Glutathione synthesis is essential for pollen germination in vitro

Overexpression of sweetpotato glutamylcysteine synthetase (IbGCS) in Arabidopsis confers tolerance to drought and salt stresses

Various environmental stresses induce the production of reactive oxygen species (ROS), which have deleterious effects on plant cells. Glutathione (GSH) is an antioxidant used to counteract reactive oxygen species. Glutathione is produced by glutamylcysteine synthetase (GCS) and glutathione synthetase (GS). However, evidence for the GCS gene in sweetpotato remains scarce. In this study, the full-length cDNA sequence of IbGCS isolated from sweetpotato cultivar Xu18 was 1566 bp in length, which encodes 521 amino acids. The qRT-PCR analysis revealed a significantly higher expression of the IbGCS in sweetpotato flowers, and the gene was induced by salinity, abscisic acid (ABA), drought, extreme temperature and heavy metal stresses. The seed germination rate, root elongation and fresh weight were promoted in T3 Arabidopsis IbGCS-overexpressing lines (OEs) in contrast to wild type (WT) plants under mannitol and salt stresses. In addition, the soil drought and salt stress experiment results indicated that IbGCS overexpression in Arabidopsis reduced the malondialdehyde (MDA) content, enhanced the levels of GCS activity, GSH and AsA content, and antioxidant enzyme activity. In summary, overexpressing IbGCS in Arabidopsis showed improved salt and drought tolerance.

SULTR2;1 Adjusts the Bolting Timing by Transporting Sulfate from Rosette Leaves to the Primary Stem

Sulfur (S) is an essential macronutrient for plant growth and metabolism. SULTR2;1 is a low-affinity sulfate transporter facilitating the long-distance transport of sulfate in Arabidopsis. The physiological function of SULTR2;1 in the plant life cycle still needs to be determined. Therefore, we analyzed the sulfate transport, S-containing metabolite accumulation and plant growth using Arabidopsis SULTR2;1 disruption lines, sultr2;1-1 and sultr2;1-2, from seedling to mature growth stages to clarify the metabolic and physiological roles of SULTR2;1. We observed that sulfate distribution to the stems was affected in sultr2;1 mutants, resulting in decreased levels of sulfate, cysteine, glutathione (GSH) and total S in the stems, flowers and siliques; however, the GSH levels increased in the rosette leaves. This suggested the essential role of SULTR2;1 in sulfate transport from rosette leaves to the primary stem. In addition, sultr2;1 mutants unexpectedly bolted earlier than the wild-type without affecting the plant biomass. Correlation between GSH levels in rosette leaves and the bolting timing suggested that the rosette leaf GSH levels or limited sulfate transport to the early stem can trigger bolting. Overall, this study demonstrated the critical roles of SULTR2;1 in maintaining the S metabolite levels in the aerial part and transitioning from the vegetative to the reproductive growth phase.

Multi-omics research on common allergens during the ripening of pollen and poplar flocs of Populus deltoides

Background

Populus deltoides is widely cultivated in China and produces a large number of pollen and poplar flocs from March to June per year. Previous studies have found that the pollen of P. deltoides contains allergens. However, studies on the ripening mechanism of pollen/poplar flocs and their common allergens are very limited.

Methods

Proteomics and metabolomics were used to study the changes of proteins and metabolites in pollen and poplar flocs of P. deltoides at different developmental stages. Allergenonline database was used to identify common allergens in pollen and poplar flocs at different developmental stages. Western blot (WB) was used to detect the biological activity of common allergens between mature pollen and poplar flocs.

Results

In total, 1400 differently expressed proteins (DEPs) and 459 different metabolites (DMs) were identified from pollen and poplar flocs at different developmental stages. KEGG enrichment analysis showed that DEPs in pollen and poplar flocs were significantly enriched in ribosome and oxidative phosphorylation signaling pathways. The DMs in pollen are mainly involved in aminoacyl-tRNA biosynthesis and arginine biosynthesis, while the DMs in poplar flocs are mainly involved in glyoxylate and dicarboxylate metabolism. Additionally, 72 common allergens were identified in pollen and poplar flocs at different developmental stages. WB showed that there were distinct binding bands between 70 and 17KD at the two groups of allergens.

Conclusions

A multitude of proteins and metabolites are closely related to the ripening of pollen and poplar flocs of Populus deltoides, and they contain common allergens between mature pollen and poplar flocs.

Thermosensitivity of pollen: a molecular perspective

A current trend in climate comprises adverse weather anomalies with more frequent and intense temperature events. Heatwaves are a serious threat to global food security because of the susceptibility of crop plants to high temperatures. Among various developmental stages of plants, even a slight rise in temperature during reproductive development proves detrimental, thus making sexual reproduction heat vulnerable. In this context, male gametophyte or pollen development stages are the most sensitive ones. High-temperature exposure induces pollen abortion, reducing pollen viability and germination rate with a concomitant effect on seed yield. This review summarizes the ultrastructural, morphological, biochemical, and molecular changes underpinning high temperature-induced aberrations in male gametophytes. Specifically, we highlight the temperature sensing cascade operating in pollen, involving reactive oxygen species (ROS), heat shock factors (HSFs), a hormones and transcriptional regulatory network. We also emphasize integrating various omics approaches to decipher the molecular events triggered by heat stress in pollen. The knowledge of genes, proteins, and metabolites conferring thermotolerance in reproductive tissues can be utilized to breed/engineer thermotolerant crops to ensure food security.

