Plant chemical defense against herbivores and pathogens: generalized defense or trade-offs?

Plants are often attacked by multiple enemies, including pathogens and herbivores. While many plant secondary metabolites show specific effects toward either pathogens or herbivores, some can affect the performance of both these groups of natural enemies and are considered to be generalized defense compounds. We tested whether aucubin and catalpol, two iridoid glycosides present in ribwort plantain (Plantago lanceolata), confer in vivo resistance to both the generalist insect herbivore Spodoptera exigua and the biotrophic fungal pathogen Diaporthe adunca using plants from P. lanceolata lines that had been selected for high- and low-leaf iridoid glycoside concentrations for four generations. The lines differed approximately three-fold in the levels of these compounds. Plants from the high-selection line showed enhanced resistance to both S. exigua and D. adunca, as evidenced by a smaller lesion size and a lower fungal growth rate and spore production, and a lower larval growth rate and herbivory under both choice and no-choice conditions. Gravimetric analysis revealed that the iridoid glycosides acted as feeding deterrents to S. exigua, thereby reducing its food intake rate, rather than having post-ingestive toxic effects as predicted from in vitro effects of hydrolysis products. We suggest that the bitter taste of iridoid glycosides deters feeding by S. exigua, whereas the hydrolysis products formed after tissue damage following fungal infection mediate pathogen resistance. We conclude that iridoid glycosides in P. lanceolata can serve as broad-spectrum defenses and that selection for pathogen resistance could potentially result in increased resistance to generalist insect herbivores and vice versa, resulting in diffuse rather than pairwise coevolution.     

Immunological cost of chemical defence and the evolution of herbivore diet breadth

Selective pressures from host plant chemistry and natural enemies may contribute independently to driving insect herbivores towards narrow diet breadths. We used the specialist caterpillar, Junonia coenia (Nymphalidae), which sequesters defensive compounds, iridoid glycosides, from its host plants to assess the effects of plant chemistry and sequestration on the larval immune response. A series of experiments using implanted glass beads to challenge immune function showed that larvae feeding on diets with high concentrations of iridoid glycosides are more likely to have their immune response compromised than those feeding on diets low in these compounds. These results indicate that larvae feeding on plants with high concentrations of toxins might be more poorly defended against parasitoids, while at the same time being better defended against predators, suggesting that predators and parasitoids can exert different selective pressures on the evolution of herbivore diet breadth.     

The interplay between toxin-releasing β-glucosidase and plant iridoid glycosides impairs larval development in a generalist caterpillar, Grammia incorrupta (Arctiidae)

Herbivores with polyphagous feeding habits must cope with a diet that varies in quality. One of the most important sources of this variation in host plant suitability is plant secondary chemistry. We examined how feeding on plants containing one such group of compounds, the iridoid glycosides, might affect the growth and enzymatic activity in a polyphagous caterpillar that feeds on over 80 plant species in 50 different families. Larvae of the polyphagous arctiid, Grammia incorrupta, were reared exclusively on one of two plant species, one of which contains iridoid glycosides (Plantago lanceolata, Plantaginaceae) while the other does not (Taraxacum officinale, Asteraceae). Larval weight was measured on the two host plants, and midgut homogenates of last instar larvae were then assayed for activity and kinetic properties of β-glucosidases, using both a standard substrate, 4-nitrophenyl-β-D-glucose (NPβGlc), and the iridoid glycoside aucubin, one of the two main iridoid glycosides in P. lanceolata. Larvae feeding on P. lanceolata weighed significantly less and developed more slowly compared to larvae on T. officinale. While the larval midgut β-glucosidase activity determined with NPβGlc was significantly decreased when fed on P. lanceolata, aucubin was substantially hydrolyzed and the larval β-glucosidase activity towards both substrates correlated negatively with larval weight. Our results demonstrate that host plants containing high concentrations of iridoid glycosides have a negative impact on larval development of this generalist insect herbivore. This is most likely due to the hydrolysis of plant glycosides in the larval midgut which results in the release of toxic aglycones. Linking the reduced larval weight to the toxin-releasing action of an iridoid glycoside cleaving β-glucosidase, our results thus support the detoxification limitation hypothesis, suggesting fitness costs for the larvae feeding solely on P. lanceolata. Thus, in addition to the adaptive regulation of midgut β-glucosidase activity, host plant switching as a behavioral adaptation might be a prerequisite for generalist herbivores that allows them to circumvent the negative effects of plant secondary compounds.     

