Résultat de la recherche
Exceptional use of sex pheromones by parasitoids of the genus
Plant elicitor peptides are conserved signals regulating direct and indirect antiherbivore defense
Insect-induced defenses occur in nearly all plants and are regulated by conserved signaling pathways. As the first described plant peptide signal, systemin regulates antiherbivore defenses in the Solanaceae, but in other plant families, peptides with analogous activity have remained elusive. In the current study, we demonstrate that a member of the maize (Zea mays) plant elicitor peptide (Pep) family, ZmPep3, regulates responses against herbivores. Consistent with being a signal, expression of the ZmPROPEP3 precursor gene is rapidly induced by Spodoptera exigua oral secretions. At concentrations starting at 5 pmol per leaf, ZmPep3 stimulates production of jasmonic acid, ethylene, and increased expression of genes encoding proteins associated with herbivory defense. These include proteinase inhibitors and biosynthetic enzymes for production of volatile terpenes and benzoxazinoids. In accordance with gene expression data, plants treated with ZmPep3 emit volatiles similar to those from plants subjected to herbivory. ZmPep3-treated plants also exhibit induced accumulation of the benzoxazinoid phytoalexin 2-hydroxy-4,7-dimethoxy-1,4-benzoxazin-3-one glucoside. Direct and indirect defenses induced by ZmPep3 contribute to resistance against S. exigua through significant reduction of larval growth and attraction of Cotesia marginiventris parasitoids. ZmPep3 activity is specific to Poaceous species; however, peptides derived from PROPEP orthologs identified in Solanaceous and Fabaceous plants also induce herbivory-associated volatiles in their respective species. These studies demonstrate that Peps are conserved signals across diverse plant families regulating antiherbivore defenses and are likely to be the missing functional homologs of systemin outside of the Solanaceae.
A specialist root herbivore exploits defensive metabolites to locate nutritious tissues
The most valuable organs of plants are often particularly rich in essential elements, but also very well defended. This creates a dilemma for herbivores that need to maximise energy intake while minimising intoxication. We investigated how the specialist root herbivore Diabrotica virgifera solves this conundrum when feeding on wild and cultivated maize plants. We found that crown roots of maize seedlings were vital for plant development and, in accordance, were rich in nutritious primary metabolites and contained higher amounts of the insecticidal 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA) and the phenolic compound chlorogenic acid. The generalist herbivores Diabrotica balteata and Spodoptera littoralis were deterred from feeding on crown roots, whereas the specialist D. virgifera preferred and grew best on these tissues. Using a 1,4-benzoxazin-3-one-deficient maize mutant, we found that D. virgifera is resistant to DIMBOA and other 1,4-benzoxazin-3-ones and that it even hijacks these compounds to optimally forage for nutritious roots.
Dispensing synthetic green leaf volatiles in maize fields increases the release of sesquiterpenes by the plants, but has little effect on the attraction of pest and beneficial insects
Maize plants respond to feeding by arthropod herbivores by producing a number of secondary plant compounds, including volatile organic compounds (VOCs). These herbivore-induced VOCs are not only known to attract natural enemies of the herbivores, but they may also prime inducible defences in neighbouring plants, resulting in stronger and faster defence responses in these VOC-exposed plants. Among the compounds that cause this priming effect, green leaf volatiles (GLVs) have received particular attention, as they are ubiquitous and rapidly emitted upon damage. In this study, we investigated their effects under realistic conditions by applying specially devised dispensers to release four synthetic GLVs at physiologically relevant concentrations in a series of experiments in maize fields. We compared the VOC emission of GLV-exposed maize plants to non-exposed plants and monitored the attraction of herbivores and predators, as well as parasitism of the caterpillar Spodoptera frugiperda, the most common herbivore in the experimental maize fields. We found that maize plants that were exposed to GLVs emitted increased quantities of sesquiterpenes compared to non-exposed plants. In several replicates, herbivorous insects, such as adult Diabrotica beetles and S. frugiperda larvae, were observed more frequently in GLV-treated plots and caused more damage to GLV-exposed plants than to non-exposed plants. Parasitism of S. frugiperda was only weakly affected by GLVs and overall parasitism rates of S. frugiperda were similar in GLV-exposed and non-exposed plots. The effects on insect presence depended on the distance from the GLV-dispensers at which the plants were located. The results are discussed in the context of strategies to improve biological control by enhancing plant-mediated attraction of natural enemies.
