Plastid thioredoxins: a "one-for-all" redox-signaling system in plants

Divergent Protein Redox Dynamics and Their Relationship with Electron Transport Efficiency during Photosynthesis Induction

Various chloroplast proteins are activated/deactivated during the light/dark cycle via the redox regulation system. Although the photosynthetic electron transport chain provides reducing power to redox-sensitive proteins via the ferredoxin (Fd)/thioredoxin (Trx) pathway for their enzymatic activity control, how the redox states of individual proteins are linked to electron transport efficiency remains uncharacterized. Here we addressed this subject with a focus on the photosynthetic induction phase. We used Arabidopsis plants, in which the amount of Fd-Trx reductase (FTR), a core component in the Fd/Trx pathway, was genetically altered. Several chloroplast proteins showed different redox shift responses toward low- and high-light treatments. The light-dependent reduction of Calvin-Benson cycle enzymes fructose 1,6-bisphosphatase (FBPase) and sedoheptulose 1,7-bisphosphatase (SBPase) was partially impaired in the FTR-knockdown ftrb mutant. Simultaneous analyses of chlorophyll fluorescence and P700 absorbance change indicated that the induction of the electron transport reactions was delayed in the ftrb mutant. FTR overexpression also mildly affected the reduction patterns of FBPase and SBPase under high-light conditions, which were accompanied by the modification of electron transport properties. Accordingly, the redox states of FBPase and SBPase were linearly correlated with electron transport rates. In contrast, ATP synthase was highly reduced even when electron transport reactions were not fully induced. Furthermore, the redox response of proton gradient regulation 5-like photosynthetic phenotype1 (PGRL1; a protein involved in cyclic electron transport) did not correlate with electron transport rates. Our results provide insights into the working dynamics of the redox regulation system and their differential associations with photosynthetic electron transport efficiency.

Overexpression of thioredoxin-like protein ACHT2 leads to negative feedback control of photosynthesis in Arabidopsis thaliana

Thioredoxin (Trx) is a small redox mediator protein involved in the regulation of various chloroplast functions by modulating the redox state of Trx target proteins in ever-changing light environments. Using reducing equivalents produced by the photosynthetic electron transport chain, Trx reduces the disulfide bonds on target proteins and generally turns on their activities. While the details of the protein-reduction mechanism by Trx have been well investigated, the oxidation mechanism that counteracts it has long been unclear. We have recently demonstrated that Trx-like proteins such as Trx-like2 and atypical Cys His-rich Trx (ACHT) can function as protein oxidation factors in chloroplasts. Our latest study on transgenic Arabidopsis plants indicated that the ACHT isoform ACHT2 is involved in regulating the thermal dissipation of light energy. To understand the role of ACHT2 in vivo, we characterized phenotypic changes specifically caused by ACHT2 overexpression in Arabidopsis. ACHT2-overexpressing plants showed growth defects, especially under high light conditions. This growth phenotype was accompanied with the impaired reductive activation of Calvin-Benson cycle enzymes, enhanced thermal dissipation of light energy, and decreased photosystem II activity. Overall, ACHT2 overexpression promoted protein oxidation that led to the inadequate activation of Calvin-Benson cycle enzymes in light and consequently induced negative feedback control of the photosynthetic electron transport chain. This study highlights the importance of the balance between protein reduction and oxidation in chloroplasts for optimal photosynthetic performance and plant growth.

Three-dimensional solution structure, dynamics and binding of thioredoxin m from Pisum sativum

Thioredoxins (TRXs) are ubiquitous small, globular proteins involved in cell redox processes. In this work, we report the solution structure of TRX m from Pisum sativum (pea), which has been determined on the basis of 1444 nuclear Overhauser effect- (NOE-) derived distance constraints. The average pairwise root-mean-square deviation (RMSD) for the 20 best structures for the backbone residues (Val7-Glu102) was 1.42 ± 0.15 Å, and 1.97 ± 0.15 Å when all heavy atoms were considered. The structure corresponds to the typical fold of TRXs, with a central five-stranded β-sheet flanked by four α-helices. Some residues had an important exchange dynamic contribution: those around the active site; at the C terminus of β-strand 3; and in the loop preceding α-helix 4. Smaller NOE values were observed at the N and C-terminal residues forming the elements of the secondary structure or, alternatively, in the residues belonging to the loops between those elements. A peptide derived from pea fructose-1,6-biphosphatase (FBPase), comprising the preceding region to the regulatory sequence of FBPase (residues Glu152 to Gln179), was bound to TRX m with an affinity in the low micromolar range, as measured by fluorescence and NMR titration experiments. Upon peptide addition, the intensities of the cross-peaks of all the residues of TRX m were affected, as shown by NMR. The value of the dissociation constant of the peptide from TRX m was larger than that of the intact FBPase, indicating that there are additional factors in other regions of the polypeptide chain of the latter protein affecting the binding to thioredoxin.

Modulation of the wheat transcriptome by TaZFP13D under well-watered and drought conditions

Maintaining global food security in the context of climate changes will be an important challenge in the next century. Improving abiotic stress tolerance of major crops such as wheat can contribute to this goal. This can be achieved by the identification of the genes involved and their use to develop tools for breeding programs aiming to generate better adapted cultivars. Recently, we identified the wheat TaZFP13D gene encoding Zinc Finger Protein 13D as a new gene improving water-stress tolerance. The current work analyzes the TaZFP13D-dependent transcriptome modifications that occur in well-watered and dehydration conditions to better understand its function during normal growth and during drought. Plants that overexpress TaZFP13D have a higher biomass under well-watered conditions, indicating a positive effect of the protein on growth. Survival rate and stress recovery after a severe drought stress are improved compared to wild-type plants. The latter is likely due the higher activity of key antioxidant enzymes and concomitant reduction of drought-induced oxidative damage. Conversely, down-regulation of TaZFP13D decreases drought tolerance and protection against drought-induced oxidative damage. RNA-Seq transcriptome analysis identified many genes regulated by TaZFP13D that are known to improve drought tolerance. The analysis also revealed several genes involved in the photosynthetic electron transfer chain known to improve photosynthetic efficiency and chloroplast protection against drought-induced ROS damage. This study highlights the important role of TaZFP13D in wheat drought tolerance, contributes to unravel the complex regulation governed by TaZFPs, and suggests that it could be a promising marker to select wheat cultivars with higher drought tolerance.

Overexpression of Grapevine VyTRXy Improves Drought Tolerance by Maintaining Photosynthesis and Enhancing the Antioxidant and Osmolyte Capacity of Plants

Drought stress profoundly affects plant growth and development, posing a significant challenge that is extensively researched in the field. Thioredoxins (TRXs), small proteins central to redox processes, are crucial to managing both abiotic and biotic stresses. In this research, the VyTRXy gene, cloned from wild Yanshan grapes, was validated as a functional TRX through enzyme activity assays. VyTRXy was found to bolster photosynthesis, augment levels of osmotic regulators, stimulate antioxidant enzyme activities, and strengthen drought resilience in transgenic plants. These enhancements were evidenced by higher survival rates, optimized photosynthetic metrics, increased proline levels, augmented chlorophyll concentration, reduced electrolyte leakage, and decreased malondialdehyde and hydrogen peroxide (H2O2) levels. Furthermore, there was a surge in the activities of enzymes such as catalase, ascorbate peroxidase, glutathione peroxidase, dehydroascorbate reductase, and glutathione reductase, along with an increased expression of TRX peroxidase. Notably, under drought stress, there was a marked elevation in the expression of stress-responsive genes, including the adversity stress-inducible expression gene (NtRD29A) and DRE-binding protein (NtDREB), in transgenic tobacco. This investigation is pivotal in the quest for drought-resistant grapevine varieties and provides significant insights into the molecular functionality of VyTRXy in enhancing grapevine drought tolerance.

