Pyridine nucleotide cycling and control of intracellular redox state in relation to poly (ADP-ribose) polymerase activity and nuclear localization of glutathione during exponential growth of Arabidopsis cells in culture

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Distinct redox state regulation in the seedling performance of Norway maple and sycamore

Norway maple and sycamore, two Acer genus species, have an important ecological value and different sensitivity to stressing factors being currently aggravated by climate change. Seedling growth is postulated to be the main barrier for successful plant establishment under the climate change scenarios. Therefore, the differences in redox regulation during the seedling performance of Norway maple and sycamore were investigated. Seeds of the two Acer species exhibited an identical high germination capacity, whereas seedling emergence was higher in sycamores. PCA analyses revealed that there is more diversification in the leaf characteristics than roots. Norway maple displayed a higher chlorophyll content index (CCI) with a similar leaf mass whereas sycamore seedlings exhibited a higher normalized difference vegetation index (NDVI), higher water content, higher root biomass and higher shoot height. Based on NDVI, sycamore seedlings appeared as very healthy plants, whereas Norway maple seedlings displayed a moderate healthy phenotype. Therefore, redox basis of seedling performance was investigated. The total pool of glutathione was four times higher in sycamore leaves than in Norway maple leaves and was reflected in highly reduced half-cell reduction potential of glutathione. Sycamore leaves contained more ascorbate because the content of its reduced form (AsA) was twice as high as in Norway maple. Therefore, the AsA/DHA ratio was balanced in sycamore leaves, reaching 1, and was halved in Norway maple leaves. Nicotinamide adenine dinucleotide phosphate content was twice as high in sycamore leaves than in Norway maples; however, its reduced form (NADPH) was predominant in Norway maple seedlings. Norway maple leaves exhibited the highest anabolic and catabolic redox charge. The higher reduction capacity and the activity of NADPH-dependent reductases in Norway maple leaves possibly resulted in higher CCI, whereas the larger root system contributed to higher NDVI in sycamore. The different methods of controlling redox parameters in Acer seedlings grown at controlled conditions provided here can be useful in understanding how tree species can cope with a changing environment in the future.

Role of Poly(ADP-Ribose) Polymerase (PARP) Enzyme in the Systemic Acquired Acclimation Induced by Light Stress in Phaseolus vulgaris L. Plants

Plants are able to acclimate to environmental constraints through functional modifications that may also occur in tissues that are not directly exposed to stress. This process is termed “systemic acquired acclimation.” The present study aims to evaluate the involvement of PolyADP-ribose) polymerase (PARP) protein in the acclimation process to high light (HL) stress in Phaseolus vulgaris plants. For this purpose, some leaves located at the top of the plant, in the apical position, were directly exposed to HL (“inducing” leaves), while others on the same plant, distal from the top, continued to be exposed to growth light (“receiving” leaves) to verify the hypothesis that an “alert” message may be transferred from injured tissues to distal ones. Biochemical and eco-physiological analyses, namely PARP activity, H₂O₂ and water- and fat-soluble antioxidants (i.e., ascorbic acid, tocopherol, glutathione (GSH), phenols, carotenoids, etc.) content, and chlorophyll fluorescence measurements were performed on both “inducing” and “receiving” leaves. Even if no change in PARP expression was found, its activity increased in “receiving” unstressed leaves in response to the light stress duration experimented by “inducing” leaves, while antioxidant capacity declined. When the “receiving” leaves were exposed to HL, the PARP activity returned to the control value, while antioxidant capacity photosynthetic electron transport rate (J_f) decreased and increased, respectively, compared to Control. Our results seem to show an acclimation pathway triggered in remote tissues not yet subjected to stress, likely involving a reactive oxygen species wave activating the PARP enzyme in a mechanism still to be clarified. In addition, the increased tolerance of plants directly exposed to HL could implicate a boosted synthesis of soluble antioxidants accompanied by a reduction of PARP activity to reduce excessive consumption of NAD(P).

Redox status of the plant cell determines epigenetic modifications under abiotic stress conditions and during developmental processes

BACKGROUND

The oxidation-reduction (redox) status of the cell influences or regulates transcription factors and enzymes involved in epigenetic changes, such as DNA methylation, histone protein modifications, and chromatin structure and remodeling. These changes are crucial regulators of chromatin architecture, leading to differential gene expression in eukaryotes. But the cell's redox homeostasis is difficult to sustain since the production of reactive oxygen species (ROS) and reactive nitrogen species (RNS) is not equal in plants at different developmental stages and under abiotic stress conditions. Exceeding optimum ROS and RNS levels leads to oxidative stress and thus alters the redox status of the cell. Consequently, this alteration modulates intracellular epigenetic modifications that either mitigate or mediate the plant growth and stress response.

AIM OF REVIEW

Recent studies suggest that the altered redox status of the cell reform the cellular functions and epigenetic changes. Recent high-throughput techniques have also greatly advanced redox-mediated gene expression discovery, but the integrated view of the redox status, and its associations with epigenetic changes and subsequent gene expression in plants are still scarce. In this review, we accordingly focus on how the redox status of the cell affects epigenetic modifications in plants under abiotic stress conditions and during developmental processes. This is a first comprehensive review on the redox status of the cell covering the redox components and signaling, redox status alters the post-translational modification of proteins, intracellular epigenetic modifications, redox interplay during DNA methylation, redox regulation of histone acetylation and methylation, redox regulation of miRNA biogenesis, redox regulation of chromatin structure and remodeling and conclusion, future perspectives and biotechnological opportunities for the future development of the plants.

KEY SCIENTIFIC CONCEPTS OF REVIEW

The interaction of redox mediators such as ROS, RNS and antioxidants regulates redox homeostasis and redox-mediated epigenetic changes. We discuss how redox mediators modulate epigenetic changes and show the opportunities for smart use of the redox status of the cell in plant development and abiotic stress adaptation. However, how a redox mediator triggers epigenetic modification without activating other redox mediators remains yet unknown.

Measuring ROS and redox markers in plant cells

Reactive oxygen species (ROS) are produced throughout plant cells as a by-product of electron transfer processes. While highly oxidative and potentially damaging to a range of biomolecules, there exists a suite of ROS-scavenging antioxidant strategies that maintain a redox equilibrium. This balance can be disrupted in the event of cellular stress leading to increased ROS levels, which can act as a useful stress signal but, in excess, can result in cell damage and death. As crop plants become exposed to greater degrees of multiple stresses due to climate change, efforts are ongoing to engineer plants with greater stress tolerance. It is therefore important to understand the pathways underpinning ROS-mediated signalling and damage, both through measuring ROS themselves and other indicators of redox imbalance. The highly reactive and transient nature of ROS makes this challenging to achieve, particularly in a way that is specific to individual ROS species. In this review, we describe the range of chemical and biological tools and techniques currently available for ROS and redox marker measurement in plant cells and tissues. We discuss the limitations inherent in current methodology and opportunities for advancement.

Nicotinamide adenine dinucleotides are associated with distinct redox control of germination in Acer seeds with contrasting physiology

Seed germination is a complex process enabling plant reproduction. Germination was found to be regulated at the proteome, metabolome and hormonal levels as well as via discrete post-translational modification of proteins including phosphorylation and carbonylation. Redox balance is also involved but less studied. Acer seeds displaying orthodox and recalcitrant characteristics were investigated to determine the levels of redox couples of nicotinamide adenine dinucleotide (NAD) phosphate (NADP) and integrated with the levels of ascorbate and glutathione. NAD and NADP concentrations were higher in Norway maple seeds and exceptionally high at the germinated stage, being the most contrasting parameter between germinating Acer seeds. In contrast, NAD(P)H/NAD(P)+ ratios were higher in sycamore seeds, thus exhibiting higher reducing power. Despite distinct concentrations of ascorbate and glutathione, both seed types attained in embryonic axes and cotyledons had similar ratios of reduced/oxidized forms of ascorbate and half-cell reduction potential of glutathione at the germinated stage. Both species accomplished germination displaying different strategies to modulate redox status. Sycamore produced higher amounts of ascorbate and maintained pyridine nucleotides in reduced forms. Interestingly, lower NAD(P) concentrations limited the regeneration of ascorbate and glutathione but dynamically drove metabolic reactions, particularly in this species, and contributed to faster germination. We suggest that NAD(P) is an important player in regulating redox status during germination in a distinct manner in Norway maple and sycamore seeds.

