Basics of blue flower colouration

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Chemical structures of the three main types of anthocyanins Figure 1 Chemical structures of the three main types of anthocyanins.jpg
Chemical structures of the three main types of anthocyanins

Blue flower colour was always associated with something unusual and desired. Blue roses especially were assumed to be a dream that cannot be realised. Blue colour in flower petals is caused by anthocyanins, which are members of flavonoid class metabolites. We can diversify three main classes of anthocyanin pigments: cyaniding type (two hydroxyl groups in the B-ring) responsible for red coloration, pelargonidin type (one hydroxyl group in the B-ring) responsible for orange colour and delphinidin type (three hydroxyl groups in the B-ring) responsible for violet/blue flower and fruits coloration. [1] The main difference in the structure of listed anthocyanins type is the number of hydroxyl groups in the B-ring of the anthocyanin. Nevertheless, in the monomeric state anthocyanins never show blue colour in the weak acidic and neutral pH. The mechanism of blue colour formation are very complicated in most cases, presence of delphinidin type pigments is not sufficient, great role play also the pH and the formation of complexes of anthocyanins with flavones and metal ions. [2]

Contents

Mechanisms

Mechanisms of blue colour formation Mechanisms of blue colour formation.png
Mechanisms of blue colour formation

Self-association is correlated with the anthocyanin concentration. When concentration is higher we can observe change in the absorbance maximum and increase of colour intensity. Molecules of anthocyanins associate together what results in stronger and darker colour. [3] [4]

Co-pigmentation stabilizes and gives protection to anthocyanins in the complexes. Co-pigments are colourless or have slightly yellow colour. Co-pigments usually are flavonoids (flavones, flavonols, flavanons, flavanols), other polyphenols, alkaloids, amino acids or organic acids. The most efficient co-pigments are flavonols like rutin or quercetin and phenolic acids like sinapic acid or ferulic acid. [5] Association of co-pigment with anthocyanin causes bathochromic effect, shift in absorption maximum to higher wavelength, in result we can observe change of the colour from red to blue. [3] This phenomenon is also called bluing effect. We can diversify two types of co-pigmentation, intermolecular and intramolecular. In the first type co-pigment is bound to anthocyanin by non-covalent bond (hydrogen bond, hydrophobic interaction, ionic interaction). [5] In the second type, we can observe covalent acylation of the glycosyl moiety of anthocyanin. Intramolecular co-pigmentation has stronger effect on the colour. This type of protective stacking is called also a sandwich type stacking and is a very common mechanism in the formation of blue flower colour. [6] Tirantin present in Clitoria ternatea (butterfly pea) and phacelianin present in Phacelia campanularia are examples of pigments with intramolecular co-pigmentation. [2]

Flower of Clitoria ternatea (left) and Phacelia campanularia (right) Figure 3 A - flower of butterfly pea (Cliteria ternatea), B - flower of Phacelia campanularia.png
Flower of Clitoria ternatea (left) and Phacelia campanularia (right)

Metal complexation: Often blue flower colour is correlated with presence of anthocyanins in the complexes with metals ions. Metalloanthocyanins are composed from anthocyanins, flavones and metal ions in the stochiometric amounts of 6:6:2, respectively. [2] Typical metals in anthocyanin complexes are iron (Fe), Magnesium (Mg), aluminium (Al), copper (Cu), potassium (K) and tin (Sn). Anthocyanins that can form complexes with metal ions are only cyanidin or delphinidin type, because at least two free hydroxyl groups in the B-ring are necessary to be present. [7]

Examples of metalloanthocyanins: Commelinin isolated from Commelina communis contains malonylawobanin (delphinidin type), flavocommelin (as a co-pigment) and magnesium ions in stochiometric ratio 6:6:2. [2]

Commelina communis Figure 4. Commelia communis.jpg
Commelina communis

Protocyanin is a blue pigment of cornflower ( Centaurea cyanus ). It is composed from succinylcyanin (anthocyanin), malonylflavone (co-pigment), iron and magnesium anions, and two calcium ions to stabilize the complex. Interesting is that the same anthocyanin when is not in complex with the metal ions is present in red rose petals. [8]

