ABSTRACT
Melatonin and serotonin are indoleamine neurotransmitters, which are best known for their roles in regulating human behavior, mood, and diseases; however, in recent years they have become increasingly recognized as important regulators of plant growth and development, from seed germination through flowering. Given both these compounds are potent antioxidants, their capacity for protecting plants against biotic and abiotic stress has been investigated in depth. In addition to oxidative stress, there also appears to be growing evidence to support the notion that both these compounds play an important role in regulating plant morphogenesis, specifically root and shoot growth and development both in vitro and in greenhouse and field conditions. These effects have been observed to date in diverse species, and several mechanisms have been proposed spanning their functions of antioxidants, interaction with previously established phytohormone and signaling networks, and transcriptional and metabolic regulation. Much is still unknown, however, with respect to their mechanisms of action, particularly as the search of a receptor continues to remain a challenging field of investigation. This chapter proposes melatonin and serotonin as a novel class of plant growth regulator, and in this context provides a brief overview of the functions and mechanisms underlying their diverse actions in plant morphogenesis.
