Plant performance is impacted by rhizosphere bacteria. These bacteria are subjected to both bottom-up control by root exudates as well as top-down control by predators, particularly protists. Protists stimulate plant growth-promoting microbes resulting in improved plant performance. However, knowledge of the mechanisms that determine the interconnections within such tripartite protist–bacteria–plant interactions remains limited. We conducted experiments examining the effects of different densities of the predatory protist Cercomonas lenta on rhizosphere bacterial communities, specifically zooming on interactions between Cercomonas lenta and key bacterial taxa, as well as interactions among key bacterial taxa. We tracked rhizosphere bacterial community composition, potential microbial interactions, and plant performance. We found that Cercomonas lenta inoculation led to an average increase in plant biomass of 92.0%. This effect was linked to an increase in plant growth-promoting rhizobacteria (Pseudomonas and Sphingomonas) and a decrease in bacteria (Chitinophaga) that negatively impact on plant growth-promoting rhizobacteria. We also found evidence for cooperative enhancements in biofilm formation within the plant growth-promoting rhizobacterial consortium. Cercomonas lenta enhanced a plant growth-promoting rhizobacterial consortium colonization by promoting its cooperative biofilm formation in the rhizosphere, leading to a 14.5% increase in phosphate solubilization that benefits plant growth. Taken together, we provide mechanistic insights into how the predatory protist Cercomonas lenta impacts plant growth, namely by stimulating plant beneficial microbes and enhancing their interactive activities such as biofilm formation. Predatory protists may therefore represent promising biological agents that can contribute to sustainable agricultural practices by promoting interactions between the plant and its microbiome.
