Tomato fruit ascorbic acid content is linked with monodehydroascorbate reductase activity and tolerance to chilling stress

Multi-regulated GDP-l-galactose phosphorylase calls the tune in ascorbate biosynthesis

Ascorbate is involved in numerous vital processes, in particular in response to abiotic but also biotic stresses whose frequency and amplitude increase with climate change. Ascorbate levels vary greatly depending on species, tissues, or stages of development, but also in response to stress. Since its discovery, the ascorbate biosynthetic pathway has been intensely studied and it appears that GDP-l-galactose phosphorylase (GGP) is the enzyme with the greatest role in the control of ascorbate biosynthesis. Like other enzymes of this pathway, its expression is induced by various environmental and also developmental factors. Although mRNAs encoding it are among the most abundant in the transcriptome, the protein is only present in very small quantities. In fact, GGP translation is repressed by a negative feedback mechanism involving a small open reading frame located upstream of the coding sequence (uORF). Moreover, its activity is inhibited by a PAS/LOV type photoreceptor, the action of which is counteracted by blue light. Consequently, this multi-level regulation of GGP would allow fine control of ascorbate synthesis. Indeed, experiments varying the expression of GGP have shown that it plays a central role in response to stress. This new understanding will be useful for developing varieties adapted to future environmental conditions.

Physiological response and tolerance of Sesuvium portulacastrum L. to low temperature stress

The plant Sesuvium portulacastrum L., commonly referred to as sea purslane, is a perennial halophytic species with significant potential for development in marine ecological restoration. However, its growth is limited in high-latitude regions with lower temperatures due to its subtropical nature. Furthermore, literature on its cold tolerance is scarce. This study, therefore, focused on sea purslane plants naturally overwintering in Ningbo (29°77'N), investigating their morphological, histological, rooting, and physiological responses to low temperatures (7 °C, 11 °C, 15 °C, and 19 °C). The findings indicated an escalation in cold damage severity with decreasing temperatures. At 7 °C, the plants failed to root and subsequently perished. In contrast, at 11 °C, root systems developed, while at 15 °C and 19 °C, the plants exhibited robust growth, outperforming the 11 °C group in terms of leaf number and root length significantly (P < 0.05). Histological analyses showed a marked reduction in leaf thickness under cold stress (P < 0.05), with disorganized leaf structure observed in the 7 °C group, whereas it remained stable at higher temperatures. No root primordia were evident in the vascular cambium of the 7 and 11 °C groups, in contrast to the 15 and 19 °C groups. Total chlorophyll content decreased with temperature, following the order: 19 °C > 15 °C > 11 °C > 7 °C. Notably, ascorbic acid levels were significantly higher in the 7 and 11 °C groups than in the 15 and 19 °C groups. Additionally, the proline concentration in the 7 °C group was approximately fourfold higher than in the 19 °C group. Activities of antioxidant enzymes-superoxide dismutase, peroxidase, and catalase-were significantly elevated in the 7 and 11 °C groups compared to the 15 and 19 °C groups. Moreover, the malondialdehyde content in the 7 °C group (36.63 ± 1.75 nmol/g) was significantly higher, about 5.5 and 9.6 times, compared to the 15 °C and 19 °C groups, respectively. In summary, 7 °C is a critical threshold for sea purslane stem segments; below this temperature, cellular homeostasis is disrupted, leading to an excessive accumulation of lipid peroxides and subsequent death due to an inability to neutralize excess reactive oxygen species. At 11 °C, although photosynthesis is impaired, self-protective mechanisms such as enhanced antioxidative systems and osmoregulation are activated. However, root development is compromised, resulting in stunted growth. These results contribute to expanding the geographic distribution of sea purslane and provide a theoretical basis for its ecological restoration in high-latitude mariculture.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-024-01429-6.

The Toxicity Differences of Fluralaner against the Red Imported Fire Ant (Solenopsis invicta) at Different Developmental Stages

The red imported fire ant (RIFA), Solenopsis invicta, is an invasive pest that causes damage to agricultural and ecological environments worldwide. Fluralaner is a new isoxazoline pesticide with the potential to become a control agent against RIFA. However, it is not clear whether S. invicta responds the same way to fluralaner at different reproductive stages. The present study firstly evaluated the toxicity of fluralaner to S. invicta at different developmental stages, finding that fourth instar larvae (LD50, 1744.23 mg/kg) and worker ants (LD50, 8.62 mg/kg) were differently susceptible to fluralaner, while the mortality rate of fourth instar larvae was significantly lower at the same concentration of 10 mg/L (5.56 ± 3.14%) than that of worker ants (62.22 ± 3.14%), demonstrating a greater tolerance to fluralaner. Subsequently, the metabolic responses of worker and larval ants to fluralaner stress (10 mg/L) were investigated using non-targeted metabolomics, which indicated that the amount of differential metabolites and the KEGG metabolic pathways enriched were different between workers and larvae when exposed to the same dose (10 mg/L) of fluralaner. Differential metabolites of larvae and worker ants under fluralaner stress were mainly concentrated in organic acids and their derivatives, lipids and lipid-like molecules, nucleosides, nucleotides, and analogues, combined with the enriched metabolic pathways, revealed that the differential metabolic responses of larvae and worker ants were mainly in energy metabolism, detoxification metabolism, and neurotransmitter ligands. Workers consumed more substrates in the arginine synthesis pathway (l-glutamic acid, l-aspartic acid, and fumaric acid) to provide energy for the detoxification (glutathione) of pesticides when exposed to fluralaner stress, and the high accumulation of l-aspartic acid induced excitotoxicity in the worker ants. Larval ants consumed more arachidonic acid to synthesize PG D2, and changes in the metabolism of antioxidants such as catechins, hesperidin, and l-ascorbic acid suggested that larvae were more capable of scavenging the ROS response than worker ants. The results of non-targeted metabolomics successfully revealed differences in the sensitivity of larvae and workers to fluralaner agents, providing insights into the fluralaner control of Solenopsis invicta.

L-Ascorbic acid metabolism and regulation in fruit crops

L-Ascorbic acid (AsA) is more commonly known as vitamin C and is an indispensable compound for human health. As a major antioxidant, AsA not only maintains redox balance and resists biological and abiotic stress but also regulates plant growth, induces flowering, and delays senescence through complex signal transduction networks. However, AsA content varies greatly in horticultural crops, especially in fruit crops. The AsA content of the highest species is approximately 1,800 times higher than that of the lowest species. There have been significant advancements in the understanding of AsA accumulation in the past 20 years. The most noteworthy accomplishment was the identification of the critical rate-limiting genes for the 2 major AsA synthesis pathways (L-galactose pathway and D-galacturonic acid pathway) in fruit crops. The rate-limiting genes of the former are GMP, GME, GGP, and GPP, and the rate-limiting gene of the latter is GalUR. Moreover, APX, MDHAR, and DHAR are also regarded as key genes in degradation and regeneration pathways. Interestingly, some of these key genes are sensitive to environmental factors, such as GGP being induced by light. The efficiency of enhancing AsA content is high by editing upstream open reading frames (uORF) of the key genes and constructing multi-gene expression vectors. In summary, the AsA metabolism has been well understood in fruit crops, but the transport mechanism of AsA and the synergistic improvement of AsA and other traits is less known, which will be the focus of AsA research in fruit crops.

Mutagenesis-based plant breeding approaches and genome engineering: A review focused on tomato

Breeding is the most important and efficient method for crop improvement involving repeated modification of the genetic makeup of a plant population over many generations. In this review, various accessible breeding approaches, such as conventional breeding and mutation breeding (physical and chemical mutagenesis and insertional mutagenesis), are discussed with respect to the actual impact of research on the economic improvement of tomato agriculture. Tomatoes are among the most economically important fruit crops consumed worldwide because of their high nutritional content and health-related benefits. Additionally, we summarize mutation-based mapping approaches, including Mutmap and MutChromeSeq, for the efficient mapping of several genes identified by random indel mutations that are beneficial for crop improvement. Difficulties and challenges in the adaptation of new genome editing techniques that provide opportunities to demonstrate precise mutations are also addressed. Lastly, this review focuses on various effective and convenient genome editing tools, such as RNA interference (RNAi), zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats (CRISPR/Cas9), and their potential for the improvement of numerous desirable traits to allow the development of better varieties of tomato and other horticultural crops.

