More lessons from linalool: insights gained from a ubiquitous floral volatile

Functional Characterization of Terpene Synthases Accounting for the Volatilized-Terpene Heterogeneity in Lathyrus odoratus Cultivar Flowers

Lathyrus odoratus (sweet pea) is an ornamental plant with exceptional floral scent, previously used as an experimental organism in the early development of Mendelian genetics. However, its terpene synthases (TPSs), which act as metabolic gatekeepers in the biosynthesis of volatile terpenoids, remain to be characterized. Auto-Headspace Solid-phase Microextraction/Gas chromatography-mass spectrometry analysis of floral volatile terpene constituents from seven sweet pea cultivars identified α-bergamotene, linalool, (-)-α-cubebene, geraniol, β-caryophyllene and β-sesquiphellandrene as the dominant compounds. RNA sequencing was performed to profile the transcriptome of L. odoratus flowers. Bioinformatic analysis identified eight TPS genes (acronymed as LoTPS) that were successfully cloned, heterologously expressed and functionally analyzed. LoTPS4 and LoTPS7, belonging to the TPS-b clade, biochemically catalyzed the formation of monoterpenes and sesquiterpenes. LoTPS3 and LoTPS8, placed in the TPS-a clade, also generated monoterpenes and sesquiterpenes, while LoTPS12 belonging to the TPS-g clade showed linalool/nerolidol synthase activity. Notably, biochemical assays of the recombinant LoTPS proteins revealed their catalytic promiscuity, and the enzymatic products were basically consistent with major volatile compounds released from sweet pea flowers. The data from our study lay the foundation for the chemical ecology, molecular genetics and biotechnological improvement of sweet pea and other legumes (Fabaceae).

Deciphering the Biotic and Climatic Factors That Influence Floral Scents: A Systematic Review of Floral Volatile Emissions

Currently, a global analysis of the information available on the relative composition of the floral scents of a very diverse variety of plant species is missing. Such analysis may reveal general patterns on the distribution and dominance of the volatile compounds that form these mixtures, and may also allow measuring the effects of factors such as the phylogeny, pollination vectors, and climatic conditions on the floral scents of the species. To fill this gap, we compiled published data on the relative compositions and emission rates of volatile organic compounds (VOCs) in the floral scents of 305 plant species from 66 families. We also gathered information on the groups of pollinators that visited the flowers and the climatic conditions in the areas of distribution of these species. This information allowed us to characterize the occurrence and relative abundances of individual volatiles in floral scents and the effects of biotic and climatic factors on floral scent. The monoterpenes trans-β-ocimene and linalool and the benzenoid benzaldehyde were the most abundant floral VOCs, in both ubiquity and predominance in the floral blends. Floral VOC richness and relative composition were moderately preserved traits across the phylogeny. The reliance on different pollinator groups and the climate also had important effects on floral VOC richness, composition, and emission rates of the species. Our results support the hypothesis that key compounds or compounds originating from specific biosynthetic pathways mediate the attraction of the main pollinators. Our results also indicate a prevalence of monoterpenes in the floral blends of plants that grow in drier conditions, which could link with the fact that monoterpene emissions protect plants against oxidative stresses throughout drought periods and their emissions are enhanced under moderate drought stress. Sesquiterpenes, in turn, were positively correlated with mean annual temperature, supporting that sesquiterpene emissions are dominated mainly by ambient temperature. This study is the first to quantitatively summarise data on floral-scent emissions and provides new insights into the biotic and climatic factors that influence floral scents.

Molecular Mechanisms of Pollination Biology

Pollination is the transfer of pollen grains from the stamens to the stigma, an essential requirement of sexual reproduction in flowering plants. Cross-pollination increases genetic diversity and is favored by selection in the majority of situations. Flowering plants have evolved a wide variety of traits that influence pollination success, including those involved in optimization of self-pollination, attraction of animal pollinators, and the effective use of wind pollination. In this review we discuss our current understanding of the molecular basis of the development and production of these various traits. We conclude that recent integration of molecular developmental studies with population genetic approaches is improving our understanding of how selection acts on key floral traits in taxonomically diverse species, and that further work in nonmodel systems promises to provide exciting insights in the years to come.

