Trichoderma harzianum ITEM 3636 colonizes peanut roots as an endophyte and protects the plants against late leaf spot

Abstract

Trichoderma harzianum ITEM 3636 has been tested against different soil-borne peanut diseases, applied as a seed treatment. The strain protected mature peanut plants suggesting a long lasting protection, and was isolated from plant roots several months after the sowing in field assays. We hypothesized that T. harzianum ITEM 3636 colonizes peanut roots as an endophyte as was demonstrated with other few strains, and in this interaction, the fungi reside in plant roots and can protect the plant against a foliar disease. The spreadability of conidia on the surface of peanut seeds and the mycelial growth inside root tissues were studied through scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM), respectively. Conidia from ITEM 3636 had an appropriate distribution on coated seed surfaces. Furthermore, the fungal mycelium colonized root surfaces forming appressorium-like structures and swollen cells. The overlay of green and red confocal images clearly showed fungal hyphae grew internally through epidermal and cortical cells. Once inside the root, T. harzianum ITEM 3636 increased the levels of jasmonic acid in roots but not of salicylic acid. Also, the effect of seed treatment in peanut plant defense against Nothopassalora personata was evaluated under greenhouse conditions. ITEM 3636 significantly decreased the number of affected leaflets, late leaf spot (LLS) incidence and severity, and induced higher levels of defense related enzymes in leaflets after challenge with the pathogen. No direct activation of JA or SA pathways was observed in the leaves of Trichoderma-induced peanuts plants. T. harzianum decreased the concentration of ABA, which may reflect that the strain might improve defense mechanisms of the plant, through ABA content.