Studies on the nature of messenger RNA in germinating wheat embryos

Seed-Stored mRNAs that Are Specifically Associated to Monosomes Are Translationally Regulated during Germination

The life cycle of many organisms includes a quiescent stage, such as bacterial or fungal spores, insect larvae, or plant seeds. Common to these stages is their low water content and high survivability during harsh conditions. Upon rehydration, organisms need to reactivate metabolism and protein synthesis. Plant seeds contain many mRNAs that are transcribed during seed development. Translation of these mRNAs occurs during early seed germination, even before the requirement of transcription. Therefore, stored mRNAs are postulated to be important for germination. How these mRNAs are stored and protected during long-term storage is unknown. The aim of this study was to investigate how mRNAs are stored in dry seeds and whether they are indeed translated during seed germination. We investigated seed polysome profiles and the mRNAs and protein complexes that are associated with these ribosomes in seeds of the model organism Arabidopsis (Arabidopsis thaliana). We showed that most stored mRNAs are associated with monosomes in dry seeds; therefore, we focus on monosomes in this study. Seed ribosome complexes are associated with mRNA-binding proteins, stress granule, and P-body proteins, which suggests regulated packing of seed mRNAs. Interestingly, ∼17% of the mRNAs that are specifically associated with monosomes are translationally up-regulated during seed germination. These mRNAs are transcribed during seed maturation, suggesting a role for this developmental stage in determining the translational fate of mRNAs during early germination.

Biology in the Dry Seed: Transcriptome Changes Associated with Dry Seed Dormancy and Dormancy Loss in the Arabidopsis GA-Insensitive sleepy1-2 Mutant

Plant embryos can survive years in a desiccated, quiescent state within seeds. In many species, seeds are dormant and unable to germinate at maturity. They acquire the capacity to germinate through a period of dry storage called after-ripening (AR), a biological process that occurs at 5-15% moisture when most metabolic processes cease. Because stored transcripts are among the first proteins translated upon water uptake, they likely impact germination potential. Transcriptome changes associated with the increased seed dormancy of the GA-insensitive sly1-2 mutant, and with dormancy loss through long sly1-2 after-ripening (19 months) were characterized in dry seeds. The SLY1 gene was needed for proper down-regulation of translation-associated genes in mature dry seeds, and for AR up-regulation of these genes in germinating seeds. Thus, sly1-2 seed dormancy may result partly from failure to properly regulate protein translation, and partly from observed differences in transcription factor mRNA levels. Two positive regulators of seed dormancy, DELLA GAI (GA-INSENSITIVE) and the histone deacetylase HDA6/SIL1 (MODIFIERS OF SILENCING1) were strongly AR-down-regulated. These transcriptional changes appeared to be functionally relevant since loss of GAI function and application of a histone deacetylase inhibitor led to decreased sly1-2 seed dormancy. Thus, after-ripening may increase germination potential over time by reducing dormancy-promoting stored transcript levels. Differences in transcript accumulation with after-ripening correlated to differences in transcript stability, such that stable mRNAs appeared AR-up-regulated, and unstable transcripts AR-down-regulated. Thus, relative transcript levels may change with dry after-ripening partly as a consequence of differences in mRNA turnover.

Characterization of cell-free protein-synthesis systems from undeveloped and developing Artemia embryos

