Changes in the subcellular distribution of glutathione during virus infection in Cucurbita pepo (L.)

Glutathione-the "master" antioxidant in the regulation of resistant and susceptible host-plant virus-interaction

The interaction between plant hosts and plant viruses is a very unique and complex process, relying on dynamically modulated intercellular redox states and the generation of reactive oxygen species (ROS). Plants strive to precisely control this state during biotic stress, as optimal redox levels enable proper induction of defense mechanisms against plant viruses. One of the crucial elements of ROS regulation and redox state is the production of metabolites, such as glutathione, or the activation of glutathione-associated enzymes. Both of these elements play a role in limiting the degree of potential oxidative damage in plant cells. While the role of glutathione and specific enzymes is well understood in other types of abiotic and biotic stresses, particularly those associated with bacteria or fungi, recent advances in research have highlighted the significance of glutathione modulation and mutations in genes encoding glutathione-associated enzymes in triggering immunity or susceptibility against plant viruses. Apparently, glutathione-associated genes are involved in precisely controlling and protecting host cells from damage caused by ROS during viral infections, playing a crucial role in the host's response. In this review, we aim to outline the significant improvements made in research on plant viruses and glutathione, specifically in the context of their involvement in susceptible and resistant responses, as well as changes in the localization of glutathione. Analyses of essential glutathione-associated enzymes in susceptible and resistant responses have demonstrated that the levels of enzymatic activity or the absence of specific enzymes can impact the spread of the virus and activate host-induced defense mechanisms. This contributes to the complex network of the plant immune system. Although investigations of glutathione during the plant-virus interplay remain a challenge, the use of novel tools and approaches to explore its role will significantly contribute to our knowledge in the field.

Glutathione Modulation in PVYNTN Susceptible and Resistant Potato Plant Interactions

Glutathione is a metabolite that plays an important role in plant response to biotic stress through its ability to remove reactive oxygen species, thereby limiting the degree of potential oxidative damage. It can couple changes in the intracellular redox state to the development, especially the defense responses, of plants. Several studies have focused on measuring glutathione levels in virus infected plants, but have not provided complete information. Therefore, we analyzed, for the first time, the content of glutathione as well as its ultrastructural distribution related to susceptible and hypersensitive potato–Potato virus Y NTN (PVY^NTN) interaction, with an aim of providing new insight into interactive responses to PVY^NTN stress. Our findings reported that the inoculation of PVY^NTN caused a dynamic increase in the content of glutathione, not only in resistance but also in susceptible reaction, especially at the first steps of plant–virus interaction. Moreover, the increase in hypersensitive response was much more dynamic, and accompanied by a significant reduction in the content of PVY^NTN. By contrast, in susceptible potato Irys, the content of glutathione decreased between 7 and 21 days after virus inoculation, which led to a significant increase in PVY^NTN concentration. Additionally, our findings clearly indicated the steady induction of two selected potato glutathione S-transferase StGSTF1 and StGSTF2 genes after PVY^NTN inoculation, regardless of the interaction type. However, the relative expression level of StGSTF1 did not significantly differ between resistant and susceptible plants, whereas the relative expression levels of StGSTF2 differed between susceptible and resistant reactions. Therefore, we proposed that StGSTF2 can act as a marker of the type of response to PVY^NTN. Our observations indicated that glutathione is an important component of signaling as well as the regulatory network in the PVY^NTN–potato pathosystem. In resistance responses to PVY^NTN, this metabolite activates plant defenses by reducing potential damage to the host plant cell, causing a reduction in virus concentration, while it can also be involved in the development of PVY^NTN elicited symptoms, as well as limiting oxidative stress, leading to systemic infection in susceptible potato plants.

Glutathione contributes to resistance responses to TMV through a differential modulation of salicylic acid and reactive oxygen species