Composition and Biosynthesis of Scent Compounds from Sterile Flowers of an Ornamental Plant Clematis florida cv. 'Kaiser'

Clematisflorida is a popular ornamental vine species known for diverse colors and shapes of its flowers but not for scent. Here we investigated the composition and biosynthesis of floral scent in ‘Kaiser’, a fragrant cultivar of C. florida that has sterile flowers. Volatile profiling revealed that flowers of ‘Kaiser’ emit more than 20 compounds, with monoterpenes being most abundant. Among the three floral organs, namely sepals, transformed-petals, and ovaries, ovaries had the highest rates of total volatile emission. To determine the molecular mechanism underlying floral scent biosynthesis in ‘Kaiser’, we sequenced a flower transcriptome and searched the transcriptome for terpene synthase genes (TPSs), which are key genes for terpene biosynthesis. Among the TPS genes identified, three were putative intact full-length genes and were designated CfTPS1, CfTPS2, and CfTPS3. Phylogenetic analysis placed CfTPS1, CfTPS2, and CfTPS3 to the TPS-g, TPS-b, and TPS-a subfamily, respectively. Through in vitro enzyme assays with Escherichia coli-expressed recombinant proteins, both CfTPS1 and CfTPS2 were demonstrated to catalyze the conversion of geranyl diphosphate to linalool, the most abundant constituent of C. florida floral scent. In addition, CfTPS1 and CfTPS2 produced the sesquiterpene nerolidol from (E,E)-farnesyl diphosphate. CfTPS3 showed sesquiterpene synthase activity and produced multiple products in vitro. All three CfTPS genes showed higher levels of expression in sepals than those in transformed-petals and ovaries. Our results show that despite being sterile, the flowers of ‘Kaiser’ have normal mechanisms for floral scent biosynthesis that make the flowers fragrant.

Evolution of a Novel and Adaptive Floral Scent in Wild Tobacco

Many plants emit diverse floral scents that mediate plant-environment interactions and attain reproductive success. However, how plants evolve novel and adaptive biosynthetic pathways for floral volatiles remains unclear. Here, we show that in the wild tobacco, Nicotiana attenuata, a dominant species-specific floral volatile (benzyl acetone, BA) that attracts pollinators and deters florivore is synthesized by phenylalanine ammonia-lyase 4 (NaPAL4), isoflavone reductase 3 (NaIFR3), and chalcone synthase 3 (NaCHAL3). Transient expression of NaFIR3 alone in N. attenuata leaves is sufficient and necessary for ectopic foliar BA emissions, and coexpressing NaIFR3 with NaPAL4 and NaCHAL3 increased the BA emission levels. Independent changes in transcription of NaPAL4 and NaCHAL3 contributed to intraspecific variations of floral BA emission. However, among species, the gain of expression of NaIFR3 resulted in the biosynthesis of BA, which was only found in N. attenuata. This study suggests that novel metabolic pathways associated with adaptation can arise via reconfigurations of gene expression.

Hop (Humulus lupulus L.) terroir has large effect on a glycosylated green leaf volatile but not on other aroma glycosides

Genetics and environment both influence the content of hop (Humulus lupulus L.) aroma compounds. The effects of these two factors on aroma glycosides, which can change the aroma profile of beer over time, were examined in a preliminary study. Twenty-three hop cultivars were grown in the northwestern United States in two locations with distinct terroirs. UPLC-MS/MS analysis of hop cone extracts revealed that growing location had a large effect on hexyl glucoside levels but only a negligible effect on levels of linalyl, raspberry ketone, and 2-phenylethyl glucoside, which were mostly affected by genetic differences. The large terroir effect on hexyl glucoside, which releases a green leaf volatile with a grassy aroma when hydrolyzed, but not on the other aroma glucosides, which have more desirable aromas when hydrolyzed, could have an impact on beer aroma profiles.

