Ascorbic acid metabolism and functions

This Special Issue was assembled to mark the 25th anniversary of the proposal of the d -mannose/ l -galactose (Smirnoff-Wheeler) ascorbate biosynthesis pathway in plants ( Wheeler et al., 1998 ). The issue aims to assess the current state of knowledge and to identify outstanding questions about ascorbate metabolism and functions in plants.

Ascorbate peroxidase plays an important role in photoacclimation in the extremophilic red alga Cyanidiococcus yangmingshanensis

Introduction

Acidothermophilic cyanidiophytes in natural habitats can survive under a wide variety of light regimes, and the exploration and elucidation of their long-term photoacclimation mechanisms promises great potential for further biotechnological applications. Ascorbic acid was previously identified as an important protectant against high light stress in Galdieria partita under mixotrophic conditions, yet whether ascorbic acid and its related enzymatic reactive oxygen species (ROS) scavenging system was crucial in photoacclimation for photoautotrophic cyanidiophytes was unclear.

Methods

The significance of ascorbic acid and related ROS scavenging and antioxidant regenerating enzymes in photoacclimation in the extremophilic red alga Cyanidiococcus yangmingshanensis was investigated by measuring the cellular content of ascorbic acid and the activities of ascorbate-related enzymes.

Results and discussion

Accumulation of ascorbic acid and activation of the ascorbate-related enzymatic ROS scavenging system characterized the photoacclimation response after cells were transferred from a low light condition at 20 μmol photons m^-2 s^-1 to various light conditions in the range from 0 to 1000 μmol photons m^-2 s^-1. The activity of ascorbate peroxidase (APX) was most remarkably enhanced with increasing light intensities and illumination periods among the enzymatic activities being measured. Light-dependent regulation of the APX activity was associated with transcriptional regulation of the chloroplast-targeted APX gene. The important role of the APX activity in photoacclimation was evidenced by the effect of the APX inhibitors on the photosystem II activity and the chlorophyll a content under the high light condition at 1000 μmol photons m^-2 s^-1. Our findings provide a mechanistic explanation for the acclimation of C. yangmingshanensis to a wide range of light regimes in natural habitats.

Ascorbate-Glutathione Cycle Genes Families in Euphorbiaceae: Characterization and Evolutionary Analysis

Ascorbate peroxidase (APX), Monodehydroascorbate Reductase (MDAR), Dehydroascorbate Reductase (DHAR) and Glutathione Reductase (GR) enzymes participate in the ascorbate-glutathione cycle, which exerts a central role in the antioxidant metabolism in plants. Despite the importance of this antioxidant system in different signal transduction networks related to development and response to environmental stresses, the pathway has not yet been comprehensively characterized in many crop plants. Among different eudicotyledons, the Euphorbiaceae family is particularly diverse with some species highly tolerant to drought. Here the APX, MDAR, DHAR, and GR genes in Ricinus communis, Jatropha curcas, Manihot esculenta, and Hevea brasiliensis were identified and characterized. The comprehensive phylogenetic and genomic analyses allowed the classification of the genes into different classes, equivalent to cytosolic, peroxisomal, chloroplastic, and mitochondrial enzymes, and revealed the duplication events that contribute to the expansion of these families within plant genomes. Due to the high drought stress tolerance of Ricinus communis, the expression patterns of ascorbate-glutathione cycle genes in response to drought were also analyzed in leaves and roots, indicating a differential expression during the stress. Altogether, these data contributed to the characterization of the expression pattern and evolutionary analysis of these genes, filling the gap in the proposed functions of core components of the antioxidant mechanism during stress response in an economically relevant group of plants.

