The role of L-ascorbic acid recycling in responding to environmental stress and in promoting plant growth

Exogenously Applied Ascorbic Acid-Mediated Changes in Osmoprotection and Oxidative Defense System Enhanced Water Stress Tolerance in Different Cultivars of Safflower (Carthamus tinctorious L.)

The present study was conducted to examine the effect of exogenously applied ascorbic acid (AsA) on osmoprotectants and the oxidative defense system in four cultivars (16171, 16183, 16207 and 16246) of safflower under well-watered and water deficit conditions. Water stress (60% field capacity) significantly decreased the shoot and root fresh and dry weights, shoot and root lengths and chlorophyll contents in all four safflower cultivars, while it increased the leaf free proline, total phenolics, total soluble proteins, hydrogen peroxide content and activities of catalase, superoxide dismutase and peroxidase enzymes. Foliar-applied (100 mg L-1 and 150 mg L-1) ascorbic acid caused a marked improvement in shoot and root fresh and dry weights, plant height, chlorophyll and AsA contents as well as the activity of peroxidase (POD) enzyme particularly under water deficit conditions. It also increased the accumulation of leaf proline, total phenolics, total soluble proteins and glycine betaine (GB) content in all four cultivars. Exogenously applied AsA lowered the contents of MDA and H2O2, and the activities of CAT and SOD enzymes. Overall, exogenously applied AsA had a positive effect on the growth of safflower plants under water deficit conditions which could be related to AsA-induced enhanced osmoprotection and regulation of antioxidant defense system.

Ascorbate and Thiamin: Metabolic Modulators in Plant Acclimation Responses

Cell compartmentalization allows incompatible chemical reactions and localised responses to occur simultaneously, however, it also requires a complex system of communication between compartments in order to maintain the functionality of vital processes. It is clear that multiple such signals must exist, yet little is known about the identity of the key players orchestrating these interactions or about the role in the coordination of other processes. Mitochondria and chloroplasts have a considerable number of metabolites in common and are interdependent at multiple levels. Therefore, metabolites represent strong candidates as communicators between these organelles. In this context, vitamins and similar small molecules emerge as possible linkers to mediate metabolic crosstalk between compartments. This review focuses on two vitamins as potential metabolic signals within the plant cell, vitamin C (L-ascorbate) and vitamin B₁ (thiamin). These two vitamins demonstrate the importance of metabolites in shaping cellular processes working as metabolic signals during acclimation processes. Inferences based on the combined studies of environment, genotype, and metabolite, in order to unravel signaling functions, are also highlighted.

Cu-based metal–organic framework HKUST-1 as effective catalyst for highly sensitive determination of ascorbic acid

In this work, a Cu-based nanosheet metal–organic framework (MOF), HKUST-1, was synthesised using a solvent method at room temperature. Its morphology, structure and composition were characterised by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), powder X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman spectroscopy, nitrogen adsorption and desorption isotherms, energy dispersive X-ray spectroscopy (EDS) and elemental analysis (EA). This material was then loaded onto the surface of an indium tin oxide (ITO) electrode to catalyse the electrochemical oxidation of ascorbic acid (AA). An equal-electron-equal-proton reaction was deduced from the pH investigation, and a diffusion-controlled process was reinforced by the dynamics study. Under optimal conditions, the oxidation peak current at +0.02 V displayed a linear relationship with the concentration of AA within the ranges of 0.01–25 and 25–265 mM, respectively. The limit of detection (LOD) was 3 μM at S/N of 3. The superb response could be ascribed to the porous nanosheet structure of HKUST-1, which enhanced both the effective surface area and the electron transfer ability significantly. Moreover, the novel AA sensor demonstrated good reproducibility, favourable stability and high sensitivity towards glucose, uric acid (UA), dopamine (DA) and several amino acids. It was also successfully applied to the real sample testing of various AA-containing tablets.

In this work, a Cu-based nanosheet metal–organic framework (MOF), HKUST-1, was synthesised using a solvent method at room temperature and it demonstrated high capability and sensitivity towards the oxidation of ascorbic acid (AA).

Exploration and quantification of ascorbate affecting peroxidase-catalyzed chromogenic reactions with a recirculating-flow catalysis detection system

This research used a recirculating-flow catalysis detection system to explore the ascorbate interference with peroxidase-catalyzed reactions and simultaneously determined peroxidase and ascorbate with a kinetic curve.

Simultaneous quantification of peroxidase and ascorbic acid in biosamples with an automatic system based on a Fe(iii)/methylthymol blue-carbon dot simulative enzyme

Simultaneous and automatic quantification of peroxidase and ascorbic acid based on one reaction system and application of a carbon dot simulative enzyme.

Morphological and Physiological Stress Responses of Lettuce to Different Intensities of Continuous Light

In this study, specific dynamic changes in growth, oxidative stress, ascorbate metabolism, and chlorophyll fluorescence were monitored during 12 days in lettuce plants exposed to continuous light (CL) of different intensities: low light (LL, 100 μmol·m-2·s-1), medium light (ML, 200 μmol·m-2·s-1), and high light (HL, 300 μmol·m-2·s-1). Lettuce plants grown under CL of higher light intensity gained greater biomass, dry weight ratio, root/shoot ratio, and specific leaf FW, but not leaf area. Both the reactive oxygen species (ROS) production and the lipid peroxidation degree, measured in terms of the malondialdehyde (MDA) levels, were progressively enhanced by increasing the light intensity of CL. Overall, the pool sizes of ascorbate (AsA) and glutathione, as well as the activities of enzymes involved in AsA metabolism, had positive correlations with light intensity under CL. Ascorbate peroxidase and dehydroascorbate reductase presented the maximal and minimal responses to light intensity, respectively, among all the studied enzymes. After 6 days under CL, ML and HL intensity caused reversible photoinhibition, represented by lower values of maximum quantum efficiency (F v /F m), effective quantum yield (ΦPSII), and photochemical quenching (qP) and a higher value of non-photochemical quenching (qN). However, this photoinhibition recovered on day 12 with increasing of F v /F m, ΦPSII, and qP. Taken together, under ML and HL conditions, greater AsA level could help maintain photosynthetic efficiency by elevating excess excitation energy dissipation, though ROS accumulation and lipid peroxidation could not be prevented in the long-term. Likewise, there was no dark period under LL condition, but no photooxidative stress was observed in lettuce. Thus, it is concluded that photooxidative stress induced by CL can be attributed to excessive daily light integral instead of circadian asynchrony.

Vitamin C in Plants: From Functions to Biofortification

Vitamin C (l-ascorbic acid) is an excellent free radical scavenger, not only for its capability to donate reducing equivalents but also for the relative stability of the derived monodehydroascorbate radical. However, vitamin C is not only an antioxidant, since it is also a cofactor for numerous enzymes involved in plant and human metabolism. In humans, vitamin C takes part in various physiological processes, such as iron absorption, collagen synthesis, immune stimulation, and epigenetic regulation. Due to the functional loss of the gene coding for l-gulonolactone oxidase, humans cannot synthesize vitamin C; thus, they principally utilize plant-based foods for their needs. For this reason, increasing the vitamin C content of crops could have helpful effects on human health. To achieve this objective, exhaustive knowledge of the metabolism and functions of vitamin C in plants is needed. In this review, the multiple roles of vitamin C in plant physiology as well as the regulation of its content, through biosynthetic or recycling pathways, are analyzed. Finally, attention is paid to the strategies that have been used to increase the content of vitamin C in crops, emphasizing not only the improvement of nutritional value of the crops but also the acquisition of plant stress resistance.

