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

The Plant Virus Tomato Spotted Wilt Orthotospovirus Benefits Its Vector Frankliniella occidentalis by Decreasing Plant Toxic Alkaloids in Host Plant Datura stramonium

The transmission of insect-borne viruses involves sophisticated interactions between viruses, host plants, and vectors. Chemical compounds play an important role in these interactions. Several studies reported that the plant virus tomato spotted wilt orthotospovirus (TSWV) increases host plant quality for its vector and benefits the vector thrips Frankliniella occidentalis. However, few studies have investigated the chemical ecology of thrips vectors, TSWV, and host plants. Here, we demonstrated that in TSWV-infected host plant Datura stramonium, (1) F. occidentalis were more attracted to feeding on TSWV-infected D. stramonium; (2) atropine and scopolamine, the main tropane alkaloids in D. stramonium, which are toxic to animals, were down-regulated by TSWV infection of the plant; and (3) F. occidentalis had better biological performance (prolonged adult longevity and increased fecundity, resulting in accelerated population growth) on TSWV-infected D. stramonium than on TSWV non-infected plants. These findings provide in-depth information about the physiological mechanisms responsible for the virus's benefits to its vector by virus infection of plant regulating alkaloid accumulation in the plant.

Transcriptome and metabonomics combined analysis revealed the energy supply mechanism involved in fruiting body initiation in Chinese cordyceps

Chinese cordyceps was one of most valuable traditional Chinese medicine fungi. To elucidate the molecular mechanisms related to energy supply mechanism involved in the initiation and formation of primordium in Chinese cordyceps, we performed the integrated metabolomic and transcriptomic analyses of it at pre-primordium period, primordium germination period and after-primordium period, respectively. Transcriptome analysis showed that many genes related to 'starch and sucrose metabolism', 'fructose and mannose metabolism', 'linoleic acid metabolism', 'fatty acids degradation' and 'glycerophospholipid metabolism' were highly up-regulated at primordium germination period. Metabolomic analysis showed many metabolites regulated by these genes in these metabolism pathways were also markedly accumulated at this period. Consequently, we inferred that carbohydrate metabolism and β-oxidation pathway of palmitic acid and linoleic acid worked cooperatively to generate enough acyl-CoA, and then entered TCA cycle to provide energy for fruiting body initiation. Overall, our finding provided important information for further exploring the energy metabolic mechanisms of realizing the industrialization of Chinese cordyceps artificial cultivation.

Induced Resistance Combined with RNA Interference Attenuates the Counteradaptation of the Western Flower Thrips

The western flower thrips, Frankliniella occidentalis Pergande, is an invasive pest that damages agricultural and horticultural crops. The induction of plant defenses and RNA interference (RNAi) technology are potent pest control strategies. This study investigated whether the anti-adaptive ability of F. occidentalis to jasmonic acid (JA)- and methyl jasmonate (MeJA)-induced defenses in kidney bean plants was attenuated after glutathione S-transferase (GST) gene knockdown. The expression of four GSTs in thrips fed JA- and MeJA-induced leaves was analyzed, and FoGSTd1 and FoGSTs1 were upregulated. Exogenous JA- and MeJA-induced defenses led to increases in defensive secondary metabolites (tannins, alkaloids, total phenols, flavonoids, and lignin) in leaves. Metabolome analysis indicated that the JA-induced treatment of leaves led to significant upregulation of defensive metabolites. The activity of GSTs increased in second-instar thrips larvae fed JA- and MeJA-induced leaves. Co-silencing with RNAi simultaneously knocked down FoGSTd1 and FoGSTs1 transcripts and GST activity, and the area damaged by second-instar larvae feeding on JA- and MeJA-induced leaves decreased by 62.22% and 55.24%, respectively. The pupation rate of second-instar larvae also decreased by 39.68% and 39.89%, respectively. Thus, RNAi downregulation of FoGSTd1 and FoGSTs1 reduced the anti-adaptive ability of F. occidentalis to JA- or MeJA-induced defenses in kidney bean plants.

