Peroxisomal NADP-isocitrate dehydrogenase is required for Arabidopsis stomatal movement

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.

NADP+-dependent isocitrate dehydrogenase as a novel target for altering carbon flux to lipid accumulation and enhancing antioxidant capacity in Tetradesmus obliquus

Regulatory complexities in lipogenesis hinder the harmonization of metabolic carbon precursors towards lipid synthesis. Exploring regulatory complexities in lipogenesis, this study identifies NADP+-dependent isocitrate dehydrogenase (IDH) in Tetradesmus obliquus as a key factor. Overexpression IDH in strains ToIDH-1 and ToIDH-2 resulted in a 1.69 and 1.64-fold increase in neutral lipids, respectively, compared to the wild type, with lipid yield reaching 234.56 and 227.17 mg/L. Notably, despite slower growth, the cellular biomass augmented to 790.67 mg/L. Metabolite analysis indicated a shift in carbon precursors from protein to lipid and carbohydrate synthesis. Morphological observations revealed increases in the volume and number of lipid droplets, alongside a change in the fatty acid profile favoring monounsaturated and saturated fatty acids. Furthermore, IDH overexpression enhanced NADPH production and antioxidant activity, thereby further boosting lipid accumulation when combined with salt stress. This study suggests a pathway for improved lipogenesis and algal growth via metabolic engineering.

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.

Potassium (K+) Starvation-Induced Oxidative Stress Triggers a General Boost of Antioxidant and NADPH-Generating Systems in the Halophyte Cakile maritima

Potassium (K+) is an essential macro-element for plant growth and development given its implication in major processes such as photosynthesis, osmoregulation, protein synthesis, and enzyme function. Using 30-day-old Cakile maritima plants as halophyte model grown under K+ deprivation for 15 days, it was analyzed at the biochemical level to determine the metabolism of reactive oxygen species (ROS), key photorespiratory enzymes, and the main NADPH-generating systems. K+ starvation-induced oxidative stress was noticed by high malondialdehyde (MDA) content associated with an increase of superoxide radical (O2•−) in leaves from K+-deficient plants. K+ shortage led to an overall increase in the activity of hydroxypyruvate reductase (HPR) and glycolate oxidase (GOX), as well as of antioxidant enzymes catalase (CAT), those of the ascorbate-glutathione cycle, peroxidase (POX), and superoxide dismutase (SOD), and the main enzymes involved in the NADPH generation in both leaves and roots. Especially remarkable was the induction of up to seven CuZn-SOD isozymes in leaves due to K+ deficiency. As a whole, data show that the K+ starvation has associated oxidative stress that boosts a biochemical response leading to a general increase of the antioxidant and NADPH-generating systems that allow the survival of the halophyte Cakile maritima.

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.

Validation of Reference Genes for Quantitative Real-Time PCR Normalization in Ananas comosus var. bracteatus During Chimeric Leaf Development and Response to Hormone Stimuli

Reverse transcription quantitative real-time PCR (RT-qPCR) is a common way to study gene regulation at the transcriptional level due to its sensibility and specificity, but it needs appropriate reference genes to normalize data. Ananas comosus var. bracteatus, with white-green chimeric leaves, is an important pantropical ornamental plant. Up to date, no reference genes have been evaluated in Ananas comosus var. bracteatus. In this work, we used five common statistics tools (geNorm, NormFinder, BestKeeper, ΔCt method, RefFinder) to evaluate 10 candidate reference genes. The results showed that Unigene.16454 and Unigene.16459 were the optimal reference genes for different tissues, Unigene.16454 and zinc finger ran-binding domain-containing protein 2 (ZRANB2) for chimeric leaf at different developmental stages, isocitrate dehydrogenase [NADP] (IDH) and triacylglycerol lipase SDP1-like (SDP) for seedlings under different hormone treatments. The comprehensive results showed IDH, pentatricopeptide repeat-containing protein (PPRC), Unigene.16454, and caffeoyl-CoA O methyltransferase 5-like (CCOAOMT) are the top-ranked stable genes across all the samples. The stability of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was the least during all experiments. Furthermore, the reliability of recommended reference gene was validated by the detection of porphobilinogen deaminase (HEMC) expression levels in chimeric leaves. Overall, this study provides appropriate reference genes under three specific experimental conditions and will be useful for future research on spatial and temporal regulation of gene expression and multiple hormone regulation pathways in Ananas comosus var. bracteatus.

