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Analyse des mécanismes de défense de la carotte (Daucus carota) face au champignon pathogène Alternaria dauci, responsable de l'alternariose ou brûlure foliaire

Abstract : Carrot is the most widely consumed root vegetable worldwide. Carrot production is strongly impacted by various diseases, including Alternaria leaf blight. This disease, caused by the necrotrophic fungus Alternaria dauci, is regarded as the most prejudicial carrot foliar disease. An important goal for carrot breeders is the enhancement of cultivars resistance towards strong and durable levels. This enhancement implies cumulating different partial resistances in one genotype. Understanding the mechanisms of Alternaria leaf blight partial resistance will help developing such strategies. In order to address this question, we developed a three pronged strategy. First, we explored the implication of carrot defense metabolites in partial resistance towards A. dauci. We showed that falcarindiol had a stronger inhibiting effect on fungal growth than 6-methoxymellein (6-MM). Moreover, there is more falcarindiol, and 6-MM accumulates more strongly after inoculation in the resistant genotype (Bolero) than in the susceptible one (Presto). These defense molecules could slow down the fungus development and thus take part in resistance. In one second part, we explored plant cell resistance towards fungal metabolites. This resistance, tested in liquid cultures amongst several cultivars (cell viability, somatic embryogenesis and enzymatic activity), shows a good correlation with the resistance levels of whole plants towards the fungus. Disease resistant genotypes showed moderate (K3) to strong (I2) fungal metabolites resistance. Thus, carrot Alternaria blight resistance mechanisms seem to include toxin resistances. In the final chapter, we explored the role of defense mechanisms in carrot partial resistance. Our first results indicate the implication of the jasmonates pathway, in particular through the overexpression of the PR4 gene in the resistant genotype K3, but not I2. In addition we explored polymorphic sites (SNP) in the sequences of four defense genes. Two genes (PAL, GLP) have a sequence polymorphism between genotypes. In the case of GLP, the SNPs highlighted allow to differentiate a susceptible genotype (H1) from the K3 genotype. During this work, we uncovered different aspect of carrot partial resistance. In particular we can hypothesize that two genotypes (I2 and K3), reach an equivalent partial resistance level, through two quite distinct mechanisms. Beyond the use for a faster breeding of carrot for resistance, these results have more basic implications, as partial resistance mechanisms are still poorly understood, including model plant pathogen interactions.
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Submitted on : Thursday, November 7, 2013 - 3:35:41 PM
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Mickaël Lecomte. Analyse des mécanismes de défense de la carotte (Daucus carota) face au champignon pathogène Alternaria dauci, responsable de l'alternariose ou brûlure foliaire. Biologie cellulaire. Université d'Angers, 2013. Français. ⟨tel-00881134⟩

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