Unraveling the regulation of floral fragrance biosynthesis

Chromosome-level genome assembly provides insights into the genome evolution and functional importance of the phenylpropanoid-flavonoid pathway in Thymus mongolicus

BACKGROUND

Thymus mongolicus (family Lamiaceae) is a Thyme subshrub with strong aroma and remarkable environmental adaptability. Limited genomic information limits the use of this plant.

RESULTS

Chromosome-level 605.2 Mb genome of T. mongolicus was generated, with 96.28% anchored to 12 pseudochromosomes. The repetitive sequences were dominant, accounting for 70.98%, and 32,593 protein-coding genes were predicted. Synteny analysis revealed that Lamiaceae species generally underwent two rounds of whole genome duplication; moreover, species-specific genome duplication was identified. A recent LTR retrotransposon burst and tandem duplication might play important roles in the formation of the Thymus genome. Using comparative genomic analysis, phylogenetic tree of seven Lamiaceae species was constructed, which revealed that Thyme plants evolved recently in the family. Under the phylogenetic framework, we performed functional enrichment analysis of the genes on nodes that contained the most gene duplication events (> 50% support) and of relevant significant expanded gene families. These genes were highly associated with environmental adaptation and biosynthesis of secondary metabolites. Combined transcriptome and metabolome analyses revealed that Peroxidases, Hydroxycinnamoyl-CoA shikimate/quinate hydroxycinnamoyl transferases, and 4-coumarate-CoA ligases genes were the essential regulators of the phenylpropanoid-flavonoid pathway. Their catalytic products (e.g., apigenin, naringenin chalcone, and several apigenin-related compounds) might be responsible for the environmental tolerance and aromatic properties of T. mongolicus.

CONCLUSION

This study enhanced the understanding of the genomic evolution of T. mongolicus, enabling further exploration of its unique traits and applications, and contributed to the understanding of Lamiaceae genomics and evolutionary biology.

Multiomics Analysis Reveals the Involvement of JsLHY in Controlling Aroma Production in Jasmine Flowers

The Jasminum sambac flower is famous for its rich fragrance. However, our knowledge of the regulatory network for its aroma formation remains largely unknown and therefore needs further study. To this end, an integrated analysis of the volatilomics and transcriptomics of jasmine flowers at different flowering stages was performed. The results revealed many candidate transcription factors (TFs) may be involved in regulating the aroma formation of jasmine, among which the MYB-related TF LATE ELONGATED HYPOCOTYL (JsLHY) was identified as a hub gene. Using the DNA affinity purification sequencing method, dual-luciferase reporter, and yeast one-hybrid assays, we demonstrate that JsLHY can bind the gene promoter regions of six aroma-related structural genes (JsBEAT1, JsTPS34, JsCNL6, JsBPBT, JsAAAT5, and Js4CL7) and directly promote their expression. In addition, suppressing JsLHY expression decreased both the expression of JsLHY-bound genes and the content of related VOCs. The present study reveals how JsLHY participates in jasmine aroma formation.

Identification of Floral Volatile Components and Expression Analysis of Controlling Gene in Paeonia ostii 'Fengdan' under Different Cultivation Conditions

In order to explore the release rule of floral volatile substances and the diurnal variation of different flower development stages of Paeonia ostii 'Fengdan' in potted and ground-planted conditions, dynamic headspace adsorption combined with gas chromatography-mass spectrometry(GC-MS) was used to analyze the dynamic changes in floral volatile components and contents. Quantitative real-time PCR (qRT-PCR) was used to analyze changes in flower fragrance-regulating genes PsPAL, PsTPSs, and PsbHLH at different flower development stages and a daily change process at the full-blooming stage. The results show that there were differences in aroma components and contents of Paeonia ostii 'Fengdan' at different flower development stages and different time quantum of every day. There were 25 and 28 aroma components identified in 7 flower development stages of tree peonies planted in pots and in the field, respectively, and 23 and 22 aroma components identified at different time quantum of the day, of which the largest and highest content was alkanes. The main characteristic aroma substances were (E)-β-ocimene, 1,3,5-trimethoxybenzene, 2,4-di-tert-butylphenol, methyl jasmonate, nerol, and cinnamyl alcohol; released amounts of the abovementioned substances varied depending on the development stage and the time of the day. The expression of flower fragrance-controlling genes (PsPAL, PsTPSs, and PsbHLH) in tree peonies varied greatly in different conditions. The results of this study provide a valuable resource to investigate floral fragrance formation in tree peonies.

SCARECROW-like GRAS protein PES positively regulates petunia floral scent production

Emission of scent volatiles by flowers is important for successful pollination and consequently, reproduction. Petunia (Petunia hybrida) floral scent is formed mainly by volatile products of the phenylpropanoid pathway. We identified and characterized a regulator of petunia scent production: the GRAS protein PHENYLPROPANOID EMISSION-REGULATING SCARECROW-LIKE (PES). Its expression increased in petals during bud development and was highest in open flowers. Overexpression of PES increased the production of floral volatiles, while its suppression resulted in scent reduction. We showed that PES upregulates the expression of genes encoding enzymes of the phenylpropanoid and shikimate pathways in petals, and of the core regulator of volatile biosynthesis ODORANT1 by activating its promoter. PES is an ortholog of Arabidopsis (Arabidopsis thaliana) PHYTOCHROME A SIGNAL TRANSDUCTION 1, involved in physiological responses to far-red (FR) light. Analyses of the effect of nonphotosynthetic irradiation (low-intensity FR light) on petunia floral volatiles revealed FR light as a scent-activating factor. While PHYTOCHROME A regulated scent-related gene expression and floral scent production under FR light, the influence of PES on volatile production was not limited by FR light conditions.

