Blue flower color development by anthocyanins: from chemical structure to cell physiology

Highlights on Anthocyanin Pigmentation and Copigmentation: A Matter of Flavonoid π-Stacking Complexation To Be Described by DFT-D

Anthocyanidins are a class of π-conjugated systems responsible for red, blue, and purple colors of plants. They exhibit the capacity of aggregation in the presence of other natural compounds including flavonols. Such complexations induce color modulation in plants, which is known as copigmentation. It is largely driven by π-interactions existing between pigments and copigments. In this work, the energies of copigmentation-complexation and self-association are systematically evaluated for an anthocyanidin/flavonol couple prototype (3-O-methylcyanidin/quercetin). To describe noncovalent interactions, DFT-D appears mandatory to reach a large accuracy. Due to the chemical complexity of this phenomenon, we also aim at assessing the relevance of both B3P86-D2 and ωB97X-D functionals. The benchmarking has shown that B3P86-D2 possesses enough accuracy when dealing with π-π interactions with respect to both spin component scaled Møller-Plesset second-order perturbation theory post Hartree-Fock method and experimental data. UV-vis absorption properties are then evaluated with time-dependent DFT for the different complexes. The use of range-separated hybrid functionals, such as ωB97X-D, helped to correctly disentangle and interpret the origin of the UV-vis experimental shifts attributed to the subtle copigmentation phenomenon.

New anthocyanidin and anthocyanin pigments from blue plumbago

Phytochemical investigations of blue plumbago ( Plumbago auriculata Poir. syn. Plumbago capensis Thunb.) flowers have led to the isolation of six new anthocyanins based on three new anthocyanidins with 5,7-dimethoxylated A-rings. Their structures were identified by 2D nuclear magnetic resonance and high-resolution mass spectrometry as the 3-O-β-galactopyranosides (1,2,4) and 3-O-α-rhamnopyranosides (3,5,6) of 5,7-dimethyldelphinidin, 5,7-dimethylpetunidin, and 5,7-dimethylmalvidin. Identification of 1-6 implies new structures for the previously reported anthocyanidins pulchellidin, europinidin, and capensinidin to be 5,7-dimethoxy-3,3',4',5'-tetrahydroxyflavylium, 5,7,3'-trimethoxy-3,4',5'-trihydroxyflavylium, and 5,7,3',5'-tetramethoxy-3,4'-dihydroxyflavylium cations, respectively. The anthocyanins (0.4 mg/g flowers) were accompanied by the dihydroflavonol taxifolin 3'-O-β-glucopyranoside (1.4 mg/g) and the flavonols 5-methylquercetin 3-O-α-rhamnopyranoside (8.8 mg/g) and 5-methylquercetin (0.4 mg/g). The anthocyanins 1-6 are the first reported natural anthocyanins with no free hydroxyl groups in their 5- and 7-positions on their A-rings. They have thus no possibility of forming the tautomeric quinonoidal bases (anhydrobases), which are related to the free hydroxyl groups in the 5- and 7-positions of previously reported anthocyanins. The genes behind the 5,7-dimethoxylated anthocyanins might be useful for making anthocyanins with special properties (colors, etc.).

Mendel, 150 years on

Mendel's paper 'Versuche über Pflanzen-Hybriden' is the best known in a series of studies published in the late 18th and 19th centuries that built our understanding of the mechanism of inheritance. Mendel investigated the segregation of seven gene characters of pea (Pisum sativum), of which four have been identified. Here, we review what is known about the molecular nature of these genes, which encode enzymes (R and Le), a biochemical regulator (I) and a transcription factor (A). The mutations are: a transposon insertion (r), an amino acid insertion (i), a splice variant (a) and a missense mutation (le-1). The nature of the three remaining uncharacterized characters (green versus yellow pods, inflated versus constricted pods, and axial versus terminal flowers) is discussed.

Chemistry and applications of flavylium compounds: a handful of colours

Flavylium compounds are versatile molecules that comprise anthocyanins, the ubiquitous colorants used by Nature to confer colour to most flowers and fruits. They have found a wide range of applications in human technology, from the millenary colour paints described by the Roman architect Vitruvius, to their use as food additives, combining colour and antioxidant effects, and even as light absorbers in solar cells aiming at a greener solar energy conversion. Their rich complexity derives in part from their ability to switch between a variety of species (flavylium cations, neutral quinoidal bases, hemiketals and chalcones, and negatively charged phenolates) by means of external stimuli, such as pH, temperature and light. This critical review describes (i) the historical advancements in the understanding of the equilibria of their chemical reaction networks; (ii) their thermodynamics and kinetics; (iii) the mechanisms underlying their colour development, such as co-pigmentation and host-guest interactions; (iv) the photophysics and photochemistry that lead to photochromism; and (v) applications in solar cells, models for optical memories, photochromic soft materials such as ionic liquids and gels, and their properties in solid state materials (274 references).

