Sequential control of phytochrome-mediated synthesis de novo of β-amylase in the cotyledons of mustard (Sinapis alba L.) seedlings.

In the cotyledons of mustard (Sinapis alba L.) seedlings irradiated from the time of sowing with continuous red light, the photoreversibility of the phytochrome-mediated increase in β-amylase activity (EC 3.2.1.2) is lost 36 h after sowing (coupling point). However, the induced increase of β-amylase activity cannot be detected before 46 h after sowing (starting point). Density labeling with deuterium oxide shows that the increase of enzyme activity in light and darkness coincides precisely with the synthesis of β-amylase protein. Thus, phytochrome mediates an increase of β-amylase synthesis de novo. Since there is no turnover detectable by density labeling, it is concluded that β-amylase of mustard cotyledons is a physiologically stable enzyme (half-life >5 d). The 10-h time gap between loss of photoreversibility and onset of light-induced β-amylase synthesis points to a relatively stable regulatory element within the signal chain ("transmitter") which links β-amylase synthesis to the primary action of phytochrome. A 12-h lag between the cessation of phytochrome action and the cessation of induced β-amylase synthesis indicates a limited lifetime of the transmitter (about 12 h). The effect of this result on the interpretation of the coupling point is discussed.