Altered profile of floral volatiles and lignin content by down-regulation of Caffeoyl Shikimate Esterase in Petunia

BACKGROUND

The floral volatile profile of Petunia x hybrida 'Mitchell diploid' (MD) is dominated by phenylpropanoids, many of which are derived from p-coumaric acid. However, the downstream processes involved in the production of caffeoyl-CoA and feruloyl-CoA from p-coumaric acid are complex, as the genes and biosynthesis steps are associated with flavonoids and lignin synthesis as well as floral volatiles benzenoid/phenylpropanoid (FVBP). Caffeoyl shikimate esterase (CSE) converts caffeoyl shikimate to caffeic acid and is considered one of the essential regulators in lignin production. Moreover, CSE in involved in phenylpropanoid production. To investigate the roles of CSE in FVBP biosynthesis, we used RNAi-mediated CSE down-regulated (ir-PhCSE) petunias.

RESULTS

Lowered CSE transcript accumulation in ir-PhCSE plants resulted in reduced lignin layers in the stems and stunted growth, suggesting a positive correlation between lignin layers and lignin content. The altered CSE level influenced the expression of many FVBP genes, including elevated transcripts of p-coumarate-3-hydroxylase (C3H), hydroxycinnamoyl transferase (HCT), and 4-coumaric acid: CoA ligase (4CL). In particular, the expression of C4H in ir-PhCSE plants was more than twice the expression in MD plants. Moreover, the production of volatile compounds was alterend in ir-PhCSE plants. Most floral volatiles decreased, and the amounts of phenylalanine and caffeic acid were significantly lower.

CONCLUSIONS

Reduced lignin layers in the stems and stunted growth in ir-PhCSE plants suggest that PhCSE is essential for lignin production and plant growth in petunia. The decreased CSE level influenced the expression of many FVBP genes, and interference of shikimate derivates altered volatile compound production. Significantly decreased caffeic acid, but not ferulic acid, in ir-PhCSE plants suggest that CSE is primarily involved in the reaction of caffeoyl shikimate. Higher C3H and C4H transcripts seem to alleviate accumulated p-coumaric acid resulting from altered CSE. Finally, alteration in C3H, HCT, and 4CL in CSE down-regulated plants suggests an interaction of the FVBP genes, leading to the regulation of floral volatiles of petunia.

NtMYB12 requires for competition between flavonol and (pro)anthocyanin biosynthesis in Narcissus tazetta tepals

The color of flowers is one of the main characteristics adopted for plants to attract pollinators to ensure the reproductive success of the plant, they are also important in their ornamental appeal in Narcissus plant. In this study, we identified a NtMYB12 locus encoding an R2R3-MYB transcription factor. Comparative transcriptome analysis of loss- and gain- of NtMYB12 tissue relative to wild-type narcissus showed NtMYB12 was mainly involved in flavonol and phenylpropanoid metabolic pathways. Biochemical evidences of dual-luciferase activity and chromatin immunoprecipitation assay supported that MYB12 directly bound to promoters of NtFLS, NtLAR, and NtDFR that were cloned by genome walking assay, and activated NtFLS and NtLAR expression but repressed NtDFR expression. More interestingly, NtMYB12 can interact with NtbHLH1 and NtWD40-1 proteins via R3 domain that were selected by transcriptome-based WGCNA and confirmed by yeast two hybrid, bimolecular fluorescence complementation and coimmunoprecipitation assay. Interaction of NtMYB12 with NtbHLH1 and NtWD40-1 forming MYB-bHLH-WD40 triplex specially activated NtDFR and NtANS expression and promoted (pro)anthocyanin accumulation, while NtMYB12 alone activated NtFLS and NtLAR expression and accumulated flavonols, but repressed NtDFR expression. These results indicated that NtMYB12 alone or NtMYB12-bHLH1-WD40-1 triplex requires for competition of metabolism fluxes between flavonol and (pro)anthocyanin biosynthesis. NtMYB12 dually functions on flavonol and proanthocyanin biogenesis via physically binding to NtFLS and NtLAR promoter activating their expression and on (pro)anthocyanin biosynthesis via NtMYB12-NtWD40-NtbHLH (MBW) triplex activating NtDFR and NtANS expression. Requirement of NtMYB12 alone or MBW complex for the competition between flavonol and anthocyanin biosynthesis results in narcissus colorized petal traits.

