Characteristics of collective and individual motions of swimming Phytophthora zoospores

Phytophthora diseases cause big threats to agriculture and eco-systems. Spreading is based on rapid dispersion of biflagellate swimming zoospores that once having reached a host initiate plant infection. Understanding their swimming mechanism and their interactions against gradients and surrounding environments becomes important. In this study, we develop a microfluidic system to investigate collective and individual motions of Phytophthora parasitica zoospores, a species that infects a broad range of host plants. Our system has the ability to generate a chemical gradient diffusing to a group of swimming zoospores and observe their swimming motions as well as the changes of the gradient at the same time. Our results show that a group of P. parasitica react differently against different doses of potassium chloride gradients: Low concentration of potassium reduces their speed and lures them away or initiates auto-aggregation, while high concentration (>3mM) causes them to change their swimming pattern to circulating around or stop moving. Moreover, observing a single zoospore swimming in water, we achieved the characteristics of its beating flagella. The correlation between zoospore velocity and its flagella motions helps us explain their reactions against the potassium gradients.