Color genes in the orchid Oncidium Gower Ramsey: identification, expression, and potential genetic instability in an interspecific cross

Multiomics analysis reveals the involvement of OnDIVARICATA 3 in controlling dynamic flower coloring of Oncidium hybridum

Flower color is one of the main quality and economic traits of ornamental plants, and a large amount of research on flower color mainly focuses on the differences between varieties, while there were few reports on the change of flower color at different developmental stages. In this study, the metabolome and transcriptome of a new strain 'XM-1' with dynamic color changes of Oncidium were analyzed. The results showed that rutin, quercetin and carotenoids metabolism decreased significantly during the change of color from yellow to white. Analyzing the correlation network between metabolites and differential expressed genes, 25 key structural genes were detected and regulated by multiple MYB-related transcription factors. The MYB-related transcription factor Cluster-100966.1_OnDIVARICATA 3 was selected for further analysis. The phylogenetic tree of DIVARICATA in different species of Orchidaceae and Arabidopsis thaliana was constructed and the most closely related members were selected for sequence comparison. The results showed that OnDIVARICATA 3 contained MYB-like conserved domains. Subcellular localization results showed that OnDIVARICATA 3 was located in the nucleus. In overexpressing OnDIVARICATA 3 transgenic hairy roots, the expression of flower color related genes FLS, ZEP, and CHYB were significantly up-regulated. In summary, this study characterized the key metabolic pathways in the formation of the dynamic flower color of Oncidium hybridum, and constructed the regulatory network of the MYB-related. These results laid a theoretical foundation for the subsequent research on flower color and genetic engineering technology breeding of Oncidium hybridum.

Color components determination and full-length comparative transcriptomic analyses reveal the potential mechanism of carotenoid synthesis during Paphiopedilum armeniacum flowering

Background

Paphiopedilum armeniacum (P. armeniacum), an ornamental plant native to China, is known for its distinctive yellow blossoms. However, the mechanisms underlying P. armeniacum flower coloration remain unclear.

Methods

We selected P. armeniacum samples from different flowering stages and conducted rigorous physicochemical analyses. The specimens were differentiated based on their chemical properties, specifically their solubilities in polar solvents. This key step enabled us to identify the main metabolite of flower color development of P. armeniacum, and to complete the identification by High-performance liquid chromatography (HPLC) based on the results. Additionally, we employed a combined approach, integrating both third-generation full-length transcriptome sequencing and second-generation high-throughput transcriptome sequencing, to comprehensively explore the molecular components involved.

Results

We combined physical and chemical analysis with transcriptome sequencing to reveal that carotenoid is the main pigment of P. armeniacum flower color. Extraction colorimetric method and HPLC were used to explore the characteristics of carotenoid accumulation during flowering. We identified 28 differentially expressed carotenoid biosynthesis genes throughout the flowering process, validated their expression through fluorescence quantification, and discovered 19 potential positive regulators involved in carotenoid synthesis. Among these candidates, three RCP2 genes showed a strong potential for governing the PDS and ZDS gene families. In summary, our study elucidates the fundamental mechanisms governing carotenoid synthesis during P. armeniacum flowering, enhancing our understanding of this process and providing a foundation for future research on the molecular mechanisms driving P. armeniacum flowering.

Untargeted Metabolomics Analysis of the Orchid Species Oncidium sotoanum Reveals the Presence of Rare Bioactive C-Diglycosylated Chrysin Derivatives

Plants are valuable sources of secondary metabolites with pharmaceutical properties, but only a small proportion of plant life has been actively exploited for medicinal purposes to date. Underexplored plant species are therefore likely to contain novel bioactive compounds. In this study, we investigated the content of secondary metabolites in the flowers, leaves and pseudobulbs of the orchid Oncidium sotoanum using an untargeted metabolomics approach. We observed the strong accumulation of C-diglycosylated chrysin derivatives, which are rarely found in nature. Further characterization revealed evidence of antioxidant activity (FRAP and DPPH assays) and potential activity against neurodegenerative disorders (MAO-B inhibition assay) depending on the specific molecular structure of the metabolites. Natural product bioprospecting in underexplored plant species based on untargeted metabolomics can therefore help to identify novel chemical structures with diverse pharmaceutical properties.

