Plant thioredoxins and glutaredoxins: identity and putative roles

New insights on thioredoxins (Trxs) and glutaredoxins (Grxs) by in silico amino acid sequence, phylogenetic and comparative structural analyses in organisms of three domains of life

Thioredoxins (Trxs) and Glutaredoxins (Grxs) regulate several cellular processes by controlling the redox state of their target proteins. Trxs and Grxs belong to thioredoxin superfamily and possess characteristic Trx/Grx fold. Several phylogenetic, biochemical and structural studies have contributed to our overall understanding of Trxs and Grxs. However, comparative study of closely related Trxs and Grxs in organisms of all domains of life was missing. Here, we conducted in silico comparative structural analysis combined with amino acid sequence and phylogenetic analyses of 65 Trxs and 88 Grxs from 12 organisms of three domains of life to get insights into evolutionary and structural relationship of two proteins. Outcomes suggested that despite diversity in their amino acids composition in distantly related organisms, both Trxs and Grxs strictly conserved functionally and structurally important residues. Also, position of these residues was highly conserved in all studied Trxs and Grxs. Notably, if any substitution occurred during evolution, preference was given to amino acids having similar chemical properties. Trxs and Grxs were found more different in eukaryotes than prokaryotes due to altered helical conformation. The surface of Trxs was negatively charged, while Grxs surface was positively charged, however, the active site was constituted by uncharged amino acids in both proteins. Also, phylogenetic analysis of Trxs and Grxs in three domains of life supported endosymbiotic origins of chloroplast and mitochondria, and suggested their usefulness in molecular systematics. We also report previously unknown catalytic motifs of two proteins, and discuss in detail about effect of abovementioned parameters on overall structural and functional diversity of Trxs and Grxs.

Differential cellular responses associated with oxidative stress and cell fate decision under nitrate and phosphate limitations in Thalassiosira pseudonana: Comparative proteomics

Diatoms are important components of marine ecosystems and contribute greatly to the world's primary production. Despite their important roles in ecosystems, the molecular basis of how diatoms cope with oxidative stress caused by nutrient fluctuations remains largely unknown. Here, an isobaric tags for relative and absolute quantitation (iTRAQ) proteomic method was coupled with a series of physiological and biochemical techniques to explore oxidative stress- and cell fate decision-related cellular and metabolic responses of the diatom Thalassiosira pseudonana to nitrate (N) and inorganic phosphate (P) stresses. A total of 1151 proteins were detected; 122 and 56 were significantly differentially expressed from control under N- and P-limited conditions, respectively. In N-limited cells, responsive proteins were related to reactive oxygen species (ROS) accumulation, oxidative stress responses and cell death, corresponding to a significant decrease in photosynthetic efficiency, marked intracellular ROS accumulation, and caspase-mediated programmed cell death activation. None of these responses were identified in P-limited cells; however, a significant up-regulation of alkaline phosphatase proteins was observed, which could be the major contributor for P-limited cells to cope with ambient P deficiency. These findings demonstrate that fundamentally different metabolic responses and cellular regulations are employed by the diatom in response to different nutrient stresses and to keep the cells viable.

Oak protein profile alterations upon root colonization by an ectomycorrhizal fungus

An increased knowledge on the real impacts of ectomycorrhizal symbiosis in forest species is needed to optimize forest sustainable productivity and thus to improve forest services and their capacity to act as carbon sinks. In this study, we investigated the response of an oak species to ectomycorrhizae formation using a proteomics approach complemented by biochemical analysis of carbohydrate levels. Comparative proteome analysis between mycorrhizal and nonmycorrhizal cork oak plants revealed no differences at the foliar level. However, the protein profile of 34 unique oak proteins was altered in the roots. Consistent with the results of the biochemical analysis, the proteome analysis of the mycorrhizal roots suggests a decreasing utilization of sucrose for the metabolic activity of mycorrhizal roots which is consistent with an increased allocation of carbohydrates from the plant to the fungus in order to sustain the symbiosis. In addition, a promotion of protein unfolding mechanisms, attenuation of defense reactions, increased nutrient mobilization from the plant-fungus interface (N and P), as well as cytoskeleton rearrangements and induction of plant cell wall loosening for fungal root accommodation in colonized roots are also suggested by the results. The suggested improvement in root capacity to take up nutrients accompanied by an increase of root biomass without apparent changes in aboveground biomass strongly re-enforces the potential of mycorrhizal inoculation to improve cork oak forest resistance capacity to cope with coming climate change.

Response of two rice cultivars differing in their sensitivity towards arsenic, differs in their expression of glutaredoxin and glutathione S transferase genes and antioxidant usage

Embodied study investigates the role of GRX and associated antioxidant enzymes in the detoxification mechanism between arsenic (As) sensitive (Usar-3) and tolerant cultivar (Pant Dhan 11) of Oryza sativa against As(III) and As(V), under GSH enriched, and GSH deprived conditions. The overall growth and physiological parameters in sensitive cultivar were lower than the tolerant cultivar, against various treatments of As(III) and As(V). The As accumulation in sensitive cv. against both As(III) and As(V) was lower than the corresponding treatments in tolerant cv. However, the As translocation against As(V) was lower (35% and 64%, resp.) than that of As(III), in both the cultivars. In sensitive cv. translocation of Zn and Cu was influenced by both As(V) and As(III) whereas, in tolerant cv. the translocation of Cu, Mn and Zn was influenced only by As(III). Translocation of Fe was negatively influenced by translocation of As in sensitive cv. and positively in tolerant cv. Strong correlation between H2O2, SOD, GRX, GR, GST and GSH/GSSG in sensitive cv. and between DHAR, APX, MDHAR and AsA in tolerant cv. demonstrates the underlying preference of GSH as electron donor for detoxification of H2O2 in sensitive cv. and AsA in tolerant cv. Higher expression of the four GRX and two GST genes in the sensitive cv. than tolerant cv, suggests that under As stress, GRX are synthesized more in the sensitive cv. than tolerant cv. Also, the expression of four GRX genes were higher against As(V) than As(III). The higher As accumulation in the tolerant cv. is due to lower GST expression, is attributed to the absence of thiolation and sequestration of As in roots, the translocation of As to shoots is higher.