Bioassay-directed analysis-based identification of relevant pyrrolizidine alkaloids

Pyrrolizidine alkaloids (PAs) are produced by various plant species and have been detected as contaminants in food and feed. Monitoring programmes should include PAs that are present in relevant matrices and that exhibit a high toxic potential. The aim of the present study was to use a bioassay-directed analysis approach to identify relevant PAs not yet included in monitoring programmes. To that end, extracts of Heliotropium europaeum and H. popovii were prepared and analysed with LC–MS/MS for the presence of 35 PAs included in monitoring programmes, as well as for genotoxic activity in the HepaRG/γH2AX assay. Europine, heliotrine and lasiocarpine were found to be the most abundant PAs. The extracts showed a higher γH2AX activity than related artificial mixtures of quantified known PAs, which might point to the presence of unknown toxic PAs. The H. europaeum extract was fractionated and γH2AX activities of individual fractions were determined. Fractions were further analysed applying LC–Orbitrap-MS analysis and Compound Discoverer software, identifying various candidate PAs responsible for the non-explained genotoxic activity. Altogether, the results obtained show that bioassay-directed analysis allows identification of candidate PAs that can be included in monitoring programmes.

Combined Transcriptome and Metabolome Analysis of Alfalfa Response to Thrips Infection

Thrips (Thysanoptera: Thripidae) is a major insect pest for alfalfa which can result in decreased plant nutrients, low yields, and even plant death. To identify the differentially expressed genes and metabolites in response to thrips in alfalfa, a combination of metabolomics and transcriptomics was employed using alfalfa (Caoyuan No. 2) with and without thrips infestation. The results showed that the flavonoid biosynthesis and isoflavonoid biosynthesis pathways were the most significantly enriched pathways in response to thrips infection, as shown by the combined transcriptome and metabolome analysis. The transcriptome results showed that SA and JA signal transduction and PAPM-triggered immunity and the MAPK signaling pathway–plant pathways played a crucial role in thrips-induced plant resistance in alfalfa. In addition, we found that thrips infestation could also induce numerous changes in plant primary metabolism, such as carbohydrate and amino acid metabolism as compared to the control. Overall, our results described here should improve fundamental knowledge of molecular responses to herbivore-inducible plant defenses and contribute to the design of strategies against thrips in alfalfa.

Seasonal variation in defence compounds: A case study on pyrrolizidine alkaloids of clones of Jacobaea vulgaris, Jacobaea aquatica and their hybrids

Concentration of plant secondary metabolites (SMs) show seasonal variations. However, it is still not well understood how these abiotic and biotic factors influence the seasonal variations of SMs. In addition, it is of interest to know if and how SMs are reallocated to the different plant organs, in particular whether SMs are reallocated to the remaining tissues when biomass is lost, e.g., during winter. Here we used Jacobaea vulgaris, Jacobaea aquatica, two F1 and four F2 hybrids that differed in their pyrrolizidine alkaloids (PAs) bouquet as a study system. A series of clones of these genotypes were investigated during their vegetative stage spanning 14 months in a semi-natural environment. We found that the total PA concentration in roots and shoots showed a gradual increase until the spring of the second year, whereafter it dropped substantially in shoots. The variation in PA composition due to seasonal changes was significant but relatively small. Senecionine-like PAs were the dominant PAs in roots, while jacobine-/erucifoline-like PAs were dominant in shoots. The variation of PA concentration was significantly correlated with temperature, day length, and plant age. A correlation analysis showed that PAs were not reallocated when biomass was lost in winter. Overall, our study showed that PA composition of each genotype changed over seasons in a different manner but seasonal variation did not overrule the differences in PA composition among genotypes.

Chemical Differentiation of Plant Latexes and Their Anti-herbivory Activity against Thrips Frankliniella occidentalis

Despite the extensive studies on latex, some fundamental questions on their chemical specialization and the factors influencing this specialization have yet to be investigated. To address this issue, latexes and their bearing tissues from diverse species were profiled by ¹HNMR and GC-MS. Additionally, the antiherbivory activity of these materials was tested against thrips (Frankliniella occidentalis Pergande, 1895). The multivariate data analysis showed a clear separation between latexes and leaves from the same species. Conversely, the chemical profiles of latexes from different species were highly similar, that is, they displayed much less metabolic species-specificity. These shared chemical profiles of latexes were reflected in their overall higher mortality index (80.4% ± 7.5) against thrips compared with their bearing tissues (55.5% ± 14.9). The metabolites correlated to the antiherbivory activity of latexes were triterpenoids and steroids. However, the activity could not be attributed to any single terpenoid. This discrepancy and the reduction of the latex activity after fractionation suggested a complementary effect of the compounds when in a mixture as represented by the latex. Additionally, aqueous fractions of several latexes were found to possess simple spectra, even with only 1 metabolite. These metabolites were determined to be organic acids that might be involved in the modulation of the rate of latex coagulation, potentially increasing the sealing and trapping effects of the latex.

