Transcriptional control of flavonoid biosynthesis by MYB-bHLH-WDR complexes

A novel R2R3-MYB from grape hyacinth, MaMybA, which is different from MaAN2, confers intense and magenta anthocyanin pigmentation in tobacco

BACKGROUND

The primary pigments in flowers are anthocyanins, the biosynthesis of which is mainly regulated by R2R3-MYBs. Muscari armeniacum is an ornamental garden plant with deep cobalt blue flowers containing delphinidin-based anthocyanins. An anthocyanin-related R2R3-MYB MaAN2 has previously been identified in M. armeniacum flowers; here, we also characterized a novel R2R3-MYB MaMybA, to determine its function and highlight similarities and differences between MaMybA and MaAN2.

RESULTS

In this study, a novel anthocyanin-related R2R3-MYB gene was isolated from M. armeniacum flowers and functionally identified. A sequence alignment showed that MaMybA contained motifs typically conserved with MaAN2 and its orthologs. However, the shared identity of the entire amino acid sequence between MaMybA and MaAN2 was 43.5%. Phylogenetic analysis showed that they were both clustered into the AN2 subgroup of the R2R3-MYB family, but not in the same branch. We also identified a IIIf bHLH protein, MabHLH1, in M. armeniacum flowers. A bimolecular fluorescence complementation assay showed that MabHLH1 interacted with MaMybA or MaAN2 in vivo; a dual luciferase assay indicated that MaMybA alone or in interaction with MabHLH1 could regulate the expression of MaDFR and AtDFR, but MaAN2 required MabHLH1 to do so. When overexpressing MaMybA in Nicotiana tabacum 'NC89', the leaves, petals, anthers, and calyx of transgenic tobacco showed intense and magenta anthocyanin pigments, whereas those of OE-MaAN2 plants had lighter pigmentation. However, the ovary wall and seed skin of OE-MaMybA tobacco were barely pigmented, while those of OE-MaAN2 tobacco were reddish-purple. Moreover, overexpressing MaMybA in tobacco obviously improved anthocyanin pigmentation, compared to the OE-MaAN2 and control plants, by largely upregulating anthocyanin biosynthetic and endogenous bHLH genes. Notably, the increased transcription of NtF3'5'H in OE-MaMybA tobacco might lead to additional accumulation of delphinidin 3-rutinoside, which was barely detected in OE-MaAN2 and control plants. We concluded that the high concentration of anthocyanin and the newly produced Dp3R caused the darker color of OE-MaMybA compared to OE-MaAN2 tobacco.

CONCLUSION

The newly identified R2R3-MYB transcription factor MaMybA functions in anthocyanin biosynthesis, but has some differences from MaAN2; MaMybA could also be useful in modifying flower color in ornamental plants.

Gene Coexpression Networks Reveal Key Drivers of Flavonoid Variation in Eleven Tea Cultivars (Camellia sinensis)

Following the recent completion of the draft genome sequence of the tea plant, high-throughput decoding of gene function, especially for those involved in complex secondary metabolic pathways, has become a major challenge. Here, we profiled the metabolome and transcriptome of 11 tea cultivars, and then illustrated a weighted gene coexpression network analysis (WGCNA)-based system biological strategy to interpret metabolomic flux, predict gene functions, and mine key regulators involved in the flavonoid biosynthesis pathway. We constructed a multilayered regulatory network, which integrated the gene coexpression relationship with the microRNA target and promoter cis-regulatory element information. This allowed us to reveal new uncharacterized TFs (e.g., MADSs, WRKYs, and SBPs) and microRNAs (including 17 conserved and 15 novel microRNAs) that are potentially implicated in different steps of the catechin biosynthesis. Furthermore, we applied metabolic-signature-based association method to capture additional key regulators involved in catechin pathway. This provides important clues for the functional characterization of five SCPL1A acyltransferase family members, which might be implicated in the production balance of anthocyanins, galloylated catechins, and proanthocyanins. Application of an "omics"-based system biology strategy should facilitate germplasm utilization and provide valuable resources for tea quality improvement.

Dynamic Changes in Metabolite Accumulation and the Transcriptome during Leaf Growth and Development in Eucommia ulmoides

Eucommia ulmoides Oliver is widely distributed in China. This species has been used mainly in medicine due to the high concentration of chlorogenic acid (CGA), flavonoids, lignans, and other compounds in the leaves and barks. However, the categories of metabolites, dynamic changes in metabolite accumulation and overall molecular mechanisms involved in metabolite biosynthesis during E. ulmoides leaf growth and development remain unknown. Here, a total of 515 analytes, including 127 flavonoids, 46 organic acids, 44 amino acid derivatives, 9 phenolamides, and 16 vitamins, were identified from four E. ulmoides samples using ultraperformance liquid chromatography-mass spectrometry (UPLC-MS) (for widely targeted metabolites). The accumulation of most flavonoids peaked in growing leaves, followed by old leaves. UPLC-MS analysis indicated that CGA accumulation increased steadily to a high concentration during leaf growth and development, and rutin showed a high accumulation level in leaf buds and growing leaves. Based on single-molecule long-read sequencing technology, 69,020 transcripts and 2880 novel loci were identified in E. ulmoides. Expression analysis indicated that isoforms in the flavonoid biosynthetic pathway and flavonoid metabolic pathway were highly expressed in growing leaves and old leaves. Co-expression network analysis suggested a potential direct link between the flavonoid and phenylpropanoid biosynthetic pathways via the regulation of transcription factors, including MYB (v-myb avian myeloblastosis viral oncogene homolog) and bHLH (basic/helix-loop-helix). Our study predicts dynamic metabolic models during leaf growth and development and will support further molecular biological studies of metabolite biosynthesis in E. ulmoides. In addition, our results significantly improve the annotation of the E. ulmoides genome.

AtWDS1 negatively regulates age-dependent and dark-induced leaf senescence in Arabidopsis

Although the involvement of ROS (reactive oxygen species) in leaf senescence is well known, the factors governing this accumulation of ROS are not fully characterized. In this study, analysis of transgenic overexpressing and knock out lines of AtWDS1 (encoding a WD repeat protein), indicates that AtWDS1 negatively regulates age-dependent and dark-induced leaf senescence. Furthermore, we observed ROS accumulation and altered tolerance of oxidative stress in atwds1 plants, as well as upregulated expression of oxidative stress-responsive genes. The location of an EGFP-AtWDS1 fusion protein in the nucleus of transformed cells and plants indicates that AtWDS1 is a nuclear protein, and, using a Dual-Luciferase assay, we showed that AtWDS1 can act as a transcription activator. However, the lack of a nuclear localization sequence in AtWDS1 suggests that its presence in the nucleus must depend on interactions with other proteins. Indeed, we found that AtWDS1 interacts directly with AtRanBPM, and that mutation of the AtRanBPM gene results in partial mislocalization of AtWDS1 in the cytoplasm. Together, these results suggest a role for AtWDS1 as a novel modulator of redox homeostasis, which responds to developmental and stress signals to regulate leaf senescence.

PdMYB118, isolated from a red leaf mutant of Populus deltoids, is a new transcription factor regulating anthocyanin biosynthesis in poplar

A new anthocyanin biosynthesis transcription factor PdMYB118, which could be used for the genetic engineering of colorful tree species, was indentified from a red leaf mutant of Populus deltoids. In higher plants, the biosynthesis of anthocyanins is regulated by several classes of transcription factors (TFs), including R2R3-MYB, bHLH and WD-repeat proteins. In this work, we isolated an MYB gene regulating anthocyanin biosynthesis from a red leaf mutant of Populus deltoids, which accumulated more anthocyanins in the leaves and showed higher expression levels of anthocyanin biosynthesis genes than did the wild type. Gene expression analyses of all TFs regulating anthocyanin biosynthesis demonstrated that only a MYB118 homologous gene, PdMYB118, was up-regulated in the mutant compared with the wide type. Subcellular localization analyses in poplar leaf mesophyll protoplasts showed that PdMYB118-YFP fusion protein was specifically located in nucleus. When transiently expressed in poplar leaf protoplasts, PdMYB118 specifically promoted the expression of anthocyanidin biosynthesis genes. Dual-luciferase assays revealed that PdMYB118 can directly activate the promoters of these genes. When overexpressed in Shanxin Yang (P. davidiana × P. bolleana), a hybrid clone commercially grown for landscaping in the northern part of China, transgenic plants overexpressing PdMYB118 produced more anthocyanins in the leaves and turned their color into redness when grown in both greenhouse and field. Consistently, transcripts of some important anthocyanidin biosynthesis genes were significantly increased in the leaves of transgenic plants. All these results indicate that PdMYB118 functions as an essential transcription factor regulating anthocyanin biosynthesis in poplar and could be used for the genetic engineering of colorful tree species.

A functional homologue of Arabidopsis TTG1 from Freesia interacts with bHLH proteins to regulate anthocyanin and proanthocyanidin biosynthesis in both Freesia hybrida and Arabidopsis thaliana

The MBW complex, consisting of MYB, basic helix-loop-helix (bHLH) and WD40 proteins, regulates multiple traits in plants, such as anthocyanin and proanthocyanidin biosynthesis and cell fate determination. The complex has been widely identified in dicot plants, whereas few studies are concentrated on monocot plants which are of crucial importance to decipher its functional diversities among angiosperms during evolution. In present study, a WD40 gene from Freesia hybrida, designated as FhTTG1, was cloned and functionally characterized. Real-time PCR analysis indicated that it was expressed synchronously with the accumulation of both proanthocyanidins and anthocyanins in Freesia flowers. Transient protoplast transfection and biomolecular fluorescence complementation (BiFC) assays demonstrated that FhTTG1 could interact with FhbHLH proteins (FhTT8L and FhGL3L) to constitute the MBW complex. Moreover, the transportation of FhTTG1 to nucleus was found to rely on FhbHLH factors. Outstandingly, FhTTG1 could highly activate the anthocyanin or proanthocyanidin biosynthesis related gene promoters when co-transfected with MYB and bHLH partners, implying that FhTTG1 functioned as a member of MBW complex to control the anthocyanin or proanthocyanidin biosynthesis in Freesia hybrida. Further ectopic expression assays in Arabidopsis ttg1-1 showed the defective phenotypes of ttg1-1 were partially restored. Molecular biological assays validated FhTTG1 might interact with the endogenous bHLH factors to up-regulate genes responsible for anthocyanin and proanthocyanidin biosynthesis and trichome formation, indicating that FhTTG1 might perform exchangeable roles with AtTTG1. These results will not only contribute to the characterization of FhTTG1 in Freesia but also shed light on the establishment of flavonoid regulatory system in monocot plants, especially in Freesia hybrida.

