Penicillium hordei acidification precipitates Bacillus subtilis lipopeptides to evade inhibition.

Interkingdom interactions are crucial for community and ecosystem function; however, the secondary metabolites mediating interactions between plant beneficial bacteria and fungi remain understudied. Beneficial 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 lines in agar. Metabolomic analysis revealed that the presence of B. subtilis enhanced the production of fungal terrestric acid and its biosynthetic intermediates, which in turn induced lipopeptide precipitation, preventing P. hordei inhibition through 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 the 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.