Direct and indirect chemical defences against insects in a multitrophic framework

Plant metabolic responses to soil herbicide residues differ under herbivory in two woodland strawberry genotypes

The use of glyphosate-based herbicides (GBHs) to control weeds has increased exponentially in recent decades, and their residues and degradation products have been found in soils across the globe. GBH residues in soil have been shown to affect plant physiology and specialised metabolite biosynthesis, which, in turn, may impact plant resistance to biotic stressors. In a greenhouse study, we investigated the interactive effects between soil GBH residues and herbivory on the performance, phytohormone concentrations, phenolic compound concentrations and volatile organic compound (VOC) emissions of two woodland strawberry (Fragaria vesca) genotypes, which were classified as herbivore resistant and herbivore susceptible. Plants were subjected to herbivory by strawberry leaf beetle (Galerucella tenella) larvae, and to GBH residues by growing in soil collected from a field site with GBH treatments twice a year over the past eight years. Soil GBH residues reduced the belowground biomass of the susceptible genotype and the aboveground biomass of both woodland strawberry genotypes. Herbivory increased the belowground biomass of the resistant genotype and the root-shoot ratio of both genotypes. At the metabolite level, herbivory induced the emission of several VOCs. Jasmonic acid, abscisic acid and auxin concentrations were induced by herbivory, in contrast to salicylic acid, which was only induced by herbivory in combination with soil GBH residues in the resistant genotype. The concentrations of phenolic compounds were higher in the resistant genotype compared to the susceptible genotype and were induced by soil GBH residues in the resistant genotype. Our results indicate that soil GBH residues can differentially affect plant performance, phytohormone concentrations and phenolic compound concentrations under herbivore attack, in a genotype-dependent manner. Soil GBH altered plant responses to herbivory, which may impact plant resistance traits and species interactions. With ongoing agrochemical pollution, we need to consider plant cultivars with better resistance to polluted soils while maintaining plant resilience under challenging environmental conditions.

Comparative metabolomics reveals how the severity of predation by the invasive insect Cydalima perspectalis modulates the metabolism re-orchestration of native Buxus sempervirens

The recent biological invasion of box tree moth Cydalima perspectalis on Buxus trees has a major impact on European boxwood stands through severe defoliation. This can hinder further regrowth and threaten survival of populations. In a mesocosm approach and controlled larval density over a 2-month period, responses of B. sempervirens essential and specialized metabolites were characterized using metabolomics, combining 1H-NMR and LC-MS/MS approaches. This is the first metabolome depiction of major Buxus responses to boxwood moth invasion. Under severe predation, remaining green leaves accumulate free amino acids (with the noticeable exception of proline). The leaf trans-4-hydroxystachydrine and stachydrine reached 10-13% and 2-3% (DW), while root content was lower but also modulated by predation level. Larval predation promoted triterpenoid and (steroidal) alkaloid synthesis and diversification, while flavonoids did not seem to have a relevant role in Buxus resistance. Our results reveal the concomitant responses of central and specialized metabolism, in relation to severity of predation. They also confirm the potential of metabolic profiling using 1H-NMR and LC-MS to detect re-orchestration of metabolism of native boxwood after severe herbivorous predation by the invasive box-tree moth, and thus their relevance for plant-insect relationships and ecometabolomics.

Effect of the mycotoxins enniatin B and deoxynivalenol on the wheat aphid Sitobion avenae and on the predatory lacewing Chrysoperla carnea

BACKGROUND

Fusarium species are responsible for Fusarium head blight (FHB) in wheat, resulting in yield losses and mycotoxin contamination. Deoxynivalenol (DON) and enniatins (ENNs) are common mycotoxins produced by Fusarium, affecting plant, animal and human health. Although DON's effects have been widely studied, limited research has explored the impact of ENNs on insects. This study examines the influence of DON and enniatin B (ENB), both singularly and in combination, on the wheat aphid Sitobion avenae and one of its predators, the lacewing Chrysoperla carnea.

RESULTS

When exposed to DON (100 mg L-1) or DON + ENB (100 mg L-1), S. avenae exhibited significantly increased mortality compared to the negative control. ENB (100 mg L-1) had no significant effect on aphid mortality. DON-treated aphids showed increasing mortality from 48 to 96 h. A dose-response relationship with DON revealed significant cumulative mortality starting at 25 mg L-1. By contrast, C. carnea larvae exposed to mycotoxins via cuticular application did not show significant differences in mortality when mycotoxins were dissolved in water but exhibited increased mortality with acetone-solubilized DON + ENB (100 mg L-1). Feeding C. carnea with aphids exposed to mycotoxins (indirect exposure) did not impact their survival or predatory activity. Additionally, the impact of mycotoxins on C. carnea was observed only with acetone-solubilized DON + ENB.

CONCLUSIONS

These findings shed light on the complex interactions involving mycotoxins, aphids and their predators, offering valuable insights for integrated pest management strategies. Further research should explore broader ecological consequences of mycotoxin contamination in agroecosystems. © 2024 Society of Chemical Industry.

Integrated Omics Analysis Reveals Key Pathways in Cotton Defense against Mirid Bug (Adelphocoris suturalis Jakovlev) Feeding

The recent dominance of Adelphocoris suturalis Jakovlev as the primary cotton field pest in Bt-cotton-cultivated areas has generated significant interest in cotton pest control research. This study addresses the limited understanding of cotton defense mechanisms triggered by A. suturalis feeding. Utilizing LC-QTOF-MS, we analyzed cotton metabolomic changes induced by A. suturalis, and identified 496 differential positive ions (374 upregulated, 122 downregulated) across 11 categories, such as terpenoids, alkaloids, phenylpropanoids, flavonoids, isoflavones, etc. Subsequent iTRAQ-LC-MS/MS analysis of the cotton proteome revealed 1569 differential proteins enriched in 35 metabolic pathways. Integrated metabolome and proteome analysis highlighted significant upregulation of 17 (89%) proteases in the α-linolenic acid (ALA) metabolism pathway, concomitant with a significant increase in 14 (88%) associated metabolites. Conversely, 19 (73%) proteases in the fructose and mannose biosynthesis pathway were downregulated, with 7 (27%) upregulated proteases corresponding to the downregulation of 8 pathway-associated metabolites. Expression analysis of key regulators in the ALA pathway, including allene oxidase synthase (AOS), phospholipase A (PLA), allene oxidative cyclase (AOC), and 12-oxophytodienoate reductase3 (OPR3), demonstrated significant responses to A. suturalis feeding. Finally, this study pioneers the exploration of molecular mechanisms in the plant-insect relationship, thereby offering insights into potential novel control strategies against this cotton pest.

Gut bacteria mediated adaptation of diamondback moth, Plutella xylostella, to secondary metabolites of host plants

IMPORTANCE

In this study, we identify an important role of gut bacteria in mediating the adaptation of diamondback moth (DBM) to plant secondary metabolites. We demonstrate that kaempferol's presence in radish seedlings greatly reduces the fitness of DBM with depleted gut biota. Reinstatement of gut biota, particularly Enterobacter sp. EbPXG5, improved insect performance by degrading kaempferol. This bacterium was common in the larval gut of DBM, lining the epithelium as a protective film. Our work highlights the role of symbiotic bacteria in insect herbivore adaptation to plant defenses and provides a practical and mechanistic framework for developing a more comprehensive understanding of insect-gut microbe-host plant co-evolution.

Flavonoid production in tomato mediates both direct and indirect plant defences against whiteflies in tritrophic interactions

BACKGROUND

The role of plant flavonoids in direct defences against chewing and sap-sucking herbivorous insects has been extensively characterized. However, little is known about flavonoid-mediated tritrophic interactions between plants, herbivorous insects and natural enemies. In this study, we investigated how flavonoids modulate plant-insect interactions in a tritrophic system involving near-isogenic lines (NILs) of cultivated tomato (Solanum lycopersicum) with high (line NIL-purple hypocotyl [PH]) and low (line NIL-green hypocotyl [GH]) flavonoid levels, with a generalist herbivore whitefly (Bemisia tabaci) and its predatory bug (Orius sauteri).

