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Plant Cell, Vol. 11, 901-910, May 1999, Copyright © 1999, American Society of Plant Physiologists

Plastid Translation Is Required for the Expression of Nuclear Photosynthesis Genes in the Dark and in Roots of the Pea lip1 Mutant

James A. Sullivana and John C. Graya
a Department of Plant Sciences and Cambridge Centre for Molecular Recognition, University of Cambridge, Downing Street, Cambridge, CB2 3EA, United Kingdom

Correspondence to: John C. Gray, jcg2{at}mole.bio.cam.ac.uk (E-mail), 44-1223-333953 (fax)


* ABSTRACT
*TOP
 *ABSTRACT
*INTRODUCTION
 *RESULTS
 *DISCUSSION
 *METHODS
 *REFERENCES

The expression of nuclear photosynthesis genes in pea seedlings requires both light and a postulated signal produced by developing plastids. The requirement for the plastid signal for the accumulation of transcripts of Lhcb1, RbcS, PetE, and AtpC genes was investigated in the pea mutant lip1, which shows light-independent photomorphogenic development. Lincomycin and erythromycin, inhibitors of plastid translation, decreased the accumulation of transcripts of nuclear photosynthesis genes in shoots of light-grown wild-type and lip1 seedlings, indicating that the plastid signal is required in the lip1 mutant. Treatment with lincomycin or erythromycin also reduced the accumulation of transcripts in shoots of dark-grown lip1 seedlings, indicating that light is not an obligate requirement for the synthesis or activity of the plastid signal. Lincomycin had a similar effect on the accumulation of Lhcb1 transcripts in dark-grown cop1-4 seedlings of Arabidopsis. Accumulation of transcripts of nuclear photosynthesis genes was also observed in roots of light-grown lip1 seedlings, and this accumulation, which was associated with the development of chloroplasts, was again dependent on plastid translation. The plastid signal therefore regulates the expression of nuclear photosynthesis genes in the dark and in roots of the lip1 mutant.


* INTRODUCTION
*TOP
 *ABSTRACT
 *INTRODUCTION
*RESULTS
 *DISCUSSION
 *METHODS
 *REFERENCES

During early development, higher plants can follow one of two developmental patterns (McNellis and Deng 1995 Down). In continuous darkness, seedlings follow skotomorphogenic (etiolated) development: etiolated seedlings have long hypocotyls, apical hooks, small unopened cotyledons that contain etioplasts, and low levels of expression of nuclear photosynthesis genes. If seedlings are grown in the light, they follow photomorphogenic (deetiolated) development. In contrast to etiolated seedlings, light-grown seedlings have short hypocotyls, no apical hooks, open and expanded cotyledons containing photosynthetically competent chloroplasts, and high levels of expression of nuclear photosynthesis genes (Staub and Deng 1996 Down).

This increased expression of nuclear photosynthesis genes also requires the presence of intact chloroplasts. It has been proposed that a plastid-derived signal is required for the expression of nuclear photosynthesis genes (reviewed in Oelmuller 1989 Down; Taylor 1989 Down; Susek and Chory 1992 Down). Treatment of mustard seedlings with norflurazon, a noncompetitive inhibitor of phytoene desaturase (Young 1991 Down), resulted in photooxidation of chloroplasts without the disruption of cytosolic ribosomes (Frosch et al. 1979 Down; Reiss et al. 1983 Down). Norflurazon-treated mustard seedlings showed a decrease in transcripts of Lhcb (for light-harvesting chlorophyll a/b binding proteins) and RbcS (for small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase) genes (Oelmuller et al. 1986 Down). Similar effects on Lhcb and RbcS transcripts were observed if chloramphenicol (an inhibitor of plastid translation) was applied to mustard seedlings during early development (Oelmuller et al. 1986 Down). Treatment of barley seedlings with tagetitoxin, an inhibitor of chloroplast transcription, also reduced the levels of Lhcb and RbcS transcripts (Rapp and Mullet 1991 Down), suggesting a role for plastid RNA in signaling.

Because the plastid signal is required continuously (Oelmuller et al. 1986 Down; Burgess and Taylor 1988 Down) and inhibitors of plastid translation, such as chloramphenicol (Oelmuller et al. 1986 Down), lincomycin (Gray et al. 1995 Down), and erythromycin (Gray et al. 1995 Down), have no effect on nuclear photosynthesis gene expression if applied later than 48 to 72 hr after germination, the plastid signal cannot be a direct product of plastid gene expression. However, because such inhibitors decrease the expression of nuclear photosynthesis genes if applied within 48 to 72 hr of germination, the synthesis of the plastid signal must involve a product of early plastid gene expression (Gray et al. 1995 Down).

