Phytochrome-mediated accumulation of chloroplast DNA in pea leaves

The structure of chloroplast DNA molecules and the effects of light on the amount of chloroplast DNA during development in Medicago truncatula

We used pulsed-field gel electrophoresis and restriction fragment mapping to analyze the structure of Medicago truncatula chloroplast DNA (cpDNA). We find most cpDNA in genome-sized linear molecules, head-to-tail genomic concatemers, and complex branched forms with ends at defined sites rather than at random sites as expected from broken circles. Our data suggest that cpDNA replication is initiated predominantly on linear DNA molecules with one of five possible ends serving as putative origins of replication. We also used 4',6-diamidino-2-phenylindole staining of isolated plastids to determine the DNA content per plastid for seedlings grown in the dark for 3 d and then transferred to light before being returned to the dark. The cpDNA content in cotyledons increased after 3 h of light, decreased with 9 h of light, and decreased sharply with 24 h of light. In addition, we used real-time quantitative polymerase chain reaction to determine cpDNA levels of cotyledons in dark- and light-grown (low white, high white, blue, and red light) seedlings, as well as in cotyledons and leaves from plants grown in a greenhouse. In white, blue, and red light, cpDNA increased initially and then declined, but cpDNA declined further in white and blue light while remaining constant in red light. The initial decline in cpDNA occurred more rapidly with increased white light intensity, but the final DNA level was similar to that in less intense light. The patterns of increase and then decrease in cpDNA level during development were similar for cotyledons and leaves. We conclude that the absence in M. truncatula of the prominent inverted repeat cpDNA sequence found in most plant species does not lead to unusual properties with respect to the structure of plastid DNA molecules, cpDNA replication, or the loss of cpDNA during light-stimulated chloroplast development.

Loss or retention of chloroplast DNA in maize seedlings is affected by both light and genotype

We examined the chloroplast DNA (cpDNA) from plastids obtained from wild type maize (Zea mays L.) seedlings grown under different light conditions and from photosynthetic mutants grown under white light. The cpDNA was evaluated by real-time quantitative PCR, quantitative DNA fluorescence, and blot-hybridization following pulsed-field gel electrophoresis. The amount of DNA per plastid in light-grown seedlings declines greatly from stalk to leaf blade during proplastid-to-chloroplast development, and this decline is due to cpDNA degradation. In contrast, during proplastid-to-etioplast development in the dark, the cpDNA levels increase from the stalk to the blade. Our results suggest that DNA replication continues in the etioplasts of the upper regions of the stalk and in the leaves. The cpDNA level decreases rapidly, however, after dark-grown seedlings are transferred to light and the etioplasts develop into photosynthetically active chloroplasts. Light, therefore, triggers the degradation of DNA in maize chloroplasts. The cpDNA is retained in the leaf blade of seedlings grown under red, but not blue light. We suggest that light signaling pathways are involved in mediating cpDNA levels, and that red light promotes replication and inhibits degradation and blue light promotes degradation. For five of nine photosynthetic mutants, cpDNA levels in expanded leaves are higher than in wild type, indicating that nuclear genotype can affect the loss or retention of cpDNA.

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.

Regulation of gene expression of ribulose bisphosphate carboxylase in greening pea leaves

Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is composed of two subunits, the chloroplastcoded large subunit (LS) and the nuclear-coded small subunit (SS). The effects of different light doses on the levels of the large subunit gene (rbcL), the rbcL and the rbcS mRNAs, and the rate of synthesis of RuBisCO were followed during greening. The rbcL gene dosage changed in response to light whereas the level of the rbcL mRNA changed independently of the gene dosage. This suggests that the expression of the rbcL gene is transcriptionally regulated and that the change in gene dosage only partially contributes to the increase in the mRNA. It appeared that the RuBisCO synthesis rate was proportional to the rbcS mRNA level rather than rbcL mRNA level. These results, taken together with the earlier observations of many researchers, suggest that RuBisCO synthesis in greening pea leaves is controlled primarily at the level of transcription of both genes and is fine-tuned at the post-transcriptional level in chloroplasts, so that the amount of LS is almost stoichiometric to that of SS.

Relations between the plastid gene dosage and the levels of 16S rRNA and rbcL gene transcripts during amyloplast to chloroplast change in mixotrophic spinach cell suspensions

Spinach cell suspension cultures maintained in photomixotrophic conditions exhibit plastids which undergo cyclic morphological transformations along a growth cycle. Ultrastructural studies show that the green chloroplasts present at the initial stage differentiate into amyloplasts during the subsequent log phase and then return to chloroplasts in stationary phase. The changes of the levels of plastid DNA (pt DNA) per cell have been determined along the growth cycle, as a percentage of total DNA by hybridization of definite amounts of total DNA to a radioactive probe of cloned pt DNA. The number of pt DNA copies have been estimated to 1125 per cell at the maximum of amyloplast development and to 5940 copies per cell at the maximum of chloroplast differentiation. Hybridizations of defined amounts of total cellular RNA to labelled probes of the 16S rDNA and of the rbcL gene allowed estimations of the variations of the corresponding cellular RNA pools. These variations are well correlated with the changes of the ptDNA cellular levels. These results show that the ptDNA gene dosage plays a central role in the regulation of the plastid transcript levels in this system.

Phytochrome regulation of mRNA levels of ribulose-1,5-bisphosphate carboxylase in etiolated rye seedlings (Secale cereale)

The effect of red light and far-red light on the appearance of mRNA for the small and large subunit of ribulose-1,5-bisphosphate carboxylase in rye seedlings has been analysed. Irradiation of etiolated seedlings with a pulse of red light increased the mRNA level of the small subunit by a factor of 7-10 and that of the large subunit by a factor of 3. A pulse of far-red light resulted in a much lower, but measurable increase. The effect of red light is reverted by an immediate far-red pulse, demonstrating the classical phytochrome response.In vitro transcription experiments with nuclei isolated from red light-treated seedlings showed that the transcription rate of the small subunit mRNA was increased only by a factor of 1.5-2.5, indicating posttranscriptional as wells as transcriptional regulations of the small subunit of ribulose-1,5-bisphosphate carboxylase.

Regulatory factors involved in gene expression (subunits of ribulose-1,5-bisphosphate carboxylase) in mustard (Sinapis alba L.) cotyledons

Phytochrome-controlled appearance of ribulose-1,5-bisphosphate carboxylase (RuBP-Case) and its subunits (large subunit LSU, small subunit SSU) was studied in the cotyledons of the mustard (Sinapis alba L.) seedling. The main results were as follows: (i) Control of RuBPCase appearance by phytochrome is a modulation of a process which is turned on by an endogenous factor between 30 and 33 h after sowing (25° C). Only 12 h later the process begins to respond to phytochrome. (ii) The rise in the level of RuBP-Case is the consequence of a strictly coordinated synthesis de novo of the subunits. (iii) While the levels of translatable mRNA for SSU are compatible with the rate of SSU synthesis the relatively high LSU mRNA levels are not reflected in the rates of in-vivo LSU or RuBPCase syntheses. (iv) Gene expression is also abolished in the case of nuclear-encoded SSU if intraplastidic translation and concomitant plastidogenesis is inhibited by chloramphenicol, pointing to a "plastidic factor" as an indispensable prerequisite for expression of the SSU gene(s). (v) Regarding the control mechanism for SSU gene expression, three factors seem to be involved: an endogenous factor which turns on gene expression, phytochrome which modulates gene expression, and the plastidic factor which is an indispensable prerequisite for the appearance of translatable SSU mRNA.