Low rates of iridoid glycoside hydrolysis in two Longitarsus leaf beetles with different feeding specialization confer tolerance to iridoid glycoside containing host plants

Iridoid glycosides are plant defence compounds that are deterrent and/or toxic for unadapted herbivores but are readily sequestered by dietary specialists of different insect orders. Hydrolysis of iridoid glycosides by β‐glucosidase leads to protein denaturation. Insect digestive β‐glucosidases thus have the potential to mediate plant–insect interactions. In the present study, mechanisms associated with iridoid glycoside tolerance are investigated in two closely‐related leaf beetle species (Coleoptera: Chrysomelidae) that feed on iridoid glycoside containing host plants. The polyphagous Longitarsus luridus Scopoli does not sequester iridoid glycosides, whereas the specialist Longitarsus tabidus Fabricius sequesters these compounds from its host plants. To study whether the biochemical properties of their β‐glucosidases correspond to the differences in feeding specialization, the number of β‐glucosidase isoforms and their kinetic properties are compared between the two beetle species. To examine the impact of iridoid glycosides on the β‐glucosidase activity of the generalist, L. luridus beetles are kept on host plants with or without iridoid glycosides. Furthermore, β‐glucosidase activities of both species are examined using an artificial β‐glucosidase substrate and the iridoid glycoside aucubin present in their host plants. Both species have one or two β‐glucosidases with different substrate affinities. Interestingly, host plant use does not influence the specific β‐glucosidase activities of the generalist. Both species hydrolyse aucubin with a much lower affinity than the standard substrate. The neutral pH reduces the β‐glucosidase activity of the specialist beetles by approximately 60% relative to its pH optimum. These low rates of aucubin hydrolysis suggest that the ability to sequester iridoid glycosides has evolved as a key to potentially preventing iridoid glycoside hydrolysis by plant‐derived β‐glucosidases.     

Biochemical detoxication: mechanism of differential tiger swallowtail tolerance to phenolic glycosides

Summary Phenolic glycosides, commonly occurring allelochemicals in the plant family Salicaceae, are differentially toxic to subspecies of the eastern tiger swallowtail and responsible for striking differences in the abilities of Papilio glaucus canadensis and P.g. glaucus to utilize the Salicaceae as food plants. This research was designed to test the hypothesis that particularly high esterase activity confers resistance to phenolic glycosides in P.g. canadensis. I conducted larval survival trials in which the phenolic glycosides salicortin and tremulacin were administered with and without inhibitors of the major detoxication enzymes. Results for P.g. canadensis showed that when esterases were inhibited, toxicity of the phenolic glycosides increased greatly. None of the inhibitors significantly increased toxicity of the compounds to P.g. glaucus. I also conducted in vitro assays of the major detoxication enzymes (polysubstrate monooxygenases, esterases, and glutathione transferases) in larval midguts. Soluble esterase activity was 3-fold higher in P.g. canadensis than in P.g. glaucus. Moreover, esterase activity was inducible by prior consumption of phenolic glycosides in P.g. canadensis but not in P.g. glaucus. Glutathione transferases may also be involved in the terminal metabolism of phenolic glycosides. This is the first verified case of detoxication of an allelochemical by esterase enzymes in herbivores. The biochemical adaptation has played an important role in the evolution of food plant preferences in P. glaucus subspecies.     