Light and the development of plastids are both crucial factors in the expression of nuclear photosynthesis genes (Taylor 1989 Down; Thompson and White 1991 Down; Arguello-Astorga and Herrera-Estrella 1998 Down; Leon et al. 1998 Down). Analysis of light-regulated promoters has identified many cis-acting light-responsive elements, although no universal element has been found in light-regulated promoters (Terzaghi and Cashmore 1995 Down; Arguello-Astorga and Herrera-Estrella 1998 Down). Analysis of promoters of nuclear photosynthesis genes has shown that elements required for light regulation cannot be separated from those required for the response to the developmental state of plastids (Bolle et al. 1994 Down; Kusnetsov et al. 1996 Down; Puente et al. 1996 Down). It has been proposed that the light and plastid signal regulatory pathways merge before the regulation of gene expression, and these pathways may act through a common trans-acting factor (Kusnetsov et al. 1996 Down; Lopez-Juez et al. 1996 Down; Puente et al. 1996 Down).

This study was designed to investigate in greater detail the proposed link between light and the plastid signal. In particular, we decided to determine whether the plastid signal regulates photosynthesis gene expression in the dark. However, under normal skotomorphogenic development, nuclear photosynthesis genes are expressed only at low levels. Therefore, we have used the pea lip1 mutant, which shows light-independent photomorphogenesis (Frances et al. 1992 Down). This mutant has many of the characteristics associated with photomorphogenic mutants previously identified in other species, including Arabidopsis (Wei and Deng 1996 Down). Dark-grown lip1 peas have short stems, expanded leaves containing partially developed plastids, and an abundance of nuclear photosynthesis gene transcripts (Frances et al. 1992 Down). The recessive lip1 mutation, which maps to a single locus, also produces pleiotropic effects throughout plant development. These include dwarfism, an inability of green plants to respond to darkness, and a change in the gibberellin GA20-to-GA19 ratio when compared with wild-type plants (Frances et al. 1992 Down; Sponsel et al. 1996 Down; Frances and Thompson 1997 Down).

To investigate the relationship between light and production of the plastid signal, we examined the effect of lincomycin and erythromycin on wild-type and lip1 seedlings. The inhibition of plastid translation decreased the transcripts of several nuclear photosynthesis genes in shoots of wild-type and lip1 light-grown seedlings. Treatment with lincomycin or erythromycin also reduced the accumulation of transcripts in shoots of dark-grown lip1 seedlings and in roots of light-grown lip1 seedlings. Lincomycin treatment also reduced the accumulation of Lhcb1 transcripts in dark-grown seedlings of the Arabidopsis photomorphogenic mutant cop1-4. This clearly demonstrates that the plastid-derived signal is synthesized and is able to regulate nuclear gene expression in the absence of light in both pea and Arabidopsis.


* RESULTS
*TOP
 *ABSTRACT
 *INTRODUCTION
 *RESULTS
*DISCUSSION
 *METHODS
 *REFERENCES

Inhibition of Plastid Translation Does Not Affect the Photomorphogenic Development of Wild-Type or lip1 Seedlings
Preliminary experiments were performed to examine the effect of the inhibition of plastid translation on pea plant development and morphology. Wild-type and lip1 seedlings were grown on either water or 0.5 mM lincomycin for 5 days in darkness followed by growth for either 2 days in the light or 2 days in the dark. Representative seedlings for each treatment are shown in Figure 1. Wild-type seedlings grown on water showed photomorphogenic development in the light; the seedlings had short stems and an expanded green shoot apex. Wild-type seedlings grown in continuous darkness on water showed skotomorphogenic development; the seedlings had long stems and a closed, unexpanded, yellow apical hook. Both light- and dark-grown lip1 seedlings germinated on water showed photomorphogenic development, as has been previously reported (Frances et al. 1992 Down). Dark-grown lip1 seedlings did not accumulate chlorophyll, due to the essential light requirement for the conversion of protochlorophyllide to chlorophyllide by protochlorophyllide reductase (Forreiter et al. 1991 Down). Treatment with lincomycin (or 0.5 mM erythromycin; data not shown) caused complete chlorosis of light-grown wild-type and lip1 seedlings but had no significant effect on the morphology of either light- or dark-grown wild-type or lip1 seedlings. This suggests that any changes in gene expression are likely to be due to the inhibition of plastid translation rather than to changes in seedling development.



