Metabolic interactions between Plasmodiophora brassicae and Arabidopsis thaliana plant

Clubroot (Plasmodiophora brassicae) is a serious agricultural problem affecting Brassica crops. It also infects Arabidopsis thaliana plants. During infection, this biotrophic pathogen manipulates the development and metabolism of its host leading to the development of galls. In turn, its own development is strongly influenced by the host. The aim of this study was to understand the metabolic interaction between A. thaliana plants and P. brassicae. An initial non-targeted approach was used to obtain metabolic `fingerprints’, which were then combined with host transcriptomic data. In addition, a targeted approach was applied focusing on carbohydrate metabolism. Hypotheses were identified using transcriptomic data and tested using mutants of A. thaliana and analysis of reporter gene expression.
Changes in plant development occurring as a consequence of clubroot infection correlated with changes in metabolic status were investigated. Following P. brassicae infection, metabolite profiles altered at the beginning of cortical infection, although plant primary growth did not show clear differences between uninfected and infected tissue at this stage. This suggests that these changes in metabolites depended on responses of the plant to infection rather changes in plant development.
The accumulation of the amino acids glutamate, aspartate and alanine are likely to be related to pathogen nutrition. Metabolites such as proline protect plants from osmotic and oxidative stress. Meanwhile, compounds associated with plant defence such as cinnamic acid and phaseic acid accumulated at 16 DPI and decreased at 26 DPI. The accumulation of vitamin B6 precursor and compounds associated with folate biosynthesis were accompanied with increasing host gene expression associated with the synthesis of these metabolites. The accumulation of other metabolites such as thiosulfate was accompanied with the repression of genes associated with their degradation. This suggests that P. brassicae has the potential to suppress the expression of host metabolism genes to obtain nutrients from the host.
Transcriptomic analysis showed that sucrose synthase (SUS) and sugar permeases were induced during gall formation. The impact of inactivating these genes (and cytosolic invertase CINV) on gall formation was examined. In wildtype plants the hypocotyl width was not affected at 16 DPI, but increased by 26 DPI. Similar results were seen in cinv1,2 and sus1-,4 plants at 16 DPI. By 26 DPI, cinv1,2 and sus1-4 plants showed a smaller hypocotyl width than Col-0 plants when uninfected, but this difference was not evident in sus1-4 plants when infected. Infected cinv1,2 plants were smaller than Col-0 plants at 26 DPI, although plasmodia colonized host cells and pathogen development was similar to that in Col-0 plants. This indicates that P. brassicae itself makes the gall a sink. Meanwhile, sweet11,12 mutants displayed slower P. brassicae development due to a change in carbohydrate partitioning. SWEET::GUS expression patterns support the hypothesis that sucrose was transported to plasmodia via these transporters.