UGT85A84 Catalyzes the Glycosylation of Aromatic Monoterpenes in Osmanthus fragrans Lour. Flowers

The monoterpenes linalool and its oxides are the key aroma-active compounds in Osmanthus fragrans Lour. flowers. The glycosides of these monoterpenes accumulate throughout flowering, leading to considerable storage of potential aroma constituents that account for the majority of non-volatile aroma compounds. However, the UDP-glycosyltransferase (UGT) responsible for the glycosylation of linalool and its oxides has not been clarified. Four candidate OfUGTs (UGT85A82, UGT85A83, UGT85AF3, and UGT85A84) with high homology to the known terpenoid UGTs were screened by transcriptome sequencing. Over-expression of the candidate OfUGTs in tobacco showed that UGT85A84 glycosylated linalool oxides in planta. Since the transcript levels of UGT85A84 were positively correlated with glycoside accumulation, the recombinant UGT85A84 protein was subjected to reactions with aglycones and sugar donors. Two formate adducts were exclusively detected in UDP-Glc with linalool and linalool oxide reactions by liquid chromatography-mass spectrometry (LC-MS), indicating that UDP-Glc was the specific sugar donor. The kinetic parameters demonstrated that UGT85A84 glycosylated both linalool and lianlool oxides in vitro. Further analysis demonstrated that the transcription levels of MEP pathway genes might play an important role in mediating terpenoid glycosylation. Our findings unraveled the mechanism underlying the glycosylation of essential aroma compounds in flowers. This study will facilitate the application of potential aroma contributors in future industries.

Wood volatiles as attractants of the confused flour beetle, Tribolium confusum (Coleoptera: Tenebrionidae)

Confused flour beetles are serious pests of stored grain products, and therefore, it is important to efficiently monitor and control their populations. Aggregation pheromones are commercially used for monitoring this beetle but their efficacy has been questioned and they may be inadequate for practical use. Food attractants as well as pheromones are commonly used for monitoring stored-product insects. However, food attractants may not be effective in the case of food handling facilities, which are already filled with food odours. The ancestors of flour beetles may have been associated with dead or decomposing woody vegetation, so we investigated the attractiveness of several wood odours to beetles using a pitfall olfactometer. The beetles were strongly attracted to all wood odours tested: Castanea crenata, Magnolia obovata, Paulownia tomentosa, Prunus jamasakura, and Zelkova serrata. The attractiveness of these wood odours was also stronger than that of the odours of the usual food of these beetles. Supercritical CO2 extracts of these species of wood were also attractive to the beetles. The Z. serrata extract was the most attractive among these extracts, and was further analysed by gas chromatography mass spectrometry. One major compound, (-)-mellein, was detected in the extract. Synthetic (±)-mellein attracted the beetles.

An unbiased approach elucidates variation in (S)-(+)-linalool, a context-specific mediator of a tri-trophic interaction in wild tobacco

Plant volatile organic compounds (VOCs) mediate many interactions, and the function of common VOCs is especially likely to depend on ecological context. We used a genetic mapping population of wild tobacco, Nicotiana attenuata, originating from a cross of 2 natural accessions from Arizona and Utah, separated by the Grand Canyon, to dissect genetic variation controlling VOCs. Herbivory-induced leaf terpenoid emissions varied substantially, while green leaf volatile emissions were similar. In a field experiment, only emissions of linalool, a common VOC, correlated significantly with predation of the herbivore Manduca sexta by native predators. Using quantitative trait locus mapping and genome mining, we identified an (S)-(+)-linalool synthase (NaLIS). Genome resequencing, gene cloning, and activity assays revealed that the presence/absence of a 766-bp sequence in NaLIS underlies the variation of linalool emissions in 26 natural accessions. We manipulated linalool emissions and composition by ectopically expressing linalool synthases for both enantiomers, (S)-(+)- and (R)-(-)-linalool, reported to oppositely affect M. sexta oviposition, in the Arizona and Utah accessions. We used these lines to test ovipositing moths in increasingly complex environments. The enantiomers had opposite effects on oviposition preference, but the magnitude of the effect depended strongly both on plant genetic background, and complexity of the bioassay environment. Our study reveals that the emission of linalool, a common VOC, differs by orders-of-magnitude among geographically interspersed conspecific plants due to allelic variation in a linalool synthase, and that the response of a specialist herbivore to linalool depends on enantiomer, plant genotype, and environmental complexity.

