TREATMENT WITH THE MYCOPARASITE PYTHIUM-OLIGANDRUM TRIGGERS INDUCTIONOF DEFENSE-RELATED REACTIONS IN TOMATO ROOTS WHEN CHALLENGED WITH FUSARIUM-OXYSPORUM F-SP RADICIS-LYCOPERSICI

The influence exerted by the mycoparasite Pythium oligandrum in trigge ring plant defense reactions was investigated using an experimental sy stem in which tomato plants were infected with the crown and root rot pathogen Fusarium oxysporum f. sp. radicis-lycopersici. To assess the antagonistic potential of P. oligandrum against F. oxysporum f. sp, ra dicis-lycopersici, the interaction between the two fungi was studied b y scanning and transmission electron microscopy (SEM and TEM, respecti vely). SEM investigations of the interaction region between the fungi demonstrated that collapse and loss of turgor of F. oxysporum f. sp. r adicis-lycopersici hyphae began soon after close contact was establish ed with P. oligandrum. Ultrastructural observations confirmed that int imate contact between hyphae of P. oligandrum and cells of the pathoge n resulted in a series of disturbances, including generalized disorgan ization of the host cytoplasm, retraction of the plasmalemma, and, fin ally, complete loss of the protoplasm. Cytochemical labeling of chitin with wheat germ agglutinin (WGA)/ovomucoid-gold complex showed that, except in the area of hyphal penetration, the chitin component of the host cell walls was structurally preserved at a time when the host cyt oplasm had undergone complete disorganization. Interestingly, the same antagonistic process was observed in planta. The specific labeling pa tterns obtained with the exoglucanase-gold and WGA-ovomucoid-gold comp lexes confirmed that P. oligandrum successfully penetrated invading ce lls of the pathogen without causing substantial cell wall alterations, shown by the intense labeling of chitin. Cytological investigations o f samples from P. oligandrum inoculated tomato roots revealed that the fungus was able to colonize root tissues without inducing extensive c ell damage. However, there was a novel finding concerning the structur al alteration of the invading hyphae, evidenced by the frequent occurr ence of empty fungal shells in root tissues. Pythium ingress in root t issues was associated with host metabolic changes, culminating in the elaboration of structural barriers at sites of potential fungal penetr ation. Striking differences in the extent of F. oxysporum f. sp. radic is-lycopersici colonization were observed between P. oligandrum-inocul ated and control tomato plants. In control roots, the pathogen multipl ied abundantly through much of the tissues, whereas in P. oligandrum-c olonized roots pathogen growth was restricted to the outermost root ti ssues. This restricted pattern of pathogen colonization was accompanie d by deposition of newly formed barriers beyond the infection sites. T hese host reactions appeared to be amplified compared to those seen in nonchallenged P. oligandrum-infected plants. Most hyphae of the patho gen that penetrated the epidermis exhibited considerable changes. Wall appositions contained large amounts of callose, in addition to be inf iltrated with phenolic compounds. The labeling pattern obtained with g old-complexed laccase showed that phenolics were widely distributed in Fusarium-challenged P. oligandrum-inoculated tomato roots. Such compo unds accumulated in the host cell walls and intercellular spaces. The wall-bound chitin component in Fusarium hyphae colonizing P. oligandru m-inoculated roots was preserved at a time when hyphae had undergone s ubstantial degradation. These observations provide the first convincin g evidence that P. oligandrum has the potential to induce plant defens e reactions in addition to acting as a mycoparasite.