An overview of volatile-mediated plant interactions with the surrounding environment. Plant-animal interactions include the attraction of pollinators and seed disseminators by floral and fruit volatiles, attraction/repellence of herbivores, and attraction of natural enemies of attacking herbivores both in atmosphere and rhizosphere. Aboveground plant-plant interactions comprise elicitation or priming of defense responses in healthy undamaged leaves of the same plant or in the neighboring unattacked plants. Belowground, these interactions include allelopathic activity on the germination and growth of competitive neighboring plants. Volatiles released from reproductive organs and roots also have antimicrobial activity thus protecting the plants from pathogen attack. In addition, isoprene, a leaf volatile confers photoprotection and thermotolerance. 

Contexts in source publication

Context 1

... attraction of animal seed dispersers ( Figure 1) (Goff and Klee, 2006). In double-choice experiments, fruit bats ( Pteropus pumilus and Ptenochirus jagori ) were able to detect and locate fruits as well as assess their state of ripeness exclusively by their odor (Luft et al. , 2003). Olfactory cues emanating from the food source might be especially important in food location for nocturnal foragers which have limited access to visual cues, as was suggested for owl monkeys ( Aotus nancymai ) (Bolen and Green, 1997). In the past two decades it has been well documented that for self-protection plants produce blends of volatile compounds in vegetative tissues in response to damage and herbivore attack. Odor blends emitted by herbivore-infested plants are complex mixtures, often composed of more than 200 different compounds, many of which occur as minor constituents (Dicke and van Loon, 2000). Emitted volatiles can directly affect herbi- vores’ physiology and behavior due to their toxic, repelling, or deterring properties (Bernasconi et al. , 1998; De Moraes et al. , 2001; Kessler and Baldwin, 2001; Vancanneyt et al. , 2001; Aharoni et al. , 2003). They can also attract enemies of attacking herbivores, such as parasitic wasps, flies or predatory mites, which can protect the signaling plant from further damage (Dicke et al. , 1990; Turlings et al. , 1990; Vet and Dicke, 1992; Pare and Tumlinson, 1997; Drukker et al. , 2000; Kessler and Baldwin, 2001). Moreover, some volatile compounds can mediate both direct and indirect defenses, deterring lepidopteran oviposition and attracting herbivore enemies as was found in Nicotiana attenuata (Figure 1) (Kessler and Baldwin, 2001). The most common volatile signals involved in direct and indirect defenses include metabolites of the lipoxygenase (LOX) pathway, the shikimic acid pathway, and products of the terpenoid pathway (monoterpenes, sesquiterpenes, homoterpenes) (see below) (Pichersky and Gershenzon, 2002). The particular response, whether it is to attract a carnivore or repel an herbivore, depends strongly on the level of plant induction (Horiuchi et al. , 2003; Heil, 2004; Gols et al. , 2003) as well as on the ability of the carnivores and parasitoids to discriminate different odor blends (Dicke, 1999a). It should, however, be noted that herbivore-induced volatiles are not always beneficial to damaged plants: in some instances, other nonspecific herbivores could be attracted by these volatile signals, resulting in increased attack for the plant (Horiuchi et al. , 2003; Bolter et al. , 1997). The emitted volatiles generally induced by elicitors in the herbivore saliva or oral secretion can be both plant and/or herbivore species specific (De Moraes et al. , 1998; (Takabayashi et al. , 1995; Dicke, 1999a). The composition of different odor blends also strongly depends on the type of damage, for example, herbivore feeding or oviposition (Hilker and Meiners, 2002; Hilker et al. , 2002). While some volatiles are constitu- tively emitted by undamaged healthy plants, the combined ac- tion of mechanical damage and low- or high-molecular-weight elicitors from attacking herbivores induce volatile emission through the release of stored compounds or lead to an increased formation of existing and/or de novo biosynthesis of new volatile compounds (Pare and Tumlinson, 1997; Turlings et al. , 1998). There is a lag period between treatment and subsequent volatile release. Some volatiles including terpenes, indole, and methyl salicylate, do not usually emit for hours after the beginning of herbivore damage. However, almost immedi- ately after wounding and the onset of herbivory plants release green leaf volatiles (GLV), six-carbon aldehydes, alcohols, and esters, which are considered as typical wound signals (Hatanaka, 1993). Within a species the volatile blends emitted upon herbivore attack differ quantitatively and qualitatively (Gouinguene et al. , 2001; Scutareanu et al. , 2003; Krips et al. , 2001) with some compounds in common (e.g., methyl salicylate and some terpenoids). The differences between volatile blends emitted from plants of one species infested by different herbivores are usually smaller than among different plant species (Takabayashi et al. , 1991). The composition of herbivore-induced plant volatiles also can be influenced by various abiotic factors, including soil and air humidity, temperature, light intensity and fertilization rate. Young corn plants, in response to herbivory by caterpillars, produced the highest level of induced volatiles when kept in a relatively dry soil, about 60% relative humidity, temperatures between 22 ◦ C and 27 ◦ C with high light intensity, and with continuous fertilization of the soil (Gouinguene and Turlings, 2002). In spite of high variability of emitted volatiles and effect of abiotic conditions on their emission, predators are able to distinguish infestations by its host from infestations by non-host closely related herbivore species. By exploit- ing herbivore-specific volatile emissions, the specialist parasitic wasp Cardiochiles nigriceps , for example, is able to distinguish between infestations by host, Heliothis virescens , and non-host, Helicoverpa zea , on phylogenetically distant species such as maize, cotton, and tobacco (De Moraes et al. , 1998). However, elevated atmospheric CO 2 concentration could weaken the plant response induced by herbivore attack leading to a distur- bance of signaling to the third trophic level (Vuorinen et al. , 2004). The tritrophic plant-herbivore-carnivore interactions (Figure 1), since first suggested in 1980 (Price et al. , 1980), are widely spread in the plant kingdom. To date this phenomenon has been reported in more than 23 plant species in combination with a diverse range of herbivore and natural enemy species (Dicke, 1999b). One of the best-studied examples for this type of tritrophic interaction includes interactions between lima bean plants ( Phaseolus lunatus ), herbivorous spider mites ( Tetranychus urticae ), and carnivorous mites ( Phytoseiulus persimilis ). Other systems are also very well characterized for this type ...