Analysis of Floral Color Differences between Different Ecological Conditions of Clematis tangutica (Maxim.) Korsh

The Clematis tangutica (Maxim.) Korsh. is a wild flowering plant that is most widely distributed on the Qinghai-Tibet Plateau, with beautiful, brightly colored flowers and good ornamental properties and adaptability. In diverse natural environments, the blossom color of C. tangutica (Maxim.) Korsh. varies greatly, although it is unclear what causes this diversity. It was examined using UPLC-MS/MS and transcriptome sequencing for the investigation of various compounds, differentially expressed genes (DEGs), and flavonoid biosynthesis-related pathways in two flowers in two ecological settings. The results showed that a total of 992 metabolites were detected, of which 425 were differential metabolites, mainly flavonoid metabolites associated with its floral color. The most abundant flavonoids, flavonols and anthocyanin metabolites in the G type were cynaroside, isoquercitrin and peonidin-3-O-glucoside, respectively. Flavonoids that differed in multiplicity in G type and N type were rhoifolin, naringin, delphinidin-3-O-rutinoside, chrysoeriol and catechin. Rhoifolin and chrysoeriol, produced in flavone and flavonol biosynthesis, two flavonoid compounds of C. tangutica (Maxim.) Korsh. with the largest difference in floral composition in two ecological environments. In two ecological environments of flower color components, combined transcriptome and metabolome analyses revealed that BZ1-1 and FG3-1 are key genes for delphinidin-3-O-rutinoside in anthocyanin biosynthesis, and HCT-5 and FG3-3 are key genes for rhoifolin and naringin in flavonoid biosynthesis and flavone and flavonol. Key genes for chlorogenic acid in flavonoid biosynthesis include HCT-6, CHS-1 and IF7MAT-1. In summary, differences in flavonoids and their content are the main factors responsible for the differences in the floral color composition of C. tangutica (Maxim.) Korsh. in the two ecological environments, and are associated with differential expression of genes related to flavonoid synthesis.

Comparative transcriptome analysis linked to key volatiles reveals molecular mechanisms of aroma compound biosynthesis in Prunus mume

BACKGROUND

Mei (Prunus mume) is the only woody plant in the genus Prunus with a floral fragrance, but the underlying mechanisms of aroma compound biosynthesis are unclear despite being a matter of considerable interest.

RESULTS

The volatile contents of the petals of two cultivars with significantly different aromas, Prunus mume 'Xiao Lve' and Prunus mume 'Xiangxue Gongfen', were characterised by GC-MS at different flowering periods, and a total of 44 volatile compounds were detected. Among these, the main substances forming the typical aroma of P. mume were identified as eugenol, cinnamyl acetate, hexyl acetate and benzyl acetate, with variations in their relative concentrations leading to sensory differences in the aroma of the two cultivars. We compiled a transcriptome database at key stages of floral fragrance formation in the two cultivars and used it in combination with differential analysis of floral volatiles to construct a regulatory network for the biosynthesis of key aroma compounds. The results indicated that PmPAL enzymes and PmMYB4 transcription factors play important roles in regulating the accumulation of key biosynthetic precursors to these compounds. Cytochrome P450s and short-chain dehydrogenases/reductases might also influence the biosynthesis of benzyl acetate by regulating production of key precursors such as benzaldehyde and benzyl alcohol. Furthermore, by analogy to genes with verified functions in Arabidopsis, we predicted that three PmCAD genes, two 4CL genes, three CCR genes and two IGS genes all make important contributions to the synthesis of cinnamyl acetate and eugenol in P. mume. This analysis also suggested that the downstream genes PmBGLU18-like, PmUGT71A16 and PmUGT73C6 participate in regulation of the matrix-bound and volatile states of P. mume aroma compounds.

CONCLUSIONS

These findings present potential new anchor points for further exploration of floral aroma compound biosynthesis pathways in P. mume, and provide new insights into aroma induction and regulation mechanisms in woody plants.

Profiling of Volatile Compounds and Associated Gene Expression in Two Anthurium Cultivars and Their F1 Hybrid Progenies

Anthurium is an important ornamental crop in the world market and its floral scent can enhance its ornamental value. To date, studies of the components and formation mechanism of the floral scent of Anthurium are relatively few. In this study, the scent profiles of two Anthurium varieties were measured by gas chromatograph-mass spectrometer (GC-MS). There were 32 volatile organic compounds (VOCs) identified in Anthurium ‘Mystral’, and the most abundant compound was eucalyptol (57.5%). Extremely small amounts of VOCs were detected in Anthurium ‘Alabama’. Compared with A. ‘Alabama’, most genes related to floral scent synthesis exhibited a higher expression in A.‘Mystral’, including AaDXS, AaDXR, AaMDS, AaHDS, AaTPS, AaDAHPS, AaADT2, AaPAL1, and AaPAL2. In order to produce new varieties of Anthurium with fragrance, 454 progenies of two crossbred combinations of A. ‘Mystral’ and A. ‘Alabama’ were obtained. Four F1 generation plants with different floral scent intensities were selected for further study. The major components of floral scent in the progenies were similar to that of the parental A.‘Mystral’ plant. The expression patterns of genes related to floral scent synthesis were consistent with the relative contents of different types of VOCs. This study revealed the profiles of volatile compounds and associated gene expression in two Anthurium cultivars and their F1 hybrids, which provided a basis for the floral scent inheritance of Anthurium andraeanum.

