The challenge of increasing vitamin C content in plant foods

Prioritizing strategies for wheat biofortification: Inspiration from underutilized species

The relationship between malnutrition and climate change is still poorly understood but a comprehensive knowledge of their interactions is needed to address the global public health agenda. Limited studies have been conducted to propose robust and economic-friendly strategies to augment the food basket with underutilized species and biofortify the staples for nutritional security. Sea-buckthorn is a known "superfood" rich in vitamin C and iron content. It is found naturally in northern hemispherical temperate Eurasia and can be utilized as a model species for genetic biofortification in cash crops like wheat. This review focuses on the impacts of climate change on inorganic (iron, zinc) and organic (vitamin C) micronutrient malnutrition employing wheat as highly domesticated crop and processed food commodity. As iron and zinc are particularly stored in the outer aleurone and endosperm layers, they are prone to processing losses. Moreover, only 5% Fe and 25% Zn are bioavailable once consumed calling to enhance the bioavailability of these micronutrients. Vitamin C converts non-available iron (Fe3+) to available form (Fe2+) and helps in the synthesis of ferritin while protecting it from degradation at the same time. Similarly, reduced phytic acid content also enhances its bioavailability. This relation urges scientists to look for a common mechanism and genes underlying biosynthesis of vitamin C and uptake of Fe/Zn to biofortify these micronutrients concurrently. The study proposes to scale up the biofortification breeding strategies by focusing on all dimensions i.e., increasing micronutrient content and boosters (vitamin C) and simultaneously reducing anti-nutritional compounds (phytic acid). Mutually, this review identified that genes from the Aldo-keto reductase family are involved both in Fe/Zn uptake and vitamin C biosynthesis and can potentially be targeted for genetic biofortification in crop plants.

Regulation of Reactive Oxygen Species during Salt Stress in Plants and Their Crosstalk with Other Signaling Molecules-Current Perspectives and Future Directions

Salt stress is a severe type of environmental stress. It adversely affects agricultural production worldwide. The overproduction of reactive oxygen species (ROS) is the most frequent phenomenon during salt stress. ROS are extremely reactive and, in high amounts, noxious, leading to destructive processes and causing cellular damage. However, at lower concentrations, ROS function as secondary messengers, playing a critical role as signaling molecules, ensuring regulation of growth and adjustment to multifactorial stresses. Plants contain several enzymatic and non-enzymatic antioxidants that can detoxify ROS. The production of ROS and their scavenging are important aspects of the plant's normal response to adverse conditions. Recently, this field has attracted immense attention from plant scientists; however, ROS-induced signaling pathways during salt stress remain largely unknown. In this review, we will discuss the critical role of different antioxidants in salt stress tolerance. We also summarize the recent advances on the detrimental effects of ROS, on the antioxidant machinery scavenging ROS under salt stress, and on the crosstalk between ROS and other various signaling molecules, including nitric oxide, hydrogen sulfide, calcium, and phytohormones. Moreover, the utilization of "-omic" approaches to improve the ROS-regulating antioxidant system during the adaptation process to salt stress is also described.

Metabolic Profiling of Sugars and Organic Acids, and Expression Analyses of Metabolism-Associated Genes in Two Yellow-Peel Pitaya Species

Sugar and organic acids are important factors determining pitaya fruit quality. However, changes in sugars and acids, and expressions of metabolism-associated genes during fruit maturation of yellow-peel pitayas are not well-documented. In this study, metabolic and expression analyses in pulps of different fruit developmental stages of ‘Wucihuanglong’ (‘WCHL’, Hylocereus undatus) and ‘Youcihuanglong’ pitaya (‘YCHL’, Hylocereus megalanthus) were used to explore the sugar and organic acid metabolic process. Total phenols and flavonoids were mainly accumulated at S1 in pitaya pulps. Ascorbic acid contents of ‘WCHL’ pitaya were higher than that of ‘YCHL’ pitaya during fruit maturation. Starch was mainly accumulated at early fruit development stages while soluble sugars were rich in late stages. Sucrose, fructose, and glucose were the main sugar components of ‘YCHL’ pitaya while glucose was dominant in ‘WCHL’ pitaya. Malic and citric acids were the main organic acids in ‘WCHL’ and ‘YCHL’ pitayas, respectively. Based on the transcriptome analyses, 118 genes involved in pitaya sugar and organic acid metabolism were obtained. Results from the correlation analyses between the expression profiling of candidate genes and the contents of sugar and organic acid showed that 51 genes had a significant correlation relationship and probably perform key role in pitaya sugar and organic acid metabolism processes. The finding of the present study provides new information for quality regulation of pitayas.

