PsTRXh1 and PsTRXh2 are both pea h-type thioredoxins with antagonistic behavior in redox imbalances

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.

Nucleoredoxin Gene TaNRX1 Positively Regulates Drought Tolerance in Transgenic Wheat (Triticum aestivum L.)

Drought is the main abiotic stress factor limiting the growth and yield of wheat (Triticum aestivum L.). Therefore, improving wheat tolerance to drought stress is essential for maintaining yield. Previous studies have reported on the important role of TaNRX1 in conferring drought stress tolerance. Therefore, to elucidate the regulation mechanism by which TaNRX1 confers drought resistance in wheat, we generated TaNRX1 overexpression (OE) and RNA interference (RNAi) wheat lines. The results showed that the tolerance of the OE lines to drought stress were significantly enhanced. The survival rate, leaf chlorophyll, proline, soluble sugar content, and activities of the antioxidant enzymes (catalase, superoxide dismutase, and peroxidase) of the OE lines were higher than those of the wild type (WT); however, the relative electrical conductivity and malondialdehyde, hydrogen peroxide, and superoxide anion levels of the OE lines were lower than those of the WT; the RNAi lines showed the opposite results. RNA-seq results showed that the common differentially expressed genes of TaNRX1 OE and RNAi lines, before and after drought stress, were mainly distributed in the plant-pathogen interaction, plant hormone signal transduction, phenylpropane biosynthesis, starch and sucrose metabolism, and carbon metabolism pathways and were related to the transcription factors, including WRKY, MYB, and bHLH families. This study suggests that TaNRX1 positively regulates drought stress tolerance in wheat.

Proteomics Analysis of E. angustifolia Seedlings Inoculated with Arbuscular Mycorrhizal Fungi under Salt Stress

To reveal the mechanism of salinity stress alleviation by arbuscular mycorrhizal fungi (AMF), we investigated the growth parameter, soluble sugar, soluble protein, and protein abundance pattern of E. angustifolia seedlings that were cultured under salinity stress (300 mmol/L NaCl) and inoculated by Rhizophagus irregularis (RI). Furthermore, a label-free quantitative proteomics approach was used to reveal the stress-responsive proteins in the leaves of E. angustifolia. The result indicates that the abundance of 75 proteins in the leaves was significantly influenced when E. angustifolia was inoculated with AMF, which were mainly involved in the metabolism, signal transduction, and reactive oxygen species (ROS) scavenging. Furthermore, we identified chorismate mutase, elongation factor mitochondrial, peptidyl-prolyl cis-trans isomerase, calcium-dependent kinase, glutathione S-transferase, glutathione peroxidase, NADH dehydrogenase, alkaline neutral invertase, peroxidase, and other proteins closely related to the salt tolerance process. The proteomic results indicated that E. angustifolia seedlings inoculated with AMF increased the secondary metabolism level of phenylpropane metabolism, enhanced the signal transduction of Ca²⁺ and ROS scavenging ability, promoted the biosynthesis of protein, accelerated the protein folding, and inhibited the degradation of protein under salt stress. Moreover, AMF enhanced the synthesis of ATP and provided sufficient energy for plant cell activity. This study implied that symbiosis of halophytes and AMF has potential as an application for the improvement of saline-alkali soils.

Redox biology response in germinating Phaseolus vulgaris seeds exposed to copper: Evidence for differential redox buffering in seedlings and cotyledon

In agriculture, heavy metal contamination of soil interferes with processes associated with plant growth, development and productivity. Here, we describe oxidative and redox changes, and deleterious injury within cotyledons and seedlings caused by exposure of germinating (Phaseolus vulgaris L. var. soisson nain hâtif) seeds to copper (Cu). Cu induced a marked delay in seedling growth, and was associated with biochemical disturbances in terms of intracellular oxidative status, redox regulation and energy metabolism. In response to these alterations, modulation of activities of antioxidant proteins (thioredoxin and glutathione reductase, peroxiredoxin) occurred, thus preventing oxidative damage. In addition, oxidative modification of proteins was detected in both cotyledons and seedlings by one- and two-dimensional electrophoresis. These modified proteins may play roles in redox buffering. The changes in activities of redox proteins underline their fundamental roles in controlling redox homeostasis. However, observed differential redox responses in cotyledon and seedling tissues showed a major capacity of the seedlings' redox systems to protect the reduced status of protein thiols, thus suggesting quantitatively greater antioxidant protection of proteins in seedlings compared to cotyledon. To our knowledge, this is the first comprehensive redox biology investigation of the effect of Cu on seed germination.