Integrating multiple omics to identify common and specific molecular changes occurring in Arabidopsis under chronic nitrate and sulfate limitations

Plants have fundamental dependences on nitrogen and sulfur and frequently have to cope with chronic limitations when their supply is sub-optimal. This study aimed at characterizing the metabolomic, proteomic, and transcriptomic changes occurring in Arabidopsis leaves under chronic nitrate (Low-N) and chronic sulfate (Low-S) limitations in order to compare their effects, determine interconnections, and examine strategies of adaptation. Metabolite profiling globally revealed opposite effects of Low-S and Low-N on carbohydrate and amino acid accumulations, whilst proteomic data showed that both treatments resulted in increases in catabolic processes, stimulation of mitochondrial and cytosolic metabolism, and decreases in chloroplast metabolism. Lower abundances of ribosomal proteins and translation factors under Low-N and Low-S corresponded with growth limitation. At the transcript level, the major and specific effect of Low-N was the enhancement of expression of defence and immunity genes. The main effect of chronic Low-S was a decrease in transcripts of genes involved in cell division, DNA replication, and cytoskeleton, and an increase in the expression of autophagy genes. This was consistent with a role of target-of-rapamycin kinase in the control of plant metabolism and cell growth and division under chronic Low-S. In addition, Low-S decreased the expression of several NLP transcription factors, which are master actors in nitrate sensing. Finally, both the transcriptome and proteome data indicated that Low-S repressed glucosinolate synthesis, and that Low-N exacerbated glucosinolate degradation. This showed the importance of glucosinolate as buffering molecules for N and S management.

Glutathione: A key player in metal chelation, nutrient homeostasis, cell cycle regulation and the DNA damage response in cadmium-exposed Arabidopsis thaliana

Glutathione (GSH) is an important player in plant responses to cadmium (Cd) through its dual function as an antioxidant and precursor for metal-chelating phytochelatins (PCs). In addition, it was shown to be involved in cell cycle regulation in Arabidopsis thaliana roots, but its involvement in this process in leaves is largely unknown and has never been evaluated in Cd-exposed plants. This study aimed to elucidate the role of GSH in leaf growth and development, metal chelation, nutrient homeostasis and cell cycle regulation in A. thaliana plants upon prolonged Cd exposure. Responses were compared between wild-type (WT) plants and three GSH-deficient mutants. Our results indicate that PC production remains important in plants exposed to Cd for an extended duration. Furthermore, an important role for GSH in regulating nutrient homeostasis in Cd-exposed plants was revealed. Cell cycle analysis demonstrated that negative effects of Cd exposure on cell division and endoreplication were more pronounced in leaves of the GSH-deficient cadmium-sensitive 2-1 (cad2-1) mutant in comparison to the WT, indicating the involvement of GSH in cell cycle regulation. Finally, a crucial role for GSH in transcriptional activation of the Cd-induced DNA damage response (DDR) was revealed, as the Cd-induced upregulation of DDR-related genes was either less pronounced or completely abolished in leaves of the GSH-deficient mutants.

Plant growth-regulating molecules as thermoprotectants: functional relevance and prospects for improving heat tolerance in food crops

Among various abiotic stresses, heat stress is one of the most damaging, threatening plant productivity and survival all over the world. Warmer temperatures due to climatic anomalies above optimum growing temperatures have detrimental impacts on crop yield potential as well as plant distribution patterns. Heat stress affects overall plant metabolism in terms of physiology, biochemistry, and gene expression. Membrane damage, protein degradation, enzyme inactivation, and the accumulation of reactive oxygen species are some of the harmful effects of heat stress that cause injury to various cellular compartments. Although plants are equipped with various defense strategies to counteract these adversities, their defensive means are not sufficient to defend against the ever-rising temperatures. Hence, substantial yield losses have been observed in all crop species under heat stress. Here, we describe the involvement of various plant growth-regulators (PGRs) (hormones, polyamines, osmoprotectants, antioxidants, and other signaling molecules) in thermotolerance, through diverse cellular mechanisms that protect cells under heat stress. Several studies involving the exogenous application of PGRs to heat-stressed plants have demonstrated their role in imparting tolerance, suggesting the strong potential of these molecules in improving the performance of food crops grown under high temperature.

Oxidative post-translational modifications in histones

Epigenetic regulation is attracting much attention because it explains many of the effects that the external environment induces in organisms. Changes in the cellular redox status and even more specifically in its nuclear redox compartment is one of these examples. Redox changes can induce modulation of the epigenetic regulation in cells. Here we present a few cases where reactive oxygen or nitrogen species induces epigenetic marks in histones. Posttranslational modification of these proteins like histone nitrosylation, carbonylation, or glutathionylation together with other mechanisms not reviewed here are the cornerstones of redox-related epigenetic regulation. We currently face a new field of research with potential important consequences for the treatment of many pathologies.

Poly(ADP-Ribose) Polymerases in Plants and Their Human Counterparts: Parallels and Peculiarities

Poly(ADP-ribosyl)ation is a rapid and transient post-translational protein modification that was described first in mammalian cells. Activated by the sensing of DNA strand breaks, poly(ADP-ribose)polymerase1 (PARP1) transfers ADP-ribose units onto itself and other target proteins using NAD⁺ as a substrate. Subsequently, DNA damage responses and other cellular responses are initiated. In plants, poly(ADP-ribose) polymerases (PARPs) have also been implicated in responses to DNA damage. The Arabidopsis genome contains three canonical PARP genes, the nomenclature of which has been uncoordinated in the past. Albeit assumptions concerning the function and roles of PARP proteins in planta have often been inferred from homology and structural conservation between plant PARPs and their mammalian counterparts, plant-specific roles have become apparent. In particular, PARPs have been linked to stress responses of plants. A negative role under abiotic stress has been inferred from studies in which a genetic or, more commonly, pharmacological inhibition of PARP activity improved the performance of stressed plants; in response to pathogen-associated molecular patterns, a positive role has been suggested. However, reports have been inconsistent, and the effects of PARP inhibitors appear to be more robust than the genetic abolition of PARP gene expression, indicating the presence of alternative targets of those drugs. Collectively, recent evidence suggests a conditionality of stress-related phenotypes of parp mutants and calls for a reconsideration of PARP inhibitor studies on plants. This review critically summarizes our current understanding of poly(ADP-ribosylation) and PARP proteins in plants, highlighting similarities and differences to human PARPs, areas of controversy, and requirements for future studies.

Glutathione compartmentalization and its role in glutathionylation and other regulatory processes of cellular pathways

Glutathione is considered the major non-protein low molecular weight modulator of redox processes and the most important thiol reducing agent of the cell. The biosynthesis of glutathione occurs in the cytosol from its constituent amino acids, but this tripeptide is also present in the most important cellular districts, such as mitochondria, nucleus, and endoplasmic reticulum, thus playing a central role in several metabolic pathways and cytoprotection mechanisms. Indeed, glutathione is involved in the modulation of various cellular processes and, not by chance, it is a ubiquitous determinant for redox signaling, xenobiotic detoxification, and regulation of cell cycle and death programs. The balance between its concentration and redox state is due to a complex series of interactions between biosynthesis, utilization, degradation, and transport. All these factors are of great importance to understand the significance of cellular redox balance and its relationship with physiological responses and pathological conditions. The purpose of this review is to give an overview on glutathione cellular compartmentalization. Information on its subcellular distribution provides a deeper understanding of glutathione-dependent processes and reflects the importance of compartmentalization in the regulation of specific cellular pathways. © 2018 BioFactors, 45(2):152-168, 2019.