Plant DNA Methylation: An Epigenetic Mark in Development, Environmental Interactions, and Evolution

DNA methylation is an epigenetic modification of the genome involved in the regulation of gene expression and modulation of chromatin structure. Plant genomes are widely methylated, and the methylation generally occurs on the cytosine bases through the activity of specific enzymes called DNA methyltransferases. On the other hand, methylated DNA can also undergo demethylation through the action of demethylases. The methylation landscape is finely tuned and assumes a pivotal role in plant development and evolution. This review illustrates different molecular aspects of DNA methylation and some plant physiological processes influenced by this epigenetic modification in model species, crops, and ornamental plants such as orchids. In addition, this review aims to describe the relationship between the changes in plant DNA methylation levels and the response to biotic and abiotic stress. Finally, we discuss the possible evolutionary implications and biotechnological applications of DNA methylation.

Anthocyanin Genes Involved in the Flower Coloration Mechanisms of Cymbidium kanran

The Orchidaceae, otherwise known as orchids, is one of the largest plant families and is renowned for its spectacular flowers and ecological adaptations. Various polymorphisms of orchid flower colour can attract pollinators and be recognised as valuable horticultural ornamentals. As one of the longest historic cultured orchids, Cymbidium kanran has been domesticated for more than 2,500 years and is an ideal species to study coloration mechanisms because of plentiful variations in floral coloration and abundant traditional varieties. In this study, we used two distinct colour-type flowers of C. kanran as experimental materials to elucidate the mechanism of flower coloration. High-performance liquid chromatography (HPLC) analysis revealed that anthocyanins in purple-red-type flowers include three types of anthocyanidin aglycones, peonidin, malvidin, and cyanidin, whereas anthocyanins are lacking in white-type flowers. Through comparative transcriptome sequencing, 102 candidate genes were identified as putative homologues of colour-related genes. Based on comprehensive correlation analysis between colour-related compounds and gene expression profiles, four candidates from 102 captured genes showed a positive correlation with anthocyanidin biosynthesis. Furthermore, transient expression of CkCHS-1, CkDFR, and CkANS by particle bombardment confirmed that recovery of their expression completed the anthocyanin pathway and produced anthocyanin compounds in white-type flowers. Collectively, this study provided a comprehensive transcriptomic dataset for Cymbidium, which significantly facilitate our understanding of the molecular mechanisms of regulating floral pigment accumulation in orchids.

Floral organ- and temperature-dependent regulation of anthocyanin biosynthesis in Cymbidium hybrid flowers

Anthocyanins are responsible for red, purple, and pink pigmentation of flowers in Cymbidium hybrids. Although anthocyanin content in all floral organs increases with flower development, they increase markedly in the tepals compared with the labella or columns. Using next-generation sequencing technology, we identified three anthocyanin biosynthesis regulatory genes, CyMYB1, CybHLH1, and CybHLH2, from Cymbidium 'Mystique'. Yeast two-hybrid analysis showed that the CyMYB1 protein can form a heterodimer with either CybHLH1 or CybHLH2. In the tepals, the expression level of CyMYB1 increased as the flower developed, whereas the high expression level of CyMYB1 was detected at the early flower developmental stages in the labella and columns, remaining constant until increasing at the late developmental stage. These expression profiles of CyMYB1 positively correlated with the profiles of anthocyanin accumulation in the tepals. When Cymbidium Sazanami 'Champion' was grown at 30 °C/25 °C, reduced anthocyanin levels were observed, specifically in the tepals, compared with those in flowers grown at 20 °C/15 °C. The transcription of CyMYB1 in the tepals was suppressed at high temperatures, and the expressions of CyDFR and CyANS were also synchronously suppressed. This study revealed that CyMYB1 activates the transcription of CyDFR and CyANS and regulates the temporal- and temperature-dependent anthocyanin accumulation in Cymbidium tepals.

Differential Accumulation of Anthocyanins in Dendrobium officinale Stems with Red and Green Peels

Dendrobium officinale stems, including red and green stems, are widely used as a dietary supplement to develop nutraceutical beverages and food products. However, there is no detailed information on pigment composition of red and green stems. Here, we investigated the content and composition of pigments in red and green stems by Ultra-performance liquid chromatography quadrupole time-of-flight mass spectrometry and assessed the differential accumulation of anthocyanins at the molecular level. The color of peels in red stems was caused by the presence of anthocyanins in epidermal cells unlike the peels of green stems. The glucoside derivatives delphinidin and cyanidin are responsible for the red color. Within the D. officinale anthocyanidin biosynthetic pathway, DoANS and DoUFGT, coding for anthocyanidin synthase and UDP-glucose flavonoid-3-O-glucosyltransferase, respectively, are critical regulatory genes related to the differential accumulation of anthocyanidin. These findings provide a more complete profile of pigments, especially anthocyanin, in D. officinale stems, and lay a foundation for producing functional foods.

