Directed disruption of the tobacco ndhB gene impairs cyclic electron flow around photosystem I

Identification of small non-coding RNAs from mitochondria and chloroplasts

Small non-protein-coding RNAs (ncRNAs) have been identified in a wide spectrum of organisms ranging from bacteria to humans. In eukarya, systematic searches for ncRNAs have so far been restricted to the nuclear or cytosolic compartments of cells. Whether or not small stable non-coding RNA species also exist in cell organelles, in addition to tRNAs or ribosomal RNAs, is unknown. We have thus generated cDNA libraries from size-selected mammalian mitochondrial RNA and plant chloroplast RNA and searched for small ncRNA species in these two types of DNA-containing cell organelles. In total, we have identified 18 novel candidates for organellar ncRNAs in these two cellular compartments and confirmed expression of six of them by northern blot analysis or RNase A protection assays. Most candidate ncRNA genes map to intergenic regions of the organellar genomes. As found previously in bacteria, the presumptive ancestors of present-day chloroplasts and mitochondria, we also observed examples of antisense ncRNAs that potentially could target organelle-encoded mRNAs. The structural features of the identified ncRNAs as well as their possible cellular functions are discussed. The absence from our libraries of abundant small RNA species that are not encoded by the organellar genomes suggests that the import of RNAs into cell organelles is of very limited significance or does not occur at all.

The role of chloroplast NAD(P)H dehydrogenase in protection of tobacco plant against heat stress

After incubation at 42 degrees C for more than 48 h, brown damages occurred on the stems of tobacco (Nicotiana tabacum L.) ndhC-ndhK-ndhJ deletion mutant (deltandhCKJ), followed by wilt of the leaves, while less the phenotype was found in its wild type (WT). Analysis of the kinetics of post-illumination rise in chlorophyll fluorescence indicated that the PSI cyclic electron flow and the chlororespiration mediated by NAD(P)H dehydrogenase (NDH) was significantly enhanced in WT under the high temperature. After leaf disks were treated with methyl viologen (MV), photosynthetic apparatus of deltandhCKJ exhibited more severe photo-oxidative damage, even bleaching of chlorophyll. Analysis of P700 oxidation and reduction showed that the NDH mediated cyclic electron flow probably functioned as an electron competitor with Mehler reaction, to reduce the accumulation of reactive oxygen species (ROS). When leaf disks were heat stressed at 42 degrees C for 6 h, the photochemical activity declined more markedly in deltandhCKJ than in WT, accompanied with more evident decrease in the amount of soluble Rubisco activase. In addition, the slow phase of millisecond-delayed light emission (ms-DLE) of chlorophyll fluorescence indicated that NDH was involved in the building-up of transthylakoid proton gradient (deltapH), while the consumption of deltapH was highly inhibited in deltandhCKJ after heat stress. Based on the results, we supposed that the cyclic electron flow mediated by NDH could be stimulated under the heat stressed conditions, to divert excess electrons via chlororespiration pathway, and sustain CO2 assimilation by providing extra deltapH, thus reducing the photooxidative damage.

Sequence of the tomato chloroplast DNA and evolutionary comparison of solanaceous plastid genomes

Tomato, Solanum lycopersicum (formerly Lycopersicon esculentum), has long been one of the classical model species of plant genetics. More recently, solanaceous species have become a model of evolutionary genomics, with several EST projects and a tomato genome project having been initiated. As a first contribution toward deciphering the genetic information of tomato, we present here the complete sequence of the tomato chloroplast genome (plastome). The size of this circular genome is 155,461 base pairs (bp), with an average AT content of 62.14%. It contains 114 genes and conserved open reading frames (ycfs). Comparison with the previously sequenced plastid DNAs of Nicotiana tabacum and Atropa belladonna reveals patterns of plastid genome evolution in the Solanaceae family and identifies varying degrees of conservation of individual plastid genes. In addition, we discovered several new sites of RNA editing by cytidine-to-uridine conversion. A detailed comparison of editing patterns in the three solanaceous species highlights the dynamics of RNA editing site evolution in chloroplasts. To assess the level of intraspecific plastome variation in tomato, the plastome of a second tomato cultivar was sequenced. Comparison of the two genotypes (IPA-6, bred in South America, and Ailsa Craig, bred in Europe) revealed no nucleotide differences, suggesting that the plastomes of modern tomato cultivars display very little, if any, sequence variation.

Sequence of the Tomato Chloroplast DNA and Evolutionary Comparison of Solanaceous Plastid Genomes

Tomato, Solanum lycopersicum (formerly Lycopersicon esculentum), has long been one of the classical model species of plant genetics. More recently, solanaceous species have become a model of evolutionary genomics, with several EST projects and a tomato genome project having been initiated. As a first contribution toward deciphering the genetic information of tomato, we present here the complete sequence of the tomato chloroplast genome (plastome). The size of this circular genome is 155,461 base pairs (bp), with an average AT content of 62.14%. It contains 114 genes and conserved open reading frames (ycfs). Comparison with the previously sequenced plastid DNAs of Nicotiana tabacum and Atropa belladonna reveals patterns of plastid genome evolution in the Solanaceae family and identifies varying degrees of conservation of individual plastid genes. In addition, we discovered several new sites of RNA editing by cytidine-to-uridine conversion. A detailed comparison of editing patterns in the three solanaceous species highlights the dynamics of RNA editing site evolution in chloroplasts. To assess the level of intraspecific plastome variation in tomato, the plastome of a second tomato cultivar was sequenced. Comparison of the two genotypes (IPA-6, bred in South America, and Ailsa Craig, bred in Europe) revealed no nucleotide differences, suggesting that the plastomes of modern tomato cultivars display very little, if any, sequence variation.

Chlororespiratory reduction 6 is a novel factor required for accumulation of the chloroplast NAD(P)H dehydrogenase complex in Arabidopsis

The chloroplast NAD(P)H dehydrogenase (NDH) complex is involved in photosystem I cyclic electron transport and chlororespiration in higher plants. An Arabidopsis (Arabidopsis thaliana) chlororespiratory reduction 6 (crr6) mutant lacking NDH activity was identified by means of chlorophyll fluorescence imaging. Accumulation of the NDH complex was impaired in crr6. Physiological characterization of photosynthetic electron transport indicated the specific defect of the NDH complex in crr6. In contrast to the CRR7 protein that was recently identified as a potential novel subunit of the NDH complex by means of the same screening, the CRR6 protein was stable under the crr2 mutant background in which the NDH complex does not accumulate. The CRR6 gene (At2g47910) encodes a novel protein without any known motif. Although CRR6 does not have any transmembrane domains, it is localized in the thylakoid membrane fraction of the chloroplast. CRR6 is conserved in phototrophs, including cyanobacteria, from which the chloroplast NDH complex has evolutionally originated, but not in Chlamydomonas reinhardtii, in which the NDH complex is absent. We believe that CRR6 is a novel specific factor for the assembly or stabilization of the NDH complex.

