Overexpression of thioredoxin h leads to enhanced activity of starch debranching enzyme (pullulanase) in barley grain

A Genome Wide Association Study Revealed Key Single Nucleotide Polymorphisms/Genes Associated With Seed Germination in Gossypium hirsutum L

Fast and uniform seed germination is essential to stabilize crop yields in agricultural production. It is important to understand the genetic basis of seed germination for improving the vigor of crop seeds. However, little is known about the genetic basis of seed vigor in cotton. In this study, we evaluated four seed germination-related traits of a core collection consisting of 419 cotton accessions, and performed a genome-wide association study (GWAS) to explore important loci associated with seed vigor using 3.66 million high-quality single nucleotide polymorphisms (SNPs). The results showed that four traits, including germination potential, germination rate, germination index, and vigor index, exhibited broad variations and high correlations. A total of 92 significantly associated SNPs located within or near 723 genes were identified for these traits, of which 13 SNPs could be detected in multiple traits. Among these candidate genes, 294 genes were expressed at seed germination stage. Further function validation of the two genes of higher expression showed that Gh_A11G0176 encoding Hsp70-Hsp90 organizing protein negatively regulated Arabidopsis seed germination, while Gh_A09G1509 encoding glutathione transferase played a positive role in regulating tobacco seed germination and seedling growth. Furthermore, Gh_A09G1509 might promote seed germination and seedling establishment through regulating glutathione metabolism in the imbibitional seeds. Our findings provide unprecedented information for deciphering the genetic basis of seed germination and performing molecular breeding to improve field emergence through genomic selection in cotton.

Interplay of starch debranching enzyme and its inhibitor is mediated by Redox-Activated SPL transcription factor

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We have first time described that pullulanase inhibitor is under control of SPL transcriptional factor in barley.

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SPL3 mediated transcriptional regulation of pullulanase inhibitor is redox-dependent.

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Antagonistic relationship between pullulanase and its inhibitors is epigenetically guided via miR156 targeted SPL3.

The germination process is of central importance across the cultivated species involving several key enzymes for mobilization of stored food reserves. Pullulanase (PUL), a starch-debranching enzyme, plays an important role in mobilizing stored endosperm food reserves during germination. Pullulanase inhibitor (PULI) hinders PUL’s activity through an unknown mechanism. Barley has one PUL and two PULI genes. During the time-dependent processes of seed germination, only PULI-1 expression shows an antagonistic relationship with that of PUL. Our data have indicated that the expression of PULI-1 is modulated by SPL (Squamosa-promoter-binding Protein Like) transcription factors, known to be targeted by miR156. We show that the binding of recombinant HvSPL3 protein to the PULI-1 promoter occurs under reducing, but not under oxidizing conditions. Replacement of Cys residues with threonine in HvSPL3 abolishes the binding, indicating an essential role of the redox state in the expression of PULI. Our findings may have important implications for the industrial use of starch.

Quality attributes for barley malt: "The backbone of beer"

Malting is the process of preparing barley for brewing through partial germination followed by drying. This process softens the grain cell wall and stimulates the production of diastatic enzymes, which convert starch into malt extract. The suitability of a barley grain for malt production depends upon a large number of quality parameters that are crucial for the identification and release of high-quality malt varieties. Maintaining tight control of these quality attributes is essential to ensure high processing efficiency and final product quality in brewery and malt house. Therefore, we have summarized the basic malting process and various physiological and biochemical quality parameters that are desirable for better malt quality. This study may provide an understanding of the process, problems faced, and opportunities to maltsters and researchers to improve the malt efficiency by altering the malting process or malt varieties.

Thioredoxin h2 contributes to the redox regulation of mitochondrial photorespiratory metabolism

Thioredoxins (TRXs) are important proteins involved in redox regulation of metabolism. In plants, it has been shown that the mitochondrial metabolism is regulated by the mitochondrial TRX system. However, the functional significance of TRX h2, which is found at both cytosol and mitochondria, remains unclear. Arabidopsis plants lacking TRX h2 showed delayed seed germination and reduced respiration alongside impaired stomatal and mesophyll conductance, without impacting photosynthesis under ambient O₂ conditions. However, an increase in the stoichiometry of photorespiratory CO₂ release was found during O₂ -dependent gas exchange measurements in trxh2 mutants. Metabolite profiling of trxh2 leaves revealed alterations in key metabolites of photorespiration and in several metabolites involved in respiration and amino acid metabolism. Decreased abundance of serine hydroxymethyltransferase and glycine decarboxylase (GDC) H and L subunits as well as reduced NADH/NAD⁺ ratios were also observed in trxh2 mutants. We further demonstrated that the redox status of GDC-L is altered in trxh2 mutants in vivo and that recombinant TRX h2 can deactivate GDC-L in vitro, indicating that this protein is redox regulated by the TRX system. Collectively, our results demonstrate that TRX h2 plays an important role in the redox regulation of mitochondrial photorespiratory metabolism.