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  		Figure 1. Phenotype of Light- and Dark-Grown Wild-Type and lip1 Seedlings Germinated on Water or Lincomycin.

Peas were germinated on water or 0.5 mM lincomycin (Lin) and grown for either 5 days in the dark followed by 2 days in the light (5D2L) or for 7 days in continuous darkness (7D). WT, wild-type.

Plastid Translation Is Required for the Expression of Nuclear Photosynthesis Genes in Shoots of Light-Grown Wild-Type and lip1 Seedlings
To investigate the effect of plastid translation on the accumulation of transcripts of photosynthesis genes, we treated wild-type and lip1 seedlings with either water or 0.5 mM lincomycin. Lincomycin has been shown previously to inhibit chloroplast protein synthesis (Ellis and Hartley 1971 Down) but has little effect on mitochondrial protein synthesis in vitro (Pope 1976 Down). Seedlings were grown for 5 days in darkness followed by 2 days in light, and total RNA was extracted from shoot tissue. Figure 2 shows the results of an RNA gel blot analysis using probes for the nuclear photosynthesis genes Lhcb1, RbcS, PetE (plastocyanin), and AtpC ({gamma} subunit of chloroplast ATP synthase). Hybridization to an rRNA probe was used as a loading control for quantification (Figure 3).



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  		Figure 2. Effect of Lincomycin on the Accumulation of Transcripts of Nuclear Photosynthesis Genes in Wild-Type and lip1 Shoots.

Wild-type Alaska (WT) and lip1 seeds were germinated on either water (W) or 0.5 mM lincomycin (L) and grown for 5 days in the dark followed by 2 days in the light (5D2L) or in continuous darkness for 7 days (7D). Shoot tissue was excised from the seedlings, and total RNA was extracted and subjected to RNA gel blot analysis using 32P-labeled probes from the nuclear photosynthesis genes Lhcb1, RbcS, PetE, and AtpC. Hybridization with a probe for rRNA was used as a loading control.



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  		Figure 3. Quantification of the Effect of Lincomycin on Transcript Accumulation in Wild-Type and lip1 Shoots.

Hybridization signals from the autoradiographs shown in Figure 2 were quantified using a laser scanning densitometer and normalized to rRNA to account for any small differences in the loading of total RNA. For each probe, the amounts of transcripts are expressed relative to wild-type plants grown on water for 5 days in the dark and 2 days in the light (5D2L), which was given an arbitrary value of 100. Abbreviations are as given in the legend to Figure 2.

Increased amounts of photosynthesis gene transcripts were observed in the shoots of light-grown lip1 seedlings grown on water compared with the shoots of wild-type seedlings. Transcripts were 1.6- to 10.6-fold higher in water-grown lip1 shoots than in the shoots of wild-type seedlings (Figure 3). Treatment with lincomycin consistently reduced the accumulation of transcripts of all photosynthesis genes examined in both light-grown wild-type and lip1 shoots but had no effect on the mRNA for polyubiquitin or high-mobility-group protein HMG-I/Y in four replicate treatments (data not shown). Essentially identical results to those presented in Figure 2 and Figure 3 were obtained in all repeated experiments. The decrease in transcript abundance due to lincomycin treatment varied among the genes examined and between wild-type and lip1 seedlings. In light-grown lincomycin-treated wild-type seedlings, transcripts decreased to 24 to 55% of the amounts in untreated seedlings (Figure 3). In light-grown lip1 seedlings, the decreases were slightly smaller (to 40 to 65% of the amounts in control seedlings).

Treatment with 0.5 mM erythromycin produced effects similar to those of lincomycin on Lhcb1 mRNA abundance in shoots of light-grown wild-type and lip1 seedlings (Figure 4). Treatment with erythromycin inhibits plastid protein synthesis but has no effect on mitochondrial translation (Tassi et al. 1983 Down). Inhibition of plastid translation results in decreased accumulation of transcripts of several photosynthesis genes in shoots of light-grown wild-type and lip1 seedlings. This indicates that pea has a requirement for plastid protein synthesis for the expression of nuclear photosynthesis genes similar to that found in mustard and tobacco (Oelmuller et al. 1986 Down; Gray et al. 1995 Down).



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  		Figure 4. Effect of Erythromycin on the Accumulation of Lhcb1 Transcripts in the Shoots and Roots of Wild-Type and lip1 Seedlings.

