Enhanced Transport of Bacteria Along Root Systems by Protists Can Impact Plant Health

Soil protists have been shown to contribute to the structure and function of the rhizosphere in a variety of ways. Protists are key contributors to nutrient cycling through the microbial loop, where biomass is digested by protists and otherwise stored nutrients are returned to the environment. Protists have also been shown to feed on plant pathogenic bacteria and alter root microbiomes in ways that may benefit plants. Recently, a mechanism involving bacterial transport, facilitated by protists, has been hypothesized to contribute to the spatial distribution of bacteria in the rhizosphere. Here, we observe the differential abilities of three soil protists: a ciliate ( sp.), a flagellate ( sp.), and a naked amoeba () to transport nitrogen-fixing to infectible root tips. Co-inoculation of protists plus resulted in the movement of bacteria, as measured by the presence of nitrogen-fixing nodules, up to 15 cm farther down the root systems when compared to plants inoculated with alone. Co-inoculation of the ciliate sp., with , resulted in shoot weights that were similar to plants that grew in nitrogen-replete potting mix. sp.-feeding style and motility likely contributed to their success at transporting bacteria through the rhizosphere. We observed that the addition of protists alone without the co-inoculum of resulted in plants with larger shoot weights than control plants. Follow-up experiments showed that protists plus their associated microbiomes were aiding in plant health, likely through means of nutrient cycling.IMPORTANCEProtists represent a significant portion of the rhizosphere microbiome and have been shown to contribute to plant health, yet they are understudied compared to their bacterial and fungal counterparts. This study elucidates their role in the rhizosphere community and suggests a mechanism by which protists can be used to move bacteria along plant roots. We found that the co-inoculation of protists with nitrogen-fixing beneficial bacteria, , resulted in nodules farther down the roots when compared to plants inoculated with S. alone, and shoot weights similar to plants that received nitrogen fertilizer. These data illustrate the ability of protists to transport viable bacteria to uninhabited regions of the root system.