We have developed and characterized translationally active cell-free systems from Artemia embryos at different developmental times. The optimized lysates from 16 h-developed embryos incorporated radiolabelled amino acids into polypeptides for up to 120 min. The polypeptides synthesized ranged in Mr from 150,000 to 10,000, suggesting that the endogenous mRNA was capable of directing the synthesis of complete polypeptides. Similar results were obtained by using lysates from early developmental stages; even the cell-free system prepared from 1 h-developed embryos was partially active in protein synthesis. Furthermore, all these lysates were capable of re-initiation, as demonstrated by inhibition of initiation with the inhibitors edeine and 7-methylguanosine 5'-triphosphate. Because we found no endogenous protein-synthetic activity in the corresponding lysates from undeveloped embryos, we have used cell-free translation systems from 0 h- and 16 h-developed Artemia embryos to analyse the mechanisms limiting protein synthesis at very early developmental stages. Undeveloped-embryo lysates supplemented with nuclease-treated reticulocyte lysate were capable of translating endogenous mRNAs to give products with a wide spectrum of Mr values, but lysates of 16 h-developed embryos supplemented in this way were not further stimulated. The nuclease-treated lysate appeared to be unnecessary 5 h after resumption of development. These results suggested that a component(s) limiting translation in the undeveloped-embryo lysate was provided by the nuclease-treated reticulocyte lysate, and that this component(s) no longer limited protein synthesis after development. In view of these results, partially fractionated reticulocyte lysates were tested for restoration of protein-synthetic activity in the undeveloped embryo lysate. A high-salt ribosomal wash devoid of ribosomal subunits, which is considered a crude polypeptide-initiation-factor preparation, also restored translation activity in the undeveloped embryo lysate and made it capable of directing the synthesis of both endogenous mRNAs and exogenous (globin) mRNA.

The nucleosomal repeat length of pea (Pisum sativum) chromatin changes during germination

Pea (Pisum sativum) nuclei have been isolated from ungerminated embryos, developing embryonic axes and seedlings. Morphological and biochemical criteria revealed that preparations were free from contaminants and that nuclei were intact. These circumstances permitted an accurate determination of nucleosomal repeat lengths, the values obtained being 175±4 base pairs for ungerminated embryos, 185±5 base pairs for 62-hours germinated embryonic axes and 185±3 base pairs for 6-day old seedlings. The results seem to indicate that the increase in repeat length is associated with the onset of transcription and/or replication of DNA.

Translation and transcription in imbibed and germinating spores of Anemia phyllitidis L.Sw

Induction of germination by gibberellin or light is not a prerequisite for protein and RNA synthesis in spores of A. phyllitidis. Imbibed but non-induced spores of this fern show a high rate of translation as well as appreciably transcription of all RNA types. The pattern of in-vitro translation of polyadenylated RNA present in the dry spore corresponds with the in-vivo translation products under non-inductive conditions. New proteins are not detectable among the in-vitro and in-vivo translation products until 48 h after a germination stimulus. Although the dark imbibition of spores is accompanied by a reduction in the lengths of polyadenylated tracts of the stored mRNA, this fraction is characterized by a remarkable stability which is undoubtedly a prerequisite for the high viability of these fern spores. The importance of these results for the general meaning of stored mRNA in spore and seed germination processes is discussed.

Germination-induced Changes in Chromosomal Proteins of Spring and Winter Wheat Embryos

The template activity of chromatin from winter wheat embryos gradually increased during germination and was regulated with some nonhistone proteins different from the two major ones, molecular weight 39k and 59k polypeptides, previously reported.To clarify chromosomal proteins which are involved in regulation of template activity of chromatin, we studied the quantitative and qualitative changes in chromosomal proteins. Differences in acid-soluble and acid-insoluble proteins between chromatins from wheat germ and embryos germinated for various times were visualized by sodium dodecyl sulfate-polyacrylamide slab gel electrophoresis.Nonhistone proteins of 39k, 41k, and 50k molecular weights were specifically present in wheat germ and in 24- or 48-hour germinated wheat embryos, thereafter greatly reduced or finally disappeared. In contrast, nonhistone protein of 37k was absent in germ and in embryos germinated for 24 hours and appeared after 48 hours of germination. Thereafter it was present in abundant amounts in 96-hour germinated winter wheat embryos and in 72-hour germinated spring embryos, corresponding to 7 and 10% of total nonhistone proteins, respectively. Histone H1, especially H1d, was slightly reduced after 48-hour germination, as much as basic nonhistone proteins having electrophoretic mobilities between H1d and H2B. Further-more, similarity and diversity of chromosomal proteins between spring and winter wheat embryos are shown in this study. A subspecies of histone H1c of spring wheat had faster electrophoretic mobility than that of winter wheat.