Cross-Talk between Hydrogen Peroxide and Nitric Oxide during Plant Development and Responses to Stress

Nitric oxide (NO) and hydrogen peroxide (H₂O₂) are gradually becoming established as critical regulators in plants under physiological and stressful conditions. Strong spatiotemporal correlations in their production and distribution have been identified in various plant biological processes. In this context, NO and H₂O₂ act synergistically or antagonistically as signals or stress promoters depending on their respective concentrations, engaging in processes such as the hypersensitive response, stomatal movement, and abiotic stress responses. Moreover, proteins identified as potential targets of NO-based modifications include a number of enzymes related to H₂O₂ metabolism, reinforcing their cross-talk. In this review, several processes of well-characterized functional interplay between H₂O₂ and NO are discussed with respect to the most recent reported evidence on hypersensitive response-induced programmed cell death, stomatal movement, and plant responses to adverse conditions and, where known, the molecular mechanisms and factors underpinning their cross-talk.

Loss of function of the chloroplast membrane K+/H+ antiporters AtKEA1 and AtKEA2 alters the ROS and NO metabolism but promotes drought stress resilience

Potassium (K⁺) exerts key physiological functions such as osmoregulation, stomatal movement, membrane transport, protein synthesis and photosynthesis among others. Previously, it was demonstrated in Arabidopsis thaliana that the loss of function of the chloroplast K⁺Efflux Antiporters KEA1 and KEA2, located in the inner envelope membrane, provokes inefficient photosynthesis. Therefore, the main goal of this study was to evaluate the potential impact of the loss of function of those cation transport systems in the metabolism of reactive oxygen and nitrogen species (ROS and RNS). Using 14-day-old seedlings from Arabidopsis double knock-out kea1kea2 mutants, ROS metabolism and NO content in roots and green cotyledons were studied at the biochemical level. The loss of function of AtKEA1 and AtKEA2 did not cause oxidative stress but it provoked an alteration of the ROS homeostasis affecting some ROS-generating enzymes. These included glycolate oxidase (GOX) and NADPH-dependent superoxide generation activity, enzymatic and non-enzymatic antioxidants and both NADP-isocitrate dehydrogenase and NADP-malic enzyme activities. NO content, analyzed by confocal laser scanning microscopy (CLSM), was negatively affected in both photosynthetic and non-photosynthetic organs in kea1kea2 mutant seedlings. Furthermore, the S-nitrosoglutathione reductase (GSNOR) protein expression and activity were downregulated in kea1kea2 mutants, whereas the tyrosine nitrated protein profile, analyzed by immunoblot, was unaffected but the relative expression of each immunoreactive band changed. Moreover, kea1kea2 mutants showed an increased photorespiratory pathway and stomata closure, thus promoting a higher resilience to drought stress. Data suggest that the chloroplast osmotic balance and integrity maintained by AtKEA1 and AtKEA2 are necessary to keep the balance of ROS/RNS metabolism. Moreover, these data open new questions about how endogenous NO generation might be affected by the K⁺/H⁺ transport located in the chloroplasts.

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.

Plant Peroxisomes: A Factory of Reactive Species

Plant peroxisomes are organelles enclosed by a single membrane whose biochemical composition has the capacity to adapt depending on the plant tissue, developmental stage, as well as internal and external cellular stimuli. Apart from the peroxisomal metabolism of reactive oxygen species (ROS), discovered several decades ago, new molecules with signaling potential, including nitric oxide (NO) and hydrogen sulfide (H2S), have been detected in these organelles in recent years. These molecules generate a family of derived molecules, called reactive nitrogen species (RNS) and reactive sulfur species (RSS), whose peroxisomal metabolism is autoregulated through posttranslational modifications (PTMs) such as S-nitrosation, nitration and persulfidation. The peroxisomal metabolism of these reactive species, which can be weaponized against pathogens, is susceptible to modification in response to external stimuli. This review aims to provide up-to-date information on crosstalk between these reactive species families and peroxisomes, as well as on their cellular environment in light of the well-recognized signaling properties of H2O2, NO and H2S.

