Enhanced glutathione metabolism is correlated with sulfur-induced resistance in Tobacco mosaic virus-infected genetically susceptible Nicotiana tabacum plants

Genome-wide analysis of tobacco NtTOM1/TOM3 gene family and identification of NtTOM1a/ NtTOM3a response to tobacco mosaic virus

BACKGROUND

TOBAMOVIRUS MULTIPLICATION 1 (TOM1) and its homolog TOBAMOVIRUS MULTIPLICATION 3 (TOM3) play a prominent role in the multiplication of tobacco mosaic virus (TMV) in higher plants. Although homologs of NtTOM1/TOM3 genes have been identified in several plant species, little is known about the characteristics and functions of NtTOM1/TOM3 at the genome-wide level in tobacco (Nicotiana tabacum L.).

RESULTS

In this study, we performed genome-wide identification and expression pattern analysis of the tobacco NtTOM1/TOM3 gene family. Twelve NtTOM1/TOM3 genes were identified and classified into four groups based on phylogenetic analysis. Sequence and conserved domain analyses showed that all these genes contained a specific DUF1084 domain. Expression pattern analysis showed that NtTOM1a, NtTOM1b, NtTOM1d, NtTOM3a, NtTOM3b, and NtTOM3d were induced by TMV at 1-, 3-, and 9 dpi, whereas the expression of other genes was not responsive to TMV at the early infection stage. TMV virion accumulation showed no obvious difference in either nttom1a or nttom3a mutants compared with the wild type. However, the virus propagation was significantly, but not completely, inhibited in the nttom1atom3a double mutant, indicating that other gene family members may function redundantly, such as NtTOM1b and NtTOM1d. In addition, overexpression of NtTOM1a or NtTOM3a also inhibited the TMV replication to some extent.

CONCLUSIONS

The present study performed genome-wide analysis of the NtTOM1/TOM3 gene family in tobacco, and identified NtTOM1a and NtTOM3a as important genes involved in TMV multiplication based on functional analysis. These results provide a theoretical basis for further improving TMV resistance in tobacco.

Transcriptome analysis and functional verification reveal the roles of copper in resistance to potato virus Y infection in tobacco

Potato virus Y (PVY) is one of the most important pathogens in the genus Potyvirus that seriously harms agricultural production. Copper (Cu), as a micronutrient, is closely related to plant immune response. In this study, we found that foliar application of Cu could inhibit PVY infection to some extent, especially at 7 days post inoculation (dpi). To explore the effect of Cu on PVY infection, transcriptome sequencing analysis was performed on PVY-infected tobacco with or without Cu application. Several key pathways regulated by Cu were identified, including plant-pathogen interaction, inorganic ion transport and metabolism, and photosynthesis. Moreover, the results of virus-induced gene silencing (VIGS) assays revealed that NbMLP423, NbPIP2, NbFd and NbEXPA played positive roles in resistance to PVY infection in Nicotiana benthamiana. In addition, transgenic tobacco plants overexpressing NtEXPA11 showed increased resistance to PVY infection. These results contribute to clarify the role and regulatory mechanism of Cu against PVY infection, and provide candidate genes for disease resistance breeding.

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.

Fine mapping and identification of two NtTOM2A homeologs responsible for tobacco mosaic virus replication in tobacco (Nicotiana tabacum L.)

BACKGROUND

Tobacco mosaic virus (TMV) is a widely distributed viral disease that threatens many vegetables and horticultural species. Using the resistance gene N which induces a hypersensitivity reaction, is a common strategy for controlling this disease in tobacco (Nicotiana tabacum L.). However, N gene-mediated resistance has its limitations, consequently, identifying resistance genes from resistant germplasms and developing resistant cultivars is an ideal strategy for controlling the damage caused by TMV.

RESULTS

Here, we identified highly TMV-resistant tobacco germplasm, JT88, with markedly reduced viral accumulation following TMV infection. We mapped and cloned two tobamovirus multiplication protein 2A (TOM2A) homeologs responsible for TMV replication using an F2 population derived from a cross between the TMV-susceptible cultivar K326 and the TMV-resistant cultivar JT88. Clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9)-mediated loss-of-function mutations of two NtTOM2A homeologs almost completely suppressed TMV replication; however, the single gene mutants showed symptoms similar to those of the wild type. Moreover, NtTOM2A natural mutations were rarely detected in 577 tobacco germplasms, and CRISPR/Cas9-mediated variation of NtTOM2A led to shortened plant height, these results indicating that the natural variations in NtTOM2A were rarely applied in tobacco breeding and the NtTOM2A maybe has an impact on growth and development.

CONCLUSIONS

The two NtTOM2A homeologs are functionally redundant and negatively regulate TMV resistance. These results deepen our understanding of the molecular mechanisms underlying TMV resistance in tobacco and provide important information for the potential application of NtTOM2A in TMV resistance breeding.

Enhanced Resistance to Viruses in Nicotiana edwardsonii 'Columbia' Is Dependent on Salicylic Acid, Correlates with High Glutathione Levels, and Extends to Plant-Pathogenic Bacteria and Abiotic Stress

Our earlier research showed that an interspecific tobacco hybrid (Nicotiana edwardsonii 'Columbia' [NEC]) displays elevated levels of salicylic acid (SA) and enhanced resistance to localized necrotic symptoms (hypersensitive response [HR]) caused by tobacco mosaic virus (TMV) and tobacco necrosis virus (TNV), as compared with another interspecific hybrid (Nicotiana edwardsonii [NE]) derived from the same parents. In the present study, we investigated whether symptomatic resistance in NEC is indeed associated with the inhibition of TMV and TNV and whether SA plays a role in this process. We demonstrated that enhanced viral resistance in NEC is manifested as both milder local necrotic (HR) symptoms and reduced levels of TMV and TNV. The presence of an adequate amount of SA contributes to the enhanced defense response of NEC to TMV and TNV, as the absence of SA resulted in seriously impaired viral resistance. Elevated levels of subcellular tripeptide glutathione (GSH) in NEC plants in response to viral infection suggest that in addition to SA, GSH may also contribute to the elevated viral resistance of NEC. Furthermore, we found that NEC displays an enhanced resistance not only to viral pathogens but also to bacterial infections and abiotic oxidative stress induced by paraquat treatments. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

A Biomass-Based Colorimetric Sulfur Dioxide Gas Sensor for Smart Packaging

Sulfur dioxide (SO₂) gas, which can effectively prohibit the growth of pathogenic microorganisms, has been internationally used in commercial food packaging to maintain high-quality food and reduce the incidence of foodborne illnesses. However, the current mainstream methods for SO₂ detection are either large and expensive instruments or synthesized chemical-based labels, which are not suitable for large-scale gas detection in food packaging. Recently, we discovered that petunia dye (PD), which is extracted from natural petunia flowers, demonstrates a highly sensitive colorimetric response to SO₂ gas with its total color difference (ΔE) modulation reaching up to 74.8 and detection limit down to 1.52 ppm. To apply the extracted petunia dye in smart packaging for real-time gas sensing and food-quality prediction, a flexible and freestanding PD-based SO₂ detection label is prepared by incorporating PD in biopolymers and assembling the films through a layer-by-layer approach. The developed label is utilized to predict grapes' quality and safety by monitoring the embedded SO₂ gas concentration. The developed colorimetric SO₂ detection label could potentially be used as an intelligent gas sensor for food status prediction in daily life, food storage, and supply chains.