Modification of flower colour by suppressing β-ring carotene hydroxylase genes in Oncidium

Oncidium 'Gower Ramsey' (Onc. GR) is a popular cut flower, but its colour is limited to bright yellow. The β-ring carotene hydroxylase (BCH2) gene is involved in carotenoid biogenesis for pigment formation. However, the role of BCH2 in Onc. GR is poorly understood. Here, we investigated the functions of three BCH2 genes, BCH-A2, BCH-B2 and BCH-C2 isolated from Onc. GR, to analyse their roles in flower colour. RT-PCR expression profiling suggested that BCH2 was mainly expressed in flowers. The expression of BCH-B2 remained constant while that of BCH-A2 gradually decreased during flower development. Using Agrobacterium tumefaciens to introduce BCH2 RNA interference (RNAi), we created transgenic Oncidium plants with down-regulated BCH expression. In the transgenic plants, flower colour changed from the bright yellow of the wild type to light and white-yellow. BCH-A2 and BCH-B2 expression levels were significantly reduced in the transgenic flower lips, which make up the major portion of the Oncidium flower. Sectional magnification of the flower lip showed that the amount of pigmentation in the papillate cells of the adaxial epidermis was proportional to the intensity of yellow colouration. HPLC analyses of the carotenoid composition of the transgenic flowers suggested major reductions in neoxanthin and violaxanthin. In conclusion, BCH2 expression regulated the accumulation of yellow pigments in the Oncidium flower, and the down-regulation of BCH-A2 and BCH-B2 changed the flower colour from bright yellow to light and white-yellow.

Orchids (Oncidium and Phalaenopsis)

This chapter describes an efficient and reproducible method for large-scale propagation of Oncidium and Phalaenopsis protocorm-like bodies (PLBs) using floral stalk sections and seeds, respectively. The propagated PLBs can be used for Agrobacterium-mediated transformation. An advanced transformation system for Oncidium and Phalaenopsis orchids has been established. This protocol demonstrates that the time during which the PLBs are cocultivated with Agrobacterium is the key to promoting transformation efficiency. Modified DNA and RNA extraction methods are also provided to diminish polysaccharide contamination and to improve the quality for further molecular analysis.

Recent advances on the development and regulation of flower color in ornamental plants

Flower color is one of the most important features of ornamental plants. Its development and regulation are influenced by many internal and external factors. Therefore, understanding the mechanism of color development and its regulation provides an important theoretical basis and premise for the cultivation and improvement of new color varieties of ornamental plants. This paper outlines the functions of petal tissue structure, as well as the distribution and type of pigments, especially anthocyanins, in color development. The progress of research on flower color regulation with a focus on physical factors, chemical factors, and genetic engineering is introduced. The shortcomings of flower color research and the potential directions for future development are explored to provide a broad background for flower color improvements in ornamental plants.

Effects of droplet-vitrification cryopreservation based on physiological and antioxidant enzyme activities of Brassidium shooting star orchid

Protocorm-like bodies (PLBs) of Brassidium Shooting Star orchid were successfully cryopreserved using droplet-vitrification method. Vitrification based cryopreservation protocol is comprised of preculture, osmoprotection, cryoprotection, cooling, rewarming, and growth recovery and each and every step contributes to the achievement of successful cryopreservation. In order to reveal the lethal and nonlethal damage produced by cryopreservation, histological observation, scanning electron microscopy (SEM), and biochemical analysis were carried out in both cryopreserved and noncryopreserved PLBs of Brassidium Shooting Star orchid comparing with the control PLBs stock culture. Histological and scanning electron microscopy analyses displayed structural changes in cryopreserved PLBs due to the impact of cryoinjury during exposure to liquid nitrogen. Total soluble protein significantly increased throughout the dehydration process and the highest value was achieved when PLBs were stored in liquid nitrogen. Ascorbate peroxidase (APX) and catalase (CAT) showed the highest enzyme activities in both dehydration and cryostorage treatments indicating that stress level of PLBs was high during these stages.

