Oxidative folding in chloroplasts

Redox regulation in chloroplast thylakoid lumen: The pmf changes everything, again

Photosynthesis is the foundation of life on Earth. However, if not well regulated, it can also generate excessive reactive oxygen species (ROS), which can cause photodamage. Regulation of photosynthesis is highly dynamic, responding to both environmental and metabolic cues, and occurs at many levels, from light capture to energy storage and metabolic processes. One general mechanism of regulation involves the reversible oxidation and reduction of protein thiol groups, which can affect the activity of enzymes and the stability of proteins. Such redox regulation has been well studied in stromal enzymes, but more recently, evidence has emerged of redox control of thylakoid lumenal enzymes. This review/hypothesis paper summarizes the latest research and discusses several open questions and challenges to achieving effective redox control in the lumen, focusing on the distinct environments and regulatory components of the thylakoid lumen, including the need to transport electrons across the thylakoid membrane, the effects of pH changes by the proton motive force (pmf) in the stromal and lumenal compartments, and the observed differences in redox states. These constraints suggest that activated oxygen species are likely to be major regulatory contributors to lumenal thiol redox regulation, with key components and processes yet to be discovered.

The thiol-disulfide exchange activity of AtPDI1 is involved in the response to abiotic stresses

BACKGROUND

Arabidopsis protein disulfide isomerase 1 (AtPDI1) has been demonstrated to have disulfide isomerase activity and to be involved in the stress response. However, whether the anti-stress function is directly related to the activities of thiol-disulfide exchange remains to be elucidated.

RESULTS

In the present study, encoding sequences of AtPDI1 of wild-type (WT) and double-cysteine-mutants were transformed into an AtPDI1 knockdown Arabidopsis line (pdi), and homozygous transgenic plants named pdi-AtPDI1, pdi-AtPDI1_m1 and pdi-AtPDI1_m2 were obtained. Compared with the WT and pdi-AtPDI1, the respective germination ratios of pdi-AtPDI1_m1 and pdi-AtPDI1_m2 were significantly lower under abiotic stresses and exogenous ABA treatment, whereas the highest germination rate was obtained with AtPDI1 overexpression in the WT (WT- AtPDI1). The root length among different lines was consistent with the germination rate; a higher germination rate was observed with a longer root length. When seedlings were treated with salt, drought, cold and high temperature stresses, pdi-AtPDI1_m1, pdi-AtPDI1_m2 and pdi displayed lower survival rates than WT and AtPDI1 overexpression plants. The transcriptional levels of ABA-responsive genes and genes encoding ROS-quenching enzymes were lower in pdi-AtPDI1_m1 and pdi-AtPDI1_m2 than in pdi-AtPDI1.

CONCLUSION

Taken together, these results clearly suggest that the anti-stress function of AtPDI1 is directly related to the activity of disulfide isomerase.

Functional redox links between lumen thiol oxidoreductase1 and serine/threonine-protein kinase STN7

In response to changing light quantity and quality, photosynthetic organisms perform state transitions, a process which optimizes photosynthetic yield and mitigates photo-damage. The serine/threonine-protein kinase STN7 phosphorylates the light-harvesting complex of photosystem II (PSII; light-harvesting complex II), which then migrates from PSII to photosystem I (PSI), thereby rebalancing the light excitation energy between the photosystems and restoring the redox poise of the photosynthetic electron transport chain. Two conserved cysteines forming intra- or intermolecular disulfide bonds in the lumenal domain (LD) of STN7 are essential for the kinase activity although it is still unknown how activation of the kinase is regulated. In this study, we show lumen thiol oxidoreductase 1 (LTO1) is co-expressed with STN7 in Arabidopsis (Arabidopsis thaliana) and interacts with the LD of STN7 in vitro and in vivo. LTO1 contains thioredoxin (TRX)-like and vitamin K epoxide reductase domains which are related to the disulfide-bond formation system in bacteria. We further show that the TRX-like domain of LTO1 is able to oxidize the conserved lumenal cysteines of STN7 in vitro. In addition, loss of LTO1 affects the kinase activity of STN7 in Arabidopsis. Based on these results, we propose that LTO1 helps to maintain STN7 in an oxidized active state in state 2 through redox interactions between the lumenal cysteines of STN7 and LTO1.