Influence of iridoid glycoside containing host plants on midgut β-glucosidase activity in a polyphagous caterpillar, Spilosoma virginica Fabricius (Arctiidae)

Iridoid glycosides are secondary plant compounds that have deterrent, growth reducing or even toxic effects on non-adapted herbivorous insects. To investigate the effects of iridoid glycoside containing plants on the digestive metabolism of a generalist herbivore, larvae of Spilosoma virginica (Lepidoptera: Arctiidae) were reared on three plant species that differ in their secondary plant chemistry: Taraxacum officinale (no iridoid glycosides), Plantago major (low iridoid glycoside content), and P. lanceolata (high iridoid glycoside content). Midguts of fifth instar larvae were assayed for the activity and kinetic properties of β-glucosidase using different substrates. Compared to the larvae on T. officinale, the β-glucosidase activity of larvae feeding on P. lanceolata was significantly lower measured with 4-nitrophenyl-β-d-glucopyranoside. Using the iridoid glycoside aucubin as a substrate, we did not find differences in the β-glucosidase activity of the larvae reared on the three plants. Heat inactivation experiments revealed the existence of a heat-labile and a more heat-stable β-glucosidase with similar Michaelis constants for 4-nitrophenyl-β-d-glucopyranoside. We discuss possible mechanisms leading to the observed decrease of β-glucosidase activity for larvae reared on P. lanceolata and its relevance for generalist herbivores in adapting to iridoid glycoside containing plant species and their use as potential host plants.     

昆虫唾液成分在昆虫与植物关系中的作用

近年来,人们对于植食性昆虫唾液的深入研究,揭示出其在昆虫与植物的相互关系和协同进化中起到非常重要的作用。植食性昆虫唾液中含有的酶类和各种有机成分,能诱导植物的一系列生化反应,而且这些反应有很强的特异性,与为害的昆虫种类甚至龄期有关。鳞翅目幼虫口腔分泌物（或反吐液）中含有的β-葡糖苷酶、葡萄糖氧化酶等酶类和挥发物诱导素等有机成分,已经证明可以诱导植物的反应; 刺吸式昆虫的取食也可以刺激植物产生反应,但其唾液内的酶类,如烟粉虱的碱性磷酸酶, 蚜虫的酚氧化酶、果胶酶和多聚半乳糖醛酸酶, 蝽类的寡聚半乳糖醛酸酶等是否发挥作用,目前还没有直接的证据。寄主植物对昆虫的唾液成分也有很大的影响,可能是昆虫对不同植物营养成分和毒性成分的适应方式。对昆虫唾液蛋白的分析表明,具有同样类型口器、食物类型接近的昆虫,唾液成分有更多的相似性。研究植食性昆虫的唾液成分,对于阐明昆虫和植物的协同进化关系、昆虫生物型的形成机理、害虫的致害机理,以及指导害虫防治等,有着一定的理论和实际意义。     

Evolutionary recruitment of a flavin-dependent monooxygenase for stabilization of sequestered pyrrolizidine alkaloids in arctiids

Pyrrolizidine alkaloids are secondary metabolites that are produced by certain plants as a chemical defense against herbivores. They represent a promising system to study the evolution of pathways in plant secondary metabolism. Recently, a specific gene of this pathway has been shown to have originated by duplication of a gene involved in primary metabolism followed by diversification and optimization for its specific function in the defense machinery of these plants. Furthermore, pyrrolizidine alkaloids are one of the best-studied examples of a plant defense system that has been recruited by several insect lineages for their own chemical defense. In each case, this recruitment requires sophisticated mechanisms of adaptations, e.g., efficient excretion, transport, suppression of toxification, or detoxification. In this review, we briefly summarize detoxification mechanism known for pyrrolizidine alkaloids and focus on pyrrolizidine alkaloid N-oxidation as one of the mechanisms allowing insects to accumulate the sequestered toxins in an inactivated protoxic form. Recent research into the evolution of pyrrolizidine alkaloid N-oxygenases of adapted arctiid moths (Lepidoptera) has shown that this enzyme originated by the duplication of a gene encoding a flavin-dependent monooxygenase of unknown function early in the arctiid lineage. The available data suggest several similarities in the molecular evolution of this adaptation strategy of insects to the mechanisms described previously for the evolution of the respective pathway in plants.     

