Protective function of chloroplast 2-cysteine peroxiredoxin in photosynthesis. Evidence from transgenic Arabidopsis

Reduced GSH Acts as a Metabolic Cue of OPDA Signaling in Coregulating Photosynthesis and Defense Activation under Stress

12-oxo-phytodienoic acid (OPDA) is a primary precursor of jasmonates, able to trigger autonomous signaling cascades that activate and fine-tune plant defense responses, as well as growth and development. However, its mechanism of actions remains largely elusive. Here we describe a dual-function messenger of OPDA signaling, reduced glutathione (GSH), that cross-regulates photosynthesis machinery and stress protection/adaptation in concert, optimizing plant plasticity and survival potential. Under stress conditions, the rapid induction of OPDA production stimulates GSH accumulation in the chloroplasts, and in turn leads to protein S-glutathionylation in modulating the structure and function of redox-sensitive enzymes such as 2-cysteine (Cys) peroxiredoxin A (2CPA), a recycler in the water-water cycle. GSH exchanges thiol-disulfides with the resolving CysR175, while donating an electron (e-, H+) to the peroxidatic CysP53, of 2CPA, which revives its reductase activity and fosters peroxide detoxification in photosynthesis. The electron flow protects photosynthetic processes (decreased total non-photochemical quenching, NPQ(T)) and maintains its efficiency (increased photosystem II quantum yield, ΦII). On the other hand, GSH also prompts retrograde signaling from the chloroplasts to the nucleus in adjusting OPDA-responsive gene expressions such as Glutathione S-Transferase 6 (GST6) and GST8, and actuating defense responses against various ecological constraints such as salinity, excess oxidants and light, as well as mechanical wounding. We thus propose that OPDA regulates a unique metabolic switch that interfaces light and defense signaling, where it links cellular and environmental cues to a multitude of plant physiological, e.g., growth, development, recovery, and acclimation, processes.

Exogenous melatonin enhances the reactive oxygen species metabolism, antioxidant defense-related gene expression, and photosynthetic capacity of Phaseolus vulgaris L. to confer salt stress tolerance

Melatonin (MT) has been reported to regulate certain plant physiological processes and promote tolerance to different environmental stresses such as salinity. Green bean (Phaseolus vulgaris L. cv. Royal Nel) seedlings were exposed to 200 mM NaCl with or without pre-treatment with 150 μM MT. Salt stress led to a lower chlorophyll content, a reduced photosynthetic activity, increased reactive oxygen species (ROS) contents, and decreased photosystem II (PSII) activity. The application of exogenous MT to green bean seedlings under salt stress improved photosynthetic activity and alleviated the oxidative damages by enhancing the activity of antioxidant enzymes. The expression of catalase (CAT1), glutathione reductase (GR), superoxide dismutase (CuZnSOD1), ascorbate peroxidase (APX), Peroxiredoxin Q (PrxQ), and 2-cysteine peroxiredoxin (2-Cys-Prx) encoding genes was significantly increased under salt stress in green bean seedling compared with the untreated control. However, plants treated with exogenous MT and NaCl had 28.8, 21.1, 26.1, 20, 26.2, and 22.4% higher CuZnSOD, CAT1, APX, GR, PrxQ, and 2-Cys-Prx transcript levels, respectively, compared to NaCl stress alone. Our study revealed the protective mechanisms mediated by exogenous MT application in NaCl stress alleviation and our findings could be used in the management of green bean cultivation in salinity-prone soils.

Roles of Exogenous α-Lipoic Acid and Cysteine in Mitigation of Drought Stress and Restoration of Grain Quality in Wheat

Cysteine (Cys) and α-lipoic acid (ALA) are naturally occurring antioxidants (sulfur-containing compounds) that can protect plants against a wide spectrum of environmental stresses. However, up to now, there are no conclusive data on their integrative roles in mitigation of drought stress in wheat plants. Here, we studied the influence of ALA at 0.02 mM (grain dipping pre-cultivation treatment) and Cys (25 and 50 ppm as a foliar application) under well watered and deficit irrigation (100% and 70% of recommended dose). The results showed that deficit irrigation markedly caused obvious cellular oxidative damage as indicated by elevating the malondialdehyde (MDA) and hydrogen peroxide content (H₂O₂). Moreover, water stressed plants exhibited multiple changes in physiological metabolism, which affected the quantitative and qualitative variables of grain yield. The enzymatic antioxidants, including superoxide dismutase (SOD), ascorbate peroxidase (APX), catalase (CAT) and peroxidase (POX) were improved by Cys application. SOD and APX had the same response when treated with ALA, but CAT and POX did not. Moreover, both studied molecules stimulated chlorophyll (Chl) and osmolytes' biosynthesis. In contrast, the Chl a/b ratio was decreased, while flavonoids were not affected by either of the examined molecules. Interestingly, all above-mentioned changes were associated with an improvement in the scavenging capacity of reactive oxygen species (ROS), leaf relative water content (RWC), grain number, total grain yield, weight of 1000 kernels, gluten index, falling number, and alveographic parameters (P, W, and P/L values). Furthermore, heatmap plot analysis revealed several significant correlations between different studied parameters, which may explore the importance of applied Cys and ALA as effective compounds in wheat cultivation under water deficit conditions.

iTRAQ-based proteome profiling revealed the role of Phytochrome A in regulating primary metabolism in tomato seedling

In plants, during growth and development, photoreceptors monitor fluctuations in their environment and adjust their metabolism as a strategy of surveillance. Phytochromes (Phys) play an essential role in plant growth and development, from germination to fruit development. FR-light (FR) insensitive mutant (fri) carries a recessive mutation in Phytochrome A and is characterized by the failure to de-etiolate in continuous FR. Here we used iTRAQ-based quantitative proteomics along with metabolomics to unravel the role of Phytochrome A in regulating central metabolism in tomato seedlings grown under FR. Our results indicate that Phytochrome A has a predominant role in FR-mediated establishment of the mature seedling proteome. Further, we observed temporal regulation in the expression of several of the late response proteins associated with central metabolism. The proteomics investigations identified a decreased abundance of enzymes involved in photosynthesis and carbon fixation in the mutant. Profound accumulation of storage proteins in the mutant ascertained the possible conversion of sugars into storage material instead of being used or the retention of an earlier profile associated with the mature embryo. The enhanced accumulation of organic sugars in the seedlings indicates the absence of photomorphogenesis in the mutant.

Wheat 2-Cys peroxiredoxin plays a dual role in chlorophyll biosynthesis and adaptation to high temperature

The molecular mechanism of high-temperature stress (HTS) response, in plants, has so far been investigated using transcriptomics, while the dynamics of HTS-responsive proteome remain unexplored. We examined the adaptive responses of the resilient wheat cultivar 'Unnat Halna' and dissected the HTS-responsive proteome landscape. This led to the identification of 55 HTS-responsive proteins (HRPs), which are predominantly involved in metabolism and defense pathways. Interestingly, HRPs included a 2-cysteine peroxiredoxin (2CP), designated Ta2CP, presumably involved in stress perception and adaptation. Complementation of Ta2CP in yeast and heterologous expression in Arabidopsis demonstrated its role in thermotolerance. Both Ta2CP silencing and overexpression inferred the involvement of Ta2CP in plant growth and chlorophyll biosynthesis. We demonstrated that Ta2CP interacts with protochlorophyllide reductase b, TaPORB. Reduced TaPORB expression was found in Ta2cp-silenced plants, while upregulation was observed in Ta2CP-overexpressed plants. Furthermore, the downregulation of Ta2CP in Taporb-silenced plants and reduction of protochlorophyllide in Ta2cp-silenced plants suggested the key role of Ta2CP in chlorophyll metabolism. Additionally, the transcript levels of AGPase1 and starch were increased in Ta2cp-silenced plants. More significantly, HTS-treated Ta2cp-silenced plants showed adaptive responses despite increased reactive oxygen species and peroxide concentrations, which might help in rapid induction of high-temperature acclimation.