Identification of VOCs in essential oils extracted using ultrasound- and microwave-assisted methods from sweet cherry flower

The floral fragrance of plants is an important indicator in their evaluation. The aroma of sweet cherry flowers is mainly derived from their essential oil. In this study, based on the results of a single-factor experiment, a Box-Behnken design was adopted for ultrasound- and microwave-assisted extraction of essential oil from sweet cherry flowers of the Brooks cultivar. With the objective of extracting the maximum essential oil yield (w/w), the optimal extraction process conditions were a liquid-solid ratio of 52 mL g^-1, an extraction time of 27 min, and a microwave power of 435 W. The essential oil yield was 1.23%, which was close to the theoretical prediction. The volatile organic compounds (VOCs) of the sweet cherry flowers of four cultivars (Brooks, Black Pearl, Tieton and Summit) were identified via headspace solid phase microextraction (SPME) and gas chromatography-mass spectrometry (GC-MS). The results showed that a total of 155 VOCs were identified and classified in the essential oil from sweet cherry flowers of four cultivars, 65 of which were shared among the cultivars. The highest contents of VOCs were aldehydes, alcohols, ketones and esters. Ethanol, linalool, lilac alcohol, acetaldehyde, (E)-2-hexenal, benzaldehyde and dimethyl sulfide were the major volatiles, which were mainly responsible for the characteristic aroma of sweet cherry flowers. It was concluded that the VOCs of sweet cherry flowers were qualitatively similar; however, relative content differences were observed in the four cultivars. This study provides a theoretical basis for the metabolism and regulation of the VOCs of sweet cherry flowers.

Plant Volatile Organic Compounds Evolution: Transcriptional Regulation, Epigenetics and Polyploidy

Volatile organic compounds (VOCs) are emitted by plants as a consequence of their interaction with biotic and abiotic factors, and have a very important role in plant evolution. Floral VOCs are often involved in defense and pollinator attraction. These interactions often change rapidly over time, so a quick response to those changes is required. Epigenetic factors, such as DNA methylation and histone modification, which regulate both genes and transcription factors, might trigger adaptive responses to these evolutionary pressures as well as regulating the rhythmic emission of VOCs through circadian clock regulation. In addition, transgenerational epigenetic effects and whole genome polyploidy could modify the generation of VOCs’ profiles of offspring, contributing to long-term evolutionary shifts. In this article, we review the available knowledge about the mechanisms that may act as epigenetic regulators of the main VOC biosynthetic pathways, and their importance in plant evolution.

Down regulation of p-coumarate 3-hydroxylase in petunia uniquely alters the profile of emitted floral volatiles

Petunia × hybrida cv ‘Mitchell Diploid’ floral volatile benzenoid/phenylpropanoid (FVBP) biosynthesis ultimately produces floral volatiles derived sequentially from phenylalanine, cinnamic acid, and p-coumaric acid. In an attempt to better understand biochemical steps after p-coumaric acid production, we cloned and characterized three petunia transcripts with high similarity to p-coumarate 3-hydroxylase (C3H), hydroxycinnamoyl-CoA:shikimate/quinate hydroxycinnamoyl transferase (HCT), and caffeoyl shikimate esterase (CSE). Transcript accumulation of PhC3H and PhHCT was highest in flower limb tissue during open flower stages. PhCSE transcript accumulation was also highest in flower limb tissue, but it was detected earlier at initial flower opening with a bell-shaped distribution pattern. Down regulation of endogenous PhC3H transcript resulted in altered transcript accumulation of many other FVBP network transcripts, a reduction in floral volatiles, and the emission of a novel floral volatile. Down regulation of PhHCT transcript did not have as large of an effect on floral volatiles as was observed for PhC3H down regulation, but eugenol and isoeugenol emissions were significantly reduced on the downstream floral volatiles. Together these results indicate that PhC3H is involved in FVBP biosynthesis and the reduction of PhC3H transcript influences FVBP metabolism at the network level. Additional research is required to illustrate PhHCT and PhCSE functions of petunia.

Two-dimensional analysis provides molecular insight into flower scent of Lilium 'Siberia'

Lily is a popular flower around the world not only because of its elegant appearance, but also due to its appealing scent. Little is known about the regulation of the volatile compound biosynthesis in lily flower scent. Here, we conducted an approach combining two-dimensional analysis and weighted gene co-expression network analysis (WGCNA) to explore candidate genes regulating flower scent production. In the approach, changes of flower volatile emissions and corresponding gene expression profiles at four flower developmental stages and four circadian times were both captured by GC-MS and RNA-seq methods. By overlapping differentially-expressed genes (DEGs) that responded to flower scent changes in flower development and circadian rhythm, 3,426 DEGs were initially identified to be candidates for flower scent production, of which 1,270 were predicted as transcriptional factors (TFs). The DEGs were further correlated to individual flower volatiles by WGCNA. Finally, 37, 41 and 90 genes were identified as candidate TFs likely regulating terpenoids, phenylpropanoids and fatty acid derivatives productions, respectively. Moreover, by WGCNA several genes related to auxin, gibberellins and ABC transporter were revealed to be responsible for flower scent production. Thus, this strategy provides an important foundation for future studies on the molecular mechanisms involved in floral scent production.