Distribution and Functions of Monodehydroascorbate Reductases in Plants: Comprehensive Reverse Genetic Analysis of Arabidopsis thaliana Enzymes

Monodehydroascorbate reductase (MDAR) is an enzyme involved in ascorbate recycling. Arabidopsis thaliana has five MDAR genes that encode two cytosolic, one cytosolic/peroxisomal, one peroxisomal membrane-attached, and one chloroplastic/mitochondrial isoform. In contrast, tomato plants possess only three enzymes, lacking the cytosol-specific enzymes. Thus, the number and distribution of MDAR isoforms differ according to plant species. Moreover, the physiological significance of MDARs remains poorly understood. In this study, we classify plant MDARs into three classes: class I, chloroplastic/mitochondrial enzymes; class II, peroxisomal membrane-attached enzymes; and class III, cytosolic/peroxisomal enzymes. The cytosol-specific isoforms form a subclass of class III and are conserved only in Brassicaceae plants. With some exceptions, all land plants and a charophyte algae, Klebsormidium flaccidum, contain all three classes. Using reverse genetic analysis of Arabidopsis thaliana mutants lacking one or more isoforms, we provide new insight into the roles of MDARs; for example, (1) the lack of two isoforms in a specific combination results in lethality, and (2) the role of MDARs in ascorbate redox regulation in leaves can be largely compensated by other systems. Based on these findings, we discuss the distribution and function of MDAR isoforms in land plants and their cooperation with other recycling systems.

Screening and Genetic Network Analysis of Genes Involved in Freezing and Thawing Resistance in DaMDHAR-Expressing Saccharomyces cerevisiae Using Gene Expression Profiling

The cryoprotection of cell activity is a key determinant in frozen-dough technology. Although several factors that contribute to freezing tolerance have been reported, the mechanism underlying the manner in which yeast cells respond to freezing and thawing (FT) stress is not well established. Therefore, the present study demonstrated the relationship between DaMDHAR encoding monodehydroascorbate reductase from Antarctic hairgrass Deschampsia antarctica and stress tolerance to repeated FT cycles (FT2) in transgenic yeast Saccharomyces cerevisiae. DaMDHAR-expressing yeast (DM) cells identified by immunoblotting analysis showed high tolerance to FT stress conditions, thereby causing lower damage for yeast cells than wild-type (WT) cells with empty vector alone. To detect FT2 tolerance-associated genes, 3′-quant RNA sequencing was employed using mRNA isolated from DM and WT cells exposed to FT (FT2) conditions. Approximately 332 genes showed ≥2-fold changes in DM cells and were classified into various groups according to their gene expression. The expressions of the changed genes were further confirmed using western blot analysis and biochemical assay. The upregulated expression of 197 genes was associated with pentose phosphate pathway, NADP metabolic process, metal ion homeostasis, sulfate assimilation, β-alanine metabolism, glycerol synthesis, and integral component of mitochondrial and plasma membrane (PM) in DM cells under FT2 stress, whereas the expression of the remaining 135 genes was partially related to protein processing, selenocompound metabolism, cell cycle arrest, oxidative phosphorylation, and α-glucoside transport under the same condition. With regard to transcription factors in DM cells, MSN4 and CIN5 were activated, but MSN2 and MGA1 were not. Regarding antioxidant systems and protein kinases in DM cells under FT stress, CTT1, GTO, GEX1, and YOL024W were upregulated, whereas AIF1, COX2, and TRX3 were not. Gene activation represented by transcription factors and enzymatic antioxidants appears to be associated with FT2-stress tolerance in transgenic yeast cells. RCK1, MET14, and SIP18, but not YPK2, have been known to be involved in the protein kinase-mediated signalling pathway and glycogen synthesis. Moreover, SPI18 and HSP12 encoding hydrophilin in the PM were detected. Therefore, it was concluded that the genetic network via the change of gene expression levels of multiple genes contributing to the stabilization and functionality of the mitochondria and PM, not of a single gene, might be the crucial determinant for FT tolerance in DaMDAHR-expressing transgenic yeast. These findings provide a foundation for elucidating the DaMDHAR-dependent molecular mechanism of the complex functional resistance in the cellular response to FT stress.

The antioxidant system in Olea europaea to enhanced UV-B radiation also depends on flavonoids and secoiridoids

