The multiple strategies of an insect herbivore to overcome plant cyanogenic glucoside defence

Plant Cyanogenic-Derived Metabolites and Herbivore Counter-Defences

The release of cyanide from cyanogenic precursors is the central core of the plant defences based on the cyanogenesis process. Although cyanide is formed as a coproduct of some metabolic routes, its production is mostly due to the degradation of cyanohydrins originating from cyanogenic glycosides in cyanogenic plants and the 4-OH-ICN route in Brassicaceae. Cyanohydrins are then hydrolysed in a reversible reaction generating cyanide, being both, cyanohydrins and cyanide, toxic compounds with potential defensive properties against pests and pathogens. Based on the production of cyanogenic-derived molecules in response to the damage caused by herbivore infestation, in this review, we compile the actual knowledge of plant cyanogenic events in the plant-pest context. Besides the defensive potential, the mode of action, and the targets of the cyanogenic compounds to combat phytophagous insects and acari, special attention has been paid to arthropod responses and the strategies to overcome the impact of cyanogenesis. Physiological and behavioural adaptations, as well as cyanide detoxification by β-cyanoalanine synthases, rhodaneses, and cyanases are common ways of phytophagous arthropods defences against the cyanide produced by plants. Much experimental work is needed to further understand the complexities and specificities of the defence-counter-defence system to be applied in breeding programs.

Development of a soybean leaf disc assay for determining oral insecticidal activity in the lepidopteran agricultural pest Helicoverpa armigera

Pest insects pose a heavy burden on global agricultural industries with small molecule insecticides being predominantly used for their control. Unwanted side effects and resistance development plagues most small molecule insecticides such as the neonicotinoids, which have been reported to be harmful to honeybees. Bioinsecticides like Bacillus thuringiensis (Bt) toxins can be used as environmentally-friendly alternatives. Arachnid venoms comprise another promising source of bioinsecticides, containing a multitude of selective and potent insecticidal toxins. Unfortunately, no standardised insect models are currently available to assess the suitability of insecticidal agents under laboratory conditions. Thus, we aimed to develop a laboratory model that closely mimics field conditions by employing a leaf disk assay (LDA) for oral application of insecticidal agents in a bioassay tray format. Neonate larvae of the cotton bollworm (Helicoverpa armigera) were fed with soybean (Glycine max) leaves that were treated with different insecticidal agents. We observed dose-dependent insecticidal effects for Bt toxin and the neonicotinoid insecticide imidacloprid, with imidacloprid exhibiting a faster response. Furthermore, we identified several insecticidal arachnid venoms that were active when co-applied with sub-lethal doses of Bt toxin. We propose the H. armigera LDA as a suitable tool for assessing the insecticidal effects of insecticidal agents against lepidopterans.

Primary and Secondary Metabolites in Lotus japonicus

Lotus japonicus is a leguminous model plant used to gain insight into plant physiology, stress response, and especially symbiotic plant-microbe interactions, such as root nodule symbiosis or arbuscular mycorrhiza. Responses to changing environmental conditions, stress, microbes, or insect pests are generally accompanied by changes in primary and secondary metabolism to account for physiological needs or to produce defensive or signaling compounds. Here we provide an overview of the primary and secondary metabolites identified in L. japonicus to date. Identification of the metabolites is mainly based on mass spectral tags (MSTs) obtained by gas chromatography linked with tandem mass spectrometry (GC-MS/MS) or liquid chromatography-MS/MS (LC-MS/MS). These MSTs contain retention index and mass spectral information, which are compared to databases with MSTs of authentic standards. More than 600 metabolites are grouped into compound classes such as polyphenols, carbohydrates, organic acids and phosphates, lipids, amino acids, nitrogenous compounds, phytohormones, and additional defense compounds. Their physiological effects are briefly discussed, and the detection methods are explained. This review of the exisiting literature on L. japonicus metabolites provides a valuable basis for future metabolomics studies.

HCN-induced embryo arrest: passion fruit as an ecological trap for fruit flies

BACKGROUND

Fruit flies are important economic pests of fruits, vegetables, and nuts all over the world. In this study, a permanent ecological trap, which was created by the ovicidal effect of phytogenic hydrogen cyanide (HCN) liberated from passion fruits due to oviposition by fruit flies and can be used in the pest management, were determined.

RESULTS

Observation of fruit fly eggs in Passiflora within the passion fruit cultivation region in southern China, from Aug 2019 to Oct 2020 showed that the exotic Passiflora attracted the native fruit flies to oviposit, but the eggs could not hatch. Using classical staging to categorize embryonic development and fumigation assays, we show that oviposition by fruit fly on passion fruits, release HCN from the cyanogenic mesocarp. Exposure of the eggs to HCN causes arrest of embryonic development and finally the death of eggs.

CONCLUSION

Our results reveal that the life cycle of fruit fly in Passiflora is interrupted at the egg stage. Consequently, we predict that this ecological trap may be permanent. Extensive cultivation of the Passiflora vine as a dead-end trap crop may be an effective avenue to reduce populations of fruit fly pests. © 2023 Society of Chemical Industry.

Dhurrin: A naturally occurring phytochemical as a weapon against insect herbivores

Dhurrin, a cyanogenic glucoside, is a plant defensive chemical synthesized from aliphatic amino acids and consists of β-d-glucopyranose linked to α-hydroxy nitrile. It is catabolized by the consecutive action of hydroxynitrilase and β-glucosidase to release hydrogen cyanide on tissue disruption during herbivory. These phytoanticipins are widely distributed across various monocot and dicot plants such as Sorghum, Macadamia, Ostrya sp., and many other plant species with ornamental, pharmaceutical, medicinal, and food value. Although repellent properties of dhurrin against herbivores are often reported, less is known about its distribution, metabolism, mode of action against insects, and application for pest control. Herein, we highlight recent updates on dhurrin distribution, biosynthesis, and catabolism along with the cyanide detoxification pathway. Additionally, this article focuses on biological activities of dhurrin against various herbivores and opportunities to explore the utilization of dhurrin as a natural pest control agent and a substitute for chemically synthesized pesticides.

UHPLC-MS/MS Analysis of the Accumulation and Excretion of Steroidal Glycoalkaloids Consumed by Potato Tuber Moth (Phthorimaea operculella) Larvae under Different Feeding Treatments

Food poisoning caused by potato glycoside alkaloids (SGA) remains a critical factor that affects potato production safety. The potato tuber moth (Phthorimaea operculella) is a notorious pest that displays good adaptability to SGA in potato tissues. Studies that explore the mechanisms underlying SGA homeostasis in potato tuber moth larvae are urgently needed. In this study, ultra-high-performance liquid chromatography (UHPLC)-triple quadrupole mass spectrometry (MS/MS) was applied to detect the dominant SGA substances α-solanine and α-chaconine in potato leaves and PTM larval tissues. From the highest to lowest SGA concentrations, the potato cultivars studied were ranked as follows: DS47, LS6, DS23 and QS9. To exclude the influence of nutrients within different potato varieties, different SGA containing (0%, 0.1%, 0.2%, 0.3% and 0.4%) the artificial diet treatment groups were added. DS47 and 0.3% SGA-containing artificial diets presented the best conditions for PTM growth, development and reproduction compared to other potato cultivars and artificial diet controls. The potato tuber moth larva tissues were dissected and the SGA content within different tissues were detected using an UHPLC machine. The results showed that α-chaconine was dispersed in the feces, midgut, hindgut, head and cuticle, and α-solanine was distributed only in the feces and midgut. Antibiotic-treated insects exhibited higher concentrations of SGA than the normal microbiome group. Furthermore, the SGA concentrations of 100 newly-hatched PTM larvae and puparia were detected, with both of them found to contain small amounts of SGA. The results showed that ecdysis and the excretion process were effective approaches used by the potato tuber moth to equilibrate internal SGA accumulation. The microorganism-decreased SGA concentrations were excited in their gut. SGA may transfer from adults to the next generation, and SGAs in PTM are inheritable. In this study, we demonstrated that the potato tuber moth possessed an effective method to preliminarily decrease high SGA accumulation in potato.

Plant Specialised Glycosides (PSGs): their biosynthetic enzymatic machinery, physiological functions and commercial potential

Plants, the primary producers of our planet, have evolved from simple aquatic life to very complex terrestrial habitat. This habitat transition coincides with evolution of enormous chemical diversity, collectively termed as 'Plant Specialised Metabolisms (PSMs)', to cope the environmental challenges. Plant glycosylation is an important process of metabolic diversification of PSMs to govern their in planta stability, solubility and inter/intra-cellular transport. Although, individual category of PSMs (terpenoids, phenylpropanoids, flavonoids, saponins, alkaloids, phytohormones, glucosinolates and cyanogenic glycosides) have been well studied; nevertheless, deeper insights of physiological functioning and genomic aspects of plant glycosylation/deglycosylation processes including enzymatic machinery (CYPs, GTs, and GHs) and regulatory elements are still elusive. Therefore, this review discussed the paradigm shift on genomic background of enzymatic machinery, transporters and regulatory mechanism of 'Plant Specialised Glycosides (PSGs)'. Current efforts also update the fundamental understanding about physiological, evolutionary and adaptive role of glycosylation/deglycosylation processes during the metabolic diversification of PSGs. Additionally, futuristic considerations and recommendations for employing integrated next-generation multi-omics (genomics, transcriptomics, proteomics and metabolomics), including gene/genome editing (CRISPR-Cas) approaches are also proposed to explore commercial potential of PSGs.

