Release Of Plant Volatiles Research Articles

Aphid-borne Viral Spread Is Enhanced by Virus-induced Accumulation of Plant Reactive Oxygen Species.

Most known plant viruses are spread from plant to plant by insect vectors. There is strong evidence that nonpersistently transmitted viruses manipulate the release of plant volatiles to attract insect vectors, thereby promoting virus spread. The mechanisms whereby aphid settling and feeding is altered on plants infected with these viruses, however, are unclear. Here we employed loss-of-function mutations in cucumber mosaic virus (CMV) and one of its host plants, tobacco (Nicotiana tabacum), to elucidate such mechanisms. We show that, relative to a CMVΔ2b strain with a deletion of the viral suppressor of RNAi 2b protein in CMV, plants infected with wild-type CMV produce higher concentrations of the reactive oxygen species (ROS) H2O2 in plant tissues. Aphids on wild-type CMV-infected plants engage in shorter probes, less phloem feeding, and exhibit other changes, as detected by electrical penetration graphing technology, relative to CMVΔ2b-infected plants. Therefore, the frequency of virus acquisition and the virus load per aphid were greater on CMV-infected plants than on CMVΔ2b-infected plants. Aphids also moved away from initial feeding sites more frequently on wild-type CMV infected versus CMVΔ2b-infected plants. The role of H2O2 in eliciting these effects on aphids was corroborated using healthy plants infused with H2O2 Finally, H2O2 levels were not elevated, and aphid behavior was unchanged, on CMV-infected RbohD-silenced tobacco plants, which are deficient in the induction of ROS production. These results suggest that CMV uses its viral suppressor of RNAi protein to increase plant ROS levels, thereby enhancing its acquisition and transmission by vector insects.

Nucleopolyhedrovirus infection enhances plant defences by increasing plant volatile diversity

ABSTRACTPlant–herbivore–entomopathogen tri-trophic interactions and biodiversity are relatively understudied topics in ecology. Particularly, the effects of entomopathogens on herbivore-induced plant volatiles and plant volatile diversity on the defensive function of plants have not been studied in detail. We used soybean (Glycine max), beet armyworm larvae (Spodoptera exigua), and nucleopolyhedrovirus (NPV) as a tri-trophic system to determine whether NPV infection can promote the emission and diversity of volatiles from plants. We also investigated whether NPV infection affects the attraction of Microplitis pallidipes, an important endoparasitoid of larval S. exigua. Uninfested soybean plants released 7 detectable volatile compounds while plants fed upon by healthy and NPV-infected S. exigua larvae released 12 and 15 volatiles, respectively. Female parasitoids were more attracted to the volatiles from plants that were fed upon by NPV-infected larvae than healthy larvae, and more attracted to the volatiles from plants that were fed upon by healthy larvae than no larvae. The selective responses of parasitoids to plant odours increased as plant volatile diversity increased. Our study suggests that the NPV infection facilitates the release of plant volatiles and enhances the defensive function of plants by increasing plant volatile diversity which in turn attracts more parasitoids. Also, this work reveals that plants might accrue two indirect benefits from NPV infection, cessation of herbivore feeding and more parasitisation.

Dynamics of belowground volatile diffusion and degradation

Above ground herbivory can induce release of plant volatiles that attract natural enemies of the herbivores. Similarly, roots can release herbivore induced volatiles that attract beneficial organisms such as entomopathogenic nematodes belowground. Unlike their aboveground counterparts, belowground volatile signals interact with solids, liquids, and gases as they move through soil pore spaces. These interactions influence belowground signaling, can create non-linear diffusion profiles, and result in surface adsorbtion and degradation of volatiles in space and time. By examining diffusion and degradation in sand-filled microcosms, we found that the diffusion profiles of E-β-caryophyllene, d-limonene, pregeijerene, α-pinene, germacrene-d, and linalool were affected by moisture and pH. Furthermore, the common plant volatile linalool was non-diffusive below ground. In addition, we discovered a novel pathway for the degradation of linalool into rapidly diffusing belowground signals. These findings suggest areas for future exploration and highlight the importance of abiotic factors when studying belowground semiochemically-based interactions such as attraction of beneficial entomopathogenic nematodes to plant roots infested by host insects.