Redox Balance-DDR-miRNA Triangle: Relevance in Genome Stability and Stress Responses in Plants

Plants are continuously faced with complex environmental conditions which can affect the oxidative metabolism and photosynthetic efficiency, thus leading to the over-production of reactive oxygen species (ROS). Over a certain threshold, ROS can damage DNA. DNA damage, unless repaired, can affect genome stability, thus interfering with cell survival and severely reducing crop productivity. A complex network of pathways involved in DNA damage response (DDR) needs to be activated in order to maintain genome integrity. The expression of specific genes belonging to these pathways can be used as indicators of oxidative DNA damage and effective DNA repair in plants subjected to stress conditions. Managing ROS levels by modulating their production and scavenging systems shifts the role of these compounds from toxic molecules to key messengers involved in plant tolerance acquisition. Oxidative and anti-oxidative signals normally move among the different cell compartments, including the nucleus, cytosol, and organelles. Nuclei are dynamically equipped with different redox systems, such as glutathione (GSH), thiol reductases, and redox regulated transcription factors (TFs). The nuclear redox network participates in the regulation of the DNA metabolism, in terms of transcriptional events, replication, and repair mechanisms. This mainly occurs through redox-dependent regulatory mechanisms comprising redox buffering and post-translational modifications, such as the thiol-disulphide switch, glutathionylation, and S-nitrosylation. The regulatory role of microRNAs (miRNAs) is also emerging for the maintenance of genome stability and the modulation of antioxidative machinery under adverse environmental conditions. In fact, redox systems and DDR pathways can be controlled at a post-transcriptional level by miRNAs. This review reports on the interconnections between the DDR pathways and redox balancing systems. It presents a new dynamic picture by taking into account the shared regulatory mechanism mediated by miRNAs in plant defense responses to stress.

Analysis of Redox Relationships in the Plant Cell Cycle: Determination of Ascorbate, Glutathione, and Poly(ADPribose)polymerase (PARP) in Plant Cell Cultures

Reactive oxygen species (ROS) and low molecular weight antioxidants, such as glutathione and ascorbate, are powerful signalling molecules that participate in the control of plant growth and development, and modulate progression through the mitotic cell cycle. Enhanced ROS accumulation or low levels of ascorbate or glutathione cause the cell cycle to arrest and halt progression especially through the G1 checkpoint. Plant cell suspension cultures have proved to be particularly useful tools for the study of cell cycle regulation. Here we provide effective and accurate methods for the measurement of changes in the cellular ascorbate and glutathione pools and the activities of related enzymes such poly(ADP-ribose)polymerase (PARP) during mitosis and cell expansion, particularly in cell suspension cultures. These methods can be used in studies seeking to improve current understanding of the roles of redox controls on cell division and cell expansion.

The Salt-Stress Response of the Transgenic Plum Line J8-1 and Its Interaction with the Salicylic Acid Biosynthetic Pathway from Mandelonitrile

Salinity is considered as one of the most important abiotic challenges that affect crop productivity. Plant hormones, including salicylic acid (SA), are key factors in the defence signalling output triggered during plant responses against environmental stresses. We have previously reported in peach a new SA biosynthetic pathway from mandelonitrile (MD), the molecule at the hub of the cyanogenic glucoside turnover in Prunus sp. In this work, we have studied whether this new SA biosynthetic pathway is also present in plum and the possible role this pathway plays in plant plasticity under salinity, focusing on the transgenic plum line J8-1, which displays stress tolerance via an enhanced antioxidant capacity. The SA biosynthesis from MD in non-transgenic and J8-1 micropropagated plum shoots was studied by metabolomics. Then the response of J8-1 to salt stress in presence of MD or Phe (MD precursor) was assayed by measuring: chlorophyll content and fluorescence parameters, stress related hormones, levels of non-enzymatic antioxidants, the expression of two genes coding redox-related proteins, and the content of soluble nutrients. The results from in vitro assays suggest that the SA synthesis from the MD pathway demonstrated in peach is not clearly present in plum, at least under the tested conditions. Nevertheless, in J8-1 NaCl-stressed seedlings, an increase in SA was recorded as a result of the MD treatment, suggesting that MD could be involved in the SA biosynthesis under NaCl stress conditions in plum plants. We have also shown that the plum line J8-1 was tolerant to NaCl under greenhouse conditions, and this response was quite similar in MD-treated plants. Nevertheless, the MD treatment produced an increase in SA, jasmonic acid (JA) and reduced ascorbate (ASC) contents, as well as in the coefficient of non-photochemical quenching (qN) and the gene expression of Non-Expressor of Pathogenesis-Related 1 (NPR1) and thioredoxin H (TrxH) under salinity conditions. This response suggested a crosstalk between different signalling pathways (NPR1/Trx and SA/JA) leading to salinity tolerance in the transgenic plum line J8-1.

Nitrate- and nitric oxide-induced plant growth in pea seedlings is linked to antioxidative metabolism and the ABA/GA balance

This study looks at the effects of potassium nitrate (KNO3) and sodium nitroprusside (SNP), a nitric oxide (NO)-donor, on the development, antioxidant defences and on the abscisic acid (ABA) and gibberellin (GA) levels in pea seedlings. Results show that 10 mM KNO3 and 50 μM SNP stimulate seedling fresh weight (FW), although this effect is not reverted by the action of 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO), a NO-scavenger. The KNO3 treatment increased peroxidase (POX) and ascorbate oxidase (AOX) activities. SNP, on the other hand, reduced monodehydroascorbate reductase (MDHAR) activity and produced a significant increase in superoxide dismutase (SOD), POX and AOX activities. The "KNO3 plus cPTIO" treatment increased ascorbate peroxidase (APX), MDHAR, glutathione reductase (GR) and SOD activities, but POX activity decreased in relation to the KNO3 treatment. The "SNP plus cPTIO" treatment increased APX and MDHAR activities, whereas a huge decrease in POX activity occurred. Both the KNO3 and the SNP treatments increased reduced ascorbate (ASC) concentrations, which reached control values in the presence of cPTIO. All treatments increased the dehydroascorbate (DHA) level in pea seedlings, leading to a decrease in the redox state of ascorbate. In the "KNO3 plus cPTIO" treatment, an increase in the redox state of ascorbate was observed. Glutathione contents, however, were higher in the presence of SNP than in the presence of KNO3. In addition, KNO3 produced an accumulation of oxidised glutathione (GSSG), especially in the presence of cPTIO, leading to a decrease in the redox state of glutathione. The effect of SNP on reduced glutathione (GSH) levels was reverted by cPTIO, suggesting that NO has a direct effect on GSH biosynthesis or turnover. Both the KNO3 and SNP treatments produced an increase in GA4 and a decrease in ABA concentrations, and this effect was reverted in the presence of the NO-scavenger. Globally, the results suggest a relationship between antioxidant metabolism and the ABA/GA balance during early seedling growth in pea. The results also suggest a role for KNO3 and NO in the modulation of GA4 and ABA levels and antioxidant metabolism in pea seedlings. Furthermore, this effect correlated with an increase in the biomass of the pea seedlings.

Reactive oxygen species in plant development

Reactive oxygen species (ROS) are produced by metabolic pathways in almost all cells. As signaling components, ROS are best known for their roles in abiotic and biotic stress-related events. However, recent studies have revealed that they are also involved in numerous processes throughout the plant life cycle, from seed development and germination, through to root, shoot and flower development. Here, we provide an overview of ROS production and signaling in the context of plant growth and development, highlighting the key functions of ROS and their interactions with plant phytohormonal networks.

Glutathione modulates the expression of heat shock proteins via the transcription factors BZIP10 and MYB21 in Arabidopsis

The contribution of glutathione (GSH) in combating environmental stress in plants has long been known. Previous reports have pointed to the involvement of GSH in inducing various heat shock proteins (HSPs), but the molecular mechanism is yet to be explored. Here, we investigate how GSH induces the expression of important HSP genes in Arabidopsis. Expression of HSP genes BiP3, HSP70B, and HSP90.1 was positively regulated by GSH, and a promoter activation assay suggested a role for GSH in their induction. Lower expression of BiP3 and HSP70B in the GSH-fed Atmyb21 mutant and of HSP90.1 in the GSH-fed Atbzip10 mutant, in comparison with GSH-fed Col-0, revealed a role for GSH in activating their promoters through the transcription factors MYB21 and BZIP10. Co-transfection of transcription factor mutant protoplasts with transcription factor constructs and HSP promoters confirmed the results. Comparative proteomics also revealed proteins whose expression was controlled by MYB21 and BZIP10 in response to GSH feeding. A co-immunoprecipitation assay demonstrated a role for GSH in modulating the level of interaction of glutathione-S-transferase with HSP70. Collectively, our results demonstrate a role for GSH in activating the promoters of BiP3 and HSP70B via MYB21 and of HSP90.1 via BZIP10.