QTL analysis for ascorbic acid content in strawberry fruit reveals a complex genetic architecture and association with GDP-L-galactose phosphorylase

Strawberry (Fragaria × ananassa) fruits are an excellent source of L-ascorbic acid (AsA), a powerful antioxidant for plants and humans. Identifying the genetic components underlying AsA accumulation is crucial for enhancing strawberry nutritional quality. Here, we unravel the genetic architecture of AsA accumulation using an F₁ population derived from parental lines 'Candonga' and 'Senga Sengana', adapted to distinct Southern and Northern European areas. To account for environmental effects, the F₁ and parental lines were grown and phenotyped in five locations across Europe (France, Germany, Italy, Poland and Spain). Fruit AsA content displayed normal distribution typical of quantitative traits and ranged five-fold, with significant differences among genotypes and environments. AsA content in each country and the average in all of them was used in combination with 6,974 markers for quantitative trait locus (QTL) analysis. Environmentally stable QTLs for AsA content were detected in linkage group (LG) 3A, LG 5A, LG 5B, LG 6B and LG 7C. Candidate genes were identified within stable QTL intervals and expression analysis in lines with contrasting AsA content suggested that GDP-L-Galactose Phosphorylase FaGGP(3A), and the chloroplast-located AsA transporter gene FaPHT4;4(7C) might be the underlying genetic factors for QTLs on LG 3A and 7C, respectively. We show that recessive alleles of FaGGP(3A) inherited from both parental lines increase fruit AsA content. Furthermore, expression of FaGGP(3A) was two-fold higher in lines with high AsA. Markers here identified represent a useful resource for efficient selection of new strawberry cultivars with increased AsA content.

Trx CDSP32-overexpressing tobacco plants improves cadmium tolerance by modulating antioxidant mechanism

The effects of overexpression of the thioredoxin-like protein CDSP32 (Trx CDSP32) on reactive oxygen species (ROS) metabolism in tobacco leaves exposed to cadmium (Cd) were studied by combining physiological measures and proteomics technology. Thus, the number of differentially expressed proteins (DEPs) in plants overexpressing the Trx CDSP32 gene in tobacco (OE) was observed to be evidently lower than that in wild-type (WT) tobacco under Cd exposure, especially the number of down-regulated DEPs. Cd exposure induced disordered ROS metabolism in tobacco leaves. Although Cd exposure inhibited the activities of superoxide dismutase (SOD), catalase (CAT), and l-ascorbate peroxidase (APX) and the expression of proteins related to the thioredoxin-peroxiredoxin (Trx-Prx) pathway, the increase in the activities of peroxidase (POD), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), glutathione reductase (GR), glutathione peroxidase (GPX), and glutathione S-transferase (GST) and their protein expression levels played an important role in the physiological response to Cd exposure. Notably, Trx CDSP32 was observed to alleviate the decrease in the expression and activities of SOD and CAT caused by Cd exposure and enhance the function of POD. Trx CDSP32 was observed to increase the H₂O₂ scavenging capacity of the ascorbic acid-glutathione (AsA-GSH) cycle and Trx-Prx pathway under Cd exposure, and it can especially regulate 2-Cys peroxiredoxin (2-Cys Prx) protein expression and thioredoxin peroxidase (TPX) activity. Thus, overexpression of the Trx CDSP32 gene can alleviate the oxidative damage that occurs in tobacco leaves under Cd exposure by modulating antioxidant defense systems.

Benzothiadiazole enhances ascorbate recycling and polyphenols accumulation in blueberry in a cultivar-dependent manner

Benzothiadiazole (BTH) is a functional analogue of salicylic acid able to induce systemic acquired resistance in many horticultural crops. The aim of the work was to investigate how BTH may affect i) fruit quality, ii) ascorbic acid (AsA) oxidation and recycling metabolism and iii) phenolic compounds accumulation, during development and ripening of berries from the two selected cultivars. Blueberry (Vaccinium corymbosum L.) plants (cv 'Brigitta' and 'Duke') were treated with 0.118 mM BTH every two weeks during ripening, then all fruits of each plant were harvested and divided in four developmental stages. Results indicated that BTH had no marked effects on fruit quality parameters. During the first developmental stage, BTH negatively affected dry matter in both cv, while soluble solids and AsA content were affected in 'Duke'. In fully ripe berries, BTH reduced dry matter in 'Duke' and enhanced soluble solids content in 'Brigitta', while diminishing titratable acidity. AsA content was positively affected by BTH in 'Duke', but not in 'Brigitta'. The effect of BTH on the enzymes involved in AsA recycling was recorded in berries at the third (fruit more than half pigmented) and fourth developmental stages. After treatment, in both cv ascorbate peroxidase (APX) activity increased in fully ripe berries, while monodehydroascorbate reductase (MDHAR) activity was stimulated at the third ripening stage. Conversely, the activities of dehydroascorbate reductase (DHAR) and glutathione reductase (GR) were enhanced only in 'Brigitta' and in 'Duke', respectively. BTH stimulated total polyphenols, flavonoid and anthocyanin accumulation in 'Brigitta' and in 'Duke' at the third and fourth ripening stages. In fully ripe berries, BTH enhanced the accumulation of delphinidins, cyanidins, petunidins and peonidins in 'Brigitta', while in 'Duke' it increased all classes of anthocyanidins, including malvidin. On the contrary, the relative proportion of the individual anthocyanins was only slightly affected by BTH treatment, mainly regarding delphinidin and malvidin at the third and fourth stage of ripening of 'Duke' and 'Brigitta', respectively. These results show that preharvest BTH application can positively impact on fruit bioactive compounds levels, affecting AsA recycling and content and increasing polyphenols accumulation in fruit, but partly depending on cv and ripening stage.

Quality of Goji Berry Fruit (Lycium barbarum L.) Stored at Different Temperatures

Goji berries are widely known for their outstanding nutritional and medicinal properties; they are usually found in the market as dried fruit or as juice because the fruit has a short shelf-life, and little information is available about its postharvest behavior at low temperatures. This study aimed to determine the storage performance of goji berry fruit by evaluating physicochemical, and sensorial attributes during storage at three different temperatures (0, 5, and 7 °C) for 12 days in a range that has not been extensively studied before. In addition, fruit respiration and ethylene production rates were also measured at the three temperatures. Fruit stored at 0 °C showed the lowest respiration rate and ethylene production (5.8 mg CO₂ kg^-1h^-1 and 0.7 µg C₂H₄ kg^-1h^-1, respectively); however, at this temperature, the incidence and severity of pitting and electrolytic leakage were the highest. In contrast, 5 °C was found to be the best storage temperature for goji berry fruit; the fruit appeared fresh and healthy, had the highest scores during sensory analysis with an acceptable general impression, and had the lowest amount of damage attributable to chilling injury, with 17.1% fruit presenting with shriveling, 12.5% pitting, 6.7% mold, and 35% electrolytic leakage on day 9 of storage. Storage of goji berries at 7 °C resulted in the lowest marketability and the highest incidence of decay. Significant differences were also found in the phytochemical attributes, vitamin C content, soluble solid content (SSC), titratable acidity (TA), SSC/TA ratio, total polyphenol content, 2,2-diphenylpicrylhydrazy (DPPH), and anthocyanin content. This study revealed that a storage temperature of 5 °C for 9 days is recommended to maintain the quality of fresh goji berry. Thus, broadening the existing knowledge of the postharvest behavior of fresh goji berries; our results can help improve the commercial life of goji berries and ensure high-quality attributes throughout distribution.

Transcriptome and Metabolome Analysis Reveals the Importance of Amino-Acid Metabolism in Spodoptera Frugiperda Exposed to Spinetoram

Pests are inevitably exposed to sublethal and lethal doses in the agroecosystem following the application of pesticides indispensable to protect food sources. The effect of spinetoram on amino-acid metabolism of fall armyworm, Spodoptera frugiperda (J.E. Smith), was investigated, at the dose of LC₁₀ and LC₉₀, by transcriptome and LC-MS/MS analysis. Using statistics-based analysis of both POS and NEG mode, a total of 715,501 metabolites in S. frugiperda were significantly changed after spinetoram treatment. The enhancement of glucose metabolism provides energy support for detoxification in larvae. The decrease in valine and isoleucine is associated with an increase in leucine, without maintaining the conservation of citric acid in the larvae. The down-regulation of phenylalanine may retard the tricarboxylic acid cycle to produce GTP. The abundance of lysine was decreased in response to spinetoram exposure, which damages the nervous system of the larvae. The abundance of arginine increases and causes non-functional contraction of the insect's muscles, causing the larva to expend excess energy. Tryptophan provides an important substrate for eliminating ROS. The changes in glutamic acid, aspartic acid, and lysine cause damage to the nerve centers of the larvae. The results of transcriptome and LC-MS/MS analysis revealed the effects of pesticide exposure on amino-acid metabolism of S. frugiperda successfully and provide a new overview of the response of insect physio-biochemistry against pesticides.