Systemic acquired resistance (SAR) is induced by pathogens and confers protection against a broad range of pathogens. Several SAR signals have been characterized, but the nature of the other unknown signalling by small metabolites in SAR remains unclear. Glutathione (GSH) has long been implicated in the defence reaction against biotic stress. However, the mechanism that GSH increases plant tolerance against virus infection is not entirely known. Here, a combination of a chemical, virus-induced gene-silencing-based genetics approach, and transgenic technology was undertaken to investigate the role of GSH in plant viral resistance in Nicotiana benthamiana. Tobacco mosaic virus (TMV) infection results in increasing the expression of GSH biosynthesis genes NbECS and NbGS, and GSH content. Silencing of NbECS or NbGS accelerated oxidative damage, increased accumulation of reactive oxygen species (ROS), compromised plant resistance to TMV, and suppressed the salicylic acid (SA)-mediated signalling pathway. Application of GSH or l-2-oxothiazolidine-4-carboxylic acid (a GSH activator) alleviated oxidative damage, decreased accumulation of ROS, elevated plant local and systemic resistance, enhanced the SA-mediated signalling pathway, and increased the expression of ROS scavenging-related genes. However, treatment with buthionine sulfoximine (a GSH inhibitor) accelerated oxidative damage, elevated ROS accumulation, compromised plant systemic resistance, suppressed the SA-mediated signalling pathway, and reduced the expression of ROS-regulating genes. Overexpression of NbECS reduced oxidative damage, decreased accumulation of ROS, increased resistance to TMV, activated the SA-mediated signalling pathway, and increased the expression of the ROS scavenging-related genes. We present molecular evidence suggesting GSH is essential for both local and systemic resistance of N. benthamiana to TMV through a differential modulation of SA and ROS.

dsRNA Molecules From the Tobacco Mosaic Virus p126 Gene Counteract TMV-Induced Proteome Changes at an Early Stage of Infection

Exogenous application of double-stranded RNA (dsRNA) in the tobacco-Tobacco mosaic virus (TMV) pathosystem was shown previously to induce resistance against TMV providing an alternative approach to transgenesis. In the present study, we employed proteomics technology to elucidate the effect of TMV on tobacco as well as the effect of exogenous application of TMV p126 dsRNA molecules (dsRNAp126) at an early stage of the tobacco-TMV interaction. The proteome of tobacco leaf at 15 min post inoculation (mpi) in the presence or absence of dsRNAp126 molecules was studied. Thirty-six tobacco proteins were differentially accumulated in TMV-infected vs. healthy tobacco leaf tissue. The identified main differential TMV-responsive proteins were found to be involved in photosynthesis, energy metabolism, stress, and defense responses. Most of the virus-induced changes in the tobacco leaf proteome were not observed in the leaves treated with dsRNAp126 + TMV. The results indicated that the protein changes induced by TMV infection were counteracted by the exogenous application of dsRNAp126 molecules. Moreover, using small RNA sequencing, we showed that the exogenously applied dsRNAp126 was efficiently processed in tobacco as early as 15 min post application (mpa) to produce small interfering RNAs (siRNAs); the dicing pattern was not affected by the presence of TMV. The presence of dsRNAp126 reduced TMV p126 RNA abundance suggesting virus titer reduction via a sequence-specific mechanism, since a non-homologous dsRNA did not protect from TMV infection nor affect TMV accumulation.

The Versatile Roles of Sulfur-Containing Biomolecules in Plant Defense-A Road to Disease Resistance

Sulfur (S) is an essential plant macronutrient and the pivotal role of sulfur compounds in plant disease resistance has become obvious in recent decades. This review attempts to recapitulate results on the various functions of sulfur-containing defense compounds (SDCs) in plant defense responses to pathogens. These compounds include sulfur containing amino acids such as cysteine and methionine, the tripeptide glutathione, thionins and defensins, glucosinolates and phytoalexins and, last but not least, reactive sulfur species and hydrogen sulfide. SDCs play versatile roles both in pathogen perception and initiating signal transduction pathways that are interconnected with various defense processes regulated by plant hormones (salicylic acid, jasmonic acid and ethylene) and reactive oxygen species (ROS). Importantly, ROS-mediated reversible oxidation of cysteine residues on plant proteins have profound effects on protein functions like signal transduction of plant defense responses during pathogen infections. Indeed, the multifaceted plant defense responses initiated by SDCs should provide novel tools for plant breeding to endow crops with efficient defense responses to invading pathogens.

