Distribution of cell layers in floral organs of chrysanthemum analyzed with periclinal chimeras carrying a transgene encoding fluorescent protein.

KEY MESSAGE: A fluorescent protein visualized distributions of cell layers in floral organs of chrysanthemum using transgenic periclinal chimeras carrying a gene encoding a fluorescent compound. Plant meristems have three cell layers: the outermost layer (L1), the second layer (L2), and the inner layer (L3). The layers are maintained during development but there is limited knowledge of the details of cell layer patterns within floral organs. In this study, we visualized the distributions of cell layers in floral organs of chrysanthemum using periclinal chimeras carrying a gene encoding a fluorescent compound in the L1 or the L2/L3 layers. The L1 layer contributed most of the epidermal cells of organs including the receptacle, petal, anther, filament, style, stigma, and ovule. The transmitting tissue in the pistil and most of the internal area of the ovule were also derived from the L1. In crossing experiments, no progeny of the L1-chimeric plants showed fluorescence, indicating that the germ cells of chrysanthemum are not derived from the L1 layer. Since anthocyanin pigment is present only in the L1-derived epidermal cells of petals, L1-specific gene integration could be used to alter flower color in commercial cultivars, with a reduced risk of transgene flow from the transgenic chrysanthemums to wild relatives.

Functional characterization of UDP-rhamnose-dependent rhamnosyltransferase involved in anthocyanin modification, a key enzyme determining blue coloration in Lobelia erinus.

Because structural modifications of flavonoids are closely related to their properties, such as stability, solubility, flavor and coloration, characterizing the enzymes that catalyze the modification reactions can be useful for engineering agriculturally beneficial traits of flavonoids. In this work, we examined the enzymes involved in the modification pathway of highly glycosylated and acylated anthocyanins that accumulate in Lobelia erinus. Cultivar Aqua Blue (AB) of L. erinus is blue-flowered and accumulates delphinidin 3-O-p-coumaroylrutinoside-5-O-malonylglucoside-3'5'-O-dihydroxycinnamoylglucoside (lobelinins) in its petals. Cultivar Aqua Lavender (AL) is mauve-flowered, and LC-MS analyses showed that AL accumulated delphinidin 3-O-glucoside (Dp3G), which was not further modified toward lobelinins. A crude protein assay showed that modification processes of lobelinin were carried out in a specific order, and there was no difference between AB and AL in modification reactions after rhamnosylation of Dp3G, indicating that the lack of highly modified anthocyanins in AL resulted from a single mutation of rhamnosyltransferase catalyzing the rhamnosylation of Dp3G. We cloned rhamnosyltransferase genes (RTs) from AB and confirmed their UDP-rhamnose-dependent rhamnosyltransferase activities on Dp3G using recombinant proteins. In contrast, the RT gene in AL had a 5-bp nucleotide deletion, resulting in a truncated polypeptide without the plant secondary product glycosyltransferase box. In a complementation test, AL that was transformed with the RT gene from AB produced blue flowers. These results suggest that rhamnosylation is an essential process for lobelinin synthesis, and thus the expression of RT has a great impact on the flower color and is necessary for the blue color of Lobelia flowers.

Recent advances in the research and development of blue flowers.

Flower color is the most important trait in the breeding of ornamental plants. In the floriculture industry, however, bluish colored flowers of desirable plants have proved difficult to breed. Many ornamental plants with a high production volume, such as rose and chrysanthemum, lack the key genes for producing the blue delphinidin pigment or do not have an intracellular environment suitable for developing blue color. Recently, it has become possible to incorporate a blue flower color trait through progress in molecular biological analysis of pigment biosynthesis genes and genetic engineering. For example, introduction of the F3'5'H gene encoding flavonoid 3',5'-hydroxylase can produce delphinidin in various flowers such as roses and carnations, turning the flower color purple or violet. Furthermore, the world's first blue chrysanthemum was recently produced by introducing the A3'5'GT gene encoding anthocyanin 3',5'-O-glucosyltransferase, in addition to F3'5'H, into the host plant. The B-ring glucosylated delphinidin-based anthocyanin that is synthesized by the two transgenes develops blue coloration by co-pigmentation with colorless flavone glycosides naturally present in the ray floret of chrysanthemum. This review focuses on the biotechnological efforts to develop blue flowers, and describes future prospects for blue flower breeding and commercialization.

