Defensive Chemistry Research Articles

Towards the optimization of botanical insecticides research: Aedes aegypti larvicidal natural products in French Guiana

Natural products have proven to be an immeasurable source of bioactive compounds. The exceptional biodiversity encountered in Amazonia, alongside a rich entomofauna and frequent interactions with various herbivores is the crucible of a promising chemodiversity. This prompted us to search for novel botanical insecticides in French Guiana. As this French overseas department faces severe issues linked to insects, notably the strong incidence of vector-borne infectious diseases, we decided to focus our research on products able to control the mosquito Aedes aegypti. We tested 452 extracts obtained from 85 species originating from 36 botanical families and collected in contrasted environments against an Ae. aegypti laboratory strain susceptible to all insecticides, and a natural population resistant to both pyrethroid and organophosphate insecticides collected in Cayenne for the most active of them. Eight species (Maytenus oblongata Reissek, Celastraceae; Costus erythrothyrsus Loes., Costaceae; Humiria balsamifera Aubl., Humiriaceae; Sextonia rubra (Mez) van der Werff, Lauraceae; Piper hispidum Sw., Piperaceae; Laetia procera (Poepp.) Eichl., Salicaceae; Matayba arborescens (Aubl.) Radlk., Sapindaceae; and Cupania scrobitulata Rich., Sapindaceae) led to extracts exhibiting more than 50% larval mortality after 48 h of exposition at 100 µg/mL against the natural population and were considered active. Selectivity and phytochemistry of these extracts were therefore investigated and discussed, and some active compounds highlighted. Multivariate analysis highlighted that solvents, plant tissues, plant family and location had a significant effect on mortality while light, available resources and vegetation type did not. Through this case study we highlighted that plant defensive chemistry mechanisms are crucial while searching for novel insecticidal products.

Chemical defense over decadal scales: Ontogenetic allocation trajectories and consequences for fitness in a foundation tree species

Abstract Expression of herbivore defense traits can change dramatically during the course of plant development. Little is known, however, about the degree of genetic or sexual variation in these ontogenetic defense trajectories or whether the trajectories themselves are adaptive, especially in long‐lived species. We used a 13‐year dataset of chemical defense traits, growth and survivorship from a common garden of trembling aspen (Populus tremuloides) genotypes to document long‐term defense trajectories and their relationship to tree fitness during juvenile and early mature stages. Overall, concentrations of the two principal classes of aspen defense compounds (salicinoid phenolic glycosides [SPGs] and condensed tannins [CTs]) decreased to differing degrees in foliage of juvenile trees and then remained relatively constant in maturity. Initial values, juvenile rates of change and average mature values all exhibited significant genetic variation for both SPGs and CTs. Relationships between defense trajectory parameters and metrics of tree fitness (growth and survivorship) depended on compound type and tree sex. Females with higher‐allocation SPG trajectories (high initial juvenile concentrations, slow juvenile declines, high mature concentrations) grew more slowly relative to females with lower‐allocation trajectories. In males, higher‐allocation SPG trajectories had a lesser effect on growth but were associated with reduced mortality. Juvenile CT trajectories were not correlated with tree fitness, but average CT concentration in maturity was positively related to growth in females. These results suggest that ontogenetic defense trajectories are adaptive and subject to natural selection. Genotypic variation and ontogeny shape tree defensive chemistry, both independently and interactively. These patterns of defense expression have the potential to structure trophic interactions and the genetic composition of forests in both space and time. A free Plain Language Summary can be found within the Supporting Information of this article.

Molecular Phylogeny and Tribal Classification of Flanged Bombardier Beetles (Carabidae: Paussinae)

Members of the carabid subfamily Paussinae are known for their explosive defensive chemistry and their associations with ants, which vary from some species being facultative predators of adult ants to others being obligate predators of ant brood. This association with ants has driven extreme morphological adaptations in some lineages. Approximately 750 species are currently classified into four tribes: Metriini, Ozaenini, Protopaussini and Paussini. Here we use molecular sequence data from five genes (28S ribosomal DNA; 18S ribosomal DNA; wingless; carbamoyl phosphate synthetase domain of the rudimentary gene; and arginine kinase) to reveal patterns of deep divergence and provide a new tribal level classification reflecting evolutionary history. We recognize and describe two new tribes, and the tribe Ozaenini is redefined. Among other traits, members of each tribe has a characteristic shape of the cuticular fold at the posterolateral angle of both elytra, known as the flange of Coanda.