Development of plant systemic resistance by beneficial rhizobacteria: Recognition, initiation, elicitation and regulation

A plant growing in nature is not an individual, but it holds an intricate community of plants and microbes with relatively stable partnerships. The microbial community has recently been demonstrated to be closely linked with plants since their earliest evolution, to help early land plants adapt to environmental threats. Mounting evidence has indicated that plants can release diverse kinds of signal molecules to attract beneficial bacteria for mediating the activities of their genetics and biochemistry. Several rhizobacterial strains can promote plant growth and enhance the ability of plants to withstand pathogenic attacks causing various diseases and loss in crop productivity. Beneficial rhizobacteria are generally called as plant growth-promoting rhizobacteria (PGPR) that induce systemic resistance (ISR) against pathogen infection. These ISR-eliciting microbes can mediate the morphological, physiological and molecular responses of plants. In the last decade, the mechanisms of microbial signals, plant receptors, and hormone signaling pathways involved in the process of PGPR-induced ISR in plants have been well investigated. In this review, plant recognition, microbial elicitors, and the related pathways during plant-microbe interactions are discussed, with highlights on the roles of root hair-specific syntaxins and small RNAs in the regulation of the PGPR-induced ISR in plants.

Development of a polyphagous leaf beetle on different host plant species and its detoxification of glucosinolates

Herbivores face a broad range of defences when feeding on plants. By mixing diets, polyphagous herbivores are assumed to benefit during their development by gaining a better nutritional balance and reducing the intake of toxic compounds from individual plant species. Nevertheless, they also show strategies to metabolically cope with plant defences. In this study, we investigated the development of the polyphagous tansy leaf beetle, Galeruca tanaceti (Coleoptera: Chrysomelidae), on mono diets consisting of one plant species [cabbage (Brassica rapa), Brassicaceae; lettuce (Lactuca sativa), or tansy (Tanacetum vulgare), Asteraceae] vs. two mixed diets, both containing tansy. Leaves of the three species were analysed for contents of water, carbon and nitrogen, the specific leaf area (SLA) and trichome density. Furthermore, we studied the insect metabolism of two glucosinolates, characteristic defences of Brassicaceae. Individuals reared on cabbage mono diet developed fastest and showed the highest survival, while the development was slowest for individuals kept on tansy mono diet. Cabbage had the lowest water content, while tansy had the highest water content, C/N ratio and trichome density and the lowest SLA. Lettuce showed the lowest C/N ratio, highest SLA and no trichomes. Analysis of insect samples with UHPLC-DAD-QTOF-MS/MS revealed that benzyl glucosinolate was metabolised to N-benzoylglycine, N-benzoylalanine and N-benzoylserine. MALDI-Orbitrap-MS imaging revealed the localisation of these metabolites in the larval hindgut region. 4-Hydroxybenzyl glucosinolate was metabolised to N-(4-hydroxybenzoyl)glycine. Our results highlight that G. tanaceti deals with toxic hydrolysis products of glucosinolates by conjugation with different amino acids, which may enable this species to develop well on cabbage. The high trichome density and/or specific plant chemistry may lower the accessibility and/or digestibility of tansy leaves, leading to a poorer beetle development on pure tansy diet or diet mixes containing tansy. Thus, diet mixing is not necessarily beneficial, if one of the plant species is strongly defended.

β-Cyanoalanine synthase protects mites against Arabidopsis defenses

Glucosinolates are antiherbivory chemical defense compounds in Arabidopsis (Arabidopsis thaliana). Specialist herbivores that feed on brassicaceous plants have evolved various mechanisms aimed at preventing the formation of toxic isothiocyanates. In contrast, generalist herbivores typically detoxify isothiocyanates through glutathione conjugation upon exposure. Here, we examined the response of an extreme generalist herbivore, the two-spotted spider mite Tetranychus urticae (Koch), to indole glucosinolates. Tetranychus urticae is a composite generalist whose individual populations have a restricted host range but have an ability to rapidly adapt to initially unfavorable plant hosts. Through comparative transcriptomic analysis of mite populations that have differential susceptibilities to Arabidopsis defenses, we identified β-cyanoalanine synthase of T. urticae (TuCAS), which encodes an enzyme with dual cysteine and β-cyanoalanine synthase activities. We combined Arabidopsis genetics, chemical complementation and mite reverse genetics to show that TuCAS is required for mite adaptation to Arabidopsis through its β-cyanoalanine synthase activity. Consistent with the β-cyanoalanine synthase role in detoxification of hydrogen cyanide (HCN), we discovered that upon mite herbivory, Arabidopsis plants release HCN. We further demonstrated that indole glucosinolates are sufficient for cyanide formation. Overall, our study uncovered Arabidopsis defenses that rely on indole glucosinolate-dependent cyanide for protection against mite herbivory. In response, Arabidopsis-adapted mites utilize the β-cyanoalanine synthase activity of TuCAS to counter cyanide toxicity, highlighting the mite's ability to activate resistant traits that enable this extreme polyphagous herbivore to exploit cyanogenic host plants.

Hydroxynitrile lyase defends Arabidopsis against Tetranychus urticae

Plant-pest interactions involve multifaceted processes encompassing a complex crosstalk of pathways, molecules, and regulators aimed at overcoming defenses developed by each interacting organism. Among plant defensive compounds against phytophagous arthropods, cyanide-derived products are toxic molecules that directly target pest physiology. Here, we identified the Arabidopsis (Arabidopsis thaliana) gene encoding hydroxynitrile lyase (AtHNL, At5g10300) as one gene induced in response to spider mite (Tetranychus urticae) infestation. AtHNL catalyzes the reversible interconversion between cyanohydrins and derived carbonyl compounds with free cyanide. AtHNL loss- and gain-of-function Arabidopsis plants showed that specific activity of AtHNL using mandelonitrile as substrate was higher in the overexpressing lines than in wild-type (WT) and mutant lines. Concomitantly, mandelonitrile accumulated at higher levels in mutant lines than in WT plants and was significantly reduced in the AtHNL overexpressing lines. After mite infestation, mandelonitrile content increased in WT and overexpressing plants but not in mutant lines, while hydrogen cyanide (HCN) accumulated in the three infested Arabidopsis genotypes. Feeding bioassays demonstrated that the AtHNL gene participated in Arabidopsis defense against T. urticae. The reduced leaf damage detected in the AtHNL overexpressing lines reflected the mite's reduced ability to feed on leaves, which consequently restricted mite fecundity. In turn, mites upregulated TuCAS1 encoding β-cyanoalanine synthase to avoid the respiratory damage produced by HCN. This detoxification effect was functionally demonstrated by reduced mite fecundity observed when dsRNA-TuCAS-treated mites fed on WT plants and hnl1 mutant lines. These findings add more players in the Arabidopsis-T. urticae interplay to overcome mutual defenses.

Unique metabolism of different glucosinolates in larvae and adults of a leaf beetle specialised on Brassicaceae

Brassicaceae plants contain glucosinolates, which are hydrolysed by myrosinases to toxic products such as isothiocyanates and nitriles, acting as defences. Herbivores have evolved various detoxification strategies, which are reviewed here. Larvae of Phaedon cochleariae (Coleoptera: Chrysomelidae) metabolise hydrolysis products of benzenic glucosinolates by conjugation with aspartic acid. In this study, we investigated whether P. cochleariae uses the same metabolic pathway for structurally different glucosinolates, whether the metabolism differs between adults and larvae and which hydrolysis products are formed as intermediates. Feeding experiments were performed with leaves of watercress (Nasturtium officinale, Brassicaceae) and pea (Pisum sativum, non-Brassicaceae), to which glucosinolates with structurally different side chains (benzenic, indole or aliphatic) or their hydrolysis products were applied. Samples were analysed by UHPLC-QTOF-MS/MS or TD–GC–MS. The same aspartic acid conjugates as previously identified in larvae were also detected as major metabolites of benzenic glucosinolates in adults. Indol-3-ylmethyl glucosinolate was mainly metabolised to N-(1H-indol-3-ylcarbonyl) glutamic acid in adults and larvae, while the metabolism of 2-propenyl glucosinolate remains unclear. The metabolism may thus proceed primarily via isothiocyanates rather than via nitriles, while the hydrolysis occurs independently of plant myrosinases. A detoxification by conjugation with these amino acids is not yet known from other Brassicaceae-feeders.

