Search

Herbivory is a major threat to virtually all plants, so adaptations to avoid herbivory will generally be selected. One potential adaptation is the ability to 'listen in' on the volatile cues emitted by plants that are experiencing herbivory and to then respond by ramping up defences. The nature of these volatile cues is poorly understood. Sagebrush ( Artemisia tridentata ) plants that were exposed to cues of experimentally damaged neighbours experienced less herbivory; this induction was most effective if emitter and receiver plants had similar volatile emission profiles, termed chemotypes. Previously, we observed that sagebrush populations that were in locations with high herbivory exhibited little diversity of volatiles compared to populations with low herbivory. Several hypotheses could produce this correlation. High risk of herbivory could have selected for individuals that converged on a common 'alarm cue' that all individuals would respond to. In this case, individuals of locally rare chemotypes that were less able to eavesdrop would experience more damage than common chemotypes when herbivores were abundant. Alternatively, low chemotypic diversity could allow higher levels of damage to plants. In this case, rare chemotypes would experience less damage than common chemotypes. We examined the chemotypes of sagebrush individuals from multiple sites and found that rare chemotypes experienced more damage than common chemotypes when herbivores were abundant. This pattern was seen among sites and among years with different densities of herbivores. This result is consistent with the hypothesis that herbivory selects for individuals that are effective communicators and shapes the communication system.

Plants produce a diversity of secondary metabolites including volatile organic compounds. Some species show discrete variation in these volatile compounds such that individuals within a population can be grouped into distinct chemotypes. A few studies reported that volatile-mediated induced resistance is more effective between plants belonging to the same chemotype and that chemotypes are heritable. The authors concluded that the ability of plants to differentially respond to cues from related individuals that share the same chemotype is a form of kin recognition. These studies assumed plants were actively responding but did not test the mechanism of resistance. A similar result was possible through the passive adsorption and reemission of repellent or toxic VOCs by plants exposed to damage-induced plant volatiles (DIPVs). Here we conducted exposure experiments with five chemotypes of sagebrush in growth chambers; undamaged receiver plants were exposed to either filtered air or DIPVs from mechanically wounded branches. Receiver plants exposed to DIPVs experienced less herbivore damage, which was correlated with increased expression of genes involved in plant defense as well as increased emission of repellent VOCs. Plants belonging to two of the five chemotypes exhibited stronger resistance when exposed to DIPVs from plants of the same chemotypes compared to when DIPVs were from plants of a different chemotype. Moreover, some plants passively absorbed DIPVs and reemitted them, potentially conferring associational resistance. These findings support previous work demonstrating that sagebrush plants actively responded to alarm cues and that the strength of their response was dependent on the chemotypes of the plants involved. This study provides further support for kin recognition in plants but also identified volatile-mediated associational resistance as a passively acquired additional defense mechanism in sagebrush., (© 2022. The Author(s).)

Animal biologists have recently focused on individual variation in behavioral traits and have found that individuals of many species have personalities. These are defined as consistent intraspecific differences in behaviors that are repeatable across different situations and stable over time. When animals sense danger, some individuals will alert neighbors with alarm calls and both calling and responding vary consistently among individuals. Plants, including sagebrush, emit volatile cues when they are attacked by herbivores and neighbors perceive these cues and reduce their own damage. We experimentally transferred volatiles between pairs of sagebrush plants to evaluate whether individuals showed consistent variation in their effectiveness as emitters and as receivers of cues, measured in terms of reduced herbivore damage. We found that 64% of the variance in chewing damage to branches over the growing season was attributable to the identity of the individual receiving the cues. This variation could have been caused by inherent differences in the plants as well as by differences in the environments where they grew and their histories. We found that 5% of the variance in chewing damage was attributable to the identity of the emitter that provided the cue. This fraction of variation was statistically significant and could not be attributed to the environmental conditions of the receiver. Effective receivers were also relatively effective emitters, indicating consistency across different situations. Pairs of receivers and emitters that were effective communicators in 2018 were again relatively effective in 2019, indicating consistency over time. These results suggest that plants have repeatable individual personalities with respect to alarm calls., (© 2022. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.

Plants that are damaged by herbivores emit complex blends of volatile compounds that often cause neighboring branches to induce resistance. Experimentally clipped sagebrush foliage emits volatiles that neighboring individuals recognize and respond to. These volatiles vary among individuals within a population. Two distinct types are most common with either thujone or camphor as the predominate compound, along with other less common types. Individuals respond more effectively to cues from the same type, suggesting that some of the informative message is contained in the compounds that differentiate the types. In this study, we characterized the chemical profiles of the two common types, and we examined differences in their microhabitats, morphologies, and incidence of attack by herbivores and pathogens. Analysis by gas chromatography coupled with mass spectrometry revealed that the camphor type had higher emissions of camphor, camphene, and tricyclene, while the thujone type emitted more α-thujone, β-thujone, (Z)-salvene, (E)-salvene, carvacrol, and various derivatives of sabinene. We were unable to detect any consistent morphological or microhabitat differences associated with the common types. However, plants of the thujone type had consistently higher rates of damage by chewing herbivores. One galling midge species was more common on thujone plants, while a second midge species was more likely to gall plants of the camphor type. The diversity of preferences of attackers may help to maintain the variation in volatile profiles. These chemical compounds that differentiate the types are likely to be informative cues and deserve further attention.

Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.

Herbivory is a major threat to virtually all plants, so adaptations to avoid herbivory will generally be selected. One potential adaptation is the ability to 'listen in' on the volatile cues emitted by plants that are experiencing herbivory and to then respond by ramping up defences. The nature of these volatile cues is poorly understood. Sagebrush (Artemisia tridentata) plants that were exposed to cues of experimentally damaged neighbours experienced less herbivory; this induction was most effective if emitter and receiver plants had similar volatile emission profiles, termed chemotypes. Previously, we observed that sagebrush populations that were in locations with high herbivory exhibited little diversity of volatiles compared to populations with low herbivory. Several hypotheses could produce this correlation. High risk of herbivory could have selected for individuals that converged on a common 'alarm cue' that all individuals would respond to. In this case, individuals of locally rare chemotypes that were less able to eavesdrop would experience more damage than common chemotypes when herbivores were abundant. Alternatively, low chemotypic diversity could allow higher levels of damage to plants. In this case, rare chemotypes would experience less damage than common chemotypes. We examined the chemotypes of sagebrush individuals from multiple sites and found that rare chemotypes experienced more damage than common chemotypes when herbivores were abundant. This pattern was seen among sites and among years with different densities of herbivores. This result is consistent with the hypothesis that herbivory selects for individuals that are effective communicators and shapes the communication system. [ABSTRACT FROM AUTHOR]