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

Determination of Volatile Organic Compounds and Endogenous Extracts and Study of Expression Patterns of TPS and BSMT in the Flowers of Seven Lilium Cultivars

Lily is one of the most important cut flowers in the world, with a rich floral fragrance. To further explore the fragrance emission mechanisms of lily cultivars, headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) and organic solvent extraction-gas chromatography-mass spectrometry (OSE-GC-MS) were used to unveil the volatile organic compounds (VOCs) and endogenous extracts of seven lily cultivars. Furthermore, real-time quantitative PCR (qRT-PCR) was used to determine the expression levels of two key genes (TPS and BSMT) related to the biosynthesis of monoterpenoids and methyl benzoate. The results show that forty-five VOCs were detected in the petals of seven lily cultivars, and the main compounds were monoterpenoids and phenylpropanoids/benzenoids. Dichloromethane was the best solvent for extracting the endogenous extracts of Lilium 'Viviana' petals and eighteen endogenous extracts were detected using dichloromethane to extract the petals of seven lily cultivars. Each compound's emission ratio (natural logarithm of the ratio of VOC content to endogenous extract content) was calculated, and linear regression analyses between emission ratios and boiling points were conducted. Significant linear negative correlations existed between the emission ratios and boiling points of compounds, and the regression equations' coefficients of determination (R2) were all greater than 0.7. TPS was expressed highly in 'Viviana', 'Pink News', and 'Palazzo', and BSMT was expressed highly in 'Pink News' and 'Palazzo'. Correlation analyses between the gene expression levels and the monoterpenoids and methyl benzoate contents found that the TPS expression levels have strong positive correlations with monoterpenoids content, while no correlations were found between the expression levels of BSMT and the contents of methyl benzoate. This study lays the foundation for research on the release patterns of VOCs in the flowers of Lilium, and the breeding of lilies for their floral fragrance.

A BAHD acyltransferase contributes to the biosynthesis of both ethyl benzoate and methyl benzoate in the flowers of Lilium oriental hybrid 'Siberia'

Lily is a popular flower worldwide due to its elegant appearance and pleasant fragrance. Floral volatiles of lily are predominated by monoterpenes and benzenoids. While a number of genes for monoterpene biosynthesis have been characterized, the molecular mechanism underlying floral benzenoid formation in lily remains unclear. Here, we report on the identification and characterization of a novel BAHD acyltransferase gene that contributes to the biosynthesis of two related floral scent benzoate esters, ethyl benzoate and methyl benzoate, in the scented Lilium oriental hybrid 'Siberia'. The emission of both methyl benzoate and ethyl benzoate in L. 'Siberia' was found to be tepal-specific, floral development-regulated and rhythmic. Through transcriptome profiling and bioinformatic analysis, a BAHD acyltransferase gene designated LoAAT1 was identified as the top candidate gene for the production of ethyl benzoate. In vitro enzyme assays and substrate feeding assays provide substantial evidence that LoAAT1 is responsible for the biosynthesis of ethyl benzoate. It was interesting to note that in in vitro enzyme assay, LoAAT1 can also catalyze the formation of methyl benzoate, which is typically formed by the action of benzoic acid methyltransferase (BAMT). The lack of an expressed putative BAMT gene in the flower transcriptome of L. 'Siberia', together with biochemical and expression evidence, led us to conclude that LoAAT1 is also responsible for, or at least contributes to, the biosynthesis of the floral scent compound methyl benzoate. This is the first report that a member of the plant BAHD acyltransferase family contributes to the production of both ethyl benzoate and methyl benzoate, presenting a new mechanism for the biosynthesis of benzoate esters.

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.

