Unravel the beneficial effects induced by Trichoderma harzianum on tomato plants: the intimate dialogue that improves growth and defense responses.

The increased need for food availability combined with the request of a more sustainable way of production, lead researchers to find alternative to chemical pesticides and fertilizers. Many Trichoderma isolates are already available as active ingredients of commercial biopesticides. Their use would reduce chemical inputs, in a perspective of sustainable agriculture, allowing natural resource conservation. The potential of Trichoderma harzianum T6776 (T6776) to be used as a biocontrol agent and as a biostimulant has been previously proven on several cultivars of tomato plants. In order to investigate the mechanisms behind these effects, the genome of T6776 was firstly sequenced and a phylogenetic analysis was performed, thus allowing to correctly identify our strain as a T. afroharzianum, a new species included into the T. harzianum species complex. Several biochemical analyses were done in order to elucidate the mechanisms responsible for growth promotion of the tomato cv. Micro Tom, and these analyses highlighted the complex metabolic change induced by T6776 in tomato. Growth promotion resulted to be a consequence of a changed sugar allocation, that influences the photosynthetic efficiency of the plants, combined with an increased level of growth related hormones, such as indoleacetic acid. Moreover, in preliminary tests, we showed that T6776 is able to increase plant tolerance to biotic and abiotic stresses. Plants grown in presence of T6776 responded better to pathogens attack, such as Fusarium oxysporum f. sp. radicis lycopersici and Alternaria solani. Likewise, the photosynthetic efficiency of salt stressed plants treated with T6776 increased, compared with untreated plants, suggesting an enhanced salt tolerance induced by T6776. The increased level of the stress related hormone jasmonic acid (JA) found in roots and xylem vessel of T6776 treated plants could in part explain the increased plants tolerance against biotic and abiotic stress. In addition, the high level of starch found in roots of treated plants could help plants to face to abiotic stress, representing an extra energy reserve. In order to identify molecules involved in root colonization process by T6776, a specific class of hypothetic effector proteins were identified and characterized in the T6776 genome. The expression levels of nine LysM encoding genes identified in the analysis were measured using qPCR in T6776 interacting with tomato roots. Analysis revealed that the expression levels of 4 LysM encoding genes increased during the early stage of the interaction between T6776 and tomato plants. These results show, for the first time, the possible involvement of LysM effectors in the interaction between a Trichoderma sp. isolate and the host plants.