Identification of r mutations conferring white flowers in the Japanese morning glory (Ipomoea nil)

Integrated Analyses of Metabolome and RNA-seq Data Revealing Flower Color Variation in Ornamental Rhododendron simsii Planchon

Rhododendron simsii Planchon is an important ornamental species in the northern hemisphere. Flower color is an important objective of Rhododendron breeding programs. However, information on anthocyanin synthesis in R. simsii is limited. In this research, the regulatory mechanism of anthocyanin biosynthesis in R. simsii was performed through the integrated analysis of metabolome and RNA-seq. A total of 805 and 513 metabolites were screened by positive and negative ionization modes, respectively, In total, 79 flavonoids contained seven anthocyanidins, 42 flavanones, 10 flavans, 13 flavones, and seven flavonols. Methylated and glycosylated derivatives took up the most. Differentially accumulated metabolites were mainly involved in "flavone and flavonol biosynthesis", "cyanoamino acid metabolism", "pyrimidine metabolism", and "phenylalanine metabolism" pathways. For flavonoid biosynthesis, different expression of shikimate O-hydroxycinnamoyltransferase, caffeoyl-CoA O-methyltransferase, flavonoid 3'-monooxygenase, flavonol synthase, dihydroflavonol 4-reductase/flavanone 4-reductase, F3'5'H, chalcone synthase, leucoanthocyanidin reductase, and 5-O-(4-coumaroyl)-D-quinate 3'-monooxygenase genes ultimately led to different accumulations of quercetin, myricetin, cyanidin, and eriodictyol. In flavone and flavonol biosynthesis pathway, differential expression of F3'5'H, flavonoid 3'-monooxygenase and flavonol-3-O-glucoside/galactoside glucosyltransferase genes led to the differential accumulation of quercetin, isovitexin, and laricitrin. This research will provide a biochemical basis for further modification of flower color and genetic breeding in R. simsii and related Rhododendron species.

Genome-wide analysis of R2R3-MYB transcription factors in Japanese morning glory

The R2R3-MYB transcription factor is one of the largest transcription factor families in plants. R2R3-MYBs play a variety of functions in plants, such as cell fate determination, organ and tissue differentiations, primary and secondary metabolisms, stress and defense responses and other physiological processes. The Japanese morning glory (Ipomoea nil) has been widely used as a model plant for flowering and morphological studies. In the present study, 127 R2R3-MYB genes were identified in the Japanese morning glory genome. Information, including gene structure, protein motif, chromosomal location and gene expression, were assigned to the InR2R3-MYBs. Phylogenetic tree analysis revealed that the 127 InR2R3-MYBs were classified into 29 subfamilies (C1-C29). Herein, physiological functions of the InR2R3-MYBs are discussed based on the functions of their Arabidopsis orthologues. InR2R3-MYBs in C9, C15, C16 or C28 may regulate cell division, flavonol biosynthesis, anthocyanin biosynthesis or response to abiotic stress, respectively. C16 harbors the known anthocyanin biosynthesis regulator, InMYB1 (INIL00g10723), and putative anthocyanin biosynthesis regulators, InMYB2 (INIL05g09650) and InMYB3 (INIL05g09651). In addition, INIL05g09649, INIL11g40874 and INIL11g40875 in C16 were suggested as novel anthocyanin biosynthesis regulators. We organized the R2R3-MYB transcription factors in the morning glory genome and assigned information to gene and protein structures and presuming their functions. Our study is expected to facilitate future research on R2R3-MYB transcription factors in Japanese morning glory.

Combined Analysis of the Transcriptome and Metabolome Revealed the Mechanism of Petal Coloration in Bauhinia variegata