The Salicylic Acid-Mediated Release of Plant Volatiles Affects the Host Choice of Bemisia tabaci.

The whitefly Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) causes serious crop losses worldwide by transmitting viruses. We have previously shown that salicylic acid (SA)-related plant defenses directly affect whiteflies. In this study, we applied exogenous SA to tomato plants in order to investigate the interaction between SA-induced plant volatiles and nonviruliferous B. tabaci B and Q or B- and Q-carrying tomato yellow leaf curl virus (TYLCV). The results showed that exogenous SA caused plants to repel nonviruliferous whiteflies, but the effect was reduced when the SA concentration was low and when the whiteflies were viruliferous. Exogenous SA increased the number and quantity of plant volatiles—especially the quantity of methyl salicylate and δ-limonene. In Y-tube olfactometer assays, methyl salicylate and δ-limonene repelled the whiteflies, but the repellency was reduced for viruliferous Q. We suggest that the release of plant volatiles as mediated by SA affects the interaction between whiteflies, plants, and viruses. Further studies are needed to determine why viruliferous Q is less sensitive than nonviruliferous Q to repellent plant volatiles.

Bio-inspired surfactants capable of generating plant volatiles.

Plants are able to synthesize, store and release lipophilic organic molecules known as plant volatiles (PVs) utilizing specific biological pathways and different enzymes which play vital roles in the plant's defence and in dealing with biotic and abiotic stress situations. The process of generation, storage and release of PVs by plants acquired during the course of evolution is a very complex phenomenon. Bio-inspired molecular design of farnesol-based surfactants facilitates similar production, storage and release of PVs. The designed molecules adsorb at air-water interface and self-aggregate into micelles in aqueous system. The structural design of the molecules allows them to self-activate in water via intramolecular cation-π interactions. The activated molecules undergo molecular rearrangements generating volatile organic molecules both at interface and inside the micelle core. The molecules adsorbed at the interface initially release the formed volatile molecules creating vacant space at interface, thus thermodynamically directing the micelle to release the manufactured volatile products.

