Plant volatiles as method of communication

Plants emit volatile compounds that can act as a communication method to insects, neighboring plants and pathogens. Plants respond to leaf and root damage by herbivores and pathogens by emitting these compounds. The volatile compounds can deter the herbivores or pathogens directly or indirectly by attracting their natural enemies to kill them. The simultaneous damage of plants by herbivores and pathogens can influence plant defense. The induced plant volatiles can also make neighboring plants ready for defense or induce defense in parts distant from the damaged area of the same plant. Belowground root herbivory can alter the defense response to aboveground leaf herbivory. In addition, most plants normally emit volatile compounds from their flowers that directly attract foraging mutualistic insects for nectar, which in turn perform the very important function of pollination for subsequent reproduction. The volatile compounds emitted from the floral and vegetative parts of plants belong to three main classes of compounds: terpenoids, phenylpropanoids/benzenoids, and C6-aldehydes (green-leaf volatiles). The volatile phytohormones methyl salicylate and methyl jasmonate serve as important signaling molecules for communication purposes, and interact with each other to optimize the plant defense response. Here we discuss and integrate the current knowledge on all types of communication between plants and insects, neighboring plants and pathogens that are mediated through plant volatiles.     

Plant age, communication, and resistance to herbivores: young sagebrush plants are better emitters and receivers

Plants progress through a series of distinct stages during development, although the role of plant ontogeny in their defenses against herbivores is poorly understood. Recent work indicates that many plants activate systemic induced resistance after herbivore attack, although the relationship between resistance and ontogeny has not been a focus of this work. In addition, for sagebrush and a few other species, individuals near neighbors that experience simulated herbivory become more resistant to subsequent attack. Volatile, airborne cues are required for both systemic induced resistance among branches and for communication among individuals. We conducted experiments in stands of sagebrush of mixed ages to determine effects of plant age on volatile signaling between branches and individuals. Young and old control plants did not differ in levels of chewing damage that they experienced. Systemic induced resistance among branches was only observed for young plants. Young plants showed strong evidence of systemic resistance only if airflow was permitted among branches; plants with only vascular connections showed no systemic resistance. We also found evidence for volatile communication between individuals. For airborne communication, young plants were more effective emitters of cues as well as more responsive receivers of volatile cues.     

Individual‐level differences in generalist caterpillar responses to a plant–plant cue

1. Plant–plant communication has been found to affect interactions between herbivores and plants in several model systems. In these systems, herbivore‐induced volatile chemical cues are emitted and perceived by other plants (receivers), which subsequently change their defensive phenotypes. Most studies have focused on how the effects of volatile cues affect plant damage, whereas herbivore performance has rarely been examined. 2. In this study, it is shown that plant–plant communication between willows reduced the growth rate, feeding rate, and conversion efficiency of some individuals but not others of a generalist caterpillar, Orgyia vetusta. 3. Using a paired, no‐choice trial design, there was substantial variation between caterpillar individuals in their response to willows that had been induced with a volatile plant–plant cue. This variation was explained by feeding parameters of the individual herbivores. Individuals behaved similarly when fed induced and non‐induced willow leaves. Specifically, growth rates of caterpillars that grew rapidly on non‐induced willow leaves were negatively affected by plant–plant cues, but growth rates of caterpillars that grew slowly on non‐induced willow leaves were not affected by the responses to volatiles from neighbouring willows. 4. Induction by volatile plant–plant cues reduced the growth rates of those individual herbivores that caused the greatest damage to willow, but had little effect on weak growers.     