Growth environment and seedling age affect constitutive and inducible defence in radiata pine

There is increasing interest in induced resistance for crop protection. This study investigated effects of growth environment on inducible defences in developing radiata pine (Pinus radiata) seedlings. Seedlings were grown in the presence or absence of Trichoderma in either a temperature‐controlled growth room or in a greenhouse. After 3, 5 or 7 months, seedlings were sprayed with 2.25 mm methyl jasmonate (MJ), 1 week before quantification of monoterpenes and total phenolics. The magnitude of induction of monoterpenes and total phenolics by MJ was inversely proportional to constitutive content, was greater in the greenhouse than the growth room and was greater at 3 months than at 7 months. This may indicate a cost‐saving strategy to limit defence induction once constitutive defence reaches a certain threshold. Growth room seedlings expressed greater constitutive resistance to terminal crook (Colletotrichum acutatum) and to diplodia dieback (Diplodia sapinea) than greenhouse plants, consistent with the relative differences in constitutive defence chemistry. Indeed, while MJ induced resistance to diplodia dieback in both environments, there was no difference in terminal crook incidence (4.1%) between MJ‐treated and untreated seedlings in the growth room. By the end of the study, growth room seedlings had c. 70% more β‐pinene in stems and 140% more total phenolics in needles than greenhouse plants but were 40% smaller, indicating a defence–growth trade‐off. Trichoderma did not have a significant effect on defence chemistry or on pathogen resistance. These results demonstrate the potential to manipulate defence in radiata pine by modification of the growth environment.

Linking plant genes to insect communities: Identifying the genetic bases of plant traits and community composition.

Community genetics aims to understand the effects of intraspecific genetic variation on community composition and diversity, thereby connecting community ecology with evolutionary biology. Thus far, research has shown that plant genetics can underlie variation in the composition of associated communities (e.g., insects, lichen and endophytes), and those communities can therefore be considered as extended phenotypes. This work, however, has been conducted primarily at the plant genotype level and has not identified the key underlying genes. To address this gap, we used genome-wide association mapping with a population of 445 aspen (Populus tremuloides) genets to identify the genes governing variation in plant traits (defence chemistry, bud phenology, leaf morphology, growth) and insect community composition. We found 49 significant SNP associations in 13 Populus genes that are correlated with chemical defence compounds and insect community traits. Most notably, we identified an early nodulin-like protein that was associated with insect community diversity and the abundance of interacting foundation species (ants and aphids). These findings support the concept that particular plant traits are the mechanistic link between plant genes and the composition of associated insect communities. In putting the "genes" into "genes to ecosystems ecology", this work enhances understanding of the molecular genetic mechanisms that underlie plant-insect associations and the consequences thereof for the structure of ecological communities.

Individual and interactive effects of herbivory on plant fitness: endopolyploidy as a driver of genetic variation in tolerance and resistance.

Previous studies have shown a causal link between mammalian herbivory, tolerance, and chemical defense in Arabidopsis thaliana, driven by the process of endoreduplication (replication of the genome without mitosis). Removal of the apical meristem by mammalian herbivores lowers auxin, which triggers entry into the endocycle. Increasing chromosome number through endoreduplication, and therefore gene copy number, provides a means of increasing gene expression promoting rapid regrowth rates, higher defensive chemistry and enhanced fitness. Here, we assess whether insect leaf-feeding elicits the same compensatory response as the removal of apical dominance. Insect feeding has been shown to downregulate auxin production, which should trigger endoreduplication. Results here support this contention; insect leaf-feeding by Trichoplusia ni elicited a compensatory response similar to that of mammalian herbivores-an ecotype-specific response consistent with the level of endoreduplication. The interactive effects of mammalian and insect herbivory were also assessed to determine whether interactions were additive (pairwise) or non-additive (diffuse) on tolerance (fitness). Specifically, results indicate that herbivory is either diffuse (a significant clipping × T. ni interaction) or pairwise (no significant interaction between clipping and T. ni herbivory), dependent upon plant genotype and compensatory ability. In general, herbivore-induced changes in plant quality appear to be responsible for the observed differences in herbivory and fitness compensation. We discuss the importance of evaluating endoreduplication among plants within a population to avoid masking the association between tolerance and resistance and the fitness consequences of multi-herbivore interactions.