Genetic engineering of novel bluer-colored chrysanthemums produced by accumulation of delphinidin-based anthocyanins.

Chrysanthemums (Chrysanthemum morifolium Ramat.) have no purple-, violet- or blue-flowered cultivars because they lack delphinidin-based anthocyanins. This deficiency is due to the absence of the flavonoid 3',5'-hydroxylase gene (F3'5'H), which encodes the key enzyme for delphinidin biosynthesis. In F3'5'H-transformed chrysanthemums, unpredictable and unstable expression levels have hampered successful production of delphinidin and reduced desired changes in flower color. With the aim of achieving delphinidin production in chrysanthemum petals, we found that anthocyanin biosynthetic gene promoters combined with a translational enhancer increased expression of some F3'5'H genes and accompanying delphinidin-based anthocyanin accumulation in transgenic chrysanthemums. Dramatic accumulation of delphinidin (up to 95%) was achieved by simple overexpression of Campanula F3'5'H controlled by a petal-specific flavanone 3-hydroxylase promoter from chrysanthemum combined with the 5'-untranslated region of the alcohol dehydrogenase gene as a translational enhancer. The flower colors of transgenic lines producing delphinidin-based anthocyanins changed from a red-purple to a purple-violet hue in the Royal Horticultural Society Colour Charts. This result represents a promising step toward molecular breeding of blue chrysanthemums.

Crystal structure of UDP-glucose:anthocyanidin 3-O-glucosyltransferase from Clitoria ternatea.

Flowers of the butterfly pea (Clitoria ternatea) accumulate a group of polyacylated anthocyanins, named ternatins, in their petals. The first step in ternatin biosynthesis is the transfer of glucose from UDP-glucose to anthocyanidins such as delphinidin, a reaction catalyzed in C. ternatea by UDP-glucose:anthocyanidin 3-O-glucosyltransferase (Ct3GT-A; AB185904). To elucidate the structure-function relationship of Ct3GT-A, recombinant Ct3GT-A was expressed in Escherichia coli and its tertiary structure was determined to 1.85 Å resolution by using X-ray crystallography. The structure of Ct3GT-A shows a common folding topology, the GT-B fold, comprised of two Rossmann-like ß/α/ß domains and a cleft located between the N- and C-domains containing two cavities that are used as binding sites for the donor (UDP-Glc) and acceptor substrates. By comparing the structure of Ct3GT-A with that of the flavonoid glycosyltransferase VvGT1 from red grape (Vitis vinifera) in complex with UDP-2-deoxy-2-fluoro glucose and kaempferol, locations of the catalytic His-Asp dyad and the residues involved in recognizing UDP-2-deoxy-2-fluoro glucose were essentially identical in Ct3GT-A, but certain residues of VvGT1 involved in binding kaempferol were found to be substituted in Ct3GT-A. These findings are important for understanding the differentiation of acceptor-substrate recognition in these two enzymes.

Combination of long-read and short-read sequencing provides comprehensive transcriptome and new insight for Chrysanthemum morifolium ray-floret colorization.