Herbivore-induced plant volatiles mediate host selection by a root herbivore

In response to herbivore attack, plants mobilize chemical defenses and release distinct bouquets of volatiles. Aboveground herbivores are known to use changes in leaf volatile patterns to make foraging decisions, but it remains unclear whether belowground herbivores also use volatiles to select suitable host plants. We therefore investigated how above- and belowground infestation affects the performance of the root feeder Diabrotica virgifera virgifera, and whether the larvae of this specialized beetle are able to use volatile cues to assess from a distance whether a potential host plant is already under herbivore attack. Diabrotica virgifera larvae showed stronger growth on roots previously attacked by conspecific larvae, but performed more poorly on roots of plants whose leaves had been attacked by larvae of the moth Spodoptera littoralis. Fittingly, D. virgifera larvae were attracted to plants that were infested with conspecifics, whereas they avoided plants that were attacked by S. littoralis. We identified (E)-β-caryophyllene, which is induced by D. virgifera, and ethylene, which is suppressed by S. littoralis, as two signals used by D. virgifera larvae to locate plants that are most suitable for their development. Our study demonstrates that soil-dwelling insects can use herbivore-induced changes in root volatile emissions to identify suitable host plants.     

Plant defence responses to volatile alert signals are population‐specific

Herbivore‐induced volatiles are widespread in plants. They can serve as alert signals that enable neighbouring leaves and plants to pre‐emptively increase defences and avoid herbivory damage. However, our understanding of the factors mediating volatile organic compound (VOC) signal interpretation by receiver plants and the degree to which multiple herbivores affect VOC signals is still limited. Here we investigated whether plant responses to damage‐induced VOC signals were population specific. As a secondary goal, we tested for interference in signal production or reception when plants were subjected to multiple types of herbivore damage. We factorially crossed the population sources of paired Phaseolus lunatus plants (same versus different population sources) with a mechanical damage treatment to one member of the pair (i.e. the VOC emitter, damaged versus control), and we measured herbivore damage to the other plant (the VOC receiver) in the field. Prior to the experiment, both emitter and receiver plants were naturally colonized by aphids, enabling us to test the hypothesis that damage from sap‐feeding herbivores interferes with VOC communication by including emitter and receiver aphid abundances as covariates in our analyses. One week after mechanical leaf damage, we removed all the emitter plants from the field and conducted fortnightly surveys of leaf herbivory. We found evidence that receiver plants responded using population‐specific ‘dialects’ where only receivers from the same source population as the damaged emitters suffered less leaf damage upon exposure to the volatile signals. We also found that the abundance of aphids on both emitter and receiver plants did not alter this volatile signalling during both production and reception despite well‐documented defence crosstalk within individual plants that are simultaneously attacked by multiple herbivores. Overall, these results show that plant communication is highly sensitive to genetic relatedness between emitter and receiver plants and that communication is resilient to herbivore co‐infestation.     

Investigations into plant biochemical wound-response pathways involved in the production of aphid-induced plant volatiles

Feeding damage to plants by insect herbivores induces the production of plant volatiles, which are attractive to the herbivores natural enemies. Little is understood about the plant biochemical pathways involved in aphid-induced plant volatile production. The aphid parasitoid Diaeretiella rapae can detect and respond to aphid-induced volatiles produced by Arabidopsis thaliana. When given experience of those volatiles, it can learn those cues and can therefore be used as a novel biosensor to detect them. The pathways involved in aphid-induced volatile production were investigated by comparing the responses of D. rapae to volatiles from a number of different transgenic mutants of A. thaliana, mutated in their octadecanoid, ethylene or salicylic acid wound-response pathways and also from wild-type plants. Plants were either undamaged or infested by the peach-potato aphid, Myzus persicae. It is demonstrated that the octadecanoid pathway and specifically the COI1 gene are required for aphid-induced volatile production. The presence of salicylic acid is also involved in volatile production. Using this model system, in combination with A. thaliana plants with single point gene mutations, has potential for the precise dissection of biochemical pathways involved in the production of aphid-induced volatiles.     

Short signalling distances make plant communication a soliloquy

Plants respond to attack by herbivores or pathogens with the release of volatile organic compounds. Neighbouring plants can receive these volatiles and consecutively induce their own defence arsenal. This ‘plant communication’, however, appears counterintuitive when it benefits independent and genetically unrelated receivers, which may compete with the emitter. As a solution to this problem, a role for volatile compounds in within-plant signalling has been predicted. We used wild-type lima bean (Phaseolus lunatus) to quantify under field conditions the distances over which volatile signals move, and thereby determine whether these cues will mainly trigger resistance in other parts of the same plant or in independent plants. Independent receiver plants exhibited airborne resistance to herbivores or pathogens at maximum distances of 50 cm from a resistance-expressing emitter. In undisturbed clusters of lima bean, over 80 per cent of all leaves that were located around a single leaf at this distance were other leaves of the same plant, whereas this percentage dropped below 50 per cent at larger distances. Under natural conditions, resistance-inducing volatiles of lima bean move over distances at which most leaves that can receive the signal still belong to the same plant.     

