Inactivation of the chloroplast ATP synthase gamma subunit results in high non-photochemical fluorescence quenching and altered nuclear gene expression in Arabidopsis thaliana

Inhibition of CO2 fixation by iodoacetamide stimulates cyclic electron flow and non-photochemical quenching upon far-red illumination

The Benson-Calvin cycle enzymes are activated in vivo when disulfide bonds are opened by reduction via the ferredoxin-thioredoxin system in chloroplasts. Iodoacetamide reacts irreversibly with free -SH groups of cysteine residues and inhibits the enzymes responsible for CO2 fixation. Here, we investigate the effect of iodoacetamide on electron transport, when infiltrated into spinach leaves. Using fluorescence and absorption spectroscopy, we show that (i) iodoacetamide very efficiently blocks linear electron flow upon illumination of both photosystems (decrease in the photochemical yield of photosystem II) and (ii) iodoacetamide favors cyclic electron flow upon light excitation specific to PSI. These effects account for an NPQ formation even faster in iodoacetamide under far-red illumination than in the control under saturating light. Such an increase in NPQ is dependent upon the proton gradient across the thylakoid membrane (uncoupled by nigericin addition) and PGR5 (absent in Arabidopsis pgr5 mutant). Iodoacetamide very tightly insulates the electron current at the level of the thylakoid membrane from any electron leaks toward carbon metabolism, therefore, providing choice conditions for the study of cyclic electron flow around PSI.

Tissue-specific expression of SMALL AUXIN UP RNA41 differentially regulates cell expansion and root meristem patterning in Arabidopsis

Among the three primary auxin-induced gene families, Auxin/Indole-3-Acetic Acid (Aux/IAA), Gretchen Hagen3 (GH3) and SMALL AUXIN UP RNA (SAUR), the function of SAUR genes remains unclear. Arabidopsis SAUR genes have been phylogenetically classified into three clades. Recent work has suggested that SAUR19 (clade II) and SAUR63 (clade I) promote cell expansion through the modulation of auxin transport. Herein, we present our work on SAUR41, a clade III SAUR gene with a distinctive expression pattern in root meristems. SAUR41 was normally expressed in the quiescent center and cortex/endodermis initials; upon auxin stimulation, the expression was provoked in the endodermal layer. During lateral root development, SAUR41 was expressed in prospective stem cell niches of lateral root primordia and in expanding endodermal cells surrounding the primordia. SAUR41-EGFP (enhanced green fluorescent protein) fusion proteins localized to the cytoplasm. Overexpression of SAUR41 from the Cauliflower mosaic virus 35S promoter led to pleiotropic auxin-related phenotypes, including long hypocotyls, increased vegetative biomass and lateral root development, expanded petals and twisted inflorescence stems. Ectopic SAUR41 proteins were able to promote auxin transport in hypocotyls. Tissue-specific expression of SAUR41 from the PIN1, WOX5, PLT2 and ACR4 promoters induced the formation of new auxin accumulation/signaling peaks above the quiescent centers, whereas tissue-specific expression of SAUR41 from the PIN2 and PLT2 promoters enhanced root gravitropic growth. Cells in the root stem cell niches of these transgenic seedlings were differentially enlarged. The distinctive expression pattern of the SAUR41 gene and the explicit function of SAUR41 proteins implied that further investigations on the loss-of-function phenotypes of this gene in root development and environmental responses are of great interest.