Chloroplast translation factor EF-Tu of Arabidopsis thaliana can be inactivated via oxidation of a specific cysteine residue

Translational elongation factor EF-Tu, which delivers aminoacyl-tRNA to the ribosome, is susceptible to inactivation by reactive oxygen species (ROS) in the cyanobacterium Synechocystis sp. PCC 6803. However, the sensitivity to ROS of chloroplast-localized EF-Tu (cpEF-Tu) of plants remains to be elucidated. In the present study, we generated a recombinant cpEF-Tu protein of Arabidopsis thaliana and examined its sensitivity to ROS in vitro. In cpEF-Tu that lacked a bound nucleotide, one of the two cysteine residues, Cys149 and Cys451, in the mature protein was sensitive to oxidation by H2O2, with the resultant formation of sulfenic acid. The translational activity of cpEF-Tu, as determined with an in vitro translation system, derived from Escherichia coli, that had been reconstituted without EF-Tu, decreased with the oxidation of a cysteine residue. Replacement of Cys149 with an alanine residue rendered cpEF-Tu insensitive to inactivation by H2O2, indicating that Cys149 might be the target of oxidation. In contrast, cpEF-Tu that had bound either GDP or GTP was less sensitive to oxidation by H2O2 than nucleotide-free cpEF-Tu. The addition of thioredoxin f1, a major thioredoxin in the Arabidopsis chloroplast, to oxidized cpEF-Tu allowed the reduction of Cys149 and the reactivation of cpEF-Tu, suggesting that the oxidation of cpEF-Tu might be a reversible regulatory mechanism that suppresses the chloroplast translation system in a redox-dependent manner.

Plastid 2-Cys peroxiredoxins are essential for embryogenesis in Arabidopsis

The redox couple formed by NADPH-dependent thioredoxin reductase C (NTRC) and 2-Cys peroxiredoxins (Prxs) allows fine-tuning chloroplast performance in response to light intensity changes. Accordingly, the Arabidopsis 2cpab mutant lacking 2-Cys Prxs shows growth inhibition and sensitivity to light stress. However, this mutant also shows defective post-germinative growth, suggesting a relevant role of plastid redox systems in seed development, which is so far unknown. To address this issue, we first analyzed the pattern of expression of NTRC and 2-Cys Prxs in developing seeds. Transgenic lines expressing GFP fusions of these proteins showed their expression in developing embryos, which was low at the globular stage and increased at heart and torpedo stages, coincident with embryo chloroplast differentiation, and confirmed the plastid localization of these enzymes. The 2cpab mutant produced white and abortive seeds, which contained lower and altered composition of fatty acids, thus showing the relevance of 2-Cys Prxs in embryogenesis. Most embryos of white and abortive seeds of the 2cpab mutant were arrested at heart and torpedo stages of embryogenesis suggesting an essential function of 2-Cys Prxs in embryo chloroplast differentiation. This phenotype was not recovered by a mutant version of 2-Cys Prx A replacing the peroxidatic Cys by Ser. Neither the lack nor the overexpression of NTRC had any effect on seed development indicating that the function of 2-Cys Prxs at these early stages of development is independent of NTRC, in clear contrast with the operation of these regulatory redox systems in leaves chloroplasts.

The ferredoxin/thioredoxin pathway constitutes an indispensable redox-signaling cascade for light-dependent reduction of chloroplast stromal proteins

To ensure efficient photosynthesis, chloroplast proteins need to be flexibly regulated under fluctuating light conditions. Thiol-based redox regulation plays a key role in reductively activating several chloroplast proteins in a light-dependent manner. The ferredoxin (Fd)/thioredoxin (Trx) pathway has long been recognized as the machinery that transfers reducing power generated by photosynthetic electron transport reactions to redox-sensitive target proteins; however, its biological importance remains unclear, because the complete disruption of the Fd/Trx pathway in plants has been unsuccessful to date. Especially, recent identifications of multiple redox-related factors in chloroplasts, as represented by the NADPH-Trx reductase C, have raised a controversial proposal that other redox pathways work redundantly with the Fd/Trx pathway. To address these issues directly, we used CRISPR/Cas9 gene editing to create Arabidopsis mutant plants in which the activity of the Fd/Trx pathway was completely defective. The mutants generated showed severe growth inhibition. Importantly, these mutants almost entirely lost the ability to reduce several redox-sensitive proteins in chloroplast stroma, including four Calvin-Benson cycle enzymes, NADP-malate dehydrogenase, and Rubisco activase, under light conditions. These striking phenotypes were further accompanied by abnormally developed chloroplasts and a drastic decline in photosynthetic efficiency. These results indicate that the Fd/Trx pathway is indispensable for the light-responsive activation of diverse stromal proteins and photoautotrophic growth of plants. Our data also suggest that the ATP synthase is exceptionally reduced by other pathways in a redundant manner. This study provides an important insight into how the chloroplast redox-regulatory system operates in vivo.

Comprehensive Expression Analyses of Plastidial Thioredoxins of Arabidopsis thaliana Indicate a Main Role of Thioredoxin m2 in Roots

Thioredoxins (TRXs) f and m are redox proteins that regulate key chloroplast processes. The existence of several isoforms of TRXs f and m indicates that these redox players have followed a specialization process throughout evolution. Current research efforts are focused on discerning the signalling role of the different TRX types and their isoforms in chloroplasts. Nonetheless, little is known about their function in non-photosynthetic plastids. For this purpose, we have carried out comprehensive expression analyses by using Arabidopsis thaliana TRXf (f1 and f2) and TRXm (m1, m2, m3 and m4) genes translationally fused to the green fluorescence protein (GFP). These analyses showed that TRX m has different localisation patterns inside chloroplasts, together with a putative dual subcellular localisation of TRX f1. Apart from mesophyll cells, these TRXs were also observed in reproductive organs, stomatal guard cells and roots. We also investigated whether photosynthesis, stomatal density and aperture or root structure were affected in the TRXs f and m loss-of-function Arabidopsis mutants. Remarkably, we immunodetected TRX m2 and the Calvin−Benson cycle fructose-1,6-bisphosphatase (cFBP1) in roots. After carrying out in vitro redox activation assays of cFBP1 by plastid TRXs, we propose that cFBP1 might be activated by TRX m2 in root plastids.

Functional division of f-type and m-type thioredoxins to regulate the Calvin cycle and cyclic electron transport around photosystem I

Redox regulation of chloroplast proteins is necessary to adjust photosynthetic performance with changes in light. The thioredoxin (Trx) system plays a central role in this process. Chloroplast-localized classical Trx is a small redox-active protein that regulates many target proteins by reducing their disulfide bonds in a light-dependent manner. Arabidopsis thaliana mutants lacking f-type Trx (trx f1f2) or m-type Trx (trx m124-2) have been reported to show delayed reduction of Calvin cycle enzymes. As a result, the trx m124-2 mutant exhibits growth defects. Here, we characterized a quintuple mutant lacking both Trx f and Trx m to investigate the functional complementarity of Trx f and Trx m. The trx f1f2 m124-2 quintuple mutant was newly obtained by crossing, and is analyzed here for the first time. The growth defects of the trx m124-2 mutant were not enhanced by the lack of Trx f. In contrast, deficiencies of both Trxs additively suppressed the reduction of Calvin cycle enzymes, resulting in a further delay in the initiation of photosynthesis. Trx f appeared to be necessary for the rapid activation of the Calvin cycle during the early induction of photosynthesis. To perform effective photosynthesis, plants seem to use both Trxs in a coordinated manner to activate carbon fixation reactions. In contrast, the PROTON GRADIENT REGULATION 5 (PGR5)-dependent cyclic electron transport around photosystem I was regulated by Trx m, but not by Trx f. Lack of Trx f did not affect the activity and regulation of the PGR5-dependent pathway. Trx f may have a higher specificity for target proteins, whereas Trx m has a variety of target proteins to regulate overall photosynthesis and other metabolic reactions in the chloroplasts.

Verification of the Relationship between Redox Regulation of Thioredoxin Target Proteins and Their Proximity to Thylakoid Membranes

Thioredoxin (Trx) is a key protein of the redox regulation system in chloroplasts, where it modulates various enzyme activities. Upon light irradiation, Trx reduces the disulfide bonds of Trx target proteins (thereby turning on their activities) using reducing equivalents obtained from the photosynthetic electron transport chain. This reduction process involves a differential response, i.e., some Trx target proteins in the stroma respond slowly to the change in redox condition caused by light/dark changes, while the ATP synthase γ subunit (CF₁-γ) located on the surface of thylakoid membrane responds with high sensitivity. The factors that determine this difference in redox kinetics are not yet known, although here, we hypothesize that it is due to each protein’s localization in the chloroplast, i.e., the reducing equivalents generated under light conditions can be transferred more efficiently to the proteins on thylakoid membrane than to stromal proteins. To explore this possibility, we anchored SBPase, one of the stromal Trx target proteins, to the thylakoid membrane in Arabidopsis thaliana. Analyses of the redox behaviors of the anchored and unanchored proteins showed no significant difference in their reduction kinetics, implying that protein sensitivity to redox regulation is determined by other factors.

