Quantitative resistance to clubroot in Brassica crops

Clubroot is a plant disease caused by the eukaryotic biotrophic pathogen Plasmodiophora brassicae in vegetables and crop plants within the Brassicaceae family. Infected plants develop large galls in the root system while the aerial plant body is dwarfed and stressed (e.g. wilting, chlorosis, premature senescence). Symptoms result from changes of the host’s primary and secondary metabolism, alterations in host stem cell maintenance and differentiation, and perturbations of vascular development. Clubroot disease is a worldwide problem and cannot be controlled effectively. Existing resistance is monogenic, unstable due to the genetic variability of the pathogen, and has already been broken down in the field. This project aims to identify robust, quantitative measures of disease development that could be exploited in breeding programmes to develop plants with durable, polygenic quantitative resistance to clubroot infection. It utilised a panel of Brassica plants (ASSYST panel) that represent the majority of genetic diversity in current Brassica crops. Initial experiments indicated that changes in above-ground plant development might act as a non-invasive measure of below-ground disease development. A large-scale phenomics screen was initiated but was unsuccessful as variation in environmental conditions within the phenomics facility resulted in highly variable symptom development. Smaller scale screens were then used to examine alternative approaches. A combination of above and below-ground biomass measurements, coupled with qRT-PCR assays of pathogen content, provided precise measurements of disease development. Of particular note was the observation that plants with visually similar gall development varied markedly in pathogen development and root-shoot ratio, indicating the presence of multiple underlying mechanisms. Metabolomics methods were also used to identify metabolic signatures of disease development in leaves and roots of susceptible and resistant cultivars. Together, these approaches have the potential to act as non-destructive measures of disease development for use in clubroot resistance breeding programmes.