An ATP synthase harboring an atypical γ-subunit is involved in ATP synthesis in tomato fruit chromoplasts

Chromoplasts are non-photosynthetic plastids specialized in the synthesis and accumulation of carotenoids. During fruit ripening, chloroplasts differentiate into photosynthetically inactive chromoplasts in a process characterized by the degradation of the thylakoid membranes, and by the active synthesis and accumulation of carotenoids. This transition renders chromoplasts unable to photochemically synthesize ATP, and therefore these organelles need to obtain the ATP required for anabolic processes through alternative sources. It is widely accepted that the ATP used for biosynthetic processes in non-photosynthetic plastids is imported from the cytosol or is obtained through glycolysis. In this work, however, we show that isolated tomato (Solanum lycopersicum) fruit chromoplasts are able to synthesize ATP de novo through a respiratory pathway using NADPH as an electron donor. We also report the involvement of a plastidial ATP synthase harboring an atypical γ-subunit induced during ripening, which lacks the regulatory dithiol domain present in plant and algae chloroplast γ-subunits. Silencing of this atypical γ-subunit during fruit ripening impairs the capacity of isolated chromoplast to synthesize ATP de novo. We propose that the replacement of the γ-subunit present in tomato leaf and green fruit chloroplasts by the atypical γ-subunit lacking the dithiol domain during fruit ripening reflects evolutionary changes, which allow the operation of chromoplast ATP synthase under the particular physiological conditions found in this organelle.

Light- and metabolism-related regulation of the chloroplast ATP synthase has distinct mechanisms and functions

The chloroplast CF0-CF1-ATP synthase (ATP synthase) is activated in the light and inactivated in the dark by thioredoxin-mediated redox modulation of a disulfide bridge on its γ subunit. The activity of the ATP synthase is also fine-tuned during steady-state photosynthesis in response to metabolic changes, e.g. altering CO2 levels to adjust the thylakoid proton gradient and thus the regulation of light harvesting and electron transfer. The mechanism of this fine-tuning is unknown. We test here the possibility that it also involves redox modulation. We found that modifying the Arabidopsis thaliana γ subunit by mutating three highly conserved acidic amino acids, D211V, E212L, and E226L, resulted in a mutant, termed mothra, in which ATP synthase which lacked light-dark regulation had relatively small effects on maximal activity in vivo. In situ equilibrium redox titrations and thiol redox-sensitive labeling studies showed that the γ subunit disulfide/sulfhydryl couple in the modified ATP synthase has a more reducing redox potential and thus remains predominantly oxidized under physiological conditions, implying that the highly conserved acidic residues in the γ subunit influence thiol redox potential. In contrast to its altered light-dark regulation, mothra retained wild-type fine-tuning of ATP synthase activity in response to changes in ambient CO2 concentrations, indicating that the light-dark- and metabolic-related regulation occur through different mechanisms, possibly via small molecule allosteric effectors or covalent modification.

Expression of the minor isoform pea ferredoxin in tobacco alters photosynthetic electron partitioning and enhances cyclic electron flow

Ferredoxins (Fds) are ferrosulfoproteins that function as low-potential electron carriers in plants. The Fd family is composed of several isoforms that share high sequence homology but differ in functional characteristics. In leaves, at least two isoforms conduct linear and cyclic photosynthetic electron transport around photosystem I, and mounting evidence suggests the existence of at least partial division of duties between these isoforms. To evaluate the contribution of different kinds of Fds to the control of electron fluxes along the photosynthetic electron transport chain, we overexpressed a minor pea (Pisum sativum) Fd isoform (PsFd1) in tobacco (Nicotiana tabacum) plants. The transplastomic OeFd1 plants exhibited variegated leaves and retarded growth and developmental rates. Photosynthetic studies of these plants indicated a reduction in carbon dioxide assimilation rates, photosystem II photochemistry, and linear electron flow. However, the plants showed an increase in nonphotochemical quenching, better control of excitation pressure at photosystem II, and no evidence of photoinhibition, implying a better dynamic regulation to remove excess energy from the photosynthetic electron transport chain. Finally, analysis of P700 redox status during illumination confirmed that the minor pea Fd isoform promotes enhanced cyclic flow around photosystem I. The two novel features of this work are: (1) that Fd levels achieved in transplastomic plants promote an alternative electron partitioning even under greenhouse light growth conditions, a situation that is exacerbated at higher light intensity measurements; and (2) that an alternative, minor Fd isoform has been overexpressed in plants, giving new evidence of labor division among Fd isoforms.

