Proteomic analysis of early-responsive redox-sensitive proteins in Arabidopsis

The nuclear sulfenome of Arabidopsis: spotlight on histone acetyltransferase GCN5 regulation through functional thiols

In aerobic life forms, reactive oxygen species (ROS) are produced by the partial reduction of oxygen during energy-generating metabolic processes. In plants, ROS production increases during periods of both abiotic and biotic stress, severely overloading the antioxidant systems. Hydrogen peroxide (H2O2) plays a central role in cellular redox homeostasis and signalling by oxidizing crucial cysteines to sulfenic acid, which is considered a biologically relevant post-translational modification (PTM). Until now, the impact of the nucleus on cellular redox homeostasis has been relatively unexplored. The regulation of histone-modifying enzymes by oxidative PTMs at redox-sensitive cysteine or tyrosine residues is particularly intriguing because it allows the integration of redox signalling mechanisms with chromatin control of transcriptional activity. One of the most extensively studied histone acetyltransferases is the conserved GENERAL CONTROL NONDEPRESSIBLE 5 (GCN5) complex. This study investigated the nuclear sulfenome in Arabidopsis thaliana by expressing a nuclear variant of the Yeast Activation Protein-1 (YAP1) probe and identified 225 potential redox-active proteins undergoing S-sulfenylation. Mass spectrometry analysis further confirmed the S-sulfenylation of GCN5 at Cys293, Cys368, and Cys400, and their functional significance and impact on the GCN5 protein-protein interaction network were assessed using cysteine-to-serine mutagenesis.

More questions than answers: insights into potential cysteine-rich receptor-like kinases redox signalling in Arabidopsis

Over the past few decades, significant advancements have been made in understanding how plasma-membrane localised receptor kinases (RKs) detect signals and activate responses to various stimuli. Numerous examples of ligand-induced receptor activation mechanisms and their downstream consequences have been characterised in detail. The crucial role of post-translational modifications (PTMs), such as the phosphorylation of receptor kinases, has been demonstrated concerning different cellular responses. Given the diverse structures and architectures of the extracellular domains (ECDs) of RKs, it is probable that various forms of PTMs also play an essential role in receptor activation, including cysteine oxidative modifications triggered by reactive oxygen species (ROS). The function of cysteine oxidative modifications as functional redox switches that modulate protein structure and function has been extensively studied across various multicellular organisms. Based on biochemical and structural characteristics, the family of cysteine-rich receptor-like kinases (CRK) emerges as excellent candidates for proteins regulated in a redox-dependent manner. This review provides a concise overview of cysteine's biochemical and structural properties in its role as a molecular redox switch. Drawing on the currently available literature, we describe how cysteine-redox signalling is maintained, particularly in plant cells. We further focus on extracellular ROS perception and the role of CRKs as promising candidates for ROS sensors in Arabidopsis thaliana. We discuss the structural and biochemical properties of CRKs, their involvement in plant growth and defence processes, and our perspective on why CRKs could be key components of the ROS sensing machinery or ROS sensors, especially regarding the dimerization abilities of CRKs. Finally, we highlight the current challenges in identifying and quantifying cysteine oxidative modifications and propose methods for detecting ROS-modified cysteines that may be promising for investigating the role of CRKs in extracellular ROS perception and signalling.

SnRK1α1-mediated RBOH1 phosphorylation regulates reactive oxygen species to enhance tolerance to low nitrogen in tomato

Nitrogen is essential for plant growth and development. SNF1-related protein kinase 1 (SnRK1) is an evolutionarily conserved protein kinase pivotal for regulating plant responses to nutrient deficiency. Here, we discovered that the expression and activity of the SnRK1 α-catalytic subunit (SnRK1α1) increased in response to low-nitrogen stress. SnRK1α1 overexpression enhanced seedling tolerance, nitrate uptake capacity, apoplastic reactive oxygen species (ROS) accumulation, and NADPH oxidase activity in tomato (Solanum lycopersicum L.) under low-nitrogen stress compared to wild type plants, while snrk1α1 mutants exhibited the opposite phenotypes. Mutation of the NADPH oxidase gene Respiratory burst oxidase homolog 1 (RBOH1) suppressed numerous nitrate uptake and metabolism genes during low-nitrogen stress. rboh1 mutants displayed lower NADPH oxidase activity, apoplastic ROS production, and seedling tolerance to low nitrogen. Silencing RBOH1 expression also compromised SnRK1α1-mediated seedling tolerance to low-nitrogen stress. SnRK1α1 interacts with and activates RBOH1 through phosphorylation of three N-terminal serine residues, leading to increased apoplastic ROS production and enhanced tolerance to low nitrogen conditions. Furthermore, RBOH1-dependent ROS oxidatively modified the transcription factor TGA4 at residue Cys-334, which increased NRT1.1 and NRT2.1 expression under low-nitrogen stress. These findings reveal a SnRK1α1-mediated signaling pathway and highlight the essential role of RBOH1-dependent ROS production in enhancing plant tolerance to low nitrogen.

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

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

The Impact of AAPH-Induced Oxidation on the Functional and Structural Properties, and Proteomics of Arachin

The aim of this study was to investigate the effect of 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH)-induced oxidation on the functional, structural properties and proteomic information of arachin. The results showed that moderate oxidation improved the water/oil holding capacity of proteins and increased the emulsifying stability, while excessive oxidation increased the carbonyl content, reduced the thiol content, altered the structure and thermal stability, and reduced most of the physicochemical properties. Through LC-QE-MS analysis, it was observed that oxidation leads to various modifications in arachin, including carbamylation, oxidation, and reduction, among others. In addition, 15 differentially expressed proteins were identified. Through gene ontology (GO) analysis, these proteins primarily affected the cellular and metabolic processes in the biological process category. Further Kyoto encyclopedia of genes and genomes (KEGG) analysis revealed that the "proteasome; protein processing in the endoplasmic reticulum (PPER)" pathway was the most significantly enriched signaling pathway during the oxidation process of arachin. In conclusion, this study demonstrated that AAPH-induced oxidation can alter the conformation and proteome of arachin, thereby affecting its corresponding functional properties. The findings of this study can potentially serve as a theoretical basis and foundational reference for the management of peanut processing and storage.

IodoTMT-labeled redox proteomics reveals the involvement of oxidative post-translational modification in response to para-hydroxybenzoic acid and hydrogen peroxide stresses in poplar

Poplar is widely planted as an economic and ecological tree species. However, accumulation of the phenolic acid allelochemical para-hydroxybenzoic acid (pHBA) in soil is a severe threat to the growth and productivity of poplar. pHBA stress leads to excessive production of reactive oxygen species (ROS). However, it is unclear which redox-sensitive proteins are involved in the pHBA-induced cellular homeostasis regulatory mechanism. We here identified reversible redox-modified proteins and modified cysteine (Cys) sites in exogenous pHBA- and hydrogen peroxide (H₂O₂)-treated poplar seedling leaves by using the iodoacetyl tandem mass tag-labeled redox proteomics method. In total, 4786 redox modification sites were identified in 3176 proteins, with 104 and 91 proteins being differentially modified at 118 and 101 Cys sites in response to pHBA and H₂O₂ stresses, respectively. The differentially modified proteins (DMPs) were predicted to be mainly localized in the chloroplast and cytoplasm, with most proteins being enzymes with catalytic activities. The KEGG enrichment analysis of these DMPs revealed that proteins related to the MAPK signaling pathway, soluble sugar metabolism, amino acid metabolism, photosynthesis, and phagosome pathways were extensively regulated by redox modifications. Moreover, combined with our previous quantitative proteomics data, 8 proteins were upregulated and oxidized under both pHBA and H₂O₂ stresses. Reversible oxidation of Cys sites in these proteins might be actively responsible for the regulation of tolerance to pHBA-induced oxidative stress. Based on the aforementioned results, a redox regulatory model activated by pHBA- and H₂O₂-induced oxidative stress was proposed. This study conducts the first redox proteomics analysis of poplar in response to pHBA stress and provides a new insight into the mechanistic framework of reversible oxidative post-translational modifications to gain a better understanding of pHBA-induced chemosensory effects on poplar.

