Glucose-6-phosphate dehydrogenase plays a central role in modulating reduced glutathione levels in reed callus under salt stress

Preharvest spraying of phenylalanine activates the sucrose and respiratory metabolism in muskmelon wounds during healing

Phenylalanine (Phe) accelerates fruit wound healing by activating phenylpropanoid metabolism. However, whether Phe affects sucrose and respiratory metabolism in fruit during wound healing remains unknown. In this research, we found that preharvest Phe spray promoted sucrose degradation and increased glucose and fructose levels by activating acid invertase (AI), neutral invertase (NI), sucrose synthase (SS) and sucrose phosphate synthase (SPS) on harvested muskmelons. The spray also activated hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), malate dehydrogenase (MDH), succinate dehydrogenase (SDH) and glucose-6-phosphate dehydrogenase (G6PDH). In addition, the spray improved energy and reducing power levels in the fruit. Taken together, preharvest Phe spray can provide carbon skeleton, energy and reducing power for wound healing by activating the sucrose metabolism, Embden-Meyerhof-Parnas (EMP) pathway, tricarboxylic acid (TCA) cycle and pentose phosphate (PPP) pathway in muskmelon wounds during healing, which is expected to be developed as a new strategy to accelerate fruit wound healing.

Effect of L-cysteine treatment to induce postharvest disease resistance of Monilinia fructicola in plum fruits and the possible mechanisms involved

Plum is an important stone fruit in China, but the fruit is easily perishable and susceptible to infection by pathogens. Traditionally, synthetic fungicides are used to control diseases. However, the side effects of fungicides should not be ignored. Cysteine, generally recognized as safe (GRAS) amino acid, has been reported to play roles in the plant abiotic stress response, but little is known about the role of cysteine to control postharvest diseases in fruits. Therefore, this study was designed to investigate the effect of L-cysteine treatment on control of postharvest brown rot in artificially inoculated plum fruits and the possible biocontrol mechanisms involved. Postharvest plum fruits were inoculated with 1, 10, 100 and 1000 mg L^-1 L-cysteine. 100 mg L^-1 L-cysteine treatment effectively controlled brown rot in artificially inoculated plum fruits by inducing resistance. Furthermore, 100 mg L^-1 L-cysteine treatment increased the activities of glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), enhanced the content of NADPH of the pentose phosphate pathway, as well as improved the contents of H₂O₂ and some amino acids in the artificially inoculated plum fruits. 100 mg L^-1 L-cysteine treatment also elevated the antioxidant content (AsA, GSH) and the antioxidant enzymes activities (APX, GR, MDAR, DHAR) of the ascorbate-glutathione (AsA-GSH) pathway. The protective effects of L-cysteine treatment on postharvest plum fruits likely be due to activating some defense-related responses of the fruit against infection. L-cysteine treatment is a safe promising method for controlling postharvest brown rot in plum fruits.

Biochemical Changes in Two Barley Genotypes Inoculated With a Beneficial Fungus Trichoderma harzianum Rifai T-22 Grown in Saline Soil

One of the most important environmental factors impacting crop plant productivity is soil salinity. Fungal endophytes have been characterised as biocontrol agents that help in plant productivity and induce resistance responses to several abiotic stresses, including salinity. In the salt-tolerant cereal crop barley (Hordeum vulgare L.), there is limited information about the metabolites and lipids that change in response to inoculation with fungal endophytes in saline conditions. In this study, gas chromatography coupled to mass spectrometry (GC-MS) and LC-electrospray ionisation (ESI)-quadrupole-quadrupole time of flight (QqTOF)-MS were used to determine the metabolite and lipid changes in two fungal inoculated barley genotypes with differing tolerance levels to saline conditions. The more salt-tolerant cultivar was Vlamingh and less salt tolerant was Gairdner. Trichoderma harzianum strain T-22 was used to treat these plants grown in soil under control and saline (200 mM NaCl) conditions. For both genotypes, fungus-colonised plants exposed to NaCl had greater root and shoot biomass, and better chlorophyll content than non-colonised plants, with colonised-Vlamingh performing better than uninoculated control plants. The metabolome dataset using GC-MS consisted of a total of 93 metabolites of which 74 were identified in roots of both barley genotypes as organic acids, sugars, sugar acids, sugar alcohols, amino acids, amines, and a small number of fatty acids. LC-QqTOF-MS analysis resulted in the detection of 186 lipid molecular species, classified into three major lipid classes-glycerophospholipids, glycerolipids, and sphingolipids, from roots of both genotypes. In Cultivar Vlamingh both metabolites and lipids increased with fungus and salt treatment while in Gairdner they decreased. The results from this study suggest that the metabolic pathways by which the fungus imparts salt tolerance is different for the different genotypes.

Eco-biochemical responses, phytoremediation potential and molecular genetic analysis of Alhagi maurorum grown in metal-contaminated soils

BACKGROUND

Alhagi maurorum Medik. (camelthorn) is a dominant desert plant indigenous in various habitats, including the Western Desert of Egypt. The plant is especially prevalent in and around economic iron ore deposits. Nutrient and heavy metal levels in A. maurorum tissues and soil samples were assessed to identify associations between heavy metal levels in plants and soil. The objective was to evaluate this species as an indicator of heavy metal pollution. Photosynthetic pigments, protein, proline, alkaloids, flavonoids, 2,2-diphenyl-1-picrylhydrazylscavenging, reduced glutathione, malondialdehyde, antioxidant enzymes, and stress-related gene expression were assessed to determine their functional roles in metal stress adaptation in ultra- and molecular structure. Additionally, the molecular genetic variation in A. maurorum samples was assessed using co-dominant sequence-related amplified polymorphism (SRAP) and inter simple sequence repeats (ISSR).