Molecular characterization of the thioredoxin system from Methanosarcina acetivorans

The thioredoxin system, composed of thioredoxin reductase (TrxR) and thioredoxin (Trx), is widely distributed in nature, where it serves key roles in electron transfer and in the defense against oxidative stress. Although recent evidence reveals Trx homologues are almost universally present among the methane-producing archaea (methanogens), a complete thioredoxin system has not been characterized from any methanogen. We examined the phylogeny of Trx homologues among methanogens and characterized the thioredoxin system from Methanosarcina acetivorans. Phylogenetic analysis of Trx homologues from methanogens revealed eight clades, with one clade containing Trxs broadly distributed among methanogens. The Methanococci and Methanobacteria each contain one additional Trx from another clade, respectively, whereas the Methanomicrobia contain an additional five distinct Trxs. Methanosarcina acetivorans, a member of the Methanomicrobia, contains a single TrxR (MaTrxR) and seven Trx homologues (MaTrx1-7), with representatives from five of the methanogen Trx clades. Purified recombinant MaTrxR had 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) reductase and oxidase activities. The apparent Km value for NADPH was 115-fold lower than that for NADH, consistent with NADPH as the physiological electron donor to MaTrxR. Purified recombinant MaTrx2, MaTrx6 and MaTrx7 exhibited dithiothreitol- and lipoamide-dependent insulin disulfide reductase activities. However, only MaTrx7, which is encoded adjacent to MaTrxR, could serve as a redox partner to MaTrxR. These results reveal that M. acetivorans harbors at least three functional and distinct Trxs, and a complete thioredoxin system composed of NADPH, MaTrxR and at least MaTrx7. This is the first characterization of a complete thioredoxin system from a methanogen, which provides a foundation to understand the system in methanogens.

Genome-wide transcriptome analysis of Arabidopsis response to sulfur dioxide fumigation

Sulfur dioxide (SO2) supplies the basic sulfur element to promote plant growth, yet at the same time it is a harmful air pollutant. Currently, the mechanisms of plant adaptation to SO2 stress are largely unknown. Pathways of SO2 metabolism, a range of networks of interacting regulatory signals and defense mechanisms triggered in resistance to SO2 stress, have not yet been clarified. We performed transcriptome analysis of Arabidopsis plants fumigated with 30 mg m(-3) SO2 for 72 h and untreated controls using microarrays. This identified 2,780 significantly up- or down-regulated genes in plants response to SO2 stress, indicating a possible genome-scale reprogramming of the transcriptome. Significant changes in the transcript abundance of genes that participated in SO2 metabolic pathways indicated that numerous sulfites were involved in sulfur assimilatory pathways directly and away from sulfite oxidative pathways. Furthermore, the up-regulation of components involved in reactive oxygen species generating and scavenging pathways demonstrated altered redox homeostasis. Transcripts encoding key components in nitric oxide biosynthesis pathways were simultaneously up-regulated by SO2 exposure. In addition, transcripts associated with putative biotic stress were also up-regulated. Therefore, SO2 evokes a comprehensive reprogramming of metabolic pathways, consistent with up-regulation of transcripts involved in tolerance and defense mechanisms, in Arabidopsis.

Nuclear accumulation of cytosolic glyceraldehyde-3-phosphate dehydrogenase in cadmium-stressed Arabidopsis roots

NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a ubiquitous enzyme involved in the glycolytic pathway. It has been widely demonstrated that mammalian GAPDH, in addition to its role in glycolysis, fulfills alternative functions mainly linked to its susceptibility to oxidative posttranslational modifications. Here, we investigated the responses of Arabidopsis (Arabidopsis thaliana) cytosolic GAPDH isoenzymes GAPC1 and GAPC2 to cadmium-induced stress in seedlings roots. GAPC1 was more responsive to cadmium than GAPC2 at the transcriptional level. In vivo, cadmium treatments induced different concomitant effects, including (1) nitric oxide accumulation, (2) cytosolic oxidation (e.g. oxidation of the redox-sensitive Green fluorescent protein2 probe), (3) activation of the GAPC1 promoter, (4) GAPC1 protein accumulation in enzymatically inactive form, and (5) strong relocalization of GAPC1 to the nucleus. All these effects were detected in the same zone of the root tip. In vitro, GAPC1 was inactivated by either nitric oxide donors or hydrogen peroxide, but no inhibition was directly provided by cadmium. Interestingly, nuclear relocalization of GAPC1 under cadmium-induced oxidative stress was stimulated, rather than inhibited, by mutating into serine the catalytic cysteine of GAPC1 (C155S), excluding an essential role of GAPC1 nitrosylation in the mechanism of nuclear relocalization, as found in mammalian cells. Although the function of GAPC1 in the nucleus is unknown, our results suggest that glycolytic GAPC1, through its high sensitivity to the cellular redox state, may play a role in oxidative stress signaling or protection in plants.

A transcriptomic study reveals differentially expressed genes and pathways respond to simulated acid rain in Arabidopsis thaliana

Acid rain, as a worldwide environmental issue, can cause serious damage to plants. In this study, we provided the first case study on the systematic responses of arabidopsis (Arabidopsis thaliana (L.) Heynh.) to simulated acid rain (SiAR) by transcriptome approach. Transcriptomic analysis revealed that the expression of a set of genes related to primary metabolisms, including nitrogen, sulfur, amino acid, photosynthesis, and reactive oxygen species metabolism, were altered under SiAR. In addition, transport and signal transduction related pathways, especially calcium-related signaling pathways, were found to play important roles in the response of arabidopsis to SiAR stress. Further, we compared our data set with previously published data sets on arabidopsis transcriptome subjected to various stresses, including wound, salt, light, heavy metal, karrikin, temperature, osmosis, etc. The results showed that many genes were overlapped in several stresses, suggesting that plant response to SiAR is a complex process, which may require the participation of multiple defense-signaling pathways. The results of this study will help us gain further insights into the response mechanisms of plants to acid rain stress.

Transgenic barley with overexpressed PTrx increases aluminum resistance in roots during germination

A transgenic barley line (LSY-11-1-1) with overexpressed Phalaris coerulescens thioredoxin gene (PTrx) was employed to measure the growth, protein oxidation, cell viability, and antioxidase activity in barley roots during germination on the presence of 2 mmol/L AlCl(3) on filter paper. The results show that (1) compared with the non-transgenic barley, LSY-11-1-1 had enhanced root growth, although both were seriously inhibited after AlCl(3) treatment; (2) the degree of protein oxidation and loss of cell viability in roots of LSY-11-1-1 were much less than those in roots of non-transgenic barley, as reflected by lower contents of protein carbonyl and Evans blue uptakes in LSY-11-1-1; (3) activities of catalase (CAT), glutathione peroxidase (GPX), ascorbate peroxidase (APX), and glutathione reductase (GR) in LSY-11-1-1 root tips were generally higher than those in non-transgenic barley root tips, although these antioxidase activities gave a rise to different degrees in both LSY-11-1-1 and non-transgenic barley under aluminum stress. These results indicate that overexpressing PTrx could efficiently protect barley roots from oxidative injury by increasing antioxidase activity, thereby quenching ROS caused by AlCl(3) during germination. These properties raise the possibility that transgenic barley with overexpressed PTrx may be used to reduce the aluminum toxicity in acid soils.