Sequestration of Plant Defense Compounds by Insects: From Mechanisms to Insect-Plant Coevolution

Plant defense compounds play a key role in the evolution of insect-plant associations by selecting for behavioral, morphological, and physiological insect adaptations. Sequestration, the ability of herbivorous insects to accumulate plant defense compounds to gain a fitness advantage, represents a complex syndrome of adaptations that has evolved in all major lineages of herbivorous insects and involves various classes of plant defense compounds. In this article, we review progress in understanding how insects selectively accumulate plant defense metabolites and how the evolution of specific resistance mechanisms to these defense compounds enables sequestration. These mechanistic considerations are further integrated into the concept of insect-plant coevolution. Comparative genome and transcriptome analyses, combined with approaches based on analytical chemistry that are centered in phylogenetic frameworks, will help to reveal adaptations underlying the sequestration syndrome, which is essential to understanding the influence of sequestration on insect-plant coevolution.

Hijacking the Mustard-Oil Bomb: How a Glucosinolate-Sequestering Flea Beetle Copes With Plant Myrosinases

Myrosinase enzymes play a key role in the chemical defense of plants of the order Brassicales. Upon herbivory, myrosinases hydrolyze the β-S-linked glucose moiety of glucosinolates, the characteristic secondary metabolites of brassicaceous plants, which leads to the formation of different toxic hydrolysis products. The specialist flea beetle, Phyllotreta armoraciae, is capable of accumulating high levels of glucosinolates in the body and can thus at least partially avoid plant myrosinase activity. In feeding experiments with the myrosinase-deficient Arabidopsis thaliana tgg1 × tgg2 (tgg) mutant and the corresponding Arabidopsis Col-0 wild type, we investigated the influence of plant myrosinase activity on the metabolic fate of ingested glucosinolates in adult P. armoraciae beetles. Arabidopsis myrosinases hydrolyzed a fraction of ingested glucosinolates and thereby reduced the glucosinolate sequestration rate by up to 50% in adult beetles. These results show that P. armoraciae cannot fully prevent glucosinolate hydrolysis; however, the exposure of adult beetles to glucosinolate hydrolysis products had no impact on the beetle's energy budget under our experimental conditions. To understand how P. armoraciae can partially prevent glucosinolate hydrolysis, we analyzed the short-term fate of ingested glucosinolates and found them to be rapidly absorbed from the gut. In addition, we determined the fate of ingested Arabidopsis myrosinase enzymes in P. armoraciae. Although we detected Arabidopsis myrosinase protein in the feces, we found only traces of myrosinase activity, suggesting that P. armoraciae can inactivate plant myrosinases in the gut. Based on our findings, we propose that the ability to tolerate plant myrosinase activity and a fast glucosinolate uptake mechanism represent key adaptations of P. armoraciae to their brassicaceous host plants.

Ecology and evolution of cycad-feeding Lepidoptera

Cycads are an ancient group of tropical gymnosperms that are toxic to most animals - including humans - though the larvae of many moths and butterflies (order: Lepidoptera) feed on cycads with apparent immunity. These insects belong to distinct lineages with varying degrees of specialisation and diverse feeding ecologies, presenting numerous opportunities for comparative studies of chemically mediated eco-evolutionary dynamics. This review presents the first evolutionary evaluation of cycad-feeding among Lepidoptera along with a comprehensive review of their ecology. Our analysis suggests that multiple lineages have independently colonised cycads from angiosperm hosts, yet only a few clades appear to have radiated following their transitions to cycads. Defensive traits are likely important for diversification, as many cycad specialists are warningly coloured and sequester cycad toxins. The butterfly family Lycaenidae appears to be particularly predisposed to cycad-feeding and several cycadivorous lycaenids are warningly coloured and chemically defended. Cycad-herbivore interactions provide a promising but underutilised study system for investigating plant-insect coevolution, convergent and divergent adaptations, and the multi-trophic significance of defensive traits; therefore the review ends by suggesting specific research gaps that would be fruitfully addressed in Lepidoptera and other cycad-feeding insects.

Novel glucosinolate metabolism in larvae of the leaf beetle Phaedon cochleariae

Plants of the Brassicales are defended by a binary system, in which glucosinolates are degraded by myrosinases, forming toxic breakdown products such as isothiocyanates and nitriles. Various detoxification pathways and avoidance strategies have been found that allow different herbivorous insect taxa to deal with the glucosinolate-myrosinase system of their host plants. Here, we investigated how larvae of the leaf beetle species Phaedon cochleariae (Coleoptera: Chrysomelidae), a feeding specialist on Brassicaceae, cope with this binary defence. We performed feeding experiments using leaves of watercress (Nasturtium officinale, containing 2-phenylethyl glucosinolate as major glucosinolate and myrosinases) and pea (Pisum sativum, lacking glucosinolates and myrosinases), to which benzenic glucosinolates (benzyl- or 4-hydroxybenzyl glucosinolate) were applied. Performing comparative metabolomics using UHPLC-QTOF-MS/MS, N-(phenylacetyl) aspartic acid, N-(benzoyl) aspartic acid and N-(4-hydroxybenzoyl) aspartic acid were identified as major metabolites of 2-phenylethyl-, benzyl- and 4-hydroxybenzyl glucosinolate, respectively, in larvae and faeces. This suggests that larvae of P. cochleariae metabolise isothiocyanates or nitriles to aspartic acid conjugates of aromatic acids derived from the ingested benzenic glucosinolates. Myrosinase measurements revealed activity only in second-instar larvae that were fed with watercress, but not in freshly moulted and starved second-instar larvae fed with pea leaves. Our results indicate that the predicted pathway can occur independently of the presence of plant myrosinases, because the same major glucosinolate-breakdown metabolites were found in the larvae feeding on treated watercress and pea leaves. A conjugation of glucosinolate-derived compounds with aspartic acid is a novel metabolic pathway that has not been described for other herbivores.

Ontogenetic changes in the targets of natural selection in three plant defenses

The evolution of plant defenses has traditionally been studied at single plant ontogenetic stages, overlooking the fact that natural selection acts continuously on organisms along their development, and that the adaptive value of phenotypes can change along ontogeny. We exposed 20 replicated genotypes of Turnera velutina to field conditions to evaluate whether the targets of natural selection on different defenses and their adaptative value change across plant development. We found that low chemical defense was favored in seedlings, which seems to be explained by the assimilation efficiency and the ability of the specialist herbivore to sequester cyanogenic glycosides. Whereas trichome density was unfavored in juvenile plants, it increased relative plant fitness in reproductive plants. At this stage we also found a positive correlative gradient between cyanogenic potential and sugar content in extrafloral nectar. We visualize this complex multi-trait combination as an ontogenetic defensive strategy. The inclusion of whole-plant ontogeny as a key source of variation in plant defense revealed that the targets and intensity of selection change along the development of plants, indicating that the influence of natural selection cannot be inferred without the assessment of ontogenetic strategies in the expression of multiple defenses.

Glucosinolate Abundance and Composition in Brassicaceae Influence Sequestration in a Specialist Flea Beetle

The horseradish flea beetle Phyllotreta armoraciae exclusively feeds on Brassicaceae, which contain glucosinolates as characteristic defense compounds. Although glucosinolates are usually degraded by plant enzymes (myrosinases) to toxic isothiocyanates after ingestion, P. armoraciae beetles sequester glucosinolates. Between and within brassicaceous plants, the glucosinolate content and composition can differ drastically. But how do these factors influence sequestration in P. armoraciae? To address this question, we performed a five-day feeding experiment with three Arabidopsis thaliana lines that differ four-fold in glucosinolate content and the composition of aliphatic and indolic glucosinolates. We quantified the amounts of ingested, sequestered, and excreted glucosinolates, and analyzed the changes in glucosinolate levels and composition in beetles before and after feeding on Arabidopsis. P. armoraciae accumulated almost all ingested glucosinolate types. However, some glucosinolates were accumulated more efficiently than others, and selected glucosinolates were modified by the beetles. The uptake of new glucosinolates correlated with a decrease in the level of stored glucosinolates so that the total glucosinolate content remained stable at around 35 nmol/mg beetle fresh weight. Beetles excreted previously stored as well as ingested glucosinolates from Arabidopsis, which suggests that P. armoraciae regulate their endogenous glucosinolate level by excretion. The metabolic fate of ingested glucosinolates, i.e. the proportions of sequestered and excreted glucosinolates, depended on glucosinolate type, content, and composition in the food plant. Overall, P. armoraciae sequestered and excreted up to 41% and 31% of the total ingested aliphatic and indolic glucosinolates from Arabidopsis, respectively. In summary, we show that glucosinolate variability in Brassicaceae influences the composition but not the level of sequestered glucosinolates in P. armoraciae beetles.

