Transcriptome analysis of Arabidopsis thaliana clubroots

The clubroot disease of Brassicaceae is caused by the obligate biotrophic protist, Plasmodiophora brassicae. Infected roots undergo a developmental switch that results in the formation of aberrant roots (clubs). To investigate host gene expression during the development of the disease the Affymetrix 22k chip were used. Two time points were chosen, an early time point, at which the pathogen has colonized the root but has induced only very limited change of host cell and root morphology. At a later time point, more than 60% of the host root cells were colonized and root morphology was drastically altered. At both time points more than 1000 genes were differentially expressed in infected versus control roots. These included genes associated with growth and cell cycle, sugar phosphate metabolism and defense. The involvement of plant hormones in club development was further supported; genes involved in auxin homeostasis, such as nitrilases and members of the GH3 family, were upregulated, whereas genes involved in cytokinin homeostasis (cytokinin synthases and cytokinin oxidases/dehydrogenases) were strongly downregulated already at the early time point. Cytokinin oxidase/dehydrogenase (CKX) overexpressing lines were disease resistant, clearly indicating the importance of cytokinin as a key factor in clubroot disease development.