Redox regulation of chloroplastic G6PDH activity by thioredoxin occurs through structural changes modifying substrate accessibility and cofactor binding

The oxidative pentose phosphate pathway in photosynthesis: a tale of two shunts

CO2 release in the light (RL) and its presumed source, oxidative pentose phosphate pathways, were found to be insensitive to CO2 concentration. The oxidative pentose phosphate pathways form glucose 6-phosphate (G6P) shunts that bypass the nonoxidative pentose phosphate reactions of the Calvin-Benson cycle. Using adenosine diphosphate glucose and uridine diphosphate glucose as proxies for labeling of G6P in the stroma and cytosol respectively, it was found that only the cytosolic shunt was active. Uridine diphosphate glucose, a proxy for cytosolic G6P, and 6-phosphogluconate (6PG) were significantly less labeled than Calvin-Benson cycle intermediates in the light. But ADP glucose, a proxy for stromal G6P, is labeled to the same degree as Calvin-Benson cycle intermediates and much greater than 6PG. A metabolically inert pool of sedoheptulose bisphosphate can slowly equilibrate keeping the label in sedoheptulose lower than in other stromal metabolites. Finally, phosphorylation of fructose 6-phosphate (F6P) in the cytosol can allow some unlabeled carbon in cytosolic F6P to dilute label in phosphenolpyruvate. The results clearly show that there is oxidative pentose phosphate pathway activity in the cytosol that provides a shunt around the nonoxidative pentose phosphate pathway reactions of the Calvin-Benson cycle and is not strongly CO2-sensitive.

Evidence for dual targeting control of Arabidopsis 6-phosphogluconate dehydrogenase isoforms by N-terminal phosphorylation

The oxidative pentose-phosphate pathway (OPPP) retrieves NADPH from glucose-6-phosphate, which is important in chloroplasts at night and in plastids of heterotrophic tissues. We previously studied how OPPP enzymes may transiently locate to peroxisomes, but how this is achieved for the third enzyme remained unclear. By extending our genetic approach, we demonstrated that Arabidopsis isoform 6-phosphogluconate dehydrogenase 2 (PGD2) is indispensable in peroxisomes during fertilization, and investigated why all PGD-reporter fusions show a mostly cytosolic pattern. A previously published interaction of a plant PGD with thioredoxin m was confirmed using Trxm2 for yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays, and medial reporter fusions (with both ends accessible) proved to be beneficial for studying peroxisomal targeting of PGD2. Of special importance were phosphomimetic changes at Thr6, resulting in a clear targeting switch to peroxisomes, while a similar change at position Ser7 in PGD1 conferred plastid import. Apparently, efficient subcellular localization can be achieved by activating an unknown kinase, either early after or during translation. N-terminal phosphorylation of PGD2 interfered with dimerization in the cytosol, thus allowing accessibility of the C-terminal peroxisomal targeting signal (PTS1). Notably, we identified amino acid positions that are conserved among plant PGD homologues, with PTS1 motifs first appearing in ferns, suggesting a functional link to fertilization during the evolution of seed plants.

The end game(s) of photosynthetic carbon metabolism

The year 2024 marks 70 years since the general outline of the carbon pathway in photosynthesis was published. Although several alternative pathways are now known, it is remarkable how many organisms use the reaction sequence described 70 yrs ago, which is now known as the Calvin-Benson cycle or variants such as the Calvin-Benson-Bassham cycle or Benson-Calvin cycle. However, once the carbon has entered the Calvin-Benson cycle and is converted to a 3-carbon sugar, it has many potential fates. This review will examine the last stages of photosynthetic metabolism in leaves. In land plants, this process mostly involves the production of sucrose provided by an endosymbiont (the chloroplast) to its host for use and transport to the rest of the plant. Photosynthetic metabolism also usually involves the synthesis of starch, which helps maintain respiration in the dark and enables the symbiont to supply sugars during both the day and night. Other end products made in the chloroplast are closely tied to photosynthetic CO2 assimilation. These include serine from photorespiration and various amino acids, fatty acids, isoprenoids, and shikimate pathway products. I also describe 2 pathways that can short circuit parts of the Calvin-Benson cycle. These final processes of photosynthetic metabolism play many important roles in plants.