The dynamics of cyanide defences in the life cycle of an aposematic butterfly: Biosynthesis versus sequestration

Heliconius butterflies are highly specialized in Passiflora plants, laying eggs and feeding as larvae only on them. Interestingly, both Heliconius butterflies and Passiflora plants contain cyanogenic glucosides (CNglcs). While feeding on specific Passiflora species, Heliconius melpomene larvae are able to sequester simple cyclopentenyl CNglcs, the most common CNglcs in this plant genus. Yet, aromatic, aliphatic, and modified CNglcs have been reported in Passiflora species and they were never tested for sequestration by heliconiine larvae. As other cyanogenic lepidopterans, H. melpomene also biosynthesize the aliphatic CNglcs linamarin and lotaustralin, and their toxicity does not rely exclusively on sequestration. Although the genes encoding the enzymes in the CNglc biosynthesis have not yet been biochemically characterized in butterflies, the cytochromes P450 CYP405A4, CYP405A5, CYP405A6 and CYP332A1 have been hypothesized to be involved in this pathway in H. melpomene. In this study, we determine how the CNglc composition and expression of the putative P450s involved in the biosynthesis of these compounds vary at different developmental stages of Heliconius butterflies. We also establish which kind of CNglcs H. melpomene larvae can sequester from Passiflora. By analysing the chemical composition of the haemolymph from larvae fed with different Passiflora diets, we show that H. melpomene is able to sequestered prunasin, an aromatic CNglcs, from P. platyloba. They are also able to sequester amygdalin, gynocardin, [C¹³/C¹⁴]linamarin and [C¹³/C¹⁴]lotaustralin painted on the plant leaves. The CNglc tetraphyllin B-sulphate from P. caerulea is not detected in the larval haemolymph, suggesting that such modified CNglcs cannot be sequestered by Heliconius. Although pupae and virgin adults contain dihydrogynocardin resulting from larval sequestration, this compound was metabolized during adulthood, and not used as nuptial gift or transferred to the offspring. Thus, we speculate that dihydrogynocardin is catabolized to recycle nitrogen and glucose, and/or to produce fitness signals during courtship. Mature adults have a higher concentration of CNglcs than any other developmental stages due to increased de novo biosynthesis of linamarin and lotaustralin. Accordingly, all CYP405As are expressed in adults, whereas larvae mostly express CYP405A4. Our results shed light on the importance of CNglcs for Heliconius biology and their coevolution with Passiflora.

Nitric Oxide, an Essential Intermediate in the Plant-Herbivore Interaction

Reactive nitrogen species (RNS), mainly nitric oxide (NO), are highly reactive molecules with a prominent role in plant response to numerous stresses including herbivores, although the information is still very limited. This perspective article compiles the current progress in determining the NO function, as either a signal molecule, a metabolic intermediate, or a toxic oxidative product, as well as the contribution of molecules associated with NO metabolic pathway in the generation of plant defenses against phytophagous arthropods, in particular to insects and acari.

Identification and evolution of glucosinolate sulfatases in a specialist flea beetle

Glucosinolates, a characteristic group of specialized metabolites found in Brassicales plants, are converted to toxic isothiocyanates upon herbivory. Several insect herbivores, including the cabbage stem flea beetle (Psylliodes chrysocephala), prevent glucosinolate activation by forming desulfo-glucosinolates. Here we investigated the molecular basis of glucosinolate desulfation in P. chrysocephala, an important pest of oilseed rape. Enzyme activity assays with crude beetle protein extracts revealed that glucosinolate sulfatase (GSS) activity is associated with the gut membrane and has narrow substrate specificity towards the benzenic glucosinolate sinalbin. In agreement with GSS activity localization in vivo, we identified six genes encoding arylsulfatase-like enzymes with a predicted C-terminal transmembrane domain, of which five showed GSS activity upon heterologous expression in insect cells. PcGSS1 and PcGSS2 used sinalbin and indol-3-ylmethyl glucosinolate as substrates, respectively, whereas PcGSS3, PcGSS4, and PcGSS5 showed weak activity in enzyme assays. RNAi-mediated knock-down of PcGSS1 and PcGSS2 expression in adult beetles confirmed their function in vivo. In a phylogenetic analysis of coleopteran and lepidopteran arylsulfatases, the P. chrysocephala GSSs formed a cluster within a coleopteran-specific sulfatase clade distant from the previously identified GSSs of the diamondback moth, Plutella xylostella, suggesting an independent evolution of GSS activity in ermine moths and flea beetles.

Tying pest insects in knots: the deployment of spider-venom-derived knottins as bioinsecticides

Spider venoms are complex chemical arsenals that contain a rich variety of insecticidal toxins. However, the major toxin class in many spider venoms is disulfide-rich peptides known as knottins. The knotted three-dimensional fold of these mini-proteins provides them with exceptional chemical and thermal stability as well as resistance to proteases. In contrast with other bioinsecticides, which are often slow-acting, spider knottins are fast-acting neurotoxins. In addition to being potently insecticidal, some knottins have exceptional taxonomic selectivity, being lethal to key agricultural pests but innocuous to vertebrates and beneficial insects such as bees. The intrinsic oral activity of these peptides, combined with the ability of aerosolized knottins to penetrate insect spiracles, has enabled them to be developed commercially as eco-friendly bioinsecticides. Moreover, it has been demonstrated that spider-knottin transgenes can be used to engineer faster-acting entomopathogens and insect-resistant crops. © 2019 Society of Chemical Industry.

A promiscuous beta-glucosidase is involved in benzoxazinoid deglycosylation in Lamium galeobdolon

In the plant kingdom beta-glucosidases (BGLUs) of the glycosidase hydrolase family 1 have essential function in primary metabolism and are particularly employed in secondary metabolism. They are essential for activation in two-component defence systems based on stabilisation of reactive compounds by glycosylation. Based on de novo assembly we isolated and functionally characterised BGLUs expressed in leaves of Lamium galeobdolon (LgGLUs). LgGLU1 could be assigned to hydrolysis of the benzoxazinoid GDIBOA (2,4-dihydroxy-1,4-benzoxazin-3-one glucoside). Within the Lamiaceae L. galeobdolon is distinguished by the presence GDIBOA in addition to the more common iridoid harpagide. Although LgGLU1 proved to be promiscuous with respect to accepted substrates, harpagide hydrolysis was not detected. Benzoxazinoids are characteristic defence compounds of the Poales but are also found in some unrelated dicots. The benzoxazinoid specific BGLUs have recently been identified for the grasses maize, wheat, rye and the Ranunculaceae Consolida orientalis. All enzymes share a general substrate ambiguity but differ in detailed substrate pattern. The isolation of the second dicot GDIBOA glucosidase LgGLU1 allowed it to analyse the phylogenetic relation of the distinct BGLUs also within dicots. The data revealed long periods of independent sequence evolution before speciation.

Plant Defensive β-Glucosidases Resist Digestion and Sustain Activity in the Gut of a Lepidopteran Herbivore

Two-component activated chemical defenses are a major part of many plants' strategies to disrupt herbivory. The activation step is often the β-glucosidase-catalyzed removal of a glucose moiety from a pro-toxin, leading to an unstable and toxic aglycone. While some β-glucosidases have been well studied, several aspects of their roles in vivo, such as their precise sites of enzymatic activity during and after ingestion, and the importance of particular isoforms in plant defense are still not fully understood. Here, plant defensive β-glucosidases from maize, white mustard and almonds were shown to resist digestion by larvae of the generalist lepidopteran Spodoptera littoralis, and the majority of the ingested activities toward both general and plant pro-toxic substrates was recovered in the frass. Among other proteins potentially involved in defense, we identified specific plant β-glucosidases and a maize β-glucosidase aggregating factor in frass from plant-fed insects using proteomic methods. We therefore found that, while S. littoralis larvae efficiently degraded bulk food protein during digestion, β-glucosidases were among a small number of plant defensive proteins that resist insect digestive proteolysis. These enzymes remain intact in the gut lumen and frass and can therefore further catalyze the activation of plant defenses after ingestion, especially in pH-neutral regions of the digestive system. As most of the ingested enzymatic activity persists in the frass, and only particular β-glucosidases were detected via proteomic analyses, our data support the involvement of specific isoforms (maize ZmGlu1 and S. alba MA1 myrosinase) in defense in vivo.

Targeting detoxification genes by phloem-mediated RNAi: A new approach for controlling phloem-feeding insect pests

Many phloem-feeding insects are considered severe pests of agriculture and are controlled mainly by chemical insecticides. Continued extensive use of these inputs is environmentally undesirable, and also leads to the development of insecticide resistance. Here, we used a plant-mediated RNA interference (RNAi) approach, to develop a new control strategy for phloem-feeding insects. The approach aims to silence "key" detoxification genes, involved in the insect's ability to neutralize defensive and toxic plant chemistry. We targeted a glutathione S-transferase (GST) gene, BtGSTs5, in the phloem-feeding whitefly Bemisia tabaci, a devastating global agricultural pest. We report three major findings. First, significant down regulation of the BtGSTs5 gene was obtained in the gut of B. tabaci when the insects were fed on Arabidopsis thaliana transgenic plants expressing dsRNA against BtGSTs5 under a phloem-specific promoter. This brings evidence that phloem-feeding insects can be efficiently targeted by plant-mediated RNAi. Second, in-silico and in-vitro analyses indicated that the BtGSTs5 enzyme can accept as substrates, hydrolyzed aliphatic- and indolic-glucosinolates, and produce their corresponding detoxified conjugates. Third, performance assays suggested that the BtGSTs5 gene silencing prolongs the developmental period of B. tabaci nymphs. Taken together, these findings suggest that BtGSTs5 is likely to play an important role in enabling B. tabaci to successfully feed on glucosinolate-producing plants. Targeting the gene by RNAi in Brassicaceae cropping systems, will likely not eliminate the pest populations from the fields but will significantly reduce their success over the growing season, support prominent activity of natural enemies, eventually allowing the establishment of stable and sustainable agroecosystem.

