Redox regulation: a broadening horizon

Mitochondria Are Important Determinants of the Aging of Seeds

Seeds enable plant survival in harsh environmental conditions, and via seeds, genetic information is transferred from parents to the new generation; this stage provides an opportunity for sessile plants to settle in new territories. However, seed viability decreases over long-term storage due to seed aging. For the effective conservation of gene resources, e.g., in gene banks, it is necessary to understand the causes of decreases in seed viability, not only where the aging process is initiated in seeds but also the sequence of events of this process. Mitochondria are the main source of reactive oxygen species (ROS) production, so they are more quickly and strongly exposed to oxidative damage than other organelles. The mitochondrial antioxidant system is also less active than the antioxidant systems of other organelles, thus such mitochondrial 'defects' can strongly affect various cell processes, including seed aging, which we discuss in this paper.

Variations in the Calvin-Benson cycle: selection pressures and optimization?

This article comments on: Arrivault S, Moraes TA, Obata T, Medeiros DB, Fernie AR, Boulouis A, Ludwig M, Lunn JE, Borghi GL, Schlereth A, Guenther M, Stitt M. 2019. Metabolite profiles reveal inter-specific variation in operation of the Calvin–Benson cycle in both C₄ and C₃ plants. Journal of Experimental Botany 70, 1843–1858.

Piecing the Puzzle Together: The Central Role of Reactive Oxygen Species and Redox Hubs in Chloroplast Retrograde Signaling

SIGNIFICANCE

Reactive oxygen species (ROS) and redox regulation are established components of chloroplast-nucleus retrograde signaling. Recent Advances: In recent years, a complex array of putative retrograde signaling molecules and novel signaling pathways have emerged, including various metabolites, chloroplast translation, mobile transcription factors, calcium, and links to the unfolded protein response. This critical mass of information now permits us to fit individual pieces into a larger picture and outline a few important stimuli and pathways.

CRITICAL ISSUES

In this review, we summarize how ROS and redox hubs directly (e.g., via hydrogen peroxide [H₂O₂]) and indirectly (e.g., by triggering the production of signaling metabolites) regulate chloroplast retrograde signaling. Indeed, evidence is accumulating that most of the presumptive signaling metabolites so far identified are produced directly by ROS (such as β-cyclocitral) or indirectly by redox- or ROS-mediated regulation of key enzymes in metabolic pathways, ultimately leading to the accumulation of certain precursors (e.g., methylerythritol cyclodiphosphate and 3'-phosphoadenosine 5'-phosphate) with signal function. Of the ROS generated in the chloroplast, only H₂O₂ is likely to leave the organelle, and recent results suggest that efficient and specific transfer of information via H₂O₂ occurs through physical association of chloroplasts with the nucleus.

FUTURE DIRECTIONS

The impact of ROS and redox regulation on chloroplast-nucleus communication is even greater than previously thought, and it can be expected that further instances of control of retrograde signaling by ROS/redox regulation will be revealed in future, perhaps including the basis for the enigmatic GUN response and translation-dependent signals.

Comparative Proteomic Analysis Unveils Critical Pathways Underlying the Role of Nitrogen Fertilizer Treatment in American Elderberry

American elderberry (Sambucus nigra subsp. canadensis) is a rapidly growing specialty crop in Missouri and eastern North America. Nitrogen (N) is a major nutrient involved in plant growth and development. However, proteome changes for different genotypes of elder in response to varying levels of N-treatment remain undefined. To reveal plant responses to N, comparative proteomic analyses were performed to determine consistent changes in three genotypes of elderberry leaves (Adams II, Bob Gordon and Wyldewood) grown under different N-fertilizer treatments. 165 proteins separated by two dimensional gel electrophoresis showed significant differences in abundance (p < 0.05 and greater than 2-fold). Principal component analysis of the abundance profiles of these proteins revealed Bob Gordon as a distinct genotype. The 165 proteins were identified by mass spectrometry and showed similar functional distributions in these genotypes underlying the N-treatment. Among the proteins identified, 23 are mainly involved in photosynthesis, protein metabolism and redox homeostasis. Their abundance profiles were not altered upon exposure to N or genotype. These results provide novel insights into plant responses to fertilizer treatment at the proteome level and could lead to a better understanding of molecular mechanisms of elderberry growth.

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

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

Maintaining homeostasis by controlled alternatives for energy distribution in plant cells under changing conditions of supply and demand

Plants depend on light energy for the generation of ATP and reductant as well as on supply of nutrients (inorganic C, N, and S compounds) to successfully produce biomass. Any excess of reducing power or lack of electron acceptors can lead to the formation of reactive oxygen species (ROS). Multiple systems are operating to avoid imbalances and subsequent oxidative stress by efficiently scavenging any formed ROS. Plants can sense an upcoming imbalance and correspondingly adapt to changed conditions not only by an increase of ROS scavengers, but also by using excess incoming light energy productively for assimilatory processes in actively metabolizing cells of growing leaves. CO₂ assimilation in chloroplasts is controlled by various redox-regulated enzymes; their activation state is strictly linked to metabolism due to the effects of small molecules on their actual activation state. Shuttle systems for indirect transfer of reducing equivalents and ATP specifically distribute the energy fluxes between compartments for optimal biomass production. Integration of metabolic and redox signals involves the cytosolic enzyme glyceraldehyde-3-P dehydrogenase (GapC) and some of its many moonlighting functions. Its redox- and metabolite-dependent interactions with the mitochondrial outer membrane, the cytoskeleton, and its occurrence in the nucleus are examples of these additional functions. Induction of the genes required to achieve an optimal response suitable for the respective conditions allows for growth when plants are exposed to different light intensities and nutrient conditions with varying rates of energy input and different assimilatory pathways for its consumption are the required in the long term. A plant-specific respiratory pathway, the alternative oxidase (AOX), functions as a site to convert excess electrons into heat. For acclimation, any imbalance is sensed and elicits signal transduction to induce the required genes. Examples for regulated steps in this sequence of events are given in this review. Continuous adjustment under natural conditions allows for adaptive responses. In contrast, sudden light stress, as employed when analyzing stress responses in lab experiments, frequently results in cell destruction. Knowledge of all the flexible regulatory mechanisms, their responsiveness, and their interdependencies is needed when plant growth is to be engineered to optimize biomass and production of any desired molecules.