The expression pattern of OsDim1 in rice and its proposed function

Development of plant tissues is dependent on numerous factors, including hormone activity, signaling, cell division, and elongation. In plants, Defective Entry into Mitosis 1 (Dim1) homologs are recognized as pivotal in leaf senescence and progress of normal growth, but their role in rice has not been functionally characterized. The findings presented in this paper suggest that OsDim1 is important in early seedling development, pollen tube elongation, and impacts rice yield components. The gene is expressed in the scutellum, endosperm, embryonic root, shoot, pollen grains and tubes, as well as in several organs of the rice flower. According to the present study findings, RNAi mediated knockdown of OsDim1 resulted in phytohormonal imbalance, reduced amylase activity, affected differentiation of embryonic root elongation zone tissues, suppressed embryonic root and shoot growth, and impaired pollen tube elongation. In contrast, overexpression of OsDim1 showed significant growth in embryonic roots and shoots, while it increased culm length, total number of tillers per plant, seed setting rate, and total number of grains per panicle compared to its wild type line. In summary, we propose OsDim1 plays an important role in seedling growth and pollen tube elongation, and has pleiotropic effects on reproductive tissues.

Recent practical researches in the development of gluten-free breads

Wheat bread is consumed globally and has played a critical role in the story of civilization since the development of agriculture. While the aroma and flavor of this staple food continue to delight and satisfy most people, some individuals have a specific allergy to wheat or a genetic disposition to celiac disease. To improve the quality of life of these patients from a dietary standpoint, food-processing researchers have been seeking to develop high-quality gluten-free bread. As the quality of wheat breads depends largely on the viscoelastic properties of gluten, various ingredients have been employed to simulate its effects, such as hydrocolloids, transglutaminase, and proteases. Recent attempts have included the use of redox regulation as well as particle-stabilized foam. In this short review, we introduce the ongoing advancements in the development of gluten-free bread, by our laboratory as well as others, focusing mainly on rice-based breads. The social and scientific contexts of these efforts are also mentioned.

Developmental dynamics of stem starch accumulation in Sorghum bicolor

Sweet sorghums were identified that accumulate up to ~9% of their total stem dry weight as starch. Starch accumulated preferentially in stem pith parenchyma in close proximity to vascular bundles. Stem starch accumulated slowly between floral initiation and anthesis and more rapidly between anthesis and 43 days post-anthesis before declining in parallel with tiller outgrowth. Genes involved in stem starch metabolism were identified through phylogenetic approaches and RNA-seq analysis of Della stem gene expression during the starch accumulation phase of development. Genes differentially expressed in stems were identified that are involved in starch biosynthesis (i.e., AGPase SS/LS, starch synthases, starch-branching enzymes), degradation (i.e., glucan-water dikinase, β-amylase, disproportionating enzyme, alpha-glucan phosphorylase) and amyloplast sugar transport (glucose-6-P translocator). Transcripts encoding AGPase SS and LS subunits with plastid localization were differentially induced during stem starch accumulation indicating that ADP-glucose for starch biosynthesis is primarily generated in stem plastids. Cytosolic heteroglucan metabolism may play a role in stem sucrose/starch accumulation because genes encoding cytosolic forms of the disproportionating enzyme and alpha-glucan phosphorylase were induced in parallel with stem sucrose/starch accumulation. Information on the stem starch pathway obtained in this study will be useful for engineering sorghum stems with elevated starch thereby improving forage quality and the efficiency of biomass conversion to biofuels and bio-products.

The Path to Thioredoxin and Redox Regulation Beyond Chloroplasts

Once the ferredoxin/thioredoxin system was established as a mechanism linking light to the post-translational regulation of chloroplast enzymes, I considered that plants might harbor a light-independent mechanism utilizing this same enzyme chemistry based on thiol-disulfide redox transitions. After reflection, it occurred to me that such a mechanism could be fundamental to seeds of cereals that undergo dramatic change following exposure to oxygen during maturation and drying. The pursuit of this idea led to the discovery of a family of extraplastidic thioredoxins, designated the h-type, that resemble animal and bacterial counterparts in undergoing enzymatic reduction with NADPH. Current evidence suggests that h-type thioredoxins function broadly throughout the plant. Here I describe how the thioredoxin h field developed, its current status and potential for contributing material benefits to society.