Pushing boundaries in above–belowground interactions

James Hutton Institute, Dundee DD2 5DA, UKBelowground organisms, such as arbuscular mycorrhizal(AM) fungi, have long been credited with altering plant fit-ness. More recently, research on belowground organismshas revealed that AM fungi also influence a wide variety ofaboveground organisms via plants (reviewed in Van Dam H Bennett 2010). Schausberger et al. (2012) demon-strate that the presence of an AM fungus in the roots of ahost plant alters volatile emissions and host plant attractive-ness to parasitoids in the presence of herbivores. Thisextends previous studies that have focused on direct inter-actions of AM with plants (e.g. mycorrhizal fungal–plant–herbivore interactions; reviewed in Gehring & Bennett2009), but have not conclusively demonstrated how below-ground organisms, and AM fungi in particular, influencethird trophic level organisms such as parasitoids (Gange,Brown & Aplin 2003; Guerrieri et al. 2004; Hempel et al.2009; Leitner et al. 2010; Hoffmann, Vierheilig & Schaus-berger 2011a,b; Wooley & Paine 2011) via the release ofplant volatiles that attract parasitoids that attack herbivoreson host plants. Until recently, these studies failed to conclu-sively document the effects of AM fungi on both volatilerelease and attraction of parasitoids. For example, Wooley& Paine (2011) and Gange, Brown & Aplin (2003) haveshown variation in parasitoid attraction to plants hostingdifferent strains and species of Glomus as compared to non-mycorrhizal plants. Hoffmann, Vierheilig & Schausberger(2011a) also showed greater preference by parasitoids foreggs oviposited on plants associated with a single AMfungus. In addition, a single AM fungus in the roots of ahost plant has been shown to positively influence parasitoidlife-history characteristics (Hempel et al. 2009; Hoffmann,Vierheilig & Schausberger 2011b). However, none of thesestudies measured volatile profiles for host plants, so parasit-oid attraction could not be directly attributed to volatiles.A study on AM fungal influenced volatile release revealeddifferences but did not test whether changes in volatilesinfluenced parasitoids (Leitner et al. 2010). One study com-bined both parasitoid attractiveness and measurement ofvolatiles, but they primarily tested effects of attraction toplants in the absence of herbivory and never made compari-sons between mycorrhizal and non-mycorrhizal plantsexperiencing herbivory (Guerrieri et al. 2004). Unlike theseprevious experiments, Schausberger et al. measured bothchanges in volatile chemistry as well as parasitoid attractionin a fully factorial design.The results presented by Shausberger et al. open up multi-plefutureopportunitiesinabove–belowgroundresearch.Thefirst of these opportunities involves identifying the mecha-nisms by which AM fungi alter parasitoid attraction. Forexample,whatarethe biochemicalortranscriptionalchangesthat occur following AM fungal colonization that result inaltered volatile profiles? Are the mechanisms suggested forAM fungal alteration of direct chemical defences the samemechanisms that alter volatile profiles? Colonization by AMfungi has been shown to turn on the salicylic acid pathwaytemporarily,aprocessthatmayprimethejasmonicacidpath-way for herbivore attack (reviewed in Pozo & Azcon-Aguilar2007).Theinductionofvolatilesislinkedtothejasmonicacidpathway (reviewed in Heil 2008), and therefore, plants maybeprimedforafasterorgreaterreleaseofvolatileswhencolo-nizedbyAMfungi.However, there may be other mechanisms by which AMfungi influence volatile release. For example, given that AMfungi increase plant biomass and fitness in the PhaseolusvulgarissystemstudiedbyShausbergeret al.(aswellasmanyothersystems),itcouldsimplybethattheincreasedresourcesprovided by the mutualism allow plants to allocate moreresourcestoplantdefensivecharacteristics(e.g.directconstit-utiveandinduceddefencesaswellasindirectdefencesviavol-atile attraction; Bennett, Alers-Garcia & Bever 2006) or thatchanges in plant size or structure in association with AMfungi benefit or hinder parasitoid searching capabilities(Gange,Brown&Aplin2003).What characteristics of the volatile blends produced in thepresence of AM fungi are attractive for parasitoids? Shaus-bergeret al.showedtherewerefewerchemicalspresentinthevolatileblendsofAMfungalplantsbeforeherbivory(relativeto plants not hosting AM fungi), but this difference disap-peared after herbivory. However, different volatile chemicalswere released from plants experiencing herbivory and colo-nizedornotbyAMfungi(seealsoLeitneret al.2010).Shaus-berger et al. did not address whether increased attraction toplants hosting AM fungi is associated with a particular vola-

Herbivore-Induced Plant Volatiles Can Serve as Host Location Cues for a Generalist and a Specialist Egg Parasitoid