Persulfidation maintains cytosolic G6PDs activity through changing tetrameric structure and competing cysteine sulfur oxidation under salt stress in Arabidopsis and tomato

Glucose-6-phosphate dehydrogenases (G6PDs) are essential regulators of cellular redox. Hydrogen sulfide (H2 S) is a small gasotransmitter that improves plant adaptation to stress; however, its role in regulating G6PD oligomerization to resist oxidative stress remains unknown in plants. Persulfidation of cytosolic G6PDs was analyzed by mass spectrometry (MS). The structural change model of AtG6PD6 homooligomer was built by chemical cross-linking coupled with mass spectrometry (CXMS). We isolated AtG6PD6C159A and SlG6PDCC155A transgenic lines to confirm the in vivo function of persulfidated sites with the g6pd5,6 background. Persulfidation occurs at Arabidopsis G6PD6 Cystine (Cys)159 and tomato G6PDC Cys155, leading to alterations of spatial distance between lysine (K)491-K475 from 42.0 Å to 10.3 Å within the G6PD tetramer. The structural alteration occurs in the structural NADP+ binding domain, which governs the stability of G6PD homooligomer. Persulfidation enhances G6PD oligomerization, thereby increasing substrate affinity. Under high salt stress, cytosolic G6PDs activity was inhibited due to oxidative modifications. Persulfidation protects these specific sites and prevents oxidative damage. In summary, H2 S-mediated persulfidation promotes cytosolic G6PD activity by altering homotetrameric structure. The cytosolic G6PD adaptive regulation with two kinds of protein modifications at the atomic and molecular levels is critical for the cellular stress response.

Thioredoxins m regulate plastid glucose-6-phosphate dehydrogenase activity in Arabidopsis roots under salt stress

Plants contain several NADPH-producing enzymes including glucose-6-phosphate dehydrogenases (G6PDH) with different sub-cellular localizations. The activity of plastidial G6PDHs is redox-regulated by thioredoxins (TRX). Although specific TRXs are known to regulate chloroplastic isoforms of G6PDH, little information is available for plastidic isoforms found in heterotrophic organs or tissues. Here, we investigated TRX regulation of the two G6PDH plastidic isoforms of Arabidopsis roots during exposure to a mild salt stress. We report that in vitro m-type TRXs are the most efficient regulators of the G6PDH2 and G6PDH3 mainly found in Arabidopsis roots. While expression of the corresponding G6PD and plastidic TRX genes was marginally affected by salt, it impaired root growth of several of the corresponding mutant lines. Using an in situ assay for G6PDH, G6PDH2 was found to be the major contributor to salt-induced increases in activity, while data from ROS assays further provide in vivo evidence that TRX m acts in redox regulation during salt stress. Taken together, our data suggest that regulation of plastid G6PDH activity by TRX m may be an important player regulating NADPH production in Arabidopsis roots undergoing salt stress.

A Chloroplast-Localized Glucose-6-Phosphate Dehydrogenase Positively Regulates Stripe Rust Resistance in Wheat

Glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme of the pentose phosphate pathway (PPP), plays a pivotal role in plant stress responses. However, the function and mechanism of G6PDHs in crop plants challenged by fungal pathogens remain poorly understood. In this study, a wheat G6DPH gene responding to infection by Puccinia striiformis f. sp. tritici (Pst), designated TaG6PDH2, was cloned and functionally identified. TaG6PDH2 expression was significantly upregulated in wheat leaves inoculated with Pst or treated with abiotic stress factors. Heterologous mutant complementation and enzymatic properties indicate that TaG6PDH2 encodes a G6PDH protein. The transient expression of TaG6PDH2 in Nicotiana benthamiana leaves and wheat protoplasts revealed that TaG6PDH2 is a chloroplast-targeting protein. Silencing TaG6PDH2 via the barley stripe mosaic virus (BSMV)-induced gene silencing (VIGS) system led to compromised wheat resistance to the Pst avirulent pathotype CYR23, which is implicated in weakened H₂O₂ accumulation and cell death. In addition, TaG6PDH2 was confirmed to interact with the wheat glutaredoxin TaGrxS4. These results demonstrate that TaG6PDH2 endows wheat with increased resistance to stripe rust by regulating reactive oxygen species (ROS) production.

