Ultrastructural and cytochemical aspects of the interaction between the mycoparasite Pythium oligandrum and soilborne plant pathogens

The interaction between the oomycete Pythium oligandrum and various soilbor ne oomycete and fungal plant pathogens (P. ultimum, P. aphanidermatum, Fusa rium oxysporum f. sp. radicis-lycopersici, Verticillium albo-atrum, Rhizoct onia solani, and Phytophthora megasperma) was studied by light and electron microscopy in order to assess the relative contribution of mycoparasitism and antibiosis in the antagonistic process. Scanning electron microscope in vestigations of the interaction regions showed that structural alterations of all pathogenic fungi and oomycetes (except for Phytophthora megasperma) occurred soon after contact with the antagonist. Light and transmission ele ctron microscope studies of the interaction region between the antagonist a nd P. ultimum revealed that intimate contact between both partners preceded a sequence of degradation events including aggregation of host cytoplasm a nd penetration of altered host hyphae. Localization of the host wall cellul ose component showed that cellulose was altered at potential penetration si tes. A similar scheme of events was observed during the interaction between P. oligandrum and F. oxysporum f. sp, radicis-lycopersici, with the except ion that complete loss of host protoplasm was associated with antagonist in vasion. The interaction between P. oligandrum and R. solani resulted in an abnormal deposition of a wall-like material at potential penetration sites for the antagonist. However, the antagonist displayed the ability to circum vent this barrier and penetrate host hyphae by locally altering the chitin component of the host hyphal wall. Interestingly, antagonist cells also sho wed extensive alteration as evidenced by the frequent occurrence of empty h yphal shells. In the case of Phytophthora megasperma, hyphal interactions d id not occur, but hyphae of the plant pathogen were damaged severely. At le ast two distinct mechanisms appear to be involved in the process of oomycet e and fungal attack by P, oligandrum: (i) mycoparasitism, mediated by intim ate hyphal interactions, and (ii) antibiosis, with alteration of the host h yphae prior to contact with the antagonist. However, the possibility that t he antagonistic process may rely on the dual action of antibiotics and hydr olytic enzymes is discussed.