Integrated analysis of metabolome and transcriptome reveals the cytoplasmic effects of CMS-D2 on pollen fertility resulting from disrupted lipid metabolism

Using cytoplasmic male sterility of Gossypium harknesii (CMS-D2) is an economical and effective method to produce cotton hybrids. However, the detrimental cytoplasmic effects of CMS-D2 on pollen fertility and fiber yields greatly limit the further development of three-line hybrid cotton in China. In this study, an integrated non-targeted metabolomics and transcriptome analysis was performed on mature pollens of maintainer line NB, isonuclear alloplasmic near-isogenic restorer lines NH and SH under two environments. A total of 820 metabolites were obtained, of which lipids and lipid-like molecules were the most, followed by organic acids derivatives, phenylpropanoids, and polyketides. Transcriptome analysis revealed significantly more differentially expressed genes (DEGs) in SH versus NH both in Anyang and Jiujiang, and most of the DEGs were significantly upregulated. Further KEGG analysis showed that most DEGs were enriched in the biosynthesis of unsaturated fatty acids, phenylalanine metabolism, and phagosome. Based on the weighted gene co-expression network analysis, totally 74 hub genes were also identified, of which three transcription factors, i.e., WRKY22, WRKY53, and ARF18 were significantly upregulated in SH and may play a negative regulatory role in pollen development by directly or indirectly regulating the jasmonic acid synthesis and signal transduction. Moreover, we found that the negative effects of CMS-D2 cytoplasm on pollen fertility were mainly due to disturbed lipid metabolism, especially the metabolic balance of unsaturated fatty acids, ultimately resulting in the decline of pollen fertility. Meanwhile, in the presence of CMS-D2 sterile cytoplasm, the cytoplasmic-nucleus interaction effects generated a substantial quantity of flavonoids involved in the fertility restoration process. This study preliminarily clarified some of the reasons for the negative effects of CMS-D2 cytoplasm on pollen fertility, and our results will provide an important theoretical reference for further breeding and improvement of three-line hybrid cotton in the future.

Proteins, Small Peptides and Other Signaling Molecules Identified as Inconspicuous but Possibly Important Players in Microspores Reprogramming Toward Embryogenesis

In this review, we describe and integrate the latest knowledge on the signaling role of proteins and peptides in the stress-induced microspore embryogenesis (ME) in some crop plants with agricultural importance (i.e., oilseed rape, tobacco, barley, wheat, rice, triticale, rye). Based on the results received from the most advanced omix analyses, we have selected some inconspicuous but possibly important players in microspores reprogramming toward embryogenic development. We provide an overview of the roles and downstream effect of stress-related proteins (e.g., β-1,3-glucanases, chitinases) and small signaling peptides, especially cysteine—(e.g., glutathione, γ-thionins, rapid alkalinization factor, lipid transfer, phytosulfokine) and glycine-rich peptides and other proteins (e.g., fasciclin-like arabinogalactan protein) on acclimation ability of microspores and the cell wall reconstruction in a context of ME induction and haploids/doubled haploids (DHs) production. Application of these molecules, stimulating the induction and proper development of embryo-like structures and green plant regeneration, brings significant improvement of the effectiveness of DHs procedures and could result in its wider incorporation on a commercial scale. Recent advances in the design and construction of synthetic peptides–mainly cysteine-rich peptides and their derivatives–have accelerated the development of new DNA-free genome-editing techniques. These new systems are evolving incredibly fast and soon will find application in many areas of plant science and breeding.

The impact of stress combination on reproductive processes in crops

Historically, extended droughts combined with heat waves caused severe reductions in crop yields estimated at billions of dollars annually. Because global warming and climate change are driving an increase in the frequency and intensity of combined water-deficit and heat stress episodes, understanding how these episodes impact yield is critical for our efforts to develop climate change-resilient crops. Recent studies demonstrated that a combination of water-deficit and heat stress exacerbates the impacts of water-deficit or heat stress on reproductive processes of different cereals and legumes, directly impacting grain production. These studies identified several different mechanisms potentially underlying the effects of stress combination on anthers, pollen, and stigma development and function, as well as fertilization. Here we review some of these findings focusing on unbalanced reactive oxygen accumulation, altered sugar concentrations, and conflicting functions of different hormones, as contributing to the reduction in yield during a combination of water-deficit and heat stress. Future studies focused on the effects of water-deficit and heat stress combination on reproduction of different crops are likely to unravel additional mechanisms, as well as reveal novel ways to develop stress combination-resilient crops. These could mitigate some of the potentially devastating impacts of this stress combination on agriculture.

Oxygen and reactive oxygen species-dependent regulation of plant growth and development

Oxygen and reactive oxygen species (ROS) have been co-opted during evolution into the regulation of plant growth, development, and differentiation. ROS and oxidative signals arising from metabolism or phytohormone-mediated processes control almost every aspect of plant development from seed and bud dormancy, liberation of meristematic cells from the quiescent state, root and shoot growth, and architecture, to flowering and seed production. Moreover, the phytochrome and phytohormone-dependent transmissions of ROS waves are central to the systemic whole plant signaling pathways that integrate root and shoot growth. The sensing of oxygen availability through the PROTEOLYSIS 6 (PRT6) N-degron pathway functions alongside ROS production and signaling but how these pathways interact in developing organs remains poorly understood. Considerable progress has been made in our understanding of the nature of hydrogen peroxide sensors and the role of thiol-dependent signaling networks in the transmission of ROS signals. Reduction/oxidation (redox) changes in the glutathione (GSH) pool, glutaredoxins (GRXs), and thioredoxins (TRXs) are important in the control of growth mediated by phytohormone pathways. Although, it is clear that the redox states of proteins involved in plant growth and development are controlled by the NAD(P)H thioredoxin reductase (NTR)/TRX and reduced GSH/GRX systems of the cytosol, chloroplasts, mitochondria, and nucleus, we have only scratched the surface of this multilayered control and how redox-regulated processes interact with other cell signaling systems.

A comprehensive integrated transcriptome and metabolome analyses to reveal key genes and essential metabolic pathways involved in CMS in kenaf

Numbers of critical genes and pathways were found from the levels of transcriptome and metabolome, which were useful information for understanding of kenaf CMS mechanism. Cytoplasmic male sterility (CMS) is a maternally inherited trait in higher plants that leads to the inability to produce or release functional pollen. However, there is lack of comprehensive studies to reveal the molecular basis of CMS occurrence in kenaf. Herein, we performed transcriptome and UPLC-MS-based metabolome analyses in the anthers of a CMS (UG93A) and its maintainer (UG93B) to sort out essential genes and metabolites responding to CMS in kenaf. Transcriptome characterized 7769 differentially expressed genes (DEGs) between these two materials, and pathway enrichment analysis indicated that these DEGs were involved mainly in pentose and glucuronate interconversions, starch and sucrose metabolism, taurine and hypotaurine metabolism. In the metabolome assay, a total of 116 significantly different metabolites (SDMs) were identified between the CMS and its maintainer line, and these SDMs were involved in eight KEGG pathways, including flavone and flavonol biosynthesis, glycerophospholipid metabolism, flavonoid biosynthesis, glycosylphosphatidylinositol-anchor biosynthesi. Integrated analyses of transcriptome and metabolome showed that 50 genes had strong correlation coefficient values (R² > 0.9) with ten metabolites enriched in six pathways; notably, most genes and metabolites of flavonoid biosynthesis pathways and flavone and flavonol biosynthesis pathways involved in flavonoids biosynthetic pathways were downregulated in CMS compared to those in maintainer. Taken together, the decreased accumulation of flavonoids resulted from the compromised biosynthesis pathways coupled with energy deficiency in the anthers may contribute largely to CMS in UG93A of kenaf.

