Antagonistic effects of floral scent in an insect–plant interaction

In southwestern USA, the jimsonweed Datura wrightii and the nocturnal moth Manduca sexta form a pollinator–plant and herbivore–plant association. Because the floral scent is probably important in mediating this interaction, we investigated the floral volatiles that might attract M. sexta for feeding and oviposition. We found that flower volatiles increase oviposition and include small amounts of both enantiomers of linalool, a common component of the scent of hawkmoth-pollinated flowers. Because (+)-linalool is processed in a female-specific glomerulus in the primary olfactory centre of M. sexta, we hypothesized that the enantiomers of linalool differentially modulate feeding and oviposition. Using a synthetic mixture that mimics the D. wrightii floral scent, we found that the presence of linalool was not necessary to evoke feeding and that mixtures containing (+)- and/or (−)-linalool were equally effective in mediating this behaviour. By contrast, females oviposited more on plants emitting (+)-linalool (alone or in mixtures) over control plants, while plants emitting (−)-linalool (alone or in mixtures) were less preferred than control plants. Together with our previous investigations, these results show that linalool has differential effects in feeding and oviposition through two neural pathways: one that is sexually isomorphic and non-enantioselective, and another that is female-specific and enantioselective.     

Plant Volatiles: Recent Advances and Future Perspectives

Volatile compounds act as a language that plants use for their communication and interaction with the surrounding environment. To date, a total of 1700 volatile compounds have been isolated from more than 90 plant families. These volatiles, released from leaves, flowers, and fruits into the atmosphere and from roots into the soil, defend plants against herbivores and pathogens or provide a reproductive advantage by attracting pollinators and seed dispersers. Plant volatiles constitute about 1% of plant secondary metabolites and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid derivatives, and amino acid derivatives. In this review we focus on the functions of plant volatiles, their biosynthesis and regulation, and the metabolic engineering of the volatile spectrum, which results in plant defense improvement and changes of scent and aroma properties of flowers and fruits.     

CYP76C1 (Cytochrome P450)-Mediated Linalool Metabolism and the Formation of Volatile and Soluble Linalool Oxides in Arabidopsis Flowers: A Strategy for Defense against Floral Antagonists

The acyclic monoterpene alcohol linalool is one of the most frequently encountered volatile compounds in floral scents. Various linalool oxides are usually emitted along with linalool, some of which are cyclic, such as the furanoid lilac compounds. Recent work has revealed the coexistence of two flower-expressed linalool synthases that produce the (S)- or (R)-linalool enantiomers and the involvement of two P450 enzymes in the linalool oxidation in the flowers of Arabidopsis thaliana. Partially redundant enzymes may also contribute to floral linalool metabolism. Here, we provide evidence that CYP76C1 is a multifunctional enzyme that catalyzes a cascade of oxidation reactions and is the major linalool metabolizing oxygenase in Arabidopsis flowers. Based on the activity of the recombinant enzyme and mutant analyses, we demonstrate its prominent role in the formation of most of the linalool oxides identified in vivo, both as volatiles and soluble conjugated compounds, including 8-hydroxy, 8-oxo, and 8-COOH-linalool, as well as lilac aldehydes and alcohols. Analysis of insect behavior on CYP76C1 mutants and in response to linalool and its oxygenated derivatives demonstrates that CYP76C1-dependent modulation of linalool emission and production of linalool oxides contribute to reduced floral attraction and favor protection against visitors and pests.     

Discrimination between enantiomers of linalool by olfactory receptor neurons in the cabbage moth Mamestra brassicae (L.)