Modifying Anthocyanins Biosynthesis in Tomato Hairy Roots: A Test Bed for Plant Resistance to Ionizing Radiation and Antioxidant Properties in Space

Gene expression manipulation of specific metabolic pathways can be used to obtain bioaccumulation of valuable molecules and desired quality traits in plants. A single-gene approach to impact different traits would be greatly desirable in agrospace applications, where several aspects of plant physiology can be affected, influencing growth. In this work, MicroTom hairy root cultures expressing a MYB-like transcription factor that regulates the biosynthesis of anthocyanins in Petunia hybrida (PhAN4), were considered as a testbed for bio-fortified tomato whole plants aimed at agrospace applications. Ectopic expression of PhAN4 promoted biosynthesis of anthocyanins, allowing to profile 5 major derivatives of delphinidin and petunidin together with pelargonidin and malvidin-based anthocyanins, unusual in tomato. Consistent with PhAN4 features, transcriptomic profiling indicated upregulation of genes correlated to anthocyanin biosynthesis. Interestingly, a transcriptome reprogramming oriented to positive regulation of cell response to biotic, abiotic, and redox stimuli was evidenced. PhAN4 hairy root cultures showed the significant capability to counteract reactive oxygen species (ROS) accumulation and protein misfolding upon high-dose gamma irradiation, which is among the most potent pro-oxidant stress that can be encountered in space. These results may have significance in the engineering of whole tomato plants that can benefit space agriculture.

Maintaining the Chloroplast Redox Balance through the PGR5-Dependent Pathway and the Trx System Is Required for Light-Dependent Activation of Photosynthetic Reactions

Light-dependent activation of chloroplast enzymes is required for the rapid induction of photosynthesis after a shift from dark to light. The thioredoxin (Trx) system plays a central role in this process. In chloroplasts, the Trx system consists of two pathways: the ferredoxin (Fd)/Trx pathway and the nicotinamide adenine dinucleotide phosphate (NADPH)-Trx reductase C (NTRC) pathway. In Arabidopsis (Arabidopsis thaliana) mutants defective in either pathway, the photoreduction of thiol enzymes was impaired, resulting in decreased carbon fixation. The close relationship between the Fd/Trx pathway and proton gradient regulation 5 (PGR5)-dependent photosystem I cyclic electron transport (PSI CET) in the induction of photosynthesis was recently elucidated. However, how the PGR5-dependent pathway is involved in the NTRC pathway is unclear, although NTRC has been suggested to physically interact with PGR5. In this study, we analyzed Arabidopsis mutants lacking either the PGR5 or the chloroplast NADH dehydrogenase-like complex (NDH)-dependent PSI CET pathway in the ntrc mutant background. The ntrc pgr5 double mutant suppressed both the growth defects and the high non-photochemical quenching phenotype of the ntrc mutant when grown under long-day conditions. By contrast, the inactivation of NDH activity with the chlororespiratory reduction 2-2 mutant failed to suppress either phenotype. We discovered that the phenotypic rescue of ntrc by pgr5 is caused by the partial restoration of Trx-dependent reduction of thiol enzymes. These results suggest that electron partitioning to the PGR5-dependent pathway and the Trx system needs to be properly regulated for the activation of the Calvin-Benson-Bassham cycle enzymes during the induction of photosynthesis.

Current Knowledge on Mechanisms Preventing Photosynthesis Redox Imbalance in Plants

Photosynthesis includes a set of redox reactions that are the source of reducing power and energy for the assimilation of inorganic carbon, nitrogen and sulphur, thus generating organic compounds, and oxygen, which supports life on Earth. As sessile organisms, plants have to face continuous changes in environmental conditions and need to adjust the photosynthetic electron transport to prevent the accumulation of damaging oxygen by-products. The balance between photosynthetic cyclic and linear electron flows allows for the maintenance of a proper NADPH/ATP ratio that is adapted to the plant’s needs. In addition, different mechanisms to dissipate excess energy operate in plants to protect and optimise photosynthesis under adverse conditions. Recent reports show an important role of redox-based dithiol–disulphide interchanges, mediated both by classical and atypical chloroplast thioredoxins (TRXs), in the control of these photoprotective mechanisms. Moreover, membrane-anchored TRX-like proteins, such as HCF164, which transfer electrons from stromal TRXs to the thylakoid lumen, play a key role in the regulation of lumenal targets depending on the stromal redox poise. Interestingly, not all photoprotective players were reported to be under the control of TRXs. In this review, we discuss recent findings regarding the mechanisms that allow an appropriate electron flux to avoid the detrimental consequences of photosynthesis redox imbalances.

Thioredoxins m are major players in the multifaceted light-adaptive response in Arabidopsis thaliana

Thioredoxins (TRXs) are well-known redox signalling players, which carry out post-translational modifications in target proteins. Chloroplast TRXs are divided into different types and have central roles in light energy uptake and the regulation of primary metabolism. The isoforms TRX m1, m2, and m4 from Arabidopsis thaliana are considered functionally related. Knowing their key position in the hub of plant metabolism, we hypothesized that the impairment of the TRX m signalling would not only have harmful consequences on chloroplast metabolism but also at different levels of plant development. To uncover the physiological and developmental processes that depend on TRX m signalling, we carried out a comprehensive study of Arabidopsis single, double, and triple mutants defective in the TRX m1, m2, and m4 proteins. As light and redox signalling are closely linked, we investigated the response to high light (HL) of the plants that are gradually compromised in TRX m signalling. We provide experimental evidence relating the lack of TRX m and the appearance of novel phenotypic features concerning mesophyll structure, stomata biogenesis, and stomatal conductance. We also report new data indicating that the isoforms of TRX m fine-tune the response to HL, including the accumulation of the protective pigment anthocyanin. These results reveal novel signalling functions for the TRX m and underline their importance for plant growth and fulfilment of the acclimation/response to HL conditions.

Biochemical Basis for Redox Regulation of Chloroplast-Localized Phosphofructokinase from Arabidopsis thaliana

Various proteins in plant chloroplasts are subject to thiol-based redox regulation, allowing light-responsive control of chloroplast functions. Most redox-regulated proteins are known to be reductively activated in the light in a thioredoxin (Trx)-dependent manner, but its regulatory network remains incompletely understood. Using a biochemical procedure, we here show that a specific form of phosphofructokinase (PFK) is a novel redox-regulated protein whose activity is suppressed upon reduction. PFK is a key enzyme in the glycolytic pathway. In Arabidopsis thaliana, PFK5 is targeted to chloroplasts and uniquely contains an insertion sequence harboring two Cys residues (Cys152 and Cys157) in the N-terminal region. Redox shift assays using a thiol-modifying reagent indicated that PFK5 is efficiently reduced by a specific type of Trx, namely, Trx-f. PFK5 enzyme activity was lowered with the Trx-f-dependent reduction. PFK5 redox regulation was bidirectional; PFK5 was also oxidized and activated by the recently identified Trx-like2/2-Cys peroxiredoxin pathway. Mass spectrometry-based peptide mapping analysis revealed that Cys152 and Cys157 are critical for the intramolecular disulfide bond formation in PFK5. The involvement of Cys152 and Cys157 in PFK5 redox regulation was further supported by a site-directed mutagenesis study. PFK5 catalyzes the reverse reaction of fructose 1,6-bisphosphatase (FBPase), which is reduced and activated specifically by Trx-f. Our data suggest that PFK5 redox regulation, together with that of FBPase, constitutes a checkpoint for switching light/dark metabolism in chloroplasts.

