Successful pathogens often induce plant developmental pathways to suppress plant defence. As a result, diseased plants frequently display symptoms such as hypertrophy, hyperplasia, hypocotyls elongation and increase of bushiness. Plasmodiophora brassicae is such a pathogen. P.brassicae is a biotrophic fungus, the causal agent of clubroot disease of brassica and Arabidopsis. This disease is characterized by the production of galls on the root of the plant. To produce the gall, the pathogen induces the local accumulation of two phytohormones: auxin and cytokinin in plant roots. These hormones activate the division and the enlargement of meristematic root cells resulting in the production of this “new organ”. Previous studies demonstrated that auxin and cytokinin are able to suppress the induction of salicylic acid (SA), which controls the main defence pathway against biotrophic pathogen. Using both pathogen-free and Arabidopsis-P.brassica experimental systems, I aim to understand the mechanism by which auxin and cytokinin interact with SA signalling in roots, and how the induction of a developmental program interferes with SA signalling. I will first examine which parts of the auxin and cytokinin pathways are required to suppress SA and to promote P.brassicae virulence. In addition to the perception of phytohormones (auxin and cytokinin for example) the induction of the developmental program is controlled by the presence of “master regulator”, often transcription factors, and epigenetic modification of the DNA. Thus, next, I will examine the involvement of the key transcription factors and epigenetic modification in the suppression of SA signalling and in P.brassicae virulence. The project will improve our knowledge on the cross talk between growth hormones and plant defence against pathogens and will contribute to develop a new generation of resistant plants.
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