Chloroplastic NAD(P)H dehydrogenase in tobacco leaves functions in alleviation of oxidative damage caused by temperature stress

In this study, the function of the NAD(P)H dehydrogenase (NDH)-dependent pathway in suppressing the accumulation of reactive oxygen species in chloroplasts was investigated. Hydrogen peroxide accumulated in the leaves of tobacco (Nicotiana tabacum) defective in ndhC-ndhK-ndhJ (DeltandhCKJ) at 42 degrees C and 4 degrees C, and in that of wild-type leaves at 4 degrees C. The maximum quantum efficiency of PSII decreased to a similar extent in both strains at 42 degrees C, while it decreased more evidently in DeltandhCKJ at 4 degrees C. The parameters linked to CO(2) assimilation, such as the photochemical efficiency of PSII, the decrease of nonphotochemical quenching following the initial rise, and the photosynthetic O(2) evolution, were inhibited more significantly in DeltandhCKJ than in wild type at 42 degrees C and were seriously inhibited in both strains at 4 degrees C. While cyclic electron flow around PSI mediated by NDH was remarkably enhanced at 42 degrees C and suppressed at 4 degrees C. The proton gradient across the thylakoid membranes and light-dependent ATP synthesis were higher in wild type than in DeltandhCKJ at either 25 degrees C or 42 degrees C, but were barely formed at 4 degrees C. Based on these results, we suggest that cyclic photophosphorylation via the NDH pathway might play an important role in regulation of CO(2) assimilation under heat-stressed condition but is less important under chilling-stressed condition, thus optimizing the photosynthetic electron transport and reducing the generation of reactive oxygen species.

Chlororespiratory reduction 6 is a novel factor required for accumulation of the chloroplast NAD(P)H dehydrogenase complex in Arabidopsis

The chloroplast NAD(P)H dehydrogenase (NDH) complex is involved in photosystem I cyclic electron transport and chlororespiration in higher plants. An Arabidopsis (Arabidopsis thaliana) chlororespiratory reduction 6 (crr6) mutant lacking NDH activity was identified by means of chlorophyll fluorescence imaging. Accumulation of the NDH complex was impaired in crr6. Physiological characterization of photosynthetic electron transport indicated the specific defect of the NDH complex in crr6. In contrast to the CRR7 protein that was recently identified as a potential novel subunit of the NDH complex by means of the same screening, the CRR6 protein was stable under the crr2 mutant background in which the NDH complex does not accumulate. The CRR6 gene (At2g47910) encodes a novel protein without any known motif. Although CRR6 does not have any transmembrane domains, it is localized in the thylakoid membrane fraction of the chloroplast. CRR6 is conserved in phototrophs, including cyanobacteria, from which the chloroplast NDH complex has evolutionally originated, but not in Chlamydomonas reinhardtii, in which the NDH complex is absent. We believe that CRR6 is a novel specific factor for the assembly or stabilization of the NDH complex.

Gene expression profile analysis in astaxanthin-induced Haematococcus pluvialis using a cDNA microarray

The unicellular green alga Haematococcus pluvialis (Volvocales) is known for the ketocarotenoid astaxanthin (3, 3'-dihydroxy-beta, beta-carotene-4, 4'-dione) accumulation, which is induced under unfavorable culture conditions. In this work, we used cDNA microarray analysis to screen differentially expressed genes in H. pluvialis under astaxanthin-inductive culture conditions, such as combination of cell exposure to high irradiance and nutrient deprivation. Among the 965 genes in the cDNA array, there are 144 genes exhibiting differential expression (twofold changes) under these conditions. A significant decrease in the expression of photosynthesis-related genes was shown in astaxanthin-accumulating cells (red cells). Defense- or stress-related genes and signal transduction genes were also induced in the red cells. A comparison of microarray and real-time PCR analysis showed good correlation between the differentially expressed genes by the two methods. Our results indicate that the cDNA microarray approach, as employed in this work, can be relied upon and used to monitor gene expression profiles in H. pluvialis. In addition, the genes that were differentially expressed during astaxanthin induction are suitable candidates for further study and can be used as tools for dissecting the molecular mechanism of this unique pigment accumulation process in the green alga H. pluvialis.

Identification of a novel protein, CRR7, required for the stabilization of the chloroplast NAD(P)H dehydrogenase complex in Arabidopsis

An Arabidopsis thaliana mutant, crr7 (chlororespiratory reduction), was isolated using chlorophyll fluorescence imaging to detect reduced activity in NAD(P)H dehydrogenase (NDH). The chloroplast NDH complex is considered to have originated from cyanobacteria in which the NDH complex is involved in respiration, photosystem I (PSI) cyclic electron transport and CO2 uptake. In higher plants the NDH complex functions in PSI cyclic electron transport within the chloroplast. Despite exhaustive biochemical approaches, the entire subunit composition of the NDH complex is unclear in both cyanobacteria and chloroplasts. In crr7 accumulation of the NDH complex was specifically impaired. In vivo analysis of electron transport supported the specific loss of the NDH complex in crr7. CRR7 (At5g39210) encodes a protein of 156 amino acids, including a putative plastid target signal, and does not contain any known motifs. In contrast to CRR2 and CRR4, involved in the expression of chloroplast ndh genes, CRR7 is conserved in cyanobacterial genomes. Although CRR7 did not contain any transmembrane domains, it localized to the membrane fraction of the chloroplast. CRR7 was unstable in the crr2-2 mutant background, in which the expression of ndhB was impaired. These results strongly suggest that CRR7 is a novel subunit of the chloroplast NDH complex.

Differential use of two cyclic electron flows around photosystem I for driving CO2-concentration mechanism in C4 photosynthesis

Whereas linear electron flow (LEF) in photosynthesis produces both ATP and NADPH, the cyclic electron flow (CEF) around photosystem I has been shown to produce only ATP. Two alternative routes have been shown for CEF; NAD(P)H dehydrogenase (NDH)- and ferredoxin:plastoquinone oxidoreductase (FQR)-dependent flows, but their physiological relevance has not been elucidated in detail. Meanwhile, because C(4) photosynthesis requires more ATP than does C(3) photosynthesis to concentrate CO(2), it has not been clear how the extra ATP is produced. In this study, to elucidate whether CEF contributes to the additional ATP needed in C(4) photosynthesis, we estimated the amounts of PGR5, which participates in FQR-dependent flow, and NDH-H, a subunit of NDH, in four C(4) species. Although the expression profiles of PGR5 did not correlate well with the additional ATP requirement, NDH was greatly expressed in mesophyll cells in the NAD-malic enzyme (ME) species, and in bundle-sheath cells in NADP-ME species, where there is a strong need for ATP in the respective cells. Our results indicate that CEF via NDH plays a central role in driving the CO(2)-concentrating mechanism in C(4) photosynthesis.

Specific function of a plastid sigma factor for ndhF gene transcription

The complexity of the plastid transcriptional apparatus (two or three different RNA polymerases and numerous regulatory proteins) makes it very difficult to attribute specific function(s) to its individual components. We have characterized an Arabidopsis T-DNA insertion line disrupting the nuclear gene coding for one of the six plastid sigma factors (SIG4) that regulate the activity of the plastid-encoded RNA polymerase PEP. This mutant shows a specific diminution of transcription of the plastid ndhF gene, coding for a subunit of the plastid NDH [NAD(P)H dehydrogenase] complex. The absence of another NDH subunit, i.e. NDHH, and the absence of a chlorophyll fluorescence transient previously attributed to the activity of the plastid NDH complex indicate a strong down-regulation of NDH activity in the mutant plants. Results suggest that plastid NDH activity is regulated on the transcriptional level by an ndhF-specific plastid sigma factor, SIG4.

