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Induction of Phenylalanine Ammonia Lyase and Pisatin by Photosensitive Psoralen Compounds ¹

^a Department of Plant Pathology, Washington State University, Pullman, Washington 99163

The psoralen compounds, xanthotoxin and 4,5', 8-trimethylpsoralen, when activated, increased phenylalanine ammonia lyase (PAL) activity and the synthesis of pisatin in excised pea pods. Pods presoaked 1 hr with 4,5',8-trimethylpsoralen and then irradiated 4 minutes with 366 nanometer ultraviolet light had twice as much PAL activity 3 hours after irradiation and 12 times as much PAL activity 20 hours after irradiation as the pods of the water-treated control. Increases in PAL activity and pisatin synthesis were not obtained with 4,5',8-trimethylpsoralen, xanthotoxin, or 366 nanometer light treatment alone. 4,5',8-Trimethylpsoralen in combination with the irradiation treatment (366 nanometers) enhanced the rate at which L-leucine is incorporated into various fractions of soluble proteins in excised pods 8 hours after treatment. This treatment decreased the rate at which orotic acid is incorporated into RNA. The increase in PAL activity induced by irradiated psoralens was prevented when 6-methylpurine (0.5 milligram per milliliter) or cycloheximide (10 micrograms per milliliter) was applied immediately following the irradiation period. Possible functions of psoralen compounds in plants are discussed.