Comparative Proteomic Analysis of Flag Leaves Reveals New Insight into Wheat Heat Adaptation

Hexaploid wheat (Triticum aestivum L.) is an important food crop but it is vulnerable to heat. The heat-responsive proteome of wheat remains to be fully elucidated because of previous technical and genomic limitations, and this has hindered our understanding of the mechanisms of wheat heat adaptation and advances in improving thermotolerance. Here, flag leaves of wheat during grain filling stage were subjected to high daytime temperature stress, and 258 heat-responsive proteins (HRPs) were identified with iTRAQ analysis. Enrichment analysis revealed that chlorophyll synthesis, carbon fixation, protein turnover, and redox regulation were the most remarkable heat-responsive processes. The HRPs involved in chlorophyll synthesis and carbon fixation were significantly decreased, together with severe membrane damage, demonstrating the specific effects of heat on photosynthesis of wheat leaves. In addition, the decrease in chlorophyll content may result from the decrease in HRPs involved in chlorophyll precursor synthesis. Further analysis showed that the accumulated effect of heat stress played a critical role in photosynthesis reduction, suggested that improvement in heat tolerance of photosynthesis, and extending heat tolerant period would be major research targets. The significantly accumulation of GSTs and Trxs in response to heat suggested their important roles in redox regulation, and they could be the promising candidates for improving wheat thermotolerance. In summary, our results provide new insight into wheat heat adaption and provide new perspectives on thermotolerance improvement.

Involvement of thiol-based mechanisms in plant development

BACKGROUND

Increasing knowledge has been recently gained regarding the redox regulation of plant developmental stages.

SCOPE OF VIEW

The current state of knowledge concerning the involvement of glutathione, glutaredoxins and thioredoxins in plant development is reviewed.

MAJOR CONCLUSIONS

The control of the thiol redox status is mainly ensured by glutathione (GSH), a cysteine-containing tripeptide and by reductases sharing redox-active cysteines, glutaredoxins (GRXs) and thioredoxins (TRXs). Indeed, thiol groups present in many regulatory proteins and metabolic enzymes are prone to oxidation, ultimately leading to post-translational modifications such as disulfide bond formation or glutathionylation. This review focuses on the involvement of GSH, GRXs and TRXs in plant development. Recent studies showed that the proper functioning of root and shoot apical meristems depends on glutathione content and redox status, which regulate, among others, cell cycle and hormone-related processes. A critical role of GRXs in the formation of floral organs has been uncovered, likely through the redox regulation of TGA transcription factor activity. TRXs fulfill many functions in plant development via the regulation of embryo formation, the control of cell-to-cell communication, the mobilization of seed reserves, the biogenesis of chloroplastic structures, the metabolism of carbon and the maintenance of cell redox homeostasis. This review also highlights the tight relationships between thiols, hormones and carbon metabolism, allowing a proper development of plants in relation with the varying environment and the energy availability.

GENERAL SIGNIFICANCE

GSH, GRXs and TRXs play key roles during the whole plant developmental cycle via their antioxidant functions and the redox-regulation of signaling pathways. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Analysis of Arabidopsis thioredoxin-h isotypes identifies discrete domains that confer specific structural and functional properties

In this study, we identified specific domains and amino acids responsible for the structural and functional properties of AtTrx-hs (Arabidopsis h-type thioredoxins). Specific domains and amino acids for the chaperone function of AtTrx-hs played a critical role in heat-shock-resistance in vivo.

Comparisons of protein profiles of beech bark disease resistant and susceptible American beech (Fagus grandifolia)

BACKGROUND

Beech bark disease is an insect-fungus complex that damages and often kills American beech trees and has major ecological and economic impacts on forests of the northeastern United States and southeastern Canadian forests. The disease begins when exotic beech scale insects feed on the bark of trees, and is followed by infection of damaged bark tissues by one of the Neonectria species of fungi. Proteomic analysis was conducted of beech bark proteins from diseased trees and healthy trees in areas heavily infested with beech bark disease. All of the diseased trees had signs of Neonectria infection such as cankers or fruiting bodies. In previous tests reported elsewhere, all of the diseased trees were demonstrated to be susceptible to the scale insect and all of the healthy trees were demonstrated to be resistant to the scale insect. Sixteen trees were sampled from eight geographically isolated stands, the sample consisting of 10 healthy (scale-resistant) and 6 diseased/infested (scale-susceptible) trees.

