Cytological effects of cellulases in the parasitism of Phytophthora parasitica by Pythium oligandrum

The ubiquitous oomycete Pythium oligandrum is a potential biocontrol agent for use against a wide range of pathogenic fungi and an inducer of plant di sease resistance. The ability of P, oligandrum to compete with root pathoge ns for saprophytic colonization of substrates may be critical for pathogen increase in soil, but other mechanisms, including antibiosis and enzyme pro duction, also may play a role in the antagonistic process. We used transmis sion electron microscopy and gold cytochemistry to analyze the intercellula r interaction between P, oligandrum and Phytophthora parasitica, Growth of P. oligandrum towards Phytophthora cells correlated with changes in the hos t, including retraction of the plasma membrane and cytoplasmic disorganizat ion. These changes were associated with the deposition onto the inner host cell surface of a cellulose-enriched material. P. oligandrum hyphae could p enetrate the thickened host cell wall and the cellulose-enriched material, suggesting that large amounts of cellulolytic enzymes were produced. Labeli ng of cellulose with gold-complexed exoglucanase showed that the integrity of the cellulose was greatly affected both along the channel of fungal pene tration and also at a distance from it. We measured cellulolytic activity o f P, oligandrum in substrate-free liquid medium. The enzymes present were a lmost as effective as those from Trichoderma viride in degrading both carbo xymethyl cellulose and Phytophthora wall-bound cellulose, P. oligandrum and its cellulolytic enzymes may be useful for biological control of oomycete pathogens, including Phytophthora and Pythium spp,, which are frequently en countered in field and greenhouse production.