ABSTRACT
Melatonin (N-acetyl-5-methoxytryptamine) is an indolic compound derived from tryptophan. Usually identified as a neurotransmitter or animal hormone, this compound was first detected in plants in 1995. Melatonin is present in all plant species studied, with large variations in levels. Melatonin has been extensively studied to ascertain its possible functions in higher plants, and various roles have been attributed to it in the physiology of these plants. The most relevant and also most studied aspect is the important role melatonin plays in stress situations, such as drought, cold, heat, salinity, chemical pollutants, herbicides, and UV radiation (abiotic stressors) and also in the biotic stress caused by plant pathogens. Related with the above, the reinforcement of plants by melatonin through their photosynthetic capacity, delaying leaf senescence in stress situations, have been studied. Also, melatonin presents some responses similar to auxin, regulating root and shoot growth, promoting new root generation (rhizogenesis), and in tropism (gravitropism). All this, makes melatonin an interesting candidate for use as a natural biostimulating substance for crops. Some data on its possible role in plant rhythms are also of great interest. All these aspects, together with the relationship studied between plant hormones and melatonin are described and discussed in this book chapter.
