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Als planten aangevallen worden door microbiële belagers, activeert de plant verschillende defensiemechanismen, waaronder de productie van zogenoemde pathogenese-gerelateerde (PR) proteïnen. Hoewel sommige van deze PR proteïnen in vitro antimicrobiële activiteit bezitten en hun accumulatie in de plant gerelateerd is aan de afweerrespons, kon nog niet voor al deze eiwitten een directe tussenkomst in de verdediging van de plant aangetoond worden. De doelstelling van dit onderzoek was meer inzicht te verkrijgen in de rol van plantdefensinen, een bepaalde familie van PR proteïnen, in de modelplant Arabidopsis thaliana. We onderzochten of deze A. thaliana plantdefensinsen (AtPDFs) vergelijkbare of verschillende in vitro activiteiten en genexpressiepatronen bezitten, en of deze defensinen (in)direct tussenkomen in de verdediging van de plant tegen pathogenen. Door gebruik te maken van een in planta productiesysteem slaagden we erin om 4 verschillende AtPDFs te zuiveren. Deze defensinen vertoonden in vitro allemaal schimmelwerende activiteiten, die echter sterk gereduceerd werden in aanwezigheid van zouten in het testmedium. Er werd aangetoond dat de meeste AtPDF genen een laag basaal expressieniveau vertoonden in bladeren, en dat sommige genen sterk tot expressie kwamen in andere organen zoals zaden en bloemen. Daarnaast werden verscheidene AtPDF genen geïnduceerd in bladeren bij behandelingen met biotische stress-gerelateerde elementen. Gebaseerd op de in vitro antifungale activiteit en expressiepatronen, selecteerden we drie “modeldefensinen”, namelijk AtPDF1.1, AtPDF1.2 en AtPDF2.3, en onderzochten hun mogelijke tussenkomst in de verdedigingsrespons van A. thaliana. We toonden aan dat transgene planten, die AtPDF1.1 tot overexpressie brachten, een verminderde gevoeligheid vertoonden voor de suikerbietpathogeen Cercospora beticola. Voor het AtPDF1.2 defensine kon echter geen directe rol in plantafweer tegen de geteste schimmels aangetoond worden. De meest belovende resultaten werden behaald met transgene planten waarvan het AtPDF2.3-expressieniveau gemoduleerd werd. AtPDF2.3 blijkt het eerste defensine waarvan de betrokkenheid in de verdediging tegen de necrotrofe schimmel Botrytis cinerea beschreven werd. Als besluit kan gesteld worden dat, gebaseerd op de antimicrobiële activiteit, de afweer-gerelateerde expressiepatronen en de behaalde resultaten met ziektetesten uitgevoerd op transgene planten, sommige plantdefensinen een directe, doch beperkte, rol kunnen spelen in de plantverdediging. 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.

Arabidopsis thaliana --- Modèle --- Models --- Relation hôte pathogène --- Host pathogen relations --- Résistance aux maladies --- Disease resistance --- Code génétique --- genetic code --- Diathèse --- Diathesis --- Academic collection --- 632.938 --- 66.098 --- 582.683.2 --- Immunology. Plants resistant to diseases and pests. Immunization of plants --- Biological processes. Biotechnology --- Cruciferae. Stocks. Cresses. Water cress. Mustards. Radishes. Rape. Turnip. Wild cabbage. Rockets, Arabidopsis. Candytuft. Shepherd's purse. Woad. Brassicaceae --- Theses --- 582.683.2 Cruciferae. Stocks. Cresses. Water cress. Mustards. Radishes. Rape. Turnip. Wild cabbage. Rockets, Arabidopsis. Candytuft. Shepherd's purse. Woad. Brassicaceae --- 66.098 Biological processes. Biotechnology --- 632.938 Immunology. Plants resistant to diseases and pests. Immunization of plants

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Arabidopsis --- Résistance induite --- Induced resistance --- Maladie des plantes --- Plant diseases --- Insecte nuisible --- Pest insects --- Génie génétique --- genetic engineering --- 582.683.2 --- 632.938 --- Cruciferae. Stocks. Cresses. Water cress. Mustards. Radishes. Rape. Turnip. Wild cabbage. Rockets, Arabidopsis. Candytuft. Shepherd's purse. Woad. Brassicaceae --- Immunology. Plants resistant to diseases and pests. Immunization of plants --- Theses --- 632.938 Immunology. Plants resistant to diseases and pests. Immunization of plants --- 582.683.2 Cruciferae. Stocks. Cresses. Water cress. Mustards. Radishes. Rape. Turnip. Wild cabbage. Rockets, Arabidopsis. Candytuft. Shepherd's purse. Woad. Brassicaceae