Three phases of energy-dependent induction of [Formula: see text] and Chl a fluorescence in Tradescantia fluminensis leaves

In plants, the short-term regulation (STR, seconds to minute time scale) of photosynthetic apparatus is associated with the energy-dependent control in the chloroplast electron transport, the distribution of light energy between photosystems (PS) II and I, activation/deactivation of the Calvin-Benson cycle (CBC) enzymes, and relocation of chloroplasts within the plant cell. In this work, using a dual-PAM technique for measuring the time-courses of P₇₀₀ photooxidation and Chl a fluorescence, we have investigated the STR events in Tradescantia fluminensis leaves. The comparison of Chl a fluorescence and [Formula: see text] induction allowed us to investigate the contribution of the trans-thylakoid pH difference (ΔpH) to the STR events. Two parameters were used as the indicators of ΔpH generation: pH-dependent component of non-photochemical quenching of Chl a fluorescence, and pH_in-dependent rate of electron transfer from plastoquinol (PQH₂) to [Formula: see text] (via the Cyt b₆f complex and plastocyanin). In dark-adapted leaves, kinetics of [Formula: see text] induction revealed three phases. Initial phase is characterized by rapid electron flow to [Formula: see text] (τ_1/2 ~ 5-10 ms), which is likely related to cyclic electron flow around PSI, while the outflow of electrons from PSI is restricted by slow consumption of NADPH in the CBC. The light-induced generation of ΔpH and activation of the CBC promote photooxidation of P₇₀₀ and concomitant retardation of [Formula: see text] reduction (τ_1/2 ~ 20 ms). Prolonged illumination induces additional slowing down of electron transfer to [Formula: see text] (τ_1/2 ≥ 30-35 ms). The latter effect is not accompanied by changes in the Chl a fluorescence parameters which are sensitive to ΔpH generation. We suggest the tentative explanation of the latter results by the reversal of Q-cycle, which causes the deceleration of PQH₂ oxidation due to the back pressure of stromal reductants.

Feeding the world: improving photosynthetic efficiency for sustainable crop production

A number of recent studies have provided strong support demonstrating that improving the photosynthetic processes through genetic engineering can provide an avenue to improve yield potential. The major focus of this review is on improvement of the Calvin-Benson cycle and electron transport. Consideration is also given to how altering regulatory process may provide an additional route to increase photosynthetic efficiency. Here we summarize some of the recent successes that have been observed through genetic manipulation of photosynthesis, showing that, in both the glasshouse and the field, yield can be increased by >40%. These results provide a clear demonstration of the potential for increasing yield through improvements in photosynthesis. In the final section, we consider the need to stack improvement in photosynthetic traits with traits that target the yield gap in order to provide robust germplasm for different crops across the globe.

Malate valves: old shuttles with new perspectives

Malate valves act as powerful systems for balancing the ATP/NAD(P)H ratio required in various subcellular compartments in plant cells. As components of malate valves, isoforms of malate dehydrogenases (MDHs) and dicarboxylate translocators catalyse the reversible interconversion of malate and oxaloacetate and their transport. Depending on the co-enzyme specificity of the MDH isoforms, either NADH or NADPH can be transported indirectly. Arabidopsis thaliana possesses nine genes encoding MDH isoenzymes. Activities of NAD-dependent MDHs have been detected in mitochondria, peroxisomes, cytosol and plastids. In addition, chloroplasts possess a NADP-dependent MDH isoform. The NADP-MDH as part of the 'light malate valve' plays an important role as a poising mechanism to adjust the ATP/NADPH ratio in the stroma. Its activity is strictly regulated by post-translational redox-modification mediated via the ferredoxin-thioredoxin system and fine control via the NADP⁺ /NADP(H) ratio, thereby maintaining redox homeostasis under changing conditions. In contrast, the plastid NAD-MDH ('dark malate valve') is constitutively active and its lack leads to failure in early embryo development. While redox regulation of the main cytosolic MDH isoform has been shown, knowledge about regulation of the other two cytosolic MDHs as well as NAD-MDH isoforms from peroxisomes and mitochondria is still lacking. Knockout mutants lacking the isoforms from chloroplasts, mitochondria and peroxisomes have been characterised, but not much is known about cytosolic NAD-MDH isoforms and their role in planta. This review updates the current knowledge on MDH isoforms and the shuttle systems for intercompartmental dicarboxylate exchange, focusing on the various metabolic functions of these valves.

ROS-Driven Oxidative Modification: Its Impact on Chloroplasts-Nucleus Communication

As a light-harvesting organelle, the chloroplast inevitably produces a substantial amount of reactive oxygen species (ROS) primarily through the photosystems. These ROS, such as superoxide anion, hydrogen peroxide, hydroxyl radical, and singlet oxygen, are potent oxidizing agents, thereby damaging the photosynthetic apparatus. On the other hand, it became increasingly clear that ROS act as beneficial tools under photo-oxidative stress conditions by stimulating chloroplast-nucleus communication, a process called retrograde signaling (RS). These ROS-mediated RS cascades appear to participate in a broad spectrum of plant physiology, such as acclimation, resistance, programmed cell death (PCD), and growth. Recent reports imply that ROS-driven oxidation of RS-associated components is essential in sensing and responding to an increase in ROS contents. ROS appear to activate RS pathways via reversible or irreversible oxidation of sensor molecules. This review provides an overview of the emerging perspective on the topic of "oxidative modification-associated retrograde signaling."

Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense

Photosynthesis, pathogen infection, and plant defense are three important biological processes that have been investigated separately for decades. Photosynthesis generates ATP, NADPH, and carbohydrates. These resources are utilized for the synthesis of many important compounds, such as primary metabolites, defense-related hormones abscisic acid, ethylene, jasmonic acid, and salicylic acid, and antimicrobial compounds. In plants and algae, photosynthesis and key steps in the synthesis of defense-related hormones occur in chloroplasts. In addition, chloroplasts are major generators of reactive oxygen species and nitric oxide, and a site for calcium signaling. These signaling molecules are essential to plant defense as well. All plants grown naturally are attacked by pathogens. Bacterial pathogens enter host tissues through natural openings or wounds. Upon invasion, bacterial pathogens utilize a combination of different virulence factors to suppress host defense and promote pathogenicity. On the other hand, plants have developed elaborate defense mechanisms to protect themselves from pathogen infections. This review summarizes recent discoveries on defensive roles of signaling molecules made by plants (primarily in their chloroplasts), counteracting roles of chloroplast-targeted effectors and phytotoxins elicited by bacterial pathogens, and how all these molecules crosstalk and regulate photosynthesis, pathogen infection, and plant defense, using chloroplasts as a major battlefield.

