Melatonin (N-acetyl-5-methoxytryptamine) was discovered in plants in 1995; since then numerous functions have been attributed to this molecule in vascular plants. In addition to its recognized role as a universal antioxidant, other relevant functions have been studied in plants such as its rhizogenic- and vegetative-growth effects, protection against leaf senescence and influences on photosynthesis and on the stomatal apparatus. Also, melatonin has a protective role in stress situations (biotic

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... and abiotic), acting as an osmoregulation and a metabolic corrector when confronted with different stresses. One of the most outstanding aspects is the involvement of melatonin as a multi-signaling molecule in plants. The dual roles of melatonin in physiological stress situations involve both its direct action (free of receptor action) as an antioxidant and its role as a regulator of gene expression. Its relationship with central elements of the plant redox network such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) and the regulation of relevant elements is discussed. All recent data on melatonin are incorporated to present an updated model, where the balance between ROS and RNS, and between these and melatonin is a regulatory key center in the responses.