Preformed and newly synthesized messenger RNA in germinating wheat embryos

In 6 h germinated wheat (Triticum aestivum L. cv. Cama) embryos, more than half of the messenger RNAs are actively involved in translation. Neither preformed nor newly synthesized poly A(+)-RNA is translated preferentially. Germination in the presence of cordycepin showed that the half-life of the templates is about 2 h and that the newly synthesized messengers are essential to support protein synthesis in the embryo from the first hours of germination. Most of the messenger RNAs in 6 h germinated embryos are newly synthesized. The polypeptides coded for by either the endogenous messenger ribonucleoproteins or purified poly A(+)-RNA from both dry and germinated embryos are qualitatively identical; minor quantitative differences can however be observed.

Messenger ribonucleoprotein complexes of cryptobiotic embryos of Artemia salina

Poly(A)-containing ribonucleoprotein (poly(A)+-RNP) particles in the post-mitochondrial supernatant of cryptobiotic embryos of Artemia salina were characterized by hybridization to [3H]-poly(U). By sucrose isopycnic centrifugation, approximately 2/3 of poly(A)+-RNPs was found to band at 1.27-1.30 (g/cm3) and the rest 1+/3 at 1.20-1.23 (g/cm3) and below 1.20 (g/cm3). The 1.27-1.30 RNPs could be separated into two density classes, 1.27-1.28 and 1.30 (g/cm3) respectively. The latter RNP class was apparently complexed with ribosomal components because they were completely converted to the former RNP class (free RNPs) by 25 mM EDTA treatment. Further, the 1.30 (g/cm3) RNPs were resolved into several RNP species having sedimentation coefficients above 50 S. which were transformed mostly to 20-30 S rnps in the presence of 25 mM EDTA. The free 20-30 S RNPs contained 8-14 S poly(A)+-RNAs, having the highest template activity in a wheat embryo cell-free system, whereas the 1.20-1.23 poly(A)+-RNPs consisted of 10 S and 16 S RNPs, both of which contained 4 S poly(A)-containing sequences without any template activity.

Changes of Template Activity and Proteins of Chromatin during Wheat Germination

Relationships between changes in template activity and composition of chromatin during germination of wheat embyros (Triticum aestivum L.) were investigated. The template activity of chromatin was determined with exogenous DNA-dependent RNA polymerase II (EC 2.7.7.6) prepared from wheat embryos. It was essentially constant for 18 hours of germination, corresponding to 2.5% of that of a native calf thymus DNA. Thereafter, the activity increased 2-fold and 5-fold at 24 and 60 hours of germination, respectively.Chromatin-associated proteins were separated into at least 22 distinct bands by sodium dodecyl sulfate gel electrophoresis throughout 60 hours of germination. Significant changes were observed in two nonhistone proteins, approximate molecular weights 59,000 and 39,000: the amount of the former was constant up to 18 hours, reduced for the period from 18 to 60 hours, and that of the latter was decreased for the period from 18 to 60 hours of germination. No change was observed in the number of histone components by acid-urea gel electrophoresis.

Messenger ribonucleoprotein particles in dry wheat and rye embryos : In vitro translation and size distribution

Extracts prepared from dry wheat (Triticum aestivum L.; var. Cama) and rye (Secale cereale L.; var. Celestijner) embryos exhibit relatively high endogenous messenger activities in an in vitro protein synthesizing system: up to 400 pmol leucine are incorporated per 50 μl reaction mixture; the messenger/ribosome ratio is estimated at about 0.03. Sucrose gradient analysis of the incubation mixture during cellfree protein synthesis shows a progressive in vitro polysome formation with the native endogenous mRNPs. The translation of these messengers, although dependent on initiation, follows linear kinetics. The native mRNP particles can be separated on sucrose gradients in 6 distinct size classes with sedimentation values varying from 25 S to 104 S. Their size is strongly reduced after deproteinization so that the naked mRNAs sediment in a range between 8 S and 35 S. The molecular weights of the proteins synthesized under direction of the native RNA particles range from 12,000 to 70,000 as estimated by polyacrylamide gel electrophoresis.