Recent Progress in the Study of Peroxiredoxin in the Harmful Algal Bloom Species Chattonella marina

Peroxiredoxin (Prx) is a relatively recently discovered antioxidant enzyme family that scavenges peroxides and is known to be present in organisms from biological taxa ranging from bacteria to multicellular eukaryotes, including photosynthetic organisms. Although there have been many studies of the Prx family in higher plants, green algae, and cyanobacteria, few studies have concerned raphidophytes and dinoflagellates, which are among the eukaryotic algae that cause harmful algal blooms (HABs). In our proteomic study using 2-D electrophoresis, we found a highly expressed 2-Cys peroxiredoxin (2-CysPrx) in the raphidophyte Chattonella marina var. antiqua, a species that induces mass mortality of aquacultured fish. The abundance of the C. marina 2-CysPrx enzyme was highest in the exponential growth phase, during which photosynthetic activity was high, and it then decreased by about a factor of two during the late stationary growth phase. This pattern suggested that 2-CysPrx is a key enzyme involved in the maintenance of high photosynthesis activity. In addition, the fact that the depression of photosynthesis by excessively high irradiance was more severe in the 2-CysPrx low-expression strain (wild type) than in the normal-expression strain (wild type) of C. marina suggested that 2-CysPrx played a critical role in protecting the cell from oxidative stress caused by exposure to excessively high irradiance. In the field of HAB research, estimates of growth potential have been desired to predict the population dynamics of HABs for mitigating damage to fisheries. Therefore, omics approaches have recently begun to be applied to elucidate the physiology of the growth of HAB species. In this review, we describe the progress we have made using a molecular physiological approach to identify the roles of 2-CysPrx and other antioxidant enzymes in mitigating environmental stress associated with strong light and high temperatures and resultant oxidative stress. We also describe results of a survey of expressed Prx genes and their growth-phase-dependent behavior in C. marina using RNA-seq analysis. Finally, we speculate about the function of these genes and the ecological significance of 2-CysPrx, such as its involvement in circadian rhythms and the toxicity of C. marina to fish.

Genome-wide identification, evolution, expression, and alternative splicing profiles of peroxiredoxin genes in cotton

Peroxiredoxin (PRX) is a ubiquitous thioredoxin-dependent peroxidase that can eliminate excessive free radicals produced by stress and protect cells from oxidative damage. PRXs are also involved in reactive oxygen species (ROS)- and redox-dependent signaling by performing redox interactions with other proteins and modify their redox status. At present, PRX family identification, evolution and regulation research has been conducted in some plants; however, systematic research about this family is lacking in cotton. In this study, a total of 44 PRXs were identified in the cotton genome. Phylogenetic and conserved active site analyses showed that the PRXs were divided into six subfamilies according to the conserved site (PxxxTxxC…S…W/F) and conserved cysteinyl residues positions. Segmental duplication and polyploid events were the main methods for PRX family expansion, and the PRXs of diploid G. arboreum were the donors of PRXs in the D subgenomes of allotetraploid G. hirsutum and G. barbadense during the evolution of the PRX family. qRT-PCR analysis confirmed that cis-acting elements play important roles in regulating the expression of PRXs. Alternative splicing events occurred in GhPRX14-D that can increased the complexity of transcripts in G. hirsutum. Subcellular localization showed that most PRX members were located in chloroplasts, the cytoplasmic membrane and the nucleus. Our results provide systematic support for a better understanding of PRXs in cotton and a starting point for further studies of the specific functions of PRXs in cotton.

CYP20-3 deglutathionylates 2-CysPRX A and suppresses peroxide detoxification during heat stress

In plants, growth-defense trade-offs occur because of limited resources, which demand prioritization towards either of them depending on various external and internal factors. However, very little is known about molecular mechanisms underlying their occurrence. Here, we describe that cyclophilin 20-3 (CYP20-3), a 12-oxo-phytodienoic acid (OPDA)-binding protein, crisscrosses stress responses with light-dependent electron reactions, which fine-tunes activities of key enzymes in plastid sulfur assimilations and photosynthesis. Under stressed states, OPDA, accumulates in the chloroplasts, binds and stimulates CYP20-3 to convey electrons towards serine acetyltransferase 1 (SAT1) and 2-Cys peroxiredoxin A (2CPA). The latter is a thiol-based peroxidase, protecting and optimizing photosynthesis by reducing its toxic byproducts (e.g., H2O2). Reduction of 2CPA then inactivates its peroxidase activity, suppressing the peroxide detoxification machinery, whereas the activation of SAT1 promotes thiol synthesis and builds up reduction capacity, which in turn triggers the retrograde regulation of defense gene expressions against abiotic stress. Thus, we conclude that CYP20-3 is a unique metabolic hub conveying resource allocations between plant growth and defense responses (trade-offs), ultimately balancing optimal growth phonotype.

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.

Overexpression of Tamarix hispida ThTrx5 Confers Salt Tolerance to Arabidopsis by Activating Stress Response Signals

Salt stress inhibits normal plant growth and development by disrupting cellular water absorption and metabolism. Therefore, understanding plant salt tolerance mechanisms should provide a theoretical basis for developing salt-resistant varieties. Here, we cloned ThTrx5 from Tamarix hispida, a salt-resistant woody shrub, and generated ThTrx5-overexpressing transgenic Arabidopsis thaliana lines. Under NaCl stress, the germination rate of overexpressing ThTrx5 lines was significantly increased relative to that of the nontransgenic line; under salt stress, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and glutathione levels and root length and fresh weight values of transgenic ThTrx5 plants were significantly greater than corresponding values for wild-type plants. Moreover, with regard to the transcriptome, comparison of differential gene expression of transgenic versus nontransgenic lines at 0 h and 3 h of salt stress exposure revealed 500 and 194 differentially expressed genes (DEGs), respectively, that were mainly functionally linked to catalytic activity and binding process. Pull-down experiments showed that ThTrx bound 2-Cys peroxiredoxin BAS1-like protein that influences stress response-associated redox, hormone signal transduction, and transcription factor functions. Therefore, this work provides important insights into ThTrx5 mechanisms that promote salt tolerance in plants.

Variability in the redox status of plant 2-Cys peroxiredoxins in relation to species and light cycle

Plant 2-Cys peroxiredoxins (2-CysPRXs) are abundant plastidial thiol-peroxidases involved in key signaling processes such as photosynthesis deactivation at night. Their functions rely on the redox status of their two cysteines and on the enzyme quaternary structure, knowledge of which remains poor in plant cells. Using ex vivo and biochemical approaches, we thoroughly characterized the 2-CysPRX dimer/monomer distribution, hyperoxidation level, and thiol content in Arabidopsis, barley, and potato in relation to the light cycle. Our data reveal that the enzyme hyperoxidization level and its distribution as a dimer and monomer vary through the light cycle in a species-dependent manner. A differential susceptibility to hyperoxidation was observed for the two Arabidopsis 2-CysPRX isoforms and among the proteins of the three species, and was associated to sequence variation in hyperoxidation resistance motifs. Alkylation experiments indicate that only a minor fraction of the 2-CysPRX pool carries one free thiol in the three species, and that this content does not change during the light period. We conclude that most plastidial 2-CysPRX forms are oxidized and propose that there is a species-dependent variability in their functions since dimer and hyperoxidized forms fulfill distinct roles regarding direct oxidation of partners and signal transmission.

Preparing plants for improved cold tolerance by priming

Cold is a major stressor, which limits plant growth and development in many parts of the world, especially in the temperate climate zones. A large number of experimental studies has demonstrated that not only acclimation and entrainment but also the experience of single short stress events of various abiotic or biotic kinds (priming stress) can improve the tolerance of plants to chilling temperatures. This process, called priming, depends on a stress "memory". It does not change cold sensitivity per se but beneficially modifies the response to cold and can last for days, months, or even longer. Elicitor factors and antagonists accumulate due to increased biosynthesis or decreased degradation either during or after the priming stimulus. Comparison of priming studies investigating improved tolerance to chilling temperatures highlighted key regulatory functions of ROS/RNS and antioxidant enzymes, plant hormones, especially jasmonates, salicylates, and abscisic acid, and signalling metabolites, such as β- and γ-aminobutyric acid (BABA and GABA) and melatonin. We conclude that these elicitors and antagonists modify local and systemic cold tolerance by integration into cold-induced signalling cascades.

The chloroplast 2-cysteine peroxiredoxin functions as thioredoxin oxidase in redox regulation of chloroplast metabolism

Thiol-dependent redox regulation controls central processes in plant cells including photosynthesis. Thioredoxins reductively activate, for example, Calvin-Benson cycle enzymes. However, the mechanism of oxidative inactivation is unknown despite its importance for efficient regulation. Here, the abundant 2-cysteine peroxiredoxin (2-CysPrx), but not its site-directed variants, mediates rapid inactivation of reductively activated fructose-1,6-bisphosphatase and NADPH-dependent malate dehydrogenase (MDH) in the presence of the proper thioredoxins. Deactivation of phosphoribulokinase (PRK) and MDH was compromised in 2cysprxAB mutant plants upon light/dark transition compared to wildtype. The decisive role of 2-CysPrx in regulating photosynthesis was evident from reoxidation kinetics of ferredoxin upon darkening of intact leaves since its half time decreased 3.5-times in 2cysprxAB. The disadvantage of inefficient deactivation turned into an advantage in fluctuating light. Physiological parameters like MDH and PRK inactivation, photosynthetic kinetics and response to fluctuating light fully recovered in 2cysprxAB mutants complemented with 2-CysPrxA underlining the significance of 2-CysPrx. The results show that the 2-CysPrx serves as electron sink in the thiol network important to oxidize reductively activated proteins and represents the missing link in the reversal of thioredoxin-dependent regulation.