Botrytis cinerea differentially induces postharvest antioxidant responses in 'Braeburn' and 'Golden Delicious' apple fruit

BACKGROUND

The fruit of two apple cultivars - 'Braeburn', which is susceptible to inoculation with Botrytis cinerea, and the less susceptible cv. 'Golden Delicious' - were investigated with respect to their response to inoculation with B. cinerea. Successful infection by B. cinerea leads to an oxidative burst and perturbation of plant redox homeostasis. To investigate the interaction between apple fruit and B. cinerea, antioxidant metabolism in fruit samples from sun-exposed and shaded sides of different tissue types was measured over time.

RESULTS

The sun-exposed tissue of 'Braeburn' had higher initial levels of total vitamin C in the peel and phenolic compounds in the flesh than 'Golden Delicious', despite its greater susceptibility to gray mold. A substantial antioxidant response was recorded in diseased 'Braeburn' fruit 14 days after inoculation, which involved an elevated superoxide dismutase activity and ascorbate peroxidase activity, a progressive oxidation of total vitamin C, and a decrease in peroxidase activity and phenolic content. Disease development was slower on the sun-exposed sides than on the shaded sides.

CONCLUSION

The two cultivars appeared to utilize different strategies to defend themselves against B. cinerea. 'Golden Delicious' almost entirely escaped infection. Preharvest exposure of apple fruit to high light / temperature stress appears to prepare them to better resist subsequent postharvest attack and disease. © 2019 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Altered regulation of porphyrin biosynthesis and protective responses to acifluorfen-induced photodynamic stress in transgenic rice expressing Bradyrhizobium japonicum Fe-chelatase

We examined the molecular regulation of porphyrin biosynthesis and protective responses in transgenic rice (Oryza sativa) expressing Bradyrhizobium japonicum Fe-chelatase (BjFeCh) after treatment with acifluorfen (AF). During the photodynamic stress imposed by AF, transcript levels of BjFeCh in transgenic plants increased greatly; moreover, transcript levels of OsFeCh2 remained almost constant, whereas in wild type (WT) plants they were considerably down-regulated. In the heme branch, transgenic plants exhibited greater levels of OsFC and HO transcripts than WT plants in the untreated stems as well as in the AF-treated leaves and stems. Both WT and transgenic plants treated with AF substantially decreased transcript levels for all the genes in the chlorophyll branch, with less decline in transgenic plants. After AF treatment, ascorbate (Asc) content and the redox Asc state greatly decreased in leaves of WT plants; however, in transgenic plants both parameters remained constant in leaves and the Asc redox state increased by 20% in stems. In response to AF, the leaves of WT plants greatly up-regulated CatA, CatB, and GST compared to those of transgenic plants, whereas, in the stems, transgenic plants showed higher levels of CatA, CatC, APXb, BCH, and VDE. Photochemical quenching, q_P, was considerably dropped by 31% and 18% in WT and transgenic plants, respectively in response to AF, whereas non-radiative energy dissipation through non-photochemical quenching increased by 77% and 38% in WT and transgenic plants, respectively. Transgenic plants treated with AF exhibited higher transcript levels of nucleus-encoded photosynthetic genes, Lhcb1 and Lhcb6, as well as levels of Lhcb6 protein compared to those of WT plants. Our study demonstrates that expression of BjFeCh in transgenic plants influences not only the regulation of porphyrin biosynthesis through maintaining higher levels of gene expression in the heme branch, but also the Asc redox function during photodynamic stress caused by AF.

Mechanisms underlying the enhanced biomass and abiotic stress tolerance phenotype of an Arabidopsis MIOX over-expresser

Myo-inositol oxygenase (MIOX) is the first enzyme in the inositol route to ascorbate (L-ascorbic acid, AsA, vitamin C). We have previously shown that Arabidopsis plants constitutively expressing MIOX have elevated foliar AsA content and displayed enhanced growth rate, biomass accumulation, and increased tolerance to multiple abiotic stresses. In this work, we used a combination of transcriptomics, chromatography, microscopy, and physiological measurements to gain a deeper understanding of the underlying mechanisms mediating the phenotype of the AtMIOX4 line. Transcriptomic analysis revealed increased expression of genes involved in auxin synthesis, hydrolysis, transport, and metabolism, which are supported by elevated auxin levels both in vitro and in vivo, and confirmed by assays demonstrating their effect on epidermal cell elongation in the AtMIOX4 over-expressers. Additionally, we detected up-regulation of transcripts involved in photosynthesis and this was validated by increased efficiency of the photosystem II and proton motive force. We also found increased expression of amylase leading to higher intracellular glucose levels. Multiple gene families conferring plants tolerance/expressed in response to cold, water limitation, and heat stresses were found to be elevated in the AtMIOX4 line. Interestingly, the high AsA plants also displayed up-regulation of transcripts and hormones involved in defense including jasmonates, defensin, glucosinolates, and transcription factors that are known to be important for biotic stress tolerance. These results overall indicate that elevated levels of auxin and glucose, and enhanced photosynthetic efficiency in combination with up-regulation of abiotic stresses response genes underly the higher growth rate and abiotic stresses tolerance phenotype of the AtMIOX4 over-expressers.

The mitochondrial oxidation resistance protein AtOXR2 increases plant biomass and tolerance to oxidative stress

This study demonstrates the existence of the oxidation resistance (OXR) protein family in plants. There are six OXR members in Arabidopsis that contain the highly conserved TLDc domain that is characteristic of this eukaryotic protein family. AtOXR2 is a mitochondrial protein able to alleviate the stress sensitivity of a yeast oxr1 mutant. It was induced by oxidative stress and its overexpression in Arabidopsis (oeOXR2) increased leaf ascorbate, photosynthesis, biomass, and seed production, as well as conferring tolerance to methyl viologen, antimycin A, and high light intensities. The oeOXR2 plants also showed higher ABA content, changes in ABA sensitivity, and modified expression of ABA- and stress-regulated genes. While the oxr2 mutants had a similar shoot phenotype to the wild-type, they exhibited increased sensitivity to stress. We propose that by influencing the levels of reactive oxygen species (ROS), AtOXR2 improves the efficiency of photosynthesis and elicits basal tolerance to environmental challenges that increase oxidative stress, allowing improved plant growth and biomass production.

