<i>Penicillium hordei</i> acidification precipitates <i>Bacillus subtilis</i> 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 <i>Penicillium</i> and <i>Bacillus</i> species can individually suppress soilborne phytopathogens and promote plant growth. Here, we showed that <i>Penicillium hordei</i> and <i>Bacillus subtilis</i> co-culture led to precipitation of <i>B. subtilis</i> lipopeptides, observed as white lines in agar. Metabolomic analysis revealed that the presence of <i>B. subtilis</i> enhanced the production of fungal terrestric acid and its biosynthetic intermediates, which in turn induced lipopeptide precipitation, preventing <i>P. hordei</i> 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 <i>P. hordei</i> to invade and overgrow the <i>B. subtilis</i> 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 <i>B. subtilis</i> 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.