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Induction of Phenylalanine Ammonia-lyase in Strawberry Leaf Disks

Action Spectra and Effects of Wounding, Sucrose, and Light ¹

Leroy L. Creasy^b

^a Department of Agricultural Chemistry, Washington State University, Pullman, Washington 99163, Department of Pomology, Cornell University, Ithaca, New York 14850

The increase in phenylalanine ammonia-lyase (PAL) activity in strawberry (Fragaria vesca var. WSU-1232) leaf disks required wounding, sucrose, and light and was cycloheximide-sensitive. In injured leaves and in leaf disks, the highest PAL activity was detected nearest the wounded tissues. Without wounding, no increase in activity was observed when leaves were cultured in sucrose and light.

The optimal concentration of sucrose for enzyme activity increase ranged from 0.15 M to 0.4 M. At the suboptimal sucrose concentration, the level of PAL activity was dependent upon the concentration of sucrose. A low but constant level of activity was detected in leaf disks maintained in 0.15 M sucrose and in darkness. Light accelerated the rate of PAL increase but did not change the total level of enzyme activity which was determined by the sucrose concentration.

Enzyme activity disappeared rapidly when leaf disks cultured in sucrose and light were transferred to darkness or to water in light. Unlike in Xanthium leaf disks, cycloheximide could not completely inhibit the decay of enzyme activity, suggesting that an inactivating system was synthesized during the induction period, and the activity of the inactivating system increased as the induction period lengthened.

The effect of light on accumulation of PAL activity appeared to be linked to photosynthesis. In the presence of 25 µM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the effect of light on enzyme increase was completely nullified. Addition of 25 µM 3-(3,4-dichlorophenyl)-1,1-dimethylurea to culture medium caused rapid decay of PAL activity from leaf disks which had been previously cultured in sucrose and light. The relation between effect of light and photosynthesis was further demonstrated by the action spectrum. Leaf disks incubated in sucrose and light of different wavelengths exhibited maximum accumulation of PAL activity at two wavelengths (475 nm and 625 nm). Action spectrum for protection against PAL decay exhibited a plateau at 475 to 525 nm and a peak at 625 nm. Action spectra for accumulation and protection against inactivation of PAL activity, therefore, appeared to be very similar to the action spectrum of photosynthesis.

¹ Scientific paper No. 4238, Project 0125 from the Agricultural Research Center, College of Agriculture, Washington State University, Pullman, Wash.