Modulation of (Homo)Glutathione Metabolism and H2O2 Accumulation during Soybean Cyst Nematode Infections in Susceptible and Resistant Soybean Cultivars

In plant immune responses, reactive oxygen species (ROS) act as signaling molecules that activate defense pathways against pathogens, especially following resistance (R) gene-mediated pathogen recognition. Glutathione (GSH), an antioxidant and redox regulator, participates in the removal of hydrogen peroxide (H2O2). However, the mechanism of GSH-mediated H2O2 generation in soybeans (Glycine max (L.) Merr.) that are resistant to the soybean cyst nematode (SCN; Heterodera glycines Ichinohe) remains unclear. To elucidate this underlying relationship, the feeding of race 3 of H. glycines with resistant cultivars, Peking and PI88788, was compared with that on a susceptible soybean cultivar, Williams 82. After 5, 10, and 15 days of SCN infection, we quantified γ-glutamylcysteine (γ-EC) and (homo)glutathione ((h)GSH), and a gene expression analysis showed that GSH metabolism in resistant cultivars differed from that in susceptible soybean roots. ROS accumulation was examined both in resistant and susceptible roots upon SCN infection. The time of intense ROS generation was related to the differences of resistance mechanisms in Peking and PI88788. ROS accumulation that was caused by the (h)GSH depletion-arrested nematode development in susceptible Williams 82. These results suggest that (h)GSH metabolism in resistant soybeans plays a key role in the regulation of ROS-generated signals, leading to resistance against nematodes.

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

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

Glutathione redox state plays a key role in flower development and pollen vigour

The importance of the glutathione pool in the development of reproductive tissues and in pollen tube growth was investigated in wild-type (WT) Arabidopsis thaliana, a reporter line expressing redox-sensitive green fluorescent protein (roGFP2), and a glutathione-deficient cad2-1 mutant (cad2-1/roGFP2). The cad2-1/roGFP2 flowers had significantly less reduced glutathione (GSH) and more glutathione disulfide (GSSG) than WT or roGFP2 flowers. The stigma, style, anther, germinated pollen grains, and pollen tubes of roGFP2 flowers had a low degree of oxidation. However, these tissues were more oxidized in cad2-1/roGFP2 flowers than the roGFP2 controls. The ungerminated pollen grains were significantly more oxidized than the germinated pollen grains, indicating that the pollen cells become reduced upon the transition from the quiescent to the metabolically active state during germination. The germination percentage was lower in cad2-1/roGFP2 pollen and pollen tube growth arrested earlier than in roGFP2 pollen, demonstrating that increased cellular reduction is essential for pollen tube growth. These findings establish that ungerminated pollen grains exist in a relatively oxidized state compared with germinating pollen grains. Moreover, failure to accumulate glutathione and maintain a high GSH/GSSG ratio has a strong negative effect on pollen germination.

Flowers and climate change: a metabolic perspective

Adverse climatic conditions at the time of flowering severely hinder crop yields and threaten the interactions between plants and their pollinators. These features depend on a common trait: the metabolism of flowers. In this Viewpoint article, we aim to provide insight into the metabolic changes that occur in flowers in response to changes in climate and emphasize that these changes severely impact the fitness of autogamous and allogamous species, plant-pollinator interactions, and overall ecosystem health. We review the biochemical processes that lead to failure of gamete development and to alterations of color, scent and nectar secretion. Then, making use of open access expression data, we examine the expression of genes that may drive these changes in response to heat and drought. Finally, we present measurements of metabolites from flowers exposed to a heat wave and discuss how the results of this short-term experiment may give rise to misleading conclusions regarding the positive effect of heat on flower fitness. We hope this article draws attention to this often-neglected dynamic and its important consequences.

Biosafety assessment of graphene nanosheets on leaf ultrastructure, physiological and yield traits of Capsicum annuum L. and Solanum melongena L

Due to limited knowledge of graphene nanosheets (GNS) on phyto-biological studies, GNS was sprayed to pepper and eggplants during the seasons 2016 and 2017 at doses 0.1, 0.2 and 0.3 GNS g L^-1 to assess their biosafety on leaf ultrastructure and agro-physiological traits. GNS was localized on plastids, cell walls and intercellular spaces of both plants. GNS-sprayed peppers characterized by giant chloroplasts with large starch granules and increment of mitochondrial number adjacent to chloroplasts. Whilst, chloroplast ultrastructure of GNS-treated eggplants appeared to be ellipsoidal-shaped with few normal sized-starch granules compared to control. The localization of GNS inside chloroplast may be activated photosynthetic pigments; thereby stimulation of fructose, sucrose and starch was displayed. The rising of hydrogen peroxide of GNS-treated leaves had beneficial role on triggering the activity of catalase, ascorbate peroxidase, glutathione peroxidase and glutathione-S-transferase. Furthermore, the reduction of hydroxyl radical and superoxide anion reflected the involvement of GNS in induction of antioxidant molecules and superoxide dismutase for modulating cell oxidative status. Thus, the lipid peroxidation and electrolyte leakage of GNS-treated plants were kept below the baseline of water-sprayed plants. Moreover, the promotions of health-promoting secondary metabolites via GNS aerosol were in close association to exacerbation of phenylalanine ammonialyase actvity. This study conclusively demonstrated that GNS did not have cytotoxic properties in pepper and eggplant cells rather healthy growth and promoted yield in the terms of number of branches plant^-1, number of fruits plant^-1 and fruit yield (ton hectare^-1) were the net result of GNS-induced metabolic regulation of the leaves physiological status.