Expression and function of the bHLH genes ALCATRAZ and SPATULA in selected Solanaceae species

The genetic mechanisms underlying fruit development have been identified in Arabidopsis and have been comparatively studied in tomato as a representative of fleshy fruits. However, comparative expression and functional analyses on the bHLH genes downstream the genetic network, ALCATRAZ (ALC) and SPATULA (SPT), which are involved in the formation of the dehiscence zone in Arabidopsis, have not been functionally studied in the Solanaceae. Here, we perform detailed expression and functional studies of ALC/SPT homologs in Nicotiana obtusifolia with capsules, and in Capsicum annuum and Solanum lycopersicum with berries. In Solanaceae, ALC and SPT genes are expressed in leaves, and all floral organs, especially in petal margins, stamens and carpels; however, their expression changes during fruit maturation according to the fruit type. Functional analyses show that downregulation of ALC/SPT genes does not have an effect on gynoecium patterning; however, they have acquired opposite roles in petal expansion and have been co-opted in leaf pigmentation in Solanaceae. In addition, ALC/SPT genes repress lignification in time and space during fruit development in Solanaceae. Altogether, some roles of ALC and SPT genes are different between Brassicaceae and Solanaceae; while the paralogs have undergone some subfunctionalization in the former they are mostly redundant in the latter.

The Dynamic Genetic-Hormonal Regulatory Network Controlling the Trichome Development in Leaves

Plant trichomes are outgrowths developed from an epidermal pavement cells of leaves and other organs. Trichomes (also called 'hairs') play well-recognized roles in defense against insect herbivores, forming a physical barrier that obstructs insect movement and mediating chemical defenses. In addition, trichomes can act as a mechanosensory switch, transducing mechanical stimuli (e.g., insect movement) into physiological signals, helping the plant to respond to insect attacks. Hairs can also modulate plant responses to abiotic stresses, such as water loss, an excess of light and temperature, and reflect light to protect plants against UV radiation. The structure of trichomes is species-specific and this trait is generally related to their function. These outgrowths are easily analyzed and their origin represents an outstanding subject to study epidermal cell fate and patterning in plant organs. In leaves, the developmental control of the trichomatous complement has highlighted a regulatory network based on four fundamental elements: (i) genes that activate and/or modify the normal cell cycle of epidermal pavement cells (i.e., endoreduplication cycles); (ii) transcription factors that create an activator/repressor complex with a central role in determining cell fate, initiation, and differentiation of an epidermal cell in trichomes; (iii) evidence that underlines the interplay of the aforesaid complex with different classes of phytohormones; (iv) epigenetic mechanisms involved in trichome development. Here, we reviewed the role of genes in the development of trichomes, as well as the interaction between genes and hormones. Furthermore, we reported basic studies about the regulation of the cell cycle and the complexity of trichomes. Finally, this review focused on the epigenetic factors involved in the initiation and development of hairs, mainly on leaves.

Assessing Anthocyanin Biosynthesis in Solanaceae as a Model Pathway for Secondary Metabolism

Solanaceae have played an important role in elucidating how flower color is specified by the flavonoid biosynthesis pathway (FBP), which produces anthocyanins and other secondary metabolites. With well-established reverse genetics tools and rich genomic resources, Solanaceae provide a robust framework to examine the diversification of this well-studied pathway over short evolutionary timescales and to evaluate the predictability of genetic perturbation on pathway flux. Genomes of eight Solanaceae species, nine related asterids, and four rosids were mined to evaluate variation in copy number of the suite of FBP enzymes involved in anthocyanin biosynthesis. Comparison of annotation sources indicated that the NCBI annotation pipeline generated more and longer FBP annotations on average than genome-specific annotation pipelines. The pattern of diversification of each enzyme among asterids was assessed by phylogenetic analysis, showing that the CHS superfamily encompasses a large paralogous family of ancient and recent duplicates, whereas other FBP enzymes have diversified via recent duplications in particular lineages. Heterologous expression of a pansy F3′5′H gene in tobacco changed flower color from pink to dark purple, demonstrating that anthocyanin production can be predictably modified using reverse genetics. These results suggest that the Solanaceae FBP could be an ideal system to model genotype-to-phenotype interactions for secondary metabolism.

Conserved Biochemical Defenses Underpin Host Responses to Oomycete Infection in an Early-Divergent Land Plant Lineage

The expansion of plants onto land necessitated the evolution of robust defense strategies to protect against a wide array of microbial invaders. Whereas host responses to microbial colonization are extensively explored in evolutionarily young land plant lineages such as angiosperms, we know relatively little about plant-pathogen interactions in early-diverging land plants thought to better represent the ancestral state. Here, we define the transcriptional and proteomic response of the early-divergent liverwort Marchantia polymorpha to infection with the oomycete pathogen Phytophthora palmivora. We uncover a robust molecular response to oomycete colonization in Marchantia that consists of conserved land plant gene families. Direct macroevolutionary comparisons of host infection responses in Marchantia and the model angiosperm Nicotiana benthamiana further reveal a shared set of orthologous microbe-responsive genes that include members of the phenylpropanoid metabolic pathway. In addition, we identify a role for the Marchantia R2R3-MYB transcription factor MpMyb14 in activating phenylpropanoid (flavonoid) biosynthesis during oomycete infection. Mpmyb14 mutants infected with P. palmivora fail to activate phenylpropanoid biosynthesis gene expression and display enhanced disease susceptibility compared to wild-type plants. Conversely, the ectopic induction of MpMyb14 led to the accumulation of anthocyanin-like pigments and dramatically enhanced liverwort resistance to P. palmivora infection. Collectively, our results demonstrate that the Marchantia response to oomycete infection displays evolutionarily conserved features indicative of an ancestral pathogen deterrence strategy centered on phenylpropanoid-mediated biochemical defenses.

The AabHLH35 Transcription Factor Identified from Anthurium andraeanum is Involved in Cold and Drought Tolerance

Anthurium andraeanum Lind. is a popular potted and cut-flower plant with an attractive spathe and foliage. It is native to tropical rainforest areas and is able to blossom throughout the year under suitable conditions. However, various abiotic stresses seriously restrict the ornamental value of A. andraeanum and increase the costs of cultivation. A dark green (dg) leaf color mutant of A. andraeanum ‘Sonate’, which accumulates high levels of anthocyanin, has shown increased vigor and tolerance to stresses during cultivation and is, thus, an ideal germplasm for studying stress tolerance in this species. Here, we show that the anthocyanin content in dg mutant plants at different stages of leaf development was higher than in wild-type (WT) plants, and the ability to tolerate under low-temperature (LT, 14 °C) stress was stronger in dg than in WT plants. RNA-Seq of cDNA libraries from young leaves of dg and WT identified AabHLH35 as a differentially expressed gene (DEG) that was significantly up-regulated in dg. Furthermore, heterologous expression of AabHLH35 improved tolerance to cold and drought stresses in Arabidopsis. These results have built an important molecular foundation for further study of stress tolerance in A. andraeanum.

Identification and differential expression analysis of anthocyanin biosynthetic genes in leaf color variants of ornamental kale

BACKGROUND

Anthocyanins perform diverse biological functions in plants and are beneficial to human health. Leaf color is the most important trait of ornamental kale and the characteristics of changes in leaf color make it an ideal material to elucidate genetic mechanisms of anthocyanins accumulation in Brassica oleracea. To elucidate the anthocyanin distribution, metabolic profiles and differentially expressed anthocyanin biosynthetic genes between different colored accessions can pave the way for understanding the genetic regulatory mechanisms of anthocyanin biosynthesis and accumulation in ornamental kale.

RESULTS

In this study, anthocyanin distributions in red- and white-leaved ornamental kale accessions were determined. Thirty-four anthocyanins were detected in the red-leaved accession. The complete set of anthocyanin biosynthetic genes in the B. oleracea reference genome was identified and differential expression analysis based on RNA-seq was conducted. Eighty-one anthocyanin biosynthetic genes were identified in the B. oleracea reference genome. The expression patterns and differential expressions of these genes in different leaf types indicated that late biosynthetic genes (BoDFR1, BoANS1 and 2, and BoUGT79B1.1), positive regulatory genes (BoTTG1, BoTT8, and Bol012528), a negative regulatory gene (BoMYBL2.1), and transport genes (BoTT19.1 and BoTT19.2) may play roles in anthocyanin accumulation in ornamental kale. A genetic regulatory network of anthocyanin accumulation in ornamental kale was constructed.

CONCLUSIONS

The distribution of pigments and anthocyanin profiles explained the leaf color phenotypes of ornamental kales. The identification of key genes and construction of genetic regulatory network in anthocyanin accumulation in ornamental kale elucidated the genetic basis of leaf color variants. These findings enhance the understanding of the genetic mechanisms and regulatory network of anthocyanin accumulation in B. oleracea, and provide a theoretical basis for breeding new cultivars of Brassica vegetables with enhanced ornamental and nutritional value.