Recent insights into antioxidant defenses of legume root nodules

Legume root nodules are sites of intense biochemical activity and consequently are at high risk of damage as a result of the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). These molecules can potentially give rise to oxidative and nitrosative damage but, when their concentrations are tightly controlled by antioxidant enzymes and metabolites, they also play positive roles as critical components of signal transduction cascades during nodule development and stress. Thus, recent advances in our understanding of ascorbate and (homo)glutathione biosynthesis in plants have opened up the possibility of enhancing N(2) fixation through an increase of their concentrations in nodules. It is now evident that antioxidant proteins other than the ascorbate-glutathione enzymes, such as some isoforms of glutathione peroxidases, thioredoxins, peroxiredoxins, and glutathione S-transferases, are also critical for nodule activity. To avoid cellular damage, nodules are endowed with several mechanisms for sequestration of Fenton-active metals (nicotianamine, phytochelatins, and metallothioneins) and for controlling ROS/RNS bioactivity (hemoglobins). The use of 'omic' technologies has expanded the list of known antioxidants in plants and nodules that participate in ROS/RNS/antioxidant signaling networks, although aspects of developmental variation and subcellular localization of these networks remain to be elucidated. To this end, a critical point will be to define the transcriptional and post-transcriptional regulation of antioxidant proteins.

Comparative genomic study of the thioredoxin family in photosynthetic organisms with emphasis on Populus trichocarpa

The recent genome sequencing of Populus trichocarpa and Vitis vinifera, two models of woody plants, of Sorghum bicolor, a model of monocot using C4 metabolism, and of the moss Physcomitrella patens, together with the availability of photosynthetic organism genomes allows performance of a comparative genomic study with organisms having different ways of life, reproduction modes, biological traits, and physiologies. Thioredoxins (Trxs) are small ubiquitous proteins involved in the reduction of disulfide bridges in a variety of target enzymes present in all sub-cellular compartments and involved in many biochemical reactions. The genes coding for these enzymes have been identified in these newly sequenced genomes and annotated. The gene content, organization and distribution were compared to other photosynthetic organisms, leading to a refined classification. This analysis revealed that higher plants and bryophytes have a more complex family compared to algae and cyanobacteria and to non-photosynthetic organisms, since poplar exhibits 49 genes coding for typical and atypical thioredoxins and thioredoxin reductases, namely one-third more than monocots such as Oryza sativa and S. bicolor. The higher number of Trxs in poplar is partially explained by gene duplication in the Trx m, h, and nucleoredoxin classes. Particular attention was paid to poplar genes with emphasis on Trx-like classes called Clot, thioredoxin-like, thioredoxins of the lilium type and nucleoredoxins, which were not described in depth in previous genomic studies.

A small family of chloroplast atypical thioredoxins

The reduction and the formation of regulatory disulfide bonds serve as a key signaling element in chloroplasts. Members of the thioredoxin (Trx) superfamily of oxidoreductases play a major role in these processes. We have characterized a small family of plant-specific Trxs in Arabidopsis (Arabidopsis thaliana) that are rich in cysteine and histidine residues and are typified by a variable noncanonical redox active site. We found that the redox midpoint potential of three selected family members is significantly less reducing than that of the classic Trxs. Assays of subcellular localization demonstrated that all proteins are localized to the chloroplast. Selected members showed high activity, contingent on a dithiol electron donor, toward the chloroplast 2-cysteine peroxiredoxin A and poor activity toward the chloroplast NADP-malate dehydrogenase. The expression profile of the family members suggests that they have distinct roles. The intermediate redox midpoint potential value of the atypical Trxs might imply adaptability to function in modulating the redox state of chloroplast proteins with regulatory disulfides.

Patterns of gene expression induced by oligoguluronates reveal conserved and environment-specific molecular defense responses in the brown alga Laminaria digitata

* Until now, no information has been available on the transcriptional response to the transduction of defense signals in brown seaweeds that leads to active resistance against pathogens or grazers. * Using a polymerase chain reaction (PCR)-based, subtractive cDNA approach combined with filter arrays we demonstrated that Laminaria digitata exhibits a rapid response to oligoguluronate elicitors. The transcription levels of several genes were validated by quantitative real-time reverse-transcription PCR and further analysed using pharmacological approaches. * Fifty upregulated genes were identified by differential screening in elicited algae over a 24-h time-course. These genes were related to oxidative stress responses, production of antimicrobial secondary compounds or cell wall strengthening. Moreover, pharmacological tests showed that intracellular signal transduction is likely to involve reactive oxygen species. A new oligoguluronate-inducible vanadium-dependent haloperoxidase (vHPO), specific to iodide was also characterized. The transcription of several vHPO genes was shown to be tightly regulated. * Taken together, our data show that early transcriptional defense responses in L. digitata are similar to those in land plants but also include novel defense responses that involve tightly regulated iodine metabolism.

The Arabidopsis putative selenium-binding protein family: expression study and characterization of SBP1 as a potential new player in cadmium detoxification processes

In Arabidopsis (Arabidopsis thaliana), the putative selenium-binding protein (SBP) gene family is composed of three members (SBP1-SBP3). Reverse transcription-polymerase chain reaction analyses showed that SBP1 expression was ubiquitous. SBP2 was expressed at a lower level in flowers and roots, whereas SBP3 transcripts were only detected in young seedling tissues. In cadmium (Cd)-treated seedlings, SBP1 level of expression was rapidly increased in roots. In shoots, SBP1 transcripts accumulated later and for higher Cd doses. SBP2 and SBP3 expression showed delayed or no responsiveness to Cd. In addition, luciferase (LUC) activity recorded on Arabidopsis lines expressing the LUC gene under the control of the SBP1 promoter further showed dynamic regulation of SBP1 expression during development and in response to Cd stress. Western-blot analysis using polyclonal antibodies raised against SBP1 showed that SBP1 protein accumulated in Cd-exposed tissues in correlation with SBP1 transcript amount. The sbp1 null mutant displayed no visible phenotype under normal and stress conditions that was explained by the up-regulation of SBP2 expression. SBP1 overexpression enhanced Cd accumulation in roots and reduced sensitivity to Cd in wild type and, more significantly, in Cd-hypersensitive cad mutants that lack phytochelatins. Similarly, in Saccharomyces cerevisiae, SBP1 expression led to increased Cd tolerance of the Cd-hypersensitive ycf1 mutant. In vitro experiments showed that SBP1 has the ability to bind Cd. These data highlight the importance of maintaining the adequate SBP protein level under healthy and stress conditions and suggest that, during Cd stress, SBP1 accumulation efficiently helps to detoxify Cd potentially through direct binding.