Evaluation of CBSX Proteins as Regulators of the Chloroplast Thioredoxin System

The chloroplast-localized cystathionine β-synthase X (CBSX) proteins CBSX1 and CBSX2 have been proposed as modulators of thioredoxins (Trxs). In this study, the contribution of CBSX proteins to the redox regulation of thiol enzymes in the chloroplast Trx system was evaluated both in vitro and in vivo. The in vitro biochemical studies evaluated whether CBSX proteins alter the specificities of classical chloroplastic Trx f and Trx m for their target proteins. However, addition of CBSX proteins did not alter the specificities of Trx f and Trx m for disulfide bond reduction of the photosynthesis-related major thiol enzymes, FBPase, SBPase, and NADP-MDH. In vivo analysis showed that CBSX-deficient mutants grew similarly to wild type plants under continuous normal light conditions and that CBSX deficiency did not affect photo-reduction of photosynthesis-related thiol enzymes by Trx system at several light intensities. Although CBSX proteins have been suggested as modulators in the chloroplast Trx system, our results did not support this model, at least in the cases of FBPase, SBPase, and NADP-MDH in leaves. However, fresh weights of the cbsx2 mutants were decreased under short day. Since Trxs regulate many proteins participating in various metabolic reactions in the chloroplast, CBSX proteins may function to regulate other chloroplast Trx target proteins, or serve as modulators in non-photosynthetic plastids of flowers. As a next stage, further investigations are required to understand the modulation of Trx-dependent redox regulation by plastidal CBSX proteins.

12-oxo-Phytodienoic Acid: A Fuse and/or Switch of Plant Growth and Defense Responses?

12-oxo-Phytodienoic acid (OPDA) is a primary precursor of (-)-jasmonic acid (JA), able to trigger autonomous signaling pathways that regulate a unique subset of jasmonate-responsive genes, activating and fine-tuning defense responses, as well as growth processes in plants. Recently, a number of studies have illuminated the physiol-molecular activities of OPDA signaling in plants, which interconnect the regulatory loop of photosynthesis, cellular redox homeostasis, and transcriptional regulatory networks, together shedding new light on (i) the underlying modes of cellular interfaces between growth and defense responses (e.g., fitness trade-offs or balances) and (ii) vital information in genetic engineering or molecular breeding approaches to upgrade own survival capacities of plants. However, our current knowledge regarding its mode of actions is still far from complete. This review will briefly revisit recent progresses on the roles and mechanisms of OPDA and information gaps within, which help in understanding the phenotypic and environmental plasticity of plants.

Biochemical insight into redox regulation of plastidial 3-phosphoglycerate dehydrogenase from Arabidopsis thaliana

Thiol-based redox regulation is a post-translational protein modification for controlling enzyme activity by switching oxidation/reduction states of Cys residues. In plant cells, numerous proteins involved in a wide range of biological systems have been suggested as the target of redox regulation; however, our knowledge on this issue is still incomplete. Here we report that 3-phosphoglycerate dehydrogenase (PGDH) is a novel redox-regulated protein. PGDH catalyzes the first committed step of Ser biosynthetic pathway in plastids. Using an affinity chromatography-based method, we found that PGDH physically interacts with thioredoxin (Trx), a key factor of redox regulation. The in vitro studies using recombinant proteins from Arabidopsis thaliana showed that a specific PGDH isoform, PGDH1, forms the intramolecular disulfide bond under nonreducing conditions, which lowers PGDH enzyme activity. MS and site-directed mutagenesis analyses allowed us to identify the redox-active Cys pair that is mainly involved in disulfide bond formation in PGDH1; this Cys pair is uniquely found in land plant PGDH. Furthermore, we revealed that some plastidial Trx subtypes support the reductive activation of PGDH1. The present data show previously uncharacterized regulatory mechanisms of PGDH and expand our understanding of the Trx-mediated redox-regulatory network in plants.

Thioredoxin-dependent control balances the metabolic activities of tetrapyrrole biosynthesis

Plastids are specialized organelles found in plants, which are endowed with their own genomes, and differ in many respects from the intracellular compartments of organisms belonging to other kingdoms of life. They differentiate into diverse, plant organ-specific variants, and are perhaps the most versatile organelles known. Chloroplasts are the green plastids in the leaves and stems of plants, whose primary function is photosynthesis. In response to environmental changes, chloroplasts use several mechanisms to coordinate their photosynthetic activities with nuclear gene expression and other metabolic pathways. Here, we focus on a redox-based regulatory network composed of thioredoxins (TRX) and TRX-like proteins. Among multiple redox-controlled metabolic activities in chloroplasts, tetrapyrrole biosynthesis is particularly rich in TRX-dependent enzymes. This review summarizes the effects of plastid-localized reductants on several enzymes of this pathway, which have been shown to undergo dithiol-disulfide transitions. We describe the impact of TRX-dependent control on the activity, stability and interactions of these enzymes, and assess its contribution to the provision of adequate supplies of metabolic intermediates in the face of diurnal and more rapid and transient changes in light levels and other environmental factors.

Plant Chloroplast Stress Response: Insights from Thiol Redox Proteomics

Significance: Plant chloroplasts generate reactive oxygen species (ROS) during photosynthesis, especially under stresses. The sulfhydryl groups of protein cysteine residues are susceptible to redox modifications, which regulate protein structure and function, and thus different signaling and metabolic processes. The ROS-governed protein thiol redox switches play important roles in chloroplasts. Recent Advances: Various high-throughput thiol redox proteomic approaches have been developed, and they have enabled the improved understanding of redox regulatory mechanisms in chloroplasts. For example, the thioredoxin-modulated antioxidant enzymes help to maintain cellular ROS homeostasis. The light- and dark-dependent redox regulation of photosynthetic electron transport, the Calvin/Benson cycle, and starch biosynthesis ensures metabolic coordination and efficient energy utilization. In addition, redox cascades link the light with the dynamic changes of metabolites in nitrate and sulfur assimilation, shikimate pathway, and biosynthesis of fatty acid hormone as well as purine, pyrimidine, and thiamine. Importantly, redox regulation of tetrapyrrole and chlorophyll biosynthesis is critical to balance the photodynamic tetrapyrrole intermediates and prevent oxidative damage. Moreover, redox regulation of diverse elongation factors, chaperones, and kinases plays an important role in the modulation of gene expression, protein conformation, and posttranslational modification that contribute to photosystem II (PSII) repair, state transition, and signaling in chloroplasts. Critical Issues: This review focuses on recent advances in plant thiol redox proteomics and redox protein networks toward understanding plant chloroplast signaling, metabolism, and stress responses. Future Directions: Using redox proteomics integrated with biochemical and molecular genetic approaches, detailed studies of cysteine residues, their redox states, cross talk with other modifications, and the functional implications will yield a holistic understanding of chloroplast stress responses.

Redox regulation by peroxiredoxins is linked to their thioredoxin-dependent oxidase function

The chloroplast contains three types of peroxiredoxins (PRXs). Recently, 2-CysPRX was associated with thioredoxin (TRX) oxidation-dependent redox regulation. Here, this analysis was expanded to include PRXQ and PRXIIE. Oxidized PRXQ was able to inactivate NADPH malate dehydrogenase and fructose-1,6-bisphosphatase most efficiently in the presence of TRX-m1 and TRX-m4. The inactivation ability of TRXs did not entirely match their reductive activation efficiency. PRXIIE was unable to function as TRX oxidase in enzyme regulation. This conclusion was further supported by the observation that PRXQ adopts the oxidized form by about 50% in leaves, supporting a possible function as a TRX oxidase similar to 2-CysPRX. Results on the oxidation state of photosystem I (P700), plastocyanin, and ferredoxin in intact leaves indicate that each type of PRX has distinct regulatory functions, and that both 2-CysPRX and PRXQ conditionally assist in adjusting the redox state of target proteins for proper activity.

Redox metabolism: the hidden player in carbon and nitrogen signaling?

While decades of research have considered redox metabolism as purely defensive, recent results show that reactive oxygen species (ROS) are necessary for growth and development. Close relationships have been found between the regulation of nitrogen metabolism and ROS in response to both carbon and nitrogen availability. Root nitrate uptake and nitrogen metabolism have been shown to be regulated by a signal from the oxidative pentose phosphate pathway (OPPP) in response to carbon signaling. As a major source of NADP(H), the OPPP is critical to maintaining redox balance under stress situations. Furthermore, recent results suggest that at least part of the regulation of the root nitrate transporter by nitrogen signaling is also linked to the redox status of the plant. This leads to the question of whether there is a more general role of redox metabolism in the regulation of nitrogen metabolism by carbon and nitrogen. This review highlights the role of the OPPP in carbon signaling and redox metabolism, and the interaction between redox and nitrogen metabolism. We discuss how redox metabolism could be an important player in the regulation of nitrogen metabolism in response to carbon/nitrogen interaction and the implications for plant adaptation to extreme environments and future crop development.