Bauhinia variegata petals are colorful, rich in anthocyanins, and have ornamental, nutritional, and medicinal value. However, the regulatory mechanism of anthocyanin accumulation in B. variegata remains unclear. In this study, a combined analysis of the metabolome and transcriptome was performed in red and white B. variegata cultivars in the early, middle, and blooming stages. A total of 46 different anthocyanins were identified, of which 27 showed marked differences in accumulation between the two cultivars, and contribute to their different petal colors. Malvidin 3-O-galactoside, peonidin 3-O-galactoside, cyanidin 3-O-glucoside, cyanidin 3-O-galactoside, and malvidin 3-O-glucoside were much more abundant in the second stage of flowering. In the blooming stage, except for the anthocyanins mentioned, delphinidin 3-O-galactoside and petunidin 3-O-galactoside were the most abundant anthocyanins in the red flowers, indicating that malvidin, peonidin, cyanidin, delphinidin, and petunidin were all responsible for the red color of petals in B. variegata. RNA sequencing identified 2,431 differentially expressed genes (DEGs), of which 26 were involved in the anthocyanin synthesis pathway. Correlations between the anthocyanin biosynthesis-related DEGs and anthocyanin contents were explored, and the DEGs involved in anthocyanin accumulation in B. variegata petals were identified. Eighteen of these DEGs encoded key catalytic enzymes, such as anthocyanidin reductase (ANR) and flavonoid-3'5'-hydroxylase (F3'5'H), and 17 of them encoded transcription factors (TFs) belonging to 14 families (including MYB, NAC, SPL, ERF, and CHR28). These results improve our understanding of the roles of anthocyanins, catalytic enzymes, and TFs in B. variegata petal-color expression.

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.

Comparative Metabolomic Analysis Reveals Distinct Flavonoid Biosynthesis Regulation for Leaf Color Development of Cymbidium sinense 'Red Sun'

The colorful leaf is an important ornamental character of Cymbidium sinense (C. sinense), especially the red leaf, which has always been attracted by breeders and consumers. However, little is documented on the formation mechanism of the red leaf of C. sinense. In this study, the changing patterns of flavonoid-related metabolites, corresponding enzyme activities and genes expression in the leaves of C. sinense ‘Red Sun’ from red to yellow and finally to green was investigated. A total of 196 flavonoid-related metabolites including 11 anthocyanins metabolites were identified using UPLC-MS/MS-based approach. In the process of leaf color change, 42 metabolites were identified as having significantly different contents and the content of 28 differential metabolites turned to zero. In anthocyanin biosynthetic pathway, content of all 15 identified metabolites showed downregulation trend in the process of leaf color change. Among the 15 metabolites, the contents of Naringenin chalcone, Pelargonidin O-acetylhexoside and Anthocyanin 3-O-beta-d-glucoside decreased to zero in the green leaf stage. The changing pattern of enzyme activity of 10 enzymes involved in the anthocyanin biosynthetic pathway showed different trends from red leaves that have turned yellow and finally green, while the expression of genes encoding these enzymes was all down-regulated in the process of leaf color change. The results of this study revealed the types of flavonoid-related metabolites and the comprehensive analysis of metabolites content, enzyme activities and genes expression providing a new reference for breeders to improve the leaf color of C. sinense ‘Red Sun’.

Anthocyanin mutants of Japanese and common morning glories exhibit normal proanthocyanidin accumulation in seed coats

Anthocyanin and proanthocyanidin biosynthesis pathways are believed to overlap. This study examined proanthocyanidin accumulation in seed coats of morning glories (Ipomoea nil and I. purpurea) carrying mutations in CHS-D, CHI, and ANS genes encoding chalcone synthase, chalcone isomerase, and anthocyanidin synthase, respectively. Chemical staining revealed that mutants accumulate proanthocyanidin normally. Thus, the tested genes are not essential to proanthocyanidin biosynthesis, but are essential to anthocyanin biosynthesis in flowers and stems. Based on the results and the I. nil draft genome sequence, the genes involved in proanthocyanidin biosynthesis, including a new copy of the flavanone 3-hydroxylase gene could be predicted. Moreover, the genome has no homologs for known enzymes involved in producing flavan-3-ols, the starter and extension units of proanthocyanidin. These results suggested that I. nil produces flavan-3-ols through an undiscovered biosynthesis pathway. To characterize proanthocyanidin pigmentation further, we conducted mutant screening using a large I. nil population. We discovered that the brown mutant lines (exhibiting brown seeds and normal anthocyanin pigmentation) do not accumulate proanthocyanidin in their seed coats. Thus, the brown mutation should be useful for further investigations into the various mechanisms controlling anthocyanin and proanthocyanidin pathways.

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.

