The Star-type Color Pattern in Petunia hybrida ‘Red Star’ Flowers is Induced by Sequence-Specific Degradation of Chalcone Synthase RNA

Chalcone-Synthase-Encoding RdCHS1 Is Involved in Flavonoid Biosynthesis in Rhododendron delavayi

Flower color is an important ornamental feature that is often modulated by the contents of flavonoids. Chalcone synthase is the first key enzyme in the biosynthesis of flavonoids, but little is known about the role of R. delavayi CHS in flavonoid biosynthesis. In this paper, three CHS genes (RdCHS1-3) were successfully cloned from R. delavayi flowers. According to multiple sequence alignment and a phylogenetic analysis, only RdCHS1 contained all the highly conserved and important residues, which was classified into the cluster of bona fide CHSs. RdCHS1 was then subjected to further functional analysis. Real-time PCR analysis revealed that the transcripts of RdCHS1 were the highest in the leaves and lowest in the roots; this did not match the anthocyanin accumulation patterns during flower development. Biochemical characterization displayed that RdCHS1 could catalyze p-coumaroyl-CoA and malonyl-CoA molecules to produce naringenin chalcone. The physiological function of RdCHS1 was checked in Arabidopsis mutants and tobacco, and the results showed that RdCHS1 transgenes could recover the color phenotypes of the tt4 mutant and caused the tobacco flower color to change from pink to dark pink through modulating the expressions of endogenous structural and regulatory genes in the tobacco. All these results demonstrate that RdCHS1 fulfills the function of a bona fide CHS and contributes to flavonoid biosynthesis in R. delavayi.

Pigmentation of soybean seed coats via a mutation that abolishes production of multiple-phased siRNAs of chalcone synthase genes

Lack of pigmentation in seed coats of soybean is caused by natural RNA silencing of chalcone synthase (CHS) genes. This phenomenon is an evolutionary consequence of structural changes in DNA that resulted in the production of double-stranded RNAs (dsRNAs) that trigger RNA degradation. Here we determined that a mutant with pigmented seed coats derived from a cultivar that lacked the pigmentation had a deletion between DNA regions ICHS1 and a cytochrome P450 gene; the deletion included GmIRCHS, a candidate gene that triggers CHS RNA silencing via production of CHS dsRNAs. We also characterized CHS short interfering RNAs (siRNAs) produced in the wild-type seed coats that had CHS RNA silencing. Phased 21-nt CHS siRNAs were detected in all 21 phases and were widely distributed in exon 2 of CHS7, which indicates commonality in the pattern of RNA degradation in natural CHS RNA silencing between distantly related species. These results with the similarities in the rearrangements found in spontaneous mutants suggest that the structural organization that generates dsRNAs that trigger phased siRNA production is vulnerable to further structural changes, which eventually abolish the induction of RNA silencing.

Metabolome and Transcriptome Analyses Reveal Flower Color Differentiation Mechanisms in Various Sophora japonica L. Petal Types

Sophora japonica L. is an important landscaping and ornamental tree species throughout southern and northern parts of China. The most common color of S. japonica petals is yellow and white. In this study, S. japonica flower color mutants with yellow and white flag petals and light purple-red wing and keel petals were used for transcriptomics and metabolomics analyses. To investigate the underlying mechanisms of flower color variation in S. japonica 'AM' mutant, 36 anthocyanin metabolites were screened in the anthocyanin-targeting metabolome. The results demonstrated that cyanidins such as cyanidin-3-O-glucoside and cyanidin-3-O-rutinoside in the 'AM' mutant were the key metabolites responsible for the red color of the wing and keel petals. Transcriptome sequencing and differentially expressed gene (DEG) analysis identified the key structural genes and transcription factors related to anthocyanin biosynthesis. Among these, F3'5'H, ANS, UFGT79B1, bHLH, and WRKY expression was significantly correlated with the cyanidin-type anthocyanins (key regulatory factors affecting anthocyanin biosynthesis) in the flag, wing, and keel petals in S. japonica at various flower development stages.

New insights into the genetic manipulation of the R2R3-MYB and CHI gene families on anthocyanin pigmentation in Petunia hybrida

Several R2R3-MYB genes control anthocyanin pigmentation in petunia, and ANTHOCYANIN-2 (AN2) is treated as the main player in petal limbs. However, the actual roles of R2R3-MYBs in the coloration of different floral tissues in the so called "darkly-veined" petunias are still not clear. The genetic background and expression of AN2 paralogs from various petunias with different color patterns were identified. All "darkly-veined" genotypes have the identical mutation in the AN2 gene, but express a different functional paralog - ANTHOCYANIN-4 (AN4) - abundantly in flowers. Constitutive overexpression of PhAN4 in this petunia resulted not only in a fully colored flower but also in a clearly visible pigmentation in the green tissue and roots, which can be rapidly increased by stress conditions. Suppression of AN4 gene resulted in discolored petals and whitish anthers. Interestingly, when a similar white flower phenotype was achieved by knockout of an essential structural gene of anthocyanin biosynthesis - CHALCONE ISOMERASE-A (CHI-A) - the plant responded directly by upregulating of another paralogs - DEEP PURPLE (DPL) and PURPLE HAZE (PHZ). Moreover, we also found that CHI-B can partially substitute for CHI-A in anthers, but not in vegetative tissues. Further, no significant effects on the longevity of white or enhanced colored flowers were observed compared with the wild type. We concluded that endogenous up-regulation of AN4 leads to the restoration of petal color in the "darkly-veined" phenotypes as a result of the breeding process under human selection, and CHI-B is a backup for CHI-A acitvity in some floral tissues.

Mechanisms underlying the formation of complex color patterns on Nigella orientalis (Ranunculaceae) petals

Complex color patterns on petals are widespread in flowering plants, yet the mechanisms underlying their formation remain largely unclear. Here, by conducting detailed morphological, anatomical, biochemical, optical, transcriptomic, and functional studies, we investigated the cellular bases, chromogenic substances, reflectance spectra, developmental processes, and underlying mechanisms of complex color pattern formation on Nigella orientalis petals. We found that the complexity of the N. orientalis petals in color pattern is reflected at multiple levels, with the amount and arrangement of different pigmented cells being the key. We also found that biosynthesis of the chromogenic substances of different colors is sequential, so that one color/pattern is superimposed on another. Expression and functional studies further revealed that a pair of R2R3-MYB genes function cooperatively to specify the formation of the eyebrow-like horizontal stripe and the Mohawk haircut-like splatters. Specifically, while NiorMYB113-1 functions to draw a large splatter region, NiorMYB113-2 functions to suppress the production of anthocyanins from the region where a gap will form, thereby forming the highly specialized pattern. Our results provide a detailed portrait for the spatiotemporal dynamics of the coloration of N. orientalis petals and help better understand the mechanisms underlying complex color pattern formation in plants.

Molecular and Metabolic Insights into Anthocyanin Biosynthesis for Spot Formation on Lilium leichtlinii var. maximowiczii Flower Petals

Plants exhibit remarkable diversity in their petal colors through biosynthesis and the accumulation of various pigments. Lilium, an important cut and potted flower, has many coloring pattern variations, including bicolors and spots. To elucidate the mechanisms regulating spot formation in Lilium leichtlinii var. maximowiczii petals, we used multiple approaches to investigate the changes in petal carotenoids, spot anthocyanins, and gene expression dynamics. This included green petals without spots (D1-Pe and D1-Sp), yellow-green petals with purple spots (D2-Pe and D2-Sp), light-orange petals with dark-purple spots (D3-Pe and D3-Sp), and orange petals with dark-purple spots (D4-Pe and D4-Sp). D3-Pe and D4-Pe contained large amounts of capsanthin and capsorubin and small amounts of zeaxanthin and violaxanthin, which contributed to the orange color. In addition to cyanidin-3-O-glucoside, pelargonidin-3-O-rutinoside, cyanidin-3-O-rutinoside, and peonidin-3-O-rutinoside may also contribute to L. leichtlinii var. maximowiczii's petal spot colors. KEGs involved in flavonoid biosyntheses, such as CHS, DFR, and MYB12, were significantly upregulated in D2-Sp and D3-Sp, compared with D1-Sp, as well as in spots, compared with petals. Upregulated anthocyanin concentrations and biosynthesis-related genes promoted spot formation and color transition. Our results provide global insight into pigment accumulation and the regulatory mechanisms underlying spot formation during flower development in L. leichtlinii var. maximowiczii.