Differential effects of actinomycin d and cordycepin in lettuce seed germination and RNA synthesis

Intact lettuce seed germination was inhibited by cordycepin but not by actinomycin D; however, when seeds were clipped at the cotyledonary end, actinomycin D partially inhibited germination. Uptake studies with intact seeds using (3)H-actinomycin D showed that it was unable to reach the embryo prior to radical protrusion. (3)H-Cordycepin uptake studies using intact seeds showed that cordycepin was able to reach the embryo during the first 3 hours of incubation and at subsequent times. The pericarp and endosperm offered resistance to penetration of cordycepin into the embryo. In contrast to actinomycin D, cordycepin markedly inhibited (3)H-uridine incorporation into RNA of intact seeds during the first 10 and 12 hours of incubation. About 60% of (3)H-adenosine incorporation into poly A-RNA was inhibited by cordycepin during 12 hours of incubation, whereas actinomycin D had little effect. RNA synthesis appears to be essential for seed germination.

In vitro translation of the long-lived messenger ribonucleic acid of dry seeds

The total RNA extracted from rye embryos (Secale cereale) and seed of the broad bean (Vicia faba), pea (Pisum sativum) and oil-seed rape (Brassica napus) exhibits low levels of template activity when incubated in a wheat-germ cell-free protein-synthesising system. The RNA from pea, rapeseed and rye embryos was fractionated by chromatography on oligo dT-cellulose columns. Most of the template-active RNA bound to the column at high ionic strength, indicating that it is polyadenylated. The remainder would not bind, even when passed through the column several times. The proteins synthesised in vitro from the template-active RNA migrated as numerous bands in polyacrylamide gels and ranged in molecular weight from 10,000 to 70,000. The banding patterns obtained were quite different for the three species of seed tested. It is concluded that dry seeds contain a store of intact, long-lived mRNA.

Early ribonucleic acid synthesis during the germination of rye (Secale cereale) embryos and the relationship to early protein synthesis

Incorporation studies with radioactive precursors showed that synthesis of protein and RNA is initiated in germinating embryos of rye within the first hour of imbibition of water. By polyacrylamide-gel fractionations of radioactive nucleic acid components, the appearance of products of transcription of the genome was shown to follow the sequence: heterogeneous (ribonuclease-sensitive) RNA, 4S and 5S RNA by 20min, 31S and 25S rRNA by 40min, and 18S RNA by 60min. "Fingerprint' analysis of T1-ribonuclease digests show that all the large oligonucleotides present in 25S and 18S RNA are present in the 31S species, indicating that 31S RNA is the precursor rRNA molecule to both 25S and 18S RNA. The importance of these early RNA syntheses and in particular the possible template function of the heterogeneous RNA is discussed in relation to the concept of long-lived mRNA and the coding for protein synthesis in the first hours of germination.

Studies on messenger and ribosomal RNA synthesis in plant tissue cultures induced to undergo synchronus cell division

Messenger and ribosomal RNA metabolism was tudied in a plant tissue culture system: cells from the quiescent tubers of Helianthus tuberosus (Jerusalem artichoke) were induced to divide synchronously and dedeifferentiate by excision and culture of explants in nutrient medium. Large accumulations of ribosomal RNA and protein started early in the 20-h lag-period preceding the first division. In pulse-labelling experiments, two types of polydisperse messenger-like RNAs were detected one with and one without a poly(adenylic acid) sequence. In the first 2 h of culture the two polydisperse RNA fractions were the predominant types of RNA synthesised. Ribosomal RNA synthesis was very low during the first 2 h, but acclerated later. Low concentrations of actinomycin-D strongly inhibited ribosomal RNA synthesis, but had little effect on the synthesis of transfer RNA or either type of polydisperse messenger-like RNA. In explants cultured with low concentrations of actinomycin-D ribosomal RNA accumulation was completely inhibited, but cell division and protein accumulation occurred, though at a reduced rate. It is concluded that the synthesis and accumulation of new ribosomal RNA which normally occur during culture are not required for the induction of cell division or for protein accumulation, i.e. the ribosomal RNA existing in the quiescent tuver cells can support protein accumulation and cell division induced by excision and culture. The quiescent tuber tissue is also shown to contain significant amounts of messenger-like RNA.