Hydrogen Peroxide and GA3 Levels Regulate the High Night Temperature Response in Pistils of Wheat (Triticum aestivum L.)

High night temperature (HNT) impairs crop productivity through the reproductive failure of gametes (pollen and pistil). Though female gametophyte (pistil) is an equal partner in the seed-set, the knowledge of the antioxidant system(s) and hormonal control of HNT tolerance or susceptibility of pistils is limited and lacking. The objectives of this study were to determine the antioxidant mechanism for homeostatic control of free radicals, and the involvement of abscisic acid (ABA) and gibberellic acid (GA₃) in HNT stress protection in the wheat pistils of contrasting wheat genotypes. We hypothesized that HNT tolerance is attributed to the homeostatic control of reactive oxygen species (ROS) and hormonal readjustment in pistils of the tolerant genotype. The ears of two contrasting wheat genotypes-HD 2329 (susceptible) and Raj 3765 (tolerant) were subjected to two HNTs (+5 °C and +8 °C) over ambient, in the absence and presence of dimethylthiourea (DMTU), a chemical trap of hydrogen peroxide (H₂O₂). Results showed that HNTs significantly increased ROS in pistils of susceptible genotype HD 2329 to a relatively greater extent compared to tolerant genotype Raj 3765. The response was similar in the presence or absence of DMTU, but the H₂O₂ values were lower in the presence of DMTU. The ROS levels were balanced by increased activity of peroxidase under HNT to a greater extent in the tolerant genotype. Cytosolic glyceraldehyde-3-phosphate dehydrogenase (GAPC) activity was inversely related to H₂O₂ production within a critical range in Raj 3765, indicating its modulation by H₂O₂ levels as no change was observed at the transcriptional level. The hormonal status showed increased ABA and decreased GA₃ contents with increasing temperature. Our study elucidates the role of H₂O₂ and GA₃ in stress tolerance of pistils of tolerant genotype where GAPC acts as a ROS sensor due to H₂O₂-mediated decrease in its activity.

Proteome-wide identification of S-sulfenylated cysteines reveals metabolic response to freezing stress after cold acclimation in Brassica napus

Redox regulation plays a wide role in plant growth, development, and adaptation to stresses. Sulfenylation is one of the reversible oxidative post-transcriptional modifications. Here we performed an iodoTMT-based proteomic analysis to identify the redox sensitive proteins in vivo under freezing stress after cold acclimation in Brassica napus. Totally, we obtained 1,372 sulfenylated sites in 714 proteins. The overall sulfenylation level displayed an increased trend under freezing stress after cold acclimation. We identified 171 differentially sulfenylated proteins (DSPs) under freezing stress, which were predicted to be mainly localized in chloroplast and cytoplasm. The up-regulated DSPs were mainly enriched in photosynthesis and glycolytic processes and function of catalytic activity. Enzymes involved in various pathways such as glycolysis and Calvin-Benson-Bassham (CBB) cycle were generally sulfenylated and the metabolite levels in these pathways was significantly reduced under freezing stress after cold acclimation. Furthermore, enzyme activity assay confirmed that the activity of cytosolic pyruvate kinase and malate dehydrogenase 2 was significantly reduced under H₂O₂ treatment. Our study provides a landscape of redox sensitive proteins in B. napus in response to freezing stress after cold acclimation, which proposes a basis for understanding the redox regulation in plant metabolic response to freezing stress after cold acclimation.

The effects of two gold-N-heterocyclic carbene (NHC) complexes in ovarian cancer cells: a redox proteomic study

PURPOSE

Ovarian cancer is the fifth leading cause of cancer-related deaths in women. Standard treatment consists of tumor debulking surgery followed by platinum and paclitaxel chemotherapy; yet, despite the initial response, about 70-75% of patients develop resistance to chemotherapy. Gold compounds represent a family of very promising anticancer drugs. Among them, we previously investigated the cytotoxic and pro-apoptotic properties of Au(NHC) and Au(NHC)2PF6, i.e., a monocarbene gold(I) complex and the corresponding bis(carbene) complex. Gold compounds are known to alter the redox state of cells interacting with free cysteine and selenocysteine residues of several proteins. Herein, a redox proteomic study has been carried out to elucidate the mechanisms of cytotoxicity in A2780 human ovarian cancer cells.

METHODS

A biotinylated iodoacetamide labeling method coupled with mass spectrometry was used to identify oxidation-sensitive protein cysteines.

RESULTS

Gold carbene complexes cause extensive oxidation of several cellular proteins; many affected proteins belong to two major functional classes: carbohydrate metabolism, and cytoskeleton organization/cell adhesion. Among the affected proteins, Glyceraldehyde-3-phosphate dehydrogenase inhibition was proved by enzymatic assays and by ESI-MS studies. We also found that Au(NHC)2PF6 inhibits mitochondrial respiration impairing complex I function. Concerning the oxidized cytoskeletal proteins, gold binding to the free cysteines of actin was demonstrated by ESI-MS analysis. Notably, both gold compounds affected cell migration and invasion.

CONCLUSIONS

In this study, we deepened the mode of action of Au(NHC) and Au(NHC)2PF6, identifying common cellular targets but confirming their different influence on the mitochondrial function.

Redox and Hormonal Changes in the Transcriptome of Grape (Vitis vinifera) Berries during Natural Noble Rot Development

Noble rot is a favorable form of the interaction between grape (Vitis spp.) berries and the phytopathogenic fungus Botrytis cinerea. The transcriptome pattern of grapevine cells subject to natural noble rot development in the historic Hungarian Tokaj wine region has not been previously published. Furmint, a traditional white Tokaj variety suited to develop great quality noble rot was used in the experiments. Exploring a subset of the Furmint transcriptome redox and hormonal changes distinguishing between noble rot and bunch rot was revealed. Noble rot is defined by an early spike in abscisic acid (ABA) accumulation and a pronounced remodeling of ABA-related gene expression. Transcription of glutathione S-transferase isoforms is uniquely upregulated, whereas gene expression of some sectors of the antioxidative apparatus (e.g., catalases, carotenoid biosynthesis) is downregulated. These mRNA responses are lacking in berries exposed to bunch rot. Our results help to explain molecular details behind the fine and dynamic balance between noble rot and bunch rot development.

Quantitative redox proteomics revealed molecular mechanisms of salt tolerance in the roots of sugar beet monomeric addition line M14

BACKGROUND

Salt stress is often associated with excessive production of reactive oxygen species (ROS). Oxidative stress caused by the accumulation of ROS is a major factor that negatively affects crop growth and yield. Root is the primary organ that senses and transmits the salt stress signal to the whole plant. How oxidative stress affect redox sensitive proteins in the roots is not known.

RESULTS

In this study, the redox proteome of sugar beet M14 roots under salt stress was investigated. Using iTRAQ reporters, we determined that salt stress caused significant changes in the abundance of many proteins (2305 at 20 min salt stress and 2663 at 10 min salt stress). Using iodoTMT reporters, a total of 95 redox proteins were determined to be responsive to salt stress after normalizing again total protein level changes. Notably, most of the differential redox proteins were involved in metabolism, ROS homeostasis, and stress and defense, while a small number play a role in transport, biosynthesis, signal transduction, transcription and photosynthesis. Transcription levels of 14 genes encoding the identified redox proteins were analyzed using qRT-PCR. All the genes were induced by salt stress at the transcriptional level.

CONCLUSIONS

Based on the redox proteomics results, we construct a map of the regulatory network of M14 root redox proteins in response to salt stress. This study further refines the molecular mechanism of salt resistance at the level of protein redox regulation.

Using DCP-Rho1 as a fluorescent probe to visualize sulfenic acid-containing proteins in living plant cells

Among the biologically relevant reactive oxygen species (ROS), hydrogen peroxide (H₂O₂) has special properties. H₂O₂ can diffuse across membranes, has a low reactivity, and is very stable. Deprotonated cysteine residues in proteins can be oxidized by H₂O₂ into a highly reactive sulfenic acid derivative (-SOH), which can react with another cysteine to form a disulfide. Under higher oxidative stress the sulfenic acid undergo further oxidation to sulfinic acid (Cys-SO₂H), which can subsequently be reduced. The sulfinic acid can be hyperoxidized to sulfonic acid (Cys-SO₃H), whose reduction is irreversible. Formation of sulfenic acids can have a role in sensing oxidative stress, signal transduction, modulating localization and activity to regulate protein functions. Therefore, there is an emerging interest in trying to understand the pool of proteins that result in these sorts of modification in response to oxidative stress. This is known as the sulfenome and several approaches have been developed in animal and plant cells to analyze the sulfenome under different stress responses. These approaches can be proteomic, molecular, immunological (i.e., antibodies), or expressing genetically encoded probes that specifically react to sulfenic modifications. In this chapter, we describe an additional approach that allows visualization of sulfenic modification in vivo. This is newly developed fluorescent probe DCP-Rho1 can be implemented in any plant cell to analyze the sulfenic modification.