Determining factors, regulation system, and domestication of anthocyanin biosynthesis in rice leaves

Wild and cultivated rice show a significant difference in anthocyanin biosynthesis in the leaf. The regulation system of anthocyanin biosynthesis in rice leaf and the causal mechanism of the difference in this biosynthesis between wild and cultivated rice remain largely unknown. In this study, a genome-wide association study and transcriptome analysis were performed to identify the determinant factors and dissect the regulatory system for anthocyanin biosynthesis in rice leaves. OsC1, OsRb and OsDFR were identified as the determinants of anthocyanin biosynthesis in rice leaves. Artificial selection of certain null mutations of OsC1 and OsRb was the main causal mechanism underlying the loss of anthocyanin pigmentation in most cultivated rice. OsP1 and the MYB-bHLH-WD40 complexes regulate anthocyanin biosynthetic genes in rice leaves with partial functional overlap. OsP1 specifically activates upstream biosynthetic genes (OsCHS, OsCHI and OsF3'H) for anthocyanin biosynthesis, whereas the ternary MYB-bHLH-WD40 complex activates all anthocyanin biosynthetic genes including OsCHS, OsCHI, OsF3'H, OsF3H, OsDFR and OsANS. OsC1 and OsRb are tissue-specific regulators that do not influence anthocyanin biosynthesis in the pericarp. Our results reveal the determinant factors, regulatory system and domestication of anthocyanin biosynthesis in rice leaves, and show the potential of engineering anthocyanin biosynthesis in rice.

A MYB/bHLH complex regulates tissue-specific anthocyanin biosynthesis in the inner pericarp of red-centered kiwifruit Actinidia chinensis cv. Hongyang

Many Actinidia cultivars are characterized by anthocyanin accumulation, specifically in the inner pericarp, but the underlying regulatory mechanism remains elusive. Here we report two interacting transcription factors, AcMYB123 and AcbHLH42, that regulate tissue-specific anthocyanin biosynthesis in the inner pericarp of Actinidia chinensis cv. Hongyang. Through transcriptome profiling analysis we identified five MYB and three bHLH transcription factors that were upregulated in the inner pericarp. We show that the combinatorial action of two of them, AcMYB123 and AcbHLH42, is required for activating promoters of AcANS and AcF3GT1 that encode the dedicated enzymes for anthocyanin biosynthesis. The presence of anthocyanin in the inner pericarp appears to be tightly associated with elevated expression of AcMYB123 and AcbHLH42. RNA interference repression of AcMYB123, AcbHLH42, AcF3GT1 and AcANS in 'Hongyang' fruits resulted in significantly reduced anthocyanin biosynthesis. Using both transient assays in Nicotiana tabacum leaves or Actinidia arguta fruits and stable transformation in Arabidopsis, we demonstrate that co-expression of AcMYB123 and AcbHLH42 is a prerequisite for anthocyanin production by activating transcription of AcF3GT1 and AcANS or the homologous genes. Phylogenetic analysis suggests that AcMYB123 or AcbHLH42 are closely related to TT2 or TT8, respectively, which determines proanthocyanidin biosynthesis in Arabidopsis, and to anthocyanin regulators in monocots rather than regulators in dicots. All these experimental results suggest that AcMYB123 and AcbHLH42 are the components involved in spatiotemporal regulation of anthocyanin biosynthesis specifically in the inner pericarp of kiwifruit.

Comparative transcriptome analyses reveal genes related to pigmentation in the petals of red and white Primula vulgaris cultivars

Primula vulgaris is an important ornamental plant species with various flower color. To explore the molecular mechanism of its color formation, comparative transcriptome analyses of the petals in red and white cultivars was performed. A total of 4451 differentially expressed genes were identified and annotated into 128 metabolic pathways. Candidate genes FLS, F3'H, DFR, ANS and AOMT in the anthocyanin pathway were expressed significantly higher in the red cultivar than the white and may be responsible for the red coloration. In the red petals, a putative transcription factors bHLH (c52273.graph_c0) was up-regulated about 14-fold, while a R2R3-MYB unigene (c36140.graph_c0) was identified as a repressor involved in anthocyanin regulation and was significantly down-regulated. In addition, the anatomy analyses and pigments composition in the red and white petals were also analyzed. The papillae on the adaxial epidermis of the red petals of P. vulgaris display a triangle-shapes, in contrast with a spherical shape for the white petals. Although flavonoids were detected in both cultivars, anthocyanins could only be identified in the red cultivar. Gossypetin and peonidin/rosinin were the most abundant pigments in red petals. This study shed light on the genetic and biochemistry mechanisms underlying the flower coloration in Primula.

The R2R3-MYB transcription factor PaMYB10 is involved in anthocyanin biosynthesis in apricots and determines red blushed skin

BACKGROUND

The majority of apricot (Prunus armeniaca L.) cultivars display orange or yellow background skin, whereas some cultivars are particularly preferred by consumers because of their red blushed skin on the background.

RESULTS

In this study, two blushed ('Jianali' and 'Hongyu') and two nonblushed ('Baixing' and 'Luntaixiaobaixing') cultivars were used to investigate the formation mechanism of blushed skin in apricots. High-performance liquid chromatography (HPLC) analysis showed that the blushed cultivars accumulated higher cyanidin-3-O-glucoside, cyanidin-3-O-rutinoside and peonidin-3-O-rutinoside levels during fruit ripening than the nonblushed cultivars. Based on coexpression network analysis (WGCNA), a putative anthocyanin-related R2R3-MYB, PaMYB10, and seven structural genes were identified from transcriptome data. The phylogenetic analysis indicated that PaMYB10 clustered in the anthocyanin-related MYB clade. Sequence alignments revealed that PaMYB10 contained a bHLH-interaction motif ([DE]Lx2[RK]x3Lx6Lx3R) and an ANDV motif. Subcellular localization analysis showed that PaMYB10 was a nuclear protein. Real-time qRT-PCR analysis demonstrated that the transcript levels of PaMYB10 and seven genes responsible for anthocyanin synthesis were significantly higher in blushed than in nonblushed apricots, which was consistent with the accumulation of anthocyanin. In addition, bagging significantly inhibited the transcript levels of PaMYB10 and the structural genes in 'Jianali' and blocked the red coloration and anthocyanin accumulation. Transient PaMYB10 overexpression in 'Luntaixiaobaixing' fruits resulted in the red blushed skin at the maturation stage.

CONCLUSIONS

Taken together, these data reveal that three anthocyanins are responsible for the blushed skin of apricots, identify PaMYB10 as a positive regulator of anthocyanin biosynthesis in apricots, and demonstrate that blush formation depends on light.

Evolutionary comparison of competitive protein-complex formation of MYB, bHLH, and WDR proteins in plants

A protein complex consisting of a MYB, basic Helix-Loop-Helix, and a WDR protein, the MBW complex, regulates five traits, namely the production of anthocyanidin, proanthocyanidin, and seed-coat mucilage, and the development of trichomes and root hairs. For complexes involved in trichome and root hair development it has been shown that the interaction of two MBW proteins can be counteracted by the respective third protein (called competitive complex formation). We examined competitive complex formation for selected MBW proteins from Arabidopsis thaliana, Arabis alpina, Gossypium hirsutum, Petunia hybrida, and Zea mays. Quantitative analyses of the competitive binding of MYBs and WDRs to bHLHs were done by pull-down assays using ProtA- and luciferase-tagged proteins expressed in human HEC cells. We found that some bHLHs show competitive complex formation whilst others do not. Competitive complex formation strongly correlated with a phylogenetic tree constructed with the bHLH proteins under investigation, suggesting a functional relevance. We demonstrate that this different behavior can be explained by changes in one amino acid and that this position is functionally relevant in trichome development but not in anthocyanidin regulation.

FtMYB8 from Tartary buckwheat inhibits both anthocyanin/Proanthocyanidin accumulation and marginal Trichome initiation

BACKGROUND

Because flavonoids and trichomes play crucial roles in plant defence, their formation requires fine transcriptional control by multiple transcription factor families. However, little is known regarding the mechanism of the R2R3-MYB transcription factors that regulate both flavonoid metabolism and trichome development.

RESULTS

Here, we identified a unique SG4-like-MYB TF from Tartary buckwheat, FtMYB8, which harbours the C2 repression motif and an additional TLLLFR repression motif. The expression profiles of FtMYB8 combined with the transcriptional activity of PFtMYB8 promoter showed that FtMYB8 mRNA mainly accumulated in roots during the true leaf stage and flowering stage and in bud trichomes and flowers, and the expression of this gene was markedly induced by MeJA, ABA and UV-B treatments but repressed by dark treatment. Overexpression of FtMYB8 in Arabidopsis reduces the accumulation of anthocyanin/proanthocyanidin by specifically inhibiting TT12 expression, which may depend on the interaction between FtMYB8 and TT8. Interestingly, this interaction may also negatively regulate the marginal trichome initiation in Arabidopsis leaves.

CONCLUSIONS

Taken together, our results suggest that FtMYB8 may fine-tune the accumulation of anthocyanin/proanthocyanidin in the roots and flowers of Tartary buckwheat by balancing the inductive effects of transcriptional activators, and probably regulate trichome distribution in the buds of Tartary buckwheat.

Uncovering anthocyanin biosynthesis related microRNAs and their target genes by small RNA and degradome sequencing in tuberous roots of sweetpotato

BACKGROUND

Compared with white-fleshed sweetpotato (WFSP), purple-fleshed sweetpotato (PFSP) is a desirable resource for functional food development because of the abundant anthocyanin accumulation in its tuberous roots. Some studies have shown that the expression regulation mediated by miRNA plays an important role in anthocyanin biosynthesis in plants. However, few miRNAs and their corresponding functions related to anthocyanin biosynthesis in tuberous roots of sweetpotato have been known.

