The case of tryptamine and serotonin in plants: a mysterious precursor for an illustrious metabolite

Indolamines are tryptophan-derived specialized metabolites that belong to the huge and ubiquitous indole alkaloids group. Serotonin and melatonin are the best-characterized members of this family, given their many hormonal and physiological roles in animals. Following their discovery in several plant families, the study of plant indolamines has flourished over the last few decades and their involvement in many important processes has been proposed, including stress responses, growth and development, and reproduction, leading to their classification by some authors as a new category of phytohormones. However, the complex puzzle of the indolamines is far from resolved, particularly the biological roles of tryptamine, the early tryptophan-derived serotonin precursor representing the central hub of downstream plant indolamines and many specialized indole alkaloids. Tryptophan decarboxylase, which catalyzes the synthesis of tryptamine, appears to strictly regulate the flux of carbon and nitrogen from the tryptophan pool into the indolamine pathway. Furthermore, tryptamine accumulates to high levels in the reproductive organs of many plant species, and therefore cannot be classed as a mere intermediate but rather as an end product with potentially important functions in fruits and seeds. This review summarizes current knowledge on the role of tryptamine and its close relative serotonin, emphasizing the need for a clear understanding of the functions of, and mutual relations between, these two indolamines and their biosynthesis pathways in plants.