Redox Homeostasis in Photosynthetic Organisms: Novel and Established Thiol-Based Molecular Mechanisms

Significance: Redox homeostasis consists of an intricate network of reactions in which reactive molecular species, redox modifications, and redox proteins act in concert to allow both physiological responses and adaptation to stress conditions. Recent Advances: This review highlights established and novel thiol-based regulatory pathways underlying the functional facets and significance of redox biology in photosynthetic organisms. In the last decades, the field of redox regulation has largely expanded and this work is aimed at giving the right credit to the importance of thiol-based regulatory and signaling mechanisms in plants. Critical Issues: This cannot be all-encompassing, but is intended to provide a comprehensive overview on the structural/molecular mechanisms governing the most relevant thiol switching modifications with emphasis on the large genetic and functional diversity of redox controllers (i.e., redoxins). We also summarize the different proteomic-based approaches aimed at investigating the dynamics of redox modifications and the recent evidence that extends the possibility to monitor the cellular redox state in vivo. The physiological relevance of redox transitions is discussed based on reverse genetic studies confirming the importance of redox homeostasis in plant growth, development, and stress responses. Future Directions: In conclusion, we can firmly assume that redox biology has acquired an established significance that virtually infiltrates all aspects of plant physiology.

Dynamic pH-induced conformational changes of the PsbO protein in the fluctuating acidity of the thylakoid lumen

The PsbO protein is an essential extrinsic subunit of photosystem II, the pigment-protein complex responsible for light-driven water splitting. Water oxidation in photosystem II supplies electrons to the photosynthetic electron transfer chain and is accompanied by proton release and oxygen evolution. While the electron transfer steps in this process are well defined and characterized, the driving forces acting on the liberated protons, their dynamics and their destiny are all largely unknown. It was suggested that PsbO undergoes proton-induced conformational changes and forms hydrogen bond networks that ensure prompt proton removal from the catalytic site of water oxidation, i.e. the Mn₄ CaO₅ cluster. This work reports the purification and characterization of heterologously expressed PsbO from green algae Chlamydomonas reinhardtii and two isoforms from the higher plant Solanum tuberosum (PsbO1 and PsbO2). A comparison to the spinach PsbO reveals striking similarities in intrinsic protein fluorescence and CD spectra, reflecting the near-identical secondary structure of the proteins from algae and higher plants. Titration experiments using the hydrophobic fluorescence probe ANS revealed that eukaryotic PsbO proteins exhibit acid-base hysteresis. This hysteresis is a dynamic effect accompanied by changes in the accessibility of the protein's hydrophobic core and is not due to reversible oligomerization or unfolding of the PsbO protein. These results confirm the hypothesis that pH-dependent dynamic behavior at physiological pH ranges is a common feature of PsbO proteins and causes reversible opening and closing of their β-barrel domain in response to the fluctuating acidity of the thylakoid lumen.

Identification and Functional Analysis of a Protein Disulfide Isomerase (AtPDI1) in Arabidopsis thaliana

Protein disulfide isomerase (PDI) catalyzes the conversion of thiol-disulfide and plays an important role in various physiological events in animals. A PDI (OaPDI) from a tropical plant was detailed studied and it was found to be involved in response of biotic stress (Gruber et al., 2007). However, the activities of PDI related to physiological functions in plants are poorly understood. In the present study, a homolog of human PDI in Arabidopsis (AtPDI1), encoded by the gene (At3g54960), was characterized. The recombinant AtPDI1 protein had disulfide isomerase activity in vitro and two pairs of conservative cysteines in catalytic domains play a crucial role in the PDI activities. Expression of AtPDI1 in Escherichia coli significantly enhanced stress tolerance of cells and the mutations of critical cysteines almost lose this function. In plants, AtPDI1 was strongly induced by abiotic stresses and exogenous abscisic acid. An ArabidopsisAtPDI1 knockdown mutant (pdi1) and overexpression lines of transgenic plants obtained by this investigation were used to further examine the function of AtPDI1. The mutant line was more sensitive to stresses than the wild-type, while overexpressing AtPDI1 increased tolerance of seedlings to abiotic stresses, with a higher germination ratio and longer length of roots than the wild-type. Our results suggested AtPDI1 played roles in anti-stresses in Arabidopsis, which relate to the activities of PDI.