Chrysanthemum morifolium is one of the most popular ornamental plants globally. Owing to its large and complex genome (around 10 Gb, segmental hexaploid), it has been difficult to obtain comprehensive transcriptome, which will promote to perform new breeding technique, such as genome editing, in C. morifolium. In this study, we used single-molecule real-time (SMRT) sequencing and RNA-seq technologies, combined them with an error-correcting process, and obtained high-coverage ray-floret transcriptome. The SMRT-seq data increased the ratio of long mRNAs containing complete open-reading frames, and the combined dataset provided a more complete transcriptomic data than those produced from either SMRT-seq or RNA-seq-derived transcripts. We finally obtained 'Sei Arabella' transcripts containing 928,645 non-redundant mRNA, which showed 96.6% Benchmarking Universal Single-Copy Orthologs (BUSCO) score. We also validated the reliability of the dataset by analyzing a mapping rate, annotation and transcript expression. Using the dataset, we searched anthocyanin biosynthesis gene orthologs and performed a qRT-PCR experiment to assess the usability of the dataset. The assessment of the dataset and the following analysis indicated that our dataset is reliable and useful for molecular biology. The combination of sequencing methods provided genetic information and a way to analyze the complicated C. morifolium transcriptome.

Identification of anthocyanin and other flavonoids from the green-blue petals of Puya alpestris (Bromeliaceae) and a clarification of their coloration mechanism.

To understand the unique green-blue color of Puya alpestris (Bromeliaceae) flowers, we investigated their constituent anthocyanin and related compounds. An anthocyanin, two undescribed flavonols, and two flavones were isolated and identified as delphinidin 3,3',5'-tri-O-ß-glucopyranoside, myricetin 3-O-[α-rhamnopyranosyl-(1 â†’ 6)-ß-glucopyranoside]-3',5'-di-O-ß-glucopyranoside, myricetin 3,3',5'-tri-O-ß-glucopyranoside, luteolin 4'-O-glucoside, and apigenin 4'-O-glucoside. Furthermore, the presence of chlorophyll has also been revealed. P. alpestris petals show a gradient color appearance: Green-blue at the tip and blue at the base. This color difference between the tip and base was used to analyze the pigment components underlying the green-blue color expression. It was found that the petal tip contains the anthocyanin, flavonols, flavones, and chlorophyll in high quantities. Furthermore, the pH of petal juice was 6.2 and 5.6 at the tip and base, respectively. In vitro reconstruction revealed the blue color expression occurred via an intermolecular copigmentation between the anthocyanin and flavones, as well as yellow color expression, which was due to an increase in the absorption at 400-450 nm of the flavonols under the higher pH conditions. Furthermore, we found that the petal extract obtaining using 50% acetone containing chlorophyll showed the same absorption spectrum as that observed for the raw petal. These results indicate that the green-blue color of P. alpestris flowers is developed via an intermolecular co-pigmentation of the anthocyanin (delphinidin 3,3',5'-tri-O-ß-glucopyranoside) with flavones, such as luteolin 4'-O-glucoside, the yellow color expression of flavonols, such as myricetin 3,3',5'-tri-O-glucoside under relatively high pH conditions in the cell sap, and the presence of chlorophyll.

Chimerism of chrysanthemum stems changes at the nodes during vegetative growth.

The shoot apical meristem (SAM) is typically divided into three cell layers: the outermost epidermal layer (L1), the subepidermal layer (L2) and the inner corpus region (L3). Structures within the cell layers are normally maintained throughout development; however, through vegetative propagation of a periclinal chimeric chrysanthemum expressing a fluorescent protein gene only in the L1 layer, we collected twelve independent shoots that had partially mosaic fluorescent inner cells (L2, L3) in addition to fluorescent epidermal cells (L1). Furthermore, the elongated tissues of nine shoots out of the twelve had no internal fluorescent cells, i.e., they had the original L1 chimerism. Observations of the fluorescence distribution suggested that the change in chimerism occurred at the nodes, indicating previously unnoticed cell layer dynamics occurring at the nodes.

Biosynthesis of malonylated flavonoid glycosides on the basis of malonyltransferase activity in the petals of Clitoria ternatea.