RESULTS

Proteins were extracted from each tree and analysed in triplicate by isoelectric focusing followed by denaturing gel electrophoresis. Gels were stained and protein spots identified and intensity quantified, then a statistical model was fit to identify significant differences between trees. A subset of BBD differential proteins were analysed by mass spectrometry and matched to known protein sequences for identification. Identified proteins had homology to stress, insect, and pathogen related proteins in other plant systems. Protein spots significantly different in diseased and healthy trees having no stand or disease-by-stand interaction effects were identified.

CONCLUSIONS

Further study of these proteins should help to understand processes critical to resistance to beech bark disease and to develop biomarkers for use in tree breeding programs and for the selection of resistant trees prior to or in early stages of BBD development in stands. Early identification of resistant trees (prior to the full disease development in an area) will allow forest management through the removal of susceptible trees and their root-sprouts prior to the onset of disease, allowing management and mitigation of costs, economic impact, and impacts on ecological systems and services.

Thiol-based redox regulation in sexual plant reproduction: new insights and perspectives

The success of sexual reproduction in plants involves (i) the proper formation of the plant gametophytes (pollen and embryo sac) containing the gametes, (ii) the accomplishment of specific interactions between pollen grains and the stigma, which subsequently lead to (iii) the fusion of the gametes and eventually to (iv) the seed setting. Owing to the lack of mobility, plants have developed specific regulatory mechanisms to control all developmental events underlying the sexual plant reproduction according to environmental challenges. Over the last decade, redox regulation and signaling have come into sight as crucial mechanisms able to manage critical stages during sexual plant reproduction. This regulation involves a complex redox network which includes reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione and other classic buffer molecules or antioxidant proteins, and some thiol/disulphide-containing proteins belonging to the thioredoxin superfamily, like glutaredoxins (GRXs) or thioredoxins (TRXs). These proteins participate as critical elements not only in the switch between the mitotic to the meiotic cycle but also at further developmental stages of microsporogenesis. They are also implicated in the regulation of pollen rejection as the result of self-incompatibility. In addition, they display precise space-temporal patterns of expression and are present in specific localizations like the stigmatic papillae or the mature pollen, although their functions and subcellular localizations are not clear yet. In this review we summarize insights and perspectives about the presence of thiol/disulphide-containing proteins in plant reproduction, taking into account the general context of the cell redox network.

Wide range of interacting partners of pea Gβ subunit of G-proteins suggests its multiple functions in cell signalling

Climate change is a major concern especially in view of the increasing global population and food security. Plant scientists need to look for genetic tools whose appropriate usage can contribute to sustainable food availability. G-proteins have been identified as some of the potential genetic tools that could be useful for protecting plants from various stresses. Heterotrimeric G-proteins consisting of three subunits Gα, Gβ and Gγ are important components of a number of signalling pathways. Their structure and functions are already well studied in animals but their potential in plants is now gaining attention for their role in stress tolerance. Earlier we have reported that over expressing pea Gβ conferred heat tolerance in tobacco plants. Here we report the interacting partners (proteins) of Gβ subunit of Pisum sativum and their putative role in stress and development. Out of 90 transformants isolated from the yeast-two-hybrid (Y2H) screening, seven were chosen for further investigation due to their recurrence in multiple experiments. These interacting partners were confirmed using β-galactosidase colony filter lift and ONPG (O-nitrophenyl-β-D-galactopyranoside) assays. These partners include thioredoxin H, histidine-containing phosphotransfer protein 5-like, pathogenesis-related protein, glucan endo-beta-1, 3-glucosidase (acidic isoform), glycine rich RNA binding protein, cold and drought-regulated protein (corA gene) and soluble inorganic pyrophosphatase 1. This study suggests the role of pea Gβ subunit in stress signal transduction and development pathways owing to its capability to interact with a wide range of proteins of multiple functions.