Eco-Evo-Devo of petal pigmentation patterning

Colourful spots, stripes and rings decorate the corolla of most flowering plants and fulfil important biotic and abiotic functions. Spatial differences in the pigmentation of epidermal cells can create these patterns. The last few years have yielded new data that have started to illuminate the mechanisms controlling the function, formation and evolution of petal patterns. These advances have broad impacts beyond the immediate field as pigmentation patterns are wonderful systems to explore multiscale biological problems: from understanding how cells make decisions at the microscale to examining the roots of biodiversity at the macroscale. These new results also reveal there is more to petal patterning than meets the eye, opening up a brand new area of investigation. In this mini-review, we summarise our current knowledge on the Eco-Evo-Devo of petal pigmentation patterns and discuss some of the most exciting yet unanswered questions that represent avenues for future research.

First genetic maps development and QTL mining in Ranunculus asiaticus L. through ddRADseq

Persian Buttercup (Ranunculus asiaticus L.; 2x=2n=16; estimated genome size: 7.6Gb) is an ornamental and perennial crop native of Asia Minor and Mediterranean basin, marketed both as cut flower or potted plant. Currently new varieties are developed by selecting plants carrying desirable traits in segregating progenies obtained by controlled mating, which are propagated through rhizomes or micro-propagated in vitro. In order to escalate selection efficiency and respond to market requests, more knowledge of buttercup genetics would facilitate the identification of markers associated with loci and genes controlling key ornamental traits, opening the way for molecular assisted breeding programs. Reduced-representation sequencing (RRS) represents a powerful tool for plant genotyping, especially in case of large genomes such as the one of buttercup, and have been applied for the development of high-density genetic maps in several species. We report on the development of the first molecular-genetic maps in R. asiaticus based on of a two-way pseudo-testcross strategy. A double digest restriction-site associated DNA (ddRAD) approach was applied for genotyping two F₁ mapping populations, whose female parents were a genotype of a so called 'ponpon' and of a 'double flower' varieties, while the common male parental ('Cipro') was a genotype producing a simple flower. The ddRAD generated a total of ~2Gb demultiplexed reads, resulting in an average of 8,3M reads per line. The sstacks pipeline was applied for the construction of a mock reference genome based on sequencing data, and SNP markers segregating in only one of the parents were retained for map construction by treating the F₁ population as a backcross. The four parental maps (two of the female parents and two of the common male parent) were aligned with 106 common markers and 8 linkage groups were identified, corresponding to the haploid chromosome number of the species. An average of 586 markers were associated with each parental map, with a marker density ranging from 1 marker/cM to 4.4 markers/cM. The developed maps were used for QTL analysis for flower color, leading to the identification of major QTLs for purple pigmentation. These results contribute to dissect on the genetics of Persian buttercup, enabling the development of new approaches for future varietal development.

Tree Peony R2R3-MYB Transcription Factor PsMYB30 Promotes Petal Blotch Formation by Activating the Transcription of the Anthocyanin Synthase Gene

Petal blotches are commonly observed in many angiosperm families and not only influence plant-pollinator interactions but also confer high ornamental value. Tree peony (Paeonia suffruticosa Andr.) is an important cut flower worldwide, but few studies have focused on its blotch formation. In this study, anthocyanins were found to be the pigment basis for blotch formation of P. suffruticosa, and peonidin-3,5-di-O-glucoside (Pn3G5G) was the most important component of anthocyanins, while the dihydroflavonol-4-reductase gene was the key factor contributing to blotch formation. Then, the R2R3-myeloblastosis (MYB) transcription factor PsMYB30 belonging to subgroup 1 was proven as a positive anthocyanin regulator with transcriptional activation and nuclear expression. Furthermore, silencing PsMYB30 in P. suffruticosa petals reduced blotch size by 37.9%, faded blotch color and decreased anthocyanin and Pn3G5G content by 23.6% and 32.9%, respectively. Overexpressing PsMYB30 increased anthocyanin content by 14.5-fold in tobacco petals. In addition, yeast one-hybrid assays, dual-luciferase assays and electrophoretic mobility shift assays confirmed that PsMYB30 could bind to the promoter of the anthocyanin synthase (ANS) gene and enhance its expression. Altogether, a novel MYB transcription factor, PsMYB30, was identified to promote petal blotch formation by activating the expression of PsANS involved in anthocyanin biosynthesis, which provide new insights for petal blotch formation in plants.

Grapevine Leafroll-Associated Virus 3 Genotype Influences Foliar Symptom Development in New Zealand Vineyards

Grapevine leafroll disease (GLD) constrains wine production worldwide. In New Zealand, the main causal agent of GLD is grapevine leafroll-associated virus 3 (GLRaV-3). To control GLD, an integrated management program is used and includes removing (roguing) GLRaV-3-infected vines from the vineyard. The classical foliar symptoms from virus-infected red-berry cultivars are leaves with dark red intervein, green veins, and downward rolling of margins. Growers use these phenotypic cues to undertake visual symptom identification (VSI) for GLD. However, the influence of the known large genetic variation among GLRaV-3 isolates on the foliar symptoms from different grapevine cultivars remains undescribed, especially in cool-climate growing environments, such as New Zealand. Over three vintages (2015, 2016, and 2017), VSI for GLD was undertaken at three field sites in New Zealand (Auckland, Hawke’s Bay, and Marlborough), each including four cultivars (Merlot, Pinot noir, Sauvignon blanc, and Pinot gris) infected with three GLRaV-3 genotypes (Groups I, VI, and X) or GLRaV-3-uninfected control plants. Throughout this study, no visual symptoms were observed on white-berry cultivars infected with GLRaV-3. For red-berry cultivars, the greatest variability in observed foliar symptoms among regional study sites, cultivars, and GLRaV-3 genotypes was observed early in the growing season. In particular, Group X had significantly delayed symptom expression across all three sites compared with Groups I and VI. As the newly infected, young vines matured in years 2 and 3, the GLRaV-3 genotype, cultivar, region, and environmental conditions had minimal influence on the accuracy of VSI, with consistently high (>95%) within-vintage identification by the end of each vintage. The results from this study strongly support the use of VSI for the GLD management of red-berry cultivar grapevines, Merlot and Pinot noir, as a reliable and cost-effective tool against GLD.

Transcriptome and carotenoid profiling of different varieties of Coffea arabica provides insights into fruit color formation

The processability and ultimate quality of coffee (C offea arabica) are determined by the composition of the matured fruits. The basis of genetic variation in coffee fruit quality could be explained by studying color formation during fruit maturation. Transcriptome profiling was conducted on matured fruits of four C. arabica varieties (orange colored fruits (ORF); purple colored fruits (PF); red colored fruits (RF) and yellow colored fruits (YF)) to identify key color-regulating genes, biosynthesis pathways and transcription factors implicated in fruit color formation. A total of 39,938 genes were identified in the transcriptomes of the four C. arabica varieties. In all, 2745, 781 and 1224 differentially expressed genes (DEGs) were detected in YF_vs_PF, YF_vs_RF and YF_vs_ORF, respectively, with 1732 DEGs conserved among the three pairwise groups. Functional annotation of the DEGs led to the detection of 28 and 82 key genes involved in the biosynthesis of carotenoids and anthocyanins, respectively. Key transcription factors bHLH, MYB, NAC, MADS, and WRKY implicated in fruit color regulation were detected. The high expression levels of gene-LOC113688784 (PSY), gene-LOC113730013 (β-CHY), gene-LOC113728842 (CCD7), gene-LOC113689681 (NCED) and gene-LOC113729473 (ABA2) in YF may have accounted for the yellow coloration. The differential expression of several anthocyanin and carotenoid-specific genes in the fruits substantially account for the purple (PF), red (RF), and orange (ORF) colorations. This study provides important insights into fruit color formation and variations in C. arabica and will help to develop coffee varieties with specific color and quality traits.

Post-transcriptional gene silencing of the chalcone synthase gene CHS causes corolla lobe-specific whiting of Japanese gentian

Post-transcriptional gene silencing of the chalcone synthase gene CHS specifically suppresses anthocyanin biosynthesis in corolla lobes and is responsible for the formation of a stripe type bicolor in Japanese gentian. The flower of Japanese gentian is a bell-shaped corolla composed of lobes and plicae, which is painted uniformly blue. However, the gentian cultivar 'Hakuju' shows bicolor phenotype (blue-white stripe corolla), in which anthocyanin accumulation is suppressed only in corolla lobes. Expression analysis indicated that steady-state levels of chalcone synthase (CHS) transcripts were remarkably reduced in corolla lobes compared with plicae during petal pigmentation initiation. However, no significant difference in expression levels of other flavonoid biosynthetic structural and regulatory genes was detected in its lobes and plicae. On feeding naringenin in white lobes, anthocyanin accumulation was recovered. Northern blotting probed with CHS confirmed the abundant accumulation of small RNAs in corolla lobes. Likewise, small RNA-seq analysis indicated that short reads from its lobes were predominantly mapped onto the 2nd exon region of the CHS gene, whereas those from the plicae were scarcely mapped. Subsequent infection with the gentian ovary ringspot virus (GORV), which had an RNA-silencing activity, showed the recovery of partial pigmentation in lobes. Hence, these results strongly suggested that suppressing anthocyanin accumulation in the lobes of bicolored 'Hakuju' was attributed to the specific degradation of CHS mRNA in corolla lobes, which was through post-transcriptional gene silencing (PTGS). Herein, we revealed the molecular mechanism of strip bicolor formation in Japanese gentian, and showed that PTGS of CHS was also responsible for flower color pattern in a floricultural plant other than petunia and dahlia.