Topology of the redox network during induction of photosynthesis as revealed by time-resolved proteomics in tobacco

Photosynthetically produced electrons provide energy for various metabolic pathways, including carbon reduction. Four Calvin-Benson cycle enzymes and several other plastid proteins are activated in the light by reduction of specific cysteines via thioredoxins, a family of electron transporters operating in redox regulation networks. How does this network link the photosynthetic chain with cellular metabolism? Using a time-resolved redox proteomic method, we have investigated the redox network in vivo during the dark–to–low light transition. We show that redox states of some thioredoxins follow the photosynthetic linear electron transport rate. While some redox targets have kinetics compatible with an equilibrium with one thioredoxin (TRXf), reduction of other proteins shows specific kinetic limitations, allowing fine-tuning of each redox-regulated step of chloroplast metabolism. We identified five new redox-regulated proteins, including proteins involved in Mg2+ transport and 1O2 signaling. Our results provide a system-level functional view of the photosynthetic redox regulation network.

Coordinated regulation of plant immunity by poly(ADP-ribosyl)ation and K63-linked ubiquitination

Poly(ADP-ribosyl)ation (PARylation) is a posttranslational modification reversibly catalyzed by poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolases (PARGs) and plays a key role in multiple cellular processes. The molecular mechanisms by which PARylation regulates innate immunity remain largely unknown in eukaryotes. Here we show that Arabidopsis UBC13A and UBC13B, the major drivers of lysine 63 (K63)-linked polyubiquitination, directly interact with PARPs/PARGs. Activation of pathogen-associated molecular pattern (PAMP)-triggered immunity promotes these interactions and enhances PARylation of UBC13. Both parp1 parp2 and ubc13a ubc13b mutants are compromised in immune responses with increased accumulation of total pathogenesis-related (PR) proteins but decreased accumulation of secreted PR proteins. Protein disulfide-isomerases (PDIs), essential components of endoplasmic reticulum quality control (ERQC) that ensure proper folding and maturation of proteins destined for secretion, complex with PARPs/PARGs and are PARylated upon PAMP perception. Significantly, PARylation of UBC13 regulates K63-linked ubiquitination of PDIs, which may further promote their disulfide isomerase activities for correct protein folding and subsequent secretion. Taken together, these results indicate that plant immunity is coordinately regulated by PARylation and K63-linked ubiquitination.

SPEAR: A proteomics approach for simultaneous protein expression and redox analysis

Oxidation and reduction of protein cysteinyl thiols serve as molecular switches, which is considered the most central mechanism for redox regulation of biological processes, altering protein structure, biochemical activity, subcellular localization, and binding affinity. Redox proteomics allows global identification of redox-modified cysteine (Cys) sites and quantification of their reversible oxidation/reduction responses, serving as a hypothesis-generating platform to stimulate redox biology mechanistic research. Here, we developed Simultaneous Protein Expression and Redox (SPEAR) analysis, a new redox-proteomics approach based on differential labeling of reversibly oxidized and reduced cysteines with light and heavy isotopic forms of commercially available isotopically-labeled N-ethylmaleimide (NEM). The presented method does not require enrichment for labeled peptides, thus enabling simultaneous quantification of Cys reversible oxidation state and protein abundance. Using SPEAR, we were able to quantify the in-vivo reversible oxidation state of thousands of cysteines across the Arabidopsis proteome under steady-state and oxidative stress conditions. Functional assignment of the identified redox-sensitive proteins demonstrated the widespread effect of oxidative conditions on various cellular functions and highlighted the enrichment of chloroplastic proteins. SPEAR provides a simple, straightforward, and cost-effective means of studying redox proteome dynamics. The presented data provide a global quantitative view of the reversible oxidation of well-known redox-regulated active sites and many novel redox-sensitive sites whose role in plant acclimation to stress conditions remains to be further explored.

Proteome-wide identification of S-sulphenylated cysteines in Brassica napus

Deregulation of reduction-oxidation (redox) metabolism under environmental stresses results in enhanced production of intracellular reactive oxygen species (ROS), which ultimately leads to post-translational modifications (PTMs) of responsive proteins. Redox PTMs play an important role in regulation of protein function and cellular signalling. By means of large-scale redox proteomics, we studied reversible cysteine modification during the response to short-term salt stress in Brassica napus (B. napus). We applied an iodoacetyl tandem mass tags (iodoTMT)-based proteomic approach to analyse the redox proteome of B. napus seedlings under control and salt-stressed conditions. We identified 1,821 sulphenylated sites in 912 proteins from all samples. A great number of sulphenylated proteins were predicted to localize to chloroplasts and cytoplasm and GO enrichment analysis of differentially sulphenylated proteins revealed that metabolic processes such as photosynthesis and glycolysis are enriched and enzymes are overrepresented. Redox-sensitive sites in two enzymes were validated in vitro on recombinant proteins and they might affect the enzyme activity. This targeted approach contributes to the identification of the sulphenylated sites and proteins in B. napus subjected to salt stress and our study will improve our understanding of the molecular mechanisms underlying the redox regulation in response to salt stress.

Guard cell redox proteomics reveals a role of lipid transfer protein in plant defense

Redox-based post-translational modifications (PTMs) involving protein cysteine residues as redox sensors are important to various physiological processes. However, little is known about redox-sensitive proteins in guard cells and their functions in stomatal immunity. In this study, we applied an integrative protein labeling method cysTMTRAQ, and identified guard cell proteins that were altered by thiol redox PTMs in response to a bacterial flagellin peptide flg22. In total, eight, seven and 20 potential redox-responsive proteins were identified in guard cells treated with flg22 for 15, 30 and 60 min, respectively. The proteins fall into several functional groups including photosynthesis, lipid binding, oxidation-reduction, and defense. Among the proteins, a lipid transfer protein (LTP)-II was confirmed to be redox-responsive and involved in plant resistance to Pseudomonas syringe pv. tomato DC3000. This study not only creates an inventory of potential redox-sensitive proteins in flg22 signal transduction in guard cells, but also highlights the biological relevance of the lipid transfer protein in plant defense against bacterial pathogens. SIGNIFICANCE: Protein redox modifications play important roles in many physiological processes. However, redox proteomics has rarely been studied in plant single cell-types. In this study, isobaric tandem mass tag-based redox proteomics technology was applied to discover redox-sensitive proteins and corresponding cysteine residues in guard cell response to a bacterial flagellin peptide flg22. Many redox-responsive proteins related to photosynthesis, lipid binding, oxidation-reduction, and defense were identified. Using reverse genetics and biochemical analyses, a lipid transfer protein was functionally characterized to be involved in plant defense against pathogens. The study highlights the utility of redox proteomics in discovering new proteins and redox modifications in important stomatal guard cell functions. Furthermore, detailed functional characterization demonstrates the biological relevance of the redox-responsive lipid transfer protein in plant pathogen defense.

Single Amino Acid Exchange in ACTIN2 Confers Increased Tolerance to Oxidative Stress in Arabidopsis der1-3 Mutant

Single-point mutation in the ACTIN2 gene of the der1-3 mutant revealed that ACTIN2 is an essential actin isovariant required for root hair tip growth, and leads to shorter, thinner and more randomly oriented actin filaments in comparison to the wild-type C24 genotype. The actin cytoskeleton has been linked to plant defense against oxidative stress, but it is not clear how altered structural organization and dynamics of actin filaments may help plants to cope with oxidative stress. In this study, we characterized root growth, plant biomass, actin organization and antioxidant activity of the der1-3 mutant under oxidative stress induced by paraquat and H₂O₂. Under these conditions, plant growth was better in the der1-3 mutant, while the actin cytoskeleton in the der1-3 carrying pro35S::GFP:FABD2 construct showed a lower bundling rate and higher dynamicity. Biochemical analyses documented a lower degree of lipid peroxidation, and an elevated capacity to decompose superoxide and hydrogen peroxide. These results support the view that the der1-3 mutant is more resistant to oxidative stress. We propose that alterations in the actin cytoskeleton, increased sensitivity of ACTIN to reducing agent dithiothreitol (DTT), along with the increased capacity to decompose reactive oxygen species encourage the enhanced tolerance of this mutant against oxidative stress.

