High doses of melatonin confer abiotic stress tolerance to phytopathogenic fungi grown in vitro

Melatonin is a secondary metabolite produced in all domains of life. Exogenous melatonin triggers defence mechanisms in plants that enhance abiotic stress tolerance. However, knowledge regarding the role of melatonin as a signal or an antioxidant in microbes is lacking. We investigated the in vitro growth responses of three phytopathogenic fungi, Sclerotinia sclerotiorum, Botrytis cinerea and Fusarium oxysporum f.sp. vasinfectum, to abiotic stress (2.5% ethanol with/without cold priming) under varying concentrations of melatonin. Melatonin at high concentrations (1000 – 2000 µM) partially restored fungal growth under stress, compared to controls, suggesting a role for melatonin in alleviating the impacts of stress exposure. Understanding how melatonin impacts fungal growth during stress conditions will be important for future applications using melatonin as a tool for crop protection.