Soil Available Water Holding Capacity Can Alter the Reproductive Performance of Mountain Pine Beetle (Coleoptera: Curculionidae) in Jack Pine (Pinales: Pinaceae) Through Phloem Nitrogen Concentration.

Mountain pine beetle (Dendroctonus ponderosae Hopkins, Coleoptera: Curculionidae, Scolytidae) has recently invaded novel jack pine (Pinus banksiana Lamb., Pinales: Pinaceae) forests in western Canada. Jack pine seems to be a suitable host for mountain pine beetle, but how growing conditions influence jack pine's quality as a host, and hence, its susceptibility for mountain pine beetle, is unknown. Specifically, how soil nutrient concentrations and available water holding capacity (AWHC) affect jack pine quality should be investigated. Host plant quality is an important determinant of mountain pine beetle host colonization and reproduction and is usually assessed by primary (nutrients) and secondary (defense chemistry) constituents of host phloem. We evaluated mountain pine beetle host acceptance and brood production by recording the percentage of female mountain pine beetle that entered the phloem and oviposited in 30 jack pine bolts from two sites that differed in soil nutrient concentrations and AWHC. We also compared the concentrations of phloem nutrients and defense monoterpenes among the selected trees and found that trees at the Low AWHC site had higher amounts of nitrogen, phosphorus, and potassium. Monoterpene concentrations did not differ among trees at the two sites. Host acceptance by and brood production of mountain pine beetle were greater in bolts from the Low AWHC site. We conclude that AWHC of the soil may influence mountain pine beetle host acceptance and brood production through altering host plant quality, particularly nitrogen in the phloem, and will potentially influence any further range expansion of the beetle in eastern North America.

Variation in growth and defence traits among plant populations at different elevations: Implications for adaptation to climate change

Abstract Alpine plants occurring at high elevation are vulnerable to ongoing climate change, yet relatively little is known about the potential for high‐elevation species to adapt to changing environmental conditions. In particular, the extent to which high‐elevation plants will be able to resist predicted increases in the intensity of biotic interactions, such as herbivory, remains unclear. Species distributed across broad elevational ranges provide an opportunity to investigate evolutionary mechanisms and traits involved in adaptation to varying abiotic and biotic environments. This study focused on the perennial alpine plant Arabis alpina and combined field surveys and climate chamber experiments to test for intraspecific genetic divergence in traits related to growth and defence against herbivores. We screened multiple populations from low, intermediate and high elevations across a broad geographic area, characterising differences in growth form, leaf structural traits, palatability for herbivores and defensive chemistry. We then quantified the proportion of variation explained by elevation and population‐level effects. Our results document within‐species genetic divergence in multiple traits relevant for adaptation to the different abiotic and biotic pressures experienced at low and high elevations. Rates of herbivore damage declined with increasing elevation in the field, but plants from high‐ and intermediate‐elevation populations were generally more palatable for specialist herbivores than those from low‐elevation populations in feeding assays. Elevational clines were also observed in several glucosinolate defence compounds, and leaf herbivory more strongly induced glucosinolates in plants from high‐elevation populations than in those from low‐elevation populations. Leaf trichome density and growth form also diverged among populations contributing to growth–defence phenotypes associated with different elevations. However, populations from similar elevations often differed significantly in both growth‐ and defence‐related traits, with trait variation often better explained by population‐level effects than by elevation alone. Synthesis. Arabis alpina exhibits patterns of genetic variation in growth and defence traits consistent with adaptation to different elevations. However, populations from similar elevations also diverge in many of these ecologically relevant traits. Together, the extent of the observed trait variation suggests that this alpine species has considerable potential to adapt to a changing biotic environment.