Inhibitor-induced oxidation of the nucleus and cytosol in Arabidopsis thaliana: implications for organelle to nucleus retrograde signalling

Concepts of organelle-to-nucleus signalling pathways are largely based on genetic screens involving inhibitors of chloroplast and mitochondrial functions such as norflurazon, lincomycin (LINC), antimycin A (ANT) and salicylhydroxamic acid. These inhibitors favour enhanced cellular oxidation, but their precise effects on the cellular redox state are unknown. Using the in vivo reduction–oxidation (redox) reporter, roGFP2, inhibitor-induced changes in the glutathione redox potentials of the nuclei and cytosol were measured in Arabidopsis thaliana root, epidermal and stomatal guard cells, together with the expression of nuclear-encoded chloroplast and mitochondrial marker genes. All the chloroplast and mitochondrial inhibitors increased the degree of oxidation in the nuclei and cytosol. However, inhibitor-induced oxidation was less marked in stomatal guard cells than in epidermal or root cells. Moreover, LINC and ANT caused a greater oxidation of guard cell nuclei than the cytosol. Chloroplast and mitochondrial inhibitors significantly decreased the abundance of LHCA1 and LHCB1 transcripts. The levels of WHY1, WHY3 and LEA5 transcripts were increased in the presence of inhibitors. Chloroplast inhibitors decreased AOXA1 mRNA levels, while mitochondrial inhibitors had the opposite effect. Inhibitors that are used to characterize retrograde signalling pathways therefore have similar general effects on cellular redox state and gene expression.

This article is part of the themed issue ‘Enhancing photosynthesis in crop plants: targets for improvement’.

H2O2 Signature and Innate Antioxidative Profile Make the Difference Between Sensitivity and Tolerance to Salt in Rice Cells

Salt tolerance is a complex trait that varies between and within species. H₂O₂ profiles as well as antioxidative systems have been investigated in the cultured cells of rice obtained from Italian rice varieties with different salt tolerance. Salt stress highlighted differences in extracellular and intracellular H₂O₂ profiles in the two cell cultures. The tolerant variety had innate reactive oxygen species (ROS) scavenging systems that enabled ROS, in particular H₂O₂, to act as a signal molecule rather than a damaging one. Different intracellular H₂O₂ profiles were also observed: in tolerant cells, an early and narrow peak was detected at 5 min; while in sensitive cells, a large peak was associated with cell death. Likewise, the transcription factor salt-responsive ethylene responsive factor 1 (TF SERF1), which is known for being regulated by H₂O₂, showed a different expression profile in the two cell lines. Notably, similar H₂O₂ profiles and cell fates were also obtained when exogenous H₂O₂ was produced by glucose/glucose oxidase (GOX) treatment. Under salt stress, the tolerant variety also exhibited rapid upregulation of K⁺ transporter genes in order to deal with K⁺/Na⁺ impairment. This upregulation was not detected in the presence of oxidative stress alone. The importance of the innate antioxidative profile was confirmed by the protective effect of experimentally increased glutathione in salt-treated sensitive cells. Overall, these results underline the importance of specific H₂O₂ signatures and innate antioxidative systems in modulating ionic and redox homeostasis for salt stress tolerance.

Redox Changes During the Cell Cycle in the Embryonic Root Meristem of Arabidopsis thaliana

AIMS

The aim of this study was to characterize redox changes in the nuclei and cytosol occurring during the mitotic cell cycle in the embryonic roots of germinating Arabidopsis seedlings, and to determine how redox cycling was modified in mutants with a decreased capacity for ascorbate synthesis.

RESULTS

Using an in vivo reduction-oxidation (redox) reporter (roGFP2), we show that transient oxidation of the cytosol and the nuclei occurred at G1 in the synchronized dividing cells of the Arabidopsis root apical meristem, with reduction at G2 and mitosis. This redox cycle was absent from low ascorbate mutants in which nuclei were significantly more oxidized than controls. The cell cycle-dependent increase in nuclear size was impaired in the ascorbate-deficient mutants, which had fewer cells per unit area in the root proliferation zone. The transcript profile of the dry seeds and size of the imbibed seeds was strongly influenced by low ascorbate but germination, dormancy release and seed aging characteristics were unaffected.

INNOVATION

These data demonstrate the presence of a redox cycle within the plant cell cycle and that the redox state of the nuclei is an important factor in cell cycle progression.

CONCLUSIONS

Controlled oxidation is a key feature of the early stages of the plant cell cycle. However, sustained mild oxidation restricts nuclear functions and impairs progression through the cell cycle leading to fewer cells in the root apical meristem. Antioxid. Redox Signal. 27, 1505-1519.

Metabolomics and Biochemical Approaches Link Salicylic Acid Biosynthesis to Cyanogenesis in Peach Plants

Despite the long-established importance of salicylic acid (SA) in plant stress responses and other biological processes, its biosynthetic pathways have not been fully characterized. The proposed synthesis of SA originates from chorismate by two distinct pathways: the isochorismate and phenylalanine (Phe) ammonia-lyase (PAL) pathways. Cyanogenesis is the process related to the release of hydrogen cyanide from endogenous cyanogenic glycosides (CNglcs), and it has been linked to plant plasticity improvement. To date, however, no relationship has been suggested between the two pathways. In this work, by metabolomics and biochemical approaches (including the use of [13C]-labeled compounds), we provide strong evidences showing that CNglcs turnover is involved, at least in part, in SA biosynthesis in peach plants under control and stress conditions. The main CNglcs in peach are prunasin and amygdalin, with mandelonitrile (MD), synthesized from phenylalanine, controlling their turnover. In peach plants MD is the intermediary molecule of the suggested new SA biosynthetic pathway and CNglcs turnover, regulating the biosynthesis of both amygdalin and SA. MD-treated peach plants displayed increased SA levels via benzoic acid (one of the SA precursors within the PAL pathway). MD also provided partial protection against Plum pox virus infection in peach seedlings. Thus, we propose a third pathway, an alternative to the PAL pathway, for SA synthesis in peach plants.

Timing of developmental reduction in epithelial glutathione redox potential is associated with increased epithelial proliferation in the immature murine intestine

BackgroundThe intracellular redox potential of the glutathione (GSH)/glutathione disulfide (GSSG) couple regulates cellular processes. In vitro studies indicate that a reduced GSH/GSSG redox potential favors proliferation, whereas a more oxidized redox potential favors differentiation. Intestinal growth depends upon an appropriate balance between the two. However, how the ontogeny of intestinal epithelial cellular (IEC) GSH/GSSG redox regulates these processes in the developing intestine has not been fully characterized in vivo.MethodsOntogeny of intestinal GSH redox potential and growth were measured in neonatal mice.ResultsWe show that IEC GSH/GSSG redox potential becomes increasingly reduced (primarily driven by increased GSH concentration) over the first 3 weeks of life. Increased intracellular GSH has been shown to drive proliferation through increased poly-ADP-ribose polymerase (PARP) activity. We show that increasing IEC poly-ADP-ribose chains can be measured over the first 3 weeks of life, indicating an increase in IEC PARP activity. These changes are accompanied by increased intestinal growth and IEC proliferation as assessed by villus height/crypt depth, intestinal length, and Ki67 staining.ConclusionUnderstanding how IEC GSH/GSSG redox potential is developmentally regulated may provide insight into how premature human intestinal redox states can be manipulated to optimize intestinal growth and adaptation.

Thioredoxin (Trxo1) interacts with proliferating cell nuclear antigen (PCNA) and its overexpression affects the growth of tobacco cell culture

Thioredoxins (Trxs), key components of cellular redox regulation, act by controlling the redox status of many target proteins, and have been shown to play an essential role in cell survival and growth. The presence of a Trx system in the nucleus has received little attention in plants, and the nuclear targets of plant Trxs have not been conclusively identified. Thus, very little is known about the function of Trxs in this cellular compartment. Previously, we studied the intracellular localization of PsTrxo1 and confirmed its presence in mitochondria and, interestingly, in the nucleus under standard growth conditions. In investigating the nuclear function of PsTrxo1 we identified proliferating cellular nuclear antigen (PCNA) as a PsTrxo1 target by means of affinity chromatography techniques using purified nuclei from pea leaves. Such protein-protein interaction was corroborated by dot-blot and bimolecular fluorescence complementation (BiFC) assays, which showed that both proteins interact in the nucleus. Moreover, PsTrxo1 showed disulfide reductase activity on previously oxidized recombinant PCNA protein. In parallel, we studied the effects of PsTrxo1 overexpression on Tobacco Bright Yellow-2 (TBY-2) cell cultures. Microscopy and flow-cytometry analysis showed that PsTrxo1 overexpression increases the rate of cell proliferation in the transformed lines, with a higher percentage of the S phase of the cell cycle at the beginning of the cell culture (days 1 and 3) and at the G2/M phase after longer times of culture (day 9), coinciding with an upregulation of PCNA protein. Furthermore, in PsTrxo1 overexpressed cells there is a decrease in the total cellular glutathione content but maintained nuclear GSH accumulation, especially at the end of the culture, which is accompanied by a higher mitotic index, unlike non-overexpressing cells. These results suggest that Trxo1 is involved in the cell cycle progression of TBY-2 cultures, possibly through its link with cellular PCNA and glutathione.