Phenotype and transcriptome analysis reveals chloroplast development and pigment biosynthesis together influenced the leaf color formation in mutants of Anthurium andraeanum 'Sonate'

Leaf color is one of the well-sought traits in breeding program for Anthurium andraeanum Lind. Knowledge of mechanisms in anthuriums to produce leaves with different shades of green would help to effectively select desirable traits. In this study, the micro- and ultra-structural and physiological features of leaves on wild type and leaf color mutants (dark green, rubescent, etiolated, albino) in A. andraeanum 'Sonate' were analyzed. Results show that chloroplasts of leaf color mutants exhibited abnormal morphology and distribution. Using next generation sequencing technology followed by de novo assembly, leaf transcriptomes comprising of 41,017 unigenes with an average sequence length of 768 bp were produced from wild type and rubescent mutant. From the 27,539 (67.1%) unigenes with annotated functions, 858 significantly differently expressed genes (DEGs) were identified, consisting of 446 up-regulated genes and 412 down-regulated genes. Genes that affect chloroplasts development and division, and chlorophyll biosynthesis were included in the down-regulated DEGs. Quantitative real-time PCR (qRT-PCR) analysis validated that the expression level of those genes was significantly lower in the rubescent, etiolated, and albino mutant compared to wild type plants, which concurs with the differences in micro- and ultra-structures and physiological features between these two types of plants. Conclusively, the leaf color formation is greatly affected by the activity of chloroplast development and pigment biosynthesis. And the possible formation pathway of leaf color mutant of A. andraeanum 'Sonate' is deduced based on our results.

Temporal and spatial regulation of anthocyanin biosynthesis provide diverse flower colour intensities and patterning in Cymbidium orchid

This study confirmed pigment profiles in different colour groups, isolated key anthocyanin biosynthetic genes and established a basis to examine the regulation of colour patterning in flowers of Cymbidium orchid. Cymbidium orchid (Cymbidium hybrida) has a range of flower colours, often classified into four colour groups; pink, white, yellow and green. In this study, the biochemical and molecular basis for the different colour types was investigated, and genes involved in flavonoid/anthocyanin synthesis were identified and characterised. Pigment analysis across selected cultivars confirmed cyanidin 3-O-rutinoside and peonidin 3-O-rutinoside as the major anthocyanins detected; the flavonols quercetin and kaempferol rutinoside and robinoside were also present in petal tissue. β-carotene was the major carotenoid in the yellow cultivars, whilst pheophytins were the major chlorophyll pigments in the green cultivars. Anthocyanin pigments were important across all eight cultivars because anthocyanin accumulated in the flower labellum, even if not in the other petals/sepals. Genes encoding the flavonoid biosynthetic pathway enzymes chalcone synthase, flavonol synthase, flavonoid 3' hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS) were isolated from petal tissue of a Cymbidium cultivar. Expression of these flavonoid genes was monitored across flower bud development in each cultivar, confirming that DFR and ANS were only expressed in tissues where anthocyanin accumulated. Phylogenetic analysis suggested a cytochrome P450 sequence as that of the Cymbidium F3'H, consistent with the accumulation of di-hydroxylated anthocyanins and flavonols in flower tissue. A separate polyketide synthase, identified as a bibenzyl synthase, was isolated from petal tissue but was not associated with pigment accumulation. Our analyses show the diversity in flower colour of Cymbidium orchid derives not from different individual pigments but from subtle variations in concentration and pattern of pigment accumulation.

Flower color patterning in pansy (Viola × wittrockiana Gams.) is caused by the differential expression of three genes from the anthocyanin pathway in acyanic and cyanic flower areas

The petals of pansy (Viola × wittrockiana Gams.) 'Mengdie' exhibit a cyanic blotched pigmentation pattern. The accumulation of anthocyanins, cyanidin and delphinidin, was detected in the upper epidermal cells of the cyanic blotches. In order to elucidate the mechanism by which cyanic blotches are formed in pansy petal, the expression level of genes involved in anthocyanin synthesis was measured and compared between cyanic blotches and acyanic areas of the flower. The use of primers in conserved regions allowed the successful isolation of six cDNA clones encoding putative anthocyanin enzymes from pansy petals. The clones isolated encoded chalcone synthase (CHS), chalcone isomerase (CHI), flavanone 3-hydroxylase (F3H), flavonoid 3'-hydroxylase (F3'H), dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS). The transcription patterns of seven genes (VwCHS, VwCHI, VwF3H, VwF3'H, VwDFR, VwF3'5'H, and VwANS) in cyanic blotches and acyanic areas of the petals at seven stages of flower development were determined by real-time quantitative PCR. Transcription of VwF3'5'H, VwDFR and VwANS was significantly increased in cyanic blotches at stages III-V of flower development, implicating these genes in the pigmentation of Viola × wittrockiana Gams. petals.

