New insights into the role of melatonin in plants and animals.

Melatonin is a hormone produced in animals by the pineal gland and in plants under stress. Melatonin research has expanded rapidly, affecting an impressive enhancement in the understanding of its functions in plants and animals. However, far less focus has been directed to clarifying the nature of melatonin dose-response relationships. Here, we provide substantial evidence of melatonin-induced biphasic dose-response relationships from a series of independent studies involving plant and animal models. The characteristics of these dose responses are similar to those of the broad toxicological and pharmacological hormesis literature. Our analysis suggests that melatonin, in coordination with the circadian rhythms, is involved in stress adaptive responses, and may act as a conditioning agent protecting organisms against subsequent health threats within an hormetic framework. Incorporation of melatonin-induced hormesis in research protocols has the potential to enhance the treatment of neuropsychiatric diseases, cancers, and other animal diseases, as well as protection against environmental stress and to increase plant productivity.