RESULTS

In this study, small RNA (sRNA) and degradome libraries from the tuberous roots of WFSP (Xushu-18) and PFSP (Xuzishu-3) were constructed, respectively. Totally, 191 known and 33 novel miRNAs were identified by sRNA sequencing, and 180 target genes cleaved by 115 known ib-miRNAs and 5 novel ib-miRNAs were identified by degradome sequencing. Of these, 121 miRNAs were differently expressed between Xushu-18 and Xuzishu-3. Integrated analysis of sRNA, degradome sequencing, GO, KEGG and qRT-PCR revealed that 26 differentially expressed miRNAs and 36 corresponding targets were potentially involved in the anthocyanin biosynthesis. Of which, an inverse correlation between the expression of ib-miR156 and its target ibSPL in WFSP and PFSP was revealed by both qRT-PCR and sRNA sequencing. Subsequently, ib-miR156 was over-expressed in Arabidopsis. Interestingly, the ib-miR156 over-expressing plants showed suppressed abundance of SPL and a purplish phenotype. Concomitantly, upregulated expression of four anthocyanin pathway genes was detected in transgenic Arabidopsis plants. Finally, a putative ib-miRNA-target model involved in anthocyanin biosynthesis in sweetpotato was proposed.

CONCLUSIONS

The results represented a comprehensive expression profiling of miRNAs related to anthocyanin accumulation in sweetpotato and provided important clues for understanding the regulatory network of anthocyanin biosynthesis mediated by miRNA in tuberous crops.

Genome-wide analysis of bHLH transcription factor family reveals their involvement in biotic and abiotic stress responses in wheat (Triticum aestivum L.)

The basic helix-loop-helix (bHLH) transcription factor family is crucial for plant development and stress responses. In this study, we identified 159 bHLH-encoding genes in the wheat (Triticum aestivum L.) genome and determined their roles in biotic and abiotic stress tolerance. Phylogenetic analyses showed that the TabHLH genes were classified into 19 groups, which shared similar gene structures and conserved motifs. A comprehensive transcriptome analysis revealed that bHLH genes were differentially expressed in diverse wheat tissues and were responsive to multiple abiotic and biotic stresses. A gene ontology analysis indicated that most bHLH proteins involved in DNA-binding activities and the gene expression regulation. Analyses of interaction networks suggested that TabHLHs mediate networks involved in multiple stress-signaling pathways. The findings of this study may help clarify the intricate transcriptional control of bHLH genes and identify putative stress-responsive genes relevant to the genetic improvement of wheat.

Transcriptome profiling reveals the roles of pigment mechanisms in postharvest broccoli yellowing

Postharvest broccoli is prone to yellowing during storage, which is the key factor leading to a reduction in value. To explore appropriate control methods, it is important to understand the mechanisms of yellowing. We analyzed the genes related to the metabolism of chlorophyll, carotenoids, and flavonoids and the transcription factors (TFs) involved in broccoli yellowing using transcriptome sequencing profiling. Broccoli stored at 10 °C showed slight yellowing on postharvest day 5 and serious symptoms on day 12. There were significant changes in chlorophyll fluorescence kinetics, mainly manifesting as a decrease in the Fv/Fm value and an increase in nonphotochemical quenching, during the yellowing process. Transcriptome sequencing profiles from samples of fresh broccoli and broccoli with slight and severe yellowing revealed 6, 5, and 4 differentially expressed genes involved in chlorophyll metabolism, carotenoid biosynthesis, and flavonoid biosynthesis, respectively. The transcription factor gene ontology categories showed that the MYB, bHLH, and bZip gene families were involved in chlorophyll metabolism. In addition, the transcription factor families included NACs and ethylene response factors (ERFs) that regulated carotenoid biosynthesis. Reverse transcription polymerase chain reaction further confirmed that bHLH66, PIF4, LOB13, NAC92, and APL were vital transcription factors that potentially regulated the CAO and HYD genes and were involved in chlorophyll metabolism and the carotenoid biosynthetic process. The flavonoid biosynthetic pathway was mainly regulated by MYBs, NACs, WRKYs, MADSs, and bZips. The results of the differentially expressed gene (DEG) and pigment content analyses indicated that the transcriptome data were accurately and positively associated with broccoli yellowing.

Construction of a high-density genetic map and identification of loci controlling purple sepal trait of flower head in Brassica oleracea L. italica

BACKGROUND

Some broccoli (Brassica oleracea L. italic) accessions have purple sepals and cold weather would deepen the purple color, while the sepals of other broccoli lines are always green even in cold winter. The related locus or gene is still unknown. In this study, a high-density genetic map was constructed based on specific locus amplified fragment (SLAF) sequencing in a doubled-haploid segregation population with 127 individuals. And mapping of the purple sepal trait in flower heads based on phenotypic data collected during three seasons was performed.

RESULTS

A genetic map was constructed, which contained 6694 SLAF markers with an average sequencing depth of 81.37-fold in the maternal line, 84-fold in the paternal line, and 15.76-fold in each individual population studied. In all of the annual data recorded, three quantitative trait loci (QTLs) were identified that were all distributed within the linkage group (LG) 1. Among them, a major locus, qPH.C01-2, located at 36.393 cM LG1, was consistently detected in all analysis. Besides this locus, another two minor loci, qPH.C01-4 and qPH.C01-5, were identified near qPH.C01-2, based on the phenotypic data from spring of 2018.

CONCLUSION

The purple sepal trait could be controlled by a major single locus and two minor loci. The genetic map and location of the purple sepal trait of flower heads provide an important foundation for mapping other compound traits and the identification of the genes related to purple sepal trait in broccoli.

MicroRNA858-mediated regulation of anthocyanin biosynthesis in kiwifruit (Actinidia arguta) based on small RNA sequencing

As important regulators, miRNAs could play pivotal roles in regulation of fruit coloring. Actinidia arguta is a newly emerged fruit tree with extensively application prospects. However, miRNAs involved in A. arguta fruit coloring are unknown. In this study, A. arguta fruit were investigated at three developmental stages by small RNAs high-throughput sequencing. A total of 482 conserved miRNAs corresponding to 526 pre-miRNAs and 581 novel miRNAs corresponding to 619 pre-miRNAs were grouped into 46 miRNA families. Target gene prediction and analysis revealed that miR858, a strongly candidate miRNA, was involved in anthocyanin biosynthesis in which contributes to fruit coloring. The anthocyanin level was determined in three A. arguta cultivars by UPLC-MS/MS (ultra-performance liquid chromatography coupled with tandem mass spectrometry). In addition, qPCR (quantitative real-time PCR), cluster analysis were conducted as well as correlation analysis. All results were combined to propose a model in which describes an association of miRNA and anthocyanin biosynthesis in A. arguta. The data presented herein is the first report on miRNA profile analysis in A. arguta, which can provide valuable information for further research into the regulation of the miRNAs in anthocyanin biosynthesis and fruit coloring.

Differential color development and response to light deprivation of fig (Ficus carica L.) syconia peel and female flower tissues: transcriptome elucidation

BACKGROUND

Color directly affects fruit quality and consumer preference. In fig syconia, the female flower tissue is contained in a receptacle. Anthocyanin pigmentation of this tissue and the peel differs temporally and spatially. A transcriptome study was carried out to elucidate key genes and transcription factors regulating differences in fig coloring.

RESULTS

Anthocyanins in the female flower tissue were identified mainly as pelargonidin-3-glucoside and cyanidin-3-rutinoside; in the peel, the major anthocyanins were cyanidin 3-O-glucoside and cyanidin-3-rutinoside. Anthocyanin content was significantly higher in the female flower tissue vs. peel before fig ripening, whereas at ripening, the anthocyanin content in the peel was 5.39 times higher than that in the female flower tissue. Light-deprivation treatment strongly inhibited peel, but not female flower tissue, anthocyanin pigmentation. RNA-Seq revealed 522 differentially expressed genes (recruited with criteria log₂ ≥ 2 and P < 0.05) at fig ripening, with 50 upregulated and 472 downregulated genes in the female flower tissue. Light deprivation upregulated 1180 and downregulated 856 genes in the peel, and upregulated 909 and downregulated 817 genes in the female flower tissue. KEGG enrichment revealed significantly changed expression in the phenylpropanoid-biosynthesis and flavonoid-biosynthesis pathways in the peel, but not in the female flower tissue, with significant repression of FcCHS, FcCHI, FcF3H, FcF3'H, FcDFR and FcUFGT transcripts. Light deprivation led to differential expression of 71 and 80 transcription factor genes in the peel and female flower tissue, respectively. Yeast one-hybrid screen revealed that FcHY5 and FcMYB114 bind the promoter regions of FcCHS and FcDFR, respectively in the flavonoid-biosynthesis pathway.

CONCLUSIONS

Phenylpropanoid- and flavonoid-biosynthesis pathways were differentially expressed spatially and temporally in the peel and female flower tissue of fig syconia; pathway expression in the peel was strongly regulated by light signal. Differentially expressed transcription factors were recruited as candidates to screen important expression regulators in the light-dependent and light-independent anthocyanin-synthesis pathway. Our study lays the groundwork for further elucidation of crucial players in fig pigmentation.

Transcriptional regulation of Lonicera japonica Thunb. during flower development as revealed by comprehensive analysis of transcription factors

BACKGROUND

Lonicera japonica Thunb. flower has been used for the treatment of various diseases for a long time and attracted many studies on its potential effects. Transcription factors (TFs) regulate extensive biological processes during plant development. As the restricted reports of L. japonica on TFs, our work was carried out to better understand the TFs' regulatory roles under different developmental stages in L. japonica.