Chemical convergence between plants and insects: biosynthetic origins and functions of common secondary metabolites

Despite the phylogenetic distance between plants and insects, these two groups of organisms produce some secondary metabolites in common. Identical structures belonging to chemical classes such as the simple monoterpenes and sesquiterpenes, iridoid monoterpenes, cyanogenic glycosides, benzoic acid derivatives, benzoquinones and naphthoquinones are sometimes found in both plants and insects. In addition, very similar glucohydrolases involved in activating two-component defenses, such as glucosinolates and cyanogenic glycosides, occur in both plants and insects. Although this trend was first noted many years ago, researchers have long struggled to find convincing explanations for such co-occurrence. In some cases, identical compounds may be produced by plants to interfere with their function in insects. In others, plant and insect compounds may simply have parallel functions, probably in defense or attraction, and their co-occurrence is a coincidence. The biosynthetic origin of such co-occurring metabolites may be very different in insects as compared to plants. Plants and insects may have different pathways to the same metabolite, or similar sequences of intermediates, but different enzymes. Further knowledge of the ecological roles and biosynthetic pathways of secondary metabolites may shed more light on why plants and insects produce identical substances.

U, Kulkarni R, Löfstedt C, Sowdhamini R, Olsson SB. A Functional Agonist of Insect Olfactory Receptors: Behavior, Physiology and Structure

Chemical signaling is ubiquitous and employs a variety of receptor types to detect the cacophony of molecules relevant for each living organism. Insects, our most diverse taxon, have evolved unique olfactory receptors with as little as 10% sequence identity between receptor types. We have identified a promiscuous volatile, 2-methyltetrahydro-3-furanone (coffee furanone), that elicits chemosensory and behavioral activity across multiple insect orders and receptors. In vivo and in vitro physiology showed that coffee furanone was detected by roughly 80% of the recorded neurons expressing the insect-specific olfactory receptor complex in the antenna of Drosophila melanogaster, at concentrations similar to other known, and less promiscuous, ligands. Neurons expressing specialized receptors, other chemoreceptor types, or mutants lacking the complex entirely did not respond to this compound. This indicates that coffee furanone is a promiscuous ligand for the insect olfactory receptor complex itself and did not induce non-specific cellular responses. In addition, we present homology modeling and docking studies with selected olfactory receptors that suggest conserved interaction regions for both coffee furanone and known ligands. Apart from its physiological activity, this known food additive elicits a behavioral response for several insects, including mosquitoes, flies, and cockroaches. A broad-scale behaviorally active molecule non-toxic to humans thus has significant implications for health and agriculture. Coffee furanone serves as a unique tool to unlock molecular, physiological, and behavioral relationships across this diverse receptor family and animal taxa.

UDP-glucosyltransferase PpUGT85A2 controls volatile glycosylation in peach

The monoterpene linalool is a major contributor to aroma and flavor in peach (Prunus persica) fruit. It accumulates during fruit ripening, where up to ~40% of the compound is present in a non-volatile glycosylated form, which affects flavor quality and consumer perception by retronasal perception during tasting. Despite the importance of this sequestration to flavor, the UDP-glycosyltransferase (UGT) responsible for linalool glycosylation has not been identified in peach. UGT gene expression during peach fruit ripening and among different peach cultivars was analyzed using RNA sequencing, and transcripts correlated with linalyl-β-d-glucoside were selected as candidates for functional analysis. Kinetic resolution of a racemic mixture of R,S-linalool was shown for PpUGT85A2, with a slight preference for S-(+)-linalool. PpUGT85A2 was shown to catalyze synthesis of linalyl-β-d-glucoside in vitro, although it did not exhibit the highest enzyme activity between tested substrates. Subcellular localization of PpUGT85A2 in the cytoplasm and nucleus was detected. Application of linalool to peach leaf disks promoted PpUGT85A2 expression and linalyl-β-d-glucoside generation. Transient expression in peach fruit and stable overexpression in tobacco and Arabidopsis resulted in significant accumulation of linalyl-β-d-glucoside in vivo. Taken together, the results indicate that PpUGT85A2 expression is a major control point predicting linalyl-β-d-glucoside content.