Plants emit complex blends of volatiles, including chiral compounds that might be detected by vertebrates and invertebrates. Insects are ideal model organisms for studying the underlying receptor neuron mechanisms involved in olfactory discrimination of enantiomers. In the present study, we have employed two-column gas chromatography linked to recordings from single olfactory receptor neurons of Mamestra brassicae, in which separation of volatiles in a polar and a chiral column was performed. We here present the response properties of olfactory receptor neurons tuned to linalool. The narrow tuning of these receptor neurons was demonstrated by their strong responses to (R)-(-)-linalool, the weaker responses to the (+)-enantiomer as well as a few structurally related compounds, and no responses to the other numerous plant released volatiles. The enantioselectivity was verified by parallel dose-response curves, that of (R)-(-)-linalool shifted 1 log unit to the left of the (S)-(+)-linalool curve. A complete overlap of the temporal response pattern was found when comparing the responses of the same strength. Analysis of the spike amplitude and waveform indicated that the responses to the two enantiomers originated from the same neuron.     

Functional evolution of biosynthetic enzymes that produce plant volatiles*

Plants synthesize volatile compounds to attract pollinators. The volatiles emitted by flowers are often complex mixtures of organic compounds; pollinators are capable of distinctly recognizing different volatile compounds. Plants also produce volatile compounds to protect themselves against herbivores and pathogens. Some of the volatile compounds produced in floral and vegetative tissues are toxic to insects and microbes. To adapt changes in the environment, plants have evolved the ability to synthesize a unique set of volatiles. Intensive studies have identified and characterized the enzymes responsible for the formation of plant volatiles. In particular, many biosynthetic genes have been isolated and their enzymatic functions have been proposed. This review describes how plants have evolved the biosynthetic pathways leading to the formation of green leaf volatiles and phenylpropene volatiles.     

Natural selection on floral volatile production in Penstemon digitalis: Highlighting the role of linalool

Natural selection is thought to have shaped the evolution of floral scent; however, unlike other floral characters, we have a rudimentary knowledge of how phenotypic selection acts on scent. We found that floral scent was under stronger selection than corolla traits such as flower size and flower color in weakly scented Penstemon digitalis. Our results suggest that to understand evolution in floral phenotypes, including scent in floral selection, studies are crucial. For P. digitalis, linalool was the direct target of selection in the scent bouquet. Therefore, we determined the enantiomeric configuration of linalool because interacting insects may perceive the enantiomers differentially. We found that P. digitalis produces only (S)-(+)-linalool and, more interestingly, it is also taken up into the nectar. Because the nectar is scented and flavored with (S)-(+)-linalool, it may be an important cue for pollinators visiting P. digitalis flowers.     

茶树诱导抗虫性的研究进展

为了抵御植食性昆虫的为害,植物在进化过程中形成了包括组成抗性和诱导抗性在内的复杂防御体系.在通过受体识别茶树害虫为害后,茶树会启动早期信号事件,继而激活茉莉酸、水杨酸、乙烯和赤霉素等植物激素信号通路,从而引起次生代谢物的积累,最终对害虫产生直接和间接抗性.基于近年来茶树害虫为害诱导的茶树防御反应及其相关调控机理的研究进...     

Breeding system of the critically endangered Lakela’s Mint and influence of plant height on pollinators and seed output

Understanding reproductive systems of rare plants is critical for conservation efforts. Lakela's Mint, Dicerandra immaculata Lakela var. immaculata, is an endangered plant endemic to an approximately 4.8-km long area in Florida, USA. We used an experimental garden and three populations of Lakela's Mint to determine: (1) what is the breeding system (autonomous, asexual, self-fertile, cross-fertile) and are insects necessary for reproduction; (2) which native and nonnative insect species visit flowers and is the frequency of visits to a plant influenced by its height; (3) does the number of flowers visited within a plant by individual insects differ among native and nonnative insect species and due to plant height; and (4) is seed output influenced by plant height? Our results indicate that the breeding system of Lakela's Mint was facultative outcrossing. Insect-pollinated flowers produced more seeds than flowers that reproduced autonomously or asexually. The honey bee Apis mellifera L., a nonnative species, was the most frequent visitor to plants and visited more flowers within plants than native pollinators, but its behavior was not influenced by plant height. Native pollinators such as Bombus impatiens Cresson were attracted more frequently to shorter plants, but visited fewer flowers than on taller plants. Despite having fewer total and pollinated flowers, shorter plants had a higher output of intact seeds than taller plants, which could be due to differences in efficiency between native and nonnative pollinators or other factors. Our results add insight into factors influencing seed output and interactions between pollinators and rare plants.     

