Time course of competence in phytochrome-controlled appearance of nuclear-encoded plastidic proteins and messenger RNAs

Low temperature regulates Arabidopsis Lhcb gene expression in a light-independent manner

Low temperature treatment of dark-grown seedlings of Arabidopsis thaliana results in a rapid increase in the amount of mRNAs encoding for the major polypeptides of the light-harvesting complex of photosystem II (Lhcb1 genes). This increase is transient and seems to be due mainly to the accumulation of Lhcb1*3 transcripts, indicating that low temperature differentially regulates the expression of the Arabidopsis Lhcb1 gene family in the dark. A 1.34 kb fragment of the Lhcb1*3 promoter is sufficient to confer low temperature regulation to a reporter gene in transgenic Arabidopsis etiolated seedlings, suggesting that the regulation is occurring at the transcriptional level. The cold-induced accumulation of Lhcb1*3 mRNA is not part of a general response to stressful conditions since no accumulation is detected in response to water stress, anaerobiosis or salt stress. The amount of Lhcb1*3 mRNA decrease in response to exogenous abscisic acid (ABA) suggesting that this phytohormone acts as a negative regulator. Moreover, the accumulation of Lhcb1*3 mRNAs in cold-treated ABA deficient etiolated seedlings is higher than that of wild-type and ABA insensitive etiolated seedlings, indicating that low temperature regulation of Lhcb1*3 is not mediated by ABA.

Multiple coordinate controls contribute to a balanced expression of ribulose-1,5-bisphosphate carboxylase/oxygenase subunits in rye leaves

In the leaves of rye (Secale cereale L.), control mechanisms acting at multiple molecular levels contribute to a coordinate expression of the subunit polypeptides of ribulose-1,5-bisphosphate carboxylase. The relevance and hierarchy of the different control steps were evaluated by comparing the time courses of changes in levels of translatable mRNA, rates of in vivo amino acid incorporation, and the turnover of subunit polypeptides after selective interference with translation at either cytoplasmic 80S ribosomes, or at the 70S ribosomes of the chloroplast, by compartment-specific inhibitors, or by the use of 70S-ribosome-deficient leaves. The latter were generated by growing the plants at a non-permissive elevated temperature of 32 degrees C. The rates of synthesis of the two ribulose-1,5-bisphosphate carboxylase subunits were most rapidly adapted to each other by translational controls. Within 0.5-2.5 h after selective inhibition of the synthesis of either subunit, that of the other subunit made in the unaffected compartment also declined by more than 90% without any marked change in its mRNA. After prolonged inhibition (24 h) of either cytoplasmic or chloroplast protein synthesis, the levels of mRNAs for both subunits were greatly diminished. In rye, the mRNA levels for both subunits changed under all experimental conditions tested in a closely parallel manner and appeared to be always maintained in a balanced, fairly constant ratio by strong coordinate controls. Even 70S-ribosome-deficient leaves contained mRNAs for both the small and the large subunits, although only in small amounts. The mRNAs for both subunits were also markedly further decreased in 70S-ribosome-deficient leaves after application of an inhibitor of cytoplasmic translation. MDMP [2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide], suggesting that the suppression of the large subunit mRNA in the plastids was not mediated through feedback effects of accumulating unassembled large subunits. Coordinate controls at both the mRNA and the translational level require a bidirectional exchange of regulatory signals between chloroplast and cytoplasm. However, these controls were not absolutely restrictive and allowed low rates of uncoupled synthesis of either large or small subunits. Large subunits made in the presence of MDMP were stable over 24 h. However, unassembled small subunits synthesized in 70S-ribosome-deficient leaves were degraded with a half-time of 10.5 h, in contrast to their behavior after integration into the holoprotein in normal leaves, where no turnover was detected. The proteolytic removal of surplus free small subunits is regarded as a final post-translational fine-tuning step to establish a balanced subunit stoichiometry in leaves.

Temporal control of phytochrome-dependent gene expression during radish seedling development

The level of two nuclear-encoded transcripts (the small subunit of ribulose-1,5-bisphosphate carboxylase and the chlorophyll a/b-binding protein of photosystem II) and two chloroplastencoded mRNAs (the large subunit of ribulose-1,5-bisphosphate carboxylase and the chloroplast 32-kDa polypeptide) were analysed during the first 4 d of radish (Raphanus sativus L.) seedling development. A single 5-min red-light pulse increased the concentrations of all transcripts by various orders of magnitude, depending on the particular mRNA. For all transcripts the maximal effect of the red light was observed when the irradiation was applied 48 h following sowing. The red-light treatment did not lead to changes in the total RNA level or to alterations in the levels of nuclear and chloroplastic DNAs; the level of β-actin mRNA was also unaffected by the light treatment. It is concluded that during the first days of development, cells of radish cotyledons undergo light-independent molecular changes, occurring at the transcriptional level, conferring on them a transient competence towards the photoreceptor phytochrome.

