Differential regulation by phytochrome of the appearance of plastidic and cytoplasmatic isoforms of glutathione reductase in mustard (Sinapis alba L.) cotyledons

An increase of glutathione reductase (GR; EC 1.6.4.2) activity during the transformation of mustard (Sinapis alba L.) cotyledons from storage organs to photosynthetically competent leaves was previously found to be controlled by light acting via phytochrome (Drumm, H., Mohr, H., Z. Naturforsch. 28c 559–563, 1973). Two isoforms of GR could be separated by disc electrophoresis. In the present study we have applied ionexchange chromatography to separate isoforms of GR during the development of the cotyledons. Furthermore, the technique of in situ photooxidation of plastids was used to distinguish between plastidic and cytoplasmatic isoforms. The isoform GR₂ is the plastidic enzyme, as shown by its sensitivity to photooxidative treatment, while GR₁ is a cytoplasmatic protein not affected by photooxidative treatment of plastids. Both isoforms are promoted by phytochrome but with different time courses. The appearance of GR₁ is independent of the integrity of the plastids, as one might expect. However, unexpectedly, the phytochrome-mediated re-appearance of GR₂ after a photooxidative treatment is much less affected by photooxidative destruction of the plastids, i.e. by the lack of a particular plastidic factor, than was to be expected from previous experience with typical plastidic proteins. An interpretation of this finding must await measurements at the level of GR₂ mRNA.Abbreviations c continuous - D darkness - FR far-red light (3.5 W·m^-2) - FPLC fast protein liquid chromatography - GR glutathione reductase (EC 1.6.4.2) - NF Norflurazon - R fed light (6.8 W·m^-2) - = Pfr/Ptot wavelength-dependent photoequilibrium of the phytochrome system     

Appearance of nitrite reductase in cotyledons of the mustard (Sinapis alba L.) seedling as affected by nitrate,phytochrome and photooxidative damage of plastids

Nitrite reductase (NIR; EC 1.7.7.1) is a central enzyme in nitrate assimilation and is localized in plastids. The present study concerns the regulation of the appearance of NIR in cotyledons of the mustard (Sinapis alba L.) seedling. It was shown that light exerts its positive control over the nitrate-mediated induction of NIR via the farred-absorbing form of phytochrome. Without nitrate the light effect cannot express itself; even though the light signal is accumulated in the cotyledons it remains totally cryptic in the absence of nitrate. Moreover, it was recognised that intact plastids are important in the control of the appearance of NIR. If the plastids are damaged by photooxidation the action of nitrate and phytochrome on NIR appearance is abolished. The appearance of nitrate reductase (NR; EC 1.6.6.1) responds similarly to photooxidative damage even though this enzyme is cytosolic. While the data strongly indicate that some plastidic signal is a prerequisite for the nitrate-induced and phytochrome-modulated appearance of NIR and NR, the possibility could not be ruled out that photooxidative damage affects the accumulation of NIR in the organelle.Abbreviations c continuous - D darkness - FR far-red light - NADP-GPD NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.1.13) - NF Norflurazon - NIR nitrite reductase (EC 1.7.7.1.) - NR nitrate reductase (EC 1.6.6.1) - Pfr phytochrome (far-red light obtained with RG9 glass filter - R red light - RG9-light long wavelenght far-red light obtained with RG9 glass filter - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - WL white light - WL_s strong white light (28 W m^-2)     

Phytochrome-mediated control of grana and stroma thylakoid formation in plastids of mustard cotyledons

The etioplast»chloroplast transition in the cotyledons of mustard seedlings (Sinapis alba L.) has been studied by electron microscopy. It was found that the active form of phytochrome, established by a red light pulse pretreatment, increases the initial rate and eliminates the lag of grana and stroma thylakoid formation after the onset of white light 60 h after sowing. The effect of a pretreatment with 15 s red light pulses is fully reversible by 756 nm light pulses. This reversibility is lost within 5 min. Evidence is presented which suggests that the time course of grana and stroma thylakoid formation is not correlated with the time course of the dispersal of the prolamellar body. The different functions of phytochrome and chlorophyll in controlling thylakoid formation are discussed.     