Glucose-6-Phosphate Dehydrogenases: The Hidden Players of Plant Physiology

Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes a metabolic hub between glycolysis and the pentose phosphate pathway (PPP), which is the oxidation of glucose-6-phosphate (G6P) to 6-phosphogluconolactone concomitantly with the production of nicotinamide adenine dinucleotide phosphate (NADPH), a reducing power. It is considered to be the rate-limiting step that governs carbon flow through the oxidative pentose phosphate pathway (OPPP). The OPPP is the main supplier of reductant (NADPH) for several "reducing" biosynthetic reactions. Although it is involved in multiple physiological processes, current knowledge on its exact role and regulation is still piecemeal. The present review provides a concise and comprehensive picture of the diversity of plant G6PDHs and their role in seed germination, nitrogen assimilation, plant branching, and plant response to abiotic stress. This work will help define future research directions to improve our knowledge of G6PDHs in plant physiology and to integrate this hidden player in plant performance.

Cytosolic glucose-6-phosphate dehydrogenases play a pivotal role in Arabidopsis seed development

Embryo development is essential for seed yield and post-germination growth. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in oxidative pentose phosphate pathway (OPPP), is widely involved in plant development and stress tolerance by providing nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the double mutant (g6pd5/6), overexpression line (G6PD5/6^OE) and complementation line (g6pd5/6^Comp) of cytosolic glucose-6-phosphate dehydrogenases (Cyt-G6PD) were used to investigate Cyt-G6PD roles in embryo development of Arabidopsis. The results showed that the germination rate of g6pd5/6 seeds was delayed in comparison with that of Col-0; moreover, 11.5% of g6pd5/6 seeds did not germinate. The dysfunction of Cyt-G6PD resulted in decreased fresh weight and primary root length of g6pd5/6 seedlings. The height and silique length of g6pd5/6 plants were also decreased. Moreover, the abortion rate of siliques and seeds of g6pd5/6 plants were increased compared with those of Col-0, G6PD5/6^OE and g6pd5/6^Comp lines. However, the dysfunction of Cyt-G6PD did not affect pollen activity; but in g6pd5/6, the embryo development was partially delayed or inhibited. The contents of fatty acids and storage proteins, two main storage materials in Arabidopsis seeds, were decreased in g6pd5/6 seeds. Exogenous application of fatty acids (C18:2; C18:3) alleviated the delayed germination of g6pd5/6 seeds. RT-qPCR results further demonstrated that the early embryo development genes were down-regulated in g6pd5/6. Taken together, Cyt-G6PD plays a pivotal role in plant seed development by regulating the transcriptions of early embryo development genes and the accumulation of storage materials (especially fatty acids).

Genome-Wide Investigation of G6PDH Gene in Strawberry: Evolution and Expression Analysis during Development and Stress

As one of the key enzymes in the pentose phosphate pathway (PPP), glucose-6-phosphate dehydrogenase (G6PDH) provides NADPH and plays an important role in plant development and stress responses. However, little information was available about the G6PDH genes in strawberry (Fragaria × ananassa). The recent release of the whole-genome sequence of strawberry allowed us to perform a genome-wide investigation into the organization and expression profiling of strawberry G6PDH genes. In the present study, 19 strawberry G6PDH genes (FaG6PDHs) were identified from the strawberry genome database. They were designated as FaG6PDH1 to FaG6PDH19, respectively, according to the conserved domain of each subfamily and multiple sequence alignment with Arabidopsis. According to their structural and phylogenetic features, the 19 FaG6PDHs were further classified into five types: Cy, P1, P1.1, P2 and PO. The number and location of exons and introns are similar, suggesting that genes of the same type are very similar and are alleles. A cis-element analysis inferred that FaG6PDHs possessed at least one stress-responsive cis-acting element. Expression profiles derived from transcriptome data analysis exhibited distinct expression patterns of FaG6PDHs genes in different developmental stages. Real-time quantitative PCR was used to detect the expression level of five types FaG6PDHs genes and demonstrated that the genes were expressed and responded to multiple abiotic stress and hormonal treatments.