Herbivore-induced plant volatiles are important host finding cues for larval parasitoids, and similarly, insect oviposition might elicit the release of plant volatiles functioning as host finding cues for egg parasitoids. We hypothesized that egg parasitoids also might utilize HIPVs of emerging larvae to locate plants with host eggs. We, therefore, assessed the olfactory response of two egg parasitoids, a generalist, Trichogramma pretiosum (Tricogrammatidae), and a specialist, Telenomus remus (Scelionidae) to HIPVs. We used a Y-tube olfactometer to tests the wasps' responses to volatiles released by young maize plants that were treated with regurgitant from caterpillars of the moth Spodoptera frugiperda (Noctuidae) or were directly attacked by the caterpillars. The results show that the generalist egg parasitoid Tr. pretiosum is innately attracted by volatiles from freshly-damaged plants 0-1 and 2-3h after regurgitant treatment. During this interval, the volatile blend consisted of green leaf volatiles (GLVs) and a blend of aromatic compounds, mono- and homoterpenes, respectively. Behavioral assays with synthetic GLVs confirmed their attractiveness to Tr. pretiosum. The generalist learned the more complex volatile blends released 6-7h after induction, which consisted mainly of sesquiterpenes. The specialist T. remus on the other hand was attracted only to volatiles emitted from fresh and old damage after associating these volatiles with oviposition. Taken together, these results strengthen the emerging pattern that egg and larval parasitoids behave in a similar way in that generalists can respond innately to HIPVs, while specialists seems to rely more on associative learning.

Timing matters: release of plant volatiles that are attractive to parasitoids

Many plant species are known to release volatile organic compounds (VOCs) in response to attack by herbivores to defend themselves by attracting natural enemies of herbivores. However, herbivores may not stay on a single plant, but may move among plants. We tested the flight response of female parasitoids to volatiles released from plants that are attacked by their host herbivores under laboratory conditions. Parasitoids significantly preferred plants that had been under attack by herbivores (currently attacked plants) over intact plants. After removal of the larvae, the plants (formerly attacked plants) were still significantly preferred by parasitoids over intact plants. Moreover, the parasitoids significantly preferred currently attacked plants to formerly attacked plants, suggesting that parasitoids may use VOCs specifically released from plants under attack to efficiently search for their hosts.

Plant volatiles

Plant volatiles are the metabolites that plants release into the air. The quantities released are not trivial. Almost one-fifth of the atmospheric CO2 fixed by land plants is released back into the air each day as volatiles. Plants are champion synthetic chemists; they take advantage of their anabolic prowess to produce volatiles, which they use to protect themselves against biotic and abiotic stresses and to provide information - and potentially disinformation - to mutualists and competitors alike. As transferors of information, volatiles have provided plants with solutions to the challenges associated with being rooted in the ground and immobile.

Silicon‐augmented resistance of plants to herbivorous insects: a review

AbstractSilicon (Si) is one of the most abundant elements in the earth's crust, although its essentiality in plant growth is not clearly established. However, the importance of Si as an element that is particularly beneficial for plants under a range of abiotic and biotic stresses is now beyond doubt. This paper reviews progress in exploring the benefits at two‐ and three‐trophic levels and the underlying mechanism of Si in enhancing the resistance of host plants to herbivorous insects. Numerous studies have shown an enhanced resistance of plants to insect herbivores including folivores, borers, and phloem and xylem feeders. Silicon may act directly on insect herbivores leading to a reduction in insect performance and plant damage. Various indirect effects may also be caused, for example, by delaying herbivore establishment and thus an increased chance of exposure to natural enemies, adverse weather events or control measures that target exposed insects. A further indirect effect of Si may be to increase tolerance of plants to abiotic stresses, notably water stress, which can in turn lead to a reduction in insect numbers and plant damage. There are two mechanisms by which Si is likely to increase resistance to herbivore feeding. Increased physical resistance (constitutive), based on solid amorphous silica, has long been considered the major mechanism of Si‐mediated defences of plants, although there is recent evidence for induced physical defence. Physical resistance involves reduced digestibility and/or increased hardness and abrasiveness of plant tissues because of silica deposition, mainly as opaline phytoliths, in various tissues, including epidermal silica cells. Further, there is now evidence that soluble Si is involved in induced chemical defences to insect herbivore attack through the enhanced production of defensive enzymes or possibly the enhanced release of plant volatiles. However, only two studies have tested for the effect of Si on an insect herbivore and third trophic level effects on the herbivore's predators and parasitoids. One study showed no effect of Si on natural enemies, but the methods used were not favourable for the detection of semiochemical‐mediated effects. Work recently commenced in Australia is methodologically and conceptually more advanced and an effect of Si on the plants' ability to generate an induced response by acting at the third trophic level was observed. This paper provides the first overview of Si in insect herbivore resistance studies, and highlights novel, recent hypotheses and findings in this area of research. Finally, we make suggestions for future research efforts in the use of Si to enhance plant resistance to insect herbivores.