RESULTS

A substantial difference in enzymatic and non-enzymatic antioxidants of A. maurorum was observed in samples collected from three sites. A. maurorum is suited to the climate in mineralized regions. Morphologically, the stem shows spines, narrow leaves, and a reduced shoot system. Anatomically, modifications included a cuticle coating on leaves and stems, sunken stomata, a compact epidermis, and a thick cortex. Significant anatomical-physiological differences were observed with varying heavy metal soil content, antioxidative enzyme activities increased as a tolerance strategy, and glutathione levels decreased in response to heavy metal toxicity. Heavy metal accumulation also affected the expression of stress-related genes. The highest levels of expression of GST, G6PDH, 6PGD, nitrate reductase 1, and sulfate transporter genes were found in plants collected from site A1. However, auxin-induced protein exhibited its highest expression in plants collected from A2. Six SRAP combinations yielded 25 scoreable markers with a polymorphism rate of 64%, and 5 ISSR markers produced 11 bands with a polymorphism rate of 36.36% for three A. maurorum genotypes. The ME1xEM7 primer combinations provided the most polymorphic information content and resolving power, making it the most useful primer for differentiating A. maurorum genotypes. SRAP markers exhibited a higher diversity index (0.24) than ISSR markers (0.16).

CONCLUSIONS

A. maurorum displayed adaptive characteristics for heavy metal sequestration from mining site soils and is proposed as a strong candidate for phytoremediation.

Genome-Wide Investigation of G6PDH Gene in Strawberry: Evolution and Expression Analysis during Development and Stress

As one of the key enzymes in the pentose phosphate pathway (PPP), glucose-6-phosphate dehydrogenase (G6PDH) provides NADPH and plays an important role in plant development and stress responses. However, little information was available about the G6PDH genes in strawberry (Fragaria × ananassa). The recent release of the whole-genome sequence of strawberry allowed us to perform a genome-wide investigation into the organization and expression profiling of strawberry G6PDH genes. In the present study, 19 strawberry G6PDH genes (FaG6PDHs) were identified from the strawberry genome database. They were designated as FaG6PDH1 to FaG6PDH19, respectively, according to the conserved domain of each subfamily and multiple sequence alignment with Arabidopsis. According to their structural and phylogenetic features, the 19 FaG6PDHs were further classified into five types: Cy, P1, P1.1, P2 and PO. The number and location of exons and introns are similar, suggesting that genes of the same type are very similar and are alleles. A cis-element analysis inferred that FaG6PDHs possessed at least one stress-responsive cis-acting element. Expression profiles derived from transcriptome data analysis exhibited distinct expression patterns of FaG6PDHs genes in different developmental stages. Real-time quantitative PCR was used to detect the expression level of five types FaG6PDHs genes and demonstrated that the genes were expressed and responded to multiple abiotic stress and hormonal treatments.

The response mechanism to salt stress in Arabidopsis transgenic lines over-expressing of GmG6PD

Glucose-6-phosphate dehydrogenase (G6PD or G6PDH) plays an important role in response to salt stress in plants. However, much less is known about G6PD proteins in soybean (Glycine max L.). Here, we found that a soybean cytosolic G6PD gene, GmG6PD7, was induced by NaCl. We generated Arabidopsis transgenic lines overexpressing GmG6PD7. The seed germination rate and primary root length of Arabidopsis thaliana over-expressing GmG6PD7 under NaCl treatment were enhanced. Salt stress induced an obvious increase of the total and cytosolic G6PD activity and the marked decrease of ROS levels in the transgenic plants. At the same time, over-expressing GmG6PD7 in Arabidopsis affected the glutathione and NADPH level and activated ROS scavengers, suggesting that GmG6PD7 contributes to increase salinity tolerance by decreasing ROS accumulation. What's more, we found GmG6PD7 overexpression led to the up-regulation of abscisic acid (ABA) degradation gene and the down-regulation of ABA synthesis and ABA-responsive genes, which finally reduced ABA content to improve seed germination rate under salinity stress. It was noteworthy that GmG6PD7 can rescue the seed and root phenotype of Arabidopsis cytosolic G6PD mutant (Atg6pd5 and Atg6pd6) under salt stress, suggesting cytosolic G6PD may have a conserved function in soybean and Arabidopsis.

Identification, Characterization, and Stress Responsiveness of Glucose-6-phosphate Dehydrogenase Genes in Highland Barley

G6PDH provides intermediate metabolites and reducing power (nicotinamide adenine dinucleotide phosphate, NADPH) for plant metabolism, and plays a pivotal role in the cellular redox homeostasis. In this study, we cloned five G6PDH genes (HvG6PDH1 to HvG6PDH5) from highland barley and characterized their encoded proteins. Functional analysis of HvG6PDHs in E. coli showed that HvG6PDH1 to HvG6PDH5 encode the functional G6PDH proteins. Subcellular localization and phylogenetic analysis indicated that HvG6PDH2 and HvG6PDH5 are localized in the cytoplasm, while HvG6PDH1, HvG6PDH3, and HvG6PDH4 are plastidic isoforms. Analysis of enzymatic activities and gene expression showed that HvG6PDH1 to HvG6PDH4 are involved in responses to salt and drought stresses. The cytosolic HvG6PDH2 is the major isoform against oxidative stress. HvG6PDH5 may be a house-keeping gene. In addition, HvG6PDH1 to HvG6PDH4 and their encoded enzymes responded to jasmonic acid (JA) and abscisic acid (ABA) treatments, implying that JA and ABA are probably critical regulators of HvG6PDHs (except for HvG6PDH5). Reactive oxygen species analysis showed that inhibition of cytosolic and plastidic G6PDH activities leads to increased H₂O₂ and O₂⁻ contents in highland barley under salt and drought stresses. These results suggest that G6PDH can maintain cellular redox homeostasis and that cytosolic HvG6PDH2 is an irreplaceable isoform against oxidative stress in highland barley.