Post-transcriptional gene silencing of CYP76AD controls betalain biosynthesis in bracts of bougainvillea

Betalain is one of four major plant pigments and shares some features with anthocyanin; however, no plant has been found to biosynthesize both pigments. Previous studies have reported that anthocyanin biosynthesis in some plants is regulated by post-transcriptional gene-silencing (PTGS), but the importance of PTGS in betalain biosynthesis remains unclear. In this study, we report the occurrence of PTGS in betalain biosynthesis in bougainvillea (Bougainvillea peruviana) 'Thimma', which produces bracts of three different color on the same plant, namely pink, white, and pink-white. This resembles the unstable anthocyanin pigmentation phenotype that is associated with PTGS, and hence we anticipated the presence of PTGS in the betalain biosynthetic pathway. To test this, we analysed pigments, gene expression, small RNAs, and transient overexpression. Our results demonstrated that PTGS of BpCYP76AD1, a gene encoding one of the betalain biosynthesis enzymes, is responsible for the loss of betalain biosynthesis in 'Thimma'. Neither the genetic background nor DNA methylation in the BpCYP76AD1 sequence could explain the induction of PTGS, implying that another locus controls the unstable pigmentation. Our results indicate that naturally occurring PTGS contributes to the diversification of color patterns not only in anthocyanin biosynthesis but also in betalain biosynthesis.

Integrated mRNA and small RNA sequencing reveals a regulatory network associated with flower color in oriental hybrid lily

Anthocyanins are one of the main components of pigments, that are responsible for a wide range of colors in plants. To clarify the regulatory mechanism of anthocyanin biosynthesis in oriental hybrid lily, UPLC/MS analysis was performed to identify the pigments in two cultivars (white and pink). Four major anthocyanins were identified in pink cultivar, and no anthocyanins were detected in white cultivar. Transcriptome and small RNA sequencing (sRNAseq) analyses were performed using tepal tissues at two floral developmental stages from the two cultivars. In total, 55,698 transcripts were assembled, among which 233 were annotated as putative anthocyanin-related transcripts. Differential expression analysis and qRT-PCR results confirmed that most of the anthocyanin-related structural genes had higher expression levels in pink cultivar than in white cultivar. Conversely, LhANR showed a significantly high expression level in white cultivar. Annotated transcription factors (TFs), including MYB activators, MYB repressors and bHLHs, that putatively inhibit or enhance the expression of anthocyanin-related genes were identified. LhMYBA1, an anthocyanin activator, was isolated, and its heterologous expression resulted in a remarkably high level of anthocyanin accumulation. Additionally, 73 differentially expressed microRNAs (miRNAs), including 23 known miRNAs, were detected through sRNAseq. The miRNA target prediction showed that several anthocyanin-related genes might be targeted by miRNAs. Expression profile analysis revealed that these miRNAs showed higher expression levels at later floral developmental stages in white cultivar than in pink cultivar. The results indicated that anthocyanin deficiency in white cultivar might be influenced by multiple levels of suppressive mechanisms, including mRNAs and sRNAs.

CRISPR/Cas9-mediated targeted mutation reveals a role for AN4 rather than DPL in regulating venation formation in the corolla tube of Petunia hybrida

Venation is a common anthocyanin pattern displayed in flowers that confers important ornamental traits to plants. An anthocyanin-related R2R3-MYB transcription factor, DPL, has been proposed to regulate corolla tube venation in petunia plants. Here, however, we provide evidence redefining the role of DPL in petunia. A CRISPR/Cas9-mediated mutation of DPL resulted in the absence of the vein-associated anthocyanin pattern above the abaxial surface of the flower bud, but not corolla tube venation, thus indicating that DPL did not regulate the formation of corolla tube venation. Alternately, quantitative real-time PCR analysis demonstrated that the spatiotemporal expression pattern of another R2R3-MYB gene, AN4, coincided with the formation of corolla tube venation in petunia. Furthermore, overexpression of AN4 promoted anthocyanin accumulation by increasing the expression of anthocyanin biosynthesis genes. CRISPR/Cas9-mediated mutation of AN4 led to an absence of corolla tube venation, suggesting that this gene in fact determines this key plant trait. Taken together, the results presented here redefine the prime regulator of corolla tube venation, paving the way for further studies on the molecular mechanisms underlying the various venation patterns in petunia.

Transcriptomic and metabolomic joint analysis reveals distinct flavonoid biosynthesis regulation for variegated testa color development in peanut (Arachis hypogaea L.)

Peanut is one of the important oil and economic crops, among which the variegated testa peanut is a unique member. The molecular mechanisms underlying the pigment synthesis in variegated testa are still unclear. Differentially expressed genes (DEGs) in the flavonoid metabolism pathway in pigmented areas indicated that there were 27 DEGs highly related to the synthesis of variegated testa color among 1,050 DEGs. Of these 27, 13 were up-regulated and 14 were down-regulated, including 3 PALs, 1 C4H, 2 CHSs, 1 F3H, 1 F3'H, 2 DFRs, 2 LARs, 2 IAAs, 4 bHLHs, and 9 MYBs. GO (Gene Ontology) analysis indicated that DEGs were similarly enriched in three branches. KEGG (Kyoto Encyclopedia of Genes and Genomes) analysis suggested flavonoid biosynthesis is the most direct metabolic pathway for the synthesis of testa variegation. The liquid chromatography–tandem mass spectrometry (LC–MS/MS) results showed that cyanidin and delphinidin were the primary metabolites that caused the color differences between the pigmented and the non-pigmented areas. Through the verification of 20 DEGs via qPCR, the results were consistent with transcriptome sequencing in four comparison groups. The results in this study lay the foundation for revealing the molecular regulation mechanisms of flavonoid synthesis in variegated testa peanut.

Ecotopic over-expression of PoCHS from Paeonia ostii altered the fatty acids composition and content in Arabidopsis thaliana

Chalcone synthase (CHS) is the key enzyme in the flavonoid biosynthetic pathway and has been studied in many plants, but the function of the CHS gene has not been well characterized in Paeonia ostii. In this study, we obtained a CHS homolog gene from P. ostii, which possessed the putative conserved amino acids of chalcone synthase by multiple alignment analysis and demonstrated the highest expression in developing seeds. In vitro assays of the recombinant PoCHS protein confirmed enzymatic activity using malonyl-CoA and 4-coumaroyl-CoA as substrates, and the optimal pH and reaction temperature were 7.5 and 40 °C, respectively. Furthermore, ectopic over-expression of PoCHS in Arabidopsis up-regulated the expression levels of genes involved in seed development (ABI), glycolysis (PKp2, PDH-E1a, and SUS2/3), and especially fatty acid biosynthesis (BCCP2, CAC2, CDS2, FatA, and FAD3). This resulted in an increased unsaturated fatty acid content, especially α-linolenic acid, in transgenic Arabidopsis seeds. In this study, we examined the functions of CHS homolog of P. ostii and demonstrated its new function in seed fatty acid biosynthesis.

A highly mutable GST is essential for bract colouration in Euphorbia pulcherrima Willd. Ex Klotsch

Background

Mutation breeding is an extraordinary tool in plant breeding to increase the genetic variability, where mutations in anthocyanin biosynthesis are targets to generate distinctive phenotypes in ornamental species. In poinsettia, ionizing radiation is routinely applied in breeding programs to obtaining a range of colours, with nearly all pink and white varieties being obtained after γ- or X-ray mutagenesis of red varieties. In the present study we performed a thorough characterization of a potential mutagenesis target gene as the main responsible for the ‘white paradox’ in poinsettia.

