Identification de gènes clés de la résistance quantitative à la bactérie pathogène Xanthomonas campestris chez Arabidopsis thaliana par analyse QTL et génétique d'association – Quantirex

Pathogens are a threat for crops and natural plant populations. A major challenge in plant breeding is to identify the genetic and molecular bases for natural resistance variation in plant species. The identification of genes underlying natural resistance variation might have enormous implications for human health by increasing crop yield and quality. Disease resistance is constituted by an elaborate, multilayered system of defense. One of the most effective systems developed at the species level to prevent pathogen penetration is called nonhost resistance. This defense system shows some similarity with the mammalian innate immunity and is associated with multiple signal transduction events. A second layer of defense, occurring in certain varieties or accessions of plants in response to certain isolates or races of a pathogen species, is the so-called race specific resistance. Much research has focused on this form of resistance which is generally inherited as a monogenic trait and is determined by the concomitant presence of a resistance (R) gene in the plant and the corresponding avirulence (avr) gene in the pathogen. A hypersensitive response (HR) is often triggered in plants when R gene products recognized avr gene products. HR is a complex, early defense response that causes necrosis and cell death to restrict the growth of a pathogen. HR is observed when plants are infected with high pathogen densities. Thus, race specific resistance in A. thaliana is usually considered a qualitative trait. In crops and natural plant populations, quantitative resistance is much more prevalent than specific resistance. Quantitative resistance corresponds to plant response to pathogen densities for which no HR is observed. While much progress in understanding the molecular and genetic bases of disease resistance in plants has been made during the past years through studies of simple forms of resistance (R genes), almost nothing is known about the molecular bases of quantitative resistance, a form of resistance prevalent in nature. The present project is aimed at understanding the molecular mechanisms underlying quantitative resistance in Arabidopsis thaliana to Xanthomonas campestris, a worldwide pathogen affecting crop yield. In order to successfully complete the long-term objective of this project, the expertise of 2 different teams in Arabidopsis pathology and their common interest in identifying novel resistance mechanisms will be used. It aims to establish a link between molecular physiology and population genetics for quickly increasing the ability to identify and validate candidate genes associated to quantitative resistance to Xcc. Adopting an interdisciplinary strategy (genomics,population genetics and functional validation) and an original method recently developped in A. thaliana to fine map candidate genes (Genome Wide Association mapping) will permit to fully take benefit of natural variation in A. thaliana to describe new quantitative resistance candidates genes. Finally, the identification of a gene conferring resistance to different strains of Xanthomonas would be a major breakthrough from both a fundamental and an applied perspective. Indeed, because A. thaliana is closely related to Brassica crop species, identified quantitative resistance genes in A. thaliana will provide good candidate genes for plant breeders.