Suppression of External NADPH Dehydrogenase-NDB1 in Arabidopsis thaliana Confers Improved Tolerance to Ammonium Toxicity via Efficient Glutathione/Redox Metabolism

Environmental stresses, including ammonium (NH₄⁺) nourishment, can damage key mitochondrial components through the production of surplus reactive oxygen species (ROS) in the mitochondrial electron transport chain. However, alternative electron pathways are significant for efficient reductant dissipation in mitochondria during ammonium nutrition. The aim of this study was to define the role of external NADPH-dehydrogenase (NDB1) during oxidative metabolism of NH₄⁺-fed plants. Most plant species grown with NH₄⁺ as the sole nitrogen source experience a condition known as “ammonium toxicity syndrome”. Surprisingly, transgenic Arabidopsis thaliana plants suppressing NDB1 were more resistant to NH₄⁺ treatment. The NDB1 knock-down line was characterized by milder oxidative stress symptoms in plant tissues when supplied with NH₄⁺. Mitochondrial ROS accumulation, in particular, was attenuated in the NDB1 knock-down plants during NH₄⁺ treatment. Enhanced antioxidant defense, primarily concerning the glutathione pool, may prevent ROS accumulation in NH₄⁺-grown NDB1-suppressing plants. We found that induction of glutathione peroxidase-like enzymes and peroxiredoxins in the NDB1-surpressing line contributed to lower ammonium-toxicity stress. The major conclusion of this study was that NDB1 suppression in plants confers tolerance to changes in redox homeostasis that occur in response to prolonged ammonium nutrition, causing cross tolerance among plants.

Physiological Significance of Plant Peroxiredoxins and the Structure-Related and Multifunctional Biochemistry of Peroxiredoxin 1

SIGNIFICANCE

Sessile plants respond to oxidative stress caused by internal and external stimuli by producing diverse forms of enzymatic and nonenzymatic antioxidant molecules. Peroxiredoxins (Prxs) in plants, including the Prx1, Prx5, Prx6, and PrxQ isoforms, constitute a family of antioxidant enzymes and play important functions in cells. Each Prx localizes to a specific subcellular compartment and has a distinct function in the control of plant growth, development, cellular metabolism, and various aspects of defense signaling. Recent Advances: Prx1, a typical Prx in plant chloroplasts, has redox-dependent multiple functions. It acts as a hydrogen peroxide (H₂O₂)-catalyzing peroxidase, a molecular chaperone, and a biological circadian marker. Prx1 undergoes a functional switching from a peroxidase to a molecular chaperone in response to oxidative stress, concomitant with the structural changes from a low-molecular-weight species to high-molecular-weight complexes mediated by the post-translational modification of its active site Cys residues. The redox status of the protein oscillates diurnally between hyperoxidation and reduction, showing a circadian rhythmic output. These dynamic structural and functional transformations mediate the effect of plant Prx1 on protecting plants from a myriad of harsh environmental stresses.

CRITICAL ISSUES

The multifunctional diversity of plant Prxs and their roles in cellular defense signaling depends on their specific interaction partners, which remain largely unidentified. Therefore, the identification of Prx-interacting proteins is necessary to clarify their physiological significance.

FUTURE DIRECTIONS

Since the functional specificity of the four plant Prx isoforms remains unclear, future studies should focus on investigating the physiological importance of each Prx isotype. Antioxid. Redox Signal. 28, 625-639.

Hub Protein Controversy: Taking a Closer Look at Plant Stress Response Hubs

Plant stress responses involve numerous changes at the molecular and cellular level and are regulated by highly complex signaling pathways. Studying protein-protein interactions (PPIs) and the resulting networks is therefore becoming increasingly important in understanding these responses. Crucial in PPI networks are the so-called hubs or hub proteins, commonly defined as the most highly connected central proteins in scale-free PPI networks. However, despite their importance, a growing amount of confusion and controversy seems to exist regarding hub protein identification, characterization and classification. In order to highlight these inconsistencies and stimulate further clarification, this review critically analyses the current knowledge on hub proteins in the plant interactome field. We focus on current hub protein definitions, including the properties generally seen as hub-defining, and the challenges and approaches associated with hub protein identification. Furthermore, we give an overview of the most important large-scale plant PPI studies of the last decade that identified hub proteins, pointing out the lack of overlap between different studies. As such, it appears that although major advances are being made in the plant interactome field, defining hub proteins is still heavily dependent on the quality, origin and interpretation of the acquired PPI data. Nevertheless, many hub proteins seem to have a reported role in the plant stress response, including transcription factors, protein kinases and phosphatases, ubiquitin proteasome system related proteins, (co-)chaperones and redox signaling proteins. A significant number of identified plant stress hubs are however still functionally uncharacterized, making them interesting targets for future research. This review clearly shows the ongoing improvements in the plant interactome field but also calls attention to the need for a more comprehensive and precise identification of hub proteins, allowing a more efficient systems biology driven unraveling of complex processes, including those involved in stress responses.

Comparative Expression Analysis of Rice and Arabidopsis Peroxiredoxin Genes Suggests Conserved or Diversified Roles Between the Two Species and Leads to the Identification of Tandemly Duplicated Rice Peroxiredoxin Genes Differentially Expressed in Seeds

BACKGROUND

Peroxiredoxins (PRXs) have recently been identified as plant antioxidants. Completion of various genome sequencing projects has provided genome-wide information about PRX genes in major plant species. Two of these -- Oryza sativa (rice) and Arabidopsis -- each have 10 PRX members. Although significant progress has been made in understanding their biological roles in Arabidopsis, those functions in rice, a model crop plant, have not been well studied.

RESULTS

We performed a comparative expression analysis of rice and Arabidopsis PRXs. Our phylogenetic analysis revealed that one subgroup contains three rice and three Arabidopsis Type-II PRXs that are expressed ubiquitously. This suggests that they are involved in housekeeping functions to process reactive oxygen species (ROS). Within the second subgroup, expression of Os1-CysPrxA (LOC_Os7g44430) and AtOs1-CysPrx is conserved in seeds while Os1-CysPrxB (LOC_Os7g44440) shows a root-preferential pattern of expression. We used transgenic plants expressing the GUS reporter gene under the control of the promoters of these two tandem duplicates to confirm their meta-expression patterns. Our GUS expression data from developing seeds and those that were germinating indicated that Os1-CysPrxB is involved in root development, as initiated from the embryo, while Os1-CysPrxA has roles in regulating endosperm development near the aleurone layer. For the third and fourth subgroups, the rice PRXs are more likely to show leaf/shoot-preferential expression, while those from Arabidopsis are significantly expressed in the flowers and seeds in addition to the leaf/shoot. To determine the biological meaning of those expression patterns that were dominantly identified in rice PRXs, we analyzed three rice genes showing leaf/shoot-preferential expression in a mutant of the light-responsive 1-deoxy-D-xylulose 5-phosphate reductoisomerase (dxr) gene and found that two of them were significantly down-regulated in the mutant.

CONCLUSION

A global expression analysis of the PRX family in rice identified tandem duplicates, Os1-CysPrxA and Os1-CysPrxB, in the 1-CysPrx subgroup that are differentially expressed in developing seeds and germinating seeds. Analysis of the cis-acting regulatory elements (CREs) revealed unique CREs responsible for embryo and root or endosperm-preferential expression. In addition, the presence of leaf/shoot-preferential PRXs in rice suggests that they are required in that crop because those plants must tolerate a higher light intensity in their normal growth environment when compared with that of Arabidopsis. Downregulation of two PRXs in the dxr mutant causing an albino phenotype, implying that those genes have roles in processing ROS produced during photosynthesis. Network analysis of four PRXs allowed us to model regulatory pathways that explain the underlying protein interaction network. This will be a useful hypothetical model for further study.

Changes in protein abundance and activity involved in freezing tolerance acquisition in winter barley (Hordeum vulgare L.)

The changes in protein abundance induced by cold hardening were analysed by 2 DE in ten doubled haploid (DH) lines of winter barley, highly differentiated with respect to freezing tolerance level. Among 45 differential proteins identified by MALDI-TOF/TOF, the majority was classified as related to photosynthesis, carbohydrate metabolism, oxidation-reduction reactions and stress response. Among the detected proteins, higher abundance of RuBisCO large and small subunits, RuBisCO activase, two Oxygen-evolving enhancer proteins, Ferredoxin-NADP reductase, Cytochrome P450-dependent fatty acid hydroxylase and 14-3-3 protein was associated with higher freezing tolerance level. Lower relative level of hypothetical ATP synthase beta subunit, uncharacterized mitochondrial protein AtMg00810 and ribosomal RNA small subunit methyltransferase G also seems to be important. The results of proteomic studies were complemented by the evaluation of photosynthetic apparatus acclimation, showing distinctive differences between the studied genotypes in the number of active PSII reaction centres (RC/CS_m). Additionally, the analysis of antioxidative enzyme activities suggests the importance of H₂O₂ as a signalling molecule possibly involved in the initiation of cold-induced plant acclimation. However, in DH lines with high freezing tolerance, H₂O₂ generation during cold hardening treatment was accompanied by more stable activity of catalase, H₂O₂-decomposing enzyme.

