pathways. Consequently, compound 6l might be developed as a candidate for treating or preventing lung cancer.

HMGR and CHS gene cloning, characterizations and tissue-specific expressions in Polygala tenuifolia Willd

Triterpenoid saponins and flavonoids have several pharmacological activities against P. tenuifolia. The 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) and chalcone synthase (CHS) are the rate-limiting enzymes of triterpenoid saponin and flavonoid biosynthesis, respectively. In this study, HMGR and CHS genes were cloned from P. tenuifolia, and their bioinformatics analyses and tissue-specific expression were investigated. The results showed that the HMGR and CHS genes were successfully cloned, separately named the PtHMGR gene (NCBI accession: MK424118) and PtCHS gene (NCBI accession: MK424117). The PtHMGR gene is 2323 bp long, has an open reading frame (ORF) of 1782 bp, and encods 593 amino acids. The PtCHS gene is 1633 bp long with an ORF of 1170 bp, encoding 389 amino acids. PtHMGR and PtCHS were both hydrophobic, not signal peptides or secreted proteins, containing 10 conserved motifs. PtHMGR and PtCHS separately showed high homology with HMGR and CHS proteins from other species, and their secondary structures mainly included α-helix and random curl. The tertiary structure of PtHMGR was highly similarity to that the template 7ULI in RCSB PDB with 92.0% coverage rate. The HMG-CoA-binding domain of PtHMGR is located at 173-572 amino acid residues, including five bound sites. The tertiary structure of PtCHS showed high consistency with the template 1I86 in RCSB PDB with 100% coverage rate, contained malonyl CoA and 4-coumaroyl-CoA linkers. The expression of PtHMGR and PtCHS is tissue-specific. PtHMGR transcripts were mainly accumulated in roots, followed by leaves, and least in stems, and were significantly positively correlated with the contents of total saponin and tenuifolin. PtCHS was highly expressed in the stems, followed by the leaves, with low expression in the roots. PtCHS transcripts showed a significant positive correlation with total flavonoids content, however, they were significantly negatively correlated with the content of polygalaxanthone III (a type of flavonoids). This study provided insight for further revealing the roles of PtHMGR and PtCHS.

Generation and characterisation of an Arabidopsis thaliana f3h/fls1/ans triple mutant that accumulates eriodictyol derivatives

BACKGROUND

Flavonoids are plant specialised metabolites, which derive from phenylalanine and acetate metabolism. They possess a variety of beneficial characteristics for plants and humans. Several modification steps in the synthesis of tricyclic flavonoids cause for the amazing diversity of flavonoids in plants. The 2-oxoglutarate-dependent dioxygenases (2-ODDs) flavanone 3-hydroxylase (F3H, synonym FHT), flavonol synthase (FLS) and anthocyanidin synthase (ANS, synonym leucoanthocyanidin dioxygenase (LDOX)), catalyse oxidative modifications to the central C ring. They are highly similar and have been shown to catalyse, at least in part, each other's reactions. FLS and ANS have been identified as bifunctional enzymes in many species, including Arabidopsis thaliana, stressing the capability of plants to bypass missing or mutated reaction steps on the way to flavonoid production. However, little is known about such bypass reactions and the flavonoid composition of plants lacking all three central flavonoid 2-ODDs.

RESULTS

To address this issue, we generated a f3h/fls1/ans mutant, as well as the corresponding double mutants and investigated the flavonoid composition of this mutant collection. The f3h/fls1/ans mutant was further characterised at the genomic level by analysis of a nanopore DNA sequencing generated genome sequence assembly and at the transcriptomic level by RNA-Seq analysis. The mutant collection established, including the novel double mutants f3h/fls1 and f3h/ans, was used to validate and analyse the multifunctionalities of F3H, FLS1, and ANS in planta. Metabolite analyses revealed the accumulation of eriodictyol and additional glycosylated derivatives in mutants carrying the f3h mutant allele, resulting from the conversion of naringenin to eriodictyol by flavonoid 3'-hydroxylase (F3'H) activity.

CONCLUSIONS

We describe the in planta multifunctionality of the three central flavonoid 2-ODDs from A. thaliana and identify a bypass in the f3h/fls1/ans triple mutant that leads to the formation of eriodictyol derivatives. As (homo-)eriodictyols are known as bitter taste maskers, the annotated eriodictyol (derivatives) and in particular the observations made on their in planta production, could provide valuable insights for the creation of novel food supplements.

Mutation in BrFLS encoding flavonol synthase induced anthocyanin accumulation in Chinese cabbage

KEY MESSAGE

BrFLS mutation promoted anthocyanin accumulation in Chinese cabbage, which was verified in four allelic mutants. Chinese cabbage is a major vegetable crop in Eastern Asia. Anthocyanin-rich vibrantly colored varieties are increasingly favored by consumers for their higher nutritional and aesthetic value compared to the typical green varieties of Chinese cabbage. Herein, we identified an anthocyanin accumulation mutant aam1 from a mutant library of EMS-mutagenized Chinese cabbage DH line 'FT', which appeared partial purple on leaves, bolting stems and floral buds. This anthocyanin accumulation trait was genetically controlled by a recessive nuclear gene, and through MutMap mapping and KASP genotyping, BraA10g030950.3C was identified as the candidate causal gene with a G202 to A202 non-synonymous SNP variation in exon 1. Three additional mutants allelic to aam1 were obtained via screening of similar-phenotype mutants from the mutant library, namely aam2/3/4, where the causal SNPs reside in the same gene as aam1, corroborating that the mutation of BraA10g030950.3C caused anthocyanin accumulation. BraA10g030950.3C encodes a flavonol synthase that catalyzes dihydroflavonols substrate into flavonols and is homologous to Arabidopsis FLS1 (AT5G08640), named BrFLS. Compared to wildtype, the expression level of BrFLS was significantly reduced in the mutants, while BrDFR, which is involved in the anthocyanin biosynthesis and competes with FLS for the common substrate dihydroflavonols, was increased. The flavonol synthase activity decreased, and dihydroflavonol 4-reductase activity was elevated. Differentially accumulated flavonoid metabolites were detected between wildtype and aam1, which were enriched primarily in flavonol and anthocyanin pathways. Our results revealed that mutations in the BrFLS gene could contribute to anthocyanin accumulation and provide a new target for Chinese cabbage color modification.

Metabolome integrated with transcriptome reveals the mechanism of three different color formations in Taxus mairei arils

Maire yew (Taxus mairei), an evergreen conifer, has high ornamental and medicinal value. The arils of this species has three different colors. However, the variation mechanisms of arils color formation remains unclear. Here, the gene expression and metabolite concentration were profiled for red (RTM), yellow (YTM), and purple (PTM) arils in different developmental stages. A total of 266 flavonoids and 35 carotenoids were identified. The predominant pigments identified in YTM were epiafzelechin, lutein, and β-Cryptoxanthin, while malvidin-3,5-di-O-glucoside and apigenin played crucial roles in PTM. And significant differential expression was observed among the HCT, DFR, LAR, ANS, crtB, NCED, and CCoAOMT genes across different color arils. During the maturation of yellow arils, the upregulation of HCT was strongly correlated with the accumulation of epiafzelechin. The diminished expression of DFR, LAR, and ANS seemed to inhibit the production of delphinidin-3-O-rutinoside. The decrease in crtB expression and concurrent increase in NCED expression potentially regulate the heightened accumulation of lutein. Meanwhile, the accumulation of β-cryptoxanthin appeared seemed to be positively influenced by NCED. As aril turning purple, the decreased expression of CCoAOMT seemed to facilitate the synthesis of apigenin. The substantial upregulation of DFR promoted the production of malvidin-3,5-di-O-glucoside. Additionally, the overexpression of MYBs may plays the important role in regulating the formation of different colored arils. In total, 14 genes were selected for qRT-PCR validation, the results indicated the reliability of the transcriptome sequences data. Our findings could provide valuable insight into the molecular breeding, development, and application of Maire yew resources.