Differentially expressed genes related to oxidoreductase activity and glutathione metabolism underlying the adaptation of Phragmites australis from the salt marsh in the Yellow River Delta, China

The common reed (Phragmites australis) is a dominant species in the coastal wetlands of the Chinese Yellow River Delta, where it tolerates a wide range of salinity. Recent environmental changes have led to the increase of soil salinity in this region, which has degraded much of the local vegetation. Clones of common reeds from the tidal marsh may have adapted to local high salinity habitat through selection on genes and metabolic pathways conferring salt tolerance. This study aims to reveal molecular mechanisms underlying salt tolerance in the tidal reed by comparing them to the salt-sensitive freshwater reed under salt stress. We employed comparative transcriptomics to reveal the differentially expressed genes (DEGs) between these two types of common reeds under different salinity conditions. The results showed that only three co-expressed genes were up-regulated and one co-expressed gene was down-regulated between the two reed types. On the other hand, 1,371 DEGs were exclusively up-regulated and 285 DEGs were exclusively down-regulated in the tidal reed compared to the control, while 115 DEGs were exclusively up-regulated and 118 DEGs were exclusively down-regulated in the freshwater reed compared to the control. From the pattern of enrichment of transcripts involved in salinity response, the tidal reed was more active and efficient in scavenging reactive oxygen species (ROS) than the freshwater reed, with the tidal reed showing significantly higher gene expression in oxidoreductase activity. Furthermore, when the reeds were exposed to salt stress, transcripts encoding glutathione metabolism were up-regulated in the tidal reed but not in the freshwater reed. DEGs related to encoding glutathione reductase (GR), glucose-6-phosphate 1-dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PD), glutathione S-transferase (GST) and L-ascorbate peroxidase (LAP) were revealed as especially highly differentially regulated and therefore represented candidate genes that could be cloned into plants to improve salt tolerance. Overall, more genes were up-regulated in the tidal reed than in the freshwater reed from the Yellow River Delta when under salt stress. The tidal reed efficiently resisted salt stress by up-regulating genes encoding for oxidoreductase activity and glutathione metabolism. We suggest that this type of common reed could be extremely useful in the ecological restoration of degraded, high salinity coastal wetlands in priority.

Identification of the Cytosolic Glucose-6-Phosphate Dehydrogenase Gene from Strawberry Involved in Cold Stress Response

Glucose-6-phosphate dehydrogenase (G6PDH) plays an important role in plant stress responses. Here, five FaG6PDH sequences were obtained in strawberry, designated as FaG6PDH-CY, FaG6PDH-P1, FaG6PDH-P1.1, FaG6PDH-P2 and FaG6PDH-P0, which were divided into cytosolic (CY) and plastidic (P) isoforms based on the bioinformatic analysis. The respective FaG6PDH genes had distinct expression patterns in all tissues and at different stages of fruit development. Notably, FaG6PDH-CY was the most highly expressed gene among five FaG6PDH members, indicating it encoded the major G6PDH isoform throughout the plant. FaG6PDH positively regulated cold tolerance in strawberry. Inhibition of its activity gave rise to greater cold-induced injury in plant. The FaG6PDH-CY transcript had a significant increase under cold stress, similar to the G6PDH enzyme activity, suggesting a principal participant in response to cold stress. Further study showed that the low-temperature responsiveness (LTR) element in FaG6PDH-CY promoter can promote the gene expression when plant encountered cold stimuli. Besides, FaG6PDH-CY was involved in regulating cold-induced activation of antioxidant enzyme genes (FaSOD, FaCAT, FaAPX and FaGR) and RBOH-dependent ROS generation. The elevated FaG6PDH-CY enhanced ROS-scavenging capability of antioxidant enzymes to suppress ROS excessive accumulation and relieved the oxidative damage, eventually improving the strawberry resistance to cold stress.

The fungal endophyte Epichloë gansuensis increases NaCl-tolerance in Achnatherum inebrians through enhancing the activity of plasma membrane H+-ATPase and glucose-6-phosphate dehydrogenase

Salt stress negatively affects plant growth, and the fungal endophyte Epichloëgansuensis increases the tolerance of its host grass species, Achnatherum inebrians, to abiotic stresses. In this work, we first evaluated the effects of E. gansuensis on glucose-6-phosphate dehydrogenase (G6PDH) and plasma membrane (PM) H⁺-ATPase activity of Achnatherum inebrians plants under varying NaCl concentrations. Our results showed that the presence of E. gansuensis increased G6PDH, PM H⁺-ATPase, superoxide dismutase and catalase activity to decrease O₂^•-, H₂O₂ and Na⁺ contents in A. inebrians under NaCl stress, resulting in enhanced salt tolerance. In addition, the PM NADPH oxidase activity and NADPH/NADP⁺ ratios were all lower in A. inebrians with E. ganusensis plants than A. inebrians plants without this endophyte under NaCl stress. In conclusion, E. gansuensis has a positive role in improving host grass yield under NaCl stress by enhancing the activity of G6PDH and PM H⁺-ATPase to decrease ROS content. This provides a new way for the selection of stress-resistant and high-quality forage varieties by the use of systemic fungal endophytes.

Functional and structural analysis of double and triple mutants reveals the contribution of protein instability to clinical manifestations of G6PD variants

Glucose-6-phosphate dehydrogenase (G6PD) deficiency is the most common polymorphism and enzymopathy in humans, affecting approximately 400 million people worldwide. Over 200 point mutations have been identified in g6pd and the molecular mechanisms underlying the severity of G6PD variants differ. We report the detailed functional and structural characterization of 11 recombinant human G6PD variants: G6PD Asahi, G6PD A, G6PD Guadalajara, G6PD Acrokorinthos, G6PD Ananindeua, G6PD A-(202), G6PD Sierra Leone, G6PD A-(680), G6PD A-(968), G6PD Mount Sinai and G6PD No name. G6PD Guadalajara, G6PD Mount Sinai and G6PD No name are inactive variants and, correlating with the observed clinical manifestations, exhibit complete loss of enzyme activity. Protein structural instability, causing a reduction in catalytic efficiency, contributes to the clinical phenotypes of all variants. In double and triple mutants sharing the G6PD A mutation, we observed cooperative interaction between two and three mutations to cause protein dysfunction. The G6PD A (Asn126Asp) mutation exhibits no effect on protein activity and stability, indicating that the additional mutations in these G6PD variants significantly contribute to enzyme deficiency. We provide insight into the molecular basis of G6PD deficiency, which can explain the severity of clinical manifestations observed in individuals with G6PD deficiency.