The crude malonyltransferase from the petals of Clitoria ternatea was characterized enzymatically to investigate its role on the biosynthetic pathways of anthocyanins and flavonol glycosides. In C. ternatea, a blue flower cultivars (DB) and mauve flower variety (WM) accumulate polyacylated anthocyanins (ternatins) and delphinidin 3-O-(6''-O-malonyl)-beta-glucoside which is one of the precursors of ternatins, respectively. Moreover, WM accumulates minor delphinidin glycosides - 3-O-beta-glucoside, 3-O-(2''-O-alpha-rhamnosyl)-beta-glucoside, 3-O-(2''-O-alpha-rhamnosyl-6''-O-malonyl)-beta-glucoside of delphinidin. These glycosidic patterns for minor anthocyanins in WM are also found among the minor flavonol glycosides in all the varieties including a white flower variety (WW) although the major flavonol glycosides are 3-O-(2''-O-alpha-rhamnosyl)-beta-glucoside, 3-O-(6''-O-alpha-rhamnosyl)-beta-glucoside, 3-O-(2'',6''-di-O-alpha-rhamnosyl)-beta-glucoside of kaempferol, quercetin, and myricetin. How do the enzymatic characteristics affect the variety of glycosidic patterns in the flavonoid glycoside biosynthesis among these varieties? While the enzyme from DB highly preferred delphinidin 3-O-beta-glucoside in the presence of malonyl-CoA, it also has a preference for other anthocyanidin 3-O-beta-glucosides. It could use flavonol 3-O-beta-glucosides in much lower specific activities than anthocyanins; however, it could not utilize 3-O-(2''-O-alpha-rhamnosyl)-beta-glucosides of anthocyanins and flavonols, and 3,3'-di- and 3,3',5'-tri-O-beta-glucoside of delphinidin - other possible precursors in ternatins biosynthesis. It highly preferred malonyl-CoA as an acyl donor in the presence of delphinidin 3-O-beta-glucoside. The crude enzymes prepared from WM and WW had the same enzymatic characteristics. These results suggested that 3-O-(2''-O-alpha-rhamnosyl-6''-O-malonyl)-beta-glucosides of flavonoids were synthesized via 3-O-(6''-O-malonyl)-beta-glucosides rather than via 3-O-(2''-O-alpha-rhamnosyl)-beta-glucosides, and that malonylation proceeded prior to glucosylation at the B-ring of delphinidin in the early biosynthetic steps towards ternatins. It seemed that the substrate specificities largely affected the difference in the accumulated amount of malonylated glycosides between anthocyanins and flavonols although they are not simply proportional to the accumulation ratio. This enzyme might join in the production of both malonylanthocyanins and flavonol malonylglycosides as a result of broad substrate specificities towards flavonoid 3-O-beta-glucosides.

Generation of blue chrysanthemums by anthocyanin B-ring hydroxylation and glucosylation and its coloration mechanism.

Various colored cultivars of ornamental flowers have been bred by hybridization and mutation breeding; however, the generation of blue flowers for major cut flower plants, such as roses, chrysanthemums, and carnations, has not been achieved by conventional breeding or genetic engineering. Most blue-hued flowers contain delphinidin-based anthocyanins; therefore, delphinidin-producing carnation, rose, and chrysanthemum flowers have been generated by overexpression of the gene encoding flavonoid 3',5'-hydroxylase (F3'5'H), the key enzyme for delphinidin biosynthesis. Even so, the flowers are purple/violet rather than blue. To generate true blue flowers, blue pigments, such as polyacylated anthocyanins and metal complexes, must be introduced by metabolic engineering; however, introducing and controlling multiple transgenes in plants are complicated processes. We succeeded in generating blue chrysanthemum flowers by introduction of butterfly pea UDP (uridine diphosphate)-glucose:anthocyanin 3',5'-O-glucosyltransferase gene, in addition to the expression of the Canterbury bells F3'5'H. Newly synthesized 3',5'-diglucosylated delphinidin-based anthocyanins exhibited a violet color under the weakly acidic pH conditions of flower petal juice and showed a blue color only through intermolecular association, termed "copigmentation," with flavone glucosides in planta. Thus, we achieved the development of blue color by a two-step modification of the anthocyanin structure. This simple method is a promising approach to generate blue flowers in various ornamental plants by metabolic engineering.

Structural basis for acceptor-substrate recognition of UDP-glucose: anthocyanidin 3-O-glucosyltransferase from Clitoria ternatea.