SIGNIFICANCE

In the study, the changes in protein abundance induced by cold hardening treatment were analysed by two-dimensional gel electrophoresis in ten doubled haploid (DH) lines of winter barley. Harnessing DH technology resulted in distinctive widening of genetic variation with respect to freezing tolerance level. Both the cold-hardening effect on the protein pattern in an individual winter barley DH line as well as the variation among the selected DH lines were investigated, which resulted in the identification of 45 differentiated proteins classified as involved in 14 metabolic pathways and cellular processes. Among them, eight proteins: (1) the precursor of RuBisCO large subunit, (2) RuBisCO small subunit (partial), (3) RuBisCO activase small isoform, (4) the precursor of Oxygen-evolving enhancer protein 1-like (predicted protein), (5) Oxygen-evolving enhancer protein 2, (6) the leaf isozyme of Ferredoxin-NADP reductase, (7) hypothetical protein M569_12509 Cytochrome P450-dependent fatty acid hydroxylase-like and (8) hypothetical protein BRADI_1g11290 (14-3-3 protein A-like) were accumulated to a higher level in leaves of cold-hardened seedlings of freezing tolerant winter barley DH lines in comparison with susceptible ones. Three others: (9) hypothetical protein BRADI_5g05668 F1 ATP synthase beta subunit-like, (10) predicted protein uncharacterized mitochondrial protein AtMg00810-like and (11) BnaA02g08010D Ribosomal RNA small subunit methyltransferase G-like were detected at lower level in freezing tolerant seedlings in comparison with susceptible genotypes. The last two were for the first time linked to cold acclimation. The results of complementary analyses indicate that PSII activity and stability of antioxidative enzymes under low temperature are also very important for freezing tolerance acquisition.

Specificity versus redundancy in the RAP2.4 transcription factor family of Arabidopsis thaliana: transcriptional regulation of genes for chloroplast peroxidases

BACKGROUND

The Arabidopsis ERFIb / RAP2.4 transcription factor family consists of eight members with highly conserved DNA binding domains. Selected members have been characterized individually, but a systematic comparison is pending. The redox-sensitive transcription factor RAP2.4a mediates chloroplast-to-nucleus redox signaling and controls induction of the three most prominent chloroplast peroxidases, namely 2-Cys peroxiredoxin A (2CPA) and thylakoid- and stromal ascorbate peroxidase (tAPx and sAPx). To test the specificity and redundancy of RAP2.4 transcription factors in the regulation of genes for chloroplast peroxidases, we compared the DNA-binding sites of the transcription factors in tertiary structure models, analyzed transcription factor and target gene regulation by qRT-PCR in RAP2.4, 2-Cys peroxiredoxin and ascorbate peroxidase T-DNA insertion lines and RAP2.4 overexpressing lines of Arabidopsis thaliana and performed promoter binding studies.

RESULTS

All RAP2.4 proteins bound the tAPx promoter, but only the four RAP2.4 proteins with identical DNA contact sites, namely RAP2.4a, RAP2.4b, RAP2.4d and RAP2.4h, interacted stably with the redox-sensitive part of the 2CPA promoter. Gene expression analysis in RAP2.4 knockout lines revealed that RAP2.4a is the only one supporting 2CPA and chloroplast APx expression. Rap2.4h binds to the same promoter region as Rap2.4a and antagonizes 2CPA expression. Like the other six RAP2.4 proteins, Rap2.4 h promotes APx mRNA accumulation. Chloroplast ROS signals induced RAP2.4b and RAP2.4d expression, but these two transcription factor genes are (in contrast to RAP2.4a) insensitive to low 2CP availability, and their expression decreased in APx knockout lines. RAP2.4e and RAP2.4f gradually responded to chloroplast APx availability and activated specifically APx expression. These transcription factors bound, like RAP2.4c and RAP2.4g, the tAPx promoter, but hardly the 2CPA promoter.

CONCLUSIONS

The RAP2.4 transcription factors form an environmentally and developmentally regulated transcription factor network, in which the various members affect the expression intensity of the others. Within the transcription factor family, RAP2.4a has a unique function as a general transcriptional activator of chloroplast peroxidase activity. The other RAP2.4 proteins mediate the fine-control and adjust the relative availability of 2CPA, sAPx and tAPx.

The effect of cold priming on the fitness of Arabidopsis thaliana accessions under natural and controlled conditions

Priming improves an organism's performance upon a future stress. To test whether cold priming supports protection in spring and how it is affected by cold acclimation, we compared seven Arabidopsis accessions with different cold acclimation potentials in the field and in the greenhouse for growth, photosynthetic performance and reproductive fitness in March and May after a 14 day long cold-pretreatment at 4 °C. In the plants transferred to the field in May, the effect of the cold pretreatment on the seed yield correlated with the cold acclimation potential of the accessions. In the March transferred plants, the reproductive fitness was most supported by the cold pretreatment in the accessions with the weakest cold acclimation potential. The fitness effect was linked to long-term effects of the cold pretreatment on photosystem II activity stabilization and leaf blade expansion. The study demonstrated that cold priming stronger impacts on plant fitness than cold acclimation in spring in accessions with intermediate and low cold acclimation potential.

Redox and Reactive Oxygen Species Network in Acclimation for Salinity Tolerance in Sugar Beet

Fine-tuned and coordinated regulation of transport, metabolism and redox homeostasis allows plants to acclimate to osmotic and ionic stress caused by high salinity. Sugar beet is a highly salt tolerant crop plant and is therefore an interesting model to study sodium chloride (NaCl) acclimation in crops. Sugar beet plants were subjected to a final level of 300 mM NaCl for up to 14 d in hydroponics. Plants acclimated to NaCl stress by maintaining its growth rate and adjusting its cellular redox and reactive oxygen species (ROS) network. In order to understand the unusual suppression of ROS accumulation under severe salinity, the regulation of elements of the redox and ROS network was investigated at the transcript level. First, the gene families of superoxide dismutase (SOD), peroxiredoxins (Prx), alternative oxidase (AOX), plastid terminal oxidase (PTOX) and NADPH oxidase (RBOH) were identified in the sugar beet genome. Salinity induced the accumulation of Cu-Zn-SOD, Mn-SOD, Fe-SOD3, all AOX isoforms, 2-Cys-PrxB, PrxQ, and PrxIIF. In contrast, Fe-SOD1, 1-Cys-Prx, PrxIIB and PrxIIE levels decreased in response to salinity. Most importantly, RBOH transcripts of all isoforms decreased. This pattern offers a straightforward explanation for the low ROS levels under salinity. Promoters of stress responsive antioxidant genes were analyzed in silico for the enrichment of cis-elements, in order to gain insights into gene regulation. The results indicate that special cis-elements in the promoters of the antioxidant genes in sugar beet participate in adjusting the redox and ROS network and are fundamental to high salinity tolerance of sugar beet.

The Plant Immunity Regulating F-Box Protein CPR1 Supports Plastid Function in Absence of Pathogens

The redox imbalanced 6 mutant (rimb6) of Arabidopsis thaliana was isolated in a genetic screening approach for mutants with defects in chloroplast-to-nucleus redox signaling. It has an atypically low activation status of the 2-Cys peroxiredoxin-A promoter in the seedling stage. rimb6 shows wildtype-like germination, seedling development and greening, but slower growth and reduced biomass in the rosette stage. Mapping of the casual mutation revealed that rimb6 carries a single nucleotide polymorphism in the gene encoding CONSTITUTIVE EXPRESSER OF PATHOGENESIS RELATED (PR) GENES 1, CPR1 (At4g12560), leading to a premature stop codon. CPR1 is known as a repressor of pathogen signaling and regulator of microtubule organization. Allelism of rimb6 and cpr1 revealed a function of CPR1 in chloroplast stress protection. Expression studies in pathogen signaling mutants demonstrated that CPR1-mediated activation of genes for photosynthesis and chloroplast antioxidant protection is, in contrast to activation of pathogen responses, regulated independently from PAD4-controlled salicylic acid (SA) accumulation. We conclude that the support of plastid function is a basic, SA-independent function of CPR1.

Characterization of the Arabidopsis thaliana 2-Cys peroxiredoxin interactome

Peroxiredoxins are ubiquitous thiol-dependent peroxidases for which chaperone and signaling roles have been reported in various types of organisms in recent years. In plants, the peroxidase function of the two typical plastidial 2-Cys peroxiredoxins (2-Cys PRX A and B) has been highlighted while the other functions, particularly in ROS-dependent signaling pathways, are still elusive notably due to the lack of knowledge of interacting partners. Using an ex vivo approach based on co-immunoprecipitation of leaf extracts from Arabidopsis thaliana wild-type and mutant plants lacking 2-Cys PRX expression followed by mass spectrometry-based proteomics, 158 proteins were found associated with 2-Cys PRXs. Already known partners like thioredoxin-related electron donors (Chloroplastic Drought-induced Stress Protein of 32kDa, Atypical Cysteine Histidine-rich Thioredoxin 2) and enzymes involved in chlorophyll synthesis (Protochlorophyllide OxidoReductase B) or carbon metabolism (Fructose-1,6-BisPhosphatase) were identified, validating the relevance of the approach. Bioinformatic and bibliographic analyses allowed the functional classification of the identified proteins and revealed that more than 40% are localized in plastids. The possible roles of plant 2-Cys PRXs in redox signaling pathways are discussed in relation with the functions of the potential partners notably those involved in redox homeostasis, carbon and amino acid metabolisms as well as chlorophyll biosynthesis.

