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The rate of synthesis of RNA during early germination of wheat and soybean embryos was investigated by ascertaining the incorporation of radioactive uridine into RNA. In wheat embryos, where the lag period preceding rapid growth is 5.5 hours, there is a 2-fold increase in RNA synthesis between 1.5 and 5.5 hours, with half of the increase occurring by 3.5 hours. In soybean axes, where the lag period is 9.5 hours, the increased rate of RNA synthesis is 5.5-fold between 1.5 and 9.5 hours, with three fourths of this increase occurring after 4 hours. Analysis of the ratio of radioactivity incorporated into the 18S and 26S rRNAs of the germinating embryos provided a further measure of the increased rates of RNA synthesis. With wheat embryos, the 26S/18S ratio increased from 1.0 at 1.5 hours to 1.5 at 3.5 hours, while with the soybean axes, distinct ribosomal patterns were obtained only after 4 hours and the 26S/18S rRNA ratio increased from 0.4 at 4 hours to 1.0 at 9 hours. The extent of methylation of the rRNA synthesized at 4 and 9 hours in the soybean axes was similar, indicating that the methylating capacity of the axes is probably not rate limiting to rRNA synthesis. In both seed embryo systems the level of UTP increased 2 to 3-fold during the lag phase of germination. With wheat embryos, the time course of the increase in UTP correlated approximately with the change in the rate of RNA synthesis. With the soybean axes, however, the increase in the rate of RNA synthesis occurred predominantly after the rise in the level of UTP.

Wheat ( Triticum aestivum L. var. Lew) embryonic axes take up externally supplied radioactive amino acid (from a solution greater than 2 millimolar) such that the specific radioactivity of the total internal amino acid rapidly reaches that of the external solution. Nevertheless, incorporation of radioactive amino acid into protein increases steadily as the concentration of external amino acid is increased, indicating that the amino acid that is precursor to protein synthesis is not in equilibrium with the total internal amino acid pool. When the external source of amino acid is removed, incorporation of radiolabeled amino acid into protein continues at a rate comparable to that of embryos maintained in the radioactive solution. In explanation of these data, it is suggested that there are two separate cytoplasmic pools of amino acids, one a protein synthesis precursor pool, and the second, an expandable pool into which exogenous radioactive amino acids are taken up. The protein synthesis pool is fed at a limited rate from the expandable pool and at a far greater rate from an endogenous source. As a consequence, the specific activity of the amino acid that is the precursor for protein synthesis is considerably below that of the total internal pool and is determined by the rate of movement into the protein synthesis pool from the expanded radioactive cytoplasmic pool. The rate of movement of amino acids from the expandable pool into the protein synthesis pool increases approximately 5-fold during the initial 4.5 hours of embryo germination. When this change is considered in analyzing the relative rates of protein synthesis, there is probably no more than a 2-fold increase in protein synthetic capacity between embryos germinated for 1.5 and 4.5 hours. The leveling off of the change in transport capacity after 4.5 hours suggests that the earlier increase in the rate of this process may be a necessary step before the embryos can begin to accelerate their growth rate.

Wheat ( Triticum aestivum L.) embryo homogenates have been fractionated into three cell fractions from which RNA was extracted and assayed for mRNA content by in vitro translation and by [ 3 H]polyuridylic acid hybridization. In dry embryos the preformed mRNAs are distributed equally between a rapidly sedimenting “pellet” fraction and a cytoplasmic “ribosomal/subribosomal” fraction. During germination 25 to 40% of the total mRNA becomes polyribosomal. The remaining 60 to 75% is retained in the pellet and ribosomal/subribosomal fractions. To compare the nucleotide sequences of the different mRNA fractions, cDNAs were transcribed from polyribosomal (A+) RNA of 40-minute imbibed embryos and from total A(+) RNA of dry embryos, and the ability of these cDNAs to hybridize with the more prevalent classes of mRNA from the different cell fractions was analyzed. The results suggest that there is no significant difference between the preformed mRNAs that move into polyribosomes and those remaining in the nonpolyribosomal fractions. In addition there appears to be no difference between the mRNAs of polyribosomes from embryos germinated for 45 minutes and 5 hours. Between 5 hours and 2 days, however, there is a considerable change in the mRNA composition of the embryos. We conclude for the prevalent classes of message, that the preformed mRNAs of the wheat embryo are not involved in temporal regulation of early development but that they function primarily to allow a rapid resumption of growth upon exposure of the embryo to water.

UDP-Sugars comprise the dominant class of nucleotide sugars in isolated soybean axes during early germination. While “dry” axes contain 1 nanomole per axis of UDP-sugars, further synthesis is initiated upon imbibition such that the concentration of total UDP-sugars reaches 8 nanomoles per axis or roughly 1 millimolar after 10 hours, when the axes begin to elongate. The GDP-sugars are essentially absent before imbibition, accumulate rapidly for 90 min to 173 picomoles per axis, then decrease somewhat, reattaining the earlier peak level shortly before growth begins. Meanwhile, the level of ADP-sugars is unchanged. These data indicate that the 10-hour lag period preceding axis growth does not result from a diminished ability to synthesize a major category of nucleotide sugars. Relative rates of synthesis of individual UDP- and GDP-sugars were determined by incorporation of [ 3 H]uridine or [ 3 H]guanosine. The distribution of label in the different classes of UDP-sugars and in the single class of GDP-sugar was quantitatively similar when analyzed before, at the onset, or during early growth. It therefore seems unlikely that synthesis of a key nucleotide sugar controls the initiation of growth. The possible relevance of nucleotide sugars to growth is discussed and new methods for enzymic analysis of picomole levels of nucleotide sugars are described.