Isoprene interferes with the attraction of bodyguards by herbaceous plants

Isoprene is the most abundant volatile compound emitted by vegetation. It influences air chemistry and is part of plant defense against abiotic stresses. However, whether isoprene influences biotic interactions between plants and other organisms has not been investigated to date. Here we show a new effect of isoprene, namely its influence on interactions between plants and insects. Herbivory induces the release of plant volatiles that attract the herbivore's enemies, such as parasitic wasps, as a kind of bodyguard. We used transgenic isoprene-emitting Arabidopsis plants in behavioral, chemical, and electrophysiological studies to investigate the effects of isoprene on ecological interactions in 2 tritrophic systems. We demonstrate that isoprene is perceived by the chemoreceptors of the parasitic wasp Diadegma semiclausum and interferes with the attraction of this parasitic wasp to volatiles from herbivore-infested plants. We verified this repellent effect on D. semiclausum female wasps by adding external isoprene to the volatile blend of wild-type plants. In contrast, the antennae of the parasitic wasp Cotesia rubecula do not perceive isoprene and the behavior of this wasp was not altered by isoprene emission. In addition, the performance of the 2 examined lepidopteran herbivores (Pieris rapae and Plutella xylostella) was not affected by isoprene emission. Therefore, attraction of parasitic wasps to host-infested herbaceous plants in the neighborhood of high isoprene emitters, such as poplar or willow, may be hampered by the isoprene emission that repels plant bodyguards.

Aphids, predators and parasitoids.

A number of studies have demonstrated the role of herbivore-induced release of plant volatiles in mediating foraging behaviour of aphid parasitoids, particularly with the parasitoid Aphidius ervi, its aphid host Acyrthosiphon pisum and the aphid food plant Vicia faba. These studies have shown that feeding by the aphid alters the composition of volatiles released by the plant and that these compounds act as synomones for the foraging parasitoid. Of particular interest is the species-specificity of the herbivore-induced synomones associated with different aphids feeding on V. faba. Aphids employ various pheromones that mediate behaviour, particularly mating and alarm responses. These pheromones play important roles in reproduction and defence against predation and parasitism. Many species of aphids reproduce sexually on their primary hosts during the autumn and the sexual females produce a pheromone that attracts males. The sex pheromones for a number of aphid species have been identified and laboratory and field studies have shown that synthetic material can act as a kairomone in attracting predators and parasitoids. The aphid alarm pheromone is released from the cornicles of aphids when they are attacked by predators or parasitoids. The activity of the main alarm pheromone component, (E)-beta-farnesene, is inhibited by the related sesquiterpene hydrocarbon beta-caryophyllene, which is reported to attract the lacewing Chrysoperla carnea. In addition, electrophysiological studies have shown that the seven-spot ladybird, Coccinella septempunctata, possesses specific olfactory receptors for (E)-beta-farnesene and beta-caryophyllene. Laboratory studies show these compounds to have behavioural activity with C. septempunctata, suggesting that they may be involved in prey location.

The influence of monoterpene synthase transformation on the odour of tobacco

Monoterpenes are an important class of terpenoids that are commonly present in plant essential oils. These can be extracted from plants and are used in the flavouring and perfumery industry. Monoterpene synthases are the key enzymes in monoterpene biosynthesis, as they catalyse the cyclisation of the ubiquitous geranyl diphosphate (GDP) to the specific monoterpene skeletons. Tobacco is one of the most studied model plants, it can easily and efficiently be transformed, and is a suitable model to study the release of plant volatiles. Thus, we have isolated monoterpene synthases from lemon, transformed tobacco with these cDNAs and have used human panelists to study the change in fragrance of the transgenic in comparison to the wild type plants. In a triangle test, we found that subjects were capable of smelling significant differences between leaf samples. However, as a result of variability in panel ratings, no significant difference between two sets of transgenic flowers and the wild type tobacco flowers was found for the generated attributes in a descriptive test.