Modulation of NADH Levels by Arabidopsis Nudix Hydrolases, AtNUDX6 and 7, and the Respective Proteins Themselves Play Distinct Roles in the Regulation of Various Cellular Responses Involved in Biotic/Abiotic Stresses

Arabidopsis Nudix hydrolases, AtNUDX6 and 7, exhibit pyrophosphohydrolase activities toward NADH and contribute to the modulation of various defense responses, such as the poly(ADP-ribosyl)ation (PAR) reaction and salicylic acid (SA)-induced Nonexpresser of Pathogenesis-Related genes 1 (NPR1)-dependent defense pathway, against biotic and abiotic stresses. However, the mechanisms by which these enzymes regulate such cellular responses remain unclear. To clarify the functional role(s) of AtNUDX6 and 7 and NADH metabolism, we examined the effects of the transient expression of the active and inactive forms of AtNUDX6 and 7 under the control of an estrogen (ES)-inducible system on various stress responses. The transient expression of active AtNUDX6 and 7 proteins suppressed NADH levels and induced PAR activity, whereas that of their inactive forms did not, indicating the involvement of NADH metabolism in the regulation of the PAR reaction. A transcriptome analysis using KO-nudx6, KO-nudx7 and double KO-nudx6/7 plants, in which intracellular NADH levels increased, identified genes (NADH-responsive genes, NRGs) whose expression levels positively and negatively correlated with NADH levels. Many NRGs did not overlap with the genes whose expression was reported to be responsive to various types of oxidants and reductants, suggesting a novel role for intracellular NADH levels as a redox signaling cue. The active and inactive AtNUDX6 proteins induced the expression of thioredoxin-h5, the activator of NPR1 and SA-induced NPR1-dependent defense genes, while the active and inactive AtNUDX7 proteins suppressed the accumulation of SA and subsequent gene expression, indicating that AtNUDX6 and 7 proteins themselves play distinct roles in stress responses.

Oxidative stress and antioxidative systems: recipes for successful data collection and interpretation

Oxidative stress and reactive oxygen species (ROS) are common to many fundamental responses of plants. Enormous and ever-growing interest has focused on this research area, leading to an extensive literature that documents the tremendous progress made in recent years. As in other areas of plant biology, advances have been greatly facilitated by developments in genomics-dependent technologies and the application of interdisciplinary techniques that generate information at multiple levels. At the same time, advances in understanding ROS are fundamentally reliant on the use of biochemical and cell biology techniques that are specific to the study of oxidative stress. It is therefore timely to revisit these approaches with the aim of providing a guide to convenient methods and assisting interested researchers in avoiding potential pitfalls. Our critical overview of currently popular methodologies includes a detailed discussion of approaches used to generate oxidative stress, measurements of ROS themselves, determination of major antioxidant metabolites, assays of antioxidative enzymes and marker transcripts for oxidative stress. We consider the applicability of metabolomics, proteomics and transcriptomics approaches and discuss markers such as damage to DNA and RNA. Our discussion of current methodologies is firmly anchored to future technological developments within this popular research field.

Glutathione--linking cell proliferation to oxidative stress

SIGNIFICANCE

The multifaceted functions of reduced glutathione (gamma-glutamyl-cysteinyl-glycine; GSH) continue to fascinate plants and animal scientists, not least because of the dynamic relationships between GSH and reactive oxygen species (ROS) that underpin reduction/oxidation (redox) regulation and signalling. Here we consider the respective roles of ROS and GSH in the regulation of plant growth, with a particular focus on regulation of the plant cell cycle. Glutathione is discussed not only as a crucial low molecular weight redox buffer that shields nuclear processes against oxidative challenge but also a flexible regulator of genetic and epigenetic functions.

RECENT ADVANCES

The intracellular compartmentalization of GSH during the cell cycle is remarkably consistent in plants and animals. Moreover, measurements of in vivo glutathione redox potentials reveal that the cellular environment is much more reducing than predicted from GSH/GSSG ratios measured in tissue extracts. The redox potential of the cytosol and nuclei of non-dividing plant cells is about -300 mV. This relatively low redox potential maintained even in cells experiencing oxidative stress by a number of mechanisms including vacuolar sequestration of GSSG. We propose that regulated ROS production linked to glutathione-mediated signalling events are the hallmark of viable cells within a changing and challenging environment.

CRITICAL ISSUES

The concept that the cell cycle in animals is subject to redox controls is well established but little is known about how ROS and GSH regulate this process in plants. However, it is increasingly likely that redox controls exist in plants, although possibly through different pathways. Moreover, redox-regulated proteins that function in cell cycle checkpoints remain to be identified in plants. While GSH-responsive genes have now been identified, the mechanisms that mediate and regulate protein glutathionylation in plants remain poorly defined.

FUTURE DIRECTIONS

The nuclear GSH pool provides an appropriate redox environment for essential nuclear functions. Future work will focus on how this essential thiol interacts with the nuclear thioredoxin system and nitric oxide to regulate genetic and epigenetic mechanisms. The characterization of redox-regulated cell cycle proteins in plants, and the elucidation of mechanisms that facilitate GSH accumulation in the nucleus are keep steps to unravelling the complexities of nuclear redox controls.

Analysis of knockout mutants reveals non-redundant functions of poly(ADP-ribose)polymerase isoforms in Arabidopsis

The enzyme poly(ADP-ribose)polymerase (PARP) has a dual function being involved both in the poly(ADP-ribosyl)ation and being a constituent of the NAD(+) salvage pathway. To date most studies, both in plant and non-plant systems, have focused on the signaling role of PARP in poly(ADP-ribosyl)ation rather than any role that can be ascribed to its metabolic function. In order to address this question we here used a combination of expression, transcript and protein localization studies of all three PARP isoforms of Arabidopsis alongside physiological analysis of the corresponding mutants. Our analyses indicated that whilst all isoforms of PARP were localized to the nucleus they are also present in non-nuclear locations with parp1 and parp3 also localised in the cytosol, and parp2 also present in the mitochondria. We next isolated and characterized insertional knockout mutants of all three isoforms confirming a complete knockout in the full length transcript levels of the target genes as well as a reduced total leaf NAD hydrolase activity in the two isoforms (PARP1, PARP2) that are highly expressed in leaves. Physiological evaluation of the mutant lines revealed that they displayed distinctive metabolic and root growth characteristics albeit unaltered leaf morphology under optimal growth conditions. We therefore conclude that the PARP isoforms play non-redundant non-nuclear metabolic roles and that their function is highly important in rapidly growing tissues such as the shoot apical meristem, roots and seeds.

Physiological and biochemical mechanisms of the ornamental Eugenia myrtifolia L. plants for coping with NaCl stress and recovery

We studied the response of Eugenia myrtifolia L. plants, an ornamental shrub native to tropical and subtropical areas, to salt stress in order to facilitate the use of these plants in Mediterranean areas for landscaping. E. myrtifolia plants implement a series of adaptations to acclimate to salinity, including morphological, physiological and biochemical changes. Furthermore, the post-recovery period seems to be detected by Eugenia plants as a new stress situation. Different physiological and biochemical changes in Eugenia myrtifolia L. plants after being subjected to NaCl stress for up to 30 days (Phase I) and after recovery from salinity (Phase II) were studied. Eugenia plants proved to be tolerant to NaCl concentrations between 44 and 88 mM, displaying a series of adaptative mechanisms to cope with salt-stress, including the accumulation of toxic ions in roots. Plants increased their root/shoot ratio and decreased their leaf area, leaf water potential and stomatal conductance in order to limit water loss. In addition, they displayed different strategies to protect the photosynthetic machinery, including the limited accumulation of toxic ions in leaves, increase in chlorophyll content, changes in chlorophyll fluorescence parameters, leaf anatomy and antioxidant defence mechanisms. Anatomical modifications in leaves, including an increase in palisade parenchyma and intercellular spaces and decrease in spongy parenchyma, served to facilitate CO2 diffusion in a situation of reduced stomatal aperture. Salinity produced oxidative stress in Eugenia plants as evidenced by oxidative stress parameters values and a reduction in APX and ASC levels. Nevertheless, SOD and GSH contents increased. The post-recovery period is detected as a new stress situation, as observed through effects on plant growth and alterations in chlorophyll fluorescence and oxidative stress parameters.