Costs of induced volatile production in maize

Herbivore‐induced plant volatiles have been shown to serve as indirect defence signals that attract natural enemies of herbivores. Parasitoids and predators exploit these plant‐provided cues to locate their victims and several herbivores are repelled by the volatiles. Recently, benefits, in terms of plant fitness, from the action of the parasitoids were shown for a few systems. However, the cost of production of herbivore‐induced volatiles for the plant remains unknown. Here, we estimate the fitness cost of the production of induced volatiles in maize, Zea mays. Plants were treated with regurgitant of Spodoptera littoralis or with the elicitor volicitin and we measured dry weight of plant parts at specific times after treatments. After a two‐week treatment period, the dry‐weight of leaves of induced plants was lower than that of un‐induced plants, suggesting a metabolic cost for induced defence. However, maize plants seem to compensate for this loss during subsequent growth, since seed production at maturity was not different for unharmed plants and plants treated with caterpillar regurgitant. For volicitin treated plants a small but significant reduction in seed production was found. It is likely that the treatments also induced the production of other defence compounds, which will contribute to the cost. Yet, a comparison of six maize inbred lines with distinct differences in volatile emissions showed a strong correlation between the intensity of induced emissions and reduction in plant performance. An analysis of the terpenoids that accumulated in the leaves of the inbred lines revealed non‐volatilised compounds are constitutively present in maize and only the volatilised compounds are induced. Interestingly, the lines that released the largest amounts of induced volatiles also contained more of the non‐volatile terpenoids. Based on these results and results from a previous study on the benefits of attracting parasitoids, we conclude that costs of induced volatile production in plants are counterbalanced by the benefits as long as natural enemies of the herbivores are present in the environment.     

Host plant scents attract rolled-leaf beetles to Neotropical gingers in a Central American tropical rain forest

Leaf volatile chemicals are known to reduce herbivory rates by repelling or intoxicating insect herbivores and by attracting the predators and parasitoids of herbivores. However, leaf volatiles may also be used by insect herbivores as cues to locate their host plants. Leaf volatiles are suggested to be important host search cues for herbivores in structurally complex and diverse habitats, such as tropical rain forests. A group of insect herbivores, the rolled-leaf beetles (Coleoptera: Chrysomelidae: Hispinae), have maintained a highly specialized interaction with Neotropical gingers (Zingiberales) for ca. 60 million years. In this study, we explored chemical attraction to host plants under controlled laboratory conditions, using four sympatric rolled-leaf beetle species, Cephaloleia dorsalis Baly, Cephaloleia erichsonii Baly, Cephaloleia fenestrata Weise, and Cephaloleia placida Baly. For each beetle species, we investigated (i) whether it was repelled or attracted by leaf scents produced by four host and four non-host plant species, including Neotropical gingers in the families Marantaceae, Costaceae, and Zingiberaceae; and (ii) its ability to use scents to detect its host plant. We found that rolled-leaf beetles can detect and are attracted by leaf volatiles from both host and non-host gingers. Additionally, when beetles were simultaneously exposed to leaf volatiles from host and non-host plants, three rolled-leaf beetle species were significantly more attracted by volatiles from their host plants than from non-hosts. Only one of the beetle species was not able to discriminate between host and non-host scents.     