Results

We identified a GST gene in poinsettia (Bract1) as an essential factor for the expression of anthocyanin-based red colouration of bracts, which presents a high phylogenetic similarity to known anthocyanin-related GSTs. Red poinsettia varieties and white mutants generated from these varieties by X-ray were analysed for polymorphisms related to the ‘white paradox’ in the species. A 4 bp mutation in a short repeat within the coding region of Bract1 is most likely responsible for the appearance of white phenotypes upon irradiation treatment. The polymorphism between wild-type and mutant alleles co-segregates with the phenotype in progeny from heterozygous red and white parents. Moreover, overexpression of Bract1 wild-type allele in Arabidopsis tt19 mutants restored the anthocyanin phenotype, while the Bract1 mutated allele showed to be non-functional.

Conclusions

The identified repeat seems to be highly unstable, since mutated plants can be easily detected among fewer than 200 shoots derived from 10 mutated plants. Our data indicate that particular short repeat sequences, similar to microsatellite sequences or so-called dynamic mutations, might be hot spots for genetic variability. Moreover, the identification of the Bract1 mutation fills a gap on the understanding on the molecular mechanism of colour formation in poinsettia.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07527-z.

Genes and genome editing tools for breeding desirable phenotypes in ornamentals

We review the main genes underlying commercial traits in cut flower species and critically discuss the possibility to apply genome editing approaches to produce novel variation and phenotypes. Promoting flowering and flower longevity as well as creating novelty in flower structure, colour range and fragrances are major objectives of ornamental plant breeding. The novel genome editing techniques add new possibilities to study gene function and breed new varieties. The implementation of such techniques, however, relies on detailed information about structure and function of genomes and genes. Moreover, improved protocols for efficient delivery of editing reagents are required. Recent results of the application of genome editing techniques to elite ornamental crops are discussed in this review. Enabling technologies and genomic resources are reviewed in relation to the implementation of such approaches. Availability of the main gene sequences, underlying commercial traits and in vitro transformation protocols are provided for the world's best-selling cut flowers, namely rose, lily, chrysanthemum, lisianthus, tulip, gerbera, freesia, alstroemeria, carnation and hydrangea. Results obtained so far are described and their implications for the improvement of flowering, flower architecture, colour, scent and shelf-life are discussed.

Transcriptomic Analysis of the Anthocyanin Biosynthetic Pathway Reveals the Molecular Mechanism Associated with Purple Color Formation in Dendrobium Nestor

Dendrobium nestor is a famous orchid species in the Orchidaceae family. There is a diversity of flower colorations in the Dendrobium species, but knowledge of the genes involved and molecular mechanism underlying the flower color formation in D. nestor is less studied. Therefore, we performed transcriptome profiling using Illumina sequencing to facilitate thorough studies of the purple color formation in petal samples collected at three developmental stages, namely-flower bud stage (F), half bloom stage (H), and full bloom stage (B) in D. nestor. In addition, we identified key genes and their biosynthetic pathways as well as the transcription factors (TFs) associated with purple flower color formation. We found that the phenylpropanoid-flavonoid-anthocyanin biosynthesis genes such as phenylalanine ammonia lyase, chalcone synthase, anthocyanidin synthase, and UDP-flavonoid glucosyl transferase, were largely up-regulated in the H and B samples as compared to the F samples. This upregulation might partly account for the accumulation of anthocyanins, which confer the purple coloration in these samples. We further identified several differentially expressed genes related to phytohormones such as auxin, ethylene, cytokinins, salicylic acid, brassinosteroid, and abscisic acid, as well as TFs such as MYB and bHLH, which might play important roles in color formation in D. nestor flower. Sturdy upregulation of anthocyanin biosynthetic structural genes might be a potential regulatory mechanism in purple color formation in D. nestor flowers. Several TFs were predicted to regulate the anthocyanin genes through a K-mean clustering analysis. Our study provides valuable resource for future studies to expand our understanding of flower color development mechanisms in D. nestor.

The MicroRNA828/MYB12 Module Mediates Bicolor Pattern Development in Asiatic Hybrid Lily (Lilium spp.) Flowers

Some Asiatic hybrid lily cultivars develop bicolor tepals, which consist of anthocyanin-pigmented upper halves and un-pigmented lower halves. MYB12, a subgroup 6 member of R2R3-MYB that positively regulates anthocyanin biosynthesis, is downregulated in the lower halves. However, MYB12 is usually expressed over entire tepal regions in numerous lily cultivars. Why MYB12 of bicolor cultivars exhibits variable expression spatially in a single tepal remains unclear. Since the lily MYB12 mRNA harbored a binding site for microRNA828 (miR828), the involvement of miR828 in variable spatial accumulation of MYB12 transcripts was evaluated. We analyzed the cleavage of MYB12 mRNA, mature miR828 accumulation, and MYB12 transcript-derived siRNA generation (microRNA-seq). In the bicolor tepals, mature miR828 was more highly accumulated in the lower halves than in the upper halves, and miR828-directed cleavage of MYB12 transcripts was observed predominantly in the lower halves. Moreover, the cleavage triggered the production of secondary siRNA from MYB12 transcripts, and the siRNAs were accumulated predominantly in the lower halves. Consequently, miR828 suppressed MYB12 transcript accumulation in the white region, and the miR828/MYB12 module participated in the development of bicolor patterns in lily flowers. The results present the first example of a microRNA mediating flower color patterns. Finally, we discuss the potential of miR828 creating flower color variations through suppressing the activity of subgroup 6 R2R3-MYB positive regulators in other species.

The Regulation of Floral Colour Change in Pleroma raddianum (DC.) Gardner

Floral colour change is a widespread phenomenon in angiosperms, but poorly understood from the genetic and chemical point of view. This article investigates this phenomenon in Pleroma raddianum, a Brazilian endemic species whose flowers change from white to purple. To this end, flavonoid compounds and their biosynthetic gene expression were profiled. By using accurate techniques (Ultra Performance Liquid Chromatography-High-Resolution Mass Spectrometry (UPLC-HRMS)), thirty phenolic compounds were quantified. Five key genes of the flavonoid biosynthetic pathway were partially cloned, sequenced, and the mRNA levels were analysed (RT-qPCR) during flower development. Primary metabolism was also investigated by gas chromatography coupled to mass spectrometry (GC-EIMS), where carbohydrates and organic acids were identified. Collectively, the obtained results suggest that the flower colour change in P. raddianum is determined by petunidin and malvidin whose accumulation coincides with the transcriptional upregulation of early and late biosynthetic genes of the flavonoid pathway, mainly CHS and ANS, respectively. An alteration in sugars, organic acids and phenolic co-pigments is observed together with the colour change. Additionally, an increment in the content of Fe³⁺ ions in the petals, from the pink to purple stage, seemed to influence the saturation of the colour.

Integrated metabolic profiling and transcriptome analysis of pigment accumulation in diverse petal tissues in the lily cultivar 'Vivian'

BACKGROUND

Petals are the colorful region of many ornamental plants. Quality traits of petal color directly affect the value of ornamental plants. Although the regulatory mechanism of flower color has been widely studied in many plants, that of lily flower color is still worth further exploration.

RESULTS

In this study, the pigmentation regulatory network in different regions of the petal of lily cultivar 'Vivian' was analyzed through tissue structure, metabolites biosynthesis, and gene expression. We found that cell morphology of the petal in un-pigmented region differed from that in pigmented region. The cell morphology tends to flatten in un-pigmented region where the color is lighter. Moreover, high level anthocyanin was found in the pigmented regions by metabonomic analysis, especially cyanidin derivatives. However, flavanones were accumulated, contrast with anthocyanin in the un-pigmented regions of lily petal. To understand the relationship of these different metabolites and lily flower color, RNA-Seq was used to analyze the differentially expressed genes-related metabolite biosynthesis. Among these genes, the expression levels of several genes-related cyanidin derivatives biosynthesis were significantly different between the pigmented and un-pigmented regions, such as LvMYB5, LvMYB7, LvF3'H, LvDFR, LvANS and Lv3GT.

CONCLUSIONS

This data will help us to further understand the regulation network of lily petal pigmentation and create different unique color species.

Disturbance of floral colour pattern by activation of an endogenous pararetrovirus, petunia vein clearing virus, in aged petunia plants

White areas of star-type bicolour petals of petunia (Petunia hybrida) are caused by post-transcriptional gene silencing (PTGS) of the key enzyme of anthocyanin biosynthesis. We observed blotched flowers and a vein-clearing symptom in aged petunia plants. To determine the cause of blotched flowers, we focused on an endogenous pararetrovirus, petunia vein clearing virus (PVCV), because this virus may have a suppressor of PTGS (VSR). Transcripts and episomal DNAs derived from proviral PVCVs accumulated in aged plants, indicating that PVCV was activated as the host plant aged. Furthermore, DNA methylation of CG and CHG sites in the promoter region of proviral PVCV decreased in aged plants, suggesting that poor maintenance of DNA methylation activates PVCV. In parallel, de novo DNA methylation of CHH sites in its promoter region was also detected. Therefore, both activation and inactivation of PVCV occurred in aged plants. The accumulation of PVCV transcripts and episomal DNAs in blotched regions and the detection of VSR activity support a mechanism in which suppression of PTGS by PVCV causes blotched flowers.

