A dehydrogenase-mediated recycling system of NADPH in plant peroxisomes

Evidence for dual targeting control of Arabidopsis 6-phosphogluconate dehydrogenase isoforms by N-terminal phosphorylation

The oxidative pentose-phosphate pathway (OPPP) retrieves NADPH from glucose-6-phosphate, which is important in chloroplasts at night and in plastids of heterotrophic tissues. We previously studied how OPPP enzymes may transiently locate to peroxisomes, but how this is achieved for the third enzyme remained unclear. By extending our genetic approach, we demonstrated that Arabidopsis isoform 6-phosphogluconate dehydrogenase 2 (PGD2) is indispensable in peroxisomes during fertilization, and investigated why all PGD-reporter fusions show a mostly cytosolic pattern. A previously published interaction of a plant PGD with thioredoxin m was confirmed using Trxm2 for yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays, and medial reporter fusions (with both ends accessible) proved to be beneficial for studying peroxisomal targeting of PGD2. Of special importance were phosphomimetic changes at Thr6, resulting in a clear targeting switch to peroxisomes, while a similar change at position Ser7 in PGD1 conferred plastid import. Apparently, efficient subcellular localization can be achieved by activating an unknown kinase, either early after or during translation. N-terminal phosphorylation of PGD2 interfered with dimerization in the cytosol, thus allowing accessibility of the C-terminal peroxisomal targeting signal (PTS1). Notably, we identified amino acid positions that are conserved among plant PGD homologues, with PTS1 motifs first appearing in ferns, suggesting a functional link to fertilization during the evolution of seed plants.

Role of glutathione S-transferases in the mode of action of herbicides that inhibit amino acid synthesis in Amaranthus palmeri

Acetolactate synthase inhibitors (ALS inhibitors) and glyphosate are two classes of herbicides that act by inhibiting an enzyme in the biosynthetic pathway of branched-chain or aromatic amino acids, respectively. Besides amino acid synthesis inhibition, both herbicides trigger similar physiological effects in plants. The main aim of this study was to evaluate the role of glutathione metabolism, with special emphasis on glutathione S-transferases (GSTs), in the mode of action of glyphosate and ALS inhibitors in Amaranthus palmeri. For that purpose, plants belonging to a glyphosate-sensitive (GLS) and a glyphosate-resistant (GLR) population were treated with different doses of glyphosate, and plants belonging to an ALS-inhibitor sensitive (AIS) and an ALS-inhibitor resistant (AIR) population were treated with different doses of the ALS inhibitor nicosulfuron. Glutathione-related contents, GST activity, and related gene expressions (glutamate-cysteine ligase, glutathione reductase, Phi GST and Tau GST) were analysed in leaves. According to the results of the analytical determinations, there were virtually no basal differences between GLS and GLR plants or between AIS and AIR plants. Glutathione synthesis and turnover did not follow a clear pattern in response to herbicides, but GST activity and gene expression (especially Phi GSTs) increased with both herbicides in treated sensitive plants, possibly related to the rocketing H2O2 accumulation. As GSTs offered the clearest results, these were further investigated with a multiple resistant (MR) population, compressing target-site resistance to both glyphosate and the ALS inhibitor pyrithiobac. As in single-resistant plants, measured parameters in the MR population were unaffected by herbicides, meaning that the increase in GST activity and expression occurs due to herbicide interactions with the target enzymes.

Mapping the castor bean endosperm proteome revealed a metabolic interaction between plastid, mitochondria, and peroxisomes to optimize seedling growth

In this work, we studied castor-oil plant Ricinus communis as a classical system for endosperm reserve breakdown. The seeds of castor beans consist of a centrally located embryo with the two thin cotyledons surrounded by the endosperm. The endosperm functions as major storage tissue and is packed with nutritional reserves, such as oil, proteins, and starch. Upon germination, mobilization of the storage reserves requires inter-organellar interplay of plastids, mitochondria, and peroxisomes to optimize growth for the developing seedling. To understand their metabolic interactions, we performed a large-scale organellar proteomic study on castor bean endosperm. Organelles from endosperm of etiolated seedlings were isolated and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Computer-assisted deconvolution algorithms were applied to reliably assign the identified proteins to their correct subcellular localization and to determine the abundance of the different organelles in the heterogeneous protein samples. The data obtained were used to build a comprehensive metabolic model for plastids, mitochondria, and peroxisomes during storage reserve mobilization in castor bean endosperm.

In Silico RNAseq and Biochemical Analyses of Glucose-6-Phosphate Dehydrogenase (G6PDH) from Sweet Pepper Fruits: Involvement of Nitric Oxide (NO) in Ripening and Modulation

Pepper (Capsicum annuum L.) fruit is a horticultural product consumed worldwide which has great nutritional and economic relevance. Besides the phenotypical changes that pepper fruit undergo during ripening, there are many associated modifications at transcriptomic, proteomic, biochemical, and metabolic levels. Nitric oxide (NO) is a recognized signal molecule that can exert regulatory functions in diverse plant processes including fruit ripening, but the relevance of NADPH as a fingerprinting of the crop physiology including ripening has also been proposed. Glucose-6-phosphate dehydrogenase (G6PDH) is the first and rate-limiting enzyme of the oxidative phase of the pentose phosphate pathway (oxiPPP) with the capacity to generate NADPH. Thus far, the available information on G6PDH and other NADPH-generating enzymatic systems in pepper plants, and their expression during the ripening of sweet pepper fruit, is very scarce. Therefore, an analysis at the transcriptomic, molecular and functional levels of the G6PDH system has been accomplished in this work for the first time. Based on a data-mining approach to the pepper genome and fruit transcriptome (RNA-seq), four G6PDH genes were identified in pepper plants and designated CaG6PDH1 to CaG6PDH4, with all of them also being expressed in fruits. While CaG6PDH1 encodes a cytosolic isozyme, the other genes code for plastid isozymes. The time-course expression analysis of these CaG6PDH genes during different fruit ripening stages, including green immature (G), breaking point (BP), and red ripe (R), showed that they were differentially modulated. Thus, while CaG6PDH2 and CaG6PDH4 were upregulated at ripening, CaG6PDH1 was downregulated, and CaG6PDH3 was slightly affected. Exogenous treatment of fruits with NO gas triggered the downregulation of CaG6PDH2, whereas the other genes were positively regulated. In-gel analysis using non-denaturing PAGE of a 50-75% ammonium-sulfate-enriched protein fraction from pepper fruits allowed for identifying two isozymes designated CaG6PDH I and CaG6PDH II, according to their electrophoretic mobility. In order to test the potential modulation of such pepper G6PDH isozymes, in vitro analyses of green pepper fruit samples in the presence of different compounds including NO donors (S-nitrosoglutathione and nitrosocysteine), peroxynitrite (ONOO-), a hydrogen sulfide (H2S) donor (NaHS, sodium hydrosulfide), and reducing agents such as reduced glutathione (GSH) and L-cysteine (L-Cys) were assayed. While peroxynitrite and the reducing compounds provoked a partial inhibition of one or both isoenzymes, NaHS exerted 100% inhibition of the two CaG6PDHs. Taken together these data provide the first data on the modulation of CaG6PDHs at gene and activity levels which occur in pepper fruit during ripening and after NO post-harvest treatment. As a consequence, this phenomenon may influence the NADPH availability for the redox homeostasis of the fruit and balance its active nitro-oxidative metabolism throughout the ripening process.

Plant stem cells under low oxygen: metabolic rewiring by phytoglobin underlies stem cell functionality

Root growth in maize (Zea mays L.) is regulated by the activity of the quiescent center (QC) stem cells located within the root apical meristem. Here, we show that despite being highly hypoxic under normal oxygen tension, QC stem cells are vulnerable to hypoxic stress, which causes their degradation with subsequent inhibition of root growth. Under low oxygen, QC stem cells became depleted of starch and soluble sugars and exhibited reliance on glycolytic fermentation with the impairment of the TCA cycle through the depressed activity of several enzymes, including pyruvate dehydrogenase (PDH). This finding suggests that carbohydrate delivery from the shoot might be insufficient to meet the metabolic demand of QC stem cells during stress. Some metabolic changes characteristic of the hypoxic response in mature root cells were not observed in the QC. Hypoxia-responsive genes, such as PYRUVATE DECARBOXYLASE (PDC) and ALCOHOL DEHYDROGENASE (ADH), were not activated in response to hypoxia, despite an increase in ADH activity. Increases in phosphoenolpyruvate (PEP) with little change in steady-state levels of succinate were also atypical responses to low-oxygen tensions. Overexpression of PHYTOGLOBIN 1 (ZmPgb1.1) preserved the functionality of the QC stem cells during stress. The QC stem cell preservation was underpinned by extensive metabolic rewiring centered around activation of the TCA cycle and retention of carbohydrate storage products, denoting a more efficient energy production and diminished demand for carbohydrates under conditions where nutrient transport may be limiting. Overall, this study provides an overview of metabolic responses occurring in plant stem cells during oxygen deficiency.