Cornflower (Centaurea cyanus) Figure 5 Cornflower (Cantaurea cyanis).jpg
Cornflower ( Centaurea cyanus )

Fuzzy metal complex pigments: In the blue coloured flowers, much often instead than metalloanthocyanins we can find non – stoichiometric metal – complex pigments stabilized by co-pigmentation. Those pigments show blue colour only in aqueous solution and are less stable than metalloanthocyanins. [2] Example of this kind of pigment is present in hydrangea sepals. Main anthocyanin here is delphinidin-3-glucoside what should result in the blue flower formation, but cultivars with red and pink flowers are also present. It is known that acidification of soil can cause change of the hydrangea flower colour from red/pink to blue/violet. Explanation of this phenomenon we can find in the molar ratio of co-pigment (acylquninic acid) to anthocyanin, which is much higher in the blue cells, also the level of Al3+ ions is higher in the blue flowers. Additionally, the pH of blue cell is around 4,1 and red is lower around 3,3. This supramolecule is relatively unstable and easily can fall apart in result of change of component concentration or pH conditions, so this can explain why blue colour in hydrangea sepals has low stability. [2]

Hydrangea with different flower colours Figure 6 Hydrangea with different flower colours.jpg
Hydrangea with different flower colours

Vacuolar pH influence on flower colour: pH theory was the first concept, that tried to explain the mystery of blue colour formation in flower petals. First observation showed that cyanine extracted from blue cornflower changes the colour in aqueous solution in different pH. In the acidic range pigment was red but in alkaline solution was blue. It leads to conclusion that increase of pH in cell vacuole should cause increase of blue coloration. This phenomenon we can observe in the morning glory (Ipomoea tricolor) and Japanese blue morning glory (Ipomoea nil) petals. During the flower development we can observe change of the flower colour form purple to blue. Morning glory has just one delphinidin type anthocyanin and the composition of it does not change during the flower development, but change of the colour is caused by increase of pH in vacuole of coloured cells from 6.6 in buds to 7.7 in fully matured flowers. During the early stage of development acidic pH is maintained by proton pumps, on the latter stage K+/H+ exchanger is responsible for vacuole alkalization. [9] [10]

Morning glory (Ipomoea tricolor) with different stages of flower development, buds have pink colour when matured flower are blue Figure 7 Morning glory (Ipomoea tricolor) whit different stages of flower development, buds have pink colour when matured flower are blue.jpg
Morning glory (Ipomoea tricolor) with different stages of flower development, buds have pink colour when matured flower are blue

Molecular basis

The anthocyanin biosynthesis pathway is now well known and most of the enzymes are characterised. In the formation of blue pigments a few enzymes have particularly important roles, in particular flavonoid 3'5'-hydroxylase (F3'5'H) and dihydroflavonol 4-reductase (DFR).

The flavonoid 3'5'H-hydroxylase is responsible for the introduction of the second and third hydroxyl group in the B-ring of dihydrokaempferol (DHK) or naringenin which are regarded as the main substrates of the reaction. Product of the reaction with DHK is dihydromyricetin (DHM), precursor for synthesis of all delphinidin type anthocyanin. Enzyme is a member of cytochrome P450 protein family (P450s). It is a very diverse group of heme-containing oxidases, which catalyse NADPH- or NADH-dependent oxidation. F3'5'H was classified into CYP75A subfamily. [11] This enzyme this is regarded as necessary for the blue pigment formation.

Dihydroflavonol 4-reductase is the oxidoreductase that catalyses in the presence of NADPH the stereospecific reduction of the keto group in position 4 of dihydroflavonols producing colourless leucoanthocyanidins as a precursor for anthocyanin formation. Enzyme can show substrate specificity with respect to the B-ring hydroxylation pattern of the dihydroflavonol and can therefore have an influence on the type of formed anthocyanin. For the blue pigment formation, necessary is enzyme, which accept dihydromyricetin (DHM) as a substrate. Product of DFR reaction with DHM in the following steps of the pathway is converted to delphinidin type blue pigments. [12]