Redox-dependent interaction between thaumatin-like protein and β-glucan influences malting quality of barley

Barley is the cornerstone of the malting and brewing industry. It is known that 250 quantitative trait loci (QTLs) of the grain are associated with 19 malting-quality phenotypes. However, only a few of the contributing genetic components have been identified. One of these, on chromosome 4H, contains a major malting QTL, QTL2, located near the telomeric region that accounts, respectively, for 28.9% and 37.6% of the variation in the β-glucan and extract fractions of malt. In the current study, we dissected the QTL2 region using an expression- and microsynteny-based approach. From a set of 22 expressed sequence tags expressed in seeds at the malting stage, we identified a candidate gene, TLP8 (thaumatin-like protein 8), which was differentially expressed and influenced malting quality. Transcript abundance and protein profiles of TLP8 were studied in different malt and feed varieties using quantitative PCR, immunoblotting, and enzyme-linked immunosorbent assay (ELISA). The experiments demonstrated that TLP8 binds to insoluble (1, 3, 1, 4)-β-D glucan in grain extracts, thereby facilitating the removal of this undesirable polysaccharide during malting. Further, the binding of TLP8 to β-glucan was dependent on redox. These findings represent a stride forward in our understanding of the malting process and provide a foundation for future improvements in the final beer-making process.

Seed thioredoxin h

Thioredoxins are nearly ubiquitous disulfide reductases involved in a wide range of biochemical pathways in various biological systems, and also implicated in numerous biotechnological applications. Plants uniquely synthesize an array of thioredoxins targeted to different cell compartments, for example chloroplastic f- and m-type thioredoxins involved in regulation of the Calvin-Benson cycle. The cytosolic h-type thioredoxins act as key regulators of seed germination and are recycled by NADPH-dependent thioredoxin reductase. The present review on thioredoxin h systems in plant seeds focuses on occurrence, reaction mechanisms, specificity, target protein identification, three-dimensional structure and various applications. The aim is to provide a general background as well as an update covering the most recent findings. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Reprogramming of Seed Metabolism Facilitates Pre-harvest Sprouting Resistance of Wheat

Pre-harvest sprouting (PHS) is a worldwide problem for wheat production and transgene antisense-thioredoxin-s (anti-trx-s) facilitates outstanding resistance. To understand the molecular details of PHS resistance, we analyzed the metabonomes of the transgenic and wild-type (control) wheat seeds at various stages using NMR and GC-FID/MS. 60 metabolites were dominant in these seeds including sugars, organic acids, amino acids, choline metabolites and fatty acids. At day-20 post-anthesis, only malate level in transgenic wheat differed significantly from that in controls whereas at day-30 post-anthesis, levels of amino acids and sucrose were significantly different between these two groups. For mature seeds, most metabolites in glycolysis, TCA cycle, choline metabolism, biosynthesis of proteins, nucleotides and fatty acids had significantly lower levels in transgenic seeds than in controls. After 30-days post-harvest ripening, most metabolites in transgenic seeds had higher levels than in controls including amino acids, sugars, organic acids, fatty acids, choline metabolites and NAD⁺. These indicated that anti-trx-s lowered overall metabolic activities of mature seeds eliminating pre-harvest sprouting potential. Post-harvest ripening reactivated the metabolic activities of transgenic seeds to restore their germination vigor. These findings provided essential molecular phenomic information for PHS resistance of anti-trx-s and a credible strategy for future developing PHS resistant crops.

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.

Grain sorghum proteomics: integrated approach toward characterization of endosperm storage proteins in kafirin allelic variants

Grain protein composition determines quality traits, such as value for food, feedstock, and biomaterials uses. The major storage proteins in sorghum are the prolamins, known as kafirins. Located primarily on the periphery of the protein bodies surrounding starch, cysteine-rich β- and γ-kafirins may limit enzymatic access to internally positioned α-kafirins and starch. An integrated approach was used to characterize sorghum with allelic variation at the kafirin loci to determine the effects of this genetic diversity on protein expression. Reversed-phase high performance liquid chromatography and lab-on-a-chip analysis showed reductions in alcohol-soluble protein in β-kafirin null lines. Gel-based separation and liquid chromatography-tandem mass spectrometry identified a range of redox active proteins affecting storage protein biochemistry. Thioredoxin, involved in the processing of proteins at germination, has reported impacts on grain digestibility and was differentially expressed across genotypes. Thus, redox states of endosperm proteins, of which kafirins are a subset, could affect quality traits in addition to the expression of proteins.