Pepper Fruit Extracts Show Anti-Proliferative Activity against Tumor Cells Altering Their NADPH-Generating Dehydrogenase and Catalase Profiles

Cancer is considered one of the main causes of human death worldwide, being characterized by an alteration of the oxidative metabolism. Many natural compounds from plant origin with anti-tumor attributes have been described. Among them, capsaicin, which is the molecule responsible for the pungency in hot pepper fruits, has been reported to show antioxidant, anti-inflammatory, and analgesic activities, as well as anti-proliferative properties against cancer. Thus, in this work, the potential anti-proliferative activity of pepper (Capsicum annuum L.) fruits from diverse varieties with different capsaicin contents (California < Piquillo < Padrón < Alegría riojana) against several tumor cell lines (lung, melanoma, hepatoma, colon, breast, pancreas, and prostate) has been investigated. The results showed that the capsaicin content in pepper fruits did not correspond with their anti-proliferative activity against tumor cell lines. By contrast, the greatest activity was promoted by the pepper tissues which contained the lowest capsaicin amount. This indicates that other compounds different from capsaicin have this anti-tumor potentiality in pepper fruits. Based on this, green fruits from the Alegría riojana variety, which has negligible capsaicin levels, was used to study the effect on the oxidative and redox metabolism of tumor cell lines from liver (Hep-G2) and pancreas (MIA PaCa-2). Different parameters from both lines treated with crude pepper fruit extracts were determined including protein nitration and protein S-nitrosation (two post-translational modifications (PTMs) promoted by nitric oxide), the antioxidant capacity, as well as the activity of the antioxidant enzymes superoxide dismutase (SOD), catalase, and glutathione peroxidase (GPX), among others. In addition, the activity of the NADPH-generating enzymes glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and NADP-isocitrate dehydrogenase (NADP-ICDH) was followed. Our data revealed that the treatment of both cell lines with pepper fruit extracts altered their antioxidant capacity, enhanced their catalase activity, and considerably reduced the activity of the NADPH-generating enzymes. As a consequence, less H₂O₂ and NADPH seem to be available to cells, thus avoiding cell proliferation and possibly triggering cell death in both cell lines.

Sharing the wealth: The versatility of proteins targeted to peroxisomes and other organelles

Peroxisomes are eukaryotic organelles with critical functions in cellular energy and lipid metabolism. Depending on the organism, cell type, and developmental stage, they are involved in numerous other metabolic and regulatory pathways. Many peroxisomal functions require factors also relevant to other cellular compartments. Here, we review proteins shared by peroxisomes and at least one different site within the cell. We discuss the mechanisms to achieve dual targeting, their regulation, and functional consequences. Characterization of dual targeting is fundamental to understand how peroxisomes are integrated into the metabolic and regulatory circuits of eukaryotic cells.

Deficiency of a peroxisomal NADP-isocitrate dehydrogenase leads to dwarf plant and defect seed in upland cotton

The NADP-isocitrate dehydrogenase-encoded gene GH_D13G1452 with a C-terminus tripeptide Proline-Lysine-Leucine was localized in the peroxisome. It was highly expressed in stems and ovules of 15 days post-anthesis and responded to multiple external stimuli in upland cotton. An upland cotton mutant (Ghpericdh) was identified by flanking sequence amplification and genome variation detection that exogenous sequence was inserted in the middle of the 12th intron of GH_D13G1452, resulting in the deficiency of gene expression. The Ghpericdh mutant displayed a dwarf plant phenotype when grown under field or greenhouse conditions, and GH_D13G1452 functioned as an incomplete dominance on plant height. The germination rate of mutant seed from greenhouse-grown plants was dramatically lower than that from field-grown plants, which indicated that GhperICDH plays a critical role in seed maturation and germination. Therefore, GH_D13G1452 is indispensable in the development of stems and seeds and functions in the adaptability of cotton to the environment. The Ghpericdh mutant provides insight into the function of peroxisomal ICDH and may contribute to the genetic improvement in cotton.

Cytosolic glucose-6-phosphate dehydrogenases play a pivotal role in Arabidopsis seed development

Embryo development is essential for seed yield and post-germination growth. Glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in oxidative pentose phosphate pathway (OPPP), is widely involved in plant development and stress tolerance by providing nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the double mutant (g6pd5/6), overexpression line (G6PD5/6^OE) and complementation line (g6pd5/6^Comp) of cytosolic glucose-6-phosphate dehydrogenases (Cyt-G6PD) were used to investigate Cyt-G6PD roles in embryo development of Arabidopsis. The results showed that the germination rate of g6pd5/6 seeds was delayed in comparison with that of Col-0; moreover, 11.5% of g6pd5/6 seeds did not germinate. The dysfunction of Cyt-G6PD resulted in decreased fresh weight and primary root length of g6pd5/6 seedlings. The height and silique length of g6pd5/6 plants were also decreased. Moreover, the abortion rate of siliques and seeds of g6pd5/6 plants were increased compared with those of Col-0, G6PD5/6^OE and g6pd5/6^Comp lines. However, the dysfunction of Cyt-G6PD did not affect pollen activity; but in g6pd5/6, the embryo development was partially delayed or inhibited. The contents of fatty acids and storage proteins, two main storage materials in Arabidopsis seeds, were decreased in g6pd5/6 seeds. Exogenous application of fatty acids (C18:2; C18:3) alleviated the delayed germination of g6pd5/6 seeds. RT-qPCR results further demonstrated that the early embryo development genes were down-regulated in g6pd5/6. Taken together, Cyt-G6PD plays a pivotal role in plant seed development by regulating the transcriptions of early embryo development genes and the accumulation of storage materials (especially fatty acids).

The moderate oxidative stress induced by glyphosate is not detected in Amaranthus palmeri plants overexpressing EPSPS

The present study aimed to determine whether glyphosate-induced oxidative stress is directly related to the action mechanism of this herbicide (5-enolpyruvylshikimate-3-phosphate synthase or EPSPS inhibition) and analyse the role of oxidative stress in glyphosate toxicity of the weed Amaranthus palmeri S. Wats. Two kinds of populations were studied using EPSPS amplification: glyphosate-sensitive and glyphosate-resistant (by gene amplification). Plants were grown hydroponically and treated with different glyphosate doses, after which several oxidative stress markers were measured in the leaves. Untreated, sensitive and resistant plants showed similar values for the analysed parameters. Treated glyphosate-sensitive plants showed an increase in shikimate, superoxide and H2O2 contents and dose-dependent lipid peroxidation and antioxidant responses; however, none of these effects were observed in resistant plants, indicating that glyphosate-induced oxidative stress is related to EPSPS inhibition. Oxidative stress is associated with an increase in the activity of peroxidases due to EPSPS inhibition, although the link between both processes remains elusive. The fact that some glyphosate doses were lethal but did not induce major oxidative damage provides evidence that glyphosate toxicity is independent of oxidative stress.

Peroxisomal Proteome Mining of Sweet Pepper (Capsicum annuum L.) Fruit Ripening Through Whole Isobaric Tags for Relative and Absolute Quantitation Analysis

Peroxisomes are ubiquitous organelles from eukaryotic cells characterized by an active nitro-oxidative metabolism. They have a relevant metabolic plasticity depending on the organism, tissue, developmental stage, or physiological/stress/environmental conditions. Our knowledge of peroxisomal metabolism from fruits is very limited but its proteome is even less known. Using sweet pepper (Capsicum annuum L.) fruits at two ripening stages (immature green and ripe red), it was analyzed the proteomic peroxisomal composition by quantitative isobaric tags for relative and absolute quantitation (iTRAQ)-based protein profiling. For this aim, it was accomplished a comparative analysis of the pepper fruit whole proteome obtained by iTRAQ versus the identified peroxisomal protein profile from Arabidopsis thaliana. This allowed identifying 57 peroxisomal proteins. Among these proteins, 49 were located in the peroxisomal matrix, 36 proteins had a peroxisomal targeting signal type 1 (PTS1), 8 had a PTS type 2, 5 lacked this type of peptide signal, and 8 proteins were associated with the membrane of this organelle. Furthermore, 34 proteins showed significant differences during the ripening of the fruits, 19 being overexpressed and 15 repressed. Based on previous biochemical studies using purified peroxisomes from pepper fruits, it could be said that some of the identified peroxisomal proteins were corroborated as part of the pepper fruit antioxidant metabolism (catalase, superoxide dismutase, ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductaseglutathione reductase, 6-phosphogluconate dehydrogenase and NADP-isocitrate dehydrogenase), the β-oxidation pathway (acyl-coenzyme A oxidase, 3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase), while other identified proteins could be considered "new" or "unexpected" in fruit peroxisomes like urate oxidase (UO), sulfite oxidase (SO), 5-methyltetrahydropteroyltriglutamate-homocysteine methyltransferase (METE1), 12-oxophytodienoate reductase 3 (OPR3) or 4-coumarate-CoA ligase (4CL), which participate in different metabolic pathways such as purine, sulfur, L-methionine, jasmonic acid (JA) or phenylpropanoid metabolisms. In summary, the present data provide new insights into the complex metabolic machinery of peroxisomes in fruit and open new windows of research into the peroxisomal functions during fruit ripening.