Cysteine modifications (oxPTM) and protein sulphenylation-mediated sulfenome expression in plants: evolutionary conserved signaling networks?

Plant resilience to oxidative stress possibly operates through the restoration of intracellular redox milieu and the activity of various posttranslationally modified proteins. Among various modes of redox regulation operative in plants cys oxPTMs are brought about by the activity of reactive oxygen species (ROS), reactive nitrogen species (RNS), and hydrogen peroxide. Cysteine oxPTMs are capable of transducing ROS-mediated long-distance hormone signaling (ABA, JA, SA) in plants. S-sulphenylation is an intermediary modification en route to other oxidative states of cysteine. In silico analysis have revealed evolutionary conservation of certain S-sulphenylated proteins across human and plants. Further analysis of protein sulphenylation in plants should be extended to the functional follow-up studies followed by site-specific characterization and case-by-case validation of protein activity. The repertoire of physiological methods (fluorescent conjugates (dimedone) and yeast AP-1 (YAP1)-based genetic probes) in the recent past has been successful in the detection of sulphenylated proteins and other cysteine-based modifications in plants. In view of a better understanding of the sulfur-based redoxome it is necessary to update our timely progress on the methodological advancements for the detection of cysteine-based oxPTM. This substantiative information can extend our investigations on plant-environment interaction thus improving crop manipulation strategies. The simulation-based computational approach has emerged as a new method to determine the directive mechanism of cysteine oxidation in plants. Thus, sulfenome analysis in various plant systems might reflect as a pinnacle of plant redox biology in the future.

Plant Proton Pumps and Cytosolic pH-Homeostasis

Proton pumps create a proton motif force and thus, energize secondary active transport at the plasma nmembrane and endomembranes of the secretory pathway. In the plant cell, the dominant proton pumps are the plasma membrane ATPase, the vacuolar pyrophosphatase (V-PPase), and the vacuolar-type ATPase (V-ATPase). All these pumps act on the cytosolic pH by pumping protons into the lumen of compartments or into the apoplast. To maintain the typical pH and thus, the functionality of the cytosol, the activity of the pumps needs to be coordinated and adjusted to the actual needs. The cellular toolbox for a coordinated regulation comprises 14-3-3 proteins, phosphorylation events, ion concentrations, and redox-conditions. This review combines the knowledge on regulation of the different proton pumps and highlights possible coordination mechanisms.

Bringing New Methods to the Seed Proteomics Platform: Challenges and Perspectives

For centuries, crop plants have represented the basis of the daily human diet. Among them, cereals and legumes, accumulating oils, proteins, and carbohydrates in their seeds, distinctly dominate modern agriculture, thus play an essential role in food industry and fuel production. Therefore, seeds of crop plants are intensively studied by food chemists, biologists, biochemists, and nutritional physiologists. Accordingly, seed development and germination as well as age- and stress-related alterations in seed vigor, longevity, nutritional value, and safety can be addressed by a broad panel of analytical, biochemical, and physiological methods. Currently, functional genomics is one of the most powerful tools, giving direct access to characteristic metabolic changes accompanying plant development, senescence, and response to biotic or abiotic stress. Among individual post-genomic methodological platforms, proteomics represents one of the most effective ones, giving access to cellular metabolism at the level of proteins. During the recent decades, multiple methodological advances were introduced in different branches of life science, although only some of them were established in seed proteomics so far. Therefore, here we discuss main methodological approaches already employed in seed proteomics, as well as those still waiting for implementation in this field of plant research, with a special emphasis on sample preparation, data acquisition, processing, and post-processing. Thereby, the overall goal of this review is to bring new methodologies emerging in different areas of proteomics research (clinical, food, ecological, microbial, and plant proteomics) to the broad society of seed biologists.

ConCysFind: a pipeline tool to predict conserved amino acids of protein sequences across the plant kingdom

BACKGROUND

Post-translational modifications (PTM) of amino acid (AA) side chains in peptides control protein structure and functionality. PTMs depend on the specific AA characteristics. The reactivity of cysteine thiol-based PTMs are unique among all proteinaceous AA. This pipeline aims to ease the identification of conserved AA of polypeptides or protein families based on the phylogenetic occurrence in the plant kingdom. The tool is customizable to include any species. The degree of AA conservation is taken as indicator for structural and functional significance, especially for PTM-based regulation. Further, this pipeline tool gives insight into the evolution of these potentially regulatory important peptides.

RESULTS

The web-based or stand-alone pipeline tool Conserved Cysteine Finder (ConCysFind) was developed to identify conserved AA such as cysteine, tryptophan, serine, threonine, tyrosin and methionine. ConCysFind evaluates multiple alignments considering the proteome of 21 plant species. This exemplar study focused on Cys as evolutionarily conserved target for multiple redox PTM. Phylogenetic trees and tables with the compressed results of the scoring algorithm are generated for each Cys in the query polypeptide. Analysis of 33 translation elongation and release factors alongside of known redox proteins from Arabidopsis thaliana for conserved Cys residues confirmed the suitability of the tool for identifying conserved and functional PTM sites. Exemplarily, the redox sensitivity of cysteines in the eukaryotic release factor 1-1 (eRF1-1) was experimentally validated.

CONCLUSION

ConCysFind is a valuable tool for prediction of new potential protein PTM targets in a broad spectrum of species, based on conserved AA throughout the plant kingdom. The identified targets were successfully verified through protein biochemical assays. The pipeline is universally applicable to other phylogenetic branches by customization of the database.

Plant Chloroplast Stress Response: Insights from Thiol Redox Proteomics

Significance: Plant chloroplasts generate reactive oxygen species (ROS) during photosynthesis, especially under stresses. The sulfhydryl groups of protein cysteine residues are susceptible to redox modifications, which regulate protein structure and function, and thus different signaling and metabolic processes. The ROS-governed protein thiol redox switches play important roles in chloroplasts. Recent Advances: Various high-throughput thiol redox proteomic approaches have been developed, and they have enabled the improved understanding of redox regulatory mechanisms in chloroplasts. For example, the thioredoxin-modulated antioxidant enzymes help to maintain cellular ROS homeostasis. The light- and dark-dependent redox regulation of photosynthetic electron transport, the Calvin/Benson cycle, and starch biosynthesis ensures metabolic coordination and efficient energy utilization. In addition, redox cascades link the light with the dynamic changes of metabolites in nitrate and sulfur assimilation, shikimate pathway, and biosynthesis of fatty acid hormone as well as purine, pyrimidine, and thiamine. Importantly, redox regulation of tetrapyrrole and chlorophyll biosynthesis is critical to balance the photodynamic tetrapyrrole intermediates and prevent oxidative damage. Moreover, redox regulation of diverse elongation factors, chaperones, and kinases plays an important role in the modulation of gene expression, protein conformation, and posttranslational modification that contribute to photosystem II (PSII) repair, state transition, and signaling in chloroplasts. Critical Issues: This review focuses on recent advances in plant thiol redox proteomics and redox protein networks toward understanding plant chloroplast signaling, metabolism, and stress responses. Future Directions: Using redox proteomics integrated with biochemical and molecular genetic approaches, detailed studies of cysteine residues, their redox states, cross talk with other modifications, and the functional implications will yield a holistic understanding of chloroplast stress responses.