Context 2

... attraction of animal seed dispersers ( Figure 1) (Goff and Klee, 2006). In double-choice experiments, fruit bats ( Pteropus pumilus and Ptenochirus jagori ) were able to detect and locate fruits as well as assess their state of ripeness exclusively by their odor (Luft et al. , 2003). Olfactory cues emanating from the food source might be especially important in food location for nocturnal foragers which have limited access to visual cues, as was suggested for owl monkeys ( Aotus nancymai ) (Bolen and Green, 1997). In the past two decades it has been well documented that for self-protection plants produce blends of volatile compounds in vegetative tissues in response to damage and herbivore attack. Odor blends emitted by herbivore-infested plants are complex mixtures, often composed of more than 200 different compounds, many of which occur as minor constituents (Dicke and van Loon, 2000). Emitted volatiles can directly affect herbi- vores’ physiology and behavior due to their toxic, repelling, or deterring properties (Bernasconi et al. , 1998; De Moraes et al. , 2001; Kessler and Baldwin, 2001; Vancanneyt et al. , 2001; Aharoni et al. , 2003). They can also attract enemies of attacking herbivores, such as parasitic wasps, flies or predatory mites, which can protect the signaling plant from further damage (Dicke et al. , 1990; Turlings et al. , 1990; Vet and Dicke, 1992; Pare and Tumlinson, 1997; Drukker et al. , 2000; Kessler and Baldwin, 2001). Moreover, some volatile compounds can mediate both direct and indirect defenses, deterring lepidopteran oviposition and attracting herbivore enemies as was found in Nicotiana attenuata (Figure 1) (Kessler and Baldwin, 2001). The most common volatile signals involved in direct and indirect defenses include metabolites of the lipoxygenase (LOX) pathway, the shikimic acid pathway, and products of the terpenoid pathway (monoterpenes, sesquiterpenes, homoterpenes) (see below) (Pichersky and Gershenzon, 2002). The particular response, whether it is to attract a carnivore or repel an herbivore, depends strongly on the level of plant induction (Horiuchi et al. , 2003; Heil, 2004; Gols et al. , 2003) as well as on the ability of the carnivores and parasitoids to discriminate different odor blends (Dicke, 1999a). It should, however, be noted that herbivore-induced volatiles are not always beneficial to damaged plants: in some instances, other nonspecific herbivores could be attracted by these volatile signals, resulting in increased attack for the plant (Horiuchi et al. , 2003; Bolter et al. , 1997). The emitted volatiles generally induced by elicitors in the herbivore saliva or oral secretion can be both plant and/or herbivore species specific (De Moraes et al. , 1998; (Takabayashi et al. , 1995; Dicke, 1999a). The composition of different odor blends also strongly depends on the type of damage, for example, herbivore feeding or oviposition (Hilker and Meiners, 2002; Hilker et al. , 2002). While some volatiles are constitu- tively emitted by undamaged healthy plants, the combined ac- tion of mechanical damage and low- or high-molecular-weight elicitors from attacking herbivores induce volatile emission through the release of stored compounds or lead to an increased formation of existing and/or de novo biosynthesis of new volatile compounds (Pare and Tumlinson, 1997; Turlings et al. , 1998). There is a lag period between treatment and subsequent volatile release. Some volatiles including terpenes, indole, and methyl salicylate, do not usually emit for hours after the beginning of herbivore damage. However, almost immedi- ately after wounding and the onset of herbivory plants release green leaf volatiles (GLV), six-carbon aldehydes, alcohols, and esters, which are considered as typical wound signals (Hatanaka, 1993). Within a species the volatile blends emitted upon herbivore attack differ quantitatively and qualitatively (Gouinguene et al. , 2001; Scutareanu et al. , 2003; Krips et al. , 2001) with some compounds in common (e.g., methyl salicylate and some terpenoids). The differences between volatile blends emitted from plants of one species infested by different herbivores are usually smaller than among different plant species (Takabayashi et al. , 1991). The composition of herbivore-induced plant volatiles also can be influenced by various abiotic factors, including soil and air humidity, temperature, light intensity and fertilization rate. Young corn plants, in response to herbivory by caterpillars, produced the highest level of induced volatiles when kept in a relatively dry soil, about 60% relative humidity, temperatures between 22 ◦ C and 27 ◦ C with high light intensity, and with continuous fertilization of the soil (Gouinguene and Turlings, 2002). In spite of high variability of emitted volatiles and effect of abiotic conditions on their emission, predators are able to distinguish infestations by its host from infestations by non-host closely related herbivore species. By exploit- ing herbivore-specific volatile emissions, the specialist parasitic wasp Cardiochiles nigriceps , for example, is able to distinguish between infestations by host, Heliothis virescens , and non-host, Helicoverpa zea , on phylogenetically distant species such as maize, cotton, and tobacco (De Moraes et al. , 1998). However, elevated atmospheric CO 2 concentration could weaken the plant response induced by herbivore attack leading to a distur- bance of signaling to the third trophic level (Vuorinen et al. , 2004). The tritrophic plant-herbivore-carnivore interactions (Figure 1), since first suggested in 1980 (Price et al. , 1980), are widely spread in the plant kingdom. To date this phenomenon has been reported in more than 23 plant species in combination with a diverse range of herbivore and natural enemy species (Dicke, 1999b). One of the best-studied examples for this type of tritrophic interaction includes interactions between lima bean plants ( Phaseolus lunatus ), herbivorous spider mites ( Tetranychus urticae ), and carnivorous mites ( Phytoseiulus persimilis ). Other systems are also very well characterized for this type ...