Activation of Protein Synthesis upon Dilution of an Arachis Cell Culture from the Stationary Phase

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.

Application of chemicals in organic solvents to dry seeds

Various chemicals were applied to dry seeds by means of organic solvents. The gibberellic acid-treated (1 mm) lettuce seeds (Lactuca sativa L.) germinated nearly 100% in the dark even after prolonged storage, and those treated with abscisic acid (1 mm or 0.5 mm) failed to germinate in the light. The seedlings emerging from morphactin-treated (1 mm) cucumber seeds (Cucumis sativus L.) exhibited profound changes in morphology. Different combinations of hormones applied to lettuce seeds caused a promotion or an inhibition of germination. Germination promotion or inhibition studies showed that the applied chemicals could be removed by washing with an organic solvent or water. Progressively larger amounts of chemicals were removed with increasing periods of washing. Thus the chemical appeared to penetrate the seed to some degree. The potential of the organic solvent method is discussed.

Germination of Phaseolus vulgaris: IV. Patterns of Protein Synthesis in Excised Axes

Soluble proteins from excised Phaseolus vulgaris axes incubated for 1 hour in (3)H or (14)C- amino acid mixtures at different times during the period leading up to initiation of cell elongation were compared by acrylamide gel electrophoresis. Differences in electrophoretic patterns were found when proteins from axes incubated during the 1st hour of imbibition were compared with proteins from axes incubated during the hour when cell elongation was initiated. These differences greatly diminished by the 2nd hour of imbibition which suggests that they were due primarily to incomplete axis imbibition. A 5-hour actinomycin D treatment which reduced amino acid incorporation by 40% in the 5th hour had no apparent effect on the electrophoretic pattern during that hour.

Embryogenesis and germination in rye (Secale cereale L.) : II. Biochemical and fine structural changes during germination

When rye embryos imbibe water they rapidly return to a condition of biochemical and structural complexity. Three stages of imbibition can be recognised: Phase I a short period (10 min) of physical wetting; Phase II a longer period (1 h) when little further imbibition occurs, followed by Phase III a continuous phase of active water uptake. The latter coincides with an increase in respiration rate and an increase both in the number of mitochondria and of cristae within them. Changes in fine structure become evident in all organelles in Phase III, after 2 h of imbibition. In the unimbibed embryo endoplasmic reticulum is present only as short crescents associated with electron lucent bodies, but in Phase III the endoplasmic reticulum proliferates to form many surrounding cirlets. After 6 h these circlets become fewer and instead the endoplasmic reticulum is seen in close association with the nuclear membrane. Concurrently incorporation of radioactive uridine and thymidine is first detectible. This suggests that the large increase in protein synthesis occurs on new ribosomes present on the reticulum associated with the nuclear membrane. For the first 6 h protein synthesis must occur either on polysomes within the dense packing of ribosomes or on these circlets of endoplasmic reticulum associated with electron lucent bodies.

Rate of RNA synthesis and tRNA end-labeling during early development of Phaseolus

Axes of Phaseolus vulgaris cease synthesis of RNA during the maturation stage of embryogeny. During the imbibition phase of germination RNA synthesis resumes after the axes reach a normal water content. In the first hour of inbibition a very low rate of incorporation of (3)H-adenosine into is RNA found, and the primary site of incorporation is the-CCA end of tRNA. At later stages of germination tRNA end-labeling accounts for a minor fraction of adenosine incorporation. The rate of RNA synthesis increases after initiation of axis elongation to a maximal rate at 18 h of germination. ATP pool-size and specific activity vary over a several-fold range during development, an important consideration in determining the rate of RNA synthesis.

[An in vivo study of detectable phytochrome in seeds of Cucurbita pepo L. in the course of different phases of germination]

In dry gourd seeds all the phytochrome is in the Pfr form. The increase of phytochrome content from the beginning of hydration involves two phases, A and B, in the embryonic axis as well as in the cotyledons. Cycloheximide does not prevent the appearance of Pr during phase A. We assume that Pr is gradually released from an inactive complex. On the other hand phase B is inhibited by cycloheximide; this could mean that a de novo synthesis of Pr occurs.Some experiments indicate that the phytochrome which is localized in the embryonic axis may be involved only in the germinating process.The phytochrome which is synthesized during phase B disappears when the seeds are irradiated with red light, while the original phytochrome does not.According to our data it seems necessary to lay down a new and precise definition of the germination process.

