Induction of defense responses in cucumber plants (Cucumis sativus L.) by the biocontrol agent Trichoderma harzianum

The potential of the biocontrol agent Trichoderma harzianum T-203 to trigge r plant defense responses was investigated by inoculating roots of cucumber seedlings with Trichoderma in an aseptic, hydroponic system. Trichoderma-t reated plants were more developed than nontreated plants throughout the exp eriment. Electron microscopy of ultrathin sections from Trichoderma-treated roots revealed penetration of Trichoderma into the roots, restricted mainl y to the epidermis and outer cortex. Strengthening of the epidermal and cor tical cell walls was observed, as was the deposition of newly formed barrie rs. These typical host reactions were found beyond the sites of potential f ungal penetration. Wall appositions contained large amounts of callose and infiltrations of cellulose. The wall-bound chitin in Trichoderma hyphae was preserved, even when the hyphae had undergone substantial disorganization. Biochemical analyses revealed that inoculation with Trichoderma initiated increased peroxidase and chitinase activities within 48 and 72 h, respectiv ely. These results were observed for both the roots and the leaves of treat ed seedlings, providing evidence that T. harzianum may induce systemic resi stance mechanisms in cucumber plants.