Expression of the chloroplast thioredoxins f and m is linked to short-term changes in the sugar and thiol status in leaves of Pisum sativum

Thioredoxins (TRXs) f and m are key components in the light regulation of photosynthetic metabolism via thiol-dithiol modulation in chloroplasts of leaves; however, little is known about the factors modulating the expression of these proteins. To investigate the effect of sugars as photosynthetic products on the expression of PsTRX f and m1 genes, sucrose and glucose were externally supplied to pea plants during the day. There was an increase in the mRNA levels of PsTRX f and m1 genes in response mainly to glucose. When leaf discs were incubated for up to 4h in the dark, glucose also led to an increase in both mRNA and protein levels of TRXs f and m, while sucrose had no substantial effect. Expression of PsDOF7, a carbon metabolism-related transcription factor gene, was also induced by glucose. Protein-DNA interaction showed that PsDOF7 binds specifically to the DOF core located in PsTRX f and m1 gene promoters. Transient expression in agroinfiltrated pea leaves demonstrated that PsDOF7 activated transcription of both promoters. The incubation of leaf discs in dithiotreitol (DTT) to increase the redox status led to a marked increase in the mRNA and protein levels of both TRXs within 4h. The increase in TRX protein levels occurred after 1h DTT feeding, implying a rapid effect of the thiol status on TRX f and m1 protein turnover rates, while transcriptional regulation took 3h to proceed. These results show that the protein levels of both TRXs are under short-term control of the sugar and thiol status in plants.

A new isoform of thioredoxin h group in potato, SbTRXh1, regulates cold-induced sweetening of potato tubers by adjusting sucrose content

In order to study the molecular mechanism of the cold-induced sweetening (CIS) of potato tubers, a novel isoform of thioredoxin h group, SbTRXh1, which was up-regulated early in the 4 °C storage of CIS-resistant potato (Solanum berthaultii) tubers, was cloned in present research. The genetic transformation of over-expression (OE) and RNA interference (RNAi) of SbTRXh1 into potato cv. E-Potato 3 (E3) was carried out to clarify its function in CIS regulation. The results showed that the transcripts of SbTRXh1 in either OE- or RNAi-tubers were strongly induced in 4 °C storage and quantitatively related to the reducing sugar (RS) accumulation, indicating that SbTRXh1 is involved in the CIS process of potato tubers. Regression analysis between the transcripts and protein contents of SbTRXh1 showed a very significant logarithmic relationship implying that the expression of SbTRXh1 may be mainly regulated at transcriptional level. Further monitoring the variation of the sugar contents in cold-stored tubers demonstrated a linear relationship between RS and sucrose (Suc). Thus, it can be inferred that SbTRXh1 may function in the Suc-RS pathway for CIS regulation of potato tubers.

KEY MESSAGE

SbTRXh1 is primarily demonstrated to be involved in the regulation of cold-induced sweetening (CIS) of potato tubers, and it may function in the Suc-RS pathway for CIS regulation.

Plastid thioredoxins f and m are related to the developing and salinity response of post-germinating seeds of Pisum sativum

Plastid thioredoxins (TRXs) f and m have long been considered to regulate almost exclusively photosynthesis-related processes. Nonetheless, some years ago, we found that type-f and m TRXs were also present in non-photosynthetic organs such as roots and flowers of adult pea plants. In the present work, using pea seedlings 2-5 days old, we have determined the mRNA expression profile of the plastid PsTRX f, m1, and m2, together with the ferredoxin NADP reductase (FNR). Our results show that these TRX isoforms are expressed in cotyledons, underlying similar expression levels in roots for PsTRX m2. We have also noted plastid TRX expression in cotyledons of etiolated seedlings of Arabidopsis thaliana lines carrying constructs corresponding to PsTRX f and m1 promoters fused to the reporter gene GUS, pointing to a role in reserve mobilization. Furthermore, the response of plastid TRXs to NaCl and their capacity in restoring the growth of a TRX-deficient yeast under saline conditions suggest a role in the tolerance to salinity. We propose that these redox enzymes take part of the reserve mobilization in seedling cotyledons and we suggest additional physiological functions of PsTRX m2 in roots and PsTRX m1 in the salinity-stress response during germination.