Role of glutathione, glutathione transferase, and glutaredoxin in regulation of redox-dependent processes

Over the last decade fundamentally new features have been revealed for the participation of glutathione and glutathione-dependent enzymes (glutathione transferase and glutaredoxin) in cell proliferation, apoptosis, protein folding, and cell signaling. Reduced glutathione (GSH) plays an important role in maintaining cellular redox status by participating in thiol-disulfide exchange, which regulates a number of cell functions including gene expression and the activity of individual enzymes and enzyme systems. Maintaining optimum GSH/GSSG ratio is essential to cell viability. Decrease in the ratio can serve as an indicator of damage to the cell redox status and of changes in redox-dependent gene regulation. Disturbance of intracellular GSH balance is observed in a number of pathologies including cancer. Consequences of inappropriate GSH/GSSG ratio include significant changes in the mechanism of cellular redox-dependent signaling controlled both nonenzymatically and enzymatically with the participation of isoforms of glutathione transferase and glutaredoxin. This review summarizes recent data on the role of glutathione, glutathione transferase, and glutaredoxin in the regulation of cellular redox-dependent processes.

Increased glutathione contributes to stress tolerance and global translational changes in Arabidopsis

Although glutathione is well known for its reactive oxygen species (ROS) scavenging function and plays a protective role in biotic stress, its regulatory function in abiotic stress still remains to be elucidated. Our previous study showed that exogenously applied reduced glutathione (GSH) could improve abiotic stress tolerance in Arabidopsis. Here, we report that endogenously increased GSH also conferred tolerance to drought and salt stress in Arabidopsis. Moreover, both exogenous and endogenous GSH delayed senescence and flowering time. Polysomal profiling results showed that global translation was enhanced after GSH treatment and by the induced increase of GSH level by salt stress. By performing transcriptomic analyses of steady-state and polysome-bound mRNAs in GSH-treated plants, we reveal that GSH has a substantial impact on translation. Translational changes induced by GSH treatment target numerous hormones and stress signaling molecules, which might contribute to the enhanced stress tolerance in GSH-treated plants. Our translatome analysis also revealed that abscisic acid (ABA), auxin and jasmonic acid (JA) biosynthesis, as well as signaling genes, were activated during GSH treatment, which has not been reported in previously published transcriptomic data. Together, our data suggest that the increased glutathione level results in stress tolerance and global translational changes.

Diurnal changes in the xanthophyll cycle pigments of freshwater algae correlate with the environmental hydrogen peroxide concentration rather than non-photochemical quenching

BACKGROUND AND AIMS

In photosynthetic organisms exposure to high light induces the production of reactive oxygen species (ROS), such as hydrogen peroxide (H2O2), which in part is prevented by non-photochemical quenching (NPQ). As one of the most stable and longest-lived ROS, H2O2 is involved in key signalling pathways in development and stress responses, although in excess it can induce damage. A ubiquitous response to high light is the induction of the xanthophyll cycle, but its role in algae is unclear as it is not always associated with NPQ induction. The aim of this study was to reveal how diurnal changes in the level of H2O2 are regulated in a freshwater algal community.

METHODS

A natural freshwater community of algae in a temporary rainwater pool was studied, comprising photosynthetic Euglena species, benthic Navicula diatoms, Chlamydomonas and Chlorella species. Diurnal measurements were made of photosynthetic performance, concentrations of photosynthetic pigments and H2O2. The frequently studied model organisms Chlamydomonas and Chlorella species were isolated to study photosynthesis-related H2O2 responses to high light.

KEY RESULTS

NPQ was shown to prevent H2O2 release in Chlamydomonas and Chlorella species under high light; in addition, dissolved organic carbon excited by UV-B radiation was probably responsible for a part of the H2O2 produced in the water column. Concentrations of H2O2 peaked at 2 µm at midday and algae rapidly scavenged H2O2 rather than releasing it. A vertical H2O2 gradient was observed that was lowest next to diatom-rich benthic algal mats. The diurnal changes in photosynthetic pigments included the violaxanthin and diadinoxanthin cycles; the former was induced prior to the latter, but neither was strictly correlated with NPQ.

CONCLUSIONS

The diurnal cycling of H2O2 was apparently modulated by the organisms in this freshwater algal community. Although the community showed flexibility in its levels of NPQ, the diurnal changes in xanthophylls correlated with H2O2 concentrations. Alternative NPQ mechanisms in algae involving proteins of the light-harvesting complex type and antioxidant protection of the thylakoid membrane by de-epoxidized carotenoids are discussed.

Redox markers for drought-induced nodule senescence, a process occurring after drought-induced senescence of the lowest leaves in soybean (Glycine max)

BACKGROUND AND AIMS

Water is an increasingly scarce resource that limits crop productivity in many parts of the world, and the frequency and severity of drought are predicted to increase as a result of climate change. Improving tolerance to drought stress is therefore important for maximizing future crop yields. The aim of this study was to compare the effects of drought on soybean (Glycine max) leaves and nodules in order to define phenotypic markers and changes in cellular redox state that characterize the stress response in different organs, and to characterize the relationships between leaf and nodule senescence during drought.

METHODS

Leaf and crown nodule metabolite pools were measured together with leaf and soil water contents, and leaf chlorophyll, total protein contents and chlorophyll a fluorescence quenching parameters in nodulated soybeans that were grown under either well-watered conditions or deprived of water for up to 21 d.

KEY RESULTS

Ureides, ascorbate, protein, chlorophyll and the ratios of variable chlorophyll a fluorescence (Fv') to maximal chlorophyll a fluorescence (Fm') fell to levels below detection in the oldest leaves after 21 d of drought. While these drought-induced responses were not observed in the youngest leaf ranks, the Fv'/Fm' ratios, pyridine nucleotide levels and the reduction state of the ascorbate pool were lower in all leaf ranks after 21 d of drought. In contrast to leaves, total nodule protein, pyridine nucleotides, ureides, ascorbate and glutathione contents increased as a result of the drought treatment. However, the nodule ascorbate pool was significantly less reduced as a result of drought. Higher levels of transcripts encoding two peroxiredoxins were detected in nodules exposed to drought stress but senescence-associated transcripts and other mRNAs encoding redox-related proteins were similar under both conditions.

CONCLUSIONS

While the physiological impact of the drought was perceived throughout the shoot, stress-induced senescence occurred only in the oldest leaf ranks. At this stage, a number of drought-induced changes in nodule metabolites were observed but no metabolite or transcript markers of senescence could be detected. It is concluded that stress-induced senescence in the lowest leaf ranks precedes nodule senescence, suggesting that leaves of low photosynthetic capacity are sacrificed in favour of nodule nitrogen metabolism.

Low concentrations of the toxin ophiobolin A lead to an arrest of the cell cycle and alter the intracellular partitioning of glutathione between the nuclei and cytoplasm

Ophiobolin A, a tetracyclic sesterpenoid produced by phytopathogenic fungi, is responsible for catastrophic losses in crop yield but its mechanism of action is not understood. The effects of ophiobolin A were therefore investigated on the growth and redox metabolism of Tobacco Bright Yellow-2 (TBY-2) cell cultures by applying concentrations of the toxin that did not promote cell death. At concentrations between 2 and 5 μM, ophiobolin A inhibited growth and proliferation of the TBY-2 cells, which remained viable. Microscopic and cytofluorimetric analyses showed that ophiobolin A treatment caused a rapid decrease in mitotic index, with a lower percentage of the cells at G1 and increased numbers of cells at the S/G2 phases. Cell size was not changed following treatment suggesting that the arrest of cell cycle progression was not the result of a block on cell growth. The characteristic glutathione redox state and the localization of glutathione in the nucleus during cell proliferation were not changed by ophiobolin A. However, subsequent decreases in glutathione and the re-distribution of glutathione between the cytoplasm and nuclei after mitosis occurring in control cells, as well as the profile of glutathionylated proteins, were changed in the presence of the toxin. The profile of poly ADP-ribosylated proteins were also modified by ophiobolin A. Taken together, these data provide evidence of the mechanism of ophiobolin A action as a cell cycle inhibitor and further demonstrate the link between nuclear glutathione and the cell cycle regulation, suggesting that glutathione-dependent redox controls in the nuclei prior to cell division are of pivotal importance.

