Intensified rape cultivation during recent years led to increased infestation with Sclerotinia sclerotiorum, a necrotophic fungus and causal pathogen of Sclerotinia stem rot disease (Fig. 1). Today no rape verieties with significant resistance against Sclerotinia are available generating yield losses of up to 50% in rape cultivation. To address this problem we initiated together with the plant breeding service company Saaten Union BioTEC GmbH (Hovedissen) a research project that investigates mechansims of plant defence against Sclerotinia and searches new resistance traits for rape breeding. At the Institute of Molecular Phytopathology we follow two major strategies integrating recources of genetic diversity among wild Brassica species and modern tools of molecular plant biotechnology. 



First, we explore various Brassica napus cultivars and wild Brassica species in their ability to resist an infection with Sclerotinia sclerotiorum. The pathogenesis is monitored on detached leaves infected with Sclerotinia-grown agar plugs by measuring the developing lesion size (Fig. 2).



Among the Brassica collection differences in resistance have been identified and individual lines representing susceptible and resistant genotypes are investigated at the gene transcritpt level. This includes targeted analysis of marker genes derived from various hormone pathways and transcriptome analysis either by Suppresison Subtractive Hybridization (SSH) or direct sequencing. The aim is to find genes whose presents or regulation correlate with increased resistance to Sclerotinia. These genes will be functionally characterized and serve as template for the generation of molecular marker, facilitating breeding the desired trait from a wild species into a commerical Brassica napus cultivar. 

The second approach employs the Arabidopsis system which is closely related to Brassica napus, a host for Sclerotinia sclerotiorum and provides manifold genetic tools. The idea is that some genes when present at high expression level (up-regulated) can confer increased resistance against Sclerotinia. To identify these genes an activation-tagged mutant population (Fig. 3) of Arabidopsis thaliana is screened. This population consists of ca. 50.000 individuals each transformed with a T-DNA containing transript enhancer elements (Weigel et al., 2000) to up-regulate neighbouring genes wherever the T-DNA is inserted in the genome. We established two high-throughput screens of this population: first, in the interaction with Sclerotinia and second, in an in vitro selection based on the tolerance to oxalic acid, a pathogenicity factor of Sclerotinia.



Resistant or tolerant plants are rescued and localisation of the T-DNAs in the genome will disclose gene targets of the enhancer elements. Knowledge gained from this approache will be used to verify gene candidates identified with the first strategy and to investigate gene homologues in resistant Brassica species. Together this project aims for the identification of new resistance traits in Brassica against Sclerotina stem rot disease and to provide molecular tools that allow transfer of theses traits into commercial rape varieties by conventional breeding or plant transformation.