Défense de thèse de doctorat en sciences biologiques

Study of the mode of action of COS-OGA, a new class of elicitors of plant innate immunity

Jury

Anne LEGREVE (UCL), Marc ONGENA (ULg), Monika HÔFTE (Universiteit Gent), Brigitte MAUCH-MANI (Université de Neufchâtel), Johan MESSIAEN, président (UNamur), Pierre VAN CUTSEM, promoteur (UNamur)

Résumé

There is an urgent need to reduce our dependence on chemical pesticides for plant protection and the stimulation of the plant innate immunity constitutes a promising alternative. The aims of this work was to study the mode of action and the activity spectrum of a novel biological elicitor of plant defenses, COS-OGA. The combination contains fungal-derived chitooligosaccharides (COS) and plant cell wall-derived oligogalacturonides (OGA) which mimic plant interaction with fungi and inform plant cells on both cell wall degradation and pathogen presence.

Preventive sprayings of FytoSave® containing 12.5 g/l COS-OGA were shown to be highly efficient against powdery mildew on grapevine, cucumber and tomato. FytoSave® effect on tomato plants in absence of pathogen showed that upon repeated COS-OGA sprayings, foliar content of the plant hormone salicylic acid (SA) increased. COS-OGA applications also led to overexpression of SA-related genes and proteins while genes linked to jasmonic acid and ethylene were not regulated. These results suggest that FytoSave® cumulatively stimulates SA-dependent systemic acquired resistance.

FytoSave® was also tested against potato late blight provoked by Phytophthora infestans and partly reduced the disease. But FytoSol, another COS-OGA composition, completely protected potato against late blight under controlled conditions using repeated applications short before pathogen infection. Both products induced the expression of defense-related genes but FytoSave® and the necrotrophic stage of P. infestans induced accumulation of SA in potato leaves while FytoSol decreased SA level and seemed to act through other hormonal pathways.

RNA-seq performed on leaves revealed a massive downregulation of potato genes by P. infestans in its biotrophic stage as well as hormonal hijacking while leaves pretreated with FytoSol and inoculated with P. infestans underwent upregulation of many genes encoding peroxidases, glutathione S-transferases, pathogenesis-related proteins, enzymes involved in cell wall turnover and receptor-like kinases, among which several wall-associated kinases. FytoSol enhanced up to variable levels abscisic acid, ethylene and oxylipin pathways but not SA. FytoSol appears to be a promising elicitor that blocks SA-related potato gene regulation by P. infestans and triggers a still unknown defense pathway.

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