Circadian regulation of chloroplastic f and m thioredoxins through control of the CCA1 transcription factor

Chloroplastic thioredoxins f and m (TRX f and TRX m) mediate light regulation of carbon metabolism through the activation of Calvin cycle enzymes. The role of TRX f and m in the activation of Calvin cycle enzymes is best known among the TRX family. However, the discoveries of new potential targets extend the functions of chloroplastic TRXs to other processes in non-photosynthetic tissues. As occurs with numerous chloroplast proteins, their expression comes under light regulation. Here, the focus is on the light regulation of TRX f and TRX m in pea and Arabidopsis during the day/night cycle that is maintained during the subjective night. In pea (Pisum sativum), TRX f and TRX m1 expression is shown to be governed by a circadian oscillation exerted at both the transcriptional and protein levels. Binding shift assays indicate that this control probably involves the interaction of the CCA1 transcription factor and an evening element (EE) located in the PsTRX f and PsTRX m1 promoters. In Arabidopsis, among the multigene family of TRX f and TRX m, AtTRX f2 and AtTRX m2 mRNA showed similar circadian oscillatory regulation, suggesting that such regulation is conserved in plants. However, this oscillation was disrupted in plants overexpressing CCA1 (cca1-ox) or repressing CCA1 and LHY (cca1-lhy). The physiological role of the oscillatory regulation of chloroplastic TRX f and TRX m in plants during the day/night cycle is discussed.

Identification and characterization of thioredoxin h isoforms differentially expressed in germinating seeds of the model legume Medicago truncatula

Thioredoxins (Trxs) h, small disulfide reductases, and NADP-thioredoxin reductases (NTRs) have been shown to accumulate in seeds of different plant species and play important roles in seed physiology. However, little is known about the identity, properties, and subcellular location of Trx h isoforms that are abundant in legume seeds. To fill this gap, in this work, we characterized the Trx h family of Medicago truncatula, a model legume, and then explored the activity and localization of Trx h isoforms accumulating in seeds. Twelve Trx h isoforms were identified in M. truncatula. They belong to the groups previously described: h1 to h3 (group I), h4 to h7 (group II), and h8 to h12 (group III). Isoforms of groups I and II were found to be reduced by M. truncatula NTRA, but with different efficiencies, Trxs of group II being more efficiently reduced than Trxs of group I. In contrast, their insulin disulfide-reducing activity varies greatly and independently of the group to which they belong. Furthermore, Trxs h1, h2, and h6 were found to be present in dry and germinating seeds. Trxs h1 and, to a lesser extent, h2 are abundant in both embryonic axes and cotyledons, while Trx h6 is mainly present in cotyledons. Thus, M. truncatula seeds contain distinct isoforms of Trx h that differ in spatial distribution and kinetic properties, suggesting that they play different roles. Because we show that Trx h6 is targeted to the tonoplast, the possible role of this isoform during germination is finally discussed.

The conformational stability and biophysical properties of the eukaryotic thioredoxins of Pisum sativum are not family-conserved

Thioredoxins (TRXs) are ubiquitous proteins involved in redox processes. About forty genes encode TRX or TRX-related proteins in plants, grouped in different families according to their subcellular localization. For instance, the h-type TRXs are located in cytoplasm or mitochondria, whereas f-type TRXs have a plastidial origin, although both types of proteins have an eukaryotic origin as opposed to other TRXs. Herein, we study the conformational and the biophysical features of TRXh1, TRXh2 and TRXf from Pisum sativum. The modelled structures of the three proteins show the well-known TRX fold. While sharing similar pH-denaturations features, the chemical and thermal stabilities are different, being PsTRXh1 (Pisum sativum thioredoxin h1) the most stable isoform; moreover, the three proteins follow a three-state denaturation model, during the chemical-denaturations. These differences in the thermal- and chemical-denaturations result from changes, in a broad sense, of the several ASAs (accessible surface areas) of the proteins. Thus, although a strong relationship can be found between the primary amino acid sequence and the structure among TRXs, that between the residue sequence and the conformational stability and biophysical properties is not. We discuss how these differences in the biophysical properties of TRXs determine their unique functions in pea, and we show how residues involved in the biophysical features described (pH-titrations, dimerizations and chemical-denaturations) belong to regions involved in interaction with other proteins. Our results suggest that the sequence demands of protein-protein function are relatively rigid, with different protein-binding pockets (some in common) for each of the three proteins, but the demands of structure and conformational stability per se (as long as there is a maintained core), are less so.

