Penicillium hordei acidification precipitates Bacillus subtilis lipopeptides to evade inhibition

Interkingdom interactions are crucial for community and ecosystem functioning, however secondary metabolites mediating interactions between plant beneficial bacteria and fungi remain understudied. Penicillium and Bacillus species can individually suppress soilborne phytopathogens and promote plant growth. Here, we showed that Penicillium hordei and Bacillus subtilis co-culture led to precipitation of B. subtilis lipopeptides, observed as white line in agar. Metabolomic analysis revealed B. subtilis triggered enhanced production of fungal terrestric acid and its biosynthetic intermediates, which induced lipopeptide precipitation to prevent P. hordei inhibition by chemical inactivation and physical barrier formation. Besides lipopeptide precipitation, terrestric acid-mediated acidification progressively reduced production of antifungal plipastatins. The lack of lipopeptide production permitted P. hordei to invade and overgrow B. subtilis colony. We demonstrated that the white line phenomenon was conserved among closely related fungi via secretion of terrestric, fulvic or barceloneic acids. Furthermore, terrestric acid at specific concentrations acted as a universal metabolite that drives B. subtilis lipopeptide precipitation even in distantly related fungi. This study provides new insights into acidification as a fungal defensive strategy that may promote co-existence with beneficial bacteria exhibiting strong antagonistic potential, thereby contributing to the formation of a stable rhizosphere community.