A Cys2His2 Zinc Finger Transcription Factor BpSZA1 Positively Modulates Salt Stress in Betula platyphylla

Zinc finger proteins (ZFPs) are widely involved in plant growth and abiotic stress responses, however, few of these proteins have been functionally characterized in tree species. In this study, we cloned and characterized the BpSZA1 gene encoding a C2H2-type ZFP from Betula platyphylla. BpSZA1 is a transcription factor localized in the nucleus, with a transcription activation domain located at the N-terminus. BpSZA1 was predominantly expressed in stems and was induced by salt. We generated transgenic birch lines displaying overexpression (OE) or RNAi silencing (Ri) of BpSZA1 and exposed these along with wild-type birch seedlings to salinity. Phenotypic and physiological parameters such as superoxide dismutase, peroxisome, H₂O₂ content, proline content, water loss rate, and malondialdehyde content were examined. Overexpression of BpSZA1 in birch conferred increased salt tolerance. Chromatin immunoprecipitation-qPCR and RNA-seq showed that BpSZA1 binds to the GAGA-motif in the promoter of downstream target genes including BpAPX1, BpAPX2, BpCAT, and Bp6PGDH to activate their transcription. BpSZA1 also participates in abscisic acid (ABA) biosynthesis, proline biosynthesis, and the ABA/jasmonic acid pathway to enhance the salt stress of B. platyphylla.

Abandoned Mine Lands Reclamation by Plant Remediation Technologies

Abandoned mine lands (AMLs), which are considered some of the most dangerous anthropogenic activities in the world, are a source of hazards relating to potentially toxic elements (PTEs). Traditional reclamation techniques, which are expensive, time-consuming and not well accepted by the general public, cannot be used on a large scale. However, plant-based techniques have gained acceptance as an environmentally friendly alternative over the last 20 years. Plants can be used in AMLs for PTE phytoextraction, phytostabilization, and phytovolatilization. We reviewed these phytoremediation techniques, paying particular attention to the selection of appropriate plants in each case. In order to assess the suitability of plants for phytoremediation purposes, the accumulation capacity and tolerance mechanisms of PTEs was described. We also compiled a collection of interesting actual examples of AML phytoremediation. On-site studies have shown positive results in terms of soil quality improvement, reduced PTE bioavailability, and increased biodiversity. However, phytoremediation strategies need to better characterize potential plant candidates in order to improve PTE extraction and to reduce the negative impact on AMLs.

Nitric Oxide (NO) Scaffolds the Peroxisomal Protein-Protein Interaction Network in Higher Plants

The peroxisome is a single-membrane subcellular compartment present in almost all eukaryotic cells from simple protists and fungi to complex organisms such as higher plants and animals. Historically, the name of the peroxisome came from a subcellular structure that contained high levels of hydrogen peroxide (H2O2) and the antioxidant enzyme catalase, which indicated that this organelle had basically an oxidative metabolism. During the last 20 years, it has been shown that plant peroxisomes also contain nitric oxide (NO), a radical molecule than leads to a family of derived molecules designated as reactive nitrogen species (RNS). These reactive species can mediate post-translational modifications (PTMs) of proteins, such as S-nitrosation and tyrosine nitration, thus affecting their function. This review aims to provide a comprehensive overview of how NO could affect peroxisomal metabolism and its internal protein-protein interactions (PPIs). Remarkably, many of the identified NO-target proteins in plant peroxisomes are involved in the metabolism of reactive oxygen species (ROS), either in its generation or its scavenging. Therefore, it is proposed that NO is a molecule with signaling properties with the capacity to modulate the peroxisomal protein-protein network and consequently the peroxisomal functions, especially under adverse environmental conditions.

Nitric oxide and hydrogen sulfide modulate the NADPH-generating enzymatic system in higher plants

Nitric oxide (NO) and hydrogen sulfide (H2S) are two key molecules in plant cells that participate, directly or indirectly, as regulators of protein functions through derived post-translational modifications, mainly tyrosine nitration, S-nitrosation, and persulfidation. These post-translational modifications allow the participation of both NO and H2S signal molecules in a wide range of cellular processes either physiological or under stressful circumstances. NADPH participates in cellular redox status and it is a key cofactor necessary for cell growth and development. It is involved in significant biochemical routes such as fatty acid, carotenoid and proline biosynthesis, and the shikimate pathway, as well as in cellular detoxification processes including the ascorbate-glutathione cycle, the NADPH-dependent thioredoxin reductase (NTR), or the superoxide-generating NADPH oxidase. Plant cells have diverse mechanisms to generate NADPH by a group of NADP-dependent oxidoreductases including ferredoxin-NADP reductase (FNR), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH), NADP-dependent malic enzyme (NADP-ME), NADP-dependent isocitrate dehydrogenase (NADP-ICDH), and both enzymes of the oxidative pentose phosphate pathway, designated as glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH). These enzymes consist of different isozymes located in diverse subcellular compartments (chloroplasts, cytosol, mitochondria, and peroxisomes) which contribute to the NAPDH cellular pool. We provide a comprehensive overview of how post-translational modifications promoted by NO (tyrosine nitration and S-nitrosation), H2S (persulfidation), and glutathione (glutathionylation), affect the cellular redox status through regulation of the NADP-dependent dehydrogenases.

Changes in Glutathione, Ascorbate, and Antioxidant Enzymes during Olive Fruit Ripening

The content of glutathione, ascorbate (ASC), and the enzymatic antioxidants, superoxide dismutase and catalase, and components of the ascorbate-glutathione cycle were investigated in the olive fruit (cv. Picual) selected at the green, turning, and mature ripening stages. The changes observed in total and reduced glutathione (GSH), oxidized glutathione (GSSG), the ratio GSH/GSSG, ASC, and antioxidant enzymes (mainly superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase) indicate a shift to a moderate cellular oxidative status during ripening and suggest a role for antioxidants in the process. The antioxidant composition of olive oils obtained from the olive fruits of the study was investigated. A model is proposed for the recycling of antioxidant polyphenols mediated by endogenous molecular antioxidants in the olive fruit.

Antioxidant Profile of Pepper (Capsicum annuum L.) Fruits Containing Diverse Levels of Capsaicinoids

Capsicum is the genus where a number of species and varieties have pungent features due to the exclusive content of capsaicinoids such as capsaicin and dihydrocapsaicin. In this work, the main enzymatic and non-enzymatic systems in pepper fruits from four varieties with different pungent capacity have been investigated at two ripening stages. Thus, a sweet pepper variety (Melchor) from California-type fruits and three autochthonous Spanish varieties which have different pungency levels were used, including Piquillo, Padrón and Alegría riojana. The capsaicinoids contents were determined in the pericarp and placenta from fruits, showing that these phenyl-propanoids were mainly localized in placenta. The activity profiles of catalase, total and isoenzymatic superoxide dismutase (SOD), the enzymes of the ascorbate–glutathione cycle (AGC) and four NADP-dehydrogenases indicate that some interaction with capsaicinoid metabolism seems to occur. Among the results obtained on enzymatic antioxidants, the role of Fe-SOD and the glutathione reductase from the AGC is highlighted. Additionally, it was found that ascorbate and glutathione contents were higher in those pepper fruits which displayed the greater contents of capsaicinoids. Taken together, all these data indicate that antioxidants may contribute to preserve capsaicinoids metabolism to maintain their functionality in a framework where NADPH is perhaps playing an essential role.