Redox regulation at the site of primary growth: auxin, cytokinin and ROS crosstalk

To maintain the activity of meristems is an absolute requirement for plant growth and development, and the role of the plant hormones auxin and cytokinin in apical meristem function is well established. Only little attention has been given, however, to the function of the reactive oxygen species (ROS) gradient along meristematic tissues and its interplay with hormonal regulatory networks. The interdependency between auxin-related, cytokinin-related and ROS-related circuits controls primary growth and development while modulating plant morphology in response to detrimental environmental factors. Because ROS interaction with redox-active compounds significantly affects the cellular redox gradient, the latter constitutes an interface for crosstalk between hormone and ROS signalling pathways. This review focuses on the mechanisms underlying ROS-dependent interactions with redox and hormonal components in shoot and root apical meristems which are crucial for meristems maintenance when plants are exposed to environmental hardships. We also emphasize the importance of cell type and the subcellular compartmentalization of ROS and redox networks to obtain a holistic understanding of how apical meristems adapt to stress.

A novel mechanism of functional cooperativity regulation by thiol redox status in a dimeric inorganic pyrophosphatase

BACKGROUND

Inorganic PPases are essential metal-dependent enzymes that convert pyrophosphate into orthophosphate. This reaction is quite exergonic and provides a thermodynamic advantage for many ATP-driven biosynthetic reactions. We have previously demonstrated that cytosolic PPase from R. microplus embryos is an atypical Family I PPase. Here, we explored the functional role of the cysteine residues located at the homodimer interface, its redox sensitivity, as well as structural and kinetic parameters related to thiol redox status.

METHODS

In this work, we used prokaryotic expression system for recombinant protein overexpression, biochemical approaches to assess kinetic parameters, ticks embryos and computational approaches to analyze and predict critical amino acids as well as physicochemical properties at the homodimer interface.

RESULTS

Cysteine 339, located at the homodimer interface, was found to play an important role in stabilizing a functional cooperativity between the two catalytic sites, as indicated by kinetics and Hill coefficient analyses of the WT-rBmPPase. WT-rBmPPase activity was up-regulated by physiological antioxidant molecules such as reduced glutathione and ascorbic acid. On the other hand, hydrogen peroxide at physiological concentrations decreased the affinity of WT-rBmPPase for its substrate (PP_i), probably by inducing disulfide bridge formation.

CONCLUSIONS

Our results provide a new angle in understanding redox control by disulfide bonds formation in enzymes from hematophagous arthropods. The reversibility of the down-regulation is dependent on hydrophobic interactions at the dimer interface.

GENERAL SIGNIFICANCE

This study is the first report on a soluble PPase where dimeric cooperativity is regulated by a redox mechanism, according to cysteine redox status.

Redox regulation in the thylakoid lumen

Higher plants need to balance the efficiency of light energy absorption and dissipative photo-protection when exposed to fluctuations in light quantity and quality. This aim is partially realized through redox regulation within the chloroplast, which occurs in all chloroplast compartments except the envelope intermembrane space. In contrast to the chloroplast stroma, less attention has been paid to the thylakoid lumen, an inner, continuous space enclosed by the thylakoid membrane in which redox regulation is also essential for photosystem biogenesis and function. This sub-organelle compartment contains at least 80 lumenal proteins, more than 30 of which are known to contain disulfide bonds. Thioredoxins (Trx) in the chloroplast stroma are photo-reduced in the light, transferring reducing power to the proteins in the thylakoid membrane and ultimately the lumen through a trans-thylakoid membrane-reduced, equivalent pathway. The discovery of lumenal thiol oxidoreductase highlights the importance of the redox regulation network in the lumen for controlling disulfide bond formation, which is responsible for protein activity and folding and even plays a role in photo-protection. In addition, many lumenal members involved in photosystem assembly and non-photochemical quenching are likely required for reduction and/or oxidation to maintain their proper efficiency upon changes in light intensity. In light of recent findings, this review summarizes the multiple redox processes that occur in the thylakoid lumen in great detail, highlighting the essential auxiliary roles of lumenal proteins under fluctuating light conditions.