Subcellular Roles of Glutathione in Mediating Plant Defense during Biotic Stress

Glutathione and reactive oxygen species (ROS) play important roles, within different cell compartments, in activating plant defense and the development of resistance. In mitochondria, the accumulation of ROS and the change of glutathione towards its oxidized state leads to mitochondrial dysfunction, activates cell death, and triggers resistance. The accumulation of glutathione in chloroplasts and peroxisomes at the early stages of plant pathogen interactions is related to increased tolerance and resistance. The collapse of the antioxidative system in these two cell compartments at the later stages leads to cell death through retrograde signaling. The cytosol can be considered to be the switchboard during biotic stress where glutathione is synthesized, equally distributed to, and collected from different cell compartments. Changes in the redox state of glutathione and the accumulation of ROS in the cytosol during biotic stress can initiate the activation of defense genes in nuclei through pathways that involve salicylic acid, jasmonic acid, auxins, and abscisic acid. This review dissects the roles of glutathione in individual organelles during compatible and incompatible bacterial, fungal, and viral diseases in plants and explores the subcelluar roles of ROS, glutathione, ascorbate, and related enzymes in the development of resistance.

Artificial elevation of glutathione contents in salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) reduces susceptibility to the powdery mildew pathogen Euoidium longipes

The effects of elevated glutathione levels on defence responses to powdery mildew (Euoidium longipes) were investigated in a salicylic acid-deficient tobacco (Nicotiana tabacum cv. Xanthi NahG) and wild-type cv. Xanthi plants, where salicylic acid (SA) contents are normal. Aqueous solutions of reduced glutathione (GSH) and its synthetic precursor R-2-oxothiazolidine-4-carboxylic acid (OTC) were injected into leaves of tobacco plants 3 h before powdery mildew inoculation. SA-deficient NahG tobacco was hyper-susceptible to E. longipes, as judged by significantly more severe powdery mildew symptoms and enhanced pathogen accumulation. Strikingly, elevation of GSH levels in SA-deficient NahG tobacco restored susceptibility to E. longipes to the extent seen in wild-type plants (i.e. enhanced basal resistance). However, expression of the SA-mediated pathogenesis-related gene (NtPR-1a) did not increase significantly in GSH or OTC-pretreated and powdery mildew-inoculated NahG tobacco, suggesting that the induction of this PR gene may not be directly involved in the defence responses induced by GSH. Our results demonstrate that artificial elevation of glutathione content can significantly reduce susceptibility to powdery mildew in SA-deficient tobacco.

Glutathione Can Compensate for Salicylic Acid Deficiency in Tobacco to Maintain Resistance to Tobacco Mosaic Virus

Earlier studies showed that the artificial elevation of endogenous glutathione (GSH) contents can markedly increase the resistance of plants against different viruses. On the other hand, salicylic acid (SA)-deficient NahG plants display enhanced susceptibility to viral infections. In the present study, the biochemical mechanisms underlying GSH-induced resistance were investigated in various tobacco biotypes displaying markedly different GSH and SA levels. The endogenous GSH levels of Nicotiana tabacum cv. Xanthi NN and N. tabacum cv. Xanthi NN NahG tobacco leaves were increased by infiltration of exogenous GSH or its synthetic precursor R-2-oxo-4-thiazolidine-carboxylic acid (OTC). Alternatively, we also used tobacco lines containing high GSH levels due to transgenes encoding critical enzymes for cysteine and GSH biosynthesis. We crossed Xanthi NN and NahG tobaccos with the GSH overproducer transgenic tobacco lines in order to obtain F1 progenies with increased levels of GSH and decreased levels of SA. We demonstrated that in SA-deficient NahG tobacco the elevation of in planta GSH and GSSG levels either by exogenous GSH or by crossing with glutathione overproducing plants confers enhanced resistance to Tobacco mosaic virus (TMV) manifested as both reduced symptoms (i.e. suppression of hypersensitive-type localized necrosis) and lower virus titers. The beneficial effects of elevated GSH on TMV resistance was markedly stronger in NahG than in Xanthi NN leaves. Infiltration of exogenous GSH and OTC or crossing with GSH overproducer tobacco lines resulted in a substantial rise of bound SA and to a lesser extent of free SA levels in tobacco, especially following TMV infection. Significant increases in expression of pathogenesis related (NtPR-1a, and NtPRB-1b), and glutathione S-transferase (NtGSTtau, and NtGSTphi) genes were evident in TMV-inoculated leaves in later stages of pathogenesis. However, the highest levels of defense gene expression were associated with SA-deficiency, rather than enhanced TMV resistance. In summary, elevated levels of glutathione in TMV-infected tobacco can compensate for SA deficiency to maintain virus resistance. Our results suggest that glutathione-induced redox changes are important components of antiviral signaling in tobacco.