Context 3

... attraction of animal seed dispersers ( Figure 1) (Goff and Klee, 2006). In double-choice experiments, fruit bats ( Pteropus pumilus and Ptenochirus jagori ) were able to detect and locate fruits as well as assess their state of ripeness exclusively by their odor (Luft et al. , 2003). Olfactory cues emanating from the food source might be especially important in food location for nocturnal foragers which have limited access to visual cues, as was suggested for owl monkeys ( Aotus nancymai ) (Bolen and Green, 1997). In the past two decades it has been well documented that for self-protection plants produce blends of volatile compounds in vegetative tissues in response to damage and herbivore attack. Odor blends emitted by herbivore-infested plants are complex mixtures, often composed of more than 200 different compounds, many of which occur as minor constituents (Dicke and van Loon, 2000). Emitted volatiles can directly affect herbi- vores’ physiology and behavior due to their toxic, repelling, or deterring properties (Bernasconi et al. , 1998; De Moraes et al. , 2001; Kessler and Baldwin, 2001; Vancanneyt et al. , 2001; Aharoni et al. , 2003). They can also attract enemies of attacking herbivores, such as parasitic wasps, flies or predatory mites, which can protect the signaling plant from further damage (Dicke et al. , 1990; Turlings et al. , 1990; Vet and Dicke, 1992; Pare and Tumlinson, 1997; Drukker et al. , 2000; Kessler and Baldwin, 2001). Moreover, some volatile compounds can mediate both direct and indirect defenses, deterring lepidopteran oviposition and attracting herbivore enemies as was found in Nicotiana attenuata (Figure 1) (Kessler and Baldwin, 2001). The most common volatile signals involved in direct and indirect defenses include metabolites of the lipoxygenase (LOX) pathway, the shikimic acid pathway, and products of the terpenoid pathway (monoterpenes, sesquiterpenes, homoterpenes) (see below) (Pichersky and Gershenzon, 2002). The particular response, whether it is to attract a carnivore or repel an herbivore, depends strongly on the level of plant induction (Horiuchi et al. , 2003; Heil, 2004; Gols et al. , 2003) as well as on the ability of the carnivores and parasitoids to discriminate different odor blends (Dicke, 1999a). It should, however, be noted that herbivore-induced volatiles are not always beneficial to damaged plants: in some instances, other nonspecific herbivores could be attracted by these volatile signals, resulting in increased attack for the plant (Horiuchi et al. , 2003; Bolter et al. , 1997). The emitted volatiles generally induced by elicitors in the herbivore saliva or oral secretion can be both plant and/or herbivore species specific (De Moraes et al. , 1998; (Takabayashi et al. , 1995; Dicke, 1999a). The composition of different odor blends also strongly depends on the type of damage, for example, herbivore feeding or oviposition (Hilker and Meiners, 2002; Hilker et al. , 2002). While some volatiles are constitu- tively emitted by undamaged healthy plants, the combined ac- tion of mechanical damage and low- or high-molecular-weight elicitors from attacking herbivores induce volatile emission through the release of stored compounds or lead to an increased formation of existing and/or de novo biosynthesis of new volatile compounds (Pare and Tumlinson, 1997; Turlings et al. , 1998). There is a lag period between treatment and subsequent volatile release. Some volatiles including terpenes, indole, and methyl salicylate, do not usually emit for hours after the beginning of herbivore damage. However, almost immedi- ately after wounding and the onset of herbivory plants release green leaf volatiles (GLV), six-carbon aldehydes, alcohols, and esters, which are considered as typical wound signals (Hatanaka, 1993). Within a species the volatile blends emitted upon herbivore attack differ quantitatively and qualitatively (Gouinguene et al. , 2001; Scutareanu et al. , 2003; Krips et al. , 2001) with some compounds in common (e.g., methyl salicylate and some terpenoids). The differences between volatile blends emitted from plants of one species infested by different herbivores are usually smaller than among different plant species (Takabayashi et al. , 1991). The composition of herbivore-induced plant volatiles also can be influenced by various abiotic factors, including soil and air humidity, temperature, light intensity and fertilization rate. Young corn plants, in response to herbivory by caterpillars, produced the highest level of induced volatiles when kept in a relatively dry soil, about 60% relative humidity, temperatures between 22 ◦ C and 27 ◦ C with high light intensity, and with continuous fertilization of the soil (Gouinguene and Turlings, 2002). In spite of high variability of emitted volatiles and effect of abiotic conditions on their emission, predators are able to distinguish infestations by its host from infestations by non-host closely related herbivore species. By exploit- ing herbivore-specific volatile emissions, the specialist parasitic wasp Cardiochiles nigriceps , for example, is able to distinguish between infestations by host, Heliothis virescens , and non-host, Helicoverpa zea , on phylogenetically distant species such as maize, cotton, and tobacco (De Moraes et al. , 1998). However, elevated atmospheric CO 2 concentration could weaken the plant response induced by herbivore attack leading to a distur- bance of signaling to the third trophic level (Vuorinen et al. , 2004). The tritrophic plant-herbivore-carnivore interactions (Figure 1), since first suggested in 1980 (Price et al. , 1980), are widely spread in the plant kingdom. To date this phenomenon has been reported in more than 23 plant species in combination with a diverse range of herbivore and natural enemy species (Dicke, 1999b). One of the best-studied examples for this type of tritrophic interaction includes interactions between lima bean plants ( Phaseolus lunatus ), herbivorous spider mites ( Tetranychus urticae ), and carnivorous mites ( Phytoseiulus persimilis ). Other systems are also very well characterized for this type ...