Linalool and linalool oxide production in transgenic carnation flowers expressing the Clarkia breweri linalool synthase gene

Most modern cut-flower cultivars, including those of carnation(Dianthus caryophyllus), lack distinct fragrance.Carnationcv. Eilat flowers produce and emit various fragrance compounds, includingbenzoic acid derivatives and sesquiterpenes, but not monoterpenes. Based onGC-MS analysis, benzoic acid, benzyl benzoate, phenylethyl benzoate, methylbenzoate, cis-3-hexenyl benzoate and -caryophylleneare the major fragrance compounds, representing ca. 60% of the total volatilesgenerated by these flowers. The level of these compounds increases dramaticallyduring petal development. To evaluate the possibility of producing monoterpenesin carnation cv. Eilat, we generated transgenic plants expressing the linaloolsynthase gene from Clarkia breweri under the regulation ofthe CaMV 35S constitutive promoter. The product of this gene catalyzes theproduction of the monoterpene linalool from geranyl diphosphate. HeadspaceGC-MSanalysis revealed that leaves and flowers of transgenic, but not controlplants,emit linalool and its derivatives, cis- andtrans-linalool oxide. GC-MS analysis of petal extractrevealed the accumulation of trans-linalool oxide but notlinalool. The emission of linalool by the transgenic flowers did not lead todetectable changes in flower scent for human olfaction.     

Floral color change and the attraction of insect pollinators in lungwort (Pulmonaria collina)

We studied the effect of floral color change on long- and short-distance attraction of insect pollinators to the herb lungwort, Pulmonaria collina. Lungwort flowers change color with age from red to blue. Young red flowers had a significantly greater pollen and nectar reward and were significantly more often unpollinated than old blue ones. Red and blue flowers both influenced long-distance attractiveness of plants, defined as the number of insect approaches towards an individual plant. After reaching a plant, flower visitors preferred to visit young red flowers. Therefore, short-distance attractiveness, defined as the number of flowers visited successively on an individual plant, was influenced mainly by the number of young red flowers. The co-occurrence of the change in reproductive ability, in amount of reward, and in flower color enabled lungwort plants to direct pollinators to reproductive, highly rewarding red flowers. The data suggest that by maintaining changed flowers lungwort plants can increase their long-distance attraction and simultaneously enhance the probability of flower visits to pre-changed flowers. Thus, we propose floral color change as a mechanism that can increase the efficiency of pollen transfer to enhance plant fitness. Received: 2 November 1998 / Accepted: 14 July 1999     

Inbreeding effects on blossom volatiles in Cucurbita pepo subsp. texana (Cucurbitaceae)

Self-pollination by plants gives rise to inbreeding depression. There is increasing recognition that plant inbreeding can have significant implications for interactions between plants and other organisms, including insects and pathogens. Many of these interactions are mediated by plant-derived volatiles, but the effects of inbreeding on volatile production have not previously been investigated. We examined variation in flower volatile production by the wild gourd Cucurbita pepo subsp. texana as a function of inbreeding, sex of the flower, and maternal line. We compared first-generation selfed progeny to outcrossed progeny to assess variation in blossom volatiles due to mating system. Our data indicate that self-pollination reduces total volatile production and changes the relative composition of individual compounds released by C. pepo subsp. texana blossoms. These findings have potentially important implications for interactions between C. pepo subsp. texana and its pollinators and herbivores-including diabroticite cucumber beetles, which vector the bacterial pathogen Erwinia tracheiphila-because previous studies have shown that a number of the individual compounds that vary with inbreeding level can influence insect behavior. We also found significant differences between the volatile profiles of male and female flowers and across maternal families.     