Wild-type Alaska (WT) and lip1 seedlings were grown on either water (W) or 0.5 mM erythromycin (E) for 5 days in the dark followed by 2 days in the light (5D2L) or in continuous darkness for 7 days (7D). Shoot and root tissues were excised from the seedlings, and total RNA was extracted and subjected to RNA gel blot analysis using a 32P-labeled probe for pea Lhcb1.

Plastid Translation Is Required for the Accumulation of Transcripts of Photosynthesis Genes in Shoots of Dark-Grown lip1 Seedlings
To investigate whether plastid translation is required for the accumulation of transcripts of photosynthesis genes in the dark, we germinated both wild-type and lip1 seedlings on water or 0.5 mM lincomycin and grew them for 7 days in continuous darkness. Shoot tissue was excised, and total RNA was extracted and subjected to RNA gel blot analysis. The results obtained are shown in Figure 2 and Figure 3.

Only very small amounts of transcripts of photosynthesis genes were detected in shoots of dark-grown wild-type seedlings grown on either water or lincomycin. However, much greater amounts of all photosynthesis gene transcripts examined were detected in shoots of dark-grown lip1 seedlings germinated on water. Transcripts were 2.3- to 47.6-fold higher in lip1 seedlings compared with wild-type seedlings. In all cases, the amounts of transcripts of photosynthesis genes in shoots of dark-grown lip1 seedlings were reduced by treatment with lincomycin. The water/lincomycin ratios for transcripts in shoots of dark-grown lip1 seedlings were 4.0 for Lhcb1, 3.1 for PetE, 3.8 for AtpC, and 17.0 for RbcS. Essentially identical results were obtained in four replicate experiments. A similar decrease in Lhcb1 mRNA was observed in shoots of dark-grown lip1 seedlings after treatment with 0.5 mM erythromycin (Figure 4). This clearly demonstrates that plastid translation is required for the expression of nuclear photosynthesis genes in the dark in shoots of lip1 seedlings.

Plastid Translation Is Required for the Accumulation of Transcripts of Photosynthesis Genes in Roots of lip1 Seedlings
Because the organ specificity of photosynthesis gene expression has been shown to be altered in some of the photomorphogenic mutants previously identified in Arabidopsis (Chory and Peto 1990 Down; Deng et al. 1991 Down), the accumulation of transcripts of photosynthesis genes in the roots of wild-type and lip1 seedlings was investigated. Both wild-type and lip1 seedlings were grown on water or 0.5 mM lincomycin, and total RNA was extracted from root tissue and subjected to RNA gel blot analysis with probes for the nuclear photosynthesis genes Lhcb1, RbcS, PetE, and AtpC. The hybridization signal of rRNA was used as a loading control. The results obtained are shown in Figure 5 and Figure 6.



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  		Figure 5. Effect of Lincomycin on the Accumulation of Transcripts of Nuclear Photosynthesis Genes in the Roots of Wild-Type and lip1 Seedlings.

Wild-type Alaska (WT) and lip1 seedlings were grown on either water (W) or 0.5 mM lincomycin (L) for 5 days in the dark followed by 2 days in the light (5D2L) or in continuous darkness for 7 days (7D). Root tissue was excised from the seedlings, and total RNA was extracted and subjected to RNA gel blot analysis using 32P-labeled probes for Lhcb1, RbcS, PetE, and AtpC. Hybridization with a probe for rRNA was used as a loading control.



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  		Figure 6. Quantification of the Effect of Lincomycin on Transcript Accumulation in Wild-Type and lip1 Roots.

Hybridization signals from the autoradiographs shown in Figure 5 were quantified using a laser scanning densitometer and normalized to rRNA to account for any small differences in the loading of total RNA. For each probe, the amounts of transcripts are expressed relative to wild-type plants grown on water for 5 days in the dark followed by 2 days in the light (5D2L), which was given an arbitrary value of 100. Other abbreviations are as given in the legend to Figure 5.

Transcripts of nuclear photosynthesis genes accumulated to a much greater extent in roots of light-grown lip1 seedlings compared with the roots of light-grown wild-type seedlings. Transcripts were 1.2- to 9.3-fold higher in lip1 seedlings than in wild-type seedlings (Figure 6). This increase in transcript abundance in lip1 roots was associated with the development of chloroplasts, which showed both a stacked thylakoid membrane system and the accumulation of starch granules (Figure 7A). Although the plastids in the roots of light-grown wild-type seedlings contained internal membranes, these were unstacked, and starch granules were not observed (cf. Figure 7A and Figure 7E).



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  		Figure 7. Transmission Electron Microscopy of Plastids from Wild-Type and lip1 Roots.