Context 4

... interactions, including small annual plants as well as long-lived trees. Infestation of lima bean leaves by spider mites triggers the release of volatiles which attract the predatory mites that prey on the spider mites (Takabayashi and Dicke, 1996). Similar to insect feeding activity, insect egg deposition can induce emission of plant volatiles which attract egg parasitoids (Anderson and Alborn, 1999; Hilker and Meiners, 2002). This occurs soon after herbivore egg deposition, allowing the plant to defend itself against pests before any damage has occurred, i.e., before the larvae have hatched from the eggs (Hilker et al. , 2002). Moreover, herbivore- and wound-induced volatiles released by the infested plant attract predators or parasitoids of the damaging herbivores in plant-caterpillar-parasitoid (Dicke and van Loon, 2000) and plant-caterpillar-predatory bug interactions (Kessler and Baldwin, 2001). The high chemical diversity within the herbivore-induced volatile mixtures complicated the identification of the compound(s) actually responsible for signaling herbivore enemies. Earlier attempts to dissect the volatile signals emitted by herbivore-damaged leaves of lima bean (Dicke et al. , 1990) and maize (Turlings et al. , 1991) failed to identify a specific compound responsible for enemy attraction, suggesting that mixtures constitute the active signal. However, it was shown that the application of individual plant volatiles, such as methyl salicylate and the C 16 -homoterpene 4,8,12-trimethyl-1,3( E ),7( E ),11- tridecatetraene [( E , E )-TMTT], in behavioral experiments can attract predatory mites (De Boer and Dicke, 2004; De Boer et al. , 2004). Recent progress in the isolation of genes encoding enzymes responsible for the formation of plant volatile compounds allowed the use of genetic engineering as a novel tool to investigate the role of individual signaling compounds in mediating tritrophic interactions. The attraction of the predatory mite Phytoseiulus persimilis to the sesquiterpene alcohol (3 S )- ( E )-nerolidol was recently demonstrated with transgenic Arabidopsis overexpressing strawberry nerolidol synthase, a terpene synthase (TPS) (Kappers et al. , 2005). These results suggested that (3 S )-( E )-nerolidol is a component of the volatile signal that attracts the predatory mites to spider mite-infested plants. Over- expression in Arabidopsis thaliana of another terpene synthase gene, the corn TPS10 gene, which forms ( E )- β -farnesene, ( E )α -bergamotene, and other herbivore-induced sesquiterpene hydrocarbons released from maize upon herbivory by lepidopteran larvae, increased attractiveness of these transgenic plant to the parasitic wasps Cotesia marginiventris (Schnee et al. , 2006). The wasps’ behavior in the olfactometer bioassays indicated that the mixture of TPS10 sesquiterpenes can be a genuine signal in attracting these parasitoids to herbivore-damaged maize. These examples show that once the biosynthetic genes for volatile formation are known, transgenic plants have a great potential to deliver individual volatiles or volatile mixtures for bioassays, which will allow the functional identification of potential volatile signals, either individually or in combinations, involved in interactions among organisms. Volatiles released from herbivore-infested plants also mediate plant-plant interactions and may induce the expression of defense genes and emission of volatiles in healthy leaves on the same plant or of neighboring unattacked plants, thus increasing their attractiveness to carnivores and decreasing their suscepti- bility to the damaging herbivores (Figure 1) (Dicke et al. , 1990; Arimura et al. , 2002, 2004b; Ruther and Kleier, 2005). Using spider mites ( Tetranychus urticae ) and predatory mites ( Phytoseiulus persimilis ), it has been shown that not only the attacked plant but also neighboring plants became more attractive to predatory mites and less susceptible to spider mites (Bruin et al ., 1992). Tetranychus urticae -infested lima bean leaf volatiles induce the expression of several genes encoding pathogenesis-related (PR) proteins, lipoxygenase (LOX), phenylalanine ammonia- lyase (PAL), and farnesyl pyrophosphate synthase (FPS) in the neighboring lima bean leaves (Arimura et al ., 2000a). This effi- cient induction of defense genes in plants exposed to herbivore- induced volatiles is the result of activation of the multifunctional signaling cascades involving ethylene and jasmonic acid (JA) (Arimura et al. , 2002) (see below). Release of herbivore-induced volatiles occurs both locally from damaged tissues and systemically from undamaged tissues and displays distinct temporal patterns (Schmelz et al. , 2001; Arimura et al. , 2004b). Nicotiana tabacum , for example, releases several herbivore-induced volatiles exclusively at night. These nocturnally emitted compounds repel female moths ( Heliothis virescens ), which search for oviposition sites during the night (De Moraes et al. , 2001). Diurnal rhythm of volatile emissions was shown from beet armyworm-damaged cotton leaves (Loughrin et al. , 1994) and lima bean leaves infested with Spodoptera littoralis (Arimura et al. , 2005). Similarly, local hybrid poplar ( Populus trichocarpa × deltoids ) leaves attacked by forest tent caterpillar and systemic non-infested leaves released very similar blends of volatiles consisting of ( E )- β -ocimene along with five or six other mono-, sesqui-, and homoterpene compounds with maximum emission during the light period (Arimura et al. , 2004b). Interestingly, ( E )- β -ocimene could also act as a possible plant-to-plant signal in uninfested lima bean plants thus up-regulating the signaling pathway of JA and ethylene (Arimura et al. , 2000, 2002). Other signaling molecules involved in intra- and inter-plant communication include methyl jasmonate (MeJA) (Farmer, 2001), methyl salicylate (MeSA) (Shulaev et al. , 1997), and (Z)-3-hexenol (Farag et al. , 2005; Ruther and Kleier, 2005). Exposure of intact maize plants to (Z)-3-hexenol induces the emission of a volatile blend, which is typically released after caterpillar feeding and attracts natural enemies of the herbivores (Ruther and Kleier, 2005). In addition to direct elicitation, exposure to volatile compounds from attacked plants may lead to priming of plant defense responses in the neighboring plants (Figure 1). Priming by volatiles prepares the plant to respond more rapidly and intensively against subsequent attack by herbivorous ...

Context 5

... interactions, including small annual plants as well as long-lived trees. Infestation of lima bean leaves by spider mites triggers the release of volatiles which attract the predatory mites that prey on the spider mites (Takabayashi and Dicke, 1996). Similar to insect feeding activity, insect egg deposition can induce emission of plant volatiles which attract egg parasitoids (Anderson and Alborn, 1999; Hilker and Meiners, 2002). This occurs soon after herbivore egg deposition, allowing the plant to defend itself against pests before any damage has occurred, i.e., before the larvae have hatched from the eggs (Hilker et al. , 2002). Moreover, herbivore- and wound-induced volatiles released by the infested plant attract predators or parasitoids of the damaging herbivores in plant-caterpillar-parasitoid (Dicke and van Loon, 2000) and plant-caterpillar-predatory bug interactions (Kessler and Baldwin, 2001). The high chemical diversity within the herbivore-induced volatile mixtures complicated the identification of the compound(s) actually responsible for signaling herbivore enemies. Earlier attempts to dissect the volatile signals emitted by herbivore-damaged leaves of lima bean (Dicke et al. , 1990) and maize (Turlings et al. , 1991) failed to identify a specific compound responsible for enemy attraction, suggesting that mixtures constitute the active signal. However, it was shown that the application of individual plant volatiles, such as methyl salicylate and the C 16 -homoterpene 4,8,12-trimethyl-1,3( E ),7( E ),11- tridecatetraene [( E , E )-TMTT], in behavioral experiments can attract predatory mites (De Boer and Dicke, 2004; De Boer et al. , 2004). Recent progress in the isolation of genes encoding enzymes responsible for the formation of plant volatile compounds allowed the use of genetic engineering as a novel tool to investigate the role of individual signaling compounds in mediating tritrophic interactions. The attraction of the predatory mite Phytoseiulus persimilis to the sesquiterpene alcohol (3 S )- ( E )-nerolidol was recently demonstrated with transgenic Arabidopsis overexpressing strawberry nerolidol synthase, a terpene synthase (TPS) (Kappers et al. , 2005). These results suggested that (3 S )-( E )-nerolidol is a component of the volatile signal that attracts the predatory mites to spider mite-infested plants. Over- expression in Arabidopsis thaliana of another terpene synthase gene, the corn TPS10 gene, which forms ( E )- β -farnesene, ( E )α -bergamotene, and other herbivore-induced sesquiterpene hydrocarbons released from maize upon herbivory by lepidopteran larvae, increased attractiveness of these transgenic plant to the parasitic wasps Cotesia marginiventris (Schnee et al. , 2006). The wasps’ behavior in the olfactometer bioassays indicated that the mixture of TPS10 sesquiterpenes can be a genuine signal in attracting these parasitoids to herbivore-damaged maize. These examples show that once the biosynthetic genes for volatile formation are known, transgenic plants have a great potential to deliver individual volatiles or volatile mixtures for bioassays, which will allow the functional identification of potential volatile signals, either individually or in combinations, involved in interactions among organisms. Volatiles released from herbivore-infested plants also mediate plant-plant interactions and may induce the expression of defense genes and emission of volatiles in healthy leaves on the same plant or of neighboring unattacked plants, thus increasing their attractiveness to carnivores and decreasing their suscepti- bility to the damaging herbivores (Figure 1) (Dicke et al. , 1990; Arimura et al. , 2002, 2004b; Ruther and Kleier, 2005). Using spider mites ( Tetranychus urticae ) and predatory mites ( Phytoseiulus persimilis ), it has been shown that not only the attacked plant but also neighboring plants became more attractive to predatory mites and less susceptible to spider mites (Bruin et al ., 1992). Tetranychus urticae -infested lima bean leaf volatiles induce the expression of several genes encoding pathogenesis-related (PR) proteins, lipoxygenase (LOX), phenylalanine ammonia- lyase (PAL), and farnesyl pyrophosphate synthase (FPS) in the neighboring lima bean leaves (Arimura et al ., 2000a). This effi- cient induction of defense genes in plants exposed to herbivore- induced volatiles is the result of activation of the multifunctional signaling cascades involving ethylene and jasmonic acid (JA) (Arimura et al. , 2002) (see below). Release of herbivore-induced volatiles occurs both locally from damaged tissues and systemically from undamaged tissues and displays distinct temporal patterns (Schmelz et al. , 2001; Arimura et al. , 2004b). Nicotiana tabacum , for example, releases several herbivore-induced volatiles exclusively at night. These nocturnally emitted compounds repel female moths ( Heliothis virescens ), which search for oviposition sites during the night (De Moraes et al. , 2001). Diurnal rhythm of volatile emissions was shown from beet armyworm-damaged cotton leaves (Loughrin et al. , 1994) and lima bean leaves infested with Spodoptera littoralis (Arimura et al. , 2005). Similarly, local hybrid poplar ( Populus trichocarpa × deltoids ) leaves attacked by forest tent caterpillar and systemic non-infested leaves released very similar blends of volatiles consisting of ( E )- β -ocimene along with five or six other mono-, sesqui-, and homoterpene compounds with maximum emission during the light period (Arimura et al. , 2004b). Interestingly, ( E )- β -ocimene could also act as a possible plant-to-plant signal in uninfested lima bean plants thus up-regulating the signaling pathway of JA and ethylene (Arimura et al. , 2000, 2002). Other signaling molecules involved in intra- and inter-plant communication include methyl jasmonate (MeJA) (Farmer, 2001), methyl salicylate (MeSA) (Shulaev et al. , 1997), and (Z)-3-hexenol (Farag et al. , 2005; Ruther and Kleier, 2005). Exposure of intact maize plants to (Z)-3-hexenol induces the emission of a volatile blend, which is typically released after caterpillar feeding and attracts natural enemies of the herbivores (Ruther and Kleier, 2005). In addition to direct elicitation, exposure to volatile compounds from attacked plants may lead to priming of plant defense responses in the neighboring plants (Figure 1). Priming by volatiles prepares the plant to respond more rapidly and intensively against subsequent attack by herbivorous ...