LC-MS/MS method for determination of seneciphylline and its metabolite, seneciphylline N-oxide in rat plasma, and its application to a rat pharmacokinetic study

A rapid and sensitive ultra-performance liquid chromatography-tandem electrospray ionization mass spectrometry (UPLC-ESI/MS) method was established and validated for simultaneous determination of seneciphylline and its main metabolite in rat plasma. The plasma sample was prepared by simple methanol-mediated precipitation. Chromatographic separation was achieved within 3 min by gradient elution using acetonitrile and water containing 0.1% formic acid as mobile phase on a Waters ACQUITY BEH C18 column (100 × 2.1 mm, i.d. 1.7  μm). Quantitation was conducted in a positive multiple reaction monitoring mode. The linearity of the method was over the range of 1-1,000 ng/mL, with the lower limit of quantification of 1 ng/mL. The intra- and inter-day precision and accuracy, extraction recovery, and matrix effect of analytes were within the acceptable limit. The analytes were stable during the process of sample collection, preparation, and analysis. The validated method was further applied to a pharmacokinetic study of seneciphylline in rats after oral and intravenous administration. The results revealed that seneciphylline was quickly absorbed into plasma (T_max , 0.23-0.32 h) and reached the maximum concentration of 0.82-1.75 μg/mL after oral administration. Both seneciphylline and seneciphylline N-oxide were eliminated from plasma quickly. The low system exposure (oral bioavailability, 5.43-10.31%) was related to the extensive metabolism in the liver and microflora.

Current Knowledge and Perspectives of Pyrrolizidine Alkaloids in Pharmacological Applications: A Mini-Review

Pyrrolizidine alkaloids (PAs) are a widespread group of secondary metabolites in plants. PAs are notorious for their acute hepatotoxicity, genotoxicity and neurological damage to humans and animals. In recent decades, the application of PAs for beneficial biological activities to cure disease has drawn greater attention. Here, we review the current knowledge regarding the pharmacological properties of PAs and discuss PAs as promising prototypes for the development of new drugs.

Antitumor activity of nervosine VII, and the crosstalk between apoptosis and autophagy in HCT116 human colorectal cancer cells

Nervosine VII is one of the known saturated pyrrolizidine alkaloids isolated from the plant of Liparis nervosa. This is first study to investigate the antitumor activity of nervosine VII in vitro, and the results indicated that nervosine VII induced autophagy and apoptosis in HCT116 human colorectal cancer cells. Mechanistic studies showed that nervosine VII-induced apoptosis was associated with the intrinsic pathway by the activation of caspase-9, -3 and -7. Autophagy induced by nervosine VII was characteristic with the regulation of autophagic markers including the increase of LC3-II and beclin 1 proteins, and the decrease of p62 protein. Nervosine VII simultaneously induced autophagy and apoptosis by activated MAPKs signaling pathway including JNK, ERK1/2 and p38, suppressing the p53 signaling pathway.

Genetic and chemical diversity of the toxic herb Jacobaea vulgaris Gaertn. (syn. Senecio jacobaea L.) in Northern Germany

Tansy ragwort, Jacobaea vulgaris Gaertn. (syn. Senecio jacobaea L.), is a common Asteraceae in Europe and Asia and known to be an invasive pest in several regions in the world. Recently it is also spreading immensely in native regions like Northern Germany. Pyrrolizidine alkaloids (PAs), which are found in high amounts in Jacobaea vulgaris, are toxic for humans and potentially lethal for grazing animals. In this study we investigated 27 populations of tansy ragwort in Northern Germany for their PA concentration and composition using liquid chromatography coupled to high resolution mass spectrometry. Furthermore, we investigated the genetic structure of selected populations using amplified length polymorphism markers. We detected 98 different PAs in the samples and considerable differences of PA composition between populations. In contrast, PA content of populations did not differ significantly. Genetic (4%) differentiation among populations was low while average genetic diversity was high (0.35). There was no correlation between genetic and geographic distance. Neither genetic markers nor chemical composition revealed any connection to the geographic pattern. As we could not detect any pattern in genetic or chemical diversity, we suggest that the existence of this diversity is a result of a broad interaction with the environment rather than that of evolutionary constraints in the current selection process driving PA composition in J. vulgaris in certain chemotypes.

