Protein Synthesis in Imbibed Seeds III. Kinetics of Amino Acid Incorporation Ribosome Activation, and Polysome Formation

Abstract

Siumtmt77ary. The kinetics of development of proteini-synthesizing capacitv in the imbibing wheat embryo, were sttudied both in vivo and in vitro. During the first 30 minutes of imbibition protein-synthesizing capacity rises rapidly, lagging about 10 minutes behind water uptake. This rise in synthesizing capacity is accompanied by an increase in polysome content. As imbibition continues, both protein-synthesizing capacity and polysome content increase. With embr,-os from aged seed, the rate of protein synthesis is initially limited by another, presuimably nonribosomal, reaction.An earlier report (4) concltuded that seed germination is accompanied by an increased capacity for protein synthesis. This conclusion was based primarily on the observation that ribosomes isolated from dry (unimbibed) Material and MethodsIn zizo Incorporation. Assay 1. Wheat embryo samples (200 mg) were imbibed for the desired times and then transferred to a test tube where they were incubated in 2.5 ,umoles potassium phosphate (pH 6.0), 40 jig chloramphenicol, 5.1 m,umoles L-leulcine-'4C (0.25 jucuiries) and water to 0.53 ml.The volume of incubation medium was such that the embryos were submerged. After incubation, 200 nmoles of L-leucine-_2C were added; free radioactivity was removed by washing with water, and the embryos were grauun(d to a homogeneous sUi'-pension in 5 % trichloroacetic acid containing 200 ,umoles L-leuicine-_2C. An appropriate aliquot of the stuspension was centrifuged; an aliquot of the supernatant was counted in Bray's solution (1) (trichloroacetic acid soluble). Another aliqulot of the suspension was centrifuged, resuspended in 5 % trichloroacetic acid, and heated for 15 minuites at 90°. After cooling for 10 minutes in ice, the precipitate was collected on a membrane filter, washed with 5 % trichloroacetic acid and counted in toluene-PPO-POPOP (trichloroacetic acid insoluble).

Isolation and Cell-free Translation of Total Messenger RNA from Germinating Castor Bean Endosperm

“…The uptake of water by dry seeds initiates a rapid transcription of RNA and ensuing protein synthesis which permits metabolic activity during germination (11,12). In the endosperm of germinating castor bean seedlings, this metabolic activity is predominantly associated with the conversion of stored triglycerides into carbohydrate (1).…”

DNA content of dormant barley leaf nuclei and the synthesis of RNA and DNA during germination

“…In embryos where cells enter early germination with the genome already replicated during embryo development such a response may be delayed. There are very convincing demonstrations that early protein synthesis in the germinating embryo is supported by pre-formed mRNA (for discussion see Walbot [4] and Walbot et al [22] and that the entire apparatus for protein synthesis is present in the dormant embryo [23,24]. However, the possibility that the DNA replication apparatus, particularly its protein component, is unable to function optimally cannot be ruled out.…”

Free poly(A) tracts complexed with protein in the cytoplasm of dried wheat embryos

“…It has been shown that the early stages of seed germination are characterized by a rapid increase of the content of polyribosomes in the embryo and by an increase of the rate of protein synthesis [1][2][3][4][5]. The formation of polyribosomes at this developmental stage does not depend on transcription [6].…”