Exogenous phytosulfokine α (PSKα) applying delays senescence and relief decay in strawberry fruits during cold storage by sufficient intracellular ATP and NADPH availability

Herein, we employed exogenous phytosulfokine α (PSKα) for delaying senescence and lessening decay in strawberry fruits during storage at 4 °C for 18 days. Our results showed that the strawberry fruits treated with 150 nM PSKα exhibited lower expression of poly-ADP-ribose polymerase 1 (PARP1) gene, leading to a higher intracellular NAD⁺ availability, beneficial for a sufficient provision of intracellular NADP⁺ with the activity of NAD kinase (NADK). Moreover, higher activities of glucose 6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and methylenetetrahydrofolate dehydrogenase (MTHFD) may be the reason for the sufficient intracellular availability of NADPH in strawberry fruits treated with 150 nM PSKα. In addition, strawberry fruits treated with 150 nM PSKα exhibited a sufficient availability of ATP resulted from higher activities of succinate dehydrogenase (SDH) and cytochrome c oxidase (CCO). Therefore, our results indicate that exogenous PSKα could be beneficial for delaying senescence and reducing decay in strawberry fruits during cold storage.

Non-AUG Translation Initiation Generates Peroxisomal Isoforms of 6-Phosphogluconate Dehydrogenase in Fungi

Proteins destined for transport to specific organelles usually contain targeting information, which are embedded in their sequence. Many enzymes are required in more than one cellular compartment and different molecular mechanisms are used to achieve dual localization. Here we report a cryptic type 2 peroxisomal targeting signal encoded in the 5′ untranslated region of fungal genes coding for 6-phosphogluconate dehydrogenase (PGD), a key enzyme of the oxidative pentose phosphate pathway. The conservation of the cryptic PTS2 motif suggests a biological function. We observed that translation from a non-AUG start codon generates an N-terminally extended peroxisomal isoform of Ustilago maydis PGD. Non-canonical initiation occurred at the sequence AGG AUU, consisting of two near-cognate start codons in tandem. Taken together, our data reveal non-AUG translation initiation as an additional mechanism to achieve the dual localization of a protein required both in the cytosol and the peroxisomes.

The Arabidopsis Plastidial Glucose-6-Phosphate Transporter GPT1 is Dually Targeted to Peroxisomes via the Endoplasmic Reticulum

Studies on Glucose-6-phosphate (G6P)/phosphate translocator isoforms GPT1 and GPT2 reported the viability of Arabidopsis (Arabidopsis thaliana) gpt2 mutants, whereas heterozygous gpt1 mutants exhibited a variety of defects during fertilization/seed set, indicating that GPT1 is essential for this process. Among other functions, GPT1 was shown to be important for pollen and embryo-sac development. Because our previous work on the irreversible part of the oxidative pentose phosphate pathway (OPPP) revealed comparable effects, we investigated whether GPT1 may dually localize to plastids and peroxisomes. In reporter fusions, GPT2 localized to plastids, but GPT1 also localized to the endoplasmic reticulum (ER) and around peroxisomes. GPT1 contacted two oxidoreductases and also peroxins that mediate import of peroxisomal membrane proteins from the ER, hinting at dual localization. Reconstitution in yeast (Saccharomyces cerevisiae) proteoliposomes revealed that GPT1 preferentially exchanges G6P for ribulose-5-phosphate (Ru5P). Complementation analyses of heterozygous +/gpt1 plants demonstrated that GPT2 is unable to compensate for GPT1 in plastids, whereas GPT1 without the transit peptide (enforcing ER/peroxisomal localization) increased gpt1 transmission significantly. Because OPPP activity in peroxisomes is essential for fertilization, and immunoblot analyses hinted at the presence of unprocessed GPT1-specific bands, our findings suggest that GPT1 is indispensable in both plastids and peroxisomes. Together with its G6P-Ru5P exchange preference, GPT1 appears to play a role distinct from that of GPT2 due to dual targeting.

Citrate valve integrates mitochondria into photosynthetic metabolism

While in heterotrophic cells and in darkness mitochondria serve as main producers of energy, during photosynthesis this function is transferred to chloroplasts and the main role of mitochondria in bioenergetics turns to be the balance of the level of phosphorylation of adenylates and of reduction of pyridine nucleotides to avoid over-energization of the cell and optimize major metabolic fluxes. This is achieved via the establishment and regulation of local equilibria of the tricarboxylic acid (TCA) cycle enzymes malate dehydrogenase and fumarase in one branch and aconitase and isocitrate dehydrogenase in another branch. In the conditions of elevation of redox level, the TCA cycle is transformed into a non-cyclic open structure (hemicycle) leading to the export of the tricarboxylic acid (citrate) to the cytosol and to the accumulation of the dicarboxylic acids (malate and fumarate). While the buildup of NADPH in chloroplasts provides operation of the malate valve leading to establishment of NADH/NAD⁺ ratios in different cell compartments, the production of NADH by mitochondria drives citrate export by establishing conditions for the operation of the citrate valve. The latter regulates the intercompartmental NADPH/NADP⁺ ratio and contributes to the biosynthesis of amino acids and other metabolic products during photosynthesis.

Salinity tolerance in barley during germination- homologs and potential genes

Salinity affects more than 6% of the world's total land area, causing massive losses in crop yield. Salinity inhibits plant growth and development through osmotic and ionic stresses; however, some plants exhibit adaptations through osmotic regulation, exclusion, and translocation of accumulated Na+ or Cl-. Currently, there are no practical, economically viable methods for managing salinity, so the best practice is to grow crops with improved tolerance. Germination is the stage in a plant's life cycle most adversely affected by salinity. Barley, the fourth most important cereal crop in the world, has outstanding salinity tolerance, relative to other cereal crops. Here, we review the genetics of salinity tolerance in barley during germination by summarizing reported quantitative trait loci (QTLs) and functional genes. The homologs of candidate genes for salinity tolerance in Arabidopsis, soybean, maize, wheat, and rice have been blasted and mapped on the barley reference genome. The genetic diversity of three reported functional gene families for salt tolerance during barley germination, namely dehydration-responsive element-binding (DREB) protein, somatic embryogenesis receptor-like kinase and aquaporin genes, is discussed. While all three gene families show great diversity in most plant species, the DREB gene family is more diverse in barley than in wheat and rice. Further to this review, a convenient method for screening for salinity tolerance at germination is needed, and the mechanisms of action of the genes involved in salt tolerance need to be identified, validated, and transferred to commercial cultivars for field production in saline soil.

Analysis of potential redundancy among Arabidopsis 6-phosphogluconolactonase isoforms in peroxisomes

Recent work revealed that PGD2, an Arabidopsis 6-phosphogluconate dehydrogenase (6-PGD) catalysing the third step of the oxidative pentose-phosphate pathway (OPPP) in peroxisomes, is essential during fertilization. Earlier studies on the second step, catalysed by PGL3, a dually targeted Arabidopsis 6-phosphogluconolactonase (6-PGL), reported the importance of OPPP reactions in plastids but their irrelevance in peroxisomes. Assuming redundancy of 6-PGL activity in peroxisomes, we examined the sequences of other higher plant enzymes. In tomato, there exist two 6-PGL isoforms with the strong PTS1 motif SKL. However, their analysis revealed problems regarding peroxisomal targeting: reporter-PGL detection in peroxisomes required construct modification, which was also applied to the Arabidopsis isoforms. The relative contribution of PGL3 versus PGL5 during fertilization was assessed by mutant crosses. Reduced transmission ratios were found for pgl3-1 (T-DNA-eliminated PTS1) and also for knock-out allele pgl5-2. The prominent role of PGL3 showed as compromised growth of pgl3-1 seedlings on sucrose and higher activity of mutant PGL3-1 versus PGL5 using purified recombinant proteins. Evidence for PTS1-independent uptake was found for PGL3-1 and other Arabidopsis PGL isoforms, indicating that peroxisome import may be supported by a piggybacking mechanism. Thus, multiple redundancy at the level of the second OPPP step in peroxisomes explains the occurrence of pgl3-1 mutant plants.

Elevated CO2 improves both lipid accumulation and growth rate in the glucose-6-phosphate dehydrogenase engineered Phaeodactylum tricornutum

BACKGROUND

Numerous studies have shown that stress induction and genetic engineering can effectively increase lipid accumulation, but lead to a decrease of growth in the majority of microalgae. We previously found that elevated CO₂ concentration increased lipid productivity as well as growth in Phaeodactylum tricornutum, along with an enhancement of the oxidative pentose phosphate pathway (OPPP) activity. The purpose of this work directed toward the verification of the critical role of glucose-6-phosphate dehydrogenase (G6PDH), the rate-limiting enzyme in the OPPP, in lipid accumulation in P. tricornutum and its simultaneous rapid growth rate under high-CO₂ (0.15%) cultivation.