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.

Peroxisomes: versatile organelles with diverse roles in plants

Peroxisomes are small, ubiquitous organelles that are delimited by a single membrane and lack genetic material. However, these simple-structured organelles are highly versatile in morphology, abundance and protein content in response to various developmental and environmental cues. In plants, peroxisomes are essential for growth and development and perform diverse metabolic functions, many of which are carried out coordinately by peroxisomes and other organelles physically interacting with peroxisomes. Recent studies have added greatly to our knowledge of peroxisomes, addressing areas such as the diverse proteome, regulation of division and protein import, pexophagy, matrix protein degradation, solute transport, signaling, redox homeostasis and various metabolic and physiological functions. This review summarizes our current understanding of plant peroxisomes, focusing on recent discoveries. Current problems and future efforts required to better understand these organelles are also discussed. An improved understanding of peroxisomes will be important not only to the understanding of eukaryotic cell biology and metabolism, but also to agricultural efforts aimed at improving crop performance and defense.

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.

Nitric oxide and hydrogen peroxide increase glucose-6-phosphate dehydrogenase activities and expression upon drought stress in soybean roots

KEY MESSAGE

Changes in glucose-6-phosphate dehydrogenase (G6PD) isoforms activities and expression were investigated in soybean roots under drought, suggesting that cytosolic G6PD plays a main role by regulating H2O2 signal and redox homeostasis. G6PD acts a vital role in plant growth, development and stress adaptation. Drought (PEG6000 treatment) could markedly increase the enzymatic activities of cytosolic G6PD (Cyt-G6PD) and compartmented G6PD (mainly plastidic P2-G6PD) in soybean roots. Application of G6PD inhibitor upon drought condition dramatically decreased the intracellular NADPH and reduced glutathione levels in soybean roots. Nitric oxide (NO) and hydrogen peroxide (H2O2) participated in the regulation of Cyt-G6PD and P2-G6PD enzymatic activities under drought stress. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, abolished the drought-induced accumulation of H2O2. The exogenous application of H2O2 and its production inhibitor (DPI) could stimulate and inhibit the NO accumulation, respectively, but not vice versa. qRT-PCR analysis confirmed that NO, as the downstream signal of H2O2, positively regulated the transcription of genes encoding Cyt-G6PD (GPD5, G6PD6, G6PD7) under drought stress in soybean roots. Comparatively, NO and H2O2 signals negatively regulated the gene expression of compartmented G6PD (GPD1, G6PD2, G6PD4), indicating that a post-transcriptional mechanism was involved in compartmented G6PD regulation. Taken together, the high Cyt-G6PD activity is essential for maintaining redox homeostasis upon drought condition in soybean roots, and the H2O2-dependent NO cascade signal is differently involved in Cyt-G6PD and compartmented G6PD regulation.

Plant peroxisomes at the crossroad of NO and H2 O2 metabolism

Plant peroxisomes are subcellular compartments involved in many biochemical pathways during the life cycle of a plant but also in the mechanism of response against adverse environmental conditions. These organelles have an active nitro-oxidative metabolism under physiological conditions but this could be exacerbated under stress situations. Furthermore, peroxisomes have the capacity to proliferate and also undergo biochemical adaptations depending on the surrounding cellular status. An important characteristic of peroxisomes is that they have a dynamic metabolism of reactive nitrogen and oxygen species (RNS and ROS) which generates two key molecules, nitric oxide (NO) and hydrogen peroxide (H₂ O₂ ). These molecules can exert signaling functions by means of post-translational modifications that affect the functionality of target molecules like proteins, peptides or fatty acids. This review provides an overview of the endogenous metabolism of ROS and RNS in peroxisomes with special emphasis on polyamine and uric acid metabolism as well as the possibility that these organelles could be a source of signal molecules involved in the functional interconnection with other subcellular compartments.