Enhanced ferredoxin-dependent cyclic electron flow around photosystem I and alpha-tocopherol quinone accumulation in water-stressed ndhB-inactivated tobacco mutants

Dissipation mechanisms of excess photon energy under water stress were studied in ndhB-inactivated tobacco (Nicotiana tabacum cv. Xanthi) mutants, which are impaired in NAD(P)H dehydrogenase-dependent cyclic electron flow around PSI. Relative leaf water content and net CO(2) assimilation decreased to 30% and almost zero, respectively, after 11-day water stress in the mutant and wild type plants. Similar reductions in PSII activity (by ca. 75%), and increases in malondialdehyde (by ca. 45%), an indicator of lipid peroxidation, were observed in both the plant groups when subjected to water stress. The stressed mutant and wild type plants showed similar P700 redox kinetics, but only the stressed mutant demonstrated an enhanced operation of the antimycin A-sensitive, ferredoxin-dependent cyclic electron flow around PSI, as indicated by a transient increase in chlorophyll fluorescence after turning off of actinic light. Further, the stressed mutant showed higher oxidation of alpha-tocopherol to alpha-tocopherol quinone, as compared with that in the stressed wild type. Thus, a deficiency in NAD(P)H dehydrogenase-dependent cyclic electron flow around PSI does not lead to oxidative damage because the mutant compensates for this deficiency by activating alternative dissipating routes of excess photon energy, such as up-regulation of ferredoxin-dependent cyclic electron flow around PSI and increased accumulation of alpha-tocopherol quinone.

Inhibitor studies on non-photochemical plastoquinone reduction and H(2) photoproduction in Chlamydomonas reinhardtii

In the absence of PSII, non-photochemical reduction of plastoquinones (PQs) occurs following NADH or NADPH addition in thylakoid membranes of the green alga Chlamydomonas reinhardtii. The nature of the enzyme involved in this reaction has been investigated in vitro by measuring chlorophyll fluorescence increase in anoxia and light-dependent O(2) uptake in the presence of methyl viologen. Based on the insensitivity of these reactions to rotenone, a type-I NADH dehydrogenase (NDH-1) inhibitor, and their sensitivity to flavoenzyme inhibitors and thiol blocking agents, we conclude to the involvement of a type-II NADH dehydrogenase (NDH-2) in PQ reduction. Intact Chlamydomonas cells placed in anoxia have the property to produce H(2) in the light by a Fe-hydrogenase which uses reduced ferredoxin as an electron donor. H(2) production also occurs in the absence of PSII thanks to the existence of a non-photochemical pathway of PQ reduction. From inhibitors effects, we suggest the involvement of a plastidial NDH-2 in PSII-independent H(2) production in Chlamydomonas. These results are discussed in relation to the absence of ndh genes in Chlamydomonas plastid genome and to the existence of 7 ORFs homologous to type-II NDHs in its nuclear genome.

The pgr1 Mutation in the Rieske Subunit of the Cytochrome b6f Complex Does Not Affect PGR5-dependent Cyclic Electron Transport around Photosystem I

Although photosystem I (PSI) cyclic electron transport is essential for plants, our knowledge of the route taken by electrons is very limited. To assess whether ferredoxin (Fd) donates electrons directly to plastoquinone (PQ) or via a Q-cycle in the cytochrome (cyt) b(6)f complex in PSI cyclic electron transport, we characterized the activity of PSI cyclic electron transport in an Arabidopsis mutant, pgr1 (proton gradient regulation). In pgr1, Q-cycle activity was hypersensitive to acidification of the thylakoid lumen because of an amino acid alteration in the Rieske subunit of the cyt b(6)f complex, resulting in a conditional defect in Q-cycle activity. In vitro assays using ruptured chloroplasts did not show any difference in the activity of PGR5-dependent PQ reduction by Fd, which functions in PSI cyclic electron transport in vivo. In contrast to the pgr5 defect, the pgr1 defect did not show any synergistic effect on the quantum yield of photosystem II in crr2-2, a mutant in which NDH (NAD(P)H dehydrogenase) activity was impaired. Furthermore, the simultaneous determination of the quantum yields of both photosystems indicated that the ratio of linear and PSI cyclic electron transport was not significantly affected in pgr1. All the results indicated that the pgr1 mutation did not affect PGR5-dependent PQ reduction by Fd. The phenotypic differences between pgr1 and pgr5 indicate that maintenance of the proper balance of linear and PSI cyclic electron transport is essential for preventing over-reduction of the stroma.

Cyclic electron flow around PSI monitored by afterglow luminescence in leaves of maize inbred lines (Zea mays L.): correlation with chilling tolerance

Maize (Zea mays L.) inbred lines of contrasting chilling sensitivity (three tolerant, three sensitive lines) were acclimated to 280 mumol photons m(-2) s(-1) white light at a 17 degrees C sub-optimal temperature. They showed no symptoms of photoinhibition, despite slight changes in photosystem II (PSII) fluorescence and thermoluminescence properties in two tolerant lines. A luminescence "afterglow" emission [Bertsch and Azzi (1965) Biochim Biophys Acta 94:15-26], inducible by a far-red (FR) illumination of unfrozen leaf discs, was detected either as a bounce in decay kinetics at constant temperatures or as a sharp thermoluminescence afterglow band at about 45 degrees C, in dark-adapted leaves. This band reflects the induction by warming of an electron pathway from stromal reductants to plastoquinones and to the Q(B) secondary acceptor of PSII, resulting in a luminescence-emitting charge recombination in the fraction of centres that were initially in the S(2/3)Q(B) non-luminescent state. A 5-h exposure of plants to growth chamber light shifted this luminescence emission towards shorter times and lower temperatures for several hours in the three chilling-tolerant lines. This downshift was not observed, or only transiently, in the three sensitive lines. In darkness, the downshifted afterglow band relaxed within hours to resume its dark-adapted location, similar for all maize lines. A faster dark re-reduction of P700(+) oxidized by FR light (monitored by 820-nm absorbance) and an increase of photochemical energy storage under FR excitation (determined by photoacoustic spectroscopy) confirmed that a cyclic pathway induced by white actinic light remained activated for several hours in the tolerant maize lines.

Pigment deficiency in nightshade/tobacco cybrids is caused by the failure to edit the plastid ATPase alpha-subunit mRNA

The subgenomes of the plant cell, the nuclear genome, the plastome, and the chondriome are known to interact through various types of coevolving macromolecules. The combination of the organellar genome from one species with the nuclear genome of another species often leads to plants with deleterious phenotypes, demonstrating that plant subgenomes coevolve. The molecular mechanisms behind this nuclear-organellar incompatibility have been elusive, even though the phenomenon is widespread and has been known for >70 years. Here, we show by direct and reverse genetic approaches that the albino phenotype of a flowering plant with the nuclear genome of Atropa belladonna (deadly nightshade) and the plastome of Nicotiana tabacum (tobacco) develops as a result of a defect in RNA editing of a tobacco-specific editing site in the plastid ATPase alpha-subunit transcript. A plastome-wide analysis of RNA editing in these cytoplasmic hybrids and in plants with a tobacco nucleus and nightshade chloroplasts revealed additional defects in the editing of species-specific editing sites, suggesting that differences in RNA editing patterns in general contribute to the pigment deficiencies observed in interspecific nuclear-plastidial incompatibilities.