Genome sequence and analysis of the Japanese morning glory Ipomoea nil

Ipomoea is the largest genus in the family Convolvulaceae. Ipomoea nil (Japanese morning glory) has been utilized as a model plant to study the genetic basis of floricultural traits, with over 1,500 mutant lines. In the present study, we have utilized second- and third-generation-sequencing platforms, and have reported a draft genome of I. nil with a scaffold N50 of 2.88 Mb (contig N50 of 1.87 Mb), covering 98% of the 750 Mb genome. Scaffolds covering 91.42% of the assembly are anchored to 15 pseudo-chromosomes. The draft genome has enabled the identification and cataloguing of the Tpn1 family transposons, known as the major mutagen of I. nil, and analysing the dwarf gene, CONTRACTED, located on the genetic map published in 1956. Comparative genomics has suggested that a whole genome duplication in Convolvulaceae, distinct from the recent Solanaceae event, has occurred after the divergence of the two sister families.

A Stowaway transposon disrupts the InWDR1 gene controlling flower and seed coloration in a medicinal cultivar of the Japanese morning glory

Floricultural cultivars of the Japanese morning glory (Ipomoea nil) carry transposons of the Tpn1 family as active spontaneous mutagens. Half of the characterized mutations related to floricultural traits were caused by insertion of Tpn1 family elements. In addition, mutations comprising insertions of several bp, presumed to be footprints generated by transposon excisions, were also found. Among these, ca-1 and ca-2 are 7-bp insertions at the same position in the InWDR1 gene, which encodes a multifunctional transcription regulator. InWDR1 enhances anthocyanin pigmentation in blue flowers and red stems, and promotes dark brown seed pigmentation as well as seed-trichome formation. The recessive ca mutants show white flowers and whitish seeds. We characterized here a white flower and whitish seed line that is used as a medicinal herb. The mutant line carries a novel ca allele named ca-3, which is the InWDR1 gene carrying an insertion of a Stowaway-like transposon, InSto1. The ca-3 allele is the first example of a mutation induced by transposons other than those in the Tpn1 family in I. nil. Because InSto1 and the 7-bp putative footprints are inserted at identical positions in InWDR1, ca-3 is likely to be the ancestor of ca-1 and ca-2. According to Japanese historical records on whitish seeds of I. nil, putative ca mutants appeared at the end of the 17th century, at the latest. This is around one hundred years before the appearance of many floricultural mutants. This suggests that ca-3 is one of the oldest mutations, and that its origin is different from that of most floricultural mutations in I. nil.

Spontaneous mutations of the UDP-glucose:flavonoid 3-O-glucosyltransferase gene confers pale- and dull-colored flowers in the Japanese and common morning glories

UDP-glucose:flavonoid 3- O -glucosyltransferase is essential for maintaining proper production quantity, acylation, and glucosylation of anthocyanin, and defects cause pale and dull flower pigmentation in morning glories. The Japanese (Ipomoea nil) and the common (I. purpurea) morning glory display bright blue and dark purple flowers, respectively. These flowers contain acylated and glucosylated anthocyanin pigments, and a number of flower color mutants have been isolated in I. nil. Of these, the duskish mutants of I. nil produce pale- and dull-colored flowers. We found that the Duskish gene encodes UDP-glucose:flavonoid 3-O-glucosyltransferase (3GT). The duskish-1 mutation is a frameshift mutation caused by a 4-bp insertion, and duskish-2 is an insertion of a DNA transposon, Tpn10, at 1.3 kb upstream of the 3GT start codon. In the duskish-2 mutant, excision of Tpn10 is responsible for restoration of the expression of the 3GT gene. The recombinant 3GT protein displays expected 3GT enzymatic activities to catalyze 3-O-glucosylation of anthocyanidins in vitro. Anthocyanin analysis of a duskish-2 mutant and its germinal revertant showing pale and normal pigmented flowers, respectively, revealed that the mutation caused around 80 % reduction of anthocyanin accumulation. We further characterized two I. purpurea mutants showing pale brownish-red flowers, and found that they carry the same frameshift mutation in the 3GT gene. Most of the flower anthocyanins in the mutants were previously found to be anthocyanidin 3-O-glucosides lacking several caffeic acid and glucose moieties that are attached to the anthocyanins in the wild-type plants. These results indicated that 3GT is essential not only for production, but also for proper acylation and glucosylation, of anthocyanin in the morning glories.

