Informed herbivore movement and interplant communication determine the effects of induced resistance in an individual-based model

Induced resistance to herbivory and the intelligent plant

Plant induced responses to environmental stressors are increasingly studied in a behavioral ecology context. This is particularly true for plant induced responses to herbivory that mediate direct and indirect defenses, and tolerance. These seemingly adaptive alterations of plant defense phenotypes in the context of other environmental conditions have led to the discussion of such responses as intelligent behavior. Here we consider the concept of plant intelligence and some of its predictions for chemical information transfer in plant interaction with other organisms. Within this framework, the flow, perception, integration, and storage of environmental information are considered tunable dials that allow plants to respond adaptively to attacking herbivores while integrating past experiences and environmental cues that are predictive of future conditions. The predictive value of environmental information and the costs of acting on false information are important drivers of the evolution of plant responses to herbivory. We identify integrative priming of defense responses as a mechanism that allows plants to mitigate potential costs associated with acting on false information. The priming mechanisms provide short- and long-term memory that facilitates the integration of environmental cues without imposing significant costs. Finally, we discuss the ecological and evolutionary prediction of the plant intelligence hypothesis.

Volatile-mediated plant-plant communication and higher-level ecological dynamics

Volatile organic compounds (VOCs) in general and herbivory-induced plant volatiles (HIPVs) in particular are increasingly understood as major mediators of information transfer between plant tissues. Recent findings have moved the field of plant communication closer to a detailed understanding of how plants emit and perceive VOCs and seem to converge on a model that juxtaposes perception and emission mechanisms. These new mechanistic insights help to explain how plants can integrate different types of information and how environmental noise can affect the transmission of information. At the same time, ever-new functions of VOC-mediated plant-plant interactions are being revealed. Chemical information transfer between plants is now known to fundamentally affect plant organismal interactions and, additionally, population, community, and ecosystem dynamics. One of the most exciting new developments places plant-plant interactions along a behavioral continuum with an eavesdropping strategy at one end and mutually beneficial information-sharing among plants within a population at the other. Most importantly and based on recent findings as well as theoretical models, plant populations can be predicted to evolve different communication strategies depending on their interaction environment. We use recent studies from ecological model systems to illustrate this context dependency of plant communication. Moreover, we review recent key findings about the mechanisms and functions of HIPV-mediated information transfer and suggest conceptual links, such as to information theory and behavioral game theory, as valuable tools for a deeper understanding of how plant-plant communication affects ecological and evolutionary dynamics.

Bringing Fundamental Insights of Induced Resistance to Agricultural Management of Herbivore Pests

In response to herbivory, most plant species adjust their chemical and morphological phenotype to acquire induced resistance to the attacking herbivore. Induced resistance may be an optimal defence strategy that allows plants to reduce metabolic costs of resistance in the absence of herbivores, allocate resistance to the most valuable plant tissues and tailor its response to the pattern of attack by multiple herbivore species. Moreover, plasticity in resistance decreases the potential that herbivores adapt to specific plant resistance traits and need to deal with a moving target of variable plant quality. Induced resistance additionally allows plants to provide information to other community members to attract natural enemies of its herbivore attacker or inform related neighbouring plants of pending herbivore attack. Despite the clear evolutionary benefits of induced resistance in plants, crop protection strategies to herbivore pests have not exploited the full potential of induced resistance for agriculture. Here, we present evidence that induced resistance offers strong potential to enhance resistance and resilience of crops to (multi-) herbivore attack. Specifically, induced resistance promotes plant plasticity to cope with multiple herbivore species by plasticity in growth and resistance, maximizes biological control by attracting natural enemies and, enhances associational resistance of the plant stand in favour of yield. Induced resistance may be further harnessed by soil quality, microbial communities and associational resistance offered by crop mixtures. In the transition to more sustainable ecology-based cropping systems that have strongly reduced pesticide and fertilizer input, induced resistance may prove to be an invaluable trait in breeding for crop resilience.

