The R2R3-MYB Factor FhMYB5 From Freesia hybrida Contributes to the Regulation of Anthocyanin and Proanthocyanidin Biosynthesis

Identification and Functional Characterization of the SaMYB113 Gene in Solanum aculeatissimum

The MYB transcription factors (TFs) have substantial functions in anthocyanin synthesis as well as being widely associated with plant responses to various adversities. In the present investigation, we found an unreported MYB TF from Solanum aculeatissimum (a wild relative of eggplant) and named it SaMYB113 in reference to its homologous gene. Bioinformatics analysis demonstrated that the open reading frame of SaMYB113 was 825 bp in length, encoding 275 amino acids, with a typical R2R3-MYB gene structure, and predicted subcellular localization in the nucleus. Analysis of the tissue-specific expression pattern through qRT-PCR showed that the SaMYB113 was expressed at a high level in young stems as well as leaves of S. aculeatissimum. Transgenic Arabidopsis and tobacco plants overexpressing SaMYB113 pertinent to the control of the 35S promoter exhibited a distinct purple color trait, suggesting a significant change in their anthocyanin content. Furthermore, we obtained three tobacco transgenic lines with significant differences in anthocyanin accumulation and analyzed the differences in anthocyanin content by LC-MS/MS. The findings demonstrated that overexpression of SaMYB113 caused tobacco to have considerably raised levels of several anthocyanin components, with the most significant increases in delphinidin-like anthocyanins and cyanidin-like anthocyanins. The qRT-PCR findings revealed significant differences in the expression levels of structural genes for anthocyanin synthesis among various transgenic lines. In summary, this study demonstrated that the SaMYB113 gene has a substantial impact on anthocyanin synthesis, and overexpression of the SaMYB113 gene leads to significant modifications to the expression levels of a variety of anthocyanin-synthesizing genes, which leads to complex changes in anthocyanin content and affects plant phenotypes. This present research offers the molecular foundation for the research of the mechanism of anthocyanin formation within plants, as well as providing some reference for the improvement of traits in solanum crops.

Study on the causes of changes in colour during Hibiscus syriacus flowering based on transcriptome and metabolome analyses

BACKGROUND

The flower colour of H. syriacus 'Qiansiban' transitions from fuchsia to pink-purple and finally to pale purple, thereby enhancing the ornamental value of the cultivars. However, the molecular mechanism underlying this change in flower colour in H. syriacus has not been elucidated. In this study, the transcriptomic data of H. syriacus 'Qiansiban' at five developmental stages were analysed to investigate the impact of flavonoid components on flower colour variation. Additionally, five cDNA libraries were constructed from H. syriacus 'Qiansiban' during critical blooming stages, and the transcriptomes were sequenced to investigate the molecular mechanisms underlying changes in flower colouration.

RESULTS

High-performance liquid chromatography‒mass spectrometry detected five anthocyanins in H. syriacus 'Qiansiban', with malvaccin-3-O-glucoside being the predominant compound in the flowers of H. syriacus at different stages, followed by petunigenin-3-O-glucoside. The levels of these five anthocyanins exhibited gradual declines throughout the flowering process. In terms of the composition and profile of flavonoids and flavonols, a total of seven flavonoids were identified: quercetin-3-glucoside, luteolin-7-O-glucoside, Santianol-7-O-glucoside, kaempferol-O-hexosyl-C-hexarbonoside, apigenin-C-diglucoside, luteolin-3,7-diglucoside, and apigenin-7-O-rutinoside. A total of 2,702 DEGs were identified based on the selected reference genome. Based on the enrichment analysis of differentially expressed genes, we identified 9 structural genes (PAL, CHS, FLS, DRF, ANS, CHI, F3H, F3'5'H, and UFGT) and 7 transcription factors (3 MYB, 4 bHLH) associated with flavonoid biosynthesis. The qRT‒PCR results were in good agreement with the high-throughput sequencing data.

CONCLUSION

This study will establish a fundamental basis for elucidating the mechanisms underlying alterations in the flower pigmentation of H. syriacus.

Metabolic and molecular mechanisms of spine color formation in Chinese red chestnut

The spines of Chinese red chestnut are red and the depth of their color gradually increases with maturity. To identify the anthocyanin types and synthesis pathways in red chestnut and to identify the key genes regulating the anthocyanin biosynthesis pathway, we obtained and analyzed the transcriptome and anthocyanin metabolism of red chestnut and its control variety with green spines at 3 different periods. GO and KEGG analyses revealed that photosynthesis was more highly enriched in green spines compared with red spines, while processes related to defense and metabolism regulation were more highly enriched in red spines. The analysis showed that the change in spine color promoted photoprotection in red chestnut, especially at the early growth stage, which resulted in the accumulation of differentially expressed genes involved in the defense metabolic pathway. The metabolome results revealed 6 anthocyanins in red spines. Moreover, red spines exhibited high levels of cyanidin, peonidin and pelargonidin and low levels of delphinidin, petunidin and malvidin. Compared with those in the control group, the levels of cyanidin, peonidin, pelargonidin and malvidin in red spines were significantly increased, indicating that the cyanidin and pelargonidin pathways were enriched in the synthesis of anthocyanins in red spines, whereas the delphinidin pathways were inhibited and mostly transformed into malvidin. During the process of flower pigment synthesis, the expression of the CHS, CHI, F3H, CYP75A, CYP75B1, DFR and ANS genes clearly increased, that of CYP73A decreased obviously, and that of PAL, 4CL and LAR both increased and decreased. Notably, the findings revealed that the synthesized anthocyanin can be converted into anthocyanidin or epicatechin. In red spines, the upregulation of BZ1 gene expression increases the corresponding anthocyanidin content, and the upregulation of the ANR gene also promotes the conversion of anthocyanin to epicatechin. The transcription factors involved in color formation included 4 WRKYs.

Functional characterization of terpene synthases SmTPS1 involved in floral scent formation in Salvia miltiorrhiza

Plants attract beneficial insects and promote pollination by releasing floral scents. Salvia miltiorrhiza, as an insect-pollinated flowering plant, which has been less studied for its floral aroma substances. This study revealed that S. miltiorrhiza flowers produce various volatile terpenoids, including five monoterpenes and ten sesquiterpenes, with the sesquiterpene compound (E)-β-caryophyllene being the most abundant, accounting for 28.1% of the total volatile terpenoids. Y-tube olfactometer experiments were conducted on the primary pollinator of S. miltiorrhiza, the Apis ceranas. The results indicated that (E)-β-caryophyllene compound had an attractive effect on the Apis ceranas. By comparing the homologous sequences with the genes of (E)-β-caryophyllene terpene synthases in other plants, the SmTPS1 gene was selected for further experiment. Subcellular localization experiments showed SmTPS1 localized in the cytoplasm, and its in vitro enzyme assay revealed that it could catalyze FPP into β-Elemene, (E)-β-caryophyllene and α-Humulene. Overexpression of SmTPS1 in S. miltiorrhiza resulted in a 5.29-fold increase in gene expression. The GC-MS analysis revealed a significant increase in the concentration of (E)-β-caryophyllene in the transgenic plants, with levels 2.47-fold higher compared to the empty vector plants. Furthermore, Y-tube olfactometer experiments showed that the transgenic plants were significantly more attractive to Apis ceranas compared to the empty vector plants. Co-expression analysis suggested that four SmMYCs (SmMYC1, SmMYC5, SmMYC10, and SmMYC11) may be involved in the transcriptional regulation of SmTPS1. The yeast one-hybrid screen and the Dual luciferase assay indicated that SmMYC10 positively regulates the expression of SmTPS1. In conclusion, this study lays a foundation for the functional analysis and transcriptional regulation of terpene synthase genes in S. miltiorrhiza.

