Induced resistance is a plant response to challenge by microorganisms or abiotic agents such that following the inducing challenge de novo resistance to pathogens is shown in normally susceptible plants (de Wit 1985) Induced resistance can be localized, when it is detected only in the area immediately adjacent to the inducing factor or systemic, where resistance occurs subsequently at sites throughout the plant. Both localized and systemically induced resistances are nonspecific.
Recently, the potential of T. harzianum T-203 to trigger plant defense responses was investigated by inoculating roots of cucumber seedlings with Trichoderma in an aseptic, hydroponic system (Yedidia et al. 1999). Trichoderma-treated plants were more developed than nontreated plants throughout the experiment. Electron microscopy of ultrathin sections from Trichoderma-treated roots revealed penetration of T. harzianum, mainly to the epidermis and outer cortex. Strengthening of the epidermal and cortical cell walls was observed, as well as deposition of newly formed barriers. These typical host reactions were found beyond the sites of potential fungal penetration. Wall appositions contained large amounts of callose and infiltrations of cellulose. Further biochemical analyses revealed that inoculation with the fungus resulted in increased peroxidase and chitinase activities in roots and leaves of treated seedlings, providing evidence that T. harzianum may induce systemic resistance mechanisms in cucumber plants (Yedidia et al. 1999).
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