Ferredoxin-linked flavoenzyme defines a family of pyridine nucleotide-independent thioredoxin reductases

Ferredoxin-dependent thioredoxin reductase was identified 35 y ago in the fermentative bacterium Clostridium pasteurianum [Hammel KE, Cornwell KL, Buchanan BB (1983) Proc Natl Acad Sci USA 80:3681-3685]. The enzyme, a flavoprotein, was strictly dependent on ferredoxin as reductant and was inactive with either NADPH or NADH. This early work has not been further pursued. We have recently reinvestigated the problem and confirmed that the enzyme, here designated ferredoxin-dependent flavin thioredoxin reductase (FFTR), is a flavoprotein. The enzyme differs from ferredoxin-thioredoxin reductase (FTR), which has a signature [4Fe-4S] cluster, but shows structural similarities to NADP-dependent thioredoxin reductase (NTR). Comparative amino acid sequence analysis showed that FFTR is present in a number of clostridial species, some of which lack both FTR and an archetypal NTR. We have isolated, crystallized, and determined the structural properties of FFTR from a member of this group, Clostridium acetobutylicum, both alone and in complex with Trx. The structures showed an elongated FFTR homodimer, each monomer comprising two Rossmann domains and a noncovalently bound FAD cofactor that exposes the isoalloxazine ring to the solvent. The FFTR structures revealed an alternative domain organization compared with NTR that enables the enzyme to accommodate Fdx rather than NADPH. The results suggest that FFTR exists in a range of conformations with varying degrees of domain separation in solution and that the stacking between the two redox-active groups for the transfer of reducing equivalents results in a profound structural reorganization. A mechanism in accord with the findings is proposed.

The Absence of Thioredoxin m1 and Thioredoxin C in Anabaena sp. PCC 7120 Leads to Oxidative Stress

Thioredoxin (Trx) family proteins perform redox regulation in cells, and they are involved in several other biological processes (e.g. oxidative stress tolerance). In the filamentous cyanobacterium Anabaena sp. PCC7120 (A. 7120), eight Trx isoforms have been identified via genomic analysis. Among these Trx isoforms, the absence of Trx-m1 and TrxC appears to result in oxidative stress in A. 7120 together with alterations of the thylakoid membrane structure and phycobiliprotein composition. To analyze the physiological changes in these Trx disruptants thoroughly, quantitative proteomics was applied. Certainly, the mutants exhibited similar alterations in the proteome including decreased relative abundance of phycobiliproteins and an increased level of proteins involved in amino acid and carbohydrate metabolism. Nevertheless, the results also indicated that the mutants exhibited changes in the relative abundance of different sets of proteins participating in reactive oxygen species detoxification, such as Fe-SOD in Δtrx-m1 and PrxQ in ΔtrxC, suggesting distinct functions of Trx-m1 and TrxC.

Thioredoxin and glutaredoxin regulate metabolism through different multiplex thiol switches

The aim of the present study was to define the role of Trx and Grx on metabolic thiol redox regulation and identify their protein and metabolite targets. The hepatocarcinoma-derived HepG2 cell line under both normal and oxidative/nitrosative conditions by overexpression of NO synthase (NOS3) was used as experimental model. Grx1 or Trx1 silencing caused conspicuous changes in the redox proteome reflected by significant changes in the reduced/oxidized ratios of specific Cys's including several glycolytic enzymes. Cys⁹¹ of peroxiredoxin-6 (PRDX6) and Cys¹⁵³ of phosphoglycerate mutase-1 (PGAM1), that are known to be involved in progression of tumor growth, are reported here for the first time as specific targets of Grx1. A group of proteins increased their Cys_RED/Cys_OX ratio upon Trx1 and/or Grx1 silencing, including caspase-3 Cys¹⁶³, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) Cys²⁴⁷ and triose-phosphate isomerase (TPI) Cys²⁵⁵ likely by enhancement of NOS3 auto-oxidation. The activities of several glycolytic enzymes were also significantly affected. Glycolysis metabolic flux increased upon Trx1 silencing, whereas silencing of Grx1 had the opposite effect. Diversion of metabolic fluxes toward synthesis of fatty acids and phospholipids was observed in siRNA-Grx1 treated cells, while siRNA-Trx1 treated cells showed elevated levels of various sphingomyelins and ceramides and signs of increased protein degradation. Glutathione synthesis was stimulated by both treatments. These data indicate that Trx and Grx have both, common and specific protein Cys redox targets and that down regulation of either redoxin has markedly different metabolic outcomes. They reflect the delicate sensitivity of redox equilibrium to changes in any of the elements involved and the difficulty of forecasting metabolic responses to redox environmental changes.

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Trx1 and Grx1 Cys redox targets are abundant among Glycolytic enzymes.

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PRDX6-Cys⁹¹ and PGAM-Cys¹⁵³ are specific targets of Grx1.

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Down regulation of thioredoxin and glutaredoxin have different metabolic outcomes.

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Glutathione synthesis and membrane lipid composition are sensitive to Trx1 and Grx1 down regulation.

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Redoxins down regulation also induce target Cys reductive changes under NOS3 overexpression.

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

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

Reactive oxygen species, oxidative signaling and the regulation of photosynthesis

Reduction-oxidation (redox) reactions, in which electrons move from a donor to an acceptor, are the functional heart of photosynthesis. It is not surprising therefore that reactive oxygen species (ROS) are generated in abundance by photosynthesis, providing a plethora of redox signals as well as functioning as essential regulators of energy and metabolic fluxes. Chloroplasts are equipped with an elaborate and multifaceted protective network that allows photosynthesis to function with high productivity even in resource-limited natural environments. This includes numerous antioxidants with overlapping functions that provide enormous flexibility in redox control. ROS are an integral part of the repertoire of chloroplast signals that are transferred to the nucleus to convey essential information concerning redox pressure within the electron transport chain. Current evidence suggests that there is specificity in the gene-expression profiles triggered by the different ROS signals, so that singlet oxygen triggers programs related to over excitation of photosystem (PS) II while superoxide and hydrogen peroxide promote the expression of other suites of genes that may serve to alleviate electron pressure on the reducing side of PSI. Not all chloroplasts are equal in their signaling functions, with some sub-populations appearing to have better contacts/access to the nucleus than others to promote genetic and epigenetic responses. While the concept that light-induced increases in ROS result in damage to PSII and photoinhibition is embedded in the photosynthesis literature, there is little consensus concerning the extent to which such oxidative damage happens in nature. Slowly reversible decreases in photosynthetic capacity are not necessarily the result of light-induced damage to PSII reaction centers.