Seleno-independent glutathione peroxidases

Glutathione peroxidases (GPXs, EC 1.11.1.9) were first discovered in mammals as key enzymes involved in scavenging of activated oxygen species (AOS). Their efficient antioxidant activity depends on the presence of the rare amino-acid residue selenocysteine (SeCys) at the catalytic site. Nonselenium GPX-like proteins (NS-GPXs) with a Cys residue instead of SeCys have also been found in most organisms. As SeCys is important for GPX activity, the function of the NS-GPX can be questioned. Here, we highlight the evolutionary link between NS-GPX and seleno-GPX, particularly the evolution of the SeCys incorporation system. We then discuss what is known about the enzymatic activity and physiological functions of NS-GPX. Biochemical studies have shown that NS-GPXs are not true GPXs; notably they reduce AOS using reducing substrates other than glutathione, such as thioredoxin. We provide evidence that, in addition to their inefficient scavenging action, NS-GPXs act as AOS sensors in various signal-transduction pathways.

A novel thioredoxin h is secreted in Nicotiana alata and reduces S-RNase in vitro

Thioredoxins type h are classified into three subgroups. The subgroup II includes thioredoxins containing an N-terminal extension, the role of which is still unclear. Although thioredoxin secretion has been observed in animal cells, there is no evidence suggesting that any thioredoxin h is secreted in plants. In this study, we report that a thioredoxin h, subgroup II, from Nicotiana alata (NaTrxh) is secreted into the extracellular matrix of the stylar transmitting tract tissue. Fractionation studies showed that NaTrxh is extracted along with well characterized secretion proteins such as S-RNases and NaTTS (N. alata transmitting tissue-specific protein). Moreover, an NaTrxh-green fluorescent fusion protein transiently expressed in Nicotiana benthamiana and Arabidopsis thaliana leaves was also secreted, showing that NaTrxh has the required information for its secretion. We performed reduction assays in vitro to identify potential extracellular targets of NaTrxh. We found that S-RNase is one of the several potential substrates of the NaTrxh in the extracellular matrix. In addition, we proved by affinity chromatography that NaTrxh specifically interacts with S-RNase. Our findings showed that NaTrxh is a new thioredoxin h in Nicotiana that is secreted as well as in animal systems. Because NaTrxh is localized in the extracellular matrix of the stylar transmitting tract and its specific interaction with S-RNase to reduce it in vitro, we suggest that this thioredoxin h may be involved either in general pollen-pistil interaction processes or particularly in S-RNase-based self-incompatibility.

Glutathione, photosynthesis and the redox regulation of stress-responsive gene expression

The ubiquitous antioxidant thiol tripeptide glutathione is present in millimolar concentrations in plant tissues and is regarded as one of the major determinants of cellular redox homeostasis. Recent research has highlighted a regulatory role for glutathione in influencing the expression of many genes important in plants' responses to both abiotic and biotic stress. Therefore, it becomes important to consider how glutathione levels and its redox state are influenced by environmental factors, how glutathione is integrated into primary metabolism and precisely how it can influence the functioning of signal transduction pathways by modulating cellular redox state. This review draws on a number of recent important observations and papers to present a unified view of how the responsiveness of glutathione to changes in photosynthesis may be one means of linking changes in nuclear gene expression to changes in the plant's external environment.

Induction of thioredoxin is required for nodule development to reduce reactive oxygen species levels in soybean roots

Nodules are formed on legume roots as a result of signaling between symbiotic partners and in response to the activities of numerous genes. We cloned fragments of differentially expressed genes in spot-inoculated soybean (Glycine max) roots. Many of the induced clones were similar to known genes related to oxidative stress, such as thioredoxin and beta-carotene hydroxylase. The deduced amino acid sequences of full-length soybean cDNAs for thioredoxin and beta-carotene hydroxylase were similar to those in other species. In situ RNA hybridization revealed that the thioredoxin gene is expressed on the pericycle of 2-d-old nodules and in the infected cells of mature nodules, suggesting that thioredoxin is involved in nodule development. The thioredoxin promoter was found to contain a sequence resembling an antioxidant responsive element. When a thioredoxin mutant of yeast was transformed with the soybean thioredoxin gene it became hydrogen peroxide tolerant. These observations prompted us to measure reactive oxygen species levels. These were decreased by 3- to 5-fold in 7-d-old and 27-d-old nodules, coincident with increases in the expression of thioredoxin and beta-carotene hydroxylase genes. Hydrogen peroxide-producing regions identified with cerium chloride were found in uninoculated roots and 2-d-old nodules, but not in 7-d-old and 27-d-old nodules. RNA interference-mediated repression of the thioredoxin gene severely impaired nodule development. These data indicate that antioxidants such as thioredoxin are essential to lower reactive oxygen species levels during nodule development.

The Arabidopsis selenium-binding protein confers tolerance to toxic levels of selenium

In the Arabidopsis genome there are three highly conserved homologues of the mammalian 56-kD selenium-binding protein (SBP). To study the function of SBP in this model plant, we used a transgenic approach by constitutively overexpressing and down-regulating the endogenous Atsbp1 gene. In the latter case, we employed both a conventional antisense method and gene silencing by intron-containing hairpin RNAs. Atsbp1-overexpressing and silenced plants were phenotypically normal, under standard growth conditions, when compared with wild type plants. Transgenic plants exhibited different growth responses to exogenously supplied selenite, which correlated with the expression levels of Atsbp1. Plants with increased Atsbp1 transcript levels showed enhanced tolerance to selenite, while plants with reduced levels were more sensitive. Our results indicate that, although Atsbp1 does not play a detectable role in the regulation of developmental processes under normal growth conditions, it appears to be involved in processes controlling tolerance of Arabidopsis to selenium toxicity.

Glutathione-associated regulation of plant growth and stress responses

A reduced form of glutathione (GSH) is considered to protect the cell from oxidative damage, based on its redox buffering action and abundance in the cell. However, in plants, the high redox potential molecule ascorbate exists at comparable or higher concentrations and is used for scavenging hydrogen peroxide as an electron donor. Recently, examples that cannot be explained simply by the antioxidant activity of GSH have been increasing in number. This article summarizes the recent findings on the glutathione-associated events in plants, in particular, growth and development including cell differentiation, cell death and senescence, pathogen resistance, and enzymatic regulation.

