Cytoplasmic flow dynamics in arbuscular mycorrhizal fungi are intrinsic and independent of plant hosts

Despite the ecological importance of arbuscular mycorrhizal (AM) fungi, it is unclear to what extent these symbionts can act autonomously from plant hosts, especially in their ability to control internal nutrient flows. We studied flows in AM fungal networks grown without plant hosts by providing myristic acid as a carbon source. Using a custom-built imaging platform, we tracked network formation of two Rhizophagus irregularis strains with and without myristic acid. We collected 5000 cytoplasmic flow videos in hyphae, and fluorescently tagged lipids to measure their speeds. We measured ∼25,000 flow trajectories and calculated flow speeds by kymograph analysis. In the presence of myristic acid but lacking a host root, AM fungi produced networks 10-times longer, covered up to 4 times more area, and showed a 50% increase in mean flow speed. Flow speeds varied drastically over time and space, with rare bursts of fast flows between 10 and 30 μm/s. Flows of fluorescently tagged lipids averaged 3 μm/s and were unaffected by myristic acid. Even one year after application, we could detect cytoplasmic flows in asymbiotic fungal hyphae close to parental spores when grown with myristic acid. Our findings suggest that cytoplasmic flows can be decoupled from hosts and challenge our current understanding of AM fungal autonomy.