Regulation and expression of the multigene family coding light-harvesting chlorophyll a/b-binding proteins of photosystem II

The current state of knowledge concerning the expression of the nuclear genes that code the light-harvesting chlorophyll a/b-binding polypeptides of photosystem II is presented. This review covers the structure of these genes, the complex multistep pathway involved in their expression, and the environmental and other factors which regulate their expression. Some of the effects of these factors are mediated, at least in part, at the level of transcription, but other effects can be explained only by the existence of multiple posttranscriptional regulatory steps.

Photoregulation of psbA transcript levels in mustard cotyledons

We have investigated the photoreceptors potentially involved in the light regulation of the transcript levels of the psbA gene coding for D1, the 32 kD QB-binding protein of PSII. In cotyledons of 4 day old mustard seedlings, increasing fluence rates of continuous white light from ca. 0.1 to 250 μmol m(-2)s(-1) (400-700 nm) lead to a five-fold increase in transcript level from ca. 0.7 to 2.8 mg/g total RNA. The blue (<500 nm) component of this light did not contribute substantially to this effect, thus ruling out cryptochrome as the receptor responsible. Although phytochrome involvement was apparent from red/far-red reversibility, even multiple red pulses failed to elicit a comparable increase in transcript level to that seen under continuous white light. Although DCMU successfully inhibited delayed fluorescence quenching, it had no effect on transcript levels, thus ruling out photoregulation via electron transport and later components of the photosynthetic system. By contrast, Norflurazon, which leads to photobleaching of chlorophyll and hence disruption of thylakoid membrane assembly, completely abolished the light effect on psbA transcript level. We infer that photoregulation of the psbA transcript is principally related to thylakoid development, which is in turn critically dependent on photoconversion of protochlorophyllide to chlorophyll, but also associated with other processes such as phytochrome-regulated LHCP availability. Photocontrol of psbA expression is discussed in relation to that of the nuclear cab and rbcS genes.

Action of light, nitrate and ammonium on the levels of NADH- and ferredoxin-dependent glutamate synthases in the cotyledons of mustard seedlings

In mustard (Sinapis alba L.) cotyledons, NADH-dependent glutamate synthase (NADH-GOGAT, EC 1.4.1.14) is only detectable during early seedling development with a peak of enzyme activity occurring between 2 and 2.5 d after sowing. With the beginning of plastidogenesis at approximately 2 d after sowing, ferredoxindependent glutamate synthase (Fd-GOGAT, EC 1.4.7.1) appears while NADH-GOGAT drops to a very low level. The enzymes were separated by anion exchange chromatography. Both enzymes are stimulated by light operating through phytochrome. However, the extent of induction is much higher in the case of Fd-GOGAT than in the case of NADH-GOGAT. Moreover, NADH-GOGAT is inducible predominantly by red light pulses, while the light induction of Fd-GOGAT operates predominantly via the high irradiance response of phytochrome. The NADH-GOGAT level is strongly increased if mustard seedlings are grown in the presence of nitrate (15 mM KNO3,15 mM NH4NO3) while the Fd-GOGAT level is only slightly affected by these treatments. No effect on NADH-GOGAT level was observed by growing the seedlings in the presence of ammonium (15 mM NH4Cl) instead of water, whereas the level of Fd-GOGAT was considerably reduced when seedlings were grown in the presence of NH4Cl. Inducibility of NADH-GOGAT by treatment with red light pulses or by transferring water-grown seedlings to NO 3 (-) -containing medium follows a temporal pattern of competence. The very low Fd-GOGAT level in mustard seedlings grown under red light in the presence of the herbicide Norflurazon, which leads to photooxidative destruction of the plastids, indicates that the enzyme is located in the plastids. The NADH-GOGAT level is, in contrast, completely independent of plastid integrity which indicates that its location is cytosolic. It is concluded that NADH-GOGAT in the early seedling development is mainly concerned with metabolizing stored glutamine whereas Fd-GOGAT is involved in ammonium assimilation.

Control by phytochrome of the appearance of ribulose-1,5-bisphosphate carboxylase and the mRNA for its small subunit

We have measured levels of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and levels of in-vitro-translatable mRNA for the small subunit (SSU) of RuBPCase up to 96 h after sowing in mustard (Sinapis alba L.) cotyledons, in order to investigate to what extent the rate of enzyme synthesis is related to the level of SSU-mRNA. Both enzyme and mRNA level are controlled strongly by phytochrome, but the rate of RuBPCase accumulation was found to be unrelated to the level of translatable SSU-mRNA. As an example, it was found that the amount of SSU-mRNA in far-red light (FR)-grown mustard seedlings doubles between 54 and 84 h after sowing while the rate of RuBPCase accumulation remains constant over this period. Since the holoenzyme shows zero turnover during this period it is concluded that the rate of enzyme synthesis remains constant although the level of SSU-mRNA increases strongly. Following an FR→dark transition, with different levels of physiologically active phytochrome (Pfr) established at the end of the light period, no correlation was found between the time course of mRNA levels in darkness and the rate of enzyme synthesis. Rather, the data indicate that there is at least one translational or post-translational regulatory step which is also phytochrome-dependent. It is concluded that coarse control of the appearance of translatable SSU-mRNA is essential for RuBPCase to appear at a high rate but that fine tuning by phytochrome of the actual appearance of RuBPCase is not transcriptional.