Different G6PDH isoforms show specific roles in acclimation to cold stress at various growth stages of barley (Hordeum vulgare) and Arabidopsis thaliana

Low temperatures (0-10 °C) represent a major physiological stress for plants, negatively affecting both their growth rates and overall growth. Cold stress may induce a wide range of negative physiological effects, from oxidative stress to photosynthetic damage. We investigated the effects of low temperatures in two different model plants, Arabidopsis thaliana and Hordeum vulgare. We tested whether the oxidative pentose phosphate pathway (OPPP) is involved in the increase of reductants' levels needed to counteract oxidative stress induced by cold. The expression, occurrence, and activity of different glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) isoforms during cold stress and plant recovery from low temperatures, were measured at different growth stages from early germinated to mature pot-grown plants. Our results showed plants exhibited changes in different stress markers; ascorbate peroxidase - APX, catalase - CAT, proline, malondialdehyde, H₂O_2, NADPH/NADP⁺. We found that a major role in cold acclimation for cytosolic- and peroxisome-located G6PDHs, and different roles for plastidial/chloroplastic isoforms. This suggests that G6PDH isoforms may regulate redox homeostasis in low temperatures, in order to support the increased and continued demand of reductants during both cold stress and recovery stages. Furthermore, we found a significant involvement of (6PGDH), strengthening the idea that the contribution of the whole oxidative pentose phosphate pathway (OPPP) is required to sustain reductant supply under cold stress.

The Phosphofructokinase Isoform AtPFK5 Is a Novel Target of Plastidic Thioredoxin-f-Dependent Redox Regulation

The chloroplast primary metabolism is of central importance for plant growth and performance. Therefore, it is tightly regulated in order to adequately respond to multiple environmental conditions. A major fluctuation that plants experience each day is the change between day and night, i.e., the change between assimilation and dissimilation. Among other mechanisms, thioredoxin-mediated redox regulation is an important component of the regulation of plastid-localized metabolic enzymes. While assimilatory processes such as the Calvin–Benson cycle are activated under illumination, i.e., under reducing conditions, carbohydrate degradation is switched off during the day. Previous analyses have identified enzymes of the oxidative pentose phosphate pathway to be inactivated by reduction through thioredoxins. In this work, we present evidence that an enzyme of the plastidic glycolysis, the phosphofructokinase isoform AtPFK5, is also inactivated through reduction by thioredoxins, namely by thioredoxin-f. With the help of chemical oxidation, mutant analyses and further experiments, the highly conserved motif CXDXXC in AtPFK5 was identified as the target sequence for this regulatory mechanism. However, knocking out this isoform in plants had only very mild effects on plant growth and performance, indicating that the complex primary metabolism in plants can overcome a lack in AtPFK5 activity.

Investigation of Heterologously Expressed Glucose-6-Phosphate Dehydrogenase Genes in a Yeast zwf1 Deletion

Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme of the oxidative part of the pentose phosphate pathway and serves as the major source of NADPH for metabolic reactions and oxidative stress response in pro- and eukaryotic cells. We here report on a strain of the model yeast Saccharomyces cerevisiae which lacks the G6PD-encoding ZWF1 gene and displays distinct growth retardation on rich and synthetic media, as well as a strongly reduced chronological lifespan. This strain was used as a recipient to introduce plasmid-encoded heterologous G6PD genes, synthesized in the yeast codon usage and expressed under the control of the native PFK2 promotor. Complementation of the hypersensitivity of the zwf1 mutant towards hydrogen peroxide to different degrees was observed for the genes from humans (HsG6PD1), the milk yeast Kluyveromyces lactis (KlZWF1), the bacteria Escherichia coli (EcZWF1) and Leuconostoc mesenteroides (LmZWF1), as well as the genes encoding three different plant G6PD isoforms from Arabidopsis thaliana (AtG6PD1, AtG6PD5, AtG6PD6). The plastidic AtG6PD1 isoform retained its redox-sensitive activity when produced in the yeast as a cytosolic enzyme, demonstrating the suitability of this host for determination of its physiological properties. Mutations precluding the formation of a disulfide bridge in AtG6PD1 abolished its redox-sensitivity but improved its capacity to complement the yeast zwf1 deletion. Given the importance of G6PD in human diseases and plant growth, this heterologous expression system offers a broad range of applications.

Nitric oxide and hydrogen peroxide increase glucose-6-phosphate dehydrogenase activities and expression upon drought stress in soybean roots

KEY MESSAGE

Changes in glucose-6-phosphate dehydrogenase (G6PD) isoforms activities and expression were investigated in soybean roots under drought, suggesting that cytosolic G6PD plays a main role by regulating H2O2 signal and redox homeostasis. G6PD acts a vital role in plant growth, development and stress adaptation. Drought (PEG6000 treatment) could markedly increase the enzymatic activities of cytosolic G6PD (Cyt-G6PD) and compartmented G6PD (mainly plastidic P2-G6PD) in soybean roots. Application of G6PD inhibitor upon drought condition dramatically decreased the intracellular NADPH and reduced glutathione levels in soybean roots. Nitric oxide (NO) and hydrogen peroxide (H2O2) participated in the regulation of Cyt-G6PD and P2-G6PD enzymatic activities under drought stress. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, abolished the drought-induced accumulation of H2O2. The exogenous application of H2O2 and its production inhibitor (DPI) could stimulate and inhibit the NO accumulation, respectively, but not vice versa. qRT-PCR analysis confirmed that NO, as the downstream signal of H2O2, positively regulated the transcription of genes encoding Cyt-G6PD (GPD5, G6PD6, G6PD7) under drought stress in soybean roots. Comparatively, NO and H2O2 signals negatively regulated the gene expression of compartmented G6PD (GPD1, G6PD2, G6PD4), indicating that a post-transcriptional mechanism was involved in compartmented G6PD regulation. Taken together, the high Cyt-G6PD activity is essential for maintaining redox homeostasis upon drought condition in soybean roots, and the H2O2-dependent NO cascade signal is differently involved in Cyt-G6PD and compartmented G6PD regulation.

NTRC and Thioredoxin f Overexpression Differentially Induces Starch Accumulation in Tobacco Leaves

Thioredoxin (Trx) f and NADPH-dependent Trx reductase C (NTRC) have both been proposed as major redox regulators of starch metabolism in chloroplasts. However, little is known regarding the specific role of each protein in this complex mechanism. To shed light on this point, tobacco plants that were genetically engineered to overexpress the NTRC protein from the chloroplast genome were obtained and compared to previously generated Trx f-overexpressing transplastomic plants. Likewise, we investigated the impact of NTRC and Trx f deficiency on starch metabolism by generating Nicotiana benthamiana plants that were silenced for each gene. Our results demonstrated that NTRC overexpression induced enhanced starch accumulation in tobacco leaves, as occurred with Trx f. However, only Trx f silencing leads to a significant decrease in the leaf starch content. Quantitative analysis of enzyme activities related to starch synthesis and degradation were determined in all of the genotypes. Zymographic analyses were additionally performed to compare the amylolytic enzyme profiles of both transplastomic tobacco plants. Our findings indicated that NTRC overexpression promotes the accumulation of transitory leaf starch as a consequence of a diminished starch turnover during the dark period, which seems to be related to a significant reductive activation of ADP-glucose pyrophosphorylase and/or a deactivation of a putative debranching enzyme. On the other hand, increased starch content in Trx f-overexpressing plants was connected to an increase in the capacity of soluble starch synthases during the light period. Taken together, these results suggest that NTRC and the ferredoxin/Trx system play distinct roles in starch turnover.