BcGR1.1, a Cytoplasmic Localized Glutathione Reductase, Enhanced Tolerance to Copper Stress in Arabidopsis thaliana

Copper is a mineral element, which is necessary for the normal growth and development of plants, but high levels of copper will seriously damage plants. Studies have shown that AtGR1 improves the tolerance of Arabidopsis to aluminum and cadmium stress. However, the role of GR in the copper stress response of plants is still unclear. Here, we identified four genes (named BcGR1.1, BcGR1.2, BcGR2.1 and BcGR2.2, respectively) encoding glutathione reductase (GR) in non-heading Chinese cabbage (Brassica campestris (syn. Brassica rapa) ssp. chinensis), which could be divided into two types based on the subcellular localization. Among them, BcGR1.1, which belonged to the cytoplasmic localization type, was significantly upregulated under copper stress. Compared to WT (the wild type), Arabidopsis thaliana heterologously overexpressed BcGR1.1 had longer roots, higher fresh weight, higher GSH levels and GSH/GSSG (oxidized form of GSH) ratio, and accumulated more superoxide dismutase and peroxidase under copper stress. However, in the AsA-GSH cycle under copper stress, the contents of AsA and AsA/DHA were significantly downregulated, and the contents of DHA and T-AsA (total AsA) were upregulated, in the BcGR1.1-overexpressing Arabidopsis. Therefore, BcGR1.1 could improve the scavenging ability of reactive oxygen species (ROS) by increasing the activity of GR, antioxidant enzymes and the utilization of AsA, and then enhance the copper stress tolerance of plants.

2-Keto-L-Gulonic Acid Improved the Salt Stress Resistance of Non-heading Chinese Cabbage by Increasing L-Ascorbic Acid Accumulation

Salt stress has long been a prominent obstacle that restricts crop growth, and increasing the L-ascorbic acid (ASA) content of crops is an effective means of alleviating this stress. 2-Keto-L-gulonic acid (2KGA) is a precursor used in industrial ASA production as well as an ASA degradation product in plants. However, to date, no study has investigated the effects of 2KGA on ASA metabolism and salt stress. Here, we evaluated the potential of using 2KGA to improve crop resistance to salt stress (100mM NaCl) through a cultivation experiment of non-heading Chinese cabbage (Brassica campestris ssp. chinensis). The results showed that the leaf and root biomass were significantly improved by 2KGA application. The levels of metabolites and enzymes related to stress resistance were increased, whereas the hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) contents were decreased. Lipid peroxidation and cell membrane damage were alleviated following 2KGA treatment. Positive correlations were found between photosynthetic pigments and organic solutes, ASA and photosynthetic pigments, and ASA and antioxidant enzymes. In contrast, negative correlations were observed between antioxidant enzymes and H₂O₂/MDA. Moreover, the expression levels of L-gulono-1,4-lactone oxidase, GDP-mannose pyrophosphorylase, dehydroascorbate reductase-3, and ascorbate peroxidase were increased by 2KGA treatment. These results suggested that exogenous 2KGA application can relieve the inhibitory effect of salt stress on plant growth, and the promotion of ASA synthesis may represent a critical underlying mechanism. Our findings have significant implications for the future application of 2KGA or its fermentation residue in agriculture.

Any Role of High-Dose Vitamin C for Septic Shock in 2021?

While the use of vitamin C as a therapeutic agent has been investigated since the 1950s, there has been substantial recent interest in the role of vitamin C supplementation in critical illness and particularly, sepsis and septic shock. Humans cannot synthesize vitamin C and rely on exogenous intake to maintain a plasma concentration of approximately 70 to 80 μmol/L. Vitamin C, in healthy humans, is involved with antioxidant function, wound healing, endothelial function, and catecholamine synthesis. Its function in the human body informs the theoretical basis for why vitamin C supplementation may be beneficial in sepsis/septic shock.Critically ill patients can be vitamin C deficient due to low dietary intake, increased metabolic demands, inefficient recycling of vitamin C metabolites, and loss due to renal replacement therapy. Intravenous supplementation is required to achieve supraphysiologic serum levels of vitamin C. While some clinical studies of intravenous vitamin C supplementation in sepsis have shown improvements in secondary outcome measures, none of the randomized clinical trials have shown differences between vitamin C supplementation and standard of care and/or placebo in the primary outcome measures of the trials. There are some ongoing studies of high-dose vitamin C administration in patients with sepsis and coronavirus disease 2019; the majority of evidence so far does not support the routine supplementation of vitamin C in patients with sepsis or septic shock.