Detection of pyrrolizidine alkaloids in jamu available on the Indonesian market and accompanying safety assessment for human consumption

The occurrence and accompanying risks of pyrrolizidine alkaloids (PAs) in Indonesian jamu were evaluated. PAs were detected in 34 out of 35 jamu containing PA-producing botanicals, in the range of 12.3-235,376 μg/kg. A total PA level of 5.9-3,421 μg/kg was found in 17 out of 23 jamu made of non-PA-producing botanicals pointing to contamination with PA-producing plants. Short-time consumption of jamu is unlikely to result in acute toxic effects, although one sample would exceed an intake of 10 μg PA/kg bw/day which may cause hepatic veno-occlusive disease (HVOD) in humans. The risk assessment for the genotoxic and carcinogenic potential of PAs revealed Margin of Exposure (MOE) values below 10,000 for 27 out of all samples analysed (46.6%), indicating a priority for risk management when assuming daily lifelong consumption. Assuming consumption for two weeks every year during a lifetime, and using Haber's rule, 13 out of 35 jamu samples containing PA-producing botanicals (37%) still pose a priority, while the jamu consisting of non-PA-producing botanicals would be of low priority (MOE>10,000). This study provides data that can support risk management actions in Indonesia to minimize the potential health risk for jamu consumers due to the occurrence of toxic PAs in these products.

Feeding intensity of insect herbivores is associated more closely with key metabolite profiles than phylogenetic relatedness of their potential hosts

Determinants of the host ranges of insect herbivores are important from an evolutionary perspective and also have implications for applications such as biological control. Although insect herbivore host ranges typically are phylogenetically constrained, herbivore preference and performance ultimately are determined by plant traits, including plant secondary metabolites. Where such traits are phylogenetically labile, insect hervivore host ranges are expected to be phylogenetically disjunct, reflecting phenotypic similarities rather than genetic relatedness among potential hosts. We tested this hypothesis in the laboratory with a Brassicaceae-specialized weevil, Ceutorhynchus cardariae Korotyaev (Coleoptera: Curculionidae), on 13 test plant species differing in their suitability as hosts for the weevil. We compared the associations between feeding by C. cardariae and either phenotypic similarity (secondary chemistry—glucosinolate profile) or genetic similarity (sequence of the chloroplast gene ndhF) using two methods—simple correlations or strengths of association between feeding by each species, and dendrograms based on either glucosinolates or ndhF sequence (i.e., a phylogram). For comparison, we performed a similar test with the oligophagous Plutella xylostella (L.) (Lepidoptera: Plutellidae) using the same plant species. We found using either method that phenotypic similarity was more strongly associated with feeding intensity by C. cardariae than genetic similarity. In contrast, neither genetic nor phenotypic similarity was significantly associated with feeding intensity on the test species by P. xylostella. The result indicates that phenotypic traits can be more reliable indicators of the feeding preference of a specialist than phylogenetic relatedness of its potential hosts. This has implications for the evolution and maintenance of host ranges and host specialization in phytophagous insects. It also has implications for identifying plant species at risk of nontarget attack by potential weed biological control agents and hence the approach to prerelease testing.

Determination of genotoxic potencies of pyrrolizidine alkaloids in HepaRG cells using the γH2AX assay

Pyrrolizidine alkaloids (PAs) are secondary metabolites from plants that have been found in substantial amounts in herbal supplements, infusions and teas. Several PAs cause cancer in animal bioassays, mediated via a genotoxic mode of action, but for the majority of the PAs, carcinogenicity data are lacking. It is assumed in the risk assessment that all PAs have the same potency as riddelliine, which is considered to be one of the most potent carcinogenic PAs in rats. This may overestimate the risks, since many PAs are expected to have lower potencies. In this study we determined the concentration-dependent genotoxicity of 37 PAs representing different chemical classes using the γH2AX in cell western assay in HepaRG human liver cells. Based on these in vitro data, PAs were grouped into different potency classes. The group with the highest potency consists particularly of open diester PAs and cyclic diester PAs (including riddelliine). The group of the least potent or non-active PAs includes the monoester PAs, non-esterified necine bases, PA N-oxides, and the unsaturated PA trachelanthamine. This study reveals differences in in vitro genotoxic potencies of PAs, supporting that the assumption that all PAs have a similar potency as riddelliine is rather conservative.