Ongoing applicative studies of plant thioredoxins

Studies triggered by the discovery of the function of thioredoxin (Trx) in photosynthesis have revealed its role throughout biology. Parallel biochemical and proteomic analyses have led to the identification of its numerous putative targets. Recently, to verify the biological significance of these targets, in vivo studies using transformants in which Trx is overexpressed or suppressed are in progress, and the transformants themselves that are being used in such studies show their potential applicative values. Moreover, Trx's mitigation of allergenicity for some proteins offers promising prospects in the food industry. Practical studies based on redox regulation, once only on the horizon, are now achieving new dimensions. This short review focuses on the industrial applications of Trx studies, the current situation, and future perspectives. The putative targets obtained by the proteomics approach in comparison with in vivo observations of the transformants are also examined. Applicative studies of glutathione, a counterpart of Trx, are also discussed briefly.

Transgenic barley: a prospective tool for biotechnology and agriculture

Barley (Hordeum vulgare L.) is one of the founder crops of agriculture, and today it is the fourth most important cereal grain worldwide. Barley is used as malt in brewing and distilling industry, as an additive for animal feed, and as a component of various food and bread for human consumption. Progress in stable genetic transformation of barley ensures a potential for improvement of its agronomic performance or use of barley in various biotechnological and industrial applications. Recently, barley grain has been successfully used in molecular farming as a promising bioreactor adapted for production of human therapeutic proteins or animal vaccines. In addition to development of reliable transformation technologies, an extensive amount of various barley genetic resources and tools such as sequence data, microarrays, genetic maps, and databases has been generated. Current status on barley transformation technologies including gene transfer techniques, targets, and progeny stabilization, recent trials for improvement of agricultural traits and performance of barley, especially in relation to increased biotic and abiotic stress tolerance, and potential use of barley grain as a protein production platform have been reviewed in this study. Overall, barley represents a promising tool for both agricultural and biotechnological transgenic approaches, and is considered an ancient but rediscovered crop as a model industrial platform for molecular farming.

Secretory type of recombinant thioredoxin h induces ER stress in endosperm cells of transgenic rice

Thioredoxin h (TRX h) functions as a reducing protein and is present in all organisms. As a new approach for inducing the endoplasmic reticulum (ER) stress, TRX h (OsTRX23) was expressed as a secretory protein using the endosperm-specific glutelin GluB-1 promoter and a signal peptide. In transgenic rice seeds, the majority of the recombinant TRX h accumulated in the ER but some was also localized to the protein body IIs (PB-IIs). The rice grain quality was dependent on the TRX h accumulation level. Increased TRX h expression resulted in aberrant phenotypes, such as chalky and shriveled features, lower seed weight and lower seed protein content. Furthermore, the accumulation of some seed storage proteins (SSPs) was significantly suppressed and the morphology of the protein bodies (PB-Is and PB-IIs) changed according to the level of TRX h. SSPs, such as 13kDa prolamin and GluA, were specifically modified via the reducing action of TRX h. These changes led to the activation of the ER stress response, which was accompanied by the expression of several chaperone proteins. Specifically, the ER stress markers BiP4 and BiP5 were significantly up-regulated by an increase in the level of TRX h. These results suggest that changes in the conformation of certain SSPs via the action of recombinant TRX h lead to an induced ER stress response in transgenic rice seeds.

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.

The Or gene enhances carotenoid accumulation and stability during post-harvest storage of potato tubers

Provitamin A carotenoids in staple crops are not very stable during storage and their loss compromises nutritional quality. To elucidate the fundamental mechanisms underlying carotenoid accumulation and stability, we investigated transgenic potato tubers that expressed the cauliflower Orange (Or) gene. We found that the Or transgene not only promoted retention of β-carotene level, but also continuously stimulated its accumulation during 5 months of cold storage. In contrast, no increased levels of carotenoids were observed in the tubers of vector-only controls or a yellow-flesh variety during the same period of storage. The increased carotenoid accumulation was found to be associated with the formation of lipoprotein-carotenoid sequestering structures, as well as with the enhanced abundance of phytoene synthase, a key enzyme in the carotenoid biosynthetic pathway. Furthermore, the provitamin A carotenoids stored were shown to be stable during simulated digestion and accessible for uptake by human intestinal absorptive cells. Proteomic analysis identified three major functional groups of proteins (i.e. heat shock proteins, glutathione-S-transferases, and carbohydrate metabolic proteins) that are potentially important in the Or-regulated carotenoid accumulation. Our results show that regulation of carotenoid sequestration capacity is an important mechanism by which carotenoid stability is regulated. Our findings suggest that induction of a proper sink structure formation in staple crops may provide the crops with a unique ability to promote and/or stabilize provitamin A accumulation during plant growth and post-harvest storage.