Effect of three safeners on sulfur assimilation and iron deficiency response in barley (Hordeum vulgare) plants

BACKGROUND

Safeners are agrochemicals used in agriculture to protect crops from herbicide injuries. They act by stimulating herbicide metabolism. As graminaceous plants, to cope with iron (Fe) deficiency, activate sulfur (S) metabolism and release huge amounts of Fe-chelating compounds, or phytosiderophores (PSs), we investigated, in barley plants (Hordeum vulgare, L.) grown in Fe deficiency, the effects of three safeners on two enzymes of S assimilation, cysteine (Cys) and glutathione (GSH), and PS release. Finally, we monitored the root Fe content in plants treated with the most effective safener.

RESULTS

Generally, all the safeners activated S metabolism and increased Cys and GSH contents. In addition, the safened plants excreted higher levels of PSs. Given that mefenpyr-diethyl (Mef) was the most effective in causing these effects, we assessed the Fe concentration in Mef-treated barley and found higher Fe levels than those in untreated plants.

CONCLUSION

The three safeners, in different ways but specifically, activated S reductive metabolism and regulated Cys and GSH contents, PS release rate and Fe content (Mef-treated barley). The results of this research provide new indications of the biochemical and physiological mechanisms involved in the safening action. © 2016 Society of Chemical Industry.

ATP-sulfurylase, sulfur-compounds, and plant stress tolerance

Sulfur (S) stands fourth in the list of major plant nutrients after N, P, and K. Sulfate (SO4 (2-)), a form of soil-S taken up by plant roots is metabolically inert. As the first committed step of S-assimilation, ATP-sulfurylase (ATP-S) catalyzes SO4 (2-)-activation and yields activated high-energy compound adenosine-5(')-phosphosulfate that is reduced to sulfide (S(2-)) and incorporated into cysteine (Cys). In turn, Cys acts as a precursor or donor of reduced S for a range of S-compounds such as methionine (Met), glutathione (GSH), homo-GSH (h-GSH), and phytochelatins (PCs). Among S-compounds, GSH, h-GSH, and PCs are known for their involvement in plant tolerance to varied abiotic stresses, Cys is a major component of GSH, h-GSH, and PCs; whereas, several key stress-metabolites such as ethylene, are controlled by Met through its first metabolite S-adenosylmethionine. With the major aim of briefly highlighting S-compound-mediated role of ATP-S in plant stress tolerance, this paper: (a) overviews ATP-S structure/chemistry and occurrence, (b) appraises recent literature available on ATP-S roles and regulations, and underlying mechanisms in plant abiotic and biotic stress tolerance, (c) summarizes ATP-S-intrinsic regulation by major S-compounds, and (d) highlights major open-questions in the present context. Future research in the current direction can be devised based on the discussion outcomes.

Compartment-specific importance of glutathione during abiotic and biotic stress

The tripeptide thiol glutathione (γ-L-glutamyl-L-cysteinyl-glycine) is the most important sulfur containing antioxidant in plants and essential for plant defense against abiotic and biotic stress conditions. It is involved in the detoxification of reactive oxygen species (ROS), redox signaling, the modulation of defense gene expression, and the regulation of enzymatic activities. Even though changes in glutathione contents are well documented in plants and its roles in plant defense are well established, still too little is known about its compartment-specific importance during abiotic and biotic stress conditions. Due to technical advances in the visualization of glutathione and the redox state through microscopical methods some progress was made in the last few years in studying the importance of subcellular glutathione contents during stress conditions in plants. This review summarizes the data available on compartment-specific importance of glutathione in the protection against abiotic and biotic stress conditions such as high light stress, exposure to cadmium, drought, and pathogen attack (Pseudomonas, Botrytis, tobacco mosaic virus). The data will be discussed in connection with the subcellular accumulation of ROS during these conditions and glutathione synthesis which are both highly compartment specific (e.g., glutathione synthesis takes place in chloroplasts and the cytosol). Thus this review will reveal the compartment-specific importance of glutathione during abiotic and biotic stress conditions.