The plant social network: Volatiles and their release

Plants have exploited the language of small chemicals for interacting with their environment more extensively than any other types of living organisms. An amazing diversity of volatile organic compounds (VOCs) released by plants play essential roles in their reproduction, defense, and communication, and influence atmospheric chemistry and climate. Plant VOCs are also used by humans as flavors, fragrances, biofuels, insecticides and pharmaceuticals. To date, the chemistry of plant volatiles is well understood, however, little is known about the biosynthesis of this diverse group of compounds and their release from the cell to the atmosphere. Presented results will show how the integration of metabolic profiling, functional genomic approach, targeted metabolic engineering with metabolic flux analysis and modeling can provide a comprehensive understanding of the regulation of flux through VOC metabolic networks. This presentation will also cover different aspects of volatile emission: from function in protecting plant reproductive organs via natural fumigation to promoting stigma development and seed yield. This presentation will also discuss molecular mechanisms responsible for deploying VOC from plant cells into the atmosphere.Support or Funding InformationThis work was supported by Grant IOS‐16555438 from the National Science Foundation and by the USDA National Institute of Food and Agriculture Hatch project 177845.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Pre‐ and post‐ingestive defenses affect larval feeding on a lethal invasive host plant

AbstractEvolutionary traps arise when organisms use novel, low‐quality or even lethal resources based on previously reliable cues. Persistence of such maladaptive interactions depends not only on how individuals locate important resources, such as host plants, but also the mechanisms underlying poor performance. Pieris macdunnoughii (Remington) (Lepidoptera: Pieridae) lays eggs on a non‐native mustard, Thlaspi arvense (L.) (Brassicaceae), which is lethal to the larvae. We first tested whether larval feeding behavior was affected before (pre‐) ingestion or following (post‐) ingestion of leaf material, indicating activity of feeding deterrents, toxins, or both in this evolutionary trap. Neonates were less likely to start feeding and eventually fed more slowly on T. arvense than on the native host plant Cardamine cordifolia (Gray) (Brassicaceae) in both laboratory and field. Starvation was a primary cause of mortality, indicating the role of a feeding deterrent. Feeding did not differ between larvae from invaded and uninvaded population. Second, T. arvense defensive chemistry is dominated by the glucosinolate sinigrin (allyl or 2‐propenyl glucosinolate). Adding sinigrin to the leaves of T. arvense and native hosts C. cordifolia and Descurainia incana (Bernhardi ex Fischer & Meyer) (Brassicaceae) delayed the onset of feeding, caused larvae to feed more slowly, and decreased survival on the native hosts. This evolutionary trap may not be driven by a novel deterrent, but rather by a change in the concentration of a deterrent found in native hosts. Many insects have adapted to evolutionary traps posed by invasive plants, incorporating the new plant into their diets. Thlaspi arvense remains lethal to P. macdunnoughii, and pre‐ingestive deterrents such as excess sinigrin may contribute to persistent maladaptation.

Carbon dioxide‐ and temperature‐mediated changes in plant defensive compounds alter food utilization of herbivores

AbstractAlthough the impact of elevated carbon dioxide and rising temperature on plants and animals has been extensively documented recently, only limited understanding exists regarding their combined effects. The objective of this research was to address the consequences of using combinations of elevated CO2 and elevated temperature on a plant's defensive chemistry, and subsequent utilization of the plant as insect food. Our results indicated that elevated CO2 and increased temperature, for the most part, act independently on the production of defensive compounds in broccoli leaves (Brassica oleracea L. var. italica). CO2 concentrations had significant effects on the foliar water content, total phenolic compounds, polyphenol oxidase and trypsin inhibitor concentrations. The herbivore Spodoptera litura (Fabricius; Lepidoptera: Noctuidae) responded to changes in the plant secondary chemistry, with larvae consuming more plant materials that had been exposed to elevated CO2. The food utilization efficiencies of second‐instar larvae were more sensitive to CO2‐treated foliage than those of the third‐ and fourth‐instar larvae. Temperature did exert a significant effect on food utilization (ECD) by the larvae. Our study will provide important information in future predictions on plant–insect interactions as a result of climate change. The study also demonstrated that since various larval stages might respond differently to climate change, this possibility needs to be considered in future forecasting and monitoring.

Fungal adaptation to plant defences through convergent assembly of metabolic modules.