Small Molecules Govern Thiol Redox Switches

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

Patterns of protein carbonylation during Medicago truncatula seed maturation

Seeds mainly acquire their physiological quality during maturation, whereas oxidative conditions reign within cells triggering protein carbonylation. To better understand the role of this protein modification in legume seeds, we compared by proteomics patterns of carbonylated proteins in maturing seeds of Medicago truncatula naturally desiccated or prematurely dried, a treatment known to impair seed quality acquisition. In both cases, protein carbonylation increased in these seeds, accompanying water removal. We identified several proteins whose extent of carbonylation varied when comparing natural desiccation and premature drying and that could therefore be responsible for the impairment of seed quality acquisition or expression. In particular, we focused on PM34, a protein specific to seeds exhibiting a high sensitivity to carbonylation and of which function in dicotyledons was not known before. PM34 proved to have a cellulase activity presumably associated with cell elongation, a process required for germination and subsequent seedling growth. We discuss the possibility that PM34 (abundance or redox state) could be used to assess crop seed quality.

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

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

Comparative effect of 28-homobrassinolide and 24-epibrassinolide on the performance of different components influencing the photosynthetic machinery in Brassica juncea L

BRs are polyhydroxylated sterol derivatives, classified as phytohormones. Plants of Brassica juncea var. Varuna were grown in pots and an aqueous solution (10^-8 M) of two brassinosteroid isomers 28-homobrassinolide (HBL) and 24-epibrassinolide (EBL) of same concentration (10^-8 M) was applied to their leaves. The treatment up-regulated the photosynthetic machinery directly by enhancing water splitting activity, photochemical quenching, non-photochemical quenching, maximum PSII efficiency, actual PSII efficiency, electron transport rate, stomatal movement, stomatal conductance, internal CO₂ concentration, transpiration rate, net photosynthetic rate and carbohydrate synthesis. Moreover, the level of biochemical enzymes (carbonic anhydrase and nitrate reductase), reactive oxygen species (superoxide and hydrogen peroxide) generation, antioxidant enzyme activity and mineral status (C, N, Mg, P, S, K), which indirectly influence the rate of photosynthesis, also improved in the treated plants. Out of the two BR analogues tested, EBL excelled in its effects over HBL.

Enhancement of Tryptic Digestibility of Milk β-Lactoglobulin Through Treatment with Recombinant Rice Glutathione/Thioredoxin and NADPH Thioredoxin Reductase/Thioredoxin Systems

β-Lactoglobulin (BLG), a member of lipocalin family, is one of the major bovine milk allergens. This protein exists as a dimer of two identical subunits and contains two intramolecular disulfide bonds that are responsible for its resistance to trypsin digestion and allergenicity. This study aimed to evaluate the effect of reduction of disulfide bonds of BLG with different rice thioredoxins (Trxs) on its digestibility and allergenicity. Therefore, the active recombinant forms of three rice Trx isoforms (OsTrx1, OsTrx20, and OsTrx23) and one rice NADPH-dependent Trx reductase isoform (OsNTRB) were expressed in Escherichia coli. Based on SDS-PAGE, HPLC analysis, and competitive ELISA, the reduction of disulfide bonds of BLG with OsNTRB/OsTrx23, OsNTRB/OsTrx1, GSH/OsTrx1, or GSH/OsTrx20 increased its trypsin digestibility and reduced its immunoreactivity. The finding of this study opens new insights for application of plant Trxs in the improvement of food protein digestibility. Especially, the use of OsTrx20 and OsTrx1 are more cost-effective than E. coli and animal Trxs due to their reduction by GSH and no need to NADPH and Trx reductase as mediator enzyme.

ROS-dependent signalling pathways in plants and algae exposed to high light: Comparisons with other eukaryotes

Like all aerobic organisms, plants and algae co-opt reactive oxygen species (ROS) as signalling molecules to drive cellular responses to changes in their environment. In this respect, there is considerable commonality between all eukaryotes imposed by the constraints of ROS chemistry, similar metabolism in many subcellular compartments, the requirement for a high degree of signal specificity and the deployment of thiol peroxidases as transducers of oxidising equivalents to regulatory proteins. Nevertheless, plants and algae carry out specialised signalling arising from oxygenic photosynthesis in chloroplasts and photoautotropism, which often induce an imbalance between absorption of light energy and the capacity to use it productively. A key means of responding to this imbalance is through communication of chloroplasts with the nucleus to adjust cellular metabolism. Two ROS, singlet oxygen (¹O₂) and hydrogen peroxide (H₂O₂), initiate distinct signalling pathways when photosynthesis is perturbed. ¹O₂, because of its potent reactivity means that it initiates but does not transduce signalling. In contrast, the lower reactivity of H₂O₂ means that it can also be a mobile messenger in a spatially-defined signalling pathway. How plants translate a H₂O₂ message to bring about changes in gene expression is unknown and therefore, we draw on information from other eukaryotes to propose a working hypothesis. The role of these ROS generated in other subcellular compartments of plant cells in response to HL is critically considered alongside other eukaryotes. Finally, the responses of animal cells to oxidative stress upon high irradiance exposure is considered for new comparisons between plant and animal cells.

Effects of Cr2O3 nanoparticles on the chlorophyll fluorescence and chloroplast ultrastructure of soybean (Glycine max)

Chromic oxide nanoparticles (Cr2O3 NPs) are widely used in commercial factories and can cause serious environmental problems. However, the mechanism behind Cr2O3 NP-induced phytotoxicity remains unknown. In this study, the effects of Cr2O3 NPs on the growth, chlorophyll fluorescence, SEM-EDS analysis, and chloroplast ultrastructure of soybean (Glycine max) were investigated to evaluate its phytotoxicity. The growth of soybean treated with various Cr2O3 NP suspensions (0.01, 0.05, 0.1, and 0.5 g L-1) was significantly inhibited. Specially, shoot and root biomass decreased by 9.9 and 46.3%, respectively. Besides, the maximum quantum yield of PSII (Fv/Fm) as well as the photochemical quenching (qP) decreased by 8-22 and 30-37%, respectively, indicating that the photosynthetic system was damaged when treated with Cr2O3 NPs. Moreover, the inhibition was confirmed by the reduction of Rubisco and MDH enzyme activity (by 54.5-86.4 and 26.7-96.5%, respectively). Overall, results indicated that the damage was caused by the destruction of chloroplast thylakoid structure, which subsequently reduced the photosynthetic rate. Our research suggests that Cr2O3 NPs can be transported and cause irreversible damage to soybean plants by inhibiting the activity of electron acceptors (NADP+) and destroying ultrastructure of chloroplasts, providing insights into plant toxicity issues. Graphical abstract ᅟ.