RNA interference-based gene silencing of phytoene synthase impairs growth, carotenoids, and plastid phenotype in Oncidium hybrid orchid

Phytoene synthase (PSY) is the first rate-limiting regulatory enzyme in the carotenoid biosynthesis pathway. In order to modify the floral color pattern by reducing carotenoid contents, a phytoene synthase-RNAi construct was delivered into protocorm-like body (PLB) of Oncidium hybrid orchid. The transgenic orchids show down-regulated level of PSY and geranyl synthase gene. They displayed semi-dwarf phenotype and brilliant green leaves. The microscopic anatomy revealed development-arrested plastids with rare grana. The total carotenoid content was decreased and the efficiency of the photosynthetic electron transport was declined. The chlorophyll level and the expression of chlorophyll biosynthetic genes, such as OgGLUTR and OgCS were dramatically reduced. HPLC analysis showed that the endogenous level of gibberellic acid and abscisic acid in the dwarf transformants are 4-fold lower than in wild type plants. In addition, chilling tolerance of the transgenic Oncidium plants was reduced. The data showed that down-regulation of PSY resulted in alterations of gene expression in enzymes involved in many metabolic pathways, such as carotenoid, gibberellic acid, abscisic acid and chlorophyll biosynthetic pathway as well as causes predominant defects in plant growth and development.

Transcriptome and proteome data reveal candidate genes for pollinator attraction in sexually deceptive orchids

BACKGROUND

Sexually deceptive orchids of the genus Ophrys mimic the mating signals of their pollinator females to attract males as pollinators. This mode of pollination is highly specific and leads to strong reproductive isolation between species. This study aims to identify candidate genes responsible for pollinator attraction and reproductive isolation between three closely related species, O. exaltata, O. sphegodes and O. garganica. Floral traits such as odour, colour and morphology are necessary for successful pollinator attraction. In particular, different odour hydrocarbon profiles have been linked to differences in specific pollinator attraction among these species. Therefore, the identification of genes involved in these traits is important for understanding the molecular basis of pollinator attraction by sexually deceptive orchids.

RESULTS

We have created floral reference transcriptomes and proteomes for these three Ophrys species using a combination of next-generation sequencing (454 and Solexa), Sanger sequencing, and shotgun proteomics (tandem mass spectrometry). In total, 121 917 unique transcripts and 3531 proteins were identified. This represents the first orchid proteome and transcriptome from the orchid subfamily Orchidoideae. Proteome data revealed proteins corresponding to 2644 transcripts and 887 proteins not observed in the transcriptome. Candidate genes for hydrocarbon and anthocyanin biosynthesis were represented by 156 and 61 unique transcripts in 20 and 7 genes classes, respectively. Moreover, transcription factors putatively involved in the regulation of flower odour, colour and morphology were annotated, including Myb, MADS and TCP factors.

CONCLUSION

Our comprehensive data set generated by combining transcriptome and proteome technologies allowed identification of candidate genes for pollinator attraction and reproductive isolation among sexually deceptive orchids. This includes genes for hydrocarbon and anthocyanin biosynthesis and regulation, and the development of floral morphology. These data will serve as an invaluable resource for research in orchid floral biology, enabling studies into the molecular mechanisms of pollinator attraction and speciation.

Downregulation of putative UDP-glucose: flavonoid 3-O-glucosyltransferase gene alters flower coloring in Phalaenopsis

