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Thioredoxins (TRXs), a large subclass of ubiquitous oxidoreductases, are involved in thiol redox regulation. Here, we performed a comprehensive analysis of TRXs in the Arabidopsis thaliana genome, revealing 41 genes encoding 18 typical and 23 atypical TRXs, and 6 genes encoding thioredoxin reductases (TRs). The high number of atypical TRXs indicates special functions in plants that mostly await elucidation. We identified an atypical class of thioredoxins called TRX-c in the genomes of photosynthetic eukaryotes. Localized to the chloroplast, TRX-c displays atypical CPLC, CHLC and CNLC motifs in the active sites. In silico analysis of the transcriptional regulations of TRXs revealed high expression of TRX-c in leaves and strong regulation under cold, osmotic, salinity and metal ion stresses., (© 2021 Federation of European Biochemical Societies.)

Forty years ago, 12-oxophytodienoic acid (12-OPDA) was reported as a prostaglandin (PG)-like metabolite of linolenic acid found in extracts of flaxseed. Since then, numerous studies have determined the role of 12-OPDA in regulating plant immunity, seed dormancy, and germination. This review summarizes our current knowledge of the regulation of 12-OPDA synthesis in the chloroplast and 12-OPDA-dependent signaling in gene expression and targeting protein functions. We describe the properties of OPDA that are linked to the activities of PGs, which are derived from arachidonic acid and act as tissue hormones in animals, including humans. The similarity of OPDA with bioactive PGs is particularly evident for the most-studied cyclopentenone, PG 15-dPGJ2. In addition to chemical approaches towards 12-OPDA synthesis, bio-organic synthesis strategies for 12-OPDA and analogous substances have recently been established. The resulting availability of OPDA will aid the identification of additional effector proteins, help in elucidating the mechanisms of OPDA sensing and transmission, and will foster the analysis of the physiological responses to OPDA in plants. There is a need to determine the compartmentation and transport of 12-OPDA and its conjugates, over long distances as well as short. It will be important to further study OPDA in animal and human cells, for example with respect to beneficial anti-inflammatory and anti-cancer activities.

Sugar beet is among the most salt-tolerant crops. This study aimed to investigate the metabolic adaptation of sugar beet to salt stress at the cellular and subcellular levels. Seedlings were grown hydroponically and subjected to stepwise increases in salt stress up to 300 mM NaCl. Highly enriched fractions of chloroplasts were obtained by non-aqueous fractionation using organic solvents. Total leaf metabolites and metabolites in chloroplasts were profiled at 3 h and 14 d after reaching the maximum salinity stress of 300 mM NaCl. Metabolite profiling by gas chromatography-mass spectrometry (GC-MS) resulted in the identification of a total of 83 metabolites in leaves and chloroplasts under control and stress conditions. There was a lower abundance of Calvin cycle metabolites under salinity whereas there was a higher abundance of oxidative pentose phosphate cycle metabolites such as 6-phosphogluconate. Accumulation of ribose-5-phosphate and ribulose-5-phosphate coincided with limitation of carbon fixation by ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Increases in glycolate and serine levels indicated that photorespiratory metabolism was stimulated in salt-stressed sugar beet. Compatible solutes such as proline, mannitol, and putrescine accumulated mostly outside the chloroplasts. Within the chloroplast, putrescine had the highest relative level and probably assisted in the acclimation of sugar beet to high salinity stress. The results provide new information on the contribution of chloroplasts and the extra-chloroplast space to salinity tolerance via metabolic adjustment in sugar beet., (© The Author 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

Legumes belong to the most important crops worldwide. They increase soil fertility due their ability to establish symbiotic associations with soil microorganisms, known as rhizobia, capable of fixing nitrogen from the atmosphere. However, they are frequently exposed to abiotic stress conditions in particular drought. Such adverse conditions impair the biological nitrogen fixation (BNF) and depend largely on the legume. Therefore, two peanut cultivars with contrasting tolerance to drought, namely the more tolerant EC-98 and the sensitive Granoleico, were investigated to elucidate the relative contribution of BNF to the tolerance to drought. The tolerant cultivar EC-98 sustained growth and BNF similar to the control condition despite the reduced water potential and photosynthesis, suggesting the functioning of distinct metabolic pathways that contributed to enhance the tolerance. The biochemical and metabolomics approaches revealed that nodules from the tolerant cultivar accumulated trehalose, proline and gamma-aminobutyric acid (GABA), metabolites with known function in protecting against drought stress. The amide metabolism was severely affected in nodules from the sensitive cultivar Granoleico as revealed by the low content of asparagine and glutamine in the drought stressed plants. The sensitive cultivar upon rehydration was unable to re-establish a metabolism similar to well-watered plants. This was evidenced by the low level of metabolites and, transcripts and specific activities of enzymes from the carbon (sucrose synthase) and nitrogen (glutamine synthetase) metabolism which decreased below the values of control plants. Therefore, the increased content of metabolites with protective functions under drought stress likely is crucial for the full restoration upon rehydration. Smaller changes of drought stress-related metabolites in nodule are another trait that contributes to the effective control of BNF in the tolerant peanut cultivar (EC-98)., (Copyright © 2017 Elsevier B.V. All rights reserved.)