RESULTS

In this study, 1316 TFs belonging to 52 families were identified from the transcriptomic data, and corresponding expression profiles during the L. japonica flower development were comprehensively analyzed. 917 (69.68%) TFs were differentially expressed. TFs in bHLH, ERF, MYB, bZIP, and NAC families exhibited obviously altered expression during flower growth. Based on the analysis of differentially expressed TFs (DETFs), TFs in MYB, WRKY, NAC and LSD families that involved in phenylpropanoids biosynthesis, senescence processes and antioxidant activity were detected. The expression of MYB114 exhibited a positive correlation with the contents of luteoloside; Positive correlation was observed among the expression of MYC12, chalcone synthase (CHS) and flavonol synthase (FLS), while negative correlation was observed between the expression of MYB44 and the synthases; The expression of LSD1 was highly correlated with the expression of SOD and the total antioxidant capacity, while the expression of LOL1 and LOL2 exhibited a negative correlation with them; Many TFs in NAC and WRKY families may be potentially involved in the senescence process regulated by hormones and reactive oxygen species (ROS). The expression of NAC19, NAC29, and NAC53 exhibited a positive correlation with the contents of ABA and H₂O₂, while the expression of WRKY53, WRKY54, and WRKY70 exhibited a negative correlation with the contents of JA, SA and ABA.

CONCLUSIONS

Our study provided a comprehensive characterization of the expression profiles of TFs during the developmental stages of L. japonica. In addition, we detected the key TFs that may play significant roles in controlling active components biosynthesis, antioxidant activity and flower senescence in L. japonica, thereby providing valuable insights into the molecular networks underlying L. japonica flower development.

Comparative Transcriptome Analysis of Celery Leaf Blades Identified an R2R3-MYB Transcription Factor that Regulates Apigenin Metabolism

Apigenin has been proven to possess many pharmacological properties, but the mechanism of regulation of apigenin biosynthesis in plants remains unclear. Apigenin is the main flavonoid in celery and is mainly accumulated in the middle stage of leaf blade development. In this study, comparative transcriptomic analysis revealed a large number of structural genes and transcription factor genes that may be involved in the apigenin metabolic pathway. On the basis of the apigenin content in different celery accessions, an R2R3-MYB transcription factor gene, named AgMYB1, was isolated from the high apigenin celery accession C014. Bioinformatics analysis indicated that AgMYB1 may be involved in flavonoid metabolism. AgMYB1 expression showed a positive relation with the expression of the apigenin accumulation marker gene FNSI and with the apigenin content in different celery tissues. Moreover, overexpression and antisense expression of AgMYB1 in transgenic celery plants significantly increased and reduced the expression of apigenin biosynthetic genes and the apigenin content, respectively. These findings suggest that AgMYB1 is involved in positive regulation of apigenin metabolism in celery.

A high-density genetic map developed by specific-locus amplified fragment (SLAF) sequencing and identification of a locus controlling anthocyanin pigmentation in stalk of Zicaitai (Brassica rapa L. ssp. chinensis var. purpurea)

BACKGROUND

Caixin and Zicaitai (Brassica rapa) belong to Southern and Central China respectively. Zicaitai contains high amount of anthocyanin in leaf and stalk resulting to the purple color. Stalk is the major edible part and stalk color is an economically important trait for the two vegetables. The aim of this study is to construct a high density genetic map using the specific length amplified fragment sequencing (SLAF-seq) technique to explore genetic basis for anthocyanin pigmentation traits via quantitative trait loci (QTL) mapping.

RESULTS

We constructed a high generation linkage map with a mapping panel of F2 populations derived from 150 individuals of parental lines "Xianghongtai 01" and "Yinong 50D" with purple and green stalk respectively. The map was constructed containing 4253 loci, representing 10,940 single nucleotide polymorphism (SNP) markers spanning 1030.04 centiMorgans (cM) over 10 linkage groups (LGs), with an average distance between markers of 0.27 cM. Quantitative trait loci (QTL) analysis revealed that a major locus on chromosome 7 and 4 minor QTLs explaining 2.69-61.21% of phenotypic variation (PVE) were strongly responsible for variation in stalk color trait. Bioinformatics analysis of the major locus identified 62 protein-coding genes. Among the major locus, there were no biosynthetic genes related to anthocyanin. However, there were several transcription factors like helix-loop-helix (bHLH) bHLH, MYB in the locus. Seven predicted candidate genes were selected for the transcription level analysis. Only bHLH49 transcription factor, was significantly higher expressed in both stalks and young leaves of Xianghongtai01 than Yinong50D. An insertion and deletion (InDel) marker developed from deletion/insertion in the promoter region of bHLH49 showed significant correlation with the stalk color trait in the F2 population.

CONCLUSION

Using the constructed high-qualified linkage map, this study successfully identified QTLs for stalk color trait. The identified valuable markers and candidate genes for anthocyanin accumulation in stalk will provide useful information for molecular regulation of anthocyanin biosynthesis. Overall our findings will lay a foundation for functional gene cloning, marker-assisted selection (MAS) and molecular breeding of important economic traits in B. rapa.

Overexpression of SmANS Enhances Anthocyanin Accumulation and Alters Phenolic Acids Content in Salvia miltiorrhiza and Salvia miltiorrhiza Bge f. alba Plantlets

Flavonoids play multiple roles in plant coloration and stress resistance and are closely associated with human health. Flavonoids and non-flavonoids (such as phenolic acids) are produced via the phenylpropanoid-derived pathway. Anthocyanidin synthase (ANS) catalyzes the synthesis of anthocyanins from leucoanthocyanidin in the flavonoids branched pathway. In this study, SmANS from Salvia miltiorrhiza was cloned and mainly localized in the endoplasmic reticulum (ER), plastids, Golgi, plasma membrane, and nucleus of tobacco epidermal cells, and was most highly expressed in purple petals in S. miltiorrhiza, whereas it showed almost no expression in white petals, green calyxes, and pistils in S. miltiorrhiza Bge f. alba. Overexpressed SmANS enhanced anthocyanin accumulation but reduced salvianolic acid B (SAB) and rosmarinic acid (RA) biosynthesis in S. miltiorrhiza and S. miltiorrhiza Bge f. alba plantlets, meanwhile, it restored the purple-red phenotype in S. miltiorrhiza Bge f. alba. These changes were due to reallocation of the metabolic flow, which was influenced by the SmANS gene. These findings indicate that SmANS not only plays a key role in anthocyanin accumulation in S. miltiorrhiza, but also acts as a “switch” for the coloration of S. miltiorrhiza Bge f. alba. This study provides baseline information for further research on flavonoids metabolism and improvement of anthocyanin or phenolic acid production by genetic engineering.

Repression of anthocyanin biosynthesis by R3-MYB transcription factors in lily (Lilium spp.)

Lily R3-MYB transcription factors are involved in negative regulation to limit anthocyanin accumulation in lily flowers and leaves and create notable color patterns on ectopically expressed petunia flowers. In eudicots, both positive and negative regulators act to precisely regulate the level of anthocyanin accumulation. The R3-MYB transcription factor is among the main factors repressing anthocyanin biosynthesis. Although, in monocots, the positive regulators have been well characterized, the negative regulators have not been examined. Two R3-MYBs, LhR3MYB1 and LhR3MYB2, which were identified in lily transcriptomes, were characterized in this study to understand the regulatory mechanisms of anthocyanin biosynthesis. LhR3MYB1 and LhR3MYB2 had a C2 suppressor motif downstream of a single MYB repeat; the similar amino acid motif appears only in AtMYBL2 among the eudicot R3-MYB proteins. Stable and transient overexpression of LhR3MYB1 and LhR3MYB2 in tobacco plants showed suppression of anthocyanin biosynthesis by both; however, suppression by LhR3MYB2 was stronger than that by LhR3MYB1. In the lily plant, the LhR3MYB2 transcript was detected in leaves with light stimulus-induced anthocyanin accumulation and in pink tepals. Although LhR3MYB1 was expressed in some, but not all tepals, its expression was not linked to anthocyanin accumulation. In addition, LhR3MYB1 expression levels in the leaves remained unchanged by the light stimulus, and LhR3MYB1 transcripts predominantly accumulated in the ovaries, which did not accumulate anthocyanins. Thus, although LhR3MYB1 and LhR3MYB2 have an ability to repress anthocyanin accumulation, LhR3MYB2 is more strongly involved in the negative regulation to limit the accumulation than that by LhR3MYB1. In addition, the overexpression of LhR3MYB2 generated notable color patterns in petunia flowers; thus, the usefulness of the LhR3MYB genes for creating unique color patterns by genetic engineering is discussed.

Functional MYB transcription factor encoding gene AN2 is associated with anthocyanin biosynthesis in Lycium ruthenicum Murray

BACKGROUND

Lycium ruthenicum Murray is an important economic plant in China and contains higher levels of anthocyanins in its fruits than other Lyciums. However, the genetic mechanism of anthocyanin production in this plant is unknown.

RESULTS

Based on previous transcriptome analysis, LrAN2 and LbAN2, encoding MYB transcription factors, were isolated from L. ruthenicum and L. barbarum, respectively. Both genes contained two introns, encoded 257 amino acids with two-Aa difference, and carried the unabridged HTH-MYB, MYB-like DNA-binding, and SANT domains. In the phylogenetic trees, LrAN2 and LbAN2 were found to be closely related to NtAN2, which regulates anthocyanin biosynthesis in tobacco. Overexpression of LrAN2 and LbAN2 induced anthocyanin biosynthesis in all tissues of tobacco. The anthocyanin content in the leaves of transgenic lines with LbAN2 was lower than LrAN2. It indicated that the function of LbAN2 was weaker than LrAN2. The AN2 transcript could be detected only in the fruits of L. ruthenicum and increased during fruit development, accompanied by anthocyanin accumulation. In natural population, the alleles LrAN2 and LrAN2 were associated strictly with L. ruthenicum and L. barbarum, respectively. Moreover, an AN2 genetic diversity study suggested that Lyciums with yellow, white, purple, and jujube red fruits were derived from L. ruthenicum.