Cultivation

In some very economically important flowers like roses, carnations and chrysanthemums despite a lot of efforts was not possible to breed the flowers with blue petals coloration. The lack of F3'5'H enzyme and hence delphinidin type anthocyanin is the reason why blue flower colour was not possible to obtain.[ citation needed ]

Blue carnations

Purple carnation cultivar Figure 8 Example of purple carnation cultivar.jpg
Purple carnation cultivar

Delphinidin accumulating carnations (Dianthus caryophyllus) were obtained by overexpression of petunia F3'5'H and DFR in the cultivars, without endogenous DFR activity. As a result, a few cultivars with different purple hue of the flowers were generated. [13] [14]

Blue roses

Blue rose cultivar "Applause" Figure 9 Example of blue rose cultivar.jpg
Blue rose cultivar "Applause"

Roses are especially difficult to obtain blue/violet flower colour. Lack of F3'5'H and unfavourable vacuole pH were the main obstacles. A lot of cultivars were screened to choose the proper one for genetic modification. Finally, flowers with violet/blue hues were obtained by overexpression of viola F3'5'H, down regulation of endogenous DFR and in the same time overexpression of iris (Iris x hollandica) DFR. In the result of those modifications flowers accumulate almost exclusively delphinidin type pigments in the petals. [15]

Additionally, hybridizers throughout the history of rose cultivation have made several strides toward producing rose varieties with colors in the lavender, violet, and mauve color families.[ citation needed ]

These colors were the height of fashion especially in the late 1800s, when the industrial revolution made synthetic color pigments inexpensive and widely available for the first time. Bright shades of royal purple, mauve, and blue naturally became extremely popular and fashionable because these colors previously were only available to extremely wealthy people. The pigments to create these colors were previously very scarce.[ citation needed ]

Several classes of garden rose were created with magenta and violet flowers during this time period to reflect the growing trend in popularity of these previously luxurious and rare colors.[ citation needed ]

Notable examples of popular historical roses which are still grown today by rosarians include the gallica rose 'Cardinal de Richelui' ; the hybrid perpetual 'Reine des violettes', and the setigera rambler rose 'Veilchenblau'.[ citation needed ]

Their violet flower colors are due to complexes of cyanidin with sugar molecules, metal complexes, and tannins naturally present in the flower petals.[ citation needed ]

In the early 1900s, a new species of rose called Rosa foetida 'bicolor' was introduced into world commerce from the middle east. It has flowers which are bright shades of butter yellow, orange, and velvety blood red, which introduced new genetic traits that serendipitously and coincidentally created a pathway toward new lavender and blue pigments that are independent of delphinidin, a blue pigment not naturally found in roses.[ citation needed ]

That pathway involves the final stages of flavanoid pigment synthesis which would normally cause flowers to appear yellow or orange. This new species carried small traces of unused genes that allowed production of another type of blue pigment called rosacyanin, which most roses evolved to stop utilizing in favor of producing flower fragrances to attract pollinators.[ citation needed ]

Rosacyanin allows roses to come in delicate mauve, lavender, and true blue shades.[ citation needed ]

These colors can be seen in the hybrid tea roses 'Sterling Silver', 'Blue Girl', and 'Blue Moon', among others, which descend from yellow roses. Most notably a popular yellow hybrid tea rose called 'Peace', which was named to commemorate the end of World War II, was used extensively in hybridizing, and fathered most of the original lavender hybrid teas.[ citation needed ]

In 2004, Japanese company Suntory produced a blue rose, named Applause. [16]

Further hybridizing advancements, scientific study, and possibly further genetic engineering will be necessary to concentrate these natural genetic factors and cofactors before roses will be able to come in deep shades of true blue.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Anthocyanidin</span>

Anthocyanidins are common plant pigments, the aglycones of anthocyanins. They are based on the flavylium cation, an oxonium ion, with various groups substituted for its hydrogen atoms. They generally change color from red through purple, blue, and bluish green as a function of pH.