Tree peony variegated flowers show a small insertion in the F3'H gene of the acyanic flower parts

BACKGROUND

The tree peony (Paeonia suffruticosa Andr.) cultivar 'Er Qiao' is appreciated for its unstable variegated flower coloration, with cyanic and acyanic flowers appearing on different branches of the same plant and occasionally in a single flower or petal. However, the variegation mechanism is still unclear.

RESULTS

In this study, we found significantly higher contents and more diverse sets of anthocyanins in the cyanic petals than in the acyanic petals. Comparative transcriptome analysis between the two flower types revealed 477 differentially expressed genes (DEGs). Quantitative real-time PCR results verified that the transcript levels of the flavonol synthase (FLS) gene were significantly increased in the acyanic petals. Furthermore, we found that a GCGGCG insertion at 246 bp in the flavonoid 3'-hydroxylase (F3'H) gene-coding region constitutes a duplication of the 241-245 bp section and was consistently found only in acyanic flowers. Sequence alignment of the F3'H gene from different plant species indicated that only the acyanic petals of 'Er Qiao' contained the GCGGCG insertion. The transformation of Arabidopsis tt7-1 lines demonstrated that the ectopic expression of F3'H-cyanic, but not F3'H-acyanic, could complement the colors in the hypocotyl and seed coat.

CONCLUSION

In summary, we found that an indel in F3'H and the upregulation of FLS drastically reduced the anthocyanin content in acyanic petals. Our results provide molecular candidates for a better understanding of the variegation mechanisms in tree peony.

Whole-Genome DNA Methylation Associated With Differentially Expressed Genes Regulated Anthocyanin Biosynthesis Within Flower Color Chimera of Ornamental Tree Prunus mume

DNA methylation is one of the best-studied epigenetic modifications involved in many biological processes. However, little is known about the epigenetic mechanism for flower color chimera of Prunus mume (Japanese apricot, mei). Using bisulfate sequencing and RNA sequencing, we analyzed the white (FBW) and red (FBR) petals collected from an individual tree of Japanese apricot cv. ‘Fuban Tiaozhi’ mei to reveal the different changes in methylation patterns associated with gene expression leading to significant difference in anthocyanins accumulation of FBW (0.012 ± 0.005 mg/g) and FBR (0.078 ± 0.013 mg/g). It was found that gene expression levels were positively correlated with DNA methylation levels within gene-bodies of FBW and FBR genomes; however, negative correlations between gene expression and DNA methylation levels were detected within promoter domains. In general, the methylation level within methylome of FBW was higher; and in total, 4,618 differentially methylated regions (DMRs) and 1,212 differentially expressed genes (DEGs) were detected from FBW vs. FBR. We also identified 82 DMR-associated DEGs, and 13 of them, including PmBAHD, PmCYP450, and PmABC, were playing critical roles in phenylalanine metabolism pathway, glycosyltransferase activity, and ABC transporter. The evidence exhibited DNA methylation may regulate gene expression resulting in flower color chimera of Japanese apricot.

Ectopic Expression of PAP1 Leads to Anthocyanin Accumulation and Novel Floral Color in Genetically Engineered Goldenrod (Solidago canadensis L.)

Floral pigmentation is of major importance to the ornamental industry, which is constantly searching for cultivars with novel colors. Goldenrod (Solidago canadensis) has monochromatic yellow carotenoid-containing flowers that cannot be modified using classical breeding approaches due to a limited gene pool. To generate Solidago with novel colors through metabolic engineering, we first developed a procedure for its regeneration and transformation. Applicability of different cytokinins for adventitious regeneration was examined in the commercial cv. Tara, with zeatin yielding higher efficiency than 6-benzylaminopurine or thidiazuron. A comparison of regeneration of commercial cvs. Tara, Golden Glory and Ivory Glory revealed Tara to be the most potent, with an efficiency of 86% (number of shoots per 100 leaf explants). Agrobacterium-based transformation efficiency was highest for cv. Golden Glory (5 independent transgenic shoots per 100 explants) based on kanamycin selection and the GUS reporter gene. In an attempt to promote anthocyanin biosynthesis, we generated transgenic Solidago expressing snapdragon (Antirrhinum majus) Rosea1 and Delila, as well as Arabidopsis thaliana PRODUCTION OF ANTHOCYANIN PIGMENT 1 (PAP1) transcription factors. Transgenic cv. Golden Glory expressing cauliflower mosaic virus 35S-driven PAP1 generated red flowers that accumulated delphinidin and its methylated derivatives, as compared to control yellow flowers in the GUS-expressing plants. The protocol described here allows efficient engineering of Solidago for novel coloration and improved agricultural traits.

Chalcone synthase is ubiquitinated and degraded via interactions with a RING-H2 protein in petals of Paeonia 'He Xie'

Flavonoids are secondary metabolites widely distributed among angiosperms, where they play diverse roles in plant growth, development, and evolution. The regulation of flavonoid biosynthesis in plants has been extensively studied at the transcriptional level, but post-transcriptional, translational, and post-translational control of flavonoid biosynthesis remain poorly understood. In this study, we analysed post-translational regulation of flavonoid biosynthesis in the ornamental plant Paeonia, using proteome and ubiquitylome profiling, in conjunction with transcriptome data. Three enzymes involved in flavonoid biosynthesis were identified as being putative targets of ubiquitin-mediated degradation. Among these, chalcone synthase (PhCHS) was shown to have the greatest number of ubiquitination sites. We examined PhCHS abundance in petals using PhCHS-specific antibody and found that its accumulation decreased at later developmental stages, resulting from 26S proteasome-mediated degradation. We further identified a ring domain-containing protein (PhRING-H2) that physically interacts with PhCHS and demonstrated that PhRING-H2 is required for PhCHS ubiquitination. Taken together, our results suggest that PhRING-H2-mediates PhCHS ubiquitination and degradation is an important mechanism of post-translational regulation of flavonoid biosynthesis in Paeonia, providing a theoretical basis for the manipulation of flavonoid biosynthesis in plants.

mRNA and miRNA Expression Analysis Reveal the Regulation for Flower Spot Patterning in Phalaenopsis 'Panda'

Phalaenopsis cultivar ‘Panda’ is a beautiful and valuable ornamental for its big flower and unique big spots on the petals and sepals. Although anthocyanins are known as the main pigments responsible for flower colors in Phalaenopsis, and the anthocyanins biosynthetic pathway in Phalaenopsis is generally well known, the detailed knowledge of anthocynins regulation within the spot and non-spot parts in ‘Panda’ flower is limited. In this study, transcriptome and small RNA libraries analysis from spot and non-spot sepal tissues of ‘Panda’ were performed, and we found PeMYB7, PeMYB11, and miR156g, miR858 is associated with the purple spot patterning in its sepals. Transcriptome analyses showed a total 674 differentially expressed genes (DEGs), with 424 downregulated and 250 upregulated (Non-spot-VS-Spot), and 10 candidate DEGs involved in anthocyanin biosynthetic pathway. The qPCR analysis confirmed that seven candidate structure genes (PeANS, PeF3′H, PeC4H, PeF3H, PeF3H1, Pe4CL2, and PeCHI) have significantly higher expressing levels in spot tissues than non-spot tissues. A total 1552 differentially expressed miRNAs (DEMs) were detected with 676 downregulated and 876 upregulated. However, microRNA data showed no DEMs targeting on anthocyanin biosynthesis structure gene, while a total 40 DEMs target transcription factor (TF) genes, which expressed significantly different level in spot via non-spot sepal, including 2 key MYB regulator genes. These results indicated that the lack of anthocyanidins in non-spot sepal may not directly be caused by microRNA suppressing anthocyanidin synthesis genes rather than the MYB genes. Our findings will help in understanding the role of miRNA molecular mechanisms in the spot formation pattern of Phalaenopsis, and would be useful to provide a reference to similar research in other species.