Comparative transcriptomic and metabolomic profiles reveal fruit peel color variation in two red pomegranate cultivars

Pomegranate (Punica granatum L.) which belongs to family Lythraceae, is one of the most important fruit crops of many tropical and subtropical regions. A high variability in fruit color is observed among different pomegranate accessions, which arises from the qualitative and quantitative differences in anthocyanins. However, the mechanism of fruit color variation is still not fully elucidated. In the present study, we investigated the red color mutation between a red-skinned pomegranate 'Hongbaoshi' and a purple-red-skinned cultivar 'Moshiliu', by using transcriptomic and metabolomic approaches. A total of 51 anthocyanins were identified from fruit peels, among which 3-glucoside and 3,5-diglucoside of cyanidin (Cy), delphinidin (Dp), and pelargonidin (Pg) were dominant. High proportion of Pg in early stages of 'Hongbaoshi' but high Dp in late stages of 'Moshiliu' were characterized. The unique high levels of Cy and Dp anthocyanins accumulating from early developmental stages accounted for the purple-red phenotype of 'Moshiliu'. Transcriptomic analysis revealed an early down-regulated and late up-regulated of anthocyanin-related structure genes in 'Moshiliu' compared with 'Hongbaoshi'. Alao, ANR was specially expressed in 'Hongbaoshi', with extremely low expression levels in 'Moshiliu'. For transcription factors R2R3-MYB, the profiles demonstrated a much higher transcription levels of three subgroup (SG) 5 MYBs and a sharp decrease in expression of SG6 MYB LOC116202527 in high-anthocyanin 'Moshiliu'. SG4 MYBs exhibited two entirely different patterns, LOC116203744 and LOC116212505 were down-regulated whereas LOC116205515 and LOC116212778 were up-regulated in 'Moshiliu' pomegranate. The results indicate that specific SG members of the MYB family might promote the peel coloration in different manners and play important roles in color mutation in pomegranate.

Functional diversity of subgroup 5 R2R3-MYBs promoting proanthocyanidin biosynthesis and their key residues and motifs in tea plant

The tea plant (Camellia sinensis) is rich in polyphenolic compounds. Particularly, flavan-3-ols and proanthocyanidins (PAs) are essential for the flavor and disease-resistance property of tea leaves. The fifth subgroup of R2R3-MYB transcription factors comprises the primary activators of PA biosynthesis. This study showed that subgroup 5 R2R3-MYBs in tea plants contained at least nine genes belonging to the TT2, MYB5, and MYBPA types. Tannin-rich plants showed an expansion in the number of subgroup 5 R2R3-MYB genes compared with other dicotyledonous and monocot plants. The MYBPA-type genes of tea plant were slightly expanded. qRT-PCR analysis and GUS staining analysis of promoter activity under a series of treatments revealed the differential responses of CsMYB5s to biotic and abiotic stresses. In particular, CsMYB5a, CsMYB5b, and CsMYB5e responded to high-intensity light, high temperature, MeJA, and mechanical wounding, whereas CsMYB5f and CsMYB5g were only induced by wounding. Three genetic transformation systems (C. sinensis, Nicotiana tabacum, and Arabidopsis thaliana) were used to verify the biological function of CsMYB5s. The results show that CsMYB5a, CsMYB5b, and CsMYB5e could promote the gene expression of CsLAR and CsANR. However, CsMYB5f and CsMYB5g could only upregulate the gene expression of CsLAR but not CsANR. A series of site-directed mutation and domain-swapping experiments were used to verify functional domains and key amino acids of CsMYB5s responsible for the regulation of PA biosynthesis. This study aimed to provide insight into the induced expression and functional diversity model of PA biosynthesis regulation in tea plants.

Small RNA and Degradome Sequencing Reveal Roles of miRNAs in the Petal Color Fading of Malus Crabapple

The Malus crabapple is an important woody ornamental plant. The fading of petals during its development significantly affects their ornamental value. Petal color is related to anthocyanin content and miRNAs play an important role in the post-transcriptional regulation of anthocyanin synthesis. However, the mechanisms underlying miRNA regulation of petal fading have rarely been studied. Transcriptome and small RNA sequencing of petals from the blooming phases of Malus. 'Indian Summer' varieties S1 (small bud), S2 (initial-flowering), and S3 (late-flowering) allowed us to identify 230 known miRNAs and 17 novel miRNAs, including 52 differentially expressed miRNAs which targeted 494 genes and formed 823 miRNA-target pairs. Based on the target gene annotation results, miRNA-target pairs were screened that may be involved in the fading process of Malus crabapple petals through three different pathways: anthocyanin synthesis, transport, and degradation, involving mcr-miR858-MYB1\MYB5 and mcr-miR396-McCHI inhibiting anthocyanin synthesis; mcr-miR167, mcr-miR390, mcr-miR535, and mcr-miR858 inhibiting anthocyanin transport from the cytoplasm to the vacuole by targeting ABC transporter genes (ABCB, ABCC, ABCD, and ABCG); and mcr-miR398 targeting the superoxide dismutase genes (CZSOD2 and CCS) to accelerate anthocyanin degradation. These findings offer a novel approach to understanding the mechanism of petal fading and serve as a reference for other plants with floral fading.

Allelic variation of terpene synthases drives terpene diversity in the wild species of the Freesia genus

Terpene synthases (TPSs) play pivotal roles in conferring the structural diversity of terpenoids, which are mainly emitted from flowers, whereas the genetic basis of the release of floral volatile terpenes remains largely elusive. Though quite similar in sequence, TPS allelic variants still function divergently, and how they drive floral terpene diversity in closely related species remains unknown. Here, TPSs responsible for the floral scent of wild Freesia species were characterized, and the functions of their natural allelic variants, as well as the causal amino acid residues, were investigated in depth. Besides the 8 TPSs previously reported in modern cultivars, 7 additional TPSs were functionally evaluated to contribute to the major volatiles emitted from wild Freesia species. Functional characterization of allelic natural variants demonstrated that allelic TPS2 and TPS10 variants changed the enzymatic capacity while allelic TPS6 variants drove the diversity of floral terpene products. Further residue substitution analysis revealed the minor residues determining the enzyme catalytic activity and product specificity. The clarification of TPSs in wild Freesia species reveals that allelic TPS variants evolved differently to determine the interspecific floral volatile terpenes in the genus and might be used for modern cultivar improvement.

A novel R2R3-MYB transcription factor FaMYB5 positively regulates anthocyanin and proanthocyanidin biosynthesis in cultivated strawberries (Fragaria × ananassa)

Flavonoids have a major contribution to the fruit quality in cultivated strawberries and are regulated by MYB, bHLH and WD40 transcriptional factors. We reported here the identification of the FaMYB5, an R2R3-MYB transcription factor, which positively regulated the accumulation of anthocyanins and proanthocyanidins through the trans-activation of the F3'H and LAR. The strawberry FaEGL3 and FaLWD1/FaLWD1-like interact with the R2R3-FaMYB5 to form an MYB-bHLH-WD40 complex (MBW), enhancing the regulatory efficiency. The R2R3-FaMYB5 was constitutively expressed in various tissues and in fruits of different developmental stages, which was strikingly contrasting to the fruit-specific expression patterns of FaMYB10. Meanwhile, R2R3-FaMYB5 failed to promote a stable accumulation of anthocyanin glycosides in the mature fruits of the myb10 mutant, mainly due to the suppressed expression of TT19. The R2R3-FaMYB5 was regulated by an antisense long noncoding RNA lncRNA-myb5. Additionally, the R2R3-FaMYB5 protein could interact with FaBT2 and was degraded through the ubiquitin/26 S proteasome pathway. Transcriptome and metabolome data showed that R2R3-FaMYB5 enhanced the gene expression and the metabolite accumulation involved in the flavonoid, phenylpropanoid and lignin biosynthesis pathways. Collectively, we conclude that the FaMYB5 is an R2R3-MYB activator involved in the composition of MBW, which positively regulates the biosynthesis of anthocyanin and proanthocyanidin. These findings provided new insights into the molecular mechanisms that regulate flavonoids in strawberry fruits.