Nano-based smart pesticide formulations: Emerging opportunities for agriculture

The incorporation of nanotechnology as a means for nanopesticides is in the early stage of development. The main idea behind this incorporation is to lower the indiscriminate use of conventional pesticides to be in line with safe environmental applications. Nanoencapsulated pesticides can provide controlled release kinetics, while efficiently enhancing permeability, stability, and solubility. Nanoencapsulation can enhance the pest-control efficiency over extended durations by preventing the premature degradation of active ingredients (AIs) under harsh environmental conditions. This review is thus organized to critically assess the significant role of nanotechnology for encapsulation of AIs for pesticides. The smart delivery of pesticides is essential to reduce the dosage of AIs with enhanced efficacy and to overcome pesticide loss (e.g., due to leaching and evaporation). The future trends of pesticide nanoformulations including nanomaterials as AIs and nanoemulsions of biopesticides are also explored. This review should thus offer a valuable guide for establishing regulatory frameworks related to field applications of these nano-based pesticides in the near future.

Terpenes and isoprenoids: a wealth of compounds for global use

Role of terpenes and isoprenoids has been pivotal in the survival and evolution of higher plants in various ecoregions. These products find application in the pharmaceutical, flavor fragrance, and biofuel industries. Fitness of plants in a wide range of environmental conditions entailed (i) evolution of secondary metabolic pathways enabling utilization of photosynthate for the synthesis of a variety of biomolecules, thereby facilitating diverse eco-interactive functions, and (ii) evolution of structural features for the sequestration of such compounds away from the mainstream primary metabolism to prevent autotoxicity. This review summarizes features and applications of terpene and isoprenoid compounds, comprising the largest class of secondary metabolites. Many of these terpene and isoprenoid biomolecules happen to be high-value bioproducts. They are essential components of all living organisms that are chemically highly variant. They are constituents of primary (quinones, chlorophylls, carotenoids, steroids) as well as secondary metabolism compounds with roles in signal transduction, reproduction, communication, climatic acclimation, defense mechanisms and more. They comprise single to several hundreds of repetitive five-carbon units of isopentenyl diphosphate (IPP) and its isomer dimethylallyl diphosphate (DMAPP). In plants, there are two pathways that lead to the synthesis of terpene and isoprenoid precursors, the cytosolic mevalonic acid (MVA) pathway and the plastidic methylerythritol phosphate (MEP) pathway. The diversity of terpenoids can be attributed to differential enzyme and substrate specificities and to secondary modifications acquired by terpene synthases. The biological role of secondary metabolites has been recognized as pivotal in the survival and evolution of higher plants. Terpenes and isoprenoids find application in pharmaceutical, nutraceutical, synthetic chemistry, flavor fragrance, and possibly biofuel industries.

De novo formation of an aggregation pheromone precursor by an isoprenyl diphosphate synthase-related terpene synthase in the harlequin bug

Many insects release volatile terpenes for chemical communication. However, the biosynthetic origin and evolution of these infochemicals are mostly unknown. We show that the harlequin bug, Murgantia histrionica, a stink bug pest (Hemiptera) of crucifer crops, produces a terpene aggregation pheromone by an enzyme that is unrelated to microbial and plant terpene synthases. M. histrionica terpene synthase activity is highly sex- and tissue-specific and makes a sesquiterpene alcohol, so far unknown in animals, as pheromone precursor. The enzyme evolved from ancestral isoprenyl diphosphate synthases and provides new evidence for de novo biosynthesis of terpenes in hemipteran insects. Knowledge of pheromone biosynthesis in stink bugs may lead to the development of new controls of these pests.