The requirement for the synthesis of RNA and DNA in early germination of wheat (Triticum aestivum var Newana) embryonic axes has been studied by incubating embryos in the presence of appropriate inhibitors and monitoring both embryo growth and the rates of specific metabolic processes. Experiments with 5-fluorouridine showed that both rRNA and DNA synthesis could be curtailed by 60 to 70% without affecting embryo growth to 24 hours. Similarly, the presence of mitomycin C and methotrexate inhibited DNA synthesis 70%, with only a small effect on growth. Experiments with a range of concentrations of cordycepin and α-amanitin indicated that mRNA synthesis could be curtailed by 30 to 40% within the first 8 hours of germination with only a small effect on embryo growth. Thus, at least the initial phases of seed embryo germination are not closely linked to the synthesis of mRNA, rRNA, or DNA. Maximal sensitivity of embryo growth was obtained with cycloheximide and 2-(4-methyl-2,6-dinitroanilino)-N-methyl propionamide, supporting the idea that protein synthesis is the macromolecular process most closely linked to early germination.

The enzyme 3'-AMP nucleotidase was purified 2,500- to 5,000-fold from extracts of an acetone powder of wheat (Triticum aestivum) embryonic axes germinated for 40 hours. Sodium dodecyl sulfate acrylamide gel electrophoresis and chromatography on Biogel-P100 indicate that the enzyme is monomeric with a molecular weight of 39,000. Extracts of embryos germinated up to 6 hours have only 1% of the 40-hour level of enzyme activity. To see if the increase to 40 hours represents de novo synthesis, extracts were compared for their ability to react with a rabbit antibody prepared against the enzyme. In immunodiffusion tests, 40-hour extracts showed a strong precipitin line coincident with that of the purified enzyme, whereas no precipitation was observed with 1-hour extracts. When the enzyme present in 40-hour extracts was partially inactivated by EDTA, it still blocked the ability of the antibody to inhibit enzyme activity. Extracts of 1-hour embryos, in contrast, were not able to block the inhibitory activity of the antibody. Embryos allowed to take up (35)SO(4) between 40 and 46 hours of germination synthesized (35)S-labeled 3'-nucleotidase. In contrast, no radioactive protein synthesized by embryos during the first 6 hours of germination coincided on gel electrophoresis with the enzyme. These results indicate that the increase in 3'-nucleotidase activity is a consequence of de novo synthesis of the enzyme.

Germinating wheat embryos (Triticum aestivum L). synthesize both ribosomal and messenger RNA at the earliest times after the onset of germination. The rates of synthesis of these two RNAs are determined at various stages in germination by an analysis of newly synthesized radioactive RNA on oligo(dT)-cellulose. The rate of messenger RNA synthesis is essentially constant throughout 18 hours of germination, while that of ribosomal RNA synthesis increases steadily, particularly after the onset of cell expansion (6 hours), reaching at 16 to 18 hours, a rate of synthesis between 5- and 20-fold greater than that observed at the earliest stages. The net effect is a relative decrease in the fraction of transcribed high molecular weight RNA that is mRNA. Throughout the first 7 hours of germination, mRNA is 25 to 30% of the transcribed fraction, whereas by 16 to 18 hours it has declined to a level of 4 to 8%.

When a stationary phase cell culture of Arachis hypogaea L. is diluted into fresh media, there occurs a 10-fold increase in the rate of protein synthesis. The kinetics of the activation of amino acid-incorporating capacity show a lag of 10 to 15 minutes with maximal activity reached at 2 hours after dilution. The activation of protein synthesis is oxygen-dependent and is accompanied by a 2- to 4-fold increase in polyribosome content, as well as by a 3- to 4-fold increase in the rate of mRNA synthesis. Ribosomal function, as ascertained by determination of ribosomal transit time, is about 2.5 times more efficient in 2-hour diluted cultures as in cells immediately after dilution. These observations indicate that a very early response in the transition of plant cell cultures from the stationary state is an increased capacity for protein synthesis. At a molecular level, this increase in protein synthetic capacity is due in part to an increased mobilization of mRNA into polyribosomes and in part to a more efficient ribosomal translational capacity.

The ATP content of isolated wheat (Triticum aestivum L. var. Polk) embryos increases 5-fold during the first 30 minutes and 10-fold during the first hour of germination to 80% of maximum. The ATP level remains at approximately 800 nanomoles per gram of tissue during the next 15 hours. ADP, AMP, and total adenosine phosphates decrease between 1 and 6.5 hours, while adenylate energy charge increases from 0.6 to 0.8 and remains constant. The rapid increase in ATP during imbibition is consistent with the energy requirement for polyribosome formation and protein synthesis during the first hours of germination. A method for determining nanomole quantities of ATP in tissue extracts by isotopic dilution of γ-32P-ATP in the hexokinase reaction is outlined.