De novo transcriptome assembly of Ipomoea nil using Illumina sequencing for gene discovery and SSR marker identification

Ipomoea nil is widely used as an ornamental plant due to its abundance of flower color, but the limited transcriptome and genomic data hinder research on it. Using illumina platform, transcriptome profiling of I. nil was performed through high-throughput sequencing, which was proven to be a rapid and cost-effective means to characterize gene content. Our goal is to use the resulting information to facilitate the relevant research on flowering and flower color formation in I. nil. In total, 268 million unique illumina RNA-Seq reads were produced and used in the transcriptome assembly. These reads were assembled into 220,117 contigs, of which 137,307 contigs were annotated using the GO and KEGG database. Based on the result of functional annotations, a total of 89,781 contigs were assigned 455,335 GO term annotations. Meanwhile, 17,418 contigs were identified with pathway annotation and they were functionally assigned to 144 KEGG pathways. Our transcriptome revealed at least 55 contigs as probably flowering-related genes in I. nil, and we also identified 25 contigs that encode key enzymes in the phenylpropanoid biosynthesis pathway. Based on the analysis relating to gene expression profiles, in the phenylpropanoid biosynthesis pathway of I. nil, the repression of lignin biosynthesis might lead to the redirection of the metabolic flux into anthocyanin biosynthesis. This may be the most likely reason that I. nil has high anthocyanins content, especially in its flowers. Additionally, 15,537 simple sequence repeats (SSRs) were detected using the MISA software, and these SSRs will undoubtedly benefit future breeding work. Moreover, the information uncovered in this study will also serve as a valuable resource for understanding the flowering and flower color formation mechanisms in I. nil.

Molecular characterization of mutations in white-flowered torenia plants

BACKGROUND

Torenia (Torenia fournieri Lind.) is a model plant increasingly exploited in studies in various disciplines, including plant engineering, biochemistry, physiology, and ecology. Additionally, cultivars with different flower colors have been bred and made commercially available. Flower color in torenia is mainly attributed to the accumulation of anthocyanins, but the molecular mechanisms inducing flower color mutations in torenia have not been well elucidated. In this study, we therefore attempted to identify the cause of white coloration in torenia by comparing the white-flowered cultivar Crown White (CrW) with Crown Violet (CrV), a violet-flowered variety.

RESULTS

In an expression analysis, no flavanone 3-hydroxylase (TfF3H) transcript accumulation was detected in CrW petals. Sequence analyses revealed that a novel long terminal repeat (LTR)-type retrotransposable element, designated as TORE1 (Torenia retrotransposon 1), is inserted into the 5'-upstream region of the TfF3H gene in CrW. A transient expression assay using torenia F3H promoters with or without TORE1 insertion showed that the TORE1 insertion substantially suppressed F3H promoter activity, suggesting that this insertion is responsible for the absence of F3H transcripts in white petals. Furthermore, a transformation experiment demonstrated that the introduction of a foreign gentian F3H cDNA, GtF3H, into CrW was able to recover pink-flower pigmentation, indicating that F3H deficiency is indeed the cause of the colorless flower phenotype in CrW. Detailed sequence analysis also identified deletion mutations in flavonoid 3'-hydroxylase (TfF3'H) and flavonoid 3',5'- hydroxylase (TfF3'5'H) genes, but these were not directly responsible for white coloration in this cultivar.

CONCLUSIONS

Taken together, a novel retrotransposable element, TORE1, inserted into the F3H 5'-upstream region is the cause of deficient F3H transcripts in white-flowered torenia, thereby leading to reduced petal anthocyanin levels. This is the first report of a retrotransposable element involved in flower color mutation in the genus Torenia.