UDP-glucose: anthocyanidin 3-O-glucosyltransferase (UGT78K6) from Clitoria ternatea catalyzes the transfer of glucose from UDP-glucose to anthocyanidins such as delphinidin. After the acylation of the 3-O-glucosyl residue, the 3'- and 5'-hydroxyl groups of the product are further glucosylated by a glucosyltransferase in the biosynthesis of ternatins, which are anthocyanin pigments. To understand the acceptor-recognition scheme of UGT78K6, the crystal structure of UGT78K6 and its complex forms with anthocyanidin delphinidin and petunidin, and flavonol kaempferol were determined to resolutions of 1.85 Å, 2.55 Å, 2.70 Å, and 1.75 Å, respectively. The enzyme recognition of unstable anthocyanidin aglycones was initially observed in this structural determination. The anthocyanidin- and flavonol-acceptor binding details are almost identical in each complex structure, although the glucosylation activities against each acceptor were significantly different. The 3-hydroxyl groups of the acceptor substrates were located at hydrogen-bonding distances to the Nε2 atom of the His17 catalytic residue, supporting a role for glucosyl transfer to the 3-hydroxyl groups of anthocyanidins and flavonols. However, the molecular orientations of these three acceptors are different from those of the known flavonoid glycosyltransferases, VvGT1 and UGT78G1. The acceptor substrates in UGT78K6 are reversely bound to its binding site by a 180° rotation about the O1-O3 axis of the flavonoid backbones observed in VvGT1 and UGT78G1; consequently, the 5- and 7-hydroxyl groups are protected from glucosylation. These substrate recognition schemes are useful to understand the unique reaction mechanism of UGT78K6 for the ternatin biosynthesis, and suggest the potential for controlled synthesis of natural pigments.

Malonylated flavonol glycosides from the petals of Clitoria ternatea.

Three flavonol glycosides, kaempferol 3-O-(2"-O-alpha-rhamnosyl-6"-O-malonyl)-beta-glucoside, quercetin 3-O-(2"-O-alpha-rhamnosyl-6"-O-malonyl)-beta-glucoside, and myricetin 3-O-(2",6"-di-O-alpha-rhamnosyl)-beta-glucoside were isolated from the petals of Clitoria ternatea cv. Double Blue, together with eleven known flavonol glycosides. Their structures were identified using UV, MS, and NMR spectroscopy. They were characterized as kaempferol and quercetin 3-(2(G)- rhamnosylrutinoside)s, kaempferol, quercetin, and myricetin 3-neohesperidosides, 3-rutinosides, and 3-glucosides in the same tissue. In addition, the presence of myricetin 3-O-(2"-O-alpha-rhamnosyl-6"-O-malonyl)-beta-glucoside was inferred from LC/MS/MS data for crude petal extracts. The flavonol compounds identified in the petals of C. ternatea differed from those reported in previous studies.

Flavonoid composition related to petal color in different lines of Clitoria ternatea.

Flavonoids in the petals of several C. ternatea lines with different petal colors were investigated with LC/MS/MS. Delphinidin 3-O-(2"-O-alpha-rhamnosyl-6"-O-malonyl)-beta-glucoside was newly isolated from the petals of a mauve line (wm) together with three known anthocyanins. They were identified structurally using UV, MS, and NMR spectroscopy. Although ternatins, a group of 15 (poly)acylated delphinidin glucosides, were identified in all the blue petal lines (WB, BM-1, 'Double Blue' and 'Albiflora'), WM accumulated delphinidin 3-O-(6"-O-malonyl)-beta-glucoside instead. The white petal line (WW) did not contain anthocyanins. Quantitative data showed that the total anthocyanin contents in WB and 'Double Blue' were ca. 8- and 10-fold higher than that in BM-1, a bud mutant of 'Double Blue', respectively. The total anthocyanin content in 'Albiflora' was less than 2 x 10(-3) times those in WB or 'Double Blue'. While all the lines contained the same set of 15 flavonol glycosides in similar relative ratios, the relative ratio of myricetin glycosides in ww and 'Albiflora' was ca. 30-70 times greater than those in the other lines. The change in flower color from blue to mauve was not due to a change in the structure of an anthocyanidin from delphinidin, but to the lack of (polyacylated) glucosyl group substitutions at both the 3'- and 5'-positions of ternatins. This implies that glucosylation at the 3'- and 5'-positions of anthocyanin is a critical step in producing blue petals in C. ternatea.