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.

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.

The level of expression of thioredoxin is linked to fundamental properties and applications of wheat seeds

Work with cereals (barley and wheat) and a legume (Medicago truncatula) has established thioredoxin h (Trx h) as a central regulatory protein of seeds. Trx h acts by reducing disulfide (S-S) groups of diverse seed proteins (storage proteins, enzymes, and enzyme inhibitors), thereby facilitating germination. Early in vitro protein studies were complemented with experiments in which barley seeds with Trx h overexpressed in the endosperm showed accelerated germination and early or enhanced expression of associated enzymes (alpha-amylase and pullulanase). The current study extends the transgenic work to wheat. Two approaches were followed to alter the expression of Trx h genes in the endosperm: (1) a hordein promoter and its protein body targeting sequence led to overexpression of Trx h5, and (2) an antisense construct of Trx h9 resulted in cytosolic underexpression of that gene (Arabidopsis designation). Underexpression of Trx h9 led to effects opposite to those observed for overexpression Trx h5 in barley-retardation of germination and delayed or reduced expression of associated enzymes. Similar enzyme changes were observed in developing seeds. The wheat lines with underexpressed Trx showed delayed preharvest sprouting when grown in the greenhouse or field without a decrease in final yield. Wheat with overexpressed Trx h5 showed changes commensurate with earlier in vitro work: increased solubility of disulfide proteins and lower allergenicity of the gliadin fraction. The results are further evidence that the level of Trx h in cereal endosperm determines fundamental properties as well as potential applications of the seed.

From proteomics to structural studies of cytosolic/mitochondrial-type thioredoxin systems in barley seeds

Thioredoxins (Trx) are ubiquitous proteins that participate in thiol disulfide reactions via two active site cysteine residues, allowing Trx to reduce disulfide bonds in target proteins. Recent progress in proteome analysis has resulted in identification of a wide range of potential target proteins for Trx, indicating that Trx plays a key role in several aspects of cell metabolism. In contrast to other organisms, plants contain multiple forms of Trx that are classified based on their primary structures and sub-cellular localization. The reduction of cytosolic and mitochondrial types of Trx is dependent on NADPH and catalyzed by NADPH-dependent thioredoxin reductase (NTR). In barley, two isoforms each of Trx and NTR have been identified and investigated using proteomics, gene expression, and structural studies. This review outlines the diverse roles suggested for cytosolic/mitochondrial-type Trx systems in cereal seeds and summarizes the current knowledge of the barley system including recent data on function, regulation, interactions, and structure. Directions for future research are discussed.

The NADPH-dependent thioredoxin reductase/thioredoxin system in germinating barley seeds: gene expression, protein profiles, and interactions between isoforms of thioredoxin h and thioredoxin reductase

The NADPH-dependent thioredoxin reductase (NTR)/thioredoxin (Trx) system catalyzes disulfide bond reduction in the cytoplasm and mitochondrion. Trx h is suggested to play an important role in seed development, germination, and seedling growth. Plants have multiple isoforms of Trx h and NTR; however, little is known about the roles of the individual isoforms. Trx h isoforms from barley (Hordeum vulgare) seeds (HvTrxh1 and HvTrxh2) were characterized previously. In this study, two NTR isoforms (HvNTR1 and HvNTR2) were identified, enabling comparison of gene expression, protein appearance, and interaction between individual NTR and Trx h isoforms in barley embryo and aleurone layers. Although mRNA encoding both Trx h isoforms is present in embryo and aleurone layers, the corresponding proteins differed in spatiotemporal appearance. HvNTR2, but not HvNTR1, gene expression seems to be regulated by gibberellic acid. Recombinant HvNTR1 and HvNTR2 exhibited virtually the same affinity toward HvTrxh1 and HvTrxh2, whereas HvNTR2 has slightly higher catalytic activity than HvNTR1 with both Trx h isoforms, and HvNTR1 has slightly higher catalytic activity toward HvTrxh1 than HvTrxh2. Notably, both NTRs reduced Trx h at the acidic conditions residing in the starchy endosperm during germination. Interspecies reactions between the barley proteins and Escherichia coli Trx or Arabidopsis thaliana NTR, respectively, occurred with 20- to 90-fold weaker affinity. This first investigation of regulation and interactions between members of the NTR/Trx system in barley seed tissues suggests that different isoforms are differentially regulated but may have overlapping roles, with HvNTR2 and HvTrxh1 being the predominant isoforms in the aleurone layer.