Photosynthetic Metabolism and Nitrogen Reshuffling Are Regulated by Reversible Cysteine Thiol Oxidation Following Nitrogen Deprivation in Chlamydomonas

As global temperatures climb to historic highs, the far-reaching effects of climate change have impacted agricultural nutrient availability. This has extended to low latitude oceans, where a deficit in both nitrogen and phosphorus stores has led to dramatic decreases in carbon sequestration in oceanic phytoplankton. Although Chlamydomonas reinhardtii, a freshwater model green alga, has shown drastic systems-level alterations following nitrogen deprivation, the mechanisms through which these alterations are triggered and regulated are not fully understood. This study examined the role of reversible oxidative signaling in the nitrogen stress response of C. reinhardtii. Using oxidized cysteine resin-assisted capture enrichment coupled with label-free quantitative proteomics, 7889 unique oxidized cysteine thiol identifiers were quantified, with 231 significantly changing peptides from 184 proteins following 2 h of nitrogen deprivation. These results demonstrate that the cellular response to nitrogen assimilation, photosynthesis, pigment biosynthesis, and lipid metabolism are regulated by reversible oxidation. An enhanced role of non-damaging oxidative pathways is observed throughout the photosynthetic apparatus that provides a framework for further analysis in phototrophs.

Glutathionylation primes soluble glyceraldehyde-3-phosphate dehydrogenase for late collapse into insoluble aggregates

Protein aggregation is a complex physiological process, primarily determined by stress-related factors revealing the hidden aggregation propensity of proteins that otherwise are fully soluble. Here we report a mechanism by which glycolytic glyceraldehyde-3-phosphate dehydrogenase of Arabidopsis thaliana (AtGAPC1) is primed to form insoluble aggregates by the glutathionylation of its catalytic cysteine (Cys149). Following a lag phase, glutathionylated AtGAPC1 initiates a self-aggregation process resulting in the formation of branched chains of globular particles made of partially misfolded and totally inactive proteins. GSH molecules within AtGAPC1 active sites are suggested to provide the initial destabilizing signal. The following removal of glutathione by the formation of an intramolecular disulfide bond between Cys149 and Cys153 reinforces the aggregation process. Physiological reductases, thioredoxins and glutaredoxins, could not dissolve AtGAPC1 aggregates but could efficiently contrast their growth. Besides acting as a protective mechanism against overoxidation, S-glutathionylation of AtGAPC1 triggers an unexpected aggregation pathway with completely different and still unexplored physiological implications.

Inhibition of TOR in Chlamydomonas reinhardtii Leads to Rapid Cysteine Oxidation Reflecting Sustained Physiological Changes

The target of rapamycin (TOR) kinase is a master metabolic regulator with roles in nutritional sensing, protein translation, and autophagy. In Chlamydomonas reinhardtii, a unicellular green alga, TOR has been linked to the regulation of increased triacylglycerol (TAG) accumulation, suggesting that TOR or a downstream target(s) is responsible for the elusive “lipid switch” in control of increasing TAG accumulation under nutrient limitation. However, while TOR has been well characterized in mammalian systems, it is still poorly understood in photosynthetic systems, and little work has been done to show the role of oxidative signaling in TOR regulation. In this study, the TOR inhibitor AZD8055 was used to relate reversible thiol oxidation to the physiological changes seen under TOR inhibition, including increased TAG content. Using oxidized cysteine resin-assisted capture enrichment coupled with label-free quantitative proteomics, 401 proteins were determined to have significant changes in oxidation following TOR inhibition. These oxidative changes mirrored characterized physiological modifications, supporting the role of reversible thiol oxidation in TOR regulation of TAG production, protein translation, carbohydrate catabolism, and photosynthesis through the use of reversible thiol oxidation. The delineation of redox-controlled proteins under TOR inhibition provides a framework for further characterization of the TOR pathway in photosynthetic eukaryotes.

Proteome-Wide Analysis of Cysteine Reactivity during Effector-Triggered Immunity

A surge in the accumulation of oxidants generates shifts in the cellular redox potential during early stages of plant infection with pathogens and activation of effector-triggered immunity (ETI). The redoxome, defined as the proteome-wide oxidative modifications of proteins caused by oxidants, has a well-known impact on stress responses in metazoans. However, the identity of proteins and the residues sensitive to oxidation during the plant immune response remain largely unknown. Previous studies of the thimet oligopeptidases TOP1 and TOP2 placed them in the salicylic acid dependent branch of ETI, with a current model wherein TOPs sustain interconnected organellar and cytosolic pathways that modulate the oxidative burst and development of cell death. Herein, we characterized the ETI redoxomes in Arabidopsis (Arabidopsis thaliana) wild-type Col-0 and top1top2 mutant plants using a differential alkylation-based enrichment technique coupled with label-free mass spectrometry-based quantification. We identified cysteines sensitive to oxidation in a wide range of protein families at multiple time points after pathogen infection. Differences were detected between Col-0 and top1top2 redoxomes regarding the identity and number of oxidized cysteines, and the amplitude of time-dependent fluctuations in protein oxidation. Our results support a determining role for TOPs in maintaining the proper level and dynamics of proteome oxidation during ETI. This study significantly expands the repertoire of oxidation-sensitive plant proteins and can guide future mechanistic studies.

RBOH-Dependent ROS Synthesis and ROS Scavenging by Plant Specialized Metabolites To Modulate Plant Development and Stress Responses

Reactive oxygen species (ROS) regulate plant growth and development. ROS are kept at low levels in cells to prevent oxidative damage, allowing them to be effective signaling molecules upon increased synthesis. In plants and animals, NADPH oxidase/respiratory burst oxidase homolog (RBOH) proteins provide localized ROS bursts to regulate growth, developmental processes, and stress responses. This review details ROS production via RBOH enzymes in the context of plant development and stress responses and defines the locations and tissues in which members of this family function in the model plant Arabidopsis thaliana. To ensure that these ROS signals do not reach damaging levels, plants use an array of antioxidant strategies. In addition to antioxidant machineries similar to those found in animals, plants also have a variety of specialized metabolites that scavenge ROS. These plant specialized metabolites exhibit immense structural diversity and have highly localized accumulation. This makes them important players in plant developmental processes and stress responses that use ROS-dependent signaling mechanisms. This review summarizes the unique properties of plant specialized metabolites, including carotenoids, ascorbate, tocochromanols (vitamin E), and flavonoids, in modulating ROS homeostasis. Flavonols, a subclass of flavonoids with potent antioxidant activity, are induced during stress and development, suggesting that they have a role in maintaining ROS homeostasis. Recent results using genetic approaches have shown how flavonols regulate development and stress responses through their action as antioxidants.

Root Adaptation to H2O2-Induced Oxidative Stress by ARF-GEF BEN1- and Cytoskeleton-Mediated PIN2 Trafficking

Abiotic stress poses constant challenges for plant survival and is a serious problem for global agricultural productivity. On a molecular level, stress conditions result in elevation of reactive oxygen species (ROS) production causing oxidative stress associated with oxidation of proteins and nucleic acids as well as impairment of membrane functions. Adaptation of root growth to ROS accumulation is facilitated through modification of auxin and cytokinin hormone homeostasis. Here, we report that in Arabidopsis root meristem, ROS-induced changes of auxin levels correspond to decreased abundance of PIN auxin efflux carriers at the plasma membrane (PM). Specifically, increase in H2O2 levels affects PIN2 endocytic recycling. We show that the PIN2 intracellular trafficking during adaptation to oxidative stress requires the function of the ADP-ribosylation factor (ARF)-guanine-nucleotide exchange factor (GEF) BEN1, an actin-associated regulator of the trafficking from the PM to early endosomes and, presumably, indirectly, trafficking to the vacuoles. We propose that H2O2 levels affect the actin dynamics thus modulating ARF-GEF-dependent trafficking of PIN2. This mechanism provides a way how root growth acclimates to stress and adapts to a changing environment.