Stromal over-reduction by high-light stress as measured by decreases in P700 oxidation by far-red light and its physiological relevance

The oxidation level of P700 induced by far-red light (DeltaA(FR)) in briefly dark-treated leaves of some sun plants decreased during the daytime and recovered at night. The dark recovery of decreased DeltaA(FR) proceeded slowly, with a half-time of about 5 h. We propose that stromal over-reduction induced by sunlight was the direct cause of the depression of DeltaA(FR). The depression of DeltaA(FR) found during the daytime was reproduced by controlled illumination with saturating light of fully dark-treated leaves. Simultaneous measurement of P700 redox and chlorophyll fluorescence showed that the depression of DeltaA(FR) was associated with dark reduction of the plastoquinone pool, which represented cyclic electron transport activity. The decrease of DeltaA(FR) in the light-stressed chloroplasts was partly reversed by treatment with 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, an inhibitor of electron transport at the cytochrome b6/f complex, and the subsequent addition of methyl viologen, an efficient electron acceptor from photosystem I (PSI), stimulated further recovery, showing that both cyclic electron flow around PSI and the charge recombination within PSI were responsible for the light-induced depression of DeltaA(FR). The dark level of blue-green fluorescence, an indicator of NAD(P)H concentration, from intact chloroplasts was increased by high-light stress, suggesting that NADPH accumulated in stroma as a result of the high-light treatment. Possible effects on photosynthetic activity of over-reduction and its physiological relevance are discussed.

A pentatricopeptide repeat protein is essential for RNA editing in chloroplasts

RNA editing is a process of RNA maturation involved in the insertion, deletion or modification of nucleotides. In organellar transcripts of higher plants, specific cytidine residues are converted into uridine residues. In many cases, editing results in the restoration of conserved amino acid residues, a process that is essential for protein function in plastids. Despite the technical breakthrough in establishing systems in vivo and in vitro for analysing RNA editing, its machinery still remains to be identified in higher plants. Here we introduce a genetic approach and report the discovery of a gene responsible for the specific RNA editing event in the chloroplast.

New vectors and marker excision systems mark progress in engineering the plastid genome of higher plants

The transformation of the plastid genome, until recently restricted to tobacco, is now being extended to a rapidly growing list of crops. This perspective provides an overview of emerging trends of technology development in the field with a focus on vector design and marker excision systems. The new tools will facilitate engineering of the photosynthetic machinery and enable novel agricultural and industrial applications.

Functional flexibility and acclimation of the thylakoid membrane

Light is an elusive substrate for the function of photosynthetic light reactions of photosynthesis in the thylakoid membrane. Therefore structural and functional dynamics, which occur in the timescale from seconds to several days, are required both at low and high light conditions. The best characterized short-time regulation mechanism at low light is a rapid state transition, resulting in higher absorption cross section of PSI at the expense of PSII. If the low light conditions continue, activation of the lhcb-genes and synthesis of the light-harvesting proteins will occur to optimize the functions of PSII and PSI. At high light, the transition to state 2 is completely inhibited, but the feedback de-excitation of absorbed energy as heat, known as the energy-dependent quenching (q(E)), is rapidly set up. It requires, at least, the DeltapH-dependent activation of violaxanthin de-epoxidase and involvement of the PsbS protein. Another crucial mechanism for protection against the high light stress is the PSII repair cycle. Furthermore, the water-water cycle, cyclic electron transfer around PSI and chlororespiration are important means induced under high irradiation, functioning mainly to avoid an excess production of reactive oxygen species.

New subunits NDH-M, -N, and -O, encoded by nuclear genes, are essential for plastid Ndh complex functioning in higher plants

In higher plants, the Ndh complex reduces plastoquinones and is involved in cyclic electron flow around photosystem I, supplying extra-ATP for photosynthesis, particularly under environmental stress conditions. Based on plastid genome sequences, the Ndh complex would contain 11 subunits (NDH-A to -K), but homologies with bacterial complex indicate the probable existence of additional subunits. To identify missing subunits, tobacco (Nicotiana tabacum) NDH-H was His tagged at its N terminus using plastid transformation. A functional Ndh subcomplex was purified by Ni(2+) affinity chromatography and its subunit composition analyzed by mass spectrometry. Five plastid encoded subunits (NDH-A, -H, -I, -J, and -K) were identified as well as three new subunits (NDH-M, -N, and -O) homologous to cyanobacterial and higher plant proteins. Arabidopsis thaliana mutants missing one of these new subunits lack a functional Ndh complex, and NDH-M and NDH-N are not detected in a tobacco transformant lacking the Ndh complex. We discuss the involvement of these three nuclear-encoded subunits in the functional integrity of the plastidial complex.

Inactivation of ycf33 Results in an Altered Cyclic Electron Transport Pathway Around Photosystem I in Synechocystis sp. PCC6803

ycf33 encodes a small protein with a molecular mass of 7.5 kDa and is found from cyanobacteria to higher plants. A ycf33 deletion mutant was constructed in Synechocystis sp. PCC6803 and characterized. The mutant showed a higher phycobilisome/chlorophyll ratio than the wild type and a higher photosystem II/photosystem I fluorescence ratio measured at 77 K. Under photoautotrophic conditions, the growth rates were not much different from those of the wild type. Cyclic electron transport activities around photosystem I were not much different between the wild type and the mutant. However, the effects of diphenyleneiodonium, an inhibitor of flavoprotein, on cyclic electron transport in the mutant were different from those in the wild type; it was severely inhibited in the wild type but not much in the mutant. Together with the effects of nitrite, which accepts electrons from ferredoxin via nitrite reductase and those of HgCl2, it was suggested that the pathway of cyclic electron transport is altered in the mutant.

Production of polyhydroxybutyrate by polycistronic expression of bacterial genes in tobacco plastid

Transgenic techniques are used to enhance and improve crop production, and their application to the production of chemical resources in plants has been under investigation. To achieve this latter goal, multiple-gene transformation is required to improve or change plant metabolic pathways; when accomplished by plant nuclear transformation, however, this procedure is costly and time consuming. We succeeded in the metabolic engineering of the tobacco plant by introducing multiple genes within a bacteria-like operon into a plastid genome. A tobacco plastid was transformed with a polycistron consisting of the spectinomycin resistance gene and three bacterial genes for the biosynthesis of the biodegradable polyester, poly[(R)-3-hydroxybutyrate] (PHB), after modification of their ribosome binding sites. DNA and RNA analysis confirmed the insertion of the introduced genes into the plastid genome and their polycistronic expression. As the result, the transplastomic tobacco accumulated PHB in its leaves. The introduced genes and the PHB productivity were maternally inherited, avoiding genetic spread by pollen diffusion, and were maintained stably in the seed progeny. Despite the low PHB productivity, this report demonstrates the feasibility of transplastomic technology for metabolic engineering. This "phyto-fermentation" system can be applied to plant production of various chemical commodities and pharmaceuticals.

Dynamic flexibility in the light reactions of photosynthesis governed by both electron and proton transfer reactions

Plant photosynthesis performs the remarkable feat of converting light energy into usable chemical forms, which involves taming highly reactive intermediates without harming plant cells. This requires an apparatus that is not only efficient and robust but also flexible in its responses to changing environmental conditions. It also requires that the output of the energy-storing reactions be matched with the demands of metabolism. This article addresses the mechanisms by which this flexibility is achieved for short-term environmental changes. We argue that chloroplasts need two types of flexible mechanisms: one for modulating the output ratio of ATP:NADPH, which involves cyclic electron flux around photosystem I; and another for changing the regulatory sensitivity of the light-harvesting antenna to electron (and proton) flow.