Strategies to maintain redox homeostasis during photosynthesis under changing conditions

Plants perform photosynthesis and assimilatory processes in a continuously changing environment. Energy production in the various cell compartments and energy consumption in endergonic processes have to be well adjusted to the varying conditions. In addition, dissipatory pathways are required to avoid any detrimental effects caused by over-reduction. A large number of short-term and long-term mechanisms interact with each other in a flexible way, depending on intensity and the type of impact. Therefore, all levels of regulation are involved, starting from energy absorption and electron flow events through to post-transcriptional control. The simultaneous presence of strong oxidants and strong reductants during oxygenic photosynthesis is the basis for regulation. However, redox-dependent control also interacts with other signal transduction pathways in order to adapt metabolic processes and redox-control to the developmental state. Examples are given here for short-term and long-term control following changes of light intensity and photoperiod, focusing on the dynamic nature of the plant regulatory systems. An integrating network of all these mechanisms exists at all levels of control. Cellular homeostasis will be maintained as long as the mechanisms for acclimation are present in sufficiently high capacities. If an impact is too rapid, and acclimation on the level of gene expression cannot occur, cellular damage and cell death are initiated.

Phylogenetic analyses identify 10 classes of the protein disulfide isomerase family in plants, including single-domain protein disulfide isomerase-related proteins

Protein disulfide isomerases (PDIs) are molecular chaperones that contain thioredoxin (TRX) domains and aid in the formation of proper disulfide bonds during protein folding. To identify plant PDI-like (PDIL) proteins, a genome-wide search of Arabidopsis (Arabidopsis thaliana) was carried out to produce a comprehensive list of 104 genes encoding proteins with TRX domains. Phylogenetic analysis was conducted for these sequences using Bayesian and maximum-likelihood methods. The resulting phylogenetic tree showed that evolutionary relationships of TRX domains alone were correlated with conserved enzymatic activities. From this tree, we identified a set of 22 PDIL proteins that constitute a well-supported clade containing orthologs of known PDIs. Using the Arabidopsis PDIL sequences in iterative BLAST searches of public and proprietary sequence databases, we further identified orthologous sets of 19 PDIL sequences in rice (Oryza sativa) and 22 PDIL sequences in maize (Zea mays), and resolved the PDIL phylogeny into 10 groups. Five groups (I-V) had two TRX domains and showed structural similarities to the PDIL proteins in other higher eukaryotes. The remaining five groups had a single TRX domain. Two of these (quiescin-sulfhydryl oxidase-like and adenosine 5'-phosphosulfate reductase-like) had putative nonisomerase enzymatic activities encoded by an additional domain. Two others (VI and VIII) resembled small single-domain PDIs from Giardia lamblia, a basal eukaryote, and from yeast. Mining of maize expressed sequence tag and RNA-profiling databases indicated that members of all of the single-domain PDIL groups were expressed throughout the plant. The group VI maize PDIL ZmPDIL5-1 accumulated during endoplasmic reticulum stress but was not found within the intracellular membrane fractions and may represent a new member of the molecular chaperone complement in the cell.

Analysis of the proteins targeted by CDSP32, a plastidic thioredoxin participating in oxidative stress responses

The chloroplastic drought-induced stress protein of 32 kDa (CDSP32) is a thioredoxin induced by environmental stress conditions. To gain insight into the function of CDSP32, we applied two strategies to analyze its targets. First, using affinity chromatography with an immobilized CDSP32 active site mutant, we identified six plastidic targets of CDSP32. Three of them are involved in photosynthetic processes: ATP-ase gamma-subunit, Rubisco and aldolase. The three others participate in the protection against oxidative damage: two peroxiredoxins, PrxQ and the BAS1 2-Cys peroxiredoxin, and a B-type methionine sulfoxide reductase. Then, we developed a novel strategy to trap targets directly in leaf extracts. The method, based on co-immunoprecipitation using extracts from plants overexpressing Wt CDSP32 or CDSP32 active site mutant, confirmed the interaction in vivo between CDSP32 and the PrxQ and BAS1 peroxiredoxins. We showed that CDSP32 is able to form heterodimeric complexes with PrxQ and that the peroxiredoxin displays CDSP32-dependent peroxidase activity. Under photooxidative stress induced by methyl viologen, plants overexpressing CDSP32 active site mutant exhibit decreased maximal PSII photochemical efficiency and retain much less chlorophyll compared with Wt plants and with plants overexpressing Wt CDSP32. We propose that the increased sensitivity results from trapping in planta of the targets involved in the protection against oxidative damage. We conclude that CDSP32, compared with other plant thioredoxins, is a thioredoxin more specifically involved in plastidic responses against oxidative stress.

Induction of PR-1 accumulation accompanied by runaway cell death in the lsd1 mutant of Arabidopsis is dependent on glutathione levels but independent of the redox state of glutathione

The lesions simulating disease (lsd) mutants of Arabidopsis spontaneously develop hypersensitive-response-like lesions in the absence of pathogens. To address the function of the redox regulator glutathione in disease resistance, we examined the relationship between endogenous glutathione and PR-1 accumulation using one of these mutants, lsd1, as a disease resistance model. Lesion formation on lsd1 was suppressed by weak light and initiated by the subsequent transition to normal light. The application of buthionine sulfoximine, a specific inhibitor of glutathione biosynthesis, suppressed conditionally induced runaway cell death and expression of the PR-1 gene, suggesting that glutathione regulates the conditional cell death and PR-1 gene expression. The application of reduced (GSH) or oxidized (GSSG) glutathione to lsd1 upregulated the level of total glutathione ([GSH]+[GSSG]) accompanied by hastened accumulation of PR-1, and the basal level of total glutathione in lsd1 was higher than that in wild-type plants. The glutathione redox state defined as [GSH]/([GSH]+[GSSG]) decreased following the conditional transition, but the suppression of this decrease by the application of GSH did not inhibit the accumulation of PR-1. Taken together, conditional PR-1 accumulation in lsd1 is regulated not by the redox state but by the endogenous level of glutathione.

Thioredoxin targets of developing wheat seeds identified by complementary proteomic approaches

The role of thioredoxin in wheat starchy endosperm was investigated utilizing two proteomic approaches. Thioredoxin targets were isolated from total KCl-soluble extracts of endosperm and flour and separated by 2-DE following (1) reduction of the extract by the NADP/thioredoxin system and labeling the newly generated sulfhydryl (SH) groups with monobromobimane (mBBr), and, in parallel, (2) trapping covalently interacting proteins on an affinity column prepared with mutant thioredoxin h in which one of the active site cysteines was replaced by serine. The two procedures were complementary: of the total targets, one-third were observed with both procedures and one-third were unique to each. Altogether 68 potential targets were identified; almost all containing conserved cysteines. In addition to confirming known interacting proteins, we identified 40 potential thioredoxin targets not previously described in seeds. A comparison of the results obtained with young endosperm (isolated 10 days after flowering) to those with mature endosperm (isolated 36 days after flowering) revealed a unique set of proteins functional in processes characteristic of each developmental stage. Flour contained 36 thioredoxin targets, most of which have been found in the isolated developing endosperm.

