Dual role of melatonin on Fusarium graminearum in vitro growth and on wheat seedlings infection

Melatonin is a tryptophan derived hormone with multiple recognized roles in animals, plants and, more recently, fungi. In a plant-fungus interaction, exogenous melatonin application can affect each organism individually and modulate their interplay. This study investigated the effect of exogenous melatonin treatments, from 0 to 5 mM, on Fusarium graminearum Schwabe 1839, one of the main pathogenic fungi of wheat, both in vitro and in planta. In vitro, on Rice Flour Agar medium, melatonin showed to act in a dose-dependent manner, inhibiting mycelial growth, and stimulating conidia germination, expression of the trichothecene biosynthetic genes TRI5 and TRI6 as well as the production of the mycotoxin deoxynivalenol (DON). The mycelial growth inhibition, highest at 5 mM, may be linked to melatonin-induced suppression of the F. graminearum tryptophan decarboxylase (TDC) gene, which is involved in auxin biosynthesis. In planta, on susceptible common wheat (Triticum aestivum L.) coleoptiles, the 5 mM melatonin treatment had opposite effects to those observed in vitro, stimulating fungal growth and down-regulating TRI5 and TRI6 expression. The increase in fungal biomass, and thus in host susceptibility, was not associated with increased DON production, but rather in the inhibition of host salicylate, jasmonate and ethylene defense pathways. These evidences support the notion that DON is not a key virulence factor on wheat coleoptiles. The in-depth analysis of the modulatory role of melatonin in fungal growth and plant defense must be carefully evaluated in each plant/pathogen interaction to optimize melatonin as a tool for managing biotic stresses in crops.