Context 6

... et al. , 2004; Kessler et al. , 2006). Maize seedlings pre-exposed to individual GLV compounds such as ( Z )-3- hexenal, ( Z )-3-hexen-1-ol, and ( Z )-3-hexenyl acetate, or to the blend of volatiles released from damaged plants, responded to wounding and beet armyworm ( Spodoptera exigua ) caterpillar regurgitant treatment with enhanced jasmonic acid production and an increased release of sesquiterpenes when compared with plants that had been similarly damaged and treated but not exposed to the volatiles (Engelberth et al. , 2004). Priming of native Nicotiana attenuata by clipped sagebrush-released volatiles resulted in lower total herbivore damage of tobacco plants and in higher mortality rate of young Manduca sexta caterpillars (Kessler et al. , 2006). These examples show that priming by volatile compounds provides a different way of responding to the threat of insect herbivory via the incomplete turning on of defense-related processes and reducing biochemical investment in defenses in receiver plants until the onset of actual attack (Engelberth et al. , 2004; Kessler et al. , 2006). While priming by volatile compounds could be one of the mechanisms involved in plant-plant signaling in nature, its underlying molecular mechanism and ecological relevance of the particular interactions still remain to be determined. The induced emission of volatiles is not limited solely to aerial parts of a plant. Plants also release volatiles from their roots with chemical and structural diversity comparable to those found in emissions from aerial plant organs. Similar to aboveground volatile compounds, root volatiles can contribute to a belowground defense system by acting as antimicrobial or antiherbivore substances, or by attracting enemies of root-feeding herbivores (Figure 1). Infection of Arabidopsis roots with either compatible bacterial ( Pseudomonas syringae strain DC3000) or fungal ( Alternaria brassicola ) pathogens or root-feeding insects ( Diuraphis noxia ) triggers the rapid emission of 1,8-cineole (Steeghs et al. , 2004). 1,8-Cineole is an oxygenated monoterpene with antimicrobial activity (Hammer et al. , 2003; Pina-Vaz et al. , 2004), the formation of which is catalyzed in Arabidopsis by a root-specific TPS (Chen et al. , 2004; Ro et al. , 2006). 1,8- Cineole may also exhibit a toxic and deterrent effect on certain insects (Tripathi et al. , 2001) thus contributing to direct plant defense. Volatiles emitted by roots of a coniferous plant Thuja occidentalis upon attack by weevil larvae Otiorhynchus sulcatus was shown to attract the entomopathogenic nematode Heterohabdi- tis megidis (Boff et al. , 2001; van Tol et al. , 2001). Similarly, the infestation of turnip roots with root-feeding larvae ( Delia radicum ) induced a systemic release of volatiles which attract the specialist parasitoid Trybliographa rapae (Neveu et al. , 2002). In the latter tritrophic system parasitoid-attracting volatiles were emitted not only by the root itself but also by undamaged leaves of a damaged plant. Unfortunately, the nature of attractants involved in these belowground plant-mediated interactions is still unknown. Recently, the sesquiterpene ( E )- -caryophyllene was identified as a first root-insect–induced belowground plant signal that strongly attracts an entomopathogenic nematode Heterorhabditis megidis under in situ laboratory and field conditions (Rasmann et al. , 2005). Maize roots released this sesquiterpene in response to feeding by larvae of the beetle Diabrotica virgifera virgifera , a maize pest that is currently invading Europe. A fivefold higher nematode infection rate of D. v. virgifera larvae was found on a maize variety that produced the signal than on a variety that did not, indicating the existence of communication between plant roots and the third trophic level in the rhizosphere. Volatiles released by roots into the soil may also exhibit allelopathic activity by reducing the germination and growth of competitive neighboring plants as was recently shown for 1.8- cineole (Figure 1) (Romagni et al. , 2000; Singh et al. , 2002). Its phytotoxic effect on seed germination and growth is probably the result of an inhibition of both nuclear and organelle DNA synthesis in the root apical meristem (Nishida et al. , 2005) and changes in the root phospholipid and sterol composition (Zunino and Zygadlo, 2005). In contrast to aboveground interactions of plant with other organisms, communication in the rhizosphere and belowground signaling are most likely limited to immediate neighbors and competitors and are restricted by the mobility of many soil organisms and the relatively low transport rates of root-emitted compounds in the soil (Van der Putten et al. , 2001; Baldwin et al. , 2002). In addition to an involvement of plant volatiles in defense and reproductive processes, volatile isoprenoids are able to protect plants from heat damage and allow them to maintain photosynthetic rates thus enhancing plant thermotolerance at elevated temperatures (Figure 1) (Sharkey et al. , 2001, Loreto et al. , 1998; Copolovici et al. , 2005; Penuelas et al. , 2005; reviewed in Sharkey and Yeh, 2001). Blocking of monoterpene emission in Quercus ilex (L.) leaves with fosmidomycin, a specific in- hibitor of the plastidial isoprenoid biosynthetic pathway, resulted in the decrease of the photosynthetic thermotolerance. However, fumigation with the relatively low atmospheric concentrations of monoterpenes partly restored the heat stress resistance (Copolovici et al. , 2005). Similarly, fumigation with exogenous isoprene of fosmidomycin-fed leaves of red oak ( Quercus rubra ) and kudzu ( Pueraria lobata [Willd.] Ohwi.) increased the ability of photosynthetic apparatus to recover from a brief high temperature exposure (Sharkey et al. , 2001). These results suggest that heat tolerance of monoterpene- and isoprene- nonemitting plants may be significantly improved by fumigation from the nearby growing emitting species during warm windless days in the Mediterranean canopies (Copolovici et al. , 2005). Although the exact mechanism by which isoprene or monoterpenes confer thermotolerance is not known, it has been proposed that at high temperature thylakoid membranes become ...

