Comparison of two peroxidases with high potential for biotechnology applications - HRP vs. APEX2

Peroxidases are essential elements in many biotechnological applications. An especially interesting concept involves split enzymes, where the enzyme is separated into two smaller and inactive proteins that can dimerize into a fully active enzyme. Such split forms were developed for the horseradish peroxidase (HRP) and ascorbate peroxidase (APX) already. Both peroxidases have a high potential for biotechnology applications. In the present study, we performed biophysical comparisons of these two peroxidases and their split analogues. The active site availability is similar for all four structures. The split enzymes are comparable in stability with their native analogues, meaning that they can be used for further biotechnology applications. Also, the tertiary structures of the two peroxidases are similar. However, differences that might help in choosing one system over another for biotechnology applications were noticed. The main difference between the two systems is glycosylation which is not present in the case of APX/sAPEX2, while it has a high impact on the HRP/sHRP stability. Further differences are calcium ions and cysteine bridges that are present only in the case of HRP/sHRP. Finally, computational results identified sAPEX2 as the systems with the smallest structural variations during molecular dynamics simulations showing its dominant stability comparing to other simulated proteins. Taken all together, the sAPEX2 system has a high potential for biotechnological applications due to the lack of glycans and cysteines, as well as due to high stability.

Management of excessive soil H+ ion induced toxicities by application of organic seaweed amendment enhances photosynthesis and resource use efficiencies in rice (Oryza sativa)

Globally, soil acidification is a serious environmental issue that reduces commercial agricultural production. Rice is subjected to nutritional stress due to acidic soil, which is a major impediment to rice production. Since acid soil threatens rice plants with soil compaction, nutrient loss, and plant stress-induced oxidative cell damage that results in affecting the photosynthetic system, restricting the availability of water, and reducing overall plant growth and productivity. Since contemporary soil acidification management strategies provide mediocre results, the use of Sargassum wightii seaweed-based biostimulants (BS) and soil amendments is sought as an environmentally friendly alternative strategy, and therefore its potential isevaluated in this study. BS was able to mediate soil quality by improving soil pH and structure along with facilitating nitrogen phytoavailability. BS also increased the activity of the antioxidant enzyme system, superoxide dismutase ((48%), peroxidase (76.6%), and ascorbate peroxidase (63.5%), aggregating the monaldehyde-mediating accumulation of osmoprotective proline in roots, that was evident from rapid initiation of root hair growth in treated seedlings. BS was also able to physiologically modulate photosynthetic activities and chlorophyll production (24.31%) in leaves, maintaining the efficiency of plant water use by regulating the stomatal conductance (0.91 mol/m/s) and the transpiration rate (13.2 mM/m/s). The BS compounds were also successful in facilitating nitrogen uptake resulting in improved plant growth (59%), tiller-panicle number, and yield (52.57%), demonstrating a resourceful nitrogen use efficiency (71.96%) previously affected by stress induced by acid soil. Therefore, the study affirms the competent potential of S. wightii-based soil amendment to be applied not only to improve soil quality, but also to increase plant production and yield.

Function of the ascorbate peroxidase (APX) in fruits and their modulation by reactive species

Ascorbate peroxidase (APX) is one of the enzymes of the ascorbate-glutathione cycle and is the key enzyme that breaks down H2O2 with the aid of ascorbate as an electron source, although other enzymes also break down H2O2 such as catalase, peroxiredoxins, among others. APX is present in all photosynthetic eukaryotes from algae to higher plants and at the cellular level, it is localized in all subcellular compartments where H2O2 is generated, including apoplast, cytosol, plastids, mitochondria, and peroxisomes, either in soluble form or attached to the organelle membranes. The APX activity can be modulated by various post-translational modifications (PTMs) including tyrosine nitration, S-nitrosation, persulfidation, and S-sulfenylation among others. This allows the connection of the H2O2 metabolism with other relevant signaling molecules such as NO and H2S thus building a complex coordination system. In both climacteric and non-climacteric fruits, APX plays a key role during the ripening process as well as during postharvest, since it participates in the regulation of both H2O2 and ascorbate levels affecting fruit quality. Currently, the exogenous application of molecules such as NO, H2S, H2O2, and more recently melatonin has been seen as a new alternative to maintain and extend the shelf life and quality of the fruits because these molecules can modulate APX activity as well as other antioxidant systems. Therefore, these molecules are being considered new biotechnological tools to improve crop quality in the horticultural industry.

