Kinetics of zoospores approaching a root using a microfluidic device

Phytophthora species are plant pathogens that cause considerable damage to agrosystems and ecosystems, and have a major impact on the economy. Infection occurs when their biflagellate zoospores move and reach a root on which they aggregate. However, the communication between the plant and the zoospores and how this communication modifies the behavior of the swimming zoospores is not fully understood. Here we use a microfluidic device incorporating a growing Arabidopsis thaliana root to study the real-time kinetics of Phytophthora parasitica zoospores approaching the root and accumulating (or aggregating) around a specific area called the elongation zone. We show that zoospore kinetics are modified only below a distance of a few hundred microns from the aggregation center, with a decrease in velocity coupled to an increase in the number of turns taken. Furthermore, we show that the rate of aggregation is constant throughout a one-hour experiment, and is dependent on zoospore density. This rate is consistent with the fact that zoospores randomly encounter the region close to the elongation zone, a result compatible with an absence of attraction beyond a few hundred microns. Finally, we show that in our configuration, this absence of attraction can be explained by a residual flow responsible for limiting the diffusion of the signal supposedly emitted by the root to a boundary layer of a few hundred microns.