Within-plant distribution of 1,4-benzoxazin-3-ones contributes to herbivore niche differentiation in maize
Plant defenses vary in space and time, which may translate into specific herbivore foraging patterns and feeding niche differentiation. To date, little is known about the effect of secondary metabolite patterning on within-plant herbivore foraging. We investigated how variation in the major maize secondary metabolites, 1,4-benzoxazin-3-one derivatives (BXDs), affects the foraging behavior of two leaf-chewing herbivores. BXD levels varied substantially within plants: Older leaves had higher levels of constitutive BXDs while younger leaves were consistently more inducible. These differences were observed independently of plant age, even though the concentrations of most BXDs declined markedly in older plants. Larvae of the well-adapted maize pest Spodoptera frugiperda preferred and grew better on young inducible leaves irrespective of plant age, while larvae of the generalist Spodoptera littoralis preferred and tended to grow better on old leaves. In BXD-free mutants, the differences in herbivore weight gain between old and young leaves were absent for both species, and leaf preferences of S. frugiperda were attenuated. In contrast, S. littoralis foraging patterns were not affected. In summary, our study shows that plant secondary metabolites differentially affect performance and foraging of adapted and non-adapted herbivores and thereby likely contribute to feeding niche differentiation.
Ultra-high pressure liquid chromatography-mass spectrometry for plant metabolomics: A systematic comparison of high-resolution quadrupole-time-of-flight and single stage Orbitrap mass spectrometers
The response of Arabidopsis to stress caused by mechanical wounding was chosen as a model to compare the performances of high resolution quadrupole-time-of-flight (Q-TOF) and single stage Orbitrap (Exactive Plus) mass spectrometers in untargeted metabolomics. Both instruments were coupled to ultra-high pressure liquid chromatography (UHPLC) systems set under identical conditions. The experiment was divided in two steps: the first analyses involved sixteen unwounded plants, half of which were spiked with pure standards that are not present in Arabidopsis. The second analyses compared the metabolomes of mechanically wounded plants to unwounded plants. Data from both systems were extracted using the same feature detection software and submitted to unsupervised and supervised multivariate analysis methods. Both mass spectrometers were compared in terms of number and identity of detected features, capacity to discriminate between samples, repeatability and sensitivity. Although analytical variability was lower for the UHPLC-Q-TOF, generally the results for the two detectors were quite similar, both of them proving to be highly efficient at detecting even subtle differences between plant groups. Overall, sensitivity was found to be comparable, although the Exactive Plus Orbitrap provided slightly lower detection limits for specific compounds. Finally, to evaluate the potential of the two mass spectrometers for the identification of unknown markers, mass and spectral accuracies were calculated on selected identified compounds. While both instruments showed excellent mass accuracy (
Minor effects of two elicitors of insect and pathogen resistance on volatile emissions and parasitism of Spodoptera frugiperda in Mexican maize fields
Synthetic elicitors can be used to induce resistance in plants against pathogens and arthropod herbivores. Such compounds may also change the emission of herbivore-induced plant volatiles, which serve as important cues for parasitic wasps to locate their hosts. Therefore, the use of elicitors in the field may affect biological control of insect pests. To test this, we treated maize seedlings growing in a subtropical field in Mexico with methyl jasmonate (MeJA), an elicitor of defense responses against many insects, and benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), an elicitor of resistance against certain pathogens. Volatile emission, herbivore infestation, pathogen infection, and plant performance (growth and grain yield) of treated and untreated maize plants were measured. Application of BTH slightly reduced volatile emission in maize, while MeJA increased the emission compared to control treatments. Despite the apparent changes in volatile emissions, the elicitor application did not consistently affect infestation by Spodoptera frugiperda larvae, the main insect pest found on the maize seedlings, and had only marginal effects on parasitism rates. Similarly, there were no treatment effects on infestation by other herbivores and pathogens. Results for the six replications that stretched over one summer and one winter season were highly variable, with parasitism rates and the species composition of the parasitoids differing significantly between seasons. This variability, as well as the severe biotic and abiotic stresses on young seedlings might explain why we measured only slight effects of elicitor application on pest incidence and biological control in this specific field study. Indeed, an additional field experiment under milder and more standardized conditions revealed that BTH induced significant resistance against Bipolaris maydis, a major pathogen in the experimental maize fields. Similar affects can be expected for herbivory and parasitism rates.
The role of indole in maize-herbivore interactions
Afin de se protéger contre les attaques d’insectes herbivores, les plantes ont développé de multiples moyens de défense, dont la libération de composés volatils induits par les herbivores (HIPVs). Ces composés volatils peuvent être utilisés par les ennemis naturels des herbivores tels que les prédateurs et les parasitoïdes. D’autre part, ils peuvent être exploités par les herbivores eux-mêmes pour localiser leurs plantes hôtes. Certains HIPVs peuvent aussi avertir les tissus non attaqués d’une même plante ou les plantes voisines d’un risque d’attaque. Le terme employé est “priming”. Les plantes averties pourront ainsi répondre plus rapidement et de manière plus efficace lorsque l’attaque se produira. Tandis que certains HIPVs ont été bien étudiés, le rôle de beaucoup d’autre reste à trouver. Par exemple, nous n’avons que peu de connaissances en ce qui concerne l’indole, un composé dominant du mélange de volatils émis par les plantes. Dans la thèse présentée ici, nous avons étudié le rôle de l’indole dans les défenses directes et indirectes du maïs grâce à l’utilisation de plantes mutantes dans la production d’indole et d’indole synthétique.