The ongoing diversification of plant defence compounds exerts dynamic selection pressures on the microorganisms that colonize plant tissues. Evolutionary processes that generate resistance towards these compounds increase microbial fitness by giving access to plant resources and increasing pathogen virulence. These processes entail sequence-based mechanisms that result in adaptive gene functions, and combinatorial mechanisms that result in novel syntheses of existing gene functions. However, the priority and interactions among these processes in adaptive resistance remain poorly understood. Using a combination of molecular genetic and computational approaches, we investigated the contributions of sequence-based and combinatorial processes to the evolution of fungal metabolic gene clusters encoding stilbene cleavage oxygenases (SCOs), which catalyse the degradation of biphenolic plant defence compounds known as stilbenes into monophenolic molecules. We present phylogenetic evidence of convergent assembly among three distinct types of SCO gene clusters containing alternate combinations of phenolic catabolism. Multiple evolutionary transitions between different cluster types suggest recurrent selection for distinct gene assemblages. By comparison, we found that the substrate specificities of heterologously expressed SCO enzymes encoded in different clusters types were all limited to stilbenes and related molecules with a 4'-OH group, and differed modestly in substrate range and activity under the experimental conditions. Together, this work suggests a primary role for genome structural rearrangement, and the importance of enzyme modularity, in promoting fungal metabolic adaptation to plant defence chemistry.

Removal of soil biota alters soil feedback effects on plant growth and defense chemistry.

Summary We examined how the removal of soil biota affects plant–soil feedback (PSF) and defense chemistry of Jacobaea vulgaris, an outbreak plant species in Europe containing the defense compounds pyrrolizidine alkaloids (PAs).Macrofauna and mesofauna, as well as fungi and bacteria, were removed size selectively from unplanted soil or soil planted with J. vulgaris exposed or not to above‐ or belowground insect herbivores. Wet‐sieved fractions, using 1000‐, 20‐, 5‐ and 0.2‐μm mesh sizes, were added to sterilized soil and new plants were grown. Sieving treatments were verified by molecular analysis of the inocula.In the feedback phase, plant biomass was lowest in soils with 1000‐ and 20‐μm inocula, and soils conditioned with plants gave more negative feedback than without plants. Remarkably, part of this negative PSF effect remained present in the 0.2‐μm inoculum where no bacteria were present. PA concentration and composition of plants with 1000‐ or 20‐μm inocula differed from those with 5‐ or 0.2‐μm inocula, but only if soils had been conditioned by undamaged plants or plants damaged by aboveground herbivores. These effects correlated with leaf hyperspectral reflectance.We conclude that size‐selective removal of soil biota altered PSFs, but that these PSFs were also influenced by herbivory during the conditioning phase.

Bacterial Associates of a Gregarious Riparian Beetle With Explosive Defensive Chemistry.

Bombardier beetles (Carabidae: Brachininae) are well known for their unique explosive defensive chemistry. These beetles are found in riparian corridors throughout the American Southwest, where they commonly form large diurnal multispecies aggregations in moist areas under rocks, in crevices, and in leaf litter. Using high throughput 16S amplicon sequencing, we provide the first microbiome survey of a bombardier beetle, Brachinus elongatulus, collected from two sites in Arizona. Two bacterial taxa were present in all individuals sampled: Enterococcus and Dysgonomonas. Enterococcus has been implicated in the production of fecal aggregation pheromone components, which have been shown to regulate aggregation in the German cockroach; it is possible that Enterococcus plays a similar role in Brachinus. Dysgonomonas was found in all the secretory cells of the defensive system and gut samples. Additional studies are needed to determine if these microbes play a role in these beetles’ unique chemical defense. Results also show that the majority of B. elongatulus individuals collected from both sites were infected with Spiroplasma. Many Spiroplasma are intracellular, vertically transmitted insect symbionts that may manipulate host reproduction (e.g., cause male-killing) or provide resistance to nematodes and/or parasitoid wasps. Defensive protection could be especially beneficial to B. elongatulus, which are frequently parasitized by horsehair worms (Nematomorpha). In sum, findings suggest several testable hypotheses on the effects bacteria may have on bombardier beetle behavior and physiology.

Temporal patterns of ungulate herbivory and phenology of aspen regeneration and defense.