Transcriptome mapping related genes encoding PR1 protein involved in necrotic symptoms to soybean mosaic virus infection

Necrosis caused by soybean mosaic virus (SMV) has not been specifically distinguished from susceptible symptoms. The molecular mechanism for the occurrence of necrosis is largely overlooked in soybean genetic research. Field evaluation reveals that SMV disease seriously influences soybean production as indicated by decreasing 22.4% ~ 77.0% and 8.8% ~ 17.0% of yield and quality production, respectively. To expand molecular mechanism behind necrotic reactions, transcriptomic data obtained from the asymptomatic, mosaic, and necrotic pools were assessed. Compared between asymptomatic and mosaic plants, 1689 and 1752 up- and down-regulated differentially expressed genes (DEGs) were specifically found in necrotic plants. Interestingly, the top five enriched pathways with up-regulated DEGs were highly related to the process of the stress response, whereas the top three enriched pathways with down-regulated DEGs were highly related to the process of photosynthesis, demonstrating that defense systems are extensively activated, while the photosynthesis systems were severely destroyed. Further, results of the phylogenetic tree based on gene expression pattern and an amino acid sequence and validation experiments discovered three PR1 genes, Glyma.15G062400, Glyma.15G062500, and Glyma.15G062700, which were especially expressed in necrotic leaves. Meanwhile, exogenous salicylic acid (SA) but not methyl jasmonate (MeJA) could induce the three PR1 gene expressions on healthy leaves. Contrastingly, exogenous SA obviously decreased the expression level of Glyma.15G062400, Glyma.15G062500, and concentration of SMV, but increased Glyma.15G062700 expression in necrotic leaves. These results showed that GmPR1 is associated with the development of SMV-induced necrotic symptoms in soybean. Glyma.15G062400, Glyma.15G062500, and Glyma.15G062700 is up-regulated in necrotic leaves at the transcriptional levels, which will greatly facilitate a better understanding of the mechanism behind necrosis caused by SMV disease.

Supplementary information

The online version contains supplementary material available at 10.1007/s11032-022-01351-3.

Organelle-specific localization of glutathione in plants grown under different light intensities and spectra

Plant ascorbate and glutathione metabolism counteracts oxidative stress mediated, for example, by excess light. In this review, we discuss the properties of immunocytochemistry and transmission electron microscopy, redox-sensitive dyes or probes and bright-field microscopy, confocal microscopy or fluorescence microscopy for the visualization and quantification of glutathione at the cellular or subcellular level in plants and the quantification of glutathione from isolated organelles. In previous studies, we showed that subcellular ascorbate and glutathione levels in Arabidopsis are affected by high light stress. The use of light-emitting diodes (LEDs) is gaining increasing importance in growing indoor crops and ornamental plants. A combination of different LED types allows custom-made combinations of wavelengths and prevents damage related to high photon flux rates. In this review we provide an overview on how different light spectra and light intensities affect glutathione metabolism at the cellular and subcellular levels in plants. Findings obtained in our most recent study demonstrate that both light intensity and spectrum significantly affected glutathione metabolism in wheat at the transcriptional level and caused genotype-specific reactions in the investigated Arabidopsis lines.

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.

Near-Isogenic Barley Lines Show Enhanced Susceptibility to Powdery Mildew Infection Following High-Temperature Stress

Barley cultivation is adversely affected by high-temperature stress, which may modulate plant defense responses to pathogens such as barley powdery mildew (Blumeria graminis f. sp. hordei, Bgh). Earlier research focused mainly on the influence of short-term heat stress (heat shock) of barley on Bgh infection. In this study, our aim was to investigate the effects of both short- and long-term heat stress (35 °C from 30 s to 5 days) on Bgh infection in the barley cultivar Ingrid and its near-isogenic lines containing different powdery mildew resistance genes (Mla12, Mlg, and mlo5) by analyzing symptom severity and Bgh biomass with RT-qPCR. The expression of selected barley defense genes (BAX inhibitor-1, Pathogenesis- related protein-1b, Respiratory burst oxidase homologue F2, and Heat shock protein 90-1) was also monitored in plants previously exposed to heat stress followed by inoculation with Bgh. We demonstrated that pre-exposure to short- and long-term heat stress negatively affects the resistance of all resistant lines manifested by the appearance of powdery mildew symptoms and increased Bgh biomass. Furthermore, prolonged heat stress (48 and 120 h) enhanced both Bgh symptoms and biomass in susceptible wild-type Ingrid. Heat stress suppressed and delayed early defense gene activation in resistant lines, which is a possible reason why resistant barley became partially susceptible to Bgh.

Sulfur-Induced Resistance against Pseudomonas syringae pv. actinidiae via Triggering Salicylic Acid Signaling Pathway in Kiwifruit

Sulfur has been previously reported to modulate plant growth and exhibit significant anti-microbial activities. However, the mechanism underlying its diverse effects on plant pathogens has not been elucidated completely. The present study conducted the two-year field experiment of sulfur application to control kiwifruit canker from 2017 to 2018. For the first time, our study uncovered activation of plant disease resistance by salicylic acid after sulfur application in kiwifruit. The results indicated that when the sulfur concentration was 1.5–2.0 kg m⁻³, the induced effect of kiwifruit canker reached more than 70%. Meanwhile, a salicylic acid high lever was accompanied by the decline of jasmonic acid. Further analysis revealed the high expression of the defense gene, especially AcPR-1, which is a marker of the salicylic acid signaling pathway. Additionally, AcICS1, another critical gene of salicylic acid synthesis, was also highly expressed. All contributed to the synthesis of increasing salicylic acid content in kiwifruit leaves. Moreover, the first key lignin biosynthetic AcPAL gene was marked up-regulated. Thereafter, accumulation of lignin content in the kiwifruit stem and the higher deposition of lignin were visible in histochemical analysis. Moreover, the activity of the endochitinase activity of kiwifruit leaves increased significantly. We suggest that the sulfur-induced resistance against Pseudomonas syringae pv. actinidiae via salicylic activates systemic acquired resistance to enhance plant immune response in kiwifruit.