Plant Cytosolic Ascorbate Peroxidase with Dual Catalytic Activity Modulates Abiotic Stress Tolerances

Ascorbic acid-glutathione (AsA-GSH) cycle represents important antioxidant defense system in planta. Here we utilized Oncidium cytosolic ascorbate peroxidase (OgCytAPX) as a model to demonstrate that CytAPX of several plants possess dual catalytic activity of both AsA and GSH, compared with the monocatalytic activity of Arabidopsis APX (AtCytAPX). Structural modeling and site-directed mutagenesis identified that three amino acid residues, Pro⁶³, Asp⁷⁵, and Tyr⁹⁷, are required for oxidization of GSH in dual substrate catalytic type. Enzyme kinetic study suggested that AsA and GSH active sites are distinctly located in cytosolic APX structure. Isothermal titration calorimetric and UV-visible analysis confirmed that cytosolic APX is a heme-containing protein, which catalyzes glutathione in addition to ascorbate. Biochemical and physiological evidences of transgenic Arabidopsis overexpressing OgCytAPX1 exhibits efficient reactive oxygen species-scavenging activity, salt and heat tolerances, and early flowering, compared with Arabidopsis overexpressing AtCytAPX. Thus results on dual activity CytAPX impose significant advantage on evolutionary adaptive mechanism in planta.

Biosynthetic Gene Pyramiding Leads to Ascorbate Accumulation with Enhanced Oxidative Stress Tolerance in Tomato

Ascorbic acid (AsA) has high antioxidant activities, and its biosynthesis has been well studied by engineering of a single structural gene (SG) in staple crops, such as tomato (Solanum lycopersicum). However, engineering the AsA metabolic pathway by multi-SG for biofortification remains unclear. In this study, pyramiding transgenic lines including GDP-Mannose 3',5'-epimerase (GME) × GDP-d-mannose pyrophosphorylase (GMP), GDP-l-Gal phosphorylase (GGP) × l-Gal-1-P phosphatase (GPP) and GME × GMP × GGP × GPP, were obtained by hybridization of four key genes to get over-expression transgenic plants (GME, GMP, GGP, and GPP) in tomato. Pyramiding lines exhibited a significant increase in total ascorbate in leaves and red fruits except for GGP × GPP. Expression analysis indicated that increased accumulation of AsA in pyramiding transgenic lines is due to multigene regulation in AsA biosynthesis. Substrate feeding in leaf and fruit suggested that AsA biosynthesis was mainly contributed by the d-Man/l-Gal pathway in leaves, while alternative pathways may contribute to AsA accumulation in tomato fruit. Pyramiding lines showed an enhanced light response, stress tolerance, and AsA transport capacity. Also, fruit shape, fruit size, and soluble solids were slightly affected by pyramiding. This study provides the first comprehensive analysis of gene pyramiding for ascorbate biosynthesis in tomato. SGs pyramiding promotes AsA biosynthesis, which in turn enhances light response and oxidative stress tolerance. Also, the data revealed an alternative ascorbate biosynthesis pathway between leaves and fruit of tomato.

A novel MYB transcription factor regulates ascorbic acid synthesis and affects cold tolerance

Dehydroascorbate reductase (DHAR) plays an important role in stress responses, but the transcriptional regulation of DHAR in response to abiotic stress is still poorly understood. In this study, we isolated a novel R2R3-type MYB transcription factor from Pyrus betulaefolia by yeast one-hybrid screening, designated as PbrMYB5. PbrMYB5 was localized in the nucleus and could bind specifically to the promoter of PbrDHAR2. PbrMYB5 was greatly induced by cold and salt but slightly by dehydration. Overexpression of PbrMYB5 in tobacco conferred enhanced tolerance to chilling stresses, whereas down-regulation of PbrMYB5 in P. betulaefolia by virus-induced gene silencing resulted in elevated chilling sensitivity. Transgenic tobacco exhibited higher expression levels of NtDHAR2 and accumulated larger amount of ascorbic acid (AsA) than the wild-type plants. Virus-induced gene silencing of PbrMYB5 in P. betulaefolia down-regulated PbrDHAR2 abundance and decreased AsA level, accompanied by an increased sensitivity to the chilling stress. Taken together, these results demonstrated that PbrMYB5 was an activator of AsA biosynthesis and may play a positive role in chilling tolerance, at least in part, due to the modulation of AsA synthesis by regulating the PbrDHAR2 expression.

The Mitochondrial Thioredoxin System Contributes to the Metabolic Responses Under Drought Episodes in Arabidopsis

Thioredoxins (Trxs) modulate metabolic responses during stress conditions; however, the mechanisms governing the responses of plants subjected to multiple drought events and the role of Trxs under these conditions are not well understood. Here we explored the significance of the mitochondrial Trx system in Arabidopsis following exposure to single and repeated drought events. We analyzed the previously characterized NADPH-dependent Trx reductase A and B double mutant (ntra ntrb) and two independent mitochondrial thioredoxin o1 (trxo1) mutant lines. Following similar reductions in relative water content (∼50%), Trx mutants subjected to two drought cycles displayed a significantly higher maximum quantum efficiency (Fv/Fm) and were less sensitive to drought than their wild-type counterparts and than all genotypes subjected to a single drought event. Trx mutant plants displayed a faster recovery after two cycles of drought, as observed by the higher accumulation of secondary metabolites and higher stomatal conductance. Our results indicate that plants exposed to multiple drought cycles are able to modulate their subsequent metabolic and physiological response, suggesting the occurrence of an exquisite acclimation in stressed Arabidopsis plants. Moreover, this differential acclimation involves the participation of a set of metabolic changes as well as redox poise alteration following stress recovery.

Ozone risk assessment of castor (Ricinus communis L.) cultivars using open top chamber and ethylenediurea (EDU)

Castor bean (Ricinus communis L.) an important non-edible oilseed crop, is a prominent feed stock towards the generation of renewable materials for industrial production which has multiple applications ranging from cosmetics to biofuels industry. India accounts for 76% of the total world production of castor oil seed. However, major concern for developing countries like India where expanding economy led to rapid increases in gases like NOx, CO and VOCs photochemically form ozone. Ozone is strong oxidant that damages agriculture, ecosystems, and materials with considerable reduction in crop yields and crop quality. One way to reduce ozone induced loss is to focus on the adapting crops to ozone exposure by selecting cultivars with demonstrated ozone resistance. An experiment was conducted for ozone risk assessment of castor cultivars to select cultivar with demonstrated resistance against ozone pollution. This study comprise an open top chamber experiment with three treatments viz. (i) control (ambient ozone concentration), (ii) enhanced ozone (average 75 ppb for 4 h daily throughout the growing season), and (iii) EDU application. Results suggested that the ozone pollution substantially affected growth and physiology of castor cultivars. Crop biomass and yield was also negatively influenced by ozone pollution. Developed defence provided strength to withstand against ozone pollution to the experimental crop cultivars. However, developed defence is cultivar specific and positively correlated with the resistance against ozone pollution. Study concluded that the damage to ozone is directly dependent on the antioxidative potential of plant species. However, ozone adaptability is based on the genetic makeup of the cultivar and yield related loss to ozone can be minimizing by selecting ozone tolerant variety as seen in cultivar Nidhi-999.

