Electrotaxis of zoospores of Phytophthora palmivora at physiologically relevant field strengths

Plant roots generate electrical fields in the rhizosphere as a consequence of their ion transport activities. We show here that zoospores of the plant pathogen Phytophthora palmivora exhibit anodal electrotaxis in electrical fields ≥0.5 V m⁻¹ comparable in size to the physiological fields around roots. An experimental protocol for applying weak electrical fields and quantifying electrotaxis is described. In this system, zoospore suspensions are isolated from the electrodes and their products using agarose bridges. Therefore, electrotaxis was not due to movement or trapping of zoospores in chemical, oxygen, pH or inhibitor gradients established by electrolysis. The electrophoretic and electroosmotic mobilities of encysted zoospores were measured. These forces did not influence the distribution of zoospores in electrotactic experiments at physiological field strengths. The electrotactic response saturated at fields above 10 V m⁻¹ was inhibited in media of osmotic strength below 400 Osmol m⁻³, was maximal at pH 7.5 and increased at high zoospore densities. These data suggest that electrotaxis may be a useful adjunct to chemotaxis in root targeting by zoospores.