CONCLUSIONS

Two AN2 alleles, from L. ruthenicum and L. barbarum, were functional MYB transcriptor regulating anthocyanin biosynthesis. The functional diversity and high expression level of LrAN2 could be the reason for high anthocyanin content in the fruit of L. ruthenicum. Lyciums with yellow, white, purple, and jujube red fruits were derived from L. ruthenicum based on AN2 sequence diversity. The results may be advantageous in identifying new varieties and breeding new cultivars.

Anthocyanin Synthesis and the Expression Patterns of bHLH Transcription Factor Family during Development of the Chinese Jujube Fruit (Ziziphus jujuba Mill.)

The basic helix–loop–helix (bHLH) family is an important transcription factor for eukaryotes and is involved in a wide range of biological activities. Among these, bHLH can interaction with WD repeat (WD40 or WDR) and V-myb avian myeloblastosis viral oncogene homolog (MYB) form a ternary complex to promote the efficient synthesis of anthocyanins. In this study, a total of 138 jujube bHLH (ZjbHLH) family members were screened from the transcriptome of the two jujube cultivars, ‘Junzao’ (JZ) and ‘Tailihong’ (TLH). Of these, 95 ZjbHLH genes were mapped to 12 chromosomes. A phylogenetic tree was constructed using 27 arabidopsis bHLH (AtbHLH) protein sequences of Arabidopsis thaliana (L.) Heynh. and 138 ZjbHLH protein sequences of jujube. The results show that the ZjbHLH family of jujube can be divided into 12 subfamilies. The three candidate genes, ZjGL3a, ZjGL3b and ZjTT8, related to anthocyanin synthesis, were classified into subgroup III. Meanwhile, ZjGL3a, ZjGL3b and ZjTT8 have high homology with the bHLH transcription factors involved in anthocyanin synthesis in other plants. In addition, it was found that the jujube ZjbHLH transcript family showed changing patterns of expression during fruit development. The relative expression levels of ZjGL3a, ZjGL3 and ZjTT8 were consistent with the changes of the anthocyanin contents in the two jujube cultivars examined. To better understand the anthocyanin synthesis pathway involved in ZjbHLH, a regulatory pathway model for anthocyanin synthesis was constructed. This model involves the processes of anthocyanin signal transduction, synthesis and transport.

LcNAC13 Physically Interacts with LcR1MYB1 to Coregulate Anthocyanin Biosynthesis-Related Genes during Litchi Fruit Ripening

Anthocyanin accumulation is crucial for the development of quality for most fruit. The mechanism underlying the regulation of anthocyanin biosynthesis by transcription factors in litchi fruit remains largely unknown. In this study, we isolated one NAC (NAM, ATAF1/2 and CUC2) TF gene, LcNAC13. Expression of LcNAC13 was upregulated as ripening proceeded, followed by the accumulation of anthocyanins. Electrophoretic mobility shift assay (EMSA) and transient expression assay showed that LcNAC13 could negatively regulate the expression of anthocyanin biosynthesis-related genes, including LcCHS1/2, LcCHI, LcF3H, LcF3’H, LcDFR, and LcMYB1. Furthermore, LcR1MYB1, as one R1-MYB type MYB, was identified to physically interact with LcNAC13 and reverse the effect of LcNAC13. Taken together, these results suggested that LcNAC13 and LcR1MYB1 may act together to antagonistically regulate anthocyanin biosynthesis during litchi fruit ripening, which helps to provide new insights into the regulatory networks of anthocyanin biosynthesis.

De novo transcriptome and phytochemical analyses reveal differentially expressed genes and characteristic secondary metabolites in the original oolong tea (Camellia sinensis) cultivar 'Tieguanyin' compared with cultivar 'Benshan'

Background

The two original plants of the oolong tea cultivar (‘Tieguanyin’) are “Wei shuo” ‘Tieguanyin’—TGY (Wei) and “Wang shuo” ‘Tieguanyin’—TGY (Wang). Another cultivar, ‘Benshan’ (BS), is similar to TGY in its aroma, taste, and genetic make-up, but it lacks the “Yin Rhyme” flavor. We aimed to identify differences in biochemical characteristics and gene expression among these tea plants.

Results

The results of spectrophotometric, high performance liquid chromatography (HPLC), and gas chromatography-mass spectrometry (GC-MS) analyses revealed that TGY (Wei) and TGY (Wang) had deeper purple-colored leaves and higher contents of anthocyanin, catechins, caffeine, and limonene compared with BS. Analyses of transcriptome data revealed 12,420 differentially expressed genes (DEGs) among the cultivars. According to a Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis, the flavonoid, caffeine, and limonene metabolic pathways were highly enriched. The transcript levels of the genes involved in these three metabolic pathways were not significantly different between TGY (Wei) and TGY (Wang), except for two unigenes encoding IMPDH and SAMS, which are involved in caffeine metabolism. The comparison of TGY vs. BS revealed eight up-regulated genes (PAL, C4H, CHS, F3’H, F3H, DFR, ANS, and ANR) and two down-regulated genes (FLS and CCR) in flavonoid metabolism, four up-regulated genes (AMPD, IMPDH, SAMS, and 5′-Nase) and one down-regulated XDH gene in caffeine metabolism; and two down-regulated genes (ALDH and HIBADH) in limonene degradation. In addition, the expression levels of the transcription factor (TF) PAP1 were significantly higher in TGY than in BS. Therefore, high accumulation of flavonoids, caffeine, and limonene metabolites and the expression patterns of their related genes in TGY might be beneficial for the formation of the “Yin Rhyme” flavor.

Conclusions

Transcriptomic, HPLC, and GC-MS analyses of TGY (Wei), TGY (Wang), and BS indicated that the expression levels of genes related to secondary metabolism and high contents of catechins, anthocyanin, caffeine, and limonene may contribute to the formation of the “Yin Rhyme” flavor in TGY. These findings provide new insights into the relationship between the accumulation of secondary metabolites and sensory quality, and the molecular mechanisms underlying the formation of the unique flavor “Yin Rhyme” in TGY.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-5643-z) contains supplementary material, which is available to authorized users.

Transcriptome Analysis Reveals the Accumulation Mechanism of Anthocyanins in Buckwheat (Fagopyrum esculentum Moench) Cotyledons and Flowers

Buckwheat (Fagopyrum esculentum) is a valuable crop which can produce multiple human beneficial secondary metabolites, for example, the anthocyanins in sprouts and flowers. However, as the predominant group of visible polyphenols in pigmentation, little is known about the molecular mechanisms underlying the anthocyanin biosynthesis within buckwheat. In this study, a comparative transcriptome analysis of green and red common buckwheat cultivars was carried out through RNA sequencing. Overall, 3727 and 5323 differently expressed genes (DEGs) were identified in flowers and cotyledons, respectively. Through GO and KEGG analysis, we revealed that DEGs in flowers and cotyledons are predominately involved in biosynthesis of anthocyanin. A total of 42 unigenes encoding 11 structural enzymes of the anthocyanin biosynthesis were identified as DEGs. We also identified some transcription factor families involved in the regulation of anthocyanin biosynthesis. Real-time qPCR validation of candidate genes was performed in flowers and cotyledons, and the results suggested that the high expression level of structural genes involved in anthocyanin biosynthetic pathway promotes anthocyanin accumulation. Our results provide the insight understanding for coloration of red common buckwheat.

Genetic control of anthocyanin pigmentation of potato tissues

BACKGROUND

The cultivated potato Solanum tuberosum L. is the fourth most important crop worldwide. Anthocyanins synthesis and accumulation in potato tissues are considered as one of important traits related to stress resistance and nutritional value. It is considered that the major regulatory gene for anthocyanin biosynthesis is R2R3 MYB-encoding gene StAN1. However, the genetic control of pigmentation of different potato tissues is substantially under investigated. The development of genetic markers for breeding of potato with specific pigmentation pattern remains an actual task.

RESULTS

We investigated 36 potato varieties and hybrids with different pigmentation of tubers and leaves. Sequence organization of regulatory R2R3 MYB (StAN1, StMYBA1, StMYB113), bHLH (StbHLH1, StJAF13) and WD40 (StWD40) genes potentially controlling anthocyanin biosynthesis has been evaluated. The results demonstrated a high variability in the StAN1 third exon and promoter region with the exception for 35 bp, containing elements for the transcription start and activation of gene expression in roots. The analysis of transcriptional activity of genes coding R2R3 MYBs, bHLHs and WD40 transcriptional factors in leaves of eight potato genotypes with different anthocyanin pigmentation was performed. The results showed a relation between the gene expression level and plant pigmentation only for the StAN1 and StWD40 genes, while other studied genes had either strong expression in all varieties and hybrids (StMYBA1, StbHLH1 and StJAF13) or they were not expressed at all (StMYB113).

CONCLUSIONS

It was found that StAN1 is the major regulatory gene controlling potato anthocyanin synthesis. However, diagnostic markers developed for the functional StAN1 alleles (StAN1777 and StAN1816) can not be used efficiently for prediction of potato pigmentation patterns. It is likely that the sequence organization of StAN1 promoter is important for anthocyanin synthesis control and the development of additional diagnostic markers is necessary.

MYB Repressors as Regulators of Phenylpropanoid Metabolism in Plants

The phenylpropanoid pathway gives rise to lignin, flavonoids, and other metabolites and is regulated by MYB transcription factors. Many R2R3-MYB transcriptional activators are known, but the prevalence of MYB repressors has only recently become recognized. This review article summarizes recent progress on function and mechanism of these MYB repressors. The characterized phenylpropanoid R2R3-MYB repressors comprise two phylogenetic clades that act on the lignin and general phenylpropanoid genes, or the flavonoid genes, respectively; anthocyanin R3-MYB repressors form a separate clade. While some flavonoid MYBs repressors can bind basic-helix-loop-helix factors and disrupt the MBW complex, for the lignin repressor MYBs interactions with promoter cis-elements have been demonstrated. The role of the conserved repression motifs that define the MYB repressors is not yet known, however.