RESULTS

By contrasting levels of tomato flavonoids (direct defence) while manipulating the presence of predators (indirect defence), we found that high production of flavonoids in tomato was associated with a higher inducibility of direct defences and a stronger plant resistance to whitefly infestation and stimulated the emissions of induced volatile organic compounds, thereby increasing the attractiveness of B. tabaci-infested plants to the predator O. sauteri. Furthermore, suppression of B. tabaci population growth and enhancement of plant growth were mediated directly by the high production of flavonoids and indirectly by the attraction of O. sauteri, and the combined effects were larger than each effect individually.

CONCLUSION

Our results show that high flavonoid production in tomato enhances herbivore-induced direct and indirect defences to better defend against herbivores in tritrophic interactions. Thus, the development of transgenic plants may present an opportunity to utilize the beneficial role of flavonoids in integrated pest management, while simultaneously maintaining or improving resistance against other pests and pathogens. © 2023 Society of Chemical Industry.

Defense Strategies of Rice in Response to the Attack of the Herbivorous Insect, Chilo suppressalis

Chilo suppressalis is a notorious pest that attacks rice, feeding throughout the entire growth period of rice and posing a serious threat to rice production worldwide. Due to the boring behavior and overlapping generations of C. suppressalis, the pest is difficult to control. Moreover, no rice variety with high resistance to the striped stem borer (SSB) has been found in the available rice germplasm, which also poses a challenge to controlling the SSB. At present, chemical control is widely used in agricultural production to manage the problem, but its effect is limited and it also pollutes the environment. Therefore, developing genetic resistance is the only way to avoid the use of chemical insecticides. This article primarily focuses on the research status of the induced defense of rice against the SSB from the perspective of immunity, in which plant hormones (such as jasmonic acid and ethylene) and mitogen-activated protein kinases (MAPKs) play an important role in the immune response of rice to the SSB. The article also reviews progress in using transgenic technology to study the relationship between rice and the SSB as well as exploring the resistance genes. Lastly, the article discusses prospects for future research on rice's resistance to the SSB.

Comparing the Above and Below-Ground Chemical Defences of Three Rumex Species Between Their Native and Introduced Provenances

Compared to their native range, non-native plants often experience reduced levels of herbivory in the introduced range. This may result in reduced pressure to produce chemical defences that act against herbivores. We measured the most abundant secondary metabolites found in Rumex spp., namely oxalates, phenols and tannins. To test this hypothesis, we compared native (UK) and introduced (NZ) provenances of three different Rumex species (R. obtusifolius, R. crispus and R. conglomeratus, Polygonaceae) to assess whether any significant differences existed in their levels of chemical defences in either leaves and roots. All three species have previously been shown to support a lower diversity of insect herbivores and experience less herbivory in the introduced range. We further examined leaf herbivory on plants from both provenances when grown together in a common garden experiment in New Zealand to test whether any differences in damage might be consistent with variation in the quantity of chemical defences. We found that two Rumex species (R. obtusifolius and R. crispus) showed no evidence for a reduction in chemical defences, while a third (R. conglomeratus) showed only limited evidence. The common garden experiment revealed that the leaves analysed had low levels of herbivory (~ 0.5%) with no differences in damage between provenances for any of the three study species. Roots tended to have a higher concentration of tannins than shoots, but again showed no difference between the provenances. As such, the findings of this study provide no evidence for lower plant investments in chemical defences, suggesting that other factors explain the success of Rumex spp. in New Zealand.

Leaf volatile and nonvolatile metabolites show different levels of specificity in response to herbivory

Plants produce diverse chemical defenses with contrasting effects on different insect herbivores. Deploying herbivore-specific responses can help plants increase their defensive efficiency. Here, we explore how variation in induced plant responses correlates with herbivore species, order, feeding guild, and level of specialization. In a greenhouse experiment, we exposed 149 plants of Salix fragilis (Linnaeus, 1753) to 22 herbivore species naturally associated with this host. The insects belonged to four orders (Coleoptera, Lepidoptera, Hemiptera, and Hymenoptera), three feeding guilds (external leaf-chewers, leaf-tying chewers, and sap-sucking), and included both dietary specialists and generalists. Following herbivory, we quantified induced changes in volatiles and nonvolatile leaf metabolites. We performed multivariate analyses to assess the correlation between herbivore order, feeding guild, dietary specialization, chewing damage by herbivores, and induced responses. The volatile composition was best explained by chewing damage and insect order, with Coleoptera and Lepidoptera eliciting significantly different responses. Furthermore, we recorded significant differences in elicited volatiles among some species within the two orders. Variation in nonvolatile leaf metabolites was mainly explained by the presence of insects, as plants exposed to herbivores showed significantly different metabolites from controls. Herbivore order also played a role to some extent, with beetles eliciting different responses than other herbivores. The induction of volatile and nonvolatile leaf metabolites shows different levels of specificity. The specificity in volatiles could potentially serve as an important cue to specialized predators or parasitoids, increasing the efficacy of volatiles as indirect defenses. By contrast, the induction of nonvolatile leaf metabolites was largely unaffected by herbivore identity. Most nonvolatile metabolites were downregulated, possibly indicating that plants redirected their resources from leaves in response to herbivory. Our results demonstrate how diverse responses to herbivores can contribute to the diversity of plant defensive strategies.

Exploring the metabolic basis of growth/defense trade-offs in complex environments with Nicotiana attenuata plants cosilenced in NaMYC2a/b expression

In response to challenges from herbivores and competitors, plants use fitness-limiting resources to produce (auto)toxic defenses. Jasmonate signaling, mediated by MYC2 transcription factors (TF), is thought to reconfigure metabolism to minimize these formal costs of defense and optimize fitness in complex environments. To study the context-dependence of this metabolic reconfiguration, we cosilenced NaMYC2a/b by RNAi in Nicotiana attenuata and phenotyped plants in the field and increasingly realistic glasshouse setups with competitors and mobile herbivores. NaMYC2a/b had normal phytohormonal responses, and higher growth and fitness in herbivore-reduced environments, but were devastated in high herbivore-load environments in the field due to diminished accumulations of specialized metabolites. In setups with competitors and mobile herbivores, irMYC2a/b plants had lower fitness than empty vector (EV) in single-genotype setups but increased fitness in mixed-genotype setups. Correlational analyses of metabolic, resistance, and growth traits revealed the expected defense/growth associations for most sectors of primary and specialized metabolism. Notable exceptions were some HGL-DTGs and phenolamides that differed between single-genotype and mixed-genotype setups, consistent with expectations of a blurred functional trichotomy of metabolites. MYC2 TFs mediate the reconfiguration of primary and specialized metabolic sectors to allow plants to optimize their fitness in complex environments.

Inactivation of RPX1 in Arabidopsis confers resistance to Plutella xylostella through the accumulation of the homoterpene DMNT

The lepidopteran crop pest Plutella xylostella causes severe constraints on Brassica cultivation. Here, we report a novel role for RPX1 (resistance to P. xylostella) in resistance to this pest in Arabidopsis thaliana. The rpx1-1 mutant repels P. xylostella larvae, and feeding on the rpx1-1 mutant severely damages the peritrophic matrix structure in the midgut of the larvae, thereby negatively affecting larval growth and pupation. This resistance results from the accumulation of defence compounds, including the homoterpene (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), due to the upregulation of PENTACYCLIC TRITERPENE SYNTHASE 1 (PEN1), which encodes a key DMNT biosynthetic enzyme. P. xylostella infestation and wounding induce RPX1 protein degradation, which may confer a rapid response to insect infestation. RPX1 inactivation and PEN1 overexpression are not associated with negative trade-offs for plant growth but have much higher seed production than the wild-type in the presence of P. xylostella infestation. This study offers a new strategy for plant molecular breeding against P. xylostella.