Insects use a diverse array of specialized terpene metabolites as pheromones in intraspecific interactions. In contrast to plants and microbes, which employ enzymes called terpene synthases (TPSs) to synthesize terpene metabolites, limited information from few species is available about the enzymatic mechanisms underlying terpene pheromone biosynthesis in insects. Several stink bugs (Hemiptera: Pentatomidae), among them severe agricultural pests, release 15-carbon sesquiterpenes with a bisabolene skeleton as sex or aggregation pheromones. The harlequin bug, Murgantia histrionica, a specialist pest of crucifers, uses two stereoisomers of 10,11-epoxy-1-bisabolen-3-ol as a male-released aggregation pheromone called murgantiol. We show that MhTPS (MhIDS-1), an enzyme unrelated to plant and microbial TPSs but with similarity to trans-isoprenyl diphosphate synthases (IDS) of the core terpene biosynthetic pathway, catalyzes the formation of (1S,6S,7R)-1,10-bisaboladien-1-ol (sesquipiperitol) as a terpene intermediate in murgantiol biosynthesis. Sesquipiperitol, a so-far-unknown compound in animals, also occurs in plants, indicating convergent evolution in the biosynthesis of this sesquiterpene. RNAi-mediated knockdown of MhTPS mRNA confirmed the role of MhTPS in murgantiol biosynthesis. MhTPS expression is highly specific to tissues lining the cuticle of the abdominal sternites of mature males. Phylogenetic analysis suggests that MhTPS is derived from a trans-IDS progenitor and diverged from bona fide trans-IDS proteins including MhIDS-2, which functions as an (E,E)-farnesyl diphosphate (FPP) synthase. Structure-guided mutagenesis revealed several residues critical to MhTPS and MhFPPS activity. The emergence of an IDS-like protein with TPS activity in M. histrionica demonstrates that de novo terpene biosynthesis evolved in the Hemiptera in an adaptation for intraspecific communication.

Linalool and linalool nerolidol synthases in roses, several genes for little scent

Roses are widely appreciated for the appearance of their flowers and for their fragrance. This latter character results from the combination of different odorant molecules among which monoterpenes are often prevalent constituents. In this study, we report the cloning and characterization of three rose monoterpene synthases. In vitro functional characterization of these enzymes showed that one is a (-)-(3R)-linalool synthase whereas the others have a dual (+)-(3S)-linalool nerolidol synthase activity. However, given that the characterized rose cultivars were only able to produce the (-)-(3R)-linalool stereoisomer, the linalool nerolidol synthases are probably not active in planta. Furthermore, these three enzymes were also characterized by a weak expression level as assessed by RT-qPCR and by the low abundance of the corresponding sequences in an EST library. This characteristic is likely to explain why linalool is generally a minor constituent in rose flowers' scents. On this basis, we propose that in roses the monoterpene biosynthesis effort is focused on the production of acyclic monoterpenes derived from geraniol through the recently characterized Nudix biosynthesis pathway, at the expense of conventional monoterpene biosynthesis via terpene synthases such as linalool or linalool nerolidol synthases.

Changes in the hippocampal and peripheral phospholipid profiles are associated with neurodegeneration hallmarks in a long-term global cerebral ischemia model: Attenuation by Linalool

Phospholipid alterations in the brain are associated with progressive neurodegeneration and cognitive impairment after acute and chronic injuries. Various types of treatments have been evaluated for their abilities to block the progression of the impairment, but effective treatments targeting long-term post-stroke alterations are not available. In this study, we analyzed changes in the central and peripheral phospholipid profiles in ischemic rats and determined whether a protective monoterpene, Linalool, could modify them. We used an in vitro model of glutamate (125 μM) excitotoxicity and an in vivo global ischemia model in Wistar rats. Linalool (0.1 μM) protected neurons and astrocytes by reducing LDH release and restoring ATP levels. Linalool was administered orally at a dose of 25 mg/kg every 24 h for a month, behavioral tests were performed, and a lipidomic analysis was conducted using mass spectrometry. Animals treated with Linalool displayed faster neurological recovery than untreated ischemic animals, accompanied by better motor and cognitive performances. These results were confirmed by the significant reduction in astrogliosis, microgliosis and COX-2 marker, involving a decrease of 24:0 free fatty acid in the hippocampus. The altered profiles of phospholipids composed of mono and polyunsaturated fatty acids (PC 36:1; 42:1 (24:0/18:1)/LPC 22:6)/LPE 22:6) in the ischemic hippocampus and the upregulation of PI 36:2 and other LCFA (long chain fatty acids) in the serum of ischemic rats were prevented by the monoterpene. Based on these data, alterations in the central and peripheral phospholipid profiles after long-term was attenuated by oral Linalool, promoting a phospholipid homeostasis, related to the recovery of brain function.