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

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

Differential response of quinoa genotypes to drought and foliage-applied H2O2 in relation to oxidative damage, osmotic adjustment and antioxidant capacity

Quinoa (Chenopodium quinoa Willd.), a highly nutritious grain crop, is resistant to abiotic stresses (drought, salinity, and cold) and offers an alternate crop to endure harsh environmental conditions under the face of climate change. Naturally, quinoa genome displays a wide degree of variabilities in drought tolerance strategies. Therefore, the present study was designed to investigate drought tolerance variations and stress tolerance enhancement in four quinoa genotypes (Pichaman, Colorado-407D, IESP and 2-Want) thorough foliage-applied H₂O₂ with the purpose of identifying suitable genotype for water limited environments. The plants were exposed to two watering regimes (75% and 30% pot WHC) and foliage-applied H₂O₂ treatments (15 mM). The drought stress significantly reduced plant growth, relative water contents, chlorophyll and carotenoids contents and increased ROS production (H₂O₂ and O₂^•-) resulting in higher oxidative damage in all quinoa genotypes. Besides, drought stress significantly enhanced the antioxidants (SOD, PPO, and PAL) activity, total soluble sugars, proline, AsA contents and increased the total accumulation of measured inorganic ions in all quinoa genotypes. The PCA analysis indicated that parameters related to osmotic adjustment and antioxidant capacity were more pronounced in 2-Want and IESP genotypes, while parameters depicting oxidative damage were higher in Colorado-407D and more specifically in Pichaman. However, foliage-applied H₂O₂ effectively improved the osmolytes accumulation, antioxidants activity and K⁺/Na⁺ ratio which increased water relations, reduced lipid peroxidation and ultimately resulted in higher plant growth. Overall, 2-Want and IESP genotypes were found relatively more drought resistant, while exogenous application of H₂O₂ can be opted for more improvement in osmotic adjustment and antioxidant system, which may further enhance drought tolerance, even in sensitive genotypes of quinoa, such as Pichaman.

Effects of Nitrogen Level during Seed Production on Wheat Seed Vigor and Seedling Establishment at the Transcriptome Level

Nitrogen fertilizer is a critical determinant of grain yield and seed quality in wheat. However, the mechanism of nitrogen level during seed production affecting wheat seed vigor and seedling establishment at the transcriptome level remains unknown. Here, we report that wheat seeds produced under different nitrogen levels (N0, N168, N240, and N300) showed significant differences in seed vigor and seedling establishment. In grain yield and seed vigor, N0 and N240 treatments showed the minimum and maximum, respectively. Subsequently, we used RNA-seq to analyze the transcriptomes of seeds and seedlings under N0 and N240 at the early stage of seedling establishment. Gene Ontology (GO) term enrichment analysis revealed that dioxygenase-activity-related genes were dramatically upregulated in faster growing seedlings. Among these genes, the top three involved linoleate 9S-lipoxygenase (Traes_2DL_D4BCDAA76, Traes_2DL_CE85DC5C0, and Traes_2DL_B5B62EE11). Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed that pathways involved in nutrient mobilization and the antioxidant system showed enhanced expression under N240. Moreover, seeds with faster growing seedlings had a higher gene expression level of α-amylase, which was consistent with α-amylase activity. Taken together, we propose a model for seedling establishment and seed vigor in response to nitrogen level during seed production.

Plant Growth Modulates Metabolites and Biological Activities in Retama raetam (Forssk.) Webb

This work focuses on the variability of Retama raetam (Forssk.) Webb bioactive compounds as a function of the plant cycle. The main results showed that it exhibited the highest percentage of polyunsaturated fatty acids, along with superior levels of vitamin C and total phenolic compounds (66.49%, 645.6 mg·100 g⁻¹ FW and 23.9 mg GAE·g⁻¹, respectively) at the vegetative stage. Instead, at the flowering and mature fruiting stages, R. raetam (Forssk.) Webb exhibited notable contents of proline (25.4 μmol·g⁻¹ DW) and carotenoids (27.2 μg·g⁻¹ FW), respectively. The gathered data concerning the antioxidant activity highlighted the effectiveness of the vegetative stage in comparison to the other periods. Actually, IC₅₀ and EC₅₀ values of the hydromethanolic extract obtained from the plant shoots at the vegetative stage were of 23, 380, 410, 1160 and 960 μg·mL⁻¹ (DPPH^• and ABTS^•+ radicals scavenging activity, reducing power, chelating power and β-carotene bleaching activity, respectively). Furthermore, the four studied stages showed appreciable antibacterial capacities against human pathogens with a higher efficiency of the vegetative stage extract. Finally, the LC-DAD-ESI/MSⁿ analysis revealed the predominance of isoflavonoids as main class of phenolic compounds and demonstrates that individual phenolic biosynthesis was clearly different as a function of plant growth. These findings highlight that reaching the optimum efficiency of R. raetam (Forssk.) Webb is closely linked to the physiological stage.

ABA-dependent salt and drought stress improve strawberry fruit quality

Strawberry crop is very sensitive to osmotic stress conditions. We investigated the effect of the stress induced by mild drought (DS) and salt (SS) stresses, on molecular, physiological, and metabolic processes in the strawberry crop (Fragaria ananassa), cv. Camarosa. The results showed that the fruit yield was not affected. Mild DS and SS also resulted in an increased content of phenolics, anthocyanins, and l-ascorbic acid, and an increased antioxidant activity. These effects were accompanied by increased levels of ABA and its derivatives (phaseic and dehydrophasic acids), alongside the upregulation of several genes involved on their synthesis. Therefore, the results obtained in this study suggest that mild DS and SS improve the functional quality of strawberry fruits through and ABA-dependent mechanism.

Effect of Exogenous General Plant Growth Regulators on the Growth of the Duckweed Lemna minor

Gibberellic acid (GA₃), indole-3-acetic acid (IAA), salicylic acid (SA), abscidic acid (ABA), jasmonic acid (JA) 1-amino cyclopropane-1-carboxylic acid (ACC) and aminoethoxyvinylglycine (AVG) are popular growth regulators of plants. However, the effects of their exogenous addition on the biomass production of aquatic plants, including Lemnoideae plants, "duckweeds," are largely unknown. In this study, the growth of Lemna minor was tested for 10 d in Hoagland medium containing each compound at different concentrations of 0-50 μM. GA₃, IAA, and SA were found to have no apparent positive effect on the growth at all concentrations tested. Conversely, ACC and JA moderately and AVG and ABA severely inhibited the growth of L. minor. Among the tested compounds, ascorbic acid had an apparent growth-promoting effect.

Characterization of a Novel Mitochondrial Ascorbate Transporter From Rat Liver and Potato Mitochondria

The Mitochondrial Ascorbic Acid Transporter (MAT) from both rat liver and potato mitochondria has been reconstituted in proteoliposomes. The protein has a molecular mass in the range of 28–35 kDa and catalyzes saturable, temperature and pH dependent, unidirectional ascorbic acid transport. The transport activity is sodium independent and it is optimal at acidic pH values. It is stimulated by proton gradient, thus supporting that ascorbate is symported with H⁺. It is efficiently inhibited by the lysine reagent pyridoxal phosphate and it is not affected by inhibitors of other recognized plasma and mitochondrial membranes ascorbate transporters GLUT1(glucose transporter-1) or SVCT2 (sodium-dependent vitamin C transporter-2). Rat protein catalyzes a cooperative ascorbate transport, being involved two binding sites; the measured K_0.5 is 1.5 mM. Taking into account the experimental results we propose that the reconstituted ascorbate transporter is not a GLUT or SVCT, since it shows different biochemical features. Data of potato transporter overlap the mammalian ones, except for the kinetic parameters non-experimentally measurable, thus supporting the MAT in plants fulfills the same transport role.

