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& Key message A histopathological study using one resistant and one susceptible clone of Eucalyptus urophylla × Eucalyptus grandis hybrid showed that the colonization of Ceratocystis fimbriata was limited by rapid and intense host defense responses such as closure of the vessel pits; formation of tyloses and gels; accumulation of amorphous material, starch, phenolic compounds, and calcium oxalate; and tissue lignification. The defense mechanisms of the resistant clone were not lethal to the pathogen because the fungus was reisolated from the diseased tissue. & Context The use of resistant genotypes of eucalypt is widely used to control Ceratocystis wilt caused by Ceratocystis fimbriata. However, little is known regarding the fungal infection process and the host defense responses. & Aims Thus, the objectives of this study were to compare the histopathological responses of one resistant and one susceptible clone of Eucalyptus urophylla × Eucalyptus grandis to artificial inoculation with C. fimbriata and to identify possible host defense responses against fungal infection. & Methods Fungal colonization was analyzed by light and scanning electron microscopy. The host defense responses to artificial fungal inoculation were evaluated through histochemical analysis and determining of the lignin concentration and lesion lengths, whereas the pathogen viability was confirmed by reisolations. & Results Both tested clones showed similar responses against fungal infection but presented defense responses with different speeds and intensities. Fungal colonization was not restricted in the tissue of plants from the susceptible clone, whereas in the resistant clone, fungal colonization was limited to the xylem vessels and parenchyma around the vessels due to closure of the vessel pits; intense formation of tyloses and gels; accumulation of amorphous material, starch, phenolic compounds, and calcium oxalate; and tissue lignification. However, the fungus was reisolated from the inoculated tissues of both clones.

The interaction between fungi and plants that form ectomycorrhizae (ECM) promotes alterations in the gene expression profiles of both organisms. Fungal genes expression related to metabolism were evaluated at the pre-symbiotic stage and during the ECM development between Scleroderma laeve and Eucalyptus grandis. Partial sequences of ATP synthase (atp6), translation elongation factor (ef1α), the RAS protein (ras), and the 17S rDNA genes were isolated. The expression of the atp6 and 17S rDNA genes during the pre-symbiotic stage showed an approximately threefold increase compared to the control. During ECM development, the expression of the 17S rDNA gene showed a 4.4-fold increase after 3 days of contact, while the expression of the atp6 gene increased 7.23-fold by the 15th day, suggesting that protein synthesis and respiratory chain activities are increased during the formation of the mantle and the Hartig net. The ras gene transcripts were only detected by RT-PCR 30 days after fungus-plant contact, suggesting that RAS-mediated signal transduction pathways are functional during the establishment of symbiosis. The present study demonstrates that alterations in gene expression occur in response to stimuli released by the plant during ECM association and increases the understanding of the association between S. laeve and E. grandis.