The Scent of the Fly

(Z)-4-undecenal (Z4-11Al) is the volatile pheromone produced by females of the vinegar fly Drosophila melanogaster. Female flies emit Z4-11Al for species-specific communication and mate-finding. A sensory panel finds that synthetic Z4-11Al has a characteristic flavour, which can be perceived even at the small amounts produced by a single female fly. Since only females produce Z4-11Al, and not males, we can reliably distinguish between single D. melanogaster males and females, according to their scent. Females release Z4-11Al at 2.4 ng/h and we readily sense 1 ng synthetic Z4-11Al in a glass of wine (0.03 nmol/L), while a tenfold concentration is perceived as a loud off-flavour. This corroborates the observation that a glass of wine is spoilt by a single D. melanogaster fly falling into it, which we here show is caused by Z4-11Al. The biological role of Z4-11Al or structurally related aldehydes in humans and the basis for this semiochemical convergence remains yet unclear.

Cloning and characterization of a monoterpene synthase gene from flowers of Camelina sativa

CsTPS1 encodes for a monoterpene synthase that contributes to the emission of a blend of volatile compounds emitted from flowers of Camelina sativa. The work describes the in vitro characterization of a monoterpene synthase and its regulatory region that we cloned from Camelina sativa (Camelina). Here, we named this gene as C. sativa terpene synthase 1 (CsTPS1). In vitro experiments performed with the CsTPS1 protein after expression and purification from Escherichia coli (E. coli) showed production of a blend of monoterpene volatile organic compounds, of which the emission was also detected in the floral bouquet of wild-type Camelina plants. Quantitative-PCR measurements revealed a high abundance of CsTPS1 transcripts in flowers and experiments performed with the GUS reporter showed high CsTPS1 expression in the pistil, in the cells of the wall of the ovary and in the stigma. Subcellular localization of the CsTPS1 protein was investigated with a GFP reporter construct that showed expression in plastids. The CsTPS1 gene identified in this study belongs to a mid-size family of 60 genes putatively codifying for TPS enzymes. This enlarged family of TPS genes suggests that Camelina has the structural framework for the production of terpenes and other secondary metabolites of relevance for the consumers.

The terpene synthase gene family in Gossypium hirsutum harbors a linalool synthase GhTPS12 implicated in direct defence responses against herbivores

Herbivore-induced terpenes have been reported to function as ecological signals in plant-insect interactions. Here, we showed that insect-induced cotton volatile blends contained 16 terpenoid compounds with a relatively high level of linalool. The high diversity of terpene production is derived from a large terpene synthase (TPS) gene family. The TPS gene family of Gossypium hirsutum and Gossypium raimondii consist of 46 and 41 members, respectively. Twelve TPS genes (GhTPS4-15) could be isolated, and protein expression in Escherichia coli revealed catalytic activity for eight GhTPS. The upregulation of the majority of these eight genes additionally supports the function of these genes in herbivore-induced volatile biosynthesis. Furthermore, transgenic Nicotiana tabacum plants overexpressing GhTPS12 were generated, which produced relatively large amounts of (3S)-linalool. In choice tests, female adults of Helicoverpa armigera laid fewer eggs on transgenic plants compared with non-transformed controls. Meanwhile, Myzus persicae preferred feeding on wild-type leaves over leaves of transgenic plants. Our findings demonstrate that transcript accumulation of multiple TPS genes is mainly responsible for the production and diversity of herbivore-induced volatile terpenes in cotton. Also, these genes might play roles in plant defence, in particular, direct defence responses against herbivores.

Expression of MEP Pathway Genes and Non-volatile Sequestration Are Associated with Circadian Rhythm of Dominant Terpenoids Emission in Osmanthus fragrans Lour. Flowers