Three-dimensional quantitative imaging of Tobacco mosaic virus and Zucchini yellow mosaic virus induced ultrastructural changes

Two-dimensional ultrastructural changes of Tobacco mosaic virus (TMV) and Zucchini yellow mosaic virus (ZYMV) in tobacco and pumpkin plants, respectively, are well studied. To provide 3D data, representative control and infected cells were reconstructed using serial sectioning and transmission electron microscopy. Quantitative data of 3D ultrastructural changes were then extracted from the cytosol and organelles by image analysis. While TMV induced the accumulation of an average of 40 virus inclusion bodies in the cytosol, which covered about 13% of the cell volume, ZYMV caused the accumulation of an average of 1752 cylindrical inclusions in the cytosol, which covered about 2.7% of the total volume of the cell. TMV infection significantly decreased the number and size of mitochondria (- 49 and - 20%) and peroxisomes (- 62 and - 28%) of the reconstructed cell. The reconstructed ZYMV-infected cell contained more (105%) and larger (109%) mitochondria when compared to the control cell. While the reconstructed TMV-infected cell contained larger (20%) and the ZYMV-infected smaller (19%) chloroplasts, both contained less chloroplasts (- 40% for TMV and - 23% for ZYMV). In chloroplasts, the volume of starch and plastoglobules increased (664% and 150% for TMV and 1324% and 1300% for ZYMV) when compared to the control. The latter was correlated with a decrease in the volume of thylakoids in the reconstructed ZYMV-infected cell (- 31%) indicating that degradation products from thylakoids are transported and stored in plastoglobules. Summing up, the data collected in this study give a comprehensive overview of 3D changes induced by TMV and ZYMV in plants.

Reactive Oxygen Species Contribute to Symptomless, Extreme Resistance to Potato virus X in Tobacco

Here we show that in tobacco (Nicotiana tabacum cultivar Samsun NN Rx1) the development of Rx1 gene-mediated, symptomless, extreme resistance to Potato virus X (PVX) is preceded by an early, intensive accumulation of the reactive oxygen species (ROS) superoxide (O2·-), evident between 1 and 6 h after inoculation and associated with increased nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activities. This suggests a direct contribution of this ROS to virus restriction during symptomless, extreme resistance. Superoxide inhibition in PVX-inoculated leaves by infiltration of antioxidants (superoxide dismutase [SOD] and catalase [CAT]) partially suppresses extreme resistance in parallel with the appearance of localized leaf necrosis resembling a hypersensitive resistance (HR) response. F1 progeny from crosses of Rx1 and ferritin overproducer (deficient in production of the ROS OH·) tobaccos also display a suppressed extreme resistance to PVX, because significantly increased virus levels are coupled to HR, suggesting a role of the hydroxyl radical (OH·) in this symptomless antiviral defense. In addition, treatment of PVX-susceptible tobacco with a superoxide-generating agent (riboflavin/methionine) results in HR-like symptoms and reduced PVX titers. Finally, by comparing defense responses during PVX-elicited symptomless, extreme resistance and HR-type resistance elicited by Tobacco mosaic virus, we conclude that defense reactions typical of an HR (e.g., induction of cell death/ROS-regulator genes and antioxidants) are early and transient in the course of extreme resistance. Our results demonstrate the contribution of early accumulation of ROS (superoxide, OH·) in limiting PVX replication during symptomless extreme resistance and support earlier findings that virus-elicited HR represents a delayed, slower resistance response than symptomless, extreme resistance.

Heat Stress Pre-Exposure May Differentially Modulate Plant Defense to Powdery Mildew in a Resistant and Susceptible Barley Genotype

Heat stress negatively affects barley production and under elevated temperatures defense responses to powdery mildew (Blumeria graminis f. sp. hordei, Bgh) are altered. Previous research has analyzed the effects of short-term (30 s to 2 h) heat stress, however, few data are available on the influence of long-term exposure to heat on powdery mildew infections. We simultaneously assessed the effects of short and long term heat pre-exposure on resistance/susceptibility of barley to Bgh, evaluating powdery mildew infection by analyzing symptoms and Bgh biomass with RT-qPCR in barley plants pre-exposed to high temperatures (28 and 35 °C from 30 s to 5 days). Plant defense gene expression after heat stress pre-exposure and inoculation was also monitored. Our results show that prolonged heat stress (24, 48 and 120 h) further enhanced Bgh susceptibility in a susceptible barley line (MvHV118-17), while a resistant line (MvHV07-17) retained its pathogen resistance. Furthermore, prolonged heat stress significantly repressed the expression of several defense-related genes (BAX inhibitor-1, Pathogenesis related-1b and Respiratory burst oxidase homologue F2) in both resistant and susceptible barley lines. Remarkably, heat-suppressed defense gene expression returned to normal levels only in MvHV07-17, a possible reason why this barley line retains Bgh resistance even at high temperatures.

Targets and Mechanisms of Geminivirus Silencing Suppressor Protein AC2

Geminiviruses are plant DNA viruses that infect a wide range of plant species and cause significant losses to economically important food and fiber crops. The single-stranded geminiviral genome encodes a small number of proteins which act in an orchestrated manner to infect the host. The fewer proteins encoded by the virus are multifunctional, a mechanism uniquely evolved by the viruses to balance the genome-constraint. The host-mediated resistance against incoming virus includes post-transcriptional gene silencing, transcriptional gene silencing, and expression of defense responsive genes and other cellular regulatory genes. The pathogenicity property of a geminiviral protein is linked to its ability to suppress the host-mediated defense mechanism. This review discusses what is currently known about the targets and mechanism of the viral suppressor AC2/AL2/transcriptional activator protein (TrAP) and explore the biotechnological applications of AC2.

I. A, Elbehi AM, Cash E, Adeyeye A. The Impact of Diet on the Involvement of Non-Coding RNAs, Extracellular Vesicles, and Gut Microbiome-Virome in Colorectal Cancer Initiation and Progression

Cancer is the major cause of morbidity and mortality in the world today. The third most common cancer and which is most diet related is colorectal cancer (CRC). Although there is complexity and limited understanding in the link between diet and CRC, the advancement in research methods have demonstrated the involvement of non-coding RNAs (ncRNAs) as key regulators of gene expression. MicroRNAs (miRNAs) which are a class of ncRNAs are key players in cancer related pathways in the context of dietary modulation. The involvement of ncRNA in cancer progression has recently been clarified throughout the last decade. ncRNAs are involved in biological processes relating to tumor onset and progression. The advances in research have given insights into cell to cell communication, by highlighting the pivotal involvement of extracellular vesicle (EV) associated-ncRNAs in tumorigenesis. The abundance and stability of EV associated ncRNAs act as a new diagnostic and therapeutic target for cancer. The understanding of the deranging of these molecules in cancer can give access to modulating the expression of the ncRNAs, thereby influencing the cancer phenotype. Food derived exosomes/vesicles (FDE) are gaining interest in the implication of exosomes in cell-cell communication with little or no understanding to date on the role FDE plays. There are resident microbiota in the colon; to which the imbalance in the normal intestinal occurrence leads to chronic inflammation and the production of carcinogenic metabolites that lead to neoplasm. Limited studies have shown the implication of various types of microbiome in CRC incidence, without particular emphasis on fungi and protozoa. This review discusses important dietary factors in relation to the expression of EV-associated ncRNAs in CRC, the impact of diet on the colon ecosystem with particular emphasis on molecular mechanisms of interactions in the ecosystem, the influence of homeostasis regulators such as glutathione, and its conjugating enzyme-glutathione S-transferase (GST) polymorphism on intestinal ecosystem, oxidative stress response, and its relationship to DNA adduct fighting enzyme-0-6-methylguanine-DNA methyltransferase. The understanding of the molecular mechanisms and interaction in the intestinal ecosystem will inform on the diagnostic, preventive and prognosis as well as treatment of CRC.