Regulation of redox homeostasis in the yeast Saccharomyces cerevisiae

An increasingly important area of research is based on sulphydryl chemistry, since the oxidation of -SH groups is one of the earliest observable events during oxidant-mediated damage and -SH groups play a critical role in the function of many macromolecular structures including enzymes, transcription factors and membrane proteins. Glutaredoxins and thioredoxins are small heat-stable oxidoreductases, conserved throughout evolution, which play key roles in maintaining the cellular redox balance. Much progress has been made in analysing these systems in the yeast Saccharomyces cerevisiae which is a very useful model eukaryote due to its ease of genetic manipulation, its compact genome, the availability of the entire genome sequence, and the current rate of progress in gene function research. Yeast, like all eukaryotes, contains a number of glutaredoxin and thioredoxin isoenzymes located in both the cytoplasm and the mitochondria. This review describes recent findings made in yeast that are leading to a better understanding of the regulation and role of redox homeostasis in eukaryotic cell metabolism.

Potato plants lacking the CDSP32 plastidic thioredoxin exhibit overoxidation of the BAS1 2-cysteine peroxiredoxin and increased lipid Peroxidation in thylakoids under photooxidative stress

The CDSP32 protein (chloroplastic drought-induced stress protein of 32 kD) is a thioredoxin participating in the defense against oxidative damage. We recently have identified in vitro the BAS1 2-Cys peroxiredoxin, a peroxide-detoxifying enzyme, as a target for CDSP32. Here, we report the characterization under stress conditions of transgenic potato (Solanum tuberosum) plants lacking CDSP32 with regard to the BAS1 redox state and the level of lipid peroxidation. Under control conditions, BAS1 is present at similar levels both in wild-type (WT) and transgenic plants. Under drought and methyl viologen treatment, CDSP32-lacking plants display, compared with WT, an increased proportion of BAS1 monomer corresponding to an overoxidized form of the protein. Leaf discs from transgenic plants treated with methyl viologen exhibit earlier degradation of BAS1 than WT plants do. Using several approaches, i.e. a probe emitting fluorescence when reacting with peroxides, high-performance liquid chromatography determination of lipid hydroxy fatty acid content, and measurement of chlorophyll thermoluminescence, we show a higher lipid peroxidation level under methyl viologen treatment in thylakoids from CDSP32-lacking plants compared with WT. These data show that CDSP32 is a critical component in the defense system against lipid peroxidation in photosynthetic membranes, likely as a physiological electron donor to the BAS1 peroxiredoxin.

Redox control of Hsp70-Co-chaperone interaction revealed by expression of a thioredoxin-like Arabidopsis protein

By using a yeast functional complementation assay, we have identified AtTDX, a new Arabidopsis thaliana gene, encoding a two-domain 42-kDa protein. The amino-terminal domain of AtTDX is closely related to the co-chaperone Hsp70-interacting protein HIP, whereas its carboxyl-terminal part contains a thioredoxin domain. Both in vivo and in vitro assays showed that AtTDX is a protein-disulfide reductase. We next found that the HIP domain of AtTDX is capable of interacting with the ATPase domain of Ssb2, a yeast heat-shock protein 70 chaperone. Strikingly, the AtTDX-Ssb2 interaction can be released under oxidative stress, a redox-dependent regulation involving the thioredoxin activity of AtTDX. A mutation inactivating the cysteine 20 of the ATPase domain of Ssb2 was found to stabilize the AtTDX-Ssb2 interaction that becomes redox-insensitive. As cysteine 20 is conserved in virtually all the Hsp70 chaperones, our results suggest that this residue might be more generally the target of redox regulations of chaperone binding activity.

Redox signaling in the chloroplast: the ferredoxin/thioredoxin system

Chloroplasts have developed a light-dependent system for the control of the activities of key enzymes involved in assimilatory (photosynthetic) and dissimilatory pathways, which allows a switch between these opposing pathways to prevent futile cycling. This regulatory system, known as the ferredoxin/thioredoxin system, consists of several proteins constituting a redox cascade that transmits the light signal perceived by chlorophyll to selected target proteins, thereby influencing their activity. A central component of the redox cascade is a novel enzyme, the ferredoxin:thioredoxin reductase, which is capable of reducing a disulfide bridge with the help of an iron-sulfur cluster. Recent developments on the elucidation of the structures of several implicated proteins and on the mechanism of signal transfer have greatly improved our understanding of this regulatory mechanism. This review describes the components of the redox cascade, the principal target proteins, and the mechanism of action of the light-signal transfer.

Redox control by dithiol-disulfide exchange in plants: I. The chloroplastic systems

In plants, the photons of light are absorbed at the level of the photosystems in the chloroplasts. The functioning of the photosynthetic electron transfer chain linked to this process is required to generate NADPH and ATP. In addition, the light signal promotes a regulatory cascade, situated in the stroma, that involves ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxins. This redox-based signal transduction chain allows fine regulation of stromal enzymes and tight control of the photosynthetic process. The molecular properties and the functioning of this redox regulatory chain will be described in this review.

Lotus japonicus gene Ljsbp is highly conserved among plants and animals and encodes a homologue to the mammalian selenium-binding proteins

We have isolated and characterized a Lotus japonicus gene (Ljsbp) encoding a putative polypeptide with striking homology to the mammalian 56-kDa selenium-binding protein (SBP). cDNA clones homologous to LjSBP were also isolated from soybean, Medicago sativa, and Arabidopsis thaliana. Comparative expression studies in L japonicus and A. thaliana showed that sbp transcripts are present in various tissues and at different levels. Especially in L japonicus nodules and seedpods and A. thaliana siliques, sbp expression appears to be developmentally up-regulated. sbp Gene transcripts were localized by in situ hybridization in the infected cells and vascular bundles of young nodules, while in mature nodules, low levels of expression were only detected in the parenchymatous cells. Expression of sbp transcripts in young seedpods and siliques was clearly visible in vascular tissues and embryos, while in embryos, low levels of expression were detected in the root epidermis and the vascular bundles. Polyclonal antibodies raised against a truncated LjSBP recombinant protein recognized a polypeptide of about 60 kDa in nodule extracts. Immunohistochemical experiments showed that accumulation of LjSBP occurred in root hairs, in the root epidermis above the nodule primordium, in the phloem of the vasculature, and abundantly in the infected cells of young nodules. Irrespective of the presence of rhizobia, expression of SBP was also observed in root tips, where it was confined in the root epidermis and protophloem cells. We hypothesize that LjSBP may have more than one physiological role and can be implicated in controlling the oxidation/reduction status of target proteins, in vesicular Golgi transport, or both.