Molecular identification and characterization of rhodaneses from the insect herbivore Pieris rapae

The association of cabbage white butterflies (Pieris spec., Lepidoptera: Pieridae) with their glucosinolate-containing host plants represents a well-investigated example of the sequential evolution of plant defenses and insect herbivore counteradaptations. The defensive potential of glucosinolates, a group of amino acid-derived thioglucosides present in plants of the Brassicales order, arises mainly from their rapid breakdown upon tissue disruption resulting in formation of toxic isothiocyanates. Larvae of P. rapae are able to feed exclusively on glucosinolate-containing plants due to expression of a nitrile-specifier protein in their gut which redirects glucosinolate breakdown to the formation of nitriles. The release of equimolar amounts of cyanide upon further metabolism of the benzylglucosinolate-derived nitrile suggests that the larvae are also equipped with efficient means of cyanide detoxification such as β-cyanoalanine synthases or rhodaneses. While insect β-cyanoalanine synthases have recently been identified at the molecular level, no sequence information was available of characterized insect rhodaneses. Here, we identify and characterize two single-domain rhodaneses from P. rapae, PrTST1 and PrTST2. The enzymes differ in their kinetic properties, predicted subcellular localization and expression in P. rapae indicating different physiological roles. Phylogenetic analysis together with putative lepidopteran rhodanese sequences indicates an expansion of the rhodanese family in Pieridae.

Origin and maintenance of chemical diversity in a species-rich tropical tree lineage

Plant secondary metabolites play important ecological and evolutionary roles, most notably in the deterrence of natural enemies. The classical theory explaining the evolution of plant chemical diversity is that new defences arise through a pairwise co-evolutionary arms race between plants and their specialized natural enemies. However, plant species are bombarded by dozens of different herbivore taxa from disparate phylogenetic lineages that span a wide range of feeding strategies and have distinctive physiological constraints that interact differently with particular plant metabolites. How do plant defence chemicals evolve under such multiple and potentially contrasting selective pressures imposed by diverse herbivore communities? To tackle this question, we exhaustively characterized the chemical diversity and insect herbivore fauna from 31 sympatric species of Amazonian Protieae (Burseraceae) trees. Using a combination of phylogenetic, metabolomic and statistical learning tools, we show that secondary metabolites that were associated with repelling herbivores (1) were more frequent across the Protieae phylogeny and (2) were found in average higher abundance than other compounds. Our findings suggest that generalist herbivores can play an important role in shaping plant chemical diversity and support the hypothesis that chemical diversity can also arise from the cumulative outcome of multiple diffuse interactions.

Cyanogenesis in Arthropods: From Chemical Warfare to Nuptial Gifts

Chemical defences are key components in insect⁻plant interactions, as insects continuously learn to overcome plant defence systems by, e.g., detoxification, excretion or sequestration. Cyanogenic glucosides are natural products widespread in the plant kingdom, and also known to be present in arthropods. They are stabilised by a glucoside linkage, which is hydrolysed by the action of β-glucosidase enzymes, resulting in the release of toxic hydrogen cyanide and deterrent aldehydes or ketones. Such a binary system of components that are chemically inert when spatially separated provides an immediate defence against predators that cause tissue damage. Further roles in nitrogen metabolism and inter- and intraspecific communication has also been suggested for cyanogenic glucosides. In arthropods, cyanogenic glucosides are found in millipedes, centipedes, mites, beetles and bugs, and particularly within butterflies and moths. Cyanogenic glucosides may be even more widespread since many arthropod taxa have not yet been analysed for the presence of this class of natural products. In many instances, arthropods sequester cyanogenic glucosides or their precursors from food plants, thereby avoiding the demand for de novo biosynthesis and minimising the energy spent for defence. Nevertheless, several species of butterflies, moths and millipedes have been shown to biosynthesise cyanogenic glucosides de novo, and even more species have been hypothesised to do so. As for higher plant species, the specific steps in the pathway is catalysed by three enzymes, two cytochromes P450, a glycosyl transferase, and a general P450 oxidoreductase providing electrons to the P450s. The pathway for biosynthesis of cyanogenic glucosides in arthropods has most likely been assembled by recruitment of enzymes, which could most easily be adapted to acquire the required catalytic properties for manufacturing these compounds. The scattered phylogenetic distribution of cyanogenic glucosides in arthropods indicates that the ability to biosynthesise this class of natural products has evolved independently several times. This is corroborated by the characterised enzymes from the pathway in moths and millipedes. Since the biosynthetic pathway is hypothesised to have evolved convergently in plants as well, this would suggest that there is only one universal series of unique intermediates by which amino acids are efficiently converted into CNglcs in different Kingdoms of Life. For arthropods to handle ingestion of cyanogenic glucosides, an effective detoxification system is required. In butterflies and moths, hydrogen cyanide released from hydrolysis of cyanogenic glucosides is mainly detoxified by β-cyanoalanine synthase, while other arthropods use the enzyme rhodanese. The storage of cyanogenic glucosides and spatially separated hydrolytic enzymes (β-glucosidases and α-hydroxynitrile lyases) are important for an effective hydrogen cyanide release for defensive purposes. Accordingly, such hydrolytic enzymes are also present in many cyanogenic arthropods, and spatial separation has been shown in a few species. Although much knowledge regarding presence, biosynthesis, hydrolysis and detoxification of cyanogenic glucosides in arthropods has emerged in recent years, many exciting unanswered questions remain regarding the distribution, roles apart from defence, and convergent evolution of the metabolic pathways involved.

Honeybees Tolerate Cyanogenic Glucosides from Clover Nectar and Flowers

Honeybees (Apis mellifera) pollinate flowers and collect nectar from many important crops. White clover (Trifolium repens) is widely grown as a temperate forage crop, and requires honeybee pollination for seed set. In this study, using a quantitative LC-MS (Liquid Chromatography-Mass Spectrometry) assay, we show that the cyanogenic glucosides linamarin and lotaustralin are present in the leaves, sepals, petals, anthers, and nectar of T. repens. Cyanogenic glucosides are generally thought to be defense compounds, releasing toxic hydrogen cyanide upon degradation. However, increasing evidence indicates that plant secondary metabolites found in nectar may protect pollinators from disease or predators. In a laboratory survival study with chronic feeding of secondary metabolites, we show that honeybees can ingest the cyanogenic glucosides linamarin and amygdalin at naturally occurring concentrations with no ill effects, even though they have enzyme activity towards degradation of cyanogenic glucosides. This suggests that honeybees can ingest and tolerate cyanogenic glucosides from flower nectar. Honeybees retain only a portion of ingested cyanogenic glucosides. Whether they detoxify the rest using rhodanese or deposit them in the hive should be the focus of further research.

Plant-Herbivore Interactions: A Case of an Extreme Generalist, the Two-Spotted Spider Mite Tetranychus urticae

Plant-herbivore interactions evolved over long periods of time, resulting in an elaborate arms race between interacting species. While specialist herbivores evolved specific strategies to cope with the defenses of a limited number of hosts, our understanding of how generalist herbivores deal with the defenses of a plethora of diverse host plants is largely unknown. Understanding the interaction between a plant host and a generalist herbivore requires an understanding of the plant's mechanisms aimed at defending itself and the herbivore's mechanisms intended to counteract diverse defenses. In this review, we use the two-spotted spider mite (TSSM), Tetranychus urticae (Koch) as an example of a generalist herbivore, as this chelicerate pest has a staggering number of plant hosts. We first establish that the ability of TSSM to adapt to marginal hosts underlies its polyphagy and agricultural pest status. We then highlight our understanding of direct plant defenses against spider mite herbivory and review recent advances in uncovering mechanisms of spider mite adaptations to them. Finally, we discuss the adaptation process itself, as it allows TSSM to overcome initially effective plant defenses. A high-quality genome sequence and developing genetic tools, coupled with an ease of mite experimental selection to new hosts, make TSSM an outstanding system to study the evolution of host range, mechanisms of pest xenobiotic resistance and plant-herbivore interactions. In addition, knowledge of plant defense mechanisms that affect mite fitness are of practical importance, as it can lead to development of new control strategies against this important agricultural pest. In parallel, understanding mechanisms of mite counter adaptations to these defenses is required to maintain the efficacy of these control strategies in agricultural practices.