Utilization of transcription factors for controlling floral morphogenesis in horticultural plants

Transcription factors play important roles not only in the development of floral organs but also in the formation of floral characteristics in various plant species. Therefore, transcription factors are reasonable targets for modifying these floral traits and generating new flower cultivars. However, it has been difficult to control the functions of transcription factors because most plant genes, including those encoding transcription factors, exhibit redundancy. In particular, it has been difficult to understand the functions of these redundant genes by genetic analysis. Thus, a breakthrough silencing method called chimeric repressor gene silencing technology (CRES-T) was developed specifically for plant transcription factors. This method transforms transcriptional activators into dominant repressors, and the artificial chimeric repressors suppress the function of transcription factors regardless of their redundancy. Among these chimeric repressors, some were found to be inappropriate for expression throughout the plant body because they resulted in deformities. For these chimeric repressors, utilization of floral organ-specific promoters overcomes this problem by avoiding expression throughout the plant body. In contrast, attachment of viral activation domain VP16 to transcriptional repressors effectively alters into transcriptional activators. This review presents the importance of transcription factors for characterizing floral traits, describes techniques for controlling the functions of transcription factors.

Generation of expressed sequence tags for discovery of genes responsible for floral traits of Chrysanthemum morifolium by next-generation sequencing technology

Background

Chrysanthemum morifolium is one of the most economically valuable ornamental plants worldwide. Chrysanthemum is an allohexaploid plant with a large genome that is commercially propagated by vegetative reproduction. New cultivars with different floral traits, such as color, morphology, and scent, have been generated mainly by classical cross-breeding and mutation breeding. However, only limited genetic resources and their genome information are available for the generation of new floral traits.

Results

To obtain useful information about molecular bases for floral traits of chrysanthemums, we read expressed sequence tags (ESTs) of chrysanthemums by high-throughput sequencing using the 454 pyrosequencing technology. We constructed normalized cDNA libraries, consisting of full-length, 3′-UTR, and 5′-UTR cDNAs derived from various tissues of chrysanthemums. These libraries produced a total number of 3,772,677 high-quality reads, which were assembled into 213,204 contigs. By comparing the data obtained with those of full genome-sequenced species, we confirmed that our chrysanthemum contig set contained the majority of all expressed genes, which was sufficient for further molecular analysis in chrysanthemums.

Conclusion

We confirmed that our chrysanthemum EST set (contigs) contained a number of contigs that encoded transcription factors and enzymes involved in pigment and aroma compound metabolism that was comparable to that of other species. This information can serve as an informative resource for identifying genes involved in various biological processes in chrysanthemums. Moreover, the findings of our study will contribute to a better understanding of the floral characteristics of chrysanthemums including the myriad cultivars at the molecular level.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-4061-3) contains supplementary material, which is available to authorized users.

Phenylpyruvate Contributes to the Synthesis of Fragrant Benzenoid-Phenylpropanoids in Petunia × hybrida Flowers

Phenylalanine (Phe) is a precursor for a large group of plant specialized metabolites, including the fragrant volatile benzenoid-phenylpropanoids (BPs). In plants, the main pathway leading to production of Phe is via arogenate, while the pathway via phenylpyruvate (PPY) is considered merely an alternative route. Unlike plants, in most microorganisms the only pathway leading to the synthesis of Phe is via PPY. Here we studied the effect of increased PPY production in petunia on the formation of BPs volatiles and other specialized metabolites originating from Phe both in flowers and leaves. Stimulation of the pathway via PPY was achieved by transforming petunia with PheA^∗ , a gene encoding a bacterial feedback insensitive bi-functional chorismate mutase/prephenate dehydratase enzyme. PheA^∗ overexpression caused dramatic increase in the levels of flower BP volatiles such as phenylacetaldehyde, benzaldehyde, benzyl acetate, vanillin, and eugenol. All three BP pathways characterized in petunia flowers were stimulated in PheA^∗ flowers. In contrast, PheA^∗ overexpression had only a minor effect on the levels of amino acids and non-volatile metabolites both in the leaves and flowers. The one exception is a dramatic increase in the level of rosmarinate, a conjugate between Phe-derived caffeate and Tyr-derived 3,4-dihydroxyphenylacetate, in PheA^∗ leaves. PheA^∗ petunia flowers may serve as an excellent system for revealing the role of PPY in the production of BPs, including possible routes directly converting PPY to the fragrant volatiles. This study emphasizes the potential of the PPY route in achieving fragrance enhancement in flowering plants.

Biotechnological Advancements for Improving Floral Attributes in Ornamental Plants

Developing new ornamental cultivars with improved floral attributes is a major goal in floriculture. Biotechnological approach together with classical breeding methods has been used to modify floral color, appearance as well as for increasing disease resistance. Transgenic strategies possess immense potential to produce novel flower phenotypes that are not found in nature. Adoption of Genetic engineering has supported the idea of floral trait modification. Ornamental plant attributes like floral color, fragrance, disease resistance, and vase life can be improved by means of genetic manipulation. Therefore, we witness transgenic plant varieties of high aesthetic and commercial value. This review focuses on biotechnological advancements in manipulating key floral traits that contribute in development of diverse ornamental plant lines. Data clearly reveals that regulation of biosynthetic pathways related to characteristics like pigment production, flower morphology and fragrance is both possible and predictable. In spite of their great significance, small number of genetically engineered varieties of ornamental plants has been field tested. Today, novel flower colors production is regarded as chief commercial benefit obtained from transgenic plants. But certain other floral traits are much more important and have high commercial potential. Other than achievements such as novel architecture, modified flower color, etc., very few reports are available regarding successful transformation of other valuable horticultural characteristics. Our review also summarized biotechnological efforts related to enhancement of fragrance and induction of early flowering along with changes in floral anatomy and morphology.