Physiological genetic variation in tomato fruit chilling tolerance during postharvest storage

Storage at low temperatures is a common practice to prolong postharvest life of fruit and vegetables with a minimal negative impact on human/environmental health. Storage at low temperatures, however, can be restricted due to produce susceptibility to non-freezing chilling temperatures, when injuries such as physiological disorders and decays may result in unmarketable produce. We have investigated tomato fruit response to postharvest chilling stress in a recombinant inbred line (RIL) population developed from a cross between a chilling-sensitive cultivated tomato (Solanum lycopersicum L.) breeding line and a chilling-tolerant inbred accession of the tomato wild species S. pimpinellifolium L. Screening of the fruit of 148 RILs under cold storage (1.5°C) indicated presence of significant variations in chilling tolerance, manifested by varying degrees of fruit injury. Two extremely contrasting groups of RILs were identified, chilling-tolerant and chilling-sensitive RILs. The RILs in the two groups were further investigated under chilling stress conditions, and several physiological parameters, including weight loss, chlorophyll fluorescence parameters Fv/Fm, and Performance Index (PI), were determined to be efficient markers for identifying response to chilling stress in postharvest fruit. The Fv/Fm values reflected the physiological damages endured by the fruit after cold storage, and PI was a sensitive marker for early changes in photosystem II function. These two parameters were early indicators of chilling response before occurrence of visible chilling injuries. Antioxidant activities and ascorbic acid content were significantly higher in the chilling-tolerant than the chilling-sensitive lines. Further, the expression of C-repeat/DREB binding factors (CBFs) genes swiftly changed within 1-hr of fruit exposure to the chilling temperature, and the SlCBF1 transcript level was generally higher in the chilling-tolerant than chilling-sensitive lines after 2-hr exposure to the low temperature. This research demonstrates the presence of potential genetic variation in fruit chilling tolerance in the tomato RIL population. Further investigation of the RIL population is underway to better understand the genetic, physiological, and biochemical mechanisms involved in postharvest fruit chilling tolerance in tomato.

Enhanced Photosynthetic Capacity, Osmotic Adjustment and Antioxidant Defenses Contribute to Improve Tolerance to Moderate Water Deficit and Recovery of Triploid Citrus Genotypes

Currently, drought stress is a major issue for crop productivity, and future climate models predict a rise in frequency and severity of drought episodes. Polyploidy has been related to improved tolerance of plants to environmental stresses. In Citrus breeding programs, the use of triploidy is an effective way to produce seedless fruits, one of the greatest consumer expectations. The current study used physiological and biochemical parameters to assess the differential responses to moderate water deficit of 3x genotypes compared to 2x genotypes belonging to the same hybridization. Both parents, the mandarin Fortune and Ellendale tangor, were also included in the experimental design, while the 2x common clementine tree was used as reference. Water deficit affects leaf water status, as well as physiological and detoxification processes. Triploid genotypes showed a better ability to maintain water status through increased proline content and photosynthetic capacity. Moreover, less oxidative damage was associated with stronger antioxidant defenses in triploid genotypes. We also found that triploidy improved the recovery capacity after a water deficit episode.

Antioxidant enzyme responses and metabolite functioning of Pisum sativum L. to sewage sludge in arid and semi-arid environments

The productivity of plants is a direct variant of the countless biotic and abiotic stresses to which a plant is exposed in an environment. This study aimed to investigate the capabilities of leguminous plant garden pea (Pisum sativum L.) to resist water deficit conditions in arid and semi-arid areas when applied with varied doses of sludge for growth response. The effect of sludge doses was evaluated on crop yield, antioxidant enzymes, viz., ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR), superoxide dismutase (SOD), and glutathione reductase (GR), and metabolites (ascorbic acid, glutathione, and total protein content). The effective sludge concentrations with respect to seed weight and crop yield were found to be in the following trend: D2 (6.25%)>D3 (12.5%)>D1 (2.5%)>D0 (control) under organic amendment (OA). Conversely, a high dose of the sludge reduced the seed weight and total crop yield. The sludge doses D2 under arid and semi-arid conditions along with organic amendments (OA) significantly enhance the antioxidant enzyme activity, whereas sludge dose D3 with OA ominously regulates the activity of these enzymes. Besides, seeds depicted a considerable increase in ascorbic acid, glutathione, and total protein content in arid and semi-arid conditions upon the application of sludge with OA. Sewage sludge as a source of nutrients indirectly enhances crop yield, antioxidant enzymes, and antioxidant metabolites. Thus, it improves the defense mechanism, reduces abnormal protein glycation, and depletes the susceptibility of protein to proteolysis.

High Throughput Analyses of Ascorbate-turnover Enzyme Activities in Rice (Oryza sativa L.) Seedlings

Ascorbate (Vitamin C) fulfills various functions in plant photosynthesis and abiotic stress tolerance. The four key enzymes involved in the ascorbate-turnover pathway are ascorbate peroxidase, ascorbate oxidase, monodehydroascorbate reductase, and dehydroascorbate reductase. Several reports have shown the pivotal roles of these enzymes in plant development and stress tolerance. Therefore, reliable and rapid assay protocols are required for researchers to investigate their enzymatic activities during plant development and stress responses. Previously published methods for analyzing these enzymatic activities rely on cuvette spectrophotometers, which can only handle one sample per test, leading to a prolonged investigation. In this protocol, we employed a 96-well microplate reader to analyze at least eight samples with two technical replicates simultaneously. We analyzed two rice (Oryza sativa L.) genotypes with distinct ascorbate oxidase and dehydroascorbate reductase activities to demonstrate the assay process, including plant growth, sample preparation, reaction setup, and data analysis. Our protocol provides a high throughput method for investigating ascorbate turnover-related enzymatic activities in plants.

A Transcriptional Analysis of the Genes Involved in the Ascorbic Acid Pathways Based on a Comparison of the Juice and Leaves of Navel and Anthocyanin-Rich Sweet Orange Varieties

Sweet oranges are an important source of ascorbic acid (AsA). In this study, the content of AsA in the juice and leaves of four orange clonal selections, different in terms of maturity time and the presence/absence of anthocyanins, was correlated with the transcription levels of the main genes involved in the biosynthesis, recycling, and degradation pathways. Within each variety, differences in the above pathways and the AsA amount were found between the analysed tissues. Variations were also observed at different stages of fruit development and maturation. At the beginning of fruit development, AsA accumulation was attributable to the synergic action of l-galactose and Myo-inositol, while the l-gulose pathway was predominant between the end of fruit development and the beginning of ripening. In leaves, the l-galactose pathway appeared to play a major role in AsA accumulation, even though higher GalUr isoform expression suggests a synergistic contribution of both pathways in this tissue. In juice, the trend of the AsA content may be related to the decrease in the transcription levels of the GME, GDH, MyoOx, and GalUr12 genes. Newhall was the genotype that accumulated the most AsA. The difference between Newhall and the other varieties seems to be attributable to the GLDH, GalUr12, APX2, and DHAR3 genes.

Regulation of Vitamin C Accumulation for Improved Tomato Fruit Quality and Alleviation of Abiotic Stress

Ascorbic acid (AsA) is an essential multifaceted phytonutrient for both the human diet and plant growth. Optimum levels of AsA accumulation combined with balanced redox homeostasis are required for normal plant development and defense response to adverse environmental stimuli. Notwithstanding its moderate AsA levels, tomatoes constitute a good source of vitamin C in the human diet. Therefore, the enhancement of AsA levels in tomato fruit attracts considerable attention, not only to improve its nutritional value but also to stimulate stress tolerance. Genetic regulation of AsA concentrations in plants can be achieved through the fine-tuning of biosynthetic, recycling, and transport mechanisms; it is also linked to changes in the whole fruit metabolism. Emerging evidence suggests that tomato synthesizes AsA mainly through the l-galactose pathway, but alternative pathways through d-galacturonate or myo-inositol, or seemingly unrelated transcription and regulatory factors, can be also relevant in certain developmental stages or in response to abiotic factors. Considering the recent advances in our understanding of AsA regulation in model and other non-model species, this review attempts to link the current consensus with novel technologies to provide a comprehensive strategy for AsA enhancement in tomatoes, without any detrimental effect on plant growth or fruit development.