Wild-type Alaska and lip1 seedlings were grown on water or 0.5 mM lincomycin as given below. Thin sections (40 to 50 nm) of root tissue were fixed in glutaraldehyde and hydrogen peroxide, embedded in Araldite, and stained with uranyl acetate before being viewed at 80 kV in a Philips C100 transmission electron microscope. t, stacked thylakoids; u, unstacked membranes. Bars = 500 nm.

(A) and (B) Plastids from lip1 seedlings grown on water (A) or lincomycin (B) for 5 days in the dark and 2 days in the light.

(C) and (D) Plastids from lip1 seedlings grown on water (C) or lincomycin (D) for 7 days in continuous darkness.

(E) and (F) Plastids from wild-type seedlings grown on water (E) or lincomycin (F) for 5 days in the dark and 2 days in the light.

(G) and (H) Plastids from wild-type seedlings grown on water (G) or lincomycin (H) for 7 days in continuous darkness.

In the roots of dark-grown wild-type or lip1 seedlings grown on water or lincomycin, transcripts were detectable only after extended exposure of autoradiographs. Plastids in roots of dark-grown wild-type and lip1 seedlings contained little internal membrane structure and were smaller than the plastids observed in roots of light-grown lip1 seedlings (Figure 7C, Figure 7D, Figure 7G, and Figure 7H). The accumulation of transcripts of photosynthesis genes in roots of light-grown lip1 seedlings was reduced after lincomycin or erythromycin treatment (Figure 4 and Figure 5). This decrease in mRNA abundance was associated with the loss of a stacked thylakoid membrane system and of internal starch grains (Figure 7B and Figure 7F). The water/lincomycin ratios for transcripts in roots of light-grown lip1 seedlings were 4.4 for Lhcb1, 5.5 for PetE, 2.4 for AtpC, and 12.8 for RbcS, similar to the effect on transcripts in shoots of dark-grown seedlings (Figure 3). Essentially identical results were obtained in three repeated experiments. This clearly demonstrates that plastid translation is required for the expression of nuclear photosynthesis genes in the roots of light-grown lip1 seedlings.

Inhibition of Plastid Translation Reduces the Accumulation of Lhcb1 mRNA in the Arabidopsis cop1-4 Photomorphogenic Mutant
To determine whether a requirement for plastid translation could be demonstrated in another photomorphogenic mutant, we investigated the effect of lincomycin on Lhcb1 mRNA accumulation in the Arabidopsis cop1-4 mutant (Deng and Quail 1992 Down). Approximately 200 seeds of cop1-4 and wild-type Arabidopsis (Wassilewskija) were sown on either water or 0.5 mM lincomycin, vernalized at 4°C overnight, and allowed to germinate in the light for 24 hr before being grown for 7 days in the light or in darkness. Total RNA was extracted from the seedlings, and RNA gel blot analysis was performed using a probe from the Arabidopsis Lhcb1*2 gene. The hybridization signal of rRNA was used as a loading control. The results obtained are shown in Figure 8.



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  		Figure 8. Effect of Lincomycin on Lhcb1 mRNA Accumulation in the Arabidopsis Photomorphogenic Mutant cop1-4.

Wild-type Arabidopsis (WT) and cop1-4 seedlings were grown on either water (W) or 0.5 mM lincomycin (L) for 7 days in the light or for 7 days in continuous darkness. Total RNA was extracted from the seedlings and subjected to RNA gel blot analysis using a 32P-labeled probe from the Arabidopsis Lhcb1*2 gene. Hybridization signals from RNA extracted from dark-grown seedlings are the result of a fivefold increase in exposure time relative to light-grown samples. Hybridization with a probe for rRNA was used as a loading control.

Growth of wild-type and cop1-4 seedlings on 0.5 mM lincomycin resulted in complete chlorosis of light-grown seedlings, as has been shown previously with wild-type and lip1 peas, but there was no apparent effect on the morphology of either light- or dark-grown seedlings (data not shown). Treatment with lincomycin decreased the accumulation of Lhcb1 mRNA in both light-grown wild-type and cop1-4 seedlings (Figure 8). Lhcb1 mRNA decreased to <10% of the amount found in untreated light-grown wild-type and cop1-4 seedlings, and it was detected only after extended exposure of autoradiographs. In dark-grown wild-type seedlings, Lhcb1 mRNA was barely detectable. In contrast, Lhcb1 mRNA accumulated in dark-grown cop1-4 seedlings, although only to ~5% of the amount observed in light-grown cop1-4 seedlings germinated on water. Lincomycin treatment of dark-grown cop1-4 seedlings markedly decreased the accumulation of Lhcb1 mRNA, resulting in a water/lincomycin ratio of 3.7 (when corrected for rRNA loading), similar to that for dark-grown lip1 seedlings. This indicates that plastid translation also is required for the accumulation of Lhcb1 mRNA in the Arabidopsis cop1-4 photomorphogenic mutant.