Sugarcane multitrophic interactions: Integrating belowground and aboveground organisms

Sugarcane is a crop of major importance used mainly for sugar and biofuel production, and many additional applications of its byproducts are being developed. Sugarcane cultivation is plagued by many insect pests and pathogens that reduce sugarcane yields overall. Recently emerging studies have shown complex multitrophic interactions in cultivated areas, such as the induction of sugarcane defense-related proteins by insect herbivory that function against fungal pathogens that commonly appear after mechanical damage. Fungi and viruses infecting sugarcane also modulate insect behavior, for example, by causing changes in volatile compounds responsible for insect attraction or repelling natural vector enemies via a mechanism that increases pathogen dissemination from infected plants to healthy ones. Interestingly, the fungus Fusarium verticillioides is capable of being vertically transmitted to insect offspring, ensuring its persistence in the field. Understanding multitrophic complexes is important to develop better strategies for controlling pathosystems affecting sugarcane and other important crops and highlights the importance of not only studying binary interactions but also adding as many variables as possible to effectively translate laboratory research to real-life conditions.

Exogenous Calcium Suppresses the Oviposition Choices of Frankliniella occidentalis (Thysanoptera: Thripidae) and Promotes the Attraction of Orius similis (Hemiptera: Anthocoridae) by Altering Volatile Blend Emissions in Kidney Bean Plants

Frankliniella occidentalis is a destructive pest of horticultural plants, while Orius similis is a natural enemy of thrips. It has been demonstrated that exogenous calcium could induce plant defenses against herbivore attack. We examined whether CaCl₂ supplementation altered the volatile emissions of kidney bean plants, which influence the oviposition preference of F. occidentalis. We also assessed the influence of volatile cues on O. similis. Using Y-tube olfactometer tests, we found that exogenous CaCl₂ treatment inhibited the selectivity of F. occidentalis but attracted O. similis. In addition, CaCl₂ treatment reduced the oviposition preference of F. occidentalis. Gas chromatography-mass spectrometry analyses revealed that CaCl₂ treatment altered the number and relative abundance of the volatile compounds in kidney bean plants and that (E)-2-hexen-1-ol, 1-octen-3-ol, β-lonone, and (E,E)-2,4-hexadienal might be potential olfactory cues. Furthermore, the results of the six-arm olfactometer test indicated that 1-octen-3-ol (10^-2 μL/μL), β-lonone (10^-2 μL/μL), and (E,E)-2,4-hexadienal (10^-3 μL/μL) repelled F. occidentalis but attracted O. similis. Overall, our results suggested that exogenous CaCl₂ treatment induced defense responses in kidney bean plants, suggesting that CaCl₂ supplementation may be a promising strategy to enhance the biological control of F. occidentalis.

Helicoverpa armigera herbivory negatively impacts Aphis gossypii populations via inducible metabolic changes

BACKGROUND

Helicoverpa armigera and Aphis gossypii are two important insect species that feed on cotton plants. These insects have distinct abilities to induce plant resistance and tolerate plant toxins, which results in interspecific competition imbalance that may be fatal to the low-tolerance A. gossypii and force these insects to develop avoidance behaviors and subsequently separate from their niche. We implemented ecological experiments to test the effects of H. armigera-induced plant resistance and behavioral avoidance in A. gossypii, and employed transcriptomics and metabolomics analyses to reveal changes in resistance genes and metabolites in plants.

RESULTS

Our results demonstrate that cotton plants induced by H. armigera cause significant inhibitory and avoidance effects on A. gossypii insect populations. Electrical penetration graph (EPG) analysis showed changes in plant resistance induced by H. armigera leading to a decreased feeding efficiency of A. gossypii. In addition, genes associated with jasmonic acid and ethylene signaling pathways were significantly up-regulated in cotton plants after H. armigera induction, which led to a significant up-regulation of metabolites inducing plant resistance. These observations were corroborated by bioactivity analysis on metabolites, which showed that jasmonic acid, gossypol and tannins have significant inhibitory effects on A. gossypii populations. In contrast, methylparaben is associated with avoidance behaviors on A. gossypii populations.

CONCLUSION

Our study suggests that the differences in the ability to induce plant resistance and tolerance between two non-predatory insects were lethal to low-tolerance A. gossypii insects, which might be a major factor determining their niche differentiation. This was further demonstrated by screening anti-insect and bio-hormonal metabolites. Our study provides a reference for investigating the evolutionary relationship between non-predatory insects and insights to implement effective insect biocontrol. © 2022 Society of Chemical Industry.

Plant Volatiles and Herbivore Induced Plant Volatiles from Chili Pepper Act as Attractant of the Aphid Parasitoid Aphelinus varipes (Hymenoptera: Aphelinidae)

Plants have evolved a number of different chemical defenses, covering nearly all classes of (secondary) metabolites, that represent a major barrier to herbivory: some are constitutive; others are induced after attacks from herbivores (HIPVs) and may elicit the attraction of predators and parasitoids. Here, we studied how the female solitary endoparasitoid Aphelinus varipes responds to plant and host aphid volatiles in a series of experiments on five commercially important vegetables that were either healthy or infested with the aphid Myzus persicae: chili pepper, eggplant, crown daisy, Chinese cabbage and cabbage. The results for the olfactory responses of A. varipes showed that the presence of M. persicae increased the attraction of the endoparasitoid to the infested plants. In a second experiment, volatiles from highly attractive and repellent plants were obtained via headspace collection to investigate volatiles from healthy and aphid-damaged plants. The results for the differences in volatile profiles in response to aphid infestation in chili pepper cultivar were dominated by the volatile blends, including α-pinene, decanal and phthalic acid, while in cabbage they were dominated by isophorone. Moreover, when HIPVs with different concentrations were compared, α-pinene at a dose rate of 100 ng/μL attracted more parasitoids, and the comparison was useful to understand the mechanisms of plant secondary volatiles during aphid infestation and to provide new resources to control this insect pest. Overall our study shows how HIPVs can bolster tritrophic interactions by enhancing the attractiveness of parasitoids.

Evolution of koinobiont parasitoid host regulation and consequences for indirect plant defence

Tritrophic interactions among plants, herbivorous insects and their parasitoids have been well studied in the past four decades. Recently, a new angle has been uncovered: koinobiont parasitoids, that allow their host to keep feeding on the plant for a certain amount of time after parasitism, indirectly alter plant responses against herbivory via the many physiological changes induced in their herbivorous hosts. By affecting plant responses, parasitoids may indirectly affect the whole community of insects interacting with plants induced by parasitized herbivores and have extended effects on plant fitness. These important findings have renewed research interests on parasitoid manipulation of their host development. Parasitoids typically arrest their host development before the last instar, resulting in a lower final weight compared to unparasitized hosts. Yet, some parasitoids prolong their host development, leading to larger herbivores that consume more plant material than unparasitized ones. Furthermore, parasitoid host regulation is plastic and one parasitoid species may arrest or promote its host growth depending on the number of eggs laid, host developmental stage and species as well as environmental conditions. The consequences of plasticity in parasitoid host regulation for plant–insect interactions have received very little attention over the last two decades, particularly concerning parasitoids that promote their host growth. In this review, we first synthesize the mechanisms used by parasitoids to regulate host growth and food consumption. Then, we identify the evolutionary and environmental factors that influence the direction of parasitoid host regulation in terms of arrestment or promotion of host growth. In addition, we discuss the implication of different host regulation types for the parasitoid’s role as agent of plant indirect defence. Finally, we argue that the recent research interests about parasitoid plant-mediated interactions would strongly benefit from revival of research on the mechanisms, ecology and evolution of host regulation in parasitoids.