Cyclic electron flow around photosystem I is essential for photosynthesis

Photosynthesis provides at least two routes through which light energy can be used to generate a proton gradient across the thylakoid membrane of chloroplasts, which is subsequently used to synthesize ATP. In the first route, electrons released from water in photosystem II (PSII) are eventually transferred to NADP+ by way of photosystem I (PSI). This linear electron flow is driven by two photochemical reactions that function in series. The cytochrome b6f complex mediates electron transport between the two photosystems and generates the proton gradient (DeltapH). In the second route, driven solely by PSI, electrons can be recycled from either reduced ferredoxin or NADPH to plastoquinone, and subsequently to the cytochrome b6f complex. Such cyclic flow generates DeltapH and thus ATP without the accumulation of reduced species. Whereas linear flow from water to NADP+ is commonly used to explain the function of the light-dependent reactions of photosynthesis, the role of cyclic flow is less clear. In higher plants cyclic flow consists of two partially redundant pathways. Here we have constructed mutants in Arabidopsis thaliana in which both PSI cyclic pathways are impaired, and present evidence that cyclic flow is essential for efficient photosynthesis.

PPR motifs of the nucleus-encoded factor, PGR3, function in the selective and distinct steps of chloroplast gene expression in Arabidopsis

Plastid gene expression is regulated by a variety of nuclear genes. We have isolated Arabidopsis thaliana proton gradient regulation 3 (pgr3) mutants, which display aberrant chlorophyll fluorescence because of defects in chloroplast gene expression. High chlorophyll fluorescence (HCF) because of a reduced level of the cytochrome b6/f complex was observed in two alleles, pgr3-1 and pgr3-2 but not in pgr3-3. In contrast, a transient increase in fluorescence after turning off the actinic light, which was ascribed to chloroplast NADPH dehydrogenase (NDH) activity, was impaired in pgr3-1 and pgr3-3 but not in pgr3-2. Both phenotypes were complemented by the introduction of a single gene, PGR3, encoding a protein containing 27 pentatrico-peptide repeat (PPR) motifs. PPR motifs are present in proteins functioning in the post-transcriptional regulation of organellar gene expression. The conserved threonine in the motif was substituted by isoleucine in the 15th and 12th PPR motifs in pgr3-1 and pgr3-2, respectively, and the conserved leucine by phenylalanine in the final incomplete motif of pgr3-3. We consider that the different domains of the PPR repeats in PGR3 might have different functions in conferring RNA stability and probably allowing translation as well as recognizing at least two distinct target RNAs.

Enhanced rates of P700(+) dark-reduction in leaves of Cucumis sativus L photoinhibited at chilling temperature

The changes in electron transport within photosystem I (PSI) were studied in detached leaves of Cucumis sativus L. during the course of irradiation with moderate white light (300 micromol photons m(-2) s(-1)) at 4 degrees C. When intact leaves were exposed to the combination of moderate light and low temperature, the amplitude of far-red light-induced P700 absorbance changes at 820 nm (deltaA(820)), a relative measure of PSI, progressively decreased as the light treatment time increased. Almost no oxidation of P700 was noticeable after 5 h. Methyl viologen accelerated the oxidation of P700 to a steady-state level and also increased the magnitudes of deltaA(820) changes in photoinhibited leaves, reflecting the rapid removal of electrons from native carriers. Photoinhibition under moderate light and chilling temperature also accelerated the rate of P700(+) reduction after far-red light excitation as the half-times of the two exponential components of P700(+) decay curves decreased relative to the control ones. A detailed analysis of the kinetics of P700(+) reduction using diuron alone or the combination of diuron and methyl viologen strongly favours an increased rate of electron donation from stromal reductants to PSI through the plastoquinone pool following photoinhibitory treatment. Importantly, the marked acceleration of P700(+) re-reduction is the consequence of the irradiation of leaf segments at low temperature and not caused by chilling stress alone.

Plastid transformation in higher plants

Plastids of higher plants are semi-autonomous organelles with a small, highly polyploid genome and their own transcription-translation machinery. This review provides an overview of the technology for the genetic modification of the plastid genome including: vectors, marker genes and gene design, the use of gene knockouts and over-expression to probe plastid function and the application of site-specific recombinases for excision of target DNA. Examples for applications in basic science include the study of plastid gene transcription, mRNA editing, photosynthesis and evolution. Examples for biotechnological applications are incorporation of transgenes in the plastid genome for containment and high-level expression of recombinant proteins for pharmaceutical and industrial applications. Plastid transformation is routine only in tobacco. Progress in implementing the technology in other crops is discussed.

The function of chloroplastic NAD(P)H dehydrogenase in tobacco during chilling stress under low irradiance

The function of chloroplastic NAD(P)H dehydrogenase (NDH) was examined by comparing a tobacco transformant (DeltandhB) in which the ndhB gene had been disrupted with its wild type, upon exposure to chilling temperature (4 degrees C) under low irradiance (100 micro mol m(-2) s(-1) PFD). During the chilling stress, the maximum photochemical efficiency of PSII (F(v)/F(m)) decreased markedly in both the wild type and DeltandhB. However, both F(v)/F(m) and P700(+), as well as the PSII-driven electron transport rate (ETR), in DeltandhB were lower than that in the wild type, implying that NDH-dependent cyclic electron flow around PSI functioned to protect the photosynthetic apparatus from chilling stress under low irradiance. Under the stress, non-photochemical quenching (NPQ), particularly the fast relaxing NPQ component (qf) and the de-epoxidized ratio of the xanthophyll cycle pigments, (A+Z)/(V+A+Z), were distinguishable in DeltandhB from those in the wild type. The lower NPQ in DeltandhB might be related to an inefficient proton gradient across thylakoid membranes (DeltapH) because of lacking an NDH-dependent cyclic electron flow around PSI at chilling temperature under low irradiance.

Electron Fluxes through Photosystem I in Cucumber Leaf Discs Probed by far-red Light

Cucumber leaf discs were illuminated at room-temperature with far-red light to photo-oxidise P700, the chlorophyll dimer in Photosystem (PS) I. The post-illumination kinetics of P700(+) re-reduction were studied in the presence of inhibitors or cofactors of photosynthetic electron transport. The re-reduction kinetics of P700(+) were well fitted as the sum of three exponentials, each with its amplitude and rate coefficient, and an initial flux (at the instant of turning off far-red light) given as the product of the two. Each initial flux is assumed equal to a steady state flux during far-red illumination. The fast phase of re-reduction, with rate coefficient k (1) approximately 10 s(-1), was completely abolished by a saturating concentration of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU); it is attributed to electron flow to P700(+) from PS II, which was stimulated to some extent by far-red light. The intermediate phase, with rate coefficient k (1) approximately 1 s(-1), was only partly diminished by methyl viologen (MV) which diverts electron flow to oxygen. The intermediate phase is attributed to electron donation from reduced ferredoxin to the intersystem pool; reduced ferredoxin could be formed: (1) directly by electron donation on the acceptor of PS I; and/or (2) indirectly by stromal reductants, in line with only a partial inhibition of the intermediate phase by MV. Duroquinol enhanced the intermediate phase in the presence of DCMU, presumably through its interaction with thylakoid membrane components leading to the partial reduction of plastoquinone. The slow phase of P700(+) re-reduction, with rate coefficient k (1) approximately 0.1 s(-1), was unaffected by DCMU and only slightly affected by MV; it could be associated with electron donation to either: (1) the intersystem chain by stromal reductants catalysed by NAD(P)H dehydrogenase slowly; or (2) plastocyanin/P700(+) by ascorbate diffusing across the thylakoid membrane to the lumen. It is concluded that a post-illumination analysis of the fluxes to P700(+) can be used to probe the pathways of electron flow to PS I in steady state illumination.