Community Structure of Phyllosphere Bacteria in Different Cultivars of Fingered Citron (Citrus medica 'Fingered') and Their Correlations With Fragrance

In recent years, plant metabolomics and microbiome studies have suggested that the synthesis and secretion of plant secondary metabolites are affected by microbial-host symbiotic interactions. In this study, six varieties of fingered citron (Citrus medica 'Fingered') are sampled to study their phyllosphere bacterial communities and volatile organic compounds (VOCs). High-throughput sequencing is used to sequence the V5-V7 region of the 16S rRNA of the fingered citron phyllosphere bacteria, and the results showed that Proteobacteria, Actinobacteria, Firmicutes, and Bacteroidetes were the dominant bacterial phylum in the phyllosphere of fingered citron. There were significant differences in the phyllosphere bacteria community between XiuZhen and the remaining five varieties. The relative abundance of Actinomycetospora was highest in XiuZhen, and Halomonas, Methylobacterium, Nocardioides, and Pseudokineococcus were also dominant. Among the remaining varieties, Halomonas was the genus with the highest relative abundance, while the relative abundances of all the other genera were low. Headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS) were used to analyze and identify the aroma compounds of six different fingered citron, and a total of 76 aroma compounds were detected in six varieties. Pinene, geraniol, and linalool were found to be the primary VOCs that affect the aroma of fingered citron based on relative odor activity value. The correlation analysis showed 55 positive and 60 negative correlations between the phyllosphere bacterial flora and aroma compounds of fingered citron. The top 10 genera in the relative abundance were all significantly associated with aroma compounds. This study provides deep insight into the relation between bacteria and VOCs of fingered citron, and this may better explain the complexity of the analysis of bacterial and metabolic interactions.

Comprehensive Flavor Analysis of Volatile Components During the Vase Period of Cut Lily (Lilium spp. 'Manissa') Flowers by HS-SPME/GC-MS Combined With E-Nose Technology

Volatile compounds could affect the flavor and ornamental quality of cut flowers, but the flavor change occurring during the vase period of the cut flower is unclear. To clarify the dynamic changes during the vase period of cut lily (Lilium spp. 'Manissa') flowers, comprehensive flavor profiles were characterized by the electronic nose (E-nose) and headspace solid-phase microextraction gas chromatography-mass spectrometry (HS-SPME/GC-MS). The response value of sensor W2W was significantly higher than other sensors, and its response value reached the highest on day 4. A total of 59 volatiles were detected in cut lilies by HS-SPME/GC-MS, mainly including aldehydes, alcohols, and esters. There were 19 volatiles with odor activity values (OAVs) greater than 1. Floral and fruity aromas were stronger, followed by a pungent scent. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) could effectively discriminate lily samples derived from different vase times on the basis of E-nose and HS-SPME-GC-MS. In summary, our study investigates the flavor change profile and the diversity of volatile compounds during the vase period of cut lilies, and lilies on day 4 after harvest exhibited excellent aroma and flavor taking into consideration of the flavor intensity and diversity. This provided theoretical guidance for the assessment of scent volatiles and flavor quality during the vase period of cut lily flowers and will be helpful for the application of cut lilies during the postharvest process.

Volatile Compounds from Flowers of Elaeagnus x submacrophylla Servett.: Extraction, Identification of Flavonoids, and Antioxidant Capacity

Beneficial to the ecosystem and with significant potential in permaculture, Elaeagnus x submacrophylla Servett. was studied here mainly for the identification of its floral odorants. After olfactory evaluation and determination of the volatile profile of freshly picked flowers by headspace/solid phase microextraction coupled with gas chromatography/mass spectrometry, an ethanolic extract was prepared and investigated for its antioxidant capacity. Unusual molecules were identified in the floral headspace, such as isochavicol or chrysanthemum acetate. The evaluation of the in vitro free radical scavenging capacity (from 0.4 to 1.3 mmol TE/g) and total phenolic content (65.1 mg GAE/g) of the extract pointed out a promising antioxidant activity, potentially related to the identification of several flavonoid glycosides. These results have to be considered in the context of the ever-increasing need to produce innovative natural extracts with notably interesting claims for the cosmetic field.

