Etude des phytoalexines chez Cucumis sativus: De leur identification chimique a leur role biologique dans la resistance contre Podosphaera xanthii

The main goal of this work was to demonstrate a clear link between the production of phytoalexin compounds and the establishment of induced resistance in cucumber, Cucumis sativus (Curcurbitaceae) against powdery mildew disease caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea). To this end, four objectives were established: (1) revision of conventional phytochemical methodologies used to study phenolic compounds (phytoalexins); (2) application of these new methodologies to isolate and identify phytoalexins produced by disease-resistant cucumber plants; (3) biochemical/molecular examination of the biosynthetic enzymes responsible for the production of these compounds and; (4) application of fluorescence and laser scanning confocal microscopy techniques to examine spatiotemporal aspects of phytoalexin synthesis in planta.; The development of new extraction and purification methods, that would be used throughout the duration of this project, made possible the isolation and structural characterization of phytoalexin compounds produced by disease-resistant cucumber plants. In particular, C-glycosyl flavonoids were identified behaving as phytoalexins within the leaves of disease-resistant plants including two new C-glycosyl flavonoid phytoalexins that were named cucumerin A and cucumerin B.; Biochemical and molecular examination of disease-resistant cucumber plants revealed the induction of mRNA transcripts and increased enzymatic activities for chalcone synthase and chalcone isomerase, two key enzymes of the flavonoid biosynthetic pathway. Moreover, it was shown that following an initial eliciting treatment, the concentration of mRNA transcripts and enzymatic activities for these enzymes peaked more rapidly within disease-resistant plants in response to subsequent eliciting treatments and fungal challenge.; Finally, examination of disease-resistant cucumber plants with fluorescence and laser scanning confocal microscopy coupled with phytochemical analyses demonstrated the accumulation of C-glycosyl flavonoid phytoalexins at the infection site and within the internal structures of the pathogen prior to its collapse. Based on the reported biological activities for C-glycosyl flavonoids and their observed sites of accumulation within the fungus, it was proposed that these compounds convey resistance to cucumber by adversely affecting the proper functioning of enzymes within the pathogen.; Collectively, this body of work demonstrated for the first time, a clear link between the production of C-glycosyl flavonoid phytoalexins and the establishment of induced resistance against powdery mildew disease in cucumber.