* DISCUSSION
*TOP
 *ABSTRACT
 *INTRODUCTION
 *RESULTS
 *DISCUSSION
*METHODS
 *REFERENCES

The pea photomorphogenic mutant lip1 aberrantly expresses nuclear photosynthesis genes in the shoots of dark-grown seedlings and in the roots of light-grown seedlings. Treatment of lip1 seedlings with inhibitors of plastid translation reduced the accumulation of transcripts of all nuclear photosynthesis genes examined in the light and the dark. Plastid translation is required during early development for the production and/or activity of the plastid signal (Oelmuller et al. 1986 Down; Gray et al. 1995 Down), which indicates that the plastid signal regulates the expression of nuclear photosynthesis genes under all conditions examined, in the light and the dark, and in roots.

Previous studies have demonstrated that the plastid signal is required continuously for the expression of nuclear photosynthesis genes (Oelmuller et al. 1986 Down; Burgess and Taylor 1988 Down). Coupled with the observation that inhibition of plastid translation only during the first 48 to 72 hr after germination affects nuclear photosynthesis gene expression (Oelmuller et al. 1986 Down; Gray et al. 1995 Down), this suggests that the plastid signal is not a direct product of plastid translation (Oelmuller et al. 1986 Down; Gray et al. 1995 Down). However, the requirement for plastid translation during early development indicates that the production and/or activity of the plastid signal requires plastid gene expression (Gray et al. 1995 Down). Indeed, it has been suggested previously that the signal synthetase may be a stable chloroplast-encoded protein (Susek and Chory 1992 Down), although this remains to be proven.

Because inhibition of plastid translation in shoots of light-grown wild-type and lip1 seedlings is capable of decreasing the accumulation of transcripts of several nuclear photosynthesis genes, the plastid signal must be required in both wild-type and lip1 shoots. It is interesting that lincomycin is not as effective in lip1 compared with the wild type for inhibition of transcript accumulation from some of the photosynthesis genes examined. Similar partial uncoupling of nuclear photosynthesis gene expression from chloroplast control has been reported in the Arabidopsis photomorphogenic mutants deetiolated2 (det2) and cop4 (Chory et al. 1991 Down; Hou et al. 1993 Down). Because there is still a requirement for plastid translation in lip1, it seems unlikely that Lip1 is directly involved in the synthesis of the plastid signal, although Lip1 still may have some role in the regulation of nuclear photosynthesis genes by the plastid signal.

The observation that the accumulation of transcripts of nuclear photosynthesis genes in shoots of dark-grown lip1 seedlings also requires plastid translation and, by inference, activity of the plastid signal, demonstrates that light is not an obligate requirement for production or activity of the plastid signal. The shoots of dark-grown lip1 seedlings contain plastids in a state intermediate between wild-type etioplasts and chloroplasts (Frances et al. 1992 Down). These plastids lack prolamellar bodies and contain an unstacked membrane system that has some resemblance to thylakoid membranes (Frances et al. 1992 Down). Because these plastids are still capable of producing the plastid signal, its synthesis therefore does not require the presence of photosynthetically active chloroplasts, a stacked thylakoid membrane system, or the synthesis of chlorophyll. Previous work has shown that the first 2 days after germination are critical for production of the plastid signal in tobacco seedlings (Gray et al. 1995 Down). Inhibition of plastid translation only during this 2-day period in dark-grown seedlings prevents the expression of nuclear photosynthesis genes when the seedlings subsequently are transferred to light (Gray et al. 1995 Down).

The observation that light is not an obligate requirement for production of the plastid signal in lip1 suggests that the plastid signal may be produced by developing plastids in the dark in wild-type plants rather than at the dark–light transition, as has been previously suggested (Mochizuki et al. 1996 Down). The recent observation that inhibition of plastid protein synthesis with streptomycin prevents expression of RbcS and POR (protochlorophyllide reductase) in dark-grown rice seedlings (Yoshida et al. 1998 Down) supports this view. It has been proposed that light and the plastid signal may act through a common trans-acting factor that binds to the promoter regions of nuclear photosynthesis genes (Bolle et al. 1994 Down; Kusnetsov et al. 1996 Down; Lopez-Juez et al. 1996 Down; Puente et al. 1996 Down). Although the experiments presented here have not addressed this hypothesis directly, the separation of the effects of the plastid signal from that of light in the lip1 mutant indicates that the signaling pathways are, at least during some stages of signal transduction, distinct from one another.