The phosphorylated redox proteome of Chlamydomonas reinhardtii: Revealing novel means for regulation of protein structure and function

Post-translational modifications (PTMs) are covalent modifications to protein residues which may alter both conformation and activity, thereby modulating signaling and metabolic processes. While PTMs have been largely investigated independently, examination into how different modification interact, or crosstalk, will reveal a more complete understanding of the reciprocity of signaling cascades across numerous pathways. Combinatorial reversible thiol oxidation and phosphorylation in eukaryotes is largely recognized, but rigorous approaches for experimental discovery are underdeveloped. To begin meaningful interrogation of PTM crosstalk in systems biology research, knowledge of targeted proteins must be advanced. Herein, we demonstrate protein-level enrichment of reversibly oxidized proteoforms in Chlamydomonas reinhardtii with subsequent phosphopeptide analysis to determine the extent of phosphorylation in the redox thiol proteome. Label-free quantification was used to quantify 3353 oxidized Cys-sites on 1457 enriched proteins, where sequential phosphopeptide enrichment measured 1094 sites of phosphorylation on 720 proteins with 23% (172 proteins) also identified as reversibly oxidized. Proteins identified with both reversible oxidation and phosphorylation were involved in signaling transduction, ribosome and translation-related machinery, and metabolic pathways. Several redox-modified Calvin-Benson cycle proteins were found phosphorylated and many kinases/phosphatases involved in phosphorylation-dependent photosynthetic state transition and stress-response pathways had sites of reversible oxidation. Identification of redox proteins serves as a crucial element in understanding stress response in photosynthetic organisms and beyond, whereby knowing the ensemble of modifications co-occurring with oxidation highlights novel mechanisms for cellular control.

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Quantified reversible oxidation on protein cysteine residues.

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Sequential phosphopeptide enrichment to define the phosphorylated redox proteome.

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Found >3000 oxidized cysteines and >1000 phosphosites in Chlamydomonas reinhardtii.

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Co-modified proteins discovered across diverse metabolic and signaling pathways.

Functional analysis by RNAi of an glutaredoxin gene in Helicoverpa armigera

Glutaredoxins play crucial roles in maintaining intracellular redox homeostasis via scavenging of excess reactive oxygen species. In this study, a glutaredoxin domain-containing cysteine-rich gene from Helicoverpa armigera, named HaGdccr, was characterized. Sequence analysis revealed that it contains a glutaredoxin domain and a conserved cysteine and shares high sequence identity with other insect genes. HaGdccr mRNA expression was highest in molting larvae of the 3rd instar and was mainly detected in the central nervous system of larvae and the wings of adults. Quantitative real-time PCR results revealed that the expression of HaGdccr was suppressed at 1 and 6 h and increased at 24 h after the larvae were treated with 4 °C and hydrogen peroxide. When the larvae were exposed to 20 °C, HaGdccr decreased at 1 h and was induced at 12 and 24 h. HaGdccr transcription level was downregulated at 2 and 12 h and upregulated at 24 h after the adults were exposed to 0 °C. However, transcript levels were increased by high temperature in both larvae and adults. After knockdown of HaGdccr by RNA interference, the expression of antioxidant genes, including thioredoxin-like (Trx-like), catalase (CAT), glutathione-S-transferase (GST), thioredoxin reductase (TrxR), and thioredoxin (Trx), was increased, whereas that of thioredoxin peroxidase (Tpx) was decreased. In addition, we found that HaGdccr knockdown enhanced the enzymatic activity of superoxide dismutase and the contents of hydrogen peroxide and ascorbate. Taken together, these results indicate that HaGdccr may play significant roles in protecting organisms against oxidative damage.

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

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

Chlorophyll Fluorescence Imaging

Chlorophyll fluorescence imaging provides a noninvasive rapid screen to assess the physiological status of a number of leaves or plants simultaneously. Although there are no standard protocols for chlorophyll fluorescence imaging, here we provide an example of routines for some of the typical measurements.

The Role of Hydrogen Peroxide in Mediating the Mechanical Wounding-Induced Freezing Tolerance in Wheat

Systemic wound response (SWR), a well-characterized systemic signaling response, plays crucial roles in plant defense responses. Progress in understanding of the SWR in abiotic stress has also been aided by the researchers. However, the function of SWR in freezing stress remains elusive. In this study, we showed that local mild mechanical wounding enhanced freezing tolerance in newly occurred systemic leaves of wheat plants (Triticum aestivum L.). Wounding significantly increased the maximal photochemical efficiency of photosystem II, net photosynthetic rate, and the activities of the antioxidant enzymes under freezing stress. Wounding also alleviated freezing-induced chlorophyll decomposition, electrolyte leakage, water lose, and membrane peroxidation. In addition, wounding-induced freezing stress mitigation was closely associated with the ratio between reduced glutathione (GSH) and oxidized glutathione (GSSG), and the ratio between ascorbate (AsA) and dehydroascorbate (DHA), as well as the contents of total soluble sugars and free amino acids. Importantly, pharmacological study showed that wounding-induced freezing tolerance was substantially arrested by pretreatment of wheat leaves with the scavenger of hydrogen peroxide (H₂O₂) or the inhibitor of NADPH oxidase (RBOH). These results support the hypothesis that local mechanical wounding-induced SWR in newly occurred leaves is largely attributed to RBOH-dependent H₂O₂ production, which may subsequently induce freezing tolerance in wheat plants. This mechanism may have a potential application to reduce the yield losses of wheat under late spring freezing conditions. Highlights: In our previous research, we found that local mechanical wounding could induce freezing tolerance in the upper systemic leaves of wheat plants. Surprisingly, in this paper, we further demonstrated that local mechanical wounding could also increase freezing resistance in newly occurred leaves of wheat plants. RBOH mediated H₂O₂ and ascorbate-glutathione cycle participate in this systemic wound response.