Modification in the ascorbate-glutathione cycle leads to a better acclimation to high light in the rose Bengal resistant mutant of Nannochloropsis oceanica

Understanding how to adapt outdoor cultures of Nannochloropsis oceanica to high light (HL) is vital for boosting productivity. The N. oceanica RB2 mutant, obtained via ethyl methanesulfonate mutagenesis, was chosen for its tolerance to Rose Bengal (RB), a singlet oxygen (1O2) generator. Compared to the wild type (WT), the RB2 mutant showed higher resilience to excess light conditions. Analyzing the ascorbate-glutathione cycle (AGC), involving ascorbate peroxidases (APX, EC 1.11.1.11), dehydroascorbate reductase (DHAR, EC 1.8.5.1), and glutathione reductase (GR, EC 1.8.1.7), in the RB2 mutant under HL stress provided valuable insights. At 250 μmol photon m-2 s-1 (HL), the WT strain displayed superoxide anion radicals (O2▪-) and hydrogen peroxide (H2O2) accumulation, increased lipid peroxidation, and cell death compared to normal light (NL) conditions (50 μmol photon m-2 s-1). The RB2 mutant didn't accumulate O2▪- and H2O2 after HL exposure, and exhibited increased APX, DHAR, and GR activities and transcript levels compared to WT and remained consistent after HL treatment. Although the RB2 mutant had a smaller ascorbate (AsA) pool than the WT, its ability to regenerate dehydroascorbate (DHA) increased post HL exposure, indicated by a higher AsA/DHA ratio. Additionally, under HL conditions, the RB2 mutant displayed an improved glutathione (GSH) regeneration rate (GSH/GSSG ratio) without changing the GSH pool size. Remarkably, H2O2 or menadione (a O2▪- donor) treatment induced cell death in the WT strain but not in the RB2 mutant. These findings emphasize the essential role of AGC in the RB2 mutant of Nannochloropsis in handling photo-oxidative stress.

Measurements of Ascorbate and Dehydroascorbate in Plants Using High-Performance Liquid Chromatography

The current concepts emphasize the fundamental role of reactive oxygen species (ROS) as signaling molecules that coordinate defense mechanisms, cell death, and the growth and development processes in plants. However, due to the inherent reactivity of ROS, achieving precise control over their levels within plant cells, both spatially and temporally, becomes important to effectively harness the potential of ROS signaling while concurrently minimizing the risk of oxidative damage. Ascorbate is an exceptional antioxidant and contributes to the antioxidant defense system in plants. Its role is further reinforced by the presence of ascorbate peroxidases and enzymes responsible for recycling ascorbate from its oxidized forms. Ascorbate metabolism plays a pivotal role in averting oxidative damage and facilitates meticulous regulation of ROS signal availability. This chapter outlines the preferred protocol for the measurement of ascorbate.

Influence of brassinosteroid and silicon on growth, antioxidant enzymes, and metal uptake of leafy vegetables under wastewater irrigation

Vegetable cultivation under sewage irrigation is a common practice mostly in developing countries due to a lack of freshwater. Long-term usage provokes heavy metals accumulation in soil and ultimately hinders the growth and physiology of crop plants and deteriorates the quality of food. A study was performed to investigate the role of brassinosteroid (BRs) and silicon (Si) on lettuce, spinach, and cabbage under lead (Pb) and cadmium (Cd) contaminated sewage water. The experiment comprises three treatments (control, BRs, and Si) applied under a completely randomized design (CRD) in a growth chamber. BRs and Si application resulted in the highest increase of growth, physiology, and antioxidant enzyme activities when applied under canal water followed by distilled water and sewage water. However, BRs and Si increased the above-determined attributes under the sewage water by reducing the Pb and Cd uptake as compared to the control. It's concluded that sewerage water adversely affected the growth and development of vegetables by increasing Pb and Cd, and foliar spray of Si and BRs could have great potential to mitigate the adverse effects of heavy metals and improve the growth. The long-term alleviating effect of BRs and Si will be evaluated in the field conditions at different ecological zones.