Appearance of nitrite-reductase mRNA in mustard seedling cotyledons is regulated by phytochrome

Nitrate reductase (NR, EC 1.6.6.1) and nitrite reductase (NIR, EC 1.7.7.1) are the key enzymes of nitrate reduction. It is well established that the appearance of these enzymes is “induced” by nitrate, and it is generally believed that NR is cytosolic while NIR is plastidic. In mustard (Sinapis alba L.) cotyledons we observed two isoforms of NIR (NIR₁ and NIR₂) using a chromato-focusing technique. Only one of them (NIR₂) disappeared when the plastids were damaged by photooxidation in the presence of Norflurazon. It is concluded that NIR₂ is plastidic while NIR₁ is extraplastidic and not affected by photooxidation of the plastids. Both isoforms appear to have the same molecular weight (60 kilodaltons, kDa). Two distinct translation products which could be immunoprecipitated with NIR antiserum produced against total NIR from mustard were observed which differed slightly in molecular weight (60 versus 63 kDa). The 63-kDa polypeptide was considered to be the precursor of NIR₂. While synthesis of NIR protein depended largely on nitrate, the levels of in-vitro-translatable NIR mRNAs were found to be either independent of nitrate and light (NIR₁) or controlled by phytochrome only (NIR₂). It appears that phytochrome strongly stimulates the level of mRNA while significant enzyme synthesis (NIR₂) takes place only in the presence of relatively large amounts of nitrate. Since an increased enzyme level was strictly correlated with an increase of immunoresponsive NIR protein it is improbable that activation of a precursor plays a role. Rather, it is concluded that, in situ, nitrate controls translation.     

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.Abbreviations CAP chloramphenicol - cFR continuous farred light - LSU large subunit of RuBPCase - NADP-GPD NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) - Pfr far-red-absorbing form of phytochrome - pSSU precursor of SSU - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - SSU small subunit of RuBPCase     

Control of the appearance of ascorbate peroxidase (EC 1.11.1.11) in mustard seedling cotyledons by phytochrome and photooxidative treatments

In photosynthetic cells the plastidic ascorbate-glutathione pathway is considered the major sequence involved in the elimination of active oxygen species. Ascorbate peroxidase (APO; EC 1.11.1.11) is an essential constituent of this pathway. In the present paper control of the appearance of APO was studied in the cotyledons of mustard (Sinapis alba L.) seedlings with the following results: (i) Two isoforms of APO (APO I, APO II) could be separated by anion-exchange chromatography; APO I is a plastidic protein, while APO II is extraplastidic, very probably cytosolic. (ii) The appearance of APO is regulated by light via phytochrome. This control is observed with both isoforms. Moreover, a strong positive control over APO II appearance (very probably over APO II synthesis) is exerted by photooxidative treatment of the plastids. (iii) Additional synthesis of extraplastidic APO II is induced by a signal created by intraplastidic pigment-photosensitized oxidative stress. The response is obligatorily oxygen-dependent and abolished by quenchers of singlet oxygen such as -tocopherol and p-benzoquinone. (iv) A short-term (4 h) photooxidative treatment suffices to saturate the signal. Signal transduction cannot be abolished or diminished by replacing the plants in non-photooxidizing conditions. Several observations indicate that control of APO synthesis by active oxygen is not an experimental artifact but a natural phenomenon.Abbreviations APO ascorbate-specific peroxidase (EC 1.11.1.11) - D darkness - FPLC fast protein liquid chromatography - FR far-red light (3.5 W · m^–2) - NF Norflurazon - R red light (6.8 W · m^–2) This research was supported by a grant from the Deutsche For-schungsgemeinschaft. B. Th. was the recipient of a stipend from the Studienstiftung des Deutschen Volkes.     

