The effects of methionine and glutathione on the promoter activities of the beta subunit gene of beta-conglycinin, a seed storage protein of soybean, were studied in a transient assay system using protoplasts derived from Arabidopsis thaliana liquid callus cultures. The promoter activity of the beta subunit gene was repressed by methionine and glutathione in a manner similar to that observed in transgenic plants. This is the first report of a transient assay system to study the regulation of gene expression by methionine and glutathione.