How Glucosinolates Affect Generalist Lepidopteran Larvae: Growth, Development and Glucosinolate Metabolism

Multiple lepidopteran larvae feed successfully on plants containing glucosinolates despite the diverse array of toxic and deterrent breakdown products, such as isothiocyanates (ITCs), formed upon plant damage. While much is known about how specialist lepidopterans metabolize and tolerate glucosinolates, there is little information about the metabolic fate of these plant defense compounds in specialized herbivores. Employing 13C- and 14C-labeled 4-methylsulfinylbutyl glucosinolate (glucoraphanin), we identified and quantified the major detoxification products of glucosinolates and ITCs in selected specialized and generalist larvae. While specialists prevented glucosinolate hydrolysis or diverted hydrolysis to form nitriles, hydrolysis in generalists proceeded to toxic ITCs, of which a portion were conjugated to glutathione. However, a large amount of ITCs remained unmodified, which may have led to the observed negative effects on growth and development. The performance of two generalist-feeding caterpillars, Spodoptera littoralis (African cotton leafworm) and Mamestra brassicae (cabbage moth) on Arabidopsis thaliana Col-0 and various glucosinolate-deficient mutants was investigated from hatching until pupation. We found that glucosinolates negatively affected larval growth and development, but not survival, with aliphatic glucosinolates having stronger effects than indolic glucosinolates, and the combination of the two glucosinolate types being even more detrimental to growth and development. Curiously, last instar larvae grew better on wild type than on non-glucosinolate-containing plant lines, but this could not be attributed to a change in detoxification rate or feeding behavior. Glucosinolates thus appear to be effective defenses against generalist lepidopteran herbivores at least during most stages of larval development. Nevertheless, the reversal of negative effects in the oldest instar is intriguing, and further investigation of this phenomenon may shed light on how generalists adjust their physiology to feed on diets with many different types of plant defense compounds.

The arms race between heliconiine butterflies and Passiflora plants - new insights on an ancient subject

Heliconiines are called passion vine butterflies because they feed exclusively on Passiflora plants during the larval stage. Many features of Passiflora and heliconiines indicate that they have radiated and speciated in association with each other, and therefore this model system was one of the first examples used to exemplify coevolution theory. Three major adaptations of Passiflora plants supported arguments in favour of their coevolution with heliconiines: unusual variation of leaf shape within the genus; the occurrence of yellow structures mimicking heliconiine eggs; and their extensive diversity of defence compounds called cyanogenic glucosides. However, the protection systems of Passiflora plants go beyond these three features. Trichomes, mimicry of pathogen infection through variegation, and production of extrafloral nectar to attract ants and other predators of their herbivores, are morphological defences reported in this plant genus. Moreover, Passiflora plants are well protected chemically, not only by cyanogenic glucosides, but also by other compounds such as alkaloids, flavonoids, saponins, tannins and phenolics. Heliconiines can synthesize cyanogenic glucosides themselves, and their ability to handle these compounds was probably one of the most crucial adaptations that allowed the ancestor of these butterflies to feed on Passiflora plants. Indeed, it has been shown that Heliconius larvae can sequester cyanogenic glucosides and alkaloids from their host plants and utilize them for their own benefit. Recently, it was discovered that Heliconius adults have highly accurate visual and chemosensory systems, and the expansion of brain structures that can process such information allows them to memorize shapes and display elaborate pre-oviposition behaviour in order to defeat visual barriers evolved by Passiflora species. Even though the heliconiine-Passiflora model system has been intensively studied, the forces driving host-plant preference in these butterflies remain unclear. New studies have shown that host-plant preference seems to be genetically controlled, but in many species there is some plasticity in this choice and preferences can even be induced. Although much knowledge regarding the coevolution of Passiflora plants and heliconiine butterflies has accumulated in recent decades, there remain many exciting unanswered questions concerning this model system.

Host plant driven transcriptome plasticity in the salivary glands of the cabbage looper (Trichoplusia ni)

Generalist herbivores feed on a wide array of plants and need to adapt to varying host qualities and defenses. One of the first insect derived secretions to come in contact with the plant is the saliva. Insect saliva is potentially involved in both the pre-digestion of the host plant as well as induction/suppression of plant defenses, yet how the salivary glands respond to changes in host plant at the transcriptional level is largely unknown. The objective of this study was to determine how the labial salivary gland transcriptome varies according to the host plant on which the insect is feeding. In order to determine this, cabbage looper (Trichoplusia ni) larvae were reared on cabbage, tomato, and pinto bean artificial diet. Labial glands were dissected from fifth instar larvae and used to extract RNA for RNASeq analysis. Assembly of the resulting sequencing reads resulted in a transcriptome library for T. ni salivary glands consisting of 14,037 expressed genes. Feeding on different host plant diets resulted in substantial remodeling of the gland transcriptomes, with 4,501 transcripts significantly differentially expressed across the three treatment groups. Gene expression profiles were most similar between cabbage and artificial diet, which corresponded to the two diets on which larvae perform best. Expression of several transcripts involved in detoxification processes were differentially expressed, and transcripts involved in the spliceosome pathway were significantly downregulated in tomato-reared larvae. Overall, this study demonstrates that the transcriptomes of the salivary glands of the cabbage looper are strongly responsive to diet. It also provides a foundation for future functional studies that can help us understand the role of saliva of chewing insects in plant-herbivore interactions.

β-Cyanoalanine Synthases and Their Possible Role in Pierid Host Plant Adaptation

Cyanide is generated in larvae of the glucosinolate-specialist Pieris rapae (Lepidoptera:Pieridae) upon ingestion of plant material containing phenylalanine-derived glucosinolates as chemical defenses. As these glucosinolates were widespread within ancient Brassicales, the ability to detoxify cyanide may therefore have been essential for the host plant shift of Pierid species from Fabales to Brassicales species giving rise to the Pierinae subfamily. Previous research identified β-cyanoalanine and thiocyanate as products of cyanide detoxification in P. rapae larvae as well as three cDNAs encoding the β-cyanoalanine synthases PrBSAS1-PrBSAS3. Here, we analyzed a total of eight species of four lepidopteran families to test if their cyanide detoxification capacity correlates with their feeding specialization. We detected β-cyanoalanine synthase activity in gut protein extracts of all six species tested, which included Pierid species with glucosinolate-containing host plants, Pierids with other hosts, and other Lepidoptera with varying food specialization. Rhodanese activity was only scarcely detectable with the highest levels appearing in the two glucosinolate-feeding Pierids. We then amplified by polymerase chain reaction (PCR) 14 cDNAs encoding β-cyanoalanine synthases from seven species. Enzyme characterization and phylogenetic analysis indicated that lepidopterans are generally equipped with one PrBSAS2 homolog with high affinity for cyanide. A second β-cyanoalanine synthase which grouped with PrBSAS3 was restricted to Pierid species, while a third variant (i.e., homologs of PrBSAS1), was only present in members of the Pierinae subfamily. These results are in agreement with the hypothesis that the host shift to Brassicales was associated with the requirement for a specialized cyanide detoxification machinery.

Spatial separation of the cyanogenic β-glucosidase ZfBGD2 and cyanogenic glucosides in the haemolymph of Zygaena larvae facilitates cyanide release

Low molecular weight compounds are typically used by insects and plants for defence against predators. They are often stored as inactive β-glucosides and kept separate from activating β-glucosidases. When the two components are mixed, the β-glucosides are hydrolysed releasing toxic aglucones. Cyanogenic plants contain cyanogenic glucosides and release hydrogen cyanide due to such a well-characterized two-component system. Some arthropods are also cyanogenic, but comparatively little is known about their system. Here, we identify a specific β-glucosidase (ZfBGD2) involved in cyanogenesis from larvae of Zygaena filipendulae (Lepidoptera, Zygaenidae), and analyse the spatial organization of cyanide release in this specialized insect. High levels of ZfBGD2 mRNA and protein were found in haemocytes by transcriptomic and proteomic profiling. Heterologous expression in insect cells showed that ZfBGD2 hydrolyses linamarin and lotaustralin, the two cyanogenic glucosides present in Z. filipendulae. Linamarin and lotaustralin as well as cyanide release were found exclusively in the haemoplasma. Phylogenetic analyses revealed that ZfBGD2 clusters with other insect β-glucosidases, and correspondingly, the ability to hydrolyse cyanogenic glucosides catalysed by a specific β-glucosidase evolved convergently in insects and plants. The spatial separation of the β-glucosidase ZfBGD2 and its cyanogenic substrates within the haemolymph provides the basis for cyanide release in Z. filipendulae. This spatial separation is similar to the compartmentalization of the two components found in cyanogenic plant species, and illustrates one similarity in cyanide-based defence in these two kingdoms of life.

