Eavesdropping on gall–plant interactions: the importance of the signaling function of induced volatiles

ABSTRACT

The galling insect manipulates the host plant tissue to its own benefit, building the gall structure where it spends during most of its life cycle. These specialist herbivore insects can induce and manipulate plant structure and metabolism throughout gall development and may affect plant volatile emission. Consequently, volatile emission from altered metabolism contribute to eavesdropping cueing. Eavesdropping can be part of adaptive strategies used by evolution for both galling insects and the entire-associated community in order to cue some interaction response. This is in contrast to some herbivores associated with delayed induced responses, altering plant metabolites during the short time while they feed. Due to the different lifestyles of the galling organism, which are associated with different plant tissues and organs (e.g leaves, flowers or fruits), a distinct diversity of organisms may eavesdrop on induced volatiles interacting with the galls. Furthermore, the eavesdropping cues may be defined according to the phenological coupling between galling organism and host plant, which results from the development of a gall structure. For instance, when plants release volatile-induced defenses after galling insects’ activity, another interactor may perceive these volatiles and change its behavior and interactions with host plants and galls. Thus, natural enemies could be attracted by different volatiles emitted by the gall tissues. Considering the duration of the life cycle of the galling organism and the gall, the temporal extent of gall-induced volatiles may include more persistent volatile cues and eavesdropping effects than the volatiles induced by non-galling herbivores. Accordingly, from chemical ecology perspective we expect that galling herbivore-induced volatiles may exhibit robust effects on neighboring-plant interactions including those ones during different plant developmental or phenological periods. Information about multitrophic interactions between insects and plants supports the additional understanding of direct and indirect effects, and allows insight into new hypotheses.

Introduction

Plants produce a blend of organic compounds, including the volatiles (VOCs), which may directly and indirectly affect other plants, herbivores, and their natural enemies, pollinators and seed dispersers.^1–3 Plant volatiles induce local, systemic, and interspecific responses, even in the absence of vascular connection, assuming self or eavesdrop cueing.⁴ Volatile cues are part of remarkable strategies of plants for growth and development, but environmental factors such as climate change and biodiversity changes may activate and modulate the activity of plant signaling at the molecular level.^5,6 Different organisms, especially plants, perceive and respond to peripheral volatiles by adjusting traits to improve reproductive performance; thus, eavesdropping or overhearing on plant volatile cues may be an adaptive strategy.^4,7 Thus, plant volatiles may be used as cues by neighbors, including other plants (competitors) herbivores, their natural enemies, as well the mutualistic partners (Figure 1), according to their traits and conditions, and thus provide information about insect-plant and/or plant–plant interactions.^9,10

Figure 1.

Hypothetical framework of plant–plant interactions by volatile organic compounds⁸ which, due to different gall life stages, could temporally permit events ranging from pollinator attraction to allopathy in order to avoid competition (black arrows). The plant volatiles are induced as defensive mechanisms in response to herbivores and consequently affect herbivore activity (dashed black arrows), directly by repelling them or indirectly by attracting herbivore enemies (e.g. parasitoids). However, assuming that herbivory induced responses on plants (wider and red arrows) are temporally extended when compared with other herbivores. According to this, in addition to the direct and indirect effects this temporal extention of responses may include effect both above and below-ground organisms and interactions related to the infested plants and their neighboring community. These gall-induced responses may affect neighboring plants by eliciting or allowing eavesdropping on olfactory cues (green arrows). Among them, elicitation from gall metabolism may enable plants, including the non-galled ones, to exhibit specific herbivore-induced responses. Particularly, it allows to hypothesize that gall cues signals to neighboring plants can be either positive – reducing the attractiveness of gallers or attracting natural enemies of galls – or negative – by indicating to the galling insects possible host plants. These signals may be used for both, pollinators, seed dispersers and parasitoids also to avoid or fing resources on plants infested by galling insects directing them to specific niches. For instance, volatile cues from induced defense may induce parasitoids to plants with galling insects presence, and other herbivores or pollinators may use thes cues to avoid galled plants with possibly lower resources available.

Herbivores modify plant metabolism according to their feeding habit (e.g. sapping or chewing) and active responsive and specific mechanisms related to herbivore traits, such as different life stage feeding and salivary compounds or herbivore-associated molecular patterns (HAMPs) exposed during herbivore damage.^11,12 Induced volatile emission in response to herbivory attack could function as direct and indirect defense by being understood as cue that induces herbivores to finish their attack or it can promote effective defense release by vegetal tissues.^9,13 Volatiles released from feeding herbivores could act as an indirect plant defense since these molecules may also attract natural enemies of herbivores and thus benefit the plant.¹⁴ However, some herbivores such as caterpillars have evolved using plant volatile cues for their own benefit, increasing the attractiveness of conspecific caterpillars to the damaged plant.^15,16,17 The blend composition of volatiles may be modified after herbivore damage, acting as herbivore-induced defenses and interfering with interactions between other organisms.^10,18 Additionally, a constant induced defense may include trade-off resolutions on plant fitness, which may reproduce or defense less than previous periods according to the cues recognizing between neighboring plants.¹⁹