Context 7

... et al. , 2004; Kessler et al. , 2006). Maize seedlings pre-exposed to individual GLV compounds such as ( Z )-3- hexenal, ( Z )-3-hexen-1-ol, and ( Z )-3-hexenyl acetate, or to the blend of volatiles released from damaged plants, responded to wounding and beet armyworm ( Spodoptera exigua ) caterpillar regurgitant treatment with enhanced jasmonic acid production and an increased release of sesquiterpenes when compared with plants that had been similarly damaged and treated but not exposed to the volatiles (Engelberth et al. , 2004). Priming of native Nicotiana attenuata by clipped sagebrush-released volatiles resulted in lower total herbivore damage of tobacco plants and in higher mortality rate of young Manduca sexta caterpillars (Kessler et al. , 2006). These examples show that priming by volatile compounds provides a different way of responding to the threat of insect herbivory via the incomplete turning on of defense-related processes and reducing biochemical investment in defenses in receiver plants until the onset of actual attack (Engelberth et al. , 2004; Kessler et al. , 2006). While priming by volatile compounds could be one of the mechanisms involved in plant-plant signaling in nature, its underlying molecular mechanism and ecological relevance of the particular interactions still remain to be determined. The induced emission of volatiles is not limited solely to aerial parts of a plant. Plants also release volatiles from their roots with chemical and structural diversity comparable to those found in emissions from aerial plant organs. Similar to aboveground volatile compounds, root volatiles can contribute to a belowground defense system by acting as antimicrobial or antiherbivore substances, or by attracting enemies of root-feeding herbivores (Figure 1). Infection of Arabidopsis roots with either compatible bacterial ( Pseudomonas syringae strain DC3000) or fungal ( Alternaria brassicola ) pathogens or root-feeding insects ( Diuraphis noxia ) triggers the rapid emission of 1,8-cineole (Steeghs et al. , 2004). 1,8-Cineole is an oxygenated monoterpene with antimicrobial activity (Hammer et al. , 2003; Pina-Vaz et al. , 2004), the formation of which is catalyzed in Arabidopsis by a root-specific TPS (Chen et al. , 2004; Ro et al. , 2006). 1,8- Cineole may also exhibit a toxic and deterrent effect on certain insects (Tripathi et al. , 2001) thus contributing to direct plant defense. Volatiles emitted by roots of a coniferous plant Thuja occidentalis upon attack by weevil larvae Otiorhynchus sulcatus was shown to attract the entomopathogenic nematode Heterohabdi- tis megidis (Boff et al. , 2001; van Tol et al. , 2001). Similarly, the infestation of turnip roots with root-feeding larvae ( Delia radicum ) induced a systemic release of volatiles which attract the specialist parasitoid Trybliographa rapae (Neveu et al. , 2002). In the latter tritrophic system parasitoid-attracting volatiles were emitted not only by the root itself but also by undamaged leaves of a damaged plant. Unfortunately, the nature of attractants involved in these belowground plant-mediated interactions is still unknown. Recently, the sesquiterpene ( E )- -caryophyllene was identified as a first root-insect–induced belowground plant signal that strongly attracts an entomopathogenic nematode Heterorhabditis megidis under in situ laboratory and field conditions (Rasmann et al. , 2005). Maize roots released this sesquiterpene in response to feeding by larvae of the beetle Diabrotica virgifera virgifera , a maize pest that is currently invading Europe. A fivefold higher nematode infection rate of D. v. virgifera larvae was found on a maize variety that produced the signal than on a variety that did not, indicating the existence of communication between plant roots and the third trophic level in the rhizosphere. Volatiles released by roots into the soil may also exhibit allelopathic activity by reducing the germination and growth of competitive neighboring plants as was recently shown for 1.8- cineole (Figure 1) (Romagni et al. , 2000; Singh et al. , 2002). Its phytotoxic effect on seed germination and growth is probably the result of an inhibition of both nuclear and organelle DNA synthesis in the root apical meristem (Nishida et al. , 2005) and changes in the root phospholipid and sterol composition (Zunino and Zygadlo, 2005). In contrast to aboveground interactions of plant with other organisms, communication in the rhizosphere and belowground signaling are most likely limited to immediate neighbors and competitors and are restricted by the mobility of many soil organisms and the relatively low transport rates of root-emitted compounds in the soil (Van der Putten et al. , 2001; Baldwin et al. , 2002). In addition to an involvement of plant volatiles in defense and reproductive processes, volatile isoprenoids are able to protect plants from heat damage and allow them to maintain photosynthetic rates thus enhancing plant thermotolerance at elevated temperatures (Figure 1) (Sharkey et al. , 2001, Loreto et al. , 1998; Copolovici et al. , 2005; Penuelas et al. , 2005; reviewed in Sharkey and Yeh, 2001). Blocking of monoterpene emission in Quercus ilex (L.) leaves with fosmidomycin, a specific in- hibitor of the plastidial isoprenoid biosynthetic pathway, resulted in the decrease of the photosynthetic thermotolerance. However, fumigation with the relatively low atmospheric concentrations of monoterpenes partly restored the heat stress resistance (Copolovici et al. , 2005). Similarly, fumigation with exogenous isoprene of fosmidomycin-fed leaves of red oak ( Quercus rubra ) and kudzu ( Pueraria lobata [Willd.] Ohwi.) increased the ability of photosynthetic apparatus to recover from a brief high temperature exposure (Sharkey et al. , 2001). These results suggest that heat tolerance of monoterpene- and isoprene- nonemitting plants may be significantly improved by fumigation from the nearby growing emitting species during warm windless days in the Mediterranean canopies (Copolovici et al. , 2005). Although the exact mechanism by which isoprene or monoterpenes confer thermotolerance is not known, it has been proposed that at high temperature thylakoid membranes become ...

