Functional analysis of Arabidopsis thaliana plant defensins

When plants are under pathogen attack, several defense mechanisms are induced, including the production of so-called pathogenesis-related (PR) proteins. Although some of these PR proteins exhibit potential in vitro antimicrobial activities and their accumulation in the plant is related with plant resistance responses, a direct functional role in defense has not been demonstrated for all. The major aim of this doctoral thesis was to gain more insight in the role of plant defensins, a family of PR proteins, in the model plant Arabidopsis thaliana. We investigated whether these A. thaliana plant defensins (AtPDFs) had comparable or different in vitro activities and gene expression patterns, and whether they were (in)directly involved in plant defense against an invading pathogen.By using an in planta high-level expression system four different AtPDFs were purified. All purified defensins showed in vitro antifungal activity against a range of fungi. However, this antifungal activity was strongly reduced in the presence of salts. Expression analysis revealed that most AtPDF genes were marginally expressed in leaves, and additionally some genes were abundantly expressed in other organs as seeds and flowers. Upon biotic stress-related treatments several AtPDF genes were induced in leaves. Based on the findings concerning their in vitro activities and expression patterns, three “model” defensins were selected, namely AtPDF1.1, AtPDF1.2 and AtPDF2.3, and their possible direct role in A. thaliana plant defense was investigated. A lower sensitivity to the heterologous sugar beet pathogen Cercospora beticola was shown in transgenic plants overexpressing AtPDF1.1. Concerning AtPDF1.2, no direct role in defense towards any of the tested pathogens could be found. More promising results were obtained with transgenic plants with modulated AtPDF2.3 expression, and it seems this is the first defensin reported to play a role in plant defense against the necrotrophic pathogen Botrytis cinerea. To conclude, in this research it was shown that some AtPDFs could play a direct, though limited, role in plant defense, based on their antimicrobial activities, defense-related expression patterns and the results obtained with transgenic plants.status: publishe