In vivo detection of protein cysteine sulfenylation in plastids

Protein cysteine thiols are post-translationally modified under oxidative stress conditions. Illuminated chloroplasts are one of the important sources of hydrogen peroxide (H₂ O₂ ) and are highly sensitive to environmental stimuli, yet a comprehensive view of the oxidation-sensitive chloroplast proteome is still missing. By targeting the sulfenic acid YAP1C-trapping technology to the plastids of light-grown Arabidopsis cells, we identified 132 putatively sulfenylated plastid proteins upon H₂ O₂ pulse treatment. Almost half of the sulfenylated proteins are enzymes of the amino acid metabolism. Using metabolomics, we observed a reversible decrease in the levels of the amino acids Ala, Asn, Cys, Gln, Glu, His, Ile, Leu, Lys, Phe, Ser, Thr and Val after H₂ O₂ treatment, which is in line with an anticipated decrease in the levels of the glycolysis and tricarboxylic acid metabolites. Through the identification of an organelle-tailored proteome, we demonstrated that the subcellular targeting of the YAP1C probe enables us to study in vivo cysteine sulfenylation at the organellar level. All in all, the identification of these oxidation events in plastids revealed that several enzymes of the amino acid metabolism rapidly undergo cysteine oxidation upon oxidative stress.

Consequences of Oxidative Stress on Plant Glycolytic and Respiratory Metabolism

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are present at low and controlled levels under normal conditions. These reactive molecules can increase to high levels under various biotic and abiotic conditions, resulting in perturbation of the cellular redox state that can ultimately lead to oxidative or nitrosative stress. In this review, we analyze the various effects that result from alterations of redox homeostasis on plant glycolytic pathway and tricarboxylic acid (TCA) cycle. Most documented modifications caused by ROS or RNS are due to the presence of redox-sensitive cysteine thiol groups in proteins. Redox modifications include Cys oxidation, disulfide bond formation, S-glutathionylation, S-nitrosylation, and S-sulfhydration. A growing number of proteomic surveys and biochemical studies document the occurrence of ROS- or RNS-mediated modification in enzymes of glycolysis and the TCA cycle. In a few cases, these modifications have been shown to affect enzyme activity, suggesting an operational regulatory mechanism in vivo. Further changes induced by oxidative stress conditions include the proposed redox-dependent modifications in the subcellular distribution of a putative redox sensor, NAD-glyceraldehyde-3P dehydrogenase and the micro-compartmentation of cytosolic glycolytic enzymes. Data from the literature indicate that oxidative stress may induce complex changes in metabolite pools in central carbon metabolism. This information is discussed in the context of our understanding of plant metabolic response to oxidative stress.

Hydrogen peroxide positively regulates brassinosteroid signaling through oxidation of the BRASSINAZOLE-RESISTANT1 transcription factor

Hydrogen peroxide (H₂O₂) is an important signaling molecule in plant developmental processes and stress responses. However, whether H₂O₂-mediated signaling crosstalks with plant hormone signaling is largely unclear. Here, we show that H₂O₂ induces the oxidation of the BRASSINAZOLE-RESISTANT1 (BZR1) transcription factor, which functions as a master regulator of brassinosteroid (BR) signaling. Oxidative modification enhances BZR1 transcriptional activity by promoting its interaction with key regulators in the auxin-signaling and light-signaling pathways, including AUXIN RESPONSE FACTOR6 (ARF6) and PHYTOCHROME INTERACTING FACTOR4 (PIF4). Genome-wide analysis shows that H₂O₂-dependent regulation of BZR1 activity plays a major role in modifying gene expression related to several BR-mediated biological processes. Furthermore, we show that the thioredoxin TRXh5 can interact with BZR1 and catalyzes its reduction. We conclude that reversible oxidation of BZR1 connects H₂O₂-mediated and thioredoxin-mediated redox signaling to BR signaling to regulate plant development.

Hydrogen peroxide and brassinosteroids (BR) both regulate plant development and stress responses. Here Tian et al. show that hydrogen peroxide can trigger oxidation of the BR-responsive BZR1 transcription factor and promote its transcriptional activity, thereby linking BR and redox signaling.

Constitutive redox and phosphoproteome changes in multiple herbicide resistant Avena fatua L. are similar to those of systemic acquired resistance and systemic acquired acclimation

Plants are routinely confronted with numerous biotic and abiotic stressors, and in response have evolved highly effective strategies of systemic acquired resistance (SAR) and systemic acquired acclimation (SAA), respectively. A much more evolutionarily recent abiotic stress is the application of herbicides to control weedy plants, and their intensive use has selected for resistant weed populations that cause substantial crop yield losses and increase production costs. Non-target site resistance (NTSR) to herbicides is rapidly increasing worldwide and is associated with alterations in generalized stress defense networks. This work investigated protein post-translational modifications associated with NTSR in multiple herbicide resistant (MHR) Avena fatua, and their commonalities with those of SAR and SAA. We used proteomic, biochemical, and immunological approaches to compare constitutive protein profiles in MHR and herbicide susceptible (HS) A. fatua populations. Phosphoproteome and redox proteome surveys showed that post-translational modifications of proteins with functions in core cellular processes were reduced in MHR plants, while those involved in xenobiotic and stress response, reactive oxygen species detoxification and redox maintenance, heat shock response, and intracellular signaling were elevated in MHR as compared to HS plants. More specifically, MHR plants contained constitutively elevated levels of three protein kinases including the lectin S-receptor-like serine/threonine-protein kinase LecRK2, a well-characterized component of SAR. Analyses of superoxide dismutase enzyme activity and protein levels did not reveal constitutive differences between MHR and HS plants. The overall results support the idea that herbicide stress is perceived similarly to other abiotic stresses, and that A. fatua NTSR shares analogous features with SAR and SAA. We speculate that MHR A. fatua's previous exposure to sublethal herbicide doses, as well as earlier evolution under a diversity of abiotic and biotic stressors, has led to a heightened state of stress preparedness that includes NTSR to a number of unrelated herbicides.

Hydrogen Peroxide Response in Leaves of Poplar (Populus simonii × Populus nigra) Revealed from Physiological and Proteomic Analyses

Hydrogen peroxide (H₂O₂) is one of the most abundant reactive oxygen species (ROS), which plays dual roles as a toxic byproduct of cell metabolism and a regulatory signal molecule in plant development and stress response. Populus simonii × Populus nigra is an important cultivated forest species with resistance to cold, drought, insect and disease, and also a key model plant for forest genetic engineering. In this study, H₂O₂ response in P. simonii × P. nigra leaves was investigated using physiological and proteomics approaches. The seedlings of 50-day-old P. simonii × P. nigra under H₂O₂ stress exhibited stressful phenotypes, such as increase of in vivo H₂O₂ content, decrease of photosynthetic rate, elevated osmolytes, antioxidant accumulation, as well as increased activities of several ROS scavenging enzymes. Besides, 81 H₂O₂-responsive proteins were identified in the poplar leaves. The diverse abundant patterns of these proteins highlight the H₂O₂-responsive pathways in leaves, including 14-3-3 protein and nucleoside diphosphate kinase (NDPK)-mediated signaling, modulation of thylakoid membrane structure, enhancement of various ROS scavenging pathways, decrease of photosynthesis, dynamics of proteins conformation, and changes in carbohydrate and other metabolisms. This study provides valuable information for understanding H₂O₂-responsive mechanisms in leaves of P. simonii × P. nigra.

Role of Reactive Oxygen Species during Cell Expansion in Leaves

Reactive oxygen species as potent regulators of leaf development poses special interest for cell expansion.

Identification of dimedone-trapped sulfenylated proteins in plants under stress

In stressed plants, the reactive oxygen species (ROS) levels rise. Key to ROS signaling research are detection and identification of the protein cysteine sulfenylation (-SOH), the ROS-mediated oxidative product of a thiol (-SH). Arabidopsis thaliana seedlings were stressed with hydrogen peroxide (H₂O₂) and the sulfenylated proteins were tagged with dimedone. Dimedone-tagged sulfenic acid proteins were visualized on a two-dimensional electrophoresis (2DE) immunoblot with an anticysteine sulfenic acid antibody and were subsequently detected by mass spectrometry. We optimized the detection method for protein sulfenylation in Arabidopsis. We conclude that dimedone can penetrate the cell wall, does not stress plants, and can "read" the changes in the protein sulfenylation pattern under oxidative stress. We observed that the number of sulfenylated proteins in plants treated with 10 mM H₂O₂ was higher than that in untreated plants. A total of 39 sulfenylated protein spots were found on 2DE immunoblots. By means of mass spectrometry, 11 sulfenylated proteins were discovered involved in primary metabolism, redox regulation, translation and signaling pathways. Hence, by combining an immunochemical 2DE strategy with mass spectrometry, we were able to identify sulfenylated proteins in H₂O₂-stressed Arabidopsis seedlings. The sulfenylated proteins can be considered for further validation as redox regulators in plants.