Proteomic changes may lead to yield alteration in maize under carbon dioxide enriched condition

In the present study, the effect of elevated CO2 on growth, physiology, yield and proteome was studied on two maize (Zea mays L.) varieties grown under Free-air CO2 enrichment. Growth in high CO2 (530 ppm) did not affect either photosynthesis or pigment contents in both varieties. Reduced MDA content, antioxidant and antioxidative enzymes levels were observed in both varieties in response to high CO2. PEHM-5 accumulated more biomass than SMH-3031 under eCO2. PEHM-5 also had more seed starch and total soluble sugar than SMH-3031. However, SMH-3031 had increased number of seed per cob than PEHM-5. Interestingly, thousand seed weight was significantly increased in PEHM-5 only, while it was decreased in SMH-3031 under eCO2. We observed increased seed size in PEHM-5, while the size of the SMH-3031 seeds remained unaltered. Leaf proteomics revealed more abundance of proteins related to Calvin cycle, protein synthesis assembly and degradation, defense and redox homeostasis in PEHM-5 that contributed to better growth and yield in elevated CO2. While in SMH-3031 leaf, proteins related to Calvin cycle, defense and redox homeostasis were less abundant in elevated CO2 resulting in average growth and yield. The results showed a differential response of two maize varieties to eCO2.

Thioredoxin targets are regulated in heterocysts of cyanobacterium Anabaena sp. PCC 7120 in a light-independent manner

In the nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120, glucose 6-phosphate dehydrogenase (G6PDH) plays an important role in producing the power for reducing nitrogenase under light conditions. Our previous study showed that thioredoxin suppresses G6PDH by reducing its activator protein OpcA, implying that G6PDH is inactivated under light conditions because thioredoxins are reduced by the photosynthetic electron transport system in cyanobacteria. To address how Anabaena sp. PCC 7120 maintains G6PDH activity even under light conditions when nitrogen fixation occurs, we investigated the redox regulation system in vegetative cells and specific nitrogen-fixing cells named heterocysts, individually. We found that thioredoxin target proteins were more oxidized in heterocysts than in vegetative cells under light conditions. Alterations in the redox regulation mechanism of heterocysts may affect the redox states of thioredoxin target proteins, including OpcA, so that G6PDH is activated in heterocysts even under light conditions.

Modulatory Role of Reactive Oxygen Species in Root Development in Model Plant of Arabidopsis thaliana

Reactive oxygen species (ROS), a type of oxygen monoelectronic reduction product, have a higher chemical activity than O₂. Although ROS pose potential risks to all organisms via inducing oxidative stress, indispensable role of ROS in individual development cannot be ignored. Among them, the role of ROS in the model plant Arabidopsis thaliana is deeply studied. Mounting evidence suggests that ROS are essential for root and root hair development. In the present review, we provide an updated perspective on the latest research progress pertaining to the role of ROS in the precise regulation of root stem cell maintenance and differentiation, redox regulation of the cell cycle, and root hair initiation during root growth. Among the different types of ROS, O₂ ^•- and H₂O₂ have been extensively investigated, and they exhibit different gradient distributions in the roots. The concentration of O₂ ^•- decreases along a gradient from the meristem to the transition zone and the concentration of H₂O₂ decreases along a gradient from the differentiation zone to the elongation zone. These gradients are regulated by peroxidases, which are modulated by the UPBEAT1 (UPB1) transcription factor. In addition, multiple transcriptional factors, such as APP1, ABO8, PHB3, and RITF1, which are involved in the brassinolide signaling pathway, converge as a ROS signal to regulate root stem cell maintenance. Furthermore, superoxide anions (O₂ ^•-) are generated from the oxidation in mitochondria, ROS produced during plasmid metabolism, H₂O₂ produced in apoplasts, and catalysis of respiratory burst oxidase homolog (RBOH) in the cell membrane. Furthermore, ROS can act as a signal to regulate redox status, which regulates the expression of the cell-cycle components CYC2;3, CYCB1;1, and retinoblastoma-related protein, thereby controlling the cell-cycle progression. In the root maturation zone, the epidermal cells located in the H cell position emerge to form hair cells, and plant hormones, such as auxin and ethylene regulate root hair formation via ROS. Furthermore, ROS accumulation can influence hormone signal transduction and vice versa. Data about the association between nutrient stress and ROS signals in root hair development are scarce. However, the fact that ROBHC/RHD2 or RHD6 is specifically expressed in root hair cells and induced by nutrients, may explain the relationship. Future studies should focus on the regulatory mechanisms underlying root hair development via the interactions of ROS with hormone signals and nutrient components.

Thioredoxin and protein-disulfide isomerase selectivity for redox regulation of proteins in Corynebacterium glutamicum

Thioredoxins (Trxs) and protein-disulfide isomerases (PDIs) are believed to play a pivotal role in ensuring the proper folding of proteins, facilitating appropriate functioning of proteins, and maintaining intracellular redox homeostasis in bacteria. Two thioredoxins (Trxs) and three thiol-disulfide isomerases (PDIs) have been annotated in Corynebacterium glutamicum. However, nothing is known about their functional diversity in the redox regulation of proteins. Thus, we here analyzed the Trx- and PDI-dependent redox shifts of ribonucleotide reductase (RNR), insulin, 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), and several thiol-dependent peroxidases by measuring enzyme activity and thiol status in vitro. We found that the two Trxs and the three PDIs had activities in the cleavage of the disulfidebond, whereas the PDIs had a lower efficiency than the two Trxs. Trx2 could activate thiol-dependent peroxidases with an efficiency comparable with that of Trx1, but the PDIs were inefficient. The redox-active Cys-X-X-Cys motif harbored in both Trxs and PDIs was essential to supply efficiently the donor of reducing equivalents for protein disulfides. In addition, stress-responsive extracytoplasmic function (ECF)-sigma factor H (SigH)-dependent Trxs and PDIs expressions were observed. These results contributed importantly to our overall understanding of reducing functionality of the Trx and PDI systems, and also highlighted the complexity and plasticity of the intracellular redox network.

New Light on Chloroplast Redox Regulation: Molecular Mechanism of Protein Thiol Oxidation

Thiol-based redox regulation is a posttranslational protein modification that plays a key role in adjusting chloroplast functions in response to changing light conditions. Redox-sensitive target proteins are reduced upon illumination, which turns on (or off in a certain case) their enzyme activities. A redox cascade via ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin has been classically recognized as the key system for transmitting the light-induced reductive signal to target proteins. By contrast, the molecular mechanism underlying target protein oxidation, which is observed during light to dark transitions, remains undetermined over the past several decades. Recently, the factors and pathways for protein thiol oxidation in chloroplasts have been reported, finally shedding light on this long-standing issue. We identified thioredoxin-like2 as one of the protein-oxidation factors in chloroplasts. This protein is characterized by its higher redox potential than that of canonical thioredoxin, that is more favorable for target protein oxidation. Furthermore, 2-Cys peroxiredoxin and hydrogen peroxide are also involved in the overall protein-oxidation machinery. Here we summarize the newly uncovered "dark side" of chloroplast redox regulation, giving an insight into how plants rest their photosynthetic activity at night.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade acts as oxidative activator of glucose-6-phosphate dehydrogenase in chloroplasts

Thiol-based redox regulation is crucial for adjusting chloroplast functions under fluctuating light environments. We recently discovered that the thioredoxin-like2 (TrxL2)/2-Cys peroxiredoxin (2CP) redox cascade supports oxidative thiol modulation by using hydrogen peroxide (H2O2) as an oxidizing force. This system plays a key role in switching chloroplast metabolism (e.g. Calvin–Benson cycle) during light to dark transitions; however, information on its function is still limited. In this study, we report a novel protein-activation mechanism based on the TrxL2/2CP redox cascade. Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes the first step of the oxidative pentose phosphate pathway (OPPP). Biochemical studies, including redox state determination and measurement of enzyme activity, suggested that the TrxL2/2CP pathway is involved in the oxidative activation of G6PDH. It is thus likely that the TrxL2/2CP redox cascade shifts chloroplast metabolism to night mode by playing a dual role, namely, down-regulation of the Calvin–Benson cycle and up-regulation of OPPP. G6PDH was also directly oxidized and activated by H2O2, particularly when H2O2 concentration was elevated. Therefore, G6PDH is thought to be finely tuned by H2O2 levels in both direct and indirect manners.