DoMYB5 and DobHLH24, Transcription Factors Involved in Regulating Anthocyanin Accumulation in Dendrobium officinale

As a kind of orchid plant with both medicinal and ornamental value, Dendrobium officinale has garnered increasing research attention in recent years. The MYB and bHLH transcription factors play important roles in the synthesis and accumulation of anthocyanin. However, how MYB and bHLH transcription factors work in the synthesis and accumulation of anthocyanin in D. officinale is still unclear. In this study, we cloned and characterized one MYB and one bHLH transcription factor, namely, D. officinale MYB5 (DoMYB5) and D. officinaleb bHLH24 (DobHLH24), respectively. Their expression levels were positively correlated with the anthocyanin content in the flowers, stems, and leaves of D. officinale varieties with different colors. The transient expression of DoMYB5 and DobHLH24 in D. officinale leaf and their stable expression in tobacco significantly promoted the accumulation of anthocyanin. Both DoMYB5 and DobHLH24 could directly bind to the promoters of D. officinale CHS (DoCHS) and D. officinale DFR (DoDFR) and regulate DoCHS and DoDFR expression. The co-transformation of the two transcription factors significantly enhanced the expression levels of DoCHS and DoDFR. DoMYB5 and DobHLH24 may enhance the regulatory effect by forming heterodimers. Drawing on the results of our experiments, we propose that DobHLH24 may function as a regulatory partner by interacting directly with DoMYB5 to stimulate anthocyanin accumulation in D. officinale.

Functional Characterization of a (E)-β-Ocimene Synthase Gene Contributing to the Defense against Spodoptera litura

Soybean is a worldwide crop that offers valuable proteins, fatty acids, and phytonutrients to humans but is always damaged by insect pests or pathogens. Plants have captured sophisticated defense mechanisms in resisting the attack of insects and pathogens. How to protect soybean in an environment- or human-friendly way or how to develop plant-based pest control is a hotpot. Herbivore-induced plant volatiles that are released by multiple plant species have been assessed in multi-systems against various insects, of which (E)-β-ocimene has been reported to show anti-insect function in a variety of plants, including soybean. However, the responsible gene in soybean is unknown, and its mechanism of synthesis and anti-insect properties lacks comprehensive assessment. In this study, (E)-β-ocimene was confirmed to be induced by Spodoptera litura treatment. A plastidic localized monoterpene synthase gene, designated as GmOCS, was identified to be responsible for the biosynthesis of (E)-β-ocimene through genome-wide gene family screening and in vitro and in vivo assays. Results from transgenic soybean and tobacco confirmed that (E)-β-ocimene catalyzed by GmOCS had pivotal roles in repelling a S. litura attack. This study advances the understanding of (E)-β-ocimene synthesis and its function in crops, as well as provides a good candidate for further anti-insect soybean improvement.

The R2R3-MYB-SG7 transcription factor CaMYB39 orchestrates surface phenylpropanoid metabolism and pathogen resistance in chickpea

Flavonoids are important plant pigments and defense compounds; understanding the transcriptional regulation of flavonoid biosynthesis may enable engineering crops with improved nutrition and stress tolerance. Here, we characterize R2R3-MYB domain subgroup 7 transcription factor CaMYB39, which regulates flavonol biosynthesis primarily in chickpea trichomes. CaMYB39 overexpression in chickpea was accompanied by a change in flux availability for the phenylpropanoid pathway, particularly flavonol biosynthesis. Lines overexpressing CaMYB39 showed higher isoflavonoid levels, suggesting its role in regulating isoflavonoid pathway. CaMYB39 transactivates the transcription of early flavonoid biosynthetic genes (EBG). FLAVONOL SYNTHASE2, an EBG, encodes an enzyme with higher substrate specificity for dihydrokaempferol than other dihydroflavonols explaining the preferential accumulation of kaempferol derivatives as prominent flavonols in chickpea. Interestingly, CaMYB39 overexpression increased trichome density and enhanced the accumulation of diverse flavonol derivatives in trichome-rich tissues. Moreover, CaMYB39 overexpression reduced reactive oxygen species levels and induced defense gene expression which aids in partially blocking the penetration efficiency of the fungal pathogen, Ascochyta rabiei, resulting in lesser symptoms, thus establishing its role against deadly Ascochyta blight (AB) disease. Overall, our study reports an instance where R2R3-MYB-SG7 member, CaMYB39, besides regulating flavonol biosynthesis, modulates diverse pathways like general phenylpropanoid, isoflavonoid, trichome density, and defense against necrotrophic fungal infection in chickpea.

Metabolome and transcriptome profiling reveal regulatory network and mechanism of flavonoid biosynthesis during color formation of Dioscorea cirrhosa L

Dioscorea cirrhosa is a plant that is used as a dye as well as in medicine. Many metabolites with pharmacological activity exist in the tubers of D. cirrhosa. However, little is known about the mechanism regulating biosynthesis in these metabolites. In this study, transcriptome and metabolome profiling were performed in four color tubers. A total of 531 metabolites, including 62 flavonoids, were identified. Epicatechin and proanthocyanin B2 were the key metabolites that exhibited high content levels in the four tubers. These metabolites were divided into nine classes with distinct change patterns. A total of 22,865 differentially expressed genes (DEGs) were identified by transcriptome analysis. Among these DEGs, we identified 67 candidate genes related to the flavonoid biosynthesis pathway and three genes that played pivotal roles in proanthocyanin (PA) synthesis. A weighted gene co-expression network analysis (WGCNA) revealed that the two modules, "MEblue" and "MEblack," were two key gene sets strongly associated with phenylpropanoid and flavonoid biosynthesis. We also found that the plant hormone signal transduction biological process exhibited activity in the late stage of tuber color formation. Additionally, we identified 37 hub transcript factors related to flavonoid biosynthesis, of which 24 were found to be highly associated with flavonoid pathway genes. In addition to the MYB-bHLH-WD40 (MBW) genes, we found that the plant hormone gene families exhibited high expression levels. This study provides a reference for understanding the synthesis of D. cirrhosa tuber metabolites at the molecular level and provides a foundation for the further development of D. cirrhosa related plant pigments as well as its further use in the pharmaceutical industry.