Identification of delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3'-O-beta-glucoside, a postulated intermediate in the biosynthesis of ternatin C5 in the blue petals of Clitoria ternatea (butterfly pea).

Ternatins are blue anthocyanins found in the petals of Clitoria ternata (butterfly pea). Among them, ternatin C5 (delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3',5'-di-O-beta-glucoside; 2) has the structure common to all the ternatins, which is characterized by its glucosylation pattern: a 3,3',5'-triglucosylated anthocyanidin. In the course of studying biosynthetic pathways of ternatins, the key enzymatic activities to produce ternatin C5 were discovered in a crude enzyme preparation from the petals of a blue petal line of C. ternatea. When this preparation was tested for activity against several delphinidin glycosides, delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3'-O-beta-glucoside (6), a postulated intermediate, was found in the reaction mixture, together with three known anthocyanins, which were spectroscopically structurally identified. As a result of structural identification, the following enzymatic activities were identified: UDP-glucose :delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3'-O-beta-glucoside 5'-O-glucosyltransferase (5'GT), UDP-glucose :delphinidin 3-O-(6''-O-malonyl)-beta-glucoside 3'-O-glucosyltransferase (3'GT), UDP-glucose :delphinidin 3-O-glucosyltransferase, and malonyl-CoA :delphinidin 3-O-beta-glucoside 6''-malonyltransferase. In a mauve petal line, which did not accumulate ternatins but delphinidin 3-O-(6''-O-malonyl)-beta-glucoside in its petal, there were neither 5'GT nor 3'GT activities. Thus, the early biosynthetic pathway of ternatins may be characterized by the stepwise transfer of two glucose residues to 3'- and 5'-position of delphinidin 3-O-(6''-O-malonyl)-beta-glucoside (1; Scheme) from UDP-glucose.

Purification and characterization of UDP-glucose: anthocyanin 3',5'-O-glucosyltransferase from Clitoria ternatea.

A UDP-glucose: anthocyanin 3',5'-O-glucosyltransferase (UA3'5'GT) (EC 2.4.1.-) was purified from the petals of Clitoria ternatea L. (Phaseoleae), which accumulate polyacylated anthocyanins named ternatins. In the biosynthesis of ternatins, delphinidin 3-O-(6''-O-malonyl)-beta-glucoside (1) is first converted to delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3'-O-beta-glucoside (2). Then 2 is converted to ternatin C5 (3), which is delphinidin 3-O-(6''-O-malonyl)-beta-glucoside-3',5'-di-O-beta-glucoside. UA3'5'GT is responsible for these two steps by transferring two glucosyl groups in a stepwise manner. Its substrate specificity revealed the regioselectivity to the anthocyanin's 3'- or 5'-OH groups. Its kinetic properties showed comparable k (cat) values for 1 and 2, suggesting the subequality of these anthocyanins as substrates. However, the apparent Km value for 1 (3.89 x 10(-5) M), which is lower than that for 2 (1.38 x 10(-4) M), renders the k(cat)/Km value for 1 smaller, making 1 catalytically more efficient than 2. Although the apparent Km value for UDP-glucose (6.18 x 10(-3) M) with saturated 2 is larger than that for UDP-glucose (1.49 x 10(-3) M) with saturated 1, the k(cat) values are almost the same, suggesting the UDP-glucose binding inhibition by 2 as a product. UA3'5'GT turns the product 2 into a substrate possibly by reversing the B-ring of 2 along the C2-C1' single bond axis so that the 5'-OH group of 2 can point toward the catalytic center.