Compartment-specific antioxidative defense in Arabidopsis against virulent and avirulent Pseudomonas syringae

The accumulation of reactive oxygen species (ROS) during biotic stress is either part of a hypersensitive response of the plant or induced directly by the pathogen. Antioxidants such as ascorbate and glutathione counteract the accumulation of ROS and are part of the defense reaction. The aim of the present study was to investigate the compartment-specific importance of ascorbate and glutathione during a virulent and avirulent Pseudomonas syringae infection in Arabidopsis thaliana. Peroxisomes were found to be the hotspot for glutathione accumulation reaching 452% and 258% of control levels 24 h postinoculation during the virulent and avirulent infection, respectively. An accumulation of ascorbate could also be observed in vacuoles during Pseudomonas syringae infection, whereas glutathione remained absent in this cell compartment. Neither glutathione nor ascorbate accumulated in the apoplast during pathogen infection demonstrating an only negligible role of these antioxidants in the apoplast during pathogen infection. Compartment-specific changes followed a recently proposed stress model with an increase of ascorbate and glutathione in most cell compartments at the early stages of infection and a strong drop at the later stage of infection when a strong accumulation of ROS and symptoms occurred in the leaves. This study highlights the importance of certain cell compartments and antioxidants in general for the protection of pathogen-induced ROS accumulation.

Subcellular distribution of glutathione precursors in Arabidopsis thaliana

Glutathione is an important antioxidant and has many important functions in plant development, growth and defense. Glutathione synthesis and degradation is highly compartment-specific and relies on the subcellular availability of its precursors, cysteine, glutamate, glycine and γ-glutamylcysteine especially in plastids and the cytosol which are considered as the main centers for glutathione synthesis. The availability of glutathione precursors within these cell compartments is therefore of great importance for successful plant development and defense. The aim of this study was to investigate the compartment-specific importance of glutathione precursors in Arabidopsis thaliana. The subcellular distribution was compared between wild type plants (Col-0), plants with impaired glutathione synthesis (glutathione deficient pad2-1 mutant, wild type plants treated with buthionine sulfoximine), and one complemented line (OE3) with restored glutathione synthesis. Immunocytohistochemistry revealed that the inhibition of glutathione synthesis induced the accumulation of the glutathione precursors cysteine, glutamate and glycine in most cell compartments including plastids and the cytosol. A strong decrease could be observed in γ-glutamylcysteine (γ-EC) contents in these cell compartments. These experiments demonstrated that the inhibition of γ-glutamylcysteine synthetase (GSH1) - the first enzyme of glutathione synthesis - causes a reduction of γ-EC levels and an accumulation of all other glutathione precursors within the cells.

Subcellular compartmentation of glutathione in dicotyledonous plants

This study describes the subcellular distribution of glutathione in roots and leaves of different plant species (Arabidopsis, Cucurbita, and Nicotiana). Glutathione is an important antioxidant and redox buffer which is involved in many metabolic processes including plant defense. Thus information on the subcellular distribution in these model plants especially during stress situations provides a deeper insight into compartment specific defense reactions and reflects the occurrence of compartment specific oxidative stress. With immunogold cytochemistry and computer-supported transmission electron microscopy glutathione could be localized in highest contents in mitochondria, followed by nuclei, peroxisomes, the cytosol, and plastids. Within chloroplasts and mitochondria, glutathione was restricted to the stroma and matrix, respectively, and did not occur in the lumen of cristae and thylakoids. Glutathione was also found at the membrane and in the lumen of the endoplasmic reticulum. It was also associated with the trans and cis side of dictyosomes. None or only very little glutathione was detected in vacuoles and the apoplast of mesophyll and root cells. Additionally, glutathione was found in all cell compartments of phloem vessels, vascular parenchyma cells (including vacuoles) but was absent in xylem vessels. The specificity of this method was supported by the reduction of glutathione labeling in all cell compartments (up to 98%) of the glutathione-deficient Arabidopsis thaliana rml1 mutant. Additionally, we found a similar distribution of glutathione in samples after conventional fixation and rapid microwave-supported fixation. Thus, indicating that a redistribution of glutathione does not occur during sample preparation. Summing up, this study gives a detailed insight into the subcellular distribution of glutathione in plants and presents solid evidence for the accuracy and specificity of the applied method.