Glucose Supply Induces PsMYB2-Mediated Anthocyanin Accumulation in Paeonia suffruticosa 'Tai Yang' Cut Flower

Tree peony (Paeonia suffruticosa) is a well-known Chinese ornamental plant with showy flower color. However, the color fading problem during vase time seriously blocks its development in the cut flower market. In this study, we found that exogenous glucose supply improved the color quality of P. suffruticosa 'Tai Yang' cut flowers with increased total soluble sugar and anthocyanin contents of petals. Besides, the promotion effect of glucose was better than the osmotic control of 3-O-methylglucose (3OMG) treatment and the glucose analog mannose treatment. The structural genes, including PsF3H, PsF3'H, PsDFR, PsAOMT, and PsUF5GT, were remarkably upregulated under glucose treatment. Meanwhile, the regulatory genes, including PsbHLH1, PsbHLH3, PsMYB2, PsWD40-1, and PsWD40-2, also showed a strong response to glucose treatment. Among these five regulatory genes, PsMYB2 showed less response to 3OMG treatment but was highly expressed under glucose and mannose treatments, indicating that PsMYB2 may have an important role in the glucose signal pathway. Ectopic overexpression of PsMYB2 in Nicotiana tabacum resulted in a strong pigmentation in petals and stamens of tobacco flowers accompanied with multiple anthocyanin biosynthetic genes upregulated. More importantly, the overexpression of PsMYB2 enhanced the ability of glucose-induced anthocyanin accumulation in Arabidopsis thaliana seedlings since PsMYB2-overexpressing Arabidopsis showed higher expression levels of AtPAL1, AtCHS, AtF3H, AtF3'H, AtDFR, and AtLDOX than those of wild type under glucose treatment. In summary, we suggested that glucose supply promoted petal coloration of P. suffruticosa 'Tai Yang' cut flower through the signal pathway, and PsMYB2 was a key component in this process. Our research made a further understanding of the mechanism that glucose-induced anthocyanin biosynthesis of P. suffruticosa cut flowers during postharvest development, laying a foundation for color retention technology development of cut flowers.

Integrated Transcriptome and Metabolome Analyses Reveal the Anthocyanin Biosynthesis Pathway in AmRosea1 Overexpression 84K Poplar

Populus alba × Populus glandulosa (84K poplar) is model material with excellent genetic engineering resource and ornamental value. In our study, AmRosea1 (Antirrhinum majus) was overexpressed in 84K poplar, and the transgenic 84K (AM) poplar with high content of anthocyanin exhibited red pigmentation leaves. The transcriptome analysis between wild type (WT) and AM showed that 170 differentially expressed genes (DEGs) (86 up-regulated and 84 down-regulated) were found, and some DEGs were involved in flavone and flavonol biosynthesis, flavonoid biosynthesis and anthocyanin biosynthesis. The metabolome analysis showed that 13 anthocyanins-related differentially accumulated metabolites (DAMs) were detected in AM. The correlation analysis between DEGs and DAMs were performed, and the results revealed that 18 DEGs, including 11 MYB genes, two BZ1 genes, one FG2 gene, one ANS gene, and three IF7MAT genes, were negatively or positively correlated with 13 DAMs. The phylogenetic analysis demonstrated that there was high homology between AmRosea1 and PagMYB113, and MYB113 co-expressed with BZ1, ANS and DFR directly. Our results elucidated the molecular mechanism of plant color change mediated by anthocyanin biosynthesis pathway, which laid the foundation for the development and utilization of colorful woody plant.

Evolution and functional diversification of R2R3-MYB transcription factors in plants

R2R3-MYB genes (R2R3-MYBs) form one of the largest transcription factor gene families in the plant kingdom, with substantial structural and functional diversity. However, the evolutionary processes leading to this amazing functional diversity have not yet been clearly established. Recently developed genomic and classical molecular technologies have provided detailed insights into the evolutionary relationships and functions of plant R2R3-MYBs. Here, we review recent genome-level and functional analyses of plant R2R3-MYBs, with an emphasis on their evolution and functional diversification. In land plants, this gene family underwent a large expansion by whole genome duplications and small-scale duplications. Along with this population explosion, a series of functionally conserved or lineage-specific subfamilies/groups arose with roles in three major plant-specific biological processes: development and cell differentiation, specialized metabolism, and biotic and abiotic stresses. The rapid expansion and functional diversification of plant R2R3-MYBs are highly consistent with the increasing complexity of angiosperms. In particular, recently derived R2R3-MYBs with three highly homologous intron patterns (a, b, and c) are disproportionately related to specialized metabolism and have become the predominant subfamilies in land plant genomes. The evolution of plant R2R3-MYBs is an active area of research, and further studies are expected to improve our understanding of the evolution and functional diversification of this gene family.

An Upgraded, Highly Saturated Linkage Map of Japanese Plum (Prunus salicina Lindl.), and Identification of a New Major Locus Controlling the Flavan-3-ol Composition in Fruits

Japanese plum fruits are rich in phenolic compounds, such as anthocyanins and flavan-3-ols, whose contents vary significantly among cultivars. Catechin (C) and epicatechin (EC) are flavan-3-ol monomers described in the fruits of this species and are associated with bitterness, astringency, antioxidant capacity, and susceptibility to enzymatic mesocarp browning. In this study, we aimed to identify quantitative trait loci (QTL) associated with the content of flavan-3-ol in Japanese plum fruits. We evaluated the content of C and EC in the mesocarp and exocarp of samples from 79 and 64 seedlings of an F1 progeny (<'98-99' × 'Angeleno'>) in the first and second seasons, respectively. We also constructed improved versions of linkage maps from '98-99' and 'Angeleno,' presently called single-nucleotide polymorphisms (SNPs) after mapping the already available GBS reads to Prunus salicina Lindl. cv. 'Sanyueli' v2.0 reference genome. These data allowed for describing a cluster of QTLs in the cultivar, 'Angeleno,' associated with the flavan-3-ol composition of mesocarp and exocarp, which explain up to 100% of the C/EC ratio. Additionally, we developed a C/EC metabolic marker, which was mapped between the markers with the highest log of odds (LOD) scores detected by the QTL analysis. The C/EC locus was located in the LG1, at an interval spanning 0.70 cM at 108.30-108.90 cM. Our results suggest the presence of a novel major gene controlling the preferential synthesis of C or EC in the Japanese plum fruits. This study is a significant advance in understanding the regulation of synthesizing compounds associated with fruit quality, postharvest, and human health promotion.

Comparative Transcriptome Analysis Identifies Key Regulatory Genes Involved in Anthocyanin Metabolism During Flower Development in Lycoris radiata

Lycoris is used as a garden flower due to the colorful and its special flowers. Floral coloration of Lycoris is a vital trait that is mainly regulated via the anthocyanin biosynthetic pathway. In this study, we performed a comparative transcriptome analysis of Lycoris radiata petals at four different flower development stages. A total of 38,798 differentially expressed genes (DEGs) were identified by RNA sequencing, and the correlation between the expression level of the DEGs and the anthocyanin content was explored. The identified DEGs are significantly categorized into 'flavonoid biosynthesis,' 'phenylpropanoid biosynthesis,' 'Tropane, piperidine and pyridine alkaloid biosynthesis,' 'terpenoid backbone biosynthesis' and 'plant hormone signal transduction' by Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. The candidate genes involved in anthocyanin accumulation in L. radiata petals during flower development stages were also identified, which included 56 structural genes (especially LrDFR1 and LrFLS) as well as 27 key transcription factor DEGs (such as C3H, GATA, MYB, and NAC). In addition, a key structural gene namely LrDFR1 of anthocyanin biosynthesis pathway was identified as a hub gene in anthocyanin metabolism network. During flower development stages, the expression level of LrDFR1 was positively correlated with the anthocyanin content. Subcellular localization revealed that LrDFR1 is majorly localized in the nucleus, cytoplasm and cell membrane. Overexpression of LrDFR1 increased the anthocyanin accumulation in tobacco leaves and Lycoris petals, suggesting that LrDFR1 acts as a positively regulator of anthocyanin biosynthesis. Our results provide new insights for elucidating the function of anthocyanins in L. radiata petal coloring during flower development.