Expression of nuclear genes as affected by treatments acting on the plastids

In a preceding paper (Oelmüller and Mohr 1986, Planta 167, 106–113) it was shown that in the cotyledons of the mustard (Sinapis alba L.) seedling the integrity of the plastid is a necessary prerequisite for phytochrome-controlled appearance of translatable mRNA for the nuclear-encoded small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase and the light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCP). It was concluded that a signal from the plastid is essential for the expression of nuclear genes involved in plastidogenesis. The present study was undertaken to characterize this postulated signal. Chloramphenicol, an inhibitor of intraplastidic protein synthesis and Norflurazon, an inhibitor of carotenoid synthesis (to bring about photooxidative sensitivity of the plastids) were applied. We obtained the following major results. (i) After a brief period of photooxidative damage a rapid decrease of the above translatable mRNAs was observed. Conclusion: the signal is short-lived and thus required continually. (ii) Once the plastids became damaged by photooxidation, no recovery with regard to nuclear gene expression was observed after a transfer to non-damaging light conditions. Conclusion: even a brief period of damage suffices to prevent production of the signal. (iii) Chloramphenicol inhibited nuclear gene expression (SSU, LHCP) and plastidic development when applied during the early stages of plastidogenesis. Once a certain stage had been reached (between 36–48 h after sowing at 25° C) nuclear gene expression became remarkably insensitive toward inhibition of intraplastidic translation. Conclusion: a certain developmental stage of the plastid must be reached before the signal is released by the plastid. (iv) Under the growth conditions we adopted in our experiments the plastids in the mesophyll cells of mustard cotyledons developed essentially between 36 and 120 (-144) h after sowing. Only during this period could translatable mRNAs for SSU and LHCP be detected. Conclusion: the signal is released by the plastids only during this time span.Abbreviations CAP Chloramphenicol (D-threo) - cFR continuous far-red light - FR far-red light (3.5 W·m^-2) - GPD glyceraldehyde-3-phosphate dehydrogenase - LHCP light-harvesting chlorophyll a/b-binding protein of photosystem II - LSU large subunit of RuBPCase - MDH malate dehydrogenase - NF Norflurazon - NIR nitrite reductase - Pfr physiologically active form of phytochrome - R red light (6.8 W·m^-2) - RG9-light long-wavelength far-red light (10 W·m^-2) - RuBPCase ribulose-1,5-bisphosphate carboxylase - SSU small subunit of RuBPCase - WL_s strong white light (28 W·m^-2) - photoequilibrium of phytochrome at wavelength      

Light control of plastogenesis and ribulosebisphosphate carboxylase levels in mustard seedling cotyledons

We have studied the problem whether the phytochrome-mediated accumulation of ribulosebisphosphate carboxylase (carboxylase; EC 4.1.1.39) in the cotyledons of sinapis alba L. is related to size, ultrastructure, or organization of the plastid compartment. We have shown that under different light conditions (e.g. continuous far-red light, continuous white light) which lead to conspicuously different plastids the time course of the enzyme levels remains precisely the same. It is concluded that the onset and the rate of carboxylase accumulation is not related to the organizational state of the plastid compartment as discernible under the electron microscope.Abbreviations P _tr far-red absorbing form of the phytochrome system - carboxylase ribulosebisphosphate carboxylase (EC 4.1.1.39)     

Carotenoid and chlorophyll biosynthesis in isolated plastids from mustard seedling cotyledons (Sinapis alba L.) during etioplast-chloroplast conversion

Etioplasts and etiochloroplasts, isolated from seedlings of white mustard (Sinapis alba L.) grown in continuous far-red light, and chloroplasts isolated from cotyledons and primary leaves of white-light-grown seedlings exhibit high prenyl-lipid-forming activities. Only the etioplasts and etiochloroplasts, and to a much lesser extent chloroplasts from cotyledons are capable of forming carotenes from isopentenyl diphosphate as substrate, whereas in chloroplasts from primary leaves no such activities could be detected. By subfractionation experiments, it could be demonstrated that the phytoene-synthase complex in etioplasts and etiochloroplasts is present in a soluble form in the stroma, whereas the subsequent enzymes, i.e. the dehydrogenase, cis-trans isomerase and cyclase are bound to both membrane fractions, the prolamellar bodies/prothylakoids and the envelopes. In good agreement with previous results using isolated chromoplasts and chloroplasts, it is concluded that the phytoene-synthase complex may change its topology from a peripheral membrane protein in non-green plastids to a tightly membrane-associated protein in chloroplasts. This change is apparently paralleled by altered functional properties which render the complex undetectable in isolated chloroplasts. Further experiments concerning the reduction of chlorophyll a containing a geranylgeranyl side chain to chlorophyll a indicate that the light-induced etioplast-chloroplast conversion is accompanied by a certain reorganization of the polyprenoid-forming enzymatic equipment.Abbreviations Chl a chlorophyll a - Chl_GG chlorophyll a containing a geranylgeranyl side chain - HPLC high-performance liquid chromatography - Tris 2-ammo-2-(hydroxymethyl)-1,3-propanediol     

Signal storage in phytochrome action on nitrate-mediated induction of nitrate and nitrite reductases in mustard seedling cotyledons