The Mediterranean crop Olea europaea is often exposed to high UV-B irradiation conditions. To understand how this species modulates its enzymatic and non-enzymatic antioxidant system under high UV-B radiation, young O. europaea plants (cultivar "Galega Vulgar") were exposed, for five days, to UV-B radiation (6.5 kJ m^-2 d^-1 and 12.4 kJ m^-2 d^-1). Our data indicate that UV-doses slightly differ in the modulation of the antioxidant protective mechanisms. Particularly, superoxide dismutase (SOD), guaiacol peroxidase (GPox) and catalase (CAT) activities increased contributing to H₂O₂ homeostasis, being more solicited by higher UV-B doses. Also, glutathione reductase (Gr) activity, ascorbate (AsA) and reduced glutathione (GSH) pools increased particularly under the highest dose, suggesting a higher mobilization of the antioxidant system in this dose. The leaf metabolites' profile of this cultivar was analysed by UHPLC-MS. Interestingly, high levels of verbascoside were found, followed by oleuropein and luteolin-7-O-glucoside. Both UV-B treatments affected mostly less abundant flavonoids (decreasing 4'-methoxy luteolin and 4' or 3'-methoxy luteolin glucoside) and hydroxycinnamic acid derivatives (HCAds, increasing β-hydroxyverbascoside). These changes show not only different mobilization with the UV-intensity, but also reinforce for the first time the protective roles of these minor compounds against UV-B, as reactive oxygen species (ROS) scavengers and UV-B shields, in complement with other antioxidant systems (e.g. AsA/GSH cycle), particularly for high UV-B doses. Secoiridoids also standout in the response to both UV-B doses, with decreases of oleuropein and increases 2''-methoxyoleuropein. Being oleuropein an abundant compound, data suggest that secoiridoids play a more important role than flavonoids and HCAds, in O. europaea protection against UV-B, possibly by acting as signalling molecules and ROS scavengers. This is the first report on the influence of UV-B radiation on the secoiridoid oleuropein, and provides a novel insight to the role of this compound in the O. europaea antioxidant defence mechanisms.

Impact of Nitric Oxide (NO) on the ROS Metabolism of Peroxisomes

Nitric oxide (NO) is a gaseous free radical endogenously generated in plant cells. Peroxisomes are cell organelles characterized by an active metabolism of reactive oxygen species (ROS) and are also one of the main cellular sites of NO production in higher plants. In this mini-review, an updated and comprehensive overview is presented of the evidence available demonstrating that plant peroxisomes have the capacity to generate NO, and how this molecule and its derived products, peroxynitrite (ONOO⁻) and S-nitrosoglutathione (GSNO), can modulate the ROS metabolism of peroxisomes, mainly throughout protein posttranslational modifications (PTMs), including S-nitrosation and tyrosine nitration. Several peroxisomal antioxidant enzymes, such as catalase (CAT), copper-zinc superoxide dismutase (CuZnSOD), and monodehydroascorbate reductase (MDAR), have been demonstrated to be targets of NO-mediated PTMs. Accordingly, plant peroxisomes can be considered as a good example of the interconnection existing between ROS and reactive nitrogen species (RNS), where NO exerts a regulatory function of ROS metabolism acting upstream of H₂O₂.

Redox Regulation of Monodehydroascorbate Reductase by Thioredoxin y in Plastids Revealed in the Context of Water Stress

Thioredoxins (TRXs) are key players within the complex response network of plants to environmental constraints. Here, the physiological implication of the plastidial y-type TRXs in Arabidopsis drought tolerance was examined. We previously showed that TRXs y1 and y2 have antioxidant functions, and here, the corresponding single and double mutant plants were studied in the context of water deprivation. TRX y mutant plants showed reduced stress tolerance in comparison with wild-type (WT) plants that correlated with an increase in their global protein oxidation levels. Furthermore, at the level of the main antioxidant metabolites, while glutathione pool size and redox state were similarly affected by drought stress in WT and trxy1y2 plants, ascorbate (AsA) became more quickly and strongly oxidized in mutant leaves. Monodehydroascorbate (MDA) is the primary product of AsA oxidation and NAD(P)H-MDA reductase (MDHAR) ensures its reduction. We found that the extractable leaf NADPH-dependent MDHAR activity was strongly activated by TRX y2. Moreover, activity of recombinant plastid Arabidopsis MDHAR isoform (MDHAR6) was specifically increased by reduced TRX y, and not by other plastidial TRXs. Overall, these results reveal a new function for y-type TRXs and highlight their role as major antioxidants in plastids and their importance in plant stress tolerance.

Differential antioxidant enzyme activity in rapid-response glyphosate-resistant Ambrosia trifida

BACKGROUND

The giant ragweed (Ambrosia trifida L.) rapid-response (RR) biotype exhibits a sacrificial form of glyphosate resistance whereby an oxidative burst in mature leaves results in foliage loss, while juvenile leaves remain uninjured. This work investigated the safening capacity of antioxidant enzymes in RR juvenile leaves following glyphosate treatment and examined cross tolerance to paraquat.