Composition of Flavonoids in the Petals of Freesia and Prediction of Four Novel Transcription Factors Involving in Freesia Flavonoid Pathway

Freesia hybrida is rich in flower colors with beautiful flower shapes and pleasant aroma. Flavonoids are vital to the color formation of its flowers. In this study, five Freesia cultivars with different flower colors were used to study on the level of accumulation of their flavonoids and expression of flavonoid-related genes and further explore new novel transcription factor (TF). Ultra-high-performance liquid chromatography and VION ion mobility quadrupole time-of-flight mass spectrometer (UPLC-Q-TOF-MS) were used to determine the flavonoids. Combined with transcriptome sequencing technology, the molecular mechanism of the flavonoid metabolism difference in Freesia was revealed. A total of 10 anthoxanthin components and 12 anthocyanin components were detected using UPLC-Q-TOF-MS. All six common anthocyanin aglycones in high plants, including cyanidin, delphinidin, petunidin, peonidin, malvidin, and pelargonidin, were detected in Freesia at first time in this study. In orange, yellow, and white cultivars, anthoxanthins gradually decreased with the opening of the petals, while in red and purple cultivars, anthoxanthins first increased and then decreased. No anthocyanin was detected in yellow and white cultivars, while anthocyanins increased with the opening of the petals and reached their maximum at the flowering stage (S3) in other three cultivars. The correlation analysis revealed that the color of Freesia petals was closely related to the composition and content of anthoxanthins and anthocyanins. Petals of five cultivars at S3 were then selected for transcriptome sequencing by using the Illumina Hiseq 4000 platform, and a total of 100,539 unigenes were obtained. There were totally 5,162 differentially expressed genes (DEGs) when the four colored cultivars were compared with the white cultivar at S3. Comparing all DEGs with gene ontology (GO), KEGG, and Pfam databases, it was found that the genes involved in the flavonoid biosynthesis pathway were significantly different. In addition, AP2, WRKY, and bHLH TF families ranked the top three among all differently expressed TFs in all DEGs. Quantitative real-time PCR (qRT-PCR) technology was used to analyze the expression patterns of the structural genes of flavonoid biosynthesis pathway in Freesia. The results showed that metabolic process was affected significantly by structural genes in this pathway, such as CHS1, CHI2, DFR1, ANS1, 3GT1, and FLS1. Cluster analysis was performed by using all annotated WRKY and AP2 TFs and the above structural genes based on their relatively expression. Four novel candidate TFs of WRKY and AP2 family were screened. Their spatiotemporal expression patterns revealed that these four novel TFs may participate in the regulation of the flavonoid biosynthesis, thus controlling its color formation in Freesia petals.

MYBA and MYBPA transcription factors co-regulate anthocyanin biosynthesis in blue-coloured berries

The regulatory network of R2R3 MYB transcription factors in anthocyanin biosynthesis is not fully understood in blue-coloured berries containing delphinidin compounds. We used blue berries of bilberry (Vaccinium myrtillus) to comprehensively characterise flavonoid-regulating R2R3 MYBs, which revealed a new type of co-regulation in anthocyanin biosynthesis between members of MYBA-, MYBPA1- and MYBPA2-subgroups. VmMYBA1, VmMYBPA1.1 and VmMYBPA2.2 expression was elevated at berry ripening and by abscisic acid treatment. Additionally, VmMYBA1 and VmMYBPA1.1 expression was strongly downregulated in a white berry mutant. Complementation and transient overexpression assays confirmed VmMYBA1 and VmMYBA2 to induce anthocyanin accumulation. Promoter activation assays showed that VmMYBA1, VmMYBPA1.1 and VmMYBPA2.2 had similar activity towards dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS), but differential regulation activity for UDP-glucose flavonoid 3-O-glucosyltransferase (UFGT) and flavonoid 3'5'-hydroxylase (F3'5'H) promoters. Silencing of VmMYBPA1.1 in berries led to the downregulation of key anthocyanin and delphinidin biosynthesis genes. Functional analyses of other MYBPA regulators, and a member of novel MYBPA3 subgroup, associated them with proanthocyanidin biosynthesis and F3'5'H expression. The existence of 18 flavonoid-regulating MYBs indicated gene duplication, which may have enabled functional diversification among MYBA, MYBPA1 and MYBPA2 subgroups. Our results provide new insights into the intricate regulation of the complex anthocyanin profile found in blue-coloured berries involving regulation of both cyanidin and delphinidin branches.

Recent Advanced Metabolic and Genetic Engineering of Phenylpropanoid Biosynthetic Pathways

The MYB transcription factors (TFs) are evolving as critical role in the regulation of the phenylpropanoid and tanshinones biosynthetic pathway. MYB TFs relate to a very important gene family, which are involved in the regulation of primary and secondary metabolisms, terpenoids, bioactive compounds, plant defense against various stresses and cell morphology. R2R3 MYB TFs contained a conserved N-terminal domain, but the domain at C-terminal sorts them different regarding their structures and functions. MYB TFs suppressors generally possess particular repressive motifs, such as pdLNLD/ELxiG/S and TLLLFR, which contribute to their suppression role through a diversity of complex regulatory mechanisms. A novel flower specific "NF/YWSV/MEDF/LW" conserved motif has a great potential to understand the mechanisms of flower development. In the current review, we summarize recent advanced progress of MYB TFs on transcription regulation, posttranscriptional, microRNA, conserved motif and propose directions to future prospective research. We further suggest there should be more focus on the investigation for the role of MYB TFs in microalgae, which has great potential for heterologous protein expression system for future perspectives.

Color variation in young and senescent leaves of Formosan sweet gum (Liquidambar formosana) by the gene regulation of anthocyanidin biosynthesis

In certain plants, leaf coloration occurs in young and senescent leaves; however, it is unclear whether these two developmental stages are controlled by the same regulatory mechanisms. Formosan sweet gum (Liquidambar formosana Hance) is a subtropical deciduous tree species that possesses attractive autumnal leaf coloration. The color of young leaves is closer to purplish red, while senescent leaves are more orange-red to dark red. It was confirmed that delphinidin and cyanidin are the two anthocyanidins that contribute to the color of Formosan sweet gum leaves, and the content of different anthocyanins influences the appearance of color. To elucidate the regulation of anthocyanidin biosynthesis, recombinant DIHYDROFLAVONOL-4-REDUCTASEs (LfDFR1 and LfDFR2) (EC 1.1.1.234) were produced, and their substrate acceptability was investigated both in vitro and in planta. The functions of flavanones and dihydroflavonols modification by FLAVONOID 3' HYDROXYLASE (LfF3'H1) (EC 1.14.14.82) and FLAVONOID 3'5' HYDROXYLASE (LfF3'5'H) (EC 1.14.14.81) were verified using a transient overexpression experiment in Nicotiana benthamiana. The results showed that LfMYB5 induced LfF3'5'H and LfMYB123 induced both LfF3'H1 and LfDFR1 in spring when the leaves were expanding, whereas LfMYB113 induced LfF3'H1, LfDFR1, and LfDFR2 in late autumn to winter when the leaves were undergoing leaf senescence. In conclusion, the color variation of Formosan sweet gum in young and senescent leaves was attributed to the composition of anthocyanidins through the transcriptional regulation of LfF3'H1 and LfF3'5'H by LfMYB5, LfMYB113, and LfMYB123.

