Photoinduction of phenylalanine deaminase in gherkin seedlings

Abstract

Exposure of dark-grown gherkin seedlings to blue light causes temporary changes in the level of phenylalanine deaminase (PADAse). Following a time lag of about 90 min the enzyme level increases; about 180 min after the beginning of irradiation it declines again. The light-induced increase in the level of PADAse is due to de-novo enzyme synthesis, since spraying of the plants with cycloheximide prior to the light treatment results in inhibition of the increase in enzyme level. The increase of the enzyme level is a function of light intensity. One of the initial processes involved is the transport of a promotive factor from the cotyledons to the hypocotyl. The decline in the PADAse level which follows the initial increase is probably caused by enhanced inactivation of the enzyme. It can be stopped with cycloheximide, indicating that the inactivation process depends on de-novo protein synthesis. The amount of previously induced PADAse and the capacity of the system to respond to a subsequent light treatment are inversely related. The light-induced changes in PADAse activity are parallel to those in the activity of cinnamic acid hydroxylase, the enzyme responsible for the subsequent step in the synthesis of hydroxycinnamic acids from L-phenylalanine. This result indicates a close connection between the induction of the two enzymes. A hypothesis in advanced which explains the effects of irradiation with blue light (“high-energy” reaction) on phenol synthesis on the basis of enzyme synthesis and enzyme inactivation.