The intracellular distribution of the components of the GET system in vascular plants

The guided entry of tail-anchored proteins (GET) pathway facilitates targeting and insertion of tail-anchored proteins into membranes. In plants, such a protein insertion machinery for the endoplasmic reticulum as well as constituents within mitochondrial and chloroplasts were discovered. Previous phylogenetic analysis revealed that Get3 sequences of Embryophyta form two clades representing cytosolic ("a") and organellar ("bc") GET3 homologs, respectively. Cellular fractionation of Arabidopsis thaliana seedlings and usage of the self-assembly GFP system in protoplasts verified the cytosolic (_ATGet3a), plastidic (_ATGet3b) and mitochondrial (_ATGet3c) localization of the different homologs. The identified plant homologs of Get1 and Get4 in A. thaliana are localized in ER and cytosol, respectively, implicating a degree of conservation of the GET pathway in A. thaliana. Transient expression of Get3 homologs of Solanum lycopersicum, Medicago × varia or Physcomitrella patens with the self-assembly GFP technique in homologous and heterologous systems verified that multiple Get3 homologs with differing subcellular localizations are common in plants. Chloroplast localized Get3 homologs were detected in all tested plant systems. In contrast, mitochondrial localized Get3 homologs were not identified in S. lycopersicum, or P. patens, while we confirmed on the example of A. thaliana proteins that mitochondrial localized Get3 proteins are properly targeted in S. lycopersicum as well.

Cytosolic Glucose-6-Phosphate Dehydrogenase Is Involved in Seed Germination and Root Growth Under Salinity in Arabidopsis

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) is the key regulatory enzyme in the oxidative pentose phosphate pathway (OPPP). The cytosolic isoforms including G6PD5 and G6PD6 account for the major part of the G6PD total activity in plant cells. Here, we characterized the Arabidopsis single null mutant g6pd5 and g6pd6 and double mutant g6pd5/6. Compared to wild type, the mutant seeds showed a reduced germination rate and root elongation under salt stress. The seeds and seedlings lacking G6PD5 and G6PD6 accumulate more reactive oxygen species (ROS) than the wild type under salt stress. Cytosolic G6PD (cy-G6PD) affected the expression of NADPH oxidases and the G6PD enzymatic activities in the mutant atrbohD/F, in which the NADPH oxidases genes are disrupted by T-DNA insertion and generation of ROS is inhibited, were lower than that in the wild type. The NADPH level in mutants was decreased under salt stress. In addition, we found that G6PD5 and G6PD6 affected the activities and transcript levels of various antioxidant enzymes in response to salt stress, especially the ascorbate peroxidase and glutathione reductase. Exogenous application of ascorbate acid and glutathione rescued the seed and root phenotype of g6pd5/6 under salt stress. Interestingly, the cytosolic G6PD negatively modulated the NaCl-blocked primary root growth under salt stress in the root meristem and elongation zone.

The relationship between carbon and nitrogen metabolism in cucumber leaves acclimated to salt stress

The study examines the effect of acclimation on carbon and nitrogen metabolism in cucumber leaves subjected to moderate and severe NaCl stress. The levels of glucose, sucrose, NADH/NAD⁺-GDH, AspAT, AlaAT, NADP⁺-ICDH, G6PDH and 6GPDH activity were determined after 24 and 72 hour periods of salt stress in acclimated and non-acclimated plants. Although both groups of plants showed high Glc and Suc accumulation, they differed with regard to the range and time of accumulation. Acclimation to salinity decreased the activities of NADP⁺-ICDH and deaminating NAD⁺-GDH compared to controls; however, these enzymes, together with the other examined parameters, showed elevated values in the stressed plants. The acclimated plants showed higher G6PDH activity than the non-acclimated plants, whereas both groups demonstrated similar 6PGDH activity. The high activities of NADH-GDH, AlaAT and AspAT observed in the examined plants could be attributed to a high demand for glutamate. The observed changes may be required for the maintenance of correct TCA cycle activity, and acclimation appeared to positively influence these adaptive processes.