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.

Cloning, functional characterization and expression analysis of LoTPS5 from Lilium 'Siberia'

Lilium 'Siberia' is a perennial herbaceous plant that is commercially significant because of its snowy white floral color and appealing scent which is mainly due to the presence of monoterpenes and benzoids compounds in floral volatile profile. In the current study, LoTPS5 was cloned and functionally characterized. Results revealed that LoTPS5 specifically generates squalene from FPP, whereas no product was produced when it was incubated with GPP or GGPP. The subcellular localization experiment showed that LoTPS5 was located in plastids. Furthermore, LoTPS5 showed its high expression in the leaf followed by petals and sepals of the flower. Moreover, the expression of LoTPS5 gradually increased from the bud stage and peak at the full-bloom stage. Besides, LoTPS5 showed a diurnal circadian rhythmic pattern with a peak in the afternoon (16:00) followed by deep night (24:00) and morning (8:00), respectively. LoTPS5 is highly responsive to mechanical wounding by rapidly elevating its mRNA transcript level. The current study will provide significant information for future studies of terpenoid and squalene biosynthesis in Lilium 'Siberia'.

Overexpression of LiTPS2 from a cultivar of lily (Lilium 'Siberia') enhances the monoterpenoids content in tobacco flowers

Lily, a famous cut flower with highly fragrance, has high ornamental and economic values. Monoterpenes are the main components contributing to its fragrance, and terpene synthase (TPS) genes play critical roles in the biosynthesis of monoterpenoids. To understand the function of TPS and to explore the molecular mechanism of floral scent in cultivar Lilium 'Siberia', transcriptomes of petal at different flowering stages and leaf were obtained by RNA sequencing and three unigenes related to TPS genes were selected for further validation. Quantitative real-time PCR showed that the expression level of LiTPS2 was greater than that of the other two TPS genes. Phylogenetic analysis indicated that LiTPS2 belonged to the TPSb subfamily, which was responsible for monoterpenes synthesis. Subcellular localization demonstrated that LiTPS2 was located in the chloroplasts. Furthermore, functional characterization showed that LiTPS2 utilized both geranyl pyrophosphate (GPP) and farnesyl pyrophosphate (FPP) to produce monoterpenoids such as linalool and sesquiterpenes like trans-nerolidol, respectively. Ectopic expression in transgenic tobacco plants suggested that the amount of linalool from the flowers of transgenic plants was 2-3 fold higher than that of wild-type plants. And the emissions of myrcene and (E)-β-ocimene were also accumulated from the flowers of LiTPS2 transgenic lines. Surprisingly, these three compounds were the main fragrance components of oriental lily hybrids. Our results indicated that LiTPS2 contributed to the production of monoterpenes and could effectively regulate the aroma of Lilium cultivars, laying the foundation for biotechnological modification of floral scent profiles.

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.

Volatile composition and classification of Lilium flower aroma types and identification, polymorphisms, and alternative splicing of their monoterpene synthase genes

Lily is a well-known ornamental plant with a diversity of fragrant types. Basic information on lily floral scent compounds has been obtained for only a few accessions, and little is known about Lilium aroma types, the terpene synthase genes that may play roles in the production of key volatiles, or the range of monoterpenes that these genes produce. In this study, 41 cultivars were analyzed for volatile emissions, and a total of 46 individual volatile compounds were identified, 16 for the first time in lilies. Lily accessions were classified into six groups according to the composition of major scent components: faint-scented, cool, fruity, musky, fruity-honey, and lily. Monoterpenes were one of the main groups of volatiles identified, and attention was focused on terpene synthase (TPS) genes, which encode enzymes that catalyze the last steps in monoterpene synthesis. Thirty-two candidate monoterpene synthase cDNAs were obtained from 66 lily cultivars, and 64 SNPs were identified. Two InDels were also shown to result from variable splicing, and sequence analysis suggested that different transcripts arose from the same gene. All identified nucleotide substitution sites were highly correlated with the amounts of myrcene emitted, and InDel site 230 was highly correlated with the emission of all major monoterpenoid components, especially (E)-β-ocimene. Heterologous expression of five cDNAs cloned from faint-scented and strong-scented lilies showed that their corresponding enzymes could convert geranyl diphosphate to (E)-β-ocimene, α-pinene, and limonene. The findings from this study provide a major resource for the assessment of lily scent volatiles and will be helpful in breeding of improved volatile components.