A Cyanide-Induced 3-Cyanoalanine Nitrilase in the Cyanide-Assimilating Bacterium Pseudomonas pseudoalcaligenes Strain CECT 5344

Pseudomonas pseudoalcaligenes CECT 5344 is a bacterium able to assimilate cyanide as a sole nitrogen source. Under this growth condition, a 3-cyanoalanine nitrilase enzymatic activity was induced. This activity was encoded by nit4, one of the four nitrilase genes detected in the genome of this bacterium, and its expression in Escherichia coli enabled the recombinant strain to fully assimilate 3-cyanoalanine. P. pseudoalcaligenes CECT 5344 showed a weak growth level with 3-cyanoalanine as the N source, unless KCN was also added. Moreover, a nit4 knockout mutant of P. pseudoalcaligenes CECT 5344 became severely impaired in its ability to grow with 3-cyanoalanine and cyanide as nitrogen sources. The native enzyme expressed in E. coli was purified up to electrophoretic homogeneity and biochemically characterized. Nit4 seems to be specific for 3-cyanoalanine, and the amount of ammonium derived from the enzymatic activity doubled in the presence of exogenously added asparaginase activity, which demonstrated that the Nit4 enzyme had both 3-cyanoalanine nitrilase and hydratase activities. The nit4 gene is located downstream of the cyanide resistance transcriptional unit containing cio1 genes, whose expression levels are under the positive control of cyanide. Real-time PCR experiments revealed that nit4 expression was also positively regulated by cyanide in both minimal and LB media. These results suggest that this gene cluster including cio1 and nit4 could be involved both in cyanide resistance and in its assimilation by P. pseudoalcaligenes CECT 5344.IMPORTANCE Cyanide is a highly toxic molecule present in some industrial wastes due to its application in several manufacturing processes, such as gold mining and the electroplating industry. The biodegradation of cyanide from contaminated wastes could be an attractive alternative to physicochemical treatment. P. pseudoalcaligenes CECT 5344 is a bacterial strain able to assimilate cyanide under alkaline conditions, thus avoiding its volatilization as HCN. This paper describes and characterizes an enzyme (Nit4) induced by cyanide that is probably involved in cyanide assimilation. The biochemical characterization of Nit4 provides a segment for building a cyanide assimilation pathway in P. pseudoalcaligenes This information could be useful for understanding, and hopefully improving, the mechanisms involved in bacterial cyanide biodegradation and its application in the treatment of cyanide-containing wastes.

Pesticide stress on plants negatively affects parasitoid fitness through a bypass of their phytophage hosts

Pesticides taken up by plants from the soil or interstitial (pore) water can cascade to higher trophic levels, which are expected to be more affected due to cumulative bottom-up effects. Knowledge about the impact of indirect exposure to pesticides on non-target terrestrial trophic chains, however, is still lacking. Therefore, we examined the direct and indirect effects of three concentrations of the herbicide 2,6-dichlorobenzonitrile (DCBN) and an insecticide with a similar molecular structure (1,4-dichlorobenzene, DCB) on the fitness traits of a tritrophic system: the wheat plant Triticum aestivum, the aphid Sitobion avenae and its specialist parasitoid Aphidius rhopalosiphi. To mimic exposure via interstitial water the toxicants were added to the growth medium of the plant. Passive dosing between the medium and a silicon layer was used to achieve constant exposure of the poorly soluble pesticides. Wheat plants exposed to both pesticides grew smaller and had reduced biomasses. Negative effects on the reproductive rate, biomass and the number of aphids were only observable at the highest concentration of DCBN. Overall parasitism rate decreased when exposed to both pesticides and parasitoid attack rates decreased at lower concentrations of DCBN and at the highest DCB concentration. The parasitoid sex ratio was extremely male-biased in the presence of DCBN. Our results demonstrate that pesticides can alter the performance of higher trophic levels by sublethal effects, through a bypass of the second trophic level. In addition, the novel test system used was suitable for detecting such carryover effects on non-target organisms.

Plant defense and herbivore counter-defense: benzoxazinoids and insect herbivores

Benzoxazinoids are a class of indole-derived plant chemical defenses comprising compounds with a 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton and their derivatives. These phytochemicals are widespread in grasses, including important cereal crops such as maize, wheat and rye, as well as a few dicot species, and display a wide range of antifeedant, insecticidal, antimicrobial, and allelopathic activities. Although their overall effects against insect herbivores are frequently reported, much less is known about how their modes of action specifically influence insect physiology. The present review summarizes the biological activities of benzoxazinoids on chewing, piercing-sucking, and root insect herbivores. We show how within-plant distribution modulates the exposure of different herbivore feeding guilds to these defenses, and how benzoxazinoids may act as toxins, feeding deterrents and digestibility-reducing compounds under different conditions. In addition, recent results on the metabolism of benzoxazinoids by insects and their consequences for plant-herbivore interactions are addressed, as well as directions for future research.

Comparative venomics of Psyttalia lounsburyi and P. concolor, two olive fruit fly parasitoids: a hypothetical role for a GH1 β-glucosidase

Venom composition of parasitoid wasps attracts increasing interest - notably molecules ensuring parasitism success on arthropod pests - but its variation within and among taxa is not yet understood. We have identified here the main venom proteins of two braconid wasps, Psyttalia lounsburyi (two strains from South Africa and Kenya) and P. concolor, olive fruit fly parasitoids that differ in host range. Among the shared abundant proteins, we found a GH1 β-glucosidase and a family of leucine-rich repeat (LRR) proteins. Olive is extremely rich in glycoside compounds that are hydrolyzed by β-glucosidases into defensive toxic products in response to phytophagous insect attacks. Assuming that Psyttalia host larvae sequester ingested glycosides, the injected venom GH1 β-glucosidase could induce the release of toxic compounds, thus participating in parasitism success by weakening the host. Venom LRR proteins are similar to truncated Toll-like receptors and may possibly scavenge the host immunity. The abundance of one of these LRR proteins in the venom of only one of the two P. lounsburyi strains evidences intraspecific variation in venom composition. Altogether, venom intra- and inter-specific variation in Psyttalia spp. were much lower than previously reported in the Leptopilina genus (Figitidae), suggesting it might depend upon the parasitoid taxa.

Coevolution of Cyanogenic Bamboos and Bamboo Lemurs on Madagascar

Feeding strategies of specialist herbivores often originate from the coevolutionary arms race of plant defenses and counter-adaptations of herbivores. The interaction between bamboo lemurs and cyanogenic bamboos on Madagascar represents a unique system to study diffuse coevolutionary processes between mammalian herbivores and plant defenses. Bamboo lemurs have different degrees of dietary specialization while bamboos show different levels of chemical defense. In this study, we found variation in cyanogenic potential (HCNp) and nutritive characteristics among five sympatric bamboo species in the Ranomafana area, southeastern Madagascar. The HCNp ranged from 209±72 μmol cyanide*g-1 dwt in Cathariostachys madagascariensis to no cyanide in Bambusa madagascariensis. Among three sympatric bamboo lemur species, the greater bamboo lemur (Prolemur simus) has the narrowest food range as it almost exclusively feeds on the highly cyanogenic C. madagascariensis. Our data suggest that high HCNp is the derived state in bamboos. The ancestral state of lemurs is most likely "generalist" while the ancestral state of bamboo lemurs was determined as equivocal. Nevertheless, as recent bamboo lemurs comprise several "facultative specialists" and only one "obligate specialist" adaptive radiation due to increased flexibility is likely. We propose that escaping a strict food plant specialization enabled facultative specialist bamboo lemurs to inhabit diverse geographical areas.

Sequestration of plant secondary metabolites by insect herbivores: molecular mechanisms and ecological consequences

Numerous insect herbivores can take up and store plant toxins as self-defense against their own natural enemies. Plant toxin sequestration is tightly linked with tolerance strategies that keep the toxins functional. Specific transporters have been identified that likely allow the herbivore to control the spatiotemporal dynamics of toxin accumulation. Certain herbivores furthermore possess specific enzymes to boost the bioactivity of the sequestered toxins. Ecologists have studied plant toxin sequestration for decades. The recently uncovered molecular mechanisms in combination with transient, non-transgenic systems to manipulate insect gene expression will help to understand the importance of toxin sequestration for food-web dynamics in nature.

Lepidopteran defence droplets - a composite physical and chemical weapon against potential predators

Insects often release noxious substances for their defence. Larvae of Zygaena filipendulae (Lepidoptera) secrete viscous and cyanogenic glucoside-containing droplets, whose effectiveness was associated with their physical and chemical properties. The droplets glued mandibles and legs of potential predators together and immobilised them. Droplets were characterised by a matrix of an aqueous solution of glycine-rich peptides (H-WG11-NH2) with significant amounts of proteins and glucose. Among the proteins, defensive proteins such as protease inhibitors, proteases and oxidases were abundant. The neurotoxin β-cyanoalanine was also found in the droplets. Despite the presence of cyanogenic glucosides, which release toxic hydrogen cyanide after hydrolysis by a specific β-glucosidase, the only β-glucosidase identified in the droplets (ZfBGD1) was inactive against cyanogenic glucosides. Accordingly, droplets did not release hydrogen cyanide, unless they were mixed with specific β-glucosidases present in the Zygaena haemolymph. Droplets secreted onto the cuticle hardened and formed sharp crystalline-like precipitates that may act as mandible abrasives to chewing predators. Hardening followed water evaporation and formation of antiparallel β-sheets of the peptide oligomers. Consequently, after mild irritation, Zygaena larvae deter predators by viscous and hardening droplets that contain defence proteins and β-cyanoalanine. After severe injury, droplets may mix with exuding haemolymph to release hydrogen cyanide.

