Phytochrome-mediated regulation of β-amylase mRNA level in mustard (Sinapis alba L.) cotyledons.

Phytochrome, activated by continuous red light, increases the amount of total polyadenylated RNA during photomorphogenesis of mustard (Sinapis alba L.) cotyledons. In-vitro translation of total polyadenylated RNA in a reticulocyte translation system has shown that the activity of translatable β-amylase mRNA is increased by phytochrome about threefold in the 3-d-old cotyledons, based on equal amounts of polyadenylated RNA, and about eightfold on a per-cotyledon basis. Cordycepin prevents the accumulation of translatable β-amylase mRNA. It is concluded that the phytochrome-mediated control of β-amylase synthesis is exerted on the level of mRNA synthesis. During seedling development in continuous red light, a phytochrome-dependent increase of β-amylase mRNA can be observed at least 6 h before the onset of β-amylase synthesis. If, after a period of enzyme synthesis, phytochrome action is interrupted by long-wavelength far-red light followed by darkness, β-amylase mRNA as well as β-amylase synthesis remain at a high level for 8-10 h and then decline sharply. It is concluded that β-amylase mRNA, having an apparent lifetime of the order of 8-10 h, can be formed under the influence of phytochrome during early seedling development but it activates β-amylase synthesis only after a lag-phase of about 8 h, when the cotyledons acquire competence to synthesize the enzyme. The consequences of these findings for the signal-transduction chain of phytochrome are discussed.