Biofertilizer induces soil disease suppression by activating pathogen suppressive protist taxa

Given the increasing demand for sustainable agricultural solutions utilizing the microbiome, particularly the use of biofertilizer (BF), it is essential to understand the mode of action and the role of predatory protists, along with their interactions with biocontrol strains and resident community members. We therefore examined these interactions through a long-term field experiment and a series of greenhouse and pot experiments. In field and greenhouse studies, we observed that Bacillus significantly stimulated the growth of Cercomonas, a genus of predatory protists, in the soil. In turn, these protists promoted the growth of Bacillus, leading to increased detection of polyketide synthase (PKS) genes and the inhibition of bacterial wilt pathogen Ralstonia solanacearum. We here reveal a positive feedback loop between the biocontrol agent Bacillus and predatory protists, which explains the biofertilizer-induced reduction of plant pathogens. These findings highlight the significance of synergistic interactions between functional microbes and predatory protists in suppressing soil-borne diseases. Moreover, it underscores the potential of incorporating predator-prey interactions into agricultural practices to foster more sustainable ecosystem development.