Sub-cellular distribution of glutathione in an Arabidopsis mutant (vtc1) deficient in ascorbate

Glutathione and ascorbate are considered the major redox buffers in plant cells. They are implicated in many reactions at the sub-cellular level. However, information about the location and quantification of glutathione in the different sub-cellular compartments is very scarce and it has been obtained mainly using organelle purification and chemical quantification. We have used a glutathione antibody to immunolabel and quantify the total glutathione in leaves from wild-type Arabidopsis thaliana (Col-0) and an A. thaliana mutant (vtc1) deficient in ascorbate. Spectrophotometrical quantification has shown that this mutant has a higher content of glutathione during plant development compared with Col-0 [Pavet V, Olmos E, Kiddle G, Mowla S, Kumar S, Antoniw J, et al. Ascorbic acid deficiency activates cell death and disease resistance responses in Arabidopsis. Plant Physiology 2005;139:1291-03]. We have observed, using immunolabelling techniques, that mitochondria showed the highest density of glutathione labelling in both Col-0 and vtc1 plants during all developmental stages and that the lowest density occurred in the chloroplasts, for both lines. However, the distribution of glutathione in the different sub-cellular compartments indicates that the chloroplasts contain about 62-75% of the total cellular glutathione and that the mitochondria represent the second greatest pool, with about 15-25% of the total cellular glutathione. It has been observed previously that the vtc1 mutant exhibits an induction of cell death and disease resistance in the face of pathogen attack. The differing distributions and concentrations of glutathione in the mitochondria of wild-type A. thaliana and the vtc1 mutant is discussed.

A temporal analysis of antioxidative defense responses in the phloem of Picea abies after attack by Ips typographus

The temporal gradation of antioxidants was investigated on the phloem tissue of Norway spruce [Picea abies (L.) Karst.] in response to weather conditions and colonization levels of Ips typographus L. (Col., Scolytidae). Two weeks after pheromone dispensers were placed on trees, the initial reaction of Norway spruce to bark beetle attack resulted in moderately lowered levels of total glutathione (tGSH) and total cysteine. Likewise, the total ascorbic acid dropped slightly below the control levels, whereas the concentration of dehydroascorbic acid increased in comparison to the first sampling date. This transient degradation and oxidation of the glutathione and ascorbate system was accompanied by moderately increased concentrations of total phenolics. One month later, the shift in antioxidant balance after moderate attack differed quantitatively from the reaction after massive attack. An intensification of antioxidant defense occurred within moderately affected bark. Total cysteine and tGSH contents were markedly raised, whereas the concentrations of total ascorbic acid and total phenolics were slightly increased by moderate attack. On the other hand, massive bark beetle colonization caused a strong decrease in tGSH and total phenolics, whereas total cysteine and total ascorbic acid values remained at control level. Dependent upon the intensity and the success of the attack, a progressive degradation of antioxidants was determined at later sampling dates, which was accompanied by an obvious oxidation of the ascorbate and glutathione pools. With an unsuccessful defense upon massive attack, the thiols and total phenolics did not reach a new steady state, but deteriorated until the end of the brood beetles' development. In contrast, the dynamic antioxidative response within the moderately affected trees indicated an acclimation stage in the middle of July. It was characterized by a higher accumulation of tGSH, total ascorbic acid and total phenolics as well as a more reduced redox state of glutathione. A sequence of changes in the endogenous levels of antioxidant defense molecules in the bark beetle-affected Norway spruce showed consistency with the general ecophysiological stress-response concept, and provided important avenues for evaluating the role and effectiveness of antioxidants in systemic acquired resistance against the complex interactive effects of bark beetle attack and environmental factors.

Analysis of cytosolic and plastidic serine acetyltransferase mutants and subcellular metabolite distributions suggests interplay of the cellular compartments for cysteine biosynthesis in Arabidopsis

In plants, the enzymes for cysteine synthesis serine acetyltransferase (SAT) and O-acetylserine-(thiol)-lyase (OASTL) are present in the cytosol, plastids and mitochondria. However, it is still not clearly resolved to what extent the different compartments are involved in cysteine biosynthesis and how compartmentation influences the regulation of this biosynthetic pathway. To address these questions, we analysed Arabidopsis thaliana T-DNA insertion mutants for cytosolic and plastidic SAT isoforms. In addition, the subcellular distribution of enzyme activities and metabolite concentrations implicated in cysteine and glutathione biosynthesis were revealed by non-aqueous fractionation (NAF). We demonstrate that cytosolic SERAT1.1 and plastidic SERAT2.1 do not contribute to cysteine biosynthesis to a major extent, but may function to overcome transport limitations of O-acetylserine (OAS) from mitochondria. Substantiated by predominantly cytosolic cysteine pools, considerable amounts of sulphide and presence of OAS in the cytosol, our results suggest that the cytosol is the principal site for cysteine biosynthesis. Subcellular metabolite analysis further indicated efficient transport of cysteine, gamma-glutamylcysteine and glutathione between the compartments. With respect to regulation of cysteine biosynthesis, estimation of subcellular OAS and sulphide concentrations established that OAS is limiting for cysteine biosynthesis and that SAT is mainly present bound in the cysteine-synthase complex.