Insect pollinators show constancy for different flower traits between the most- and less-preferred plants: a case study of the long-proboscid tangle-veined fly

• The coevolution of insect pollinators and their host plants is a typical example of natural selection; however, how insect pollinators avoid overdependence on one peculiar plant remains unclear. As most insect pollinators exhibit a diet breadth when showing flower constancy, determining the difference and similarity of most and less preferred flowers by insect pollinators may be helpful to understand their trade-off between flower constancy and overdependence.
• This was addressed using the long-proboscid tangle-veined fly (Nemetrinus spp.). Dietary investigation indicates that the flies show constancy for the morphological characteristic of the Delphinium caeruleum, which is the most preferred plant for this Nemestrinidae fly that has blue, long-tubed flowers.
• In a colour selection experiment, focal individuals showed obvious preference for white, which is the colour of less preferred flowers by the fly in the natural environment. In a scent selection experiment, focal individuals showed obvious preference for D. caeruleum and Dracocephalum heterophyllum but avoidance to Dasiphora fruticosa and Dasiphora davurica. This indicates that long-proboscid tangle-veined flies can forage on other flowers, despite the existing constancy for D. caeruleum, as long as they do not hate the scent. It seems that long-proboscid tangle-veined flies can maximise foraging efficiency by showing constancy for the morphological characteristic of the most preferred plant and for the scent and colour of less-preferred plants.
• The trade-off of long-proboscid tangle-veined flies in the selection of nectar sources may be an adaptation to the risk of overdependence on one plant in evolution.

     

Dioecism of the genusCallicarpa (Verbenaceae) in the Bonin (Ogasawara) Islands

The morphology and function of the flowers of three species of the genusCallicarpa, C. glabra, C. nisnimurae andC. subpubescens, endemic to the Bonin Islands, revealed that all three species were dioecious. Male plants had short-styled flowers with sterile ovaries, while female plants had long-styled flowers with non-germinating inaperturate pollen grains. Such dioecism has never been reported from the family Verbenaceae. This curious sexual system with non-germinating pollen grains may have been maintained as a reward to insect pollinators.     

Spatial and temporal dispersion patterns of pollinators and their relationship to the flowering strategy of Yucca whipplei (Agavaceae)

Summary A field investigation of the mutualistic interaction between a monocarpic perennial plant, Yucca whipplei, and its host-specific pollinator and seed predator, Tegeticula maculata (Lepidoptera: Prodoxidae), was conducted to determine how the resource utilization pattern and population dynamics of the pollinator have influenced the evolution of the flowering and fruiting pattern of the plant. Although the temporal pattern of emergence of pollinators results in a relatively close tracking of flower abundance within a season, the ratio of pollinators to open flowers does vary significantly within a season, as well as between seasons. At any point in time during the flowering season, the population of adult yucca moths is distributed evenly among the available flowers, so that the number of pollinators on an inflorescence is directly proportional to the number of open flowers available. The relative isolation of individual flowering plants appears to have little effect on the distribution of pollinators among inflorescences. The number of fruits initiated on a plant is directly proportional to the number of flowers produced, and is also partially determined by the time of flowering. Yucca whipplei always produces many more flowers than fruits. Most flowers are not fertilized, and the plants also generally abort and abscise immature fruits after flowering. Fruit production of at least some plants, however, appeared limited by pollination. It is also expected that in some years the relative abundance of pollinators will be low enough that most plants will be pollinator-limited. It is suggested that the pattern of flowering and fruiting of this species has evolved in response to the unpredictability of pollinator availability, both within and between seasons. Resource uncertainty and selection acting on the male component of fitness may also be involved.     

Biochemical and physiological mechanisms underlying effects of Cucumber mosaic virus on host‐plant traits that mediate transmission by aphid vectors

