Soil protists can actively redistribute beneficial bacteria alongMedicago truncatularoots

The rhizosphere is the region of soil directly influenced by plant roots. The microbial community in the rhizosphere includes fungi, protists, and bacteria, all of which play a significant role in plant health. The beneficial bacteriumSinorhizobium melilotiinfects growing root hairs on nitrogen-starved leguminous plants. Infection leads to the formation of a root nodule, whereS. meliloticonverts atmospheric nitrogen to ammonia, a usable form of nitrogen for plants.S. melilotiis often found in biofilms and travels slowly along the roots, leaving developing root hairs at the growing root tips uninfected. Soil protists are an important component of the rhizosphere system who prey on soil bacteria and have been known to egest undigested phagosomes. We show that the soil protist,Colpoda sp., can transportS. melilotidownMedicago truncatularoots. By using pseudo-3D soil microcosms, we directly observed the presence of fluorescently labelledS. melilotialongM. truncatularoots and tracked the displacement of the fluorescence signal over time. Two weeks after co-inoculation, this signal was detected 52 mm, on average, farther down the roots whenColpoda sp. was also present compared with the experimental treatment that contained bacteria but not protists. Direct counts also showed that protists are required for viable bacteria to reach the deeper sections of root systems in our microcosms. Facilitating bacterial transport may be an important mechanism whereby soil protists promote plant health. As a sustainable agriculture biotechnology, protist-facilitated transport has the potential to boost efficacy of bacterial inoculants, thereby helping growers avoid overuse of nitrogen fertilizers and enhance performance of climate-smart, no-till farming practices.