Comparative transcriptome analysis and flavonoid profiling of floral mutants reveals CmMYB11 regulating flavonoid biosynthesis in chrysanthemum

Flavonoids, of which the major groups are flavones, flavonols, and anthocyanins, confer a variety of colors on plants. Bud sports with variation of floral colors occur occasionally during chrysanthemum cultivation. Although it has been reported that methylation at the promoter of CmMYB6 was related to anthocyanin contents, the regulatory networks of flavonoid biosynthesis still remain largely unknown in mutation of chrysanthemum. We compared phenotypes, pigment composition and transcriptomes in two chrysanthemum cultivars, 'Anastasia Dark Green' and 'Anastasia Pink', and regenerated bud sports of these cultivars with altered floral colors. Increased anthocyanins turned the 'Anastasia Dark Green' mutant red, while decreased anthocyanins turned the 'Anastasia Pink' mutant white. Moreover, total flavonoids were reduced in both mutants. Multiple flavonoid biosynthetic genes and regulatory genes encoding MYBs and bHLHs transcription factors were differentially expressed in pairwise comparisons of transcriptomes in 'Anastasia Dark Green' or 'Anastasia Pink' and their mutants at different flowering stages. Among these regulatory genes, the expression patterns of CmMYB6 and CmbHLH2 correlated to changes of anthocyanin contents, and down-regulation of CmMYB11 correlated to decreased total flavonoid contents in two mutants. CmMYB11 was shown to directly activate the promoter activities of CmCHS2, CmCHI, CmDFR, CmANS, CmFNS, and CmFLS. Furthermore, overexpression of CmMYB11 increased both flavonols and anthocyanins in tobacco petals. Our work provides new insights into regulatory networks involved in flavonoid biosynthesis and coloration in chrysanthemum.

Myricetin: a potential plant-derived anticancer bioactive compound-an updated overview

The globe is currently confronting a global fight against the deadliest cancer sickness. Chemotherapy, hormonal therapy, surgery, and radiation therapy are among cancer treatment options. Still, these treatments can induce patient side effects, including recurrence, multidrug resistance, fever, and weakness. As a result, the scientific community is always working on natural phytochemical substances. Numerous phytochemical compounds, including taxol analogues, vinca alkaloids such as vincristine and vinblastine, and podophyllotoxin analogues, are currently undergoing testing and have shown promising results against a number of the deadliest diseases, as well as considerable advantages due to their safety and low cost. According to research, secondary plant metabolites such as myricetin, a flavonoid in berries, herbs, and walnuts, have emerged as valuable bio-agents for cancer prevention. Myricetin and its derivatives have antiinflammatory, anticancer, apoptosis-inducing, and anticarcinogenic properties and can prevent cancer cell proliferation. Multiple studies have found that myricetin has anticancer characteristics in various malignancies, including colon, breast, prostate, bladder, and pancreatic cancers. Current knowledge of the anticancer effects of myricetin reveals its promise as a potentially bioactive chemical produced from plants for the prevention and treatment of cancer. This review aimed to study the numerous bioactivities, mode of action, and modification of several cellular processes that myricetin possesses to impede the spread of cancer cells. This review also addresses the challenges and future prospects of using myricetin as a anticancer drug.

Insight into the Molecular Mechanism of Flower Color Regulation in Rhododendron latoucheae Franch: A Multi-Omics Approach

Rhododendron latoucheae Franch. (R. latoucheae) is a valuable woody plant known for its high ornamental value. While purple flowers are a distinct and attractive variant phenotype of R. latoucheae, the underlying mechanism regulating its flower color is still poorly understood. To investigate the molecular regulatory mechanism responsible for the variation in flower color, we selected plants with white-pink and purple petals as the object and conducted analyses of metabolites, key genes, and transcription factors associated with flower color. A combined metabolome-transcriptome analysis was performed, and the expression of key genes was subsequently verified through qRT-PCR experiments. The results of our study demonstrated a significant enrichment of differential metabolites in the flavonoid metabolic pathway. Changes in anthocyanin content followed the same trend as the observed flower color variations, specifically showing significant correlations with the contents of malvidin-3-O-glucoside, dihydromyricetin, gallocatechin, and peonidin-3-O-glucoside. Furthermore, we identified three key structural genes (F3GT1, LAR, ANR) and four transcription factors (bHLH130, bHLH41, bHLH123, MYB4) that are potentially associated with the biosynthesis of flavonoid compounds, thereby influencing the appearance of purple flower color in R. latoucheae.

From Nature to Lab: A Review of Secondary Metabolite Biosynthetic Pathways, Environmental Influences, and In Vitro Approaches

Secondary metabolites are gaining an increasing importance in various industries, such as pharmaceuticals, dyes, and food, as is the need for reliable and efficient methods of procuring these compounds. To develop sustainable and cost-effective approaches, a comprehensive understanding of the biosynthetic pathways and the factors influencing secondary metabolite production is essential. These compounds are a unique type of natural product which recognizes the oxidative damage caused by stresses, thereby activating the defence mechanism in plants. Various methods have been developed to enhance the production of secondary metabolites in plants. The elicitor-induced in vitro culture technique is considered an efficient tool for studying and improving the production of secondary metabolites in plants. In the present review, we have documented various biosynthetic pathways and the role of secondary metabolites under diverse environmental stresses. Furthermore, a practical strategy for obtaining consistent and abundant secondary metabolite production via various elicitation agents used in culturing techniques is also mentioned. By elucidating the intricate interplay of regulatory factors, this review paves the way for future advancements in sustainable and efficient production methods for high-value secondary metabolites.

Separation of Antioxidant-Rich Alternanthera Sessilis Red Extracts by Sephadex LH-20 and Identification of Polyphenols Using HPLC-QToF-MS/MS

This study aimed to fractionate Alternanthera sessilis Red (ASR) crude extracts and determine their antioxidant activities as well as the related active components in the whole plant. ASR was extracted with water and ethanol, and further separated using a Sephadex LH-20 column. Following the assessments of the polyphenolic contents and antioxidant activities of crude extracts (H2 OASR and EtOHASR ) and fractions, a HPLC-QToF analysis was performed on the crude extracts and selected fractions (H2 OASR FII and EtOHASR FII). Three water fractions (H2 OASR FI, FII and FIII) and four ethanolic fractions (EtOHASR FI, FII, FIII and FIV) were derived from their crude extracts, respectively. EtOHASR FII exhibited the greatest total phenolic content (120.41 mg GAE/g fraction), total flavonoid content (223.07 mg RE/g fraction), and antioxidant activities (DPPH IC50 =159.43 μg/mL; FRAP=1.93 mmol Fe2+ /g fraction; TEAC=0.90 mmol TE/g fraction). Correlation analysis showed significant (p<0.01) positive correlations between both TPC (r=0.748-0.970) and TFC (r=0.686-0.949) with antioxidant activities in the crude extracts and fractions. Flavonoids were the major compounds in the four selected samples tentatively identified using HPLC-QToF-MS/MS, with the highest number of 30 polyphenol compounds detected in the most active fraction, EtOHASR FII.

Phytomelatonin interferes with flavonols biosynthesis to regulate ROS production and stomatal closure in tobacco

Flavonols are well-known antioxidants that prevent stomatal closure via interfering with ROS signaling. Phytomelatonin regulates stomatal closure, but the signaling pathways are still largely unknown. Here, we investigated the role of flavonols in phytomelatonin-mediated stomatal closure in tobacco plants. The application of melatonin induced stomatal closure through NADPH oxidase-mediated ROS production. Transgenic tobacco plants overexpressing soybean GmSNAT1 (coding for serotonin N-acetyltransferase that catalyzes the penultimate step in phytomelatonin biosynthesis) had higher phytomelatonin concentration, accumulated more ROS in guard cells and were more sensitive to melatonin-induced stomatal closure than the wild-type plants, which was associated with the higher expression of PMTR1-homologous genes. Exogenous melatonin decreased flavonol concentrations in guard cells and the expression of flavonoid-related genes in wild-type and transgenic tobacco plants, and these inhibitory effects were more obvious in GmSNAT1-overexpressing plants than the wild type. However, the melatonin-mediated stomatal closure and ROS production were diminished by the application of kaempferol (a type of flavonol). Additionally, transgenic tobacco plants with increased expression of NtFLS (encoding flavonol synthase) were less sensitive to melatonin-induced stomatal closure. In conclusion, phytomelatonin hampers the biosynthesis of flavonols in guard cells, which results in high concentration of ROS and induces stomatal closure in tobacco plants.