Alternative photosystem I-driven electron transport routes: mechanisms and functions

In addition to the linear electron transport, several alternative Photosystem I-driven (PS I) electron pathways recycle the electrons to the intersystem electron carriers mediated by either ferredoxin:NADPH reductase, NAD(P)H dehydrogenase, or putative ferredoxin:plastoquinone reductase. The following functions have been proposed for these pathways: adjustment of ATP/NADPH ratio required for CO(2) fixation, generation of the proton gradient for the down-regulation of Photosystem II (PS II), and ATP supply the active transport of inorganic carbon in algal cells. Unlike ferredoxin-dependent cyclic electron transport, the pathways supported by NAD(P)H can function in the dark and are likely involved in chlororespiratory-dependent energization of the thylakoid membrane. This energization may support carotenoid biosynthesis and/or maintain thylakoid ATPase in active state. Active operation of ferredoxin-dependent cyclic electron transport requires moderate reduction of both the intersystem electron carriers and the acceptor side of PS I, whereas the rate of NAD(P)H-dependent pathways under light depends largely on NAD(P)H accumulation in the stroma. Environmental stresses such as photoinhibition, high temperatures, drought, or high salinity stimulated the activity of alternative PS I-driven electron transport pathways. Thus, the energetic and regulatory functions of PS I-driven pathways must be an integral part of photosynthetic organisms and provides additional flexibility to environmental stress.

A nucleus-encoded factor, CRR2, is essential for the expression of chloroplast ndhB in Arabidopsis

The chloroplast NDH complex, NAD(P)H dehydrogenase, reduces the plastoquinone pool non-photochemically and is involved in cyclic electron flow around photosystem I (PSI). A transient increase in chlorophyll fluorescence after turning off actinic light is a result of NDH activity. We focused on this subtle change in chlorophyll fluorescence to isolate nuclear mutants affected in chloroplast NDH activity in Arabidopsis by using chlorophyll fluorescence imaging. crr2-1 and crr2-2 (chlororespiratory reduction) are recessive mutant alleles in which accumulation of the NDH complex is impaired. Except for the defect in NDH activity, photosynthetic electron transport was unaffected. CRR2 encodes a member of the plant combinatorial and modular protein (PCMP) family consisting of more than 200 genes in Arabidopsis. CRR2 functions in the intergenic processing of chloroplast RNA between rps7 and ndhB, which is possibly essential for ndhB translation. We have determined the function of a PCMP family member, indicating that the family is closely related to pentatrico-peptide PPR proteins involved in the maturation steps of organellar RNA.

Deletion of the tobacco plastid psbA gene triggers an upregulation of the thylakoid-associated NAD(P)H dehydrogenase complex and the plastid terminal oxidase (PTOX)

We have constructed a tobacco psbA gene deletion mutant that is devoid of photosystem II (PSII) complex. Analysis of thylakoid membranes revealed comparable amounts, on a chlorophyll basis, of photosystem I (PSI), the cytochrome b6f complex and the PSII light-harvesting complex (LHCII) antenna proteins in wild-type (WT) and DeltapsbA leaves. Lack of PSII in the mutant, however, resulted in over 10-fold higher relative amounts of the thylakoid-associated plastid terminal oxidase (PTOX) and the NAD(P)H dehydrogenase (NDH) complex. Increased amounts of Ndh polypeptides were accompanied with a more than fourfold enhancement of NDH activity in the mutant thylakoids, as revealed by in-gel NADH dehydrogenase measurements. NADH also had a specific stimulating effect on P700+ re-reduction in the DeltapsbA thylakoids. Altogether, our results suggest that enhancement of electron flow via the NDH complex and possibly other alternative electron transport routes partly compensates for the loss of PSII function in the DeltapsbA mutant. As mRNA levels were comparable in WT and DeltapsbA plants, upregulation of the alternative electron transport pathways (NDH complex and PTOX) occurs apparently by translational or post-translational mechanisms.

Endosymbiosis and the design of eukaryotic electron transport

The bioenergetic organelles of eukaryotic cells, mitochondria and chloroplasts, are derived from endosymbiotic bacteria. Their electron transport chains (ETCs) resemble those of free-living bacteria, but were tailored for energy transformation within the host cell. Parallel evolutionary processes in mitochondria and chloroplasts include reductive as well as expansive events: On one hand, bacterial complexes were lost in eukaryotes with a concomitant loss of metabolic flexibility. On the other hand, new subunits have been added to the remaining bacterial complexes, new complexes have been introduced, and elaborate folding patterns of the thylakoid and mitochondrial inner membranes have emerged. Some bacterial pathways were reinvented independently by eukaryotes, such as parallel routes for quinol oxidation or the use of various anaerobic electron acceptors. Multicellular organization and ontogenetic cycles in eukaryotes gave rise to further modifications of the bioenergetic organelles. Besides mitochondria and chloroplasts, eukaryotes have ETCs in other membranes, such as the plasma membrane (PM) redox system, or the cytochrome P450 (CYP) system. These systems have fewer complexes and simpler branching patterns than those in energy-transforming organelles, and they are often adapted to non-bioenergetic functions such as detoxification or cellular defense.

Arabidopsis thaliana plants lacking the PSI-D subunit of photosystem I suffer severe photoinhibition, have unstable photosystem I complexes, and altered redox homeostasis in the chloroplast stroma

The PSI-D subunit of photosystem I is a hydrophilic subunit of about 18 kDa, which is exposed to the stroma and has an important function in the docking of ferredoxin to photosystem I. We have used an antisense approach to obtain Arabidopsis thaliana plants with only 5-60% of PSI-D. No plants were recovered completely lacking PSI-D, suggesting that PSI-D is essential for a functional PSI in plants. Plants with reduced amounts of PSI-D showed a similar decrease in all other subunits of PSI including the light harvesting complex, suggesting that in the absence of PSI-D, PSI cannot be properly assembled and becomes degraded. Plants with reduced amounts of PSI-D became light-stressed even in low light although they exhibited high non-photochemical quenching (NPQ). The high NPQ was generated by upregulating the level of violaxanthin de-epoxidase and PsbS, which are both essential components of NPQ. Interestingly, the lack of PSI-D affected the redox state of thioredoxin. During the normal light cycle thioredoxin became increasingly oxidized, which was observed as decreasing malate dehydrogenase activity over a 4-h light period. This result shows that photosynthesis was close to normal the first 15 min, but after 2-4 h photoinhibition dominated as the stroma progressively became less reduced. The change in the thiol disulfide redox state might be fatal for the PSI-D-less plants, because reduction of thioredoxin is one of the main switches for the initiation of CO2 assimilation and photoprotection upon light exposure.

A role of the -35 element in the initiation of transcription at psbA promoter in tobacco plastids

Most plastid promoters recognized by bacteria-like plastid RNA polymerase (PEP) are similar to E. coli sigma(70)-type promoters comprising "-35" and "-10" elements. Among them, psbA promoter is unique in bearing additional elements between the conserved -35 and -10 elements. The psbA promoter activity is differentially maintained in the mature chloroplasts where the activity of most PEP promoters declines. Previously, we identified two types of PEP activities in wheat seedlings [Satoh et al. (1999) Plant J. 18: 407]; PEP present in the mature chloroplasts of the leaf tip (tip-type PEP) can initiate transcription from the -35-destructed psbA promoter, but the -35 element is essential for transcription by PEP present in immature chloroplasts of the leaf base (base-type PEP). To reveal which type of PEP functions in various types of plastids in tobacco, we analyzed the tobacco psbA promoter by means of a transplastomic approach. The promoter core context (-42 to +9) was sufficient for developmental regulation of the psbA promoter activity. The -35 promoter element was important for transcription initiation at the psbA promoter in all types of plastids, including chloroplasts in mature leaves, leucoplasts in roots, etioplasts in etiolated cotyledons. The conclusion is that the PEP bearing a promoter preference, similar to the wheat base-type PEP, functions dominantly in tobacco chloroplasts.