Osmanthus fragrans Lour. is one of the top 10 traditional ornamental flowers in China famous for its unique fragrance. Preliminary study proved that the terpenoids including ionone, linalool, and ocimene and their derivatives are the dominant aroma-active compounds that contribute greatly to the scent bouquet. Pollination observation implies the emission of aromatic terpenoids may follow a circadian rhythm. In this study, we investigated the variation of volatile terpenoids and its potential regulators. The results showed that both volatile and non-volatile terpenoids presented circadian oscillation with high emission or accumulation during the day and low emission or accumulation during the night. The volatile terpenoids always increased to reach their maximum values at 12:00 h, while free and glycosylated compounds continued increasing throughout the day. The depletion of non-volatile pool might provide the substrates for volatile emission at 0:00-6:00, suggesting the sequestration of non-volatile compounds acted like a buffer regulating emission of terpenoids. Further detection of MEP pathway genes demonstrated that their expressions increased significantly in parallel with the evident increase of both volatile and non-volatile terpenoids during the day, indicating that the gene expressions were also closely associated with terpenoid formation. Thus, the expression of MEP pathway genes and internal sequestration both played crucial roles in modulating circadian rhythm of terpenoid emission in O. fragrans.

UV-B irradiation differentially regulates terpene synthases and terpene content of peach

Plants generate protective molecules in response to ultraviolet (UV) light. In laboratory experiments, 48 h UV-B irradiation of peach fruits and leaves reduced the flavour-related monoterpene linalool by 60%. No isoprene was detected, but other terpenoids increased significantly, including a threefold accumulation of the sesquiterpene (E,E)-α-farnesene, which was also increased by jasmonic acid treatment. RNA sequencing revealed altered transcript levels for two terpene synthases (TPSs): PpTPS1, a TPS-g subfamily member, decreased by 86% and PpTPS2, a TPS-b subfamily member, increased 80-fold. Heterologous expression in Escherichia coli and transient overexpression in tobacco and peach fruits showed PpTPS1 was localized in plastids and associated with production of linalool, while PpTPS2 was responsible for (E,E)-α-farnesene biosynthesis in the cytoplasm. Candidate regulatory genes for these responses were identified. Commercial peach production in Asia involves fruit bagging to maintain marketable yield and quality. TPS gene expression and volatile terpenoid production in field experiments, using bags transmitting high UV-B radiation, showed similar effects on peach volatiles to those from laboratory experiments. Bags transmitting less UV-B light ameliorated the reduction in the flavour volatile linalool, indicating that flavour components of peach fruits can be modulated by selecting an appropriate source of environmental screening material.

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.

Metabolic Products of Linalool and Modulation of GABAA Receptors

Terpenoids are major subcomponents in aroma substances which harbor sedative physiological potential. We have demonstrated that various monoterpenoids such as the acyclic linalool enhance GABAergic currents in an allosteric manner in vitro upon overexpression of inhibitory α1β2 GABA_A receptors in various expression systems. However, in plants or humans, i.e., following intake via inhalation or ingestion, linalool undergoes metabolic modifications including oxygenation and acetylation, which may affect the modulatory efficacy of the generated linalool derivatives. Here, we analyzed the modulatory potential of linalool derivatives at α1β2γ2 GABA_A receptors upon transient overexpression. Following receptor expression control, electrophysiological recordings in a whole cell configuration were used to determine the chloride influx upon co-application of GABA EC₁₀₋₃₀ together with the modulatory substance. Our results show that only oxygenated linalool metabolites at carbon 8 positively affect GABAergic currents whereas derivatives hydroxylated or carboxylated at carbon 8 were rather ineffective. Acetylated linalool derivatives resulted in non-significant changes of GABAergic currents. We can conclude that metabolism of linalool reduces its positive allosteric potential at GABA_A receptors compared to the significant potentiation effects of the parent molecule linalool itself.

The Sexual Advantage of Looking, Smelling, and Tasting Good: The Metabolic Network that Produces Signals for Pollinators

A striking feature of the angiosperms that use animals as pollen carriers to sexually reproduce is the great diversity of their flowers with regard to morphology and traits such as color, odor, and nectar. These traits are underpinned by the synthesis of secondary metabolites such as pigments and volatiles, as well as carbohydrates and amino acids, which are used by plants to lure and reward animal pollinators. We review here the knowledge of the metabolic network that supports the biosynthesis of these compounds and the behavioral responses that these molecules elicit in the animal pollinators. Such knowledge provides us with a deeper insight into the ecology and evolution of plant-pollinator interactions, and should help us to better manage these ecologically essential interactions in agricultural ecosystems.