Résultat de la recherche
The effect of root‐associated microbes on plant growth and chemical defence traits across two contrasted elevations
Variable effects on growth and defense traits for plant ecotypic differentiation and phenotypic plasticity along elevation gradients
Along ecological gradients, phenotypic differentiation can arise through natural selection on trait diversity and magnitude, and environment‐driven plastic changes. The magnitude of ecotypic differentiation versus phenotypic plasticity can vary depending on the traits under study. Using reciprocal transplant‐common gardens along steep elevation gradients, we evaluated patterns of ecotypic differentiation and phenotypic plasticity of several growth and defense‐related traits for two coexisting but unrelated plant species, Cardamine pratensis and Plantago major. For both species, we observed ecotypic differentiation accompanied by plasticity in growth‐related traits. Plants grew faster and produced more biomass when placed at low elevation. In contrast, we observed fixed ecotypic differentiation for defense and resistance traits. Generally, low‐elevation ecotypes produced higher chemical defenses regardless of the growing elevation. Yet, some plasticity was observed for specific compounds, such as indole glucosinolates. The results of this study may suggest that ecotypic differentiation in defense traits is maintained by costs of chemical defense production, while plasticity in growth traits is regulated by temperature‐driven growth response maximization.
The effect of climate and soil microorganisms on plants growth-defence strategies
Contrairement à la plupart des autres organismes, les plantes sont immobiles. Elles ne peuvent donc échapper aux facteurs de stress et sont soumises à d’énormes pressions de l’environnement. L'une des stratégies utilisées par les plantes pour faire face à ces stress environnementaux, tant biotiques qu'abiotiques, consiste à allouer des ressources entre différentes fonctions spécifiques, les principales étant la croissance et la défense. Malgré l’intérêt croissant porté par la recherche dans ce domaine, la compréhension des processus éco-évolutifs responsables de l’allocation des ressources entre croissance et défense reste fragmentaire. Les microbes présents dans le sol, comprenant les bactéries et champignons, sont des candidats idéaux pour atténuer le stress environnemental. L'omniprésence des microbes et leur longue histoire de coévolution avec les plantes les rendent souvent indispensable pour ces dernières. En raison de leur double capacité à agir sur la productivité ainsi que sur les défenses contre les herbivores, des associations spécifiques sont mise en place par les plantes pour obtenir des avantages distincts. La thèse présentée ici tente d'éclaircir la compréhension des interactions entre plantes et microorganismes en fonction des variations des conditions environnementales. Elle comporte trois objectifs principaux. Tout d'abord, j'ai étudié comment les relations entre microorganismes associées aux racines (RAMs) et climat affectent le phénotype de croissance et de défense de Plantago major le long d’un gradient d'altitude (Chapitres I et II). J'ai mis en évidence le fait que les conditions climatiques régulaient les traits de croissance de P. major, alors que les traits de défense étaient plutôt génétiquement fixés (Chapitre I). Par la suite, j’ai constaté que les RAMs provenant de la même altitude que les plantes, favorisaient la croissance des populations de Plantago major, alors que les défenses chimiques étaient globalement plus élevées lorsque des microbes de faible altitude étaient présents (chapitre II). Pour finir, au Chapitre III, je me suis intéressé aux tendances macro-évolutives des taux de colonisation des plantes par les champignons mycorhiziens arbusculaires (AMF) et à la réactivité de la croissance et de la défense des plantes qui en résulte. Pour ce faire, j'ai comparé 24 espèces de Plantago L. dans un environnement commun avec et sans AMF. J'ai constaté que la variation interspécifique de la colonisation des plantes par les AMF, ainsi que la réactivité de la croissance et de défense des plantes aux AMF étaient contrôlés par l'histoire évolutive des plantes et par la convergence des niches climatiques à des degrés divers (Chapitre III). Ma thèse tend à améliorer notre compréhension de la réponse stratégique des plantes à la variation des conditions environnementales en affectant des ressources vers la croissance ou la défense en fonction de la présence de microorganismes bénéfiques du sol. La nouveauté de ces travaux réside principalement dans l’étude combinée des facteurs climatiques et biotiques