Protein HYTRA1 secreted by Trichoderma plays a key role in induced systemic resistance

Trichoderma harzianum T22 is one of the Trichoderma isolates most used as active ingredient in commercial bio-fungicides and bio-fertilizers. In addition to their mycoparasitic abilities, many Trichoderma strains can colonize and grow in association with plant roots and can significantly increase plant growth, development and systemic resistance to pathogen attack. During Trichoderma–plant interaction, a plant response has been confirmed as responsible for at least part of the protection effect, but the molecular mechanisms involved are still largely unknown. The protein Hytra1 secreted by Trichoderma was tested for its ability to induce a hypersensitive reaction (HR) and induced systemic resistance in tomato plants. Hytra1 infiltration elicited a strong HR on tomato leaves and could trigger plant defence reactions subsequent to pathogen infections both locally and systemically. Physiological analyses of tomato leaves treated with Hytra1 showed that this hydrophobin can induce an oxidative burst in plant cells. Low Hytra1 concentrations also triggered activation of the antioxidant system that controls accumulation of reactive oxygen species (superoxide anions and peroxides). This modulation plays a role in transduction of the oxidative signal, leading to formation of signal molecules (lipoperoxides) that trigger the accumulation of defence molecules (e.g. riscitin) and PR proteins. Real-time PCR analysis of RNA extracted from tomato plants 24 hours after treatment with 0.31 micron Hytra1 showed strong activation of PR4 transcription, while PR1 expression was induced to a lesser extent in the same conditions. Steady-state mRNA levels of PR4 and PR1 were about 13-fold and 5-fold higher, respectively, than in controls.