Sublethal and lethal effects of the imidacloprid on the metabolic characteristics based on high-throughput non-targeted metabolomics in Aphis gossypii Glover

Sublethal effect considered as an emerging factor to assess the environmental risk of insecticides, which can impact the insects on both physiology and behavior. Lethal exposure can be causing near immediate mortality. Pests are inevitably exposed to sublethal and lethal dose in the agroecosystem following application of pesticides. Insecticides, widely used for the control of insect pests, are irreplaceable in insect pest management. The effects of imidacloprid by the method of high-throughput non-targeted metabolomics was investigated in Aphis gossypii Glover exposed to LC₁₀ and LC₉₀ doses of the imidacloprid, and the control group was treated with the same condition without imidacloprid. Pairwise comparisons showed that 111 metabolites changed significantly, 60 in the LC₁₀ group, and 66 in the LC₉₀ group compared to the control group, while only 16 changes in the LC₁₀ were same with that in LC₉₀ group. Among the changed metabolites, a total of 16 metabolites were identified as potential biomarkers, which represented the most influential pathways including glycolysis and gluconeogenesis, alanine, aspartate, and glutamate metabolism, ascorbate and aldarate metabolism, glutathione metabolism, phenylalanine metabolism, tyrosine metabolism, caffeine metabolism and parkinson's disease (PD), which could account for the sublethal and lethal effects on A. gossypii. These modified metabolic pathways demonstrated that high energy consumption, excitotoxicity and oxidative stress (OS) were appeared in both LC₁₀ and LC₉₀ groups, while PD was detected only in the LC₉₀ group. The results of non-targeted metabolomics revealed the effects of neonicotinoid pesticide exposure on A. gossypii successfully, and provided a deep insight into the influenced physiology by the stress of neonicotinoid pesticide in the insect.

Triploidy in Citrus Genotypes Improves Leaf Gas Exchange and Antioxidant Recovery From Water Deficit

The triploidy has proved to be a powerful approach breeding programs, especially in Citrus since seedlessness is one of the main consumer expectations. Citrus plants face numerous abiotic stresses including water deficit, which negatively impact growth and crop yield. In this study, we evaluated the physiological and biochemical responses to water deficit and recovery capacity of new triploid hybrids, in comparison with diploid hybrids, their parents ("Fortune" mandarin and "Ellendale" tangor) and one clementine tree used as reference. The water deficit significantly decreased the relative water content (RWC) and leaf gas exchange (P _net and g _s ) and it increased the levels of oxidative markers (H₂O₂ and MDA) and antioxidants. Compared to diploid varieties, triploid hybrids limited water loss by osmotic adjustment as reflected by higher RWC, intrinsic water use efficiency (iWUE P_net/g_s ) iWUE and leaf proline levels. These had been associated with an effective thermal dissipation of excess energy (NPQ) and lower oxidative damage. Our results showed that triploidy in citrus enhances the recovery capacity after a water deficit in comparison with diploids due to better carboxylation efficiency, restored water-related parameters and efficient antioxidant system.

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.

Genome-Wide Analysis of MDHAR Gene Family in Four Cotton Species Provides Insights into Fiber Development via Regulating AsA Redox Homeostasis

Monodehydroasorbate reductase (MDHAR) (EC1.6.5.4), a key enzyme in ascorbate-glutathione recycling, plays important roles in cell growth, plant development and physiological response to environmental stress via control of ascorbic acid (AsA)-mediated reduction/oxidation (redox) regulation. Until now, information regarding MDHAR function and regulatory mechanism in Gossypium have been limited. Herein, a genome-wide identification and comprehensive bioinformatic analysis of 36 MDHAR family genes in four Gossypium species, Gossypium arboreum, G. raimondii, G. hirsutum, and G. barbadense, were performed, indicating their close evolutionary relationship. Expression analysis of GhMDHARs in different cotton tissues and under abiotic stress and phytohormone treatment revealed diverse expression features. Fiber-specific expression analysis showed that GhMDHAR1A/D, 3A/D and 4A/D were preferentially expressed in fiber fast elongating stages to reach peak values in 15-DPA fibers, with corresponding coincident observances of MDHAR enzyme activity, AsA content and ascorbic acid/dehydroascorbic acid (AsA/DHA) ratio. Meanwhile, there was a close positive correlation between the increase of AsA content and AsA/DHA ratio catalyzed by MDHAR and fiber elongation development in different fiber-length cotton cultivars, suggesting the potential important function of MDHAR for fiber growth. Following H₂O₂ stimulation, GhMDHAR demonstrated immediate responses at the levels of mRNA, enzyme, the product of AsA and corresponding AsA/DHA value, and antioxidative activity. These results for the first time provide a comprehensive systemic analysis of the MDHAR gene family in plants and the four cotton species and demonstrate the contribution of MDHAR to fiber elongation development by controlling AsA-recycling-mediated cellular redox homeostasis.

Application of Genome Editing in Tomato Breeding: Mechanisms, Advances, and Prospects

Plants regularly face the changing climatic conditions that cause biotic and abiotic stress responses. The abiotic stresses are the primary constraints affecting crop yield and nutritional quality in many crop plants. The advances in genome sequencing and high-throughput approaches have enabled the researchers to use genome editing tools for the functional characterization of many genes useful for crop improvement. The present review focuses on the genome editing tools for improving many traits such as disease resistance, abiotic stress tolerance, yield, quality, and nutritional aspects of tomato. Many candidate genes conferring tolerance to abiotic stresses such as heat, cold, drought, and salinity stress have been successfully manipulated by gene modification and editing techniques such as RNA interference, insertional mutagenesis, and clustered regularly interspaced short palindromic repeat (CRISPR/Cas9). In this regard, the genome editing tools such as CRISPR/Cas9, which is a fast and efficient technology that can be exploited to explore the genetic resources for the improvement of tomato and other crop plants in terms of stress tolerance and nutritional quality. The review presents examples of gene editing responsible for conferring both biotic and abiotic stresses in tomato simultaneously. The literature on using this powerful technology to improve fruit quality, yield, and nutritional aspects in tomato is highlighted. Finally, the prospects and challenges of genome editing, public and political acceptance in tomato are discussed.

Silencing of ascorbate oxidase results in reduced growth, altered ascorbic acid levels and ripening pattern in melon fruit

Ascorbate oxidase (AO, EC 1.10.3.3) is a copper-containing enzyme localized at the apoplast, where it catalyzes the oxidation of ascorbic acid (AA) to dehydroascorbic acid (DHA) via monodehydroascorbic acid (MDHA) intermediate. Despite it has been extensively studied, no biological roles have been definitively ascribed. To understand the role of AO in plant metabolism, fruit growth and physiology, we suppressed AO expression in melon (Cucumis melo L.) fruit. Reduction of AO activity increased AA content in melon fruit, which is the result of repression of AA oxidation and simultaneous induction of certain biosynthetic and recycling genes. As a consequence, ascorbate redox state was altered in the apoplast. Interestingly, transgenic melon fruit displayed increased ethylene production rate coincided with elevated levels of 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase (ACO, EC 1.14.17.4) activity and gene expression, which might contribute to earlier ripening. Moreover, AO suppressed transgenic melon fruit exhibited a dramatic arrest in fruit growth, due to a simultaneous decrease in fruit cell size and in plasmalemma (PM) ATPase activity. All the above, support for the first time, the in vivo AO participation in the rapid fruit growth of Cucurbitaceae and further suggest an alternative route for AA increase in ripening fruit.