Contrasting patterns of nucleotide diversity for four conifers of Alpine European forests

A candidate gene approach was used to identify levels of nucleotide diversity and to identify genes departing from neutral expectations in coniferous species of the Alpine European forest. Twelve samples were collected from four species that dominate montane and subalpine forests throughout Europe: Abies alba Mill, Larix decidua Mill, Pinus cembra L., and Pinus mugo Turra. A total of 800 genes, originally resequenced in Pinus taeda L., were resequenced across 12 independent trees for each of the four species. Genes were assigned to two categories, candidate and control, defined through homology-based searches to Arabidopsis. Estimates of nucleotide diversity per site varied greatly between polymorphic candidate genes (range: 0.0004–0.1295) and among species (range: 0.0024–0.0082), but were within the previously established ranges for conifers. Tests of neutrality using stringent significance thresholds, performed under the standard neutral model, revealed one to seven outlier loci for each species. Some of these outliers encode proteins that are involved with plant stress responses and form the basis for further evolutionary enquiries.

Molecular crosstalk between PAMP-triggered immunity and photosynthesis

The innate immune system allows plants to respond to potential pathogens in an appropriate manner while minimizing damage and energy costs. Photosynthesis provides a sustained energy supply and, therefore, has to be integrated into the defense against pathogens. Although changes in photosynthetic activity during infection have been described, a detailed and conclusive characterization is lacking. Here, we addressed whether activation of early defense responses by pathogen-associated molecular patterns (PAMPs) triggers changes in photosynthesis. Using proteomics and chlorophyll fluorescence measurements, we show that activation of defense by PAMPs leads to a rapid decrease in nonphotochemical quenching (NPQ). Conversely, NPQ also influences several responses of PAMP-triggered immunity. In a mutant impaired in NPQ, apoplastic reactive oxygen species production is enhanced and defense gene expression is differentially affected. Although induction of the early defense markers WRKY22 and WRKY29 is enhanced, induction of the late markers PR1 and PR5 is completely abolished. We propose that regulation of NPQ is an intrinsic component of the plant's defense program.

Transcriptional organization of the large and the small ATP synthase operons, atpI/H/F/A and atpB/E, in Arabidopsis thaliana chloroplasts

The ATP synthase is a ubiquitous enzyme which is found in bacteria and eukaryotic organelles. It is essential in the photosynthetic and respiratory processes, by transforming the electrochemical proton gradient into ATP energy via proton transport across the membranes. In Escherichia coli, the atp genes coding for the subunits of the ATP synthase enzyme are grouped in the same transcriptional unit, while in higher plants the plastid atp genes are organized into a large (atpI/H/F/A) and a small (atpB/E) atp operon. By using the model plant Arabidopsis thaliana, we have investigated the strategy evolved in chloroplasts to overcome the physical separation of the atp gene clusters and to coordinate their transcription. We show that all the identified promoters in the two atp operons are PEP dependent and require sigma factors for specific recognition. Our results indicate that transcription of the two atp operons is initiated by at least one common factor, the essential SIG2 factor. Our data show that SIG3 and SIG6 also participate in transcription initiation of the large and the small atp operon, respectively. We propose that SIG2 might be the factor responsible for coordinating the basal transcription of the plastid atp genes and that SIG3 and SIG6 might serve to modulate plastid atp expression with respect to physiological and environmental conditions. However, we observe that in the sigma mutants (sig2, sig3 and sig6) the deficiency in the recognition of specific atp promoters is largely balanced by mRNA stabilization and/or by activation of otherwise silent promoters, indicating that the rate-limiting step for expression of the atp operons is mostly post-transcriptional.