Context 8

... et al. , 2004; Kessler et al. , 2006). Maize seedlings pre-exposed to individual GLV compounds such as ( Z )-3- hexenal, ( Z )-3-hexen-1-ol, and ( Z )-3-hexenyl acetate, or to the blend of volatiles released from damaged plants, responded to wounding and beet armyworm ( Spodoptera exigua ) caterpillar regurgitant treatment with enhanced jasmonic acid production and an increased release of sesquiterpenes when compared with plants that had been similarly damaged and treated but not exposed to the volatiles (Engelberth et al. , 2004). Priming of native Nicotiana attenuata by clipped sagebrush-released volatiles resulted in lower total herbivore damage of tobacco plants and in higher mortality rate of young Manduca sexta caterpillars (Kessler et al. , 2006). These examples show that priming by volatile compounds provides a different way of responding to the threat of insect herbivory via the incomplete turning on of defense-related processes and reducing biochemical investment in defenses in receiver plants until the onset of actual attack (Engelberth et al. , 2004; Kessler et al. , 2006). While priming by volatile compounds could be one of the mechanisms involved in plant-plant signaling in nature, its underlying molecular mechanism and ecological relevance of the particular interactions still remain to be determined. The induced emission of volatiles is not limited solely to aerial parts of a plant. Plants also release volatiles from their roots with chemical and structural diversity comparable to those found in emissions from aerial plant organs. Similar to aboveground volatile compounds, root volatiles can contribute to a belowground defense system by acting as antimicrobial or antiherbivore substances, or by attracting enemies of root-feeding herbivores (Figure 1). Infection of Arabidopsis roots with either compatible bacterial ( Pseudomonas syringae strain DC3000) or fungal ( Alternaria brassicola ) pathogens or root-feeding insects ( Diuraphis noxia ) triggers the rapid emission of 1,8-cineole (Steeghs et al. , 2004). 1,8-Cineole is an oxygenated monoterpene with antimicrobial activity (Hammer et al. , 2003; Pina-Vaz et al. , 2004), the formation of which is catalyzed in Arabidopsis by a root-specific TPS (Chen et al. , 2004; Ro et al. , 2006). 1,8- Cineole may also exhibit a toxic and deterrent effect on certain insects (Tripathi et al. , 2001) thus contributing to direct plant defense. Volatiles emitted by roots of a coniferous plant Thuja occidentalis upon attack by weevil larvae Otiorhynchus sulcatus was shown to attract the entomopathogenic nematode Heterohabdi- tis megidis (Boff et al. , 2001; van Tol et al. , 2001). Similarly, the infestation of turnip roots with root-feeding larvae ( Delia radicum ) induced a systemic release of volatiles which attract the specialist parasitoid Trybliographa rapae (Neveu et al. , 2002). In the latter tritrophic system parasitoid-attracting volatiles were emitted not only by the root itself but also by undamaged leaves of a damaged plant. Unfortunately, the nature of attractants involved in these belowground plant-mediated interactions is still unknown. Recently, the sesquiterpene ( E )- -caryophyllene was identified as a first root-insect–induced belowground plant signal that strongly attracts an entomopathogenic nematode Heterorhabditis megidis under in situ laboratory and field conditions (Rasmann et al. , 2005). Maize roots released this sesquiterpene in response to feeding by larvae of the beetle Diabrotica virgifera virgifera , a maize pest that is currently invading Europe. A fivefold higher nematode infection rate of D. v. virgifera larvae was found on a maize variety that produced the signal than on a variety that did not, indicating the existence of communication between plant roots and the third trophic level in the rhizosphere. Volatiles released by roots into the soil may also exhibit allelopathic activity by reducing the germination and growth of competitive neighboring plants as was recently shown for 1.8- cineole (Figure 1) (Romagni et al. , 2000; Singh et al. , 2002). Its phytotoxic effect on seed germination and growth is probably the result of an inhibition of both nuclear and organelle DNA synthesis in the root apical meristem (Nishida et al. , 2005) and changes in the root phospholipid and sterol composition (Zunino and Zygadlo, 2005). In contrast to aboveground interactions of plant with other organisms, communication in the rhizosphere and belowground signaling are most likely limited to immediate neighbors and competitors and are restricted by the mobility of many soil organisms and the relatively low transport rates of root-emitted compounds in the soil (Van der Putten et al. , 2001; Baldwin et al. , 2002). In addition to an involvement of plant volatiles in defense and reproductive processes, volatile isoprenoids are able to protect plants from heat damage and allow them to maintain photosynthetic rates thus enhancing plant thermotolerance at elevated temperatures (Figure 1) (Sharkey et al. , 2001, Loreto et al. , 1998; Copolovici et al. , 2005; Penuelas et al. , 2005; reviewed in Sharkey and Yeh, 2001). Blocking of monoterpene emission in Quercus ilex (L.) leaves with fosmidomycin, a specific in- hibitor of the plastidial isoprenoid biosynthetic pathway, resulted in the decrease of the photosynthetic thermotolerance. However, fumigation with the relatively low atmospheric concentrations of monoterpenes partly restored the heat stress resistance (Copolovici et al. , 2005). Similarly, fumigation with exogenous isoprene of fosmidomycin-fed leaves of red oak ( Quercus rubra ) and kudzu ( Pueraria lobata [Willd.] Ohwi.) increased the ability of photosynthetic apparatus to recover from a brief high temperature exposure (Sharkey et al. , 2001). These results suggest that heat tolerance of monoterpene- and isoprene- nonemitting plants may be significantly improved by fumigation from the nearby growing emitting species during warm windless days in the Mediterranean canopies (Copolovici et al. , 2005). Although the exact mechanism by which isoprene or monoterpenes confer thermotolerance is not known, it has been proposed that at high temperature thylakoid membranes become ...