Lilium floral fragrance: A biochemical and genetic resource for aroma and flavor

Hybrid Lilium (common name lily) cultivars are among the top produced domestic fresh cut flowers and potted plants in the US today. Many hybrid Lilium cultivars produce large and showy flowers that emit copious amounts of volatile molecules, which can negatively affect a consumer's appreciation or limit use of the plant product. There are few publications focused on the biochemistry, genetics, and/or molecular regulation of floral volatile biosynthesis for Lilium cultivars. In an initial pursuit to provide breeders with molecular markers for floral volatile biosynthesis, a total of five commercially available oriental and oriental-trumpet hybrid Lilium cultivars were selected for analytical characterization of floral volatile emission. In total, 66 volatile molecules were qualified and quantitated among all cultivars. Chemical classes of identified volatiles include monoterpene hydrocarbons, monoterpene alcohols and aldehydes, phenylpropanoids, benzenoids, fatty-acid-derived, nitrogen-containing, and amino-acid-derived compounds. In general, the floral volatile profiles of the three oriental-trumpet hybrids were dominated by monoterpene hydrocarbons, monoterpene alcohols and aldehydes, while the two oriental hybrids were dominated by monoterpene alcohols and aldehydes and phenylpropanoids, respectively. Tepal tissues (two petal whirls) emitted the vast majority of total volatile molecules compared to the reproductive organs of the flowers. Tepal volatile profiles were cultivar specific with a high degree of distinction, which indicates the five cultivars chosen will provide an excellent differential genetic environment for gene discovery through comparative transcriptomics in the future. Cloning and assaying transcript accumulation from four floral volatile biosynthetic candidates provided few immediate or obvious trends with floral volatile emission.

Circadian Rhythms in Floral Scent Emission

To successfully recruit pollinators, plants often release attractive floral scents at specific times of day to coincide with pollinator foraging. This timing of scent emission is thought to be evolutionarily beneficial to maximize resource efficiency while attracting only useful pollinators. Temporal regulation of scent emission is tied to the activity of the specific metabolic pathways responsible for scent production. Although floral volatile profiling in various plants indicated a contribution by the circadian clock, the mechanisms by which the circadian clock regulates timing of floral scent emission remained elusive. Recent studies using two species in the Solanaceae family provided initial insight into molecular clock regulation of scent emission timing. In Petunia hybrida, the floral volatile benzenoid/phenylpropanoid (FVBP) pathway is the major metabolic pathway that produces floral volatiles. Three MYB-type transcription factors, ODORANT 1 (ODO1), EMISSION OF BENZENOIDS I (EOBI), and EOBII, all of which show diurnal rhythms in mRNA expression, act as positive regulators for several enzyme genes in the FVBP pathway. Recently, in P. hybrida and Nicotiana attenuata, homologs of the Arabidopsis clock gene LATE ELONGATED HYPOCOTYL (LHY) have been shown to have a similar role in the circadian clock in these plants, and to also determine the timing of scent emission. In addition, in P. hybrida, PhLHY directly represses ODO1 and several enzyme genes in the FVBP pathway during the morning as an important negative regulator of scent emission. These findings facilitate our understanding of the relationship between a molecular timekeeper and the timing of scent emission, which may influence reproductive success.

Petunia × hybrida floral scent production is negatively affected by high-temperature growth conditions

Increasing temperatures due to changing global climate are interfering with plant-pollinator mutualism, an interaction facilitated mainly by floral colour and scent. Gas chromatography-mass spectroscopy analyses revealed that increasing ambient temperature leads to a decrease in phenylpropanoid-based floral scent production in two Petunia × hybrida varieties, P720 and Blue Spark, acclimated at 22/16 or 28/22 °C (day/night). This decrease could be attributed to down-regulation of scent-related structural gene expression from both phenylpropanoid and shikimate pathways, and up-regulation of a negative regulator of scent production, emission of benzenoids V (EOBV). To test whether the negative effect of increased temperature on scent production can be reduced in flowers with enhanced metabolic flow in the phenylpropanoid pathway, we analysed floral volatile production by transgenic 'Blue Spark' plants overexpressing CaMV 35S-driven Arabidopsis thaliana production of anthocyanin pigments 1 (PAP1) under elevated versus standard temperature conditions. Flowers of 35S:PAP1 transgenic plants produced the same or even higher levels of volatiles when exposed to a long-term high-temperature regime. This phenotype was also evident when analysing relevant gene expression as inferred from sequencing the transcriptome of 35S:PAP1 transgenic flowers under the two temperature regimes. Thus, up-regulation of transcription might negate the adverse effects of temperature on scent production.