Onset of deoxyribonucleic Acid synthesis in germinating wheat embryos

Germinating wheat embryos (Triticum vulgare var. Florence) synthesize proteins before the onset of DNA synthesis. The onset of DNA replication occurs at about 15 hours of germination and was shown to depend on proteins synthesized before 9 hours of germination with the use of blasticidin S, a specific inhibitor of protein synthesis. A 10-fold increase in the activity of DNA-dependent DNA polymerase was found in extracts derived from germinated embryos, as compared to the activity found in extracts from ungerminated embryos.

Root cell ultrastructure of Zea mays embryo during early stages of germination

The ultrastructure of root cells of the germinating corn embryo has been studied during the first 72 hours of soaking. The most spectacular ultrastructural modifications occur in the nucleus. In the dry seed, the chromatin is heavily condensed and complete dispersion occurs during the first 8 hr of germination. The nucleolus appears as a compact structure in the dormant embryo, and as a uniform granular structure after 3 hr. At the 8th hour, large nucleolar vacuoles appear filled with material structurally similar to chromatin. Later on, the nucleolus is composed of a central, fibrillo-granular region surrounded by a thin, peripheral, granular region and fewer nucleolar vacuoles are found.In a previous autoradiographic study (Deltour, 1970), it was shown that the onset of RNA synthesis in these cells occurs 4 hr after soaking. From that time to the 8th hour, uridine-(3)H is incorporated exclusively into the chromatin. Incorporation of radioactive uridine into the nucleolus begins only after the 8th hour.It is interesting that the onset of RNA synthesis in the chromatin occurs simultaneously with the dispersion of this cell component, and that the appearance of vacuoles in the nucleolus is correlated with the beginning if uridine incorporation into this organelle.The following ultrastructural changes take place in the cytoplasm; (a) the lamellae system of proplastids increases slightly; (b) phytoferritin granules present in the proplastids of the dry seed disappear very rapidly; (c) polysomes appear 72 hr after soaking; (d) the spherosomes which are essentially localized in the vicinity of the wall in the dormant embryo become uniformly distributed throughout the cytoplasm at the 72nd hr.

Nucleic Acid Metabolism in Germinating Onion: I. Changes in Root Tip Nucleic Acid during Germination

Nucleic acid synthesis in the G(1) cell population of the 1-millimeter apex of the Allium cepa embryo was studied during the initial 73 hours of germination. Quantitative data indicate that the total amount of RNA per cell began to increase after 18 hours of germination while the initial DNA per cell increase did not occur until some 20 hours later. Polyacrylamide gel electrophoresis patterns of (3)H-uridine-labeled total nucleic acid samples indicated that synthesis of all detectable RNA fractions present in the pre-emergent 1-millimeter apex (i.e., cytoplasmic and "chloroplast-like" RNA) began at approximately the same time (18 hours). Synthesis of the various cytoplasmic RNA fractions continued throughout the germination period. Data indicating synthesis of the "chloroplast-like" RNA were obtained only for the initial 36 hours of germination. Specific radioactivity of (3)H-uridine-labeled total nucleic acid increased during the first 41.5 hours of germination but then decreased while the accumulation of RNA per cell continued to increase throughout the 73-hour period. In addition, a method is described which reduced the bacterial contamination of Allium seed to a level not detectable by incorporation of radioactive precursors into bacterial ribosomal RNA.