Herbivory effects: plant trait modifications induced throughout herbivore development

Galls represent an interesting structure developing from the host plant tissue after induction mainly by insects and resulting in changes in host plant metabolism.^20,21 Using unknown mechanical and chemical stimuli, the galling insects induce galls with different tissues from those of the host plant. These tissues are a gain for the galling insect in relation to its free-life ancestors, giving better nutrition and defense.^22,23,24 Thus, the galling insects manipulate the host plant morphogenesis and metabolism for their own benefit, while completing their development using host plants resources.^25,26,27

The intimate relationship between galling insects and host plant organs can lead to gall formation with different and specialized tissues such as lignified and nutritive tissues.²³ The nutritive tissue can be induced by different taxa of galling organisms and is responsible for their nutrition.²⁸ Primary compounds (e.g. carbohydrates, proteins and lipids) are commonly detected in this nutritive tissue by galling insects and used in their diet.²⁹ However, secondary compounds (e.g. phenolics derivatives) are distributed in the outer cortices of galls, commonly associated with mechanical tissue.^30,31 These secondary compounds in the outer cortices of galls can provide defenses against natural enemies.^32,33 As a plant organ induced by insects,²¹ galls can accumulate large amounts of defensive compounds,³⁴ with herbivore induction being responsible for most of the defensive responses.^12,35 Within this context, the constitutive and induced plant defense strategies may be changed by herbivores, including the gall system interactions, which activate elicitors from the myriad signaling pathways related to direct and indirect plant defenses.³⁶

All herbivores alter plant metabolism during feeding activity, but some herbivores such as galling insects are more likely to induce or manipulate plant tissues during an extended period of feeding during their development.^27,37 However, galling activity depends on the chemical and mechanical reactive host plant tissues, as well as on synchronization between the life cycle of the galling organism and host plant conditions (e.g. phenology).^38,39 The galling stimuli for the manipulation and development of the gall seem to be constant over its life cycle.⁴⁰ The optimal defense theories predict less costs from inducible defenses than from constitutive ones because those are produced only when they are actually needed.⁴¹ However, the time lag after biotic or abiotic cue and the optimal adjust of plant defensive mechanisms still is considered a strong disadvantage of inducible defenses.^42,43 Thus, considering that time as an important factor in plant defense strategies,⁴⁴ such volatile emissions, we believe that galling-plant interactions may constantly influence other insect-plant interactions – including mutualist and antagonistic ones – in natural communities (Figure 1).

Histochemical and physiological assays have helped to elucidate the chemical interactions between the dietary requirements of the galls and the chemical defensive arsenal of the host plants.³¹ nonvolatile plant chemistry is known to be altered in the gall and surrounding leaf tissue.⁴⁵ Thus, the chemical changes in plant traits due to gall activity are associated with the feeding and protection strategies of galling insects in order to complete their developmental stages.^39,46 Some of these strategies are complex and induce chemical trait changes in plant tissues including nectar availability from galls, which support the protection of galling insects.⁴⁷ The development of galling insects depends on the nutritional status, and particularly on the physiology and phenology of their host plants⁴⁸ as they recognize the reactive tissue.³⁸ An adequate nutritional status, as well as synchronized galling insects’ life cycles and plant phenology, are required for gall initiation and development,⁴⁹ in addition to strategies used to explore the host plant resources leading to gall phenotype.³⁹ The gall phenotype may depend on the length of the life cycle of the galling insects. Insects with short life cycles appear to develop less structurally and metabolically complex galls than galling insects with long life cycles, regardless of inductor taxon, as shown by their structural and physiological profiles.^25,48 Thus, the longer the galling insect lives in the gall, the greater its chance of interacting with the community of animals around.