Effects of ozone treatment on the antioxidant capacity of postharvest strawberry

Strawberries are highly popular around the world because of their juicy flesh and unique taste. However, they are delicate and extremely susceptible to peroxidation of their membrane lipids during storage, which induces water loss and rotting of the fruit. This study investigated the effects of ozone treatment on the physiological traits, active oxygen metabolism, and the antioxidant properties of postharvest strawberry. The results revealed that the weight loss (WL) and respiration rate (RR) of strawberry were inhibited by ozone treatment (OT), while the decline of firmness (FIR) and total soluble solids (TSS) were delayed. Ozone also reduced the generation rate of superoxide radical anions , and the content of hydrogen peroxide (H₂O₂) enhanced the activity of superoxidase (SOD), catalase (CAT), ascorbate peroxidase (APX), and monodehydroascorbate reductase (MDHAR), as well as promoted the accumulation of ascorbic acid (ASA), glutathione (GSH), and ferric reducing/antioxidant power (FRAP). In addition, a total of 29 antioxidant-related proteins were changed between the OT group and control (CK) group as detected by label-free proteomics during the storage time, and the abundance associated with ASA–GSH cycle was higher in the OT group at the later stage of storage, and the qRT-PCR results were consistent with those of proteomics. The improvement of the antioxidant capacity of postharvest strawberry treated with ozone may be achieved by enhancing the activity of the antioxidant enzymes and increasing the expression of the antioxidant proteins related to the ascorbic acid–glutathione (ASA–GSH) cycle.

Strawberries are highly popular around the world because of their juicy flesh and unique taste.

Triploid Citrus Genotypes Have a Better Tolerance to Natural Chilling Conditions of Photosynthetic Capacities and Specific Leaf Volatile Organic Compounds

Low temperatures during winter are one of the main constraints for citrus crop. Polyploid rootstocks can be used for improving tolerance to abiotic stresses, such as cold stress. Because the produced fruit are seedless, using triploid scions is one of the most promising approaches to satisfy consumer expectations. In this study, we evaluated how the triploidy of new citrus varieties influences their sensitivity to natural chilling temperatures. We compared their behavior to that of diploid citrus, their parents (Fortune mandarin and Ellendale tangor), and one diploid clementine tree, as reference, focusing on photosynthesis parameters, oxidative metabolism, and volatile organic compounds (VOC) in leaves. Triploid varieties appeared to be more tolerant than diploid ones to natural low temperatures, as evidenced by better photosynthetic properties (Pnet, gs, Fv/Fm , ETR/P net ratio), without relying on a better antioxidant system. The VOC levels were not influenced by chilling temperatures; however, they were affected by the ploidy level and atypical chemotypes were found in triploid varieties, with the highest proportions of E-β-ocimene and linalool. Such compounds may contribute to better stress adaptation.

Physiological and proteomic responses of reactive oxygen species metabolism and antioxidant machinery in mulberry (Morus alba L.) seedling leaves to NaCl and NaHCO3 stress

In this paper, the effects of 100 mM NaCl and NaHCO₃ stress on reactive oxygen species (ROS) and physiological and proteomic aspects of ROS metabolism in mulberry seedling leaves were studied. The results showed that NaCl stress had little effect on photosynthesis and respiration of mulberry seedling leaves. Superoxide dismutase (SOD) activity and the expression of related proteins in leaves increased by varying degrees, and accumulation of superoxide anion (O₂·^-) not observed. Under NaHCO₃ stress, photosynthesis and respiration were significantly inhibited, while the rate of O₂·^- production rate and H₂O₂ content increased. The activity of catalase (CAT) and the expression of CAT (W9RJ43) increased under NaCl stress. In response to NaHCO₃ stress, the activity and expression of CAT were significantly decreased, but the ability of H₂O₂ scavenging of peroxidase (POD) was enhanced. The ascorbic acid-glutathione (AsA-GSH) cycle in mulberry seedling leaves was enhancement in both NaCl and NaHCO₃ stress. The expression of 2-Cys peroxiredoxin BAS1 (2-Cys Prx BAS1), together with thioredoxin F (TrxF), thioredoxin O1 (TrxO1), thioredoxin-like protein CITRX (Trx CITRX), and thioredoxin-like protein CDSP32 (Trx CDSP32) were significantly increased under NaCl stress. Under NaHCO₃ stress, the expression of the electron donor of ferredoxin-thioredoxin reductase (FTR), together with Trx-related proteins, such as thioredoxin M (TrxM), thioredoxin M4 (TrxM4), thioredoxin X (TrxX), TrxF, and Trx CSDP32 were significantly decreased, suggesting that the thioredoxin-peroxiredoxin (Trx-Prx) pathway's function of scavenging H₂O₂ of in mulberry seedling leaves was inhibited. Taken together, under NaCl stress, excessive production of O₂·^- mulberry seedlings leaves was inhibited, and H₂O₂ was effectively scavenged by CAT, AsA-GSH cycle and Trx-Prx pathway. Under NaHCO₃ stress, despite the enhanced functions of POD and AsA-GSH cycle, the scavenging of O₂·^- by SOD was not effective, and that of H₂O₂ by CAT and Trx-Prx pathway were inhibited; and in turn, the oxidative damage to mulberry seedling leaves could not be reduced.

Genome-wide analysis in Hevea brasiliensis laticifers revealed species-specific post-transcriptional regulations of several redox-related genes

MicroRNA-mediated post-transcriptional regulation has been reported on ROS production and scavenging systems. Although microRNAs first appeared highly conserved among plant species, several aspects of biogenesis, function and evolution of microRNAs were shown to differ. High throughput transcriptome and degradome analyses enable to identify small RNAs and their mRNA targets. A non-photosynthetic tissue particularly prone to redox reactions, laticifers from Hevea brasiliensis, revealed species-specific post-transcriptional regulations. This paper sets out to identify the 407 genes of the thirty main redox-related gene families harboured by the Hevea genome. There are 161 redox-related genes expressed in latex. Thirteen of these redox-related genes were targeted by 11 microRNAs. To our knowledge, this is the first report on a mutation in the miR398 binding site of the cytosolic CuZnSOD. A working model was proposed for transcriptional and post-transcriptional regulation with respect to the predicted subcellular localization of deduced proteins.

Analyzing the Function of Catalase and the Ascorbate-Glutathione Pathway in H2O2 Processing: Insights from an Experimentally Constrained Kinetic Model

SIGNIFICANCE

Plant stress involves redox signaling linked to reactive oxygen species such as hydrogen peroxide (H₂O₂), which can be generated at high rates in photosynthetic cells. The systems that process H₂O₂ include catalase (CAT) and the ascorbate-glutathione pathway, but interactions between them remain unclear. Modeling can aid interpretation and pinpoint areas for investigation. Recent Advances: Based on emerging data and concepts, we introduce a new experimentally constrained kinetic model to analyze interactions between H₂O₂, CAT, ascorbate, glutathione, and NADPH. The sensitivity points required for accurate simulation of experimental observations are analyzed, and the implications for H₂O₂-linked redox signaling are discussed.

CRITICAL ISSUES

We discuss several implications of the modeled results, in particular the following. (i) CAT and ascorbate peroxidase can share the load in H₂O₂ processing even in optimal conditions. (ii) Intracellular H₂O₂ concentrations more than the low μM range may rarely occur. (iii) Ascorbate redox turnover is largely independent of glutathione until ascorbate peroxidation exceeds a certain value. (iv) NADPH availability may determine glutathione redox status through its influence on monodehydroascorbate reduction. (v) The sensitivity of glutathione status to oxidative stress emphasizes its potential suitability as a sensor of increased H₂O₂.

FUTURE DIRECTIONS

Important future questions include the roles of other antioxidative systems in interacting with CAT and the ascorbate-glutathione pathway as well as the nature and significance of processes that achieve redox exchange between different subcellular compartments. Progress in these areas is likely to be favored by integrating kinetic modeling analyses into experimentally based programs, allowing each approach to inform the other.

Vitamin C Content in Fruits: Biosynthesis and Regulation

Throughout evolution, a number of animals including humans have lost the ability to synthesize ascorbic acid (ascorbate, vitamin C), an essential molecule in the physiology of animals and plants. In addition to its main role as an antioxidant and cofactor in redox reactions, recent reports have shown an important role of ascorbate in the activation of epigenetic mechanisms controlling cell differentiation, dysregulation of which can lead to the development of certain types of cancer. Although fruits and vegetables constitute the main source of ascorbate in the human diet, rising its content has not been a major breeding goal, despite the large inter- and intraspecific variation in ascorbate content in fruit crops. Nowadays, there is an increasing interest to boost ascorbate content, not only to improve fruit quality but also to generate crops with elevated stress tolerance. Several attempts to increase ascorbate in fruits have achieved fairly good results but, in some cases, detrimental effects in fruit development also occur, likely due to the interaction between the biosynthesis of ascorbate and components of the cell wall. Plants synthesize ascorbate de novo mainly through the Smirnoff-Wheeler pathway, the dominant pathway in photosynthetic tissues. Two intermediates of the Smirnoff-Wheeler pathway, GDP-D-mannose and GDP-L-galactose, are also precursors of the non-cellulosic components of the plant cell wall. Therefore, a better understanding of ascorbate biosynthesis and regulation is essential for generation of improved fruits without developmental side effects. This is likely to involve a yet unknown tight regulation enabling plant growth and development, without impairing the cell redox state modulated by ascorbate pool. In certain fruits and developmental conditions, an alternative pathway from D-galacturonate might be also relevant. We here review the regulation of ascorbate synthesis, its close connection with the cell wall, as well as different strategies to increase its content in plants, with a special focus on fruits.