Photosynthetic fuel for heterologous enzymes: the role of electron carrier proteins

Plants, cyanobacteria, and algae generate a surplus of redox power through photosynthesis, which makes them attractive for biotechnological exploitations. While central metabolism consumes most of the energy, pathways introduced through metabolic engineering can also tap into this source of reducing power. Recent work on the metabolic engineering of photosynthetic organisms has shown that the electron carriers such as ferredoxin and flavodoxin can be used to couple heterologous enzymes to photosynthetic reducing power. Because these proteins have a plethora of interaction partners and rely on electrostatically steered complex formation, they form productive electron transfer complexes with non-native enzymes. A handful of examples demonstrate channeling of photosynthetic electrons to drive the activity of heterologous enzymes, and these focus mainly on hydrogenases and cytochrome P450s. However, competition from native pathways and inefficient electron transfer rates present major obstacles, which limit the productivity of heterologous reactions coupled to photosynthesis. We discuss specific approaches to address these bottlenecks and ensure high productivity of such enzymes in a photosynthetic context.

Glutathione, Glutaredoxins, and Iron

SIGNIFICANCE

Glutathione (GSH) is the most abundant cellular low-molecular-weight thiol in the majority of organisms in all kingdoms of life. Therefore, functions of GSH and disturbed regulation of its concentration are associated with numerous physiological and pathological situations. Recent Advances: The function of GSH as redox buffer or antioxidant is increasingly being questioned. New functions, especially functions connected to the cellular iron homeostasis, were elucidated. Via the formation of iron complexes, GSH is an important player in all aspects of iron metabolism: sensing and regulation of iron levels, iron trafficking, and biosynthesis of iron cofactors. The variety of GSH coordinated iron complexes and their functions with a special focus on FeS-glutaredoxins are summarized in this review. Interestingly, GSH analogues that function as major low-molecular-weight thiols in organisms lacking GSH resemble the functions in iron homeostasis.

CRITICAL ISSUES

Since these iron-related functions are most likely also connected to thiol redox chemistry, it is difficult to distinguish between mechanisms related to either redox or iron metabolisms.

FUTURE DIRECTIONS

The ability of GSH to coordinate iron in different complexes with or without proteins needs further investigation. The discovery of new Fe-GSH complexes and their physiological functions will significantly advance our understanding of cellular iron homeostasis. Antioxid. Redox Signal. 27, 1235-1251.

Persulfidation proteome reveals the regulation of protein function by hydrogen sulfide in diverse biological processes in Arabidopsis

Hydrogen sulfide-mediated signaling pathways regulate many physiological and pathophysiological processes in mammalian and plant systems. The molecular mechanism by which hydrogen sulfide exerts its action involves the post-translational modification of cysteine residues to form a persulfidated thiol motif, a process called protein persulfidation. We have developed a comparative and quantitative proteomic analysis approach for the detection of endogenous persulfidated proteins in wild-type Arabidopsis and L-CYSTEINE DESULFHYDRASE 1 mutant leaves using the tag-switch method. The 2015 identified persulfidated proteins were isolated from plants grown under controlled conditions, and therefore, at least 5% of the entire Arabidopsis proteome may undergo persulfidation under baseline conditions. Bioinformatic analysis revealed that persulfidated cysteines participate in a wide range of biological functions, regulating important processes such as carbon metabolism, plant responses to abiotic and biotic stresses, plant growth and development, and RNA translation. Quantitative analysis in both genetic backgrounds reveals that protein persulfidation is mainly involved in primary metabolic pathways such as the tricarboxylic acid cycle, glycolysis, and the Calvin cycle, suggesting that this protein modification is a new regulatory component in these pathways.

Climate Change, CO2, and Defense: The Metabolic, Redox, and Signaling Perspectives

Ongoing human-induced changes in the composition of the atmosphere continue to stimulate interest in the effects of high CO₂ on plants, but its potential impact on inducible plant defense pathways remains poorly defined. Recently, several studies have reported that growth at elevated CO₂ is sufficient to induce defenses such as the salicylic acid pathway, thereby increasing plant resistance to pathogens. These reports contrast with evidence that defense pathways can be promoted by photorespiration, which is inhibited at high CO₂. Here, we review signaling, metabolic, and redox processes modulated by CO₂ levels and discuss issues to be resolved in elucidating the relationships between primary metabolism, inducible defense, and biotic stress resistance.

Comparative Transcriptome Analyses of Gene Expression Changes Triggered by Rhizoctonia solani AG1 IA Infection in Resistant and Susceptible Rice Varieties

Rice sheath blight, caused by Rhizoctonia solani, is one of the most devastating diseases for stable rice production in most rice-growing regions of the world. Currently, studies of the molecular mechanism of rice sheath blight resistance are scarce. Here, we used an RNA-seq approach to analyze the gene expression changes induced by the AG1 IA strain of R. solani in rice at 12, 24, 36, 48, and 72 h. By comparing the transcriptomes of TeQing (a moderately resistant cultivar) and Lemont (a susceptible cultivar) leaves, variable transcriptional responses under control and infection conditions were revealed. From these data, 4,802 differentially expressed genes (DEGs) were identified. Gene ontology and pathway enrichment analyses suggested that most DEGs and related metabolic pathways in both rice genotypes were common and spanned most biological activities after AG1 IA inoculation. The main difference between the resistant and susceptible plants was a difference in the timing of the response to AG1 IA infection. Photosynthesis, photorespiration, and jasmonic acid and phenylpropanoid metabolism play important roles in disease resistance, and the relative response of disease resistance-related pathways in TeQing leaves was more rapid than that of Lemont leaves at 12 h. Here, the transcription data include the most comprehensive list of genes and pathway candidates induced by AG1 IA that is available for rice and will serve as a resource for future studies into the molecular mechanisms of the responses of rice to AG1 IA.

Oxidative Stress

Oxidative stress is two sided: Whereas excessive oxidant challenge causes damage to biomolecules, maintenance of a physiological level of oxidant challenge, termed oxidative eustress, is essential for governing life processes through redox signaling. Recent interest has focused on the intricate ways by which redox signaling integrates these converse properties. Redox balance is maintained by prevention, interception, and repair, and concomitantly the regulatory potential of molecular thiol-driven master switches such as Nrf2/Keap1 or NF-κB/IκB is used for system-wide oxidative stress response. Nonradical species such as hydrogen peroxide (H₂O₂) or singlet molecular oxygen, rather than free-radical species, perform major second messenger functions. Chemokine-controlled NADPH oxidases and metabolically controlled mitochondrial sources of H₂O₂ as well as glutathione- and thioredoxin-related pathways, with powerful enzymatic back-up systems, are responsible for fine-tuning physiological redox signaling. This makes for a rich research field spanning from biochemistry and cell biology into nutritional sciences, environmental medicine, and molecular knowledge-based redox medicine.