DIET QUALITY AFFECTS WARNING COLORATION INDIRECTLY: EXCRETION COSTS IN A GENERALIST HERBIVORE

Aposematic herbivores are under selection pressure from their host plants and predators. Although many aposematic herbivores exploit plant toxins in their own secondary defense, dealing with these harmful compounds might underlay costs. We studied whether the allocation of energy to detoxification and/or sequestration of host plant defense chemicals trades off with warning signal expression. We used a generalist aposematic herbivore Parasemia plantaginis (Arctiidae), whose adults and larvae show extensive phenotypic and genetic variation in coloration. We reared larvae from selection lines for small and large larval warning signals on Plantago lanceolata with either low or high concentration of iridoid glycosides (IGs). Larvae disposed of IGs effectively; their body IG content was low irrespective of their diet. Detoxification was costly as individuals reared on the high IG diet produced fewer offspring. The IG concentration of the diet did not affect larval coloration (no trade-off) but the wings of females were lighter orange (vs. dark red) when reared on the high IG diet. Thus, the difference in plant secondary chemicals did not induce variation in the chemical defense efficacy of aposematic individuals but caused variation in reproductive output and warning signals of females.     

昆虫对植物次生物质的代谢适应机制及其对昆虫抗药性的意义

植物次生物质(plant secondary metabolites)对昆虫的取食行为、生长发育及繁殖可以产生不利影响,甚至对昆虫可以产生毒杀作用。为了应对植物次生物质的不利影响,昆虫通过对植物次生物质忌避取食、解毒代谢等多种机制,而对寄主植物产生适应性。其中,昆虫的解毒代谢酶包括昆虫细胞色素P450酶系（P450s）及谷胱甘肽硫转移酶（GSTs）等,在昆虫对植物次生物质的解毒代谢及对寄主植物的适应性中发挥了重要作用。昆虫的解毒酶系统不仅可以代谢植物次生物质,还可能代谢化学杀虫剂,因而昆虫对寄主植物的适应性与其对杀虫剂的耐药性甚至抗药性密切相关。昆虫细胞色素P450s和GSTs等代谢解毒酶活性及相关基因的表达可以被植物次生物质影响,这不仅使昆虫对寄主植物的防御产生了适应性,还影响了昆虫对杀虫剂的解毒代谢,因而改变昆虫的耐药性或抗药性。掌握昆虫对植物次生物质的代谢适应机制及其在昆虫抗药性中的作用,对于明确昆虫的抗药性机制具有重要的参考意义。本文综述了植物次生物质对昆虫的影响、昆虫对寄主植物次生物质的代谢机制、昆虫对植物次生物质的代谢适应性对昆虫耐药性及抗药性的影响等方面的研究进展。     

Microbial degradation of plant toxins

Plants produce a variety of secondary metabolites in response to biotic and abiotic stresses. Although they have many functions, a subclass of toxic secondary metabolites mainly serve plants as deterring agents against herbivores, insects, or pathogens. Microorganisms present in divergent ecological niches, such as soil, water, or insect and rumen gut systems have been found capable of detoxifying these metabolites. As a result of detoxification, microbes gain growth nutrients and benefit their herbivory host via detoxifying symbiosis. Here, we review current knowledge on microbial degradation of toxic alkaloids, glucosinolates, terpenes, and polyphenols with an emphasis on the genes and enzymes involved in breakdown pathways. We highlight that the insect-associated microbes might find application in biotechnology and become targets for an alternative microbial pest control strategy.     

Gut microbes of mammalian herbivores facilitate intake of plant toxins

The foraging ecology of mammalian herbivores is strongly shaped by plant secondary compounds (PSCs) that defend plants against herbivory. Conventional wisdom holds that gut microbes facilitate the ingestion of toxic plants; however, this notion lacks empirical evidence. We investigated the gut microbiota of desert woodrats (Neotoma lepida), some populations of which specialise on highly toxic creosote bush (Larrea tridentata). Here, we demonstrate that gut microbes are crucial in allowing herbivores to consume toxic plants. Creosote toxins altered the population structure of the gut microbiome to facilitate an increase in abundance of genes that metabolise toxic compounds. In addition, woodrats were unable to consume creosote toxins after the microbiota was disrupted with antibiotics. Last, ingestion of toxins by naïve hosts was increased through microbial transplants from experienced donors. These results demonstrate that microbes can enhance the ability of hosts to consume PSCs and therefore expand the dietary niche breadth of mammalian herbivores.     

