Plants produce complex mixtures of secondary metabolites to defend themselves from pathogens. Among these defenses are metabolites produced de novo, phytoalexins, and constitutive metabolites, phytoanticipins. As a counter-attack, pathogenic fungi are able to transform such plant defenses utilizing detoxifying enzymes. This thesis investigates the metabolism of two important cruciferous phytoalexins (brassinin (33) and camalexin (39)) by the phytopathogenic fungus Botrytis cinerea and the metabolism of cruciferous phytoanticipins (glucosinolates and derivatives) by three economically important fungi of crucifers Alternaria brassicicola, Rhizoctonia solani and Sclerotinia sclerotiorum to investigate their role in cruciferous defense. In the first part of this thesis, the transformations of brassinin (33) and camalexin (39) by B. cinerea were investigated. During these studies a number of new metabolites were isolated, their chemical structures were determined using spectroscopic techniques, and further confirmed by synthesis. Camalexin (39) was transformed via oxidative degradation and brassinin (33) was hydrolyzed to indoly-3-methanamine (49). The metabolic products did not show detectable antifungal activity against B. cinerea, which indicated that these transformations were detoxification processes. Camalexin (39) was found to be more antifungal than brassinin (33). In the second part of this thesis, the metabolism of glucobrassicin (86), 1-methoxyglucobrassicin (87), 4-methoxyglucobrassicin (90), phenylglucosinolate (65), and benzylglucosinolate (66), the corresponding desulfoglucosinolates and derivatives by three fungal pathogens (A. brassicicola, R. solani and S. sclerotiorum) was investigated and their antifungal activity against the same pathogens was tested. Aryl iii glucosinolates 65 and 66 were metabolized by A. brassicicola but not by R. solani or S. sclerotiorum, whereas indolylglucosinolates were not metabolized by any pathogen. Indolyl desulfoglucosinolates (159 and 233) were transformed by R. solani and S. sclerotiorum to the corresponding carboxylic acids and indolyl acetonitriles 40, 102, and 103 were also metabolized to the corresponding carboxylic acids by all pathogens. None of the glucosinolates or their desulfo derivatives showed antifungal activity, but some of their metabolites showed low to very high antifungal activities. Among these metabolites, diindolyl-3-methane (113) showed the highest antifungal activity, and benzyl isothiocyanate (170) showed higher inhibitory effect against R. solani and S. sclerotiorum, but did not inhibit the growth of A. brassicicola. The cell-free extracts of A. brassicicola, R. solani, and S. sclerotiorum were tested for myrosinase activity against several glucosinolates. The cell-free extracts of mycelia of A. brassicicola displayed higher myrosinase activity for sinigrin (131), phenyl and benzyl glucosinolates 65 and 66, but lower activities for glucobrassicin (86) and 1-methoxyglucobrassicin (87); no myrosinase activity was detected in mycelia of either R. solani or S. sclerotiorum.

Creeping bentgrass (Agrostis stolonifera L.) is commonly used on golf course greens and fairways in cool-humid regions but is plagued by numerous fungal diseases, one of which is dollar spot disease (Sclerotinia homoeocarpa F. T. Bennett). Dollar spot occurs frequently throughout the growing season requiring biweekly fungicide applications for complete control. The objective of this study was to investigate methods of reducing the number of fungicide applications needed to maintain dollar spot at acceptable levels through dew removal and potential mechanisms of resistance in bentgrass. In the first study, a combination of mowing three times a week and dragging by hose the remaining four days to remove dew was used in an attempt to reduce disease severity. The main effect of this combination treatment was not significant (p>0.05) and did not reduce the number of fungicide applications compared to normal mowing three times a week. However, dollar spot was managed curatively with 20-80% fewer applications compared to a normal preventative fungicide program. In the second experiment, two experimental germplasms with varying disease resistance were tested for the possible production of antifungal compounds known as phytoanticipins. Preliminary results indicate the resistant line may contain compounds not present in the susceptible line.

La transmission aux semences est l’un des moyens les plus efficaces de survie et de dispersion pour les champignons phytopathogènes. Les semences ainsi contaminées sont altérées dans leur germination et leur viabilité. De ce fait, nous avons cherché à identifier des mécanismes moléculaires qui pourraient être impliqués dans cette capacité de transmission en utilisant le pathosystème modèle Alternaria brassicicola - Arabidopsis thaliana. Pour cela, nous avons analysé la réponse d’A. brassicicola soumis à différentes contraintes in vitro et in vivo :l’exposition à des métabolites de défenses de la famille des Brassicacées (brassinine,camalexine et isothiocyanate) et à des perturbations de la balance hydrique (dessiccation,sorbitol and PEG) ainsi que lors de la colonisation de la semence à partir des siliques. Nous avons montré que la cible probable des phytoalexines indoliques était la mitochondrie avec notamment une altération de la respiration et du potentiel membranaire mitochondrial après une courte exposition. Nos analyses ont aussi révélé que plusieurs protéines de type hydrophilines-like ou en lien avec formation des eisosomes, semblaient être impliquées dans la réponse au stress hydrique. Nous avons également montré que l’expression de la majorité des gènes codant ces protéines était dépendante d’au moins une des trois protéines kinases,AbSch9, AbNik1 and AbHog1. Enfin, nos analyses in planta ont permis d’identifier un mécanisme inattendu, impliquant le remodelage de la chromatine comme élément potentiel,de la régulation de l’expression génique du champignon lors de l’infection. / Seed transmission is one of the most effective means of survival and dispersal for plant pathogenic fungi. The contaminated seeds are altered in their germination and viability. As a result, we have sought to identify molecular mechanisms that could be involved in this transmission capacity using the Alternaria brassicicola - Arabidopsis thaliana pathosystem model. To do this, we analyzed the response of A. brassicicola subjected to different stresses in vitro and in vivo: exposure to defence metabolites of the Brassicaceae family (brassininin, camalexin and isothiocyanate) and to perturbations of the water balance (desiccation, sorbitol and PEG) as well as during seed colonization from silicics. We have shown that the likely target of indolic phytoalexins is mitochondria, including impaired respiration and mitochondrial membrane potential after short exposure. Our analyses also revealed that several hydrophiline-like proteins or proteins related to eisosome formation appeared to be involved in the response to water stress. We have also shown that the expression of the majority of genes encoding these proteins is dependent on at least one of the three protein kinases, AbSch9, AbNik1 and AbHog1. Finally, our in planta analyses identified an unexpected mechanism, involving the remodelling of chromatin as a potential element in regulating the gene expression of the fungus during infection.