An R2R3-MYB Transcription Factor Regulates Eugenol Production in Ripe Strawberry Fruit Receptacles

Eugenol is a volatile phenylpropanoid that contributes to flower and ripe fruit scent. In ripe strawberry (Fragaria × ananassa) fruit receptacles, eugenol is biosynthesized by eugenol synthase (FaEGS2). However, the transcriptional regulation of this process is still unknown. We have identified and functionally characterized an R2R3 MYB transcription factor (emission of benzenoid II [FaEOBII]) that seems to be the orthologous gene of PhEOBII from Petunia hybrida, which contributes to the regulation of eugenol biosynthesis in petals. The expression of FaEOBII was ripening related and fruit receptacle specific, although high expression values were also found in petals. This expression pattern of FaEOBII correlated with eugenol content in both fruit receptacle and petals. The expression of FaEOBII was repressed by auxins and activated by abscisic acid, in parallel to the ripening process. In ripe strawberry receptacles, where the expression of FaEOBII was silenced, the expression of cinnamyl alcohol dehydrogenase1 and FaEGS2, two structural genes involved in eugenol production, was down-regulated. A subsequent decrease in eugenol content in ripe receptacles was also observed, confirming the involvement of FaEOBII in eugenol metabolism. Additionally, the expression of FaEOBII was under the control of FaMYB10, another R2R3 MYB transcription factor that regulates the early and late biosynthetic genes from the flavonoid/phenylpropanoid pathway. In parallel, the amount of eugenol in FaMYB10-silenced receptacles was also diminished. Taken together, these data indicate that FaEOBII plays a regulating role in the volatile phenylpropanoid pathway gene expression that gives rise to eugenol production in ripe strawberry receptacles.

Enhanced formation of aromatic amino acids increases fragrance without affecting flower longevity or pigmentation in Petunia × hybrida

Purple Petunia × hybrida V26 plants accumulate fragrant benzenoid-phenylpropanoid molecules and anthocyanin pigments in their petals. These specialized metabolites are synthesized mainly from the aromatic amino acids phenylalanine. Here, we studied the profile of secondary metabolites of petunia plants, expressing a feedback-insensitive bacterial form of 3-deoxy-di-arabino-heptulosonate 7-phosphate synthase enzyme (AroG*) of the shikimate pathway, as a tool to stimulate the conversion of primary to secondary metabolism via the aromatic amino acids. We focused on specialized metabolites contributing to flower showy traits. The presence of AroG* protein led to increased aromatic amino acid levels in the leaves and high phenylalanine levels in the petals. In addition, the AroG* petals accumulated significantly higher levels of fragrant benzenoid-phenylpropanoid volatiles, without affecting the flowers' lifetime. In contrast, AroG* abundance had no effect on flavonoids and anthocyanins levels. The metabolic profile of all five AroG* lines was comparable, even though two lines produced the transgene in the leaves, but not in the petals. This implies that phenylalanine produced in leaves can be transported through the stem to the flowers and serve as a precursor for formation of fragrant metabolites. Dipping cut petunia stems in labelled phenylalanine solution resulted in production of labelled fragrant volatiles in the flowers. This study emphasizes further the potential of this metabolic engineering approach to stimulate the production of specialized metabolites and enhance the quality of various plant organs. Furthermore, transformation of vegetative tissues with AroG* is sufficient for induced production of specialized metabolites in organs such as the flowers.

Floral volatiles: from biosynthesis to function

Floral volatiles have attracted humans' attention since antiquity and have since then permeated many aspects of our lives. Indeed, they are heavily used in perfumes, cosmetics, flavourings and medicinal applications. However, their primary function is to mediate ecological interactions between flowers and a diverse array of visitors, including pollinators, florivores and pathogens. As such, they ultimately ensure the plants' reproductive and evolutionary success. To date, over 1700 floral volatile organic compounds (VOCs) have been identified. Interestingly, they are derived from only a few biochemical networks, which include the terpenoid, phenylpropanoid/benzenoid and fatty acid biosynthetic pathways. These pathways are intricately regulated by endogenous and external factors to enable spatially and temporally controlled emission of floral volatiles, thereby fine-tuning the ecological interactions facilitated by floral volatiles. In this review, we will focus on describing the biosynthetic pathways leading to floral VOCs, the regulation of floral volatile emission, as well as biological functions of emitted volatiles.

Comparative analysis of flower volatiles from nine citrus at three blooming stages

Volatiles from flowers at three blooming stages of nine citrus cultivars were analyzed by headspace-solid phase microextraction (HS-SPME)-GC-MS. Up to 110 volatiles were detected, with 42 tentatively identified from citrus flowers for the first time. Highest amounts of volatiles were present in fully opened flowers of most citrus, except for pomelos. All cultivars were characterized by a high percentage of either oxygenated monoterpenes or monoterpene hydrocarbons, and the presence of a high percentage of nitrogen containing compounds was also observed. Flower volatiles varied qualitatively and quantitatively among citrus types during blooming. Limonene was the most abundant flower volatile only in citrons; α-citral and β-citral ranked 2nd and 3rd only for Bergamot, and unopened flowers of Ponkan had a higher amount of linalool and β-pinene while much lower amount of γ-terpinene and p-cymene than Satsuma. Taking the average of all cultivars, linalool and limonene were the top two volatiles for all blooming stages; β-pinene ranked 3rd in unopened flowers, while indole ranked 3rd for half opened and fully opened flower volatiles. As flowers bloomed, methyl anthranilate increased while 2-hexenal and p-cymene decreased. In some cases, a volatile could be high in both unopened and fully opened flowers but low in half opened ones. Through multivariate analysis, the nine citrus cultivars were clustered into three groups, consistent with the three true citrus types. Furthermore, an influence of blooming stages on clustering was observed, especially with hybrids Satsuma and Huyou. Altogether, it was suggested that flower volatiles can be suitable markers for revealing the genetic relationships between citrus cultivars but the same blooming stage needs to be strictly controlled.