Effects of Low-Temperature Stress and Brassinolide Application on the Photosynthesis and Leaf Structure of Tung Tree Seedlings

The tung tree is an important woody oil tree species. Tung oil extracted from the tung fruit seeds is used in the manufacture of environmentally friendly paint. This study investigated the effects of the application of brassinolide (BR) under different temperature conditions on the chlorophyll content, photosynthesis, chlorophyll fluorescence, leaf structure, and chloroplast ultrastructure in Vernicia fordii and Vernicia montana. The conditions used were 8°C-Control (low temperature and no BR), 8°C-BR (low temperature and BR application), 28°C-Control (normal temperature and no BR), and 28°C-BR (normal temperature and BR application), and effects were monitored from 5 to 15 days after the treatments (DAT). The results showed that the low temperature treatment (8°C-Control) significantly reduced the net photosynthetic rate (P_n ), stomatal conductance (G_s ), maximum fluorescence (F_m ), maximum photochemical efficiency (F _v/F _m), and actual photochemical and quantum efficiency (Φ _PSII ) compared to the control condition (28°C-Control). However, the external application of BR alleviated the negative effects of low-temperature stress to some degree for all the above parameters for both species tested, except for P _n and G _s at 15 DAT. There were no significant differences in most of the parameters in either species between the 28°C-Control and 28°C-BR treatments. At 10 and 15 DAT of low-temperature stress, the 8°C-Control treatment significantly reduced leaf cell tense ratio (CTR) and increased spongy ratio (SR) compared to the 28°C-Control, whereas BR application alleviated the adverse effects. Moreover, the 8°C-Control treatment significantly destroyed the chloroplast structure, loosening the thylakoids until they disintegrated, while exogenous spraying of BR protected the chloroplast structure and enabled it to function properly in both species. Our results suggested that long-term low temperatures significantly reduced the photosynthetic efficiency of tung tree seedlings, affecting the formation of the internal structure of plant leaves and destroying the integrity and function of the chloroplast. To prevent this, external application of BR to tung tree seedlings could enhance the photosynthetic potential of tung trees by maintaining the stability of the leaf structure, morphology, and function, and alleviating the damage caused by cold injury. The results also showed that V. fordii seedlings are more resistant to low temperatures than V. montana seedlings.

Comparison of the responses to NaCl stress of three tomato introgression lines

We aimed to examine the response of three tomato introgression lines (IL925.3, IL925.5 and IL925.6) to NaCl stress. These lines originated from a cross between M82 (Solanum lycopersicum) and the wild salttolerant tomato Solanum pennellii, each line containing a different fragment of the S.pennellii genome. Salt-sensitive phenotypes related to plant growth and physiology, and the response of antioxidants, pigments and antioxidant enzymes were measured. In general, salt stress decreased the fresh weight of leaves, leaf area and leaf number and an increase of Na⁺ accumulation in aerial parts was observed, which caused a reduction in the absorption of K⁺ and Ca²⁺. Salt stress also induced a decrease in chlorophyll, carotenoids and lipid peroxidation (MDA) and an increase in anthocyanins and reduced ascorbate, although some differences were seen between the lines, for example for carotenoid levels. Guaiacol peroxidase, catalase and glutathione reductase activity enhanced in aerial parts of the lines, but again some differences were seen between the three lines. It is concluded that IL925.5 might be the most sensitive line to salt stress as its dry weight loss was the greatest in response to salt and this line showed the highest Na⁺ ion accumulation in leaves.

Redox imbalance contributed differently to membrane damage of cucumber leaves under water stress and Fusarium infection

Redox-associated events are important in plant development and responses to environmental stresses. In this study, we investigated spatial redox responses of cucumber (Cucumis sativus L.) leaves to biotic stress (Fusarium infection) or abiotic stress (water stress). Plants were grown under hydroponic conditions and either treated with polyethylene glycol to simulate drought or infected with Fusarium oxysporum f. sp. cucumerinum. Both water stress and Fusarium infection restricted cucumber growth and were associated with cellular plasma-membrane damage, reactive oxygen species accumulation, and changes in antioxidants; however, the responses to each stress were distinctive. Under water stress, H₂O₂ generation at the leaf edge increased 29.7% compared with that at the centre but with Fusarium infection there was a relative 10.4% decrease at the edge. These changes correlated with changes in antioxidants and linked enzyme activities. The key sources of variation in oxidative events were defined by principal component analysis of all of the data and redox balance evaluations. We suggest that these spatial differences under water stress and Fusarium infection arise from discrete regulatory mechanisms, reflecting either developmental effect over the leaf regions or systemic anti-oxidative events occurred following infection.

Genotypic and environmental effects on the level of ascorbic acid, phenolic compounds and related gene expression during pineapple fruit development and ripening

Pineapple (Ananas comosus (L.) Merr.) is a non-climacteric tropical fruit whose ripening could be accompanied by oxidative processes and the concurrent activation of enzymatic and non-enzymatic reactive oxygen species (ROS) scavenging systems. To better understand the variability of these processes among climatic environments or genotypes in pineapple, the temporal expression dynamics for genes encoding oxidative and antioxidative stress enzymes were analyzed by real-time RT-PCR during fruit development and ripening, among three cultivars: Queen Victoria, Flhoran 41 and MD-2 hybrid, and in two climatic areas. Pineapple development and ripening involved changes in the levels of transcripts encoding for polyphenol oxidase and transcripts involved in the first steps of the phenylpropanoid pathway and in the balance of ROS, especially those encoding for ascorbate peroxydase and metallothioneins, regardless of the cultivar. Our results confirm the same dynamic in gene expression from the two environmental crop areas, however climatic conditions influenced the level of the expression of the major transcripts studied that were linked to these oxidative and antioxidant metabolisms. MT3a and MT3b transcripts were not influenced by genetic factor. The genetic effect was not significant on the various transcripts linked to the first steps of the phenylpropanoid pathway and to phenol oxidation, except 4CL ones. In ripe pineapple, highly significant relationships were found between the contents in antioxidant metabolites, i.e., ascorbic acid and total phenolic compounds, and the transcript levels of genes involved in the enzymatic ROS-scavenging system and in the biosynthesis or regeneration of ROS-scavenging compounds, like phenylpropanoids, ascorbic acid, metallothioneins.

Nitro-oxidative metabolism during fruit ripening

Pepper (Capsicum annuum L.) and tomato (Solanum lycopersicum L.), which belong to the Solanaceae family, are among the most cultivated and consumed fleshy fruits worldwide and constitute excellent sources of many essential nutrients, such as vitamins A, C, and E, calcium, and carotenoids. While fruit ripening is a highly regulated and complex process, tomato and pepper have been classified as climacteric and non-climacteric fruits, respectively. These fruits differ greatly in shape, color composition, flavor, and several other features which undergo drastic changes during the ripening process. Such ripening-related metabolic and developmental changes require extensive alterations in many cellular and biochemical processes, which ultimately leads to fully ripe fruits with nutritional and organoleptic features that are attractive to both natural dispersers and human consumers. Recent data show that reactive oxygen and nitrogen species (ROS/RNS) are involved in fruit ripening, during which molecules, such as hydrogen peroxide (H2O2), NADPH, nitric oxide (NO), peroxynitrite (ONOO-), and S-nitrosothiols (SNOs), interact to regulate protein functions through post-translational modifications. In light of these recent discoveries, this review provides an update on the nitro-oxidative metabolism during the ripening of two of the most economically important fruits, discusses the signaling roles played by ROS/RNS in controlling this complex physiological process, and highlights the potential biotechnological applications of these substances to promote further improvements in fruit ripening regulation and nutritional quality. In addition, we suggest that the term 'nitro-oxidative eustress' with regard to fruit ripening would be more appropriate than nitro-oxidative stress, which ultimately favors the consolidation of the plant species.