A plant pathogen modulates the effects of secondary metabolites on the performance and immune function of an insect herbivore

Host plant chemical composition critically shapes the performance of insect herbivores feeding on them. Some insects have become specialized on plant secondary metabolites, and even use them to their own advantage such as defense against predators. However, infection by plant pathogens can seriously alter the interaction between herbivores and their host plants. We tested whether the effects of the plant secondary metabolites, iridoid glycosides (IGs), on the performance and immune response of an insect herbivore are modulated by a plant pathogen. We used the IG‐specialized Glanville fritillary butterfly Melitaea cinxia, its host plant Plantago lanceolata, and the naturally occurring plant pathogen, powdery mildew Podosphaera plantaginis, as model system. Pre‐diapause larvae were fed on P. lanceolata host plants selected to contain either high or low IGs, in the presence or absence of powdery mildew. Larval performance was measured by growth rate, survival until diapause, and by investment in immunity. We assessed immunity after a bacterial challenge in terms of phenoloxidase (PO) activity and the expression of seven pre‐selected insect immune genes (qPCR). We found that the beneficial effects of constitutive leaf IGs, that improved larval growth, were significantly reduced by mildew infection. Moreover, mildew presence downregulated one component of larval immune response (PO activity), suggesting a physiological cost of investment in immunity under suboptimal conditions. Yet, feeding on mildew‐infected leaves caused an upregulation of two immune genes, lysozyme and prophenoloxidase. Our findings indicate that a plant pathogen can significantly modulate the effects of secondary metabolites on the growth of an insect herbivore. Furthermore, we show that a plant pathogen can induce contrasting effects on insect immune function. We suspect that the activation of the immune system toward a plant pathogen infection may be maladaptive, but the actual infectivity on the larvae should be tested.     

Proteomics in Myzus persicae: effect of aphid host plant switch

Chemical ecology is the study of how particular chemicals are involved in interactions of organisms with each other and with their surroundings. In order to reduce insect attack, plants have evolved a variety of defence mechanisms, both constitutive and inducible, while insects have evolved strategies to overcome these plant defences (such as detoxification enzymes). A major determinant of the influence of evolutionary arms races is the strategy of the insect: generalist insect herbivores, such as Myzus persicae aphid, need more complex adaptive mechanisms since they need to respond to a large array of different plant defensive chemicals. Here we studied the chemical ecology of M. persicae associated with different plant species, from Brassicaceae and Solanaceae families. To identify the involved adaptation systems to cope with the plant secondary substances and to assess the differential expression of these systems, a proteomic approach was developed. A non-restrictive approach was developed to identify all the potential adaptation systems toward the secondary metabolites from host plants. The complex protein mixtures were separated by two-dimensional electrophoresis methods and the related spots of proteins significantly varying were selected and identified by mass spectrometry (ESI MS/MS) coupled with data bank investigations. Fourteen aphid proteins were found to vary according to host plant switch; ten of them were down regulated (proteins involved in glycolysis, TCA cycle, protein and lipid synthesis) while four others were overexpressed (mainly related to the cytoskeleton). These techniques are very reliable to describe the proteome from organisms such as insects in response to particular environmental change such as host plant species of herbivores.     

How do herbivorous insects cope with noxious secondary plant compounds in their diet?