RESULTS

In this study, G6PDH was identified as a target for algal strain improvement, wherein G6PDH gene was successfully overexpressed and antisense knockdown in P. tricornutum, and systematic comparisons of the photosynthesis performance, algal growth, lipid content, fatty acid profiles, NADPH production, G6PDH activity and transcriptional abundance were performed. The results showed that, due to the enhanced G6PDH activity, transcriptional abundance and NAPDH production, overexpression of G6PDH accompanied by high-CO₂ cultivation resulted in a much higher of both lipid content and growth in P. tricornutum, while knockdown of G6PDH greatly decreased algal growth as well as lipid accumulation. In addition, the total proportions of saturated and unsaturated fatty acid, especially the polyunsaturated fatty acid eicosapentaenoic acid (EPA; C20:5, n-3), were highly increased in high-CO₂ cultivated G6PDH overexpressed strains.

CONCLUSIONS

The successful of overexpression and antisense knockdown of G6PDH well demonstrated the positive influence of G6PDH on algal growth and lipid accumulation in P. tricornutum. The improvement of algal growth, lipid content as well as polyunsaturated fatty acids in high-CO₂ cultivated G6PDH overexpressed P. tricornutum suggested this G6PDH overexpression-high CO₂ cultivation pattern provides an efficient and economical route for algal strain improvement to develop algal-based biodiesel production.

Sweet Pepper (Capsicum annuum L.) Fruits Contain an Atypical Peroxisomal Catalase That is Modulated by Reactive Oxygen and Nitrogen Species

During the ripening of sweet pepper (Capsicum annuum L.) fruits, in a genetically controlled scenario, enormous metabolic changes occur that affect the physiology of most cell compartments. Peroxisomal catalase gene expression decreases after pepper fruit ripening, while the enzyme is also susceptible to undergo post-translational modifications (nitration, S-nitrosation, and oxidation) promoted by reactive oxygen and nitrogen species (ROS/RNS). Unlike most plant catalases, the pepper fruit enzyme acts as a homodimer, with an atypical native molecular mass of 125 to 135 kDa and an isoelectric point of 7.4, which is higher than that of most plant catalases. These data suggest that ROS/RNS could be essential to modulate the role of catalase in maintaining basic cellular peroxisomal functions during pepper fruit ripening when nitro-oxidative stress occurs. Using catalase from bovine liver as a model and biotin-switch labeling, in-gel trypsin digestion, and nanoliquid chromatography coupled with mass spectrometry, it was found that Cys377 from the bovine enzyme could potentially undergo S-nitrosation. To our knowledge, this is the first report of a cysteine residue from catalase that can be post-translationally modified by S-nitrosation, which makes it especially important to find the target points where the enzyme can be modulated under either physiological or adverse conditions.

Thermodynamic buffering, stable non-equilibrium and establishment of the computable structure of plant metabolism

The equilibria of coenzyme nucleotides and substrates established in plant cells generate simple rules that govern the plant metabolome and provide optimal conditions for the non-equilibrium fluxes of major metabolic processes such as ATP synthesis, CO₂ fixation, and mitochondrial respiration. Fast and abundant enzymes, such as adenylate kinase, carbonic anhydrase or malate dehydrogenase, provide constant substrate flux for these processes. These "buffering" enzymes follow the Michaelis-Menten (MM) kinetics and operate near equilibrium. The non-equilibrium "engine" enzymes, such as ATP synthase, Rubisco or the respiratory complexes, follow the modified version of MM kinetics due to their high concentration and low concentration of their substrates. The equilibrium reactions serve as control gates for the non-equilibrium flux through the engine enzymes establishing the balance of the fluxes of load and consumption of metabolic components. Under the coordinated operation of buffering and engine enzymes, the concentrations of free and Mg-bound adenylates and of free Mg²⁺ are set, serving as feedback signals from the adenylate metabolome. Those are linked to various cell energetics parameters, including membrane potentials. Also, internal levels of reduced and oxidized pyridine nucleotides are established in the coordinated operation of malate dehydrogenase and respiratory components, with proton concentration as a feedback from pyridine nucleotide pools. Non-coupled pathways of respiration serve to equilibrate the levels of pyridine nucleotides, adenylates, and as a pH stat. This stable non-equilibrium organizes the fluxes of energy spatially and temporally, controlling the rates of major metabolic fluxes that follow thermodynamically and kinetically defined computational principles.

Role of organic acids in the integration of cellular redox metabolism and mediation of redox signalling in photosynthetic tissues of higher plants

Organic acids play a crucial role in numerous metabolic processes accompanied by transfer of electrons and protons and linked to the reduction/oxidation of major redox couples in plant cells, such as NAD, NADP, glutathione, and ascorbate. Fluxes through the pathways metabolizing organic acids modulate redox states in cell compartments, contribute to generation of reactive oxygen and nitrogen species, and mediate signal transduction processes. Organic acid metabolism not only functions to equilibrate the redox potential in plant cells but also to transfer redox equivalents between cell compartments supporting various metabolic processes. The most important role in this transfer belongs to different forms of malate dehydrogenase interconverting malate and oxaloacetate or forming pyruvate (malic enzymes). During photosynthesis malate serves as a major form of transfer of redox equivalents from chloroplasts to the cytosol and other compartments via the malate valve. On the other hand, mitochondria, via alterations of their redox potential, become a source of citrate that can be transported to the cytosol and support biosynthesis of amino acids. Citrate is also an important retrograde signalling compound that regulates transcription of several genes including those encoding the alternative oxidase. The alternative oxidase, which is activated by increased redox potential and by pyruvate, is, in turn, important for the maintenance of redox potential in mitochondria. The roles of organic acids in establishing redox equilibrium, supporting ionic gradients on membranes, acidification of the extracellular medium, and regulation of production of reactive oxygen and nitrogen species are discussed.

Peroxisomal plant metabolism - an update on nitric oxide, Ca2+ and the NADPH recycling network

Plant peroxisomes are recognized organelles that - with their capacity to generate greater amounts of H₂O₂ than other subcellular compartments - have a remarkable oxidative metabolism. However, over the last 15 years, new information has shown that plant peroxisomes contain other important molecules and enzymes, including nitric oxide (NO), peroxynitrite, a NADPH-recycling system, Ca²⁺ and lipid-derived signals, such as jasmonic acid (JA) and nitro-fatty acid (NO₂-FA). This highlights the potential for complex interactions within the peroxisomal nitro-oxidative metabolism, which also affects the status of the cell and consequently its physiological processes. In this review, we provide an update on the peroxisomal interactions between all these molecules. Particular emphasis will be placed on the generation of the free-radical NO, which requires the presence of Ca²⁺, calmodulin and NADPH redox power. Peroxisomes possess several NADPH regeneration mechanisms, such as those mediated by glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH) proteins, which are involved in the oxidative phase of the pentose phosphate pathway, as well as that mediated by NADP-isocitrate dehydrogenase (ICDH). The generated NADPH is also an essential cofactor across other peroxisomal pathways, including the antioxidant ascorbate-glutathione cycle and unsaturated fatty acid β-oxidation, the latter being a source of powerful signaling molecules such as JA and NO₂-FA.

The Craft of Peroxisome Purification-A Technical Survey Through the Decades

Purification technologies are one of the working horses in organelle proteomics studies as they guarantee the separation of organelle-specific proteins from the background contamination by other subcellular compartments. The development of methods for the separation of organelles was a major prerequisite for the initial detection and characterization of peroxisome as a discrete entity of the cell. Since then, isolated peroxisomes fractions have been used in numerous studies in order to characterize organelle-specific enzyme functions, to allocate the peroxisome-specific proteome or to unravel the organellar membrane composition. This review will give an overview of the fractionation methods used for the isolation of peroxisomes from animals, plants and fungi. In addition to "classic" centrifugation-based isolation methods, relying on the different densities of individual organelles, the review will also summarize work on alternative technologies like free-flow-electrophoresis or flow field fractionation which are based on distinct physicochemical parameters. A final chapter will further describe how different separation methods and quantitative mass spectrometry have been used in proteomics studies to assign the proteome of PO.