MYB transcription factors GmMYBA2 and GmMYBR function in a feedback loop to control pigmentation of seed coat in soybean

Soybean has undergone extensive selection pressures for seed nutrient composition and seed color during domestication, but the major genetic loci controlling seed coat color have not been completely understood, and the transcriptional regulation relationship among the loci remains elusive. Here, two major regulators, GmMYBA2 and GmMYBR, were functionally characterized as an anthocyanin activator and repressor, respectively. Ectopic expression of GmMYBA2 in soybean hairy roots conferred the enhanced accumulation of delphinidin and cyanidin types of anthocyanins in W1t and w1T backgrounds, respectively, through activating anthocyanin biosynthetic genes in the reported loci. The seed coat pigmentation of GmMYBA2-overexpressing transgenic plants in the W1 background mimicked the imperfect black phenotype (W1/w1, i, R, t), suggesting that GmMYBA2 was responsible for the R locus. Molecular and biochemical analysis showed that GmMYBA2 interacted with GmTT8a to directly activate anthocyanin biosynthetic genes. GmMYBA2 and GmMYBR might form a feedback loop to fine-tune seed coat coloration, which was confirmed in transgenic soybeans. Both GmTT8a and GmMYBR that were activated by GmMYBA2 in turn enhanced and obstructed the formation of the GmMYBA2-GmTT8a module, respectively. The results revealed the sophisticated regulatory network underlying the soybean seed coat pigmentation loci and shed light on the understanding of the seed coat coloration and other seed inclusions.

MYB-Mediated Regulation of Anthocyanin Biosynthesis

Anthocyanins are natural water-soluble pigments that are important in plants because they endow a variety of colors to vegetative tissues and reproductive plant organs, mainly ranging from red to purple and blue. The colors regulated by anthocyanins give plants different visual effects through different biosynthetic pathways that provide pigmentation for flowers, fruits and seeds to attract pollinators and seed dispersers. The biosynthesis of anthocyanins is genetically determined by structural and regulatory genes. MYB (v-myb avian myeloblastosis viral oncogene homolog) proteins are important transcriptional regulators that play important roles in the regulation of plant secondary metabolism. MYB transcription factors (TFs) occupy a dominant position in the regulatory network of anthocyanin biosynthesis. The TF conserved binding motifs can be combined with other TFs to regulate the enrichment and sedimentation of anthocyanins. In this study, the regulation of anthocyanin biosynthetic mechanisms of MYB-TFs are discussed. The role of the environment in the control of the anthocyanin biosynthesis network is summarized, the complex formation of anthocyanins and the mechanism of environment-induced anthocyanin synthesis are analyzed. Some prospects for MYB-TF to modulate the comprehensive regulation of anthocyanins are put forward, to provide a more relevant basis for further research in this field, and to guide the directed genetic modification of anthocyanins for the improvement of crops for food quality, nutrition and human health.

The MIR-Domain of PbbHLH2 Is Involved in Regulation of the Anthocyanin Biosynthetic Pathway in "Red Zaosu" (PyrusBretschneideri Rehd.) Pear Fruit

The N-terminal of Myc-like basic helix-loop-helix transcription factors (bHLH TFs) contains an interaction domain, namely the MYB-interacting region (MIR), which interacts with the R2R3-MYB proteins to regulate genes involved in the anthocyanin biosynthetic pathway. However, the functions of MIR-domain bHLHs in this pathway are not fully understood. In this study, PbbHLH2 containing the MIR-domain was identified and its function investigated. The overexpression of PbbHLH2 in ”Zaosu” pear peel increased the anthocyanin content and the expression levels of late biosynthetic genes. Bimolecular fluorescence complementation showed that PbbHLH2 interacted with R2R3-MYB TFs PbMYB9, 10, and 10b in onion epidermal cells and confirmed that MIR-domain plays important roles in the interaction between the MIR-domain bHLH and R2R3-MYB TFs. Moreover, PbbHLH2 bound and activated the dihydroflavonol reductase promoter in yeast one-hybrid (Y1H) and dual-luciferase assays. Taken together these results suggested that the MIR domain of PbbHLH2 regulated anthocyanin biosynthesis in pear fruit peel.

R2R3-MYB Transcription Factor NtMYB330 Regulates Proanthocyanidin Biosynthesis and Seed Germination in Tobacco (Nicotiana tabacum L.)

Proanthocyanidins (PAs) are important phenolic compounds and PA biosynthesis is regulated by a ternary MBW complex consisting of a R2R3-MYB regulator, a bHLH factor and a WDR protein. In this study, a tobacco R2R3-MYB factor NtMYB330 was characterized as the PA-specific regulator in which the PA biosynthesis was promoted in the flowers of NtMYB330-overexpressing lines while decreased in the flowers of ntmyb330 mutants. NtMYB330 can interact with flavonoid-related bHLH partner NtAn1b and WDR protein NtAn11-1, and the NtMYB330-NtAn1b complex is required to achieve strong transcriptional activation of the PA-related structural genes NtDFR1, NtANS1, NtLAR1 and NtANR1. Our data reveal that NtMYB330 regulates PA biosynthesis in seeds and affects seed germination, in which NtMYB330-overexpressing lines showed higher PA accumulations in seed coats and inhibited germination, while ntmyb330 mutants had reduced seed coat PAs and improved germination. NtMYB330 affects seed germination possibly through two mechanisms: modulating seed coat PAs to affect coat-imposed dormancy. In addition, NtMYB330 regulates the expressions of abscisic acid (ABA) and gibberellin acid (GA) signaling-related genes, affecting ABA-GA crosstalk and seed germination. This study reveals that NtMYB330 specifically regulates PA biosynthesis via formation of the MBW complex in tobacco flowers and affects germination through adjustment of PA concentrations and ABA/GA signaling in tobacco seeds.

Auxin-Responsive R2R3-MYB Transcription Factors HcMYB1 and HcMYB2 Activate Volatile Biosynthesis in Hedychium coronarium Flowers

Auxin, an important plant hormone, induces the biosynthesis of various secondary metabolites by modulating the expression of auxin-responsive genes. In the ornamental plant Hedychium coronarium, linalool and methyl benzoate are biosynthesized by the terpene synthase (TPS) HcTPS5 and the benzoic/salicylic acid methyltransferase (BSMT) HcBSMT2, respectively. However, the transcriptional regulation of this process remains unclear. Here, we identified and functionally characterized the R2R3-MYB transcription factors HcMYB1 and HcMYB2 in regulating the biosynthesis of these floral aroma compounds. HcMYB1 and HcMYB2 are specifically expressed in flowers, their expression is correlated with the emission of volatile compounds in flowers, and is induced by auxin. Moreover, HcMYB1 and HcMYB2 interact with the HcBSMT2 promoter region. HcMYB2 activates the expression of the linalool synthase gene HcTPS5. In flowers with HcMYB1 or HcMYB2 silenced, the levels of floral scent compounds were significantly reduced, and HcBSMT2 and HcTPS5 were downregulated compared with the wild type. Moreover, HcMYB1 form protein-protein interaction with key scent-related HcIAA4 protein to regulate floral aroma production. Taken together, these results indicate that HcMYB1 and HcMYB2 play crucial roles in regulating the formation of scent compounds in Hedychium coronarium (H. coronarium) flowers in response to auxin signaling.