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 Author 2017. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

The methodological developments of in vivo and in vitro protein labeling and subsequent detection enable sensitive and specific detection of redox modifications. Such methods are presently applied to diverse cells and tissues, subproteomes and developmental as well as environmental conditions. The chloroplast proteome is particularly suitable for such kind of studies, because redox regulation of chloroplast proteins is well established, many plastid proteins are abundant, redox network components have been inventoried in great depth, and functional consequences explored. Thus the repertoire of redox-related posttranslational modifications on the one hand side and their abundance on the other pose a challenge for the near future to understand their contribution to physiological regulation. The various posttranslational redox modifications are introduced, followed by a description of the available proteomics methods. The significance of the redox-related posttranslational modification is exemplarily worked out using established examples from photosynthesis. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock., (Copyright © 2016. Published by Elsevier B.V.)

Glutathione peroxidases (Gpxs) are antioxidant enzymes not studied so far in legume nodules, despite the fact that reactive oxygen species are produced at different steps of the symbiosis. The function of two Gpxs that are highly expressed in nodules of the model legume Lotus japonicus was examined. Gene expression analysis, enzymatic and nitrosylation assays, yeast cell complementation, in situ mRNA hybridization, immunoelectron microscopy, and LjGpx-green fluorescent protein (GFP) fusions were used to characterize the enzymes and to localize each transcript and isoform in nodules. The LjGpx1 and LjGpx3 genes encode thioredoxin-dependent phospholipid hydroperoxidases and are differentially regulated in response to nitric oxide (NO) and hormones. LjGpx1 and LjGpx3 are nitrosylated in vitro or in plants treated with S-nitrosoglutathione (GSNO). Consistent with the modification of the peroxidatic cysteine of LjGpx3, in vitro assays demonstrated that this modification results in enzyme inhibition. The enzymes are highly expressed in the infected zone, but the LjGpx3 mRNA is also detected in the cortex and vascular bundles. LjGpx1 is localized to the plastids and nuclei, and LjGpx3 to the cytosol and endoplasmic reticulum. Based on yeast complementation experiments, both enzymes protect against oxidative stress, salt stress, and membrane damage. It is concluded that both LjGpxs perform major antioxidative functions in nodules, preventing lipid peroxidation and other oxidative processes at different subcellular sites of vascular and infected cells. The enzymes are probably involved in hormone and NO signalling, and may be regulated through nitrosylation of the peroxidatic cysteine essential for catalytic function., (© The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.)

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.

Background: Efficient light acclimation of photosynthetic cells is a basic and important property of plants. The process of acclimation depends on transformation of retrograde signals in gene expression, transcript accumulation and de novo protein synthesis. While signalling cues, transcriptomes and some involved players have been characterized, an integrated view is only slowly emerging, and information on the translational level is missing. Transfer of low (8 μmol quanta.m(-2).s(-1)) or normal light (80 μmol quanta.m(-2).s(-1)) acclimated 30 d old Arabidopsis thaliana plants to high light (800 μmol quanta.m(-2).s(-1)) triggers retrograde signals. Using this established approach, we sought to link transcriptome data with de novo synthesized proteins by in vivo labelling with (35)S methionine and proteome composition., Results: De novo synthesized protein and proteome patterns could reliably be matched with newly annotated master gels. Each molecular level could be quantified for a set of 41 proteins. Among the proteins preferentially synthesized in plants transferred to high light were enzymes including carbonic anhydrase, fructose-1,6-bisphosphate aldolase, O-acetyl serine thiol lyase, and chaperones, while low rates upon transfer to high light were measured for e.g. dehydroascorbate reductase, glyceraldehyde-3-phosphate dehydrogenase and CuZn superoxide dismutase, and opposite responses between 10-fold and 100-fold light increment for e.g. glutamine synthetase and phosphoglycerate kinase., Conclusions: The results prove the hypothesis that transcript abundance is poorly linked to de novo protein synthesis due to profound regulation at the level of translation. This vertical systems biology approach enables to quantitatively and kinetically link the molecular levels for scrutinizing signal processing and response generation.