Overexpression of AhpC enhances stress tolerance and N2-fixation in Anabaena by upregulating stress responsive genes

The study explores the significance of peroxides in regulating the CO2- and N2-fixation capacities in Anabaena sp. PCC7120. To this end Anabaena strains were generated carrying an extra copy of ahpC (An+ahpC) or by deleting from their endogenous functional ahpC (AnΔahpC). AhpC levels were 2.2- to 6.0-fold higher in An+ahpC than in wild type. An+ahpC revealed 1.4- to 2-fold upregulation of photosystems I and II, nitrogenase, superoxide dismutase and catalase activities while same activities were 1.3- to 2.5-fold downregulated in the insertional mutant (AnΔahpC) compared to the wild type. Peroxide, superoxide and malondialdehyde contents were low in An+ahpC and high in AnΔahpC. Growth was inhibited in AnΔahpC by approximately 40-60% compared to a 33-40% enhanced growth in An+ahpC under selected stresses. Most interestingly, heterocyst frequency was increased in An+ahpC. In order to address transcriptional and posttranscriptional effects, transcripts of genes including groEL, fld, kat, gor, gst, dps, bfr, tf, sodA, dnaK, prx, uspA, pcs and apx were quantified and found to be increased 1.33- to 7.70-fold in unstressed and 1.76- to 13.80-fold in stressed An+ahpC. In a converse manner, they were downregulated by 1.20- to 7.50-fold in unstressed and 1.23 to 10.20-fold in stressed AnΔahpC. It is concluded that the level of AhpC controls a major set of metabolic and developmental genes in normal and stress conditions and thus likely is in the core of the redox regulatory system of Anabaena.

Redox-Dependent Conformational Dynamics of Decameric 2-Cysteine Peroxiredoxin and its Interaction with Cyclophilin 20-3

2-Cysteine peroxiredoxins (2-CysPrxs) switch between functions as a thiol peroxidase, chaperone, an interaction partner and possibly a proximity-based oxidase in a redox-dependent manner. In photosynthetic eukaryotes, 2-CysPrx localizes to the plastid, functions in the context of photosynthesis and enables an ascorbate peroxidase-independent water-water cycle for detoxifying H₂O₂ The high degree of evolutionary conservation of 2-CysPrx suggests that the switching is an essential characteristic and needed to transduce redox information to downstream pathways and regulation. The study aimed at exploring the dissociation behavior of 2-CysPrx and its interactions with cyclophilin depending on bulk phase conditions. Isothermal titration microcalorimetry (ITC), dynamic light scattering and size exclusion chromatography (SEC) proved the previously suggested model that reduced 2-CysPrx below a critical transition concentration (CTC) exists in its dimeric state, and above the CTC adopts the decameric state. The presence of cyclophilin 20-3 (Cyp20-3) affected the CTC of a 2-CysPrx decamer suggesting interaction which was further quantified by direct titration of 2-CysPrx with Cyp20-3, and in overlays. Finally catalytic inactivation assays showed the higher catalytic efficiency of 2-CysPrx at pH 8 compared with pH 7.2, but also revealed increased inactivation by hyperoxidation at pH 8. Interestingly, calculation of the average turnover number until inactivation gave rather similar values of 243 and 268 catalytic cycles at pH 8 and pH 7.2, respectively. These quantitative data support a model where 2-CysPrx and Cyp20-3, by interaction, form a redox-sensitive regulatory module in the chloroplast which is under control of the photosynthesis-linked stromal pH value, the redox state and additional stromal protein factor(s).

Redox- and Reactive Oxygen Species-Dependent Signaling into and out of the Photosynthesizing Chloroplast

Photosynthesis is a high-rate redox metabolic process that is subjected to rapid changes in input parameters, particularly light. Rapid transients of photon capture, electron fluxes, and redox potentials during photosynthesis cause reactive oxygen species (ROS) to be released, including singlet oxygen, superoxide anion radicals, and hydrogen peroxide. Thus, the photosynthesizing chloroplast functions as a conditional source of important redox and ROS information, which is exploited to tune processes both inside the chloroplast and, following retrograde release or processing, in the cytosol and nucleus. Analyses of mutants and comparative transcriptome profiling have led to the identification of these processes and associated players and have allowed the specificity and generality of response patterns to be defined. The release of ROS and oxidation products, envelope permeabilization (for larger molecules), and metabolic interference with mitochondria and peroxisomes produce an intricate ROS and redox signature, which controls acclimation processes. This photosynthesis-related ROS and redox information feeds into various pathways (e.g. the mitogen-activated protein kinase and OXI1 signaling pathways) and controls processes such as gene expression and translation.

Proteomic and Physiological Studies Provide Insight into Photosynthetic Response of Rice (Oryza sativa L.) Seedlings to Microgravity

The mechanisms whereby how photosynthesis is regulated and maintained under conditions of microgravity remain incompletely understood. Herein, we took a combination of proteomic and physiological approaches to examine the response of rice (Oryza sativa L.) seedlings to spaceflight conditions. Our results show that both PSI fluorescence emission peak and P700 absorbance amplitude are severely decreased in spaceflight seedlings under microgravity. This is consistent with an observed significant reduction in PSI efficiency (ϕI ). To further analyze global changes of protein profiles under microgravity, isobaric tags for relative and absolute quantization (iTRAQ) labeling technology were deployed. Four hundred fifty-four differentially expressed proteins were identified by comparison of spaceflight and ground control. Of proteins relevant to photosynthesis, 34 were downregulated and 4 were upregulated. The significantly downregulated ones are essential components of PSI, NDH and the Cytb6 f complex. This downregulation of PSI proteins and/or protein structure changes may cause the overall reduction in PSI activity. Intriguingly, although abundance of some PSII proteins was altered under microgravity, no significant changes in PSII activity were detected. Taken together, our results suggest that PSI, rather than PSII being usually much more sensitive to environmental stresses, is more susceptible to spaceflight conditions in rice seedlings.

Comparative proteomics of wild type, An+ahpC and An∆ahpC strains of Anabaena sp. PCC7120 demonstrates AhpC mediated augmentation of photosynthesis, N2-fixation and modulation of regulatory network of antioxidative proteins

Alkylhydroperoxide reductase (AhpC), a 1-Cys peroxiredoxin is well known for maintaining the cellular homeostasis. Present study employs proteome approach to analyze and compare alterations in proteome of Anabaena PCC7120 in overexpressing (An+ahpC), deletion (An∆ahpC) and its wild type. 2-DE based analysis revealed that the major portion of identified protein belongs to energy metabolism, protein folding, modification and stress related proteins and carbohydrate metabolism. The two major traits discernible from An+ahpC were (i) augmentation of photosynthesis and nitrogen fixation (ii) modulation of regulatory network of antioxidative proteins. Increased accumulation of proteins of light reaction, dark reaction, pentose phosphate pathway and electron transfer agent FDX for nitrogenase in An+ahpC and their simultaneous downregulation in AnΔahpC demonstrates its role in augmenting photosynthesis and nitrogen fixation. Proteomic data was nicely corroborated with physiological, biochemical parameters displaying upregulation of nitrogenase (1.6 fold) PSI (1.08) and PSII (2.137) in An+ahpC. Furthermore, in silico analysis not only attested association of AhpC with peroxiredoxins but also with other players of antioxidative defense system viz. thioredoxin and thioredoxin reductase. Above mentioned findings are in agreement with 33-40% and 40-60% better growth performance of An+ahpC over wild type and An∆ahpC respectively under abiotic stresses, suggesting its role in maintenance of metabolic machinery under stress.

SIGNIFICANCE

Present work explores key role of AhpC in mitigating stress in Anabaena PCC7120 through combined proteomic, biochemical and in silico investigations. This study is the first attempt to analyze and compare alterations in proteome of Anabaena PCC7120 following addition (overexpressing strain An+ahpC) and deletion (mutant An∆ahpC) of AhpC against its wild type. The effort resulted in two major traits in An+ahpC as (i) augmentation of photosynthesis and nitrogen fixation (ii) modulation of regulatory network of antioxidative proteins.