A low-temperature-responsive element involved in the regulation of the Arabidopsis thaliana At1g71850/At1g71860 divergent gene pair

The bidirectional promoter of the Arabidopsis thaliana gene pair At1g71850/At1g71860 harbors low-temperature-responsive elements, which participate in anti-correlated transcription regulation of the driving genes in response to environmental low temperature. A divergent gene pair is defined as two adjacent genes organized head to head in opposite orientation, sharing a common promoter region. Divergent gene pairs are mainly coexpressed, but some display opposite regulation. The mechanistic basis of such anti-correlated regulation is not well understood. Here, the regulation of the Arabidopsis thaliana gene pair At1g71850/At1g71860 was investigated. Semi-quantitative RT-PCR and Genevestigator analyses showed that while one of the pair was upregulated by exposure to low temperature, the same treatment downregulated the other. Promoter::GUS fusion transgenes were used to show that this behavior was driven by a bidirectional promoter, which harbored an as-1 motif, associated with the low-temperature response; mutation of this sequence produced a significant decrease in cold-responsive expression. With regard to the as-1 motif in the native orientation repressing the promoter's low-temperature responsiveness, the same as-1 motif introduced in the reverse direction showed a slight enhancement in the promoter's responsiveness to low-temperature exposure, indicating that the orientation of the motif was important for the promoter's activity. These findings provide new insights into the complex transcriptional regulation of bidirectional gene pairs as well as plant stress response.

Tuning Cysteine Reactivity and Sulfenic Acid Stability by Protein Microenvironment in Glyceraldehyde-3-Phosphate Dehydrogenases of Arabidopsis thaliana

AIMS

Cysteines and H2O2 are fundamental players in redox signaling. Cysteine thiol deprotonation favors the reaction with H2O2 that generates sulfenic acids with dual electrophilic/nucleophilic nature. The protein microenvironment surrounding the target cysteine is believed to control whether sulfenic acid can be reversibly regulated by disulfide formation or irreversibly oxidized to sulfinates/sulfonates. In this study, we present experimental oxidation kinetics and a quantum mechanical/molecular mechanical (QM/MM) investigation to elucidate the reaction of H2O2 with glycolytic and photosynthetic glyceraldehyde-3-phosphate dehydrogenase from Arabidopsis thaliana (cytoplasmic AtGAPC1 and chloroplastic AtGAPA, respectively).

RESULTS

Although AtGAPC1 and AtGAPA have almost identical 3D structure and similar acidity of their catalytic Cys149, AtGAPC1 is more sensitive to H2O2 and prone to irreversible oxidation than AtGAPA. As a result, sulfenic acid is more stable in AtGAPA.

INNOVATION

Based on crystallographic structures of AtGAPC1 and AtGAPA, the reaction potential energy surface for Cys149 oxidation by H2O2 was calculated by QM. In both enzymes, sulfenic acid formation was characterized by a lower energy barrier than sulfinate formation, and sulfonate formation was prevented by very high energy barriers. Activation energies for both oxidation steps were lower in AtGAPC1 than AtGAPA, supporting the higher propensity of AtGAPC1 toward irreversible oxidation.

CONCLUSIONS

QM/MM calculations coupled to fingerprinting analyses revealed that two Arg of AtGAPA (substituted by Gly and Val in AtGAPC1), located at 8-15 Å distance from Cys149, are the major factors responsible for sulfenic acid stability, underpinning the importance of long-distance polar interactions in tuning sulfenic acid stability in native protein microenvironments.

Changes in protein expression of pacific oyster Crassostrea gigas exposed in situ to urban sewage

The composition and concentration of substances in urban effluents are complex and difficult to measure. These contaminants elicit biological responses in the exposed organisms. Proteomic analysis is a powerful tool in environmental toxicology by evidencing alterations in protein expression due to exposure to contaminants and by providing a useful framework for the development of new potential biomarkers. The aim of this study was to determine changes in protein expression signatures (PES) in the digestive gland of oysters Crassostrea gigas transplanted to two farming areas (LIS and RIB) and to one area contaminated by sanitary sewage (BUC) after 14 days of exposure. This species is one of the most cultivated molluscs in the world. The identified proteins are related to the cytoskeleton (CKAP5 and ACT2), ubiquitination pathway conjugation (UBE3C), G protein-coupled receptor and signal transduction (SVEP1), and cell cycle/division (CCNB3). CKAP5 showed higher expression in oysters kept at BUC in comparison with those kept at the farming areas, while ACT2, UBE3C, SVEP1, and CCNB3 were suppressed. The results suggest that these changes might lead to DNA damage, apoptosis, and interference with the immune system in oyster C. gigas exposed to sewage and give initial information on PES of C. gigas exposed to sanitary sewage, which can subsequently be useful in the development of more sensitive tools for biomonitoring coastal areas, particularly those devoted mainly to oyster farming activities.

Cysteines under ROS attack in plants: a proteomics view

Plants generate reactive oxygen species (ROS) as part of their metabolism and in response to various external stress factors, potentially causing significant damage to biomolecules and cell structures. During the course of evolution, plants have adapted to ROS toxicity, and use ROS as signalling messengers that activate defence responses. Cysteine (Cys) residues in proteins are one of the most sensitive targets for ROS-mediated post-translational modifications, and they have become key residues for ROS signalling studies. The reactivity of Cys residues towards ROS, and their ability to react to different oxidation states, allow them to appear at the crossroads of highly dynamic oxidative events. As such, a redox-active cysteine can be present as S-glutathionylated (-SSG), disulfide bonded (S-S), sulfenylated (-SOH), sulfinylated (-SO2H), and sulfonylated (-SO3H). The sulfenic acid (-SOH) form has been considered as part of ROS-sensing pathways, as it leads to further modifications which affect protein structure and function. Redox proteomic studies are required to understand how and why cysteines undergo oxidative post-translational modifications and to identify the ROS-sensor proteins. Here, we update current knowledge of cysteine reactivity with ROS. Further, we give an overview of proteomic techniques that have been applied to identify different redox-modified cysteines in plants. There is a particular focus on the identification of sulfenylated proteins, which have the potential to be involved in plant signal transduction.

Quantifying reversible oxidation of protein thiols in photosynthetic organisms

Photosynthetic organisms use dynamic post-translational modifications to survive and adapt, which include reversible oxidative modifications of protein thiols that regulate protein structure, function, and activity. Efforts to quantify thiol modifications on a global scale have relied upon peptide derivatization, typically using isobaric tags such as TMT, ICAT, or iTRAQ that are more expensive, less accurate, and provide less proteome coverage than label-free approaches--suggesting the need for improved experimental designs for studies requiring maximal coverage and precision. Herein, we present the coverage and precision of resin-assisted thiol enrichment coupled to label-free quantitation for the characterization of reversible oxidative modifications on protein thiols. Using C. reinhardtii and Arabidopsis as model systems for algae and plants, we quantified 3662 and 1641 unique cysteinyl peptides, respectively, with median coefficient of variation (CV) of 13% and 16%. Further, our method is extendable for the detection of protein abundance changes and stoichiometries of cysteine oxidation. Finally, we demonstrate proof-of-principle for our method, and reveal that exogenous hydrogen peroxide treatment regulates the C. reinhardtii redox proteome by increasing or decreasing the level of oxidation of 501 or 67 peptides, respectively. As protein activity and function is controlled by oxidative modifications on protein thiols, resin-assisted thiol enrichment coupled to label-free quantitation can reveal how intracellular and environmental stimuli affect plant survival and fitness through oxidative stress.

Proteomic identification of early salicylate- and flg22-responsive redox-sensitive proteins in Arabidopsis

Accumulation of reactive oxygen species (ROS) is one of the early defense responses against pathogen infection in plants. The mechanism about the initial and direct regulation of the defense signaling pathway by ROS remains elusive. Perturbation of cellular redox homeostasis by ROS is believed to alter functions of redox-sensitive proteins through their oxidative modifications. Here we report an OxiTRAQ-based proteomic study in identifying proteins whose cysteines underwent oxidative modifications in Arabidopsis cells during the early response to salicylate or flg22, two defense pathway elicitors that are known to disturb cellular redox homeostasis. Among the salicylate- and/or flg22-responsive redox-sensitive proteins are those involved in transcriptional regulation, chromatin remodeling, RNA processing, post-translational modifications, and nucleocytoplasmic shuttling. The identification of the salicylate-/flg22-responsive redox-sensitive proteins provides a foundation from which further study can be conducted toward understanding biological significance of their oxidative modifications during the plant defense response.