High light acclimation depends on retrograde control of nuclear gene expression. Retrograde regulation uses multiple signalling pathways and thus exploits signal patterns. To maximally challenge the acclimation system, Arabidopsis thaliana plants were either adapted to 8 (low light (L-light)) or 80 µmol quanta m(-2) s(-1) (normal light (N-light)) and subsequently exposed to a 100- and 10-fold light intensity increase, respectively, to high light (H-light, 800 µmol quanta m(-2) s(-1)), for up to 6 h. Both L → H- and N → H-light plants efficiently regulated CO2 assimilation to a constant level without apparent damage and inhibition. This experimental set-up was scrutinized for time-dependent regulation and efficiency of adjustment. Transcriptome profiles revealed that N-light and L-light plants differentially accumulated 2119 transcripts. After 6 h in H-light, only 205 remained differently regulated between the L → H- and N → H-light plants, indicating efficient regulation allowing the plants to reach a similar transcriptome state. Time-dependent analysis of transcripts as markers for signalling pathways, and of metabolites and hormones as possibly involved transmitters, suggests that oxylipins such as oxophytodienoic acid and jasmonic acid, metabolites and redox cues predominantly control the acclimation response, whereas abscisic acid, salicylic acid and auxins play an insignificant or minor role.

Soil salinity severely affects plant growth and agricultural productivity. AtbZIP24 encodes a bZIP transcription factor that is induced by salt stress in Arabidopsis thaliana but suppressed in the salt-tolerant relative Lobularia maritima. Transcriptional repression of AtbZIP24 using RNA interference improved salt tolerance in A. thaliana. Under non-stress growth conditions, transgenic A. thaliana lines with decreased AtbZIP24 expression activated the expression of stress-inducible genes involved in cytoplasmic ion homeostasis and osmotic adjustment: the Na(+) transporter AtHKT1, the Na(+)/H(+) antiporter AtSOS1, the aquaporin AtPIP2.1, and a glutamine synthetase. In addition, candidate target genes of AtbZIP24 with functions in plant growth and development were identified such as an argonaute (AGO1)-related protein and cyclophilin AtCYP19. The salt tolerance in transgenic plants correlated with reduced Na(+) accumulation in leaves. In vivo interaction of AtbZIP24 as a homodimer was shown using fluorescence energy transfer (FRET) with cyan fluorescent protein (CFP) and yellow fluorescent protein (YFP) as fused FRET pairs. Translational fusion of AtbZIP24 with GFP showed subcellular localization of the protein in nucleus and cytoplasm in plants grown under control conditions whereas in response to salt stress AtbZIP24 was preferentially targeted to the nucleus. It is concluded that AtbZIP24 is an important regulator of salt stress response in plants. The modification of transcriptional control by regulatory transcription factors provides a useful strategy for improving salt tolerance in plants.

Salt stress is an environmental factor that severely impairs plant growth and productivity. Salinity-induced transcript accumulation was monitored in the salt-sensitive Arabidopsis thaliana and the related salt-tolerant Lobularia maritima using cDNA-arrays with expressed sequence tags derived from a cDNA subtraction library of salt-stressed L. maritima. The expression profiles revealed differences of the steady state transcript regulation in A. thaliana and L. maritima in response to salt stress. The differentially expressed transcripts include those involved in the control of gene expression as a transcription factor II homologue as well as signal transduction elements such as a serine/threonine protein kinase, a SNF1-related protein kinase AKIN10 homologue, and protein phosphatase 2C. Other ESTs with differential regulation patterns included transcripts encoding proteins with function in general stress responses and defense and included a peroxidase, dehydrins, enzymes of lipid and nitrogen metabolism, and functionally unclassified proteins. In a more detailed analysis the basic leucine zipper transcription factor AtbZIP24 showed differential transcript abundance in A. thaliana and L. maritima in response to salt stress. Transgenic AtbZIP24-RNAi lines showed improved growth and development under salt stress that was correlated with changed Cl(-) accumulation. The data indicate that AtbZIP24 functions as a transcriptional repressor in salt-stressed A. thaliana that negatively regulates growth and development under salinity in context of controlling Cl(-) homeostasis. Monitoring the differential and tissue specific global regulation of gene expression during adaptation to salinity in salt-sensitive and halotolerant plants is a promising and powerful approach to identify novel elements of plant salt stress adaptation.

Recent research has established redox-dependent thiol modification of proteins as a major regulatory layer superimposed on most cell functional categories in plants. Modern proteomics and forward as well as reverse genetics approaches have enabled the identification of a high number of novel targets of redox regulation. Redox-controlled processes range from metabolism to transport, transcription and translation. Gene activity regulation by transcription factors such as TGA, Athb-9 and RAP2 directly or indirectly is controlled by the redox state. Knowledge on putative redox sensors such as the peroxiredoxins, on redox transmitters including thioredoxins and glutaredoxins and biochemical mechanisms of their linkage to the metabolic redox environment has emerged as the framework of a functional redox regulatory network. Its basic principle is similar in eukaryotic cells and particularly complex in the photosynthesizing chloroplast. Methods and knowledge are now at hand to develop a quantitative understanding of redox signalling and the redox regulatory network in the eukaryotic cell.