Inhibition of ice recrystallization and cryoprotective activity of wheat proteins in liver and pancreatic cells

Efficient cryopreservation of cells at ultralow temperatures requires the use of substances that help maintain viability and metabolic functions post-thaw. We are developing new technology where plant proteins are used to substitute the commonly-used, but relatively toxic chemical dimethyl sulfoxide. Recombinant forms of four structurally diverse wheat proteins, TaIRI-2 (ice recrystallization inhibition), TaBAS1 (2-Cys peroxiredoxin), WCS120 (dehydrin), and TaENO (enolase) can efficiently cryopreserve hepatocytes and insulin-secreting INS832/13 cells. This study shows that TaIRI-2 and TaENO are internalized during the freeze-thaw process, while TaBAS1 and WCS120 remain at the extracellular level. Possible antifreeze activity of the four proteins was assessed. The "splat cooling" method for quantifying ice recrystallization inhibition activity (a property that characterizes antifreeze proteins) revealed that TaIRI-2 and TaENO are more potent than TaBAS1 and WCS120. Because of their ability to inhibit ice recrystallization, the wheat recombinant proteins TaIRI-2 and TaENO are promising candidates and could prove useful to improve cryopreservation protocols for hepatocytes and insulin-secreting cells, and possibly other cell types. TaENO does not have typical ice-binding domains, and the TargetFreeze tool did not predict an antifreeze capacity, suggesting the existence of nontypical antifreeze domains. The fact that TaBAS1 is an efficient cryoprotectant but does not show antifreeze activity indicates a different mechanism of action. The cryoprotective properties conferred by WCS120 depend on biochemical properties that remain to be determined. Overall, our results show that the proteins' efficiencies vary between cell types, and confirm that a combination of different protection mechanisms is needed to successfully cryopreserve mammalian cells.

Interactions between 2-Cys peroxiredoxins and ascorbate in autophagosome formation during the heat stress response in Solanum lycopersicum

2-Cys peroxiredoxins (2-CPs) function in the removal of hydrogen peroxide and lipid peroxides but their precise roles in the induction of autophagy have not been characterized. Here we show that heat stress, which is known to induce oxidative stress, leads to the simultaneous accumulation of transcripts encoding 2-CPs and autophagy proteins, as well as autophagosomes, in tomato (Solanum lycopersicum) plants. Virus-induced gene silencing of the tomato peroxiredoxin genes 2-CP1, 2-CP2, and 2-CP1/2 resulted in an increased sensitivity of tomato plants to heat stress. Silencing 2-CP2 or 2-CP1/2 increased the levels of transcripts associated with ascorbate biosynthesis but had no effect on the glutathione pool in the absence of stress. However, the heat-induced accumulation of transcripts associated with the water-water cycle was compromised by the loss of 2-CP1/2 functions. The transcript levels of autophagy-related genes ATG5 and ATG7 were higher in plants with impaired 2-CP1/2 functions, and the formation of autophagosomes increased, together with an accumulation of oxidized and insoluble proteins. Silencing of ATG5 or ATG7 increased the levels of 2-CP transcripts and protein but decreased heat stress tolerance. These results demonstrate that 2-CPs fulfil a pivotal role in heat stress tolerance in tomato, via interactions with ascorbate-dependent pathways and autophagy.

Thiol-Based Peroxidases and Ascorbate Peroxidases: Why Plants Rely on Multiple Peroxidase Systems in the Photosynthesizing Chloroplast?

Photosynthesis is a highly robust process allowing for rapid adjustment to changing environmental conditions. The efficient acclimation depends on balanced redox metabolism and control of reactive oxygen species release which triggers signaling cascades and potentially detrimental oxidation reactions. Thiol peroxidases of the peroxiredoxin and glutathione peroxidase type, and ascorbate peroxidases are the main peroxide detoxifying enzymes of the chloroplast. They use different electron donors and are linked to distinct redox networks. In addition, the peroxiredoxins serve functions in redox regulation and retrograde signaling. The complexity of plastid peroxidases is discussed in context of suborganellar localization, substrate preference, metabolic coupling, protein abundance, activity regulation, interactions, signaling functions, and the conditional requirement for high antioxidant capacity. Thus the review provides an opinion on the advantage of linking detoxification of peroxides to different enzymatic systems and implementing mechanisms for their inactivation to enforce signal propagation within and from the chloroplast.

Site-directed mutagenesis substituting cysteine for serine in 2-Cys peroxiredoxin (2-Cys Prx A) of Arabidopsis thaliana effectively improves its peroxidase and chaperone functions

BACKGROUND AND AIMS

The 2-Cys peroxiredoxin (Prx) A protein of Arabidopsis thaliana performs the dual functions of a peroxidase and a molecular chaperone depending on its conformation and the metabolic conditions. However, the precise mechanism responsible for the functional switching of 2-Cys Prx A is poorly known. This study examines various serine-to-cysteine substitutions on α-helix regions of 2-Cys Prx A in Arabidopsis mutants and the effects they have on the dual function of the protein.

METHODS

Various mutants of 2-Cys Prx A were generated by replacing serine (Ser) with cysteine (Cys) at different locations by site-directed mutagenesis. The mutants were then over-expressed in Escherichia coli. The purified protein was further analysed by size exclusion chromatography, polyacrylamide gel electrophoresis, circular dichroism spectroscopy and transmission electron microscopy (TEM) and image analysis. Peroxidase activity, molecular chaperone activity and hydrophobicity of the proteins were also determined. Molecular modelling analysis was performed in order to demonstrate the relationship between mutation positions and switching of 2-Cys Prx A activity.

KEY RESULTS

Replacement of Ser(150) with Cys(150) led to a marked increase in holdase chaperone and peroxidase activities of 2-Cys Prx A, which was associated with a change in the structure of an important domain of the protein. Molecular modelling demonstrated the relationship between mutation positions and the switching of 2-Cys Prx A activity. Examination of the α2 helix, dimer-dimer interface and C-term loop indicated that the peroxidase function is associated with a fully folded α2 helix and easy formation of a stable reduced decamer, while a more flexible C-term loop makes the chaperone function less likely.

CONCLUSIONS

Substitution of Cys for Ser at amino acid location 150 of the α-helix of 2-Cys Prx A regulates/enhances the dual enzymatic functions of the 2-Cys Prx A protein. If confirmed in planta, this leads to the potential for it to be used to maximize the functional utility of 2-Cys Prx A protein for improved metabolic functions and stress resistance in plants.

Comparative Proteomic Analysis of Brassica napus in Response to Drought Stress

Drought is one of the most widespread stresses leading to retardation of plant growth and development. We examined proteome changes of an important oil seed crop, canola (Brassica napus L.), under drought stress over a 14-day period. Using iTRAQ LC-MS/MS, we identified 1976 proteins expressed during drought stress. Among them, 417 proteins showed significant changes in abundance, and 136, 244, 286, and 213 proteins were differentially expressed in the third, seventh, 10th, and 14th day of stress, respectively. Functional analysis indicated that the number of proteins associated with metabolism, protein folding and degradation, and signaling decreased, while those related to energy (photosynthesis), protein synthesis, and stress and defense increased in response to drought stress. The seventh and 10th-day profiles were similar to each other but with more post-translational modifications (PTMs) at day 10. Interestingly, 181 proteins underwent PTMs; 49 of them were differentially changed in drought-stressed plants, and 33 were observed at the 10th day. Comparison of protein expression changes with those of gene transcription showed a positive correlation in B. napus, although different patterns between transcripts and proteins were observed at each time point. Under drought stress, most protein abundance changes may be attributed to gene transcription, and PTMs clearly contribute to protein diversity and functions.

Selective silencing of 2Cys and type-IIB Peroxiredoxins discloses their roles in cell redox state and stress signaling

Peroxiredoxins (Prx) catalyse the reduction of hydrogen peroxide (H2O2) and, in association with catalases and other peroxidases, may participate in signal transduction by regulating intercellular H2O2 concentration that in turn can control gene transcription and cell signaling. Using virus-induced-gene-silencing (VIGS), 2-Cys Peroxiredoxin (2CysPrx) family and type-II Peroxiredoxin B (PrxIIB) gene were silenced in Nicotiana benthamiana, to study the impact that the loss of function of each Prx would have in the antioxidant system under control (22 °C) and severe heat stress conditions (48 °C). The results showed that both Prxs, although in different organelles, influence the regeneration of ascorbate to a significant extent, but with different purposes. 2CysPrx affects abscisic acid (ABA) biosynthesis through ascorbate, while PrxIIB does it probably through the xanthophyll cycle. Moreover, 2CysPrx is key in H2O2 scavenging and in consequence in the regulation of ABA signaling downstream of reactive oxygen species and PrxIIB provides an important assistance for H2O2 peroxisome scavenges.