Effects of Different Light Wavelengths on Fruit Quality and Gene Expression of Anthocyanin Biosynthesis in Blueberry (Vaccinium corymbosm)

Different light wavelengths display diverse effects on fruit quality formation and anthocyanin biosynthesis. Blueberry is a kind of fruit rich in anthocyanin with important economic and nutritional values. This study explored the effects of different light wavelengths (white (W), red (R), blue (B) and yellow (Y)) on fruit quality and gene expression of anthocyanin biosynthesis in blueberry. We found that the B and W treatments attained the maximum values of fruit width, fruit height and fruit weight in blueberry fruits. The R treatment attained the maximum activities of superoxide dismutase (SOD) and peroxidase (POD), and the Y treatment displayed the maximum contents of ascorbic acid (AsA), glutathione (GSH) and total phenol in fruits, thus improving blueberry-fruit antioxidant capacity. Interestingly, there were differences in the solidity-acid ratio of fruit under different light-wavelength treatments. Moreover, blue light could significantly improve the expression levels of anthocyanin biosynthesis genes and anthocyanin content in fruits. Correlation and principal component analysis showed that total acid content and antioxidant enzymes were significantly negatively correlated with anthocyanin content in blueberry fruits. These results provide new insights for the application of light wavelength to improve blueberry fruit quality and anthocyanin content.

Integrative Transcriptomic and Metabolomic Analysis Reveals the Molecular Mechanism of Red Maple (Acer rubrum L.) Leaf Coloring

This study employed a combination of ultraviolet spectrophotometry, LC-ESI-MS/MS system, and RNA-sequencing technology; the extracts and isolation of total RNA from the red and yellow leaf strains of red maple (Acer rubrum L.) at different developmental stages were subjected to an intercomparison of the dynamic content of chlorophyll and total anthocyanin, flavonoid metabolite fingerprinting, and gene expression. The metabonomic results indicated that one hundred and ninety-two flavonoids were identified, which could be classified into eight categories in the red maple leaves. Among them, 39% and 19% were flavones and flavonols, respectively. The metabolomic analysis identified 23, 32, 24, 24, 38, and 41 DAMs in the AR1018r vs. AR1031r comparison, the AR1018r vs. AR1119r comparison, the AR1031r vs. AR1119r comparison, the AR1018y vs. AR1031y comparison, the AR1018y vs. AR1119y comparison, and the AR1031y vs. AR1119y comparison, respectively. In total, 6003 and 8888 DEGs were identified in AR1018r vs. AR1031r comparison and in the AR1018y vs. AR1031y comparison, respectively. The GO and KEGG analyses showed that the DEGs were mainly involved in plant hormone signal transduction, flavonoid biosynthesis, and other metabolite metabolic processes. The comprehensive analysis revealed that caffeoyl-CoA 3-O-methyltransferase (Cluster-28704.45358 and Cluster-28704.50421) was up-regulated in the red strain but down-regulated in the yellow strain, while Peonidin 3-O-glucoside chloride and Pelargonidin 3-O-beta-D-glucoside were up-regulated in both the red and yellow strains. By successfully integrating the analyses on the behavior of pigment accumulation, dynamics of flavonoids, and differentially expressed genes with omics tools, the regulation mechanisms underlying leaf coloring in red maple at the transcriptomic and metabolomic levels were demonstrated, and the results provide valuable information for further research on gene function in red maple.

Exogenous abscisic acid and sodium nitroprusside regulate flavonoid biosynthesis and photosynthesis of Nitraria tangutorum Bobr in alkali stress

Abscisic acid (ABA) and nitric oxide (NO) are involved in mediating abiotic stress-induced plant physiological responses. Nitraria tangutorum Bobr is a typical salinized desert plant growing in an arid environment. In this study, we investigated the effects of ABA and NO on N.tangutorum seedlings under alkaline stress. Alkali stress treatment caused cell membrane damage, increased electrolyte leakage, and induced higher production of reactive oxygen species (ROS), which caused growth inhibition and oxidative stress in N.tangutorum seedlings. Exogenous application of ABA (15μm) and Sodium nitroprusside (50μm) significantly increased the plant height, fresh weight, relative water content, and degree of succulency in N.tangutorum seedlings under alkali stress. Meanwhile, the contents of ABA and NO in plant leaves were significantly increased. ABA and SNP can promote stomatal closure, decrease the water loss rate, increase leaf surface temperature and the contents of osmotic regulator proline, soluble protein, and betaine under alkali stress. Meanwhile, SNP more significantly promoted the accumulation of chlorophyll a/b and carotenoids, increased quantum yield of photosystem II (φPSII) and electron transport rate (ETRII) than ABA, and decreased photochemical quenching (qP), which improved photosynthetic efficiency and accelerated the accumulation of soluble sugar, glucose, fructose, sucrose, starch, and total sugar. However, compared with exogenous application of SNP in the alkaline stress, ABA significantly promoted the transcription of NtFLS/NtF3H/NtF3H/NtANR genes and the accumulation of naringin, quercetin, isorhamnetin, kaempferol, and catechin in the synthesis pathway of flavonoid metabolites, and isorhamnetin content was the highest. These results indicate that both ABA and SNP can reduce the growth inhibition and physiological damage caused by alkali stress. Among them, SNP has a better effect on the improvement of photosynthetic efficiency and the regulation of carbohydrate accumulation than ABA, while ABA has a more significant effect on the regulation of flavonoid and anthocyanin secondary metabolite accumulation. Exogenous application of ABA and SNP also improved the antioxidant capacity and the ability to maintain Na⁺/K⁺ balance of N. tangutorum seedlings under alkali stress. These results demonstrate the beneficial effects of ABA and NO as stress hormones and signaling molecules that positively regulate the defensive response of N. tangutorum to alkaline stress.

GbFLSa overexpression negatively regulates proanthocyanin biosynthesis

Flavonoids are important secondary metabolites with extensive pharmacological functions. Ginkgo biloba L. (ginkgo) has attracted extensive attention because of its high flavonoid medicinal value. However, little is understood about ginkgo flavonol biosynthesis. Herein, we cloned the full-length gingko GbFLSa gene (1314 bp), which encodes a 363 amino acid protein that has a typical 2-oxoglutarate (2OG)-Fe(II) oxygenase region. Recombinant GbFLSa protein with a molecular mass of 41 kDa was expressed in Escherichia coli BL21(DE3). The protein was localized to the cytoplasm. Moreover, proanthocyanins, including catechin, epicatechin, epigallocatechin and gallocatechin, were significantly less abundant in transgenic poplar than in nontransgenic (CK) plants. In addition, dihydroflavonol 4-reductase, anthocyanidin synthase and leucoanthocyanidin reductase expression levels were significantly lower than those of their CK counterparts. GbFLSa thus encodes a functional protein that might negatively regulate proanthocyanin biosynthesis. This study helps elucidate the role of GbFLSa in plant metabolism and the potential molecular mechanism of flavonoid biosynthesis.

Key metabolites and mechanistic insights in forchlorfenuron controlling kiwifruit development

Forchlorfenuron (CPPU) is a plant growth regulator widely applied on kiwifruit to improve yield, however, there are rarely reports on its effects on the nutrients of kiwifruits. Based on UHPLC-Q-TOF-MS, the effects of CPPU on metabolism profile and nutrient substances of two kiwifruit varieties during development were investigated by non-targeted metabolomics. A total of 115 metabolites were identified, and 29 differential metabolites were confirmed and quantified using certified reference standards. Metabolic profile indicated that CPPU promoted kiwifruit development during the main expansion stages at the molecular level, and the effects varied slightly for different varieties. In the early and middle stages of kiwifruit development, the anthocyanin, flavone and flavonol biosynthesis were down-regulated in both varieties, and flavanols biosynthesis was down-regulated only in Hayward variety. Arginine biosynthesis was down-regulated at all stages till the harvest. Although the synthesis of these nutrient substances in kiwifruits was mostly down-regulated by CPPU, the negative effects became mild at harvest time, and positively, the significant increase of sucrose and decrease of organic acids at harvest time could help to improve the taste of kiwifruits.

Transcription and Metabolism Pathways of Anthocyanin in Purple Shamrock (Oxalis triangularis A.St.-Hil.)