Chloroplast thioredoxin systems dynamically regulate photosynthesis in plants

Photosynthesis is a highly regulated process in photoautotrophic cells. The main goal of the regulation is to keep the basic photosynthetic reactions, i.e. capturing light energy, conversion into chemical energy and production of carbohydrates, in balance. The rationale behind the evolution of strong regulation mechanisms is to keep photosynthesis functional under all conditions encountered by sessile plants during their lifetimes. The regulatory mechanisms may, however, also impair photosynthetic efficiency by overriding the photosynthetic reactions in controlled environments like crop fields or bioreactors, where light energy could be used for production of sugars instead of dissipation as heat and down-regulation of carbon fixation. The plant chloroplast has a high number of regulatory proteins called thioredoxins (TRX), which control the function of chloroplasts from biogenesis and assembly of chloroplast machinery to light and carbon fixation reactions as well as photoprotective mechanisms. Here, we review the current knowledge of regulation of photosynthesis by chloroplast TRXs and assess the prospect of improving plant photosynthetic efficiency by modification of chloroplast thioredoxin systems.

Proteomic Analyses of Thioredoxins f and m Arabidopsis thaliana Mutants Indicate Specific Functions for These Proteins in Plants

A large number of plastidial thioredoxins (TRX) are present in chloroplast and the specificity versus the redundancy of their functions is currently under discussion. Several results have highlighted the fact that each TRX has a specific target protein and thus a specific function. In this study we have found that in vitro activation of the fructose-1,6-bisphosphatase (FBPase) enzyme is more efficient when f1 and f2 type thioredoxins (TRXs) are used, whilst the m3 type TRX did not have any effect. In addition, we have carried out a two-dimensional electrophoresis-gel to obtain the protein profiling analyses of the trxf1, f2, m1, m2, m3 and m4 Arabidopsis mutants. The results revealed quantitative alteration of 86 proteins and demonstrated that the lack of both the f and m type thioredoxins have diverse effects on the proteome. Interestingly, 68% of the differentially expressed proteins in trxf1 and trxf2 mutants were downregulated, whilst 75% were upregulated in trxm1, trxm2, trxm3 and trxm4 lines. The lack of TRX f1 provoked a higher number of down regulated proteins. The contrary occurred when TRX m4 was absent. Most of the differentially expressed proteins fell into the categories of metabolic processes, the Calvin–Benson cycle, photosynthesis, response to stress, hormone signalling and protein turnover. Photosynthesis, the Calvin–Benson cycle and carbon metabolism are the most affected processes. Notably, a significant set of proteins related to the answer to stress situations and hormone signalling were affected. Despite some studies being necessary to find specific target proteins, these results show signs that are suggest that the f and m type plastidial TRXs most likely have some additional specific functions.

Application of Data-Independent Acquisition Approach to Study the Proteome Change from Early to Later Phases of Tomato Pathogenesis Responses

Plants and pathogens are entangled in a continual arms race. Plants have evolved dynamic defence and immune mechanisms to resist infection and enhance immunity for second wave attacks from the same or different types of pathogenic species. In addition to evolutionarily and physiological changes, plant-pathogen interaction is also highly dynamic at the molecular level. Recently, an emerging quantitative mass spectrometry-based proteomics approach named data-independent acquisition (DIA), has been developed for the analysis of the proteome in a high-throughput fashion. In this study, the DIA approach was applied to quantitatively trace the change in the plant proteome from the early to the later stage of pathogenesis progression. This study revealed that at the early stage of the pathogenesis response, proteins directly related to the chaperon were regulated for the defence proteins. At the later stage, not only the defence proteins but also a set of the pathogen-associated molecular pattern-triggered immunity (PTI) and effector triggered immunity (ETI)-related proteins were highly induced. Our findings show the dynamics of the plant regulation of pathogenesis at the protein level and demonstrate the potential of using the DIA approach for tracing the dynamics of the plant proteome during pathogenesis responses.

Versatility of Cyclophilins in Plant Growth and Survival: A Case Study in Arabidopsis

Cyclophilins (CYPs) belong to a peptidyl-prolyl cis-trans isomerase family, and were first characterized in mammals as a target of an immunosuppressive drug, cyclosporin A, preventing proinflammatory cytokine production. In Arabidopsis, 29 CYPs and CYP-like proteins are found across all subcellular compartments, involved in various physiological processes including transcriptional regulation, organogenesis, photosynthetic and hormone signaling pathways, stress adaptation and defense responses. These important but diverse activities of CYPs must be reflected by their versatility as cellular and molecular modulators. However, our current knowledge regarding their mode of actions is still far from complete. This review will briefly revisit recent progresses on the roles and mechanisms of CYPs in Arabidopsis studies, and information gaps within, which help understanding the phenotypic and environmental plasticity of plants.

An L,L-diaminopimelate aminotransferase mutation leads to metabolic shifts and growth inhibition in Arabidopsis

Lysine (Lys) connects the mitochondrial electron transport chain to amino acid catabolism and the tricarboxylic acid cycle. However, our understanding of how a deficiency in Lys biosynthesis impacts plant metabolism and growth remains limited. Here, we used a previously characterized Arabidopsis mutant (dapat) with reduced activity of the Lys biosynthesis enzyme L,L-diaminopimelate aminotransferase to investigate the physiological and metabolic impacts of impaired Lys biosynthesis. Despite displaying similar stomatal conductance and internal CO2 concentration, we observed reduced photosynthesis and growth in the dapat mutant. Surprisingly, whilst we did not find differences in dark respiration between genotypes, a lower storage and consumption of starch and sugars was observed in dapat plants. We found higher protein turnover but no differences in total amino acids during a diurnal cycle in dapat plants. Transcriptional and two-dimensional (isoelectric focalization/SDS-PAGE) proteome analyses revealed alterations in the abundance of several transcripts and proteins associated with photosynthesis and photorespiration coupled with a high glycine/serine ratio and increased levels of stress-responsive amino acids. Taken together, our findings demonstrate that biochemical alterations rather than stomatal limitations are responsible for the decreased photosynthesis and growth of the dapat mutant, which we hypothesize mimics stress conditions associated with impairments in the Lys biosynthesis pathway.

Crystal Structure of Chloroplastic Thioredoxin f2 from Chlamydomonas reinhardtii Reveals Distinct Surface Properties

Protein disulfide reduction by thioredoxins (TRXs) controls the conformation of enzyme active sites and their multimeric complex formation. TRXs are small oxidoreductases that are broadly conserved in all living organisms. In photosynthetic eukaryotes, TRXs form a large multigenic family, and they have been classified in different types: f, m, x, y, and z types are chloroplastic, while o and h types are located in mitochondria and cytosol. In the model unicellular alga Chlamydomonas reinhardtii, the TRX family contains seven types, with f- and h-types represented by two isozymes. Type-f TRXs interact specifically with targets in the chloroplast, controlling photosynthetic carbon fixation by the Calvin⁻Benson cycle. We solved the crystal structures of TRX f2 and TRX h1 from C. reinhardtii. The systematic comparison of their atomic features revealed a specific conserved electropositive crown around the active site of TRX f, complementary to the electronegative surface of their targets. We postulate that this surface provides specificity to each type of TRX.

Characterization of TrxC, an Atypical Thioredoxin Exclusively Present in Cyanobacteria

Cyanobacteria form a diverse group of oxygenic photosynthetic prokaryotes considered to be the antecessor of plant chloroplast. They contain four different thioredoxins isoforms, three of them corresponding to m, x and y type present in plant chloroplast, while the fourth one (named TrxC) is exclusively found in cyanobacteria. TrxC has a modified active site (WCGLC) instead of the canonical (WCGPC) present in most thioredoxins. We have purified it and assayed its activity but surprisingly TrxC lacked all the classical activities, such as insulin precipitation or activation of the fructose-1,6-bisphosphatase. Mutants lacking trxC or over-expressing it were generated in the model cyanobacterium Synechocystis sp. PCC 6803 and their phenotypes have been analyzed. The ΔtrxC mutant grew at similar rates to WT in all conditions tested although it showed an increased carotenoid content especially under low carbon conditions. Overexpression strains showed reduced growth under the same conditions and accumulated lower amounts of carotenoids. They also showed lower oxygen evolution rates at high light but higher Fv’/Fm’ and Non-photochemical-quenching (NPQ) in dark adapted cells, suggesting a more oxidized plastoquinone pool. All these data suggest that TrxC might have a role in regulating photosynthetic adaptation to low carbon and/or high light conditions.

