Effects and modes of action of COS-OGA based elicitors against late and early blight on Solanaceae

The development of alternatives to the use of chemical pesticides constitutes the current challenge of the control of plant diseases. The stimulation of plant immune system using plant defense elicitor constitutes an interesting approach. COS-OGA is a novel plant defense elicitor combining fungal-derived chitooligosaccharides (COS) and plant cell wall-derived oligogalacturonides (OGA). A first formulation of this product, FytoSave®, is now registered in Europe to control powdery mildew in grapevine and cucurbit. Another composition of COS-OGA, FytoSol, is under development by Fytofend SA to enlarge the action range of this elicitor. This thesis aims at studying the potential use of the elicitor COS-OGA to protect Solanaceae against Phytophthora infestans and Alternaria solani. Bioassays were performed to evaluate the efficacy of two formulations of the elicitor, FytoSave® and FytoSol, and to understand their modes of action. Preventive sprayings of both formulations were shown to reduce early and late blight of potato and tomato. However, the levels of protection differed according to the formulation of the elicitor and the plant-pathogen interactions, in particular the types (species and strain) of pathogens and their lifestyle, the genotypes of plant species and the environmental conditions. FytoSol is more efficient against the hemibiotrophic and necrotrophic pathogens than FytoSave®. The modes of action of both formulations were investigated by quantification of defense gene expression and enzymatic activities. Our work confirms that FytoSave® activate a defense response through the salicylic pathway in tomato in interaction with A. solani and in potato in interaction with P. infestans. The protective effect of FytoSol was higher but, despite a significant augmentation of endogenous SA in tomato treated leaves, the molecular pathways induced by this elicitor is yet not so clear. The results of this thesis demonstrated that plant defense elicitors interact differently with plant depending on the plant-pathogen interactions. Further studies using whole transcriptome shotgun sequencing and proteomics are required to better understand the modes of action of plant defense elicitors. These information are important to optimize protective effect of elicitors in order to reinforce the interest to their use for the integrated control and encourage their practical use in agriculture.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 201