Involvement of a plastid terminal oxidase in plastoquinone oxidation as evidenced by expression of the Arabidopsis thaliana enzyme in tobacco

Chlororespiration has been defined as a respiratory electron transport chain in interaction with photosynthetic electron transport involving both non-photochemical reduction and oxidation of plastoquinones. Different enzymatic activities, including a plastid-encoded NADH dehydrogenase complex, have been reported to be involved in the non-photochemical reduction of plastoquinones. However, the enzyme responsible for plasquinol oxidation has not yet been clearly identified. In order to determine whether the newly discovered plastid oxidase (PTOX) involved in carotenoid biosynthesis acts as a plastoquinol oxidase in higher plant chloroplasts, the Arabidopsis thaliana PTOX gene (At-PTOX) was expressed in tobacco under the control of a strong constitutive promoter. We showed that At-PTOX is functional in tobacco chloroplasts and strongly accelerates the non-photochemical reoxidation of plastoquinols; this effect was inhibited by propyl gallate, a known inhibitor of PTOX. During the dark to light induction phase of photosynthesis at low irradiances, At-PTOX drives significant electron flow to O(2), thus avoiding over-reduction of plastoquinones, when photo- synthetic CO(2) assimilation was not fully induced. We proposed that PTOX, by modulating the redox state of intersystem electron carriers, may participate in the regulation of cyclic electron flow around photosystem I.

PGR5 is involved in cyclic electron flow around photosystem I and is essential for photoprotection in Arabidopsis

During photosynthesis, plants must control the utilization of light energy in order to avoid photoinhibition. We isolated an Arabidopsis mutant, pgr5 (proton gradient regulation), in which downregulation of photosystem II photochemistry in response to intense light was impaired. PGR5 encodes a novel thylakoid membrane protein that is involved in the transfer of electrons from ferredoxin to plastoquinone. This alternative electron transfer pathway, whose molecular identity has long been unclear, is known to function in vivo in cyclic electron flow around photosystem I. We propose that the PGR5 pathway contributes to the generation of a Delta(pH) that induces thermal dissipation when Calvin cycle activity is reduced. Under these conditions, the PGR5 pathway also functions to limit the overreduction of the acceptor side of photosystem I, thus preventing photosystem I photoinhibition.

In vivo interactions between photosynthesis, mitorespiration, and chlororespiration in Chlamydomonas reinhardtii

Interactions between photosynthesis, mitochondrial respiration (mitorespiration), and chlororespiration have been investigated in the green alga Chlamydomonas reinhardtii using flash illumination and a bare platinum electrode. Depending on the physiological status of algae, flash illumination was found to induce either a fast (t(1/2) approximately 300 ms) or slow (t(1/2) approximately 3 s) transient inhibition of oxygen uptake. Based on the effects of the mitorespiratory inhibitors myxothiazol and salicyl hydroxamic acid (SHAM), and of propyl gallate, an inhibitor of the chlororespiratory oxidase, we conclude that the fast transient is due to the flash-induced inhibition of chlororespiration and that the slow transient is due to the flash-induced inhibition of mitorespiration. By measuring blue-green fluorescence changes, related to the redox status of the pyridine nucleotide pool, and chlorophyll fluorescence, related to the redox status of plastoquinones (PQs) in C. reinhardtii wild type and in a photosystem I-deficient mutant, we show that interactions between photosynthesis and chlororespiration are favored when PQ and pyridine nucleotide pools are reduced, whereas interactions between photosynthesis and mitorespiration are favored at more oxidized states. We conclude that the plastid oxidase, similar to the mitochondrial alternative oxidase, becomes significantly engaged when the PQ pool becomes highly reduced, and thereby prevents its over-reduction.

Indications for chlororespiration in relation to light regime in the marine diatom Thalassiosira weissflogii

The marine diatom Thalassiosira weissflogii was cultured under a light regime simulating the daily rise and fall of the sun. The light regime caused a daily cycle in non-photochemical quenching. Remarkable were the changes in fluorescence directly after a light-to-dark transition that occurred in addition to the changes induced by non-photochemical quenching. A transient non-photochemical reduction of PQ and of Q(A) was indicated by a transient increase in apparent F(o) and by changes in the shape of the fluorescence induction curve. The observed changes developed approximately the first 100-120 s after a light-to-dark transition and could be reversed by the application of far-red illumination. Chlororespiration is thought to cause the reduction of PQ and, as the PQ-pool is in equilibrium with Q(A), also a reduction of Q(A). The function and ecological relevance of chlororespiration are discussed.

The amino acid sequence of a plastid protein is developmentally regulated by RNA editing

RNA editing in plant organelles post-transcriptionally alters single nucleotides by C-to-U or U-to-C conversions at highly specific sites. Plant editing is generally viewed as a repair mechanism acting at the transcript level by restoring conserved amino acid residues. Here we report that an editing reaction within the ndhB transcript (encoding a plastid NAD(P)H dehydrogenase subunit) is strictly dependent on active photosynthesis. Employing non-photosynthetic mutants, we show that in the absence of photosynthesis, the site remains unedited, whereas it is fully edited when the photosynthetic apparatus is intact. Moreover, the site also remains unedited during the etiolated stage of seedling development, suggesting that two different NdhB proteins are synthesized under photosynthetic versus non-photosynthetic conditions. This is the first case where RNA editing in plants appears to regulate gene expression qualitatively, resulting in the production of two different proteins from one and the same gene in a developmental stage-dependent manner.

Cyclic electron flow around photosystem I in C(3) plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex

Cyclic electron flow around photosystem (PS) I has been widely described in vitro in chloroplasts or thylakoids isolated from C(3) plant leaves, but its occurrence in vivo is still a matter of debate. Photoacoustic spectroscopy and kinetic spectrophotometry were used to analyze cyclic PS I activity in tobacco (Nicotiana tabacum cv Petit Havana) leaf discs illuminated with far-red light. Only a very weak activity was measured in air with both techniques. When leaf discs were placed in anaerobiosis, a high and rapid cyclic PS I activity was measured. The maximal energy storage in far-red light increased to 30% to 50%, and the half-time of the P(700) re-reduction in the dark decreased to around 400 ms; these values are comparable with those measured in cyanobacteria and C(4) plant leaves in aerobiosis. The stimulatory effect of anaerobiosis was mimicked by infiltrating leaves with inhibitors of mitochondrial respiration or of the chlororespiratory oxidase, therefore, showing that changes in the redox state of intersystem electron carriers tightly control the rate of PS I-driven cyclic electron flow in vivo. Measurements of energy storage at different modulation frequencies of far-red light showed that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco mutant deficient in the plastid Ndh complex was kinetically different from that of the wild type, the cycle being slower in the former leaves. We conclude that the Ndh complex is required for rapid electron cycling around PS I.