RESULTS

Basal antioxidant enzyme activities were similar between glyphosate-susceptible (GS) and RR biotypes. Lipid peroxidation was first detected in RR mature leaves at 8 h after treatment (HAT) and by 32 HAT was 5.3 and 21.1 times greater than that in RR juvenile leaves and GS leaves, respectively. Preceding lipid peroxidation in the RR biotype at 2 and 4 HAT, the only increase in enzymatic activity was observed in ascorbate-glutathione cycle enzymes in RR juvenile leaves, particularly ascorbate peroxidase, dehydroascorbate reductase, and glutathione reductase. Sensitivity to paraquat was similar between biotypes.

CONCLUSION

The RR biotype is not inherently more tolerant to oxidative stress. The difference in tissue damage between RR juvenile and mature leaves following glyphosate treatment is attributable at least partially to the transient increase in antioxidant enzyme expression in juvenile leaves (0-8 HAT), but may also be attributable to lower overall RR induction in juvenile leaves compared with mature leaves. © 2018 Society of Chemical Industry.

Ascorbate degradation in tomato leads to accumulation of oxalate, threonate and oxalyl threonate

Ascorbate content in plants is controlled by its synthesis from carbohydrates, recycling of the oxidized forms and degradation. Of these pathways, ascorbate degradation is the least studied and represents a lack of knowledge that could impair improvement of ascorbate content in fruits and vegetables as degradation is non-reversible and leads to a depletion of the ascorbate pool. The present study revealed the nature of degradation products using [¹⁴ C]ascorbate labelling in tomato, a model plant for fleshy fruits; oxalate and threonate are accumulated in leaves, as is oxalyl threonate. Carboxypentonates coming from diketogulonate degradation were detected in relatively insoluble (cell wall-rich) leaf material. No [¹⁴ C]tartaric acid was found in tomato leaves. Ascorbate degradation was stimulated by darkness, and the degradation rate was evaluated at 63% of the ascorbate pool per day, a percentage that was constant and independent of the initial ascorbate or dehydroascorbic acid concentration over periods of 24 h or more. Furthermore, degradation could be partially affected by the ascorbate recycling pathway, as lines under-expressing monodehydroascorbate reductase showed a slight decrease in degradation product accumulation.

Differential molecular response of monodehydroascorbate reductase and glutathione reductase by nitration and S-nitrosylation

The ascorbate-glutathione cycle is a metabolic pathway that detoxifies hydrogen peroxide and involves enzymatic and non-enzymatic antioxidants. Proteomic studies have shown that some enzymes in this cycle such as ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), and glutathione reductase (GR) are potential targets for post-translational modifications (PMTs) mediated by nitric oxide-derived molecules. Using purified recombinant pea peroxisomal MDAR and cytosolic and chloroplastic GR enzymes produced in Escherichia coli, the effects of peroxynitrite (ONOO(-)) and S-nitrosoglutathione (GSNO) which are known to mediate protein nitration and S-nitrosylation processes, respectively, were analysed. Although ONOO(-) and GSNO inhibit peroxisomal MDAR activity, chloroplastic and cytosolic GR were not affected by these molecules. Mass spectrometric analysis of the nitrated MDAR revealed that Tyr213, Try292, and Tyr345 were exclusively nitrated to 3-nitrotyrosine by ONOO(-). The location of these residues in the structure of pea peroxisomal MDAR reveals that Tyr345 is found at 3.3 Å of His313 which is involved in the NADP-binding site. Site-directed mutagenesis confirmed Tyr345 as the primary site of nitration responsible for the inhibition of MDAR activity by ONOO(-). These results provide new insights into the molecular regulation of MDAR which is deactivated by nitration and S-nitrosylation. However, GR was not affected by ONOO(-) or GSNO, suggesting the existence of a mechanism to conserve redox status by maintaining the level of reduced GSH. Under a nitro-oxidative stress induced by salinity (150mM NaCl), MDAR expression (mRNA, protein, and enzyme activity levels) was increased, probably to compensate the inhibitory effects of S-nitrosylation and nitration on the enzyme. The present data show the modulation of the antioxidative response of key enzymes in the ascorbate-glutathione cycle by nitric oxide (NO)-PTMs, thus indicating the close involvement of NO and reactive oxygen species metabolism in antioxidant defence against nitro-oxidative stress situations in plants.