A novel cis-acting element, ESP, contributes to high-level endosperm-specific expression in an oat globulin promoter

To examine the genetic controls of endosperm (ES) specificity, several cereal seed storage protein (SSP) promoters were isolated and studied using a transient expression analysis system. An oat globulin promoter (AsGlo1) capable of driving strong ES-specific expression in barley and wheat was identified. Progressive 5' deletions and cis element mutations demonstrated that the mechanism of specificity in the AsGlo1 promoter was distinct from that observed in glutelin and prolamin promoters. A novel interrupted palindromic sequence, ACATGTCATCATGT, was required for ES specificity and substantially contributed to expression strength of the AsGlo1 promoter. This sequence was termed the endosperm specificity palindrome (ESP) element. The GCN4 element, which has previously been shown to be required for ES specificity in cereal SSP promoters, had a quantitative role but was not required for tissue specificity. The 960-bp AsGlo1 promoter and a 251-bp deletion containing the ESP element also drove ES-specific expression in stably transformed barley. Reporter gene protein accumulated at very high levels (10% of total soluble protein) in ES tissues of plants transformed with an AsGlo1:GFP construct. Expression strength and tissue specificity were maintained over five transgenic generations. These attributes make the AsGlo1 promoter an ideal promoter for biotechnology applications. In conjunction with previous findings, our data demonstrate that there is more than one genetically distinct mechanism by which ES specificity can be achieved in cereal SSP promoters, and also suggest that there is redundancy between transcriptional and post-transcriptional tissue specificity mechanisms in cereal globulin genes.

Thioredoxins, glutaredoxins, and glutathionylation: new crosstalks to explore

Oxidants are widely considered as toxic molecules that cells have to scavenge and detoxify efficiently and continuously. However, emerging evidence suggests that these oxidants can play an important role in redox signaling, mainly through a set of reversible post-translational modifications of thiol residues on proteins. The most studied redox system in photosynthetic organisms is the thioredoxin (TRX) system, involved in the regulation of a growing number of target proteins via thiol/disulfide exchanges. In addition, recent studies suggest that glutaredoxins (GRX) could also play an important role in redox signaling especially by regulating protein glutathionylation, a post-translational modification whose importance begins to be recognized in mammals while much less is known in photosynthetic organisms. This review focuses on oxidants and redox signaling with particular emphasis on recent developments in the study of functions, regulation mechanisms and targets of TRX, GRX and glutathionylation. This review will also present the complex emerging interplay between these three components of redox-signaling networks.

A complete ferredoxin/thioredoxin system regulates fundamental processes in amyloplasts

A growing number of processes throughout biology are regulated by redox via thiol-disulfide exchange. This mechanism is particularly widespread in plants, where almost 200 proteins have been linked to thioredoxin (Trx), a widely distributed small regulatory disulfide protein. The current study extends regulation by Trx to amyloplasts, organelles prevalent in heterotrophic plant tissues that, among other biosynthetic activities, catalyze the synthesis and storage of copious amounts of starch. Using proteomics and immunological methods, we identified the components of the ferredoxin/Trx system (ferredoxin, ferredoxin-Trx reductase, and Trx), originally described for chloroplasts, in amyloplasts isolated from wheat starchy endosperm. Ferredoxin is reduced not by light, as in chloroplasts, but by metabolically generated NADPH via ferredoxin-NADP reductase. However, once reduced, ferredoxin appears to act as established for chloroplasts, i.e., via ferredoxin-Trx reductase and a Trx (m-type). A proteomics approach in combination with affinity chromatography and a fluorescent thiol probe led to the identification of 42 potential Trx target proteins, 13 not previously recognized, including a major membrane transporter (Brittle-1 or ADP-glucose transporter). The proteins function in a range of processes in addition to starch metabolism: biosynthesis of lipids, amino acids, and nucleotides; protein folding; and several miscellaneous reactions. The results suggest a mechanism whereby light is initially recognized as a thiol signal in chloroplasts, then as a sugar during transit to the sink, where it is converted again to a thiol signal. In this way, amyloplast reactions in the grain can be coordinated with photosynthesis taking place in leaves.

Disulfide proteome yields a detailed understanding of redox regulations: A model study of thioredoxin-linked reactions in seed germination

Accumulating evidence suggests that redox regulations play important roles in a broad spectrum of biological processes. Recently, Yano et al. developed a disulfide proteome technique that comprehensively visualizes redox change in proteins. In this paper, using the disulfide proteome, we examined rice bran and identified fragments of embryo-specific protein and dienelactone hydrolase as putative targets of thioredoxin. Also, monitoring of the endogenous and recombinant effects of thioredoxin on rice bran proteins and supporting in vivo observations propose a mechanism of redox regulation in seed germination, in which thioredoxin activates cysteine protease with a concurrent unfolding of its substrate, the embryo-specific protein. Our findings suggest that thioredoxin controls the lifetime of specific proteins effectively by regulating the redox reactions coordinately. The model study demonstrates that the disulfide proteome technique is useful not only for identifying targets of thioredoxin, but also for clarify the detailed mechanism of redox regulation.