Thioredoxin-insensitive plastid ATP synthase that performs moonlighting functions

The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and acts as a key feedback regulatory component of photosynthesis. Arabidopsis possesses two homologues of the regulatory γ subunit of the ATP synthase, encoded by the ATPC1 and ATPC2 genes. Using a series of mutants, we show that both these subunits can support photosynthetic ATP synthesis in vivo with similar specific activities, but that in wild-type plants, only γ(1) is involved in ATP synthesis in photosynthesis. The γ(1)-containing ATP synthase shows classical light-induced redox regulation, whereas the mutant expressing only γ(2)-ATP synthase (gamma exchange-revised ATP synthase, gamera) shows equally high ATP synthase activity in the light and dark. In situ redox titrations demonstrate that the regulatory thiol groups on γ(2)-ATP synthase remain reduced under physiological conditions but can be oxidized by the strong oxidant diamide, implying that the redox potential for the thiol/disulphide transition in γ(2) is substantially higher than that for γ(1). This regulatory difference may be attributed to alterations in the residues near the redox-active thiols. We propose that γ(2)-ATP synthase functions to catalyze ATP hydrolysis-driven proton translocation in nonphotosynthetic plastids, maintaining a sufficient transthylakoid proton gradient to drive protein translocation or other processes. Consistent with this interpretation, ATPC2 is predominantly expressed in the root, whereas modifying its expression results in alteration of root hair development. Phylogenetic analysis suggests that γ(2) originated from ancient gene duplication, resulting in divergent evolution of functionally distinct ATP synthase complexes in dicots and mosses.

A mutation of OSOTP 51 leads to impairment of photosystem I complex assembly and serious photo-damage in rice

Gene expression in chloroplasts is regulated by many nuclear-encoded proteins. In this study, we isolated a rice (Oryza sativa subsp. japonica) mutant osotp51 with significant reduction in photosystem I (PSI). The osotp51 is extremely sensitive to light and accumulates a higher level of reactive oxygen species. Its leaves are almost albino when grown at 40 μmol photons/m(2) per s. However, grown at 4 μmol photons/m(2) per s, osotp51 has a similar phenotype to the wild-type. 77K chlorophyll fluorescence analysis showed a blue shift in the highest peak emission from PSI in osotp51. In addition, the level of PSI and PSII dimer is dramatically reduced in osotp51. OSOTP 51 encodes a pentatricopeptide repeats protein, homologous to organelle transcript processing 51 in Arabidopsis. Loss-of-function OSOTP51 affects intron splicing of a number of plastid genes, particularly the ycf3 coding a protein involved in the assembly of PSI complex. OSOTP51 is functionally conserved in higher plants. The mutation of osotp51 indirectly leads to a widespread change in the structure and functions of PSI, results in severe photoinhibition, and finally dies, even when grown under very low light intensity.

The cytosolic kinases STY8, STY17, and STY46 are involved in chloroplast differentiation in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), transit peptides for chloroplast-destined preproteins can be phosphorylated by the protein kinases STY8, STY17, and STY46. In this study, we have investigated the in vitro properties of these plant-specific kinases. Characterization of the mechanistic functioning of STY8 led to the identification of an essential threonine in the activation segment, which is phosphorylated by an intramolecular mechanism. STY8 is inhibited by specific tyrosine kinase inhibitors, although it lacked the ability to phosphorylate tyrosine residues in vitro. In vivo analysis of sty8, sty17, and sty46 Arabidopsis knockout/knockdown mutants revealed a distinct function of the three kinases in the greening process and in the efficient differentiation of chloroplasts. Mutant plants displayed not only a delayed accumulation of chlorophyll but also a reduction of nucleus-encoded chloroplast proteins and a retarded establishment of photosynthetic capacity during the first 6 h of deetiolation, supporting a role of cytosolic STY kinases in chloroplast differentiation.