Plant defense and herbivore counter-defense: benzoxazinoids and insect herbivores

Benzoxazinoids are a class of indole-derived plant chemical defenses comprising compounds with a 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one skeleton and their derivatives. These phytochemicals are widespread in grasses, including important cereal crops such as maize, wheat and rye, as well as a few dicot species, and display a wide range of antifeedant, insecticidal, antimicrobial, and allelopathic activities. Although their overall effects against insect herbivores are frequently reported, much less is known about how their modes of action specifically influence insect physiology. The present review summarizes the biological activities of benzoxazinoids on chewing, piercing-sucking, and root insect herbivores. We show how within-plant distribution modulates the exposure of different herbivore feeding guilds to these defenses, and how benzoxazinoids may act as toxins, feeding deterrents and digestibility-reducing compounds under different conditions. In addition, recent results on the metabolism of benzoxazinoids by insects and their consequences for plant-herbivore interactions are addressed, as well as directions for future research.

Metabolism, excretion and avoidance of cyanogenic glucosides in insects with different feeding specialisations

Cyanogenic glucosides (CNglcs) are widespread plant defence compounds releasing toxic hydrogen cyanide when hydrolysed by specific β-glucosidases after plant tissue damage. In contrast to specialist herbivores that have mechanisms to avoid toxicity from CNglcs, it is generally assumed that non-adapted herbivores are negatively affected by CNglcs. Recent evidence, however, implies that the defence potential of CNglcs towards herbivores may not be as effective as previously anticipated. Here, performance, metabolism and excretion products of insects not adapted to CNglcs were analysed, including species with different degrees of dietary specialisation (generalists, specialists) and different feeding modes (leaf-snipping lepidopterans, piercing-sucking aphids). Insects were reared either on cyanogenic or acyanogenic plants or on an artificial cyanogenic diet. Lepidopteran generalists (Spodoptera littoralis, Spodoptera exigua, Mamestra brassicae) were compared to lepidopteran glucosinolate-specialists (Pieris rapae, Pieris brassicae, Plutella xylostella), and a generalist aphid (Myzus persicae) was compared to an aphid glucosinolate-specialist (Lipaphis erysimi). All insects were tolerant to cyanogenic plants; in lepidopterans tolerance was mainly due to excretion of intact CNglcs. The two Pieris species furthermore metabolized aromatic CNglcs to amino acid conjugates (Cys, Gly, Ser) and derivatives of these, which is similar to the metabolism of benzylglucosinolates in these species. Aphid species avoided uptake of CNglcs during feeding. Our results imply that non-adapted insects tolerate plant CNglcs either by keeping them intact for excretion, metabolizing them, or avoiding uptake.

The Cystine Knot Is Responsible for the Exceptional Stability of the Insecticidal Spider Toxin ω-Hexatoxin-Hv1a

The inhibitor cystine knot (ICK) is an unusual three-disulfide architecture in which one of the disulfide bonds bisects a loop formed by the two other disulfide bridges and the intervening sections of the protein backbone. Peptides containing an ICK motif are frequently considered to have high levels of thermal, chemical and enzymatic stability due to cross-bracing provided by the disulfide bonds. Experimental studies supporting this contention are rare, in particular for spider-venom toxins, which represent the largest diversity of ICK peptides. We used ω-hexatoxin-Hv1a (Hv1a), an insecticidal toxin from the deadly Australian funnel-web spider, as a model system to examine the contribution of the cystine knot to the stability of ICK peptides. We show that Hv1a is highly stable when subjected to temperatures up to 75 °C, pH values as low as 1, and various organic solvents. Moreover, Hv1a was highly resistant to digestion by proteinase K and when incubated in insect hemolymph and human plasma. We demonstrate that the ICK motif is essential for the remarkable stability of Hv1a, with the peptide’s stability being dramatically reduced when the disulfide bonds are eliminated. Thus, this study demonstrates that the ICK motif significantly enhances the chemical and thermal stability of spider-venom peptides and provides them with a high level of protease resistance. This study also provides guidance to the conditions under which Hv1a could be stored and deployed as a bioinsecticide.

Detoxification of hostplant's chemical defence rather than its anti-predator co-option drives β-glucosidase-mediated lepidopteran counteradaptation

The evolutionary plant-herbivore arms race sometimes gives rise to remarkably unique adaptation strategies. Here we report one such strategy in the lepidopteran herbivore Manduca sexta against its hostplant Nicotiana attenuata's major phytotoxins, 17-hydroxygeranyllinalool diterpene glycoside, lyciumoside IV and its malonylated forms. We show that alkalinity of larval regurgitant non-enzymatically demalonylates the malonylated forms to lyciumoside IV. Lyciumoside IV is then detoxified in the midgut by β-glucosidase 1-catalysed deglycosylation, which is unusual, as typically the deglycosylation of glycosylated phytochemicals by insects results in the opposite: toxin activation. Suppression of deglucosylation by silencing larval β-glucosidase 1 by plant-mediated RNAi causes moulting impairments and mortality. In the native habitat of N. attenuata, β-glucosidase 1 silencing also increases larval unpalatability to native predatory spiders, suggesting that the defensive co-option of lyciumoside IV may be ecologically advantageous. We infer that M. sexta detoxifies this allelochemical to avoid its deleterious effects, rather than co-opting it against predators.

Taste detection of the non-volatile isothiocyanate moringin results in deterrence to glucosinolate-adapted insect larvae

Isothiocyanates (ITCs), released from Brassicales plants after hydrolysis of glucosinolates, are known for their negative effects on herbivores but mechanisms have been elusive. The ITCs are initially present in dissolved form at the site of herbivore feeding, but volatile ITCs may subsequently enter the gas phase and all ITCs may react with matrix components. Deterrence to herbivores resulting from topically applied volatile ITCs in artificial feeding assays may hence lead to ambiguous conclusions. In the present study, the non-volatile ITC moringin (4-(α-L-rhamnopyranosyloxy)benzyl ITC) and its glucosinolate precursor glucomoringin were examined for effects on behaviour and taste physiology of specialist insect herbivores of Brassicales. In feeding bioassays, glucomoringin was not deterrent to larvae of Pieris napi (Lepidoptera: Pieridae) and Athalia rosae (Hymenoptera: Tenthredinidae), which are adapted to glucosinolates. Glucomoringin stimulated feeding of larvae of the related Pieris brassicae (Lepidoptera: Pieridae) and also elicited electrophysiological activity from a glucosinolate-sensitive gustatory neuron in the lateral maxillary taste sensilla. In contrast, the ITC moringin was deterrent to P. napi and P. brassicae at high levels and to A. rosae at both high and low levels when topically applied to cabbage leaf discs (either 12, 120 or 1200 nmol moringin per leaf disc of 1cm diameter). Survival of A. rosae was also significantly reduced when larvae were kept on leaves treated with moringin for several days. Furthermore, moringin elicited electrophysiological activity in a deterrent-sensitive neuron in the medial maxillary taste sensillum of P. brassicae, providing a sensory mechanism for the deterrence and the first known ITC taste response of an insect. In simulated feeding assays, recovery of moringin was high, in accordance with its non-volatile nature. Our results demonstrate taste-mediated deterrence of a non-volatile, natural ITC to glucosinolate-adapted insects.

Mechanisms and ecological consequences of plant defence induction and suppression in herbivore communities

BACKGROUND

Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

SCOPE

The objective of this review is to evaluate (1) which strategies plants have evolved to cope with herbivores and (2) which traits herbivores have evolved that enable them to counter these defences. The primary focus is on the induction and suppression of plant defences and the review outlines how the palette of traits that determine induction/suppression of, and resistance/susceptibility of herbivores to, plant defences can give rise to exploitative competition and facilitation within ecological communities "inhabiting" a plant.

CONCLUSIONS

Herbivores have evolved diverse strategies, which are not mutually exclusive, to decrease the negative effects of plant defences in order to maximize the conversion of plant material into offspring. Numerous adaptations have been found in herbivores, enabling them to dismantle or bypass defensive barriers, to avoid tissues with relatively high levels of defensive chemicals or to metabolize these chemicals once ingested. In addition, some herbivores interfere with the onset or completion of induced plant defences, resulting in the plant's resistance being partly or fully suppressed. The ability to suppress induced plant defences appears to occur across plant parasites from different kingdoms, including herbivorous arthropods, and there is remarkable diversity in suppression mechanisms. Suppression may strongly affect the structure of the food web, because the ability to suppress the activation of defences of a communal host may facilitate competitors, whereas the ability of a herbivore to cope with activated plant defences will not. Further characterization of the mechanisms and traits that give rise to suppression of plant defences will enable us to determine their role in shaping direct and indirect interactions in food webs and the extent to which these determine the coexistence and persistence of species.