Melatonin: Another Phytohormone?

Melatonin is known to be produced by plants and to exert numerous effects, which overlap with actions of known phytohormones, including auxins, ethylene, abscisic, jasmonic and salicylic acids. It exhibits growth effects, alleviates stress by heat, cold, drought, and toxic chemicals, counteracts infections by bacterial or fungal pathogens, favours wound healing, delays senescence and acts as an antioxidant and photoprotectant. Stressors and intense radiation frequently induce substantial increases in melatonin. High levels are particularly found in oily seeds. The criteria are discussed which have to be fulfilled before melatonin might be classified as a phytohormone. These include, in particular, the identification of high-affinity binding sites, of components of signal transduction pathways, the determination of freely movable melatonin and its movements within the organism.