Separation of Plant 6-Phosphogluconate Dehydrogenase (6PGDH) Isoforms by Non-denaturing Gel Electrophoresis

6-Phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) catalyzes the third and irreversible reaction of the pentose phosphate pathway (PPP). It carries out the oxidative decarboxylation of the 6-phosphogluconate to yield ribulose-5-phosphate, carbon dioxide and NADPH. In higher plants, 6PGDH has several subcellular localizations including cytosol, chloroplast, mitochondria and peroxisomes ( Corpas et al., 1998 ; Krepinsky et al., 2001 ; Mateos et al., 2009 ; Fernández-Fernández and Corpas, 2016; Hölscher et al., 2016 ). Using Arabidopsis thaliana as plant model and sweet pepper (Capsicum annuum L.) fruits as a plant with agronomical interest, this protocol illustrates how to prepare the plant extracts for the separation of the potential 6PGDH isoforms by electrophoresis on 6% polyacrylamide non-denaturing gels. Thus, this method allows detecting three 6PGDH isoforms in Arabidopsis seedlings and two 6PGDH isoforms in sweet pepper fruits.

Involvement of ABA- and H2O2-dependent cytosolic glucose-6-phosphate dehydrogenase in maintaining redox homeostasis in soybean roots under drought stress

The roles of abscisic acid (ABA) and hydrogen peroxide (H2O2) in inducing glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) activity and the possible roles of G6PDH in regulating ascorbate-glutathione (AsA-GSH) cycle were investigated in soybean (Glycine max L.) roots under drought stress. Drought caused a marked increase of the total and cytosolic G6PDH activities and triggered a rapid ABA and H2O2 accumulation in soybean roots. Exogenous ABA or H2O2 treatment elevated the total and cytosolic G6PDH activities, whereas suppressing ABA or H2O2 production inhibited the drought-induced increase in total and cytosolic G6PDH activities, suggesting that ABA and H2O2 are required for drought-induced increase of total G6PDH activity, namely cytosolic G6PDH activity. Furthermore, ABA induced H2O2 production by stimulating NADPH oxidase activity under drought stress. Moreover, drought significantly increased the contents of AsA and GSH and the activities of key enzymes in AsA-GSH cycle, while application of G6PDH inhibitor to seedlings significantly reduced the above effect induced by drought. Taken together, these results indicate that H2O2 acting as a downstream signaling molecule of ABA mediates drought-induced increase in cytosolic G6PDH activity, and that enhanced cytosolic G6PDH activity maintains cellular redox homeostasis by regulating AsA-GSH cycle in soybean roots.

NADPH production, a growth marker, is stimulated by maslinic acid in gilthead sea bream by increased NADP-IDH and ME expression

NADPH plays a central role in reductive biosynthesis of membrane lipids, maintenance of cell integrity, protein synthesis and redox balance maintenance. Hence, NADPH is involved in the growth and proliferation processes. In addition, it has been shown that changes in nutritional conditions produced changes in NADPH levels and growth rate. Maslinic acid (MA), a pentacyclic triterpene of natural origin, is able to stimulate NADPH production, through regulation of the two oxidative phase dehydrogenases of the pentose phosphate pathway. Our main objective was to study the effects of MA on the kinetic behaviour and on the molecular expression of two NADPH-generating systems, NADP-dependent isocitrate dehydrogenase (NADP-IDH) and malic enzyme (ME), in the liver and white muscle of gilthead sea bream (Sparus aurata). Four groups of 12g of a mean body mass were fed for 210days in a fish farm, with diets containing 0 (control), and 0.1g of MA per kg of diet. Two groups were fed ad libitum (C-AL and MA-AL) and another's two, with restricted diet of 1% of fish weight (C-R and MA-R). Results showed that MA significantly increased the main kinetic parameter of the NADPH-forming enzymes (NADP-IDH and ME). In this sense, specific activity, maximum velocity, catalytic efficiency and activity ratio values were higher in MA conditions than control groups. Moreover, these changes were observed in both feeding regimen, AL and R. Meanwhile, the Michaelis constant changed mainly in groups fed with the MA and restricted diet, these changes are related to the best substrate affinity by enzyme. Moreover, in the Western-blot result, we found that MA increased both protein levels studied, this behaviour being consistent with the regulation of the number of enzyme molecules. All results, indicate that MA, independently of the fed regimen, could potentially be a nutritional additive for fish as it improved the metabolic state of fish, as consequence of increased activity and expression of NADP-IDH and ME enzymes.

Saponin-Based Nanoemulsification Improves the Antioxidant Properties of Vitamin A and E in AML-12 Cells

Our work aimed to investigate the protective effects of saponin-based nanoemulsions of vitamin A and E against oxidative stress-induced cellular damage in AML-12 cells. Saponin nanoemulsions of vitamin A (SAN) and vitamin E (SEN) were prepared by high-pressure homogenization and characterized in terms of size, zeta potential, and polydispersity index. SEN and SAN protect AML-12 cells against oxidative stress-induced cellular damage more efficiently via scavenging reactive oxygen species (ROS), and reducing DNA damage, protein carbonylation, and lipid peroxidation. These results provide valuable information for the development of nanoemulsion-based delivery systems that would improve the antioxidant properties of vitamin A and E.

ROS Generation in Peroxisomes and its Role in Cell Signaling

In plant cells, as in most eukaryotic organisms, peroxisomes are probably the major sites of intracellular H₂O₂ production, as a result of their essentially oxidative type of metabolism. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in eukaryotic cells. The generation of the important messenger molecule hydrogen peroxide (H₂O₂) by animal and plant peroxisomes and the presence of catalase in these organelles has been known for many years, but the generation of superoxide radicals (O₂^·- ) and the occurrence of the metalloenzyme superoxide dismutase was reported for the first time in peroxisomes from plant origin. Further research showed the presence in plant peroxisomes of a complex battery of antioxidant systems apart from catalase. The evidence available of reactive oxygen species (ROS) production in peroxisomes is presented, and the different antioxidant systems characterized in these organelles and their possible functions are described. Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes. The regulation of ROS production in peroxisomes can take place by post-translational modifications of those proteins involved in their production and/or scavenging. In recent years, different studies have been carried out on the proteome of ROS metabolism in peroxisomes. Diverse evidence obtained indicates that peroxisomes are an important cellular source of different signaling molecules, including ROS, involved in distinct processes of high physiological importance, and might play an important role in the maintenance of cellular redox homeostasis.

Nitrogen Assimilation, Abiotic Stress and Glucose 6-Phosphate Dehydrogenase: The Full Circle of Reductants

Glucose 6 phosphate dehydrogenase (G6PDH; EC 1.1.1.49) is well-known as the main regulatory enzyme of the oxidative pentose phosphate pathway (OPPP) in living organisms. Namely, in Planta, different G6PDH isoforms may occur, generally localized in cytosol and plastids/chloroplasts. These enzymes are differently regulated by distinct mechanisms, still far from being defined in detail. In the last decades, a pivotal function for plant G6PDHs during the assimilation of nitrogen, providing reductants for enzymes involved in nitrate reduction and ammonium assimilation, has been described. More recently, several studies have suggested a main role of G6PDH to counteract different stress conditions, among these salinity and drought, with the involvement of an ABA depending signal. In the last few years, this recognized vision has been greatly widened, due to studies clearly showing the non-conventional subcellular localization of the different G6PDHs, and the peculiar regulation of the different isoforms. The whole body of these considerations suggests a central question: how do the plant cells distribute the reductants coming from G6PDH and balance their equilibrium? This review explores the present knowledge about these mechanisms, in order to propose a scheme of distribution of reductants produced by G6PDH during nitrogen assimilation and stress.

Defects in Peroxisomal 6-Phosphogluconate Dehydrogenase Isoform PGD2 Prevent Gametophytic Interaction in Arabidopsis thaliana

We studied the localization of 6-phosphogluconate dehydrogenase (PGD) isoforms of Arabidopsis (Arabidopsis thaliana). Similar polypeptide lengths of PGD1, PGD2, and PGD3 obscured which isoform may represent the cytosolic and/or plastidic enzyme plus whether PGD2 with a peroxisomal targeting motif also might target plastids. Reporter-fusion analyses in protoplasts revealed that, with a free N terminus, PGD1 and PGD3 accumulate in the cytosol and chloroplasts, whereas PGD2 remains in the cytosol. Mutagenesis of a conserved second ATG enhanced the plastidic localization of PGD1 and PGD3 but not PGD2. Amino-terminal deletions of PGD2 fusions with a free C terminus resulted in peroxisomal import after dimerization, and PGD2 could be immunodetected in purified peroxisomes. Repeated selfing of pgd2 transfer (T-)DNA alleles yielded no homozygous mutants, although siliques and seeds of heterozygous plants developed normally. Detailed analyses of the C-terminally truncated PGD2-1 protein showed that peroxisomal import and catalytic activity are abolished. Reciprocal backcrosses of pgd2-1 suggested that missing PGD activity in peroxisomes primarily affects the male gametophyte. Tetrad analyses in the quartet1-2 background revealed that pgd2-1 pollen is vital and in vitro germination normal, but pollen tube growth inside stylar tissues appeared less directed. Mutual gametophytic sterility was overcome by complementation with a genomic construct but not with a version lacking the first ATG. These analyses showed that peroxisomal PGD2 activity is required for guided growth of the male gametophytes and pollen tube-ovule interaction. Our report finally demonstrates an essential role of oxidative pentose-phosphate pathway reactions in peroxisomes, likely needed to sustain critical levels of nitric oxide and/or jasmonic acid, whose biosynthesis both depend on NADPH provision.

