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Veyrat, Nathalie
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Veyrat, Nathalie
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- PublicationAccès libreThe role of indole in maize-herbivore interactions(2014)
; 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. - PublicationAccès libreMinor effects of two elicitors of insect and pathogen resistance on volatile emissions and parasitism of Spodoptera frugiperda in Mexican maize fields(2012)
; ; ; D’Alessandro, MarcoSynthetic 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. - PublicationAccès libreDispensing 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(2011)
; ; D’Alessandro, MarcoMaize 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. - PublicationAccès libreExceptional Use of Sex Pheromones by Parasitoids of the Genus Cotesia: Males Are Strongly Attracted to Virgin Females, but Are No Longer Attracted to or Even Repelled by Mated Females
; ; Sex pheromones have rarely been studied in parasitoids, and it remains largely unknown how male and female parasitoids locate each other. We investigated possible attraction (and repellency) between the sexes of two braconid wasps belonging to the same genus, the gregarious parasitoid, Cotesia glomerata (L.), and the solitary parasitoid, Cotesia marginiventris (Cresson). Males of both species were strongly attracted to conspecific virgin females. Interestingly, in C. glomerata, the males were repelled by mated females, as well as by males of their own species. This repellency of mated females was only evident hours after mating, implying a change in pheromone composition. Males of C. marginiventris were also no longer attracted, but not repelled, by mated females. Females of both species showed no attraction to the odors of conspecific individuals, male or female, and C. glomerata females even appeared to be repelled by mated males. Moreover, the pheromones were found to be highly specific, as males were not attracted by females of the other species. Males of Cotesia glomerata even avoided the pheromones of female Cotesia marginiventris, indicating the recognition of non-conspecific pheromones. We discuss these unique responses in the context of optimal mate finding strategies in parasitoids.