Cascading effects of caffeine intake by primary consumers to the upper trophic level

Secondary metabolites are central to understanding the evolution of plant-animal interactions. Direct effects on phytophagous animals are well-known, but how secondary consumers adjust their behavioural and physiological responses to the herbivore's diet remains more scarcely explored for some metabolites. Caffeine is a neuroactive compound that affects both the behaviour and physiology of several animal species, from humans to insects. It is an alkaloid present in nectar, leaves and even sap of numerous species of plants where it plays a role in chemical defences against herbivores and pathogens. Caffeine effects have been overlooked in generalist herbivores that are not specialized in coffee or tea plants. Using a host-parasitoid system, we show that caffeine intake at a relatively low dose affects longevity and fecundity of the primary consumer, but also indirectly of the secondary one, suggesting that this alkaloid and/or its effects can be transmitted through trophic levels and persist in the food chain. Parasitism success was lowered by ≈16% on hosts fed with caffeine, and parasitoids of the next generation that have developed in hosts fed on caffeine showed a reduced longevity, but no differences in mass and size were found. This study helps at better understanding how plant secondary metabolites, such as caffeine involved in plant-animal interactions, could affect primary consumers, could have knock-on effects on upper trophic levels over generations, and could modify interspecific interactions in multitrophic systems.

Exogenous Application of Gallic Acid Induces the Direct Defense of Tea Plant Against Ectropis obliqua Caterpillars

Gallic acid (GA), an important polyphenolic compound in the plant, is a well-known antioxidant, antihyperglycemic, and anti-lipid peroxidative agent. Recently, GA treatment exhibited ameliorative effects on plants in response to some abiotic stresses. However, the elicitation effect of GA on plant defense against herbivorous insects has not yet been reported. In this study, we found that the exogenous application of GA induced the direct defense of tea plant (Camellia sinensis) against tea geometrid (Ectropis obliqua) larvae, through activating jasmonic acid (JA) signaling and phenylpropanoid pathways. These signaling cascades resulted in the efficient induction of several defensive compounds. Among them, astragalin, naringenin, and epigallocatechin-3-gallate were the three of the most active anti-feeding compounds. However, the exogenous GA treatment did not affect the preference of E. obliqua female moths and larval parasitoid Apanteles sp. Our study suggests that GA may serve as an elicitor that triggers a direct defense response against tea geometrid larvae in tea plants. This study will help to deepen the understanding of the interaction between plants and phytophagous insects and also provide theoretical and technical guidance for the development of plant defense elicitors.

Limited increase in asynchrony between the onset of spring green-up and the arrival of a long-distance migratory bird

For many migrant bird species around the world, climate change has been shown to induce changes in the timings of arrival and the onset of spring food availability at breeding sites. However, whether such changes enlarged asynchrony between the timings of spring arrival of long-distance migratory birds and onset of vegetation greenness increase remain controversial. We used a 29-year phenological dataset to investigate the temporal changes in spring first-sighting date (FSD) of a long-distance migratory bird (barn swallow, Hirundo rustica), from observations at 160 local breeding sites across northern China, and the vegetation green-up onset date (VGD), determined from satellite observations of vegetation greenness. We found that both FSD and VGD trended earlier at over two-thirds of the breeding sites. FSD significantly advanced at 26.9% of the sites, and VGD significantly advanced at 23.8% of the sites. The degree of asynchrony between FSD and VGD changed significantly at one-third of the breeding sites (22.5% with an increase versus 11.3% with a decrease), leading to a limited increase of phenological mismatch. We speculated that climate change did not disrupt the climatic connections between most breeding sites and corresponding non-breeding sites (wintering grounds and migration routes). Our findings suggest that climate change may not greatly increase phenological mismatch between first arrival date of barn swallows and VGD at breeding sites. Importantly, this study should serve as a cue to encourage ecologists and conservation biologists to expand the context under which to explore the ecological consequences of phenological shifts beyond asynchrony, such as individual survival, population demography and ecosystem-level consequences.

In a belowground multitrophic interaction, Trichoderma harzianum induces maize root herbivore tolerance against Phyllophaga vetula

BACKGROUND

Trichoderma spp. are soil fungi that interact with plant roots and associated biota such as other microorganisms and soil fauna. However, information about their interactions with root-feeding insects is limited. Here, interactions between Trichoderma harzianum and the root-feeding insect Phyllophaga vetula, a common insect pest in maize agroecosystems, were examined.

RESULTS

Applications of T. harzianum and P. vetula to the root system increased and decreased maize growth, respectively. Induced tolerance against herbivore attack was provided by T. harzianum maintaining a robust and functional root system as evidenced by the increased uptake of Cu, Ca, Mg, Na and K. Herbivore tolerance also coincided with changes in the emission of root volatile terpenes known to induce indirect defense responses and attract natural enemies of the herbivore. More importantly, T. harzianum induced de novo emission of several sesquiterpenes such as β-caryophyllene and δ-cadinene. In addition, single and combined applications of T. harzianum and P. vetula altered the sucrose content of the roots. Finally, T. harzianum produced 6-pentyl-2H-pyran-2-one (6-PP) a volatile compound that may act as an antifeedant-signaling compound mitigating root herbivory by P. vetula.

CONCLUSION

Our results provide novel information about belowground multitrophic plant-microbe-arthropod interactions between T. harzianum and P. vetula in the maize rhizosphere resulting in alterations in maize phenotypic plant responses, inducing root herbivore tolerance.

Plant cytochrome P450s: Role in stress tolerance and potential applications for human welfare

Cytochrome P450s (CYPs) are a versatile group of enzymes and one of the largest families of proteins, controlling various physiological processes via biosynthetic and detoxification pathways. CYPs perform multiple roles through a critical irreversible enzymatic reaction in which an oxygen atom is inserted within hydrophobic molecules, converting them into the reactive and hydro soluble components. During evolution, plants have acquired significantly more number of CYPs and represent about 1% of the encoded genes . CYPs are highly conserved proteins involved in growth, development and tolerance against biotic and abiotic stresses. Furthermore, CYPs reinforce plants' molecular and chemical defense mechanisms by regulating the biosynthesis of secondary metabolites, enhancing reactive oxygen species (ROS) scavenging and controlling biosynthesis and homeostasis of phytohormones, including abscisic acid (ABA) and jasmonates. Thus, they are the critical targets of metabolic engineering for enhancing plant defense against environmental stresses. Additionally, CYPs are also used as biocatalysts in the fields of pharmacology and phytoremediation. Herein, we highlight the role of CYPs in plant stress tolerance and their applications for human welfare.

The high adaptability of Hyphantria cunea larvae to cinnamic acid involves in detoxification, antioxidation and gut microbiota response

Hyphantria cunea is one of the most destructive invasive agricultural and forest pests worldwide. In order to better understand the adaptation mechanism of H. cunea larvae to secondary metabolites of their highly diversified host plants, the physiological function and detoxification ability of midgut, as well as the gut microbial community were investigated in H. cunea larvae fed with cinnamic acid-treated artificial diets. Our results showed that cinnamic acid treatment could not affect the growth and food utilization of H. cunea larvae, as evidenced by a non-significantly altered larval body weight and efficiency of conversion of ingested food. Evaluation of oxidative stress-related parameters (e.g. malondialdehyde and hydrogen peroxide) and midgut histopathology also clearly confirmed that cinnamic acid treatment caused no significant oxidative damage and pathological changes in the larval midgut. Variance analysis showed that cinnamic acid treatment significantly increased the content of non-enzymatic antioxidants (ascorbic acid and glutathione), the activity of antioxidant enzymes (superoxide dismutase and peroxidase) and detoxification enzyme (carboxylate esterase), as well as the abundance of several gut microbiota at the genus level (Hydrogenophaga and Acinetobacter) involved in the organic substance degradation in larval midgut. Further Pearson's correlation analysis revealed that these strongly altered gut microbiota at the genus level appeared to be significantly correlated with the detoxification and antioxidation parameters. These findings demonstrate the high adaptability of H. cunea larvae to cinnamic acid involves in detoxification, antioxidation and gut microbiota response, and indicate the existence of an extremely effective counter-defense mechanism for H. cunea larvae against the secondary metabolites of host plants.