Context 9

... as sedentary organisms, have to adjust to the surrounding environment during their life cycle. To compensate for their immobility, plants have evolved various mechanisms for their interactions with the environment including the release of arrays of volatile compounds from their leaves, flowers and fruits into the atmosphere and from roots into the soil. At present, a total of 1700 volatile compounds have been described from more than 90 plant families (Knudsen and Gershenzon, 2006). These volatiles constitute about 1% of plant secondary metabolites known to date and are mainly represented by terpenoids, phenylpropanoids/benzenoids, fatty acid and amino acid derivatives (Dudareva et al. , 2004). Volatile compounds are typically lipophilic liquids with high vapor pressures and can cross membranes freely and be released into the atmosphere or soil in the absence of a diffusion barrier (Pichersky et al. , 2006). The chemical composition of plant-emitted volatile blends and their intensity can carry information about the plants’ physiological status and the stresses they have been subjected to. The primary functions of airborne volatiles are to defend plants against herbivores and pathogens or to provide a reproductive advantage by attracting pollinators and seed dispersers (Reinhard et al. , 2004; Pichersky and Gershenzon, 2002). Volatiles emitted from vegetative tissues, as a part of the plant defense system, can directly repel (De Moraes et al. , 2001; Kessler and Baldwin, 2001) or intoxicate (Vancanneyt et al. , 2001) microbes and animals, or attract natural predators of attacking herbivores, indirectly protecting the plant via tritrophic interactions (Mercke et al. , 2004; Arimura et al. , 2004b; Degen et al. , 2004). By releasing volatiles, a signaling plant not only reduces the number of attacking herbivores (Kessler and Baldwin, 2001) but can also warn the neighboring plants about the herbivore or pathogen attack (Shulaev et al., 1997). These warnings induce the expression of defense genes or emission of volatiles in neighboring plants (Arimura et al. , 2000; Birkett et al. , 2000; Ruther and Kleier, 2005; Farag et al. , 2005) or prime these plants to respond faster to future herbivore attack (Engelberth et al. , 2004; Kessler et al. , 2006). Volatiles emitted from roots can contribute to a belowground defense system by acting as antimicrobial or antiherbivore substances, or by attracting enemies of root- feeding herbivores (Rasmann et al. , 2005). Over the past decade there has been significant progress in plant volatile research as a result of the increasing sensitivity of analytical instrumentation and improvements in molecular and biochemical approaches, which has led to a better understanding of function, biosynthesis and regulation of plant volatiles. Here we review the functions of plant volatiles, their biosynthesis and regulation, and metabolic engineering leading to improvement of plant defense, scent and aroma of flowers and fruits. Due to the complexity of plant volatile research and its many different aspects ranging from genomics and biochemistry to ecology as well as the use of many different plant systems, this review is not intended to be entirely comprehensive. Instead, we will focus on a few major topics with examples drawn from selected systems. In nature, all organisms are under selective pressure to maxi- mize their reproductive success. Within the plant kingdom more than a quarter of a million species belong to flowering plants, most of which are animal pollinated. To attract pollinators and seed disseminators and thus to ensure reproductive and evolutionary success, many of these flowering species release diverse blends of volatile compounds from their flowers and fruits in addition to visual and tactile cues (Figure 1) (Buchmann and Nabhan, 1996; Dudareva and Pichersky, 2000). While flowers could be identical in their color or shape, there are no two floral scents that are exactly the same due to a large diversity of volatile compounds and their relative abundances and interactions within the scent bouquet (Knudsen and Tollsten, 1993; Knudsen et al. , 1993). Thus, floral scent is a signal which pollinators can use to discriminate a particular flower whose nectar and/or pollen is the reward. In addition to attracting insects to flowers and guiding them to food resources within the flower, floral volatiles are essential in allowing insects to discriminate among plant species and even among individual flowers of a single species. Floral scent bouquets may contain from one to 100 volatiles, but most species emit between 20 and 60 different compounds (Knudsen and Gershenzon, 2006). The total amount of emitted floral volatiles varies from the low picogram range to more than 30 μ g/h with the largest amounts produced by flowers of various beetle- and moth-pollinated species (Knudsen and Gershenzon, 2006). Closely related plant species that rely on different types of insects for pollination produce different odors, reflecting the olfactory sensitivities or preferences of the pollinators (Henderson, 1986; Raguso and Pichersky, 1995). By pro- viding species-specific signals, flower fragrances facilitate an insect’s ability to learn particular food sources, thereby increasing its foraging efficiency. Within a species, the level of scent emission changes in response to endogenous diurnal rhythms, flower age, pollination status and environmental conditions such as light, temperature, and moisture status (Dudareva et al. , 2004). The perceptual properties of any mixture of volatile compounds are different from the perceptual properties of their individual constituents and are a function of the number of compounds, their relative concentrations, the intensity of the scent that arises from all the compounds within a bouquet and an animal’s previous experiences with odors (Wright and Smith, 2004a, 2004b). To date, there is very little information about how insects respond to individual components found in floral scents, even though it is known that they are able to ...

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... complex floral scent mixtures. Also, it is still unclear whether insect pollinators use only a few compounds present in a scent for floral identification, or whether they use information from all the scent compounds. However, recently it has been shown that honeybees are capable of using all of the floral volatiles to discriminate subtle differences in the scent of four snapdragon cultivars emitting the same volatile compounds but at different levels (Wright et al. , 2005). Moreover, the ability of honeybees to distinguish between cultivars expands with the increasing intensity of floral scent. In contrast to bees, moths use odor cues over longer dis- tances (Dobson, 1994) and therefore odor quantity has to be higher in order to be detected. Moth-pollinated Clarkia breweri flowers emit a strong sweet floral scent rich in linalool and aromatic esters in contrast to closely related bee-pollinated Clarkia concinna flowers which emit less compounds and at lower levels (Raguso and Pichersky, 1995). Similarly, hawk moth pollinated Petunia axillaries flowers release high levels of several compounds and elicit significantly higher responses from the Manduca sexta antenna than flowers of the closely related bee-pollinated Petunia integrifolia , which emit almost exclusively benzaldehyde (Hoballah et al. , 2005). When the antenna- responses to the individual components of the floral scent were analyzed using gas chromatography coupled with electroantennogram detection (GC-EAD), all three major compounds of P. axillaries scent, benzaldehyde, benzyl alcohol, and methylbenzoate, elicit very high responses (Hoballah et al. , 2005). EAD-recordings from the nocturnal hawk moth Sphinx perelegans yielded preferentially strong responses to the same three compounds (Raguso and Light, 1998), suggesting that many moths may use the same compounds to locate nectar sources, although rank orders of sensitivity may differ. Moreover, odor emission from moth-pollinated flowers correlates with nocturnal moth activity and flower-volatile composition is adapted to the antennal perception of these pollinators (Hoballah et al. , 2005). Many floral volatiles have antimicrobial or antifungal activity (DeMoraes et al. , 2001; Friedman et al. , 2002; Hammer et al. , 2003), and could also act to protect valuable reproductive plant organs from pathogens (Figure 1). Volatiles involved in antimicrobial defense are often produced in pistils and/or nectaries, as was shown for linalool and linalool oxide in flowers of Clarkia species (Pichersky et al. , 1994; Dudareva et al. , 1996) and for sesquiterpene and monoterpene formation in Arabidopsis flowers (Chen et al. , 2003; Tholl et al. , 2005). Volatile compounds emitted from fruits determine the over- all aroma properties and taste, and thus could play a role ...