Subcellular immunocytochemical analysis detects the highest concentrations of glutathione in mitochondria and not in plastids

The tripeptide glutathione is a major antioxidant and redox buffer with multiple roles in plant metabolism. Glutathione biosynthesis is restricted to the cytosol and the plastids and the product is distributed to the various organelles by unknown mechanisms. In the present study immunogold cytochemistry based on anti-glutathione antisera and transmission electron microscopy was used to determine the relative concentration of glutathione in different organelles of Arabidopsis thaliana leaf and root cells. Glutathione-specific labelling was detected in all cellular compartments except the apoplast and the vacuole. The highest glutathione content was surprisingly not found in plastids, which have been described before as a major site of glutathione accumulation, but in mitochondria which lack the capacity for glutathione biosynthesis. Mitochondria of both leaf and root cells contained 7-fold and 4-fold, respectively, higher glutathione levels than plastids while the density of glutathione labelling in the cytosol, nuclei, and peroxisomes was intermediate. The accuracy of the glutathione labelling is supported by two observations. First, pre-adsorption of the anti-glutathione antisera with glutathione reduced the density of the gold particles in all organelles to background levels. Second, the overall glutathione-labelling density was reduced by about 90% in leaves of the glutathione-deficient Arabidopsis mutant pad2-1 and increased in transgenic plants with enhanced glutathione accumulation. Hence, there was a strong correlation between immunocytochemical and biochemical data of glutathione accumulation. Interestingly, the glutathione labelling of mitochondria in pad2-1 remained very similar to wild-type plants thus suggesting that the high mitochondrial glutathione content is maintained in a situation of permanent glutathione-deficiency at the expense of other glutathione pools. High and constant levels of glutathione in mitochondria appear to be particularly important in cell survival strategies and it is predicted that mitochondria must have highly competitive mitochondrial glutathione uptake systems. The present results underline the suggestion that subcellular glutathione concentrations are not controlled by a global mechanism but are controlled on an individual basis and it is therefore not possible to conclude from global biochemical glutathione analysis on the status of the various organellar pools.

Virus-induced changes in the subcellular distribution of glutathione precursors in Cucurbita pepo (L.)

Changes in glutathione contents occur in plants during environmental stress situations, such as pathogen attack, as the formation of reactive oxygen species leads to the activation of the antioxidative defence system. As glutathione is synthesized out of its constituents cysteine, glycine, and glutamate the availability of these components will limit glutathione synthesis in plants especially during stress situations and therefore the ability of the plant to fight oxidative stress. To gain a deeper insight into possible limitations of glutathione synthesis during pathogen attack the present investigations were aimed to study how the subcellular distribution of glutathione precursors correlates with the subcellular distribution of glutathione during virus attack in plants. Selective antibodies against cysteine, glutamate, and glycine were used to study the impact of Zucchini yellow mosaic virus (ZYMV) infection on glutathione precursor contents within different cell compartments of cells from Cucurbita pepo (L.) plants with the transmission electron microscope (TEM). Generally, levels of cysteine and glutamate were found to be strongly decreased in most cell compartments of younger and older leaves including glutathione-producing cell compartments such as plastids and the cytosol. The strongest decrease of cysteine was found in plastids (- 54 %) and mitochondria (- 51 %) of younger leaves and in vacuoles (- 37 %) and plastids (- 29 %) of older leaves. The strongest decrease of glutamate in younger leaves occurred in peroxisomes (- 67 %) and nuclei (- 58 %) and in peroxisomes (- 64 %) and plastids (- 52 %) of the older ones. Glycine levels were found to be strongly decreased (- 63 % in mitochondria and - 53 % in plastids) in most cell compartments of older leaves and strongly increased (about 50 % in plastids and peroxisomes) in all cell compartments of the younger ones. These results indicate that low glycine contents in the older leaves were responsible for low levels of glutathione in these organs during ZYMV infection rather than limited amounts of cysteine or glutamate. Glutathione precursors were virtually absent in cell walls and intercellular spaces and play therefore no important role during ZYMV attack in the apoplast. While glutamate was absent in vacuoles, elevated levels of glycine (up to 30 %) and decreased cysteine contents (up to - 37 %) were observed in vacuoles during ZYMV infection. The impact of the present results on the current knowledge about glutathione synthesis and degradation on the cellular level during ZYMV infection are discussed.