Distinct electron transfer from ferredoxin–thioredoxin reductase to multiple thioredoxin isoforms in chloroplasts

Thiol-based redox regulation is considered to support light-responsive control of various chloroplast functions. The redox cascade via ferredoxin–thioredoxin reductase (FTR)/thioredoxin (Trx) has been recognized as a key to transmitting reducing power; however, Arabidopsis thaliana genome sequencing has revealed that as many as five Trx subtypes encoded by a total of 10 nuclear genes are targeted to chloroplasts. Because each Trx isoform seems to have a distinct target selectivity, the electron distribution from FTR to multiple Trxs is thought to be the critical branch point for determining the consequence of chloroplast redox regulation. In the present study, we aimed to comprehensively characterize the kinetics of electron transfer from FTR to 10 Trx isoforms. We prepared the recombinant FTR protein from Arabidopsis in the heterodimeric form containing the Fe–S cluster. By reconstituting the FTR/Trx system in vitro, we showed that FTR prepared here was enzymatically active and suitable for uncovering biochemical features of chloroplast redox regulation. A series of redox state determinations using the thiol-modifying reagent, 4-acetamido-4′-maleimidylstilbene-2,2′-disulfonate, indicated that all chloroplast Trx isoforms are commonly reduced by FTR; however, significantly different efficiencies were evident. These differences were apparently correlated with the distinct midpoint redox potentials among Trxs. Even when the experiments were performed under conditions of hypothetical in vivo stoichiometry of FTR and Trxs, a similar trend in distinguishable electron transfers was observed. These data highlight an aspect of highly organized circuits in the chloroplast redox regulation network.

The Unprecedented Versatility of the Plant‎ Thioredoxin System

Thioredoxins are ubiquitous enzymes catalyzing reversible disulfide-bond formation to regulate structure and function of many proteins in diverse organisms. In recent years, reverse genetics and biochemical approaches were used to resolve the functions, specificities, and interactions of the different thioredoxin isoforms and reduction systems in planta and revealed the most versatile thioredoxin system of all organisms. Here we review the emerging roles of the thioredoxin system, namely the integration of thylakoid energy transduction, metabolism, gene expression, growth, and development under fluctuating environmental conditions. We argue that these new developments help us to understand why plants organize such a divergent composition of thiol redox networks and provide insights into the regulatory hierarchy that operates between them.

Post-translational Modifications in Regulation of Chloroplast Function: Recent Advances

Post-translational modifications (PTMs) of proteins enable fast modulation of protein function in response to metabolic and environmental changes. Phosphorylation is known to play a major role in regulating distribution of light energy between the Photosystems (PS) I and II (state transitions) and in PSII repair cycle. In addition, thioredoxin-mediated redox regulation of Calvin cycle enzymes has been shown to determine the efficiency of carbon assimilation. Besides these well characterized modifications, recent methodological progress has enabled identification of numerous other types of PTMs in various plant compartments, including chloroplasts. To date, at least N-terminal and Lys acetylation, Lys methylation, Tyr nitration and S-nitrosylation, glutathionylation, sumoylation and glycosylation of chloroplast proteins have been described. These modifications impact DNA replication, control transcriptional efficiency, regulate translational machinery and affect metabolic activities within the chloroplast. Moreover, light reactions of photosynthesis as well as carbon assimilation are regulated at multiple levels by a number of PTMs. It is likely that future studies will reveal new metabolic pathways to be regulated by PTMs as well as detailed molecular mechanisms of PTM-mediated regulation.

The role of conserved Cys residues in Brassica rapa auxin amidohydrolase: Cys139 is crucial for the enzyme activity and Cys320 regulates enzyme stability

Brassica rapa auxin amidohydrolase (BrILL2) participates in the homeostasis of the plant hormones auxins by hydrolyzing the amino acid conjugates of auxins, thereby releasing the free active form of hormones. Herein, the potential role of the two conserved Cys residues of BrILL2 (at sequence positions 139 and 320) has been investigated by using interdisciplinary approaches and methods of molecular biology, biochemistry, biophysics and molecular modelling. The obtained results show that both Cys residues participate in the regulation of enzyme activity. Cys320 located in the satellite domain of the enzyme is mainly responsible for protein stability and regulation of enzyme activity through polymer formation, as has been revealed by enzyme kinetics and differential scanning calorimetry analysis of the BrILL2 wild type and mutants C320S and C139S. Cys139 positioned in the active site of the catalytic domain is involved in the coordination of one Mn(2+) ion of the bimetal center and is crucial for the enzymatic activity. Although the point mutation Cys139 to Ser causes the loss of enzyme activity, it does not affect the metal binding to the BrILL2 enzyme, as has been shown by isothermal titration calorimetry, circular dichroism spectropolarimetry and differential scanning calorimetry data. MD simulations (200 ns) revealed a different active site architecture of the BrILL2C139S mutant in comparison to the wild type enzyme. Additional possible reasons for the inactivity of the BrILL2C139S mutant have been discussed based on MD simulations and MM-PBSA free energy calculations of BrILL2 enzyme complexes (wt and C139S mutant) with IPA-Ala as a substrate.

Functional Significance of NADPH-Thioredoxin Reductase C in the Antioxidant Defense System of Cyanobacterium Anabaena sp. PCC 7120

The redox regulation system is widely accepted as a crucial mechanism for controlling the activities of various metabolic enzymes. In addition to thioredoxin reductase/thioredoxin cascades, NADPH-thioredoxin reductase C (NTRC), a hybrid protein formed by an NADPH-thioredoxin reductase domain and a thioredoxin (Trx) domain, is present in chloroplasts and in most cyanobacteria species. Although several target proteins and physiological functions of NTRC in chloroplasts have been characterized, little is known about NTRC functions in cyanobacteria. Therefore, we investigated the molecular basis and physiological significance of NTRC-dependent redox regulation in the filamentous heterocyst-forming nitrogen-fixing cyanobacterium Anabaena sp. PCC 7120 (Anabaena 7120). Initially, we identified six candidate NTRC targets in Anabaena 7120 using NTRC affinity chromatography. Subsequently, we compared the efficiency of reducing-equivalent transfer from NTRC and Trx-m1 to the NTRC target protein 2-Cys peroxiredoxin. Biochemical analyses revealed that compared with Trx-m1, NTRC more efficiently transfers reducing equivalents to 2-Cys peroxiredoxin. Subsequently, we constructed and analyzed an ntrC knockout strain in Anabaena 7120. The mutant showed impaired growth under oxidative stress conditions and lower concentrations of reduced 2-Cys peroxiredoxin in cells. Taken together, the present in vitro and in vivo results indicate that NTRC is a significant electron donor for 2-Cys peroxiredoxin and plays a pivotal role in antioxidant defense systems in Anabaena 7120 cells.