Recovery of plastids from photooxidative damage: Significance of a plastidic factor

It was inferred from previous findings that a plastid-derived factor (plastidic factor) is involved in the transcriptional control of nuclear genes coding for proteins destined for the chloroplast. Photooxidative damage to the plastid destroys the ability of the organelle to give off this factor. Cytosolic enzyme levels are not impaired if plastids are damaged, and morphogenesis of seedlings is normal. The only exception found so far is nitrate reductase, a cytosolic enzyme, which is regulated by the cellas if it were a plastidic protein. In the present study we have shown that the plastids in the mesophyll of mustard (Sinapis alba L.) cotyledons, damaged by 3 h photooxidation in red light (6.8 W·m(-2)) and then returned to darkness or to continuous, non-photooxidative far-red light (cFR), recover from photooxidative damage. The rate of recovery is stimulated by phytochrome (operationally, cFR). Since the cytosolic enzyme nitrate reductase is affected by the different treatments in principally the same way as the levels of plastidic enzymes, we conclude that it is recovery of the plastids' ability to give off the plastidic factor rather than structural recovery which leads to recovery of gene expression and protein (and chlorophyll) re-accumulation. The extent of recovery varied according to the enzyme and this variation could be explained by different plastidic-factor requirements for gene expression. This explanation was confirmed by measurements of translatable mRNAs. It was found that LHCP-gene expression (light-harvesting chlorophyll a/b-binding protein of photosystem II) is far more sensitive to photooxidative damage of the plastids than SSU-gene expression (small subunit of ribulose-1.5-bisphosphate carboxylase). Correspondingly, recovery is expressed to a much greater extent in the latter than in the former case.

Temporal and light control of plastid transcript levels for proteins involved in photosynthesis during mustard (Sinapis alba L.) seedling development

Transcript levels of four plastid genes encoding constituents of the photosynthetic apparatus were assessed in cotyledons of developing mustard (Sinapis alba L.) seedlings. These genes, encoding the P700 apoproteins of photosystem I, the alpha subunit of the extrinsic CF1 moiety of the plastid ATP synthase complex, and the cytochromes f and b 6, have been localized and mapped previously on mustard chloroplast DNA (G. Dietrich and G. Link, 1985 Curr. Genet. 9, 683-692). Dot blot and Northern hybridization analysis provides evidence that in dark-grown seedlings transcript levels of all four genes rise between 30 h and 72 h after sowing and thereafter fall again, pointing to the existence of an endogenous, light-independent, developmental program. In light-grown seedlings, an additional enhancement of transcript levels beyond, the dark values becomes noticeable at approx. 30-36 h and then continues throughout the subsequent "light-responsive" phase until 96 h after sowing. This is consistent with a photoregulated modulation mechanism operating once "competence" has been reached. Enhanced transcript accumulation occurs following continuous illumination by either white light or (photosynthetically inefficient) far-red light thought to operate mainly through phytochrome. However, the degree of light enhancement for the transcript specifying the P700 apoprotiens is higher with white light than with far-red light, implying involvement of additional photoreceptor(s) mediating this response. In addition to the endogenous gross regulation and light enhancement, a fine regulation of transcript levels seems to operate, as indicated by temporal variations of two related transcripts originating from the cytochrome-f gene region. The observed developmental and photocontrolled changes in specific transcript levels for photosynthesis proteins are only reflected in part by changes in total RNA content and do not appear due to light-dark differences in plastid-DNA copy number during mustard seedling development.

Inhibition and promotion by light of the accumulation of translatable mRNA of the light-harvesting chlorophyll a/b-binding protein of photosystem II

The amount of in-vitro translatable mRNA of the light-harvesting chlorophyll a/b-binding protein (LHCP) of photosystem II strongly increases in darkness (D) after a 5-min red-light pulse while continuous illumination of mustard seedlings with far-red (FR), red or white light leads only to a slight increase in the amount of translatable LHCP-mRNA. No increase can be observed after a long-wavelength FR (RG9-light) pulse. However, a FR pretreatment prior to the RG9-light pulse strongly increase LHCP-mRNA accumulation in subsequent D. This is not observed in the case of the mRNA for the small subunit of ribulose-1.5-bisphosphate carboxylase. The increase of LHCP-mRNA in D after a FR pretreatment can be inhibited by a reillumination of the seedlings with FR. The inhibition of LHCP-mRNA accumulation during continuous illumination with FR and the strong increase in D following a FR illumination was found to be independent of chlorophyll biosynthesis since no correlation between chlorophyll biosynthesis and translatable LHCP-mRNA levels could be detected. Even strong changes in the amount of intermediates of chlorophyll biosynthesis caused by application of levulinic acid or 5-aminolevulinic acid did not affect LHCP-mRNA levels. Therefore, we conclude that the appearance of LHCP-mRNA is inhibited during continuous illumination, even though illumination leads to a storage of a light singal which promotes accumulation of translatable LHCP-mRNA in D.