Identification of MBW Complex Components Implicated in the Biosynthesis of Flavonoids in Woodland Strawberry

Flavonoids belong to the family of polyphenolic secondary metabolites and contribute to fruit quality traits. It has been shown that MBW complexes (MYB-bHLH-WD40) regulate the flavonoids biosynthesis in different plants, but only a limited number of MBW complexes have been identified in strawberry species in general. In this study, we identified 112 R2R3-MYB proteins in woodland strawberry; 12 of them were found to have potential functions in regulating flavonoids biosynthesis by phylogenetic analysis. qRT-PCR assays showed that FvMYB3, FvMYB9, FvMYB11, FvMYB22, FvMYB64, and FvMYB105 mostly expressed at green stage of fruit development, aligned with proanthocyanidins accumulation; FvMYB10 and FvMYB41 showed higher expression levels at turning and ripe stages, aligned with anthocyanins accumulation. These results suggest that different MYBs might be involved in flavonoids biosynthesis at specific stages. Furthermore, FvMYB proteins were demonstrated to interact with FvbHLH proteins and induce expression from the promoters of CHS2 and DFR2 genes, which encode key enzymes in flavonoids biosynthesis. The co-expression of FvMYB and FvbHLH proteins in strawberry fruits also promoted the accumulation of proanthocyanidins. These findings confirmed and provided insights into the biofunction of MBW components in the regulation of flavonoid biosynthesis in woodland strawberry.

Tomato BZR/BES transcription factor SlBZR1 positively regulates BR signaling and salt stress tolerance in tomato and Arabidopsis

Brassinosteroids (BRs) play critical roles in plant growth and development, as well as in responses to abiotic stresses. The BRASSINAZOLE RESISTANT 1 (BZR1) and BRI1-EMS-SUPPRESSOR 1 (BES1) families of transcription factors have been elucidated largely in Arabidopsis and rice but not in other plant species. Here, we studied the functional characterization of a tomato (Solanum lycopersicum) BZR homolog gene, SlBZR1, in BR-regulated plant growth and tolerance to salt stress. SlBZR1 was highly expressed in the flowers and developing fruits of tomato. Both SlBZR1 and SlBZR1D (proline to leucine mutation at the 239th amino acid of SlBZR1) were transcriptional repressors and localized in the nucleus. SlBZR1 or SlBZR1D could interact with SlMYB30, SlMYBL2, SlPIF4, SlHAT1, SlIWS1 and SlREF6 in tomato. Overexpression of SlBZR1D enhanced the BR response and improved tolerance to salt stress in Arabidopsis, consistent with the phenotype of the Arabidopsis bes1-D mutant. Moreover, SlBZR1D-overexpressing tomato lines showed a short plant height, smaller and curly leaves, and delayed flowering. Additionally, SlBZR1D positively regulated salt tolerance in tomato and upregulated the expression of multiple stress-related genes. Our study provides new insights for understanding the function and mechanism of BZR transcription factors in BR-regulated plant growth and abiotic stress responses.

The spatio-temporal biosynthesis of floral flavonols is controlled by differential phylogenetic MYB regulators in Freesia hybrida

Floral flavonols play specific pivotal roles in pollinator attraction, pollen germination and fertility, in addition to other functions in vegetative organs. For many plants, the process of flavonol biosynthesis in late flower development stages and in mature flower tissues is poorly understood, in contrast to early flower development stages. It is thought that this process may be regulated independently of subgroup 7 R2R3 MYB (SG7 MYB) transcription factors. In this study, two FLS genes were shown to be expressed synchronously with the flower development-specific and tissue-specific biosynthesis of flavonols in Freesia hybrida. FhFLS1 contributed to flavonol biosynthesis in early flower buds, toruses and calyxes, and was regulated by four well-known SG7 MYB proteins, designated as FhMYBFs, with at least partial regulatory redundancy. FhFLS2 accounted for flavonols in late developed flowers and in the petals, stamens and pistils, and was targeted directly by non SG7 MYB protein FhMYB21L2. In parallel, AtMYB21 and AtMYB24 also activated AtFLS1, a gene highly expressed in Arabidopsis anthers and pollen, indicating the conserved regulatory roles of MYB21 against FLS genes in these two evolutionarily divergent angiosperm plants. Our results reveal a novel regulatory and synthetic mechanism underlying flavonol biosynthesis in floral organs and tissues which may be exploited to investigate supplementary roles of flavonols in flowers.

Cloning and functional characterization of a carotenoid cleavage dioxygenase 2 gene in safranal and crocin biosynthesis from Freesia hybrida

Safranal and crocin, commonly derived from the oxidative cleavage reaction of zeaxanthin in plants, are two kinds of apocarotenoids with versatile functions, which were only found in limited number of plant species. In this study, both metabolites were detected and varied concomitantly with the expression of carotenoid cleavage dioxygenase (CCD) genes in Freesia hybrida, Red River® and Ambiance cultivars. The newly isolated CCD, denoted here as FhCCD2, was phylogenetically clustered with other reported saffron CCD2s. Besides, ten introns were also observed in the genomic DNA sequence of FhCCD2 and the presence of N-terminal transporter peptide suggested its plastidial sub-localization. Biochemical analysis showed that the FhCCD2 cleaved zeaxanthin at the 7, 8 and 7', 8' double bonds to generate intermediates prerequisite for the biosynthesis of safranal and crocin. Further, gene transient expression analysis showed that the promoter of FhCCD2 was functional in Ambiance as well as Red River® cultivars, even with slight variation in their promoter sequence. At present, CCD2 proteins have only been found in Freesia and Crocus genus of Iridaceae family. Phylogenetic and intron position analysis infer that CCD2 perhaps emerged after the intron loss during evolutionary process of CCD1 or their shared ancestry.

The proanthocyanin-related transcription factors MYBC1 and WRKY44 regulate branch points in the kiwifruit anthocyanin pathway

The groups of plant flavonoid metabolites termed anthocyanins and proanthocyanins (PA) are responsible for pigmentation in seeds, flowers and fruits. Anthocyanins and PAs are produced by a pathway of enzymes which are transcriptionally regulated by transcription factors (TFs) that form the MYB-bHLH-WD40 (MBW) complex. In this study, transcriptomic analysis of purple-pigmented kiwifruit skin and flesh tissues identified MYBC1, from subgroup 5 of the R2R3 MYB family, and WRKY44 (highly similar to Arabidopsis TTG2) as candidate activators of the anthocyanin pathway. Transient over-expression of MYBC1 and WRKY44 induced anthocyanin accumulation in tobacco leaves. Dual luciferase promoter activation assays revealed that both MYBC1 and WRKY44 were able to strongly activate the promoters of the kiwifruit F3'H and F3'5'H genes. These enzymes are branch points of the pathway which specifies the type of anthocyanin accumulated. Stable over-expression of MYBC1 and WRKY44 in kiwifruit calli activated the expression of F3'5'H and PA-related biosynthetic genes as well as increasing levels of PAs. These results suggest that while previously characterised anthocyanin activator MYBs regulate the overall anthocyanin biosynthesis pathway, the PA-related TFs, MYBC1 and WRKY44, more specifically regulate key branch points. This adds a layer of regulatory control that potentially balances anthocyanin and PA levels.

