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

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 14C-tryptophan was recovered as 14C-indoleacetic acid, 14C-tryptamine, 14C-5-hydroxytryptophan, 14C-serotonin and 14C-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.