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caroline.pandin@inra.fr

Title : Exploration of mecanisms involved in the bioprotection of Agaricus bisporus by Bacillus subtilis QST713 biofilms.

Keywords : Bacillus velezensis, Biofilm, Biocontrol, Agaricus bisporus, Compost microbiota, Trichoderma aggressivum

Abstract : Worldwide, food losses amount for about one-third of food for human consumption, 1.3 billion tons per year (FAO). A large fraction of these losses is due to microbiological alterations. The use of phytosanitary products remains today the most widely used solution in agriculture to limit these losses. However, with the EcoPhyto 2 plan, the French government aims to reduce the use of chemical pesticides by 50% by 2025, in particular by promoting the emergence of biocontrol. To develop this approach, it is necessary to understand the underlying mechanisms. The different modes of action of biocontrol by the microorganisms described are the stimulation of the natural defenses of the plants, the production of antimicrobial substances and the nutritional competition. The originality of this project is to integrate the biofilm mode of life into bioprotection mechanisms (spatial and nutritional competition, release of antimicrobial principles). In the French sector of the button mushrooms (Agaricus bisporus) culture, the biocontrol agent used since 2008 by
more than 80% of the sector, is Bacillus subtilis QST713.This biofungicide shows a clear efficacy against Trichoderma aggressivum, the main mold causing economic losses during the cultivation of A. bisporus. To accompany the sector in this biological pathway, we have sequenced and studied the genome of this strain, in order to determine its biocontrol potential and its ability to form biofilms. We also evaluated the impact of this biofungicide on the dynamics of microbial communities in A. bisporus culture compost exposed or not to T. aggressivum. Finally, the study of the cellular reprogramming of this biocontrol agent during the culture in axenic micromodels allowed us a better understanding of the substrates colonization phenomenon and the inhibition of undesirable flora. This project enriched the knowledge of the biocontrol mechanisms used in the mushroom industry and may allow a possible application to other agricultural sectors.

This thesis is funded by the DIM ASTREA program of the Regional Council of Ile-de-France and was carried out within the Micalis Institute (Microbiology of Food for Health), Joint Research Unit (UMR1319) associating INRA and AgroParisTech and being part of Paris-Saclay University.