What can we learn from clubroots: alterations in host roots and hormone homeostasis caused by Plasmodiophora brassicae

Abstract

The clubroot disease of cruciferous crops is caused by an obligate biotrophic protist, Plasmodiophora brassicae. The disease is characterized by the development of large root galls accompanied by changes in source-sink relations and the hormonal balance within the plant. Since the disease is difficult to control, it is of high economic interest to understand the events leading to gall formation. In this review we will give an overview on the current knowledge of changes brought about in the host root by this obligate biotrophic pathogen. Emphasis will be on the regulation of changes in plant hormone homeostasis, mainly auxins and cytokinins; the possible role of secondary metabolites, especially indole glucosinolates, in gall formation and auxin homeostasis will be discussed. Also, results from mutant analysis and microarrays using the model plant Arabidopsis thaliana are presented.

Analysis of global host gene expression during the primary phase of the Arabidopsis thaliana–Plasmodiophora brassicae interaction

“…Several studies indicate that there is an increase in cytokinins at late time points during root gall formation (Devos et al 2006;Siemens et al 2006;Ludwig-Müller and Schuller 2008). The current microarray results confirm the involvement of cytokinins at the early infection stage.…”

“…The jar1 mutant was more sensitive to clubroot infection, according to Siemens et al (2002), and was also confirmed to be susceptible to the Australian population of P. brassicae. Induction of one lipoxygenase gene at 23 DAI has been shown by Siemens et al (2006), indicating a potential role for jasmonic acid during gall development (Ludwig-Müller and Schuller 2008). In contrast, several genes involved in the ethylene pathway (1-aminocyclopropane-1-carboxylate synthase 2 and a putative ethylene receptor, ETR2) were downregulated at the early time point.…”

“…Only one ABA-responsive protein induced as a defence response was upregulated at 10 DAI and downregulated at 7 dAI. The data might indicate that these plant hormones, which are thought to be important for later infection stages (Siemens et al 2006;Ludwig-Müller and Schuller 2008), are not important for the early phase of development of P. brassicae in A. thaliana.…”

“…The role of hormones such as jasmonic acid, ethylene, cytokinins and auxins during clubroot development has recently been reviewed by Ludwig-Müller and Schuller (2008). Several studies indicate that there is an increase in cytokinins at late time points during root gall formation (Devos et al 2006;Siemens et al 2006;Ludwig-Müller and Schuller 2008).…”

“…Only 20 protein spots in total were differentially regulated including proteins involved in lignin biosynthesis, cytokinin metabolism, glycolysis, intracellular calcium homeostasis and the detoxification of reactive oxygen species (ROS). Furthermore, a review of the interactions of P. brassicae with its hosts (Ludwig-Müller and Schuller 2008) once again emphasised that although the components of early signalling in the interaction between A. thaliana and leaf pathogens are well understood, there are relatively few studies with root pathogens and indeed none conducted with P. brassicae. A comparative study between resistance and susceptibility is not possible for the interaction of P. brassicae with A. thaliana, as almost all the A. thaliana ecotypes tested so far are susceptible to P. brassicae (Siemens et al 2002;Agarwal et al 2009).…”