Dual Protein Localization to the Envelope and Thylakoid Membranes Within the Chloroplast

The chloroplast houses various metabolic processes essential for plant viability. This organelle originated from an ancestral cyanobacterium via endosymbiosis and maintains the three membranes of its progenitor. Among them, the outer envelope membrane functions mainly in communication with cytoplasmic components while the inner envelope membrane houses selective transport of various metabolites and the biosynthesis of several compounds, including membrane lipids. These two envelope membranes also play essential roles in import of nuclear-encoded proteins and in organelle division. The third membrane, the internal membrane system known as the thylakoid, houses photosynthetic electron transport and chemiosmotic phosphorylation. The inner envelope and thylakoid membranes share similar lipid composition. Specific targeting pathways determine their defined proteomes and, thus, their distinct functions. Nonetheless, several proteins have been shown to exist in both the envelope and thylakoid membranes. These proteins include those that play roles in protein transport, tetrapyrrole biosynthesis, membrane dynamics, or transport of nucleotides or inorganic phosphate. In this review, we summarize the current knowledge about proteins localized to both the envelope and thylakoid membranes in the chloroplast, discussing their roles in each membrane and potential mechanisms of their dual localization. Addressing the unanswered questions about these dual-localized proteins should help advance our understanding of chloroplast development, protein transport, and metabolic regulation.

Beneficial Roles of Melatonin on Redox Regulation of Photosynthetic Electron Transport and Synthesis of D1 Protein in Tomato Seedlings under Salt Stress

Melatonin is important in the protection of plants suffering various forms of abiotic stress. The molecular mechanisms underlying the melatonin-mediated protection of their photosynthetic machinery are not completely resolved. This study investigates the effects of exogenous melatonin applications on salt-induced damage to the light reaction components of the photosynthetic machinery of tomato seedlings. The results showed that melatonin pretreatments can help maintain growth and net photosynthetic rate (PN) under salt stress conditions. Pretreatment with melatonin increased the effective quantum yield of photosystem II (ΦPSII), the photochemical quenching coefficient (qP) and the proportion of PSII centers that are "open" (qL) under saline conditions. In this way, damage to the photosynthetic electron transport chain (PET) in photosystem II (PSII) was mitigated. In addition, melatonin pretreatment facilitated the repair of PSII by maintaining the availability of D1 protein that was otherwise reduced by salinity. The ROS levels and the gene expressions of the chloroplast TRXs and PRXs were also investigated. Salt stress resulted in increased levels of reactive oxygen species (ROS), which were mitigated by melatonin. In tomato leaves under salt stress, the expressions of PRXs and TRXf declined but the expressions of TRXm1/4 and TRXm2 increased. Melatonin pretreatment promoted the expression of TRXf and the abundances of TRXf and TRXm gene products but had no effects on the expressions of PRXs. In summary, melatonin improves the photosynthetic activities of tomato seedlings under salt stress. The mechanism could be that: (1) Melatonin controls ROS levels and prevents damaging elevations of ROS caused by salt stress. (2) Melatonin facilitates the recovery of PET and D1 protein synthesis, thus enhancing the tolerance of photosynthetic activities to salinity. (3) Melatonin induces the expression of TRXf and regulates the abundance of TRXf and TRXm gene products, which may facilitate repair of the light reaction parts of the photosynthetic machinery.