Molecular Evolutionary and Expression Pattern Analysis of AKR Genes Shed New Light on GalUR Functional Characteristics in Brassica rapa

The aldo-keto reductase (AKR) superfamily plays a major role in oxidation-reduction in plants. _D-galacturonic acid reductase (GalUR), an ascorbic acid (AsA) biosynthetic enzyme, belongs to this superfamily. However, the phylogenetic relationship and evolutionary history of the AKR gene family in plants has not yet been clarified. In this study, a total of 1268 AKR genes identified in 36 plant species were used to determine this phylogenetic relationship. The retention, structural characteristics, and expression patterns of AKR homologous genes in Brassica rapa and Arabidopsis thaliana were analyzed to further explore their evolutionary history. We found that the AKRs originated in algae and could be divided into A and B groups according to the bootstrap value; GalURs belonged to group A. Group A AKR genes expanded significantly before the origin of angiosperms. Two groups of AKR genes demonstrated functional divergence due to environmental adaptability, while group A genes were more conservative than those in group B. All 12 candidate GalUR genes were cloned, and their expression patterns under stress were analyzed, in Pak-choi. These genes showed an obvious expression divergence under multiple stresses, and BrcAKR22 exhibited a positive correlation between its expression trend and AsA content. Our findings provide new insights into the evolution of the AKR superfamily and help build a foundation for further investigations of GalUR’s functional characteristics.

Identification of quantitative trait loci and candidate genes for primary metabolite content in strawberry fruit

Improvement of nutritional and organoleptic quality of fruits is a key goal in current strawberry breeding programs. The ratio of sugars to acids is a determinant factor contributing to fruit liking, although different sugars and acids contribute in varying degrees to this complex trait. A segregating F1 population of 95 individuals, previously characterized for several fruit quality characters, was used to map during 2 years quantitative trait loci (QTL) for 50 primary metabolites, l-ascorbic acid (L-AA) and other related traits such as soluble solid content (SSC), titratable acidity (TA), and pH. A total of 133 mQTL were detected above the established thresholds for 44 traits. Only 12.9% of QTL were detected in the 2 years, suggesting a large environmental influence on primary metabolite content. An objective of this study was the identification of key metabolites that were associated to the overall variation in SSC and acidity. As it was observed in previous studies, a number of QTL controlling several metabolites and traits were co-located in homoeology group V (HG V). mQTL controlling a large variance in raffinose, sucrose, succinic acid, and L-AA were detected in approximate the same chromosomal regions of different homoeologous linkage groups belonging to HG V. Candidate genes for selected mQTL are proposed based on their co-localization, on the predicted function, and their differential gene expression among contrasting F1 progeny lines. RNA-seq analysis from progeny lines contrasting in L-AA content detected 826 differentially expressed genes and identified Mannose-6-phosphate isomerase, FaM6PI1, as a candidate gene contributing to natural variation in ascorbic acid in strawberry fruit.

Genome-scale metabolic reconstructions of multiple Salmonella strains reveal serovar-specific metabolic traits

Salmonella strains are traditionally classified into serovars based on their surface antigens. While increasing availability of whole-genome sequences has allowed for more detailed subtyping of strains, links between genotype, serovar, and host remain elusive. Here we reconstruct genome-scale metabolic models for 410 Salmonella strains spanning 64 serovars. Model-predicted growth capabilities in over 530 different environments demonstrate that: (1) the Salmonella accessory metabolic network includes alternative carbon metabolism, and cell wall biosynthesis; (2) metabolic capabilities correspond to each strain's serovar and isolation host; (3) growth predictions agree with 83.1% of experimental outcomes for 12 strains (690 out of 858); (4) 27 strains are auxotrophic for at least one compound, including L-tryptophan, niacin, L-histidine, L-cysteine, and p-aminobenzoate; and (5) the catabolic pathways that are important for fitness in the gastrointestinal environment are lost amongst extraintestinal serovars. Our results reveal growth differences that may reflect adaptation to particular colonization sites.