Endogenous hydrogen sulfide (H2S) is up-regulated during sweet pepper (Capsicum annuum L.) fruit ripening. In vitro analysis shows that NADP-dependent isocitrate dehydrogenase (ICDH) activity is inhibited by H2S and NO

Like nitric oxide (NO), hydrogen sulfide (H₂S) has been recognized as a new gasotransmitter which plays an important role as a signaling molecule in many physiological processes in higher plants. Although fruit ripening is a complex process associated with the metabolism of reactive oxygen species (ROS) and nitrogen oxygen species (RNS), little is known about the potential involvement of endogenous H₂S. Using sweet pepper (Capsicum annuum L.) as a model non-climacteric fruit during the green and red ripening stages, we studied endogenous H₂S content and cytosolic l-cysteine desulfhydrase (L-DES) activity which increased by 14% and 28%, respectively, in red pepper fruits. NADPH is a redox compound and key cofactor required for cell growth, proliferation and detoxification. We studied the NADPH-regenerating enzyme, NADP-isocitrate dehydrogenase (NADP-ICDH), whose activity decreased by 34% during ripening. To gain a better understanding of its potential regulation by H₂S, we obtained a 50-75% ammonium sulfate-enriched protein fraction containing the NADP-ICDH protein; with the aid of in vitro assays in the presence of H₂S, we observed that 2 and 10 mM NaHS used as H₂S donors resulted in a decrease of up to 36% and 45%, respectively, in NADP-ICDH activity, which was unaffected by reduced glutathione (GSH). On the other hand, peroxynitrite (ONOO^-), S-nitrosocyteine (CysNO) and DETA-NONOate, with the last two acting as NO donors, also inhibited NADP-ICDH activity. In silico analysis of the tertiary structure of sweet pepper NADP-ICDH activity (UniProtKB ID A0A2G2Y555) suggests that residues Cys133 and Tyr450 are the most likely potential targets for S-nitrosation and nitration, respectively. Taken together, the data reveal that the increase in the H₂S production capacity of red fruits is due to higher L-DES activity during non-climacteric pepper fruit ripening. In vitro assays appear to show that H₂S inhibits NADP-ICDH activity, thus suggesting that this enzyme may be regulated by persulfidation, as well as by S-nitrosation and nitration. NO and H₂S may therefore regulate NADPH production and consequently cellular redox status during pepper fruit ripening.

Conserved and differential transcriptional responses of peroxisome associated pathways to drought, dehydration and ABA

Plant peroxisomes are important components of cellular antioxidant networks, dealing with ROS generated by multiple metabolic pathways. Peroxisomes respond to environmental and cellular conditions by changing their size, number, and proteomic content. To investigate the role of peroxisomes in response to drought, dehydration and ABA treatment we took an evolutionary and comparative genomics approach. Colonisation of land required evolution of dehydration tolerance in the absence of subsequent anatomical adaptations. Therefore, the model bryophyte Physcomitrella patens, the model dicot Arabidopsis thaliana and wheat (Tricitcum aestivum), a globally important cereal crop were compared. Three sets of genes namely 'PTS1 genes' (a proxy for genes encoding peroxisome targeted proteins), PEX genes (involved in peroxisome biogenesis) and genes involved in plant antioxidant networks were identified in all 3 species and their expression compared under drought (dehydration) and ABA treatment. Genes encoding enzymes of β-oxidation and gluconeogenesis, antioxidant enzymes including catalase and glutathione reductase and PEX3 and PEX11 isoforms showed conserved up-regulation, and peroxisome proliferation was induced by ABA in moss. Interestingly, expression of some of these genes differed between drought sensitive and resistant genotypes of wheat in line with measured photosynthetic and biochemical differences. These results point to an underappreciated role for peroxisomes in drought response.

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.

Calmodulin antagonist affects peroxisomal functionality by disrupting both peroxisomal Ca2+ and protein import

Ca²⁺ is a second messenger in many physiological and phytopathological processes. Peroxisomes are subcellular compartments with an active oxidative and nitrosative metabolism. Previous studies have demonstrated that peroxisomal nitric oxide (NO) generation is dependent on Ca²⁺ and calmodulin (CaM). Here, we used Arabidopsis thaliana transgenic seedlings expressing cyan fluorescent protein (CFP) containing a type 1 peroxisomal-targeting signal motif (PTS1; CFP-PTS1), which enables peroxisomes to be visualized in vivo, and also used a cell-permeable fluorescent probe for Ca²⁺ Analysis by confocal laser-scanning microscopy (CLSM) enabled us to visualize the presence of endogenous Ca²⁺ in the peroxisomes of both roots and guard cells. The presence of Ca²⁺ in peroxisomes and the import of CFP-PTS1 are drastically disrupted by both CaM antagonist and glutathione (GSH). Furthermore, the activity of three peroxisomal enzymes (catalase, glycolate oxidase and hydroxypyruvate reductase) containing PTS1 was clearly affected in these conditions, with a decrease of between 41 and 51%. In summary, data show that Ca²⁺ and CaM are strictly necessary for protein import and normal functionality of peroxisomal enzymes, including antioxidant and photorespiratory enzymes, as well as for NO production.