japonica

Monodehydroascorbate reductase (MDHAR; EC 1.6.5.4) is a key enzyme in the reactive oxygen species (ROS) detoxification system of plants. The participation of MDHAR in ascorbate (AsA) recycling in the ascorbate-glutathione cycle is important in the acquired tolerance of crop plants to abiotic environmental stresses. Thus, MDHAR represents a strategic target protein for the improvement of crop yields. Although physiological studies have intensively characterized MDHAR, a structure-based functional analysis is not available. Here, a cytosolic MDHAR (OsMDHAR) derived from Oryza sativa L. japonica was expressed using Escherichia coli strain NiCo21 (DE3) and purified. The purified OsMDHAR showed specific enzyme activity (approximately 380 U per milligram of protein) and was crystallized using the hanging-drop vapour-diffusion method at pH 8.0 and 298 K. The crystal diffracted to 1.9 Å resolution and contained one molecule in the asymmetric unit (the Matthews coefficient VM is 1.98 Å(3) Da(-1), corresponding to a solvent content of 38.06%) in space group P41212 with unit-cell parameters a = b = 81.89, c = 120.4 Å. The phase of the OsMDHAR structure was resolved by the molecular-replacement method using a ferredoxin reductase from Acidovorax sp. strain KKS102 (PDB entry 4h4q) as a model.

Reduced mitochondrial and ascorbate-glutathione activity after artificial ageing in soybean seed

The effect of artificial ageing on the relationship between mitochondrial activities and the antioxidant system was studied in soybean seeds (Glycine max L. cv. Zhongdou No. 27). Ageing seeds for 18d and 41d at 40°C reduced germination from 99% to 52% and 0%, respectively. In comparison to the control, malondialdehyde content and leachate conductivity in aged seeds increased and were associated with membrane damage. Transmission electron microscopy and Percoll density gradient centrifugation showed that aged seeds mainly contained poorly developed mitochondria in which respiration and marker enzymes activities were significantly reduced. Heavy mitochondria isolated from the interface of the 21% and 40% Percoll were analyzed. Mitochondrial antioxidant enzymes activities including superoxide dismutase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase were significantly reduced in aged seeds. A decrease in total ascorbic acid (ASC) and glutathione (GSH) content as well as the reduced/oxidized ratio of ASC and GSH in mitochondria with prolonged ageing showed that artificial ageing reduced ASC-GSH cycle activity. These results suggested an elevated reactive oxygen species (ROS) level in the aged seeds, which was confirmed by measurements of superoxide radical and hydrogen peroxide levels. We conclude that mitochondrial dysfunction in artificially aged seeds is due to retarded mitochondrial and ASC-GSH cycle activity and elevated ROS accumulation.

Reduced mitochondrial and ascorbate-glutathione activity after artificial ageing in soybean seed

The effect of artificial ageing on the relationship between mitochondrial activities and the antioxidant system was studied in soybean seeds (Glycine max L. cv. Zhongdou No. 27). Ageing seeds for 18d and 41d at 40°C reduced germination from 99% to 52% and 0%, respectively. In comparison to the control, malondialdehyde content and leachate conductivity in aged seeds increased and were associated with membrane damage. Transmission electron microscopy and Percoll density gradient centrifugation showed that aged seeds mainly contained poorly developed mitochondria in which respiration and marker enzymes activities were significantly reduced. Heavy mitochondria isolated from the interface of the 21% and 40% Percoll were analyzed. Mitochondrial antioxidant enzymes activities including superoxide dismutase, ascorbate peroxidase, glutathione reductase, monodehydroascorbate reductase, and dehydroascorbate reductase were significantly reduced in aged seeds. A decrease in total ascorbic acid (ASC) and glutathione (GSH) content as well as the reduced/oxidized ratio of ASC and GSH in mitochondria with prolonged ageing showed that artificial ageing reduced ASC-GSH cycle activity. These results suggested an elevated reactive oxygen species (ROS) level in the aged seeds, which was confirmed by measurements of superoxide radical and hydrogen peroxide levels. We conclude that mitochondrial dysfunction in artificially aged seeds is due to retarded mitochondrial and ASC-GSH cycle activity and elevated ROS accumulation.