Analysis of the maize polyubiquitin-1 promoter heat shock elements and generation of promoter variants with modified expression characteristics

The maize polyubiquitin-1 (Ubi-1) promoter is one of a few select promoters used to express foreign genes in monocots, such that recombinant proteins can be produced at commercially viable levels. Modifying the activity, specificity and responsiveness of such promoters provides a means to achieve desired levels and patterns of expression of genes encoding target products. Ubi-1 is constitutively expressed but is further induced by heat shock. The promoter contains two overlapping sequences with similarity to defined heat shock elements and we show that these sequences are also present upstream of the Ubi-1 homologue isolated from teosinte. Both the maize and teosinte promoters can mediate a heat shock response in transgenic maize. We have dissected the overlapping maize Ubi-1 promoter heat shock elements and demonstrate that the 3' element is required to mediate a heat shock response. The Ubi-1 promoter is particularly active in tissues consisting of rapidly dividing cells, and within the seed it is strongly biased towards driving expression in the embryo. However, replacement of the heat shock elements with a trimer of a basic domain/leucine zipper factor binding site of a pea lectin promoter shifts the balance in seed expression towards the endosperm. The Ubi-1 variants described here differ in their overall activity in the seed, but they all show potential for driving high levels of heterologous gene expression in maize.

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.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

Cy5 maleimide labelling for sensitive detection of free thiols in native protein extracts: identification of seed proteins targeted by barley thioredoxin h isoforms

Barley thioredoxin h isoforms HvTrxh1 and HvTrxh2 differ in temporal and spatial distribution and in kinetic properties. Target proteins of HvTrxh1 and HvTrxh2 were identified in mature seeds and in seeds after 72 h of germination. Improvement of the established method for identification of thioredoxin-targeted proteins based on two-dimensional electrophoresis and fluorescence labelling of thiol groups was achieved by application of a highly sensitive Cy5 maleimide dye and large-format two-dimensional gels, resulting in a 10-fold increase in the observed number of labelled protein spots. The technique also provided information about accessible thiol groups in the proteins identified in the barley seed proteome. In total, 16 different putative target proteins were identified from 26 spots using tryptic in-gel digestion, matrix-assisted laser-desorption ionization-time-of-flight MS and database search. HvTrxh1 and HvTrxh2 were shown to have similar target specificity. Barley alpha-amylase/subtilisin inhibitor, previously demonstrated to be reduced by both HvTrxh1 and HvTrxh2, was among the identified target proteins, confirming the suitability of the method. Several alpha-amylase/trypsin inhibitors, some of which are already known as target proteins of thioredoxin h, and cyclophilin known as a target protein of m-type thioredoxin were also identified. Lipid transfer protein, embryospecific protein, three chitinase isoenzymes, a single-domain glyoxalase-like protein and superoxide dismutase were novel identifications of putative target proteins, suggesting new physiological roles of thioredoxin h in barley seeds.

Protein phosphorylation in amyloplasts regulates starch branching enzyme activity and protein-protein interactions

Protein phosphorylation in amyloplasts and chloroplasts of Triticum aestivum (wheat) was investigated after the incubation of intact plastids with gamma-(32)P-ATP. Among the soluble phosphoproteins detected in plastids, three forms of starch branching enzyme (SBE) were phosphorylated in amyloplasts (SBEI, SBEIIa, and SBEIIb), and both forms of SBE in chloroplasts (SBEI and SBEIIa) were shown to be phosphorylated after sequencing of the immunoprecipitated (32)P-labeled phosphoproteins using quadrupole-orthogonal acceleration time of flight mass spectrometry. Phosphoamino acid analysis of the phosphorylated SBE forms indicated that the proteins are all phosphorylated on Ser residues. Analysis of starch granule-associated phosphoproteins after incubation of intact amyloplasts with gamma-(32)P-ATP indicated that the granule-associated forms of SBEII and two granule-associated forms of starch synthase (SS) are phosphorylated, including SSIIa. Measurement of SBE activity in amyloplasts and chloroplasts showed that phosphorylation activated SBEIIa (and SBEIIb in amyloplasts), whereas dephosphorylation using alkaline phosphatase reduced the catalytic activity of both enzymes. Phosphorylation and dephosphorylation had no effect on the measurable activity of SBEI in amyloplasts and chloroplasts, and the activities of both granule-bound forms of SBEII in amyloplasts were unaffected by dephosphorylation. Immunoprecipitation experiments using peptide-specific anti-SBE antibodies showed that SBEIIb and starch phosphorylase each coimmunoprecipitated with SBEI in a phosphorylation-dependent manner, suggesting that these enzymes may form protein complexes within the amyloplast in vivo. Conversely, dephosphorylation of immunoprecipitated protein complex led to its disassembly. This article reports direct evidence that enzymes of starch metabolism (amylopectin synthesis) are regulated by protein phosphorylation and indicate a wider role for protein phosphorylation and protein-protein interactions in the control of starch anabolism and catabolism.