Cadmium induced mitochondrial redox changes in germinating pea seed

Mitochondria play an essential role in producing the energy required for seedling growth following imbibition. Heavy metals, such as cadmium impair mitochondrial functioning in part by altering redox regulation. The activities of two protein redox systems present in mitochondria, thioredoxin (Trx) and glutaredoxin (Grx), were analysed in the cotyledons and embryo of pea (Pisum sativum L.) germinating seeds exposed to toxic Cd concentration. Compared to controls, Cd-treated germinating seeds showed a decrease in total soluble protein content, but an increase in -SH content. Under Cd stress conditions, Grx and glutathione reductase (GR) activities as well as glutathione (GSH) concentrations decreased both in cotyledons and the embryo. Similar results were obtained with the Trx system: Trx and NADPH-dependent thioredoxin reductase (NTR) activities were not stimulated, whereas total NAD(P) contents diminished in the embryo. However, Cd enhanced the levels of all components of the Trx system in the cotyledons. On the other hand, Cd caused a significant increase in oxidative stress parameters such as the redox ratio of coenzymes (oxidized to reduced forms) and NAD(P)H oxidase activities. These results indicate that Cd induces differential redox responses on different seed tissues. We suggest that neither Grx system nor Trx one may improve the redox status of mitochondrial thiols in the embryo of germinating pea seeds exposed to Cd toxicity, but in the cotyledons the contribution of Trx/NTR/NADPH can be established in despite the vulnerability of the coenzyme pools due to enzymatic oxidation.

An h-type thioredoxin functions in tobacco defense responses to two species of viruses and an abiotic oxidative stress

Various thioredoxin (Trx) proteins have been identified in plants. However, many of the physiological roles played by these proteins remain to be elucidated. We cloned a TRXh-like gene predicted to encode an h-type Trx in tobacco (Nicotiana tabacum) and designated it NtTRXh3, based on the biochemical activity of the NtTRXh3 protein. Overexpression of NtTRXh3 conferred resistance to Tobacco mosaic virus and Cucumber mosaic virus, both of which showed reduced multiplication and pathogenicity in NtTRXh3-overexpressing plants compared with controls. NtTRXh3 overexpression also enhanced tobacco resistance to oxidative stress induced by paraquat, an herbicide that inhibits the production of reducing equivalents by chloroplasts. The NtTRXh3 protein localized exclusively to chloroplasts in coordination with the maintenance of cellular reducing conditions, which accompanied an elevation in the glutathione/glutathione disulfide couple ratio. NtTRXh3 gene expression and NtTRXh3 protein production were necessary for these defensive responses, because they were all arrested when NtTRXh3 was silenced and the production of NtTRXh3 protein was abrogated. These results suggest that NtTRXh3 is involved in the resistance of tobacco to virus infection and abiotic oxidative stress.

Evidence of non-functional redundancy between two pea h-type thioredoxins by specificity and stability studies

The largest group of plant thioredoxins (TRXs) consists of the so-called h-type; their great number raises questions about their specific or redundant roles in plant cells. Pisum sativum thioredoxin h1 (PsTRXh1) and Pisum sativum thioredoxin h2 (PsTRXh2) are both h-type TRXs from pea (Pisum sativum) previously identified and biochemically characterized. While both are involved in redox regulation and show a high-sequence identity (60%), they display different behavior during in vitro and in vivo assays. In this work, we show that these two proteins display different specificity in the capturing of protein targets in vitro, by the use of a new stringent method. PsTRXh2 interacted with classical antioxidant proteins, whereas PsTRXh1 showed a completely different pattern of targeted proteins, and was able to capture a transcription factor. We also showed that the two proteins display very different thermal and chemical stabilities. We suggest that the differences in thermal and chemical stability point to a distinct and characteristic pattern of protein specificity.