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.

Protein Extraction, Enrichment and MALDI MS and MS/MS Analysis from Bitter Orange Leaves (Citrus aurantium)

Citrus aurantium is a widespread tree in the Mediterranean area, and it is mainly used as rootstock for other citrus. In the present study, a vacuum infiltration centrifugation procedure, followed by solid phase extraction matrix-assisted laser desorption ionization tandem mass spectrometry (SPE MALDI MS/MS) analysis, was adopted to isolate proteins from leaves. The results of mass spectrometry (MS) profiling, combined with the top-down proteomics approach, allowed the identification of 78 proteins. The bioinformatic databases TargetP, SignalP, ChloroP, WallProtDB, and mGOASVM-Loc were used to predict the subcellular localization of the identified proteins. Among 78 identified proteins, 20 were targeted as secretory pathway proteins and 36 were predicted to be in cellular compartments including cytoplasm, nucleus, and cell membrane. The largest subcellular fraction was the secretory pathway, accounting for 25% of total proteins. Gene Ontology (GO) of Citrus sinensis was used to simplify the functional annotation of the proteins that were identified in the leaves. The Kyoto Encyclopedia of Genes and Genomes (KEGG) showed the enrichment of metabolic pathways including glutathione metabolism and biosynthesis of secondary metabolites, suggesting that the response to a range of environmental factors is the key processes in citrus leaves. Finally, the Lipase GDSL domain-containing protein GDSL esterase/lipase, which is involved in plant development and defense response, was for the first time identified and characterized in Citrus aurantium.

Influence of Tobacco Plant on Macronutrient Levels in Sandy Soils

Tobacco (Nicotiana tabacum L.) is associated with great uptake of soil macronutrients. Following the need to understand the macronutrients levels before and after tobacco cultivation, research was conducted in loamy sand soil of Sikonge, Urambo, and sand soil of Tabora, Tanzania. The initial macronutrients levels in the soil were compared with those measured after unfertilized and fertilized tobacco. Results showed that unfertilized tobacco plant influences the increase of nicotine to the rhizosphere, the macronutrients Ca (135%) > N (25%), decrease in the order of S (81%) > P (49%) > Mg (12%) > K (11%). The sole effect of nitrogen–phosphorus–potassium (NPK) and calcium–ammonium–nitrate (CAN) 27% fertilizers increased further nicotine, Ca (25%) > N (20%) > S (8%) > Mg (4%) > P (3%), and decreased K (3%) in the rhizosphere. Both tobacco plant and NPK + CAN fertilizers on the rhizosphere increased Ca (193%) > N (50%) and decreased S (80%) > P (48%) > K (14%) > Mg (8%). Leaf concentrations in fertilized tobacco increased Ca (197%) > K (28%) > P (27%) > S (26%) > N (18%) > Mg (12%). Therefore, tobacco plant increases soil N and Ca but decreases P, K, Mg, and S.

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.

Proteomic and Phosphoproteomic Analysis in Tobacco Mosaic Virus-Infected Tobacco (Nicotiana tabacum)

Tobacco mosaic virus (TMV) is a common source of biological stress that significantly affects plant growth and development. It is also useful as a model in studies designed to clarify the mechanisms involved in plant viral disease. Plant responses to abiotic stress were recently reported to be regulated by complex mechanisms at the post-translational modification (PTM) level. Protein phosphorylation is one of the most widespread and major PTMs in organisms. Using immobilized metal ion affinity chromatography (IMAC) enrichment, high-pH C18 chromatography fraction, and high-accuracy mass spectrometry (MS), a set of proteins and phosphopeptides in both TMV-infected tobacco and control tobacco were identified. A total of 4905 proteins and 3998 phosphopeptides with 3063 phosphorylation sites were identified. These 3998 phosphopeptides were assigned to 1311 phosphoproteins, as some proteins carried multiple phosphorylation sites. Among them, 530 proteins and 337 phosphopeptides corresponding to 277 phosphoproteins differed between the two groups. There were 43 upregulated phosphoproteins, including phosphoglycerate kinase, pyruvate phosphate dikinase, protein phosphatase 2C, and serine/threonine protein kinase. To the best of our knowledge, this is the first phosphoproteomic analysis of leaves from a tobacco cultivar, K326. The results of this study advance our understanding of tobacco development and TMV action at the protein phosphorylation level.

Disruption of the methionine cycle and reduced cellular gluthathione levels underlie potex-potyvirus synergism in Nicotiana benthamiana

Infection caused by the synergistic interaction of two plant viruses is typically manifested by severe symptoms and increased accumulation of either virus. In potex-potyviral synergism, the potyviral RNA silencing suppressor helper component proteinase (HCPro) is known to enhance the pathogenicity of the potexvirus counterpart. In line with this, Potato virus X (PVX; genus Potexvirus) genomic RNA (gRNA) accumulation and gene expression from subgenomic RNA (sgRNA) are increased in Nicotiana benthamiana by Potato virus A (PVA; genus Potyvirus) HCPro expression. Recently, we have demonstrated that PVA HCPro interferes with the host cell methionine cycle by interacting with its key enzymes S-adenosyl-l-methionine synthetase (SAMS) and S-adenosyl-l-homocysteine hydrolase (SAHH). To study the involvement of methionine cycle enzymes in PVX infection, we knocked down SAMS and SAHH. Increased PVX sgRNA expression between 3 and 9 days post-infiltration (dpi) and upregulation of (-)-strand gRNA accumulation at 9 dpi were observed in the SAHH-silenced background. We found that SAMS and SAHH silencing also caused a significant reduction in glutathione (GSH) concentration, specifically in PVX-infected plants between 2 and 9 dpi. Interestingly, HCPro expression in PVX-infected plants caused an even stronger reduction in GSH levels than did SAMS + SAHH silencing and a similar level of reduction was also achieved by knocking down GSH synthetase. PVX sgRNA expression was increased in the GSH synthetase-silenced background. GSH is a major antioxidant of plant cells and therefore GSH shortage may explain the strong oxidative stress and severe symptoms observed during potex-potyvirus mixed infection.

Enhanced Arabidopsis disease resistance against Botrytis cinerea induced by sulfur dioxide

Sulfur dioxide (SO₂) is a common air pollutant that has complex impacts on plants. The effect of prior exposure to 30mgm^-3 SO₂ on defence against Botrytis cinerea (B. cinerea) in Arabidopsis thaliana and the possible mechanisms of action were investigated. The results indicated that pre-exposure to 30mgm^-3 SO₂ resulted in significantly enhanced resistance to B. cinerea infection. SO₂ pre-treatment significantly enhanced the activities of defence-related enzymes including phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), β-1,3-glucanase (BGL) and chitinase (CHI). Transcripts of the defence-related genes PAL, PPO, PR2, and PR3, encoding PAL, PPO, BGL and CHI, respectively, were markedly elevated in Arabidopsis plants pre-exposed to SO₂ and subsequently inoculated with B. cinerea (SO₂+ treatment group) compared with those that were only treated with SO₂ (SO₂) or inoculated with B. cinerea (CK+). Moreover, SO₂ pre-exposure also led to significant increases in the expression levels of MIR393, MIR160 and MIR167 in Arabidopsis. Meanwhile, the expression of known targets involved in the auxin signalling pathway, was negatively correlated with their corresponding miRNAs. Additionally, the transcript levels of the primary auxin-response genes GH3-like, BDL/IAA12, and AXR3/IAA17 were markedly repressed. Our findings indicate that 30mgm^-3 SO₂ pre-exposure enhances disease resistance against B. cinerea in Arabidopsis by priming defence responses through enhancement of defence-related gene expression and enzyme activity, and miRNA-mediated suppression of the auxin signalling pathway.

