A pressure gradient facilitates mass flow in the oomycete Achlya bisexualis

We have used a single cell pressure probe and observed movement of microinjected oil droplets to investigate mass flow in the oomycete Achlya bisexualis. To facilitate these experiments, split Petri dishes that had media containing different sorbitol concentrations (and hence a different osmotic potential) on each side of the dish were inoculated with a single zoospore. An initial germ tube grew out from this and formed a mycelium that extended over both sides of the Petri dish. Hyphae growing on the 0 M sorbitol side of the dish had a mean turgor ( ± sem) of 0.53 ± 0.03 MPa (n = 13) and on the 0.3 M sorbitol side had a mean turgor ( ± sem) of 0.3 ± 0.027 MPa (n = 9). Oil droplets that had been microinjected into the hyphae moved towards the lower turgor area of the mycelia (i.e. retrograde movement when microinjected into hyphae on the 0 M sorbitol side of the split Petri dish and anterograde movement when microinjected into hyphae on the 0.3 M sorbitol side of the Petri dish). In contrast, the movement of small refractile vesicles occurred in both directions irrespective of the pressure gradient. Experiments with neutral red indicate that the dye is able to move through the mycelia from one side of a split Petri dish to the other, suggesting that there is no compartmentation. This study shows that hyphae that are part of the same mycelia can have different turgor pressures and that this pressure gradient can drive mass flow.