Thylakoid membrane oxidoreductase LTO1/AtVKOR is involved in ABA-mediated response to osmotic stress in Arabidopsis

Arabidopsis lumen thiol oxidoreductase 1 (LTO1) - the At4g35760 gene product - was previously found to be related to reactive oxygen species (ROS) accumulation. Here, we show that ROS accumulated in a mutant Arabidopsis line (lto1-2, mutant of LTO1/AtVKOR) under osmotic stress at a higher level than that observed in wild-type and transgenic complemented plants of the lto1-2 mutant (lto1-2C, transgenic complemented plants of lto1-2). Because ROS accumulation in osmotic stress is triggered by abscisic acid (ABA), an ABA-responsive gene, Annexin 1 (AnnAt1), was selected to study the response. Osmotic stress or exogenous ABA can significantly upregulate the transcription of AnnAt1 in wild-type and lto1-2C plants. Only a slight change in the transcriptional abundance of AnnAt1 was observed under osmotic stress in the lto1-2 mutant, but exogenous ABA application could increase the expression of AnnAt1, which suggested that exogenous ABA had a partial complementation role. Because the transcription of AnnAt1 is regulated by ABRE (ABA-responsive elements) binding proteins (AREBs)/ABRE binding factors (ABFs), the expression of AREBs/ABFs was also analyzed. The transcription of AREBs/ABFs in the lto1-2 mutant was not induced by osmotic stress but was significantly upregulated by exogenous ABA, which significantly differs from the wild-type and lto1-2C plant responses. Similarly, the expression of another ABA-responsive gene, RD29B (responsive to desiccation stress gene 29B), in the lto1-2 mutant was also upregulated by exogenous ABA. The partial complementation of mutants by ABA indicated that the ABA signal transduction pathway was not significantly affected in the lto1-2 mutant. Taken together, these results suggest that LTO1 is involved in ABA-mediated response to osmotic stress, possibly by affecting the biosynthesis of endogenous ABA.

Dual protein trafficking to secretory and non-secretory cell compartments: clear or double vision?

Approximately 18% of Arabidopsis thaliana proteins encode a signal peptide for translocation to the endoplasmic reticulum (ER), the gateway of the eukaryotic secretory pathway. However, it was recently discovered that some ER proteins can undergo both co-translational import into the ER/secretory pathway and trafficking to compartments outside of the secretory pathway. This phenomenon is observed among members of the protein disulfide isomerase (PDI) family, which are traditionally regarded as ER enzymes involved in protein folding. Although classical PDIs possess an N-terminal signal peptide and a C-terminal ER retention signal, some also dual localize to secretory and non-secretory compartments, including mammalian PDI ERp57, Chlamydomonas reinhardtii PDI RB60, and A. thaliana AtPDI2. ERp57 is present in both the ER and nucleus where it influences gene transcription. RB60 localizes to the ER and chloroplast where it modulates the redox state of polyadenylate-binding protein RB47. AtPDI2, which interacts with transcription factor MEE8, localizes to the ER-secretory pathway and the nucleus. A model proposing secretory trafficking of AtPDI2 and nuclear co-translocation of an AtPDI2-MEE8 complex illustrates the diversity of dual targeting mechanisms, the multifunctional roles of some PDIs, and the potential co-translocation of other proteins to multiple subcellular compartments.

Violaxanthin de-epoxidase disulphides and their role in activity and thermal stability

Violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin at the lumen side of the thylakoids during exposure to intense light. VDE consists of a cysteine-rich N-terminal domain, a lipocalin-like domain and a negatively charged C-terminal domain. That the cysteines are important for the activity of VDE is well known, but in what way is less understood. In this study, wild-type spinach VDE was expressed in E. coli as inclusion bodies, refolded and purified to give a highly active and homogenous preparation. The metal content (Fe, Cu, Ni, Mn, Co and Zn) was lower than 1 mol% excluding a metal-binding function of the cysteines. To investigate which of the 13 cysteines that could be important for the function of VDE, we constructed mutants where the cysteines were replaced by serines, one by one. For 12 out of 13 mutants the activity dropped by more than 99.9%. A quantification of free cysteines showed that only the most N-terminal of these cysteines was in reduced form in the native VDE. A disulphide pattern in VDE of C9-C27, C14-C21, C33-C50, C37-C46, C65-C72 and C118-C284 was obtained after digestion of VDE with thermolysin followed by mass spectroscopy analysis of reduced versus non-reduced samples. The residual activity found for the mutants showed a variation that was consistent with the results obtained from mass spectroscopy. Reduction of the disulphides resulted in loss of a rigid structure and a decrease in thermal stability of 15 °C.