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.

Plastidic glucose-6-phosphate dehydrogenases are regulated to maintain activity in the light

Glucose-6-phosphate dehydrogenase (G6PDH) can initiate the glucose-6-phosphate (G6P) shunt around the Calvin-Benson cycle. To understand the regulation of flux through this pathway, we have characterized the biochemical parameters and redox regulation of the three functional plastidic isoforms of Arabidopsis G6PDH. When purified, recombinant proteins were measured, all three exhibited significant substrate inhibition by G6P but not NADP⁺, making the determination of enzyme kinetic parameters complex. We found that the half-saturation concentration of G6PDH isoform 1 is increased under reducing conditions. The other two isoforms exhibit less redox regulation, however, isoform 2 is strongly inhibited by NADPH. Redox regulation of G6PDH1 can be partially reversed by hydrogen peroxide or protected against by the presence of its substrate, G6P. Overall, our results support the conclusion that G6PDH can have significant activity throughout the day and can be dynamically regulated to allow or prevent flux through the glucose-6-phosphate shunt.

Mechanism(s) of action of heavy metals to investigate the regulation of plastidic glucose-6-phosphate dehydrogenase

The regulation of recombinant plastidic glucose-6P dehydrogenase from Populus trichocarpa (PtP2-G6PDH - EC 1.1.1.49) was investigated by exposing wild type and mutagenized isoforms to heavy metals. Nickel and Cadmium caused a marked decrease in PtP2-G6PDH WT activity, suggesting their poisoning effect on plant enzymes; Lead (Pb⁺⁺) was substantially ineffective. Copper (Cu⁺⁺) and Zinc (Zn⁺⁺) exposition resulted in strongest decrease in enzyme activity, thus suggesting a physiological competition with Magnesium, a well-known activator of G6PDH activity. Kinetic analyses confirmed a competitive inhibition by Copper, and a mixed inhibition by (Cd⁺⁺). Mutagenized enzymes were differently affected by HMs: the reduction of disulfide (C¹⁷⁵–C¹⁸³) exposed the NADP⁺ binding sites to metals; C¹⁴⁵ participates to NADP⁺ cofactor binding; C¹⁹⁴ and C²⁴² are proposed to play a role in the regulation of NADP⁺/NADPH binding. Copper (and possibly Zinc) is able to occupy competitively Magnesium (Mg⁺⁺) sites and/or bind to NADP⁺, resulting in a reduced access of NADP⁺ sites on the enzyme. Hence, heavy metals could be used to describe specific roles of cysteine residues present in the primary protein sequence; these results are discussed to define the biochemical mechanism(s) of inhibition of plant plastidic G6PDH.

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.

The Deep Thioredoxome in Chlamydomonas reinhardtii: New Insights into Redox Regulation

Thiol-based redox post-translational modifications have emerged as important mechanisms of signaling and regulation in all organisms, and thioredoxin plays a key role by controlling the thiol-disulfide status of target proteins. Recent redox proteomic studies revealed hundreds of proteins regulated by glutathionylation and nitrosylation in the unicellular green alga Chlamydomonas reinhardtii, while much less is known about the thioredoxin interactome in this organism. By combining qualitative and quantitative proteomic analyses, we have comprehensively investigated the Chlamydomonas thioredoxome and 1188 targets have been identified. They participate in a wide range of metabolic pathways and cellular processes. This study broadens not only the redox regulation to new enzymes involved in well-known thioredoxin-regulated metabolic pathways but also sheds light on cellular processes for which data supporting redox regulation are scarce (aromatic amino acid biosynthesis, nuclear transport, etc). Moreover, we characterized 1052 thioredoxin-dependent regulatory sites and showed that these data constitute a valuable resource for future functional studies in Chlamydomonas. By comparing this thioredoxome with proteomic data for glutathionylation and nitrosylation at the protein and cysteine levels, this work confirms the existence of a complex redox regulation network in Chlamydomonas and provides evidence of a tremendous selectivity of redox post-translational modifications for specific cysteine residues.

