Tryptophan is a precursor for melatonin and serotonin biosynthesis in in vitro regenerated St. John's wort (Hypericum perforatum L. cv. Anthos) plants

Evidence of a pathway for the biosynthesis of the mammalian neurohormones melatonin and serotonin in in vitro regenerated plantlets of St. John's wort (Hypericum perforatum cv. Anthos) is presented. Isotope tracer experiments were performed on plantlets regenerated from thidiazuron-induced stem explants and grown on MS basal medium for 2 months. Radiolabel from C-14-tryptophan was recovered as C-14-indoleacetic acid, C-14-tryptamine, C-14-5-hydroxytryptophan, C-14-serotonin and C-14-melatonin in the treated St. John's wort plantlets. Chromatographic peak identity was confirmed by high performance liquid chromatography-mass spectrometry-mass spectrometry and quantification of melatonin by radioimmunoassay. Significantly more radiolabel was recovered in serotonin relative to melatonin under low light conditions with this ratio being reversed under increased lighting, indicating that the rate of flow through this biosynthetic pathway is regulated, at least in part, by light.