| Summary: | The recently-emerged field of chemical genomics is centered on the use of small molecules to perturb biological systems as a means of investigating their function. In order to employ this approach for the study of plant-pathogen interactions, I established an assay in which Arabidopsis thaliana seedlings are grown in liquid media in 96-well plates. Inoculation of these seedlings with a virulent strain of the bacterial phytopathogen Pseudomonas syringae resulted in macroscopic bleaching of the cotyledons of these seedlings. This symptom was used as the basis for high-throughput chemical genomic screens aimed at identifying small molecules that protect Arabidopsis seedlings from infection. One of the first chemicals identified through this screen was the sulfanilamide compound sulfamethoxazole (Smex). This compound was later shown to also reduce the susceptibility of both Arabidopsis and wheat to infection by the fungal pathogen Fusarium graminearum, suggesting a broad spectrum of activity. More detailed investigations of Smex indicated that the protective activity of this compound did not derive from antimicrobial effects, and that this activity was not executed through common defence-related signalling pathways. The folate biosynthetic pathway enzyme dihydropteroate synthase is a known target of sulfanilamides, and it does appear to contribute to Smex-induced disease resistance, albeit in a folate-independent manner. In order to identify downstream mediators of Smex activity, I initiated two forward genetic screens intended to recover mutants with altered sensitivity to Smex in a seedling growth assay. Interestingly, while these screens yielded mutants with striking Smex sensitivity phenotypes, disease resistance phenotypes were not altered. Gene expression profiling of Arabidopsis tissues treated with Smex prior to bacterial inoculation suggested that this compound generally affects lipid signalling. Altogether, it is evident that Smex elicits a complex set of responses in Arabidopsis with apparently non-overlapping phenotypic outputs.
|
|---|