Redox regulation is a central control element in cell metabolism. It is employed to adjust photosynthesis and the antioxidant defence system of leaves to the prevailing environment. During recent years progress has been made in describing the redox-dependent alterations in metabolism, the thiol/disulfide proteome, the redox-dependent and cross-talking signalling pathways and the target genes of redox regulation. Some transcription factors have been identified as proteins that perform thiol/disulfide transitions linked to the redox-regulation of specific plant promoters. In addition first mathematical models have been designed to simulate antioxidant defence and predict its response. Taken together, a profound experimental data set has been generated which allows to approach a systems biology type of understanding of antioxidant defence in photosynthesising cells in the near future. Since oxidative stress is likely to limit plant growth under stress, such a systematic understanding of antioxidant defence will help to define novel targets for breeding stress-tolerant plants.

During early seedling development of oil seed plants, the transition from lipid based heterotrophic to photoautotrophic carbohydrate metabolism is accompanied with a biphasic control of the chloroplast antioxidant system. In continuous light, organellar peroxiredoxins (Prx) and thylakoid-bound ascorbate peroxidase (tAPx) are activated early in seedling development, while stromal ascorbate peroxidase (sAPx), Cu/Zn-superoxide dismutase-2 (Csd2) and monodehydroascorbate reductase (MDHAR) and the cytosolic peroxiredoxins PrxIIB, PrxIIC and PrxIID are fully activated between 2.5 and 3 days after radicle emergence (DARE). Discontinuous light synchronized the expression of chloroplast antioxidant enzymes, but defined diurnally specific typeII-Prx-patterns in the cytosol and initiated chloroplast senescence around 2.5 DARE. Carbohydrate feeding uncoupled sAPx expression from the light pattern. In contrast, sucrose-feeding did not significantly impact on Prx transcript amounts. It is concluded that upon post-germination growth Prxs are activated endogenously to provide early antioxidant protection, which is supported by the Halliwell-Asada-Cycle, whose expressional activation depends on metabolic signals provided only later in development or in day-night-cycles.

Background: To coordinate metabolite fluxes and energy availability, plants adjust metabolism and gene expression to environmental changes through employment of interacting signalling pathways., Results: Comparing the response of Arabidopsis wild-type plants with that of the mutants adg1, pgr1 and vtc1 upon altered CO2-availability, the regulatory role of the cellular energy status, photosynthetic electron transport, the redox state and concentration of ascorbate and glutathione and the assimilatory force was analyzed in relation to the transcript abundance of stress-responsive nuclear encoded genes and psaA and psbA encoding the reaction centre proteins of photosystem I and II, respectively. Transcript abundance of Bap1, Stp1, psaA and psaB was coupled with seven metabolic parameters. Especially for psaA and psaB, the complex analysis demonstrated that the assumed PQ-dependent redox control is subordinate to signals linked to the relative availability of 3-PGA and DHAP, which define the assimilatory force. For the transcripts of sAPx and Csd2 high correlations with the calculated redox state of NADPH were observed in pgr1, but not in wild-type, suggesting that in wild-type plants signals depending on thylakoid acidification overlay a predominant redox-signal. Strongest correlation with the redox state of ascorbate was observed for 2CPA, whose transcript abundance regulation however was almost insensitive to the ascorbate content demonstrating dominance of redox regulation over metabolite sensing., Conclusion: In the mutants, signalling pathways are partially uncoupled, demonstrating dominance of metabolic control of photoreaction centre expression over sensing the redox state of the PQ-pool. The balance between the cellular redox poise and the energy signature regulates sAPx and Csd2 transcript abundance, while 2CPA expression is primarily redox-controlled.

Redox chemistry is a key feature of life. Oxidized substrates are reduced to synthesize functional molecules; reduced substrates are oxidized for energy supply. In addition, cells must fight against uncontrolled oxidation of essential constituents, a process that continuously occurs in an atmosphere of 21% O2. The redox situation is further complicated in plants with their highly reactive photosynthetic metabolism. To this end it is now well established that redox regulation is a central element in adjusting plant metabolism and development to the prevailing environmental conditions. This review introduces general redox chemistry and the main components of the cellular redox network, namely pyridine nucleotides, ascorbate, glutathione, lipoic acid, tocopherol, thioredoxins, glutaredoxins, peroxiredoxins, and other thiol proteins. Examples for redox sensing, transduction, redox-regulated enzymes and transcription, and the function of regulatory circuits are presented. Emphasis is placed on redox regulation of photosynthesis, which is the best understood metabolism governed by redox control on essentially all levels, ranging from gene transcription to translation, assembly and turnover, as well as short-term adaptation by state transition and enzyme activity. Increasing evidence shows the importance of redox regulation in the context of transport, plant development, and programmed cell death.