Guidance of zoospores by potassium gradient sensing mediates aggregation

Abstract

The biflagellate zoospores of some phytopathogenic Phytophthora species spontaneously aggregate within minutes in suspension. We show here that Phytophthora parasitica zoospores can form aggregates in response to a K + gradient with a particular geometric arrangement. Using time-lapse live imaging in macro- and microfluidic devices, we defined (i) spatio-temporal and concentration-scale changes in the gradient, correlated with (ii) the cell distribution and (iii) the metrics of zoospore motion (velocity, trajectory). In droplets, we found that K + -induced aggregates resulted from a single biphasic temporal sequence involving negative chemotaxis followed by bioconvection over a K + gradient concentration scale [0–17 mM]. Each K + -sensing cell moved into a region in which potassium concentration is below the threshold range of 1–4 mM, resulting in swarming. Once a critical population density had been achieved, the zoospores formed a plume that migrated downward, with fluid advection in its wake and aggregate formation on the support surface. In the microfluidic device, the density of zoospores escaping potassium was similar to that achieved in droplets. We discuss possible sources of K + gradients in the natural environment (zoospore population, microbiota, plant roots, soil particles), and implications for the events preceding inoculum formation on host plants.

Transcriptomic and Ultrastructural Signatures of K+-Induced Aggregation in Phytophthora parasitica Zoospores

“…Consistent with previous studies conducted on bacteria and other Phytophthora species [7,61,70], the rapid zoospore aggregation observed in this study states the effect of K + gradient on P. parasitica coordinated behavior and reinforces the involvement of electro-and/or chemo-reception in K + flow sensing and zoospore aggregation. As suggested by the verapamil pharmacological assay, inhibition of voltage-gated Ca 2+ channels, which are responsible for the activation of BK channels [25], resulted in inhibition of zoospore aggregation, but not motility impairment.…”

“…Conversely-and in agreement with a previous study [7]-application of other cations, such as Na + , in the same range of concentration, did not lead to zoospore aggregation (Video S3). Ultrastructural analyses conducted on K + -induced aggregates revealed the occurrence of cell-to-cell associations together with the deposition of intercellular matrix-like material (Figure 2A-D).…”

“…Fifteen minutes after K + treatment, the most of cells exhibited the characteristic ellipsoidal shape of zoospores rather than the circular conformation typical of cysts ( Figure S1A and Video S2). Moreover, as previously observed by Galiana et al (2019) [7], most of cells showed anticlockwise rotation movement or a short and erratic displacement indicative of the presence of flagella at that time point ( Figure S1B and Video S2). Measurement of average speed, deduced from zoospore trajectories, indicated that 84% of cells moved at a speed higher than 7 µm/s, a threshold below which a cell was considered to be motionless and potentially encysted.…”

“…Zoospore motion was investigated as previously described by Galiana et al (2019) [7] using the FIJI TrackMate plugin. Videos were analyzed with the following parameters: estimated blob diameter, 12 µm and threshold, 10 µm; automatic initial thresholding; linking max distance, 20 µm; gap-closing max distance, 20 µm; gap-closing max frame, 2; spot filtering, duration of track above 2 s. For each remaining trajectory, we calculated the mean velocity.…”

Phytophthora zoospores: From perception of environmental signals to inoculum formation on the host-root surface

“…2005 [9] . Panels C and C′ show P. parasitica zoospores swimming in water (C) and the corresponding trajectory patterns delineated using the TrackMate plugin [19] as per the procedure detailed in Galiana et al [15] . The trajectories indicate the randomness in swimming speed and direction of zoospores under no constraints.…”