Melatonin: A New Plant Hormone and/or a Plant Master Regulator?

Melatonin is a pleiotropic molecule with many diverse actions in plants. It is considered primarily an antioxidant with important actions in the control of reactive oxygen and nitrogen species (ROS and RNS), among other free radicals, and harmful oxidative molecules present in plant cells. In addition, plant melatonin is involved in multiple physiological actions, such as growth, rooting, seed germination, photosynthesis, and protection against abiotic and/or biotic stressors. The recent identification of the first plant melatonin receptor opened the door to this regulatory molecule being considered a new plant hormone. However, due to the diversity of its actions, melatonin has also been proposed as a plant master regulator. Here, we discuss the most recent data in respect to both perspectives. Plant melatonin is involved in multiple physiological actions, including growth, rooting, seed germination, photosynthesis, osmoregulation, and protection against abiotic and biotic stressors. Plant melatonin not only acts as an antioxidant, but also induces substantial changes in gene expression in many physiological processes. Recently, the first plant melatonin receptor was identified and characterized in Arabidopsis thaliana. Many data indicate that plant melatonin could be considered a new plant hormone, although its multiple actions also point to it being an important master regulator of redox homeostasis in plants.