Transcriptomic profile of tobacco in response to Tomato zonate spot orthotospovirus infection

BACKGROUND

Tomato zonate spot virus (TZSV), a dominant species of thrips-transmitted orthotospoviruses in Yunnan and Guangxi provinces in China, causes significant loss of yield in lots of crops and is a major threat to incomes of rural families. However, the detailed molecular mechanism of crop disease caused by TZSV remains obscure.

METHODS

Next-generation sequencing (NGS)-based transcriptome analysis (RNA-seq) was performed to investigate and compare the gene expression changes in systemic leaves of tobacco upon infection with TZSV and mock-inoculated plants as a control.

RESULTS

De novo assembly and analysis of tobacco transcriptome data by RNA-Seq identified 135,395 unigenes. 2102 differentially expressed genes (DEGs) were obtained in tobacco with TZSV infection, among which 1518 DEGs were induced and 584 were repressed. Gene Ontology enrichment analysis revealed that these DEGs were associated with multiple biological functions, including metabolic process, oxidation-reduction process, photosynthesis process, protein kinase activity. The KEGG pathway analysis of these DEGs indicated that pathogenesis caused by TZSV may affect multiple processes including primary and secondary metabolism, photosynthesis and plant-pathogen interactions.

CONCLUSION

Our global survey of transcriptional changes in TZSV infected tobacco provides crucial information into the precise molecular mechanisms underlying pathogenesis and symptom development. This is the first report on the relationships in the TZSV-plant interaction using transcriptome analysis. Findings of present study will significantly help enhance our understanding of the complicated mechanisms of plant responses to orthotospoviral infection.

Post-Translational Regulation of the Dicing Activities of Arabidopsis DICER-LIKE 3 and 4 by Inorganic Phosphate and the Redox State

In Arabidopsis thaliana, small interfering RNAs (siRNAs) generated by two Dicer isoforms, DCL3 and DCL4, function in distinct epigenetic processes, i.e. RNA-directed DNA methylation and post-transcriptional gene silencing, respectively. Plants often respond to their environment by producing a distinct set of small RNAs; however, the mechanism for controlling the production of different siRNAs from the same dsRNA substrate remains unclear. We established a simple biochemical method to visualize the dsRNA-cleaving activities of DCL3 and DCL4 in cell-free extracts prepared from Arabidopsis seedlings. Here, we demonstrate that different nutrient statuses of a host plant affect the post-translational regulation of the dicing activity of DCL3 and DCL4. Phosphate deficiency inhibited DCL3, and the activity of DCL3 was directly activated by inorganic phosphate. Sulfur deficiency inhibited DCL4 but not DCL3, and the activity of DCL4 was recovered by supplementation of the cell-free extracts with reductants containing a thiol group. Immunopurified DCL4 was activated by recombinant Arabidopsis thioredoxin-h1 with dithiothreitol. Therefore, DCL4 is subject to redox regulation. These results demonstrate that post-translational regulation of DCL activities fine-tunes the balance between branches of the gene silencing pathway according to the growth environment.

Differential expression of miRNAs and associated gene targets in grapevine leafroll-associated virus 3-infected plants

MicroRNAs (miRNAs) are a class of endogenous small non-coding RNAs (sRNA) that play an essential role in the regulation of target mRNAs expressed during plant development and in response to stress. MicroRNA expression profiling has helped to identify miRNAs that regulate a range of processes, including the plant's defence response to pathogens. In this study, differential miRNA expression in own-rooted Vitis vinifera cv. Cabernet Sauvignon plants infected with grapevine leafroll-associated virus 3 was investigated with microarrays and next-generation sequencing (NGS) of sRNA and mRNA. These high-throughput approaches identified several differentially expressed miRNAs. Four miRNAs, identified by both approaches, were validated by stemloop RT-PCRs. Three of the predicted targets of the differentially expressed miRNAs were also differentially expressed in the transcriptome data of infected plants, and were validated by RT-qPCR. Identification of these miRNAs and their targets can lead to a better understanding of host-pathogen interactions involved in grapevine leafroll disease and the identification of possible targets for virus resistance.

iTRAQ protein profile analysis provides integrated insight into mechanisms of tolerance to TMV in tobacco (Nicotiana tabacum)

To further investigate the mechanism of the plant tolerance to tobacco mosaic virus (TMV) infection, tobacco NC89 (N) hypersensitive to TMV and its natural mutant Yuyan8 (Y) with tolerance to TMV were employed for differential accumulation proteome analysis. There were 260 specifically accumulated proteins in Yuyan8 after 24 h inoculation (Yd), and the accumulations of 285 proteins inherent in Y have changed after TMV infection. Equally, there were 183 specifically accumulated proteins in NC89 after 24 h inoculation (Nd), and 132 proteins inherent in N have changed after TMV infection. These differential proteins were respectively enriched in two pathways, of which photosynthesis pathway was the common pathway in two varieties. In photoreaction system, the accumulations of differential proteins, especially D1 protein, were not decreased in Yd compared to Nd. The results indicated that maintaining the stability of D1 protein and reasonable utilization of the energy was the essential for tolerance to TMV infection. It was also revealed that 14-3-3 protein and PR4 was specific expressed, and the expression of LRR was enhanced in Yd, suggesting that regulation of defense protein mediated by 14-3-3 protein quickly activated resistance system and enhanced the plant tolerance to TMV infection.

SIGNIFICANCE

This is the first work that the molecular basis of tobacco tolerance was discussed basic on proteomic investigation performed on wild type and its natural mutant. Our results lay the foundation for development of molecular breeding and further proteome research in tobacco.

