UV irradiation stimulates expression of the gene encoding the key enzyme chalcone synthase (CHS), which leads to the generation of protective flavonoids in parsley cell cultures. CHS transcripts increase after 3 to 4 hr, and early genes are involved in the signal transduction to the CHS promoter. By using the fluorescent differential display technique in a large-scale screening, several early UV light-induced genes were isolated. Of these, a novel glutathione S-transferase (PcGST1) is induced within 2 hr and precedes CHS expression. Overexpression of PcGST1 in transformed cell lines containing a CHS promoter/luciferase reporter (CHS-LUC) affected the onset of LUC transcription. Supplementing these cell lines with glutathione immediately stimulated CHS-LUC expression within 2 hr in dark-incubated cells and resulted in a biphasic induction profile in UV-irradiated cells. Our data indicate the involvement of glutathione and PcGST1 in early events of a UV light-dependent signal transduction pathway to CHS. In this context, the oxidative status of a cell acts as a central regulating element.