Selenium modulates cadmium-induced ultrastructural and metabolic changes in cucumber seedlings

Intensive insight into the potential mechanisms of Se-induced Cd tolerance in cucumber seedlings is essential for further improvement of vegetable crop cultivation and breeding to obtain high yields and quality in Cd-contaminated soil. To reveal the ultrastructural and metabolic differences in Se-induced Cd tolerance, we examined the ultrastructures of chloroplasts and root cells and characterised 155 differentially expressed metabolites under Cd and/or Se stress using gas chromatography-mass spectrometry (GC-MS)-based metabolomics. Exogenous Se greatly relieved Cd-caused injuries to the ultrastructures of cucumber leaves and roots; for example, the shapes of chloroplasts treated with Cd + Se improved or even began to return to normal, the nuclei of root cells began to regenerate better and the chromatin was well-distributed compared with plants treated with Cd alone. Metabolite profiling revealed several intermediates of glycolysis and the tricarboxylic acid (TCA) cycle; also, some amino acids were up-accumulated in Cd + Se-treated cucumber seedlings and down-accumulated in Cd-treated cucumber seedlings, such as pyruvic acid, galactose, lactose, glutaric acid and alanine in leaves, glucose-6-phosphate and serine in roots, and lactic acid and glycine in both leaves and roots. These metabolites may play dominant roles in developing Se-mediated Cd tolerance. Moreover, a high level of sugars and polyols, amino acids and organic acids were up-accumulated in Cd-treated plants. Meanwhile, our data suggest that high accumulation of fructose, α-ketoglutaric acid, shikimic acid, fumaric acid and succinic acid in roots is a Cd-specific response, indicating that these metabolites are vital for cucumbers to develop Cd resistance. This study extends the current understanding of the mechanisms of Se in abating Cd contamination in cucumber and demonstrates that metabolomics profiling provides a more comprehensive view of the response of plants to heavy metals.

Intensive insight into the potential mechanisms of Se-induced Cd tolerance in cucumber seedlings is essential for further improvement of vegetable crop cultivation and breeding to obtain high yields and quality in Cd-contaminated soil.

Involvement of active MKK9-MAPK3/MAPK6 in increasing respiration in salt-treated Arabidopsis callus

Mitogen-activated protein kinase kinase 9 (MKK9) is an upstream activator of mitogen-activated protein kinase 3 (MAPK3) and MAPK6 in planta. To investigate MKK9 roles in mitochondrial respiration in Arabidopsis, MKK9^DD, the active allele with mutations of Thr-201 and Ser-205 to Asp, and MKK9^KR, the allele lacking MKK9 activity with a mutation of Lys-76 to Arg, were used. Results showed that the total respiratory rate (V_t), alternative pathway capacity (V_alt) and cytochrome pathway capacity (V_cyt) increased under 0-100 mM NaCl treatments but decreased under 150-300 mM NaCl treatments in Col-0 callus. However, the activation of MKK9 by dexamethasone (DEX) increased V_t, V_alt and V_cyt under 200 mM NaCl treatment; moreover, V_alt showed more increase than V_cyt. The activation of MKK9 in MKK9^DD callus sharply increased AOX protein expression under normal and NaCl conditions, but the increase was not observed in MKK9^KR callus. Further results indicated that MAPK3 and MAPK6 were involved in the MKK9-induced increase of AOX protein levels. qRT-PCR results showed that MKK9-MAPK3/MAPK6 was involved in the NaCl-induced AOX1b and AOX1d expression, but only MKK9-MAPK3 was necessary for AOX2 expression; in addition, MAPK3 regulated the AOX1a transcription in an MKK9-independent manner. MKK9 positively regulated SOD and CAT activities by affecting MAPK3 and MAPK6 and negatively regulated APX and POD activities by affecting MAPK3. Moreover, MKK9 functions as a positive factor in H₂O₂ accumulation under salt stress. The regulation of ethylene on alternative respiration was also associated with MKK9 under salt stress. Taken together, the MKK9-MAPK3/MAPK6 pathway plays a pivotal role in increasing alternative respiration in the salt-treated Arabidopsis callus.

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.

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.

Involvement of G6PD5 in ABA response during seed germination and root growth in Arabidopsis

BACKGROUND

Glucose-6-phosphate dehydrogenase (G6PDH or G6PD) functions in supply of NADPH, which is required for plant defense responses to stresses. However, whether G6PD functions in the abscisic acid (ABA) signaling pathway remains to be elucidated. In this study, we investigated the involvement of the cytosolic G6PD5 in the ABA signaling pathway in Arabidopsis.

RESULTS

We characterized the Arabidopsis single null mutant g6pd5. Phenotypic analysis showed that the mutant is more sensitive to ABA during seed germination and root growth, whereas G6PD5-overexpressing plants are less sensitive to ABA compared to wild type (WT). Furthermore, ABA induces excessive accumulation of reactive oxygen species (ROS) in mutant seeds and seedlings. G6PD5 participates in the reduction of H₂O₂ to H₂O in the ascorbate-glutathione cycle. In addition, we found that G6PD5 suppressed the expression of Abscisic Acid Insensitive 5 (ABI5), the major ABA signaling component in dormancy control. When G6PD5 was overexpressed, the ABA signaling pathway was inactivated. Consistently, G6PD5 negatively modulates ABA-blocked primary root growth in the meristem and elongation zones. Of note, the suppression of root elongation by ABA is triggered by the cell cycle B-type cyclin CYCB1.

CONCLUSIONS

This study showed that G6PD5 is involved in the ABA-mediated seed germination and root growth by suppressing ABI5.

