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

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Abstract
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.