ZOOSPORE-ZOOSPORE AND ZOOSPORE-ROOT INTERACTIONS OF PHYTOPHTHORA AND PYTHIUM SPECIES TA CAMPBELL NAR GOW BM MORRIS MC OSBORNE SJ SHEPHERD Department of Molecular and Cell Biology, University of Aberdeen, Institute of Medical Sciences, Foresterhill, Aberdeen, AB9 2ZD, UK Background and objectives Zoospores of oomycete pathogens accumulate at spatially precise locations around host and non-host plant roots. Since roots are known to exude low molecular weight solutes chemotaxis towards the nutrient gradient in the rhizosphere has been assumed to represent the major mechanism leading to zoospore accumulation at the root surface. Accumulation at a given location is related to (a) active swimming towards that site and (b) immobilisation of the zoospores by induced encystment. In this study we have designed experiments to attempt to discriminate between these separate aspects of zoospore behaviour that both can result in zoospore accretion. Roots also generate weak electrical fields in the rhizosphere that are sufficiently large to induce electrotaxis and encystment of zoospores [ 1]. Here we describe data that further explore the relationship between the natural electrical fields around roots and the localisation of zoospores at the root surface. In addition, we have shown that aggregates of zoospores of Phytophthora palmivora are attractive to free-swimming zoospores (adelphotaxis or autoaggregation) but were not attractive to zoospores of Pythium species [2]. Therefore zoospore accumulation at the surface at a root is the net result of a number of factors including chemotaxis, chemically induced encystment, electrotaxis, electrically induced encystment and autoaggregation. The aim of this study was therefore to attempt to evaluate the role and relative importance of these various spatial cues in the rhizosphere. Materials and methods Zoospore behaviour and swimming patterns were studied using videomicroscopical analysis [2]. Electrophysiological procedures are as described previously [1]. Chemotaxis was assessed by calculating the net swimming vector in relation to a point source of nutrients within a capillary tube. The nutrients that were tested were the major amino acids and sugars detected for cocoa root exudates as analysed by HPLC. Movements of zoospores around cyst autoaggregates and the direction of outgrowth of germ tubes from cysts was again calculated by use of an image analysis system [3]. Results and conclusions The spatial patterns of zoospore accretion at the root surface correlated well with the electrotactic in vitro behaviour of zoospores and the electrical profile of roots mapped with a voltage-sensitive vibrating microelectrode. Zoospores of Py. aphanidermatum were cathodotactic and accumulated at cathodic regions of perennial rye grass roots - namely wound sites and distal regions to the root hair zone to rear of the zone of elongation. Zoospores ofph. palmivora were anodotactic and accumulated at anodic root sites and were not attracted to wounds. Local electrical fields applied via a 10 pm tip diameter capillary could attract or repel zoospores and stimulate their encystment. Focal electrodes applied at root surfaces could override endogenous cues and recruit zoospores to regions of roots that were otherwise repellent. These data suggest that zoospore responses to endogenous root-driven electrical fields are an important aspect of the root-zoospore interaction. Little evidence was found for directional swimming of zoospores in response to most single sugar or single amino acid gradients. Chemically induced encystment rather than bonefide chemotaxis is therefore unlikely to be the primary mechanism accounting for attraction to nutrients. Instead, induced encystment of zoospores is likely to be an important aspect of the mechanism that accounts for zoospore aggregation at plant surfaces. Interpretation of "swim-in" tests using capillary tubes containing nutrient sources is also likely to be affected by the adelphotactic responses of zoospores towards zoospores that are induced to encyst. Here we show that adelphotaxis and subsequent adelphotropism of the germ tubes formed by cysts is species and genus specific. These adelphotactic and adelphotropic responses may aid recruitment and build up of inoculum potential at the infection court and result in competitive exclusion of competing species of zoosporic fungi. References [1] Morris BM, B Reid, NAR Gow, 1992. Plant Cell & Environment 15, 645-53 [2] Morris BM, NAR Gow, 1993. Phytopathology 83, 877-82. [3] Reid B, Morris BM, NAR Gow, 1995. Experimental Mycology 19, 202-13.