Biotic priming with Pseudomonas fluorescens induce drought stress tolerance in Abelmoschus esculentus (L.) Moench (Okra)

Drought is a major abiotic stress which negatively affects plant growth and yield. The decrease in rainfall and ensuing drought affect crop production to a larger extent. The present investigation was aimed to evaluate the drought stress mitigation potential of Abelmoschus esculentus (L.) Moench (okra) plants primed with Pseudomonas fluorescens (PF). Okra seeds were primed with 10-7 CFU of PF, germinated and the plants were exposed to drought stress for 7 days, and the recovery potential was assessed after re-watering the plants. Physiological and biochemical parameters were evaluated during stress and recovery. PF treated plants mitigated the effect of drought stress by increasing relative water content (RWC), accumulated metabolites such as sugar, free amino acids and enhanced the activity of non enzymatic antioxidants; phenolics, ascorbate (AsA) and glutathione (GSH) and reactive oxygen species scavenging enzyme like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and guaicol peroxidase (GPX). Drought stress related membrane damage and protein denaturation were also alleviated by PF treatment. During re-watering, PF treated plants regained RWC, total sugar, total amino acid, protein, AsA, GSH, phenolics, SOD, CAT, APX and GPX to appreciable levels. Thus, this study suggests that PF can be used as an agent to effectively mitigate drought stress in okra plants.

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

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

A novel family of proline/serine-rich proteins, which are phospho-targets of stress-related mitogen-activated protein kinases, differentially regulates growth and pathogen defense in Arabidopsis thaliana

The molecular actions of mitogen-activated protein kinases (MAPKs) are ultimately accomplished by the substrate proteins where phosphorylation affects their molecular properties and function(s), but knowledge regarding plant MAPK substrates is currently still fragmentary. Here, we uncovered a previously uncharacterized protein family consisting of three proline/serine-rich proteins (PRPs) that are substrates of stress-related MAPKs. We demonstrated the importance of a MAPK docking domain necessary for protein-protein interaction with MAPKs and consequently also for phosphorylation. The main phosphorylated site was mapped to a residue conserved between all three proteins, which when mutated to a non-phosphorylatable form, differentially affected their protein stability. Together with their distinct gene expression patterns, this differential accumulation of the three proteins upon phosphorylation probably contributes to their distinct function(s). Transgenic over-expression of PRP, the founding member, led to plants with enhanced resistance to Pseudomonas syringae pv. tomato DC3000. Older plants of the over-expressing lines have curly leaves and were generally smaller in stature. This growth phenotype was lost in plants expressing the phosphosite variant, suggesting a phosphorylation-dependent effect. Thus, this novel family of PRPs may be involved in MAPK regulation of plant development and / or pathogen resistance responses. As datamining associates PRP expression profiles with hypoxia or oxidative stress and PRP-overexpressing plants have elevated levels of reactive oxygen species, PRP may connect MAPK and oxidative stress signaling.

Pollen wall degradation in the Brassicaceae permits cell emergence after pollination

PREMISE OF THE STUDY

Despite attempts to degrade the sporopollenin in pollen walls, this material has withstood a hundred years of experimental treatments and thousands of years of environmental attack in insects and soil. We present evidence that sporopollenin, nonetheless, locally degrades only minutes after pollination in Arabidopsis thaliana flowers, and describe here a two-part pollen germination mechanism in A. thaliana involving both chemical weakening of the exine wall and swelling of the underlying intine.

METHODS

We explored naturally occurring components from pollen and stigma surfaces and found a tripartite mix of hydrogen peroxide, peroxidase and catalase enzymes (all at high levels at the pollination interface) to be experimentally sufficient to degrade the sporopollenin of some Brassicaceae family members.

KEY RESULTS

At pollination, factors carried on the pollen surface may mix with factors on the stigma surface in a reaction that locally oxidizes the exine pollen wall. Hydrogen peroxide, catalases, and peroxidases are biologically present at the right time and place and, when mixed experimentally, are sufficient to degrade the walls of susceptible pollen.

CONCLUSIONS

Our work on native biochemistry for breaching sporopollenin suggests new research directions in pollen aperture evolution and could aid efforts to analyze sporopollenin's composition, needed for application of this corrosion-resistant, but long-intractable material.

Untargeted metabolomic analysis of tomato pollen development and heat stress response

Pollen development metabolomics. Developing pollen is among the plant structures most sensitive to high temperatures, and a decrease in pollen viability is often associated with an alteration of metabolite content. Most of the metabolic studies of pollen have focused on a specific group of compounds, which limits the identification of physiologically important metabolites. To get a better insight into pollen development and the pollen heat stress response, we used a liquid chromatography-mass spectrometry platform to detect secondary metabolites in pollen of tomato (Solanum lycopersicum L.) at three developmental stages under control conditions and after a short heat stress at 38 °C. Under control conditions, the young microspores accumulated a large amount of alkaloids and polyamines, whereas the mature pollen strongly accumulated flavonoids. The heat stress treatment led to accumulation of flavonoids in the microspore. The biological role of the detected metabolites is discussed. This study provides the first untargeted metabolomic analysis of developing pollen under a changing environment that can serve as reference for further studies.

Expression of a Drosophila glutathione transferase in Arabidopsis confers the ability to detoxify the environmental pollutant, and explosive, 2,4,6-trinitrotoluene

The explosive 2,4,6-trinitrotoluene (TNT) is a significant, global environmental pollutant that is both toxic and recalcitrant to degradation. Given the sheer scale and inaccessible nature of contaminated areas, phytoremediation may be a viable clean-up approach. Here, we have characterized a Drosophila melanogaster glutathione transferase (DmGSTE6) which has activity towards TNT. Recombinantly expressed, purified DmGSTE6 produces predominantly 2-glutathionyl-4,6-dinitrotoluene, and has a 2.5-fold higher Maximal Velocity (V_max ), and five-fold lower Michaelis Constant (K_m ) than previously characterized TNT-active Arabidopsis thaliana (Arabidopsis) GSTs. Expression of DmGSTE6 in Arabidopsis conferred enhanced resistance to TNT, and increased the ability to remove TNT from contaminated soil relative to wild-type plants. Arabidopsis lines overexpressing TNT-active GSTs AtGST-U24 and AtGST-U25 were compromised in biomass production when grown in the absence of TNT. This yield drag was not observed in the DmGSTE6-expressing Arabidopsis lines. We hypothesize that increased levels of endogenous TNT-active GSTs catalyse excessive glutathionylation of endogenous substrates, depleting glutathione pools, an activity that DmGST may lack. In conclusion, DmGSTE6 has activity towards TNT, producing a compound with potential for further biodegradation. Selecting or manipulating plants to confer DmGSTE6-like activity could contribute towards development of phytoremediation strategies to clean up TNT from polluted military sites.