Inbreeding in horsenettle (Solanum carolinense) alters night-time volatile emissions that guide oviposition by Manduca sexta moths

Plant volatiles serve as key foraging and oviposition cues for insect herbivores as well as their natural enemies, but little is known about how genetic variation within plant populations influences volatile-mediated interactions among plants and insects. Here, we explore how inbred and outbred plants from three maternal families of the native weed horsenettle (Solanum carolinense) vary in the emission of volatile organic compounds during the dark phase of the photoperiod, and the effects of this variation on the oviposition preferences of Manduca sexta moths, whose larvae are specialist herbivores of Solanaceae. Compared with inbred plants, outbred plants consistently released more total volatiles at night and more individual compounds—including some previously reported to repel moths and attract predators. Female moths overwhelmingly chose to lay eggs on inbred (versus outbred) plants, and this preference persisted when olfactory cues were presented in the absence of visual and contact cues. These results are consistent with our previous findings that inbred plants recruit more herbivores and suffer greater herbivory under field conditions. Furthermore, they suggest that constitutive volatiles released during the dark portion of the photoperiod can convey accurate information about plant defence status (and/or other aspects of host plant quality) to foraging herbivores.     

Induced plant defense via volatile production is dependent on rhizobial symbiosis

Nitrogen-fixing rhizobia can substantially influence plant–herbivore interactions by altering plant chemical composition and food quality. However, the effects of rhizobia on plant volatiles, which serve as indirect and direct defenses against arthropod herbivores and as signals in defense-associated plant–plant and within-plant signaling, are still unstudied. We measured the release of jasmonic acid (JA)-induced volatiles of rhizobia-colonized and rhizobia-free lima bean plants (Fabaceae: Phaseolus lunatus L.) and tested effects of their respective bouquets of volatile organic compounds (VOCs) on a specialist insect herbivore (Mexican bean beetle; Coccinellidae: Epilachna varivestis Mulsant) in olfactometer choice trials. In a further experiment, we showed that VOC induction by JA reflects the plant responses to mechanical wounding and insect herbivory. Following induction with JA, rhizobia-colonized plants released significantly higher amounts of the shikimic acid-derived compounds, whereas the emission of compounds produced via the octadecanoid, mevalonate and non-mevalonate pathways was reduced. These changes affected the choice behavior of beetles as the preference of non-induced plants was much more pronounced for plants that were colonized by rhizobia. We showed that indole likely represents the causing agent for the observed repellent effects of jasmonic acid-induced VOCs of rhizobia-colonized lima bean plants. Our study demonstrates a rhizobia-triggered efficacy of induced plant defense via volatiles. Due to these findings, we interpret rhizobia as an integral part of legume defenses against herbivores.     

Integration of two herbivore‐induced plant volatiles results in synergistic effects on plant defence and resistance

Plants can use induced volatiles to detect herbivore‐ and pathogen‐attacked neighbors and prime their defenses. Several individual volatile priming cues have been identified, but whether plants are able to integrate multiple cues from stress‐related volatile blends remains poorly understood. Here, we investigated how maize plants respond to two herbivore‐induced volatile priming cues with complementary information content, the green leaf volatile (Z)‐3‐hexenyl acetate (HAC) and the aromatic volatile indole. In the absence of herbivory, HAC directly induced defence gene expression, whereas indole had no effect. Upon induction by simulated herbivory, both volatiles increased jasmonate signalling, defence gene expression, and defensive secondary metabolite production and increased plant resistance. Plant resistance to caterpillars was more strongly induced in dual volatile‐exposed plants than plants exposed to single volatiles.. Induced defence levels in dual volatile‐exposed plants were significantly higher than predicted from the added effects of the individual volatiles, with the exception of induced plant volatile production, which showed no increase upon dual‐exposure relative to single exposure. Thus, plants can integrate different volatile cues into strong and specific responses that promote herbivore defence induction and resistance. Integrating multiple volatiles may be beneficial, as volatile blends are more reliable indicators of future stress than single cues.     