Application of nitrate leads to an induction of nitrate reductase (NR; EC 1.6.6.1) and nitrite reductase (NIR; EC 1.7.7.1) in the cotyledons of dark-grown mustard (Sinapis alba L.) seedlings, and this induction can strongly be promoted by a far-red-light pretreatment — operating through phytochrome — prior to nitrate application. This light treatment is almost ineffective — as far as enzyme appearance is concerned — if no nitrate is given. When nitrate is applied, the stored light signal potentiates the appearance of NR and NIR in darkness, even in the absence of active phytochrome, to the same extent as continuous far-red light. This action of previously stored light signal lasts for approx. 12 h.Storage of the light signal was measured for NR and NIR. The process shows enzyme-specific differences. Storage occurs in the absence as well as in the presence of nitrate, i.e. irrespective of whether or not enzyme synthesis takes place. The kinetics of signal transduction and signal storage indicate that the formation and action of the stored signal are a bypass to the process of direct signal transduction. Signal storage is possibly a means of enabling the plant to maintain the appropriate levels of NR and NIR during the dark period of the natural light/dark cycle.Abbreviations cD continuous darkness - cFR continuous far-red light - D darkness - FR far-red light - NIR nitrite reductase (EC 1.7.7.1) - NR nitrate reductase (EC 1.6.6.1) - Pfr phytochrome (far-red absorbing) - Pr phytochrome (red absorbing) - R red light - RG9-light long wavelength far-red light obtained with RG9 glass filter - - Ptot total phytochrome (Pr+Pfr) Professor Wilhelm Nultsch mit guten Wünschen zum 60. Geburtstag     

Photooxidative destruction of chloroplasts and its consequences for expression of nuclear genes

Expression of nuclear genes involved in plastidogenesis is known to be controlled by light via phytochrome. Examples are the small subunit (SSU) of ribulose-1,5-bisphosphate carboxylase and the light harvesting chlorophyll a/b binding protein of photosystem II (LHCP). In the present study we show that, beside phytochrome, the integrity of the plastid is essential for the expression of the pertinent nuclear genes as measured at the level of translatable mRNA. When the plastids are severely damaged by photooxidation in virtually carotenoid-free mustard (Sinapis alba L.) seedling cotyledons (made carotenoid-free by the application of Norflurazon, NF), almost no SSU, no SSU precursor, LHCP and LHCP precursor can be detected by immunological assays, and almost no translatable mRNA of SSU and LHCP can be found, although the levels and rates of phytochrome-mediated syntheses of representative cytoplasmic, mitochondrial and glyoxisomal enzymes are not adversely affected and morphogenesis of the mustard seedling proceeds normally (Reiß et al. 1983; Planta 159, 518–528). Norflurazon per se has no effect on the amount of translatable mRNA of SSU and LHCP as shown by irradiation of NF-treated seedlings with far-red light (FR) which strongly activates phytochrome but does not cause photooxidation in the plastids. It is concluded that a signal from the plastid is required to allow the phytochrome-mediated appearance of translatable mRNA for SSU and LHCP. Seedlings not treated with NF show a higher level of translatable mRNA_LHCP in red light (RL) compared to FR, whereas the mRNA_SSU levels are the same in RL and FR. These facts indicate that the level of translatable mRNA_LHCP is adversely affected if the apoprotein is not incorporated into the thylakoid membrane.Abbreviations FR far-red light (3.5 W m^-2) - LHCP light harvesting chlorophyll a/b binding protein of photosystem II - LSU large subunit of RuBPCase - NF Norflurazon - RL red light (6.8 W m^-2) - RuBPCase ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) - SSU small subunit of RuBPCase - WL white light (28 W m^-2)     

Down-regulation of phytochrome mRNA abundance by red light and benzyladenine in etiolated cucumber cotyledons

Northern blot analysis revealed that a single 4.2 kb phytochrome mRNA species was detectable in cotyledons excised from five-day-old etiolated cucumber seedlings. Intact etiolated five-day-old cucumber seedlings were given a red light or benzyladenine treatment, and cotyledons were harvested at various times following treatment. The abundance of phytochrome mRNA in the cotyledons was quantitated using ³²P-labeled RNA probes and slot blot analysis. By 2 h after irradiation the phytochrome mRNA level was reduced to 40% of the initial abundance and reaccumulation began by 3 h after irradiation. Reaccumulation of phytochrome mRNA to the time-zero dark control level was achieved by 10 h after treatment. A decrease in phytochrome mRNA abundance was evident by 2 h after benzyladenine treatment, and a maximal reduction to 45% of the time-zero dark control was attained by 4 h after treatment. No recovery of the phytochrome mRNA level was evident by 8 h after benzyladenine treatment. The abundance of actin mRNA was unaffected by benzyladenine treatment.     