Impedance Flow Cytometry: A Novel Technique in Pollen Analysis

INTRODUCTION

An efficient and reliable method to estimate plant cell viability, especially of pollen, is important for plant breeding research and plant production processes. Pollen quality is determined by classical methods, like staining techniques or in vitro pollen germination, each having disadvantages with respect to reliability, analysis speed, and species dependency. Analysing single cells based on their dielectric properties by impedance flow cytometry (IFC) has developed into a common method for cellular characterisation in microbiology and medicine during the last decade. The aim of this study is to demonstrate the potential of IFC in plant cell analysis with the focus on pollen.

METHOD

Developing and mature pollen grains were analysed during their passage through a microfluidic chip to which radio frequencies of 0.5 to 12 MHz were applied. The acquired data provided information about the developmental stage, viability, and germination capacity. The biological relevance of the acquired IFC data was confirmed by classical staining methods, inactivation controls, as well as pollen germination assays.

RESULTS

Different stages of developing pollen, dead, viable and germinating pollen populations could be detected and quantified by IFC. Pollen viability analysis by classical FDA staining showed a high correlation with IFC data. In parallel, pollen with active germination potential could be discriminated from the dead and the viable but non-germinating population.

CONCLUSION

The presented data demonstrate that IFC is an efficient, label-free, reliable and non-destructive technique to analyse pollen quality in a species-independent manner.

Nitric Oxide and Reactive Oxygen Species Mediate Metabolic Changes in Barley Seed Embryo during Germination

The levels of nitric oxide (NO) and reactive oxygen species (ROS), ATP/ADP ratios, reduction levels of ascorbate and glutathione, expression of the genes encoding proteins involved in metabolism of NO and activities of the enzymes involved in fermentation and in metabolism of NO and ROS were studied in the embryos of germinating seeds of two barley (Hordeum vulgare L.) cultivars differing in dormancy level. The level of NO production continuously increased after imbibition while the level of nitrosylated SH-groups in proteins increased. This corresponded to the decrease of free SH-groups in proteins. At early stage of germination (0-48 h post imbibition) the genes encoding class 1 phytoglobin (the protein scavenging NO) and S-nitrosoglutathione reductase (scavenging S-nitrosoglutathione) were markedly expressed. More dormant cultivar exhibited lower ATP/ADP and ascorbate/dehydroascorbate ratios and lower lactate and alcohol dehydrogenase activities, while the production of NO and nitrosylation of proteins was higher as compared to the non-dormant cultivar. The obtained data indicate that at the onset of germination NO is actively generated causing nitrosylation of SH-groups and a switch from respiration to fermentation. After radicle protrusion the metabolism changes in a more reducing type as recorded by ratio of reduced and oxidized glutathione and ascorbate. The turnover of NO by the scavenging systems (phytoglobin, S-nitrosoglutathione reductase and interaction with ROS) might contribute to the maintenance of redox and energy balance of germinating seeds and lead to alleviation of dormancy.

Unveiling the Redox Control of Plant Reproductive Development during Abiotic Stress

Plants being sessile in nature are often challenged to various abiotic stresses including temperature fluctuations, water supply, salinity, and nutrient availability. Exposure of plants to such environmental perturbations result in the formation of reactive oxygen species (ROS) in cells. To scavenge ROS, enzymatic and molecular antioxidants are produced at a cellular level. ROS act as a signaling entity at lower concentrations maintaining normal growth and development, but if their levels increase beyond certain threshold, they produce toxic effects in plants. Some developmental stages, such as development of reproductive organs are more sensitive to abiotic stress than other stages of growth. As success of plant reproductive development is directly correlated with grain yield, stresses coinciding with reproductive phase results in the higher yield losses. In this article, we summarize the redox control of plant reproductive development, and elaborate how redox homeostasis is compromised during abiotic stress exposure. We highlight why more emphasis should be given to understand redox control of plant reproductive organ development during abiotic stress exposure96to engineer crops with better crop yield. We specifically discuss the role of ROS as a signaling molecule and its cross-talk with other signaling molecules such as hormones and sugars.

Glutathione--linking cell proliferation to oxidative stress

SIGNIFICANCE

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

RECENT ADVANCES

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

CRITICAL ISSUES

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

FUTURE DIRECTIONS

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

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

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

Involvement of thiol-based mechanisms in plant development

BACKGROUND

Increasing knowledge has been recently gained regarding the redox regulation of plant developmental stages.

SCOPE OF VIEW

The current state of knowledge concerning the involvement of glutathione, glutaredoxins and thioredoxins in plant development is reviewed.

MAJOR CONCLUSIONS

The control of the thiol redox status is mainly ensured by glutathione (GSH), a cysteine-containing tripeptide and by reductases sharing redox-active cysteines, glutaredoxins (GRXs) and thioredoxins (TRXs). Indeed, thiol groups present in many regulatory proteins and metabolic enzymes are prone to oxidation, ultimately leading to post-translational modifications such as disulfide bond formation or glutathionylation. This review focuses on the involvement of GSH, GRXs and TRXs in plant development. Recent studies showed that the proper functioning of root and shoot apical meristems depends on glutathione content and redox status, which regulate, among others, cell cycle and hormone-related processes. A critical role of GRXs in the formation of floral organs has been uncovered, likely through the redox regulation of TGA transcription factor activity. TRXs fulfill many functions in plant development via the regulation of embryo formation, the control of cell-to-cell communication, the mobilization of seed reserves, the biogenesis of chloroplastic structures, the metabolism of carbon and the maintenance of cell redox homeostasis. This review also highlights the tight relationships between thiols, hormones and carbon metabolism, allowing a proper development of plants in relation with the varying environment and the energy availability.

