The presence of DNA in ribosome-deficient plastids of heat-bleached rye leaves

Efficient embryogenesis and regeneration in freshly isolated and cultured wheat (Triticum aestivum L.) microspores without stress pretreatment

The major advantage of doubled haploids in plant breeding is the immediate achievement of complete homozygosity. Desired genotypes are thus fixed in one generation, reducing time and cost for cultivar or inbred development. Among the different technologies to produce doubled haploids, microspore embryogenesis is by far the most common. It usually requires reprogramming of microspores by stress such as cold, heat, and starvation, followed by embryo development under stress-free conditions. We report here the development of a simple and efficient isolated microspore culture system for producing doubled haploid wheat plants in a wide spectrum of genotypes, in which embryogenic microspores and embryos are formed without any apparent stress treatment. Microspores were isolated from fresh spikes in a nutrient-free medium by stirring and cultured in medium A2 in the dark at 25 degrees C. Once embryogenic microspores were formed, ovaries and phytohormones were added directly to the cultures without changing the medium. The cultures were incubated in the dark at 25-27 degrees C until the formation of embryos and then the embryos were transferred to regeneration medium. The regeneration frequency and percentage of green plants increased significantly using this protocol compared to the shed microspore culture method.

Loss of chloroplast transcripts for proteins associated with photosystem II: an early event during heat-bleaching in Euglena gracilis

A shift in the ratio of chlorophyll (Chl) a and Chl b is an early indicator of heat bleaching in Euglena gracilis. This observation prompted us to consider whether or not changes in steady-state levels of chloroplast transcripts and in transcriptional activity could limit the synthesis of Chl a-binding proteins in bleaching plastids. We found that the mature transcripts for CP47 and CP43, the Chl a-binding apoproteins of the proximal antenna of photosystem II, decline sharply very early during bleaching. Our study also shows that transcription of psbB and psbC, the chloroplast genes encoding CP47 and CP43, remains essentially unchanged during the same interval. We conclude that posttranscriptional events, such as mRNA stability, could play a major role in initiating an irreversible loss of chloroplast function in Euglena at a moderately elevated temperature. Lack of these transcripts would eventually impair the assembly of photosystem II in thylakoids.

Ribosome-deficient plastids affect transcription of light-induced nuclear genes: genetic evidence for a plastid-derived signal

Transcription of ten nuclear genes was analysed in the albostrians mutant of barley (Hordeum vulgare L.). The lack of plastid ribosomes in white seedlings of this mutant results in a complex alteration of nuclear gene expression at the transcriptional level. We found a strong reduction in the accumulation of mRNAs transcribed from nuclear genes encoding chloroplast enzymes involved in the Calvin cycle, the chlorophyll a/b binding protein, and the cytosolic enzyme nitrate reductase. In contrast, the levels of transcripts of the genes encoding the cytosolic glycolytic enzymes glyceraldehyde phosphate dehydrogenase and phosphoglycerate kinase were slightly enhanced. Accumulation of chalcone synthase mRNA even reaches much higher levels in white than in green leaves. Ribosome-deficient plastids were combined by crossing with a nuclear genotype heterozygous for the albostrians allele. Analysis of transcript levels in F1 plants having the same nuclear genotype and differing only with respect to their content of normally developed chloroplasts versus undifferentiated mutant plastids, provided strong genetic evidence for the plastid being the origin of a signal (chain) involved in regulation of nuclear gene expression. Results of run-on transcription in isolated nuclei demonstrated that the plastid signal acts at the level of transcription; it does not interfere with gene regulation in general. Mechanisms triggering nuclear gene expression in response to light operate in white mutant leaves: the very low levels of mRNAs derived from nuclear genes encoding chloroplast proteins and the strongly enhanced level of chalcone synthase mRNA were both light inducible. Also the negative regulation of leaf thionein gene expression by light is observed in white albostrians seedlings.(ABSTRACT TRUNCATED AT 250 WORDS)

Unassembled polypeptides of the plastidic ribosomes in heat-treated 70S-ribosome-deficient rye leaves

The polypeptides of the subunits of 70S ribosomes isolated from rye (Secale cereale L.) leaf chloroplasts were analyzed by two-dimensional polyacrylamide gel electrophoresis. The 50S subunit contained approx. 33 polypeptides in the range of relative molecular mass (Mr) 13000-36000, the 30S subunit contained approx. 25 polypeptides in the range of Mr 13000-40500. Antisera raised against the individual isolated ribosomal subunits detected approx. 17 polypeptides of the 50S and 10 polypeptides of the 30S subunit in the immunoblotting assay. By immunoblotting with these antisera the major antigenic ribosomal polypeptides (r-proteins) of the chloroplasts were clearly and specifically visualized also in separations of leaf extracts or soluble chloroplast supernatants. In extracts from rye leaves grown at 32° C, a temperature which is non-permissive for 70S-ribosome formation, or in supernatants from ribosome-deficient isolated plastids, six plastidic r-proteins were visualized by immunoblotting with the anti-50S-serum and two to four plastidic r-proteins were detected by immunoblotting with the anti-30S-serum, while other r-proteins that reacted with our antisera were missing. Those plastidic r-proteins that were present in 70S-ribosome-deficient leaves must represent individual unassembled ribosomal polypeptides that were synthesized on cytoplasmic 80S ribosomes. For the biogenesis of chloroplast ribosomes the mechanism of coordinate regulation appear to be less strict than those known for the biogenesis of bacterial ribosomes, thus allowing a marked accumulation of several unassembled ribosomal polypeptides of cytoplasmic origin.

