Molecular and biochemical characterisation of antagonistic mechanisms of the biocontrol agent Bacillus cereus B25 inhibiting the growth of the phytopathogen Fusarium verticillioides P03 during their direct interaction in vitro

Fusarium verticillioides (Fv) is a mycotoxin-producing phytopathogen causing ear and root rot in maize. Bacillus cereus strain B25 is a maize endophyte and an effective biological control agent against Fv. The B25 genome contains several genes associated with fungal antagonism, including lytic enzymes (chitinases A and B, chitosanase and glycoside hydrolase), siderophores (petrobactin and bacillibactin), antibiotics (surfactin), and biofilm. This study aimed to elucidate which B25 antagonistic mechanisms are implicated in Fv biocontrol. The expression of twelve B25 genes was evaluated by qRT-PCR after 3-30 h of confrontation between B25 and Fv. All B25 genes tested were induced, mainly between 6 and 24 h, while none was induced at 3 h. Lytic enzyme genes were induced at different times, with three of them occurring at 12 h. Chitinase and chitosanase activities were induced after 6 and 12 h, respectively, and gradually increased until 30 h. Biofilm formation was detected only when B25 interacted with Fv. Images of the B25-Fv interaction and measurements at 24 and 30 h showed little branching or abnormal mycelial growth, suggesting cell wall damage. In conclusion, when B25 is confronted against Fv, all B25 antagonistic mechanisms tested appear to be involved in Fv growth control.