Increased accumulation and decreased catabolism of anthocyanins in red grape cell suspension culture following magnesium treatment

Anthocyanins are the largest and best studied group of plant pigments. However, not very much is known about the fate of these phenolic pigments after they have accumulated in the cell vacuoles of plant tissues. We have previously shown that magnesium treatment of ornamentals during the synthesis of anthocyanins in the flowers or foliage caused an increase in the pigment concentration. In this study, we characterized the effect of magnesium on the accumulation of anthocyanin in red cell suspension originating from Vitis vinifera cv. Gamay Red grapes. Magnesium treatment of the cells caused a 2.5- to 4.5-fold increase in anthocyanin concentration, with no substantial induction of the biosynthetic genes. This treatment inhibited the degradation of anthocyanins occurring in the cells, and changed the ratio between different anthocyanins determining cell color, with an increase in the relative concentration of the less stable pigment molecules. The process by which magnesium treatment affects anthocyanin accumulation is still not clear. However, the results presented suggest at least part of its effect on anthocyanin accumulation stems from inhibition of the pigments' catabolism. When anthocyanin biosynthesis was inhibited, magnesium treatments prevented the constant degradation of anthocyanins in the cell suspension. Future understanding of the catabolic processes undergone by anthocyanins in plants may enable more efficient inhibition of this process and increased accumulation of these pigments, and possibly of additional phenolic compounds.

Plant polyphenols: chemical properties, biological activities, and synthesis

Eating five servings of fruits and vegetables per day! This is what is highly recommended and heavily advertised nowadays to the general public to stay fit and healthy! Drinking green tea on a regular basis, eating chocolate from time to time, as well as savoring a couple of glasses of red wine per day have been claimed to increase life expectancy even further! Why? The answer is in fact still under scientific scrutiny, but a particular class of compounds naturally occurring in fruits and vegetables is considered to be crucial for the expression of such human health benefits: the polyphenols! What are these plant products really? What are their physicochemical properties? How do they express their biological activity? Are they really valuable for disease prevention? Can they be used to develop new pharmaceutical drugs? What recent progress has been made toward their preparation by organic synthesis? This Review gives answers from a chemical perspective, summarizes the state of the art, and highlights the most significant advances in the field of polyphenol research.

Genetic engineering of flavonoid pigments to modify flower color in floricultural plants

Recent advances in genetic transformation techniques enable the production of desirable and novel flower colors in some important floricultural plants. Genetic engineering of novel flower colors is now a practical technology as typified by commercialization of a transgenic blue rose and blue carnation. Many researchers exploit knowledge of flavonoid biosynthesis effectively to obtain unique flower colors. So far, the main pigments targeted for flower color modification are anthocyanins that contribute to a variety of colors such as red, pink and blue, but recent studies have also utilized colorless or faint-colored compounds. For example, chalcones and aurones have been successfully engineered to produce yellow flowers, and flavones and flavonols used to change flower color hues. In this review, we summarize examples of successful flower color modification in floricultural plants focusing on recent advances in techniques.

Co-pigmentation and flavonoid glycosyltransferases in blue Veronica persica flowers