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.

A Role for RNS in the Communication of Plant Peroxisomes with Other Cell Organelles?

Plant peroxisomes are organelles with a very active participation in the cellular regulation of the metabolism of reactive oxygen species (ROS). However, during the last two decades peroxisomes have been shown to be also a relevant source of nitric oxide (NO) and other related molecules designated as reactive nitrogen species (RNS). ROS and RNS have been mainly associated to nitro-oxidative processes; however, some members of these two families of molecules such as H₂O₂, NO or S-nitrosoglutathione (GSNO) are also involved in the mechanism of signaling processes mainly through post-translational modifications. Peroxisomes interact metabolically with other cell compartments such as chloroplasts, mitochondria or oil bodies in different pathways including photorespiration, glyoxylate cycle or β-oxidation, but peroxisomes are also involved in the biosynthesis of phytohormones including auxins and jasmonic acid (JA). This review will provide a comprehensive overview of peroxisomal RNS metabolism with special emphasis in the identified protein targets of RNS inside and outside these organelles. Moreover, the potential interconnectivity between peroxisomes and other plant organelles, such as mitochondria or chloroplasts, which could have a regulatory function will be explored, with special emphasis on photorespiration.

Alleviation of Cr(VI)-induced oxidative stress in maize (Zea mays L.) seedlings by NO and H2S donors through differential organ-dependent regulation of ROS and NADPH-recycling metabolisms

Chromium (Cr) contamination in soil is a growing concern in relation to sustainable agricultural production and food safety. Nitric oxide (NO) and, more recently, hydrogen sulfide (H₂S) are considered to be new signalling molecules with biotechnological applications in the agronomical sector. Using 9-day-old maize (Zea mays) seedlings exposed to 200μM Cr(VI), the potential mitigating effects of exogenous NO and H₂S on chromium-induced stress in maize seedlings were investigated in roots, cotyledons and coleoptiles. Analysis of Cr content, lipid peroxidation, antioxidant enzymes (catalase and superoxide dismutase isozymes), peroxisomal H₂O₂-producing glycolate oxidase and the main NADPH-regenerating system revealed that chromium causes oxidative stress, leading to a general increase in these activities in coleptiles and roots, with the latter organ being the most affected. However, cotyledons behaved in an opposite manner. Moreover, exogenous applications of NO and H₂S to Cr-stressed maize seedlings triggered a significant response, involving the virtual restoration of the values for all these activities to those observed in unstressed seedlings, although their specific impact on ROS and NADPH-recycling metabolisms depends on the seedling organ involved. Taken together, the data indicate that gas transmitters, NO and H₂S, which act as a defence against the negative effects of hexavalent chromium contamination, are alternative compounds with potential biotechnological applications.

Transcriptome Analysis of Two Species of Jute in Response to Polyethylene Glycol (PEG)- induced Drought Stress

Drought stress results in significant crop yield losses. Comparative transcriptome analysis between tolerant and sensitive species can provide insights into drought tolerance mechanisms in jute. We present a comprehensive study on drought tolerance in two jute species-a drought tolerant species (Corchorus olitorius L., GF) and a drought sensitive species (Corchorus capsularis L., YY). In total, 45,831 non-redundant unigenes with average sequence length of 1421 bp were identified. Higher numbers of differentially expressed genes (DEGs) were discovered in YY (794) than in GF (39), implying that YY was relatively more vulnerable or hyper-responsive to drought stress at the molecular level; the two main pathways, phenylpropanoid biosynthesis and peroxisome pathway, significantly involved in scavenging of reactive oxygen species (ROS) and 14 unigenes in the two pathways presented a significant differential expression in response to increase of superoxide. Our classification analysis showed that 1769 transcription factors can be grouped into 81 families and 948 protein kinases (PKs) into 122 families. In YY, we identified 34 TF DEGs from and 23 PK DEGs, including 19 receptor-like kinases (RLKs). Most of these RLKs were downregulated during drought stress, implying their role as negative regulators of the drought tolerance mechanism in jute.

