Résultat de la recherche
Tritrophic interactions on cultivated maize and its wild ancestor "teosinte"
Modern maize plants (Zea mays ssp. mays, Poaceae) are characterized by large cobs that contain juicy grains, although they have not always had these characteristics. Approximately 9000 years ago, maize was domesticated from teosinte (Z. mays ssp. parviglumis), its closest wild ancestor, which produces much less and much smaller seeds. Teosinte still grows in the wild in Mexico, while maize is produced all over the world. Continuous selection for improved yield and quality has had a cost for the plant in terms of the loss or alteration of other potentially useful traits, such as resistance to pathogens and herbivorous insects. This thesis focuses on the resistance of maize and teosinte against insect pests, in particular with respect to the emission of herbivore-induced volatiles. This feature is considered an indirect defense trait, as the volatiles can betray the presence of prey or hosts to predators and parasitoids, natural enemies of herbivorous insects. Among these natural enemies are parasitoid wasps, of which females have an ovipositor with which they can lay single or multiple eggs in individual hosts. When the wasp larvae develop inside the host they will eventually kill it, potentially benefitting the plant.
At first, we assessed in nature which insects occur on teosinte. Fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae), is reported as an important maize pest in the Americas and is frequently attacked by many species of parasitoids. However, little is known about the presence of this herbivore and its associated parasitoids on teosinte.
In a laboratory setting, we then assessed whether maize and teosinte emit a similar blend of volatiles when induced by lepidopteran herbivores. We also evaluated the attractiveness of the odor blends to females of two species of parasitoid wasps. Although the odor blends appeared similar upon gas chromatography analysis, the foraging parasitoids responded differentially to them. We obtained similar results when testing real plants and extracts of collected odors, indicating that these extracts can be used to identify the key compounds that are responsible for parasitoid attraction.
There is surprisingly little field evidence for fitness benefits for plants due to the action of natural enemies. This is still an important point in the discussion on the possible indirect defense role of inducible plant volatiles, especially in the case of parasitoids that, unlike predators, do not directly kill their hosts. Therefore, we evaluated how parasitoid wasps can affect plant performance in a semi-natural setting in Mexico. The presence of parasitoid wasps in field tents containing teosinte plants and fall armyworm significantly reduced herbivore damage, which, for the smallest plants, resulted in a reduction in plant mortality. These findings support the notion that plants may benefit from the presence of parasitoids and may help to resolve the current debate on the defensive function of herbivore-induced volatiles.
To further explore the importance of volatiles for the attraction of parasitoids under field conditions, we studied maize lox10 mutants, impaired in the biosynthesis of green leaf volatiles (GLVs). These volatiles, responsible for the smell of cut grass, are commonly emitted by plants when wounded or attacked by herbivores. Previously, laboratory studies have shown that GLVs can be used as foraging cues by predators and parasitoids. However, our results imply that GLVs are not of key importance for parasitoid attraction in the field.
Collectively, these studies provide novel insights into the importance of herbivore-induced volatiles for the attraction of parasitoid wasps, and their indirect importance for plant growth and survival. We identified differences between maize and its wild ancestor, teosinte, with respect to the attraction of parasitoids that could possibly be exploited for ecologically sound methods to better protect maize against insect pests.
Attractiveness of Constitutive and Herbivore-Induced Sesquiterpene Blends of Maize to the Parasitic Wasp Cotesia marginiventris (Cresson)
Plant volatile compounds induced by herbivore attack have been demonstrated to provide a signal to herbivore enemies such as parasitic wasps that use these volatiles to locate their hosts. However, in addition to herbivore-induced volatiles, plants often release volatiles constitutively. We assessed the interaction between herbivore-induced and constitutively released volatiles of maize in the attraction of the wasp Cotesia marginiventris that parasitizes herbivorous lepidopteran larvae feeding on maize. Experiments were carried out with olfactometers in which the sources of volatiles were transgenic Arabidopsis thaliana plants overexpressing maize sesquiterpene synthases that produce blends of herbivore-induced or constitutive compounds. We found that the constitutive volatiles of maize terpene synthase 8 (TPS8) were attractive to C. marginiventris, just like the herbivore-induced volatiles of TPS10 studied earlier. A mixture of both the TPS8 and TPS10 volatile blends, however, was more effective in parasitoid attraction, indicating that constitutively released sesquiterpenes enhance the attraction of those induced by herbivores. While C. marginiventris did not distinguish among the volatiles of TPS8, TPS10, nor those of another maize sesquiterpene synthase (TPS5), when these blends were combined, their attractiveness to the wasp appeared to increase with the complexity of the blend.
Direct and indirect plant defenses are not suppressed by endosymbionts of a specialist root herbivore
Attractiveness of constitutive and herbivore-induced sesquiterpene blends of maize to the parasitic wasp
Plant volatile compounds induced by herbivore attack have been demonstrated to provide a signal to herbivore enemies such as parasitic wasps that use these volatiles to locate their hosts. However, in addition to herbivore-induced volatiles, plants often release volatiles constitutively. We assessed the interaction between herbivore-induced and constitutively released volatiles of maize in the attraction of the wasp Cotesia marginiventris that parasitizes herbivorous lepidopteran larvae feeding on maize. Experiments were carried out with olfactometers in which the sources of volatiles were transgenic Arabidopsis thaliana plants overexpressing maize sesquiterpene synthases that produce blends of herbivore-induced or constitutive compounds. We found that the constitutive volatiles of maize terpene synthase 8 (TPS8) were attractive to C. marginiventris, just like the herbivore-induced volatiles of TPS10 studied earlier. A mixture of both the TPS8 and TPS10 volatile blends, however, was more effective in parasitoid attraction, indicating that constitutively released sesquiterpenes enhance the attraction of those induced by herbivores. While C. marginiventris did not distinguish among the volatiles of TPS8, TPS10, nor those of another maize sesquiterpene synthase (TPS5), when these blends were combined, their attractiveness to the wasp appeared to increase with the complexity of the blend.