Potassium modulates Phytophthora zoospore motility and flagellar beating through dose-dependent effects

Biflagellate zoospores are critical agents in the spread of Phytophthora spp., plant pathogens causing major agronomic and economic losses worldwide. These motile cells navigate soil water films toward plant roots using taxis triggered by environmental cues. Here, we investigate how potassium modulates zoospore motility by analyzing trajectory and flagellar beating parameters across KCl concentrations. Using high-speed imaging, we apply kymograph analysis and Fourier transform of the cell body direction over time to quantify flagellar dynamics, while low-speed imaging provides trajectory characteristics. Our results demonstrate that increasing potassium concentration significantly reduces swimming speed while enhancing directional changes. We establish a direct correlation between altered swimming speed and changes in flagellar beating parameters. Our findings indicate that potassium differentially affects anterior and posterior flagella beating. Furthermore, potassium exerts multiple effects, influencing beat amplitude, wave propagation speed and direction- change frequency. This reveals potassium as a key modulator of zoospore motility, triggering a shift from exploratory swimming to confined, reoriented movement. By uncovering the biomechanical responses of zoospores to potassium, this study advances our understanding of how surrounding signals regulate pathogen dispersal in the environment.