Anthocyanins are water-soluble pigments that can impart various colors to plants. Purple shamrock (Oxalis triangularis) possesses unique ornamental value due to its purple leaves. In this study, three anthocyanins, including malvidin 3-O-(4-O-(6-O-malonyl-glucopyranoside)-rhamnopyranosyl)-5-O-(6-O-malonyl-glucopyranoside), delphinidin-3-O-rutinoside and malvidin-3,5-di-O-glucoside, were characterized with ultra-performance liquid chromatography-electrospray ionization tandem mass spectrometry (UPLC-ESI-MS/MS) in purple shamrock. To investigate the molecular mechanism of anthocyanin biosynthesis in green shamrock (Oxalis corymbosa) and purple shamrock, RNA-seq and qRT-PCR were performed, and the results showed that most of the anthocyanin biosynthetic and regulatory genes were up-regulated in purple shamrock. Then, dark treatment and low temperature treatment experiments in purple shamrock showed that both light and low temperature can induce the biosynthesis of anthocyanins.

Diverse Physiological Roles of Flavonoids in Plant Environmental Stress Responses and Tolerance

Flavonoids are characterized as the low molecular weight polyphenolic compounds universally distributed in planta. They are a chemically varied group of secondary metabolites with a broad range of biological activity. The increasing amount of evidence has demonstrated the various physiological functions of flavonoids in stress response. In this paper, we provide a brief introduction to flavonoids' biochemistry and biosynthesis. Then, we review the recent findings on the alternation of flavonoid content under different stress conditions to come up with an overall picture of the mechanism of involvement of flavonoids in plants' response to various abiotic stresses. The participation of flavonoids in antioxidant systems, flavonoid-mediated response to different abiotic stresses, the involvement of flavonoids in stress signaling networks, and the physiological response of plants under stress conditions are discussed in this review. Moreover, molecular and genetic approaches to tailoring flavonoid biosynthesis and regulation under abiotic stress are addressed in this review.

An insight into the potent medicinal plant Phyllanthus amarus Schum. and Thonn

Phyllanthus amarus Schum. and Thonn., a globally distributed herb is known for its several therapeutic potentials. P. amarus has a long history of use in the traditional system of medicine for over 2000 years owing to its wide array of secondary metabolites that confer significant medicinal attributes. Research on various aspects including ethnobotany, phytochemistry to bioactivity, or pharmacological studies has been conducted over the past several decades on this potent herb. P. amarus extracts have shown a broad range of pharmacological activities like hepatoprotective, antioxidant, antiviral, antimicrobial, antidiabetic, anti-inflammatory, anticancer, antimalarial, nephroprotective, diuretic, and several other properties. The present review compiles and covers literature and research of several groups across past decades to date and focuses on how the therapeutic significance of this plant can be further explored for future research either as herbal formulations, alternative medicine, or in the pharmaceutical industry.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13237-022-00409-z.

Physiological and transcriptomic analysis uncovers salinity stress mechanisms in a facultative crassulacean acid metabolism plant Dendrobium officinale

Dendrobium officinale is a precious medicinal Chinese herb that employs facultative crassulacean acid metabolism (CAM) and has a high degree of abiotic stress tolerance, but the molecular mechanism underlying the response of this orchid to abiotic stresses is poorly understood. In this study, we analyzed the root microstructure of D. officinale plantlets and verified the presence of chloroplasts by transmission electron microscopy. To obtain a more comprehensive overview of the molecular mechanism underlying their tolerance to abiotic stress, we performed whole-transcriptome sequencing of the roots of 10-month-old plantlets exposed to salt (NaCl) treatment in a time-course experiment (0, 4 and 12 h). The total of 7376 differentially expressed genes that were identified were grouped into three clusters (P < 0.05). Metabolic pathway analysis revealed that the expression of genes related to hormone (such as auxins, cytokinins, abscisic acid, ethylene and jasmonic acid) biosynthesis and response, as well as the expression of genes related to photosynthesis, amino acid and flavonoid metabolism, and the SOS pathway, were either up- or down-regulated after salt treatment. Additionally, we identified an up-regulated WRKY transcription factor, DoWRKY69, whose ectopic expression in Arabidopsis promoted seed germination under salt tress. Collectively, our findings provide a greater understanding of the salt stress response mechanisms in the roots of a facultative CAM plant. A number of candidate genes that were discovered may help plants to cope with salt stress when introduced via genetic engineering.

Integrated analysis of multi-omics and fine-mapping reveals a candidate gene regulating pericarp color and flavonoids accumulation in wax gourd (Benincasa hispida)

Wax gourd (Benincasa hispida), a popular fruit of the Cucurbitaceae (cucurbits) family, contains many nutrients with health benefits and is widely grown in China and other tropical areas. In this study, a wax gourd mutant hfc12 with light-color pericarp was obtained through ethane methylsulfonate (EMS) mutagenesis. Integrative analysis of the metabolome and transcriptome identified 31 differentially accumulated flavonoids (DAFs; flavonoids or flavonoid glycosides) and 828 differentially expressed genes (DEGs) between the hfc12 mutant and wild-type 'BWT'. Furthermore, BSA-seq and kompetitive allele specific PCR (KASP) analysis suggested that the light-color pericarp and higher flavonoid content was controlled by a single gene BhiPRR6 (Bhi12M000742), a typical two-component system (TCS) pseudo-response regulator (PRR). Genetic analysis detected only one nonsynonymous mutation (C-T) in the second exon region of the BhiPRR6. Weighted correlation network analysis (WGCNA) identified the downstream target genes of BhiPRR6, probably regulated by light and were intermediated in the regulatory enzyme reaction of flavonoid biosynthetic pathway. Thus, these results speculated that the transcription factor BhiPRR6, interacting with multiple genes, regulates the absorption of light signals and thereby changes the pericarp color and synthesis of flavonoids in wax gourd.

Comparative Transcriptome Analysis of Purple and Green Non-Heading Chinese Cabbage and Function Analyses of BcTT8 Gene

Non-heading Chinese cabbage (Brassica campestris ssp. chinensis) is an important vegetative crop in the south of China. As an antioxidant, anthocyanin is the major quality trait for vegetables with purple leaves or petioles. However, the molecular biosynthetic mechanism of anthocyanin in non-heading Chinese cabbage has not been explained exclusively. In this study, two non-heading Chinese cabbage with contrasting colors in the leaves were used as the materials for RNA-seq. A total of 906 DEGs were detected, and we found that the anthocyanin and flavonoid biosynthetic pathways are significantly enriched in the purple NHCC. The transcriptome result was verified by RT-qPCR. Though bioinformatics analysis, BcTT8 was selected as the candidate gene for the regulation of anthocyanin synthesis, and the characterization of BcTT8 was elucidated by the functional analyses. The results proved that BcTT8 is a nucleus protein and phylogenetically close to the TT8 protein from Brassica. After silencing BcTT8, the total anthocyanin content of pTY-BcTT8 plants decreased by 42.5%, and the relative expression levels of anthocyanin pathway genes BcDFR, BcLODX and BcUF3GT-1 were significantly downregulated, while the transcription level of BcFLS was significantly upregulated. Compared with the wild type, the transgenic Arabidopsis showed obvious violet in the cotyledons part, and the anthocyanin biosynthetic genes such as AtDFR and AtLODX were significantly upregulated. In conclusion, BcTT8 is critical in the anthocyanin synthesis process of non-heading Chinese cabbage. Our findings illustrated the molecular mechanism of anthocyanin biosynthesis in non-heading Chinese cabbage.

Association of Bitter Metabolites and Flavonoid Synthesis Pathway in Jujube Fruit

Jujube is rich in nutrients and can be eaten fresh or made into dried fruit, candied fruit, and preserved fruit. Its slightly bitter peel affects nutritional value and commercial value, but the mechanism of the formation of bitter substances is still unclear. We dynamically analyzed the biosynthesis of jujube peel bitterness and related nutrient metabolites through the transcriptome and metabolome. The results demonstrated that flavonoids were the main bitter substances in 'Junzao' jujube fruit skins and a total of 11,106 differentially expressed genes and 94 differentially abundant flavonoid metabolites were identified. Expression patterns of genes in the flavonoid synthesis pathway showed that flavonol synthase (FLS) expression was significantly correlated with quercetin content. Transient overexpression and virus induced gene silencing (VIGS) of ZjFLS1 and ZjFLS2 in jujube fruits and sour jujube seedlings significantly affected flavonol accumulation, especially the content of quercetin-3-O-rutinoside. Moreover, in vitro enzymatic reactions showed that ZjFLS1 and ZjFLS2 could catalyze the formation of quercetin from dihydroquercetin. These findings indicate that ZjFLS gene is the key gene in the biosynthesis of bitter substances in jujube fruit skins and provide basis for the research on the development of functional nutrients in jujube and the synthesis mechanism of bitter compounds.