Volatile DMNT directly protects plants against Plutella xylostella by disrupting the peritrophic matrix barrier in insect midgut

Insect pests negatively affect crop quality and yield; identifying new methods to protect crops against insects therefore has important agricultural applications. Our analysis of transgenic Arabidopsis thaliana plants showed that overexpression of pentacyclic triterpene synthase 1, encoding the key biosynthetic enzyme for the natural plant product (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), led to a significant resistance against a major insect pest, Plutella xylostella. DMNT treatment severely damaged the peritrophic matrix (PM), a physical barrier isolating food and pathogens from the midgut wall cells. DMNT repressed the expression of PxMucin in midgut cells, and knocking down PxMucin resulted in PM rupture and P. xylostella death. A 16S RNA survey revealed that DMNT significantly disrupted midgut microbiota populations and that midgut microbes were essential for DMNT-induced killing. Therefore, we propose that the midgut microbiota assists DMNT in killing P. xylostella. These findings may provide a novel approach for plant protection against P. xylostella.

Chemical defence in Brassicaceae against pollen beetles revealed by metabolomics and flower bud manipulation approaches

Divergence of chemical plant defence mechanisms within the Brassicaceae can be utilized to identify means against specialized pest insects. Using a bioassay-driven approach, we (a) screened 24 different Brassica napus cultivars, B. napus resyntheses and related brassicaceous species for natural plant resistance against feeding adults of the pollen beetle (Brassicogethes aeneus), (b) tested for gender-specific feeding resistance, (c) analysed the flower bud metabolomes by a non-targeted approach and (d) tested single candidate compounds for their antifeedant activity. (a) In no-choice assays, beetles were allowed to feed on intact plants. Reduced feeding activity was mainly observed on Sinapis alba and Barbarea vulgaris but not on B. napus cultivars. (b) Males fed less and discriminated more in feeding than females. (c) Correlation of the metabolite abundances with the beetles' feeding activity revealed several glucosinolates, phenylpropanoids, flavonoids and saponins as potential antifeedants. (d) These were tested in dual-bud-choice assays developed for medium-throughput compound screening. Application of standard compounds on single oilseed rape flower buds revealed highly deterrent effects of glucobarbarin, oleanolic acid and hederagenin. These results help to understand chemical plant defence in the Brassicaceae and are of key importance for further breeding strategies for insect-resistant oilseed rape cultivars.

Profiling Volatile Terpenoids from Calabrian Pine Stands Infested by the Pine Processionary Moth

Terpenoids make up the biggest and most diversified class of chemical substances discovered in plants, encompassing over 40,000 individual compounds. In conifers, the production of terpenoids, either as oleoresin or emitted as volatile compounds, play an important role in the physical and chemical defence responses against pathogens and herbivores. In the present work, we examined, for the first time to the best of our knowledge, the terpenic defensive relations of Calabrian pine (Pinus nigra subsp. laricio (Poiret) Maire), facing the attack of the pine processionary moth (Thaumetopoea pityocampa (Denis and Schiffermüller, 1775)), brought about in the open on adult plant individuals growing at two distinct forest sites. Among the volatile terpenoids emitted from pine needles, bornyl acetate [(4,7,7-trimethyl-3-bicyclo[2.2.1]heptanyl) acetate] was the most frequently and selectively associated with the infestation, increasing during the period of most intense trophic activity of the caterpillars (defoliation), and decreasing thereafter. Although further work is needed to clarify whether the observed response reflects defence reactions and/or they are involved in communication among the infested plants and their biotic environment, the present results boost the currently growing interest in the isolation and characterization of plant secondary metabolites that can be used to control pests, pathogens, and weeds.

Age stage, two-sex life table of Habrobracon hebetor (Braconidae) on Spodoptera exigua (Noctuidae) reared on different sugar beet genotypes

The present study was carried out to unveil interactive relevance among consecutive and alternate members of a tritrophic system comprised of sugar beet genotypes, beet armyworm, Spodoptera exigua (Hübner), and its parasitoid, Habrobracon hebetor (Say) using demographic parameters. To do so, H. hebetor was reared on S. exigua fed on 10 sugar beet genotypes, including SB26; SB27; SB29; SB33; SB34; (7112*SB36)*Sh-1-HSF-5; FC220; FC301; SBSI006; and HM 1339RZ in a growth chamber at 25 ± 1 °C, 60 ± 5% RH, and 16:8 (L: D) h photoperiod. The data was analyzed based on the age-stage, two-sex life table theory. Our results revealed high variation in duration of different life stages of H. hebetor on S. exigua reared on different sugar beet genotypes examined. The shortest (10.605 days) and longest (13.721 days) pre-adult period of H. hebetor was on S. exigua reared on SB26 and SB34, respectively. The longest (17.2 days) and shortest adult longevity (7.5 days) was on S. exigua reared on SB26 and SB27, respectively. The highest values of the intrinsic rate of increase (r) (0.209 day-1) and finite rate of increase (λ) (1.233 day-1) were observed on S. exigua reared on SB34 and their lowest values (0.159 and 1.172 day-1, respectively) were recorded on SB27. Resistant and susceptible genotypes to S. exigua, FC301 and (7112*SB36)*Sh-1-HSF-5, respectively, were only genotypes on which H. hebetor had greater and approximately equal r compared with S. exigua. This finding indicates high capability of H. hebetor to be successfully employed against S. exigua on sugar beet genotypes which are extremely different in resistance to this pest.

Herbivore Gender Effects on Volatile Induction in Aspen and on Olfactory Responses in Leaf Beetles

Hybrid aspen (Populus tremula × tremuloides Michx.) is a fast-growing tree species used for short-rotation forestry in northern latitudes. Aspen species have a rich herbivore fauna, including defoliating leaf beetles that induce emissions of volatile organic compounds (VOCs) when feeding on aspen leaves. We investigated the differential induction of VOCs by male and female Phratora laticollis leaf beetles feeding on hybrid aspen and the differences in the orientation of beetles in response to gender-specific induced VOCs. The hypotheses for the study were (1) the VOCs in the headspace of plants infested with beetles of the two genders individually and in mixed aggregates would vary subtly, and (2) foraging adult beetles would be able to detect differences in VOC blends and use them to fine-tune their orientation choices. In Y-tube bioassays, both females and males preferred VOCs from leaves damaged by one gender (females or males) over undamaged leaves. However, if leaves were damaged by a two-gender population, neither females nor males indicated a preference over volatiles of undamaged leaves. Leaves damaged by both beetle genders simultaneously had significantly increased green leaf volatile (GLV), benzenoid and homoterpene emissions compared to undamaged leaves. Emissions of these compounds possibly indicate higher herbivore pressure and a higher risk of attack by parasitoids and predators and could thus be the cause of the lack of beetle preference. Our findings provide new basic information on gender-based host plant selection by herbivores and may be helpful in the development of sustainable biogenic VOC-based herbivore-control methods for intensive short-rotation hybrid aspen production.

Plant resistance does not compromise parasitoid-based biocontrol of a strawberry pest

Plant nutritional  quality can influence interactions between herbivores and their parasitoids. While most previous work has focused on a limited set of secondary plant metabolites, the tri-trophic effects of overall phenotypic resistance have been understudied. Furthermore, the joint effects of secondary and primary metabolites on parasitoids are almost unexplored. In this study, we compared the performance and survival of the parasitoid species Asecodes parviclava Thompson on wild woodland strawberry (Fragaria vesca L.) genotypes showing variation in resistance against the parasitoid's host, the strawberry leaf beetle (Galerucella tenella L.). Additionally, we related the metabolic profiles of these plant genotypes to the tritrophic outcomes in order to identify primary and secondary metabolites involved in regulating plant potential to facilitate parasitism. We found that parasitoid performance was strongly affected by plant genotype, but those differences in plant resistance to the herbivore were not reflected in parasitoid survival. These findings could be explained in particular by a significant link between parasitoid survival and foliar carbohydrate levels, which appeared to be the most important compounds for parasitism success. The fact that plant quality strongly affects parasitism should be further explored and utilized in plant breeding programs for a synergistic application in sustainable pest management.