Ascorbate-mediated regulation of growth, photoprotection, and photoinhibition in Arabidopsis thaliana

The requirements for ascorbate for growth and photosynthesis were assessed under low (LL; 250 µmol m-2 s-1) or high (HL; 1600 µmol m-2 s-1) irradiance in wild-type Arabidopsis thaliana and two ascorbate synthesis mutants (vtc2-1 and vtc2-4) that have 30% wild-type ascorbate levels. The low ascorbate mutants had the same numbers of leaves but lower rosette area and biomass than the wild type under LL. Wild-type plants experiencing HL had higher leaf ascorbate, anthocyanin, and xanthophyll pigments than under LL. In contrast, leaf ascorbate levels were not increased under HL in the mutant lines. While the degree of oxidation measured using an in vivo redox reporter in the nuclei and cytosol of the leaf epidermal and stomatal cells was similar under both irradiances in all lines, anthocyanin levels were significantly lower in the low ascorbate mutants than in the wild type under HL. Differences in the photosynthetic responses of vtc2-1 and vtc2-4 mutants were observed. Unlike vtc2-1, the vtc2-4 mutants had wild-type zeaxanthin contents. While both low ascorbate mutants had lower levels of non-photochemical quenching of chlorophyll a fluorescence (NPQ) than the wild type under HL, qPd values were greater only in vtc2-1 leaves. Ascorbate is therefore essential for growth but not for photoprotection.

The induction of salt stress tolerance by propyl gallate treatment in green microalga Dunaliella bardawil, through enhancing ascorbate pool and antioxidant enzymes activity

The effect of propyl gallate (PG), a synthetic antioxidant, on antioxidant responses and salinity tolerance was investigated in the cells of the green microalga, Dunaliella bardawil. Algal suspensions grown at three salinity levels of 1, 2, and 3 M NaCl were incubated with 1 mM of PG. The number of cells was significantly lower in all PG-treated cells compared to untreated controls. Despite PG-induced cell death, the fresh weight of all PG-treated cells was considerably higher than controls. PG-treated cells had enhanced antioxidant capacity because of increased levels of Chlorophyll a, β-carotene, reduced ascorbate, protein, and enzymatic activities, but accumulated lower levels of malonyldialdehyde and hydrogen peroxide compared to untreated cells. The results suggest that PG acts as a signal molecule both directly by reducing of free radical oxidants and indirectly by augmenting ascorbate pool levels, β-carotene production, and antioxidant enzymes activity to boost the capacity of antioxidant systems and radical oxygen species scavenging. Therefore, induction of salt stress tolerance by PG in D. bardawil is associated with metabolic adjustments through activation or synthesis of both enzymatic and non-enzymatic molecules involved in antioxidant systems.

Is monodehydroascorbate reductase activity in leaf tissue critical for the maintenance of yield in tomato?

Ascorbate redox metabolism and growth have been shown to be linked and related to the activity of enzymes that produce or remove the radical monodehydroascorbate, the semi-oxidized form of ascorbate (ascorbate oxidase or peroxidase and monodehydroascorbate reductase respectively). Previous work in cherry tomato has revealed correlations between monodehydroascorbate reductase and ascorbate oxidase activity and fruit yield: decreased whole plant MDHAR activity decreases yield while decreased whole plant ascorbate oxidase activity increases yield under unfavourable environmental conditions. We aimed to investigate if similar effects on yield are obtained in a large-fruited variety of tomato, Moneymaker. Furthermore we wished to establish whether previously observed effects on yield in cherry tomato following changes in whole plant enzyme activity could be reproduced by reducing MDHAR activity in fruit only by using a fruit-specific promoter in cherry tomato (West Virginia 106). In Moneymaker, RNAi lines for monodehydroascorbate reductase did not show significant yield decrease compared to control lines when plants were grown under optimal or non-optimal conditions of carbon stress generated by mature leaf removal. In addition, we show that a decrease in monodehydroascorbate reductase activity in fruit of cherry tomato had no effect on yield compared to a reduction in whole-plant monodehydroascorbate reductase activity: we therefore show that whole plant MDHAR activity is necessary to maintain yield in cherry tomato, suggesting that the carbon source in autotrophic tissue is more important than fruit sink activity. The present data also revealed differences between cherry and large fruited tomato that could be linked to a source of genetic variability in the response to monodehydroascorbate metabolism in tomato: maybe the domestication of tomato towards large-fruited lines could have affected the importance of MDHAR in yield maintenance.

Perturbations in carotenoid and porphyrin status result in differential photooxidative stress signaling and antioxidant responses

We examined differential photooxidative stress signaling and antioxidant responses in rice plants treated with norflurazon (NF) and oxyfluorfen (OF), which are inhibitors of carotenoid and porphyrin biosynthesis, respectively. Plants treated with OF markedly increased levels of cellular leakage and malondialdehyde, compared with NF-treated plants, showing that OF plants suffered greater oxidative damage with respect to membrane integrity. The enhanced production of H₂O₂ in response to OF, but not NF, indicates the important role of H₂O₂ in activation of photooxidative stress signaling in OF plants. In response to NF and OF, the increased levels of free salicylic acid as well as maintenance of the redox ratio of ascorbate and glutathione pools to a certain level are considered to be crucial factors in the protection against photooxidation. Plants treated with OF greatly up-regulated catalase (CAT) activity and Cat transcript levels, compared with NF-treated plants. Interestingly, NF plants showed no noticeable increase in oxidative metabolism, although they did show considerable increases in ascorbate peroxidase (APX) and peroxidase activities and transcript levels of APX, as in OF plants. Our results suggest that perturbations in carotenoid and porphyrin status by NF and OF can be sensed by differential photooxidative stress signaling, such as that involving H₂O₂, redox state of ascorbate and glutathione, and salicylic acid, which may be responsible for at least part of the induction of ROS-scavenging enzymes.

Genome-wide association study to identify candidate loci and genes for Mn toxicity tolerance in rice

Manganese (Mn) is an essential micro-nutrient for plants, but flooded rice fields can accumulate high levels of Mn²⁺ leading to Mn toxicity. Here, we present a genome-wide association study (GWAS) to identify candidate loci conferring Mn toxicity tolerance in rice (Oryza sativa L.). A diversity panel of 288 genotypes was grown in hydroponic solutions in a greenhouse under optimal and toxic Mn concentrations. We applied a Mn toxicity treatment (5 ppm Mn²⁺, 3 weeks) at twelve days after transplanting. Mn toxicity caused moderate damage in rice in terms of biomass loss and symptom formation despite extremely high shoot Mn concentrations ranging from 2.4 to 17.4 mg g^-1. The tropical japonica subpopulation was more sensitive to Mn toxicity than other subpopulations. Leaf damage symptoms were significantly correlated with Mn uptake into shoots. Association mapping was conducted for seven traits using 416741 single nucleotide polymorphism (SNP) markers using a mixed linear model, and detected six significant associations for the traits shoot manganese concentration and relative shoot length. Candidate regions contained genes coding for a heavy metal transporter, peroxidase precursor and Mn²⁺ ion binding proteins. The significant marker SNP-2.22465867 caused an amino acid change in a gene (LOC_Os02g37170) with unknown function. This study demonstrated significant natural variation in rice for Mn toxicity tolerance and the possibility of using GWAS to unravel genetic factors responsible for such complex traits.

