Plant pathogenic microswimmers

Zoospores are flagellated spores, asexual and motile cells, produced by different organisms including oomycetes (pseudo fungi). Oomycetes of the genus Phytophthora are plant pathogens (e.g. potato blight). The zoospores of oomycetes are biflagellate (they carry a smooth flagellum and a flagellum covered with hairs); some of them are able to swim in aqueous environments by using chemotaxis.
We are currently working on the species P. parasitica with Eric Galiana and Agnès Attard at the Institut Sophia Agrobiotech (INRA-CNRS-UCA). P. parasitica infects several plant species, and is considered as a model for the study of the genus Phytophthora.
 

Swimming zoospores in ion gradients

The biflagellate zoospores of some phytopathogenic Phytophthora species spontaneously aggregate within minutes in suspension. In different experiments we showed 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).
Zoospores exposed to a potassium gradient aggregate. The central region is low in potassium concentration. The chamber is 1 mm wide.

Zoospores: how they swim, how they turn

Despite the relevance of zoospore spreading in the epidemics of plant diseases, it is not known how these zoospores swim and steer with two opposite beating flagella. Combining experiments and modeling, we show how these two flagella contribute to generate thrust when beating together, and identify the mastigonemes-attached anterior flagellum as the main source of thrust.

Zoom sur une zoospore
Zoom sur une zoospore

Early interactions between roots and zoospores

Knowledge about plant‐pathogen interactions focuses on the penetration and colonization steps of infection. A molecular dialogue during these steps is crucial to the outcome of the disease. Upstream signaling events remain to be defined. These events are attraction of the pathogen toward the host, adhesion, aggregation and association with bacteria. We name here this sequence of events: the first events of infection (FEI). The aim is to define plant signals that drive FEI from attraction of swimming Phytophthora zoospores to their aggregation and association with bacteria prior to penetration. We test the hypothesis that ion fluxes operating around the elongation zone of roots regulate rhizospheric microbial behavior during FEI. By combining plant genetics, membrane physiology, and physics, we want to understand how plant ion fluxes contribute to rhizosphere modifications, and to what extent they are a signal contributing to FEI and disease fate. Zoospores aggregated on a root (left) and reconstructed trajectories of zoospores around the root (right).
 
 
Zoospores aggregating on a root. 30 s movie (left) and reconstructed trajectories using ImageJ (right).