Effect of chloramphenicol and lincomycin on chloroplast DNA amplification in greening pea leaves

The light-induced increase in chloroplast DNA was not inhibited by two inhibitors of protein synthesis on 70S polysomes, chloramphenicol and lincomycin, in greening pea leaves. The changes in chloroplast DNA were observed by fluorescence microscopy and measured by hybridization to specific cloned probes. The results suggest that the light-induced increase in chloroplast DNA proceeds without de novo protein synthesis in the chloroplast, in agreement with those with mutants and cultured leaf tissue.

Plastid and nuclear DNA synthesis are not coupled in suspension cells ofNicotiana tabacum

The relationship between nuclear and plastid DNA synthesis in cultured tobacco cells was measured by following(3)H-thymidine incorporation into total cellular DNA in the absence or presence of specific inhibitors. Plastid DNA synthesis was determined by hybridization of total radiolabeled cellular DNA to cloned chloroplast DNA.Cycloheximide, an inhibitor of nuclear encoded cytoplasmic protein synthesis, caused a rapid and severe inhibition of nuclear DNA synthesis and a delayed inhibition of plastid DNA synthesis. By contrast, chloramphenicol which only inhibits plastid and mitochondrial protein production, shows little inhibition of either nuclear or plastid DNA synthesis even after 24 h of exposure to the cells.The inhibition of nuclear DNA synthesis by aphidicolin, which specifically blocks the nuclear DNA polymeraseα, has no significant effect on plastid DNA formation. Conversely, the restraint of plastid DNA synthesis exerted by low levels of ethidium bromide has no effect on nuclear DNA synthesis.These results show that the synthesis of plastid and nuclear DNA are not coupled to one another. However, both genomes require the formation of cytoplasmic proteins for their replication, though our data suggest that different proteins regulate the biosynthesis of nuclear and plastid DNA.

Synthesis of the large subunit of ribulose-1,5-bisphosphate carboxylase in anin vitro partially definedE. coli system

Thein vitro DNA- or RNA-directed synthesis of the large subunit (LS) of spinach chloroplast ribulose-1,5-biphosphate carboxylase (RuP2C) has been examined in a highly definedE. coli transcription-translation system. Spinach chloroplast DNA, RNA and recombinant plasmids containing the spinach chloroplast LS gene (rbcL) have been used as templates in thein vitro system and a quantitative assay has been developed to measure LS formation. Thein vitro formed product contains formylmethionine at the N-terminal position and sediments primarily as a monomer. There is no detectable enzymatic activity associated with thein vitro product. To determine where theE. coli RNA polymerase used in these systems initiates, we have examined the transcripts produced by this enzymein vitro. Measurements of run-off transcripts indicate thatE. coli RNA polymerase initiates at the same position on the gene as is seenin vivo. In addition, the complete nucleotide sequence of therbcL gene including previously unsequenced 3' and 5' flanking regions has been determined. The sequence agrees, except at two nucleotide positions, with previously published sequencing data for this gene (Zurawski, G, Perrot, B, Bottomley, W, Whitfeld, PR, 1981. Nucleic Acids Res. 9:3251-3270).

Capacity for RNA synthesis in 70S ribosome-deficient plastids of heat-bleached rye leaves

In the leaves of rye seedlings (Secale cereale L.) grown at an elevated temperature of 32°C the formation of plastidic 70S ribosomes is specifically prevented. The resulting plastid ribosome-deficient leaves, which are chlorotic in light, represent a system for the identification of translation products of the 80S ribosomes among the chloroplastic proteins. Searching for the primary heat-sensitive event causing the 70S ribosome-deficiency, the thermostability of the chloroplastic capacity for RNA synthesis was investigated. The RNA polymerase activity of isolated normal chloroplasts from 22°-grown rye leaves was not inactivated in vitro at temperatures between 30° and 40°C. The ribosome-deficient plastids purified from bleached 32°-grown leaf parts contained significant RNA polymerase activity which was, however, lower than in functional chloroplasts. After application of [(3)H]uridine to intact leaf tissues [(3)H]uridine incorporation was found in ribosome-deficient plastids of 32°C-grown leaves. The amount of incorporation was similar to that in the control chloroplasts from 22°C-grown leaves. According to these results, it is unlikely that the non-permissive temperature (32°C) causes a general inactivation of the chloroplastic RNA synthesis in rye leaves.