Sex difference in monocrotaline-induced developmental toxicity and fetal hepatotoxicity in rats

Pyrrolizidine alkaloids (PAs) are a class of hepatic toxins widely existing in plants. Cytochromes P450 (CYP) mediates PA bioactivation and toxicities in mammals. It has been reported that PAs can induce developmental toxicity, but systematic research is lacking. In this study, we investigated developmental toxicity of monocrotaline (MCT) in rats. Pregnant rats were administered with MCT (20 mg/kg) intragastrically from gestation day 9 to 20, followed by determination of changes in fetal growth, hepatic morphology, serum biochemical indices, and indicators of hepatocytes apoptosis. MCT was found to induce developmental toxicity and fetal hepatotoxicity, particularly in female fetuses. Metabolic activation was also studied by examination of bioactivation efficiency of MCT in fetal liver microsomes, serum MCT, pyrrole-protein adduction derived from MCT, and hepatic CYP3 A expression of fetuses in vivo. Male fetuses showed greater basal MCT bioactivation than that of female fetuses, but continuous exposure to MCT caused a selective CYP3 A induction in female fetuses, which may contribute to the sex difference in MCT-induced developmental toxicity.

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

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

Pyrrolizidine Alkaloids: Biosynthesis, Biological Activities and Occurrence in Crop Plants

Pyrrolizidine alkaloids (PAs) are heterocyclic secondary metabolites with a typical pyrrolizidine motif predominantly produced by plants as defense chemicals against herbivores. They display a wide structural diversity and occur in a vast number of species with novel structures and occurrences continuously being discovered. These alkaloids exhibit strong hepatotoxic, genotoxic, cytotoxic, tumorigenic, and neurotoxic activities, and thereby pose a serious threat to the health of humans since they are known contaminants of foods including grain, milk, honey, and eggs, as well as plant derived pharmaceuticals and food supplements. Livestock and fodder can be affected due to PA-containing plants on pastures and fields. Despite their importance as toxic contaminants of agricultural products, there is limited knowledge about their biosynthesis. While the intermediates were well defined by feeding experiments, only one enzyme involved in PA biosynthesis has been characterized so far, the homospermidine synthase catalyzing the first committed step in PA biosynthesis. This review gives an overview about structural diversity of PAs, biosynthetic pathways of necine base, and necic acid formation and how PA accumulation is regulated. Furthermore, we discuss their role in plant ecology and their modes of toxicity towards humans and animals. Finally, several examples of PA-producing crop plants are discussed.

Methyl Jasmonate Changes the Composition and Distribution Rather than the Concentration of Defence Compounds: a Study on Pyrrolizidine Alkaloids

In this study we investigated the effect of methyl jasmonate (MeJA) application on pyrrolizidine alkaloid (PA) concentration and composition of two closely related Jacobaea species. In addition, we examined whether MeJA application affected herbivory of the polyphagous leaf feeding herbivore Spodoptera exigua. A range of concentrations of MeJA was added to the medium of Jacobaea vulgaris and J. aquatica tissue culture plants grown under axenic conditions. PA concentrations were measured in roots and shoots using LC-MS/MS. In neither species MeJA application did affect the total PA concentration at the whole plant level. In J. vulgaris the total PA concentration decreased in roots but increased in shoots. In J. aquatica a similar non-significant trend was observed. In both Jacobaea species MeJA application induced a strong shift from senecionine- to erucifoline-like PAs, while the jacobine- and otosenine-like PAs remained largely unaffected. The results show that MeJA application does not necessarily elicits de novo synthesis, but rather leads to PA conversion combined with reallocation of certain PAs from roots to shoots. S. exigua preferred feeding on control leaves of J. aquatica over MeJA treated leaves, while for J. vulgaris both the control and MeJA treated leaves were hardly eaten. This suggests that the MeJA-induced increase of erucifoline-like PAs can play a role in resistance of J. aquatica to S. exigua. In J. vulgaris resistance to S. exigua may already be high due to the presence of jacobine-like PAs or other resistance factors.