Mechanism(s) of action of heavy metals to investigate the regulation of plastidic glucose-6-phosphate dehydrogenase

The regulation of recombinant plastidic glucose-6P dehydrogenase from Populus trichocarpa (PtP2-G6PDH - EC 1.1.1.49) was investigated by exposing wild type and mutagenized isoforms to heavy metals. Nickel and Cadmium caused a marked decrease in PtP2-G6PDH WT activity, suggesting their poisoning effect on plant enzymes; Lead (Pb⁺⁺) was substantially ineffective. Copper (Cu⁺⁺) and Zinc (Zn⁺⁺) exposition resulted in strongest decrease in enzyme activity, thus suggesting a physiological competition with Magnesium, a well-known activator of G6PDH activity. Kinetic analyses confirmed a competitive inhibition by Copper, and a mixed inhibition by (Cd⁺⁺). Mutagenized enzymes were differently affected by HMs: the reduction of disulfide (C¹⁷⁵–C¹⁸³) exposed the NADP⁺ binding sites to metals; C¹⁴⁵ participates to NADP⁺ cofactor binding; C¹⁹⁴ and C²⁴² are proposed to play a role in the regulation of NADP⁺/NADPH binding. Copper (and possibly Zinc) is able to occupy competitively Magnesium (Mg⁺⁺) sites and/or bind to NADP⁺, resulting in a reduced access of NADP⁺ sites on the enzyme. Hence, heavy metals could be used to describe specific roles of cysteine residues present in the primary protein sequence; these results are discussed to define the biochemical mechanism(s) of inhibition of plant plastidic G6PDH.

Luxurious Nitrogen Fertilization of Two Sugar Cane Genotypes Contrasting for Lignin Composition Causes Changes in the Stem Proteome Related to Carbon, Nitrogen, and Oxidant Metabolism but Does Not Alter Lignin Content

Sugar cane is an important crop for sugar and biofuel production. Its lignocellulosic biomass represents a promising option as feedstock for second-generation ethanol production. Nitrogen fertilization can affect differently tissues and its biopolymers, including the cell-wall polysaccharides and lignin. Lignin content and composition are the most important factors associated with biomass recalcitrance to convert cell-wall polysaccharides into fermentable sugars. Thus it is important to understand the metabolic relationship between nitrogen fertilization and lignin in this feedstock. In this study, a large-scale proteomics approach based on GeLC-MS/MS was employed to identify and relatively quantify proteins differently accumulated in two contrasting genotypes for lignin composition after excessive nitrogen fertilization. From the ∼1000 nonredundant proteins identified, 28 and 177 were differentially accumulated in response to nitrogen from IACSP04-065 and IACSP04-627 lines, respectively. These proteins were associated with several functional categories, including carbon metabolism, amino acid metabolism, protein turnover, and oxidative stress. Although nitrogen fertilization has not changed lignin content, phenolic acids and lignin composition were changed in both species but not in the same way. Sucrose and reducing sugars increased in plants of the genotype IACSP04-065 receiving nitrogen.

Overexpression of E3 Ubiquitin Ligase Gene AdBiL Contributes to Resistance against Chilling Stress and Leaf Mold Disease in Tomato

Ubiquitination is a common regulatory mechanism, playing a critical role in diverse cellular and developmental processes in eukaryotes. However, a few reports on the functional correlation between E3 ubiquitin ligases and reactive oxygen species (ROS) or reactive nitrogen species (RNS) metabolism in response to stress are currently available in plants. In the present study, the E3 ubiquitin ligase gene AdBiL (Adi3 Binding E3 Ligase) was introduced into tomato line Ailsa Craig via Agrobacterium-mediated method. Transgenic lines were confirmed for integration into the tomato genome using PCR. Transcription of AdBiL in various transgenic lines was determined using real-time PCR. Evaluation of stress tolerance showed that T₁ generation of transgenic tomato lines showed only mild symptoms of chilling injury as evident by higher biomass accumulation and chlorophyll content than those of non-transformed plants. Compared with wild-type plants, the contents of AsA, AsA/DHA, GSH and the activity of GaILDH, γ-GCS and GSNOR were increased, while H₂O₂, [Formula: see text], MDA, NO, SNOs, and GSNO accumulations were significantly decreased in AdBiL overexpressing plants in response to chilling stress. Furthermore, transgenic tomato plants overexpressing AdBiL showed higher activities of enzymes such as G6PDH, 6PGDH, NADP-ICDH, and NADP-ME involved in pentose phosphate pathway (PPP). The transgenic tomato plants also exhibited an enhanced tolerance against the necrotrophic fungus Cladosporium fulvum. Tyrosine nitration protein was activated in the plants infected with leaf mold disease, while the inhibition could be recovered in AdBiL gene overexpressing lines. Taken together, our results revealed a possible physiological role of AdBiL in the activation of the key enzymes of AsA-GSH cycle, PPP and down-regulation of GSNO reductase, thereby reducing oxidative and nitrosative stress in plants. This study demonstrates an optimized transgenic strategy using AdBiL gene for crop improvement against biotic and abiotic stress factors.

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.

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.

A comparative analysis of transcriptomic, biochemical, and physiological responses to elevated ozone identifies species-specific mechanisms of resilience in legume crops

Current concentrations of tropospheric ozone ([O3]) pollution negatively impact plant metabolism, which can result in decreased crop yields. Interspecific variation in the physiological response of plants to elevated [O3] exists; however, the underlying cellular responses explaining species-specific differences are largely unknown. Here, a physiological screen has been performed on multiple varieties of legume species. Three varieties of garden pea (Pisum sativum L.) were resilient to elevated [O3]. Garden pea showed no change in photosynthetic capacity or leaf longevity when exposed to elevated [O3], in contrast to varieties of soybean (Glycine max (L.) Merr.) and common bean (Phaseolus vulgaris L.). Global transcriptomic and targeted biochemical analyses were then done to examine the mechanistic differences in legume responses to elevated [O3]. In all three species, there was an O3-mediated reduction in specific leaf weight and total non-structural carbohydrate content, as well as increased abundance of respiration-related transcripts. Differences specific to garden pea included a pronounced increase in the abundance of GLUTATHIONE REDUCTASE transcript, as well as greater contents of foliar glutathione, apoplastic ascorbate, and sucrose in elevated [O3]. These results suggest that garden pea may have had greater capacity for detoxification, which prevented net losses in CO2 fixation in an elevated [O3] environment.

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.

