Identification of quantitative trait loci controlling partial clubroot resistance in new mapping populations of Arabidopsis thaliana.

To date, mechanisms of partial quantitative resistance, under polygenic control, remain poorly understood, studies of the molecular basis of disease resistance have mainly focused on qualitative variation under oligogenic control. However, oligogenic conferred resistance is rapidly overcome by the pathogen and knowledge of the relationship between qualitative and quantitative resistance is necessary to develop durably resistant cultivars. In this study, we exploited the Arabidopsis thaliana-Plasmodiophora brassicae pathosystem to decipher the genetic architecture determining partial resistance. This soil-borne pathogen causes clubroot, one of the economically most important diseases of Brassica crops in the world. A quantitative trait locus (QTL) approach was carried out using two segregating populations (F(2) and recombinant inbred lines) from crosses between the partially resistant accession Burren and the susceptible accession Columbia. Four additive QTLs (one moderate and three minor) controlling partial resistance to clubroot were identified, all the resistance alleles being derived from the partially resistant parent. In addition, four epistatic regions, which have no additive effect on resistance, were also found to be involved in partial resistance. An examination of candidate genes suggested that a potentially diverse array of mechanisms is related to the different QTLs. By fine-mapping and cloning these regions, the mechanisms involved in partial resistance will be identified.