Influence of natural and synthetic vitamin C (ascorbic acid) on primary and secondary metabolites and associated metabolism in quinoa (Chenopodium quinoa Willd.) plants under water deficit regimes

Phytoextracts are being widely used these days as a source of bioactive compounds for mitigating the harmful effects of abiotic stresses including drought stress. In this study, it was assessed how far foliar applied pure synthetic ascorbic acid (AsA) or natural sweet orange juice (OJ) enriched with AsA could mitigate the drought stress induced adverse effects on growth and some key metabolic processes in quinoa (Chenopodium quinoa Willd.; cultivar V₉) plants. Two weeks old quinoa seedlings were subjected to varying irrigation regimes as control [100% field capacity (FC)] and drought stress (60% FC, 40% FC and 20% FC). After one month of water deficit treatments, various levels of ascorbic acid (150 mg L^-1 AsA or 25% OJ) besides control [distilled water (DW) and no spray (NS)] were applied as a foliar spray. After 15 days of AsA application, different physio-biochemical attributes were measured. The results showed that water deficit markedly decreased plant growth, relative water content (RWC), photosynthetic rate, total carotenoids (CAR) and total flavonoids, while it increased relative membrane permeability (RMP), intrinsic AsA content, hydrogen peroxide (H₂O₂), malondialdehyde (MDA), glycinebetaine (GB), total phenolics, total soluble proteins (TSP), total free amino acids, activities of key antioxidant enzymes [superoxide dismutase (SOD), peroxidase (POD)], total soluble sugars (TSS), reducing (RS) and non-reducing sugars (NRS). Most obvious results of most of these parameters were observed at 40% and 20% FC. Foliar-applied pure 150 mg L^-1 AsA and 25% OJ were found to be very effective in improving plant growth, RMP, photosynthetic rate, CAR, proline, AsA, MDA, GB, TSP, free amino acids, SOD, POD, TSS, RS, NRS and total flavonoids. It was noticed that 25% OJ enriched with AsA and other essential nutrients and biomolecules was as efficient as 150 mg L^-1 AsA in reducing the adverse effects of drought stress on quinoa plants. So, it was concluded that OJ, a cheaper source of vitamin C, can be used as a mitigating agent for improving drought tolerance in plants under drought-prone environments.

Melatonin in plant signalling and behaviour

Melatonin is an indoleamine neurotransmitter that has recently become well established as an important multi-functional signalling molecule in plants. These signals have been found to induce several important physiological responses that may be interpreted as behaviours. The diverse processes in which melatonin has been implicated in plants have expanded far beyond the traditional roles for which it has been implicated in mammals, which include sleep, tropisms and reproduction. These functions, however, appear to also be important melatonin mediated processes in plants, though the mechanisms underlying these functions have yet to be fully elucidated. Mediation or redirection of plant physiological processes induced by melatonin can be summarised as a series of behaviours including, among others: herbivore defence, avoidance of undesirable circumstances or attraction to opportune conditions, problem solving and response to environmental stimulus. As the mechanisms of melatonin action are elucidated, its involvement in plant growth, development and behaviour is likely to expand beyond the aspects discussed in this review and hold promise for applications in diverse fundamental and applied plant sciences including conservation, cryopreservation, morphogenesis, industrial agriculture and natural health products.

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 Evolution of GDP-D-Mannose Epimerase (GME), a Key Gene in Plant Ascorbic Acid Biosynthesis

The widespread ascorbic acid (AsA) plays a vital role in plant development and abiotic stress tolerance, but AsA concentration varies greatly among different plants. GDP-D-mannose epimerase (GME), which catalyzes GDP-D-mannose to GDP-L-galactose or GDP-L-gulose, is a key enzyme in plant AsA biosynthesis pathway. Functions and expression patterns of GME have been well studied in previous works, however, little information is known about the evolutionary patterns of the gene. In this study, GME gene structure, corresponding conserved protein motifs and evolutionary relationships were systematically analyzed. A total of 111 GME gene sequences were retrieved from 59 plant genomes, which representing almost all the major lineages of Viridiplantae: dicotyledons, monocotyledons, gymnosperms, pteridophytes, bryophytes, and chlorophytes. Results showed that homologs of GME were widely present in Viridiplantae. GME gene structures were conservative in higher plants, while varied greatly in the basal subgroups of the phylogeny including lycophytes, bryophytes, and chlorophytes, suggesting GME gene structure might have undergone severe differentiation at lower plant and then gradually fixed as plant evolution. The basic motifs of GME were strongly conserved throughout Viridiplantae, suggesting the conserved function of the protein. Molecular evolution analysis showed that strong purifying selection was the predominant force in the evolution of GME. A few branches and sites under episodic diversifying selection were identified and most of the branches located in the subgroup of chlorphytes, indicating episodic diversifying selection at a few branches and sites may play a role in the evolution of GME and diversifying selection may have occurred at the early stage of Viridiplantae. Our results provide novel insights into functional conservation and the evolution of GME.

Comparative transcriptome analysis of duckweed (Landoltia punctata) in response to cadmium provides insights into molecular mechanisms underlying hyperaccumulation

Cadmium (Cd) is a detrimental environmental pollutant. Duckweeds have been considered promising candidates for Cd phytoremediation. Although many physiological studies have been conducted, the molecular mechanisms underlying Cd hyperaccumulation in duckweeds are largely unknown. In this study, clone 6001 of Landoltia punctata, which showed high Cd tolerance, was obtained by large-scale screening of over 200 duckweed clones. Subsequently, its growth, Cd flux, Cd accumulation, and Cd distribution characteristics were investigated. To further explore the global molecular mechanism, a comprehensive transcriptome analysis was performed. For RNA-Seq, samples were treated with 20 μM CdCl₂ for 0, 1, 3, and 6 days. In total, 9,461, 9,847, and 9615 differentially expressed unigenes (DEGs) were discovered between Cd-treated and control (0 day) samples. DEG clustering and enrichment analysis identified several biological processes for coping with Cd stress. Genes involved in DNA repair acted as an early response to Cd, while RNA and protein metabolism would be likely to respond as well. Furthermore, the carbohydrate metabolic flux tended to be modulated in response to Cd stress, and upregulated genes involved in sulfur and ROS metabolism might cause high Cd tolerance. Vacuolar sequestration most likely played an important role in Cd detoxification in L. punctata 6001. These novel findings provided important clues for molecular assisted screening and breeding of Cd hyperaccumulating cultivars for phytoremediation.