GENERAL SIGNIFICANCE

GSH, GRXs and TRXs play key roles during the whole plant developmental cycle via their antioxidant functions and the redox-regulation of signaling pathways. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Low glutathione regulates gene expression and the redox potentials of the nucleus and cytosol in Arabidopsis thaliana

Reduced glutathione (GSH) is considered to exert a strong influence on cellular redox homeostasis and to regulate gene expression, but these processes remain poorly characterized. Severe GSH depletion specifically inhibited root meristem development, while low root GSH levels decreased lateral root densities. The redox potential of the nucleus and cytosol of Arabidopsis thaliana roots determined using roGFP probes was between -300 and -320 mV. Growth in the presence of the GSH-synthesis inhibitor buthionine sulfoximine (BSO) increased the nuclear and cytosolic redox potentials to approximately -260 mV. GSH-responsive genes including transcription factors (SPATULA, MYB15, MYB75), proteins involved in cell division, redox regulation (glutaredoxinS17, thioredoxins, ACHT5 and TH8) and auxin signalling (HECATE), were identified in the GSH-deficient root meristemless 1-1 (rml1-1) mutant, and in other GSH-synthesis mutants (rax1-1, cad2-1, pad2-1) as well as in the wild type following the addition of BSO. Inhibition of auxin transport had no effect on organ GSH levels, but exogenous auxin decreased the root GSH pool. We conclude that GSH depletion significantly increases the redox potentials of the nucleus and cytosol, and causes arrest of the cell cycle in roots but not shoots, with accompanying transcript changes linked to altered hormone responses, but not oxidative stress.

Transcriptomics of desiccation tolerance in the streptophyte green alga Klebsormidium reveal a land plant-like defense reaction

BACKGROUND

Water loss has significant effects on physiological performance and survival rates of algae. However, despite the prominent presence of aeroterrestrial algae in terrestrial habitats, hardly anything is known about the molecular events that allow aeroterrestrial algae to survive harsh environmental conditions. We analyzed the transcriptome and physiology of a strain of the alpine aeroterrestrial alga Klebsormidium crenulatum under control and strong desiccation-stress conditions.

PRINCIPAL FINDINGS

For comparison we first established a reference transcriptome. The high-coverage reference transcriptome includes about 24,183 sequences (1.5 million reads, 636 million bases). The reference transcriptome encodes for all major pathways (energy, carbohydrates, lipids, amino acids, sugars), nearly all deduced pathways are complete or missing only a few transcripts. Upon strong desiccation, more than 7000 transcripts showed changes in their expression levels. Most of the highest up-regulated transcripts do not show similarity to known viridiplant proteins, suggesting the existence of some genus- or species-specific responses to desiccation. In addition, we observed the up-regulation of many transcripts involved in desiccation tolerance in plants (e.g. proteins similar to those that are abundant in late embryogenesis (LEA), or proteins involved in early response to desiccation ERD), and enzymes involved in the biosynthesis of the raffinose family of oligosaccharides (RFO) known to act as osmolytes). Major physiological shifts are the up-regulation of transcripts for photosynthesis, energy production, and reactive oxygen species (ROS) metabolism, which is supported by elevated cellular glutathione content as revealed by immunoelectron microscopy as well as an increase in total antiradical power. However, the effective quantum yield of Photosystem II and CO2 fixation decreased sharply under the applied desiccation stress. In contrast, transcripts for cell integrative functions such as cell division, DNA replication, cofactor biosynthesis, and amino acid biosynthesis were down-regulated.

SIGNIFICANCE

This is the first study investigating the desiccation transcriptome of a streptophyte green alga. Our results indicate that the cellular response is similar to embryophytes, suggesting that embryophytes inherited a basic cellular desiccation tolerance from their streptophyte predecessors.

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 response of antioxidants to drought stress in Arabidopsis

Compartment specific changes in ascorbate and glutathione contents were studied during drought stress in Arabidopsis thaliana Col-0 and in ascorbate and glutathione deficient mutants vtc2-1 and pad2-1, respectively, over a time period of 10 days. The results of this study revealed a strong decrease of glutathione contents in both mutants (up to 52% in mitochondria of pad2-1 and 40% in nuclei of vtc2-1) at early time points when drought stress was not yet measurable in leaves even though the soil showed a drop in relative water contents. These results indicate that glutathione is used at early time points to signal drought stress from roots to leaves. Such roles could not be confirmed for ascorbate which remained unchanged in most cell compartments until very late stages of drought. During advanced drought stress the strong depletion of ascorbate and glutathione in chloroplasts (up to 50% in Col-0 and vtc2-1) and peroxisomes (up to 56% in Col-0) could be correlated with a strong accumulation of H2O2. The strong increase of H2O2 and ascorbate in vacuoles (up to 111%) in wildtype plants indicates that ascorbate plays an important role for the detoxification of ROS in vacuoles during drought stress.

The metabolic basis of pollen thermo-tolerance: perspectives for breeding

Crop production is highly sensitive to elevated temperatures. A rise of a few degrees above the optimum growing temperature can lead to a dramatic yield loss. A predicted increase of 1-3 degrees in the twenty first century urges breeders to develop thermo-tolerant crops which are tolerant to high temperatures. Breeding for thermo-tolerance is a challenge due to the low heritability of this trait. A better understanding of heat stress tolerance and the development of reliable methods to phenotype thermo-tolerance are key factors for a successful breeding approach. Plant reproduction is the most temperature-sensitive process in the plant life cycle. More precisely, pollen quality is strongly affected by heat stress conditions. High temperature leads to a decrease of pollen viability which is directly correlated with a loss of fruit production. The reduction in pollen viability is associated with changes in the level and composition of several (groups of) metabolites, which play an important role in pollen development, for example by contributing to pollen nutrition or by providing protection to environmental stresses. This review aims to underline the importance of maintaining metabolite homeostasis during pollen development, in order to produce mature and fertile pollen under high temperature. The review will give an overview of the current state of the art on the role of various pollen metabolites in pollen homeostasis and thermo-tolerance. Their possible use as metabolic markers to assist breeding programs for plant thermo-tolerance will be discussed.