A chalcone isomerase-like protein enhances flavonoid production and flower pigmentation

Flavonoids are major pigments in plants, and their biosynthetic pathway is one of the best-studied metabolic pathways. Here we have identified three mutations within a gene that result in pale-colored flowers in the Japanese morning glory (Ipomoea nil). As the mutations lead to a reduction of the colorless flavonoid compound flavonol as well as of anthocyanins in the flower petal, the identified gene was designated enhancer of flavonoid production (EFP). EFP encodes a chalcone isomerase (CHI)-related protein classified as a type IV CHI protein. CHI is the second committed enzyme of the flavonoid biosynthetic pathway, but type IV CHI proteins are thought to lack CHI enzymatic activity, and their functions remain unknown. The spatio-temporal expression of EFP and structural genes encoding enzymes that produce flavonoids is very similar. Expression of both EFP and the structural genes is coordinately promoted by genes encoding R2R3-MYB and WD40 family proteins. The EFP gene is widely distributed in land plants, and RNAi knockdown mutants of the EFP homologs in petunia (Petunia hybrida) and torenia (Torenia hybrida) had pale-colored flowers and low amounts of anthocyanins. The flavonol and flavone contents in the knockdown petunia and torenia flowers, respectively, were also significantly decreased, suggesting that the EFP protein contributes in early step(s) of the flavonoid biosynthetic pathway to ensure production of flavonoid compounds. From these results, we conclude that EFP is an enhancer of flavonoid production and flower pigmentation, and its function is conserved among diverse land plant species.

A WD40-repeat protein controls proanthocyanidin and phytomelanin pigmentation in the seed coats of the Japanese morning glory

The protein complex composed of the transcriptional regulators containing R2R3-MYB domains, bHLH domains, and WDR in plants controls various epidermal traits, including anthocyanin and proanthocyanidin pigmentation, trichome and root hair formation, and vacuolar pH. In the Japanese morning glory (Ipomoea nil), InMYB1 having R2R3-MYB domains and InWDR1 containing WDR were shown to regulate anthocyanin pigmentation in flowers, and InWDR1 was reported to control dark-brown pigmentation and trichome formation on seed coats. Here, we report that the seed pigments of I. nil mainly comprise proanthocyanidins and phytomelanins and that these pigments are drastically reduced in the ivory seed coats of an InWDR1 mutant. In addition, a transgenic plant of the InWDR1 mutant carrying the active InWDR1 gene produced dark-brown seeds, further confirming that InWDR1 regulates seed pigmentation. Early steps in anthocyanin and proanthocyanidin biosynthetic pathways are thought to be common. In the InWDR1 mutant, none of the structural genes for anthocyanin biosynthesis that showed reduced expression in the white flowers were down-regulated in the ivory seeds, which suggests that InWDR1 may activate different sets of the structural genes for anthocyanin biosynthesis in flowers and proanthocyanidin production in seeds. As in the flowers, however, we noticed that the expression of InbHLH2 encoding a bHLH regulator was down-regulated in the seeds of the InWDR1 mutant. We discuss the implications of these results with respect to the proanthocyanidin biosynthesis in the seed coats.

Simultaneous post-transcriptional gene silencing of two different chalcone synthase genes resulting in pure white flowers in the octoploid dahlia

Garden dahlias (Dahlia variabilis) are autoallooctoploids with redundant genes producing wide color variations in flowers. There are no pure white dahlia cultivars, despite its long breeding history. However, the white areas of bicolor flower petals appear to be pure white. The objective of this experiment was to elucidate the mechanism by which the pure white color is expressed in the petals of some bicolor cultivars. A pigment analysis showed that no flavonoid derivatives were detected in the white areas of petals in a star-type cultivar 'Yuino' and the two seedling cultivars 'OriW1' and 'OriW2' borne from a red-white bicolor cultivar, 'Orihime', indicating that their white areas are pure white. Semi-quantitative RT-PCR showed that in the pure white areas, transcripts of two chalcone synthases (CHS), DvCHS1 and DvCHS2 which share 69% nucleotide similarity with each other, were barely detected. Premature mRNA of DvCHS1 and DvCHS2 were detected, indicating that these two CHS genes are silenced post-transcriptionally. RNA gel blot analysis revealed that small interfering RNAs (siRNAs) derived from CHSs were produced in these pure white areas. By high-throughput sequence analysis of small RNAs in the pure white areas with no mismatch acceptance, small RNAs were mapped to two alleles of DvCHS1 and two alleles of DvCHS2 expressed in 'Yuino' petals. Therefore, we concluded that simultaneous siRNA-mediated post-transcriptional gene silencing of redundant CHS genes results in the appearance of pure white color in dahlias.