An extracellular disulfide bond forming protein (DsbF) from Mycobacterium tuberculosis: structural, biochemical, and gene expression analysis

Disulfide bond forming (Dsb) proteins ensure correct folding and disulfide bond formation of secreted proteins. Previously, we showed that Mycobacterium tuberculosis DsbE (Mtb DsbE, Rv2878c) aids in vitro oxidative folding of proteins. Here, we present structural, biochemical, and gene expression analyses of another putative Mtb secreted disulfide bond isomerase protein homologous to Mtb DsbE, Mtb DsbF (Rv1677). The X-ray crystal structure of Mtb DsbF reveals a conserved thioredoxin fold although the active-site cysteines may be modeled in both oxidized and reduced forms, in contrast to the solely reduced form in Mtb DsbE. Furthermore, the shorter loop region in Mtb DsbF results in a more solvent-exposed active site. Biochemical analyses show that, similar to Mtb DsbE, Mtb DsbF can oxidatively refold reduced, unfolded hirudin and has a comparable pK(a) for the active-site solvent-exposed cysteine. However, contrary to Mtb DsbE, the Mtb DsbF redox potential is more oxidizing and its reduced state is more stable. From computational genomics analysis of the M. tuberculosis genome, we identified a potential Mtb DsbF interaction partner, Rv1676, a predicted peroxiredoxin. Complex formation is supported by protein coexpression studies and inferred by gene expression profiles, whereby Mtb DsbF and Rv1676 are upregulated under similar environments. Additionally, comparison of Mtb DsbF and Mtb DsbE gene expression data indicates anticorrelated gene expression patterns, suggesting that these two proteins and their functionally linked partners constitute analogous pathways that may function under different conditions.

Thioredoxins and glutaredoxins: unifying elements in redox biology

Since their discovery as a substrate for ribonucleotide reductase (RNR), the role of thioredoxin (Trx) and glutaredoxin (Grx) has been largely extended through their regulatory function. Both proteins act by changing the structure and activity of a broad spectrum of target proteins, typically by modifying redox status. Trx and Grx are members of families with multiple and partially redundant genes. The number of genes clearly increased with the appearance of multicellular organisms, in part because of new types of Trx and Grx with orthologs throughout the animal and plant kingdoms. The function of Trx and Grx also broadened as cells achieved increased complexity, especially in the regulation arena. In view of these progressive changes, the ubiquitous distribution of Trx and the wide occurrence of Grx enable these proteins to serve as indicators of the evolutionary history of redox regulation. In so doing, they add a unifying element that links the diverse forms of life to one another in an uninterrupted continuum. It is anticipated that future research will embellish this continuum and further elucidate the properties of these proteins and their impact on biology. The new information will be important not only to our understanding of the role of Trx and Grx in fundamental cell processes but also to future societal benefits as the proteins find new applications in a range of fields.

AtCXXS: atypical members of the Arabidopsis thaliana thioredoxin h family with a remarkably high disulfide isomerase activity

The Arabidopsis thaliana thioredoxin subgroup h III is composed of four members and includes the two monocysteinic (CXXS) thioredoxins encoded by the genome. We show that AtCXXS1 is the ortholog of monocysteinic thioredoxins present in all higher plants. In contrast, unicellular algae and the moss Physcomitrella patens do not encode monocysteinic thioredoxin. AtCXXS2, the second monocysteinic thioredoxin of Arabidopsis has no ortholog in any other higher plants. It probably appeared recently by duplications of a dicysteinic thioredoxin of the same subgroup h III. Both monocysteinic thioredoxins show a low disulfide reductase activity in vitro but are very efficient as disulfide isomerases in RNAse refolding tests. The possible interactions of these proteins with the glutathione glutaredoxin pathway are discussed on the basis of recent papers.

Crystal structures of barley thioredoxin h isoforms HvTrxh1 and HvTrxh2 reveal features involved in protein recognition and possibly in discriminating the isoform specificity

H-type thioredoxins (Trxs) constitute a particularly large Trx sub-group in higher plants. Here, the crystal structures are determined for the two barley Trx h isoforms, HvTrxh1 and HvTrxh2, in the partially radiation-reduced state to resolutions of 1.7 A, and for HvTrxh2 in the oxidized state to 2.0 A. The two Trxs have a sequence identity of 51% and highly similar fold and active-site architecture. Interestingly, the four independent molecules in the crystals of HvTrxh1 form two relatively large and essentially identical protein-protein interfaces. In each interface, a loop segment of one HvTrxh1 molecule is positioned along a shallow hydrophobic groove at the primary nucleophile Cys40 of another HvTrxh1 molecule. The association mode can serve as a model for the target protein recognition by Trx, as it brings the Met82 Cgamma atom (gamma position as a disulfide sulfur) of the bound loop segment in the proximity of the Cys40 thiol. The interaction involves three characteristic backbone-backbone hydrogen bonds in an antiparallel beta-sheet-like arrangement, similar to the arrangement observed in the structure of an engineered, covalently bound complex between Trx and a substrate protein, as reported by Maeda et al. in an earlier paper. The occurrence of an intermolecular salt bridge between Glu80 of the bound loop segment and Arg101 near the hydrophobic groove suggests that charge complementarity plays a role in the specificity of Trx. In HvTrxh2, isoleucine corresponds to this arginine, which emphasizes the potential for specificity differences between the coexisting barley Trx isoforms.