Concerted biosynthesis of an insect elicitor of plant volatiles.

A variety of agricultural plant species, including corn, respond to insect herbivore damage by releasing large quantities of volatile compounds and, as a result, become highly attractive to parasitic wasps that attack the herbivores. An elicitor of plant volatiles, N-(17-hydroxylinolenoyl)-L-glutamine, named volicitin and isolated from beet armyworm caterpillars, is a key component in plant recognition of damage from insect herbivory. Chemical analysis of the oral secretion from beet armyworms that have fed on 13C-labeled corn seedlings established that the fatty acid portion of volicitin is plant derived whereas the 17-hydroxylation reaction and the conjugation with glutamine are carried out by the caterpillar by using glutamine of insect origin. Ironically, these insect-catalyzed chemical modifications to linolenic acid are critical for the biological activity that triggers the release of plant volatiles, which in turn attract natural enemies of the caterpillar.

Identification of Semiochemicals Released During Aphid Feeding That Attract Parasitoid Aphidius ervi

Herbivore induced release of plant volatiles mediating the foraging behavior of the aphid parasitoid Aphidius ervi was investigated using the pea aphid, Acyrthosiphon pisum, feeding on broad bean, Vicia faba. Behavioral responses were studied using an olfactometer and a wind tunnel. Volatiles obtained by air entrainment of aphid infested plants were more attractive to A. ervi than those from uninfested plants, in both behavioral bioassays. GC-EAG of both extracts showed a number of peaks associated with responses by A. ervi, but with some differences between extracts. Compounds giving these peaks were tentatively identified by GC-MS and confirmed by comparison with authentic samples on GC, using two columns of different polarity. The activity of pure compounds was further investigated by EAG and wind tunnel assays. Results showed that, of the compounds tested, 6-methyl-5-hepten-2-one was the most attractive for A. ervi females, with linalool, (Z)-3-hexen-1-yl acetate, (E)-β-ocimene, (Z)-3-hexen-1-ol, and (E)-β-farnesene all eliciting significantly more oriented flight behavior than a solvent control. Foraging experience significantly increased parasitoid responses to these compounds, with the exception of (E)-β-farnesene. Time-course GC analysis showed that feeding of A. pisum on V. faba induced or increased the release of several compounds. Release of two of these compounds (6-methyl-5-hepten-2-one and geranic acid) was not induced by the nonhost black bean aphid, Aphis fabae. During the analysis period, production of (E)-β-ocimene remained constant, but 6-methyl-5-hepten-2-one, linalool, geranic acid, and (E)-β-farnesene appeared during the first day after A. pisum infestation and increased in concentration with increasing time of aphid feeding.

Plant Volatile Signals in Response to Herbivore Feeding

A surge in release of volatiles by several plant species can be observed in response to insect feeding. Oral secretions from these feeding herbivores provide the initial chemical signal that triggers the release of plant volatiles; one or more elicitors from the oral secretion allow the plant to identify and differentiate herbivore feeding from mere mechanical wounding. Elicitor(s), in combination with mechanical wounding, trigger the release of compounds both locally and systemically. These volatiles, which may be a blend of constitutive and induced compounds, vary in their relative and absolute concentration over time. They serve as easily detectable and distinctive chemical cues for predators and parasitoids of the herbivores feeding on the plants. Volatile compounds released from herbivore infested plants include the monoterpenes and sesquiterpenes of the isoprenoid pathway, green leaf volatiles of the fatty acid/lipoxygenase pathway and aromatic metabolites, such as indole and methyl salicylate, of the shikimic acid/tryptophan pathway.