Nitro-Fatty Acids in Plant Signaling: Nitro-Linolenic Acid Induces the Molecular Chaperone Network in Arabidopsis

Nitro-fatty acids (NO2-FAs) are the product of the reaction between reactive nitrogen species derived of nitric oxide (NO) and unsaturated fatty acids. In animal systems, NO2-FAs are considered novel signaling mediators of cell function based on a proven antiinflammatory response. Nevertheless, the interaction of NO with fatty acids in plant systems has scarcely been studied. Here, we examine the endogenous occurrence of nitro-linolenic acid (NO2-Ln) in Arabidopsis and the modulation of NO2-Ln levels throughout this plant's development by mass spectrometry. The observed levels of this NO2-FA at picomolar concentrations suggested its role as a signaling effector of cell function. In fact, a transcriptomic analysis by RNA-seq technology established a clear signaling role for this molecule, demonstrating that NO2-Ln was involved in plant defense response against different abiotic-stress conditions, mainly by inducing heat shock proteins and supporting a conserved mechanism of action in both animal and plant defense processes. Bioinformatics analysis revealed that NO2-Ln was also involved in the response to oxidative stress conditions, mainly depicted by H2O2, reactive oxygen species, and oxygen-containing compound responses, with a high induction of ascorbate peroxidase expression. Closely related to these results, NO2-Ln levels significantly rose under several abiotic-stress conditions such as wounding or exposure to salinity, cadmium, and low temperature, thus validating the outcomes found by RNA-seq technology. Jointly, to our knowledge, these are the first results showing the endogenous presence of NO2-Ln in Arabidopsis (Arabidopsis thaliana) and supporting the strong signaling role of these molecules in the defense mechanism against different abiotic-stress situations.

In Silico Analysis of Arabidopsis thaliana Peroxisomal 6-Phosphogluconate Dehydrogenase

NADPH, whose regeneration is critical for reductive biosynthesis and detoxification pathways, is an essential component in cell redox homeostasis. Peroxisomes are subcellular organelles with a complex biochemical machinery involved in signaling and stress processes by molecules such as hydrogen peroxide (H2O2) and nitric oxide (NO). NADPH is required by several peroxisomal enzymes involved in β-oxidation, NO, and glutathione (GSH) generation. Plants have various NADPH-generating dehydrogenases, one of which is 6-phosphogluconate dehydrogenase (6PGDH). Arabidopsis contains three 6PGDH genes that probably are encoded for cytosolic, chloroplastic/mitochondrial, and peroxisomal isozymes, although their specific functions remain largely unknown. This study focuses on the in silico analysis of the biochemical characteristics and gene expression of peroxisomal 6PGDH (p6PGDH) with the aim of understanding its potential function in the peroxisomal NADPH-recycling system. The data show that a group of plant 6PGDHs contains an archetypal type 1 peroxisomal targeting signal (PTS), while in silico gene expression analysis using affymetrix microarray data suggests that Arabidopsis p6PGDH appears to be mainly involved in xenobiotic response, growth, and developmental processes.

Protein-protein interaction network analysis and identifying regulation microRNAs in asthmatic children

BACKGROUND

Asthma is a chronic inflammatory airway disease, the incidence of which has increased recently. In order to identify the potential biomarkers for allergic asthma therapy, microarray data were analysed to find meaningful information.

METHODS

Microarray data GSE18965 were downloaded from Gene Expression Ominibus (GEO), including seven asthmatic epithelium samples from children with allergic asthma and nine healthy controls. Limma package was used to detect differentially expressed genes (DEGs) and the criteria were |log fold change|>0.5 and p value<0.05. We used Database for Annotation, Visualization and Integrated Discovery (DAVID) tool to perform GO function and KEGG pathway analysis. STRING database was used to construct protein-protein interaction (PPI) network. MicroRNA (miRNA) regulation network was constructed according to miRecords database.

RESULTS

We identified 274 DEGs in asthma epithelium samples comparing with healthy controls. There were 123 up-regulated DEGs and 151 down-regulated DEGs. PPI network analysis showed that TSPO, G6PD and TXN had higher degree. miRNA regulation network demonstrated that miR-16 and miR-15a had higher degree. The target genes of miRNAs were significantly enriched in the apoptosis function.

CONCLUSIONS

TSPO, G6PD and TXN, miR-16, miR-15a and apoptosis may be used as the targets for children's allergic asthma therapy.

Spatial and temporal regulation of the metabolism of reactive oxygen and nitrogen species during the early development of pepper (Capsicum annuum) seedlings

BACKGROUND AND AIMS

The development of seedlings involves many morphological, physiological and biochemical processes, which are controlled by many factors. Some reactive oxygen and nitrogen species (ROS and RNS, respectively) are implicated as signal molecules in physiological and phytopathological processes. Pepper (Capsicum annuum) is a very important crop and the goal of this work was to provide a framework of the behaviour of the key elements in the metabolism of ROS and RNS in the main organs of pepper during its development.

METHODS

The main seedling organs (roots, hypocotyls and green cotyledons) of pepper seedlings were analysed 7, 10 and 14 d after germination. Activity and gene expression of the main enzymatic antioxidants (catalase, ascorbate-glutathione cycle enzymes), NADP-generating dehydrogenases and S-nitrosoglutathione reductase were determined. Cellular distribution of nitric oxide ((·)NO), superoxide radical (O2 (·-)) and peroxynitrite (ONOO(-)) was investigated using confocal laser scanning microscopy.

KEY RESULTS

The metabolism of ROS and RNS during pepper seedling development was highly regulated and showed significant plasticity, which was co-ordinated among the main seedling organs, resulting in correct development. Catalase showed higher activity in the aerial parts of the seedling (hypocotyls and green cotyledons) whereas roots of 7-d-old seedlings contained higher activity of the enzymatic components of the ascorbate glutathione cycle, NADP-isocitrate dehydrogenase and NADP-malic enzyme.

CONCLUSIONS

There is differential regulation of the metabolism of ROS, nitric oxide and NADP dehydrogenases in the different plant organs during seedling development in pepper in the absence of stress. The metabolism of ROS and RNS seems to contribute significantly to plant development since their components are involved directly or indirectly in many metabolic pathways. Thus, specific molecules such as H2O2 and NO have implications for signalling, and their temporal and spatial regulation contributes to the success of seedling establishment.

Glucose-6-phosphate dehydrogenase and alternative oxidase are involved in the cross tolerance of highland barley to salt stress and UV-B radiation

In this study, a new mechanism involving glucose-6-phosphate dehydrogenase (G6PDH) and alternative pathways (AP) in salt pretreatment-induced tolerance of highland barley to UV-B radiation was investigated. When highland barley was exposed to UV-B radiation, the G6PDH activity decreased but the AP capacity increased. In contrast, under UV-B+NaCl treatment, the G6PDH activity was restored to the control level and the maximal AP capacity and antioxidant enzyme activities were reached. Glucosamine (Glucm, an inhibitor of G6PDH) obviously inhibited the G6PDH activity in highland barley under UV-B + NaCl treatment and a similar pattern was observed in reduced glutathione (GSH) and ascorbic acid (Asc) contents. Similarly, salicylhydroxamic acid (SHAM, an inhibitor of AOX) significantly reduced the AP capacity in highland barley under UV-B + NaCl treatment. The UV-B-induced hydrogen peroxide (H2O2) accumulation was also followed. Further studies indicated that non-functioning of G6PDH or AP under UV-B+NaCl + Glucm or UV-B + NaCl + SHAM treatment also caused damages in photosynthesis and stomatal movement. Western blot analysis confirmed that the alternative oxidase (AOX) and G6PDH were dependent each other in cross tolerance to UV-B and salt. The inhibition of AP or G6PDH activity resulted in a significant accumulation or reduction of NADPH content, respectively, under UV-B+NaCl treatment in highland barley leaves. Taken together, our results indicate that AP and G6PDH mutually regulate and maintain photosynthesis and stomata movement in the cross adaptation of highland barley seedlings to UV-B and salt by modulating redox homeostasis and NADPH content.