Volatile Exchange between Undamaged Plants - a New Mechanism Affecting Insect Orientation in Intercropping

Changes in plant volatile emission can be induced by exposure to volatiles from neighbouring insect-attacked plants. However, plants are also exposed to volatiles from unattacked neighbours, and the consequences of this have not been explored. We investigated whether volatile exchange between undamaged plants affects volatile emission and plant-insect interaction. Consistently greater quantities of two terpenoids were found in the headspace of potato previously exposed to volatiles from undamaged onion plants identified by mass spectrometry. Using live plants and synthetic blends mimicking exposed and unexposed potato, we tested the olfactory response of winged aphids, Myzus persicae. The altered potato volatile profile deterred aphids in laboratory experiments. Further, we show that growing potato together with onion in the field reduces the abundance of winged, host-seeking aphids. Our study broadens the ecological significance of the phenomenon; volatiles carry not only information on whether or not neighbouring plants are under attack, but also information on the emitter plants themselves. In this way responding plants could obtain information on whether the neighbouring plant is a competitive threat and can accordingly adjust their growth towards it. We interpret this as a response in the process of adaptation towards neighbouring plants. Furthermore, these physiological changes in the responding plants have significant ecological impact, as behaviour of aphids was affected. Since herbivore host plants are potentially under constant exposure to these volatiles, our study has major implications for the understanding of how mechanisms within plant communities affect insects. This knowledge could be used to improve plant protection and increase scientific understanding of communication between plants and its impact on other organisms.     

Herbivore-induced Blueberry Volatiles and Intra-plant Signaling

Herbivore-induced plant volatiles (HIPVs) are commonly emitted from plants after herbivore attack^1,2. These HIPVs are mainly regulated by the defensive plant hormone jasmonic acid (JA) and its volatile derivative methyl jasmonate (MeJA)^3,4,5. Over the past 3 decades researchers have documented that HIPVs can repel or attract herbivores, attract the natural enemies of herbivores, and in some cases they can induce or prime plant defenses prior to herbivore attack. In a recent paper⁶, I reported that feeding by gypsy moth caterpillars, exogenous MeJA application, and mechanical damage induce the emissions of volatiles from blueberry plants, albeit differently. In addition, blueberry branches respond to HIPVs emitted from neighboring branches of the same plant by increasing the levels of JA and resistance to herbivores (i.e., direct plant defenses), and by priming volatile emissions (i.e., indirect plant defenses). Similar findings have been reported recently for sagebrush⁷, poplar⁸, and lima beans⁹..Here, I describe a push-pull method for collecting blueberry volatiles induced by herbivore (gypsy moth) feeding, exogenous MeJA application, and mechanical damage. The volatile collection unit consists of a 4 L volatile collection chamber, a 2-piece guillotine, an air delivery system that purifies incoming air, and a vacuum system connected to a trap filled with Super-Q adsorbent to collect volatiles^5,6,10. Volatiles collected in Super-Q traps are eluted with dichloromethane and then separated and quantified using Gas Chromatography (GC). This volatile collection method was used n my study⁶ to investigate the volatile response of undamaged branches to exposure to volatiles from herbivore-damaged branches within blueberry plants. These methods are described here. Briefly, undamaged blueberry branches are exposed to HIPVs from neighboring branches within the same plant. Using the same techniques described above, volatiles emitted from branches after exposure to HIPVs are collected and analyzed.     

Roles of the volatile terpene, 1,8-cineole,in plant–herbivore interactions: a foraging odor cue as well as a toxin?

Olfaction is an important sense for many animals, yet its role in foraging by herbivores is poorly known. Many plants contain volatile compounds, such as terpenes, that are not only volatile but can be toxic if ingested. Volatile terpenes can be used by herbivores to assess leaf quality, but there is little evidence for whether they are also used as a searching cue. We applied the giving-up density (GUD) framework to examine fine-scale foraging by two free-ranging mammalian herbivores, the brush-tail possum (Trichosurus vulpecula) and the swamp wallaby (Wallabia bicolor), using patches with food and an inedible matrix that varied in content of a volatile terpene, 1,8-cineole. We tested the effect of (1) increasing dietary cineole concentration, and (2) masking the food odor by adding cineole to the inedible matrix, thus overriding the smell released by the diet. In both species GUD was affected by dietary cineole; a high cineole concentration raised GUD, consistent with its role as a toxin. There was a significant effect of masking on GUD for wallabies but not for possums, suggesting that odor was an important foraging cue at the feeding patch only for the former. Differences in ecological niche and diet may explain this pattern. We suggest that herbivores, such as the swamp wallaby, opportunistically eavesdrop on plant volatiles, i.e., take advantage of the signal proffered for a different function. The cost of this eavesdropping for plants, however, is presumably counteracted by other ecological benefits of these volatiles, including a reduction in leaf consumption as a function of toxicity.     