The observation that transcripts of nuclear photosynthesis genes did not accumulate in roots of dark-grown lip1 seedlings indicates that the lip1 mutation does not affect the organ specificity of nuclear photosynthesis gene expression. It is therefore likely that the mechanism of organ-specific expression of nuclear photosynthesis genes is distinct from those involved in light and plastid signal regulation. This is supported by the observation that the tissue-specific regulation of the pea plastocyanin gene (PetE) takes place at the transcriptional level, whereas light-regulated expression appears to involve a post-transcriptional mechanism (Pwee and Gray 1993 Down; Helliwell et al. 1997 Down). The increased accumulation of transcripts of photosynthesis genes in roots of light-grown seedlings, however, suggests that lip1 may affect the development of plastids in roots. Indeed, this increase in transcript abundance in roots of light-grown lip1 seedlings was associated with the development of chloroplasts containing a stacked thylakoid membrane system. The plastids in the roots of light-grown wild-type seedlings also showed partial development of a thylakoid membrane system, although the membranes were not stacked; this may represent an earlier stage of chloroplast development. Therefore, lip1 appears to affect the development of plastids in light-grown roots. However, because plastids in roots of dark-grown lip1 seedlings are indistinguishable from those found in wild-type seedlings, some degree of organ specificity in the control of plastid development must remain in lip1 seedlings.

The observation that the accumulation of Lhcb1 mRNA in dark-grown Arabidopsis cop1-4 seedlings was also reduced by inhibition of plastid translation suggests that the plastid signal may be acting downstream of COP1 in the light-regulatory signal transduction pathway. Because treatment with inhibitors of plastid translation had no observable effect upon the morphological development of pea or Arabidopsis seedlings, it is unlikely that the plastid signal acts directly on COP1 function. One possible explanation is that the plastid signal acts to modulate the activity of a CIP (for COP1-interacting protein) either directly, through protein–protein interactions, or by binding to the promoter regions of target genes and preventing the CIP from activating transcription.

Dark-grown lip1 and cop1 seedlings provide useful experimental systems in which to investigate production of the plastid signal in the absence of light. Many of the previous studies investigating the plastid signal used the herbicide norflurazon, an inhibitor of carotenoid biosynthesis (Young 1991 Down), to examine the effects of the plastid signal on gene expression (e.g., Oelmuller et al. 1986 Down; Bolle et al. 1994 Down; Kusnetsov et al. 1996 Down; Puente et al. 1996 Down). After growth in light, the absence of carotenoids causes photooxidation of chloroplasts and downregulation of nuclear photosynthesis genes (Mayfield and Taylor 1984 Down; Oelmuller et al. 1986 Down). However, by using norflurazon, one cannot address the effects of, or the requirement for, the plastid signal during growth in darkness, because the destruction of chloroplasts (and presumably the plastid factor) will take place only upon transfer to light. The treatment of dark-grown lip1 and cop1 seedlings with inhibitors of plastid translation, such as lincomycin or erythromycin, allows the effects of the plastid signal in the dark to be investigated. The identification and cloning of Lip1 may provide further insight into how the light and plastid signal transduction pathways interact during development.


* METHODS
*TOP
 *ABSTRACT
 *INTRODUCTION
 *RESULTS
 *DISCUSSION
 *METHODS
*REFERENCES

Plant Material and Growth Conditions
Seeds of wild-type pea plants (Pisum sativum cv Alaska) and the photomorphogenic mutant lip1 were obtained from the John Innes germ-plasm collection (John Innes Centre, Norwich, UK) and from W.F. Thompson (North Carolina State University, Raleigh, NC). Seeds of Arabidopsis cop1-4 were obtained from X.-W. Deng (Yale University, New Haven, CT). Seeds from these stocks were grown to maturity in a greenhouse and allowed to self-fertilize to produce seed for subsequent experiments.