On the Evolution and Functional Diversity of Terpene Synthases in the Pinus Species: A Review

In the biosynthesis of terpenoids, the ample catalytic versatility of terpene synthases (TPS) allows the formation of thousands of different molecules. A steadily increasing number of sequenced plant genomes invariably show that the TPS gene family is medium to large in size, comprising from 30 to 100 functional members. In conifers, TPSs belonging to the gymnosperm-specific TPS-d subfamily produce a complex mixture of mono-, sesqui-, and diterpenoid specialized metabolites, which are found in volatile emissions and oleoresin secretions. Such substances are involved in the defence against pathogens and herbivores and can help to protect against abiotic stress. Oleoresin terpenoids can be also profitably used in a number of different fields, from traditional and modern medicine to fine chemicals, fragrances, and flavours, and, in the last years, in biorefinery too. In the present work, after summarizing the current views on the biosynthesis and biological functions of terpenoids, recent advances on the evolution and functional diversification of plant TPSs are reviewed, with a focus on gymnosperms. In such context, an extensive characterization and phylogeny of all the known TPSs from different Pinus species is reported, which, for such genus, can be seen as the first effort to explore the evolutionary history of the large family of TPS genes involved in specialized metabolism. Finally, an approach is described in which the phylogeny of TPSs in Pinus spp. has been exploited to isolate for the first time mono-TPS sequences from Pinus nigra subsp. laricio, an ecologically important endemic pine in the Mediterranean area.

Combined transcriptome and metabolome analysis identifies defence responses in spider mite-infested pepper (Capsicum annuum)

Plants regulate responses towards herbivory through fine-tuning of defence-related hormone production, expression of defence genes, and production of secondary metabolites. Jasmonic acid (JA) plays a key role in plant-herbivorous arthropod interactions. To understand how pepper (Capsicum annuum) responds to herbivory, leaf transcriptomes and metabolomes of two genotypes different in their susceptibility to spider mites were studied. Mites induced both JA and salicylic acid (SA) signalling. However, mite infestation and exogenous JA resulted in distinct transcriptome profiles. Compared with JA, mites induced fewer differentially expressed genes involved in metabolic processes (except for genes involved in the phenylpropanoid pathway) and lipid metabolic processes. Furthermore, pathogen-related defence responses including WRKY transcription factors were more strongly induced upon mite infestation, probably as a result of induced SA signalling. Untargeted analysis of secondary metabolites confirmed that JA treatment induced larger changes in metabolism than spider mite infestation, resulting in higher terpenoid and flavonoid production. The more resistant genotype exhibited a larger increase in endogenous JA and volatile and non-volatile secondary metabolites upon infestation, which could explain its stronger defence. Reasoning that in JA-SA antagonizing crosstalk, SA defences are prioritized over JA defences, we hypothesize that lack of SA-mediated repression of JA-induced defences could result in gain of resistance towards spider mites in pepper.

Phenotypic and molecular characterization of Hessian fly resistance in diploid wheat, Aegilops tauschii

BACKGROUND

The Hessian fly (Mayetiola destructor), belonging to the gall midge family (Cecidomyiidae), is a devastating pest of wheat (Triticum aestivum) causing significant yield losses. Despite identification and characterization of numerous Hessian fly-responsive genes and associated biological pathways involved in wheat defense against this dipteran pest, their functional validation has been challenging. This is largely attributed to the large genome, polyploidy, repetitive DNA, and limited genetic resources in hexaploid wheat. The diploid progenitor Aegilops tauschii, D-genome donor of modern-day hexaploid wheat, offers an ideal surrogate eliminating the need to target all three homeologous chromosomes (A, B and D) individually, and thereby making the functional validation of candidate Hessian fly-responsive genes plausible. Furthermore, the well-annotated sequence of Ae. tauschii genome and availability of genetic resources amenable to manipulations makes the functional assays less tedious and time-consuming. However, prior to utilization of this diploid genome for downstream studies, it is imperative to characterize its physical and molecular responses to Hessian fly.

RESULTS

In this study we screened five Ae. tauschii accessions for their response to the Hessian fly biotypes L and vH13. Two lines were identified that exhibited a homozygous resistance response to feeding by both Hessian fly biotypes. Studies using physical measurements and neutral red staining showed that the resistant Ae. tauschii accessions resembled hexaploid wheat in their phenotypic responses to Hessian fly, that included similarities in larval developmental stages, leaf and plant growth, and cell wall permeability. Furthermore, molecular responses, characterized by gene expression profiling using quantitative real-time PCR, in select resistant Ae. tauschii lines also revealed similarities with resistant hexaploid wheat.

CONCLUSIONS

Phenotypic and molecular characterization of Ae. tauschii to Hessian fly infestation revealed resistant accessions that shared similarities to hexaploid wheat. Resembling the resistant hexaploid wheat, the Ae. tauschii accessions mount an early defense strategy involving defense proteins including lectins, secondary metabolites and reactive oxygen species (ROS) radicals. Our results reveal the suitability of the diploid progenitor for use as an ideal tool for functional genomics research in deciphering the wheat-Hessian fly molecular interactions.

The MYB transcription factor Emission of Methyl Anthranilate 1 stimulates emission of methyl anthranilate from Medicago truncatula hairy roots

Plants respond to herbivore or pathogen attacks by activating specific defense programs that include the production of bioactive specialized metabolites to eliminate or deter the attackers. Volatiles play an important role in the interaction of a plant with its environment. Through transcript profiling of jasmonate-elicited Medicago truncatula cells, we identified Emission of Methyl Anthranilate (EMA) 1, a MYB transcription factor that is involved in the emission of the volatile compound methyl anthranilate when expressed in M. truncatula hairy roots, giving them a fruity scent. RNA sequencing (RNA-Seq) analysis of the fragrant roots revealed the upregulation of a methyltransferase that was subsequently characterized to catalyze the O-methylation of anthranilic acid and was hence named M. truncatula anthranilic acid methyl transferase (MtAAMT) 1. Given that direct activation of the MtAAMT1 promoter by EMA1 could not be unambiguously demonstrated, we further probed the RNA-Seq data and identified the repressor protein M. truncatula plant AT-rich sequence and zinc-binding (MtPLATZ) 1. Emission of Methyl Anthranilate 1 binds a tandem repeat of the ACCTAAC motif in the MtPLATZ1 promoter to transactivate gene expression. Overexpression of MtPLATZ1 in transgenic M. truncatula hairy roots led to transcriptional silencing of EMA1, indicating that MtPLATZ1 may be part of a negative feedback loop to control the expression of EMA1. Finally, application of exogenous methyl anthranilate boosted EMA1 and MtAAMT1 expression dramatically, thus also revealing a positive amplification loop. Such positive and negative feedback loops seem to be the norm rather than the exception in the regulation of plant specialized metabolism.

Liquid and Vapour Phase of Lavandin (Lavandula × intermedia) Essential Oil: Chemical Composition and Antimicrobial Activity

Essential oils from Lavandula genus and the obtained hybrids are widely used for different purposes such as perfume production in the cosmetic field and for its biological properties. This is the first study on the liquid and vapour phase of Lavandula × intermedia "Grosso" essential oil grown in the Lazio Region, Italy, investigated using headspace coupled to gas chromatography and mass spectrometry (HS-GC/MS). The results showed the most abundant components were linalool and linalyl acetate, followed by 1,8-cineole and terpinen-4-ol, while lavandulyl acetate and borneol were identified as minor compounds, maintaining the same proportion in both the liquid and vapour phase. Furthermore, we tested lavandin liquid and vapour phase essential oil on gram-negative bacteria (Escherichia coli, Acinetobacter bohemicus, and Pseudomonas fluorescens) and gram-positive bacteria (Bacillus cereus and Kocuria marina).