The synthesis and distribution of ribonucleic acid in developing archegonia of Pteridium aquilinum

Feeding gametophytes of Pteridium aquilinum with tritiated uridine, followed by autoradiography, revealed two waves of incorporation of the nucleoside into nuclei during oognesis. The first, affecting the nucleus of the primary cell of the archegonium, is interpreted as indicating the activation of the genes initiating oogenesis. The second, seen in the nucleus of the maturing egg, is believed to be concerned with the differentiation of the egg itself.The results of the autoradiography, corroborated by enzyme digestion and fluorescence microscopy, also demonstrated a high concentration of ribonucleic acid towards the periphery of the mature egg, some possibly located in nucleolus-like bodies found only in this region of the cytoplasm. It is suggested that part of this cytoplasmic ribonucleic acid may represent genetic information remaining untranslated until after fertilization.No evidence was found of any asymmetry in the distribution of the ribonucleic acid in mature eggs which might account for the polarity of developing zygotes.

Dissociation of ribosomes and seed germination

Ribosomes from rice embryos (Oryza sativa) were dissociated into ribosomal subunits in vitro by systematic reduction of the Mg(2+) concentration. Ribosomes from imbibed (28 C) embryos were more easily dissociated than those from nonimbibed embryos. This was not observed with ribosomes from either imbibed, nonviable embryos, or from embryos imbibed at 0 C. Ribosomes from embryos which had been imbided and subsequently dehydrated resembled ribosomes from nonimbibed embryos in their resistance to dissociation. The change in the resistance to dissociation was essentially complete after the first 20 minutes of imbibition at 28 C, and accompanied activation in vivo of protein synthesis as determined by amino acid incorporation in vitro. Ribosomes from either imbibed or nonimbibed embryos could be dissociated into subunits by 0.5 m KCl. These subunits were separated by density gradient centrifugation, and, if recombined, were active for polyphenylalanine synthesis in vitro. The individual subunits prepared from nonimbibed embryos could be replaced by the corresponding subunit fraction from imbibed embryos without loss of capacity to support polyphenylalanine synthesis. The change in the ease of dissociation of ribosomes appears to be a physiological process, and its possible relationship to the initiation of protein synthesis during seed germination is discussed.

Respiration and Protein Synthesis in Dormant and After-ripened Seeds of Avena fatua

Dormant seeds of Avena fatua, which do not germinate when allowed to imbibe water, have a respiration rate only about 20% less than that of imbibed nondormant (after-ripened) seeds in the period before actual germination and are capable of synthesizing protein at a rate comparable to that of the nondormant seeds. An increase of protein synthesis is observed in nondormant seeds at the beginning of root protrusion. Autoradiography of seeds administered (3)H-leucine shows that protein synthesis occurs in the axis part of the embryo, the scutellum, the coleorhiza, and the aleurone layer. Dormancy in seeds is not a state of general inactivity; rather, it must be due to some specific metabolic block.

Cell-free protein synthesis with polysomes from germinating Petunia pollen grains

1. A cell-free protein synthesizing system was developed, which is able to incorporate amino acids into protein programmed by polysomes from pollen grains. 2. The enzyme system used was a 100,000 g supernatant from rat liver. 3. The characteristics of this mixed "plant-animal" system are described 4. Ungerminated pollen contains only monosomes which have no incorporation activity in the cell-free system. 5. Formation of polysomes starts immediately with the germination accompanied by decrease of the monosomal pool. 6. The rapid synthesis of polysomes suggests the presence of a preformed, masked form of messenger-RNA (mmRNA) in the resting pollen grain. 7. The instantaneous activation of protein synthesis enables pollen germination to proceed to a stage at which the pollen genome is not yet activated.

Hormones in the Translational Control of Early Germination in Wheat Embryos

Gibberellic and abscisic acid seem to exert their effects on germination by acting in some way at the level of translation. The two hormones may act at different sites.

Ultrastructural changes in cells of pea embryo radicles during germination

Radicles (1 mm long) of pea embryo axes, excised from dormant seeds and from seeds soaked for 80 min, 8, 12, and 48 hr, were fixed in glutaraldehyde and postfixed in OsO(4). The nucleus was only slightly changed by 48 hr soaking, but cytoplasmic organelles underwent developmental changes. There was proliferation of endoplasmic reticulum, the appearance of dictyosomes, and an inward migration of lipid bodies. Most of these changes were observed within 8 hr after soaking started. Plastids, mitochondria, protein bodies, and protein crystalline bodies were also identified and their developmental changes were followed.