Before germination, pea seeds were surface sterilized for 2 min with 70% (v/v) ethanol followed by a 10-min treatment in 10% (v/v) sodium hypochlorite solution. The seeds were washed several times in sterile distilled water before being soaked for 4 hr at room temperature in sterile distilled water, 0.5 mM lincomycin (Melford Laboratories Ltd., Ipswich, UK), or 0.5 mM erythromycin (Melford Laboratories Ltd.). After soaking, the seeds were sown onto Whatman 3MM chromatography paper in sterile magenta vessels containing 10 mL of water, lincomycin, or erythromycin solutions, as described above. The vessels were wrapped in two layers of aluminium foil, and the seeds were allowed to imbibe overnight at 4°C before being placed in a constant temperature growth room at 22°C for 5 days. After 5 days, the vessels were either unwrapped and placed for 2 days in photosynthetically active irradiance of 100 µmol m-2 sec-1 (5 days of dark and 2 days of light) or left in the dark for another 2 days (7 days of dark).

For experiments with Arabidopsis, seeds were surface sterilized as described above before being sown onto half-strength Murashige and Skoog medium (Sigma) containing 1% (w/v) agar and either water or 0.5 mM lincomycin in 9-cm Petri dishes. Seeds were vernalized overnight at 4°C and allowed to germinate in the light for 24 hr before being grown for an additional 7 days in photosynthetically active irradiance of 100 µmol m-2 sec-1 or left in continuous darkness for 7 days.

Total RNA Extraction and Gel Blot Analysis
Total RNA was extracted separately from shoot and root tissues from pea seedlings or from whole Arabidopsis seedlings by using Tripure isolation reagent (Boehringer Mannheim) according to the manufacturer's protocol. Approximately 10 µg of total RNA was separated by electrophoresis on a 1.2% agarose gel and blotted to GeneScreen Plus membrane (New England Nuclear Research Products, Boston, MA), as previously described (Helliwell et al. 1997 Down). Radiolabeled probes were produced from cDNAs encoding pea RbcS (Anderson and Smith 1986 Down), pea Lhcb1 (Cline et al. 1989 Down), pea PetE (Last and Gray 1989 Down), pea AtpC (Napier et al. 1992 Down), pea polyubiquitin (Watts and Moore 1992 Down), and pea HMG-I/Y (Webster et al. 1997 Down), and from an Arabidopsis genomic DNA fragment containing Lhcb1*2 (Leutwiler et al. 1986 Down), by using random hexanucleotide primers and {alpha}-32P-dATP (Feinberg and Vogelstein 1983 Down). Hybridization with each of these 32P-labeled probes was followed by probe removal and rehybridization, as described by Helliwell et al. 1997 Down. As a control for loading and transfer efficiency, the membranes were incubated with a 32P-labeled probe for the pea rRNA gene cluster (Jorgensen et al. 1987 Down).

Autoradiographic images were obtained by exposing the membranes to X-ograph Blue X-ray autoradiography film (X-ograph Ltd., Malmesbury, UK). Several exposures for different lengths of time were made for each membrane. Measurements of the hybridization signals were made using a laser scanning densitometer (model 300s; Molecular Dynamics, Sunnyvale, CA) with Imagequant software (Molecular Dynamics). The values obtained were normalized to the intensity of the 25S and 18S rRNA bands. Quantitative analysis was performed only on those autoradiographs within the linear response range.

Transmission Electron Microscopy
Root tissue from wild-type and lip1 seedlings was fixed in glutaraldehyde and hydrogen peroxide, as described by Peracchia and Mittler 1972 Down. Thick tissue sections were washed in Pipes buffer, treated with 2% osmium ferricyanide and 2 mM calcium chloride, and bulk stained with uranyl acetate before being embedded in araldite, as described by Navaratnam et al. 1998 Down. Thin sections (40 to 50 nm) were cut using a diamond knife on a microtome (Ultracut E; Reichert, Vienna, Austria) and viewed at 80 kV in a transmission electron microscope (model CM100; Philips, Eindhoven, The Netherlands).


* ACKNOWLEDGMENTS

We thank Bill Thompson and those at the John Innes pea germ-plasm center for their gifts of wild-type Alaska and lip1 peas, David Lonsdale and Tony Moore for the gift of plasmids, and Xing-Wang Deng for the Arabidopsis cop1-4 seed. We also thank Jeremy Skepper and Janet Powell at the Cambridge Multi-Imaging Centre for the preparation of tissue and assistance with transmission electron microscopy, and Martyn Seekings for his invaluable assistance with the growth of the peas used for these experiments. J.A.S. was supported by a Biotechnology and Biological Sciences Research Council research studentship.

Received August 19, 1998; accepted February 18, 1999.


* REFERENCES
*TOP
 *ABSTRACT
 *INTRODUCTION
 *RESULTS
 *DISCUSSION
 *METHODS
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