Clonal growth of sagebrush (Artemisia tridentata) (Asteraceae) and its relationship to volatile communication

To increase systemic resistance to herbivory, some clonal plants transmit internal cues among clonal ramets to prevent widespread damage to genets. Sagebrush (Artemisia tridentata) (Asteraceae) is known to use volatile cues to induce resistance within and between plants (so‐called volatile communication). In the present study, we observed the extent and frequency of clonal growth in a natural sagebrush population in western North America to understand the influence of clonal growth on volatile communication. We used genetic analysis involving microsatellite markers and excavation of the root systems. In addition, we characterized the volatile profiles from the headspace of sagebrush ramets. Excavation of the root system of sagebrush plants revealed that sagebrush propagates clonally below ground and that daughter ramets grow near the mother stem. Volatiles were variable among genetically different ramets, although clonal ramets (genetically identical ramets) released similar volatiles, suggesting a genetic basis for volatile similarity. Sagebrush has been shown to be most responsive to volatiles released from artificially produced clones and suffers less herbivore damage as a result. Therefore, these results, taken into consideration together, imply that volatile communication may occur among genetically identical ramets under natural conditions, and that volatile similarity between the releaser and receiver may be recognized by the receiver and increase resistance against herbivory.     

Prolonged exposure is required for communication in sagebrush

Volatile communication allows plants to coordinate systemic induced resistance against herbivores. The mechanisms responsible and nature of the cues remain poorly understood. It is unknown how plants distinguish between reliable cues and misinformation. Previous experiments in which clipped sagebrush branches were bagged suggested that cues are emitted or remain active for up to 3 days. We conducted experiments using plastic bags to block emission of cues at various times following experimental clipping. We also collected headspace volatiles from clipped and unclipped branches for 1 h, transferred those volatiles to assay branches, and incubated the assays for either 1 or 6 h. We found that assay branches that received volatile cues for less than 1 h following clipping of neighbors failed to induce resistance. Assay branches that received volatile cues for more than 1 h experienced reduced herbivory throughout the season. Branches incubated for 6 h with volatiles that had been collected during the first hour following clipping showed induced resistance. These results indicate that sagebrush must receive cues for an extended time (>1 h) before responding; they suggest that the duration of cue reception is an important and overlooked process in communication allowing plants to avoid unreliable, ephemeral cues.     

Exposure of Unwounded Plants to Chemical Cues Associated with Herbivores Leads to Exposure-Dependent Changes in Subsequent Herbivore Attack

Although chemical predator cues often lead to changes in the anti-predator behavior of animal prey, it is not clear whether non-volatile herbivore kairomones (i.e. incidental chemical cues produced by herbivore movement or metabolism but not produced by an attack) trigger the induction of defense in plants prior to attack. I found that unwounded plants (Brassica nigra) that were regularly exposed to kairomones from snails (mucus and feces produced during movement of Helix aspersa) subsequently experienced reduced rates of attack by snails, unlike unwounded plants that received only one initial early exposure to snail kairomones. A follow-up experiment found that mucus alone did not affect snail feeding on previously harvested B. oleracea leaves, suggesting that changes in herbivory on B. nigra were due to changes in plant quality. The finding that chemicals associated with herbivores leads to changes in palatability of unwounded plants suggests that plants eavesdrop on components of non-volatile kairomones of their snail herbivores. Moreover, this work shows that the nature of plant exposure matters, supporting the conclusion that plants that have not been attacked or wounded nonetheless tailor their use of defenses based on incidental chemical information associated with herbivores and the timing with which cues of potential attack are encountered.