Overexpression of LiDXS and LiDXR From Lily (Lilium 'Siberia') Enhances the Terpenoid Content in Tobacco Flowers

Lilium, the famous and significant cut flower, emits a variety of volatile organic compounds, which mainly contain monoterpenes, such as myrcene, (E)-β-ocimene, and linalool. To understand the molecular mechanism of monoterpene synthesis in Lilium, we cloned two potential genes in the methylerythritol 4-phosphate pathway, namely LiDXS and LiDXR, from the strong-flavored oriental Lilium 'Siberia' using a homology-based PCR strategy. The expression levels of LiDXS and LiDXR were consistent with the emission and accumulation of monoterpenes in different floral organs and during the floral development, indicating that these two genes may play key roles in monoterpene synthesis. Subcellular localization demonstrated that LiDXS and LiDXR are expressed in the chloroplasts. Ectopic expression in transgenic tobacco suggested that the flowers of LiDXS and LiDXR transgenic lines accumulated substantially more diterpene, sclareol, compared to the plants transformed with empty vector. Surprisingly, increased content of the monoterpene, linalool and sesquiterpene, caryophyllene, were detected in the LiDXR transgenic lines, whereas the emission of caryophyllene, increased in one of the LiDXS transgenic tobacco lines, indicating that these two genes play significant roles in the synthesis of floral volatiles in the transgenic plants. These results demonstrate that LiDXR can contribute to monoterpene biosynthesis in Lilium 'Siberia'; however, the role of LiDXS in the biosynthesis of monoterpenes needs further study.

Identification of differentially expressed genes in flower, leaf and bulb scale of Lilium oriental hybrid 'Sorbonne' and putative control network for scent genes

BACKGROUND

Lily is an economically important plant, with leaves and bulbs consisting of overlapping scales, large ornamental flowers and a very large genome. Although it is recognized that flowers and bulb scales are modified leaves, very little is known about the genetic control and biochemical differentiation underlying lily organogenesis and development. Here we examined the differentially expressed genes in flower, leaf and scale of lily, using RNA-sequencing, and identified organ-specific genes, including transcription factors, genes involved in photosynthesis in leaves, carbohydrate metabolism in bulb scales and scent and color production in flowers.

RESULTS

Over 11Gb data were obtained and 2685, 2296, and 1709 differentially expressed genes were identified in the three organs, with 581, 662 and 977 unique DEGs in F-vs-S, L-vs-S and L-vs-F comparisons. By functional enrichment analysis, genes likely to be involved in biosynthetic pathways leading to floral scent production, such as 1-deoxy-D-xylulose-5-phosphate synthase (DXS), 3-ketoacyl-CoA thiolase (KAT), hydroperoxide lyase (HPL), geranylgeranyl pyrophosphate (GGPP) 4-hydroxy-3-methylbut-2-en-1-yl diphosphate (HDS) and terpene synthase (TPS), and floral color genes, such as dihydroflavonol 4-reductase (DFR), chalcone synthase (CHS), chalcone isomerase (CHI), flavonol synthase (FLS) were identified. Distinct groups of genes that participate in starch and sucrose metabolism, such as sucrose synthase (SS), invertase (INV), sucrose phosphate synthase (SPS), starch synthase (SSS), starch branching enzyme (SBE), ADP-glucose pyrophosphorylase (AGP) andβ-amylase (BAM) and photosynthesis genes (Psa, Psb, Pet and ATP) were also identified. The expression of six floral fragrance-related DGEs showed agreement between qRT-PCR results and RPKM values, confirming the value of the data obtained by RNA-seq. We obtained the open reading frame of the terpene synthase gene from Lilium 'Sorbonne', designated LsTPS, which had 99.55% homology to transcript CL4520.Contig5_All. In addition, 54, 48 and 50 differently expressed transcription factor were identified by pairwise comparisons between the three organs and a regulatory network for monoterpene biosynthesis was constructed.