Galls are able to modify herbivore-induced responses against other herbivores due to changes in plant tissue,^37,50 but the VOC emission from gall-infested plants can vary greatly among species of galling insects on the same plant.⁵¹ In Baccharis dracunculifolia, galls are induced by the psyllid Baccharopelma dracunculifoliae to produce 3.5% more VOCs than usual;⁵² Pistacia atlantica galled by the aphid Slavum wertheimae produced approximately 2.5 times more volatiles than ungalled plants; and galls induced by the aphids Baizongia pistaciae on Pistacia palaestina exhibited independent metabolism producing and accumulating monoterpenes.³³ These examples emphasize the ability of galling insects to alter the biochemical function of the host plant, possibly protecting them against their own natural enemies.⁵³ Due to plant immobility, the volatiles induced by galling insects may be an efficient strategy to avoid galls’ natural enemies or plant’s herbivores, specifically considering the possible manipulation of VOC release.⁵⁴ Changes in volatiles induced by galls can increase or decrease the amount of VOCs emitted (Figure 1), but evidence suggests that gall formation mainly affects the emission of terpenoids and VOCs from the lipoxygenase pathway, common volatile cues involved in direct and indirect defenses.^50,51 Therefore, VOCs essentially mediate diverse ecological interactions but the understanding of VOCs induced by galling activity remains unclear, especially considering long-term effects in addition to interactions of other insects with the host plant and plant phenology. To better explore the host plant resources the galling insect must synchronize their colonization activities with the host plant phenology, in an essential step of the interaction establishment. This fine-tuned synchronization and the continuous galling stimuli can lead to univoltine life cycle along one year time,³⁹ thus extending the long term of effects VOCs induced by the galls.

The ecological role of gall-emitted volatiles as cues for surrounding insect-plant interactions

The chewers and phloem-sucking herbivores may induce not only immediate, but also delayed defense responses, and alter plant metabolites during feeding or on an extended time after feeding.^19,43,55 However, galling herbivores may induce and manipulate plant tissues throughout their developmental processes and may also induce delayed responses. Released VOCs are temporarily able to react with other molecules or organisms^5,56 and their lifetime varies (minutes to hours) depending on environmental conditions such as air pollution.⁵⁷ However, galling insects affect plant traits and maintain modifications for a longer time than other non-galling insect herbivores.

In a general way, the development of galls depends on the continuous chemical, physical and feeding stimuli of the galling organism^28,58 and how much the plant tissues are reactiveness to the galling organisms.^38,39 As an example, Bystracoccus mataybae (Eriococcidae) can induce gall in both stems and leaflets of Matayba guianensis (Sapindaceae). These galls induced by the same galling organism showed different morphological and anatomical traits, a clear indicative of constraints imposed by the different organ of the host plant.⁵⁹ Despite this, the histochemical profile detected in the leaflet gall tissues is established in the first steps of gall development, which is maintained until gall maturation by the feeding action of the galling insect.⁶⁰ Another example horn-shaped gall induced by Cecidomyiidae on leaflets of Copaifera langsdorffii (Fabaceae) has an univoltine life cycle along one-year time. This galling insect continuously stimulates the host tissue and consequently, the gall showed a complex structural and histochemical profile.²⁵ Therefore, considering galling herbivores as constant inducers during their developmental processes, new approaches comparing effects between galling and non-galling herbivores could expect the effects of galls on plant metabolism to be constantly inducing or avoiding plant defenses, in contrast to non-galling herbivores.

Complementarily, the idea that surrounding changes in the environment may affect insect-plant interactions is well accepted, especially regarding plant volatile cueing which may be disrupted due the noise from other possible environmental interactive volatiles.^61,62 Considering the herbivore-induced volatile effects on other organisms which interact with the surrounding community, galling insects are expected to affect the dynamics of their host plants (Figure 1) and a large set of interactions including other herbivores,⁶³ pollinators¹⁷ or neighboring plants.⁷ For instance, based on the morphological variations selected by plant-galling interactions, different sizes, colors, and fresh floral fragrances can be incorporated into populations affecting their community-level interactions, such as plant-pollinator-herbivore interactions.¹⁰ Changes in floral characters related to the attraction of pollinators can affect the floral visitors’ behavior and the frequency of their visits and may culminate in the pollinator exchange.⁶⁴ Changes in flower coloration have been poorly studied within the biology of pollination and, even less frequently, within the context of interaction with galling insects. However, the composition or concentration of VOCs may also be altered by the action of herbivores on vegetative or reproductive vegetative tissue.³

Gall induction and development depends not only on specific host traits,^27,48 but also on galling recognition by their natural enemies and other competing herbivores possibly through the eavesdropping.^37,65 Since different organisms recognizes the galls, several positive or negative effects may be encompassed on host plants interactions (Figure 1). Additionally, the galling insects may use a wide range of volatile cues released by plants to avoid those that pinpoint non-ideal periods for gall induction.⁶⁶ It has also been observed that some monoterpenes serve as olfactory cues for host location by the gall wasp Antistrophus rufus, helping the female to find optimal host plant species and to choose better ovipositional ones.⁶⁷ The gall wasp Antistrophus rufus induces host plant volatiles which attract male wasps and enhance their mating with partners.⁶⁸ In this system, the intraspecific competition between male gall-forming insects and females could be reduced by two mechanisms: (I) galls recognizing and locating host plants by volatile compounds also released from reactive and healthy plant tissue – including healthy plants on this scenery – and (II) galling activity suppressing the normal release of plant volatiles.^12,37 Focusing on intraspecific galling insects interactions, it is expected that changes in volatiles release – both from the induced healthy plants and those suppressed by gallers – may inactivate olfactory cues for intraspecific galling insects and consequently reduce the male competition for females (Figure 1).