Redox-dependent chaperone/peroxidase function of 2-Cys-Prx from the cyanobacterium Anabaena PCC7120: role in oxidative stress tolerance

BACKGROUND

Cyanobacteria, progenitors of plant chloroplasts, provide a suitable model system for plants to study adaptation towards different abiotic stresses. Genome of the filamentous, heterocystous, nitrogen-fixing cyanobacterium Anabaena PCC7120 harbours a single gene (alr4641) encoding a typical 2-Cys-Peroxiredoxins (2-Cys-Prxs). 2-Cys-Prxs are thiol-based peroxidases that also function as molecular chaperones in plants and other systems. The Alr4641 protein from Anabaena PCC7120 shows high level biochemical similarities with the plant 2-Cys-Prx. The physiological role played by the Alr4641 protein in Anabaena was addressed in this study.

RESULTS

In Anabaena PCC7120, alr4641 transcript /Alr4641 protein was induced in response to abiotic stresses and its promoter was active in the vegetative cells as well as heterocysts. The wild-type Alr4641 protein or Alr4641 lacking the peroxidatic cysteine (Alr4641C56S) or the resolving cysteine (Alr4641C178S) existed as higher oligomers in their native form. The wild-type or the mutant Alr4641 proteins showed similar chaperone activity, but only the wild-type protein exhibited peroxidase activity indicating that unlike peroxidase activity, chaperone activity was not dependent on cysteines. In contrast to other 2-Cys-Prxs, chaperone/peroxidase activity of Alr4641 was dependent on its redox state and not oligomerization status. Alr4641 could protect plasmid DNA from oxidative damage and physically associate with NADPH-dependent thioredoxin reductase (NTRC). Like 2-Cys-Prxs from plants (e.g. rice), Alr4641 could detoxify various peroxides using NTRC as reductant. On exposure to H2O2, recombinant Anabaena PCC7120 strain over-expressing Alr4641 (An4641+) showed reduced content of reactive oxygen species (ROS), intact photosynthetic functions and consequently better survival than the wild-type Anabaena PCC7120, indicating that Alr4641 can protect Anabaena from oxidative stress.

CONCLUSIONS

The peroxidase/chaperone function of Alr4641, its inherent transcriptional/translational induction under different abiotic stresses and localization in both vegetative cells and heterocysts could be an adaptive strategy to battle various oxidative stresses that Anabaena encounters during its growth. Moreover, the recombinant Anabaena strain over expressing Alr4641 showed higher resistance to oxidative stress, suggesting its potential to serve as stress-tolerant biofertilizers in rice fields.

Characterization of gene expression associated with drought avoidance and tolerance traits in a perennial grass species

To understand molecular mechanisms of perennial grass adaptation to drought stress, genes associated with drought avoidance or tolerance traits were identified and their expression patterns were characterized in C4 hybrid bermudagrass [Cynodon dactylon (L.) Pers.×C. transvaalensis Burtt Davy, cv. Tifway] and common bermudagrass (C. dactylon, cv. C299). Plants of drought-tolerant 'Tifway' and drought-sensitive 'C299' were exposed to drought for 5 d (mild stress) and 10 d (severe stress) by withholding irrigation in a growth chamber. 'Tifway' maintained significantly lower electrolyte leakage and higher relative water content than 'C299' at both 5 and 10 d of drought stress. Four cDNA libraries via suppression subtractive hybridization analysis were constructed and identified 277 drought-responsive genes in the two genotypes at 5 and 10 d of drought stress, which were mainly classified into the functional categories of stress defense, metabolism, osmoregulation, membrane system, signal and regulator, structural protein, protein synthesis and degradation, and energy metabolism. Quantitative-PCR analysis confirmed the expression of 36 drought up-regulated genes that were more highly expressed in drought-tolerant 'Tifway' than drought-sensitive 'C299', including those for drought avoidance traits, such as cuticle wax formation (CER1 and sterol desaturase), for drought tolerance traits, such as dehydration-protective proteins (dehydrins, HVA-22-like protein) and oxidative stress defense (superoxide dismutase, dehydroascorbate reductase, 2-Cys peroxiredoxins), and for stress signaling (EREBP-4 like protein and WRKY transcription factor). The results suggest that the expression of genes for stress signaling, cuticle wax accumulation, antioxidant defense, and dehydration-protective protein accumulation could be critically important for warm-season perennial grass adaptation to long-term drought stress.

2-Cys peroxiredoxin responds to low temperature and other cues in Caragana jubata, a plant species of cold desert of Himalaya

A 2-Cys peroxiredoxin cDNA (CjPrx) was isolated and characterized from Caragana jubata, a temperate/alpine plant species of high altitude cold desert of Himalaya and Eurasia. The cDNA obtained was 1,064 bp long consisting of an open reading frame of 789 bp encoding 262 amino acids. The calculated molecular mass of the mature protein was 28.88 kDa and pI was 5.84. Deduced amino acid sequence of CjPrx shared a high degree homology with 2-CysPrx proteins from other plants. CjPrx had both the PRX_type 2-Cys domain and thioredoxin-like superfamily domains. CjPrx contained 26.72% α-helices, 6.87% β-turns, 20.61% extended strands and 45.80% random coils, and was a hydrophilic protein. Expression of CjPrx was modulated by low temperature, methyl jasmonate (MJ), salicylic acid and drought stress, but no significant change was observed in response to abscisic acid treatment. Among all the treatments, a strong up-regulation of CjPrx was observed in response to MJ treatment.

Linking chloroplast antioxidant defense to carbohydrate availability: the transcript abundance of stromal ascorbate peroxidase is sugar-controlled via ascorbate biosynthesis

All genes encoding chloroplast antioxidant enzymes are nuclear-encoded and posttranscriptionally targeted to chloroplasts. The transcript levels of most of them decreased upon sucrose feeding like the transcript levels of many genes encoding components of the photosynthetic electron transport chain. However, the transcript abundance of stromal ascorbate peroxidase (s-APX; At4g08390) increased. Due to mild sugar application conditions, the plants kept the phosphorylation status of the ADP+ATP pool and the redox states of the NADPH+NADP+ and the ascorbate pools under control, which excludes them as signals in s-APX regulation. Correlation with ascorbate pool size regulation and comparison of transcript abundance regulation in the starch-biosynthetic mutant adg1, the ascorbate biosynthesis mutant vtc1, and the abscisic acid (ABA) biosynthetic mutant aba2 showed a link between sugar induction of s-APX and ascorbate biosynthesis.

Plastid thioredoxins: a "one-for-all" redox-signaling system in plants

The sessile nature of plants forces them to face an ever-changing environment instead of escape from hostile conditions as animals do. In order to overcome this survival challenge, a fine monitoring and controlling of the status of the photosynthetic electron transport chain and the general metabolism is vital for these organisms. Frequently, evolutionary plant adaptation has consisted in the appearance of multigenic families, comprising an array of enzymes, structural components, or sensing, and signaling elements, in numerous occasions with highly conserved primary sequences that sometimes make it difficult to discern between redundancy and specificity among the members of a same family. However, all this gene diversity is aimed to sort environment-derived plant signals to efficiently channel the external incoming information inducing a right physiological answer. Oxygenic photosynthesis is a powerful source of reactive oxygen species (ROS), molecules with a dual oxidative/signaling nature. In response to ROS, one of the most frequent post-translational modifications occurring in redox signaling proteins is the formation of disulfide bridges (from Cys oxidation). This review is focused on the role of plastid thioredoxins (pTRXs), proteins containing two Cys in their active site and largely known as part of the plant redox-signaling network. Several pTRXs types have been described so far, namely, TRX f, m, x, y, and z. In recent years, improvements in proteomic techniques and the study of loss-of-function mutants have enabled us to grasp the importance of TRXs for the plastid physiology. We will analyze the specific signaling function of each TRX type and discuss about the emerging role in non-photosynthetic plastids of these redox switchers.

Physiological and proteomic analyses of drought stress response in Holm oak provenances

Responses to drought stress by water withholding have been studied in 1 year old Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) seedlings from seven provenances from Andalusia (southern Spain). Several physiological parameters, including predawn xylem water potentials and relative water content in soil, roots, and leaves as well as maximum quantum efficiency and yield of PSII were evaluated for 28 days in both irrigated and nonirrigated seedlings. The leaf proteome map of the two provenances that show the extreme responses (Seville, GSE, is the most susceptible, while Almerı́a, SSA, is the least susceptible) was obtained. Statistically significant variable spots among provenances and treatments were subjected to MALDI-TOF/TOF-MS/MS analysis for protein identification. In response to drought stress, ~12.4% of the reproducible spots varied significantly depending on the treatment and the population. These variable proteins were mainly chloroplastic and belonged to the metabolism and defense/stress functional categories. The 2-DE protein profile of nonirrigated seedlings was similar in both provenances. Physiological and proteomics data were generally in good agreement. The general trend was a decrease in protein abundance upon water withholding in both provenances, mainly in those involved in ATP synthesis and photosynthesis. This decrease, moreover, was most marked in the most susceptible population compared with the less susceptible one.