Genome-wide analysis of MpBHLH12, a IIIf basic helix-loop-helix transcription factor of Marchantia polymorpha

The evolution of plants on land required adaptation to UV radiation and dry environments, and involved the appearance and/or rewiring of genetic connections, known as gene regulatory networks (GRNs), which consist of one or more transcription factors (TFs). The liverwort, Marchantia polymorpha, is a basal land plant, with a recently sequenced genome. The number of genes encoding basic helix-loop-helix (bHLH) family members is considerably higher in M. polymorpha than in charophyte green algae, suggesting the contribution of bHLH proteins to the evolution of GRNs associated with the adaptation of plants to land. Although an understanding of the evolutionary aspects of GRNs is fundamental for elucidating the mechanisms of environmental adaptation, the evolution of GRNs that led to land adaptation in plants remains poorly understood. In this study, we isolated a single gene encoding a IIIf bHLH TF from M. polymorpha, MpBHLH12. Transgenic M. polymorpha constitutively overexpressing MpBHLH12 showed smaller and fewer gemma cups than wild type, suggesting that MpBHLH12 is involved in the regulation of morphological development. Transcriptomic analysis of MpBHLH12 overexpressor (MpBHLH12ox) lines revealed an overlap with the GRN of MpMYB14, which regulates the biosynthesis of anthocyanins and phenolic compounds. However, MpBHLH12ox did not show anthocyanin accumulation. Results of the transient reporter assay suggest that MpBHLH12 could function in repression rather than activation. Our findings suggest that although the IIIf bHLH MpBHLH12 shows highest amino acid similarity with IIIf bHLH clade and is involved in developmental process and partly biosynthesis of phenolic compounds in M. polymorpha like Arabidopsis IIIf bHLH, the GRN involving MpBHLH12 would be distinct one from those of the IIIf bHLH TFs of seed plants.

Advance of the negative regulation of anthocyanin biosynthesis by MYB transcription factors

Anthocyanins are secondary metabolites derived from the specific branch of the flavonoid pathway, responsible for red, purple and blue coloration display in the flowers and fruits. The functions of anthocyanins are diverse, including acting as visual signals to pollinators, defense against biotic and abiotic stresses. Thus, anthocyanins have been the most intensely studied secondary metabolite pathway. From model plants to horticultural crops, numerous studies have resulted in the discovery of highly conserved MYB-bHLH-WDR (MBW) transcriptional complex for the regulation of anthocyanin biosynthesis in plants. Recent discoveries have revealed that the anthocyanin biosynthesis pathway is also controlled by MYB repressors. Here we focus on the research progress into the role of MYB repressors in anthocyanin biosynthesis. In particular, we will discuss their functions and relationship to the MBW complex in the control of anthocyanin accumulation. In addition, an integrated regulatory network of anthocyanin biosynthesis controlled by MYB repressors and MBW activation complex is built based on the significant progress.

MdGSTF6, activated by MdMYB1, plays an essential role in anthocyanin accumulation in apple

Anthocyanins are biosynthesized on the cytosolic surface of the endoplasmic reticulum and then transported into the vacuole for storage. Glutathione S-transferases (GSTs) are considered to be responsible for the transport of anthocyanins into the vacuole. However, the regulatory mechanisms of GSTs in plants are still unclear. Here, we performed a genome-wide analysis and identified 69 GST genes in apple. The expression of MdGSTF6 was positively correlated with the anthocyanin content (r = 0.949) during 'Yanfu 8' fruit development. The overexpression of MdGSTF6 in the Arabidopsis thaliana tt19 mutant resulted in seedlings of 35S::MdGSTF6-GFP/tt19 that could accumulate anthocyanin and rescue its phenotype, suggesting that MdGSTF6 was an anthocyanin transporter. The silencing of MdGSTF6 affected anthocyanin accumulation in apple fruit. Moreover, the knockdown of MdGSTF6 by RNA interference in cultured 'Gala' seedlings inhibited anthocyanin accumulation. The interaction experiments showed that MdMYB1 could bind directly to the MdGSTF6 promoter to transcriptionally activate its expression. Collectively, our results demonstrate that MdGSTF6 encodes an important GST transporter of anthocyanins in apple fruit and provide evidence for the associated regulatory mechanisms. Therefore, MdMYB1 can not only regulate anthocyanin synthesis, but also control the transport of anthocyanin in apples. This information may be useful for further clarifying the regulation of anthocyanin transport in apple.

A Novel R2R3-MYB Transcription Factor Contributes to Petal Blotch Formation by Regulating Organ-Specific Expression of PsCHS in Tree Peony (Paeonia suffruticosa)

Flower color patterns play critical roles in plant-pollinator interactions and represent one of the most common adaptations during angiosperm evolution. However, the molecular mechanisms underlying flower color pattern formation are less understood in non-model organisms. The aim of this study was to identify genes involved in the formation of petal blotches in tree peony (Paeonia suffruticosa) through transcriptome profiling and functional experiments. We identified an R2R3-MYB gene, PsMYB12, representing a distinct R2R3-MYB subgroup, with a spatiotemporal expression pattern tightly associated with petal blotch development. We further demonstrated that PsMYB12 interacts with a basic helix-loop-helix (bHLH) and a WD40 protein in a regulatory complex that directly activates PsCHS expression, which is also specific to the petal blotches. Together, these findings advance our understanding of the molecular mechanisms of pigment pattern formation beyond model plants. They also benefit molecular breeding of tree peony cultivars with novel color patterns and promote germplasm innovation.

The regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple

The plant hormone ethylene is critical for climacteric fruit ripening, while glucose and anthocyanins determine the fruit quality of climacteric fruits such as apple. Understanding the exact molecular mechanism for this process is important for elucidating the interconnection of ethylene and fruit quality. Overexpression of apple MdbHLH3 gene, an anthocyanin-related basic helix-loop-helix transcription factor (bHLH TF) gene, promotes ethylene production, and transgenic apple plantlets and trees exhibit ethylene-related root developmental abnormalities, premature leaf senescence, and fruit ripening. Biochemical analyses demonstrate that MdbHLH3 binds to the promoters of three genes that are involved in ethylene biosynthesis, including MdACO1, MdACS1, and MdACS5A, activating their transcriptional expression, thereby promoting ethylene biosynthesis. High glucose-inhibited U-box-type E3 ubiquitin ligase MdPUB29, the ortholog of Arabidopsis AtPUB29 in apple, influences the expression of ethylene biosynthetic genes and ethylene production by direct ubiquitination of the MdbHLH3 protein. Our findings provide new insights into the ubiquitination of MdbHLH3 by glucose-inhibited ubiquitin E3 ligase MdPUB29 in the regulation of ethylene biosynthesis as well as indicate that the regulatory module MdPUB29-MdbHLH3 connects ethylene biosynthesis with fruit quality in apple.

Structural and functional divergence of the Mpc1 genes in wheat and barley

Background

The members of the Triticeae tribe are characterised by the presence of orthologous and homoeologous gene copies regulating flavonoid biosynthesis. Among transcription factors constituting a regulatory MBW complex, the greatest contribution to the regulation of flavonoid biosynthetic pathway is invested by R2R3-Myb-type TFs. Differently expressed R2R3-Myb copies activate the synthesis of various classes of flavonoid compounds in different plant tissues. The aim of this research was the identification, comparison and analysis of full-length sequences of the duplicated R2R3-Myb Mpc1 (Myb protein c1) gene copies in barley and wheat genomes.

Results

The Mpc1 genes were identified in homoeologous group 4 and 7 chromosomes: a total of 3 copies in barley (Hordeum vulgare L.) and 8 copies in bread wheat (Triticum aestivum L.) genomes. All Mpc1 genes have a similar two-exon structure, and almost all of them are transcriptionally active. The calculation of the divergence time revealed that first duplication between 4 and 7 chromosomes of the common ancestor of the Triticeae tribe occurred about 35–46 million years ago (MYA); the last duplication arised about 16–19 MYA before the divergence Triticum and Hordeum genera The connection between gene expression and the appearance of anthocyanin pigmentation was found for three genes from homoeologous group 4 chromosomes: TaMpc1-A2 (5AL) in wheat coleoptile, HvMpc1-H2 (4HL) in barley lemma and aleurone layer, and HvMpc1-H3 (4HL) in barley aleurone layer. TaMpc1-D4 (4DL) from the wheat genome showed a strong level of expression regardless of the colour of coleoptile or pericarp. It is assumed, that this gene regulates the biosynthesis of uncoloured flavonoids in analysed tissues.

Conclusions

The regulatory R2R3-Myb genes involved in anthocyanin synthesis were identified and characterised in Triticeae tribe species. Genes designated HvMpc1-H2 and HvMpc1-H3 appeared to be the main factors underlying intraspecific variation of H. vulgare by lemma and aleurone colour. TaMpc1-A2 is the co-regulator of the Mpc1–1 genes in bread wheat genome controlling anthocyanin synthesis in coleoptile.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1378-3) contains supplementary material, which is available to authorized users.

Analysis of Metabolite Accumulation Related to Pod Color Variation of Caragana intermedia

Caragana intermedia, a leguminous shrub widely distributed in cold and arid regions, is rich in secondary metabolites and natural active substances, with high nutritional and medical values. It is interesting that the pods of C. intermedia often show different colors among individual plants. In this study, 10-, 20- and 30-day-old red and green pods of C. intermedia were used to identify and characterize important metabolites associated with pod color. A total 557 metabolites, which could be classified into 21 groups, were detected in the pod extracts using liquid chromatography coupled with ESI-triple quadrupole-linear ion trap mass spectrometer (LC-ESI-MS/MS). Metabolomics analysis revealed significant differences in 15 groups of metabolites between red and green pods, including amino acids, nucleotide derivatives, flavonoids, and phytohormones. Metabolic pathway analysis showed that the shikimic acid and the phytohormone metabolic pathways were extraordinarily active in red pods, and the difference between red and green pods was obvious. Moreover, red pods showed remarkable flavonoids, cytokinins, and auxin accumulation, and the content of total flavonoids and proanthocyanidins in 30-day-old red pods was significantly higher than that in green pods. This metabolic profile contributes to valuable insights into the metabolic regulation mechanism in different color pods.

