The electrotactic responses of plant pathogenic fungi
Plant roots generate electrical fields in the rhizosphere as a consequence of their ion transport activities. In this thesis it is shown that zoospores of Phytophthora palmivora and Pythium aphanidermatum exhibit electrotaxis in electrical fields ≤ 50 mV/cm, comparable in size to the physiological fields around plant roots. P.palmivora zoospores showed anodotaxis and P.aphanidermatum zoospores cathodotaxis. An experimental protocol for applying weak electrical fields is described. In this sytem zoospores 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 zoospores were measured. These forces did not influence the distribution of zoospores in electrotactic experiments at physiological field strengths. Applied electrical fields elicited topotaxis and klinokinesis of zoospores of P.palmivora. These responses may be one mechanism whereby zoospores accumulate preferentially at specific areas on plant roots. Electro-topotaxis is likely to be the result of differential charges on the anterior and posterior flagella of zoospores, causing electrophoretic orientation in an electrical field.