Identification of global ferredoxin interaction networks in Chlamydomonas reinhardtii

Ferredoxins (FDXs) can distribute electrons originating from photosynthetic water oxidation, fermentation, and other reductant-generating pathways to specific redox enzymes in different organisms. The six FDXs identified in Chlamydomonas reinhardtii are not fully characterized in terms of their biological function. In this report, we present data from the following: (a) yeast two-hybrid screens, identifying interaction partners for each Chlamydomonas FDX; (b) pairwise yeast two-hybrid assays measuring FDX interactions with proteins from selected biochemical pathways; (c) affinity pulldown assays that, in some cases, confirm and even expand the interaction network for FDX1 and FDX2; and (d) in vitro NADP(+) reduction and H2 photo-production assays mediated by each FDX that verify their role in these two pathways. Our results demonstrate new potential roles for FDX1 in redox metabolism and carbohydrate and fatty acid biosynthesis, for FDX2 in anaerobic metabolism, and possibly in state transition. Our data also suggest that FDX3 is involved in nitrogen assimilation, FDX4 in glycolysis and response to reactive oxygen species, and FDX5 in hydrogenase maturation. Finally, we provide experimental evidence that FDX1 serves as the primary electron donor to two important biological pathways, NADPH and H2 photo-production, whereas FDX2 is capable of driving these reactions at less than half the rate observed for FDX1.

Physiological and proteomics analyses of Holm oak (Quercus ilex subsp. ballota [Desf.] Samp.) responses to Phytophthora cinnamomi

Phytophthora cinnamomi is one of the agents that trigger the decline syndrome in Quercus spp., this being a serious threat to Mediterranean Holm oak forest sustainability and reforestation programs. Quercus ilex responses to Phytophthora cinnamomi have been studied in one-year olds seedlings from two Andalucía provenances, assessing the physiological water status and photosynthesis-related parameters. Upon inoculation with mycelium a reduction in water content, chlorophyll fluorescence, stomatal conductance and gas exchange was observed along a 90 days post inoculation period in both provenances. The reduction was higher in the most susceptible (SSA) provenance, than in the most tolerant (PCO), being these typical plant responses to drought stress. Leaf protein profiles were analyzed in non-inoculated and inoculated seedlings from the two provenances by using a 2-DE coupled to MS proteomics strategy. Ninety seven proteins changing in abundance in response to the inoculation were successfully identified after MALDI-TOF-TOF analyses. The largest group of variable identified proteins were chloroplasts ones, and they were involved in the photosynthesis, Calvin cycle and carbohydrate metabolism. It was noted that a general tendency was a decrease in the protein abundance as a consequence of the inoculation, being it less accused in the least susceptible, the Northern provenance (PCO), than in the most susceptible, the Southern provenance (SSA). This trend is clearly manifested in photosynthesis, amino acid metabolism and stress/defence proteins. On the contrary, some proteins related to starch biosynthesis, glycolysis and stress related peroxiredoxin showed an increase upon inoculation. These changes in protein abundance were correlated to the estimated physiological parameters and have been frequently observed in plants subjected to drought stress.

Down-regulation of catalase activity allows transient accumulation of a hydrogen peroxide signal in Chlamydomonas reinhardtii

In photosynthetic organisms, excess light is a stress that induces production of reactive oxygen species inside the chloroplasts. As a response, the capacity of antioxidative defence mechanisms increases. However, when cells of Chlamydomonas reinhardtii were shifted from dark to high light, a reversible partial inactivation of catalase activity was observed, which correlated with a transient increase in the level of H2 O2 in the 10 μm range. This concentration range seems to be necessary to activate H2 O2 -dependent signalling pathways stimulating the expression of H2 O2 responsive genes, such as the heat shock protein HSP22C. Catalase knock-down mutants had lost the transient accumulation of H2 O2 , suggesting that a decrease in catalase activity was the key element for establishing a transient H2 O2 burst. Catalase was inactivated by a one-electron event consistent with the reduction of a single cysteine. We propose that under high light intensity, the redox state of the photosynthetic electron transport chain is sensed and transmitted to the cytosol to regulate the catalase activity. This allows a transient accumulation of H2 O2 , inducing a signalling event that is transmitted to the nucleus to modulate the expression of chloroplast-directed protection enzymes.

Leaf proteome profiling of transgenic mint infected with Alternaria alternata

The genus Mentha has been widely used in food, flavor, culinary, cosmetic and pharmaceutical industries. Substantial damage to this crop happened regularly due to environmental stresses like metal toxicity and pathogen attack. Here, an approach has been taken to raise transgenic mint over-expressing γ-glutamyl-cysteine synthetase (γ-ECS), the rate-limiting enzyme of GSH biosynthesis, resulted enhanced GSH content and its in planta expression confers significant tolerance towards abiotic/biotic stresses viz. metal toxicity - Cd, Zn as well as against infection of Alternaria alternata and Rhizoctonia solani. A differential proteomic analysis through 2-DE and MALDI TOF-TOF MSMS was performed to focus on the altered abundance of functionally important protein species in control and infected transgenic mint. Results showed a significant variation in the protein profile of the infected transgenic plant as compared to the wild/control transgenic counterpart. In addition to protein species related to stress and defense, redox regulation, transcription factors and energy & metabolism, protein species related to signaling and gene regulation as well as cell division also showed differential accumulation in infected transgenic. Hence, proteomics can be used as a tool to decipher the mechanism of action of GSH in providing tolerance against a necrotrophic fungus, A. alternata in transgenic mint.

BIOLOGICAL SIGNIFICANCE

The reported work describes a comparative proteomics of non-model unsequenced plants like Mentha. There is a comparative protein profile between transgenic and its wild counterparts under control and infected condition. The work has an impact in crop proteomics and also tries to explain the application of proteomic approach to decipher the mechanism by which a foreign metabolite mediates stress tolerance in plant under control and infected condition. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Proteomics analysis of date palm leaves affected at three characteristic stages of brittle leaf disease

Proteomics analysis has been performed in leaf tissue from field date palm trees showing the brittle leaf disease (BLD) or maladie des feuilles cassantes, the main causal agent of the date palm decline in south Tunisia. To study the evolution of the disease, proteins from healthy and affected leaves taken at three disease stages (S1, S2 and S3) were trichloroacetic acid acetone extracted and subjected to two-dimensional gel electrophoresis (5-8 pH range). Statistical analysis showed that the protein abundance profile is different enough to differentiate the affected leaves from the healthy ones. Fifty-eight variable spots were successfully identified by matrix-assisted laser desorption ionization time of flight, 60 % of which corresponded to chloroplastic ones being involved in the photosynthesis electronic chain and ATP synthesis, metabolic pathways implicated in the balance of the energy, and proteases. Changes in the proteome start at early disease stage (S1), and are greatest at S2. In addition to the degradation of the ribulose-1.5-bisphosphate carboxylase oxygenase in affected leaflets, proteins belonging to the photosynthesis electronic chain and ATP synthesis decreased following the disease, reinforcing the relationship between BLD and manganese deficiency. The manganese-stabilizing proteins 33 kDa, identified in the present work, can be considered as protein biomarkers of the disease, especially at early disease step.

Putting the pieces together: high-performance LC-MS/MS provides network-, pathway-, and protein-level perspectives in Populus

High-performance mass spectrometry (MS)-based proteomics enabled the construction of a detailed proteome atlas for Populus, a woody perennial plant model organism. Optimization of experimental procedures and implementation of current state-of-the-art instrumentation afforded the most detailed look into the predicted proteome space of Populus, offering varying proteome perspectives: (1) network-wide, (2) pathway-specific, and (3) protein-level viewpoints. Together, enhanced protein retrieval through a detergent-based lysis approach and maximized peptide sampling via the dual-pressure linear ion trap mass spectrometer (LTQ Velos), have resulted in the identification of 63,056 tryptic peptides. The technological advancements, specifically spectral-acquisition and sequencing speed, afforded the deepest look into the Populus proteome, with peptide abundances spanning 6 orders of magnitude and mapping to ∼25% of the predicted proteome space. In total, tryptic peptides mapped to 11,689 protein assignments across four organ-types: mature (fully expanded, leaf plastichronic index (LPI) 10-12) leaf, young (juvenile, LPI 4-6) leaf, root, and stem. To resolve protein ambiguity, identified proteins were grouped by sequence similarity (≥ 90%), thereby reducing the protein assignments into 7538 protein groups. In addition, this large-scale data set features the first systems-wide survey of protein expression across different Populus organs. As a demonstration of the precision and comprehensiveness of the semiquantitative analysis, we were able to contrast two stages of leaf development, mature versus young leaf. Statistical comparison through ANOVA analysis revealed 1432 protein groups that exhibited statistically significant (p ≤ 0.01) differences in protein abundance. Experimental validation of the metabolic circuitry expected in mature leaf (characterized by photosynthesis and carbon fixation) compared with young leaf (characterized by rapid growth and moderate photosynthetic activities) strongly testifies to the credibility of the approach. Instead of quantitatively comparing a few proteins, a systems view of all the changes associated with a given cellular perturbation could be made.