Conservation and Expression Patterns Divergence of Ascorbic Acid d-mannose/l-galactose Pathway Genes in Brassica rapa

Ascorbic acid (AsA) participates in diverse biological processes, is regulated by multiple factors and is a potent antioxidant and cellular reductant. The D-Mannose/L-Galactose pathway is a major plant AsA biosynthetic pathway that is highly connected within biosynthetic networks, and generally conserved across plants. Previous work has shown that, although most genes of this pathway are expressed under standard growth conditions in Brassica rapa, some paralogs of these genes are not. We hypothesize that regulatory evolution in duplicate AsA pathway genes has occurred as an adaptation to environmental stressors, and that gene retention has been influenced by polyploidation events in Brassicas. To test these hypotheses, we explored the conservation of these genes in Brassicas and their expression patterns divergence in B. rapa. Similar retention and a high degree of gene sequence similarity were identified in B. rapa (A genome), B. oleracea (C genome) and B. napus (AC genome). However, the number of genes that encode the same type of enzymes varied among the three plant species. With the exception of GMP, which has nine genes, there were one to four genes that encoded the other enzymes. Moreover, we found that expression patterns divergence widely exists among these genes. (i) VTC2 and VTC5 are paralogous genes, but only VTC5 is influenced by FLC. (ii) Under light treatment, PMI1 co-regulates the AsA pool size with other D-Man/L-Gal pathway genes, whereas PMI2 is regulated only by darkness. (iii) Under NaCl, Cu(2+), MeJA and wounding stresses, most of the paralogs exhibit different expression patterns. Additionally, GME and GPP are the key regulatory enzymes that limit AsA biosynthesis in response to these treatments. In conclusion, our data support that the conservative and divergent expression patterns of D-Man/L-Gal pathway genes not only avoid AsA biosynthesis network instability but also allow B. rapa to better adapt to complex environments.

Light avoidance reduces ascorbic acid accumulation in the peel of Citrus fruit

Citrus fruits are highly consumed worldwide and represent one of the most important sources of ascorbic acid (AsA). However, information about the molecular mechanisms regulating AsA accumulation in Citrus fruit and the effects of environmental factors is scarce. In this study we have investigated the effect of fruit shading on AsA content and the expression of AsA biosynthetic, degrading and recycling genes in fruits of different Citrus species. Immature-green fruits were covered at the end of the cell enlargement phase and AsA concentration in the flavedo declined and remained at low levels as compared with light-exposed fruits. Fruit shading marginally altered the expression of genes from the l-galactose pathway and this effect was variable in the four Citrus species. However, specific isoforms (GalUR8 or GalUR12) from the l-galacturonic acid pathway were significantly repressed paralleling the reduction in AsA concentration. No significant effect of shading was detected in transcription of genes of the myo-inositol and l-gulose pathways as well as recycling and degradation. Collectively, results indicate that light avoidance inhibited accumulation of AsA in the flavedo of Citrus fruits and suggest that the l-galacturonic acid pathway has a relevant contribution to AsA content in this tissue.

Patterns of evolutionary conservation of ascorbic acid-related genes following whole-genome triplication in Brassica rapa

Ascorbic acid (AsA) is an important antioxidant in plants and an essential vitamin for humans. Extending the study of AsA-related genes from Arabidopsis thaliana to Brassica rapa could shed light on the evolution of AsA in plants and inform crop breeding. In this study, we conducted whole-genome annotation, molecular-evolution and gene-expression analyses of all known AsA-related genes in B. rapa. The nucleobase-ascorbate transporter (NAT) gene family and AsA l-galactose pathway genes were also compared among plant species. Four important insights gained are that: 1) 102 AsA-related gene were identified in B. rapa and they mainly diverged 12-18 Ma accompanied by the Brassica-specific genome triplication event; 2) during their evolution, these AsA-related genes were preferentially retained, consistent with the gene dosage hypothesis; 3) the putative proteins were highly conserved, but their expression patterns varied; and 4) although the number of AsA-related genes is higher in B. rapa than in A. thaliana, the AsA contents and the numbers of expressed genes in leaves of both species are similar, the genes that are not generally expressed may serve as substitutes during emergencies. In summary, this study provides genome-wide insights into evolutionary history and mechanisms of AsA-related genes following whole-genome triplication in B. rapa.