Similarly, when we transpose this idea to interactions involving predators (or parasitoids), the predatory pressure is reduced when there is less volatile emission acting as olfactory cues for prey location. For instance, Solidago altissima exhibited enhanced defenses and reduced susceptibility to insect feeding damage when previously exposed to volatiles from the galling insect Eurosta solidaginis.⁶⁹ It is not clear whether plant reaction to gall induction is related to herbivore defenses or if the modifications of plant metabolism actively guide the galls (e.g. avoid defensive effects).^37,65 In mango leaves, there is an enhancement of VOCs in gall fly-susceptible cultivars compared to resistant ones, clearly relating VOC emission to gall fly susceptibility.⁷⁰ Moreover, Silphium laciniatum galled by Antistrophus rufus increases the plant volatile production employed as an olfactory cue by the parasitoid females (Euritoma lutea).⁷¹ Consequently, this gall-parasitoid interaction helps plant fitness by reducing galling insect infestation.^5,72

Salicylate accumulation by galls may act on volatile suppression, as well as on resource limitation and salivary components, which consequently make herbivores less affected by plant defense.³⁷ Thus, plant defense suppression by herbivores is widely unexplored and its particular effects may lead us to believe that galling suppressors act as ecosystem engineers. The beetle Microrhopala vittata prefers to colonize Solidago altissima plants that were galled by Rhopalomyia solidaginis, suggesting a possible advantage of aggregating on galled plants.⁵⁰ On P. atlantica, the galling aphid Slavum wertheimae, which enhances release of leaf volatile terpenes, also reduces plant palatability and consequently reduces herbivory of galled tissue.³³ Both of these studies revealed a significant impact of galls on host plant quality by suggesting through metabolic changes that insect preference and interacting behavior is plant mediated by gall facilitation effects on plant palatability.^33,50 Thus, the different developmental gall stages and plant genotypes show that the gall–plant interaction is a synergistic process.^73,74 Additionally, the phenological association between plant and galling organism life cycles represents a new frontier of chemical ecology, which determines how the VOCs released during the host plant phenological stages interact with the galling insects, natural enemies or host plant interactors such as pollinators.

Accordingly, the herbivore-induced volatiles vary due to different stimuli, and concomitantly depend on herbivore habit and damage, allowing wide possible interaction responses. From an ecological point of view, plant community may mediate effects from galling herbivores to all possible organisms they interact. Therefore, in order to highlight our perspective about galling herbivore-induced defenses may affect ecological interactions differently from non-galling ones, we emphasize that galling insects may induce volatiles during all processes of gall development – not only during feeding – and consequently affect a large set of interactions since roots on soil until flowers on plant top (Figure 1). Nevertheless, it refers to question such: how are host plants chemically restructured by galling organisms? And, how do they provide cues for neighboring organisms and their respective interactions?

Answering these questions could include understanding on plant defenses induced by galling herbivores, and its effects on nearby ecological interactions. To better understand the ecological role of volatile cues from galls is necessary (i) to compare volatile emission between galling and non-galling herbivores, (ii) to record the induced volatile emission from galled tissue during different times along gallers development, and (iii) to measure if and how the neighboring interactions are affected along time while gall develops and induces defenses on plants. Therefore, we strongly suggest that new research efforts could be focused on understanding how volatile cues mediate ecological interactions in real-world environments and on the ecological and evolutionary implications of chemical cues from phenotypes within and among plant populations. Although the questions still need to be answered, mainly regarding the suppressive mechanisms associated with trait-mediated effects, the knowledge about multitrophic interactions between insects and plants supports the understanding of direct and indirect effects and allows insight into new hypotheses.

Funding Statement

This work was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico [301246/2016-5];Conselho Nacional de Desenvolvimento Científico e Tecnológico [Universal nº 425130/2018-5]; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior[PELD/CAPES/CNPq/UFU nº 88887.137914/2017-00 and PNPD/UFVJM nº 88887.352134/2019-00].

Acknowledgments

We would like to thank Moshe Inbar from University of Haifa – Israel for criticism and considerations during manuscript preparation. GJB is grateful to PELD/CAPES/CNPq/UFU for postdoctoral fellowships (nº 88887.137914/2017-00) and CAPES/PNPD/UFVJM (nº 88887.352134/2019-00) and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for funding (Universal nº 425130/2018-5). DCO is grateful to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for a produtivity fellowship (nº 301246/2016-5). All authors are grateful for color illustration support provided by Ana Carolina Monetta.

Declaration of interest statement

The authors declare there is no any financial interest that arisen from direct application of this paper.