Ectopic phytocystatin expression increases nodule numbers and influences the responses of soybean (Glycine max) to nitrogen deficiency

Cysteine proteases and cystatins have many functions that remain poorly characterised, particularly in crop plants. We therefore investigated the responses of these proteins to nitrogen deficiency in wild-type soybeans and in two independent transgenic soybean lines (OCI-1 and OCI-2) that express the rice cystatin, oryzacystatin-I (OCI). Plants were grown for four weeks under either a high (5 mM) nitrate (HN) regime or in the absence of added nitrate (LN) in the absence or presence of symbiotic rhizobial bacteria. Under the LN regime all lines showed similar classic symptoms of nitrogen deficiency including lower shoot biomass and leaf chlorophyll. However, the LN-induced decreases in leaf protein and increases in root protein tended to be smaller in the OCI-1 and OCI-2 lines than in the wild type. When LN-plants were grown with rhizobia, OCI-1 and OCI-2 roots had significantly more crown nodules than wild-type plants. The growth nitrogen regime had a significant effect on the abundance of transcripts encoding vacuolar processing enzymes (VPEs), LN-dependent increases in VPE2 and VPE3 transcripts in all lines. However, the LN-dependent increases of VPE2 and VPE3 transcripts were significantly lower in the leaves of OCI-1 and OCI-2 plants than in the wild type. These results show that nitrogen availability regulates the leaf and root cysteine protease, VPE and cystatin transcript profiles in a manner that is in some cases influenced by ectopic OCI expression. Moreover, the OCI-dependent inhibition of papain-like cysteine proteases favours increased nodulation and enhanced tolerance to nitrogen limitation, as shown by the smaller LN-dependent decreases in leaf protein observed in the OCI-1 and OCI-2 plants relative to the wild type.

The metabolomics of oxidative stress

Oxidative stress resulting from increased availability of reactive oxygen species (ROS) is a key component of many responses of plants to challenging environmental conditions. The consequences for plant metabolism are complex and manifold. We review data on small compounds involved in oxidative stress, including ROS themselves and antioxidants and redox buffers in the membrane and soluble phases, and we discuss the wider consequences for plant primary and secondary metabolism. While metabolomics has been exploited in many studies on stress, there have been relatively few non-targeted studies focused on how metabolite signatures respond specifically to oxidative stress. As part of the discussion, we present results and reanalyze published datasets on metabolite profiles in catalase-deficient plants, which can be considered to be model oxidative stress systems. We emphasize the roles of ROS-triggered changes in metabolites as potential oxidative signals, and discuss responses that might be useful as markers for oxidative stress. Particular attention is paid to lipid-derived compounds, the status of antioxidants and antioxidant breakdown products, altered metabolism of amino acids, and the roles of phytohormone pathways.

Allyl isothiocyanate depletes glutathione and upregulates expression of glutathione S-transferases in Arabidopsis thaliana

Allyl isothiocyanate (AITC) is a phytochemical associated with plant defense in plants from the Brassicaceae family. AITC has long been recognized as a countermeasure against external threats, but recent reports suggest that AITC is also involved in the onset of defense-related mechanisms such as the regulation of stomatal aperture. However, the underlying cellular modes of action in plants remain scarcely investigated. Here we report evidence of an AITC-induced depletion of glutathione (GSH) and the effect on gene expression of the detoxification enzyme family glutathione S-transferases (GSTs) in Arabidopsis thaliana. Treatment of A. thaliana wild-type with AITC resulted in a time- and dose-dependent depletion of cellular GSH. AITC-exposure of mutant lines vtc1 and pad2-1 with elevated and reduced GSH-levels, displayed enhanced and decreased AITC-tolerance, respectively. AITC-exposure also led to increased ROS-levels in the roots and loss of chlorophyll which are symptoms of oxidative stress. Following exposure to AITC, we found that GSH rapidly recovered to the same level as in the control plant, suggesting an effective route for replenishment of GSH or a rapid detoxification of AITC. Transcriptional analysis of genes encoding GSTs showed an upregulation in response to AITC. These findings demonstrate cellular effects by AITC involving a reversible depletion of the GSH-pool, induced oxidative stress, and elevated expression of GST-encoding genes.

ROS-mediated redox signaling during cell differentiation in plants

BACKGROUND

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

SCOPE OF REVIEW

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

MAJOR CONCLUSIONS

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

GENERAL SIGNIFICANCE

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

Defects in a new class of sulfate/anion transporter link sulfur acclimation responses to intracellular glutathione levels and cell cycle control

We previously identified a mutation, suppressor of mating type locus3 15-1 (smt15-1), that partially suppresses the cell cycle defects caused by loss of the retinoblastoma tumor suppressor-related protein encoded by the MAT3 gene in Chlamydomonas reinhardtii. smt15-1 single mutants were also found to have a cell cycle defect leading to a small-cell phenotype. SMT15 belongs to a previously uncharacterized subfamily of putative membrane-localized sulfate/anion transporters that contain a sulfate transporter domain and are found in a widely distributed subset of eukaryotes and bacteria. Although we observed that smt15-1 has a defect in acclimation to sulfur-limited growth conditions, sulfur acclimation (sac) mutants, which are more severely defective for acclimation to sulfur limitation, do not have cell cycle defects and cannot suppress mat3. Moreover, we found that smt15-1, but not sac mutants, overaccumulates glutathione. In wild-type cells, glutathione fluctuated during the cell cycle, with highest levels in mid G1 phase and lower levels during S and M phases, while in smt15-1, glutathione levels remained elevated during S and M. In addition to increased total glutathione levels, smt15-1 cells had an increased reduced-to-oxidized glutathione redox ratio throughout the cell cycle. These data suggest a role for SMT15 in maintaining glutathione homeostasis that impacts the cell cycle and sulfur acclimation responses.

Ectopic phytocystatin expression leads to enhanced drought stress tolerance in soybean (Glycine max) and Arabidopsis thaliana through effects on strigolactone pathways and can also result in improved seed traits

Ectopic cystatin expression has long been used in plant pest management, but the cysteine protease, targets of these inhibitors, might also have important functions in the control of plant lifespan and stress tolerance that remain poorly characterized. We therefore characterized the effects of expression of the rice cystatin, oryzacystatin-I (OCI), on the growth, development and stress tolerance of crop (soybean) and model (Arabidopsis thaliana) plants. Ectopic OCI expression in soybean enhanced shoot branching and leaf chlorophyll accumulation at later stages of vegetative development and enhanced seed protein contents and decreased the abundance of mRNAs encoding strigolactone synthesis enzymes. The OCI-expressing A. thaliana showed a slow-growth phenotype, with increased leaf numbers and enhanced shoot branching at flowering. The OCI-dependent inhibition of cysteine proteases enhanced drought tolerance in soybean and A. thaliana, photosynthetic CO2 assimilation being much less sensitive to drought-induced inhibition in the OCI-expressing soybean lines. Ectopic OCI expression or treatment with the cysteine protease inhibitor E64 increased lateral root densities in A. thaliana. E64 treatment also increased lateral root densities in the max2-1 mutants that are defective in strigolactone signalling, but not in the max3-9 mutants that are defective in strigolactone synthesis. Taken together, these data provide evidence that OCI-inhibited cysteine proteases participate in the control of growth and stress tolerance through effects on strigolactones. We conclude that cysteine proteases are important targets for manipulation of plant growth, development and stress tolerance, and also seed quality traits.

