Melatonin as an Endogenous Plant Regulatory Signal: Debates and Perspectives

Abstract

Melatonin (N-acetyl-5-methoxytryptamine) exists in plants, although it is commonly known as a neurohormone in animals. Indeed, the melatonin level is very high in some medicinal plants and changes developmental stage specifically, indicating that it plays specific physiological roles. Plant melatonin may play unique roles in plants as well as similar functions in animals. Furthermore, exogenously applied melatonin affects developmental processes during both vegetative and reproductive growth. In this study, current knowledge regarding plant melatonin is reviewed and its implications and problems are discussed.

Melatonin, an underestimated natural substance with great potential for agricultural application

“…In rice plants tryptophan is converted to melatonin via tryptamine but not 5-hydroxytryptophan as in animals in the following process: tryptophan decarboxylase (TDC) converts tryptophan to tryptamine which is catalyzed to serotonin by tryptamine-5-hydroxylase (T5H), next serotonin N-acetyltransferase (AANAT) catalyzes conversion of serotonin to N-acetylserotonin, and this is changes to MEL by N-acetylserotonin O-methyltransferese (ASMT). Genes of TDC, T5H and ASMT have been cloned and expressed in rice while gene of AANAT has not been characterized in plants yet (Kang et al 2009;Park 2011, Park et al 2013a. It is suggested that in plants the pathway of this indoleamine can be more complex than it was thought earlier ).…”

“…Among many functions which MEL performs in plants, its role as an antioxidant molecule and a growth promoter is most supported by experimental evidence (Paredes et al 2009;Park 2011).…”

“…MEL in plants can regulate rhythmic physiological processes (Tal et al 2011;Park 2011). Level of this indoleamin increases at night and decreases during the day MEL which is why it is called the hormone of darkness in animals (Reiter 1991a, b).…”

“…The effects of MEL may be indirect by influencing the auxin levels but during reproductive development it may act as a transition signal indicating the time for seed germination (Park 2011). Can MEL act as a hormone independently, or in concert with auxin and its own precursors and metabolites?…”

“…It seems that resumption of the research on classic plant models is necessary in order to clarify the role and mechanism of MEL action (i) as an independent plant growth regulator, (ii) as a factor mediating the activity of other substances influencing plant growth or (iii) as a substance involved in growth regulation but whose activity generally is ascribed to other compounds (Park 2011).…”