influençant à la fois la croissance et la défense des plantes. Les conclusions qui en découlent pourraient inspirer des recherches plus approfondies sur la manière dont les plantes s'adaptent localement aux conditions biotiques et abiotiques dans les écosystèmes. ABSTRACT Unlike most other living organisms on this planet, plants are immobile. Given their sessile nature, plants are unable to escape from stressors, thus plants face enormous environmental pressures. One way, plants cope with environmental stress, both at the biotic and abiotic level, is by finely allocating resources into different functions, among which the majors are growth and defence. Despite the rising research focus on this matter, a global understanding of the eco-evolutionary processes responsible for plant resources allocation into growth and defence is still vague. Soil-borne microbes, which include bacteria and fungi, are promising candidates to alleviate such environmental stress acting on plants. The omnipresence of microbes and their long co-evolutionary history with plants qualify them as extremely valuable for plant life. Due to the microbes’ dual function in enhancing plant productivity and defence against herbivores, across ecological gradients, plants may associate with specific microbes to obtain distinct benefits. The thesis presented here is an attempt to shed light on how plants interact with microorganisms across changing environments. The thesis is composed of three major goals. First, I investigated and dissected the interaction of root-associate microbes (RAMs) and climate in shaping Plantago major growth and defence phenotype across an elevation gradient at the intraspecific level (Chapters I and II). I found that climatic conditions regulate P. major growth traits, while defensive traits where rather genetically fixed (Chapter I). Subsequently, I found that local elevation RAMs promoted the growth of Plantago major populations, while chemical defences were overall higher when low elevation microbes were present (Chapter II). Finally, in Chapter III, I unravelled macro-evolutionary trends in plant colonization levels of arbuscular mycorrhizal fungi (AMF) and the resulting plant growth and defence responsiveness. To do this, I compared 24 species of Plantago L. in a common environment with and without AMF. I found that plant interspecific variation in AMF colonization, and growth and defence plant responsiveness to AMF were driven by both plant's evolutionary history and climatic niche convergence at different levels (Chapter III). My thesis enhanced our understanding of how plants strategically respond to variation in environmental conditions by diverting resources into growth or defence in the presence of soil-borne beneficial microorganisms. The novelty of combining both climatic and biotic factor influencing both plants growth and defence at different scales of life organization may inspire further and deeper investigation on how plant locally adapt to biotic and abiotic conditions across ecosystems.
Biological Control beneath the Feet: A Review of Crop Protection against Insect Root Herbivores
Mycorrhizal Fungi Enhance Resistance to Herbivores in Tomato Plants with Reduced Jasmonic Acid Production
Arbuscular mycorrhizal (AM) fungi favor plant growth by improving nutrient acquisition, but also by increasing their resistance against abiotic and biotic stressors, including herbivory. Mechanisms of AM fungal mediated increased resistance include a direct effect of AM fungi on plant vigor, but also a manipulation of the hormonal cascades, such as the systemic activation of jasmonic acid (JA) dependent defenses. However, how AM fungal inoculation and variation in the endogenous JA production interact to produce increased resistance against insect herbivores remains to be further elucidated. To address this question, three genotypes of Solanum lycopersicum L., a JA-biosynthesis deficient mutant, a JA over-accumulating mutant, and their wild-type were either inoculated with AM fungi or left un-inoculated. Plant growth-related traits and resistance against Spodoptera littoralis (Boisduval) caterpillars, a major crop pest, were measured. Overall, we found that deficiency in JA production reduced plant development and were the least resistant against S. littoralis. Moreover, AM fungi increased plant resistance against S. littoralis, but such beneficial effect was more pronounced in JA-deficient plant than on JA over-accumulating plants. These results highlight that AM fungi-driven increased plant resistance is negatively affected by the ability of plants to produce JA and that AM fungi complement JA-mediated endogenous plant defenses in this system.