Towards Initial Indications for a Thiol-Based Redox Control of Arabidopsis 5-Aminolevulinic Acid Dehydratase

Thiol-based redox control is one of the important posttranslational mechanisms of the tetrapyrrole biosynthesis pathway. Many enzymes of the pathway have been shown to interact with thioredoxin (TRX) and Nicotinamide adenine dinucleotide phosphate (NADPH)-dependent thioredoxin reductase C (NTRC). We examined the redox-dependency of 5-aminolevulinic acid dehydratase (ALAD), which catalyzed the conjugation of two 5-aminolevulinic acid (ALA) molecules to porphobilinogen. ALAD interacted with TRX f, TRX m and NTRC in chloroplasts. Consequently, less ALAD protein accumulated in the trx f1, ntrc and trx f1/ntrc mutants compared to wild-type control resulting in decreased ALAD activity. In a polyacrylamide gel under non-reducing conditions, ALAD monomers turned out to be present in reduced and two oxidized forms. The reduced and oxidized forms of ALAD differed in their catalytic activity. The addition of TRX stimulated ALAD activity. From our results it was concluded that (i) deficiency of the reducing power mainly affected the in planta stability of ALAD; and (ii) the reduced form of ALAD displayed increased enzymatic activity.

Determining the Rate-Limiting Step for Light-Responsive Redox Regulation in Chloroplasts

Thiol-based redox regulation ensures light-responsive control of chloroplast functions. Light-derived signal is transferred in the form of reducing power from the photosynthetic electron transport chain to several redox-sensitive target proteins. Two types of protein, ferredoxin-thioredoxin reductase (FTR) and thioredoxin (Trx), are well recognized as the mediators of reducing power. However, it remains unclear which step in a series of redox-relay reactions is the critical bottleneck for determining the rate of target protein reduction. To address this, the redox behaviors of FTR, Trx, and target proteins were extensively characterized in vitro and in vivo. The FTR/Trx redox cascade was reconstituted in vitro using recombinant proteins from Arabidopsis. On the basis of this assay, we found that the FTR catalytic subunit and f-type Trx are rapidly reduced after the drive of reducing power transfer, irrespective of the presence or absence of their downstream target proteins. By contrast, three target proteins, fructose 1,6-bisphosphatase (FBPase), sedoheptulose 1,7-bisphosphatase (SBPase), and Rubisco activase (RCA) showed different reduction patterns; in particular, SBPase was reduced at a low rate. The in vivo study using Arabidopsis plants showed that the Trx family is commonly and rapidly reduced upon high light irradiation, whereas FBPase, SBPase, and RCA are differentially and slowly reduced. Both of these biochemical and physiological findings suggest that reducing power transfer from Trx to its target proteins is a rate-limiting step for chloroplast redox regulation, conferring distinct light-responsive redox behaviors on each of the targets.

The NADPH-Dependent Thioredoxin Reductase C-2-Cys Peroxiredoxin Redox System Modulates the Activity of Thioredoxin x in Arabidopsis Chloroplasts

The chloroplast redox network is composed of a complex set of thioredoxins (Trxs), reduced by ferredoxin (Fdx) via a Fdx-dependent Trx reductase (FTR), and an NADPH-dependent Trx reductase with a joint Trx domain, NTRC, which efficiently reduces 2-Cys peroxiredoxins (2-Cys Prxs). Recently, it was proposed that the redox balance of 2-Cys Prxs maintains the redox state of f-type Trxs, thus allowing the proper redox regulation of Calvin-Benson cycle enzymes such as fructose 1,6-bisphosphatase (FBPase). Here, we have addressed whether the action of 2-Cys Prxs is also exerted on Trx x. To that end, an Arabidopsis thaliana quadruple mutant, ntrc-trxx-Δ2cp, which is knocked out for NTRC and Trx x, and contains severely decreased levels of 2-Cys Prxs, was generated. In contrast to ntrc-trxx, which showed a severe growth inhibition phenotype and poor photosynthetic performance, the ntrc-trxx-Δ2cp mutant showed a significant recovery of growth rate and photosynthetic efficiency, indicating that the content of 2-Cys Prxs is critical for the performance of plants lacking both NTRC and Trx x. Light-dependent reduction of FBPase was severely impaired in mutant plants lacking NTRC or NTRC plus Trx x, despite the fact that neither NTRC nor Trx x is an effective reductant of this enzyme. However, FBPase reduction was recovered in the ntrc-trxx-Δ2cp mutant. Our results show that the redox balance of 2-Cys Prxs, which is mostly dependent on NTRC, modulates the activity of Trx x in a similar way as f-type Trxs, thus suggesting that the activity of these Trxs is highly interconnected.

Small Molecules Govern Thiol Redox Switches

Oxygenic photosynthesis gave rise to a regulatory mechanism based on reversible redox-modifications of enzymes. In chloroplasts, such on-off switches separate metabolic pathways to avoid futile cycles. During illumination, the redox interconversions allow for rapidly and finely adjusting activation states of redox-regulated enzymes. Noncovalent effects by metabolites binding to these enzymes, here addressed as 'small molecules', affect the rates of reduction and oxidation. The chloroplast enzymes provide an example for a versatile regulatory principle where small molecules govern thiol switches to integrate redox state and metabolism for an appropriate response to environmental challenges. In general, this principle can be transferred to reactive thiols involved in redox signaling, oxidative stress responses, and in disease of all organisms.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade supports oxidative thiol modulation in chloroplasts

Thiol-based redox regulation is central to adjusting chloroplast functions under varying light conditions. A redox cascade via the ferredoxin-thioredoxin reductase (FTR)/thioredoxin (Trx) pathway has been well recognized to mediate the light-responsive reductive control of target proteins; however, the molecular basis for reoxidizing its targets in the dark remains unidentified. Here, we report a mechanism of oxidative thiol modulation in chloroplasts. We biochemically characterized a chloroplast stroma-localized atypical Trx from Arabidopsis, designated as Trx-like2 (TrxL2). TrxL2 had redox-active properties with an unusually less negative redox potential. By an affinity chromatography-based method, TrxL2 was shown to interact with a range of chloroplast redox-regulated proteins. The direct discrimination of thiol status indicated that TrxL2 can efficiently oxidize, but not reduce, these proteins. A notable exception was found in 2-Cys peroxiredoxin (2CP); TrxL2 was able to reduce 2CP with high efficiency. We achieved a complete in vitro reconstitution of the TrxL2/2CP redox cascade for oxidizing redox-regulated proteins and draining reducing power to hydrogen peroxide (H₂O₂). We further addressed the physiological relevance of this system by analyzing protein-oxidation dynamics. In Arabidopsis plants, a decreased level of 2CP led to the impairment of the reoxidation of redox-regulated proteins during light-dark transitions. A delayed response of protein reoxidation was concomitant with the prolonged accumulation of reducing power in TrxL2. These results suggest an in vivo function of the TrxL2/2CP redox cascade for driving oxidative thiol modulation in chloroplasts.

Regulation by S-nitrosylation of the Calvin-Benson cycle fructose-1,6-bisphosphatase in Pisum sativum

Redox regulation is of great importance in chloroplasts. Many chloroplast enzymes, such as those belonging to the Calvin-Benson cycle (CBC), have conserved regulatory cysteines which form inhibitory disulphide bridges when physiological conditions become unfavourable. Amongst these enzymes, cFBP1, the CBC fructose-1,6-bisphosphatase (FBPase) isoform, is well known to be redox activated by thioredoxin f through the reduction of a disulphide bridge involving Cys153 and Cys173. Moreover, data obtained during recent years point to S-nitrosylation as another redox post-translational modification putatively regulating an increasing number of plant enzymes, including cFBP1. In this study we have shown that the Pisum sativum cFBP1 can be efficiently S-nitrosylated by GSNO and SNAP, triggering the formation of the regulatory disulphide. Using in vivo experiments with P. sativum we have established that cFBP1 S-nitrosylation only occurs during the light period and we have elucidated by activity assays with Cys-to-Ser mutants that this enzyme may be inactivated through the S-nitrosylation of Cys153. Finally, in the light of the new data, we have proposed an extended redox-regulation model by integrating the S-nitrosylation and the TRX f-mediated regulation of cFBP1.