Glycosylation is one of the key modification steps for plants to produce a broad spectrum of flavonoids with various structures and colors. A survey of flavonoids in the blue flowers of Veronica persica Poiret (Lamiales, Scrophulariaceae), which is native of Eurasia and now widespread worldwide, led to the identification of highly glycosylated flavonoids, namely delphinidin 3-O-(2-O-(6-O-p-coumaroyl-glucosyl)-6-O-p-coumaroyl-glucoside)-5-O-glucoside (1) and apigenin 7-O-(2-O-glucuronosyl)-glucuronide (2), as two of its main flavonoids. Interestingly, the latter flavone glucuronide (2) caused a bathochromic shift on the anthocyanin (1) toward a blue hue in a dose-dependent manner, showing an intermolecular co-pigment effect. In order to understand the molecular basis for the biosynthesis of this glucuronide, we isolated a cDNA encoding a UDP-dependent glycosyltransferase (UGT88D8), based on the structural similarity to flavonoid 7-O-glucuronosyltransferases (F7GAT) from Lamiales plants. Enzyme assays showed that the recombinant UGT88D8 protein catalyzes the 7-O-glucuronosylation of apigenin and its related flavonoids with preference to UDP-glucuronic acid as a sugar donor. Furthermore, we identified and functionally characterized a cDNA encoding another UGT, UGT94F1, as the anthocyanin 3-O-glucoside-2''-O-glucosyltransferase (A3Glc2''GlcT), according to the structural similarity to sugar-sugar glycosyltransferases classified to the cluster IV of flavonoid UGTs. Preferential expression of UGT88D8 and UGT94F1 genes in the petals supports the idea that these UGTs play an important role in the biosynthesis of key flavonoids responsible for the development of the blue color of V. persica flowers.

A spontaneous dominant-negative mutation within a 35S::AtMYB90 transgene inhibits flower pigment production in tobacco

BACKGROUND

In part due to the ease of visual detection of phenotypic changes, anthocyanin pigment production has long been the target of genetic and molecular research in plants. Specific members of the large family of plant myb transcription factors have been found to play critical roles in regulating expression of anthocyanin biosynthetic genes and these genes continue to serve as important tools in dissecting the molecular mechanisms of plant gene regulation.

FINDINGS

A spontaneous mutation within the coding region of an Arabidopsis 35S::AtMYB90 transgene converted the activator of plant-wide anthocyanin production to a dominant-negative allele (PG-1) that inhibits normal pigment production within tobacco petals. Sequence analysis identified a single base change that created a premature nonsense codon, truncating the encoded myb protein. The resulting mutant protein lacks 78 amino acids from the wild type C-terminus and was confirmed as the source of the white-flower phenotype. A putative tobacco homolog of AtMYB90 (NtAN2) was isolated and found to be expressed in flower petals but not leaves of all tobacco plants tested. Using transgenic tobacco constitutively expressing the NtAN2 gene confirmed the NtAN2 protein as the likely target of PG-1-based inhibition of tobacco pigment production.

CONCLUSIONS

Messenger RNA and anthocyanin analysis of PG-1Sh transgenic lines (and PG-1Sh x purple 35S::NtAN2 seedlings) support a model in which the mutant myb transgene product acts as a competitive inhibitor of the native tobacco NtAN2 protein. This finding is important to researchers in the field of plant transcription factor analysis, representing a potential outcome for experiments analyzing in vivo protein function in test transgenic systems that over-express or mutate plant transcription factors.

The chemical mechanism for Al3+ complexing with delphinidin: a model for the bluing of hydrangea sepals

The blooms of many hydrangea cultivars can be red or blue, with the color depending on the soil pH. This dependence reflects the availability of Al(3+) to the plant under acidic conditions, as Al(3+) changes the color of the anthocyanin pigment in hydrangea sepals from red to blue. A chemical model, Al(3+) and delphinidin in acidic ethanol, was developed to understand the spectral characteristics and bluing of the hydrangea sepals. Delphinidin as its flavylium cation leads to red solutions in the model system. In the presence of Al(3+), the Al(3+) removes H(+) ions from delphinidin, transforming delphinidin's flavylium cation to its blue quinoidal base anion which complexes with the Al(3+). To further stabilize this complex, a second flavylium cation stacks on top of the complexed quinoidal base anion, creating a bathochromic shift of the cation's spectral signature and accentuating the blue color. This Al(3+)-delphinidin entity forms in adequate concentration for bluing only if there is a sufficient excess of Al(3+), the exact excess being a function of pH and concentration. The role of Al(3+) in bluing is not just to form a primary complex with delphinidin, but also to create a template for the stacking of delphinidin (or possibily co-pigments).

