Factors influencing the pathogenesis of zoosporic fungi
Plant roots generate weak electrical fields in the rhizosphere due to the movement of ions, particularly protons, into and out of their tissues. It has been postulated that zoosporic fungi pathogenic to plants may utilise these electrical fields in targeting suitable host plants to infect. An experimental system is described which permits the application of low voltage electrical fields to zoospore suspensions. The resulting electrotactic response is shown to occur at field strengths of a magnitude similar to those generated by plant roots. In addition, pH taxis and electrophoresis/electroosmosis are shown not to occur in the system used. Electrotaxis is influenced by electrical field strength, ambient pH, and zoospore population density. Zoospores of Phytophthora palmivora swim consistently to the anode of an applied electrical field. Zoospore populations of a relatively high density form multi-cell clumps, or auto-aggregates. This is a result of active taxis and is shown to be density-dependent and influenced by pH. In addition, auto-aggregation is genus-specific and requires calcium ions to occur. It is postulated that auto-aggregation may enhance zoospore population survival. In addition, observations suggest that aggregation on roots, as well as being Ca2+-dependent, acts to enhance inoculum potential, i.e. to increase the number of zoospores reaching a plant root. The electrical activity of various plant roots is assessed. The primary host of Ph. palmivora, Theobroma cacao (cocoa), is shown consistently to display an outward electrical current (proton flow) at its zone of elongation, thus rendering this region anodic (positively charged). The same is true of some non-host plants e.g. Lolium perenne (perennial rye grass), Hordeum vulgare (barley) and Eucalyptus pilularis (eucalyptus).