The interaction of the obligate biotrophic pathogen Plasmodiophora brassicae with host plants of the family Brassicaceae leads to the formation of root galls which create a strong metabolic sink, and reprograms host growth and development. Ultimately, the plant succumbs to the loss of both root nutrients and water, and assimilates are redirected from leaves to roots. Central regulators of these changes are the plant hormones auxins and cytokinins. Plant hormones such as abscisic acid might be involved in drought protection of the host roots during late stages of pathogen development. However, other signalling molecules might play important roles in defence reactions. It is long known that salicylic acid is induced in plants during defence responses with a methyl systemic signal that confers resistance to the upper parts of the plant, but whether this plays a role in the establishment of the clubroot pathogen is not yet clear. Jasmonic acid and ethylene might also play a role in the intricate relationship between the obligate biotrophic pathogen and its host.