Higher sensitivity of pad2-1 and vtc2-1 mutants to cadmium is related to lower subcellular glutathione rather than ascorbate contents

Cadmium (Cd) interferes with ascorbate and glutathione metabolism as it induces the production of reactive oxygen species (ROS), binds to glutathione due to its high affinity to thiol groups, and induces the production of phytochelatins (PCs) which use glutathione as a precursor. In this study, changes in the compartment specific distribution of ascorbate and glutathione were monitored over a time period of 14 days in Cd-treated (50 and 100 μM) Arabidopsis Col-0 plants, and two mutant lines deficient in glutathione (pad2-1) and ascorbate (vtc2-1). Both mutants showed higher sensitivity to Cd than Col-0 plants. Strongly reduced compartment specific glutathione, rather than decreased ascorbate contents, could be correlated with the development of symptoms in these mutants suggesting that higher sensitivity to Cd is related to low glutathione contents rather than low ascorbate contents. On the subcellular level it became obvious that long-term treatment of wildtype plants with Cd induced the depletion of glutathione and ascorbate contents in all cell compartments except chloroplasts indicating an important protective role for antioxidants in chloroplasts against Cd. Additionally, we could observe an immediate decrease of glutathione and ascorbate in all cell compartments 12 h after Cd treatment indicating that glutathione and ascorbate are either withdrawn from or not redistributed into other organelles after their production in chloroplasts, cytosol (production centers for glutathione) and mitochondria (production center for ascorbate). The obtained data is discussed in respect to recently proposed stress models involving antioxidants in the protection of plants against environmental stress conditions.

A conserved KIN17 curved DNA-binding domain protein assembles with SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE7 to adapt Arabidopsis growth and development to limiting copper availability

Proper copper (Cu) homeostasis is required by living organisms to maintain essential cellular functions. In the model plant Arabidopsis (Arabidopsis thaliana), the SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE7 (SPL7) transcription factor participates in reprogramming global gene expression during Cu insufficiency in order to improve the metal uptake and prioritize its distribution to Cu proteins of major importance. As a consequence, spl7 null mutants show morphological and physiological disorders during Cu-limited growth, resulting in lower fresh weight, reduced root elongation, and chlorosis. On the other hand, the Arabidopsis KIN17 homolog belongs to a well-conserved family of essential eukaryotic nuclear proteins known to be stress activated and involved in DNA and possibly RNA metabolism in mammals. In the study presented here, we uncovered that Arabidopsis KIN17 participates in promoting the Cu deficiency response by means of a direct interaction with SPL7. Moreover, the double mutant kin17-1 spl7-2 displays an enhanced Cu-dependent phenotype involving growth arrest, oxidative stress, floral bud abortion, and pollen inviability. Taken together, the data presented here provide evidence for SPL7 and KIN17 protein interaction as a point of convergence in response to both Cu deficiency and oxidative stress.

Successful fertilization requires the presence of at least one major O-acetylserine(thiol)lyase for cysteine synthesis in pollen of Arabidopsis

The synthesis of cysteine (Cys) is a master control switch of plant primary metabolism that coordinates the flux of sulfur with carbon and nitrogen metabolism. In Arabidopsis (Arabidopsis thaliana), nine genes encode for O-acetylserine(thiol)lyase (OAS-TL)-like proteins, of which the major isoforms, OAS-TL A, OAS-TL B, and OAS-TL C, catalyze the formation of Cys by combining O-acetylserine and sulfide in the cytosol, the plastids, and the mitochondria, respectively. So far, the significance of individual OAS-TL-like enzymes is unresolved. Generation of all major OAS-TL double loss-of-function mutants in combination with radiolabeled tracer studies revealed that subcellular localization of OAS-TL proteins is more important for efficient Cys synthesis than total cellular OAS-TL activity in leaves. The absence of oastl triple embryos after targeted crosses indicated the exclusiveness of Cys synthesis by the three major OAS-TLs and ruled out alternative sulfur fixation by other OAS-TL-like proteins. Analyses of oastlABC pollen demonstrated that the presence of at least one functional OAS-TL isoform is essential for the proper function of the male gametophyte, although the synthesis of histidine, lysine, and tryptophan is dispensable in pollen. Comparisons of oastlABC pollen derived from genetically different parent plant combinations allowed us to separate distinct functions of Cys and glutathione in pollen and revealed an additional role of glutathione for pollen germination. In contrast, female gametogenesis was not affected by the absence of major OAS-TLs, indicating significant transport of Cys into the developing ovule from the mother plant.

A novel method for efficient in vitro germination and tube growth of Arabidopsis thaliana pollen

In addition to its importance in studies of plant reproduction and fertility, pollen is as widely employed as a model system of cell growth and development. This work demands robust, reproducible methods to induce pollen germination and morphologically normal growth of pollen tubes in vitro. Despite numerous advantages of Arabidopsis thaliana as a model plant, such experiments on pollen germination and pollen tube growth have often proved challenging. Our new method employs a physical cellulosic membrane, overlying an agarose substrate. By modulating the substrate composition, we provide important insights into the mechanisms promoting pollen growth both in vitro and in vivo. This effective new technical approach to A. thaliana pollen germination and tube growth results in swift, consistent and unprecedented levels of germination to over 90%. It can also promote rapid growth of long, morphologically normal pollen tubes. This technical development demonstrates that exogenous spermidine and a cellulosic substrate are key factors in stimulating germination. It has potential to greatly assist the study of reproduction in A. thaliana and its closest relatives, not only for the study of germination levels and pollen tube growth dynamics by microscopy, but also for biochemical and molecular analysis of germinating pollen.

Thiol-based redox regulation in sexual plant reproduction: new insights and perspectives

The success of sexual reproduction in plants involves (i) the proper formation of the plant gametophytes (pollen and embryo sac) containing the gametes, (ii) the accomplishment of specific interactions between pollen grains and the stigma, which subsequently lead to (iii) the fusion of the gametes and eventually to (iv) the seed setting. Owing to the lack of mobility, plants have developed specific regulatory mechanisms to control all developmental events underlying the sexual plant reproduction according to environmental challenges. Over the last decade, redox regulation and signaling have come into sight as crucial mechanisms able to manage critical stages during sexual plant reproduction. This regulation involves a complex redox network which includes reactive oxygen species (ROS), reactive nitrogen species (RNS), glutathione and other classic buffer molecules or antioxidant proteins, and some thiol/disulphide-containing proteins belonging to the thioredoxin superfamily, like glutaredoxins (GRXs) or thioredoxins (TRXs). These proteins participate as critical elements not only in the switch between the mitotic to the meiotic cycle but also at further developmental stages of microsporogenesis. They are also implicated in the regulation of pollen rejection as the result of self-incompatibility. In addition, they display precise space-temporal patterns of expression and are present in specific localizations like the stigmatic papillae or the mature pollen, although their functions and subcellular localizations are not clear yet. In this review we summarize insights and perspectives about the presence of thiol/disulphide-containing proteins in plant reproduction, taking into account the general context of the cell redox network.

