Sustainment of Uninterrupted Increase of β-amylase Activity in Mustard (Sinapis alba L.) Cotyledons during Light/Dark Cycles

Abstract

Summary In the cotyledons of mustard (Sinapis alba L.) seedlings, phytochrome-mediated increase in the rate of de novo synthesis of β-amylase (E.C. 3.2.1.2) under continuous red light involves a stable regulatory element (≪transmitter≫) in the signal transduction chain which has a life time of about 12 h and acts in an allor -none fashion. Therefore continuous red light can be quantitatively substituted by alternate cycles of red light and darkness, each of 12 h duration. However, when phytochrome is reverted to the Pr form before the onset of the daily dark cycle, seedlings require a 17 h light/7 h dark period to sustain uninterrupted β-amylase synthesis as in continuous red light. The same result is obtained with a 6 h red light/6 h dark period. Using inhibitors of transcription and translation it is established that the transmitter is a stable β-amylase mRNA. The uninterrupted increase in β-amylase level in light/dark cycles results from the continued presence of stable β-amylase mRNA in the dark periods and the resumption of β-amylase mRNA synthesis at the onset of the next light period. These results indicate that under natural conditions β-amylase accumulation in mustard cotyledons would proceed normally through the daily night period.