A Salutary Role of Reactive Oxygen Species in Intercellular Tunnel-Mediated Communication

The reactive oxygen species, generally labeled toxic due to high reactivity without target specificity, are gradually uncovered as signaling molecules involved in a myriad of biological processes. But one important feature of ROS roles in macromolecule movement has not caught attention until recent studies with technique advance and design elegance have shed lights on ROS signaling for intercellular and interorganelle communication. This review begins with the discussions of genetic and chemical studies on the regulation of symplastic dye movement through intercellular tunnels in plants (plasmodesmata), and focuses on the ROS regulatory mechanisms concerning macromolecule movement including small RNA-mediated gene silencing movement and protein shuttling between cells. Given the premise that intercellular tunnels (bridges) in mammalian cells are the key physical structures to sustain intercellular communication, movement of macromolecules and signals is efficiently facilitated by ROS-induced membrane protrusions formation, which is analogously applied to the interorganelle communication in plant cells. Although ROS regulatory differences between plant and mammalian cells exist, the basis for ROS-triggered conduit formation underlies a unifying conservative theme in multicellular organisms. These mechanisms may represent the evolutionary advances that have enabled multicellularity to gain the ability to generate and utilize ROS to govern material exchanges between individual cells in oxygenated environment.

M-type thioredoxins are involved in the xanthophyll cycle and proton motive force to alter NPQ under low-light conditions in Arabidopsis

M-type thioredoxins are required to regulate zeaxanthin epoxidase activity and to maintain the steady-state level of the proton motive force, thereby influencing NPQ properties under low-light conditions in Arabidopsis. Non-photochemical quenching (NPQ) helps protect photosynthetic organisms from photooxidative damage via the non-radiative dissipation of energy as heat. Energy-dependent quenching (qE) is a major constituent of NPQ. However, the mechanism underlying the regulation of qE is not well understood. In this study, we demonstrate that the m-type thioredoxins TRX-m1, TRX-m2, and TRX-m4 (TRX-ms) interact with the xanthophyll cycle enzyme zeaxanthin epoxidase (ZE) and are required for maintaining the redox-dependent stabilization of ZE by regulating its intermolecular disulfide bridges. Reduced ZE activity and accumulated zeaxanthin levels were observed under TRX-ms deficiency. Furthermore, concurrent deficiency of TRX-ms resulted in a significant increase in proton motive force (pmf) and acidification of the thylakoid lumen under low irradiance, perhaps due to the significantly reduced ATP synthase activity under TRX-ms deficiency. The increased pmf, combined with acidification of the thylakoid lumen and the accumulation of zeaxanthin, ultimately contribute to the elevated stable qE in VIGS-TRX-m2m4/m1 plants under low-light conditions. Taken together, these results indicate that TRX-ms are involved in regulating NPQ-dependent photoprotection in Arabidopsis.

NTRC-dependent redox balance of 2-Cys peroxiredoxins is needed for optimal function of the photosynthetic apparatus

Chloroplasts harbor a complex redox network formed by two systems, the FTR- thioredoxins (Trxs), which relies on photoreduced ferredoxin (Fd), and the NADPH-dependent Trx reductase C NTRC. Thus, an important issue in chloroplast biology is to establish the relationship between these redox pathways. Here we propose that the Fd-FTR-Trxs and NTRC redox systems are integrated via the redox balance of 2-Cys peroxiredoxins (Prxs), which therefore has a key role in chloroplast function. NTRC controls the redox balance of 2-Cys Prxs, which maintains the reducing capacity of the pool of chloroplast Trxs and, consequently, proper regulation of photosynthetic carbon assimilation enzymes. Therefore, redox regulation of chloroplast enzymes and hydrogen peroxide reduction are linked by the action of the NTRC-2-Cys Prxs system.

Thiol-dependent redox regulation allows the rapid adaptation of chloroplast function to unpredictable changes in light intensity. Traditionally, it has been considered that chloroplast redox regulation relies on photosynthetically reduced ferredoxin (Fd), thioredoxins (Trxs), and an Fd-dependent Trx reductase (FTR), the Fd-FTR-Trxs system, which links redox regulation to light. More recently, a plastid-localized NADPH-dependent Trx reductase (NTR) with a joint Trx domain, termed NTRC, was identified. NTRC efficiently reduces 2-Cys peroxiredoxins (Prxs), thus having antioxidant function, but also participates in redox regulation of metabolic pathways previously established to be regulated by Trxs. Thus, the NTRC, 2-Cys Prxs, and Fd-FTR-Trxs redox systems may act concertedly, but the nature of the relationship between them is unknown. Here we show that decreased levels of 2-Cys Prxs suppress the phenotype of the Arabidopsis thaliana ntrc KO mutant. The excess of oxidized 2-Cys Prxs in NTRC-deficient plants drains reducing power from chloroplast Trxs, which results in low efficiency of light energy utilization and impaired redox regulation of Calvin–Benson cycle enzymes. Moreover, the dramatic phenotype of the ntrc-trxf1f2 triple mutant, lacking NTRC and f-type Trxs, was also suppressed by decreased 2-Cys Prxs contents, as the ntrc-trxf1f2-Δ2cp mutant partially recovered the efficiency of light energy utilization and exhibited WT rate of CO₂ fixation and growth phenotype. The suppressor phenotype was not caused by compensatory effects of additional chloroplast antioxidant systems. It is proposed that the Fd-FTR-Trx and NTRC redox systems are linked by the redox balance of 2-Cys Prxs, which is crucial for chloroplast function.

Thioredoxin and NADPH-Dependent Thioredoxin Reductase C Regulation of Tetrapyrrole Biosynthesis

In chloroplasts, thioredoxin (TRX) isoforms and NADPH-dependent thioredoxin reductase C (NTRC) act as redox regulatory factors involved in multiple plastid biogenesis and metabolic processes. To date, less is known about the functional coordination between TRXs and NTRC in chlorophyll biosynthesis. In this study, we aimed to explore the potential functions of TRX m and NTRC in the regulation of the tetrapyrrole biosynthesis (TBS) pathway. Silencing of three genes, TRX m1, TRX m2, and TRX m4 (TRX ms), led to pale-green leaves, a significantly reduced 5-aminolevulinic acid (ALA)-synthesizing capacity, and reduced accumulation of chlorophyll and its metabolic intermediates in Arabidopsis (Arabidopsis thaliana). The contents of ALA dehydratase, protoporphyrinogen IX oxidase, the I subunit of Mg-chelatase, Mg-protoporphyrin IX methyltransferase (CHLM), and NADPH-protochlorophyllide oxidoreductase were decreased in triple TRX m-silenced seedlings compared with the wild type, although the transcript levels of the corresponding genes were not altered significantly. Protein-protein interaction analyses revealed a physical interaction between the TRX m isoforms and CHLM. 4-Acetoamido-4-maleimidylstilbene-2,2-disulfonate labeling showed the regulatory impact of TRX ms on the CHLM redox status. Since CHLM also is regulated by NTRC (Richter et al., 2013), we assessed the concurrent functions of TRX m and NTRC in the control of CHLM. Combined deficiencies of three TRX m isoforms and NTRC led to a cumulative decrease in leaf pigmentation, TBS intermediate contents, ALA synthesis rate, and CHLM activity. We discuss the coordinated roles of TRX m and NTRC in the redox control of CHLM stability with its corollary activity in the TBS pathway.

Cyclophilin 20-3 is positioned as a regulatory hub between light-dependent redox and 12-oxo-phytodienoic acid signaling

The jasmonate family of phytohormones plays central roles in plant development and stress acclimation. However, the regulatory modes of their signaling circuitry remain largely unknown. Here we describe that cyclophilin 20-3 (CYP20-3), a binding protein of (+)-12-oxo-phytodienoic acid (OPDA), crisscrosses stress responses with light-dependent redox reactions, which fine-tunes the activity of key enzymes in the plastid photosynthetic carbon assimilation and sulfur assimilation pathways. Under stressed states, OPDA - accumulated in the chloroplasts - binds and promotes CYP20-3 to transfer electron (e^-) from thioredoxins (i.e., type-f2 and -x) to 2-Cys peroxiredoxin B (2-CysPrxB) or serine acetyltransferase 1 (SAT1). Reduction (activation) of 2-CysPrxB then optimizes peroxide detoxification and carbon metabolisms in the photosynthesis, whereas the activation of SAT1 stimulates sulfur assimilation which in turn coordinates redox-resolved nucleus gene expressions in defense responses against biotic and abiotic stresses. Thus, we conclude that CYP20-3 is positioned as a unique metabolic hub in the interface between photosynthesis (light) and OPDA signaling, where controls resource (e^-) allocations between plant growth and defense responses.