# Deciphering the modes of action of bacterial biocontrol agents, candidates for control of Sclerotinia sclerotiorum on oilseed rape

Margot Grimonpont, Marta Gallart, Louise F Thatcher, Estelle Turc, Adrien Lies, Noadya Monnier, Philippe Nicot, Marc Bardin

► To cite this version:

Margot Grimonpont, Marta Gallart, Louise F Thatcher, Estelle Turc, Adrien Lies, et al. Deciphering the modes of action of bacterial biocontrol agents, candidates for control of Sclerotinia sclerotiorum on oilseed rape. Commission "Young IOBC-WPRS", PhD Symposium "Biological Control of Arthropod Pests and Plant Diseases", International organisation for Biological Control, Apr 2024, Cabrils, Spain. hal-05122338

HAL Id: hal-05122338

https://hal.inrae.fr/hal-05122338v1

Submitted on 20 Jun 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Deciphering the modes of action of bacterial biocontrol agents, candidates for control of *Sclerotinia sclerotiorum* on oilseed rape

¹Margot Grimonpont, ²Marta Gallart, ²Louise F. Thatcher, ³Estelle Turc, ³Adrien Lies,

³Noadya Monnier, ¹Philippe Nicot, ¹Marc Bardin

¹ INRAE, Pathologie Végétale, F-84140, Montfavet, France and

²Commonwealth Scientific and Industrial Research Organisation, Black Mountain, Australian Capital Territory, Australia

³Groupe Elephant Vert France, 77700 Serris, France

margot.grimonpont@inrae.fr

**Abstract:** Oilseed rape (*Brassica napus*), one of the main cultivated oleaginous crops worldwide is susceptible to white mold caused by the fungal pathogen *Sclerotinia sclerotiorum*. Control of this necrotrophic and polyphagous fungal pathogen relies mainly on chemicals. Therefore, the development of biocontrol agents appears as a promising option to contribute to a more sustainable production of rapeseed. Previous work by our team has identified two bacteria with particularly promising activity against *S. sclerotiorum* under controlled conditions and in the field. A biocontrol solution based on these bacteria is currently being developed in close collaboration with a company. In order to further develop this solution, we are undertaking the study of the mode(s) of action of these bacteria. Better knowledge of the biocontrol mechanisms of strains and of the abiotic conditions favourable to the expression of the modes of action are required to develop biocontrol agents that are more effective. As part of my PhD, my objectives are to assess the ability of the both candidate bacteria to synthesize metabolites of interest, to identify the metabolites produced, to evaluate the conditions for their synthesis, and to assess the direct effect of these metabolites on the fungus and on the defence mechanisms of the plant.

**Key words:** *Sclerotinia sclerotiorum*, white mold, biocontrol, efficacy, cell-free supernatant

**Summary:** The use of bacteria as biocontrol agents has received considerable attention in the current context due to their ability to naturally suppress plant pathogens by a variety of mechanisms. Plant pathogens can be controlled by a variety of modes of action, including direct antibiosis, parasitism of the pathogen, competition for the ecological niche and/or for nutrients. However, other indirect modes of action can also reduce disease, such as interfering with pathogenesis or stimulating plant defences (Puopolo et al., 2022). Various studies have shown that most plant beneficial bacteria used as biocontrol agents are capable of synthetizing a plethora of compounds with multiple modes of actions, some of them having the ability to inhibit fungal growth. For instance, studies on biocontrol strategies for the management of *Sclerotinia sclerotium* have been reported by Wang et al. (2024) and show inhibition of spore germination, inhibition of mycelial growth and/or hyphal lysis of the pathogen through the production of antifungal compounds.
In this context, one of my objectives is to assess the ability of the two candidate bacteria to synthesize metabolites of interest, to identify the metabolites produced and to assess the direct effect of these metabolites on the fungus.

To this end, the *Sclerotinia*-inhibiting properties of bacterial cell-free supernatant (CFS) have been investigated. *In vitro* assays showed a significant reduction in the mycelial development of the pathogen and its ability to produce sclerotia. Experiments were carried out under controlled conditions on detached rapeseed leaves treated with these CFS. One day after treatment the pathogen was inoculated with a mycelial plug and incubated at 21°C, the temperature which favors its development. Results showed that the cell free supernatant provided a very good protection reaching between 69 and 100% on detached leaves.

To understand the underlying mechanism, the CFS were fractionated using different separation techniques to identify the fractions most active against fungi. Semi-purified fractions were tested using *in vitro* bioassays. High-throughput screening based on imaging was carried out with the fractions obtained to identify ones containing antifungal compounds (Sabburg *et al.* 2021). LC-MS analyses were then performed on the most active fractions to search for active metabolites in the selected fractions.

Further studies are underway to assess the potential of the cell-free supernatant to induce plant defence. To this end, the effect of bacteria and acellular supernatant on rapeseed plant defence mechanisms will be compared.

## References

* Puopolo, G. (Ed.), 2022. Microbial Biocontrol Agents: Developing Effective Biopesticides. CABI, GB, 45-56.

* Wang, S.-Y. *et al.* Biocontrol strategies for the management of *Sclerotinia sclerotiorum* in Wang, S.-Y., Jiang, Y.-H., Chen, X., Herrera-Balandrano, D.D., Simoes, M.F., Shi, X.-C., Laborda, P., 2024. Biocontrol strategies for the management of *Sclerotinia sclerotiorum* in Brassica species: A review. Physiological and Molecular Plant Pathology 102239.

* Sabburg, R., Gregson, A., Urquhart, A.S., Aitken, E.A.B., Smith, L., Thatcher, L.F., Gardiner, D.M., 2021. A method for high-throughput image-based antifungal screening. Journal of Microbiological Methods 190, 106342.