Enzymatic characterization and crystal structure analysis of Chlamydomonas reinhardtii dehydroascorbate reductase and their implications for oxidative stress

Dehydroascorbate reductase (DHAR) is a key enzyme for glutathione (GSH)-dependent reduction of dehydroascorbate (DHA) to recycled ascorbate (AsA) in plants, and plays a major role against the toxicity of reactive oxygen species (ROS). Previously, we proposed that the increase of AsA regeneration via enhanced DHAR activity modulates the ascorbate-glutathione cycle activity against photooxidative stress in Chlamydomonas reinhardtii. In the present work, we use site-directed mutagenesis and crystal structure analysis to elucidate the molecular basis of how C. reinhardtii DHAR (CrDHAR1) is involved in the detoxification mechanisms. Mutagenesis data show that the D21A, D21N and C22A mutations result in severe loss of the enzyme's function, suggesting crucial roles of Asp-21 and Cys-22 in substrate binding and catalysis. The mutant K11A also exhibits reduced redox activity (∼50%). The crystal structure of apo CrDHAR1 further provides insights into the proposed mechanism centering on the strictly conserved Cys-22, which is suggested to initiate the redox reactions of DHA and GSH. Furthermore, in vitro oxidation of the recombinant CrDHAR1 in the presence of 1 mM H₂O₂ has minor effects on the K_m for the substrates but significantly reduces the k_cat. The enzyme's activity and its mRNA abundance in the C. reinhardtii cells are increased by treatment with 0.2-1 mM H₂O₂ but decreased when H₂O₂ is ≥ 1.5 mM. The latter decrease is accompanied by oxidative damage and lower AsA concentrations. These biochemical and physiological data provide new insights into the catalytic mechanism of CrDHAR1, which protects the C. reinhardtii cells from oxidative stress-induced toxicity.

Plant growth under water/salt stress: ROS production; antioxidants and significance of added potassium under such conditions

Plants are confronted with a variety of environmenmtal stresses resulting in enhanced production of ROS. Plants require a threshold level of ROS for vital functions and any change in their concentration alters the entire physiology of plant. Delicate balance of ROS is maintained by an efficient functioning of intriguing indigenous defence system called antioxidant system comprising enzymatic and non enzymatic components. Down regulation of antioxidant system leads to ROS induced oxidative stress causing damage to important cellular structures and hence anomalies in metabolism. Proper mineral nutrition, in addition to other agricultural practices, forms an important part for growth and hence the yield. Potassium (K) is a key macro-element regulating growth and development through alterations in physiological and biochemical attributes. K has been reported to result into accumulation of osmolytes and augmentation of antioxidant components in the plants exposed to water and salt stress. In the present review an effort has been made to revisit the old findings and the current advances in research regarding the role of optimal, suboptimal and deficient K soil status on growth under normal and stressful conditions. Effect of K deficiency and sufficiency is discussed and the information about the K mediated antioxidant regulation and plant response is highlighted.

Identification of drought-responsive microRNAs in tomato using high-throughput sequencing

Drought is a major abiotic stress affecting crop productivity and quality. As a class of noncoding RNA, microRNA (miRNA) plays important roles in plant growth, development, and stress response. However, their response and roles in tomato drought stress is largely unknown. Here, by using high-throughput sequencing, we compared the miRNA profiles before and after drought treatment in two tomato genotypes: M82, a drought-sensitive cultivated tomato (Solanum lycopersicum), and IL2-5, a drought-tolerant introgression line derived from M82 and the tomato wild species S. pennellii (LA0716). A total of 108 conserved and 208 novel miRNAs were identified, among them, 32 and 68 were significantly changed in expression after stress. Further, 1936 putative target genes were predicted for those differentially-expressed miRNAs. Gene ontology and pathway analysis showed that many of the target genes were involved in stress resistance, such as genes in GO terms including response to stress, defense response, response to stimulus, phosphorylation, and signal transduction. Our results suggested that miRNAs play an essential role in the drought response of tomato. This work will help to further characterize specific miRNAs functioning in drought tolerance.

Understanding Fe2+ toxicity and P deficiency tolerance in rice for enhancing productivity under acidic soils

Plants experience low phosphorus (P) and high iron (Fe) levels in acidic lowland soils that lead to reduced crop productivity. A better understanding of the relationship between these two stresses at molecular and physiological level will lead to development of suitable strategies to increase crop productivity in such poor soils. Tolerance for most abiotic stresses including P deficiency and Fe toxicity is a quantitative trait in rice. Recent studies in the areas of physiology, genetics, and overall metabolic pathways in response to P deficiency of rice plants have improved our understanding of low P tolerance. Phosphorous uptake and P use efficiency are the two key traits for improving P deficiency tolerance. In the case of Fe toxicity tolerance, QTLs have been reported but the identity and role played by underlying genes is just emerging. Details pertaining to Fe deficiency tolerance in rice are well worked out including genes involved in Fe sensing and uptake. But, how rice copes with Fe toxicity is not clearly understood. This review focuses on the progress made in understanding these key environmental stresses. Finally, an opinion on the key genes which can be targeted for this stress is provided.

Abscisic acid treatment alleviates cadmium toxicity in purple flowering stalk (Brassica campestris L. ssp. chinensis var. purpurea Hort.) seedlings

The aim of this research was to investigate how exogenous abscisic acid (ABA) alleviates cadmium (Cd) toxicity in purple flowering stalk (Brassica campestris L. ssp. chinensis) and evaluate whether it could be a potential choice for phytoremediation. Purple flowering stalk seedlings were cultivated in a hydroponic system with Cd at various concentrations (0-100 μmol L^-1) as controls and Cd plus ABA as the treatment in the growth media. The soluble proteins, chlorophyll contents and the activity of the antioxidant enzyme system were determined by previously established biochemical methods. The contents of soluble protein and chlorophyll, and the activities of superoxide dismutase (SOD, EC 1. 15.1.1), peroxidase (POD, EC 1.11.1.7), ascorbic peroxidase (APX, EC 1.11.1.11), glutathione reductase (GR, EC 1.8.1.7) and superoxide anion (O2^·-) increased with the increase of external Cd concentrations, and then decreased in both Cd and Cd+ABA treatments, with higher activities of enzymes but lower level of O2^·- in Cd+ABA than those in Cdonly treatments. It indicated that a stress adaptation mechanism was employed at lower Cd concentrations. The contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), increased with the increase of Cd concentrations in the growth medium, with the highest levels in the treatment of 100 μmol L^-1 Cd with lower levels in respective Cd+ABAtreatments than the Cd only treatmetns. Plants treated with 100 μmol L^-1 Cd plus ABA showed a 60% decrease in Cd content in the leaves but a 259% increase in Cd content in the roots. In summary, exogenous ABA might alleviate Cd toxicity in purple flowering stalk mainly by reducing the reactive oxygen species (ROS) though activing the antioxidant enzyme system and accumulating more Cd in roots.

The manipulation of gene expression and the biosynthesis of Vitamin C, E and folate in light-and dark-germination of sweet corn seeds

This study investigates the potential interrelationship between gene expression and biosynthesis of vitamin C, E and folate in sweet corn sprouts. Germination of sweet corn kernels was conducted in light and dark environments to determine if this relationship was regulated by photo-illumination. Results indicated that light and dark environments affected the DHAR, TMT and GTPCH expression and that these genes were the predominant genes of vitamin C, E and folate biosynthesis pathways respectively during the germination. Levels of vitamin C and folate increased during the germination of sweet corn seeds while vitamin E had a declining manner. Sweet corn sprouts had higher vitamin C and E levels as well as relevant gene expression levels in light environment while illumination had little influence on the folate contents and the gene expression levels during the germination. These results indicate that there might be a collaborative relationship between vitamin C and folate regulation during sweet corn seed germination, while an inhibitive regulation might exist between vitamin C and E.