Exploring the active site of plant glutaredoxin by site-directed mutagenesis

Six mutants (Y26A, C27S, Y29F, Y29P, C30S and Y26W/Y29P) have been engineered in order to explore the active site of poplar glutaredoxin (Grx) (Y26CPYC30). The cysteinic mutants indicate that Cys 27 is the primary nucleophile. Phe is a good substitute for Tyr 29, but the Y29P mutant was inactive. The Y26A mutation caused a moderate loss of activity. The YCPPC and WCPPC mutations did not improve the reactivity of Grx with the chloroplastic NADP-malate dehydrogenase, a well known target of thioredoxins (Trxs). The results are discussed in relation with the known biochemical properties of Grx and Trx.

Orthologs in Arabidopsis thaliana of the Hsp70 interacting protein Hip

The Hsp70-interacting protein Hip binds to the adenosine triphosphatase domain of Hsp70, stabilizing it in the adenosine 5'-diphosphate-ligated conformation and promoting binding of target polypeptides. In mammalian cells, Hip is a component of the cytoplasmic chaperone heterocomplex that regulates signal transduction via interaction with hormone receptors and protein kinases. Analysis of the complete genome sequence of the model flowering plant Arabidopsis thaliana revealed 2 genes encoding Hip orthologs. The deduced sequence of AtHip-1 consists of 441 amino acid residues and is 42% identical to human Hip. AtHip-1 contains the same functional domains characterized in mammalian Hip, including an N-terminal dimerization domain, an acidic domain, 3 tetratricopeptide repeats flanked by a highly charged region, a series of degenerate GGMP repeats, and a C-terminal region similar to the Sti1/Hop/p60 protein. The deduced amino acid sequence of AtHip-2 consists of 380 amino acid residues. AtHip-2 consists of a truncated Hip-like domain that is 46% identical to human Hip, followed by a C-terminal domain related to thioredoxin. AtHip-2 is 63% identical to another Hip-thioredoxin protein recently identified in Vitis labrusca (grape). The truncated Hip domain in AtHip-2 includes the amino terminus, the acidic domain, and tetratricopeptide repeats with flanking charged region. Analyses of expressed sequence tag databases indicate that both AtHip-1 and AtHip-2 are expressed in A thaliana and that orthologs of Hip are also expressed widely in other plants. The similarity between AtHip-1 and its mammalian orthologs is consistent with a similar role in plant cells. The sequence of AtHip-2 suggests the possibility of additional unique chaperone functions.

PLANT MITOCHONDRIA AND OXIDATIVE STRESS: Electron Transport, NADPH Turnover, and Metabolism of Reactive Oxygen Species

The production of reactive oxygen species (ROS), such as O2- and H2O2, is an unavoidable consequence of aerobic metabolism. In plant cells the mitochondrial electron transport chain (ETC) is a major site of ROS production. In addition to complexes I-IV, the plant mitochondrial ETC contains a non-proton-pumping alternative oxidase as well as two rotenone-insensitive, non-proton-pumping NAD(P)H dehydrogenases on each side of the inner membrane: NDex on the outer surface and NDin on the inner surface. Because of their dependence on Ca2+, the two NDex may be active only when the plant cell is stressed. Complex I is the main enzyme oxidizing NADH under normal conditions and is also a major site of ROS production, together with complex III. The alternative oxidase and possibly NDin(NADH) function to limit mitochondrial ROS production by keeping the ETC relatively oxidized. Several enzymes are found in the matrix that, together with small antioxidants such as glutathione, help remove ROS. The antioxidants are kept in a reduced state by matrix NADPH produced by NADP-isocitrate dehydrogenase and non-proton-pumping transhydrogenase activities. When these defenses are overwhelmed, as occurs during both biotic and abiotic stress, the mitochondria are damaged by oxidative stress.

Binding features of chloroplast fructose-1,6-bisphosphatase-thioredoxin interaction

It has been proposed that a hydrophobic groove surrounded by positively charged amino acids on thioredoxin (Trx) serves as the recognition and docking site for the interaction of Trx with target proteins. This model for Trx-protein interactions fits well with the Trx-mediated fructose-1,6-bisphosphatase (FBPase) activation, where a protruding negatively charged loop of FBPase would bind to this Trx groove, in a process involving both electrostatic and hydrophobic interactions. This model facilitates the prediction of Trx amino acid residues likely to be involved in enzyme binding. Site-directed mutagenesis of some of these amino acids, in conjunction with measurements of the FBPase activation capacity of the wild type and mutated Trxs, was used to check the model and provided evidence that lysine-70 and arginine-74 of pea Trx m play an essential role in FBPase binding. The binding parameters for the interaction between chloroplast FBPase and the wild type pea Trxs f and m, as well as mutated pea Trx m, determined by equilibrium dialysis in accordance with the Koshland-Nemethy-Filmer model of saturation kinetics, provided additional support for the role of these basic Trx residues in the interaction with FBPase. These data, in conjunction with the midpoint redox potential (E(m)) determinations of Trxs, support the hydrophobic groove model for the interaction between chloroplast FBPase and Trx. This model predicts that differences in the FBPase activation capacity of Trxs arise from their different binding abilities.

Association of glutathione with flowering in Arabidopsis thaliana

In order to study the relationship between GSH and flowering, wild-type and late-flowering mutant, fca-1, of Arabidopsis thaliana were treated with L-buthionine sulfoximine (BSO), a specific inhibitor of GSH biosynthesis, under long-day conditions. BSO treatment of the fca-1 mutant starting at 17 d after imbibition promoted flowering. However, when the treatment was started at 12 d after imbibition, BSO treatment at 10(-4) M resulted in an inhibition of flowering. This inhibitory effect of BSO on flowering was abolished by GSH treatment at 10(-4) M, although GSH treatment at an increased concentration of 10(-3) M clearly delayed flowering. In contrast, BSO treatment of wild-type plants starting at 12 d after imbibition promoted flowering, whose effect was abolished by GSH application. In the fca-1 mutant, whose endogenous GSH levels were high, chilling treatment lowered the GSH levels and promoted flowering, as was the case in the BSO treatment. An A. thaliana mutant, cad2-1, which has a defect in GSH biosynthesis also exhibited late flowering. The late-flowering phenotype of this mutant tended to be strengthened by BSO and abolished by GSH treatment. These results suggest that flowering is associated with the rate of GSH biosynthesis and/or the levels of GSH in A. thaliana.