Solute transporters in plant thylakoid membranes: Key players during photosynthesis and light stress

Plants utilize sunlight to drive photosynthetic energy conversion in the chloroplast thylakoid membrane. Here are located four major photosynthetic complexes, about which we have great knowledge in terms of structure and function. However, much less we know about auxiliary proteins, such as transporters, ensuring an optimum function and turnover of these complexes. The most prominent thylakoid transporter is the proton-translocating ATP-synthase. Recently, four additional transporters have been identified in the thylakoid membrane of Arabidopsis thaliana, namely one copper-transporting P-ATPase, one chloride channel, one phosphate transporter, and one ATP/ADP carrier. Here, we review the current knowledge on the function and physiological role of these transporters during photosynthesis and light stress in plants. Subsequently, we make a survey on the outlook of thylakoid activities awaiting identification of responsible proteins. Such knowledge is necessary to understand the thylakoid network of transporters, and to design strategies for bioengineering crop plants in the future.

FdC1, a novel ferredoxin protein capable of alternative electron partitioning, increases in conditions of acceptor limitation at photosystem I

In higher plants, [2Fe-2S] ferredoxin (Fd) proteins are the unique electron acceptors from photosystem I (PSI). Fds are soluble, and distribute electrons to many enzymes, including Fd:NADP(H) reductase (FNR), for the photoreduction of NADP(+). In addition to well studied [2Fe-2S] Fd proteins, higher plants also possess genes for significantly different, as yet uncharacterized Fd proteins, with extended C termini (FdCs). Whether these FdC proteins function as photosynthetic electron transfer proteins is not known. We examined whether these proteins play a role as alternative electron acceptors at PSI, using quantitative RT-PCR to follow how their expression changes in response to acceptor limitation at PSI, in mutant Arabidopsis plants lacking 90-95% of photosynthetic [2Fe-2S] Fd. Expression of the gene encoding one FdC protein, FdC1, was identified as being strongly up-regulated. We confirmed that this protein was chloroplast localized and increased in abundance on PSI acceptor limitation. We purified the recombinant FdC1 protein, which exhibited a UV-visible spectrum consistent with a [2Fe-2S] cluster, confirmed by EPR analysis. Measurements of electron transfer show that FdC1 is capable of accepting electrons from PSI, but cannot support photoreduction of NADP(+). Whereas FdC1 was capable of electron transfer with FNR, redox potentiometry showed that it had a more positive redox potential than photosynthetic Fds by around 220 mV. These results indicate that FdC1 electron donation to FNR is prevented because it is thermodynamically unfavorable. Based on our data, we speculate that FdC1 has a specific function in conditions of acceptor limitation at PSI, and channels electrons away from NADP(+) photoreduction.

Radical-induced cell death1 and similar to RCD one1 and the stress-induced morphogenetic response

Plants exposed to abiotic stress show a range of morphogenetic responses, sometimes termed the stress-induced morphogenetic response (SIMR). SIMR is principally composed of three components: inhibition of cell elongation, alterations in cell differentiation, and stimulus of cell division in localized areas. An explanation proposed for SIMR has been increased accumulation of reactive oxygen species (ROS) and alterations in hormone signaling. Mutations in the Arabidopsis thaliana RADICAL-INDUCED CELL DEATH1 (RCD1) gene have altered abiotic stress responses and ROS accumulation. Even in the absence of exogenous stress, these plants show many morphological changes also seen in SIMR. In the September issue of Plant Physiology we reported an in depth analysis of the phenotype of rcd1-3 plants as well as the phenotype of a mutations in the previously uncharacterized paralog of RCD1, SIMILAR TO RCD ONE1 (SRO1). sro1-1 plants have mild morphological changes and abiotic stress response defects while rcd1-3; sro1-1 double mutant plants have severe developmental defects, including less cell elongation. In this Addendum, we hypothesize that rcd1, sro1 and rcd1; sro1 mutant plants are under constitutive stress, and that this stress is responsible for at least some of the developmental defects seen in these plants.