Limit dextrinase from germinating barley has endotransglycosylase activity, which explains its activation by maltodextrins

Limit dextrinase (EC 3.2.1.41) from germinating barley (Hordeum vulgare L) can be activated by millimolar concentrations of linear maltodextrins with a degree of polymerisation > or = 2. The activation was assay-dependent; it was detected using assays based on the solubilisation of cross-linked dyed pullulan but not in assays that directly measured cleavage events such as the formation of new reducing termini. This strongly suggested that maltodextrins did not increase the catalytic rate of limit dextrinase i.e. this is not a true activation. On the other hand, considerable activation was noted in assays that measured pullulan degradation by reduction in viscosity. Taken together, this suggested that maltodextrins altered the mode of action of limit dextrinase, causing more rapid decreases in viscosity or greater solubilisation of dye-linked pullulan fragments per cleavage event. The proposed mechanism of activation by alteration in action pattern was reminiscent of initial work in the discovery of xyloglucan endotransglycosylase. Therefore, the ability of limit dextrinase to catalyse transglycosylation reactions into pullulan was tested and confirmed by an assay based on the incorporation of a fluorescently labelled maltotriose derivative into higher-molecular-weight products. The transglycosylation reaction was dependent on limit dextrinase activity and was enhanced in more highly purified preparations of limit dextrinase. Transglycosylation was inhibited by unlabelled maltotriose. How transglycosylation accounts for the apparent activation of limit dextrinase by maltodextrins and the physiological relevance of this novel reaction are discussed.

Stable transformation of rice (Oryza sativa L.) via microprojectile bombardment of highly regenerative, green tissues derived from mature seed

A highly efficient and reproducible transformation system for rice ( Oryza sativa L. cv. Taipei 309) was developed using microprojectile bombardment of highly regenerative, green tissues. These tissues were induced from mature seeds on NB-based medium containing 2,4-dichlorophenoxyacetic acid (2,4-D), 6-benzylaminopurine (BAP) and high concentrations of cupric sulfate under dim light conditions; germinating shoots and roots were completely removed. Highly regenerative, green tissues were proliferated on the same medium and used as transformation targets. From 431 explants bombarded with transgenes [i.e. a hygromycin phosphotransferase ( hpt) gene plus one of a wheat thioredoxin h ( wtrxh), a barley NADP-thioredoxin reductase ( bntr), a maize Mutator transposable element ( mudrB) or beta-glucuronidase ( uidA; gus) gene], 28 independent transgenic events were obtained after an 8- to 12-week selection period, giving a 6.5% transformation frequency. Of the 28 independent events, 17 (61%) were regenerable. Co-transformation of the second introduced transgene was detected in 81% of the transgenic lines tested. Stable integration and expression of the foreign genes in T(0) plants and T(1) progeny were confirmed by DNA hybridization, western blot analyses and germination tests.

The thioredoxin superfamily in Chlamydomonas reinhardtii

The thioredoxin (TRX) superfamily includes redox proteins such as thioredoxins, glutaredoxins (GRXs) and protein disulfide isomerases (PDI). These proteins share a common structural motif named the thioredoxin fold. They are involved in disulfide oxido-reduction and/or isomerization. The sequencing of the Arabidopsisgenome revealed an unsuspected multiplicity of TRX and GRX genes compared to other organisms. The availability of full Chlamydomonasgenome sequence offers the opportunity to determine whether this multiplicity is specific to higher plant species or common to all photosynthetic eukaryotes. We have previously shown that the multiplicity is more limited in Chlamydomonas for TRX and GRX families. We extend here our analysis to the PDI family. This paper presents a comparative analysis of the TRX, GRX and PDI families present in Arabidopsis,Chlamydomonas and Synechocystis. The putative subcellular localization of each protein and its relative expression level, based on EST data, have been investigated. This analysis provides a large overview of the redox regulatory systems present in Chlamydomonas. The data are discussed in view of recent results suggesting a complex cross-talk between the TRX, GRX and PDI redox regulatory networks.