Frontiers of torenia research: innovative ornamental traits and study of ecological interaction networks through genetic engineering

Advances in research in the past few years on the ornamental plant torenia (Torenia spps.) have made it notable as a model plant on the frontier of genetic engineering aimed at studying ornamental characteristics and pest control in horticultural ecosystems. The remarkable advantage of torenia over other ornamental plant species is the availability of an easy and high-efficiency transformation system for it. Unfortunately, most of the current torenia research is still not very widespread, because this species has not become prominent as an alternative to other successful model plants such as Arabidopsis, snapdragon and petunia. However, nowadays, a more global view using not only a few selected models but also several additional species are required for creating innovative ornamental traits and studying horticultural ecosystems. We therefore introduce and discuss recent research on torenia, the family Scrophulariaceae, for secondary metabolite bioengineering, in which global insights into horticulture, agriculture and ecology have been advanced. Floral traits, in torenia particularly floral color, have been extensively studied by manipulating the flavonoid biosynthetic pathways in flower organs. Plant aroma, including volatile terpenoids, has also been genetically modulated in order to understand the complicated nature of multi-trophic interactions that affect the behavior of predators and pollinators in the ecosystem. Torenia would accordingly be of great use for investigating both the variation in ornamental plants and the infochemical-mediated interactions with arthropods.

The R2R3-MYB-like regulatory factor EOBI, acting downstream of EOBII, regulates scent production by activating ODO1 and structural scent-related genes in petunia

Flower scent is a highly dynamic trait, under developmental, spatial, and diurnal regulation. The mechanism governing scent production is only beginning to be unraveled. In petunia (Petunia hybrida), EMISSION OF BENZENOIDS II (EOBII) controls transcription of both the shikimate pathway-regulating MYB factor ODORANT1 (ODO1) and phenylpropanoid scent-related structural genes. A promoter-activation screen identified an R2R3-MYB-like regulatory factor of phenylpropanoid volatile biosynthesis acting downstream of EOBII, designated EOBI. EOBI silencing led to downregulation of ODO1 and numerous structural scent-related genes from both the shikimate and phenylpropanoid pathways. The ability of EOBI to directly activate ODO1, as revealed by electrophoretic mobility shift assay and yeast one-hybrid analysis, place EOBI upstream of ODO1 in regulating substrate availability for volatile biosynthesis. Interestingly, ODO1-silenced transgenic petunia flowers accumulated higher EOBI transcript levels than controls, suggesting a complex feedback loop between these regulatory factors. The accumulation pattern of EOBI transcript relative to EOBII and ODO1, and the effect of up/downregulation of EOBII on transcript levels of EOBI and ODO1, further support these factors' hierarchical relationships. The dependence of scent production on EOBI expression and its direct interaction with both regulatory and structural genes provide evidence for EOBI's wide-ranging involvement in the production of floral volatiles.

A peroxisomally localized acyl-activating enzyme is required for volatile benzenoid formation in a Petuniaxhybrida cv. 'Mitchell Diploid' flower

Floral volatile benzenoid/phenylpropanoid (FVBP) biosynthesis is a complex and coordinate cellular process executed by petal limb cells of a Petunia×hybrida cv. 'Mitchell Diploid' (MD) plant. In MD flowers, the majority of benzenoid volatile compounds are derived from a core phenylpropanoid pathway intermediate by a coenzyme A (CoA) dependent, β-oxidative scheme. Metabolic flux analysis, reverse genetics, and biochemical characterizations of key enzymes in this pathway have supported this putative concept. However, the theoretical first enzymatic reaction, which leads to the production of cinnamoyl-CoA, has only been physically demonstrated in a select number of bacteria like Streptomyces maritimus through mutagenesis and recombinant protein production. A transcript has been cloned and characterized from MD flowers that shares high homology with an Arabidopsis thaliana transcript ACYL-ACTIVATING ENZYME11 (AtAAE11) and the S. maritimus ACYL-COA:LIGASE (SmEncH). In MD, the PhAAE transcript accumulates in a very similar manner as bona fide FVBP network genes, i.e. high levels in an open flower petal and ethylene regulated. In planta, PhAAE is localized to the peroxisome. Upon reduction of PhAAE transcript through a stable RNAi approach, transgenic flowers emitted a reduced level of all benzenoid volatile compounds. Together, the data suggest that PhAAE may be responsible for the activation of t-cinnamic acid, which would be required for floral volatile benzenoid production in MD.