Isolation and Characterization of Three Chalcone Synthase Genes in Pecan (Carya illinoinensis)

Phenolics are a group of important plant secondary metabolites that have been proven to possess remarkable antioxidant activity and to be beneficial for human health. Pecan nuts are an excellent source of dietary phenolics. In recent years, many studies have focused on the separation and biochemical analysis of pecan phenolics, but the molecular mechanisms of phenolic metabolism in pecans have not been fully elucidated, which significantly hinders quality breeding research for this plant. Chalcone synthase (CHS) plays crucial roles in phenolic biosynthesis. In this study, three Carya illinoinensisCHSs (CiCHS1, CiCHS2, and CiCHS3), were isolated and analyzed. CiCHS2 and CiCHS3 present high expression levels in different tissues, and they are also highly expressed at the initial developmental stages of kernels in three pecan genotypes. A correlation analysis was performed between the phenolic content and CHSs expression values during kernel development. The results indicated that the expression variations of CiCHS2 and CiCHS3 are significantly related to changes in total phenolic content. Therefore, CiCHSs play crucial roles in phenolic components synthesis in pecan. We believe that the isolation of CiCHSs is helpful for understanding phenolic metabolism in C. illinoinensis, which will improve quality breeding and resistance breeding studies in this plant.

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.

Inhibition of post-transcriptional gene silencing of chalcone synthase genes in petunia picotee petals by fluacrypyrim

In petals of picotee petunia (Petunia hybrida) cultivars, margin-specific post-transcriptional gene silencing (PTGS) of chalcone synthase A (CHSA) inhibits anthocyanin biosynthesis, resulting in marginal white tissue formation. In this study, we found that a low molecular mass compound, fluacrypyrim, inhibits PTGS of CHSA, and we explored the site-specific PTGS mechanism of operation. Fluacrypyrim treatment abolished the picotee pattern and eliminated site-specific differences in the levels of anthocyanin-related compounds, CHSA expression, and CHSA small interfering RNA (siRNA). In addition, fluacrypyrim abolished the petunia star-type pattern, which is also caused by PTGS of CHSA. Fluacrypyrim treatment was effective only at the early floral developmental stage and predominantly eliminated siRNA derived from CHS genes; i.e. siRNA derived from other genes remained at a comparable level. Fluacrypyrim probably targets the induction of PTGS that specifically operates for CHS genes in petunia picotee flowers, rather than common PTGS maintenance mechanisms that degrade mRNAs and generate siRNA. Upon treatment, the proportion of colored tissue increased due to a shift of the border between white and colored sites toward the margin in a time- and dose-dependent manner. These findings imply that the fluacrypyrim-targeted PTGS induction is completed gradually and its strength is attenuated from the margins to the center of petunia picotee petals.

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.

Anthocyanin Degrading and Chlorophyll Accumulation Lead to the Formation of Bicolor Leaf in Ornamental Kale

Ornamental kale is a popular decorative plant. We identified a peculiar bicolor leaf double haploid line, with green margins and red centers. The development of bicolor leaves can be divided into three stages: S1, S2, and S3. To probe the reason for bicolor formation, we analyzed the anthocyanin and chlorophyll contents, detected the changes in indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin 3 (GA3), sugar, and starch contents, and identified the differentially expressed genes (DEGs) using RNA-seq. Results showed that the bicolor leaf phenotype is gradually formed with anthocyanin degrading and chlorophyll accumulation. Anthocyanin content is lower in the green margin (S3_S) than in the red center (S3_C) part at S3. IAA content was positively correlated with anthocyanin content during the bicolor leaf development. During anthocyanin degrading from S1 to S2, cinnamate-4-hydroxylase (C4H) and transport inhibitor response 1 (TIR1) were downregulated, while lateral organ boundaries domain 39 (LBD39) was upregulated. Two peroxidases, two β-glucosidases (BGLU), LBD39, LBD37, detoxifying efflux carrier 35 (DTX35), three no apical meristem (NAC) transcription factors (TFs), and 15 WRKY DNA-binding protein (WRKY) TFs were downregulated in S3_S vs. S3_C. The bicolor phenotype was mainly linked to anthocyanin degrading and chlorophyll accumulation, and that anthocyanin degrading resulted from reduced anthocyanin biosynthesis and increased anthocyanin degradation.

Floral pigmentation pattern in Oriental hybrid lily (Lilium spp.) cultivar 'Dizzy' is caused by transcriptional regulation of anthocyanin biosynthesis genes

Flower color patterns are the result of spatially and temporally restricted pigment deposition, and clarifying the mechanisms responsible for restricted pigment deposition is a topic of broad interest for both theoretical and practical reasons. The Oriental hybrid lily cultivar 'Dizzy' develops red stripes along the tepal midribs; in order to clarify the genetic basis of these stripes, we isolated most of the genes related to anthocyanin accumulation from 'Dizzy' tepals and compared their expression levels between the red stripe region and the white marginal region of the tepals. RNA-seq revealed a complete set of genes necessary for anthocyanin biosynthesis and transport, including anthocyanidin 3-O-glucosyltransferase and glutathione S-transferase. Most of these genes were expressed at higher rates in the red stripe region than in the white region, suggesting that transcriptional regulation of these genes is primarily responsible for the spatially restricted anthocyanin deposition in 'Dizzy' tepals. Subgroup 6 R2R3-MYB is a major factor regulating anthocyanin biosynthesis: RNA-seq clarified three subgroup 6 R2R3-MYB genes expressed in 'Dizzy' tepals, of which MYB12 was predominantly expressed. Expression of MYB12 was six-fold higher in the red-pigmented region than in the white region. Thus, MYB12 is more likely to be involved in the regulation of the restricted anthocyanin deposition in 'Dizzy', even though MYB12 is expressed in the entire tepal region of many Oriental hybrid lily cultivars. Diversity of the expression profiles of MYB12 among lily cultivars and species is also discussed.

Substantial Epigenetic Variation Causing Flower Color Chimerism in the Ornamental Tree Prunus mume Revealed by Single Base Resolution Methylome Detection and Transcriptome Sequencing

Epigenetic changes caused by methylcytosine modification participate in gene regulation and transposable element (TE) repression, resulting in phenotypic variation. Although the effects of DNA methylation and TE repression on flower, fruit, seed coat, and leaf pigmentation have been investigated, little is known about the relationship between methylation and flower color chimerism. In this study, we used a comparative methylomic⁻transcriptomic approach to explore the molecular mechanism responsible for chimeric flowers in Prunus mume "Danban Tiaozhi". High-performance liquid chromatography-electrospray ionization mass spectrometry revealed that the variation in white (WT) and red (RT) petal tissues in this species is directly due to the accumulation of anthocyanins, i.e., cyanidin 3,5-O-diglucoside, cyanidin 3-O-glucoside, and peonidin 3-O-glucoside. We next mapped the first-ever generated methylomes of P. mume, and found that 11.29⁻14.83% of the genomic cytosine sites were methylated. We also determined that gene expression was negatively correlated with methylcytosine level in general, and uncovered significant epigenetic variation between WT and RT. Furthermore, we detected differentially methylated regions (DMRs) and DMR-related genes between WT and RT, and concluded that many of these genes, including differentially expressed genes (DEGs) and transcription factor genes, are critical participants in the anthocyanin regulatory pathway. Importantly, some of the associated DEGs harbored TE insertions that were also modified by methylcytosine. The above evidence suggest that flower color chimerism in P. mume is induced by the DNA methylation of critical genes and TEs.

Molecular dissection of distinct symptoms induced by tomato chlorosis virus and tomato yellow leaf curl virus based on comparative transcriptome analysis

The viral infection of plants may cause various physiological symptoms associated with the reprogramming of plant gene expression. However, the molecular mechanisms and associated genes underlying disease symptom development in plants infected with viruses are largely unknown. In this study, we employed RNA sequencing for in-depth molecular characterization of the transcriptional changes associated with the development of distinct symptoms induced by tomato chlorosis virus (ToCV) and tomato yellow leaf curl virus (TYLCV) in tomato. Comparative analysis of differentially expressed genes revealed that ToCV and TYLCV induced distinct transcriptional changes in tomato and resulted in the identification of important genes responsible for the development of symptoms of ToCV (i.e., chlorosis and anthocyanin accumulation) and TYLCV (i.e., yellowing, stunted growth, and leaf curl). Our comprehensive transcriptome analysis can provide molecular strategies to reduce the severity of disease symptoms as well as new insights for the development of virus-resistant crops.