Expression and characterization of a cytosolic glucose 6 phosphate dehydrogenase isoform from barley (Hordeum vulgare) roots

In plant cells, glucose 6 phosphate dehydrogenase (G6PDH-EC 1.1.1.49) regulates the oxidative pentose phosphate pathway (OPPP), a metabolic route involved in the production of NADPH for various biosynthetic processes and stress response. In this study, we report the overexpression of a cytosolic G6PDH isoform from barley (Hordeum vulgare) roots in bacteria, and the biochemical characterization of the purified recombinant enzyme (HvCy-G6PDH). A full-length cDNA coding for a cytosolic isoform of G6PDH was isolated, and the sequence was cloned into pET3d vector; the protein was overexpressed in Escherichia coli BL21 (DE3) and purified by anion exchange and affinity chromatography. The kinetic properties were calculated: the recombinant HvCy-G6PDH showed KMs and KINADPH comparable to those observed for the enzyme purified from barley roots; moreover, the analysis of NADPH inhibition suggested a competitive mechanism. Therefore, this enzyme could be utilised for the structural and regulatory characterization of this isoform in higher plants.

Transcriptomic analysis of the primary roots of Alhagi sparsifolia in response to water stress

BACKGROUND

Alhagi sparsifolia is a typical desert phreatophyte and has evolved to withstand extreme dry, cold and hot weather. While A. sparsifolia represents an ideal model to study the molecular mechanism of plant adaption to abiotic stress, no research has been done in this aspect to date. Here we took advantage of Illumina platform to survey transcriptome in primary roots of A. sparsifolia under water stress conditions in aim to facilitate the exploration of its genetic basis for drought tolerance.

METHODOLOGY AND PRINCIPAL FINDINGS

We sequenced four primary roots samples individually collected at 0, 6, 24 and 30h from the A. sparsifolia seedlings in the course of 24h of water stress following 6h of rehydration. The resulting 38,763,230, 67,511,150, 49,259,804 and 54,744,906 clean reads were pooled and assembled into 33,255 unigenes with an average length of 1,057 bp. All-unigenes were subjected to functional annotation by searching against the public databases. Based on the established transcriptome database, we further evaluated the gene expression profiles in the four different primary roots samples, and identified numbers of differently expressed genes (DEGs) reflecting the early response to water stress (6h vs. 0h), the late response to water stress (24h vs. 0h) and the response to post water stress rehydration (30h vs. 24h). Moreover, the DEGs specifically regulated at 6, 24 and 30h were captured in order to depict the dynamic changes of gene expression during water stress and subsequent rehydration. Functional categorization of the DEGs indicated the activation of oxidoreductase system, and particularly emphasized the significance of the 'Glutathione metabolism pathway' in response to water stress.

CONCLUSIONS

This is the first description of the genetic makeup of A. sparsifolia, thus providing a substantial contribution to the sequence resources for this species. The identified DEGs offer a deep insight into the molecular mechanism of A. sparsifolia in response to water stress, and merit further investigation.

A cytosolic glucose-6-phosphate dehydrogenase gene, ScG6PDH, plays a positive role in response to various abiotic stresses in sugarcane

As one of the key enzymes in the oxidative pentose phosphate pathway, glucose-6-phosphate dehydrogenase (G6PDH) plays a role in response to abiotic stresses and pathogenesis. Here, a full-length cDNA was obtained, designed as ScG6PDH from sugarcane. The ScG6PDH gene is 1,646 bp long with a 1,524-bp long ORF encoding 507 amino acid residues. Analysis of a phylogenetic tree indicated that this gene is a member of the cytosolic G6PDH gene family, which is consistent with results from a subcellular localization experiment. Based on a real-time quantitative RT-PCR performed under salt, drought, heavy metal (CdCl₂) and low temperature (4°C) treatments, the transcription levels of the ScG6PDH gene were higher compared with transcription levels where these treatments were not imposed, suggesting a positive response of this gene to these environmental stresses. Furthermore, G6PDH activity was stimulated under 4°C, CdCl₂, NaCl and PEG treatments, but the increments varied with treatment and sampling time, implying positive response to abiotic stresses, similar to the transcript of the G6PDH gene. Ion conductivity measurements and a histochemical assay provided indirect evidence of the involvement of the ScG6PDH gene in defense reactions to the above-mentioned abiotic stresses.

Hydrogen sulfide is involved in maintaining ion homeostasis via regulating plasma membrane Na+/H+ antiporter system in the hydrogen peroxide-dependent manner in salt-stress Arabidopsis thaliana root

Hydrogen sulfide (H2S) and hydrogen peroxide (H2O2) function as the signaling molecules in plants responding to salt stresses. The present study presents a signaling network involving H2S and H2O2 in salt resistance pathway of the Arabidopsis root. Arabidopsis roots were sensitive to 100 mM NaCl treatment, which displayed a great increase in electrolyte leakage (EL) and Na(+)/K(+) ratio under salt stress. The treatment of H2S donors sodium hydrosulfide (NaHS) enhanced the salt tolerance by maintaining a lower Na(+)/K(+) ratio. In addition, the inhibition of root growth under salt stress was removed by H2S. Further studies indicated that H2O2 was involved in H2S-induced salt tolerance pathway. H2S induced the production of the endogenous H2O2 via regulating the activities of glucose-6-phosphate dehydrogenase (G6PDH) and plasma membrane (PM) NADPH oxidase, with the treatment with dimethylthiourea (DMTU, an ROS scavenger), diphenylene iodonium (DPI, a PM NADPH oxidase inhibitor), or glycerol (G6PDH inhibitor) removing the effect of H2S. Treatment with amiloride (an inhibitor of PM Na(+)/H(+) antiporter) and vanadate (an inhibitor of PM H(+)-ATPase) also inhibited the activity of H2S on Na(+)/K(+) ratio. Through an analysis of quantitative real-time polymerase chain reaction and Western blot, we found that H2S promoted the genes expression and the phosphorylation level of PM H(+)-ATPase and Na(+)/H(+) antiporter protein level. However, when the endogenous H2O2 level was inhibited by DPI or DMTU, the effect of H2S on the PM Na(+)/H(+) antiporter system was removed. Taken together, H2S maintains ion homeostasis in the H2O2-dependent manner in salt-stress Arabidopsis root.