High resolution imaging of subcellular glutathione concentrations by quantitative immunoelectron microscopy in different leaf areas of Arabidopsis

Glutathione is an important antioxidant and redox buffer in plants. It fulfills many important roles during plant development, defense and is essential for plant metabolism. Even though the compartment specific roles of glutathione during abiotic and biotic stress situations have been studied in detail there is still great lack of knowledge about subcellular glutathione concentrations within the different leaf areas at different stages of development.

In this study a method is described that allows the calculation of compartment specific glutathione concentrations in all cell compartments simultaneously in one experiment by using quantitative immunogold electron microscopy combined with biochemical methods in different leaf areas of Arabidopsis thaliana Col-0 (center of the leaf, leaf apex, leaf base and leaf edge). The volume of subcellular compartments in the mesophyll of Arabidopsis was found to be similar to other plants. Vacuoles covered the largest volume within a mesophyll cell and increased with leaf age (up to 80% in the leaf apex of older leaves). Behind vacuoles, chloroplasts covered the second largest volume (up to 20% in the leaf edge of the younger leaves) followed by nuclei (up to 2.3% in the leaf edge of the younger leaves), mitochondria (up to 1.6% in the leaf apex of the younger leaves), and peroxisomes (up to 0.3% in the leaf apex of the younger leaves). These values together with volumes of the mesophyll determined by stereological methods from light and electron micrographs and global glutathione contents measured with biochemical methods enabled the determination of subcellular glutathione contents in mM.

Even though biochemical investigations did not reveal differences in global glutathione contents, compartment specific differences could be observed in some cell compartments within the different leaf areas. Highest concentrations of glutathione were always found in mitochondria, where values in a range between 8.7 mM (in the apex of younger leaves) and 15.1 mM (in the apex of older leaves) were found. The second highest amount of glutathione was found in nuclei (between 5.5 mM and 9.7 mM in the base and the center of younger leaves, respectively) followed by peroxisomes (between 2.6 mM in the edge of younger leaves and 4.8 mM in the base of older leaves, respectively) and the cytosol (2.8 mM in the edge of younger and 4.5 mM in the center of older leaves, respectively). Chloroplasts contained rather low amounts of glutathione (between 1 mM and 1.4 mM). Vacuoles had the lowest concentrations of glutathione (0.01 mM and 0.14 mM) but showed large differences between the different leaf areas. Clear differences in glutathione contents between the different leaf areas could only be found in vacuoles and mitochondria revealing that glutathione in the later cell organelle accumulated with leaf age to concentrations of up to 15 mM and that concentrations of glutathione in vacuoles are quite low in comparison to the other cell compartments.

Thioredoxin and glutaredoxin systems in plants: molecular mechanisms, crosstalks, and functional significance

Thioredoxins (Trx) and glutaredoxins (Grx) constitute families of thiol oxidoreductases. Our knowledge of Trx and Grx in plants has dramatically increased during the last decade. The release of the Arabidopsis genome sequence revealed an unexpectedly high number of Trx and Grx genes. The availability of several genomes of vascular and nonvascular plants allowed the establishment of a clear classification of the genes and the chronology of their appearance during plant evolution. Proteomic approaches have been developed that identified the putative Trx and Grx target proteins which are implicated in all aspects of plant growth, including basal metabolism, iron/sulfur cluster formation, development, adaptation to the environment, and stress responses. Analyses of the biochemical characteristics of specific Trx and Grx point to a strong specificity toward some target enzymes, particularly within plastidial Trx and Grx. In apparent contradiction with this specificity, genetic approaches show an absence of phenotype for most available Trx and Grx mutants, suggesting that redundancies also exist between Trx and Grx members. Despite this, the isolation of mutants inactivated in multiple genes and several genetic screens allowed the demonstration of the involvement of Trx and Grx in pathogen response, phytohormone pathways, and at several control points of plant development. Cytosolic Trxs are reduced by NADPH-thioredoxin reductase (NTR), while the reduction of Grx depends on reduced glutathione (GSH). Interestingly, recent development integrating biochemical analysis, proteomic data, and genetics have revealed an extensive crosstalk between the cytosolic NTR/Trx and GSH/Grx systems. This crosstalk, which occurs at multiple levels, reveals the high plasticity of the redox systems in plants.

Expression of the affinity tags, glutathione-S-transferase and maltose-binding protein, in tobacco chloroplasts

Chloroplast transformation offers an exciting platform for the safe, inexpensive and large-scale production of recombinant proteins in plants. An important advantage for the isolation of proteins produced in the chloroplast would be the use of affinity tags for rapid purification by affinity chromatography. To date, only His-tags have been used. In this study, we have tested the feasibility of expressing two additional affinity tags: glutathione-S-transferase (GST) and a His-tagged derivative of the maltose-binding protein (His₆-MBP). By using the chloroplast 16S rRNA promoter and 5' untranslated region of phage T7 gene 10, GST and His₆-MBP were expressed in homoplastomic tobacco plants at approximately 7% and 37% of total soluble protein, respectively. GST could be purified by one-step-affinity purification using a glutathione column. Much better recoveries were obtained for His₆-MBP by using a twin-affinity purification procedure involving first immobilised nickel followed by binding to amylose. Interestingly, expression of GST led to cytoplasmic male sterility. Overall, our work expands the tools available for purifying recombinant proteins from the chloroplast.

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.

Respiratory burst oxidases: the engines of ROS signaling

Reactive oxygen species (ROS) play a key signal transduction role in cells. They are involved in the regulation of growth, development, responses to environmental stimuli and cell death. The level of ROS in cells is determined by interplay between ROS producing pathways and ROS scavenging mechanisms, part of the ROS gene network of plants. Recent studies identified respiratory burst oxidase homologues (RBOHs) as key signaling nodes in the ROS gene network of plants integrating a multitude of signal transduction pathways with ROS signaling. The ability of RBOHs to integrate calcium signaling and protein phosphorylation with ROS production, coupled with genetic studies demonstrating their involvement in many different biological processes in cells, places RBOHs at the center of the ROS network of cells and demonstrate their important function in plants.

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.