Comparison of ascorbic acid biosynthesis in different tissues of three non-heading Chinese cabbage cultivars

Ascorbic acid (L-AsA) is an important antioxidant in plants and humans. Vegetables are one of the main sources of ascorbic acid for humans. For instance, non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino) is considered as one of the most important vegetables in south China. To elucidate the mechanism by which AsA accumulates, we systematically investigated the expression profiles of D-mannose/L-galactose pathway-related genes. We also investigated the recycling-related genes and AsA contents in different tissues of three non-heading Chinese cabbage cultivars, 'Suzhouqing', 'Wutacai' and 'Erqing' containing different amounts of AsA. Our results showed that six genes [D-mannose-6-phosphate isomerase 1 (PMI1), GDP-L-galactose phosphorylase 1 (GGP1), GGP2, GGP4, GDP-mannose-3', 5'-epimerase1 (GME1), and GME2] were expressed at high level and ascorbate oxidase (AAO) was expressed at low level. This expression pattern contributes, at least partially, to higher AsA accumulation in the leaves and petioles than in the roots. Eight genes (PMI1, GME, GGP, L-galactose-1-phosphate phosphatase, L-galactose dehydrogenase, L-galactono-1, 4-lactone dehydrogenase, monodehydroascorbate reductase 1, and glutathione reductase1) were also expressed at high level; AAO and ascorbate peroxidase (APX) were expressed at low level. This expression pattern may similarly contribute to higher AsA accumulation in 'Wutacai' and 'Suzhouqing' than in 'Erqing'. Therefore, the high expression levels of PMI, GME, and GGP and the low expression level of AAO contributed to the high AsA accumulation in non-heading Chinese cabbage.

Expression profiling of genes involved in ascorbate biosynthesis and recycling during fleshy root development in radish

Ascorbate is a primary antioxidant and an essential enzyme cofactor in plants, which has an important effect on the development of plant root system. To investigate the molecular mechanisms of ascorbate accumulation during root development and reveal the key genes of the ascorbate biosynthesis and recycling pathways, the expression of 16 related genes together with ascorbate abundance were analyzed in the flesh and skin of radish (Raphanus sativus L.) fleshy root. The content of ascorbate decreased with root growth in both the flesh and skin. Expression of GDP-d-mannose pyrophosphorylase, GDP-d-mannose-3',5'-epimerase and d-galacturonate reductase were also decreased and correlated with ascorbate levels in the flesh. In the skin, the expression of GDP-d-mannose pyrophosphorylase and l-galactose dehydrogenase was correlated with ascorbate levels. These results suggested that ascorbate accumulation is affected mainly by biosynthesis rather than recycling in radish root, and the l-galactose pathway may be the major biosynthetic route of ascorbate, and moreover, the salvage pathway may also contribute to ascorbate accumulation. The data suggested that GDP-d-mannose pyrophosphorylase could play an important role in the regulation of ascorbate accumulation during radish fleshy taproot development.

Salicylic acid increases the contents of glutathione and ascorbate and temporally regulates the related gene expression in salt-stressed wheat seedlings

Exogenous salicylic acid (SA) significantly improved abiotic tolerance in higher plants, and ascorbate (ASA) and glutathione (GSH) play important roles in abiotic tolerance. In this study, SA (0.5mM) markedly increased the contents of ASA and GSH in SA-treated plants during salt stress (250mM NaCl). The transcript levels of the genes encoding ASA and GSH cycle enzymes were measured using quantitative real-time PCR. The results indicated that, during salt stress, exogenous SA significantly enhanced the transcripts of glutathione peroxidase (GPX1), phospholipid hydroperoxide glutathione peroxidase (GPX2) and dehydroascorbate reductase (DHAR) genes at 12h, glutathione reductase (GR) at 24h, 48h and 72h, glutathione-S-transferase 1 (GST1), 2 (GST2), monodehydroascorbate reductase (MDHAR) and glutathione synthetase (GS) at the 48h and 72h after salt stress, respectively. The results implied that SA temporally regulated the transcript levels of the genes encoding ASA-GSH cycle enzymes, resulting in the increased contents of GSH and ASA and enhanced salt tolerance.