<i>Hydrangea macrophylla</i> Species of flowering plant in the family Hydrangeaceae

Hydrangea macrophylla is a species of flowering plant in the family Hydrangeaceae, native to Japan. It is a deciduous shrub growing to 2 m (7 ft) tall by 2.5 m (8 ft) broad with large heads of pink or blue flowers in summer and autumn. Common names include bigleaf hydrangea, French hydrangea, lacecap hydrangea, mophead hydrangea, and hortensia. It is widely cultivated in many parts of the world in many climates. It is not to be confused with H. aspera 'Macrophylla'.

<span class="mw-page-title-main">Blue rose</span> Genetically engineered variety of rose

A blue rose is a flower of the genus Rosa that presents blue-to-violet pigmentation instead of the more common red, white, or yellow. Blue roses are often used to symbolize mystery or the unattainable, since they do not exist in nature because of genetic limitations. In 2004, researchers used genetic modification to create roses that contain the blue pigment delphinidin.

<span class="mw-page-title-main">Delphinidin</span> Chemical compound

Delphinidin is an anthocyanidin, a primary plant pigment, and also an antioxidant. Delphinidin gives blue hues to flowers in the genera Viola and Delphinium. It also gives the blue-red color of the grape variety Cabernet Sauvignon, and can be found in cranberries and Concord grapes as well as pomegranates, and bilberries.

Protocyanin is an anthocyanin pigment that is responsible for the red colouration of roses, but in cornflowers is blue. The pigment was first isolated in 1913 from the blue cornflower, and the identical pigment was isolated from a red rose in 1915. The difference in colour had been explained as a difference in flower-petal pH, but the pigment in the blue cornflower has been shown to be a supermolecular pigment consisting of anthocyanin, flavone, one ferric ion, one magnesium and two calcium ions forming a copigmentation complex.

<span class="mw-page-title-main">Biological pigment</span> Substances produced by living organisms

Biological pigments, also known simply as pigments or biochromes, are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments. Many biological structures, such as skin, eyes, feathers, fur and hair contain pigments such as melanin in specialized cells called chromatophores. In some species, pigments accrue over very long periods during an individual's lifespan.

<span class="mw-page-title-main">Betalain</span> Class of chemical compounds

Betalains are a class of red and yellow tyrosine-derived pigments found in plants of the order Caryophyllales, where they replace anthocyanin pigments. Betalains also occur in some higher order fungi. They are most often noticeable in the petals of flowers, but may color the fruits, leaves, stems, and roots of plants that contain them. They include pigments such as those found in beets.

In enzymology, a dihydrokaempferol 4-reductase (EC 1.1.1.219) is an enzyme that catalyzes the chemical reaction

<span class="mw-page-title-main">Anthoxanthin</span>

Anthoxanthins are a type of flavonoid pigments in plants. Anthoxanthins are water-soluble pigments which range in color from white or colorless to a creamy to yellow, often on petals of flowers. These pigments are generally whiter in an acid medium and yellowed in an alkaline medium. They are very susceptible to color changes with minerals and metal ions, similar to anthocyanins.

<span class="mw-page-title-main">Anthocyanin</span> Class of chemical compounds

Anthocyanins, also called anthocyans, are water-soluble vacuolar pigments that, depending on their pH, may appear red, purple, blue, or black. In 1835, the German pharmacist Ludwig Clamor Marquart gave the name Anthokyan to a chemical compound that gives flowers a blue color for the first time in his treatise "Die Farben der Blüthen". Food plants rich in anthocyanins include the blueberry, raspberry, black rice, and black soybean, among many others that are red, blue, purple, or black. Some of the colors of autumn leaves are derived from anthocyanins.

<span class="mw-page-title-main">Phenolic content in wine</span> Wine chemistry

The phenolic content in wine refers to the phenolic compounds—natural phenol and polyphenols—in wine, which include a large group of several hundred chemical compounds that affect the taste, color and mouthfeel of wine. These compounds include phenolic acids, stilbenoids, flavonols, dihydroflavonols, anthocyanins, flavanol monomers (catechins) and flavanol polymers (proanthocyanidins). This large group of natural phenols can be broadly separated into two categories, flavonoids and non-flavonoids. Flavonoids include the anthocyanins and tannins which contribute to the color and mouthfeel of the wine. The non-flavonoids include the stilbenoids such as resveratrol and phenolic acids such as benzoic, caffeic and cinnamic acids.