Pre-harvest treatments of pepper plants with nitrophenolates increase crop yield and enhance nutritive and bioactive compounds in fruits at harvest and during storage

Pepper plants (Capsicum annuum L. cv Lamuyo) were treated with a mix of nitrophenolates either by foliar spray or in the irrigation system, and its effect on crop yield and the content of nutritive and bioactive compounds in fruit were analysed at harvest and after post-harvest storage. Treatments were applied at 2-week intervals from the development of first floral bunch (1 March) to end of August. Pepper fruits were harvested at commercial ripening stage (red surface colour) along the growth cycle (from May to September). Total yield (cumulative kilogram per plant) was increased ( 4.5% more) by nitrophenolate treatments due to significant increases in fruit mass, although the number of fruits per plant was unaffected. Pepper fruit quality (weight, firmness and pericarp thickness), its content in nutritive (sugars and organic acids) and bioactive compounds (vitamin C and total phenolics) and antioxidant activity were also enhanced by nitrophenolate treatments at the three harvested dates assayed (end May, mid July and end August). In addition, all these parameters were maintained at higher levels in treated peppers during storage, while diminutions in these parameters occurred generally in control fruit. Thus, nitrophenolate treatments were able to improve crop yield as well as the nutritional value and antioxidant properties of peppers at harvest and after fruit storage.

Influence of silicon on maize roots exposed to antimony - growth and antioxidative response

Pollution of antimony (Sb) raises a serious environmental problem. Although this non-essential element can be taken up by roots and accumulated in plant tissues in relatively high concentrations, there is still lack of knowledge about the effect of Sb on biochemical and metabolic processes in plants. It was shown that application of silicon (Si) can decrease the toxicity of other heavy metals and toxic elements in various plants. The aim of this study was to assess how Si influences the growth and antioxidative response of young Zea mays L. roots exposed to elevated concentrations of Sb. Antimony reduced the root growth and induced oxidative stress and activated antioxidant defense mechanisms in maize. Silicon addition to Sb treated roots decreased oxidative stress symptoms documented by lower lipid peroxidation, proline accumulation, and decreased activity of antioxidative enzymes (ascorbate peroxidase, EC 1.11.1.11; catalase, EC 1.11.1.6; and guaiacol peroxidase, EC 1.11.1.7). Although neither positive nor negative effect of Si has been observed on root length and biomass, changes in the oxidative response of plants exposed to Sb indicate a possible mitigation role of Si on Sb toxicity in plants.

Increased antioxidant capacity in tomato by ectopic expression of the strawberry D-galacturonate reductase gene

Increasing L-ascorbic acid (AsA, vitamin C) content in fruits is a common goal in current breeding programs due to its beneficial effect on human health. Attempts to increase AsA content by genetic engineering have resulted in variable success likely due to AsA's complex regulation. Here, we report the effect of ectopically expressing in tomato the D-galacturonate reductase (FaGalUR) gene from strawberry, involved in AsA biosynthesis, either under the control of the constitutive 35S or the tomato fruit-specific polygalucturonase (PG) promoters. Although transgenic lines showed a moderate increase on AsA content, complex changes in metabolites were found in transgenic fruits. Metabolomic analyses of ripe fruits identified a decrease in citrate, glutamate, asparagine, glucose, and fructose, accompanied by an increase of sucrose, galactinol, and chlorogenic acid. Significant metabolic changes also occurred in leaves of 35S-FaGalUR lines, which showed higher non-photochemical fluorescence quenching (NPQ), indicative of a higher constitutive photo-protective capacity. Overall, overexpression of FaGalUR increased total antioxidant capacity in fruits and the results suggest a tight control of AsA content, probably linked to a complex regulation of cellular redox state and metabolic adjustment.