Integration of Transcriptome and Metabolome Reveals the Formation Mechanism of Red Stem in Prunus mume

Prunus mume var. purpurea, commonly known as "Red Bone", is a special variety with pink or purple-red xylem. It is famous due to gorgeous petals and delightful aromas, playing important roles in urban landscaping. The regulation mechanism of color formation in P. mume var. purpurea stem development is unclear. Here, we conducted a comprehensive analysis of transcriptome and metabolome in WYY ('Wuyuyu' accession, red stem) and FLE ('Fei Lve' accession, green stem), and found a total of 256 differential metabolites. At least 14 anthocyanins were detected in WYY, wherein cyanidin 3,5-O-diglucoside and peonidin3-O-glucoside were significantly accumulated through LC-MS/MS analysis. Transcriptome data showed that the genes related to flavonoid-anthocyanin biosynthesis pathways were significantly enriched in WYY. The ratio of dihydroflavonol 4-reductase (DFR) and flavonol synthase (FLS) expression levels may affect metabolic balance in WYY, suggesting a vital role in xylem color formation. In addition, several transcription factors were up-regulated, which may be the key factors contributing to transcriptional changes in anthocyanin synthesis. Overall, the results provide a reference for further research on the molecular mechanism of xylem color regulation in P. mume and lay a theoretical foundation for cultivating new varieties.

Comparison of Metabolome and Transcriptome of Flavonoid Biosynthesis in Two Colors of Coreopsis tinctoria Nutt

Coreopsis tinctoria Nutt. (C. tinctoria) has a long history of application and high economic and medicinal value. Flavonoids, the main active components of C. tinctoria, are widely studied in pharmacology and food development. However, the flavonoid biosynthesis pathway in C. tinctoria is unclear. In this study, we comprehensively compared the transcriptomes and metabolite profiles of two colors of C. tinctoria flowers (LS and JS) at different flowering stages. A total of 165 flavonoids (46 flavonoids, 42 flavonols, 22 anthocyanins, 18 chalcones, 12 dihydroflavonols, nine isoflavones, eight dihydroflavonoids, six flavanols, and two tannins) were identified in LS and JS at different flowering stages. Thirty-three metabolites (11 anthocyanins, 11 flavonols, seven flavonoids, two dihydroflavonols, one dihydroflavone, and one chalcone) were found to be statistically significantly different in the LS vs. JS groups. LS flowers accumulated higher levels of 10 anthocyanins (seven cyanidins and three pelargonidins) than JS flowers. Furthermore, candidate genes related to the regulation of flavonoid and anthocyanin synthesis were identified and included 28 structural genes (especially F3H, Cluster-28756.299649, and 3GT, Cluster-28756.230942) in LS and JS, six key differentially expressed transcription factors (especially MYB90a, Cluster-28756.143139) in LS and JS, and 17 other regulators (mainly including transporter proteins and others) in LS. Our results provide valuable information for further studies on the mechanism underlying flavonoid biosynthesis in C. tinctoria.

MdSCL8 as a Negative Regulator Participates in ALA-Induced FLS1 to Promote Flavonol Accumulation in Apples

Apples (Malus domestica) are rich in flavonols, and 5-aminolevulinic acid (ALA) plays an important role in the regulation of plant flavonoid metabolism. To date, the underlying mechanism of ALA promoting flavonol accumulation is unclear. Flavonol synthase (FLS) is a key enzyme in flavonol biosynthesis. In this study, we found that ALA could enhance the promoter activity of MdFLS1 in the ‘Fuji’ apple and improve its expression. With MdFLS1 as bait, we screened a novel transcription factor MdSCL8 by the Yeast One-Hybrid (Y1H) system from the apple cDNA library which we previously constructed. Using luciferase reporter assay and transient GUS activity assay, we verified that MdSCL8 inhibits the activity of MdFLS1 promoter and hinders MdFLS1 expression, thus reducing flavonol accumulation in apple. ALA significantly inhibited MdSCL8 expression. Therefore, ALA promoted the expression of MdFLS1 and the consequent flavonol accumulation probably by down-regulating MdSCL8. We also found that ALA significantly enhanced the gene expression of MdMYB22 and MdHY5, two positive regulators of MdFLS. We further demonstrated that MdMYB22 interacts with MdHY5, but neither of them interacts with MdSCL8. Taken together, our data suggest MdSCL8 as a novel regulator of MdFLS1 and provide important insights into mechanisms of ALA-induced flavonol accumulation in apples.

TMT-based comparative proteomic analysis of the male-sterile mutant ms01 sheds light on sporopollenin production and pollen development in wucai (Brassica campestris L.)

A spontaneous male-sterile mutant ms01 was discovered from the excellent high-generation inbred line 'hx12-6-3' in wucai. Compared with wild-type 'hx12-6-3', ms01 displayed complete male sterility with degenerated stamens and no pollen. In this study, cytological observation revealed that the tapetum of the anthers of ms01 had degraded in advance, and microspore development had stagnated in the mononuclear stage, ultimately resulting in completely aborted pollen. Genetic analysis indicated that the sterility of ms01 was controlled by a single recessive nuclear gene. In the differential proteomic analysis of 'hx12-6-3' and ms01 flower buds using a tandem mass tags-based approach, a comparison of two stages (stage a and stage e) revealed 1272 differentially abundant proteins (DAPs). The abnormal variation of the anther cuticle, pollen coat, and sporopollenin production were effected by lipid metabolism and phenylpropanoid biosynthesis in the mutant ms01. Further analysis elucidated that pollen development was associated with amino acid metabolism, protein synthesis and degradation, carbohydrate metabolism, flavonoid biosynthesis and glutathione metabolism. These results provide novel insights into the molecular mechanism of GMS (genic male sterility) in wucai. SIGNIFICANCE: ms01, as the first indentified spontaneous male-sterile mutant in wucai, plays a significant role in the initial study of GMS (genic male sterility). In our study, the key DAPs related to anther and pollen development were obtained by TMT-based comparative proteomic analysis. We found that the abnormal accumulation of H₂O₂ might induce premature degradation of the tapetum, causing anther metabolism disorder and pollen abortion. This process involved multiple DAPs and formed a complex regulatory network that generated a series of physiological metabolic alterations, ultimately leading to male sterility. Our results provide a theoretical foundation for further research on the complex anther and pollen development process.

CmMYB9a activates floral coloration by positively regulating anthocyanin biosynthesis in chrysanthemum

KEY MESSAGE

An R2R3-MYB transcription factor, CmMYB9a, activates floral coloration in chrysanthemum by positively regulating CmCHS, CmDFR and CmFNS, but inhibiting the expression of CmFLS. Chrysanthemum is one of the most popular ornamental plants worldwide. Flavonoids, such as anthocyanins, flavones, and flavonols, are important secondary metabolites for coloration and are involved in many biological processes in plants, like petunia, snapdragon, Gerbera hybrida, as well as chrysanthemum. However, the metabolic regulation of flavonoids contributing to chrysanthemum floral coloration remains largely unexplored. Here, an R2R3-MYB transcription factor, CmMYB9a, was found to be involved in flavonoid biosynthesis. Phylogenetic analysis and amino acid sequence analysis suggested that CmMYB9a belonged to subgroup 7. Transient overexpression of CmMYB9a in flowers of chrysanthemum cultivar 'Anastasia Pink' upregulated the anthocyanin-related and flavone-related genes and downregulated CmFLS, which led to the accumulation of anthocyanins and flavones. We further demonstrated that CmMYB9a independently activates the expression of CmCHS, CmDFR and CmFNS, but inhibits the expression of CmFLS. Overexpression of CmMYB9a in tobacco resulted in increased anthocyanins and decreased flavonols in the petals by upregulating NtDFR and downregulating NtFLS. These results suggest that CmMYB9a facilitates metabolic flux into anthocyanin and flavone biosynthesis. Taken together, this study functionally characterizes the role of CmMYB9a in regulating the branched pathways of flavonoids in chrysanthemum flowers.