Metabolic Integration of Spectral and Chemical Cues Mediating Plant Responses to Competitors and Herbivores

Light quality and chemicals in a plant's environment can provide crucial information about the presence and nature of antagonists, such as competitors and herbivores. Here, we evaluate the roles of three sources of information-shifts in the red:far red (R:FR) ratio of light reflected off of potentially competing neighbors, induced metabolic changes to damage by insect herbivores, and induced changes to volatile organic compounds emitted from herbivore-damaged neighboring plants-to affect metabolic responses in the tall goldenrod, Solidago altissima. We address the hypothesis that plants integrate the information available about competitors and herbivory to optimize metabolic responses to interacting stressors by exposing plants to the different types of environmental information in isolation and combination. We found strong interactions between the exposure to decreased R:FR light ratios and damage on the induction of secondary metabolites (volatile and non-volatile) in plants. Similarly, the perception of VOCs emitted from neighboring plants was altered by the simultaneous exposure to spectral cues from neighbors. These results suggest that plants integrate spectral and chemical environmental cues to change the production and perception of volatile and non-volatile compounds and highlight the role of plant context-dependent metabolic responses in mediating population and community dynamics.

Integrating plant-to-plant communication and rhizosphere microbial dynamics: ecological and evolutionary implications and a call for experimental rigor

The perception of airborne chemical signals by plants can trigger reconfigurations of their metabolism that alter their biotic interactions. While plant-to-plant chemical communication has primarily been studied in the context of eliciting defenses to herbivores and pathogens, recent work suggests that it can also affect plants’ interactions with their rhizosphere microbiomes. In this perspective, we discuss the potential for integrating the fields of plant-to-plant communication and microbial ecology to understand the chemical ecology of plant−microbiome interactions. As an introduction for microbial ecologists, we highlight mechanistic knowledge gaps in plant volatile organic compound (VOC) perception and provide recommendations for avoiding common experimental errors that have plagued the plant communication field. Lastly, we discuss potential implications of plant VOCs structuring rhizosphere microbiomes, particularly effects on plant community and evolutionary dynamics. As we continue to discover links between plant metabolism and their microbiomes—from molecular to community scales—we hope that this perspective will provide both motivation and words of caution for researchers working at the intersection of these two fields.

Risk of herbivory negatively correlates with the diversity of volatile emissions involved in plant communication

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.

Pieris brassicae eggs trigger interplant systemic acquired resistance against a foliar pathogen in Arabidopsis

Recognition of plant pathogens or herbivores activate a broad-spectrum plant defense priming in distal leaves against potential future attacks, leading to systemic acquired resistance (SAR). Additionally, attacked plants can release aerial or below-ground signals that trigger defense responses, such as SAR, in neighboring plants lacking initial exposure to pathogen or pest elicitors. However, the molecular mechanisms involved in interplant defense signal generation in sender plants and decoding in neighboring plants are not fully understood. We previously reported that Pieris brassicae eggs induce intraplant SAR against the foliar pathogen Pseudomonas syringae in Arabidopsis thaliana. Here we extend this effect to neighboring plants by discovering an egg-induced interplant SAR via mobile root-derived signal(s). The generation of an egg-induced interplant SAR signal requires pipecolic acid (Pip) pathway genes ALD1 and FMO1 but occurs independently of salicylic acid (SA) accumulation in sender plants. Furthermore, reception of the signal leads to accumulation of SA in the recipient plants. In response to insect eggs, plants may induce interplant SAR to prepare for potential pathogen invasion following feeding-induced wounding or to keep neighboring plants healthy for hatching larvae. Our results highlight a previously uncharacterized below-ground plant-to-plant signaling mechanism and reveals genetic components required for its generation.

Population-wide shifts in herbivore resistance strategies over succession

As a strategic cost-saving alternative to constitutive resistance, induction of resistance against herbivores in plants can be especially beneficial when enemies are scarce or variable in abundance. Although probably describing the two ends of a continuum, constitutive and induced resistance strategies have long been observed to trade off within species. Examining these traits among populations along a successional gradient can help explain how temporally variable environments can maintain genetic variation and how ecosystem processes are affected by shifting plant resistance trait expression over time. Here we leverage large experimental plots that represent a chronosequence of succession up to 15 yr in combination with common garden experiments to examine changes in the selective environment and genetic differences in tall goldenrod's (Solidago altissima) constitutive and induced resistance. We show that resistance against a specialist herbivore Trirhabda virgata was inducible in the plants originating from midsuccession, which coincides with the largest loads of herbivores. The flavonoid compound content of the leaves varied with successional stage of the population of origin, which is indicative of constitutive differences in secondary metabolite production. Finally, there was a clear trade-off between constitutive and induced resistance. Our study indicates that selection for resistance traits within a population can be highly variable over time and likely result in genetically determined shifts of resistance strategies over relatively short time periods via genotype sorting.