Anthocyanin is the primary pigment contributing to red, violet, and blue flower color formation. The solubility of anthocyanins is enhanced by UDP glucose: flavonoid 3-O-glucosyltransferase (UFGT) through transfer of the glucosyl moiety from UDP-glucose to 3-hydroxyl group to produce the first stable pigments. To assess the possibility that UFGT is involved in the flower color formation in Phalaenopsis, the transcriptional activities of PeUFGT3, and other flower color-related genes in developing red or white flower buds were examined using RT-PCR analysis. In contrast with chalcone synthase, chalcone isomerase, and anthocyanidin synthase genes, PeUFGT3 transcriptional activity was higher expressed in the red color of Phalaenopsis cultivars. In the red labellum of Phalaenopsis 'Luchia Lady', PeUFGT3 also showed higher expression levels than that in the white perianth. PeUFGT3 was predominantly expressed in the red region of flower among various Phalaenopsis cultivars. To investigate the role of PeUFGT3 in red flower color formation, PeUFGT3 was specifically knocked down using RNA interference technology via virus inducing gene silencing in Phalaenopsis. The PeUFGT3-suppressed Phalaenopsis exhibited various levels of flower color fading that was well correlated with the extent of reduced level of PeUFGT3 transcriptional activity. Furthermore, there was a significant decrease in anthocyanin content in the PeUFGT3-suppressed Phalaenopsis flowers. The decrease of anthocyanin content due to PeUFGT3 gene silencing possibly caused the faded flower color in PeUFGT3-suppressed Phalaenopsis. Consequently, these results suggested that the glycosylation-related gene PeUFGT3 plays a critical role in red color formation in Phalaenopsis.

The regulation of carotenoid pigmentation in flowers

Carotenoids fulfill many processes that are essential for normal growth and development in plants, but they are also responsible for the breathtaking variety of red-to-yellow colors we see in flowers and fruits. Although such visual diversity helps to attract pollinators and encourages herbivores to distribute seeds, humans also benefit from the aesthetic properties of flowers and an entire floriculture industry has developed on the basis that new and attractive varieties can be produced. Over the last decade, much has been learned about the impact of carotenoid metabolism on flower color development and the molecular basis of flower color. A number of different regulatory mechanisms have been described ranging from the transcriptional regulation of genes involved in carotenoid synthesis to the control of carotenoid storage in sink organs. This means we can now explain many of the natural colorful varieties we see around us and also engineer plants to produce flowers with novel and exciting varieties that are not provided by nature.

Differential expression of carotenoid-related genes determines diversified carotenoid coloration in floral tissues of Oncidium cultivars

Three cultivars of Oncidium orchid with varied coloration, such as Oncidium Gower Ramsey (yellow), Sunkist (orange), and White Jade (white), were analyzed for carotenoid metabolites and gene expression of carotenoid-biosynthetic genes. The HPLC analysis revealed that yellow Gower Ramsey accumulates violaxanthin, 9-cis-violaxanthin and neoxanthin, orange Sunkist accumulates an additional beta-carotene, and White Jade is devoid of carotenoid compounds. Molecular characterization indicated that the three Oncidium cultivars exhibited varied expression pattern and level in carotenoid-biosynthetic pathway. Among them, high expression level of beta-hydroxylase (OgHYB) and zeaxanthin epoxidase (OgZEP) was displayed in yellow Gower Ramsey, relative to the down-regulation of OgHYB and OgZEP exhibited in orange Sunkist, which results in the accumulation of beta-carotene and orange coloration in floral tissues. However, White Jade is caused by the up-regulation of OgCCD1 (Carotenoid Cleavage Dioxygenase 1), which catabolizes carotenoid metabolites. Methylation assay of OgCCD1 promoter in White Jade and Gower Ramsey revealed that a high level of DNA methylation was present in OgCCD1 promoter region of Gower Ramsey. Transient expression of OgCCD1 in yellow lip tissues of Gower Ramsey by bombardment confirmed its function of disintegrating carotenoid compounds. Our results suggest an evolutionary significance that genetic variation of carotenoid-related genes in Oncidium generates the complexity of floral pigmentation and consequently provides the profound varieties in Oncidium population.

Genetic engineering of novel flower colors in floricultural plants: recent advances via transgenic approaches

Since the first successful genetic engineering of flower color in petunia, several new techniques have been developed and applied to modify flower color not only in model plants but also in floricultural plants. A typical example is the commercial violet-flowered carnation "Moondust series" developed by Suntry Ltd. and Florigene Ltd. More recently, blue-flowered roses have been successfully produced and are expected to be commercially available in the near future. In recent years, successful modification of flower color by sophisticated regulation of flower-pigment metabolic pathways has become possible. In this chapter, we review recent advances in flower color modification by genetic engineering, especially focusing on the methodology. We have included our own recent results on successful production of flower-color-modified transgenic plants in a model plant, tobacco and an ornamental plant, gentian. Based on these results, genetic engineering of flower color for improvement of floricultural plants is discussed.