Increase of chromium tolerance in Scenedesmus acutus after sulfur starvation: Chromium uptake and compartmentalization in two strains with different sensitivities to Cr(VI)

In photosynthetic organisms sulfate constitutes the main sulfur source for the biosynthesis of GSH and its precursor Cys. Hence, sulfur availability can modulate the capacity to cope with environmental stresses, a phenomenon known as SIR/SED (Sulfur Induced Resistance or Sulfur Enhanced Defence). Since chromate may compete for sulfate transport into the cells, in this study chromium accumulation and tolerance were investigated in relation to sulfur availability in two strains of the unicellular green alga Scenedesmus acutus with different Cr-sensitivities. Paradoxically, sulfur deprivation has been demonstrated to induce a transient increase of Cr-tolerance in both strains. Sulfur deprivation is known to enhance the sulfate uptake/assimilation pathway leading to important consequences on Cr-tolerance: (i) reduced chromate uptake due to the induction of high affinity sulfate transporters (ii) higher production of cysteine and GSH which can play a role both through the formation of unsoluble complexes and their sequestration in inert compartments. To investigate the role of the above mentioned mechanisms, Cr accumulation in total cells and in different cell compartments (cell wall, membranes, soluble and miscellaneous fractions) was analyzed in both sulfur-starved and unstarved cells. Both strains mainly accumulated chromium in the soluble fraction, but the uptake was higher in the wild-type. In this type a short period of sulfur starvation before Cr(VI) treatment lowered chromium accumulation to the level observed in the unstarved Cr-tolerant strain, in which Cr uptake seems instead less influenced by S-starvation, since no significant decrease was observed. The increase in Cr-tolerance following S-starvation seems thus to rely on different mechanisms in the two strains, suggesting the induction of a mechanism constitutively active in the Cr-tolerant strain, maybe a high affinity sulfate transporter also in the wild-type. Changes observed in the cell wall and membrane fractions suggest a strong involvement of these compartments in Cr-tolerance increase following S-starvation.

Spatial gene expression analysis in tomato hypocotyls suggests cysteine as key precursor of vascular sulfur accumulation implicated in Verticillium dahliae defense

Verticillium dahliae is a prominent generator of plant vascular wilting disease and sulfur (S)-enhanced defense (SED) mechanisms contribute to its in-planta elimination. The accumulation of S-containing defense compounds (SDCs) including elemental S (S(0) ) has been described based on the comparison of two near-isogenic tomato (Solanum lycopersicum) lines differing in fungal susceptibility. To better understand the effect of S nutrition on V. dahliae resistance both lines were supplied with low, optimal or supraoptimal sulfate-S. An absolute quantification demonstrated a most effective fungal elimination due to luxury plant S nutrition. High-pressure liquid chromatography (HPLC) showed a strong regulation of Cys levels and an S-responsive GSH pool rise in the bulk hypocotyl. High-frequency S peak accumulations were detected in vascular bundles of resistant tomato plants after fungal colonization by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Global transcriptomic analysis suggested that early steps of the primary S metabolism did not promote the SDCs synthesis in the whole hypocotyl as gene expression was downregulated after infection. Enhanced S fertilization mostly alleviated the repressive fungal effect but did not reverse it. Upregulation of glutathione (GSH)-associated genes in bulk hypocotyls but not in vascular bundles indicated a global antioxidative role of GSH. To finally assign the contribution of S metabolism-associated genes to high S(0) accumulations exclusively found in the resistant tomato line, a spatial gene expression approach was applied. Laser microdissection of infected vascular bundles revealed a switch toward transcription of genes connected with cysteine (Cys) synthesis. The upregulation of LeOASTLp1 suggests a role for Cys as key precursor for local S accumulations (possibly S(0) ) in the vascular bundles of the V. dahliae-resistant tomato line.

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.

Glutathione application affects the transcript profile of genes in Arabidopsis seedling

Glutathione (GSH), a tripeptide thiol compound has multiple functions in plants. Recent works suggested that GSH plays a regulatory role in signaling in plants as part of their adaptation to stress. To better understand the role of GSH as a regulatory molecule, 14 days old Arabidopsis thaliana seedlings were treated with 5mM of GSH for 4h. Changes in gene expression patterns were studied by cDNA microarray analysis. The expression of 453 genes was significantly changed compared to the untreated control, of which 261 genes were up-regulated and 192 genes were down-regulated. Genes from several groups were affected, including those of sulfur metabolism, degradation and synthesis of macromolecules and transcription factors. Up-regulation of genes involved in responses to biotic stresses, or in jasmonate or salicylic acid synthesis and their signaling, suggests that GSH triggers genes that help protect the plants during stresses. In addition, GSH down regulated genes involved in plant growth and development, like those involved in cell wall synthesis and its extension, and genes associated with auxin and cytokinins response, which are related to growth and development of the plants. The results suggest that GSH might have a role in response to biotic stress by initiating defense responses and modifying plants' growth and development in an effort to tune their sessile lifestyle of plants to environmental constraints.

Functional analysis of Arabidopsis mutants points to novel roles for glutathione in coupling H(2)O(2) to activation of salicylic acid accumulation and signaling

AIMS

Through its interaction with H(2)O(2), glutathione is a candidate for transmission of signals in plant responses to pathogens, but identification of signaling roles is complicated by its antioxidant function. Using a genetic approach based on a conditional catalase-deficient Arabidopsis mutant, cat2, this study aimed at establishing whether GSH plays an important functional role in the transmission of signals downstream of H(2)O(2).

RESULTS

Introducing the cad2 or allelic mutations in the glutathione synthesis pathway into cat2 blocked H(2)O(2)-triggered GSH oxidation and accumulation. While no effects on NADP(H) or ascorbate were observed, and H(2)O(2)-induced decreases in growth were maintained, blocking GSH modulation antagonized salicylic acid (SA) accumulation and SA-dependent responses. Other novel double and triple mutants were produced and compared with cat2 cad2 at the levels of phenotype, expression of marker genes, nontargeted metabolite profiling, accumulation of SA, and bacterial resistance. Most of the effects of the cad2 mutation on H(2)O(2)-triggered responses were distinct from those produced by mutations for GLUTATHIONE REDUCTASE1 (GR1) or NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), and were linked to compromised induction of ISOCHORISMATE SYNTHASE1 (ICS1) and ICS1-dependent SA accumulation.

INNOVATION

A novel genetic approach was used in which GSH content or antioxidative capacity was independently modified in an H(2)O(2) signaling background. Analysis of new double and triple mutants allowed us to infer previously undescribed regulatory roles for GSH.

CONCLUSION

In parallel to its antioxidant role, GSH acts independently of NPR1 to allow increased intracellular H(2)O(2) to activate SA signaling, a key defense response in plants.

Regulation of basal and oxidative stress-triggered jasmonic acid-related gene expression by glutathione

Glutathione is a determinant of cellular redox state with roles in defence and detoxification. Emerging concepts suggest that this compound also has functions in cellular signalling. Here, we report evidence that glutathione plays potentially important roles in setting signalling strength through the jasmonic acid (JA) pathway. Firstly, we show that basal expression of JA-related genes is correlated with leaf glutathione content when the latter is manipulated either genetically or pharmacologically. Secondly, analyses of an oxidative stress signalling mutant, cat2, reveal that up-regulation of the JA pathway triggered by intracellular oxidation requires accompanying glutathione accumulation. Genetically blocking this accumulation in a cat2 cad2 line largely annuls H2 O2 -induced expression of JA-linked genes, and this effect can be rescued by exogenously supplying glutathione. While most attention on glutathione functions in biotic stress responses has been focused on the thiol-regulated protein NPR1, a comparison of JA-linked gene expression in cat2 cad2 and cat2 npr1 double mutants provides evidence that glutathione acts through other components to regulate the response of this pathway to oxidative stress. Our study provides new information implicating glutathione as a factor determining basal JA gene expression and suggests novel glutathione-dependent control points that regulate JA signalling in response to intracellular oxidation.