L-Ascorbic acid metabolism during fruit development in an ascorbate-rich fruit crop chestnut rose (Rosa roxburghii Tratt)

Chestnut rose (Rosa roxburghii Tratt) is a fruit crop that contains unusually high levels of l-ascorbic acid (AsA; ∼1300 mg 100g(-1) FW). To explore the mechanisms underlying AsA metabolism, we investigated the distribution and abundance of AsA during fruit development. We also analyzed gene expression patterns, enzyme activities, and content of metabolites related to AsA biosynthesis and recycling. AsA first accumulated during late fruit development and continued to accumulate during ripening, with the highest accumulation rate near fruit maturity. The redox state of AsA in fruit was also enhanced during late fruit development, while leaf and other tissues had much lower levels of AsA and the redox state of AsA was lower. In mature fruit, AsA was mainly distributed in the cytoplasm of the mesocarp. Correlation analysis suggested that the gene expression patterns, enzyme activities, and related metabolite concentrations involved in the l-galactose pathway showed relatively high correlations with the accumulation rate of AsA. The gene expression pattern and activity of dehydroascorbate reductase (DHAR, EC 1.8.5.1) correlated strongly with AsA concentration, possibly indicating the crucial role of DHAR in the accumulation of high levels of AsA in chestnut rose fruit. Over expression of DHAR in Arabidopsis significantly increased the reduced AsA content and redox state. This was more effective than over expression of the l-galactose pathway gene GDP-d-mannose-3,5-epimerase (EC 5.1.3.18). These findings will enhance understanding of the molecular mechanisms regulating accumulation of AsA in chestnut rose.

Differential transcriptional regulation of L-ascorbic acid content in peel and pulp of citrus fruits during development and maturation

Citrus fruits are an important source of ascorbic acid (AsA) for human nutrition, but the main pathways involved in its biosynthesis and their regulation are still not fully characterized. To study the transcriptional regulation of AsA accumulation, expression levels of 13 genes involved in AsA biosynthesis, 5 in recycling and 5 in degradation were analyzed in peel and pulp of fruit of two varieties with different AsA concentration: Navel orange (Citrus sinensis) and Satsuma mandarin (Citrus unshiu). AsA accumulation in peel and pulp correlated with the transcriptional profiling of the L-galactose pathway genes, and the myo-inositol pathway appeared to be also relevant in the peel of immature-green orange. Differences in AsA content between varieties were associated with differential gene expression of GDP-mannose pyrophosphorylase (GMP), GDP-L-galactose phosphorylase (GGP) and L-galactose-1-phosphate phosphatase (GPP), myo-inositol oxygenase in peel, and GGP and GPP in pulp. Relative expressions of monodehydroascorbate reductase 3 (MDHAR3) and dehydroascorbate reductase1 (DHAR1) correlated with AsA accumulation during development and ripening in peel and pulp, respectively, and were more highly expressed in the variety with higher AsA contents. Collectively, results indicated a differential regulation of AsA concentration in peel and pulp of citrus fruits that may change during the different stages of fruit development. The L-galactose pathway appears to be predominant in both tissues, but AsA concentration is regulated by complex mechanisms in which degradation and recycling also play important roles.

Editorial: from plant biotechnology to bio-based products

From plant biotechnology to bio-based products - this Special Issue of Biotechnology Journal is dedicated to plant biotechnology and is edited by Prof. Eva Stöger (University of Natural Resources and Life Sciences, Vienna, Austria). The Special Issue covers a wide range of topics in plant biotechnology, including metabolic engineering of biosynthesis pathways in plants; taking advantage of the scalability of the plant system for the production of innovative materials; as well as the regulatory challenges and society acceptance of plant biotechnology.

Proteomic analysis of non-heading Chinese cabbage infected with Hyaloperonospora parasitica

Downy mildew is a serious fungal disease in non-heading Chinese cabbage (Brassica campestris ssp. chinensis Makino) that is caused by Hyaloperonospora parasitica, which infects members of the Brassicaceae family. For breeding improvement, researchers must understand the defence mechanisms employed by non-heading Chinese cabbage to combat H. parasitica infection. Using 2-DE protein analysis, we compared the proteomes from leaves of non-heading Chinese cabbage seedlings that were infected with H. parasitica or that were only treated with water at different time points post-infection. By MS analysis, 91 protein spots with significant differences in abundance (>2-fold, p<0.05) were identified in mock- and H. parasitica-inoculated leaves. Next, a resistance strategy for incompatible interactions was proposed. This network consisted of several functional components, including enhanced ethylene biosynthesis and energy supply, balanced ROS production and scavenging, accelerated protein metabolism and photorespiratory, reduced photosynthesis, and induced photosystem repair. These findings increase our knowledge of incompatible interactions between plants and pathogens and also provide new insight regarding the function of plant molecular processes, which should assist in the discovery of new strategies for pathogen control.