Overexpression, purification and enzymatic characterization of a recombinant plastidial glucose-6-phosphate dehydrogenase from barley (Hordeum vulgare cv. Nure) roots

In plant cells, the plastidial glucose 6-phosphate dehydrogenase (P2-G6PDH, EC 1.1.1.49) represents one of the most important sources of NADPH. However, previous studies revealed that both native and recombinant purified P2-G6PDHs show a great instability and a rapid loss of catalytic activity. Therefore it has been difficult to describe accurately the catalytic and physico-chemical properties of these isoforms. The plastidial G6PDH encoding sequence from barley roots (Hordeum vulgare cv. Nure), devoid of a long plastidial transit peptide, was expressed as recombinant protein in Escherichia coli, either untagged or with an N-terminal his-tag. After purification from both the soluble fraction and inclusion bodies, we have explored its kinetic parameters, as well as its sensitivity to reduction. The obtained results are consistent with values determined for other P2-G6PDHs previously purified from barley roots and from other land plants. Overall, these data shed light on the catalytic mechanism of plant P2-G6PDH, summarized in a proposed model in which the sequential mechanism is very similar to the mammalian cytosolic G6PDH. This study provides a rational basis to consider the recombinant barley root P2-G6PDH as a good model for further kinetic and structural studies.

Glucose-6-phosphate dehydrogenase plays a pivotal role in tolerance to drought stress in soybean roots

KEY MESSAGE : Two soybean cultivars showed markedly different drought tolerance. G6PDH plays a central role in the process of H ( 2 ) O ( 2 ) regulated GR, DHAR, and MDHAR activities to maintain GSH and Asc levels. Glucose-6-phosphate dehydrogenase (G6PDH) plays a pivotal role in plant resistance to environmental stresses. In this study, we investigated the role of G6PDH in modulating redox homeostasis under drought stress induced by polyethylene glycol 6000 (PEG6000) in two soybean cultivars JINDOU21 (JD-21) and WDD00172 (WDD-172). The G6PDH activity markedly increased and reached a maximum at 96 h in JD-21 and 72 h in WDD-172 during PEG6000 treatments, respectively. Glucosamine (Glucm, a G6PDH inhibitor) obviously inhibited G6PDH activity in both soybeans under PEG6000 treatments. After PEG6000 treatment, JD-21 showed higher tolerance than WDD-172 not only in higher activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione reductase (GR), dehydroascorbate reductase (DHAR), and monodehydroascorbate reductase (MDHAR), but also in higher content of glutathione (GSH) and ascorbate (Asc). And we found that hydrogen peroxide (H(2)O(2)) regulated the cell length in root elongation zone. Diphenylene iodonium (DPI, a plasma membrane NADPH oxidase inhibitor) counteracted the PEG6000-induced H(2)O(2) accumulation and decreased the activities of GR, DHAR, and MDHAR as well as GSH and Asc content. Furthermore, exogenous application of H(2)O(2) increased the GR, DHAR, and MDHAR activities that were decreased by Glucm under drought stress. Western blot analysis showed that the G6PDH expression was stimulated by PEG6000 and buthionine sulfoximine (BSO, glutathione biosynthesis inhibitor), and blocked by Glucm, DPI and N-acetyl-L-cysteine (NAC, GSH precursor) in both cultivars. Taken together, our evidence indicates that G6PDH plays a central role in the process of H(2)O(2) regulated GR, DHAR, and MDHAR activities to maintain GSH and Asc levels.

Tissue metabolic responses to salt stress in wild and cultivated barley

A thorough understanding of the mechanisms underlying barley salt tolerance and exploitation of elite genetic resource are essential for utilizing wild barley germplasm in developing barley varieties with salt tolerance. In order to reveal the physiological and molecular difference in salt tolerance between Tibetan wild barley (Hordeum spontaneum) and cultivated barley (Hordeum vulgare), profiles of 82 key metabolites were studies in wild and cultivated barley in response to salinity. According to shoot dry biomass under salt stress, XZ16 is a fast growing and salt tolerant wild barley. The results of metabolite profiling analysis suggested osmotic adjustment was a basic mechanism, and polyols played important roles in developing salt tolerance only in roots, and high level of sugars and energy in roots and active photosynthesis in leaves were important for barley to develop salt tolerance. The metabolites involved in tolerance enhancement differed between roots and shoots, and also between genotypes. Tibetan wild barley, XZ16 had higher chlorophyll content and higher contents of compatible solutes than CM72, while the cultivated barley, CM72 probably enhanced its salt tolerance mainly through increasing glycolysis and energy consumption, when the plants were exposed to high salinity. The current research extends our understanding of the mechanisms involved in barley salt tolerance and provides possible utilization of Tibetan wild barley in developing barley cultivars with salt tolerance.

NADP-dependent isocitrate dehydrogenase from Arabidopsis roots contributes in the mechanism of defence against the nitro-oxidative stress induced by salinity

NADPH regeneration appears to be essential in the mechanism of plant defence against oxidative stress. Plants contain several NADPH-generating dehydrogenases including isocitrate dehydrogenase (NADP-ICDH), glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME). In Arabidopsis seedlings grown under salinity conditions (100 mM NaCl) the analysis of physiological parameters, antioxidant enzymes (catalase and superoxide dismutase) and content of superoxide radical (O₂^∙−), nitric oxide (NO), and peroxynitrite (ONOO⁻) indicates a process of nitro-oxidative stress induced by NaCl. Among the analysed NADPH-generating dehydrogenases under salinity conditions, the NADP-ICDH showed the maximum activity mainly attributable to the root NADP-ICDH. Thus, these data provide new insights on the relevance of the NADP-ICDH which could be considered as a second barrier in the mechanism of response against the nitro-oxidative stress generated by salinity.