Myc-like transcriptional factors in wheat: structural and functional organization of the subfamily I members

BACKGROUND

Myc-like regulatory factors carrying the basic helix-loop-helix (bHLH) domain belong to a large superfamily of transcriptional factors (TFs) present in all eukaryotic kingdoms. In plants, the representatives of this superfamily regulate diverse biological processes including growth and development as well as response to various stresses. As members of the regulatory MBW complexes, they participate in biosynthesis of flavonoids. In wheat, only one member (TaMyc1) of the Myc-like TFs family has been studied, while structural and functional organization of further members remained uncharacterized. From two Myc-subfamilies described recently in the genomes of Triticeae tribe species, we investigated thoroughly the members of the subfamily I which includes the TaMyc1 gene.

RESULTS

Comparison of the promoter regions of the Myc subfamily I members in wheat suggested their division into two groups (likely homoeologous sets): TaMyc-1 (TaMyc-A1/TaMyc1, TaMyc-B1, TaMyc-D1) and TaMyc-2 (TaMyc-A2 and TaMyc-D2). It was demonstrated that the TaMyc-D1 copy has lost its functionality due to the frame shift mutation. The study of functional features of the other four copies suggested some of them to be involved in the biosynthesis of anthocyanins. In particular, TaMyc-B1 is assumed to be a co-regulator of the gene TaC1-A1 (encoding R2R3-Myb factor) in the MBW regulatory complex activating anthocyanin synthesis in wheat coleoptile. The mRNA levels of the TaMyc-A1, TaMyc-B1, TaMyc-A2 and TaMyc-D2 genes increased significantly in wheat seedlings exposed to osmotic stress. Salinity stress induced expression of TaMyc-B1 and TaMyc-A2, while TaMyc-A1 was repressed.

CONCLUSIONS

The features of the structural and functional organization of the members of subfamily I of Myc-like TFs in wheat were determined. Myc-like co-regulator (TaMyc-B1) of anthocyanin synthesis in wheat coleoptile was described for the first time. The Myc-encoding genes presumably involved in response to drought and salinity were determined in wheat. The results obtained are important for further manipulations with Myc genes, aimed on increasing wheat adaptability.

PbCOP1.1 Contributes to the Negative Regulation of Anthocyanin Biosynthesis in Pear

The synthesis of anthocyanin in pear (Pyrus bretschneideri) fruit is regulated by light. However, little is known about the molecular mechanisms of pear fruit coloring mediated by upstream light-signaling regulators. Here, the photoresponse factors CONSTITUTIVE PHOTOMORPHOGENIC (COP) 1.1 and 1.2 were cloned from ‘Red Zaosu’ peel to study their functions in pear fruit coloring. The overexpression vectors pBI121-PbCOP1.1 and pBI121-PbCOP1.2 were constructed to analyze their effects on anthocyanin synthesis in pear fruit. A protein sequence alignment and phylogenetic tree analysis revealed that PbCOP1 proteins are highly homologous with those of other species. An analysis of tissue differential expression showed that the greatest expression levels of PbCOP1s occurred in the leaves. Their expression levels increased in the leaves during development, when the leaves changed from red to green. The overexpression of PbCOP1s in the peel resulted in reduced anthocyanin synthesis at the injection sites. A quantitative PCR analysis of the injection sites showed that PbCOP1.1 significantly inhibited the expression of the anthocyanin synthesis-related genes CHI, DFR, UFGT2, bHLH3, HY5 and GST. Based on the above results, we hypothesize that PbCOP1.1 is an anthocyanin synthetic inhibitory factor of pear coloration.

Characterization of a novel litchi R2R3-MYB transcription factor that involves in anthocyanin biosynthesis and tissue acidification

BACKGROUND

Maturation of litchi (Litchi chinensis) fruit is characterized by dramatic changes in pigments in the pericarp and flavor compounds in the aril. Among them, the biosynthesis of anthocyanins is most noticeable. Previous studies showed that LcMYB1 and LcbHLH transcription factors participated in regulating the anthocyanin biosynthesis in litchi. However, the roles of other MYB factors remain unclear.

RESULTS

In this study, we cloned and characterized the function of LcMYB5, a novel R2R3-MYB identified from litchi transcriptome. Although LcMYB5 was constitutively expressed in litchi tissues and its expressions was not correlated with tissue coloration, overexpression of LcMYB5 resulted in enhanced biosynthesis of anthocyanins in tobacco and petunia concurrent with the up-regulation of their endogenous bHLHs and key structural genes in anthocyanin precursor biosynthesis. These results indicate that LcMYB5 is an R2R3 transcriptional factor regulates anthocyanin biosynthesis either by directly activating the expression of key structural genes such as DFR or by indirectly up regulating the expressions of endogenous bHLH regulators. More interestingly, the pH values in petals and leaves from transgenic lines were significant lower than those in both untransformed tobacco and petunia, indicating LcMYB5 is also associated with pH regulation. The expressions of LcMYB5 and its bHLH partner LcbHLH1 were consistent with the expression of putative tissue acidification gene LcPH1, and the changes in malic acid provided further evidence for the close relationship between LcMYB5 and tissue acidification.

CONCLUSIONS

Taking together, our study indicated that LcMYB5 is involved in not only anthocyanin biosynthesis but also tissue acidification.

Three LcABFs are Involved in the Regulation of Chlorophyll Degradation and Anthocyanin Biosynthesis During Fruit Ripening in Litchi chinensis

During litchi (Litchi chinensis Sonn.) fruit ripening, two major physiological changes, degreening (Chl degradation) and pigmentation (anthocyanin biosynthesis), are visually apparent. However, the specific factor triggering this important transition is still unclear. In the present study, we found that endogenous ABA content increased sharply when Chl breakdown was initiated and the ABA level peaked just before the onset of anthocyanin accumulation, suggesting that ABA plays an important role during litchi fruit pigmentation. We characterized three ABSCISIC ACID RESPONSE ELEMENT-BINDING FACTORs (LcABF1/2/3) belonging to group A of the basic leucine zipper (bZIP) transcription factors previously shown to be involved in ABA signaling under abiotic stress. LcABF1 transcripts increased at the onset of Chl degradation, and the expression of LcABF3 accumulated in parallel with anthocyanin biosynthesis. In addition, dual luciferase and yeast one-hybrid assays indicated that LcABF1/2 recognized ABA-responsive elements in the promoter region of Chl degradation-related genes (PAO and SGR), while LcABF2/3 bound the promoter region of LcMYB1 and anthocyanin biosynthesis-related structural genes. Indeed, Nicotiana benthamiana leaves transiently expressing LcABF1/2 showed a senescence phenomenon with Chl degradation, and LcABF3 overexpression increased the accumulation of anthocyanin via activation of LcMYB1, which is the key determinant of anthocyanin biosynthesis. These data indicate that LcABF1/2/3 are important transcriptional regulators of ABA-dependent litchi fruit ripening involved in both Chl degradation and anthocyanin biosynthesis.

Methyl Salicylate Enhances Flavonoid Biosynthesis in Tea Leaves by Stimulating the Phenylpropanoid Pathway

The phytohormone salicylic acid (SA) is a secondary metabolite that regulates plant growth, development and responses to stress. However, the role of SA in the biosynthesis of flavonoids (a large class of secondary metabolites) in tea (Camellia sinensis L.) remains largely unknown. Here, we show that exogenous methyl salicylate (MeSA, the methyl ester of SA) increased flavonoid concentration in tea leaves in a dose-dependent manner. While a moderate concentration of MeSA (1 mM) resulted in the highest increase in flavonoid concentration, a high concentration of MeSA (5 mM) decreased flavonoid concentration in tea leaves. A time-course of flavonoid concentration following 1 mM MeSA application showed that flavonoid concentration peaked at 2 days after treatment and then gradually declined, reaching a concentration lower than that of control after 6 days. Consistent with the time course of flavonoid concentration, MeSA enhanced the activity of phenylalanine ammonia-lyase (PAL, a key enzyme for the biosynthesis of flavonoids) as early as 12 h after the treatment, which peaked after 1 day and then gradually declined upto 6 days. qRT-PCR analysis of the genes involved in flavonoid biosynthesis revealed that exogenous MeSA upregulated the expression of genes such as CsPAL, CsC4H, Cs4CL, CsCHS, CsCHI, CsF3H, CsDFR, CsANS and CsUFGT in tea leaves. These results suggest a role for MeSA in modulating the flavonoid biosynthesis in green tea leaves, which might have potential implications in manipulating the tea quality and stress tolerance in tea plants.

Evolutionary Analysis of MBW Function by Phenotypic Rescue in Arabidopsis thaliana

The MBW complex consisting of the three proteins R2R3MYB, bHLH and WDR regulates five traits in Arabidopsis thaliana including trichome and root hair patterning, seed coat color, anthocyanidin production and seed coat mucilage release. The WDR gene TTG1 regulates each trait in specific combinations with different bHLH and R2R3MYB proteins. In this study we analyze to what extent the biochemical properties of the MBW proteins contribute to trait specificity by expressing them in appropriate A. thaliana mutants. We show that the rescue behavior of A. thaliana bHLH and R2R3MYB protein is sufficient to explain the function as derived previously from mutant analysis. When extending this rescue approach using MBW proteins from other species we find that proteins involved in anthocyanidin regulation typically show a rescue of the anthocyanidin phenotype but not of the other traits. Finally, we correlate the rescue abilities of MBW protein from different species with the A. thaliana proteins.