CONCLUSIONS

Analysis of differentially expressed genes in flower, leaf and bulb scale of lily, using second generation sequencing technology, yielded detailed information on lily metabolic differentiation in three organs. Analysis of the expression of flower scent biosynthesis genes has provided a model for the regulation of the pathway and identified a candidate gene encoding an enzyme catalyzing the final step in scent production. These digital gene expression profiles provide a valuable and informative database for the further identification and analysis of structural genes and transcription factors in different lily organs and elucidation of their function.

Scent from Jasminum grandiflorum flowers: Investigation of the change in linalool enantiomers at various developmental stages using chemical and molecular methods

Jasminum species are among the most preferred fresh cut flowers in India since ancient times. The plant produces small and fragrant flowers, which are of great demand in the preparation of fragrant garlands and also in perfume industries. Floral volatile of Jasminum grandiflorum L. (Family: Oleaceae) was extracted using solid-phase microextraction and analyzed in enantioselective gas chromatography. Chemical classes of identified volatiles revealed the presence of terpenoids, phenylpropanoids, and fatty acid derivatives. Marker constituent of flower volatiles, linalool was selected for analytical characterization on ethyl- and acetyl-β-cyclodextrin stationary phase. (R)-(-)-Linalool was found as major enantiomer in volatiles of floral buds whereas (S)-(+)-linalool predominated in the volatiles of matured flowers. Simultaneously, a quantitative real-time PCR was performed to find the gene expression of linalool synthase to investigate the mechanism of enantiomeric inversion. The emission pattern of (R)-(-)-linalool at different flower developmental stages was well correlated (P = 0.01) with the gene expression of the cloned linalool synthase from J. grandiflorum. We observed that the successive change in (R)- to (S)-linalool ratio from bud to mature flower was mainly due to the enantio- specific transformation and temporal decline of (R)-linalool producing gene in J. grandiflorum. This enantiomeric change also leads to the difference in flower aroma. Furthermore, this is probably the reason behind consumer's acceptance for jasmine buds rather than bloomed flowers in cut flower segments.

Light Quality Dependent Changes in Morphology, Antioxidant Capacity, and Volatile Production in Sweet Basil (Ocimum basilicum)

Narrow-bandwidth light treatments may be used to manipulate plant growth, development and metabolism. In this report LED-based light treatments were used to affect yield and metabolic content of sweet basil (Ocimum basilicum L. cv "Ceasar") grown in controlled environments. This culinary herb produces an aroma highly appreciated by consumers, primarily composed of terpenes/terpenoids, phenylpropanoids, and fatty-acid- derived volatile molecules. Basil plants were grown under narrow-bandwidth light conditions, and leaf area, height, mass, antioxidant capacity and volatile emissions were measured at various time points. The results indicate reproducible significant differences in specific volatiles, and in biochemical classes of volatiles, compared to greenhouse grown plants. For example, basil plants grown under blue/red/yellow or blue/red/green wavelengths emit higher levels of a subset of monoterpenoid volatiles, while a blue/red/far-red treatment leads to higher levels of most sesquiterpenoid volatile molecules. Specific light treatments increase volatile content, mass, and antioxidant capacity. The results show that narrow-bandwidth illumination can induce discrete suites of volatile classes that affect sensory quality in commercial herbs, and may be a useful tool in improving commercial production.