<span class="mw-page-title-main">Petunidin</span> Chemical compound

Petunidin (Pt), like Europinidin and Malvidin, is derived from Delphinidin and is an O-methylated anthocyanidin of the 3-hydroxy type. It is a natural organic compound, a dark-red or purple water-soluble pigment found in many redberries including chokeberries, Saskatoon berries or different species of grape, and also part of the pigments responsible for the petal colors in many flowers. This pigment gives the Indigo Rose tomatoes the majority of their deep purple color when the fruits are exposed to sunlight. The name of the molecule itself is derived from the word Petunia.

<span class="mw-page-title-main">Wine color</span> Wine characteristic

The color of wine is one of the most easily recognizable characteristics of wines. Color is also an element in wine tasting since heavy wines generally have a deeper color. The accessory traditionally used to judge the wine color was the tastevin, a shallow cup allowing one to see the color of the liquid in the dim light of a cellar. The color is an element in the classification of wines.

<span class="mw-page-title-main">Metalloanthocyanin</span>

A metalloanthocyanin is a chemical complex giving color to petals of certain plants.

<span class="mw-page-title-main">Myrtillin</span> Chemical compound

Myrtillin is an anthocyanin. It is the 3-glucoside of delphinidin. It can be found in all green plants, most abundantly in black beans, blackcurrant, blueberry, huckleberry, bilberry leaves and in various myrtles, roselle plants, and Centella asiatica plant. It is also present in yeast and oatmeal. The sumac fruit's pericarp owes its dark red colour to anthocyanin pigments, of which chrysanthemin, myrtillin and delphinidin have yet been identified.

<span class="mw-page-title-main">Phlobaphene</span>

Phlobaphenes are reddish, alcohol-soluble and water-insoluble phenolic substances. They can be extracted from plants, or be the result from treatment of tannin extracts with mineral acids. The name phlobaphen come from the Greek roots φλoιὀς (phloios) meaning bark and βαφή (baphe) meaning dye.

<span class="mw-page-title-main">Violdelphin</span> Chemical compound

Violdelphin is an anthocyanin, a plant pigment, has been found in the purplish blue flower of Aconitum chinense, in the blue flowers in the genus Campanula and in the blue flowers of Delphinium hybridum. It is a flavenoid natural product, incorporating two p-hydroxy benzoic acid residues, one rutinoside and two glucosides associated with a delphinidin.

<span class="mw-page-title-main">Anthochlor pigments</span> Group of plant metabolites

Anthochlor pigments are a group of secondary plant metabolites and with carotenoids and some flavonoids produce yellow flower colour. Both, chalcones and aurones are known as anthochlor pigments. Anthochlor pigments serve as UV nectar guides in some plants. Important anthochlor pigments accumulating plants are from the genus Coreopsis, Snapdragon or Bidens ferulifolia.

White flower colour is related to the absence or reduction of the anthocyanidin content. Unlike other colors, white colour is not induced by pigments. Several white plant tissues are principally equipped with the complete machinery for anthocyanin biosynthesis including the expression of regulatory genes. Nevertheless, they are unable to accumulate red or blue pigments, for example Dahlia ´Seattle´ petals showing a white tip. Several studies have revealed a further reduction of the anthocyanidin to colorless epicatechin by the enzyme anthocyanidin reductase (ANR).

<span class="mw-page-title-main">Orange petunia</span> Genetically modified petunia variety

Orange petunias or A1-DFR petunias are genetically modified organisms which contain a transgene from maize that colors the petunia flowers orange. First created in a 1987 experiment at the Max Planck Institute for Plant Breeding Research in Cologne, the petunias were subsequently released into the wild but were not commercialized. In 2015 orange petunias were discovered in Helsinki by botanist Teemu Teeri, leading to a regulatory response dubbed the petunia carnage of 2017 in which plant sellers were directed to destroy the modified petunia plants rather than sell them. The United States Department of Agriculture approved the sale of orange petunias in the United States in January 2021.

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Further reading