Impairment in photosynthesis induced by CAT inhibition depends on the intensity of photorespiration and peroxisomal APX expression in rice

We have previously shown that rice plants silenced for peroxisomal ascorbate peroxidase (OsAPX4-RNAi) display higher resilience to photosynthesis under oxidative stress and photorespiratory conditions. However, the redox mechanisms underlying that intriguing response remain unknown. Here, we tested the hypothesis that favorable effects triggered by peroxisomal APX deficiency on photosynthesis resilience under CAT inhibition are dependent on the intensity of photorespiration associated with the abundance of photosynthetic and redox proteins. Non-transformed (NT) and OsAPX4-RNAi silenced rice plants were grown under ambient (AC) or high CO2 (HC) conditions and subjected to 3-amino-1,2,4-triazole (3-AT)-mediated CAT activity inhibition. Photosynthetic measurements evidenced that OsAPX4-RNAi plants simultaneously exposed to CAT inhibition and HC lost the previously acquired advantage in photosynthesis resilience displayed under AC. Silenced plants exposed to environment photorespiration and CAT inhibition presented lower photorespiration as indicated by smaller Gly/Ser and Jo/Jc ratios and glycolate oxidase activity. Interestingly, when these silenced plants were exposed to HC and CAT-inhibition, they exhibited an inverse response compared to AC in terms of photorespiration indicators, associated with higher accumulation of proteins. Multivariate and correlation network analyses suggest that the proteomics changes induced by HC combined with CAT inhibition are substantially different between NT and OsAPX4-RNAi plants. Our results suggest that the intensity of photorespiration and peroxisomal APX-mediated redox signaling are tightly regulated under CAT inhibition induced oxidative stress, which can modulate the photosynthetic efficiency, possibly via a coordinated regulation of protein abundance and rearrangement, ultimately triggered by crosstalk involving H2O2 levels related to CAT and APX activities in peroxisomes.

Physiological and biochemical role of nickel in nodulation and biological nitrogen fixation in Vigna unguiculata L. Walp

Studies on the role of nickel (Ni) in photosynthetic and antioxidant metabolism, as well as in flavonoid synthesis and biological fixation nitrogen in cowpea crop are scarce. The aim of this study was to elucidate the role of Ni in metabolism, photosynthesis and nodulation of cowpea plants. A completely randomized experiment was performed in greenhouse, with cowpea plants cultivated under 0, 0.5, 1, 2, or 3 mg kg-1 Ni, as Ni sulfate. In the study the following parameters were evaluated: activity of urease, nitrate reductase, superoxide dismutase, catalase and ascorbate peroxidase; concentration of urea, n-compounds, photosynthetic pigments, flavonoids, H2O2 and MDA; estimative of gas exchange, and biomass as plants, yield and weight of 100 seeds. At whole-plant level, Ni affected root biomass, number of seeds per pot, and yield, increasing it at 0.5 mg kg-1 and leading to inhibition at 2-3 mg kg-1 (e.g. number of seeds per pot and nodulation). The whole-plant level enhancement by 0.5 mg Ni kg-1 occurred along with increased photosynthetic pigments, photosynthesis, ureides, and catalase, and decreased hydrogen peroxide concentration. This study presents fundamental new insights regarding Ni effect on N metabolism, and nodulation that can be helpful to increase cowpea yield. Considering the increasing population and its demand for staple food, these results contribute to the enhancement of agricultural techniques that increase crop productivity and help to maintain human food security.

