Biofilm Formation and Synthesis of Antimicrobial Compounds by the Biocontrol Agent Bacillus velezensis QST713 in an Agaricus bisporus Compost Micromodel

Abstract

Bacillus velezensis QST713 is widely used as a biological control agent for crop protection and disease suppression. This strain is used industrially in France for the protection of Agaricus bisporus against Trichoderma aggressivum f. europaeum, which causes green mold disease. The efficacy of this biocontrol process was evaluated in a previous study, yet the mode of its action has not been explored under production conditions. In order to decipher the underlying biocontrol mechanisms for effective biofilm formation by strain QST713 in the compost and for the involvement of antimicrobial compounds, we developed a simplified micromodel for the culture of A. bisporus during its early culture cycle. By using this micromodel system, we studied the transcriptional response of strain QST713 in the presence or absence of A. bisporus and/or T. aggressivum in axenic industrial compost. We report the overexpression of several genes of the biocontrol agent involved in biofilm formation in the compost compared to their expression during growth in broth compost extract either in the exponential growth phase (the epsC, blsA, and tapA genes) or in the stationary growth phase (the tapA gene), while a gene encoding a flagellar protein (hag) was underexpressed. We also report the overexpression of Bacillus velezensis QST713 genes related to surfactin (srfAA) and fengycin (fenA) production in the presence of the fungal pathogen in the compost. IMPORTANCE Biocontrol agents are increasingly used to replace chemical pesticides to prevent crop diseases. In the button mushroom field in France, the use of Bacillus velezensis QST713 as a biocontrol agent against the green mold Trichoderma aggressivum has been shown to be efficient. However, the biocontrol mechanisms effective in the Agaricus bisporus/Trichoderma aggressivum/Bacillus velezensis QST713 pathosystem are still unknown. Our paper focuses on the exploration of the bioprotection mechanisms of the biocontrol agent Bacillus velezensis QST713 during culture of the button mushroom (Agaricus bisporus) in a micromodel culture system to study the specific response of strain QST713 in the presence of T. aggressivum and/or A. bisporus.

Plant metabolomics to the benefit of crop protection and growth stimulation

“…Promoting biofilm formation when developing bioinoculants might thus help to enhance their biocontrol or biostimulant activities. Similar observations have also been made with plant-associated Bacillus strains that overproduce antimicrobial compounds in biofilms with the up-regulation at a transcriptional level of fenA involved in the synthesis of the antifungal cyclic lipopeptide, fengycin; bmyA, involved in the synthesis of the antifungal compound, bacillomycin D; and dhbA, involved in the synthesis of the siderophore, bacillibactin (Pandin et al, 2019). Furthermore, the production of Bacillus bioactive secondary metabolites and developmental processes like biofilm formation and sporulation are modified upon biotic interactions of bacilli PGPR with rhizosphere-inhabiting competitors or fungal pathogens (Andri c, Meyer, & Ongena, 2020).…”

Bacillus Responses to Plant-Associated Fungal and Bacterial Communities

“…Thus, Bacillus cells could specifically sense the presence of fungal competitors and accordingly overproduce appropriate antifungal BSMs to outcompete the interacting fungi. Moreover, besides modulating the production of fengycins and iturins, some strains of B. velezensis (SQR9, FZB42, and QST713) and B. subtilis (B9-5) may overproduce surfactins when sensing phytopathogenic fungi (Li et al, 2014;Chowdhury et al, 2015b;DeFilippi et al, 2018;Pandin et al, 2019). In support to this hypothesis, surfactin production of B. velezensis FZB42 was highly induced in the presence of fungal pathogen R. solani in the lettuce rhizosphere where it was found as the main produced compound (Chowdhury et al, 2015b).…”

Crop Protection against Botrytis cinerea by Rhizhosphere Biological Control Agent Bacillus velezensis XT1

“…With regard to fungal mycelium growth inhibition, several strains of B. velezensis have been reported to suppress the in vitro growth of fungal phytopathogens such as the biotrophic/saprotrophic Trichoderma , the hemibiotrophic Fusarium and the necrotrophics Alternaria and Monilinia [ 64 , 65 , 66 ]. In two previous studies, we demonstrated that strain XT1 inhibits the mycelium growth of Alternaria alternata , Fusarium oxysporum , Magnaporthe oryzae , Sclerotinia sclerotiorum and Thanatephorus cucumeris by over 40%.…”