Research from the last two decades increased our understanding of the beneficial effects of Trichoderma to plants regarding 1) root branching and absorptive potential, 2) usage of organic amendments and fertilizers, 3) growth and development, and 4) adaptation to abiotic and biotic challenges. In summary, many species including T. virens, T. atroviride and T. longibrachiatum may help plants to survive better and enhance productivity in a safe and eco-friendly manner. The impact of these fungi has been assessed under field conditions, which correlated with yield increases in important cereal, fruit and vegetable crops, including maize, wheat, soybean, tomato, grape and lettuce (2).
In recent years, a major question has been how Trichoderma adjusts its metabolism according to the highly variable ecological niches and nutritional resources encountered. Apparently, secretion of potent enzymes such as cellulases, chitinases, and peptidases, are the hallmark that enables exploitation of dead wood and decaying leaf, root and stem materials and underscores its ability to parasitize phytopathogen fungi. However, it appears that the repression of genes encoding fungal degradative enzymes enables root colonization by Trichoderma and thus the fungus recognizes healthy roots by means of their exudation profiles (3).
Decoding the Trichoderma chemical message
Trichoderma is a biofactory of organic substances and releases volatiles, plant hormones, secondary metabolites and small peptides whose molecular composition depends on several factors including the fungal species, nutrient availability and the interaction with microorganism and plants. These info-chemicals can be perceived by roots through free diffusion within the soil and organic matter and during physical contact between the hyphae with the root epidermis, or at later stages, where the fungus spreads to inner cortical cells (4-6).
The first apparent change in the rhizosphere as a consequence of Trichoderma presence is the pH acidification (7). It may explain its highly efficient performance to solubilize sparingly soluble phosphates that accounts for a better plant nutrition. As the fungus grows, volatile emissions are thought to sensitize roots and enable long distance root-fungal recognition. 6-pentyl-2H-pyran-2-one (6-PP) is the main volatile from T. atroviride blends, which triggers root branching in Arabidopsis via changes in auxin and ethylene transport and response, respectively (4). 1-decene has been found to repress defense, stress and disease response genes, which facilitates fungal spread in root tissues (6). T. virens and T. asperellum releases auxins, a class of phytohormones with roles in plant growth and immunity that may be directly related to their biostimulant properties (8, 9).
Physical recognition may trigger further reactions in both the fungal and plant partners. Chitin, a major constituent of fungal cell walls has long been considered an elicitor that triggers defensive reactions in plants. Other molecules such as small peptides as well as membrane or cytoplasmic fungal proteins may further alert roots to be prepared for the interaction in order to avoid deleterious effects, making it much more competitive (10). Through the proliferation of lateral and adventitious roots, plants exploit better the mineral and water resources and are more resistant to abiotic stress, and these processes are effectively induced by Trichoderma.