Auxin plays an essential role during plant growth and development. In plant cells, the IAA signal is recognized through two different types of receptors. The first one, ABP1, is responsible for activation of H+/ATPases leading to acidification, cell wall loosening and finally cell elongation. In the second signalling pathway IAA binds to F-box proteins, e.g. TIR1, AFB1 and AFB2, which are part of an ubiquitin ligase pathway that degrades Aux/IAA repressor proteins. This affects the expression of auxin responsive genes, among them members of the GH3 family.
In addition to IAA signalling, conjugation of IAA plays an important role during the regulation of auxin homeostasis. The GH3 auxin conjugate synthetases remove active IAA from the total auxin pool and store it as amino acid conjugates. We investigate the role of auxin signalling and conjugation in the increased cell divisions and elongations taking place during the clubroot disease by expression analyses of ABP1, TIR1, AFB2 and GH3s in Arabidopsis during the infection with Plasmodiophora. Infection tests with T-DNA knockout mutants for different GH3 genes show a higher susceptibility against Plasmodiophora. In addition, experiments with selected promotor::GUS lines will examine the influence of Plasmodiophora on IAA homeostasis.