Interaction of Arabidopsis Thaliana with Plasmodiophora Brassicae

Plasmodiophora brassicae is a protistan pathogen that attacks roots of brassicaceous plant species causing devastating disease. Resistance is characterised by restriction of the pathogen and susceptibility by the development of severely malformed roots (‘clubroots’) and stunting of the plant that is associated with alterations in the synthesis of cytokinin and auxin hormones. We are examining the susceptible response in Arabidopsis and whether suppression of key resistance factors by the pathogen contributes to susceptibility. The interaction is being studied using a number of approaches including microscopy of the infection process and development of the pathogen within roots and host gene expression analysis. Quantitative PCR was used to confirm the timing of infection of roots and showed that infection occurred at day four and colonisation increased thereafter to high levels by 23 days after inoculation by which time roots were showing systemic abnormalities. To investigate the basis of this compatible interaction we have conducted a time course experiment following infection of a susceptible ecotype of Arabidopsis (Col-0) to examine whole genome geneexpression changes in the host. Differential gene expression analysis of inoculated versus control roots showed that a higher number of genes had altered expression levels at day four compared to that at day seven and at day ten. At day four the expression levels of several genes known to be important for recognition and signal transduction in resistant interactions and genes involved in the biosynthesis of lignin, phenylpropanoids and ethylene were suppressed. Suppression by P. brassicae of specific plant defence responses appears to be a key component of susceptibility in this system.<br>