Transcriptional profiling of canker-resistant transgenic sweet orange (Citrus sinensis Osbeck) constitutively overexpressing a spermidine synthase gene

Citrus canker disease caused by Xanthomonas citri subsp. citri (Xcc) is one of the most devastating diseases affecting the citrus industry worldwide. In our previous study, the canker-resistant transgenic sweet orange (Citrus sinensis Osbeck) plants were produced via constitutively overexpressing a spermidine synthase. To unravel the molecular mechanisms underlying Xcc resistance of the transgenic plants, in the present study global transcriptional profiling was compared between untransformed line (WT) and the transgenic line (TG9) by hybridizing with Affymetrix Citrus GeneChip. In total, 666 differentially expressed genes (DEGs) were identified, 448 upregulated, and 218 downregulated. The DEGs were classified into 33 categories after Gene ontology (GO) annotation, in which 68 genes are in response to stimulus and involved in immune system process, 12 genes are related to cell wall, and 13 genes belong to transcription factors. These genes and those related to starch and sucrose metabolism, glutathione metabolism, biosynthesis of phenylpropanoids, and plant hormones were hypothesized to play major roles in the canker resistance of TG9. Semiquantitative RT-PCR analysis showed that the transcript levels of several candidate genes in TG9 were significantly higher than in WT both before and after Xcc inoculation, indicating their potential association with canker disease.

Hibiscus chlorotic ringspot virus coat protein upregulates sulfur metabolism genes for enhanced pathogen defense

In both Hibiscus chlorotic ringspot virus (HCRSV)-infected and HCRSV coat protein (CP) agroinfiltrated plant leaves, we showed that sulfur metabolism pathway related genes-namely, sulfite oxidase (SO), sulfite reductase, and adenosine 5'-phosphosulfate kinase-were upregulated. It led us to examine a plausible relationship between sulfur-enhanced resistance (SED) and HCRSV infection. We broadened an established method to include different concentrations of sulfur (0S, 1S, 2S, and 3S) to correlate them to symptom development of HCRSV-infected plants. We treated plants with glutathione and its inhibitor to verify the SED effect. Disease resistance was induced through elevated glutathione contents during HCRSV infection. The upregulation of SO was related to suppression of symptom development induced by sulfur treatment. In this study, we established that HCRSV-CP interacts with SO which, in turn, triggers SED and leads to enhanced plant resistance. Thus, we have discovered a new function of SO in the SED pathway. This is the first report to demonstrate that the interaction of a viral protein and host protein trigger SED in plants. It will be interesting if such interaction applies generally to other host-pathogen interactions that will lead to enhanced pathogen defense.

Phytoalexin transgenics in crop protection--fairy tale with a happy end?

Phytoalexins are pathogen induced low molecular weight compounds with antimicrobial activities derived from secondary metabolism. Following their identification, phytoalexins were directly incorporated into the network of plant defense responses. Due to their heterogeneity, the metabolic pathways involved in phytoalexin formation and in particular the regulatory mechanisms remained elusive. Consequently, research focus shifted to the characterization of other components of plant immunity such as defense signaling and resistance mechanisms, including components of systemic acquired and induced systemic resistance, effector and pathogen-associated molecular pattern triggered immunity as well as R-gene resistance. Despite the obtained knowledge on these immunity mechanisms, genetic engineering employing these mechanisms and classical breeding reached too low improvements in crop protection, probably because classical breeding focused on yield performance and taste, rather than pathogen resistance. The increasing demand for disease resistant crop species and the aim to reduce pesticide application therefore requires alternative approaches. Recent advances in the understanding of phytoalexin function, biosynthesis and regulation, in combination with novel methods of molecular engineering and advances in instrumental analysis, returned attention to phytoalexins as a potent target for improving crop protection. Based on this, the advantages as well as potential bottlenecks for molecular approaches of modulating inducible phytoalexins to improve crop protection are discussed.

Sulfate supply influences compartment specific glutathione metabolism and confers enhanced resistance to Tobacco mosaic virus during a hypersensitive response

Sufficient sulfate supply has been linked to the development of sulfur induced resistance or sulfur enhanced defense (SIR/SED) in plants. In this study we investigated the effects of sulfate (S) supply on the response of genetically resistant tobacco (Nicotiana tabacum cv. Samsun NN) to Tobacco mosaic virus (TMV). Plants grown with sufficient sulfate (+S plants) developed significantly less necrotic lesions during a hypersensitive response (HR) when compared to plants grown without sulfate (-S plants). In +S plants reduced TMV accumulation was evident on the level of viral RNA. Enhanced virus resistance correlated with elevated levels of cysteine and glutathione and early induction of a Tau class glutathione S-transferase and a salicylic acid-binding catalase gene. These data indicate that the elevated antioxidant capacity of +S plants was able to reduce the effects of HR, leading to enhanced virus resistance. Expression of pathogenesis-related genes was also markedly up-regulated in +S plants after TMV-inoculation. On the subcellular level, comparison of TMV-inoculated +S and -S plants revealed that +S plants contained 55-132 % higher glutathione levels in mitochondria, chloroplasts, nuclei, peroxisomes and the cytosol than -S plants. Interestingly, mitochondria were the only organelles where TMV-inoculation resulted in a decrease of glutathione levels when compared to mock-inoculated plants. This was particularly obvious in -S plants, where the development of necrotic lesions was more pronounced. In summary, the overall higher antioxidative capacity and elevated activation of defense genes in +S plants indicate that sufficient sulfate supply enhances a preexisting plant defense reaction resulting in reduced symptom development and virus accumulation.

Up-regulation of antioxidants in tobacco by low concentrations of H₂O₂ suppresses necrotic disease symptoms

Pretreatment of tobacco leaves with low concentrations (5 to 10 mM) of H₂O₂ suppressed hypersensitive-type necrosis associated with resistance to Tobacco mosaic virus (TMV) or Pseudomonas syringae pv. phaseolicola. The same pretreatment resulted in suppression of normosensitive necrosis associated with susceptibility to Botrytis cinerea. This type of H₂O₂-mediated, induced disease symptom resistance correlated with enhanced host antioxidant capacity, i.e., elevated enzymatic activities of catalase (CAT), ascorbate peroxidase (APX), and guaiacol peroxidase (POX) after viral and bacterial infections. Induction of genes that encode the antioxidants superoxide dismutase (SOD), CAT, and APX was also enhanced early after TMV infection. Artificial application of SOD and CAT suppressed necroses caused by viral, bacterial, or fungal pathogens similarly as H₂O₂ pretreatment, implying that H₂O₂-mediated symptom resistance operates through enhancement of plant antioxidant capacity. Pathogen multiplication was not significantly affected in H₂O₂-pretreated plants. Salicylic acid (SA), a central component of plant defense, does not seem to function in this type of H₂O₂-mediated symptom resistance, indicated by unchanged levels of free and bound SA and a lack of early up-regulation of an SA glucosyltransferase gene in TMV-infected H₂O₂-pretreated tobacco. Taken together, H₂O₂-mediated, induced resistance to necrotic symptoms in tobacco seems to depend on enhanced antioxidant capacity.