Modulation of O-mediated retrograde signaling by the PLEIOTROPIC RESPONSE LOCUS 1 (PRL1) protein, a central integrator of stress and energy signaling

Shortly after the release of singlet oxygen ((1)O(2)) in chloroplasts, changes in nuclear gene expression occur in the conditional flu mutant of Arabidopsis that reveal a rapid transfer of signals from the plastid to the nucleus. Extensive genetic screens aimed at identifying constituents involved in (1)O(2)-mediated plastid-to-nucleus signaling have failed to identify extraplastidic signaling components. This finding suggests that (1)O(2)-mediated signals are not translocated to the nucleus via a single linear pathway, but rather through a signaling network that is difficult to block by single mutations. The complexity of this signaling network has been tackled by mutagenizing a transgenic flu line expressing the luciferase reporter gene under the control of the promoter of a (1)O(2)-responsive AAA-ATPase gene (At3g28580) and isolating second site mutants that constitutively express the reporter gene at a high level. One of the mutants was shown by map-based cloning and sequencing to contain a single amino acid change in the PLEIOTROPIC RESPONSE LOCUS 1 (PRL1) protein. PRL1 suppresses the expression of AAA-ATPase and other (1)O(2)-responsive genes. PRL1 seems to play a major role in modulating responses of plants to environmental changes by interconnecting (1)O(2)-mediated retrograde signaling with other signaling pathways.

Subunits of the plastid ClpPR protease complex have differential contributions to embryogenesis, plastid biogenesis, and plant development in Arabidopsis

The plastid ClpPR protease complex in Arabidopsis thaliana consists of five catalytic ClpP and four noncatalytic ClpR subunits. An extensive analysis of the CLPR family and CLPP5 is presented to address this complexity. Null alleles for CLPR2 and CLPR4 showed delayed embryogenesis and albino embryos, with seedling development blocked in the cotyledon stage; this developmental block was overcome under heterotrophic conditions, and seedlings developed into small albino to virescent seedlings. By contrast, null alleles for CLPP5 were embryo lethal. Thus, the ClpPR proteins make different functional contributions. To further test for redundancies and functional differences between the ClpR proteins, we overexpressed full-length cDNAs for ClpR1, R2, R3, R4 in clpr1, clpr2 and clpr4 mutants. This showed that overexpression of ClpR3 can complement for the loss of ClpR1, but not for the loss of ClpR2 or ClpR4, indicating that ClpR3 can functionally substitute ClpR1. By contrast, ClpR1, R2 and R4 could not substitute each other. Double mutants of weak CLPR1 and 2 alleles were seedling lethal, showing that a minimum concentration of different ClpR proteins is essential for Clp function. Microscopy and large-scale comparative leaf proteome analyses of a CLPR4 null allele demonstrate a central role of Clp protease in chloroplast biogenesis and protein homeostasis; substrates are discussed. Lack of transcriptional and translational feedback regulation within the CLPPR gene family indicates that regulation of Clp activity occurs through Clp complex assembly and substrate delivery.

Causes of decreased photosynthetic rate and metabolic capacity in water-deficient leaf cells: a critical evaluation of mechanisms and integration of processes

BACKGROUND

Water deficit (WD) decreases photosynthetic rate (A) via decreased stomatal conductance to CO(2) (g(s)) and photosynthetic metabolic potential (A(pot)). The relative importance of g(s) and A(pot), and how they are affected by WD, are reviewed with respect to light intensity and to experimental approaches.