Fatty-acid synthesis in plastids from maturing safflower and linseed cotyledons

Plastids isolated from maturing, nongreen safflower (Carthamus tinctorius L.) cotyledons yielded unesterified fatty acids as the predominant product of fatty-acid synthesis from [1-¹⁴C]acetate. Exogenous reduced pyridine nucleotides were not required for this synthesis, but [1-¹⁴C]acetate incorporation was absolutely dependent on addition of ATP. Linseed (Linum usitatissimum L.) cotyledons are green during development and plastids isolated from them resembled leaf chloroplasts with developed grana. In contrast to the safflower plastids, those from linseed were able to carry out fatty-acid synthesis at low irradiances without the addition of either pyridine nucleotides or ATP. Intact linseed cotyledons were capable of net photosynthesis at rates up to 95 mol·mg^-1 chlorophyll·h^-1. However, the low-light environment inside the linseed capsule (approx. 15% of external) means that photosynthesis will not contribute appreciably to the carbon economy of the developing seed and its main role may be to supply cofactors for fatty-acid synthesis.Abbreviations ACP acyl carrier protein - DHAP dihydroxyacetone phosphate - PC phosphatidylcholine - PEP phosphoenolpyruvate - UFA unesterified fatty acids     

Effect of light on the development of glyoxysomal functions in the cotyledons of mustard (Sinapis alba L.) seedlings

The degradation of storage fat in the cotyledons of mustard seedlings is unaffected by phytochrome and photosynthesis (irradiation with continuous red or far-red light from sowing of the seeds) although light imposes a strong constraint on the translocation of organic matter from the cotyledons into the seedling axis. Likewise, the development and disappearance of glyoxysomal enzyme activities (isocitrate lyase, malate synthase, citrate synthase) takes place independently of light. It is concluded that the mobilization of storage fat (fatcarbohydrate transformation) is independent of photomorphogenesis. The surplus of carbohydrate produced from fat in the light seems to be converted to starch grains in the plastids, which function as a secondary storage pool in the cotyledons.Abbreviations CS citrate synthase - ICL isocitrate lyase - MS malate synthase     

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.Abbreviations and symbols c continuous - D darkness - Fd-GOGAT ferredoxin-dependent glutamate synthase (EC 1.4.7.1) - FR far-red light (3.5 W·m^-2) - NADH-GOGAT NADH-dependent glutamate synthase (EC 1.4.1.14) - Pfr far-red absorbing form of phytochrome - Ptot total phytochrome - R red light (6.8 W· m^-2) - RG9-light long wavelength FR (10 W·m^-2, _RG9<0.01) - () Pfr/Ptot=wavelength-dependent photoequilibrium of the phytochrome system     

Chloroplast photooxidation affects the accumulation of cytosolic mRNAs encoding chloroplast proteins in maize

Maize (Zea mays L.) seedlings were grown in the presence or absence of an herbicide, norflurazon (4-chloro-5-(methylamino)-2-(,,-trifluoro-m-tolyl)-pyridazinone), which prevents the accumulation of colored carotenoids. In the absence of carotenoids, plants grown in high light incur extensive photooxidative damage to their plastids, but relatively little damage elsewhere. Growth in very low light minimizes chlorophyll photooxidation and allows chloroplast development to proceed. We have previously reported that mRNA encoding light-harvesting chlorophyll a/b protein (LHCP) fails to accumulate in high-light-grown carotenoid-deficient seedlings, but accumulates normally in carotenoid-deficient seedlings grown in low light. Here we extend these results by examining the levels of translatable mRNAs encoding seven additional nuclear-encoded chloroplast proteins. When norflurazon-treated seedlings were grown in low light for 8 d and then transferred to high light for 24 h, three cytosolic mRNAs (plastocyanin, Rieske Fe–S protein, and the 33-kdalton (kDa) subunit of the photosystem II O₂-evolving complex) decreased to less than 1% the amount found in untreated seedlings. Two other mRNAs (NADP malic enzyme, EC 1.1.1.40, and the 23-kDa subunit of the photosystem II O₂-evolving complex) decreased significantly but not to levels as low as the first three. Levels of translatable mRNA for two other chloroplast proteins (pyruvate orthophosphate dikinase, EC 2.7.9.1, and ferredoxin NADP oxidoreductase, EC 1.18.1.2) were not reduced in nonflurazon-treated seedlings after 24 h in high light, but did not show the normal light-induced increase found in untreated plants. Photooxidative damage in the chloroplast thus affects the accumulation of a number of cytosolic mRNAs encoding proteins destined for the chloroplast.Abbreviations Da dalton - FNR ferredoxin NADP oxidoreductase - LHCP light-harvesting chlorophyll a/b-binding protein - poly(A)RNA polyadenylated RNA - PPDK pyruvate orthophosphate dikinase - PSII photosystem II - SDSPAGE sodium dodecyl sulfate-polyacrylamide gel electrophoresis - SSu small subunit (of ribulose-1,5-bisphosphate carboxylase)