[Defense de thèse] 10 July 2023 - Estelle Turc: "Contribution au développement d'un agent de biocontrôle contre Sclerotinia sclerotiorum sur colza", Plant Pathology Unit, INRAE

Date and place

INRAE, Plant Pathology, Domaine Saint Maurice, 67 All. des Chênes, 84140 Avignon, 10 July 2023 at 9.30 a.m.
Closed session

Discipline

Agricultural sciences

Laboratory

Plant Pathology Unit, INRAE

Framing

• Thesis supervisor: Marc Bardin
• Thesis supervisors: Philippe Nicot & Noadya Monnier

Composition of the jury

• Dr Philippe Jacques, Professor, University of Liège, Rapporteur
• Dr Elsa Ballini, Senior Lecturer, Institut Agronomique de Montpellier, Rapporteur
• Dr Véronique Broussolle, Research Director, INRAE SQPOV Examiner
• Dr Sonia Rippa, Research Engineer, University of Compiègne Examiner
• Dr Marc Tchamitchian, Director of Research, University of Avignon Examiner
• Dr Marc Bardin, Research Director, INRAE, Plant Pathology Unit Thesis supervisor
• Dr Philippe Nicot, Research Fellow, INRAE, Plant Pathology Unit Thesis co-supervisor
• Dr Noadya Monnier, Head of Research and Development, Eléphant Vert Group Thesis co-supervisor

Summary of the thesis

Sclerotinia sclerotiorum is a plant pathogen responsible for sclerotinia or white rot on many plant species. This fungus causes considerable economic losses in economically important crops such as oilseed rape, with yields falling by up to 30%. At present, the fight against this disease in this crop is mainly based on the use of chemical treatments, but alternative and/or complementary methods to chemical substances need to be developed. Two bacterial strains with biocontrol potential against S. sclerotiorumwere selected during preliminary tests carried out under controlled conditions and in the field. The aim of this thesis is to carry out the first stages in the development of a biocontrol product that is antiSclerotinia for foliar treatment of oilseed rape. The specific objectives of the thesis are as follows: (i) to study the mode of action of the bacterial strains selected, (ii) to evaluate the effect of factors such as dose, isolates of the pathogenic fungus, rapeseed genotypes and temperature on the protective efficacy of the bacteria against S. sclerotiorum on rapeseed, (iii) optimising fermenter production and the formulation of selected strains.

To achieve this, a complete genome analysis was carried out, followed by functional validation of modes of action such as antibiosis and induction of rapeseed defences. These modes of action were studied in vitro and in planta using biological, biochemical and molecular techniques. The results show that both bacterial strains are anti-biotic. The results suggest that the production of microbial secondary metabolites may be involved. Stimulation of the oilseed rape plant's defences was also demonstrated through activation of the jasmonic acid/ethylene pathway, the antioxidant system, glucosinolate metabolism and salicylic acid.

The influence of efficacy factors such as the dose, the susceptibility of isolates of the pathogenic fungus and the oilseed rape genotype was assessed using comparative tests. in vitroThe results of these trials were obtained from semi-controlled trials and field trials. The application rate and temperature have a significant effect on the efficacy of biocontrol agents. The dose selected makes it possible to guarantee stable protective efficacy while limiting the potential costs of the final product. Temperature also has an effect on the production of microbial secondary metabolites. Differences in the sensitivity of S. sclerotiorum have been demonstrated in-vitro against bacterial strains. However, this effect has not been demonstrated in planta. The oilseed rape variety influences the effectiveness of one of the biocontrol agents.

Optimising the production of bacteria in fermenters involved selecting a source of nitrogen and carbon, while taking care to choose media with a low economic impact. This optimisation of production involved increasing the scale of production from a microplate culture to a 30 L fermenter. Work on formulation has led to the development of extemporaneous adjuvants in oily and aqueous form, composed of co-formulants with a low ecotoxicological and toxicological impact. Physico-chemical parameters such as the wettability of these adjuvants were analysed and the protective efficacy of bacterial strains in combination with the adjuvants was tested. The development of an oily H 2-1 adjuvant and an aqueous A 1-5 adjuvant resulted in levels of wettability and bacterial strain efficacy comparable to those of a market reference product. These results are therefore promising for the continued development of these strains as biocontrol products.