Recent advances in flower color variation and patterning of Japanese morning glory and petunia

The Japanese morning glory (Ipomoea nil) and petunia (Petunia hybrida), locally called "Asagao" and "Tsukubane-asagao", respectively, are popular garden plants. They have been utilized as model plants for studying the genetic basis of floricultural traits, especially anthocyanin pigmentation in flower petals. In their long history of genetic studies, many mutations affecting flower pigmentation have been characterized, and both structural and regulatory genes for the anthocyanin biosynthesis pathway have been identified. In this review, we will summarize recent advances in the understanding of flower pigmentation in the two species with respect to flower hue and color patterning. Regarding flower hue, we will describe a novel enhancer of flavonoid production that controls the intensity of flower pigmentation, new aspects related to a flavonoid glucosyltransferase that has been known for a long time, and the regulatory mechanisms of vacuolar pH being a key determinant of red and blue coloration. On color patterning, we describe particular flower patterns regulated by epigenetic and RNA-silencing mechanisms. As high-quality whole genome sequences of the Japanese morning glory and petunia wild parents (P. axillaris and P. inflata, respectively) were published in 2016, further study on flower pigmentation will be accelerated.

Development of basic technologies for improvement of breeding and cultivation of Japanese gentian

Japanese gentians are the most important ornamental flowers in Iwate Prefecture and their breeding and cultivation have been actively conducted for half a century. With its cool climate and large hilly and mountainous area, more than 60% of gentian production in Japan occurs in Iwate Prefecture. Recent advances in gentian breeding and cultivation have facilitated the efficient breeding of new cultivars; disease control and improved cultivation conditions have led to the stable production of Japanese gentians. Molecular biology techniques have been developed and applied in gentian breeding, including the diagnosis of viral diseases and analysis of physiological disorders to improve gentian production. This review summarizes such recent approaches that will assist in the development of new cultivars and support cultivation. More recently, new plant breeding techniques, including several new biotechnological methods such as genome editing and viral vectors, have also been developed in gentian. We, therefore, present examples of their application to gentians and discuss their advantages in future studies of gentians.

Spatiotemporal Transcriptome Analysis Provides Insights into Bicolor Tepal Development in Lilium "Tiny Padhye"

The bicolor Asiatic hybrid lily cultivar "Tiny Padhye" is an attractive variety because of its unique color pattern. During its bicolor tepal development, the upper tepals undergo a rapid color change from green to white, while the tepal bases change from green to purple. However, the molecular mechanisms underlying these changes remain largely uncharacterized. To systematically investigate the dynamics of the lily bicolor tepal transcriptome during development, we generated 15 RNA-seq libraries from the upper tepals (S2-U) and basal tepals (S1-D, S2-D, S3-D, and S4-D) of Lilium "Tiny Padhye." Utilizing the Illumina platform, a total of 295,787 unigenes were obtained from 713.12 million high-quality paired-end reads. A total of 16,182 unigenes were identified as differentially expressed genes during tepal development. Using Kyoto Encyclopedia of Genes and Genomes pathway analysis, candidate genes involved in the anthocyanin biosynthetic pathway (61 unigenes), and chlorophyll metabolic pathway (106 unigenes) were identified. Further analyses showed that most anthocyanin biosynthesis genes were transcribed coordinately in the tepal bases, but not in the upper tepals, suggesting that the bicolor trait of "Tiny Padhye" tepals is caused by the transcriptional regulation of anthocyanin biosynthetic genes. Meanwhile, the high expression level of chlorophyll degradation genes and low expression level of chlorophyll biosynthetic genes resulted in the absence of chlorophylls from "Tiny Padhye" tepals after flowering. Transcription factors putatively involved in the anthocyanin biosynthetic pathway and chlorophyll metabolism in lilies were identified using a weighted gene co-expression network analysis and their possible roles in lily bicolor tepal development were discussed. In conclusion, these extensive transcriptome data provide a platform for elucidating the molecular mechanisms of bicolor tepals in lilies and provide a basis for similar research in other closely related species.

Efficient Generation of diRNAs Requires Components in the Posttranscriptional Gene Silencing Pathway

It has been reported that double-stranded break (DSB)-induced small RNAs (diRNAs) are generated via the RNA-directed DNA methylation pathway and function in DSB repair in Arabidposis. However, important questions remain regarding the biogenesis and function of diRNAs. Here, we used CRISPR/Cas9- or TALEN-triggered DSBs to characterize diRNAs in Arabidopsis and rice. We found that 21-nt diRNAs were generated from a 35S promoter::GU-US reporter transgene targeted by CRISPR/Cas9. Unexpectedly, Pol II transcription of the transgene was required for efficient diRNA production and the level of diRNA accumulation correlated with the expression level of the transgene. diRNAs were not detected from CRISPR/Cas9- or TALEN-induced DSBs within the examined endogenous genes in Arabidopsis or rice. We also found that DCL4 and RDR6 that are known to be involved in posttranscriptional gene silencing were required to generate diRNAs. Our results suggest that DSBs are necessary but not sufficient for efficient diRNA generation and a high level of diRNAs is not necessary for DSB repair.

Virus-induced gene silencing (VIGS) in Lilium leichtlinii using the Cucumber mosaic virus vector

Lilies (Lilium) are among the most important floriculture crops, and to accelerate research regarding lily genetics, the development of reverse-genetics tools is necessary. However, Agrobacterium-mediated transformation in Lilium is time-consuming, since the plants require several years to progress from acclimation to flowering. Thus, virus-induced gene silencing (VIGS) is an attractive method for assaying gene function. In the present study, we modified a lily-derived strain of Cucumber mosaic virus (CMV-HL) as a VIGS vector and evaluated its effectiveness for inducing gene silencing in Lilium leichtlinii by introducing L. leichtlinii phytoene desaturase (LlPDS) gene fragments into an intercistronic region between the 3a and 3b genes of the CMV-HL RNA3 genome. At 30 days after inoculation (dpi) with LlPDS-containing CMV-HL, photo-bleaching was observed in the upper leaves of L. leichtlinii, and at 57 dpi, we observed that the natural orange color in flower tepals had faded. Reduced LlPDS expression and the detection of small interfering LlPDS RNA indicated that the color changes were the result of LlPDS gene silencing. In addition, the leaves also exhibited a mild photo-bleaching phenotype in the following year. Therefore, our results indicate that CMV-HL spreads systemically in the leaves and flowers of Lilium during the first year of infection, as well as in new shoots during the following year, and that the vector system can be successfully applied to induce short-term endogenous gene silencing in lilies.

RNA-seq-based evaluation of bicolor tepal pigmentation in Asiatic hybrid lilies (Lilium spp.)

BACKGROUND

Color patterns in angiosperm flowers are produced by spatially and temporally restricted deposition of pigments. Identifying the mechanisms responsible for restricted pigment deposition is a topic of broad interest. Some dicots species develop bicolor petals, which are often caused by the post-transcriptional gene silencing (PTGS) of chalcone synthase (CHS) genes. An Asiatic hybrid lily (Lilium spp.) cultivar Lollypop develops bicolor tepals with pigmented tips and white bases. Here, we analyzed the global transcription of pigmented and non-pigmented tepal parts from Lollypop, to determine the main transcriptomic differences.

RESULTS

De novo assembly of RNA-seq data yielded 49,239 contigs (39,426 unigenes), which included a variety of novel transcripts, such as those involved in flavonoid-glycosylation and sequestration and in regulation of anthocyanin biosynthesis. Additionally, 1258 of the unigenes exhibited significantly differential expression between the tepal parts (false discovery rates <0.05). The pigmented tepal parts accumulated more anthocyanins, and unigenes annotated as anthocyanin biosynthesis genes (e.g., CHS, dihydroflavonol 4-reductase, and anthocyanidin synthase) were expressed 7-30-fold higher than those in non-pigmented parts. These results indicate that the transcriptional regulation of biosynthesis genes is more likely involved in the development of bicolor lily tepals rather than the PTGS of CHS genes. In addition, the expression level of a unigene homologous to LhMYB12, which often regulates full-tepal anthocyanin pigmentation in lilies, was >2-fold higher in the pigmented parts. Thus, LhMYB12 should be involved in the transcriptional regulation of the biosynthesis genes in bicolor tepals. Other factors that potentially suppress or enhance the expression of anthocyanin biosynthesis genes, including a WD40 gene, were identified, and their involvement in bicolor development is discussed.

CONCLUSIONS

Our results indicate that the bicolor trait of Lollypop tepals is caused by the transcriptional regulation of anthocyanin biosynthesis genes and that the transcription profile of LhMYB12 provides a clue for elucidating the mechanisms of the trait. The tepal transcriptome constructed in this study will accelerate investigations of the genetic controls of anthocyanin color patterns, including the bicolor patterns, of Lilium spp.