The transmission of insect‐vectored diseases entails complex interactions among pathogens, hosts and vectors. Chemistry plays a key role in these interactions; yet, little work has addressed the chemical ecology of insect‐vectored diseases, especially in plant pathosystems. Recently, we documented effects of Cucumber mosaic virus (CMV) on the phenotype of its host (Cucurbita pepo) that influence plant‐aphid interactions and appear conducive to the non‐persistent transmission of this virus. CMV reduces host‐plant quality for aphids, causing rapid vector dispersal. Nevertheless, aphids are attracted to the elevated volatile emissions of CMV‐infected plants. Here, we show that CMV infection (1) disrupts levels of carbohydrates and amino acids in leaf tissue (where aphids initially probe plants and acquire virions) and in the phloem (where long‐term feeding occurs) in ways that reduce plant quality for aphids; (2) causes constitutive up‐regulation of salicylic acid; (3) alters herbivore‐induced jasmonic acid biosynthesis as well as the sensitivity of downstream defences to jasmonic acid; and (4) elevates ethylene emissions and free fatty acid precursors of volatiles. These findings are consistent with previously documented patterns of aphid performance and behaviour and provide a foundation for further exploration of the genetic mechanisms responsible for these effects and the evolutionary processes that shape them.     

Floral scent of Canada thistle and its potential as a generic insect attractant

The flowers of Canada thistle, Cirsium arvense (L.), attract a wide range of insects, including pollinators and herbivorous species. This attraction is primarily mediated by floral odor, which offers potential for developing generic insect attractants based on odor. In this study, we have analyzed the chemical composition of the volatiles produced by Canada thistle flowers. Nineteen floral compounds were identified in the headspace, including phenylacetaldehyde (55%), methyl salicylate (14%), dimethyl salicylate (8%), pyranoid linalool oxide (4.5%), and benzaldehyde (3.5%). Other minor compounds include benzyl alcohol, methylbenzoate, linalool, phenylethyl alcohol, furanoid linalool oxide, p-anisaldehyde, 2,6-dimethyl-1,3,5,7-octatetraene, benzylacetate, benzyl tiglate, (E,E)-alpha-farnesene, benzyl benzoate, isopropyl myristate, and 2-phenylethyl ester benzoic acid. The relative attractiveness of various doses of the main floral volatile compound phenylacetaldehyde (i.e., 10, 100, 200, and 400 mg) was tested for insect attraction. Both the total catch and the biodiversity of insect species trapped increased as the loading of phenylacetaldehyde increased. Volatiles were chosen from the odors from the flowers of Canada thistle and formulated and tested in the field. An 11-component blend was the most attractive of several floral blends tested. These findings indicate that chemical components of flower odors of Canada thistle can serve as a generic insect attractant for monitoring of invasive pest species.     

Gene Coexpression Analysis Reveals Complex Metabolism of the Monoterpene Alcohol Linalool in Arabidopsis Flowers

The cytochrome P450 family encompasses the largest family of enzymes in plant metabolism, and the functions of many of its members in Arabidopsis thaliana are still unknown. Gene coexpression analysis pointed to two P450s that were coexpressed with two monoterpene synthases in flowers and were thus predicted to be involved in monoterpenoid metabolism. We show that all four selected genes, the two terpene synthases (TPS10 and TPS14) and the two cytochrome P450s (CYP71B31 and CYP76C3), are simultaneously expressed at anthesis, mainly in upper anther filaments and in petals. Upon transient expression in Nicotiana benthamiana, the TPS enzymes colocalize in vesicular structures associated with the plastid surface, whereas the P450 proteins were detected in the endoplasmic reticulum. Whether they were expressed in Saccharomyces cerevisiae or in N. benthamiana, the TPS enzymes formed two different enantiomers of linalool: (−)-(R)-linalool for TPS10 and (+)-(S)-linalool for TPS14. Both P450 enzymes metabolize the two linalool enantiomers to form different but overlapping sets of hydroxylated or epoxidized products. These oxygenated products are not emitted into the floral headspace, but accumulate in floral tissues as further converted or conjugated metabolites. This work reveals complex linalool metabolism in Arabidopsis flowers, the ecological role of which remains to be determined.     