A Moss 2-Oxoglutarate/Fe(II)-Dependent Dioxygenases (2-ODD) Gene of Flavonoids Biosynthesis Positively Regulates Plants Abiotic Stress Tolerance

Flavonoids, the largest group of polyphenolic secondary metabolites present in all land plants, play essential roles in many biological processes and defense against abiotic stresses. In the flavonoid biosynthesis pathway, flavones synthase I (FNSI), flavanone 3-hydroxylase (F3H), flavonol synthase (FLS), and anthocyanidin synthase (ANS) all belong to 2-oxoglutarate/Fe(II)-dependent dioxygenases (2-ODDs) family, which catalyzes the critical oxidative reactions to form different flavonoid subgroups. Here, a novel 2-ODD gene was cloned from Antarctic moss Pohlia nutans (Pn2-ODD1) and its functions were investigated both in two model plants, Physcomitrella patens and Arabidopsis thaliana. Heterologous expression of Pn2-ODD1 increased the accumulation of anthocyanins and flavonol in Arabidopsis. Meanwhile, the transgenic P. patens and Arabidopsis with expressing Pn2-ODD1 exhibited enhanced tolerance to salinity and drought stresses, with larger gametophyte sizes, better seed germination, and longer root growth. Heterologous expression of Pn2-ODD1 in Arabidopsis also conferred the tolerance to UV-B radiation and oxidative stress by increasing antioxidant capacity. Therefore, we showed that Pn2-ODD1 participated in the accumulation of anthocyanins and flavonol in transgenic plants, and regulated the tolerance to abiotic stresses in plants, contributing to the adaptation of P. nutans to the polar environment.

The Flavonoid Biosynthesis Network in Plants

Flavonoids are an important class of secondary metabolites widely found in plants, contributing to plant growth and development and having prominent applications in food and medicine. The biosynthesis of flavonoids has long been the focus of intense research in plant biology. Flavonoids are derived from the phenylpropanoid metabolic pathway, and have a basic structure that comprises a C15 benzene ring structure of C6-C3-C6. Over recent decades, a considerable number of studies have been directed at elucidating the mechanisms involved in flavonoid biosynthesis in plants. In this review, we systematically summarize the flavonoid biosynthetic pathway. We further assemble an exhaustive map of flavonoid biosynthesis in plants comprising eight branches (stilbene, aurone, flavone, isoflavone, flavonol, phlobaphene, proanthocyanidin, and anthocyanin biosynthesis) and four important intermediate metabolites (chalcone, flavanone, dihydroflavonol, and leucoanthocyanidin). This review affords a comprehensive overview of the current knowledge regarding flavonoid biosynthesis, and provides the theoretical basis for further elucidating the pathways involved in the biosynthesis of flavonoids, which will aid in better understanding their functions and potential uses.

Elucidation of myricetin biosynthesis in Morella rubra of the Myricaceae

Flavonols are health-promoting bioactive compounds important for plant defense and human nutrition. Quercetin (Q) and kaempferol (K) biosynthesis have been studied extensively while little is known about myricetin (M) biosynthesis. The roles of flavonol synthases (FLSs) and flavonoid 3',5'-hydroxylase (F3'5'H) in M biosynthesis in Morella rubra, a member of the Myricaceae rich in M-based flavonols, were investigated. The level of MrFLS transcripts alone did not correlate well with the accumulation of M-based flavonols. However, combined transcript data for MrFLS1 and MrF3'5'H showed a good correlation with the accumulation of M-based flavonols in different tissues of M. rubra. Recombinant MrFLS1 and MrFLS2 proteins showed strong activity with dihydroquercetin (DHQ), dihydrokaempferol (DHK), and dihydromyricetin (DHM) as substrates, while recombinant MrF3'5'H protein preferred converting K to M, amongst a range of substrates. Tobacco (Nicotiana tabacum) overexpressing 35S::MrFLSs produced elevated levels of K-based and Q-based flavonols without affecting M-based flavonol levels, while tobacco overexpressing 35S::MrF3'5'H accumulated significantly higher levels of M-based flavonols. We conclude that M accumulation in M. rubra is affected by gene expression and enzyme specificity of FLS and F3'5'H as well as substrate availability. In the metabolic grid of flavonol biosynthesis, the strong activity of MrF3'5'H with K as substrate additionally promotes metabolic flux towards M in M. rubra.

Differential Phenolic Compounds and Hormone Accumulation Patterns between Early- and Mid-Maturing Sweet Cherry (Prunus avium L.) Cultivars during Fruit Development and Ripening

Color acquisition is one of the most distinctive features of fruit development and ripening processes. The color red is closely related to the accumulation of polyphenolic compounds, mainly anthocyanins, during sweet cherry fruit maturity. In non-climacteric fruit species like sweet cherry, the maturity process is mainly controlled by the phytohormone abscisic acid (ABA), though other hormones may also play a role. However, the coordinated stage-specific production of polyphenolic compounds and their relation with hormone content variations have not been studied in depth in sweet cherry fruits. To further understand the accumulation dynamics of these compounds (hormones and metabolites) during fruit development, two sweet cherry cultivars ("Lapins" and "Glenred") with contrasting maturity timing phenotypes were analyzed using targeted metabolic analysis. The ultra-high performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS) approach revealed that phenolic acids, flavonols, and flavan-3-ols accumulated mainly until the straw-yellow stage in the early-maturing cultivar, while accumulation was mainly at the green stage in the mid-maturing cultivar, suggesting a cultivar-dependent stage-specific production of secondary metabolites. In the mid-maturing cultivar, anthocyanins were detected only from the red stage onward, whereas detection began at the pink stage in the early-maturing cultivar. ABA negatively correlated (p-value < 0.05) with the flavonols and flavan-3-ols in both cultivars. ABA and anthocyanin content increased at the same time in the early-season cultivar. In contrast, anthocyanins accumulated and the pink color initiation started several days after the ABA increase in the mid-maturing cultivar. Differential accumulation patterns of GA₄, a ripening antagonizing hormone, between the cultivars could explain this difference. These results showed that both red-colored cultivars presented different accumulation dynamics of phenolic compounds and plant hormones during fruit development, suggesting underlying differences in the sweet cherry fruit color evolution.

EkFLS overexpression promotes flavonoid accumulation and abiotic stress tolerance in plant

Flavonoids with great medicinal value play an important role in plant individual growth and stress resistance. Flavonol synthetase (FLS) is one of the key enzymes to synthesize flavonoids. However, the role of the FLS gene in flavonoid accumulation and tolerance to abiotic stresses, as well as its mechanism has not yet been investigated systematically in plants. The aim of this research is to evaluate the effect of FLS overexpression on the accumulation of active ingredients and stress resistance in Euphorbia kansui Liou. The results showed that when the EkFLS gene was overexpressed in Arabidopsis thaliana, the accumulation of flavonoids was improved. In addition, when the wild-type and EkFLS overexpressed Arabidopsis plants were treated with ABA and MeJA, compared with WT Arabidopsis, EkFLS overexpressed Arabidopsis promoted stomatal aperture to influence photosynthesis of the plants, which in turn can promote stress resistance. Meanwhile, under MeJA, NaCl, and PEG treatment, EkFLS overexpressed in Arabidopsis induced higher accumulation of flavonoids, which significantly enhanced peroxidase (POD) and superoxide dismutase (SOD) activities that can scavenge reactive oxygen species in cells to protect the plant. These results indicated that EkFLS overexpression is strongly correlated to the increase of flavonoid synthesis and therefore the tolerance to abiotic stresses in plants, providing a theoretical basis for further improving the quality of medicinal plants and their resistance to abiotic stresses simultaneously.

Comparative transcriptome between male fertile and male sterile alfalfa (Medicago varia)

Male sterility is an important factor in improving crop quality and yield through heterosis breeding. In this study, we analyzed the transcriptomes of male fertile (MF) and male sterile (MS) alfalfa flower buds using the Illumina HiSeq™ 4000 platform. A total of 54.05 million clean reads were generated and assembled into 65,777 unigenes with an average length of 874 bp. The differentially expressed genes (DEGs) between the MF and MS flowers at three stages of pollen development were identified, and there were 3832, 5678 and 5925 DEGs respectively in stages 1, 2 and 3. GO and KEGG functional enrichment analysis revealed 12, 12, 6 and 12 key branch-point genes involved in circadian rhythm, transcription factors, pollen development and flavonoid biosynthesis. Our findings provide novel insights into the mechanism of male sterility in alfalfa.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01026-x.