SCOPE AND CONCLUSIONS

With progressive WD, A decreases as g(s) falls. Under low light during growth and WD, A is stimulated by elevated CO(2), showing that metabolism (A(pot)) is not impaired, but at high light A is not stimulated, showing inhibition. At a given intercellular CO(2) concentration (C(i)) A decreases, showing impaired metabolism (A(pot)). The C(i) and probably chloroplast CO(2) concentration (C(c)), decreases and then increases, together with the equilibrium CO(2) concentration, with greater WD. Estimation of C(c) and internal (mesophyll) conductance (g(i)) is considered uncertain. Photosystem activity is unaffected until very severe WD, maintaining electron (e(-)) transport (ET) and reductant content. Low A, together with photorespiration (PR), which is maintained or decreased, provides a smaller sink for e(-)(,) causing over-energization of energy transduction. Despite increased non-photochemical quenching (NPQ), excess energy and e(-) result in generation of reactive oxygen species (ROS). Evidence is considered that ROS damages ATP synthase so that ATP content decreases progressively with WD. Decreased ATP limits RuBP production by the Calvin cycle and thus A(pot). Rubisco activity is unlikely to determine A(pot). Sucrose synthesis is limited by lack of substrate and impaired enzyme regulation. With WD, PR decreases relative to light respiration (R(L)), and mitochondria consume reductant and synthesise ATP. With progressing WD at low A, R(L) increases C(i) and C(c). This review emphasises the effects of light intensity, considers techniques, and develops a qualitative model of photosynthetic metabolism under WD that explains many observations: testable hypotheses are suggested.

Functions of chloroplastic adenylate kinases in Arabidopsis

Adenosine monophosphate kinase (AMK; adenylate kinase) catalyses the reversible formation of ADP by the transfer of one phosphate group from ATP to AMP, thus equilibrating adenylates. The Arabidopsis (Arabidopsis thaliana) genome contains 10 genes with an adenylate/cytidylate kinase signature; seven of these are identified as putative adenylate kinases. Encoded proteins of at least two members of this Arabidopsis adenylate kinase gene family are targeted to plastids. However, when the individual genes are disrupted, the phenotypes of both mutants are strikingly different. Although absence of AMK2 causes only 30% reduction of total adenylate kinase activity in leaves, there is loss of chloroplast integrity leading to small, pale-looking plantlets from embryo to seedling development. In contrast, no phenotype for disruption of the second plastid adenylate kinase was found. From this analysis, we conclude that AMK2 is the major activity for equilibration of adenylates and de novo synthesis of ADP in the plastid stroma.

Intertwined translational regulations set uneven stoichiometry of chloroplast ATP synthase subunits

The (C)F1 sector from H(+)-ATP synthases comprises five subunits: alpha, beta, gamma, delta and epsilon, assembled in a 3:3:1:1:1 stoichiometry. Here, we describe the molecular mechanism ensuring this unique stoichiometry, required for the functional assembly of the chloroplast enzyme. It relies on a translational feedback loop operating in two steps along the assembly pathway of CF1. In Chlamydomonas, production of the nucleus-encoded subunit gamma is required for sustained translation of the chloroplast-encoded subunit beta, which in turn stimulates the expression of the chloroplast-encoded subunit alpha. Translational downregulation of subunits beta or alpha, when not assembled, is born by the 5'UTRs of their own mRNAs, pointing to a regulation of translation initiation. We show that subunit gamma, by assembling with alpha(3)beta(3) hexamers, releases a negative feedback exerted by alpha/beta assembly intermediates on translation of subunit beta. Moreover, translation of subunit alpha is transactivated by subunit beta, an observation unprecedented in the biogenesis of organelle proteins.

Singlet oxygen affects the activity of the thylakoid ATP synthase and has a strong impact on its gamma subunit

Singlet oxygen is reported to have the most potent damaging effect upon the photosynthetic machinery. Usually this reactive oxygen molecule acts in concert with other ROS types under stressful conditions. To understand the specific role of singlet oxygen we took advantage of the conditional flu mutant of Arabidopsis thaliana. In flu, the negative feedback loop is abolished, which blocks chlorophyll biosynthesis in the dark. Therefore high amounts of free protochlorophyllide accumulate during darkness. If flu gets subsequently illuminated, free protochlorophyllide acts as a photosensitiser leading almost exclusively to high amounts of (1)O2. Analysing the thylakoid protein pattern by using 2D PAGE and subsequent MALDI-TOF analysis, we could show, in addition to previous described effects on photosystem II, that singlet oxygen has a massive impact on the thylakoid ATP synthase, especially on its gamma subunit. Additionally, it could be shown that the activity of the ATP synthase is reduced upon singlet oxygen exposure and that the rate of non-photochemical quenching is affected in flu mutants exposed to (1)O2.