Genetic engineering of novel flower colour by suppression of anthocyanin modification genes in gentian

Ornamental gentian plants have vivid-blue flowers. The main factor contributing to the flower colour is the accumulation of a polyacylated delphinidin 'gentiodelphin' in their petals. Although in vitro studies proposed that acylation plays an important role in the stability and development of gentian blue colour, the in vivo stability of the polyacylated anthocyanin was not clearly demonstrated. Thus, to reveal the importance of anthocyanin modification, especially acylation, and to engineer new colours of gentian flowers, we used chimeric RNAi technology to produce transgenic gentian plants with downregulated anthocyanin 5,3'-aromatic acyltransferase (5/3'AT) and flavonoid 3',5'-hydroxylase (F3'5'H) activities, which are both essential enzymes for gentiodelphin biosynthesis. Two lines of flower colour-modified plants were obtained from fifteen transgenic gentian plants. Clone no. 1 exhibited a lilac flower colour and clone no. 15 exhibited pale-blue flowers. RNA gel blot analysis confirmed that both transgenic lines had markedly suppressed 5/3'AT transcripts, whereas clone no. 15 had fewer F3'5'H transcripts than clone no. 1 and untransformed control plants. HPLC analysis of anthocyanin compositions showed that downregulation of the 5/3'AT gene led to increased accumulation of non-acylated anthocyanins, as expected. These results demonstrated that genetic engineering to reduce the accumulation of polyacylated anthocyanins could cause modulations of flower colour.

Recent progress of flower colour modification by biotechnology

Genetically-modified, colour-altered varieties of the important cut-flower crop carnation have now been commercially available for nearly ten years. In this review we describe the manipulation of the anthocyanin biosynthesis pathway that has lead to the development of these varieties and how similar manipulations have been successfully applied to both pot plants and another cut-flower species, the rose. From this experience it is clear that down- and up-regulation of the flavonoid and anthocyanin pathway is both possible and predictable. The major commercial benefit of the application of this technology has so far been the development of novel flower colours through the development of transgenic varieties that produce, uniquely for the target species, anthocyanins derived from delphinidin. These anthocyanins are ubiquitous in nature, and occur in both ornamental plants and common food plants. Through the extensive regulatory approval processes that must occur for the commercialization of genetically modified organisms, we have accumulated considerable experimental and trial data to show the accumulation of delphinidin based anthocyanins in the transgenic plants poses no environmental or health risk.

A vacuolar iron transporter in tulip, TgVit1, is responsible for blue coloration in petal cells through iron accumulation

Blue color in flowers is due mainly to anthocyanins, and a considerable part of blue coloration can be attributed to metal-complexed anthocyanins. However, the mechanism of metal ion transport into vacuoles and subsequent flower color development has yet to be fully explored. Previously, we studied the mechanism of blue color development specifically at the bottom of the inner perianth in purple tulip petals of Tulipa gesneriana cv. Murasakizuisho. We found that differences in iron content were associated with the development of blue- and purple-colored cells. Here, we identify a vacuolar iron transporter in T. gesneriana (TgVit1), and characterize the localization and function of this transporter protein in tulip petals. The amino acid sequence of TgVit1 is 85% similar that of the Arabidopsis thaliana vacuolar iron transporter AtVIT1, and also showed similarity to the AtVIT1 homolog in yeast, Ca(2+)-sensitive cross-complementer 1 (CCC1). The gene TgVit1 was expressed exclusively in blue-colored epidermal cells, and protein levels increased with increasing mRNA expression and blue coloration. Transient expression experiments revealed that TgVit1 localizes to the vacuolar membrane, and is responsible for the development of the blue color in purple cells. Expression of TgVit1 in yeast rescued the growth defect of ccc1 mutant cells in the presence of high concentrations of FeSO(4). Our results indicate that TgVit1 plays an essential role in blue coloration as a vacuolar iron transporter in tulip petals. These results suggest a new role for involvement of a vacuolar iron transporter in blue flower color development.

Synchrony between flower opening and petal-color change from red to blue in morning glory, Ipomoea tricolor cv. Heavenly Blue

Petal color change in morning glory Ipomoea tricolor cv. Heavenly Blue, from red to blue, during the flower-opening period is due to an unusual increase in vacuolar pH (pHv) from 6.6 to 7.7 in colored epidermal cells. We clarified that this pHv increase is involved in tonoplast-localized Na+/H+ exchanger (NHX). However, the mechanism of pHv increase and the physiological role of NHX1 in petal cells have remained obscure. In this study, synchrony of petal-color change from red to blue, pHv increase, K+ accumulation, and cell expansion growth during flower-opening period were examined with special reference to ItNHX1. We concluded that ItNHX1 exchanges K+, but not Na+, with H+ to accumulate an ionic osmoticum in the vacuole, which is then followed by cell expansion growth. This function may lead to full opening of petals with a characteristic blue color.