The role of serotonin and melatonin in plant morphogenesis:: Regulation of auxin-induced root organogenesis in in vitro-cultured explants of St. John's wort (Hypericum perforatum L.)

St. John's wort (Hypericum perforatum cv. Anthos) is a medicinal plant with historical and anecdotal evidence of efficacy as an anti-depressant. Recent research has demonstrated an active biosynthetic pathway leading to the production of the mammalian neurohormone melatonin in St. John's wort plantlets. The objective of the current study with to assess the physiological role of melatonin and related indoleamines in plant morphogenesis. In the initial experiments, two of the indoleamines, serotonin and melatonin, were supplemented to the culture medium. In subsequent research,six inhibitors of auxin and indoleamine action or transport, 2,3,5-triiodobenzoic acid, p-chlorophenoxyisobutyric acid, p-chlorophenyl-alanine, D-amphetamine, fluoxetine (Prozac (TM)), and methylphenidate (Ritalin (TM)), were included in a culture medium in the presence or absence of the auxin, indoleacetic acid (IAA). The rate of de novo root and shoot organogenesis and the endogenous concentrations of auxin and indoleamines were determined in cultured explants. Significant reductions in de novo root regeneration were found to correspond with decreases in the pool of both IAA and melatonin. Art increase in the endogenous concentration of melatonin was correlated with an increase in de novo root formation, and increased serotonin levels corresponded to increased shoot formation on the explants. Our findings provide the first evidence that a balance of the endogenous concentration of serotonin and melatonin may modulate plant morphogenesis in vitro.