Changes in Gene Expression and Metabolite Profiles in Platanus acerifolia Leaves in Response to Feeding Damage Caused by Corythucha ciliata

The sycamore lace bug, Corythucha ciliata (Say) is a highly invasive pest insect that feeds on sycamore trees (Platanus spp.) worldwide. The interaction between Platanus species and this insect pest has not yet been studied at the molecular level. Therefore, a recent study was conducted to compare the gene expression and metabolite profiles of Platanus acerifolia leaves in response to C. ciliata feeding damage after 24 and 48 h. We employed high throughput RNA sequencing (RNA- seq) to identify a total of 2,828 significantly differentially expressed genes (DEGs) after C. ciliata feeding. In addition, 303 unigenes were found to be up-regulated at both time points. Moreover, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that monoterpenoid biosynthesis, the linoleic acid metabolism pathway, and alpha- linolenic acid metabolism were the most prominent pathways among the DEGs. Further analysis of the metabolite profiles showed that nine metabolites were significantly different before and after C. ciliata damage. In addition, we analyzed DEGs detected in the P. acerifolia and C. ciliata interaction using Mapman. The terpene synthase gene family was also identified. We suggest that the results obtained from DEGs and metabolite analysis can provide important information for the identification of genes involved in the P. acerifolia–C. ciliata interaction, which might be necessary for controlling C. ciliata efficiently.

Multiple molecular defense strategies in Brachypodium distachyon surmount Hessian fly (Mayetiola destructor) larvae-induced susceptibility for plant survival

The Hessian fly is a destructive pest of wheat causing severe economic damage. Numerous genes and associated biological pathways have been implicated in defense against Hessian fly. However, due to limited genetic resources, compounded with genome complexity, functional analysis of the candidate genes are challenging in wheat. Physically, Brachypodium distachyon (Bd) exhibits nonhost resistance to Hessian fly, and with a small genome size, short life cycle, vast genetic resources and amenability to transformation, it offers an alternate functional genomic model for deciphering plant-Hessian fly interactions. RNA-sequencing was used to reveal thousands of Hessian fly-responsive genes in Bd one, three, and five days after egg hatch. Genes encoding defense proteins, stress-regulating transcription factors, signaling kinases, and secondary metabolites were strongly up-regulated within the first 24 hours of larval feeding indicating an early defense, similar to resistant wheat. Defense was mediated by a hypersensitive response that included necrotic lesions, up-regulated ROS-generating and -scavenging enzymes, and H₂O₂ production. Suppression of cell wall-associated proteins and increased cell permeability in Bd resembled susceptible wheat. Thus, Bd molecular responses shared similarities to both resistant and susceptible wheat, validating its suitability as a model genome for undertaking functional studies of candidate Hessian fly-responsive genes.

Modulation of plant-mediated interactions between herbivores of different feeding guilds: Effects of parasitism and belowground interactions

Herbivory affects subsequent herbivores, mainly regulated by the phytohormones jasmonic (JA) and salicylic acid (SA). Additionally, organisms such as soil microbes belowground or parasitoids that develop inside their herbivorous hosts aboveground, can change plant responses to herbivory. However, it is not yet well known how organisms of trophic levels other than herbivores, below- and above-ground, alter the interactions between insect species sharing a host plant. Here, we investigated whether the parasitoid Aphidius colemani and different soil microbial communities (created through plant-soil feedbacks) affect the JA and SA signalling pathways in response to the aphid Myzus persicae and the thrips Frankliniella occidentalis, as well as subsequent thrips performance. Our results show that the expression of the JA-responsive gene CaPINII in sweet pepper was more suppressed by aphids than by parasitised aphids. However, parasitism did not affect the expression of CaPAL1, a biosynthetic gene of SA. Furthermore, aphid feeding enhanced thrips performance compared with uninfested plants, but this was not observed when aphids were parasitised. Soils where different plant species were previously grown, did not affect plant responses or the interaction between herbivores. Our study shows that members of the third trophic level can modify herbivore interactions by altering plant physiology.

Origanum vulgare Terpenoids Induce Oxidative Stress and Reduce the Feeding Activity of Spodoptera littoralis

Terpenoids are toxic compounds produced by plants as a defense strategy against insect herbivores. We tested the effect of Origanum vulgare terpenoids on the generalist herbivore Spodoptera littoralis and the response of the plant to herbivory. Terpenoids were analyzed by GC-FID and GC-MS and quantitative gene expression (qPCR) was evaluated on selected plant genes involved in both terpene biosynthesis. The insect detoxification response to terpenes was evaluated by monitoring antioxidant enzymes activity and expression of insect genes involved in terpene detoxification. O. vulgare terpenoid biosynthesis and gene expression was modulated by S. littoralis feeding. The herbivore-induced increased level of terpenoids (particularly carvacrol and p-cymene) interacted with the herbivore by decreasing larval survival and growth rate. The assimilation by S. littoralis of more than 50% of ingested terpenes correlated with the possible toxic effects of O. vulgare terpenoids. In choice test experiments, carvacrol and γ-terpinene mediated the larval feeding preferences, wherease the prolonged feeding on O. vulgare terpenoids (particularly on γ-terpinene) exerted relevant antinutritional effects on larvae. S. littoralis was found to react to O. vulgare terpenoids by increasing its antioxidant enzymes activities and gene expression, although this was not sufficient to sustain the toxicity of O. vulgare terpenoids.

Identification and functional analysis of two P450 enzymes of Gossypium hirsutum involved in DMNT and TMTT biosynthesis

The homoterpenes (3E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene (TMTT) are major herbivore-induced plant volatiles that can attract predatory or parasitic arthropods to protect injured plants from herbivore attack. In this study, DMNT and TMTT were confirmed to be emitted from cotton (Gossypium hirsutum) plants infested with chewing caterpillars or sucking bugs. Two CYP genes (GhCYP82L1 and GhCYP82L2) involved in homoterpene biosynthesis in G. hirsutum were newly identified and characterized. Yeast recombinant expression and enzyme assays indicated that the two GhCYP82Ls are both responsible for the conversion of (E)-nerolidol to DMNT and (E,E)-geranyllinalool to TMTT. The two heterologously expressed proteins without cytochrome P450 reductase fail to convert the substrates to homoterpenes. Quantitative real-time PCR (qPCR) analysis suggested that the two GhCYP82L genes were significantly up-regulated in leaves and stems of G. hirsutum after herbivore attack. Subsequently, electroantennogram recordings showed that electroantennal responses of Microplitis mediator and Peristenus spretus to DMNT and TMTT were both dose dependent. Laboratory behavioural bioassays showed that females of both wasp species responded positively to DMNT and males and females of M. mediator could be attracted by TMTT. The results provide a better understanding of homoterpene biosynthesis in G. hirsutum and of the potential influence of homoterpenes on the behaviour of natural enemies, which lay a foundation to study genetically modified homoterpene biosynthesis and its possible application in agricultural pest control.

Epichloë Fungal Endophytes and Plant Defenses: Not Just Alkaloids

Although the role of fungal alkaloids in protecting grasses associated with Epichloë fungal endophytes has been extensively documented, the effects of the symbiont on the host plant's immune responses have received little attention. We propose that, in addition to producing protective alkaloids, endophytes enhance plant immunity against chewing insects by promoting endogenous defense responses mediated by the jasmonic acid (JA) pathway. We advance a model that integrates this dual effect of endophytes on plant defenses and test its predictions by means of a standard meta-analysis. This analysis supports a role of Epichloë endophytes in boosting JA-mediated plant defenses. We discuss the ecological scenarios where this effect of endophytes on plant defenses would be most beneficial for increasing plant fitness.