Integrative iTRAQ-based proteomic and transcriptomic analysis reveals the accumulation patterns of key metabolites associated with oil quality during seed ripening of Camellia oleifera

Camellia oleifera (C. oleifera) is one of the four major woody oil-bearing crops in the world and has relatively high ecological, economic, and medicinal value. Its seeds undergo a series of complex physiological and biochemical changes during ripening, which is mainly manifested as the accumulation and transformation of certain metabolites closely related to oil quality, especially flavonoids and fatty acids. To obtain new insights into the underlying molecular mechanisms, a parallel analysis of the transcriptome and proteome profiles of C. oleifera seeds at different maturity levels was conducted using RNA sequencing (RNA-seq) and isobaric tags for relative and absolute quantification (iTRAQ) complemented with gas chromatography-mass spectrometry (GC-MS) data. A total of 16,530 transcripts and 1228 proteins were recognized with significant differential abundances in pairwise comparisons of samples at various developmental stages. Among these, 317 were coexpressed with a poor correlation, and most were involved in metabolic processes, including fatty acid metabolism, α-linolenic acid metabolism, and glutathione metabolism. In addition, the content of total flavonoids decreased gradually with seed maturity, and the levels of fatty acids generally peaked at the fat accumulation stage; these results basically agreed with the regulation patterns of genes or proteins in the corresponding pathways. The expression levels of proteins annotated as upstream candidates of phenylalanine ammonia-lyase (PAL) and chalcone synthase (CHS) as well as their cognate transcripts were positively correlated with the variation in the flavonoid content, while shikimate O-hydroxycinnamoyltransferase (HCT)-encoding genes had the opposite pattern. The increase in the abundance of proteins and mRNAs corresponding to alcohol dehydrogenase (ADH) was associated with a reduction in linoleic acid synthesis. Using weighted gene coexpression network analysis (WGCNA), we further identified six unique modules related to flavonoid, oil, and fatty acid anabolism that contained hub genes or proteins similar to transcription factors (TFs), such as MADS intervening keratin-like and C-terminal (MIKC_MADS), type-B authentic response regulator (ARR-B), and basic helix-loop-helix (bHLH). Finally, based on the known metabolic pathways and WGCNA combined with the correlation analysis, five coexpressed transcripts and proteins composed of cinnamyl-alcohol dehydrogenases (CADs), caffeic acid 3-O-methyltransferase (COMT), flavonol synthase (FLS), and 4-coumarate: CoA ligase (4CL) were screened out. With this exploratory multiomics dataset, our results presented a dynamic picture regarding the maturation process of C. oleifera seeds on Hainan Island, not only revealing the temporal specific expression of key candidate genes and proteins but also providing a scientific basis for the genetic improvement of this tree species.

Simultaneous changes in anthocyanin, chlorophyll, and carotenoid contents produce green variegation in pink-leaved ornamental kale

BACKGROUND

Anthocyanin, chlorophyll, and carotenoid pigments are widely distributed in plants, producing various colors. Ornamental kale (Brassica oleracea var. acephala DC) which has colorful inner leaves is an ideal plant to explore how these three pigments contribute to leaf color. The molecular mechanisms of the coloration in ornamental kale could provide reference for exploring the mechanisms of pigmentation in other plants.

RESULTS

In this study, we sequenced the transcriptome and determined the pigment contents of an unusual cultivar of ornamental kale with three different types of leaf coloration: pink (C3), light pink (C2), and variegated pink-green (C1). A total of 23,965 differentially expressed genes were detected in pairwise comparisons among the three types of leaves. The results indicate that Bo9g058630 coding dihydroflavonol 4-reductase (DFR) and Bo3g019080 coding shikimate O-hydroxycinnamoyltransferase (HCT) acted in anthocyanin biosynthesis in pink leaves. Bo1g053420 coding pheophorbidase (PPD) and Bo3g012430 coding 15-cis-phytoene synthase (crtB) were identified as candidate genes for chlorophyll metabolism and carotenoid biosynthesis, respectively. The transcription factors TT8, MYBL2, GATA21, GLK2, and RR1 might participate in triggering the leaf color change in ornamental kale. Anthocyanin content was highest in C3 and lowest in C1. Chlorophyll and carotenoid contents were lowest in C2 and highest in C1.

CONCLUSIONS

Based on these findings, we suspected that the decrease in anthocyanin biosynthesis and the increase in chlorophyll and carotenoid biosynthesis might be the reason for the leaf changing from pink to variegate pink-green in this unusual cultivar. Our research provides insight into the molecular mechanisms of leaf coloration in ornamental kale, contributing to a theoretical foundation for breeding new varieties.

Flavonone 3-hydroxylase Relieves Bacterial Leaf Blight Stress in Rice via Overaccumulation of Antioxidant Flavonoids and Induction of Defense Genes and Hormones

Efficient accumulation of flavonoids is important for increased tolerance to biotic stress. Although several plant defense mechanisms are known, the roles of many pathways, proteins, and secondary metabolites in stress tolerance are unknown. We generated a flavanone 3-hydroxylase (F3H) overexpressor rice line and inoculated Xanthomonas Oryzae pv. oryzae and compared the control and wildtype inoculated plants. In addition to promoting plant growth and developmental maintenance, the overexpression of F3H increased the accumulation of flavonoids and increased tolerance to bacterial leaf blight (BLB) stress. Moreover, leaf lesion length was higher in the infected wildtype plants compared with infected transgenics. Kaempferol and quercetin, which scavenge reactive oxygen species, overaccumulated in transgenic lines compared with wildtypes in response to pathogenic infection, detected by scanning electron microscopy and spectrophotometry. The induction of F3H altered the antioxidant system and reduced the levels of glutathione peroxidase activity and malondialdehyde (MDA) contents in the transgenic lines compared with the wildtypes. Downstream gene regulation analysis showed that the expression of F3H increased the regulation of flavonol synthase (FLS), dihydroflavonol 4-reductase (DFR), and slender rice mutant (SLR1) during BLB stress. The analysis of SA and JA signaling revealed an antagonistic interaction between both hormones and that F3H induction significantly promoted SA and inhibited JA accumulation in the transgenic lines. SA-dependent nonexpressor pathogenesis-related (NPR1) and Xa1 showed significant upregulation in the infected transgenic lines compared with the infected control and wildtype lines. Thus, the overexpression of F3H was essential for increasing BLB stress tolerance.

Overexpression of an Apocynum venetum flavonols synthetase gene confers salinity stress tolerance to transgenic tobacco plants

Soil salinity is a major limiting factor for agricultural production, threatening food security worldwide. A thorough understanding of the mechanisms underlying plant responses is required to effectively counter its deleterious effects on crop productivity. Total flavonoid accumulation reportedly improves salinity tolerance in many crops. Therefore, we isolated the full-length cDNA of a flavonol synthetase (FLS) gene from Apocynum venetum (AvFLS). The gene contained a 1008-bp open reading frame encoding a protein composed of 335 amino acid residues. Multiple sequence alignment showed that the AvFLS protein was highly homologous to FLSs from other plants. AvFLS was expressed in leaves, stems, roots, flowers, and germinated seeds. Expression pattern analysis revealed that AvFLS was significantly induced by salinity stress. AvFLS overexpression in tobacco positively affected the development and growth of transgenic plants under salinity stress: root and seedling growth were inhibited to a lesser extent, while seed germination rate increased. Additionally, the overexpression of AvFLS under salinity stress resulted in an increase in total flavonoid content (1.63 mg g-1 in wild-type samples and 4.63 mg g-1 on average in transgenic samples), which accompanied the increase in the activity of antioxidant enzymes and inhibited the production of reactive oxygen species. Further, AvFLS-overexpressing transgenic tobacco plants absorbed more K+ than wild type plants, leading to an increased K+/Na+ ratio, which in turn contributed to the maintenance of Na+/K+ homeostasis. These findings suggest that an AvFLS-induced increase in total flavonoid content enhanced plant salinity tolerance, implying the importance of AvFLS gene responses to salinity stress.