Molecular characterization of the Oncidium orchid HDR gene encoding 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase, the last step of the methylerythritol phosphate pathway

Two pathways are used by higher plants for the biosynthesis of isoprenoid precursors: the mevalonate pathway in the cytosol and a 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway in the plastids, with 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate reductase (HDR) catalyzing the last step in the MEP pathway. In order to understand the contribution of MEP pathway in isoprenoid biosynthesis of Oncidium orchid, a full-length cDNA corresponding to HDR from the flower tissues of Oncidium Gower Ramsey was cloned. The deduced OncHDR amino acid sequence contains a plastid signal peptide at the N-terminus and four conserved cysteine residues. RT-PCR analysis of HDR in Oncidium flowering plants revealed ubiquitous expression in organs and tissues, with preferential expression in the floral organs. Phylogenetic analysis revealed evolutionary conservation of the encoding HDR protein sequence. The genomic sequence of the HDR in Oncidium is similar to that in Arabidopsis, grape, and rice in structure. Successful complementation by OncHDR of an E. coli hdr(-) mutant confirmed its function. Transgenic tobacco carrying the OncHDR promoter-GUS gene fusion showed expression in most tissues, as well as in reproductive organs, as revealed by histochemical staining. Light induced strong GUS expression driven by the OncHDR promoter in transgenic tobacco seedlings. Taken together, our data suggest a role for OncHDR as a light-activated gene.

Floral isolation, specialized pollination, and pollinator behavior in orchids

Floral isolation is a form of prepollination reproductive isolation mediated by floral morphology (morphological isolation) and pollinator behavior (ethological isolation). Here we review mechanisms and evolutionary consequences of floral isolation in various pollination systems. Furthermore, we compare key features of floral isolation, i.e., pollinator sharing and specialization in pollination, in different orchid pollination systems. In orchid pollination, pollinator sharing is generally low, indicating strong floral isolation. The pollinators' motivation to visit flowers (specifically) can be due to both foraging or reproductive behavior. In both types of behavior, innate preferences for floral signals can be quickly overruled by learning. In pollination systems in which reproductive behavior of pollinators triggers flower visits, lower pollinator sharing was evident compared with systems with foraging behavior, probably because pollinators displaying reproductive behavior show higher fidelity in their visitation patterns. Orchids pollinated through reproductive behavior also use fewer pollinators than orchids pollinated through foraging behavior. No association between specialization and pollinator sharing was found. Thus, generalized pollination does not impede floral isolation, as orchids with many pollinators may nonetheless have low pollinator sharing. Specialization in pollination was, however, linked to orchid species richness in our analysis. Flower size, spur, and column morphology are most important for morphological isolation, and floral scent is most important for ethological isolation. These traits may be based on few genes, implying that floral isolation can be brought about by few genes of large effect.

Characterization and promoter activity of chromoplast specific carotenoid associated gene (CHRC) from Oncidium Gower Ramsey

Tissue-specific promoters are required for plant molecular breeding to drive a target gene in the appropriate location in plants. A chromoplast-specific, carotenoid-associated gene (OgCHRC) and its promoter (Pchrc) were isolated from Oncidium orchid and characterized. Northern blot analysis revealed that OgCHRC is specifically expressed in flowers, not in roots and leaves. Transient expression assay of Pchrc by bombardment transformation confirmed its differential expression pattern in floral tissues of different horticulture plants and cell-type location in conical papillate cells of adaxial epidermis of flower. These results suggest that Pchrc could serve as a useful tool in ornamental plant biotechnology to modify flower color.

Molecular mechanisms of floral mimicry in orchids

Deceptive plants do not produce floral rewards, but attract pollinators by mimicking signals of other organisms, such as food plants or female insects. Such floral mimicry is particularly common in orchids, in which flower morphology, coloration and odour play key roles in deceiving pollinators. A better understanding of the molecular bases for these traits should provide new insights into the occurrence, mechanisms and evolutionary consequences of floral mimicry. It should also reveal the molecular bases of pollinator-attracting signals, in addition to providing strategies for manipulating insect behaviour in general. Here, we review data on the molecular bases for traits involved in floral mimicry, and we describe methodological advances helpful for the functional evaluation of key genes.