Plant Secondary Metabolite Diversity and Species Interactions

Ever since the first plant secondary metabolites (PSMs) were isolated and identified, questions about their ecological functions and diversity have been raised. Recent advances in analytical chemistry and complex data computation, as well as progress in chemical ecology from mechanistic to functional and evolutionary questions, open a new box of hypotheses. Addressing these hypotheses includes the measurement of complex traits, such as chemodiversity, in a context-dependent manner and allows for a deeper understanding of the multifunctionality and functional redundancy of PSMs. Here we review a hypothesis framework that addresses PSM diversity on multiple ecological levels (α, β, and γ chemodiversity), its variation in space and time, and the potential agents of natural selection. We use the concept of chemical information transfer as mediator of antagonistic and mutualistic interaction to interpret functional and microevolutionary studies and create a hypothesis framework for understanding chemodiversity as a factor driving ecological processes.

Predator-prey interactions in a ladybeetle-aphid system depend on spatial scale

The outcome of species interactions may manifest differently at different spatial scales; therefore, our interpretation of observed interactions will depend on the scale at which observations are made. For example, in ladybeetle-aphid systems, the results from small-scale cage experiments usually cannot be extrapolated to landscape-scale field observations. To understand how ladybeetle-aphid interactions change across spatial scales, we evaluated predator-prey interactions in an experimental system. The experimental habitat consisted of 81 potted plants and was manipulated to facilitate analysis across four spatial scales. We also simulated a spatially explicit metacommunity model parallel to the experiment. In the experiment, we found that the negative effect of ladybeetles on aphids decreased with increasing spatial scales. This pattern can be explained by ladybeetles strongly suppressing aphids at small scales, but not colonizing distant patches fast enough to suppress aphids at larger scales. In the experiment, the positive effects of aphids on ladybeetles were strongest at three-plant scale. In a model scenario where predators did not have demographic dynamics, we found, consistent with the experiment, that both the effects of ladybeetles on aphids and the effects of aphids on ladybeetles decreased with increasing spatial scales. These patterns suggest that dispersal was the primary cause of ladybeetle population dynamics in our experiment: aphids increased ladybeetle numbers at smaller scales because ladybeetles stayed in a patch longer and performed area-restricted searches after encountering aphids; these behaviors did not affect ladybeetle numbers at larger spatial scales. The parallel experimental and model results illustrate how predator-prey interactions can change across spatial scales, suggesting that our interpretation of observed predator-prey dynamics would differ if observations were made at different scales. This study demonstrates how studying ecological interactions at a range of scales can help link the results of small-scale ecological experiments to landscape-scale ecological problems.

Functional reduction in pollination through herbivore-induced pollinator limitation and its potential in mutualist communities

Plant-pollinator interactions are complex because they are affected by both interactors' phenotypes and external variables. Herbivory is one external variable that can have divergent effects on the individual and the population levels depending on specific phenotypic plastic responses of a plant to herbivory. In the wild tomato, Solanum peruvianum, herbivory limits pollinator visits, which reduces individual plant fitness due to herbivore-induced chemical defenses and signaling on pollinators (herbivore-induced pollinator limitation). We showed these herbivory-induced decreases in pollination to individual plants best match a Type II functional-response curve. We then developed a general model that shows these individual fitness reductions from herbivore-induced changes in plant metabolism can indirectly benefit overall populations and community resilience. These results introduce mechanisms of persistence in antagonized mutualistic communities that were previously found prone to extinction in theoretical models. Results also imply that emergent ecological dynamics of individual fitness reductions may be more complex than previously thought.