Nitric Oxide and Hydrogen Peroxide Mediate Wounding-Induced Freezing Tolerance through Modifications in Photosystem and Antioxidant System in Wheat

Mechanical wounding is a common stress caused by herbivores or manual and natural manipulations, whereas its roles in acclimation response to a wide spectrum of abiotic stresses remain unclear. The present work showed that local mechanical wounding enhanced freezing tolerance in untreated systemic leaves of wheat plants (Triticum aestivum L.), and meanwhile the signal molecules hydrogen peroxide (H₂O₂) and nitric oxide (NO) were accumulated systemically. Pharmacological study showed that wounding-induced NO synthesis was substantially arrested by pretreatment with scavengers of reactive oxygen species and an inhibitor of NADPH oxidase (respiratory burst oxidase homolog, RBOH). On the contrary, wounding-induced H₂O₂ accumulation was not sensitive to NO synthetic inhibitors or scavenger, indicating that H₂O₂ acts upstream of NO in wounding signal transduction pathways. Cytochemical and vascular tissues localizations approved that RBOH-dependent H₂O₂ acts as long-distance signal in wounding response. Transcriptome analysis revealed that 279 genes were up-regulated in plants treated with wounding and freezing, but not in plants treated with freezing alone. Importantly, freezing- and wounding-induced genes were significantly enriched in the categories of "photosynthesis" and "signaling." These results strongly supported that primary mechanical wounding can induce freezing tolerance in wheat through the systemic accumulation of NO and H₂O₂, and further modifications in photosystem and antioxidant system.

Integrated role of ROS and Ca+2 in blue light-induced chloroplast avoidance movement in leaves of Hydrilla verticillata (L.f.) Royle

Directional chloroplast photorelocation is a major physio-biochemical mechanism that allows these organelles to realign themselves intracellularly in response to the intensity of the incident light as an adaptive response. Signaling processes involved in blue light (BL)-dependent chloroplast movements were investigated in Hydrilla verticillata (L.f.) Royle leaves. Treatments with antagonists of actin filaments [2,3,5-triiodobenzoic acid (TIBA)] and microtubules (oryzalin) revealed that actin filaments, but not microtubules, play a pivotal role in chloroplast movement. Involvement of reactive oxygen species (ROS) in controlling chloroplast avoidance movement has been demonstrated, as exogenous H₂O₂ not only accelerated chloroplast avoidance but also could induce chloroplast avoidance even in weak blue light (WBL). Further support came from experiments with different ROS scavengers, i.e., dimethylthiourea (DMTU), KI, and CuCl₂, which inhibited chloroplast avoidance, and from ROS localization using specific stains. Such avoidance was also partially inhibited by ZnCl₂, an inhibitor of NADPH oxidase (NOX) as well as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), a photosynthetic electron transport chain (ETC) inhibitor at PS II. However, methyl viologen (MV), a PS I ETC inhibitor, rather accelerated avoidance response. Exogenous calcium (Ca⁺²) induced avoidance even in WBL while inhibited chloroplast accumulation partially. On the other hand, chloroplast movements (both accumulation and avoidance) were blocked by Ca⁺² antagonists, La³⁺ (inhibitor of plasma membrane Ca⁺² channel) and ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA, Ca⁺² chelator) while LiCl that affects Ca⁺² release from endosomal compartments did not show any effect. A model on integrated role of ROS and Ca⁺² (influx from apolastic space) in actin-mediated chloroplast avoidance has been proposed.

Deficiency in phytochrome A alters photosynthetic activity, leaf starch metabolism and shoot biomass production in tomato

Photosynthesis is a key process that promotes plant growth and development. Light provides photosynthetic organisms with a major source of energy to fix carbon dioxide into organic matter. Of the entire visible light spectrum, red/blue light are known to maximise photosynthetic performance and are thus essential for proper growth and development of plants. Red and blue light stimulate synthesis of chlorophyll and orchestrate the positioning of leaves and chloroplasts for optimal utilisation of light, both of which are critical for photosynthesis. The response of plants to external light cues is accomplished via finely tuned complex photoreceptors and signaling mechanisms which enable them to continually monitor light availability and quality for optimal utilisation of light energy towards enhancing their growth. Higher plants contain a suite of photoreceptor proteins that allow them to perceive red, blue/UV-A and UV-B light. Analyses of the phyA mutant of tomato deficient in the red-light photoreceptor phytochrome A (phyA), showed reduced photosynthetic activity of isolated chloroplasts along with decreased shoot biomass in adult plants. The regulation of leaf transitory starch in the mutant was also altered as compared to the wild type (cv Moneymaker). Our results suggest a possible role for phyA in these processes in tomato.

Metabolite transport and associated sugar signalling systems underpinning source/sink interactions

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilisation. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement.

Secretomic Insight into Glucose Metabolism of Aspergillus brasiliensis in Solid-State Fermentation

The genus Aspergillus is ubiquitous in nature and includes various species extensively exploited industrially due to their ability to produce and secrete a variety of enzymes and metabolites. Most processes are performed in submerged fermentation (SmF); however, solid-state fermentation (SSF) offers several advantages, including lower catabolite repression and substrate inhibition and higher productivity and stability of the enzymes produced. This study aimed to explain the improved metabolic behavior of A. brasiliensis ATCC9642 in SSF at high glucose concentrations through a proteomic approach. Online respirometric analysis provided reproducible samples for secretomic studies when the maximum CO₂ production rate occurred, ensuring consistent physiological states. Extracellular extracts from SSF cultures were treated by SDS-PAGE, digested with trypsin, and analyzed by LC-MS/MS. Of 531 sequences identified, 207 proteins were analyzed. Twenty-five were identified as the most abundant unregulated proteins; 87 were found to be up-regulated and 95 were down-regulated with increasing glucose concentration. Of the regulated proteins, 120 were enzymes, most involved in the metabolism of carbohydrates (51), amino acids (23), and nucleotides (9). This study shows the high protein secretory activity of A. brasiliensis under SSF conditions. High glucose concentration favors catabolic activities, while some stress-related proteins and those involved in proteolysis are down-regulated.