Cyclic electron flow around photosystem I in C(3) plants. In vivo control by the redox state of chloroplasts and involvement of the NADH-dehydrogenase complex

Cyclic electron flow around photosystem (PS) I has been widely described in vitro in chloroplasts or thylakoids isolated from C(3) plant leaves, but its occurrence in vivo is still a matter of debate. Photoacoustic spectroscopy and kinetic spectrophotometry were used to analyze cyclic PS I activity in tobacco (Nicotiana tabacum cv Petit Havana) leaf discs illuminated with far-red light. Only a very weak activity was measured in air with both techniques. When leaf discs were placed in anaerobiosis, a high and rapid cyclic PS I activity was measured. The maximal energy storage in far-red light increased to 30% to 50%, and the half-time of the P(700) re-reduction in the dark decreased to around 400 ms; these values are comparable with those measured in cyanobacteria and C(4) plant leaves in aerobiosis. The stimulatory effect of anaerobiosis was mimicked by infiltrating leaves with inhibitors of mitochondrial respiration or of the chlororespiratory oxidase, therefore, showing that changes in the redox state of intersystem electron carriers tightly control the rate of PS I-driven cyclic electron flow in vivo. Measurements of energy storage at different modulation frequencies of far-red light showed that anaerobiosis-induced cyclic PS I activity in leaves of a tobacco mutant deficient in the plastid Ndh complex was kinetically different from that of the wild type, the cycle being slower in the former leaves. We conclude that the Ndh complex is required for rapid electron cycling around PS I.

Mercury inhibits the non-photochemical reduction of plastoquinone by exogenous NADPH and NADH: evidence from measurements of the polyphasic chlorophyll a fluorescence rise in spinach chloroplasts

Chlorophyll a fluorescence rise kinetics (from 50 mus to 1 s) were used to investigate the non-photochemical reduction of the plastoquinone (PQ) pool in osmotically broken spinach chloroplasts (Spinacia oleracea L.). Incubation of the chloroplasts in the presence of exogenous NADPH or NADH resulted in significant changes in the shape of the fluorescence transient reflecting an NAD(P)H-dependent accumulation of reduced PQ in the dark, with an extent depending on the concentration of NAD(P)H and the availability of oxygen; the dark reduction of the PQ pool was saturated at lower NAD(P)H concentrations and reached a higher level when the incubation took place under anaerobic conditions than when it occurred under aerobic conditions. Under both conditions NADPH was more effective than NADH in reducing PQ, however only at sub-saturating concentrations. Neither antimycin A nor rotenone were found to alter the effect of NAD(P)H. The addition of mercury chloride to the chloroplast suspension decreased the NAD(P)H-dependent dark reduction of the PQ pool, with the full inhibition requiring higher mercury concentrations under anaerobic than under aerobic conditions. This is the first time that this inhibitory role of mercury is reported for higher plants. The results demonstrate that in the dark the redox state of the PQ pool is regulated by the reduction of PQ via a mercury-sensitive NAD(P)H-PQ oxidoreductase and the reoxidation of reduced PQ by an O(2)-dependent pathway, thus providing additional evidence for the existence of a chlororespiratory electron transport chain in higher plant chloroplasts.

Chlororespiration

Chlororespiration has been defined as a respiratory electron transport chain (ETC) in interaction with the photosynthetic ETC in thylakoid membranes of chloroplasts. The existence of chlororespiration has been disputed during the last decade, with the initial evidence mainly obtained with intact algal cells being possibly explained by redox interactions between chloroplasts and mitochondria. The discovery in higher-plant chloroplasts of a plastid-encoded NAD(P)H-dehydrogenase (Ndh) complex, homologous to the bacterial complex I, and of a nuclear-encoded plastid terminal oxidase (PTOX), homologous to the plant mitochondrial alternative oxidase, brought molecular support to the concept of chlororespiration. The functionality of these proteins in non-photochemical reduction and oxidation of plastoquinones (PQs), respectively, has recently been demonstrated. In thylakoids of mature chloroplasts, chlororespiration appears to be a relatively minor pathway compared to linear photosynthetic electron flow from H2O to NADP+. However, chlororespiration might play a role in the regulation of photosynthesis by modulating the activity of cyclic electron flow around photosystem I (PS I). In non-photosynthetic plastids, chlororespiratory electron carriers are more abundant and may play a significant bioenergetic role.

Photoinhibition and light-induced cyclic electron transport in ndhB(-) and psaE(-) mutants of Synechocystis sp. PCC 6803

The ndhB(-) and psaE(-) mutants of the cyanobacterium Synechocystis sp. PCC 6803 are partly deficient in PSI-driven cyclic electron transport. We compared photoinhibition in these mutants to the wild type to test the hypothesis that PSI cyclic electron transport protects against photoinhibition. Photoinhibitory treatment greatly accelerated PSI cyclic electron transport in the wild type and also in both the mutants. The psaE(-) mutant showed rates of PSI cyclic electron transport similar to the wild type under all conditions tested. The ndhB(-) mutant showed much lower rates of PSI cyclic electron transport than the wild type following brief dark adaptation but exceeded wild type rates after exposure to photoinhibitory light. The wild type and both mutants showed similar rates of photoinhibition damage and photoinhibition repair at PSII. Photoinhibition at PSI was much slower than at PSII and was also similar between the wild type and both mutants, despite the known instability of PSI in the psaE(-) mutant. We conclude that photoinhibitory light induces sufficient PSI-driven cyclic electron transport in both the ndhB(-) and psaE(-) mutants to fulfill any role that cyclic electron transport plays in protection against photoinhibition.

Increased sensitivity of photosynthesis to antimycin A induced by inactivation of the chloroplast ndhB gene. Evidence for a participation of the NADH-dehydrogenase complex to cyclic electron flow around photosystem I

Tobacco (Nicotiana tabacum var Petit Havana) ndhB-inactivated mutants (ndhB-) obtained by plastid transformation (E.M. Horvath, S.O. Peter, T. Joët, D. Rumeau, L. Cournac, G.V. Horvath, T.A. Kavanagh, C. Schäfer, G. Peltier, P. MedgyesyHorvath [2000] Plant Physiol 123: 1337-1350) were used to study the role of the NADH-dehydrogenase complex (NDH) during photosynthesis and particularly the involvement of this complex in cyclic electron flow around photosystem I (PSI). Photosynthetic activity was determined on leaf discs by measuring CO2 exchange and chlorophyll fluorescence quenchings during a dark-to-light transition. In the absence of treatment, both non-photochemical and photochemical fluorescence quenchings were similar in ndhB- and wild type (WT). When leaf discs were treated with 5 microM antimycin A, an inhibitor of cyclic electron flow around PSI, both quenchings were strongly affected. At steady state, maximum photosynthetic electron transport activity was inhibited by 20% in WT and by 50% in ndhB-. Under non-photorespiratory conditions (2% O2, 2,500 microL x L(-1) CO2), antimycin A had no effect on photosynthetic activity of WT, whereas a 30% inhibition was observed both on quantum yield of photosynthesis assayed by chlorophyll fluorescence and on CO2 assimilation in ndhB-. The effect of antimycin A on ndhB- could not be mimicked by myxothiazol, an inhibitor of the mitochondrial cytochrome bc1 complex, therefore showing that it is not related to an inhibition of the mitochondrial electron transport chain but rather to an inhibition of cyclic electron flow around PSI. We conclude to the existence of two different pathways of cyclic electron flow operating around PSI in higher plant chloroplasts. One of these pathways, sensitive to antimycin A, probably involves ferredoxin plastoquinone reductase, whereas the other involves the NDH complex. The absence of visible phenotype in ndhB- plants under normal conditions is explained by the complement of these two pathways in the supply of extra-ATP for photosynthesis.