A strategy for the identification of proteins targeted by thioredoxin

Thioredoxins are 12-kDa proteins functional in the regulation of cellular processes throughout the animal, plant, and microbial kingdoms. Growing evidence with seeds suggests that an h-type of thioredoxin, reduced by NADPH via NADP-thioredoxin reductase, reduces disulfide bonds of target proteins and thereby acts as a wakeup call in germination. A better understanding of the role of thioredoxin in seeds as well as other systems could be achieved if more were known about the target proteins. To this end, we have devised a strategy for the comprehensive identification of proteins targeted by thioredoxin. Tissue extracts incubated with reduced thioredoxin are treated with a fluorescent probe (monobromobimane) to label sulfhydryl groups. The newly labeled proteins are isolated by conventional two-dimensional electrophoresis: (i) nonreducing/reducing or (ii) isoelectric focusing/reducing SDS/PAGE. The isolated proteins are identified by amino acid sequencing. Each electrophoresis system offers an advantage: the first method reveals the specificity of thioredoxin in the reduction of intramolecular vs. intermolecular disulfide bonds, whereas the second method improves the separation of the labeled proteins. By application of both methods to peanut seed extracts, we isolated at least 20 thioredoxin targets and identified 5-three allergens (Ara h2, Ara h3, and Ara h6) and two proteins not known to occur in peanut (desiccation-related and seed maturation protein). These findings open the door to the identification of proteins targeted by thioredoxin in a wide range of systems, thereby enhancing our understanding of its function and extending its technological and medical applications.

The S-locus receptor kinase is inhibited by thioredoxins and activated by pollen coat proteins

The self-incompatibility response in Brassica allows recognition and rejection of self-pollen by the stigmatic papillae. The transmembrane S-locus receptor kinase (SRK), a member of the receptor-like kinase superfamily in plants, mediates recognition of self-pollen on the female side, whereas the S-locus cysteine-rich protein (SCR) is the male component of the self-incompatibility response. SCR is presumably located in the pollen coat, and is thought to be the SRK ligand. Although many receptor-like kinases have been isolated in plants, the mechanisms of signal transduction mediated by these molecules remain largely unknown. Here we show that SRK is phosphorylated in vivo within one hour of self-pollination. We also show that, in vitro, autophosphorylation of SRK is prevented by the stigma thioredoxin THL1 in the absence of a ligand. This inhibition is released in a haplotype-specific manner by the addition of pollen coat proteins. Our data indicate that SRK is inhibited by thioredoxins and activated by pollen coat proteins.

Cloning and expression of a distinct subclass of plant thioredoxins

mRNAs encoding a novel thioredoxin were isolated from pollen RNA of Lolium perenne (LpTrx), Hordeum bulbosum (HbTrx), Phalaris coerulescens (PTrx) and Secale cereale (ScTrx). The cDNAs contain a single ORF of 393 bp encoding a protein of 131 amino acids. The predicted proteins showed highest homology to plant thioredoxins of the h class yet form a distinct subgroup that is characterized by a high level of sequence conservation (95.4-97.7% identity). GenBank searches revealed additional members of this subclass in tomato, soybean, rice and pine. LpTrx and PTrx were expressed as recombinant proteins in Escherichia coli and tested for thioredoxin activity. Both proteins displayed typical thioredoxin activity in the nonspecific insulin reduction assay, however, were not reduced by E. coli NADPH-dependant thioredoxin reductase.

Characterization of determinants for the specificity of Arabidopsis thioredoxins h in yeast complementation

The disruption of the two thioredoxin genes in Saccharomyces cerevisiae leads to a complex phenotype, including the inability to use methionine sulfoxide as sulfur source, modified cell cycle parameters, reduced H(2)O(2) tolerance, and inability to use sulfate as sulfur source. Expression of one of the multiple Arabidopsis thaliana thioredoxins h in this mutant complements only some aspects of the phenotype, depending on the expressed thioredoxin: AtTRX2 or AtTRX3 induce methionine sulfoxide assimilation and restore a normal cell cycle. In addition AtTRX2 also confers growth on sulfate but no H(2)O(2) tolerance. In contrast, AtTRX3 does not confer growth on sulfate but induces H(2)O(2) tolerance. We have constructed hybrid proteins between these two thioredoxins and show that all information necessary for sulfate assimilation is present in the C-terminal part of AtTRX2, whereas some information needed for H(2)O(2) tolerance is located in the N-terminal part of AtTRX3. In addition, mutation of the atypical redox active site WCPPC to the classical site WCGPC restores some growth on sulfate. All these data suggest that the multiple Arabidopsis thioredoxins h originate from a totipotent ancestor with all the determinants necessary for interaction with the different thioredoxin target proteins. After duplications each member evolved by losing or masking some of the determinants.

Characterization of proteins in latex of the opium poppy (Papaver somniferum) using two-dimensional gel electrophoresis and microsequencing

The opium poppy (Papaver somniferum) belongs to the group of latex-containing plants. Latex is the milky-like fluid within laticifer cells. In this study, poppy latex was analyzed with respect to ultrastructure, alkaloid, and protein content. The main goal of this project was the examination of the proteins by two-dimensional gel electrophoresis. In a proteomics approach, we investigated two main fractions of the latex, namely the cytosolic serum and the sedimented fraction containing the alkaloid-accumulating vesicles. Of the serum, representing the protein-rich part of the latex, 75 spots were analyzed by internal peptide microsequencing, followed by a database searching. For 69 proteins a function could be assigned due to homology to known proteins, whereas six spots could not be identified. Furthermore, codeinone reductase, a representative of the specific enzyme system in morphine biosynthesis, could be detected within the cytosolic serum fraction. In the vesicle-containing pellet, 23 protein spots were analyzed. An attempt was also made to separate the vesicle pellet by density centrifugation, followed by investigation of the alkaloid content, ultrastructure, and protein pattern. This study describes the first database of soluble proteins present in the latex of P. somniferum

PLANT THIOREDOXIN SYSTEMS REVISITED

Thioredoxins, the ubiquitous small proteins with a redox active disulfide bridge, are important regulatory elements in plant metabolism. Initially recognized as regulatory proteins in the reversible light activation of key photosynthetic enzymes, they have subsequently been found in the cytoplasm and in mitochondria. The various plant thioredoxins are different in structure and function. Depending on their intracellular location they are reduced enzymatically by an NADP-dependent or by a ferredoxin (light)-dependent reductase and transmit the regulatory signal to selected target enzymes through disulfide/dithiol interchange reactions. In this review we summarize recent developments that have provided new insights into the structures of several components and into the mechanism of action of the thioredoxin systems in plants.