L-ascorbic acid metabolism in an ascorbate-rich kiwifruit (Actinidia. Eriantha Benth.) cv. 'White' during postharvest

Kiwifruit (Actinidia eriantha Benth.) 'White', a novel cultivar with higher L-ascorbic acid (AsA) level, is registered in China. Changes in AsA, related metabolites, enzymatic activity, and gene expression associated with AsA biosynthesis and recycling process were investigated in this paper. The results indicated that AsA biosynthesis through L-galactose pathway supplemented by D-galacturonic acid pathway and AsA recycling collectively contributed to accumulating and remaining higher AsA level in kiwifruit cv. 'White' during postharvest. Moreover, L-galactose dehydrogenase (GalDH) activity and relative expressions of the genes encoding GDP-D-mannose pyrophosphorylase (GMP), L-galactose-1-P phosphatase (GPP), GDP-L-galactose phosphorylase (GGP), GalDH and D-galacturonate reductase (GalUR) were important for regulation of AsA biosynthesis, and the activity and expression of dehydroascorbate reductase (DHAR) were primarily responsible for regulation of AsA recycling in kiwifruit 'White' during postharvest.

A Systems Biology Study in Tomato Fruit Reveals Correlations between the Ascorbate Pool and Genes Involved in Ribosome Biogenesis, Translation, and the Heat-Shock Response

Changing the balance between ascorbate, monodehydroascorbate, and dehydroascorbate in plant cells by manipulating the activity of enzymes involved in ascorbate synthesis or recycling of oxidized and reduced forms leads to multiple phenotypes. A systems biology approach including network analysis of the transcriptome, proteome and metabolites of RNAi lines for ascorbate oxidase, monodehydroascorbate reductase and galactonolactone dehydrogenase has been carried out in orange fruit pericarp of tomato (Solanum lycopersicum). The transcriptome of the RNAi ascorbate oxidase lines is inversed compared to the monodehydroascorbate reductase and galactonolactone dehydrogenase lines. Differentially expressed genes are involved in ribosome biogenesis and translation. This transcriptome inversion is also seen in response to different stresses in Arabidopsis. The transcriptome response is not well correlated with the proteome which, with the metabolites, are correlated to the activity of the ascorbate redox enzymes-ascorbate oxidase and monodehydroascorbate reductase. Differentially accumulated proteins include metacaspase, protein disulphide isomerase, chaperone DnaK and carbonic anhydrase and the metabolites chlorogenic acid, dehydroascorbate and alanine. The hub genes identified from the network analysis are involved in signaling, the heat-shock response and ribosome biogenesis. The results from this study therefore reveal one or several putative signals from the ascorbate pool which modify the transcriptional response and elements downstream.

Antioxidant Phytochemicals in Fresh Produce: Exploitation of Genotype Variation and Advancements in Analytical Protocols

Horticultural commodities (fruit and vegetables) are the major dietary source of several bioactive compounds of high nutraceutical value for humans, including polyphenols, carotenoids and vitamins. The aim of the current review was dual. Firstly, toward the eventual enhancement of horticultural crops with bio-functional compounds, the natural genetic variation in antioxidants found in different species and cultivars/genotypes is underlined. Notably, some landraces and/or traditional cultivars have been characterized by substantially higher phytochemical content, i.e., small tomato of Santorini island (cv. "Tomataki Santorinis") possesses appreciably high amounts of ascorbic acid (AsA). The systematic screening of key bioactive compounds in a wide range of germplasm for the identification of promising genotypes and the restoration of key gene fractions from wild species and landraces may help in reducing the loss of agro-biodiversity, creating a healthier "gene pool" as the basis of future adaptation. Toward this direction, large scale comparative studies in different cultivars/genotypes of a given species provide useful insights about the ones of higher nutritional value. Secondly, the advancements in the employment of analytical techniques to determine the antioxidant potential through a convenient, easy and fast way are outlined. Such analytical techniques include electron paramagnetic resonance (EPR) and infrared (IR) spectroscopy, electrochemical, and chemometric methods, flow injection analysis (FIA), optical sensors, and high resolution screening (HRS). Taking into consideration that fruits and vegetables are complex mixtures of water- and lipid-soluble antioxidants, the exploitation of chemometrics to develop "omics" platforms (i.e., metabolomics, foodomics) is a promising tool for researchers to decode and/or predict antioxidant activity of fresh produce. For industry, the use of optical sensors and IR spectroscopy is recommended to estimate the antioxidant activity rapidly and at low cost, although legislation does not allow its correlation with health claims.

Differentially expressed protein and gene analysis revealed the effects of temperature on changes in ascorbic acid metabolism in harvested tea leaves

Tea is an important non-alcoholic beverage worldwide. Tea quality is determined by numerous secondary metabolites in harvested tea leaves, including tea polyphenols, theanine, caffeine, and ascorbic acid (AsA). AsA metabolism in harvested tea leaves is affected by storage and transportation temperature. However, the molecular mechanisms underlying AsA metabolism in harvested tea leaves exposed to different storage and transportation temperature conditions remain unclear. Here we performed RP-HPLC to detect dynamic changes in AsA content in tea leaves subjected to high- (38 °C), low- (4 °C), or room-temperature (25 °C) treatments. The AsA distribution and levels in the treated tea leaves were analyzed using cytological-anatomical characterization methods. The differentially expressed CsAPX1 and CsDHAR2 proteins, which are involved in the AsA recycling pathway, were identified from the corresponding proteomic data using iTRAQ. We also analyzed the expression profiles of 18 genes involved in AsA metabolism, including CsAPX1 and CsDHAR2. AsA was mainly distributed in tea leaf mesophyll cells. High- and low-temperature treatments upregulated the CsAPX1 and CsDHAR2 proteins and induced CsAPX and CsDHAR2 gene expression. These results indicated that the CsAPX1 and CsDHAR2 proteins might have critical roles in AsA recycling in tea leaves. Our results provide a foundation for the in-depth investigation of AsA metabolism in tea leaves during storage and transportation, and they will promote better tea flavor in tea production.

Molecular cloning, in-silico characterization and functional validation of monodehydroascorbate reductase gene in Eleusine coracana

Ascorbic acid is a ubiquitous water soluble antioxidant that plays a critical role in plant growth and environmental stress tolerance. It acts as a free radical scavenger as well as a source of reducing power for several cellular processes. Because of its pivotal role in regulating plant growth under optimal as well as sub-optimal conditions, it becomes obligatory for plants to maintain a pool of reduced ascorbic acid. Several cellular processes help in maintaining the reduced ascorbic acid pool, by regulating its synthesis and regeneration processes. Current study demonstrates that monodehydroascorbate reductase is an important enzyme responsible for maintaining the reduced ascorbate pool, by optimizing the recycling of oxidized ascorbate. Cloning and functional characterization of this important stress inducible gene is of great significance for its imperative use in plant stress management. Therefore, we have cloned and functionally validated the role of monodehydroascorbate reductase gene (mdar) from a drought tolerant variety of Eleusine coracana. The cloned Ecmdar gene comprises of 1437bp CDS, encoding a 478 amino acid long polypeptide. The active site analysis showed presence of conserved Tyr₃₄₈ residue, facilitating the catalytic activity in electron transfer mechanism. qPCR expression profiling of Ecmdar under stress indicated that it is an early responsive gene. The analysis of Ecmdar overexpressing Arabidopsis transgenic lines suggests that monodehydroascorbate reductase acts as a key stress regulator by modulating the activity of antioxidant enzymes to strengthen the ROS scavenging ability and maintains ROS homeostasis. Thus, it is evident that Ecmdar is an important gene for cellular homeostasis and its over-expression could be successfully used to strengthen stress tolerance in crop plants.

Genetic Control of Ascorbic Acid Biosynthesis and Recycling in Horticultural Crops

Ascorbic acid (AsA) is an essential compound present in almost all living organisms that has important functions in several aspects of plant growth and development, hormone signaling, as well as stress defense networks. In recent years, the genetic regulation of AsA metabolic pathways has received much attention due to its beneficial role in human diet. Despite the great variability within species, genotypes, tissues and developmental stages, AsA accumulation is considered to be controlled by the fine orchestration of net biosynthesis, recycling, degradation/oxidation, and/or intercellular and intracellular transport. To date, several structural genes from the AsA metabolic pathways and transcription factors are considered to significantly affect AsA in plant tissues, either at the level of activity, transcription or translation via feedback inhibition. Yet, all the emerging studies support the notion that the steps proceeding through GDP-_L-galactose phosphorylase and to a lesser extent through GDP-_D-mannose-3,5-epimerase are control points in governing AsA pool size in several species. In this mini review, we discuss the current consensus of the genetic regulation of AsA biosynthesis and recycling, with a focus on horticultural crops. The aspects of AsA degradation and transport are not discussed herein. Novel insights of how this multifaceted trait is regulated are critical to prioritize candidate genes for follow-up studies toward improving the nutritional value of fruits and vegetables.