Plant growth under water/salt stress: ROS production; antioxidants and significance of added potassium under such conditions

Plants are confronted with a variety of environmenmtal stresses resulting in enhanced production of ROS. Plants require a threshold level of ROS for vital functions and any change in their concentration alters the entire physiology of plant. Delicate balance of ROS is maintained by an efficient functioning of intriguing indigenous defence system called antioxidant system comprising enzymatic and non enzymatic components. Down regulation of antioxidant system leads to ROS induced oxidative stress causing damage to important cellular structures and hence anomalies in metabolism. Proper mineral nutrition, in addition to other agricultural practices, forms an important part for growth and hence the yield. Potassium (K) is a key macro-element regulating growth and development through alterations in physiological and biochemical attributes. K has been reported to result into accumulation of osmolytes and augmentation of antioxidant components in the plants exposed to water and salt stress. In the present review an effort has been made to revisit the old findings and the current advances in research regarding the role of optimal, suboptimal and deficient K soil status on growth under normal and stressful conditions. Effect of K deficiency and sufficiency is discussed and the information about the K mediated antioxidant regulation and plant response is highlighted.

Induced plant defences in biological control of arthropod pests: a double-edged sword

Biological control is an important ecosystem service delivered by natural enemies. Together with breeding for plant defence, it constitutes one of the most promising alternatives to pesticides for controlling herbivores in sustainable crop production. Especially induced plant defences may be promising targets in plant breeding for resistance against arthropod pests. Because they are activated upon herbivore damage, costs are only incurred when defence is needed. Moreover, they can be more specific than constitutive defences. Nevertheless, inducible defence traits that are harming plant pest organisms may interfere with biological control agents, such as predators and parasitoids. Despite the vast fundamental knowledge on plant defence mechanisms and their effects on natural enemies, our understanding of the feasibility of combining biological control with induced plant defence in practice is relatively poor. In this review, we focus on arthropod pest control and present the most important features of biological control with natural enemies and of induced plant defence. Furthermore, we show potential synergies and conflicts among them and, finally, identify gaps and list opportunities for their combined use in crop protection. We suggest that breeders should focus on inducible resistance traits that are compatible with the natural enemies of arthropod pests, specifically traits that help communities of natural enemies to build up. © 2017 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Bottom-up effects on herbivore-induced plant defences: a case study based on compositional patterns of rhizosphere microbial communities

Below-ground soil microorganisms can modulate above-ground plant-insect interactions. It still needs to be determined whether this is a direct effect of single species or an indirect effect of shifts in soil microbial community assemblages. Evaluation of the soil microbiome as a whole is critical for understanding multi-trophic interactions, including those mediated by volatiles involving plants, herbivorous insects, predators/parasitoids and microorganisms. We implemented a regulated system comprising Nerium oleander plants grown in soil initially containing a sterile/non sterile inoculum, herbivore Aphis nerii and predator Chrysoperla carnea. After aphid attack, plants emitted a characteristic blend of volatiles derived from two biosynthetic classes: fatty acid catabolites and aromatic-derived products. Three aliphatic compounds were mainly detected in plants grown in the inoculated microbial soil, a blend which was preferentially chosen by C. carnea adult females. The contrasting effect of the initial inocula was attributed to the different microbial consortia developed in each treatment. We argue that differences in the relative abundance of the active microbial communities in the rhizosphere correlate with those in the emission of selected volatile compounds by attacked plants. The mechanisms involved in how the functional soil microbiome modulates inducible indirect defence of plants are discussed.

Constraints on the evolution of resistance to gall flies in Solidago altissima: resistance sometimes costs more than it is worth

Plant populations frequently maintain submaximal levels of resistance to natural enemies, even in the presence of substantial genetic variation for resistance. Identifying constraints on the evolution of increased resistance has been a major goal of researchers of plant-herbivore interactions. In a glasshouse study, we measured relative costs and benefits of resistance of tall goldenrod (Solidago altissima) to the gall-inducing tephritid Eurosta solidaginis. We exposed multiple ramets of 26 goldenrod genets to nutrient or shade stress and to oviposition by E. solidaginis. The presence of a gall cost a ramet an average of 1743 seeds, but the cost differed 10-fold across environments. Plant genets varied widely for an induced 'hypersensitive' response in which meristem cells become necrotic and kill E. solidaginis hatchlings before gall induction. There was no evidence that this highly effective resistance trait carried an allocation cost. However, the response carried a risk of autotoxicity, as necrosis killed the apex of 37% of the ungalled ramets. On average, a damaged apex cost each ramet 5015 seeds. Autotoxicity may constrain the resistance of S. altissima to an intermediate level, and variation in environmental conditions may alter the relative costs and benefits of resistance and tolerance, thus maintaining genetic variation within goldenrod populations.

The effect of structurally related metabolites on insect herbivores: A case study on pyrrolizidine alkaloids and western flower thrips

Plant specialised metabolites (SMs) are very diverse in terms of both their number and chemical structures with more than 200,000 estimated compounds. This chemical diversity occurs not only among different groups of compounds but also within the groups themselves. In the context of plant-insect interactions, the chemical diversity within a class of structurally related metabolites is generally also related to their bioactivity. In this study, we tested firstly whether individual SMs within the group of pyrrolizidine alkaloids (PAs) differ in their effects on insect herbivores (western flower thrips, Frankliniella occidentalis). Secondly, we tested combinations of PA N-oxides to determine whether they are more active than their individual components. We also evaluated the bioactivity of six PA free bases and their corresponding N-oxides. At concentrations similar to that in plants, several PAs reduced thrip's survival but the effect also differed strongly among PAs. In general, PA free bases caused a lower survival than their corresponding N-oxides. Among the tested PA free bases, we found jacobine and retrorsine to be the most active against second instar larvae of thrips, followed by erucifoline and seneciphylline, while senecionine and monocrotaline did not exhibit significant dose-dependent effects on thrip's survival. In the case of PA N-oxides, we found that only senecionine N-oxide and jacobine N-oxide reduced thrip's survival, although the effect of senecionine N-oxide was weak. Combinations of PA N-oxides showed no synergistic effects. These findings indicate the differences observed in the effect of structurally related SMs on insect herbivores. It is of limited value to study the bioactivity of combined groups, such as PAs, without taking their composition into account.

Plant-mediated effects on an insect-pathogen interaction vary with intraspecific genetic variation in plant defences

Baculoviruses are food-borne microbial pathogens that are ingested by insects on contaminated foliage. Oxidation of plant-derived phenolics, activated by insect feeding, can directly interfere with infections in the gut. Since phenolic oxidation is an important component of plant resistance against insects, baculoviruses are suggested to be incompatible with plant defences. However, plants among and within species invest differently in a myriad of chemical and physical defences. Therefore, we hypothesized that among eight soybean genotypes, some genotypes would be able to maintain both high resistance against an insect pest and high efficacy of a baculovirus. Soybean constitutive (non-induced) and jasmonic acid (JA)-induced (anti-herbivore response) resistance was measured against the fall armyworm Spodoptera frugiperda (weight gain, leaf consumption and utilization). Indicators of phenolic oxidation were measured as foliar phenolic content and peroxidase activity. Levels of armyworm mortality inflicted by baculovirus (SfMNPV) did not vary among soybean genotypes when the virus was ingested with non-induced foliage. Ingestion of the virus on JA-induced foliage reduced armyworm mortality, relative to non-induced foliage, on some soybean genotypes. Baculovirus efficacy was lower when ingested with foliage that contained higher phenolic content and defensive properties that reduced armyworm weight gain and leaf utilization. However, soybean genotypes that defended the plant by reducing consumption rate and strongly deterred feeding upon JA-induction did not reduce baculovirus efficacy, indicating that these defences may be more compatible with baculoviruses to maximize plant protection. Differential compatibility of defence traits with the third trophic level highlights an important cost/trade-off associated with plant defence strategies.