BIOLOGICAL SIGNIFICANCE

This study reported the proteomic analysis of the incompatible interactions between non-heading Chinese cabbage and downy mildew using 2-DE and MS. In total, 91 protein spots that were related to the resistance response were identified. These proteins were assigned to different functional categories, such as amino acid and carbohydrate metabolism, photosynthesis and photorespiration, protein metabolism, signal transduction, redox homeostasis, and ethylene biosynthesis. Meanwhile, several key proteins were determined to be associated with ethylene signalling, ROS scavenging and resistance-related proteins. Consistent with these results, the expression of ethylene biosynthesis genes and response genes, as well as the activity of antioxidant enzymes, increased after inoculation. These findings provide new insight for further understanding the molecular mechanisms of plant resistance.

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.

Ascorbic acid synthesis and metabolism in maize are subject to complex and genotype-dependent feedback regulation during endosperm development

L-ascorbic acid (vitamin C) is an antioxidant and electron donor whose metabolism in plants is under strict feedback control. The factors that influence L-ascorbic acid accumulation in staple crops are only partially understood. One way to gain insight into the regulation of L-ascorbic acid metabolism is to investigate the endogenous pathways in various genetic backgrounds and characterize their interactions with transgenes encoding relevant enzymes. In an initial step, we investigated the developmental profile of L-ascorbic acid accumulation in the endosperm of three diverse maize genotypes and a transgenic line expressing rice dehydroascorbate reductase, which enhances L-ascorbic acid recycling. We determined the transcript levels of all the key genes in the L-ascorbic acid metabolic pathways as well as the specific levels of ascorbic acid and dehydroascorbate. L-ascorbic acid levels were high 20 days after pollination and declined thereafter. We found significant genotype-dependent variations in the transcript levels of some genes, with particular complexity in the ascorbic acid recycling pathway. Our data will help to elucidate the complex mechanisms underlying the regulation of L-ascorbic acid metabolism in plants, particularly the impact of genetic background on the strict regulation of ascorbic acid metabolism in endosperm cells.

Proteomic analysis of strawberry achenes reveals active synthesis and recycling of L-ascorbic acid

Although the commonly named strawberry fruit (Fragaria×ananassa) is the sum of achenes and receptacles, the true fruit in the botanical sense is the achene. Here we report the protein changes occurring in the achene when developing from immature to mature stage. We have used 2-DE followed by image analysis, and protein identification by PMF combined with MS/MS, to investigate the protein variations associated to this transition. From a total of 331 spots analyzed, the corresponding 315 proteins have been identified. Differentially accumulated proteins between immature and mature achenes mostly reflect the physiological events associated to seed development and maturation, with only a few changes related to the development of the dry pericarp. We have focused our attention on vitamin C biosynthesis. Interestingly, GDP-mannose 3',5'-epimerase, a key enzyme in the l-ascorbate biosynthesis pathway, and ascorbate peroxidase, involved in l-ascorbic acid oxidation, accumulate in immature achenes. The higher amount of these enzymes found in the green achene is coincident with a higher content of l-ascorbate, and higher expression levels of these and other gene encoding enzymes of the l-ascorbic acid biosynthesis pathway. Altogether our results suggest an important role of l-ascorbic acid at the early developmental stage of the achene.

BIOLOGICAL SIGNIFICANCE

In this manuscript we report the identification of the most abundant proteins in strawberry (F.×ananassa) achenes at early and late stages of development, thus providing a proteomic view of the events that occur during the development of this organ. Despite the importance of strawberry as a commercial fruit, the molecular changes governing its growth and ripening processes are largely unknown. The lack of information is even greater in the case of the achenes, which are the true fruit and play a critical role in the developmental process of the receptacle. Our original proteomic study reported here, restricted to the achenes, completes the previous transcriptomic (very limited) and metabolomic maps of this organ, adding clarity to the role of the achene in the global ripening process. The results obtained not only complement the previous "omics" studies significantly, but also open new key questions that deserve further research (role of hormones). We finally focus on the biosynthesis of l-ascorbic acid, which appears to be tightly regulated by some specific pathways, and whose content is important in the achene. The information provided here will be of interest not only for the groups studying strawberry, but also for many other groups interested in the fruit ripening process, as well as for groups studying the regulation of l-ascorbic acid content in different plant tissues.