Differential expression of MYB gene (OgMYB1) determines color patterning in floral tissue of Oncidium Gower Ramsey

The yellow coloration pattern in Oncidium floral lip associated with red sepal and petal tissues is an ideal model to study coordinate regulation of anthocyanin synthesis. In this study, chromatography analysis revealed that the red coloration in floral tissues was composed of malvidin-3-O-galactoside, peonidin-3-O-glucoside, delphinidin-3-O-glucoside and cyanidin-3-O-glucoside compounds. By contrary, these pigments were not detected in yellow lip tissue. Four key genes involved in anthocyanin biosynthetic pathway, i.e. chalcone synthase (OgCHS), chalcone isomerase (OgCHI), dihydroflavonol 4-reductase (OgDFR) and anthocyanidin synthase (OgANS) were isolated and their expression patterns were characterized. Northern blot analysis confirmed that although they are active during floral development, OgCHI and OgDFR genes are specifically down-regulated in yellow lip tissue. Bombardment with OgCHI and OgDFR genes into lip tissue driven by a flower-specific promoter, Pchrc (chromoplast-specific carotenoid-associated gene), demonstrated that transient expression of these two genes resulted in anthocyanin production in yellow lip. Further analysis of a R2R3 MYB transcription factor, OgMYB1, revealed that although it is actively expressed during floral development, it is not expressed in yellow lip tissue. Transient expression of OgMYB1 in lip tissues by bombardment can also induce formation of red pigments through the activation of OgCHI and OgDFR transcription. These results demonstrate that differential expression of OgMYB1 is critical to determine the color pattern of floral organ in Oncidium Gower Ramsey.

Expression of carotenoid biosynthesis genes during carrot root development

Carotenogenesis has been extensively studied in fruits and flower petals. Transcriptional regulation is thought to be the major factor in carotenoid accumulation in these organs. However, little is known about regulation in root organs. The root carotenoid content of carrot germplasm varies widely. The present study was conducted to investigate transcriptional regulation of carotenoid biosynthesis genes in relation to carotenoid accumulation during early carrot root development and up to 3 months after sowing. HPLC carotenoid content analysis and quantitative RT-PCR were compared to quantify the expression of eight genes encoding carotenoid biosynthesis enzymes during the development of white, yellow, orange, and red carrot roots. The genes chosen encode phytoene synthase (PSY1 and PSY2), phytoene desaturase (PDS), zeta-carotene desaturase (ZDS1 and ZDS2), lycopene epsilon-cyclase (LCYE), lycopene beta-cyclase (LCYB1), and zeaxanthin epoxidase (ZEP). All eight genes were expressed in the white cultivar even though it did not contain carotenoids. By contrast with fruit maturation, the expression of carotenogenic genes began during the early stages of development and then progressively increased for most of these genes during root development as the total carotenoid level increased in coloured carrots. The high expression of genes encoding LCYE and ZDS noted in yellow and red cultivars, respectively, might be consistent with the accumulation of lutein and lycopene, respectively. The results showed that the accumulation of total carotenoids during development and the accumulation of major carotenoids in the red and yellow cultivars might partially be explained by the transcriptional level of genes directing the carotenoid biosynthesis pathway.

Carotenoid biosynthesis in the primitive red alga Cyanidioschyzon merolae

Cyanidioschyzon merolae is considered to be one of the most primitive of eukaryotic photosynthetic organisms. To obtain insights into the origin and evolution of the pathway of carotenoid biosynthesis in eukaryotic plants, the carotenoid content of C. merolae was ascertained, genes encoding enzymes of carotenoid biosynthesis in this unicellular red alga were identified, and the activities of two candidate pathway enzymes of particular interest, lycopene cyclase and beta-carotene hydroxylase, were examined. C. merolae contains perhaps the simplest assortment of chlorophylls and carotenoids found in any eukaryotic photosynthetic organism: chlorophyll a, beta-carotene, and zeaxanthin. Carotenoids with epsilon-rings (e.g., lutein), found in many other red algae and in green algae and land plants, were not detected, and the lycopene cyclase of C. merolae quite specifically produced only beta-ringed carotenoids when provided with lycopene as the substrate in Escherichia coli. Lycopene beta-ring cyclases from several bacteria, cyanobacteria, and land plants also proved to be high-fidelity enzymes, whereas the structurally related epsilon-ring cyclases from several plant species were found to be less specific, yielding products with beta-rings as well as epsilon-rings. C. merolae lacks orthologs of genes that encode the two types of beta-carotene hydroxylase found in land plants, one a nonheme diiron oxygenase and the other a cytochrome P450. A C. merolae chloroplast gene specifies a polypeptide similar to members of a third class of beta-carotene hydroxylases, common in cyanobacteria, but this gene did not produce an active enzyme when expressed in E. coli. The identity of the C. merolae beta-carotene hydroxylase therefore remains uncertain.