DYn-2 Based Identification of Arabidopsis Sulfenomes

Identifying the sulfenylation state of stressed cells is emerging as a strategic approach for the detection of key reactive oxygen species signaling proteins. Here, we optimized an in vivo trapping method for cysteine sulfenic acids in hydrogen peroxide (H2O2) stressed plant cells using a dimedone based DYn-2 probe. We demonstrated that DYn-2 specifically detects sulfenylation events in an H2O2 dose- and time-dependent way. With mass spectrometry, we identified 226 sulfenylated proteins after H2O2 treatment of Arabidopsis cells, residing in the cytoplasm (123); plastid (68); mitochondria (14); nucleus (10); endoplasmic reticulum, Golgi and plasma membrane (7) and peroxisomes (4). Of these, 123 sulfenylated proteins have never been reported before to undergo cysteine oxidative post-translational modifications in plants. All in all, with this DYn-2 approach, we have identified new sulfenylated proteins, and gave a first glance on the locations of the sulfenomes of Arabidopsis thaliana.

Redox proteomics of tomato in response to Pseudomonas syringae infection

Unlike mammals with adaptive immunity, plants rely on their innate immunity based on pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) for pathogen defense. Reactive oxygen species, known to play crucial roles in PTI and ETI, can perturb cellular redox homeostasis and lead to changes of redox-sensitive proteins through modification of cysteine sulfhydryl groups. Although redox regulation of protein functions has emerged as an important mechanism in several biological processes, little is known about redox proteins and how they function in PTI and ETI. In this study, cysTMT proteomics technology was used to identify similarities and differences of protein redox modifications in tomato resistant (PtoR) and susceptible (prf3) genotypes in response to Pseudomonas syringae pv tomato (Pst) infection. In addition, the results of the redox changes were compared and corrected with the protein level changes. A total of 90 potential redox-regulated proteins were identified with functions in carbohydrate and energy metabolism, biosynthesis of cysteine, sucrose and brassinosteroid, cell wall biogenesis, polysaccharide/starch biosynthesis, cuticle development, lipid metabolism, proteolysis, tricarboxylic acid cycle, protein targeting to vacuole, and oxidation-reduction. This inventory of previously unknown protein redox switches in tomato pathogen defense lays a foundation for future research toward understanding the biological significance of protein redox modifications in plant defense responses.

ROS-mediated redox signaling during cell differentiation in plants

BACKGROUND

Reactive oxygen species (ROS) have emerged in recent years as important regulators of cell division and differentiation.

SCOPE OF REVIEW

The cellular redox state has a major impact on cell fate and multicellular organism development. However, the exact molecular mechanisms through which ROS manifest their regulation over cellular development are only starting to be understood in plants. ROS levels are constantly monitored and any change in the redox pool is rapidly sensed and responded upon. Different types of ROS cause specific oxidative modifications, providing the basic characteristics of a signaling molecule. Here we provide an overview of ROS sensors and signaling cascades that regulate transcriptional responses in plants to guide cellular differentiation and organ development.

MAJOR CONCLUSIONS

Although several redox sensors and cascades have been identified, they represent only a first glimpse on the impact that redox signaling has on plant development and growth.

GENERAL SIGNIFICANCE

We provide an initial evaluation of ROS signaling cascades involved in cell differentiation in plants and identify potential avenues for future studies. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Proteomic and biochemical analyses show a functional network of proteins involved in antioxidant defense of the Arabidopsis anp2anp3 double mutant

Disentanglement of functional complexity associated with plant mitogen-activated protein kinase (MAPK) signaling has benefited from transcriptomic, proteomic, phosphoproteomic, and genetic studies. Published transcriptomic analysis of a double homozygous recessive anp2anp3 mutant of two MAPK kinase kinase (MAPKKK) genes called Arabidopsis thaliana Homologues of Nucleus- and Phragmoplast-localized Kinase 2 (ANP2) and 3 (ANP3) showed the upregulation of stress-related genes. In this study, a comparative proteomic analysis of anp2anp3 mutant against its respective Wassilevskaja ecotype (Ws) wild type background is provided. Such differential proteomic analysis revealed overabundance of core enzymes such as FeSOD1, MnSOD, DHAR1, and FeSOD1-associated regulatory protein CPN20, which are involved in the detoxification of reactive oxygen species in the anp2anp3 mutant. The proteomic results were validated at the level of single protein abundance by Western blot analyses and by quantitative biochemical determination of antioxidant enzymatic activities. Finally, the functional network of proteins involved in antioxidant defense in the anp2anp3 mutant was physiologically linked with the increased resistance of mutant seedlings against paraquat treatment.

Redox regulation of transcription factors in plant stress acclimation and development

SIGNIFICANCE

The redox regulatory signaling network of the plant cell controls and co-regulates transcriptional activities, thereby enabling adjustment of metabolism and development in response to environmental cues, including abiotic stress.

RECENT ADVANCES

Our rapidly expanding knowledge on redox regulation of plant transcription is driven by methodological advancements such as sensitive redox proteomics and in silico predictions in combination with classical targeted genetic and molecular approaches, often in Arabidopsis thaliana. Thus, transcription factors (TFs) are both direct and indirect targets of redox-dependent activity modulation. Redox control of TF activity involves conformational switching, nucleo-cytosolic partitioning, assembly with coregulators, metal-S-cluster regulation, redox control of upstream signaling elements, and proteolysis.

CRITICAL ISSUES

While the significance of redox regulation of transcription is well established for prokaryotes and non-plant eukaryotes, the momentousness of redox-dependent control of transcription in plants still receives insufficient awareness and, therefore, is discussed in detail in this review.

FUTURE DIRECTIONS

Improved proteome sensitivity will enable characterization of low abundant proteins and to simultaneously address the various post-translational modifications such as nitrosylation, hydroxylation, and glutathionylation. Combining such approaches by gradually increasing biotic and abiotic stress strength is expected to result in a systematic understanding of redox regulation. In the end, only the combination of in vivo, ex vivo, and in vitro results will provide conclusive pictures on the rather complex mechanism of redox regulation of transcription.

Chloroplastic thioredoxin-f and thioredoxin-m1/4 play important roles in brassinosteroids-induced changes in CO2 assimilation and cellular redox homeostasis in tomato

Chloroplast thioredoxins (TRXs) and glutathione function as redox messengers in the regulation of photosynthesis. In this work, the roles of chloroplast TRXs in brassinosteroids (BRs)-induced changes in cellular redox homeostasis and CO2 assimilation were studied in the leaves of tomato plants. BRs-deficient d (^im) plants showed decreased transcripts of TRX-f, TRX-m2, TRX-m1/4, and TRX-x, while exogenous BRs significantly induced CO2 assimilation and the expression of TRX-f, TRX-m2, TRX-m1/4, and TRX-x. Virus-induced gene silencing (VIGS) of the chloroplast TRX-f, TRX-m2, TRX-m1/4, and TRX-y genes individually increased membrane lipid peroxidation and accumulation of 2-Cys peroxiredoxin dimers, and decreased the activities of the ascorbate-glutathione cycle enzymes and the ratio of reduced glutathione to oxidized glutathione (GSH/GSSG) in the leaves. Furthermore, partial silencing of TRX-f, TRX-m2, TRX-m1/4, and TRX-y resulted in decreased expression of genes involved in the Benson-Calvin cycle and decreased activity of the associated enzymes. Importantly, the BRs-induced increase in CO2 assimilation and the increased expression and activities of antioxidant- and photosynthesis-related genes and enzymes were compromised in the partially TRX-f- and TRX-m1/4-silenced plants. All of these results suggest that TRX-f and TRX-m1/4 are involved in the BRs-induced changes in CO2 assimilation and cellular redox homeostasis in tomato.