Dans le premier chapitre, nous avons étudié le rôle de l’indole en tant que signal de défense. Nous fournissons la preuve que l’indole est essentiel pour le “priming” d’autres HIPVs au sein d’une même plante mais qu’il agit aussi comme signal de communication entre différentes plantes afin de les préparer à une possible attaque. Dans le deuxième chapitre, nous avons étudié l’effet de l’indole sur un insecte herbivore généraliste, Spodoptera littoralis. Nous démontrons que l’indole agit en tant de défense directe chez le maïs en repoussant les adultes et les chenilles de cette espèce et en réduisant la survie des chenille et le succès reproducteur des adultes. Dans le troisième chapitre, nous avons étudié l’importance de l’indole au niveau du troisième niveau trophique. Nous avons trouvé que malgré une attraction de certains parasitoïdes, une exposition à l’indole protège les chenilles de l’espèce S. littoralis en augmentant leur résistance contre les parasitoïdes. Dans le quatrième chapitre, nous avons étudié la spécificité des effets trouvés dans les deux chapitres précédents. Nous avons trouvé que ni le degré de spécialisation pour les plantes hôtes, ni l’origine phylogénétique, ni l’association avec des plantes produisantde l’indole ne déterminent la réponse des insectes herbivores et des ennemis naturels à l’indole. Nous concluons que le rôle de l’indole est dépendant des espèces.
D’une manière générale, cette thèse contribue à une meilleure compréhension du rôle de l’indole dans les intéractions entre les plantes, les insectes herbivores et les ennemis naturels; elle confirme le rôle multiple des composés volatils dans les intéractions tri-trophiques., In order to counter herbivore attacks, plants have developed a multitude of defence strategies, including the release herbivore-induced plant volatiles (HIPVs). HIPVs can be used as foraging cues by natural enemies of the herbivores, including predators and parasitoids. In addition, they can also be exploited by herbivores themselves to localize their host plants. Some HIPVs even prime non-attacked plant tissues or neighbouring plants to respond faster and more strongly to subsequent attacks. Whereas some HIPVs have been well studied, the role of many others remains unclear. For instance, little is known about indole, a major constituent of the herbivore-induced volatile blend. In the present thesis, we studied the role of indole in direct and indirect defences in maize using indole deficient mutants and synthetic indole.
In Chapter 1 we investigated the role of indole as a plant defence signal. We provide evidence that indole is essential for within-plant priming of other HIPVs and acts as a between-plant signal that primes non-attacked neighbours. In Chapter 2, we investigated the impact of indole on the generalist herbivore Spodoptera littoralis. We demonstrate that volatile indole acts as a direct defence in maize by repelling S. littoralis moths and caterpillars and by reducing the survival of early instar caterpillars and the reproductive output of adults. In Chapter 3, we studied the importance of indole on the third trophic level. We found that, although indole attracts certain parasitoids, indole-exposure protects S. littoralis caterpillars by increasing their resistance against parasitism. In Chapter 4, we investigated the specificity of the effects found in chapters 2 and 3. We found that neither the degree of host plant specialization nor the phylogenetic origin or the association with indole-producing plants determines the response of herbivores and natural enemies to the volatile, and that the role of indole is highly species-specific.
Overall, this thesis contributes to a better understanding of the role of indole in interactions between plants, herbivore insects and natural enemies and highlights the diverse roles of HIPVs in tritrophic interactions.
Herbivore-induced maize leaf volatiles affect attraction and feeding behaviour of
Plants under herbivore attack emit volatile organic compounds (VOCs) that can serve as foraging cues for natural enemies. Adult females of Lepidoptera, when foraging for host plants to deposit eggs, are commonly repelled by herbivore-induced VOCs, probably to avoid competition and natural enemies. Their larval stages, on the other hand, have been shown to be attracted to inducible VOCs. We speculate that this contradicting behaviour of lepidopteran larvae is due to a need to quickly find a new suitable host plant if they have fallen to the ground. However, once they are on a plant they might avoid the sites with fresh damage to limit competition and risk of cannibalism by conspecifics, as well as exposure to natural enemies. To test this we studied the effect of herbivore-induced VOCs on the attraction of larvae of the moth Spodoptera littoralis and on their feeding behaviour. The experiments further considered the importance of previous feeding experience on the responses of the larvae. It was confirmed that herbivore-induced VOCs emitted by maize plants are attractive to the larvae, but exposure to the volatiles decreased the growth rate of caterpillars at early developmental stages. Larvae that had fed on maize previously were more attracted by VOCs of induced maize than larvae that had fed on artificial diet. At relatively high concentrations synthetic green leaf volatiles, indicative of fresh damage, also negatively affected the growth rate of caterpillars, but not at low concentrations. In all cases, feeding by the later stages of the larvae was not affected by the VOCs. The results are discussed in the context of larval foraging behaviour under natural conditions, where there may be a trade-off between using available host plant signals and avoiding competitors and natural enemies.