Violaxanthin de-epoxidase disulphides and their role in activity and thermal stability

Violaxanthin de-epoxidase (VDE) catalyses the conversion of violaxanthin to zeaxanthin at the lumen side of the thylakoids during exposure to intense light. VDE consists of a cysteine-rich N-terminal domain, a lipocalin-like domain and a negatively charged C-terminal domain. That the cysteines are important for the activity of VDE is well known, but in what way is less understood. In this study, wild-type spinach VDE was expressed in E. coli as inclusion bodies, refolded and purified to give a highly active and homogenous preparation. The metal content (Fe, Cu, Ni, Mn, Co and Zn) was lower than 1 mol% excluding a metal-binding function of the cysteines. To investigate which of the 13 cysteines that could be important for the function of VDE, we constructed mutants where the cysteines were replaced by serines, one by one. For 12 out of 13 mutants the activity dropped by more than 99.9%. A quantification of free cysteines showed that only the most N-terminal of these cysteines was in reduced form in the native VDE. A disulphide pattern in VDE of C9-C27, C14-C21, C33-C50, C37-C46, C65-C72 and C118-C284 was obtained after digestion of VDE with thermolysin followed by mass spectroscopy analysis of reduced versus non-reduced samples. The residual activity found for the mutants showed a variation that was consistent with the results obtained from mass spectroscopy. Reduction of the disulphides resulted in loss of a rigid structure and a decrease in thermal stability of 15 °C.

The conservative cysteines in transmembrane domain of AtVKOR/LTO1 are critical for photosynthetic growth and photosystem II activity in Arabidopsis

Thylakoid protein vitamin K epoxide reductase (AtVKOR/LTO1) is involved in oxidoreduction. The deficiency of this compound causes pleiotropic defects in Arabidopsis thaliana, such as severely stunted growth, smaller sized leaves, and delay of flowering. Transgenic complementation of wild-type AtVKOR (VKORWT) to vkor mutant lines ultimately demonstrates that the phenotype changes are due to this gene. However, whether AtVKOR functions in Arabidopsis through its protein oxidoreduction is unknown. To further study the redox-active sites of AtVKOR in vivo, a series of plasmids containing cysteine-mutant VKORs were constructed and transformed into vkor deficient lines. Compared with transgenic AtVKORWT plants, the size of the transgenic plants with a single conservative cysteine mutation (VKORC109A, VKORC116A, VKORC195A, and VKORC198A) were smaller, and two double-cysteine mutations (VKORC109AC116A and VKORC195AC198A) showed significantly stunted growth, similar with the vkor mutant line. However, mutations of two non-conservative cysteines (VKORC46A and VKORC230A) displayed little obvious changes in the phenotypes of Arabidopsis. Consistently, the maximum and actual efficiency of photosystem II (PSII) in double-cysteine mutation plants decreased significantly to the level similar to that of the vkor mutant line both under normal growth light and high light. A significantly decreased amount of D1 protein and increased accumulation of reactive oxygen species were observed in two double-cysteine mutations under high light. All of the results above indicated that the conservative cysteines in transmembrane domains were the functional sites of AtVKOR in Arabidopsis and that the oxidoreductase activities of AtVKOR were directly related to the autotrophic photosynthetic growth and PSII activity of Arabidopsis thaliana.

Plastidic type I signal peptidase 1 is a redox-dependent thylakoidal processing peptidase

Thylakoids are the photosynthetic membranes in chloroplasts and cyanobacteria. The aqueous phase inside the thylakoid known as the thylakoid lumen plays an essential role in the photosynthetic electron transport. The presence and significance of thiol-disulfide exchange in this compartment have been recognized but remain poorly understood. All proteins found free in the thylakoid lumen and some proteins associated to the thylakoid membrane require an N-terminal targeting signal, which is removed in the lumen by a membrane-bound serine protease called thylakoidal processing peptidase (TPP). TPP is homologous to Escherichia coli type I signal peptidase (SPI) called LepB. Genetic data indicate that plastidic SPI 1 (Plsp1) is the main TPP in Arabidopsis thaliana (Arabidopsis) although biochemical evidence had been lacking. Here we demonstrate catalytic activity of bacterially produced Arabidopsis Plsp1. Recombinant Plsp1 showed processing activity against various TPP substrates at a level comparable to that of LepB. Plsp1 and LepB were also similar in the pH optima, sensitivity to arylomycin variants and a preference for the residue at -3 to the cleavage site within a substrate. Plsp1 orthologs found in angiosperms contain two unique Cys residues located in the lumen. Results of processing assays suggested that these residues were redox active and formation of a disulfide bond between them was necessary for the activity of recombinant Arabidopsis Plsp1. Furthermore, Plsp1 in Arabidopsis and pea thylakoids migrated faster under non-reducing conditions than under reducing conditions on SDS-PAGE. These results underpin the notion that Plsp1 is a redox-dependent signal peptidase in the thylakoid lumen.