iTRAQ protein profile analysis of Citrus sinensis roots in response to long-term boron-deficiency

Seedlings of Citrus sinensis were fertilized with boron (B)-deficient (0μM H3BO3) or -sufficient (10μM H3BO3) nutrient solution for 15weeks. Thereafter, iTRAQ analysis was employed to compare the abundances of proteins from B-deficient and -sufficient roots. In B-deficient roots, 164 up-regulated and 225 down-regulated proteins were identified. These proteins were grouped into the following functional categories: protein metabolism, nucleic acid metabolism, stress responses, carbohydrate and energy metabolism, cell transport, cell wall and cytoskeleton metabolism, biological regulation and signal transduction, and lipid metabolism. The adaptive responses of roots to B-deficiency might include following several aspects: (a) decreasing root respiration; (b) improving the total ability to scavenge reactive oxygen species (ROS); and (c) enhancing cell transport. The differentially expressed proteins identified by iTRAQ are much larger than those detected using 2D gel electrophoresis, and many novel B-deficiency-responsive proteins involved in cell transport, biological regulation and signal transduction, stress responses and other metabolic processes were identified in this work. Our results indicate remarkable metabolic flexibility of citrus roots, which may contribute to the survival of B-deficient plants. This represents the most comprehensive analysis of protein profiles in response to B-deficiency.

BIOLOGICAL SIGNIFICANCE

In this study, we identified many new proteins involved in cell transport, biological regulation and signal transduction, stress responses and other metabolic processes that were not previously known to be associated with root B-deficiency responses. Therefore, our manuscript represents the most comprehensive analysis of protein profiles in response to B-deficiency and provides new information about the plant response to B-deficiency. This article is part of a Special Issue entitled: Translational Plant Proteomics.

Regulation of fruit ascorbic acid concentrations during ripening in high and low vitamin C tomato cultivars

BACKGROUND

To gain insight into the regulation of fruit ascorbic acid (AsA) pool in tomatoes, a combination of metabolite analyses, non-labelled and radiolabelled substrate feeding experiments, enzyme activity measurements and gene expression studies were carried out in fruits of the 'low-' and 'high-AsA' tomato cultivars 'Ailsa Craig' and 'Santorini' respectively.

RESULTS

The two cultivars exhibited different profiles of total AsA (totAsA, AsA + dehydroascorbate) and AsA accumulation during ripening, but both displayed a characteristic peak in concentrations at the breaker stage. Substrate feeding experiments demonstrated that the L-galactose pathway is the main AsA biosynthetic route in tomato fruits, but that substrates from alternative pathways can increase the AsA pool at specific developmental stages. In addition, we show that young fruits display a higher AsA biosynthetic capacity than mature ones, but this does not lead to higher AsA concentrations due to either enhanced rates of AsA breakdown ('Ailsa Craig') or decreased rates of AsA recycling ('Santorini'), depending on the cultivar. In the later stages of ripening, differences in fruit totAsA-AsA concentrations of the two cultivars can be explained by differences in the rate of AsA recycling activities. Analysis of the expression of AsA metabolic genes showed that only the expression of one orthologue of GDP-L-galactose phosphorylase (SlGGP1), and of two monodehydroascorbate reductases (SlMDHAR1 and SlMDHAR3) correlated with the changes in fruit totAsA-AsA concentrations during fruit ripening in 'Ailsa Craig', and that only the expression of SlGGP1 was linked to the high AsA concentrations found in red ripe 'Santorini' fruits.

CONCLUSIONS

Results indicate that 'Ailsa Craig' and 'Santorini' use complementary mechanisms to maintain the fruit AsA pool. In the low-AsA cultivar ('Ailsa Craig'), alternative routes of AsA biosynthesis may supplement biosynthesis via L-galactose, while in the high-AsA cultivar ('Santorini'), enhanced AsA recycling activities appear to be responsible for AsA accumulation in the later stages of ripening. Gene expression studies indicate that expression of SlGGP1 and two orthologues of SlMDHAR are closely correlated with totAsA-AsA concentrations during ripening and are potentially good candidates for marker development for breeding and selection.