The regulation of rhizosphere bacterial community structure and function by the plant-beneficial protist Naegleria

Protists are known to impact plant performance, making them promising targets for developing sustainable agronomic solutions. However, the mechanism underlying interactions between protists and plant-beneficial bacteria in supporting plant growth remains largely unknown. To help fill this knowledge gap, we examined direct and indirect impacts of protists on the pathogen density and plant growth via a series of in vitro and pot experiments. First, we examined the impact of the two protists (Naegleria sp. and Cercomonas sp.) on individual bacterial strains, including plant pathogen and the propagation strain. After that, we constructed a SynComs (composed of 7 common rhizosphere bacteria) and tested the extent to which the SynComs with protists could impact pathogen density and plant growth. We found the SynComs together with Naegleria exhibited stronger capacities in inhibiting plant pathogen and promoting plant growth, compared with SynComs alone. Naegleria enhanced the biofilm formation capability of SynComs with enriching Bacillus in liquid culture experiments, although Naegleria reduced the biofilm formation of the Bacillus when Bacillus were provided as sole food source for Naegleria. We also tracked the population dynamics of individual SynComs members and found that Naegleria lead to an increase of Bacillus and promoted the correlation between Bacillus and Pseudomonas . Finally, we explored gene expression of biofilm formation and secondary metabolites to validate previous results by metatranscriptomics, and results showed that Naegleria indeed upregulated the expression of the genes related to biofilm formation and secondary metabolites biosynthesis. Our study highlights amoeba Naegleria can promote biofilm information of SynComs by enriching the abundance of plant-beneficial bacteria and promoting the production of secondary metabolites within microbial communities. This study provides valuable insights for utilizing the interaction between predatory protists and rhizosphere microbial communities to optimize plant performance in sustainable agriculture.