Herbivorous insects use a variety of physiological mechanisms to cope with noxious (i.e., unpalatable and/or toxic) compounds in their food plants. Here, I review what is known about this coping process, focusing on one species of caterpillar, the tobacco hornworm (Manduca sexta). Herbivorous insects possess both preingestive (i.e., chemosensory) and postingestive response mechanisms for detecting plant secondary compounds. Stimulation of either class of detection mechanism inhibits feeding rapidly by reducing biting rate and/or bite size. This aversive response is highly adaptive during encounters with secondary plant compounds that are toxic. The insect's dilemma is that many harmless or mildly toxic compounds also activate the aversive response. To overcome this dilemma, herbivorous insects employ at least three mechanisms for selectively deactivating their aversive response to relatively harmless secondary plant compounds: (1) the presence of carbohydrates can mask the unpalatable taste of some secondary plant compounds; (2) prolonged dietary exposure to some unpalatable secondary plant compounds can initiate long-term adaptation mechanisms in the peripheral and central gustatory system; and (3) dietary exposure to toxic compounds can induce production of P450 detoxication enzymes. Thus, herbivorous insects utilize an integrated suite of physiological mechanisms to detect potentially toxic compounds in foods, and then selectively adapt to those that do not pose a serious threat to their growth and survivorship.     

Na+/K+-ATPase resistance and cardenolide sequestration: basal adaptations to host plant toxins in the milkweed bugs (Hemiptera: Lygaeidae: Lygaeinae)

Despite sequestration of toxins being a common coevolutionary response to plant defence in phytophagous insects, the macroevolution of the traits involved is largely unaddressed. Using a phylogenetic approach comprising species from four continents, we analysed the ability to sequester toxic cardenolides in the hemipteran subfamily Lygaeinae, which is widely associated with cardenolide-producing Apocynaceae. In addition, we analysed cardenolide resistance of their Na⁺/K⁺-ATPases, the molecular target of cardenolides. Our data indicate that cardenolide sequestration and cardenolide-resistant Na⁺/K⁺-ATPase are basal adaptations in the Lygaeinae. In two species that shifted to non-apocynaceous hosts, the ability to sequester was secondarily reduced, yet Na⁺/K⁺-ATPase resistance was maintained. We suggest that both traits evolved together and represent major coevolutionary adaptations responsible for the evolutionary success of lygaeine bugs. Moreover, specialization on cardenolides was not an evolutionary dead end, but enabled this insect lineage to host shift to cardenolide-producing plants from distantly related families.     

Cyanogenic glucosides in the biological warfare between plants and insects: the Burnet moth-Birdsfoot trefoil model system

Cyanogenic glucosides are important components of plant defense against generalist herbivores due to their bitter taste and the release of toxic hydrogen cyanide upon tissue disruption. Some specialized herbivores, especially insects, preferentially feed on cyanogenic plants. Such herbivores have acquired the ability to metabolize cyanogenic glucosides or to sequester them for use in their own predator defense. Burnet moths (Zygaena) sequester the cyanogenic glucosides linamarin and lotaustralin from their food plants (Fabaceae) and, in parallel, are able to carry out de novo synthesis of the very same compounds. The ratio and content of cyanogenic glucosides is tightly regulated in the different stages of the Zygaena filipendulae lifecycle and the compounds play several important roles in addition to defense. The transfer of a nuptial gift of cyanogenic glucosides during mating of Zygaena has been demonstrated as well as the possible involvement of hydrogen cyanide in male assessment and nitrogen metabolism. As the capacity to de novo synthesize cyanogenic glucosides was developed independently in plants and insects, the great similarities of the pathways between the two kingdoms indicate that cyanogenic glucosides are produced according to a universal route providing recruitment of the enzymes required. Pyrosequencing of Z. filipendulae larvae de novo synthesizing cyanogenic glucosides served to provide a set of good candidate genes, and demonstrated that the genes encoding the pathway in plants and Z. filipendulae are not closely related phylogenetically. Identification of insect genes involved in the biosynthesis and turn-over of cyanogenic glucosides will provide new insights into biological warfare as a determinant of co-evolution between plants and insects.     