Immunocytochemical localization of Pisum sativum TRXs f and m in non-photosynthetic tissues

Plants are the organisms containing the most complex multigenic family for thioredoxins (TRX). Several types of TRXs are targeted to chloroplasts, which have been classified into four subgroups: m, f, x, and y. Among them, TRXs f and m were the first plastidial TRXs characterized, and their function as redox modulators of enzymes involved in carbon assimilation in the chloroplast has been well-established. Both TRXs, f and m, were named according to their ability to reduce plastidial fructose-1,6-bisphosphatase (FBPase) and malate dehydrogenase (MDH), respectively. Evidence is presented here based on the immunocytochemistry of the localization of f and m-type TRXs from Pisum sativum in non-photosynthetic tissues. Both TRXs showed a different spatial pattern. Whilst PsTRXm was localized to vascular tissues of all the organs analysed (leaves, stems, and roots), PsTRXf was localized to more specific cells next to xylem vessels and vascular cambium. Heterologous complementation analysis of the yeast mutant EMY63, deficient in both yeast TRXs, by the pea plastidial TRXs suggests that PsTRXm, but not PsTRXf, is involved in the mechanism of reactive oxygen species (ROS) detoxification. In agreement with this function, the PsTRXm gene was induced in roots of pea plants in response to hydrogen peroxide.

Localization in roots and flowers of pea chloroplastic thioredoxin f and thioredoxin m proteins reveals new roles in nonphotosynthetic organs

Plant thioredoxins (TRXs) are involved in redox regulation of a wide variety processes and usually exhibit organ specificity. We report strong evidence that chloroplastic TRXs are localized in heterotrophic tissues and suggest some ways in which they might participate in several metabolic and developmental processes. The promoter regions of the chloroplastic f and m1 TRX genes were isolated from a pea (Pisum sativum) plant genomic bank. Histochemical staining for beta-glucuronidase (GUS) in transgenic homozygous Arabidopsis (Arabidopsis thaliana) plants showed preferential expression of the 444-bp PsTRXf1 promoter in early seedlings, stems, leaves, and roots, as well as in flowers, stigma, pollen grains, and filaments. GUS activity under the control of the 1,874-bp PsTRXm1 promoter was restricted to the leaves, roots, seeds, and flowers. To gain insight into the translational regulation of these genes, a series of deletions of 5' elements in both TRX promoters were analyzed. The results revealed that a 126-bp construct of the PsTRXf2 promoter was unable to reproduce the expression pattern observed with the full promoter. The differences in expression and tissue specificity between PsTRXm1 and the deleted promoters PsTRXm2 and PsTRXm3 suggest the existence of upstream positive or negative regulatory regions that affect tissue specificity, sucrose metabolism, and light regulation. PsTRXm1 expression is finely regulated by light and possibly by other metabolic factors. In situ hybridization experiments confirmed new localizations of these chloroplastic TRX transcripts in vascular tissues and flowers, and therefore suggest possible new functions in heterotrophic tissues related to cell division, germination, and plant reproduction.

Thioredoxin and Redox Control within the New Concept of Oxidative Signaling

During the last decade, plant thioredoxins (TRX) h-type have been shown to be implicated in several new roles like the protection against the oxidative stress by their ability to reduce some antioxidant proteins as peroxiredoxins (PRX) or methionine-sulphoxide-reductases (MSR). However, the concept of the oxidative stress is changing and this fact raises the question of the TRX roles in this new context. In the January issue of Plant Physiology, we have presented two TRXsh from Pisum sativum differently involved in the control of the redox status. PsTRXh1 is an h-type TRX that acts by reducing classical antioxidant proteins. PsTRXh2 seems to be also involved in redox control, however it could act contrary to its counterpart h1. Both proteins may play antagonistic roles in pea in order to lead a better control of the redox status.