Profiling of drought-responsive microRNA and mRNA in tomato using high-throughput sequencing

BACKGROUND

Abiotic stresses cause severe loss of crop production. Among them, drought is one of the most frequent environmental stresses, which limits crop growth, development and productivity. Plant drought tolerance is fine-tuned by a complex gene regulatory network. Understanding the molecular regulation of this polygenic trait is crucial for the eventual success to improve plant yield and quality. Recent studies have demonstrated that microRNAs play critical roles in plant drought tolerance. However, little is known about the microRNA in drought response of the model plant tomato. Here, we described the profiling of drought-responsive microRNA and mRNA in tomato using high-throughput next-generation sequencing.

RESULTS

Drought stress was applied on the seedlings of M82, a drought-sensitive cultivated tomato genotype, and IL9-1, a drought-tolerant introgression line derived from the stress-resistant wild species Solanum pennellii LA0716 and M82. Under drought, IL9-1 performed superior than M82 regarding survival rate, H2O2 elimination and leaf turgor maintenance. A total of four small RNA and eight mRNA libraries were constructed and sequenced using Illumina sequencing technology. 105 conserved and 179 novel microRNAs were identified, among them, 54 and 98 were differentially expressed upon drought stress, respectively. The majority of the differentially-expressed conserved microRNAs was up-regulated in IL9-1 whereas down-regulated in M82. Under drought stress, 2714 and 1161 genes were found to be differentially expressed in M82 and IL9-1, respectively, and many of their homologues are involved in plant stress, such as genes encoding transcription factor and protein kinase. Various pathways involved in abiotic stress were revealed by Gene Ontology and pathway analysis. The mRNA sequencing results indicated that most of the target genes were regulated by their corresponding microRNAs, which suggested that microRNAs may play essential roles in the drought tolerance of tomato.

CONCLUSION

In this study, numerous microRNAs and mRNAs involved in the drought response of tomato were identified using high-throughput sequencing, which will provide new insights into the complex regulatory network of plant adaption to drought stress. This work will also help to exploit new players functioning in plant drought-stress tolerance.

Ascorbate pool, sugars and organic acids in black currant (Ribes nigrum L.) berries are strongly influenced by genotype and post-flowering temperature

BACKGROUND

Marked effects of the climatic environment on fruit chemical composition have often been demonstrated in field experiments. However, complex covariations of several climatic factors in the natural environment complicate the interpretation of such experiments and the identification of the causal factors. This can be better achieved in a phytotron where the various climatic factors can be varied systematically. Therefore, we grew four black currant cultivars of contrasting origin in a phytotron under controlled post-flowering temperature and photoperiod conditions and analysed the berries for their ascorbic acid, sugar and organic acid contents.

RESULTS

The analyses revealed significant effects of genotype on all investigated compounds. Particularly large cultivar differences were observed in the concentrations of l-ascorbic acid (AA) and sucrose. The concentrations of both AA and dehydroascorbic acid (DHAA), as well as the concentrations of all major sugars, decreased consistently with an increasing temperature over the temperature range 12-24 °C. Fructose and glucose were the predominant sugars with concentrations several fold higher than that for sucrose. AA was the main contributor to the total ascorbate pool in black currant berries. The AA/DHAA ratio varied from 5.6 to 10.3 among the studied cultivars. The concentration of citric acid, which was the predominant organic acid in black currant berries, increased with an increasing temperature, whereas the opposite trend was observed for malic and shikimic acid. Quninic acid was always present at relatively low concentrations. By contrast, photoperiod had no significant effect on berry content of any of the investigated compounds.

CONCLUSION

It is concluded that the post-flowering temperature has marked effects on the concentration of important chemical compounds responsible for taste and nutritional value of black currant berries, whereas photoperiod has no such effect in the studied cultivars. © 2016 Society of Chemical Industry.

Tomato SlGGP-LIKE gene participates in plant responses to chilling stress and pathogenic infection

Plants are always exposed to abiotic and biotic stresses which can adversely affect their growth and development. As an important antioxidant, AsA plays a vital role in plant defence against damage caused by stresses. In this study, we cloned a tomato GDP-_L-galactose phosphorylase-like (SlGGP-LIKE) gene and investigated its role in resistance to abiotic and biotic stresses by using antisense transgenic (AS) tomato lines. The AsA content in AS plants was lower than that in WT plants. Under chilling stress, the growth of AS plants was inhibited significantly, and they yielded higher levels of ROS, REC and MDA but demonstrated weaker APX activity than that shown by WT plants. Additionally, the declined values of Pn, Fv/Fm, oxidisable P700, and D1 protein content of PSII in AS lines were significant. Furthermore, the effect on xanthophyll cycle of AS plants was more severe than that on WT plants, and the ratio of zeaxanthin (Z)/(V + A + Z) and (Z + 0.5 A)/(V + A + Z) in AS lines was lower than that in WT plants. In spite of chilling stress, under Pseudomonas syringae pv.tomato (Pst) DC3000 strain infection, AS plants showed lesser bacterial cell growth and dead cells than those shown by WT plants. This finding indicated that AS plants demonstrated stronger resistance against pathogenic infection. Results suggest that SlGGP-LIKE gene played an important role in plant defence against chilling stress and pathogenic infection.

Ascorbic Acid-A Potential Oxidant Scavenger and Its Role in Plant Development and Abiotic Stress Tolerance

Over-production of reactive oxygen species (ROS) in plants under stress conditions is a common phenomenon. Plants tend to counter this problem through their ability to synthesize ROS neutralizing substances including non-enzymatic and enzymatic antioxidants. In this context, ascorbic acid (AsA) is one of the universal non-enzymatic antioxidants having substantial potential of not only scavenging ROS, but also modulating a number of fundamental functions in plants both under stress and non-stress conditions. In the present review, the role of AsA, its biosynthesis, and cross-talk with different hormones have been discussed comprehensively. Furthermore, the possible involvement of AsA-hormone crosstalk in the regulation of several key physiological and biochemical processes like seed germination, photosynthesis, floral induction, fruit expansion, ROS regulation and senescence has also been described. A simplified and schematic AsA biosynthetic pathway has been drawn, which reflects key intermediates involved therein. This could pave the way for future research to elucidate the modulation of plant AsA biosynthesis and subsequent responses to environmental stresses. Apart from discussing the role of different ascorbate peroxidase isoforms, the comparative role of two key enzymes, ascorbate peroxidase (APX) and ascorbate oxidase (AO) involved in AsA metabolism in plant cell apoplast is also discussed particularly focusing on oxidative stress perception and amplification. Limited progress has been made so far in terms of developing transgenics which could over-produce AsA. The prospects of generation of transgenics overexpressing AsA related genes and exogenous application of AsA have been discussed at length in the review.

Influence of storage temperature and low-temperature conditioning on the levels of health-promoting compounds in Rio Red grapefruit

Commercial operations use low‐temperature conditioning of citrus fruits to reduce the incidence of chilling injury (CI) during cold storage and quarantine treatments. Rio Red grapefruits (Citrus paradisi Macf) were stored for 12 weeks at 11°C or 5°C; an additional set was temperature conditioned at 16°C for 7 days before storing at 5°C (CD). Every 3 weeks, samples were assessed for chilling injury (CI) and health‐promoting compounds such as ascorbic acid, carotenoids, limonoids, flavonoids, and furocoumarins. Low‐temperature conditioning significantly reduced CI but did not affect the total soluble solids, acidity, and ripening ratio. After 12 weeks of storage, grapefruits showed no significant differences in lycopene, narirutin, poncirin, furocoumarins, and radical scavenging activity in all the three treatments. Limonin was significantly higher (p < .05) in CD fruits, nomilin was significantly higher in fruits stored at 11°C, whereas fruits stored at 5°C had lower levels of naringin, neohesperidin, and didymin after 12 weeks of storage. Low‐temperature conditioning treatment helped fruits to retain similar or higher levels of most of the health‐promoting compounds by the end of storage period while maintaining better quality than the nonconditioned fruits.