Biosynthesis and enantioselectivity in the production of the lilac compounds in Actinidia arguta flowers

Biosynthesis of the lilac alcohols and alcohol epoxides from linalool in ‘Hortgem Tahi’ kiwifruit (Actinidiaarguta) flowers was investigated by incubating flowers with rac-linalool, rac-[4,4,10,10,10-²H₅]linalool, (R)-8-hydroxylinalool and (R)-8-oxolinalool. All substrates were incorporated into the lilac alcohols although the (R)-configured compounds are not normally present in flowers. Biosynthesis of the lilac alcohol epoxides from rac-1,2-epoxy[4,4,10,10,10-²H₅]linalool and rac-[4′,4′, 8′, 8′,8′-²H₅]lilac aldehyde epoxide, rather than the lilac alcohols, was examined. Both substrates were non-enantioselectively converted to the lilac alcohol epoxides, suggesting two biosynthetic pathways for these compounds, contrary to previous reports. Their ability to process unnatural substrates indicates that A.arguta flowers have a greater biosynthetic capability than is suggested by their phytochemical composition. Linalool, the lilac compounds, and their biosynthetic intermediates were measured in the pistils, stamen, petals and sepals to determine if localisation in different organs contributed to only (S)-linalool being processed to the lilac compounds. Both linalool enantiomers were present in all organs, while most (97%) of the lilac compounds, and their precursors, were found in the petals. (S)-Linalool was not depleted from the flower petals, with respect to (R)-linalool, during the time of maximum production of the metabolites of (S)-linalool.     

AN EXPERIMENTAL STUDY OF THE FLORAL ECOLOGY OF DODECATHEON MEADIA

An established colony of Dodecatheon meadia on glacial drift in southeastern Wisconsin was observed for its insect pollination interrelationships. Twelve randomly distributed plants from which insects were excluded failed to produce seed, while plants exposed to insect visits set abundant seed. Female solitary bees of 2 species, viz., Augochloropsis metallica fulgida (Smith) and Lasioglossum forbesii (Robertson), and queens and workers of 7 species of Bombus were observed collecting pollen from the pendant anther cone by rapid wing vibration, while hanging inverted from the cone tip. This behavior—cinematographically recorded—was found fundamentally identical to that of females of the same species of solitary bees and of queens and workers of 5 species of Bombus on Solanum dulcamara, and of females of Lasioglossum forbesii (Robertson) and workers of 2 species of Bombus on Lycopersicum esculentum Mill. No other insects were found to pollinate these morphologically similar flowers in this manner, and crepuscular pollinators were not observed. Experimental modifications of form, function, and position of Dodecatheon flowers were offered to the pollinators, and behavioral changes of the insects were noted. Constituents of insect pollen loads were examined, and relationships of the pollinators to the flora of the region were noted. The detailed ecological study of insect pollen vector behavior as an aid in discovering biosystematic relationships of plant populations is suggested.     

Use of visual and olfactory cues of flowers of two brassicaceous species by insect pollinators

1. Pollinating insects exploit visual and olfactory cues associated with flower traits indicative of flower location and reward quality. Pollination can induce changes in these flower-associated cues, thereby influencing the behaviour of flower visitors. 2. This study investigated the main cues exploited by the syrphid fly Episyrphus balteatus and the butterfly Pieris brassicae when visiting flowers of Brassica nigra and Raphanus sativus plants. Whether pollen is used as a cue and whether pollination-induced changes affect flower volatile emission and the behavioural responses of the two pollinator species were also studied. 3. Pollinator preference was investigated by offering visual and olfactory cues individually as well as simultaneously in two-choice bioassays. Plant treatments included emasculation, hand-pollination and untreated control plants. The composition of flower volatiles from pollinated and unpollinated control plants was analysed by gas chromatography-mass spectrometry. 4. Both pollinators exhibited a strong bias for visual cues over olfactory cues. Neither pollinator used pollen as a cue. However, E. balteatus discriminated between newly opened and long-open flowers at short distance only when pollen was available. Flower visits by pollinators were influenced by pollination-induced changes in B. nigra but not R. sativus flowers. Pieris brassicae only responded to pollination-induced changes when visual and olfactory cues were offered simultaneously. The blend of volatiles emitted by B. nigra, but not R. sativus inflorescences was affected by pollination. 5. Collectively, the findings of this study show that different pollinators exploit different visual and olfactory traits when searching for flowers of two brassicaceous plant species.