Insight into the potential application of polyphenol-rich dietary intervention in degenerative disease management

In recent times, a great number of plants have been studied in order to identify new components with nutraceutical properties, among which are polyphenols. Dietary polyphenols represent a large group of bioactive molecules widely found in the food of plant origin and they have been found able to prevent the onset and progression of degenerative diseases, and to reduce and control their symptoms. These health protective effects have been mainly related to their antioxidant and anti-inflammatory properties. However, it must be considered that the application of isolated polyphenols as nutraceuticals is quite limited due to their poor systemic distribution and relative bioavailability. The present review highlights the potential effect of dietary intervention with polyphenol-rich food and plant extracts in patients with cancer, diabetes and neurodegenerative, autoimmune, cardiovascular and ophthalmic diseases, as well as the possible molecular mechanisms of action suggested in numerous studies with animal models.

A Review of Strawberry Photobiology and Fruit Flavonoids in Controlled Environments

Rapid technology development in controlled environment (CE) plant production has been applied to a large variety of plants. In recent years, strawberries have become a popular fruit for CE production because of their high economic and nutritional values. With the widespread use of light-emitting diode (LED) technology in the produce industry, growers can manipulate strawberry growth and development by providing specific light spectra. Manipulating light intensity and spectral composition can modify strawberry secondary metabolism and highly impact fruit quality and antioxidant properties. While the impact of visible light on secondary metabolite profiles for other greenhouse crops is well documented, more insight into the impact of different light spectra, from UV radiation to the visible light spectrum, on strawberry plants is required. This will allow growers to maximize yield and rapidly adapt to consumer preferences. In this review, a compilation of studies investigating the effect of light properties on strawberry fruit flavonoids is provided, and a comparative analysis of how light spectra influences strawberry's photobiology and secondary metabolism is presented. The effects of pre-harvest and post-harvest light treatments with UV radiation and visible light are considered. Future studies and implications for LED lighting configurations in strawberry fruit production for researchers and growers are discussed.

5,7,3',4'-Tetrahydroxyflav-2-en-3-ol 3-O-glucoside, a new biosynthetic precursor of cyanidin 3-O-glucoside in the seed coat of black soybean, Glycine max

The seed coat of mature black soybean, Glycine max, accumulates a high amount of cyanidin 3-O-glucoside (Cy3G), which is the most abundant anthocyanin in nature. In the pod, it takes two months for the seed coat color change from green to black. However, immature green beans rapidly adopt a black color within one day when the shell is removed. We analyzed the components involved in the color change of the seed coat and detected a new precursor of Cy3G, namely 5,7,3',4'-tetrahydroxyflav-2-en-3-ol 3-O-glucoside (2F3G). Through quantitative analysis using purified and synthetic standard compounds, it was clarified that during this rapid color change, an increase in the Cy3G content was observed along with the corresponding decrease in the 2F3G content. Chemical conversion from 2F3G to Cy3G at pH 5 with air and ferrous ion was observed. Our findings allowed us to propose a new biosynthetic pathway of Cy3G via a colorless glucosylated compound, 2F3G, which was oxidized to give Cy3G.

The moss flavone synthase I positively regulates the tolerance of plants to drought stress and UV-B radiation

Flavonoids are extensively distributed secondary metabolites in land plants. They play a critical role in plant evolution from aquatic to terrestrial and plant adaption to ultraviolet radiation. However, the downstream branching pathway of flavonoids and its regulatory mechanism in bryophytes, which are the most ancient of terrestrial plants, remain unclear. Here, a type I flavone synthase (PnFNSI) was characterized from the Antarctic moss Pohlia nutans. PnFNSI was primarily distributed in the cytoplasm, as detected by subcellular localization. PnFNSI could catalyze the conversion of naringenin to apigenin with an optimal temperature between 15 and 20 °C in vitro. Overexpression of PnFNSI in Arabidopsis alleviated the growth restriction caused by naringenin and accumulated apigenin product. PnFNSI-overexpressing plants showed enhanced plant tolerance to drought stress and UV-B radiation. PnFNSI also increased the enzyme activities and gene transcription levels of reactive oxygen species (ROS) scavengers, protecting plants against oxidative stress. Moreover, overexpression of PnFNSI enhanced the flavone biosynthesis pathway in Arabidopsis. Therefore, this moss FNSI-type enzyme participates in flavone metabolism, conferring protection against drought stress and UV-B radiation.

Metabolome and transcriptome analyses reveal chlorophyll and anthocyanin metabolism pathway associated with cucumber fruit skin color

BACKGROUND

Fruit skin color play important role in commercial value of cucumber, which is mainly determined by the content and composition of chlorophyll and anthocyanins. Therefore, understanding the related genes and metabolomics involved in composition of fruit skin color is essential for cucumber quality and commodity value.

RESULTS

The results showed that chlorophyll a, chlorophyll b and carotenoid content in fruit skin were higher in Lv (dark green skin) than Bai (light green skin) on fruit skin. Cytological observation showed more chloroplast existed in fruit skin cells of Lv. A total of 162 significantly different metabolites were found between the fruit skin of the two genotypes by metabolome analysis, including 40 flavones, 9 flavanones, 8 flavonols, 6 anthocyanins, and other compounds. Crucial anthocyanins and flavonols for fruit skin color, were detected significantly decreased in fruit skin of Bai compared with Lv. By RNA-seq assay, 4516 differentially expressed genes (DEGs) were identified between two cultivars. Further analyses suggested that low expression level of chlorophyll biosynthetic genes, such as chlM, por and NOL caused less chlorophylls or chloroplast in fruit skin of Bai. Meanwhile, a predicted regulatory network of anthocyanin biosynthesis was established to illustrate involving many DEGs, especially 4CL, CHS and UFGT.

CONCLUSIONS

This study uncovered significant differences between two cucumber genotypes with different fruit color using metabolome and RNA-seq analysis. We lay a foundation to understand molecular regulation mechanism on formation of cucumber skin color, by exploring valuable genes, which is helpful for cucumber breeding and improvement on fruit skin color.

Functional analysis of ZmCOP1 and ZmHY5 reveals conserved light signaling mechanism in maize and Arabidopsis

Plants have evolved light signaling mechanisms to optimally adapt developmental patterns to the ambient light environments. CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1) and LONG HYPOCOTYL5 (HY5) are two critical components in the light signaling pathway in Arabidopsis thaliana. COP1 acts as an E3 ubiquitin ligase that targets positive regulators, such as HY5, leading to their degradation in darkness. However, functional analysis of the COP1-HY5 module in maize (Zea mays) has not been reported. Here, we investigated the expression patterns and roles of the COP1 and HY5 orthologs, ZmCOP1 and ZmHY5, in regulating photomorphogenesis. These two genes have high amino acid identities with their Arabidopsis homolog and were both regulated by light. Subcellular localization assay showed that ZmCOP1 was distributed in the cytosol and ZmHY5 localized in the nucleus. Exogenous expression of ZmCOP1 rescued the physiological defects of the cop1-4 mutant, and expression of ZmHY5 complemented the long hypocotyl phenotype of the hy5-215 mutant in Arabidopsis. Yeast two-hybrid and fluorescence resonance energy transfer assays showed that ZmCOP1 interacted with ZmHY5. Our study gains insight into the conserved function and regulatory mechanism of the COP1-HY5 signaling pathway in maize and Arabidopsis.

Identification and analysis of phenylpropanoid biosynthetic genes and phenylpropanoid accumulation in watercress (Nasturtium officinale R. Br.)

Watercress (Nasturtium officinale R. Br.) is a cruciferous plant consumed by people worldwide. This vegetable contains numerous health-benefiting compounds; however, gene information and metabolomic profiling of individual parts for this plant species are scarce. In this study, we investigated the expression patterns of phenylpropanoid biosynthetic genes and the content of phenylpropanoids in different parts of watercress. We identified 11 genes encoding enzymes related to the phenylpropanoid biosynthetic pathway and analyzed the expression patterns of these genes in the leaves, stems, roots, flowers, and seeds of watercress. The result showed that most of the genes were expressed at the highest levels in the flowers. HPLC analysis performed in samples from these same parts revealed the presence of seven phenylpropanoid-derived compounds. The content of total phenylpropanoids was the highest in flowers, followed by that in the leaves, whereas the lowest level was generally detected in the stems. Rutin was the most abundant phenylpropanoid in all plant segments, while quercetin was detected only in the flowers and roots. This study provides useful information for further molecular and functional research involving N. officinale and closely related species.