Effects of class-specific,synthetic, and natural proteinase inhibitors on life-history traits of the cotton bollworm Helicoverpa armigera

Herbivorous insects have more difficulty obtaining proteins from their food than do predators and parasites. The scarcity of proteins in their diet requires herbivores to feed voraciously, thus heavily damaging their host plants. Plants respond to herbivory by producing defense compounds, which reduce insect growth, retard development, and increase mortality. Herbivores use both pre- and postdigestive response mechanisms to detect and avoid plant defense compounds. Proteinase inhibitors (PIs) are one example of plant compounds produced as a direct defense against herbivory. Many insects can adapt to PIs when these are incorporated into artificial diets. However, little is known about the effect of PIs on diet choice and feeding behavior. We monitored the diet choice, life-history traits, and gut proteinase activity of Helicoverpa armigera larvae using diets supplemented with synthetic and natural PIs. In choice experiments, both neonates and fourth-instar larvae preferred the control diet over PI-supplemented diets, to varying degrees. Larvae that fed on PI-supplemented diets weighed less than those that fed on the control diet and produced smaller pupae. Trypsin-specific PIs had a stronger effect on mean larval weight than did other PIs. A reduction of trypsin activity but not of chymotrypsin activity was observed in larvae fed on PI-supplemented diets. Therefore, behavioral avoidance of feeding on plant parts high in PIs could be an adaptation to minimize the impact of this plant's defensive strategy.     

Toxic cardenolides: chemical ecology and coevolution of specialized plant-herbivore interactions

Cardenolides are remarkable steroidal toxins that have become model systems, critical in the development of theories for chemical ecology and coevolution. Because cardenolides inhibit the ubiquitous and essential animal enzyme Na?/K?-ATPase, most insects that feed on cardenolide-containing plants are highly specialized. With a huge diversity of chemical forms, these secondary metabolites are sporadically distributed across 12 botanical families, but dominate the Apocynaceae where they are found in > 30 genera. Studies over the past decade have demonstrated patterns in the distribution of cardenolides among plant organs, including all tissue types, and across broad geographic gradients within and across species. Cardenolide production has a genetic basis and is subject to natural selection by herbivores. In addition, there is strong evidence for phenotypic plasticity, with the biotic and abiotic environment predictably impacting cardenolide production. Mounting evidence indicates a high degree of specificity in herbivore-induced cardenolides in Asclepias. While herbivores of cardenolide-containing plants often sequester the toxins, are aposematic, and possess several physiological adaptations (including target site insensitivity), there is strong evidence that these specialists are nonetheless negatively impacted by cardenolides. While reviewing both the mechanisms and evolutionary ecology of cardenolide-mediated interactions, we advance novel hypotheses and suggest directions for future work.     

Insect-resistant transgenic plants in a multi-trophic context

So far, genetic engineering of plants in the context of insect pest control has involved insertion of genes that code for toxins, and may be characterized as the incorporation of biopesticides into classical plant breeding. In the context of pesticide usage in pest control, natural enemies of herbivores have received increasing attention, because carnivorous arthropods are an important component of insect pest control. However, in plant breeding programmes, natural enemies of herbivores have largely been ignored, although there are many examples that show that plant breeding affects the effectiveness of biological control. Negative influences of modified plant characteristics on carnivorous arthropods may induce population growth of new, even more harmful pest species that had no pest status prior to the pesticide treatment. Sustainable pest management will only be possible when negative effects on non-target, beneficial arthropods are minimized. In this review, we summarize the effects of insect-resistant crops and insect-resistant transgenic crops, especially Bt crops, from a food web perspective. As food web components, we distinguish target herbivores, non-target herbivores, pollinators, parasitoids and predators. Below-ground organisms such as Collembola, nematodes and earthworms should also be included in risk assessment studies, but have received little attention. The toxins produced in Bt plants retain their toxicity when bound to the soil, so accumulation of these toxins is likely to occur. Earthworms ingest the bound toxins but are not affected by them. However, earthworms may function as intermediaries through which the toxins are passed on to other trophic levels. In studies where effects of insect-resistant (Bt) plants on natural enemies were considered, positive, negative and no effects have been found. So far, most studies have concentrated on natural enemies of target herbivores. However, Bt toxins are structurally rearranged when they bind to midgut receptors, so that they are likely to lose their toxicity inside target herbivores. What happens to the toxins in non-target herbivores, and whether these herbivores may act as intermediaries through which the toxins may be passed on to the natural enemies, remains to be studied.