Enhanced salt-induced antioxidative responses involve a contribution of polyamine biosynthesis in grapevine plants

The possible involvement of polyamines in the salt stress adaptation was investigated in grapevine (Vitis vinifera L.) plantlets focusing on photosynthesis and oxidative metabolism. Salt stress resulted in the deterioration of plant growth and photosynthesis, and treatment of plantlets with methylglyoxal-bis(guanylhydrazone) (MGBG), a S-adenosylmethionine decarboxylase (SAMDC) inhibitor, enhanced the salt stress effect. A decrease in PSII quantum yield (Fv/Fm), effective PSII quantum yield (Y(II)) and coefficient of photochemical quenching (qP) as well as increases in non-photochemical quenching (NPQ) and its coefficient (qN) was observed by these treatments. Salt and/or MGBG treatments also triggered an increase in lipid peroxidation and reactive oxygen species (ROS) accumulation as well as an increase of superoxide dismutase (SOD) and peroxidase (POX) activities, but not ascorbate peroxidase (APX) activity. Salt stress also resulted in an accumulation of oxidized ascorbate (DHA) and a decrease in reduced glutathione. MGBG alone or in combination with salt stress increased monodehydroascorbate reductase (MDHAR), SOD and POX activities and surprisingly no accumulation of DHA was noticed following treatment with MGBG. These salt-induced responses correlated with the maintaining of high level of free and conjugated spermidine and spermine, whereas a reduction of agmatine and putrescine levels was observed, which seemed to be amplified by the MGBG treatment. These results suggest that maintaining polyamine biosynthesis through the enhanced SAMDC activity in grapevine leaf tissues under salt stress conditions could contribute to the enhanced ROS scavenging activity and a protection of photosynthetic apparatus from oxidative damages.

Poly(ADP-ribose)polymerase activity controls plant growth by promoting leaf cell number

A changing global environment, rising population and increasing demand for biofuels are challenging agriculture and creating a need for technologies to increase biomass production. Here we demonstrate that the inhibition of poly (ADP-ribose) polymerase activity is a promising technology to achieve this under non-stress conditions. Furthermore, we investigate the basis of this growth enhancement via leaf series and kinematic cell analysis as well as single leaf transcriptomics and plant metabolomics under non-stress conditions. These data indicate a regulatory function of PARP within cell growth and potentially development. PARP inhibition enhances growth of Arabidopsis thaliana by enhancing the cell number. Time course single leaf transcriptomics shows that PARP inhibition regulates a small subset of genes which are related to growth promotion, cell cycle and the control of metabolism. This is supported by metabolite analysis showing overall changes in primary and particularly secondary metabolism. Taken together the results indicate a versatile function of PARP beyond its previously reported roles in controlling plant stress tolerance and thus can be a useful target for enhancing biomass production.

Suppression of the external mitochondrial NADPH dehydrogenase, NDB1, in Arabidopsis thaliana affects central metabolism and vegetative growth

Ca(2+)-dependent oxidation of cytosolic NADPH is mediated by NDB1, which is an external type II NADPH dehydrogenase in the plant mitochondrial electron transport chain. Using RNA interference, the NDB1 transcript was suppressed by 80% in Arabidopsis thaliana plants, and external Ca(2+)-dependent NADPH dehydrogenase activity became undetectable in isolated mitochondria. This was linked to a decreased level of NADP(+) in rosettes of the transgenic lines. Sterile-grown transgenic seedlings displayed decreased growth specifically on glucose, and respiratory metabolism of (14)C-glucose was increased. On soil, NDB1-suppressing plants had a decreased vegetative biomass, but leaf maximum quantum efficiency of photosystem II and CO2 assimilation rates, as well as total respiration, were similar to the wild-type. The in vivo alternative oxidase activity and capacity were also similar in all genotypes. Metabolic profiling revealed decreased levels of sugars, citric acid cycle intermediates, and amino acids in the transgenic lines. The NDB1-suppression induced transcriptomic changes associated with protein synthesis and glucosinolate and jasmonate metabolism. The transcriptomic changes also overlapped with changes observed in a mutant lacking ABAINSENSITIVE4 and in A. thaliana overexpressing stress tolerance genes from rice. The results thus indicate that A. thaliana NDB1 modulates NADP(H) reduction levels, which in turn affect central metabolism and growth, and interact with defense signaling.

Ectopic expression of cytosolic superoxide dismutase and ascorbate peroxidase leads to salt stress tolerance in transgenic plums

To fortify the antioxidant capacity of plum plants, genes encoding cytosolic antioxidants ascorbate peroxidase (cytapx) and Cu/Zn-superoxide dismutase (cytsod) were genetically engineered in these plants. Transgenic plum plants expressing the cytsod and/or cytapx genes in cytosol have been generated under the control of the CaMV35S promoter. High levels of cytsod and cytapx gene transcripts suggested that the transgenes were constitutively and functionally expressed. We examined the potential functions of cytSOD and cytAPX in in vitro plum plants against salt stress (100 mm NaCl). Several transgenic plantlets expressing cytsod and/or cytapx showed an enhanced tolerance to salt stress, mainly lines C5-5 and J8-1 (expressing several copies of sod and apx, respectively). Transformation as well as NaCl treatments influenced the antioxidative metabolism of plum plantlets, including enzymatic and nonenzymatic antioxidants. Transgenic plantlets exhibited higher contents of nonenzymatic antioxidants glutathione and ascorbate than nontransformed control, which correlated with lower accumulation of hydrogen peroxide. Overall, our results suggest that transformation of plum plants with genes encoding antioxidant enzymes enhances the tolerance to salinity.

Ophiobolin A, a sesterterpenoid fungal phytotoxin, displays higher in vitro growth-inhibitory effects in mammalian than in plant cells and displays in vivo antitumor activity

Ophiobolin A, a sesterterpenoid produced by plant pathogenic fungi, was purified from the culture extract of Drechslera gigantea and tested for its growth-inhibitory activity in both plant and mammalian cells. Ophiobolin A induced cell death in Nicotiana tabacum L. cv. Bright Yellow 2 (TBY-2) cells at concentrations ≥10 µM, with the TBY-2 cells showing typical features of apoptosis-like cell death. At a concentration of 5 µM, ophiobolin A did not affect plant cell viability but prevented cell proliferation. When tested on eight cancer cell lines, concentrations <1 µM of ophiobolin A inhibited growth by 50% after 3 days of culture irrespective of their multidrug resistance (MDR) phenotypes and their resistance levels to pro-apoptotic stimuli. It is, thus, unlikely that ophiobolin A exerts these in vitro growth-inhibitory effects in cancer cells by activating pro-apoptotic processes. Highly proliferative human keratinocytes appeared more sensitive to the growth-inhibitory effects of ophiobolin A than slowly proliferating ones. Ophiobolin A also displayed significant antitumor activity at the level of mouse survival when assayed at 10 mg/kg in the B16F10 mouse melanoma model with lung pseudometastases. Ophiobolin A could, thus, represent a novel scaffold to combat cancer types that display various levels of resistance to pro-apoptotic stimuli and/or various MDR phenotypes.

Redox modulation of plant developmental regulators from the class I TCP transcription factor family

TEOSINTE BRANCHED1-CYCLOIDEA-PROLIFERATING CELL FACTOR1 (TCP) transcription factors participate in plant developmental processes associated with cell proliferation and growth. Most members of class I, one of the two classes that compose the family, have a conserved cysteine at position 20 (Cys-20) of the TCP DNA-binding and dimerization domain. We show that Arabidopsis (Arabidopsis thaliana) class I proteins with Cys-20 are sensitive to redox conditions, since their DNA-binding activity is inhibited after incubation with the oxidants diamide, oxidized glutathione, or hydrogen peroxide or with nitric oxide-producing agents. Inhibition can be reversed by treatment with the reductants dithiothreitol or reduced glutathione or by incubation with the thioredoxin/thioredoxin reductase system. Mutation of Cys-20 in the class I protein TCP15 abolished its redox sensitivity. Under oxidizing conditions, covalently linked dimers were formed, suggesting that inactivation is associated with the formation of intermolecular disulfide bonds. Inhibition of class I TCP protein activity was also observed in vivo, in yeast (Saccharomyces cerevisiae) cells expressing TCP proteins and in plants after treatment with redox agents. This inhibition was correlated with modifications in the expression of the downstream CUC1 gene in plants. Modeling studies indicated that Cys-20 is located at the dimer interface near the DNA-binding surface. This places this residue in the correct orientation for intermolecular disulfide bond formation and explains the sensitivity of DNA binding to the oxidation of Cys-20. The redox properties of Cys-20 and the observed effects of cellular redox agents both in vitro and in vivo suggest that class I TCP protein action is under redox control in plants.