Ungulate herbivory can create strong top-down effects on forest recruitment, especially after fire. Defense strategies of tree species against ungulate herbivory include escape through vertical growth and resistance through the production of defense compounds. Using afour-way fence design and camera traps we characterized the differential impacts of ungulate herbivores (deer, elk, cattle) on aspen forest recruitment and plant defense responses and how they vary depending on the timing of herbivory. Aspen height growth was greatest between June and August and ungulate use of aspen was highest in July and August. Threeyears after fire, height of aspendiffered among fence treatments with full ungulate exclusion > deer-only plots > native ungulate plots > fenceless plots: 108 ± 4cm, 94 ± 4cm, 89 ± 4cm, and 65 ± 4cm, respectively. Fenceless plots had the highest rates of removal of apical meristems by the end of 2014 and 2015 (61% and 53%, respectively). Native ungulate plots, and deer-only plots both had similar removal of apical meristems in 2014 (37% and 39%, respectively). The highest phenolic glycoside concentrations were associated with an 80% reduction in meristem removal and four-fold greater aspen height by the end of summer. Low nitrogen was associated with an 86% reduction in apical meristem removal and threefold greater aspen height. In conclusion, our study suggests that high ungulate abundance can have detrimental impacts on forest recruitment and that high aspen defense chemistry and lower leaf N deters ungulate herbivory, especially in the late summer.

Application of Direct Analysis in Real Time–High Resolution Mass Spectrometry to Investigations of Induced Plant Chemical Defense Mechanisms—Revelation of Negative Feedback Inhibition of an Alliinase

Several plants of agricultural and medicinal importance utilize defense chemistry that involves deployment of highly labile, reactive, and lachrymatory organosulfur molecules. However, this chemistry is difficult to investigate because the compounds are often short-lived and prone to degradation under the conditions required for analysis by common analytical techniques. This issue has complicated efforts to study the defense chemistry of plants that exploit the use of sulfur in their defense arsenals. This work illustrates how direct analysis in real time-high resolution mass spectrometry (DART-HRMS) can be used to track organosulfur defense compound chemistry under mild conditions. Petiveria alliacea was used as a model plant that exploits the enzyme alliinase to generate induced organosulfur compounds in response to herbivory. Tracking of the organosulfur compounds it produces and quantifying them by DART-HRMS using isotopically labeled analogues revealed a feedback inhibition loop through which the activities of the alliinase are stymied shortly after their activation. The results show that the downstream thiosulfinate products petivericin (100 μM) and pyruvate (8.4 mM) inhibit alliinase activity by 60% and 29%, respectively, after 1 h, and a mixture of the two inhibited alliinase activity by 65%. By 2 h, alliinase activity in the presence of these alliinase-derived products had ceased completely. Because thiosulfinate, pyruvate, and lachrymatory sulfine compounds are produced via the same alliinase-derived sulfenic acid intermediate, the inhibition of alliinase activity by increasing concentrations of downstream products shows how production of these defense compounds is modulated in real time in response to a tissue breach. These findings provide a framework within which heretofore unexplained phenomena observed in the defense chemistry of P. alliacea, onion, garlic, and other plants can be explained, as well as an approach by which to track labile compounds and enzymatic activity by DART-HRMS.

Clonal Saplings of Trembling Aspen Do Not Coordinate Defense Induction.

Induction of plant chemical defenses in response to insect feeding may be localized to the site of damage or expressed systemically, mediated by signal transduction throughout the plant. Such systemic induction processes have been widely investigated in plants with single stems, but rarely in clonal plants comprised of multiple ramets with vascular connections. For a clonal tree species such as trembling aspen (Populus tremuloides Michx), integration of induced defense within clones could be adaptive, as clones are spatially extensive and susceptible to outbreak herbivores. We used pairs of aspen saplings with shared roots, replicated from three genotypes, to determine whether defense-induction signals are communicated within clones. One ramet in each pair was subjected to a damage treatment (feeding by Lymantria dispar, followed by mechanical damage), and subsequent changes in leaf defensive chemistry were measured in both ramets. Responses to damage varied by defense type: condensed tannins (CTs) increased in damaged ramets but not in connected undamaged ramets, whereas salicinoid phenolic glycosides (SPGs) were not induced in any ramets. Genotypes varied in their levels of CTs, but not in their levels of SPGs, and responded similarly to damage treatment. These results suggest that, even with both vascular and volatile information available, young aspen ramets do not induce defenses based on signals or metabolites from other ramets. Thus, unlike other clonal plant species, aspen do not appear to coordinate defense induction within clones. Lack of coordinated early induction in aspen may be related to the function of CTs in tolerance, rather than resistance.

