Guidance of zoospores by potassium gradient sensing mediates aggregation

Biflagellate zoospores of some phytopathogenic Phytophthora species spontaneously aggregate within few minutes in suspension. Depending on species auto-aggregate formation results from bioconvection or from a sequence bioconvection-positive chemotaxis. In this study, we show that P. parasitica zoospores may form aggregates upon application of a K+ gradient in particular geometric arrangements. Based on the use of macro- and microfluidic devices, in addition to time-lapse live imaging both in the vertical and horizontal planes, we defined (i) the spatiotemporal and concentration scale evolution within the gradient in correlation with (ii) cell distribution and (iii) metrics of zoospore motion (velocity, trajectory). The results indicated that K+-induced aggregates result from a single bi-phasic temporal sequence involving negative chemotaxis and then bioconvection in a K+ gradient concentration scale [0-17 mM]. Each K+-sensing cell undergoes a forward-to-backward movement within a threshold range of 1-4mM, thereby forming progressively a swarm. Once a critical population density is achieved zoospores form a plume which undergoes a downward migration leading to aggregate formation on the support surface. We discuss putative sources for K+ gradient generation in natural environment (zoospore population, microbiota, plant roots, soil particles), and implication for events preceding inoculum formation on host plant.