Enzymatic Browning in Sugar Beet Leaves (Beta vulgaris L.): Influence of Caffeic Acid Derivatives, Oxidative Coupling, and Coupled Oxidation

Sugar beet (Beta vulgaris L.) leaves of 8 month (8m) plants showed more enzymatic browning than those of 3 month (3m). Total phenolic content increased from 4.6 to 9.4 mg/g FW in 3m and 8m, respectively, quantitated by reverse-phase-ultrahigh-performance liquid chromatography-ultraviolet-mass spectrometry (RP-UHPLC-UV-MS). The PPO activity was 6.7 times higher in extracts from 8m than from 3m leaves. Substrate content increased from 0.53 to 2.45 mg/g FW in 3m and 8m, respectively, of which caffeic acid glycosyl esters were most important, increasing 10-fold with age. Caffeic acid glycosides and vitexin derivatives were no substrates. In 3m and 8m, nonsubstrate-to-substrate ratios were 8:1 and 3:1, respectively. A model system showed browning at 3:1 ratio due to formation of products with extensive conjugated systems through oxidative coupling and coupled oxidation. The 8:1 ratio did not turn brown as oxidative coupling occurred without much coupled oxidation. We postulate that differences in nonsubstrate-to-substrate ratio and therewith extent of coupled oxidation explain browning.

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.

Ascorbic acid deficiency leads to increased grain chalkiness in transgenic rice for suppressed of L-GalLDH

The grain chalkiness of rice (Oryza sativa L.), which determines the rice quality and price, is a major concern in rice breeding. Reactive oxygen species (ROS) plays a critical role in regulating rice endosperm chalkiness. Ascorbic acid (Asc) is a major plant antioxidant, which strictly regulates the levels of ROS. l-galactono-1, 4-lactone dehydrogenase (L-GalLDH, EC 1.3.2.3) is an enzyme that catalyzes the last step of Asc biosynthesis in higher plants. Here we show that the L-GalLDH-suppressed transgenic rice, GI-1 and GI-2, which have constitutively low (between 30% and 50%) leaf and grain Asc content compared with the wild-type (WT), exhibit significantly increased grain chalkiness. Further examination showed that the deficiency of Asc resulted in a higher lipid peroxidation and H₂O₂ content, accompanied by a lower hydroxyl radical scavenging rate, total antioxidant capacity and photosynthetic ability. In addition, changes of the enzyme activities and gene transcript abundances related to starch synthesis were also observed in GI-1 and GI-2 grains. The results we presented here suggest a close correlation between Asc deficiency and grain chalkiness in the L-GalLDH-suppressed transgenics. Asc deficiency leads to the accumulation of H₂O₂, affecting antioxidant capacity and photosynthetic function, changing enzyme activities and gene transcript abundances related to starch synthesis, finally leading to the increased grain chalkiness.

Nutritional, Biophysical and Physiological Characteristics of Wild Rocket Genotypes As Affected by Soilless Cultivation System, Salinity Level of Nutrient Solution and Growing Period

With the aim of defining the best management of nutrient solution (NS) in a soilless system for obtaining high quality baby-leaf rocket, the present study focuses on two wild rocket genotypes ("Nature" and "Naturelle"), grown in a greenhouse under two Southern Italy growing conditions-autumn-winter (AW) and winter-spring (WS)-using two soilless cultivation systems (SCS)-at two electrical conductivity values (EC) of NS. The SCSs used were the Floating System (FS) and Ebb and Flow System (EFS) and the EC values were 2.5 and 3.5 dS m-1 (EC2.5; EC3.5) for the AW cycle and 3.5 and 4.5 dS m-1 (EC3.5; EC4.5) for the WS cycle. The yield, bio-physical, physiological and nutritional characteristics were evaluated. Higher fresh (FY) (2.25 vs. 1.50 kg m-2) and dry (DY) (230.6 vs. 106.1 g m-2) weight yield, leaf firmness (dry matter, 104.3 vs. 83.2 g kg-1 FW; specific leaf area, 34.8 vs. 24.2 g cm-2) and antioxidant compounds (vitamin C, 239.0 vs. 152.7 mg kg-1 FW; total phenols, 997 vs. 450 mg GAE mg kg-1 FW; total glucosinulates-GLSs, 1,078.8 vs. 405.7 mg kg-1 DW; total antioxidant capacity-TAC, 11,534 vs. 8,637 μmol eq trolox kg-1 FW) and lower nitrates (1,470 vs. 3,460 mg kg-1 FW) were obtained under WS conditions. The seasonal differences were evident on the GLS profile: some aliphatic GLSs (gluconapoleiferin, glucobrassicanapin) and indolic 4-OH-glucobrassicin were only expressed in WS conditions, while indolic glucobrassicin was only detected in the AW period. Compared with EFS, FS improved leaf firmness, visual quality, antioxidant content (TAC, +11.6%) and reduced nitrate leaf accumulation (-37%). "Naturelle" performed better than "Nature" in terms of yield, visual quality and nutritional profile, with differences more evident under less favorable climatic conditions and when the cultivars were grown in FS. Compared to EC2.5, the EC3.5 treatment did not affect DY while enhancing firmness, visual quality, and antioxidant compounds (TAC, +8%), and reducing the nitrate content (-47%). The EC4.5 treatment reduced FY and DY and the antioxidant content. Despite seasonal climatic condition variability, FS and the moderate salinity level of NS (3.5 dS m-1) can be suggested as optimum.

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.

Subcellular localization of Arabidopsis arogenate dehydratases suggests novel and non-enzymatic roles

Arogenate dehydratases (ADTs) catalyze the final step in phenylalanine biosynthesis in plants. The Arabidopsis thaliana genome encodes a family of six ADTs capable of decarboxylating/dehydrating arogenate into phenylalanine. Using cyan fluorescent protein (CFP)-tagged proteins, the subcellular localization patterns of all six A. thaliana ADTs were investigated in intact Nicotiana benthamiana and A. thaliana leaf cells. We show that A. thaliana ADTs localize to stroma and stromules (stroma-filled tubules) of chloroplasts. This localization pattern is consistent with the enzymatic function of ADTs as many enzymes required for amino acid biosynthesis are primarily localized to chloroplasts, and stromules are thought to increase metabolite transport from chloroplasts to other cellular compartments. Furthermore, we provide evidence that ADTs have additional, non-enzymatic roles. ADT2 localizes in a ring around the equatorial plane of chloroplasts or to a chloroplast pole, which suggests that ADT2 is a component of the chloroplast division machinery. In addition to chloroplasts, ADT5 was also found in nuclei, again suggesting a non-enzymatic role for ADT5. We also show evidence that ADT5 is transported to the nucleus via stromules. We propose that ADT2 and ADT5 are moonlighting proteins that play an enzymatic role in phenylalanine biosynthesis and a second role in chloroplast division or transcriptional regulation, respectively.