Dithiol disulphide exchange in redox regulation of chloroplast enzymes in response to evolutionary and structural constraints

Redox regulation of chloroplast enzymes via disulphide reduction is believed to control the rates of CO₂ fixation. The study of the thioredoxin reduction pathways and of various target enzymes lead to the following guidelines.

Redox status in a model of cancer stem cells

Reversible oxidation of Cys residues is a crucial element of redox homeostasis and signaling. According to a popular concept in oxidative stress signaling, the oxidation of targets of signals can only take place following an overwhelming of the cellular antioxidant capacity. This concept, however, ignores the activation of feedback mechanisms possibly leading to a paradoxical effect. In a model of cancer stem cells (CSC), stably overexpressing the TAZ oncogene, we observed that the increased formation of oxidants is associated with a globally more reduced state of proteins. Redox proteomics revealed that several proteins, capable of undergoing reversible redox transitions, are indeed more reduced while just few are more oxidized. Among the proteins more oxidized, G6PDH emerges as both more expressed and activated by oxidation. This accounts for the observed more reduced state of the NADPH/NADP⁺ couple. The dynamic redox flux generating this apparently paradoxical effect is rationalized in a computational system biology model highlighting the crucial role of G6PDH activity on the rate of redox transitions eventually leading to the reduction of reversible redox switches.

Mixed results with mixed disulfides

A period of research with Helmut Sies in the 1980s is recalled. Our experiments aimed at an in-depth understanding of metabolic changes due to oxidative challenges under near-physiological conditions, i.e. perfused organs. A major focus were alterations of the glutathione and the NADPH/NADP(+) system by different kinds of oxidants, in particular formation of glutathione mixed disulfides with proteins. To analyze mixed disulfides, a test was adapted which is widely used until today. The observations in perfused rat livers let us believe that glutathione-6-phosphate dehydrogenase (G6PDH), i.a. might be activated by glutathionylation. Although we did not succeed to verify this hypothesis for the special case of G6PDH, the regulation of enzyme/protein activities by glutathionylation today is an accepted posttranslational mechanism in redox biology in general. Our early experimental approaches are discussed in the context of present knowledge.

Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants

Glucose 6 phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is well-known as the main regulatory enzyme of the oxidative pentose phosphate pathway (OPPP) in living organisms. Namely, in Planta, different G6PDH isoforms may occur, generally localized in cytosol and plastids/chloroplasts. These enzymes are differently regulated by distinct mechanisms, still far from being defined in detail. In the last decades, a pivotal function for plant G6PDHs during the assimilation of nitrogen, providing reductants for enzymes involved in nitrate reduction and ammonium assimilation, has been described. More recently, several studies have suggested a main role of G6PDH to counteract different stress conditions, among these salinity and drought, with the involvement of an ABA depending signal. In the last few years, this recognized vision has been greatly widened, due to studies clearly showing the non-conventional subcellular localization of the different G6PDHs, and the peculiar regulation of the different isoforms. The whole body of these considerations suggests a central question: how do the plant cells distribute the reductants coming from G6PDH and balance their equilibrium? This review explores the present knowledge about these mechanisms, in order to propose a scheme of distribution of reductants produced by G6PDH during nitrogen assimilation and stress.

The Path to Thioredoxin and Redox Regulation in Chloroplasts

After a brief discussion of my graduate work at Duke University, I describe a series of investigations on redox proteins at the University of California, Berkeley. Starting with ferredoxin from fermentative bacteria, the Berkeley research fostered experiments that uncovered a pathway for fixing CO2 in bacterial photosynthesis. The carbon work, in turn, opened new vistas, including the discovery that thioredoxin functions universally in regulating the Calvin-Benson cycle in oxygenic photosynthesis. These experiments, which took place over a 50-year period, led to the formulation of a set of biological principles and set the stage for research demonstrating a role for redox in the regulation of previously unrecognized processes extending far beyond photosynthesis.