Plant peroxisomes: A nitro-oxidative cocktail

Although peroxisomes are very simple organelles, research on different species has provided us with an understanding of their importance in terms of cell viability. In addition to the significant role played by plant peroxisomes in the metabolism of reactive oxygen species (ROS), data gathered over the last two decades show that these organelles are an endogenous source of nitric oxide (NO) and related molecules called reactive nitrogen species (RNS). Molecules such as NO and H2O2 act as retrograde signals among the different cellular compartments, thus facilitating integral cellular adaptation to physiological and environmental changes. However, under nitro-oxidative conditions, part of this network can be overloaded, possibly leading to cellular damage and even cell death. This review aims to update our knowledge of the ROS/RNS metabolism, whose important role in plant peroxisomes is still underestimated. However, this pioneering approach, in which key elements such as β-oxidation, superoxide dismutase (SOD) and NO have been mainly described in relation to plant peroxisomes, could also be used to explore peroxisomes from other organisms.

Reactive oxygen species signaling and stomatal movement: Current updates and future perspectives

Reactive oxygen species (ROS), a by-product of aerobic metabolism were initially studied in context to their damaging effect but recent decades witnessed significant advancements in understanding the role of ROS as signaling molecules. Contrary to earlier views, it is becoming evident that ROS production is not necessarily a symptom of cellular dysfunction but it might represent a necessary signal in adjusting the cellular machinery according to the altered conditions. Stomatal movement is controlled by multifaceted signaling network in response to endogenous and environmental signals. Furthermore, the stomatal aperture is regulated by a coordinated action of signaling proteins, ROS-generating enzymes, and downstream executors like transporters, ion pumps, plasma membrane channels, which control the turgor pressure of the guard cell. The earliest hallmarks of stomatal closure are ROS accumulation in the apoplast and chloroplasts and thereafter, there is a successive increase in cytoplasmic Ca2+ level which rules the multiple kinases activity that in turn regulates the activity of ROS-generating enzymes and various ion channels. In addition, ROS also regulate the action of multiple proteins directly by oxidative post translational modifications to adjust guard cell signaling. Notwithstanding, an active progress has been made with ROS signaling mechanism but the regulatory action for ROS signaling processes in stomatal movement is still fragmentary. Therefore, keeping in view the above facts, in this mini review the basic concepts and role of ROS signaling in the stomatal movement have been presented comprehensively along with recent highlights.

Plant peroxisomes: recent discoveries in functional complexity, organelle homeostasis, and morphological dynamics

Peroxisomes are essential for life in plants. These organelles house a variety of metabolic processes that generate and inactivate reactive oxygen species. Our knowledge of pathways and mechanisms that depend on peroxisomes and their constituent enzymes continues to grow, and in this review we highlight recent advances in understanding the identity and biological functions of peroxisomal enzymes and metabolic processes. We also review how peroxisomal matrix and membrane proteins enter the organelle from their sites of synthesis. Peroxisome homeostasis is regulated by specific degradation mechanisms, and we discuss the contributions of specialized autophagy and a peroxisomal protease to the degradation of entire peroxisomes and peroxisomal enzymes that are damaged or superfluous. Finally, we review how peroxisomes can flexibly change their morphology to facilitate inter-organellar contacts.

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.

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.

What is the role of hydrogen peroxide in plant peroxisomes?

Plant peroxisomes are unusual subcellular compartments with an apparent simple morphology but with complex metabolic activity. The presence of signal molecules, such as hydrogen peroxide (H(2)O(2)) and nitric oxide inside plant peroxisomes have added new functions in the cross-talk events among organelles and cells under physiological and stress conditions. Moreover, recent advances in proteomic analyses of plant peroxisomes have identified new protein candidates involved in several novel metabolic pathways. With all these new data, the present concise manuscript will focus on the relevance of the peroxisomal H(2)O(2) and its two main antioxidant enzymes, catalase and membrane-bound ascorbate peroxidase, which regulate its level and consequently its potential functions.