Characterization of antioxidant enzymes and peroxisomes of olive (Olea europaea L.) fruits

The presence of peroxisomes in olive (Olea europaea L.) fruits and different antioxidant enzymes occurring in this plant tissue is reported for the first time. Ultrastructural analysis showed that olive cells were characterized by the presence of large vacuoles and lipid drops. Plastids, mitochondria and peroxisomes were placed near the cell wall, showing some type of association with it. Olive fruit peroxisomes were purified by sucrose density-gradient centrifugation, and catalase, glutathione reductase and ascorbate peroxidase were found in peroxisomes. In olive fruit tissue the presence of a battery of antioxidant enzymes was demonstrated, including catalase, four superoxide dismutase isozymes (mainly an Fe-SOD plus 2 Cu,Zn-SOD and a Mn-SOD), all the enzymes of the ascorbate-glutathione cycle, reduced and oxidized glutathione, ascorbate, and four NADPH-recycling dehydrogenases. The knowledge of the full composition of antioxidants (enzymatic and non-enzymatic) in olive fruits is crucial to be able to understand the processes regulating the antioxidant composition of olive oil.

Fungal peroxisomes as biosynthetic organelles

Peroxisomes are nearly ubiquitous single-membrane organelles harboring multiple metabolic pathways beside their prominent role in the β-oxidation of fatty acids. Here we review the diverse metabolic functions of peroxisomes in fungi. A variety of fungal metabolites are at least partially synthesized inside peroxisomes. These include the essential co-factor biotin but also different types of secondary metabolites. Peroxisomal metabolites are often derived from acyl-CoA esters for example β-oxidation intermediates. In several ascomycetes a subtype of peroxisomes has been identified that is metabolically inactive but is required to plug the septal pores of wounded hyphae. Thus, peroxisomes are versatile organelles that can adapt their function to the life style of an organism. This remarkable variability suggests that the full extent of the biosynthetic capacity of peroxisomes is still elusive. Moreover, in fungi peroxisomes are non-essential under laboratory conditions making them attractive organelles for biotechnological approaches and the design of novel metabolic pathways in customized peroxisomes.

Carotenoid responses to environmental stimuli: integrating redox and carbon controls into a fruit model

Carotenoids play an important role in plant adaptation to fluctuating environments as well as in the human diet by contributing to the prevention of chronic diseases. Insights have been gained recently into the way individual factors, genetic, environmental or developmental, control the carotenoid biosynthetic pathway at the molecular level. The identification of the rate-limiting steps of carotenogenesis has paved the way for programmes of breeding, and metabolic engineering, aimed at increasing the concentration of carotenoids in different crop species. However, the complexity that arises from the interactions between the different factors as well as from the coordination between organs remains poorly understood. This review focuses on recent advances in carotenoid responses to environmental stimuli and discusses how the interactions between the modulation factors and between organs affect carotenoid build-up. We develop the idea that reactive oxygen species/redox status and sugars/carbon status can be considered as integrated factors that account for most effects of the major environmental factors influencing carotenoid biosynthesis. The discussion highlights the concept of carotenoids or carotenoid-derivatives as stress signals that may be involved in feedback controls. We propose a conceptual model of the effects of environmental and developmental factors on carotenoid build-up in fruits.

Changes in RNA Splicing in Developing Soybean (Glycine max) Embryos

Developing soybean seeds accumulate oils, proteins, and carbohydrates that are used as oxidizable substrates providing metabolic precursors and energy during seed germination. The accumulation of these storage compounds in developing seeds is highly regulated at multiple levels, including at transcriptional and post-transcriptional regulation. RNA sequencing was used to provide comprehensive information about transcriptional and post-transcriptional events that take place in developing soybean embryos. Bioinformatics analyses lead to the identification of different classes of alternatively spliced isoforms and corresponding changes in their levels on a global scale during soybean embryo development. Alternative splicing was associated with transcripts involved in various metabolic and developmental processes, including central carbon and nitrogen metabolism, induction of maturation and dormancy, and splicing itself. Detailed examination of selected RNA isoforms revealed alterations in individual domains that could result in changes in subcellular localization of the resulting proteins, protein-protein and enzyme-substrate interactions, and regulation of protein activities. Different isoforms may play an important role in regulating developmental and metabolic processes occurring at different stages in developing oilseed embryos.

Dual targeting of peroxisomal proteins

Cellular compartmentalization into organelles serves to separate biological processes within the environment of a single cell. While some metabolic reactions are specific to a single organelle, others occur in more than one cellular compartment. Specific targeting of proteins to compartments inside of eukaryotic cells is mediated by defined sequence motifs. To achieve multiple targeting to different compartments cells use a variety of strategies. Here, we focus on mechanisms leading to dual targeting of peroxisomal proteins. In many instances, isoforms of peroxisomal proteins with distinct intracellular localization are encoded by separate genes. But also single genes can give rise to differentially localized proteins. Different isoforms can be generated by use of alternative transcriptional start sites, by differential splicing or ribosomal read-through of stop codons. In all these cases different peptide variants are produced, of which only one carries a peroxisomal targeting signal. Alternatively, peroxisomal proteins contain additional signals that compete for intracellular targeting. Dual localization of proteins residing in both the cytoplasm and in peroxisomes may also result from use of inefficient targeting signals. The recent observation that some bona fide cytoplasmic enzymes were also found in peroxisomes indicates that dual targeting of proteins to both the cytoplasm and the peroxisome might be more widespread. Although current knowledge of proteins exhibiting only partial peroxisomal targeting is far from being complete, we speculate that the metabolic capacity of peroxisomes might be larger than previously assumed.

Antioxidant systems from Pepper (Capsicum annuum L.): involvement in the response to temperature changes in ripe fruits

Sweet pepper is susceptible to changes in the environmental conditions, especially temperatures below 15 °C. In this work, two sets of pepper fruits (Capsicum annuum L.) which underwent distinct temperature profiles in planta were investigated. Accordingly, two harvesting times corresponding to each set were established: Harvest 1, whose fruits developed and ripened at 14.9 °C as average temperature; and Harvest 2, with average temperature of 12.4 °C. The oxidative metabolism was analyzed in all fruits. Although total ascorbate content did not vary between Harvests, a shift from the reduced to the oxidized form (dehydroascorbate), accompanied by a higher ascorbate peroxidase activity, was observed in Harvest 2 with respect to Harvest 1. Moreover, a decrease of the ascorbate-generating enzymatic system, the γ-galactono-lactone dehydrogenase, was found at Harvest 2. The activity values of the NADP-dependent dehydrogenases analyzed seem to indicate that a lower NADPH synthesis may occur in fruits which underwent lower temperature conditions. In spite of the important changes observed in the oxidative metabolism in fruits subjected to lower temperature, no oxidative stress appears to occur, as indicated by the lipid peroxidation and protein oxidation profiles. Thus, the antioxidative systems of pepper fruits seem to be involved in the response against temperature changes.

Chloroplast-localized 6-phosphogluconate dehydrogenase is critical for maize endosperm starch accumulation

Plants have duplicate versions of the oxidative pentose phosphate pathway (oxPPP) enzymes with a subset localized to the chloroplast. The chloroplast oxPPP provides NADPH and pentose sugars for multiple metabolic pathways. This study identified two loss-of-function alleles of the Zea mays (maize) chloroplast-localized oxPPP enzyme 6-phosphogluconate dehydrogenase (6PGDH). These mutations caused a rough endosperm seed phenotype with reduced embryo oil and endosperm starch. Genetic translocation experiments showed that pgd3 has separate, essential roles in both endosperm and embryo development. Endosperm metabolite profiling experiments indicated that pgd3 shifts redox-related metabolites and increases reducing sugars similar to starch-biosynthetis mutants. Heavy isotope-labelling experiments indicates that carbon flux into starch is altered in pgd3 mutants. Labelling experiments with a loss of cytosolic 6PGDH did not affect flux into starch. These results support the known role for plastid-localized oxPPP in oil synthesis and argue that amyloplast-localized oxPPP reactions are integral to endosperm starch accumulation in maize kernels.