Sulfur fertilization and fungal infections affect the exchange of H(2)S and COS from agricultural crops

The emission of gaseous sulfur (S) compounds by plants is related to several factors, such as the plant S status or fungal infection. Hydrogen sulfide (H(2)S) is either released or taken up by the plant depending on the ambient air concentration and the plant demand for S. On the contrary, carbonyl sulfide (COS) is normally taken up by plants. In a greenhouse experiment, the dependence of H(2)S and COS exchange with ambient air on the S status of oilseed rape (Brassica napus L.) and on fungal infection with Sclerotinia sclerotiorum was investigated. Thiol contents were determined to understand their influence on the exchange of gaseous S compounds. The experiment revealed that H(2)S emissions were closely related to pathogen infections as well as to S nutrition. S fertilization caused a change from H(2)S consumption by S-deficient oilseed rape plants to a H(2)S release of 41 pg g(-1) (dw) min(-1) after the addition of 250 mg of S per pot. Fungal infection caused an even stronger increase of H(2)S emissions with a maximum of 1842 pg g(-1) (dw) min(-1) 2 days after infection. Healthy oilseed rape plants acted as a sink for COS. Fungal infection caused a shift from COS uptake to COS releases. The release of S-containing gases thus seems to be part of the response to fungal infection. The roles the S-containing gases may play in this response are discussed.

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.

Glutathione in plants: an integrated overview

Plants cannot survive without glutathione (γ-glutamylcysteinylglycine) or γ-glutamylcysteine-containing homologues. The reasons why this small molecule is indispensable are not fully understood, but it can be inferred that glutathione has functions in plant development that cannot be performed by other thiols or antioxidants. The known functions of glutathione include roles in biosynthetic pathways, detoxification, antioxidant biochemistry and redox homeostasis. Glutathione can interact in multiple ways with proteins through thiol-disulphide exchange and related processes. Its strategic position between oxidants such as reactive oxygen species and cellular reductants makes the glutathione system perfectly configured for signalling functions. Recent years have witnessed considerable progress in understanding glutathione synthesis, degradation and transport, particularly in relation to cellular redox homeostasis and related signalling under optimal and stress conditions. Here we outline the key recent advances and discuss how alterations in glutathione status, such as those observed during stress, may participate in signal transduction cascades. The discussion highlights some of the issues surrounding the regulation of glutathione contents, the control of glutathione redox potential, and how the functions of glutathione and other thiols are integrated to fine-tune photorespiratory and respiratory metabolism and to modulate phytohormone signalling pathways through appropriate modification of sensitive protein cysteine residues.

Cysteine homeostasis plays an essential role in plant immunity

• Cysteine is the metabolic precursor of essential biomolecules such as vitamins, cofactors, antioxidants and many defense compounds. The last step of cysteine metabolism is catalysed by O-acetylserine(thiol)lyase (OASTL), which incorporates reduced sulfur into O-acetylserine to produce cysteine. In Arabidopsis thaliana, the main OASTL isoform OAS-A1 and the cytosolic desulfhydrase DES1, which degrades cysteine, contribute to the cytosolic cysteine homeostasis. • Meta-analysis of the transcriptomes of knockout plants for OAS-A1 and for DES1 show a high correlation with the biotic stress series in both cases. • The study of the response of knockout mutants to plant pathogens shows that des1 mutants behave as constitutive systemic acquired resistance mutants, with high resistance to biotrophic and necrotrophic pathogens, salicylic acid accumulation and WRKY54 and PR1 induction, while oas-a1 knockout mutants are more sensitive to biotrophic and necrotrophic pathogens. However, oas-a1 knockout mutants lack the hypersensitive response associated with the effector-triggered immunity elicited by Pseudomonas syringae pv. tomato DC3000 avrRpm1. • Our results highlight the role of cysteine as a crucial metabolite in the plant immune response.

Subcellular distribution of glutathione and its dynamic changes under oxidative stress in the yeast Saccharomyces cerevisiae

Glutathione is an important antioxidant in most prokaryotes and eukaryotes. It detoxifies reactive oxygen species and is also involved in the modulation of gene expression, in redox signaling, and in the regulation of enzymatic activities. In this study, the subcellular distribution of glutathione was studied in Saccharomyces cerevisiae by quantitative immunoelectron microscopy. Highest glutathione contents were detected in mitochondria and subsequently in the cytosol, nuclei, cell walls, and vacuoles. The induction of oxidative stress by hydrogen peroxide (H₂O₂) led to changes in glutathione-specific labeling. Three cell types were identified. Cell types I and II contained more glutathione than control cells. Cell type II differed from cell type I in showing a decrease in glutathione-specific labeling solely in mitochondria. Cell type III contained much less glutathione contents than the control and showed the strongest decrease in mitochondria, suggesting that high and stable levels of glutathione in mitochondria are important for the protection and survival of the cells during oxidative stress. Additionally, large amounts of glutathione were relocated and stored in vacuoles in cell type III, suggesting the importance of the sequestration of glutathione in vacuoles under oxidative stress.

Subcellular distribution of glutathione in the gametophyte

Glutathione is an important antioxidant and redox buffer in plants. Despite its crucial roles in plant metabolism and defense in the sporophyte, its roles in the gametophyte are largely unexplored. Recently, we demonstrated that glutathione synthesis is essential for pollen germination in vitro. In this study, we extend these results and focus on the subcellular distribution of glutathione in pollen grains and compare it to the situation in the sporophyte. Glutathione was equally distributed within mitochondria, plastids, nuclei and the cytosol in the gametophyte -- in contrast to youngest fully developed leaves and root tips of the sporophyte, where glutathione was highest in the mitochondria, followed by nuclei, cytosol, peroxisomes and plastids in decreasing concentration. Glutathione was not detected in vacuoles. We can conclude that glutathione synthesis is essential for pollen germination in vitro and that the subcellular distribution of glutathione in the gametophyte differs significantly from the sporophyte.

Glutathione

Glutathione is a simple sulfur compound composed of three amino acids and the major non-protein thiol in many organisms, including plants. The functions of glutathione are manifold but notably include redox-homeostatic buffering. Glutathione status is modulated by oxidants as well as by nutritional and other factors, and can influence protein structure and activity through changes in thiol-disulfide balance. For these reasons, glutathione is a transducer that integrates environmental information into the cellular network. While the mechanistic details of this function remain to be fully elucidated, accumulating evidence points to important roles for glutathione and glutathione-dependent proteins in phytohormone signaling and in defense against biotic stress. Work in Arabidopsis is beginning to identify the processes that govern glutathione status and that link it to signaling pathways. As well as providing an overview of the components that regulate glutathione homeostasis (synthesis, degradation, transport, and redox turnover), the present discussion considers the roles of this metabolite in physiological processes such as light signaling, cell death, and defense against microbial pathogen and herbivores.