Species-specific plant\u2013soil feedbacks alter herbivore-induced gene expression and defense chemistry in Plantago lanceolata

Plants actively interact with antagonists and beneficial organisms occurring in the above- and belowground domains of terrestrial ecosystems. In the past decade, studies have focused on the role of plant–soil feedbacks (PSF) in a broad range of ecological processes. However, PSF and its legacy effects on plant defense traits, such as induction of defense-related genes and production of defensive secondary metabolites, have not received much attention. Here, we study soil legacy effects created by twelve common grassland plant species on the induction of four defense-related genes, involved in jasmonic acid signaling, related to chewing herbivore defense (LOX2, PPO7), and in salicylic acid signaling, related to pathogen defense (PR1 and PR2) in Plantago lanceolata in response to aboveground herbivory by Mamestra brassicae. We also assessed soil legacy and herbivory effects on the production of terpenoid defense compounds (the iridoid glycosides aucubin and catalpol) in P. lanceolata. Our results show that both soil legacy and herbivory influence phenotypes of P. lanceolata in terms of induction of Pl PPO7 and Pl LOX2, whereas the expression of Pl PR1 and Pl PR2-1 is not affected by soil legacies, nor by herbivory. We also find species-specific soil legacy effects on the production of aucubin. Moreover, P. lanceolata accumulates more catalpol when they are grown in soils conditioned by grass species. Our study highlights that PSF can influence aboveground plant–insect interactions through the impacts on plant defense traits and suggests that aboveground plant defense responses can be determined, at least partly, by plant-specific legacy effects induced by belowground organisms.

Ectomycorrhizal fungal species differentially affect the induced defensive chemistry of lodgepole pine.

Plants interact simultaneously with multiple organisms, including ectomycorrhizal (EM) fungal symbionts which benefit plants by facilitating resource acquisition. Yet, their role in induced plant defenses that rely on the allocation of plant resources has received little attention. We investigated whether EM fungi can affect the induction of defense-related monoterpenes in greenhouse-grown lodgepole pine (Pinus contorta var. latifolia) seedlings, and whether such effects differed between EM fungal species occurring alone or in combination. Fungal interactions on growth media were also assessed to complement the greenhouse study. Our study revealed that the production of certain monoterpenes is influenced by the fungal species colonizing pine roots. Furthermore, pine seedlings did not necessarily benefit from having associations with multiple EM fungi, as we found contrasting effects of single vs. multiple species of fungi on induced monoterpene responses. Finally, monoterpene responses were altered when early-colonizing species inhibited the colonization or development of later-arriving species. We conclude that the presence of EM fungi can impact host susceptibility to insect and pathogen attack, suggesting that seedlings establishing in areas lacking fungi that promote the induction of tree defense chemicals may suffer from increased susceptibility to future pest damage.

Influence of drought on plant performance through changes in belowground tritrophic interactions.

Climate change is predicted to increase the risk of drought in many temperate agroecosystems. While the impact of drought on aboveground plant‐herbivore‐natural enemy interactions has been studied, little is known about its effects on belowground tritrophic interactions and root defense chemistry. We investigated the effects of low soil moisture on the interaction between maize, the western corn rootworm (WCR, Diabrotica virgifera), and soil‐borne natural enemies of WCR. In a manipulative field experiment, reduced soil moisture and WCR attack reduced plant performance and increased benzoxazinoid levels. The negative effects of WCR on cob dry weight and silk emergence were strongest at low moisture levels. Inoculation with entomopathogenic nematodes (EPNs, Heterorhabditis bacteriophora) was ineffective in controlling WCR, and the EPNs died rapidly in the warm and dry soil. However, ants of the species Solenopsis molesta invaded the experiment, were more abundant in WCR‐infested pots and predated WCR independently of soil moisture. Ant presence increased root and shoot biomass and was associated with attenuated moisture‐dependent effects of WCR on maize cob weight. Our study suggests that apart from directly reducing plant performance, drought can also increase the negative effects of root herbivores such as WCR. It furthermore identifies S. molesta as a natural enemy of WCR that can protect maize plants from the negative impact of herbivory under drought stress. Robust herbivore natural enemies may play an important role in buffering the impact of climate change on plant‐herbivore interactions.