[PhD defence] 12/09/2025 - Margot Grimonpont: "Deciphering the modes of action of biocontrol agents for white rot in oilseed rape" (UR INRAE PV)

Ms Margot GRIMONPONT will defend her thesis entitled "Deciphering the modes of action of biocontrol agents for white rot of rapeseed", directed by Mr Marc BARDIN, on Friday 12 September 2025.

Laboratory

• UPR 407 - PV - Plant pathology

Composition of the jury

Mr Marc BARDIN	Avignon University	Thesis supervisor
Ms Noadya MONNIER	Eléphant Vert Group France	Thesis co-supervisor
Ms Eléonore ATTARD	University of Pau	Examiner
Mr Ali SIAH	ISA-Junia	Examiner
Ms Elodie VANDELLE	University of Verona, Italy	Rapporteur
Mr Marc ONGENA	University of Liège	Rapporteur

Summary

The phytopathogenic fungus Sclerotinia sclerotiorum, the causal agent of white rot, infects a wide range of host plants and forms resistance structures that enable it to survive in the soil over the long term. Added to this is the emergence of fungicide-resistant strains and increasing restrictions on their use for environmental and health reasons. All these factors make managing this disease particularly difficult. Against this backdrop, the development of new biocontrol agents appears to be a promising solution to complement integrated protection strategies. However, microbial biocontrol products are often perceived by farmers as not very effective and unreliable in real-life conditions. This lack of stability in protection is generally linked to a still limited knowledge of the factors influencing their efficacy, in particular their modes of action. Given the promising results obtained previously with two strains of Bacillus velezensis against S. sclerotiorum on oilseed rape, under controlled conditions and in the field, this work aimed to characterise their modes of action in detail in order to optimise and guarantee their protective efficacy. To achieve this, four hypotheses were explored: (1) bacteria have a direct effect on S. sclerotiorum via the synthesis of metabolites with antifungal activity; (2) the cells or metabolites synthesised induce defence mechanisms in oilseed rape (Brassica napus); (3) microclimatic factors, such as temperature, influence the implementation of these modes of action and ultimately the effectiveness of protection against S. sclerotiorum on oilseed rape; (4) an automated tool for monitoring infection on oilseed rape leaves over time is used to better characterise the modes of action of biocontrol agents. Various complementary methodological approaches were used to test these hypotheses. The main results obtained highlight the essential contribution of lipopeptides to the direct anti-Sclerotinia activity observed. This result is supported by the observation that maximum protection of oilseed rape is achieved at the temperature corresponding to the peak synthesis of the three main families of lipopeptides (iturins, fengycins, surfactins) by the bacteria. However, bioguided fractionation revealed the presence of additional candidate metabolites, some of which have no homology with molecules frequently described in the literature or even referenced. Treatment of oilseed rape leaves with the bacteria or their metabolites induced overexpression of certain genes involved in the establishment of plant defences, suggesting induction of resistance in oilseed rape. Finally, an automated image acquisition and analysis system was used to identify and discriminate between various relevant variables (reduction in the number of infected leaves, delay in the appearance of symptoms, slowing of fungal progression), suggesting distinct modes of action depending on the treatments applied. These results highlight the major role played by the synthesis of direct metabolites, in particular lipopeptides, and the induction of resistance in the plant in the mode of action of these bacteria. They also suggest the existence of a still largely unexplored reservoir of biomolecules of interest synthesised by these bacterial biocontrol agents. However, the dynamics of the different modes of action and their exact role in the observed efficacy remain to be explored.

Keywords Sclerotinia sclerotiorum, Bacillus velezensis, biocontrol, modes of action, fermentation products, metabolomics, lipopeptides, Modes of action, bacteria, mycology, oilseed rape