MYB repressors and MBW activation complex collaborate to fine-tune flower coloration in Freesia hybrida

Floral anthocyanin has multiple ecological and economic values, its biosynthesis largely depends on the conserved MYB-bHLH-WD40 (MBW) activation complex and MYB repressors hierarchically with the MBW complex. In contrast to eudicots, the MBW regulatory network model has not been addressed in monocots because of the lack of a suitable system, as grass plants exhibit monotonous floral pigmentation patterns. Presently, the MBW regulatory network was investigated in a non-grass monocot plant, Freesia hybrida. FhMYB27 and FhMYBx with different functional manners were confirmed to be anthocyanin related R2R3 and R3 MYB repressors, respectively. Particularly, FhMYBx could obstruct the formation of positive MBW complex by titrating bHLH proteins, whereas FhMYB27 mainly defected the activator complex into suppressor via its repression domains in C-terminus. Furthermore, the hierarchical and feedback regulatory loop was verified, indicating the synergistic and sophisticated regulatory network underlying Freesia anthocyanin biosynthesis was quite similar to that reported in eudicot plants.

Yueqing Li, Xiaotong Shan, et al. study the MYB-bHLH-WD40 (MBW) regulatory network in a non-grass monocot plant, Freesia hybrida. They report two anthocyanin related MYB repressors FhMYB27 and FhMYBx and verified their involvement in a functional feedback loop with MBW to regulate anthocyanin biosynthesis.

MYB21 interacts with MYC2 to control the expression of terpene synthase genes in flowers of Freesia hybrida and Arabidopsis thaliana

Previously, linalool was found to be the most abundant component among the cocktail of volatiles released from flowers of Freesia hybrida. Linalool formation is catalysed by monoterpene synthase TPS1. However, the regulatory network developmentally modulating the expression of the TPS1 gene in Freesia hybrida remains unexplored. In this study, three regulatory genes, FhMYB21L1, FhMYB21L2, and FhMYC2, were screened from 52 candidates. Two MYB transcription factor genes were synchronously expressed with FhTPS1 and could activate its expression significantly when overexpressed, and the binding of FhMYB21L2 to the MYBCORE sites in the FhTPS1 promoter was further confirmed, indicating a direct role in activation. FhMYC2 showed an inverse expression pattern compared with FhTPS1; its expression led to a decreased binding of FhMYB21 to the FhTPS1 promoter to reduce its activation capacity when co-expressed, suggesting a role for an MYB-bHLH complex in the regulation of the FhTPS1 gene. In Arabidopsis, both MYB21 and MYC2 regulators were shown to activate the expression of sesquiterpene synthase genes, and the regulatory roles of AtMYB21 and AtMYC2 in the expression of the linalool synthase gene were also confirmed, implying conserved functions of the MYB-bHLH complex in these two evolutionarily divergent plants. Moreover, the expression ratio between MYB21 and MYC2 orthologues might be a determinant factor in floral linalool emission.

The Conserved and Particular Roles of the R2R3-MYB Regulator FhPAP1 from Freesia hybrida in Flower Anthocyanin Biosynthesis

Anthocyanin biosynthesis is mainly controlled by MYB-bHLH-WD40 (MBW) complexes that modulate the expression of anthocyanin biosynthetic genes (ABGs). The MYB regulators involved in anthocyanin biosynthesis arose early during plant evolution and thus might function divergently in different evolutionary lineages. Although the anthocyanin-promoting R2R3-MYB regulators in eudicots have been comprehensively explored, little consensus has been reached about functional discrepancies versus conservation among MYB regulators from different plant lineages. Here, we integrated transcriptome analysis, gene expression profiles, gain-of-function experiments and transient protoplast transfection assays to functionally characterize the monocot Freesia hybrida anthocyanin MYB regulator gene FhPAP1, which showed correlations with late ABGs. FhPAP1 could activate ABGs as well as TT8-clade genes FhTT8L, AtTT8 and NtAN1 when overexpressed in Freesia, Arabidopsis and tobacco, respectively. Consistently, FhPAP1 could interact with FhTT8L and FhTTG1 to form the conserved MBW complex and shared similar target genes with its orthologs from Arabidopsis. Most prominently, FhPAP1 displayed higher transactivation capacity than its homologs in Arabidopsis and tobacco, which was instantiated in its powerful regulation on ABGs. Moreover, we found that FhPAP1 might be the selected gene during the domestication and rapid evolution of the wild Freesia species to generate intensive flower pigmentation. These results showed that while the MBW complex was highly evolutionarily conserved between tested monocot and core eudicot plants, participating MYB regulators showed functional differences in transactivation capacity according to their activation domain and played important roles in the flower coloration domestication and evolution of angiosperms.

Transcription Factor VviMYB86 Oppositely Regulates Proanthocyanidin and Anthocyanin Biosynthesis in Grape Berries

Proanthocyanidins (PAs) and anthocyanins are two vital groups of flavonoid compounds for grape berries and red wines. Several transcription factors (TFs) have been identified to be involved in regulating PA and anthocyanin biosynthesis in grape berries. However, research on TFs with different regulatory mechanisms for these two biosynthesis branches in grapes remains limited. In this study, we identified an R2R3-MYB TF, VviMYB86, whose spatiotemporal gene expression pattern in grape berries coincided well with PA accumulation but contrasted with anthocyanin synthesis. Both in vivo and in vitro experiments verified that VviMYB86 positively regulated PA biosynthesis, primarily by upregulating the expression of the two leucoanthocyanidin reductase (LAR) genes in the Arabidopsis protoplast system, as well as in VviMYB86-overexpressing grape callus cultured under 24 h of darkness. Moreover, VviMYB86 was observed to repress the anthocyanin biosynthesis branch in grapes by downregulating the transcript levels of VviANS and VviUFGT. Overall, VviMYB86 is indicated to have a broad effect on flavonoid synthesis in grape berries. The results of this study will help elucidate the regulatory mechanism governing the expression of the two LAR genes in grape berries and provide new insights into the regulation of PA and anthocyanin biosynthesis in grape berries.

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.

Functional Differentiation of Duplicated Flavonoid 3-O-Glycosyltransferases in the Flavonol and Anthocyanin Biosynthesis of Freesia hybrida

Flavonols and anthocyanins are two widely distributed groups of flavonoids that occurred apart during plant evolution and biosynthesized by shared specific enzymes involved in flavonoid metabolism. UDP-glucose, flavonoid 3-O-glycosyltransferase (UF3GT), is one of the common enzymes which could catalyze the glycosylation of both flavonol and anthocyanidin aglycons simultaneously in vitro. However, whether and how UF3GT paralogous genes function diversely at the biochemical and transcriptional levels are largely unknown. Recently, Fh3GT1 was identified to be a member of UF3GTs in Freesia hybrida. However, its expression patterns and enzymatic characteristics could not coincide well with flavonol accumulation. In an attempt to characterize other flavonoids, especially flavonol glycosyltransferase genes in Freesia, three closest candidate UFGT genes-Fh3GT2, Fh3GT3, and Fh3GT4-were mined from the Freesia transcriptomic database and isolated from the flowers of the widely distributed Freesia cultivar, Red River^®. Based on bioinformatic analysis and enzymatic assays, Fh3GT2 turned out to be another bona fide glycosyltransferase gene. Biochemical analysis further proved that Fh3GT2 preferentially glucosylated kaempferol while Fh3GT1 controlled the glucosylation of quercetin and anthocyanidins. In addition, transfection assays demonstrated that Fh3GT2 could be mainly activated by the flavonol regulator FhMYBF1 or the anthocyanin regulator FhPAP1, whereas Fh3GT1 could only be activated by FhPAP1. These findings suggested that Fh3GTs might have functionally diverged in flavonoid biosynthesis at both the biochemical and transcriptional levels.