Redox regulation and antioxidative defence in Arabidopsis leaves viewed from a systems biology perspective

Redox regulation is a central control element in cell metabolism. It is employed to adjust photosynthesis and the antioxidant defence system of leaves to the prevailing environment. During recent years progress has been made in describing the redox-dependent alterations in metabolism, the thiol/disulfide proteome, the redox-dependent and cross-talking signalling pathways and the target genes of redox regulation. Some transcription factors have been identified as proteins that perform thiol/disulfide transitions linked to the redox-regulation of specific plant promoters. In addition first mathematical models have been designed to simulate antioxidant defence and predict its response. Taken together, a profound experimental data set has been generated which allows to approach a systems biology type of understanding of antioxidant defence in photosynthesising cells in the near future. Since oxidative stress is likely to limit plant growth under stress, such a systematic understanding of antioxidant defence will help to define novel targets for breeding stress-tolerant plants.

Nfu2: a scaffold protein required for [4Fe-4S] and ferredoxin iron-sulphur cluster assembly in Arabidopsis chloroplasts

Nfu proteins are candidates to act as scaffold protein in vivo for iron-sulphur cluster biogenesis. In this work, Nfu2 protein function in the chloroplast was investigated in vivo using T-DNA insertion lines disrupted in AtNfu2 gene. Both alleles characterized presented the same dwarf phenotype due to photosynthetic and metabolic limitations. Nfu2 cDNA expression in nfu2.1 mutant rescued this phenotype. Photosynthesis study of these mutants revealed an altered photosystem I (PSI) activity together with a decrease in PSI amount confirmed by immunodetection experiments, and leading to an over reduction of the plastoquinol pool. Decrease of plastid 4Fe-4S sulphite reductase activity correlates with PSI amount decrease and supports an alteration of 4Fe-4S cluster biogenesis in nfu2 chloroplasts. The decrease of electron flow from the PSI is combined with a decrease in ferredoxin amount in nfu2 mutants. Our results are therefore in favour of a requirement of Nfu2 protein for 4Fe-4S and 2Fe-2S ferredoxin cluster assembly, conferring to this protein an important function for plant growth and photosynthesis as demonstrated by nfu2 mutant phenotype. As glutamate synthase and Rieske Fe-S proteins are not affected in nfu2 mutants, these data indicate that different pathways are involved in Fe-S biogenesis in Arabidopsis chloroplasts.

The nuclear factor HCF145 affects chloroplast psaA-psaB-rps14 transcript abundance in Arabidopsis thaliana

The high chlorophyll fluorescence (hcf)145 mutant of Arabidopsis thaliana is specifically affected in photosystem (PS)I function as judged from spectroscopic analysis of PSII and PSI activity. The defect is because of a severe deficiency of PSI core subunits, whereas levels of the four outer antenna subunits of PSI were less reduced in hcf145. Pulse labelling of chloroplast proteins indicated that synthesis of the two largest PSI reaction-centre polypeptides, Psa (photosystem I subunit) A and PsaB, is significantly affected by the mutation. A comparison of stationary transcript levels with rates of transcription demonstrates that hcf145 induces a decreased stability and, probably, transcription of the tricistronic psaA-psaB-rps (small-subunit ribosomal protein)14 mRNA, which is generated by the plastid-encoded RNA polymerase. Translation inhibition experiments excluded translational defects as primary cause of impaired mRNA stability. Larger primary transcripts, which also contain sequences of the ycf3 (hypothetical chloroplast reading frame) gene located upstream of the psaA-psaB-rps14 operon and generated by the action of the nuclear-encoded RNA polymerase, are not targeted by the mutation. Real-time reverse transcription (RT)-PCR analysis has successfully been applied to quantify defined intervals of the tricistronic transcript and it was established that the psaA region is less stable than the rps14 region in hcf145. The hcf145 gene has been mapped on the upper part of chromosome 5.