Phytochemical Background Mediates Effects of Pyrrolizidine Alkaloids on Western Flower Thrips

Plants produce an extremely diverse array of metabolites that mediate many aspects of plant-environment interactions. In the context of plant-herbivore interactions, it is as yet poorly understood how natural backgrounds shape the bioactivity of individual metabolites. We tested the effects of a methanol extract of Jacobaea plants and five fractions derived from this extract, on survival of western flower thrips (WFT). When added to an artificial diet, the five fractions all resulted in a higher WFT survival rate than the methanol extract. In addition, their expected combined effect on survival, assuming no interaction between them, was lower than that of the methanol extract. The bioactivity was restored when the fractions were combined again in their original proportion. These results strongly suggest synergistic interactions among the fractions on WFT survival rates. We then tested the effects of two pyrrolizidine alkaloids (PAs), free base retrorsine and retrorsine N-oxide, alone and in combination with the five shoot fractions on WFT survival. The magnitude of the effects of the two PAs depended on the fraction to which they were added. In general, free base retrorsine was more potent than retrorsine N-oxide, but this was contingent on the fraction to which these compounds were added. Our results support the commonly held, though seldom tested, notion that the efficacy of plant metabolites with respect to plant defence is dependent on their phytochemical background. It also shows that the assessment of bioactivity cannot be decoupled from the natural chemical background in which these metabolites occur.

Pyrrolizidine alkaloids from Liparis nervosa with antitumor activity by modulation of autophagy and apoptosis

Seven pyrrolizidine alkaloids, nervosine X-XV and nervosine VII N-oxide, together with a reaction product, namely chloride-(N-chloromethyl nervosine VII), were isolated from Liparis nervosa. Their structures were elucidated by extensive spectroscopic analyses. Most of these compounds were investigated for their cytotoxicity in vitro against HCT116 human cancer cell line, and the results showed that chloride-(N-chloromethyl nervosine VII) induced tumor cell death in a dose-dependent manner. Furthermore, the mechanisms underlying its cytotoxicity were investigated, including apoptosis and autophagy. Apoptosis in HCT116 cells was associated with up-regulation of caspase-3 and -9 expressions by activation of the mitochondrial pathway. The autophagy inducing effect was associated with the regulation of autophagic markers, including LC3-II, p62, and Beclin 1. Mechanistic studies showed that JNK, ERK1/2, and p38 MAPKs signaling cascades play an important role in chloride-(N-chloromethyl nervosine VII) induced autophagy and apoptosis.

Thrips advisor: exploiting thrips-induced defences to combat pests on crops

Plants have developed diverse defence mechanisms to ward off herbivorous pests. However, agriculture still faces estimated crop yield losses ranging from 25% to 40% annually. These losses arise not only because of direct feeding damage, but also because many pests serve as vectors of plant viruses. Herbivorous thrips (Thysanoptera) are important pests of vegetable and ornamental crops worldwide, and encompass virtually all general problems of pests: they are highly polyphagous, hard to control because of their complex lifestyle, and they are vectors of destructive viruses. Currently, control management of thrips mainly relies on the use of chemical pesticides. However, thrips rapidly develop resistance to these pesticides. With the rising demand for more sustainable, safer, and healthier food production systems, we urgently need to pinpoint the gaps in knowledge of plant defences against thrips to enable the future development of novel control methods. In this review, we summarize the current, rather scarce, knowledge of thrips-induced plant responses and the role of phytohormonal signalling and chemical defences in these responses. We describe concrete opportunities for breeding resistance against pests such as thrips as a prototype approach for next-generation resistance breeding.

Interactions between Plant Metabolites Affect Herbivores: A Study with Pyrrolizidine Alkaloids and Chlorogenic Acid

The high structural diversity of plant metabolites suggests that interactions among them should be common. We investigated the effects of single metabolites and combinations of plant metabolites on insect herbivores. In particular we studied the interacting effects of pyrrolizidine alkaloid (PAs), and chlorogenic acid (CGA), on a generalist herbivore, Frankliniella occidentalis. We studied both the predominantly occurring PA N-oxides and the less frequent PA free bases. We found antagonistic effects between CGA and PA free bases on thrips mortality. In contrast PA N-oxides showed synergistic interactions with CGA. PA free bases caused a higher thrips mortality than PA N-oxides while the reverse was through for PAs in combination with CGA. Our results provide an explanation for the predominate storage of PA N-oxides in plants. We propose that antagonistic interactions represent a constraint on the accumulation of plant metabolites, as we found here for Jacobaea vulgaris. The results show that the bioactivity of a given metabolite is not merely dependent upon the amount and chemical structure of that metabolite, but also on the co-occurrence metabolites in, e.g., plant cells, tissues and organs. The significance of this study is beyond the concerns of the two specific groups tested here. The current study is one of the few studies so far that experimentally support the general conception that the interactions among plant metabolites are of great importance to plant-environment interactions.