Thioredoxin-dependent regulatory networks in chloroplasts under fluctuating light conditions

Plants have adopted a number of mechanisms to restore redox homeostasis in the chloroplast under fluctuating light conditions in nature. Chloroplast thioredoxin systems are crucial components of this redox network, mediating environmental signals to chloroplast proteins. In the reduced state, thioredoxins control the structure and function of proteins by reducing disulfide bridges in the redox active site of a protein. Subsequently, an oxidized thioredoxin is reduced by a thioredoxin reductase, the two enzymes together forming a thioredoxin system. Plant chloroplasts have versatile thioredoxin systems, including two reductases dependent on ferredoxin and NADPH as reducing power, respectively, several types of thioredoxins, and the system to deliver thiol redox signals to the thylakoid membrane and lumen. Light controls the activity of chloroplast thioredoxin systems in two ways. First, light reactions activate the thioredoxin systems via donation of electrons to oxidized ferredoxin and NADP⁺, and second, light induces production of reactive oxygen species in chloroplasts which deactivate the components of the thiol redox network. The diversity and partial redundancy of chloroplast thioredoxin systems enable chloroplast metabolism to rapidly respond to ever-changing environmental conditions and to raise plant fitness in natural growth conditions.

Oxidative protein-folding systems in plant cells

Plants are unique among eukaryotes in having evolved organelles: the protein storage vacuole, protein body, and chloroplast. Disulfide transfer pathways that function in the endoplasmic reticulum (ER) and chloroplasts of plants play critical roles in the development of protein storage organelles and the biogenesis of chloroplasts, respectively. Disulfide bond formation requires the cooperative function of disulfide-generating enzymes (e.g., ER oxidoreductase 1), which generate disulfide bonds de novo, and disulfide carrier proteins (e.g., protein disulfide isomerase), which transfer disulfides to substrates by means of thiol-disulfide exchange reactions. Selective molecular communication between disulfide-generating enzymes and disulfide carrier proteins, which reflects the molecular and structural diversity of disulfide carrier proteins, is key to the efficient transfer of disulfides to specific sets of substrates. This review focuses on recent advances in our understanding of the mechanisms and functions of the various disulfide transfer pathways involved in oxidative protein folding in the ER, chloroplasts, and mitochondria of plants.

Operation of trans-thylakoid thiol-metabolizing pathways in photosynthesis

Thiol oxidation to disulfides and the reverse reaction, i.e., disulfide reduction to free thiols, are under the control of catalysts in vivo. Enzymatically assisted thiol-disulfide chemistry is required for the biogenesis of all energy-transducing membrane systems. However, until recently, this had only been demonstrated for the bacterial plasma membrane. Long considered to be vacant, the thylakoid lumen has now moved to the forefront of photosynthesis research with the realization that its proteome is far more complicated than initially anticipated. Several lumenal proteins are known to be disulfide bonded in Arabidopsis, highlighting the importance of sulfhydryl oxidation in the thylakoid lumen. While disulfide reduction in the plastid stroma is known to activate several enzymatic activities, it appears that it is the reverse reaction, i.e., thiol oxidation that is required for the activity of several lumen-resident proteins. This paradigm for redox regulation in the thylakoid lumen has opened a new frontier for research in the field of photosynthesis. Of particular significance in this context is the discovery of trans-thylakoid redox pathways controlling disulfide bond formation and reduction, which are required for photosynthesis.