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Thesis started octobre 1st, 2012, defended october 22th, 2015
Fund: CJS INRA
Direction: Maria Manzanares & Antoine Gravot

Abstract:

The use of resistant varieties, in combination with adequate agricultural practices, is a promising strategy to control clubroot, a Brassica root disease caused by Plasmodiophora brassicae. However, few resistant varieties are currently available and most of them harbour qualitative resistances that have been overcome. The use of quantitative resistance, by combining different genetic factors or Quantitative Trait Loci (QTL), could enhance the potential of resistance durability. Understanding both the mode of action and the effect on physiological responses induced during the interaction can optimize the choice and the deployment of these QTL. In this context, the aim of this work is to characterise cellular mechanisms underlying two QTLs controlling partial quantitative resistance to clubroot in the Arabidopsis Bur-0 accession. The kinetic expression of the resistance was assessed through histopathological and molecular approaches and the effect of the two QTL was validated by using Heterogeneous Inbred Families, obtained from a cross between Bur-0 x Col-0 (susceptible). Salicylate and jasmonate-dependent defense responses and secondary indolic metabolites were studied in infected roots of the different genotypes. Results showed that partial resistance was characterised by a reduction of both clubroot symptoms and multiplication of the pathogen during the secondary phase of infection. The mode of action of the two QTL was distinct: the QTL Pb-At1 decreased galls development whereas the QTL Pb-At5.2 diminished both symptoms development and pathogen multiplication. Transcriptional and metabolomic analysis showed that the QTL Pb-At5.2 controlled both the salicylate pathway and the accumulation of camalexin during infection. The implication of these defense responses in partial quantitative resistance was confirmed by pharmacological and genetic studies. In vitro approaches showed that camalexin antagonise auxin-responses suggesting that this phytoalexin could play a signalling role. The transfer of these results to other Brassica species could, in the future, contribute to the optimization of the construction and the deployment of resistant varieties.