Artificial elevation of glutathione affects symptom development in ZYMV-infected Cucurbita pepo L. plants

Styrian oil pumpkin seedlings (Cucurbita pepo L. subsp. pepo var. styriaca GREB: .) were treated for 48 h with 1 mM OTC (L-2-oxothiazolidine-4-carboxylic acid) in order to artificially increase cellular glutathione content. They were inoculated with zucchini yellow mosaic virus (ZYMV) 10 days later. The effects of OTC treatment and ZYMV infection on glutathione levels were examined at the subcellular level by immunogold labeling of glutathione using a transmission electron microscope (TEM). These effects were further tested at the whole-tissue level by high performance liquid chromatography (HPLC). Such tests were carried out a) on roots, cotyledons and the first true leaves immediately after OTC treatment in order to analyze to which extent OTC increases glutathione levels in different cell compartments as well as in the whole organ; and b) in older and younger leaves and in roots three weeks after ZYMV inoculation in order to study how possible effects of OTC on symptom development would correlate with glutathione levels at the subcellular level and in the whole organ. Immunocytological and biochemical investigations revealed that, 48 h after OTC treatment, glutathione content had increased in all investigated organs, up to 144% in peroxisomes of cotyledons. Three weeks after ZYMV inoculation, glutathione labeling density had significantly increased within intact cells of infected leaves, up to 124% in the cytosol of younger leaves. Roots showed decreased amounts of glutathione in the TEM. Biochemical studies revealed that OTC treatment resulted in 41 and 51% higher glutathione content in older and younger ZYMV-infected leaves, respectively, in comparison to untreated and ZYMV-infected plants. Evaluation of symptom development at this point revealed that all untreated ZYMV-infected plants had symptoms, whereas only 42% of OTC-treated ZYMV-infected plants showed signs of symptoms. Quantification of ZYMV particles revealed that all organs of OTC-treated and ZYMV-infected plants contained significantly decreased amounts of ZYMV particles over a period of five weeks when compared to the same organs of untreated ZYMV-infected plants. We can conclude that OTC treatment and subsequently elevated glutathione contents within Styrian oil pumpkin plants led to a strong decrease in virus content, which was accompanied by a suppression of ZYMV-induced symptoms as well as reduced and delayed symptom development within plants exhibiting symptoms.

Intracellular adaptations of glutathione content in Cucurbita pepo L. induced by treatment with reduced glutathione and buthionine sulfoximine

The intracellular effects of GSH (reduced glutathione) and BSO (buthionine sulfoximine) treatment on glutathione content were investigated with immunogold labeling in individual cellular compartments of Cucurbita pepo L. seedlings. Generally, GSH treatment led to increased levels of glutathione in roots and leaves (up to 3.5-fold in nuclei), whereas BSO treatment significantly decreased glutathione content in all organs. Transmission electron microscopy revealed that glutathione levels in mitochondria, which showed the highest glutathione labeling density of all compartments, remained generally unaffected by both treatments. Since glutathione within mitochondria is involved in the regulation of cell death, these results indicate that high and stable levels of glutathione in mitochondria play an important role in cell survival strategies. BSO treatment significantly decreased glutathione levels (1) in roots by about 78% in plastids and 60.8% in the cytosol and (2) in cotyledons by about 55% in the cytosol and 38.6% in plastids. After a short recovery period, glutathione levels were significantly increased in plastids and the cytosol of root tip cells (up to 3.7-fold) and back to control values in cotyledons. These results indicate that plastids, either alone or together with the cytosol, are the main center of glutathione synthesis in leaves as well as in roots. After GSH treatment for 24 h, severe ultrastructural damage related to increased levels of glutathione was found in roots, in all organelles except mitochondria. Possible negative effects of GSH treatment leading to the observed ultrastructural damage are discussed.