Glucose-6-Phosphate Dehydrogenase Regulation in Anoxia Tolerance of the Freshwater Crayfish Orconectes virilis

Glucose-6-phosphate dehydrogenase (G6PDH), the enzyme which catalyzes the rate determining step of the pentose phosphate pathway (PPP), controls the production of nucleotide precursor molecules (R5P) and powerful reducing molecules (NADPH) that support multiple biosynthetic functions, including antioxidant defense. G6PDH from hepatopancreas of the freshwater crayfish (Orconectes virilis) showed distinct kinetic changes in response to 20 h anoxic exposure. K(m) values for both substrates decreased significantly in anoxic crayfish; K(m) NADP(+) dropped from 0.015 ± 0.008 mM to 0.012 ± 0.008 mM, and K(m) G6P decreased from 0.13 ± 0.02 mM to 0.08 ± 0.007 mM. Two lines of evidence indicate that the mechanism involved is reversible phosphorylation. In vitro incubations that stimulated protein kinase or protein phosphatase action mimicked the effects on anoxia on K(m) values, whereas DEAE-Sephadex chromatography showed the presence of two enzyme forms (low- and high-phosphate) whose proportions changed during anoxia. Incubation studies implicated protein kinase A and G in mediating the anoxia-responsive changes in G6PDH kinetic properties. In addition, the amount of G6PDH protein (measured by immunoblotting) increased by ∼60% in anoxic hepatopancreas. Anoxia-induced phosphorylation of G6PDH could contribute to modifying carbon flow through the PPP under anoxic conditions, potentially maintaining NADPH supply for antioxidant defense during prolonged anoxia-induced hypometabolism.

cGMP regulates hydrogen peroxide accumulation in calcium-dependent salt resistance pathway in Arabidopsis thaliana roots

3',5'-cyclic guanosine monophosphate (cGMP) is an important second messenger in plants. In the present study, roles of cGMP in salt resistance in Arabidopsis roots were investigated. Arabidopsis roots were sensitive to 100 mM NaCl treatment, displaying a great increase in electrolyte leakage and Na(+)/K(+) ratio and a decrease in gene expression of the plasma membrane (PM) H(+)-ATPase. However, application of exogenous 8Br-cGMP (an analog of cGMP), H(2)O(2) or CaCl(2) alleviated the NaCl-induced injury by maintaining a lower Na(+)/K(+) ratio and increasing the PM H(+)-ATPase gene expression. In addition, the inhibition of root elongation and seed germination under salt stress was removed by 8Br-cGMP. Further study indicated that 8Br-cGMP-induced higher NADPH levels for PM NADPH oxidase to generate H(2)O(2) by regulating glucose-6-phosphate dehydrogenase (G6PDH) activity. The effect of 8Br-cGMP and H(2)O(2) on ionic homeostasis was abolished when Ca(2+) was eliminated by glycol-bis-(2-amino ethyl ether)-N,N,N',N'-tetraacetic acid (EGTA, a Ca(2+) chelator) in Arabidopsis roots under salt stress. Taken together, cGMP could regulate H(2)O(2) accumulation in salt stress, and Ca(2+) was necessary in the cGMP-mediated signaling pathway. H(2)O(2), as the downstream component of cGMP signaling pathway, stimulated PM H(+)-ATPase gene expression. Thus, ion homeostasis was modulated for salt tolerance.

Abscisic acid effects on activity and expression of barley (Hordeum vulgare) plastidial glucose-6-phosphate dehydrogenase

Total glucose-6-phosphate dehydrogenase (G6PDH) activity, protein abundance, and transcript levels of G6PDH isoforms were measured in response to exogenous abscisic acid (ABA) supply to barley (Hordeum vulgare cv Nure) hydroponic culture. Total G6PDH activity increased by 50% in roots treated for 12 h with exogenous 0.1 mM ABA. In roots, a considerable increase (35%) in plastidial P2-G6PDH transcript levels was observed during the first 3 h of ABA treatment. Similar protein variations were observed in immunoblotting analyses. In leaves, a 2-fold increase in total G6PDH activity was observed after ABA treatment, probably related to an increase in the mRNA level (increased by 50%) and amount of protein (increased by 85%) of P2-G6PDH. Together these results suggest that the plastidial P2-isoform plays an important role in ABA-treated barley plants.

Glucose-6-phosphate dehydrogenase-dependent hydrogen peroxide production is involved in the regulation of plasma membrane H+-ATPase and Na+/H+ antiporter protein in salt-stressed callus from Carex moorcroftii

Glucose-6-phosphate dehydrogenase (G6PDH) is important for the activation of plant resistance to environmental stresses, and ion homeostasis is the physiological foundation for living cells. In this study, we investigated G6PDH roles in modulating ion homeostasis under salt stress in Carex moorcroftii callus. G6PDH activity increased to its maximum in 100 mM NaCl treatment and decreased with further increased NaCl concentrations. K+/Na+ ratio in 100 mM NaCl treatment did not exhibit significant difference compared with the control; however, in 300 mM NaCl treatment, it decreased. Low-concentration NaCl (100 mM) stimulated plasma membrane (PM) H+-ATPase and NADPH oxidase activities as well as Na+/H+ antiporter protein expression, whereas high-concentration NaCl (300 mM) decreased their activity and expression. When G6PDH activity and expression were reduced by glycerol treatments, PM H+-ATPase and NADPH oxidase activities, Na+/H+ antiporter protein level and K+/Na+ ratio dramatically decreased. Simultaneously, NaCl-induced hydrogen peroxide (H₂O₂) accumulation was abolished. Exogenous application of H₂O₂ increased G6PDH, PM H+-ATPase and NADPH oxidase activities, Na+/H+ antiporter protein expression and K+/Na+ ratio in the control and glycerol treatments. Diphenylene iodonium (DPI), the NADPH oxidase inhibitor, which counteracted NaCl-induced H₂O₂ accumulation, decreased G6PDH, PM H+-ATPase and NADPH oxidase activities, Na+/H+ antiporter protein level and K+/Na+ ratio. Western blot result showed that G6PDH expression was stimulated by NaCl and H₂O₂, and blocked by DPI. Taken together, G6PDH is involved in H₂O₂ accumulation under salt stress. H₂O₂, as a signal, upregulated PM H+-ATPase activity and Na+/H+ antiporter protein level, which subsequently resulted in the enhanced K+/Na+ ratio. G6PDH played a central role in the process.