Investigation of a miRNA-Induced Gene Silencing Technique in Petunia Reveals Alterations in miR173 Precursor Processing and the Accumulation of Secondary siRNAs from Endogenous Genes

MIGS (miRNA-induced gene silencing) is a straightforward and efficient gene silencing technique in Arabidopsis. It works by exploiting miR173 to trigger the production of phasiRNAs (phased small interfering RNAs). MIGS can be used in plant species other than Arabidopsis by co-expression of miR173 and target gene fragments fused to an upstream miR173 target site. However, the efficiency and technical mechanisms have not been thoroughly investigated in other plants. In this work, two vectors, pMIGS-chs and pMIGS-pds, were constructed and transformed into petunia plants. The transgenic plants showed CHS (chalcone synthase) and PDS (phytoene desaturase) gene-silencing phenotypes respectively, indicating that MIGS functions in petunia. MIGS-chs plants were used to investigate the mechanisms of this technique in petunia. Results of 5′- RACE showed that the miR173 target site was cleaved at the expected position and that endogenous CHS genes were cut at multiple positions. Small RNA deep sequencing analysis showed that the processing of Arabidopsis miR173 precursors in MIGS-chs transgenic petunia plants did not occur in exactly the same way as in Arabidopsis, suggesting differences in the machinery of miRNA processing between plant species. Small RNAs in-phase with the miR173 cleavage register were produced immediately downstream from the cleavage site and out-of-phase small RNAs were accumulated at relatively high levels from processing cycle 5 onwards. Secondary siRNAs were generated from multiple sites of endogenous CHS-A and CHS-J genes, indicating that miR173 cleavage induced siRNAs have the same ability to initiate siRNA transitivity as the siRNAs functioning in co-suppression and hpRNA silencing. On account of the simplicity of vector construction and the transitive amplification of signals from endogenous transcripts, MIGS is a good alternative gene silencing method for plants, especially for silencing a cluster of homologous genes with redundant functions.

A small indel mutation in an anthocyanin transporter causes variegated colouration of peach flowers

The ornamental peach cultivar 'Hongbaihuatao (HBH)' can simultaneously bear pink, red, and variegated flowers on a single tree. Anthocyanin content in pink flowers is extremely low, being only 10% that of a red flower. Surprisingly, the expression of anthocyanin structural and potential regulatory genes in white flowers was not significantly lower than that in both pink and red flowers. However, proteomic analysis revealed a GST encoded by a gene-regulator involved in anthocyanin transport (Riant)-which is expressed in the red flower, but almost undetectable in the variegated flower. The Riant gene contains an insertion-deletion (indel) polymorphism in exon 3. In white flowers, the Riant gene is interrupted by a 2-bp insertion in the last exon, which causes a frameshift and a premature stop codon. In contrast, both pink and red flowers that arise from bud sports are heterozygous for the Riant locus, with one functional allele due to the 2-bp deletion or a novel 1-bp insertion. Southern blot analysis indicated that the Riant gene occurs in a single copy in the peach genome and it is not interrupted by a transposon. The function of the Riant gene was confirmed by its ectopic expression in the Arabidopsis tt19 mutant, where it complements the anthocyanin phenotype, but not the proanthocyanidin pigmentation in seed coat. Collectively,these results indicate that a small indel mutation in the Riant gene, which is not the result of a transposon insertion or excision, causes variegated colouration of peach flowers.

GmCOL1a and GmCOL1b Function as Flowering Repressors in Soybean Under Long-Day Conditions

CONSTANS (CO) has a central role in the photoperiod response mechanism in Arabidopsis. However, the functions of legume CO genes in controlling flowering remain unknown. Here, we analyze the expression patterns of E1, E2 and GmCOL1a/1b using near-isogenic lines (NILs), and we further analyze flowering-related genes in gmcol1b mutants and GmCOL1a-overexpressing plants. Our data showed that both E3 and E4 up-regulate E1 expression, with the effect of E3 on E1 being greater than the effect of E4 on E1. E2 was up-regulated by E3 and E4 but down-regulated by E1. GmCOL1a/1b were up-regulated by E1, E2, E3 and E4. Although the spatial and temporal patterns of GmCOL1a/1b expression were more similar to those of AtCOL2 than to those of AtCO, gmcol1b mutants flowered earlier than wild-type plants under long-day (LD) conditions, and the overexpression of GmCOL1a caused late flowering under LD or natural conditions. In addition, GmFT2a/5a, E1 and E2 were down-regulated in GmCOL1a-overexpressing plants under LD conditions. Because E1/2 influences the expression of GmCOL1a, and vice versa, we conclude that these genes may function as part of a negative feedback loop, and GmCOL1a/b genes may serve as suppressors in photoperiodic flowering in soybean under LD conditions.

Transcriptome sequencing of purple petal spot region in tree peony reveals differentially expressed anthocyanin structural genes

The pigmented cells in defined region of a petal constitute the petal spots. Petal spots attract pollinators and are found in many angiosperm families. Several cultivars of tree peony contain a single red or purple spot at the base of petal that makes the flower more attractive for the ornamental market. So far, the understanding of the molecular mechanism of spot formation is inadequate. In this study, we sequenced the transcriptome of the purple spot and the white non-spot of tree peony flower. We assembled and annotated 67,892 unigenes. Comparative analyses of the two transcriptomes showed 1,573 differentially expressed genes, among which 933 were up-regulated, and 640 were down-regulated in the purple spot. Subsequently, we examined four anthocyanin structural genes, including PsCHS, PsF3'H, PsDFR, and PsANS, which expressed at a significantly higher level in the purple spot than in the white non-spot. We further validated the digital expression data using quantitative real-time PCR. Our result uncovered transcriptome variance between the spot and non-spot of tree peony flower, and revealed that the co-expression of four anthocyanin structural genes was responsible for spot pigment in tree peony. The data will further help to unravel the genetic mechanism of peony flower spot formation.

GmmiR156b overexpression delays flowering time in soybean

Soybean [Glycine max (L.) Merr.] is an important crop used for human consumption, animal feed and biodiesel fuel. Wering time and maturity significantly affect soybean grain yield. In Arabidopsis thaliana, miR156 has been proposed to regulate the transition from the juvenile to the adult phase of shoot development, which is accompanied by changes in vegetative morphology and an increase in reproductive potential. However, the molecular mechanisms underlying miR156 function in soybean flowering remain unknown. Here, we report that the overexpression of GmmiR156b delays flowering time in soybean. GmmiR156b may target SPL orthologs and negatively regulate GmSPLs, thereby delaying flowering in soybean under LD and natural conditions. GmmiR156b down-regulates several known flowering time regulators in soybean, such as GmAP1 (a, b, c), GmLFY2, GmLFY2, GmFULs, GmSOC1s, GmFT5a, and GmmiR172. These data show that a similar miR156-SPL regulatory module was conserved in the soybean flowering pathway. However, GmFULs, GmSOC1a and GmSOC1b were significantly suppressed under LD conditions but not under SD conditions, which is different in Arabidopsis that these genes were down-regulated irrespective of photoperiod. In addition, GmmiR156b was up-regulated by E1, E2 (GmGI), E3 and E4, which control flowering time and maturity in soybean, and suppressed E1 (E1-Like) and E2 (E2-Like) genes under LD conditions. These data indicated that the miR156-SPL regulatory module was also with some degree of divergent in soybean flowering pathway.

Three R2R3-MYB transcription factors regulate distinct floral pigmentation patterning in Phalaenopsis spp

Orchidaceae are well known for their fascinating floral morphologic features, specialized pollination, and distinctive ecological strategies. With their long-lasting flowers of various colors and pigmentation patterning, Phalaenopsis spp. have become important ornamental plants worldwide. In this study, we identified three R2R3-MYB transcription factors PeMYB2, PeMYB11, and PeMYB12. Their expression profiles were concomitant with red color formation in Phalaenopsis spp. flowers. Transient assay of overexpression of three PeMYBs verified that PeMYB2 resulted in anthocyanin accumulation, and these PeMYBs could activate the expression of three downstream structural genes Phalaenopsis spp. Flavanone 3-hydroxylase5, Phalaenopsis spp. Dihydroflavonol 4-reductase1, and Phalaenopsis spp. Anthocyanidin synthase3. In addition, these three PeMYBs participated in the distinct pigmentation patterning in a single flower, which was revealed by virus-induced gene silencing. In the sepals/petals, silencing of PeMYB2, PeMYB11, and PeMYB12 resulted in the loss of the full-red pigmentation, red spots, and venation patterns, respectively. Moreover, different pigmentation patterning was regulated by PeMYBs in the sepals/petals and lip. PeMYB11 was responsive to the red spots in the callus of the lip, and PeMYB12 participated in the full pigmentation in the central lobe of the lip. The differential pigmentation patterning was validated by RNA in situ hybridization. Additional assessment was performed in six Phalaenopsis spp. cultivars with different color patterns. The combined expression of these three PeMYBs in different ratios leads to a wealth of complicated floral pigmentation patterning in Phalaenopsis spp.