The effect of light pretreatments on phytochrome-mediated induction of anthocyanin and of phenylalanine ammonia-lyase.

Induction by light of phenylalanine ammonia-lyase (PAL; EC 4.3.1.5) and of anthocyanin in cotyledons of the mustard (Sinapis alba L.) seedling is strongly affected by a light pretreatment which operates through phytochrome. If PAL or anthocyanin is induced by a light pulse, the effectiveness of phytochrome (Pfr) is strongly increased by a light pretreatment; however, if the increase of the PAL level or synthesis of anthocyanin is elicited by continuous far-red light (operating via phytochrome in the 'High Irradiance Response'), effectiveness of light is strongly reduced by the same light pretreatment. This reduction of effectiveness is correlated with a decrease of total phytochrome (Ptot) caused by the light pretreatment. It is argued that the observations are compatible only with the 'open phytochrome-receptor model' as suggested by Schäfer (J. Mathem. Biol. 2, 41-56, 1975). The peaks of the time courses of the PAL levels under continous far-red light are located at 48 h after sowing and do not depend on the original level of phytochrome. The decrease of the PAL levels beyond 48 h after sowing takes place independently of phytochrome and of the actual level of PAL.