Identification of citrus APX gene family and their response to CYVCV infection

Ascorbate peroxidase (APX) is one of the most important antioxidant enzymes in the reactive oxygen metabolic pathway of plants. The role of APX under biotic and abiotic stress conditions has been explored, but the response pattern of APX under biotic stresses is relatively less known. In this study, seven CsAPXs gene family members were identified based on the sweet orange (Citrus sinensis) genome and subjected to evolutionary and structural analysis using bioinformatics software. The APX genes of lemon (ClAPXs) were cloned and showed a high conservation to CsAPXs by sequences alignment. In citrus yellow vein clearing virus (CYVCV)-infected Eureka lemons (C. limon) at 30th day post inoculation, APX activity and accumulation of hydrogen peroxide (H₂O₂) and malondialdehyde were measured to be 3.63, 2.29, and 1.73 times to that of the healthy control. The expression levels of 7 ClAPX genes in different periods of CYVCV-infected Eureka lemon were analyzed. Notably, ClAPX1, ClAPX5, and ClAPX7 showed higher expression levels compared to healthy plants, while ClAPX2, ClAPX3, and ClAPX4 showed lower expression levels. Functional identification of ClAPX1 in Nicotiana benthamiana showed that increasing the expression of ClAPX1 could significantly reduce the accumulation of H₂O₂, and it was verified that ClAPX1 is located in the plasma membrane of the cell. The present study provided information on the evolution and function of citrus APXs and revealed for the first time their response pattern to CYVCV infection.

Functions of NO and H2S Signal Molecules Against Plant Abiotic Stress

Nitric oxide (NO) and hydrogen sulfide (H₂S) are two recognized signal molecules in higher plants involved in a wide range of physiological processes and the mechanisms of response against adverse environmental conditions. These molecules can interact to provide an adequate response to palliate the negative impact exerted by stressful conditions, particularly by regulating key components of the metabolism of reactive oxygen species (ROS) to avoid their overproduction and further oxidative damage which, finally, affects cellular functioning. NO and H₂S can exert the regulation over the function of susceptible proteins by posttranslational modifications (PTMs) including nitration, S-nitrosation, and persulfidation but also through the regulation of gene expression by the induction of specific transcription factors which modulate the expression of genes encoding proteins related to stress resistance. This chapter encompasses a wide perspective of the signaling and functional relationships between NO and H₂S to modulate the overproduction of reactive oxygen species, particularly under abiotic stress conditions.

Quantitative Measurements of Biochemical and Molecular Markers of Oxidative Stress Signaling and Responses

Increases in cellular oxidation are a part of most plant responses to challenging conditions and are commonly described as oxidative stress. While this phenomenon is closely related to the accumulation of reactive oxygen species, these latter compounds can be difficult to measure. Complementary measurements to assess cellular redox state are, therefore, very useful in studies of plant responses to stress. Here, we detail protocols for three complementary approaches that can be used to assess the intensity of oxidative stress. These involve quantification of marker transcripts, assays of the extractable activities of major antioxidative enzymes, and measurement of antioxidant buffers. We confirm experimentally that the data obtained by such approaches can provide reliable information on the intensity of oxidative stress.

Ion content, antioxidant enzyme activity and transcriptional response under salt stress and recovery condition in the halophyte grass Aeluropus littoralis

OBJECTIVE

In contrast to glycophytes, halophyte plants have evolved unique morphological and physiological mechanisms to deal with abiotic stress. This study presents the physiological responses of Aeluropus littoralis, a halophyte grass, to salt stress and recovery conditions on the molecular level.

RESULTS

Elemental analysis showed that Na⁺ concentration increased in the analyzed tissue during salt stress application, and declined at recovery condition. With the exception of root tissue, comparable trends of K⁺, Ca²⁺, and Mg²⁺ concentrations were observed (decreased during salt stress, increased during recovery). Salinity led to an increase in total chlorophyll (Chl), Chl a, and carotenoids content, while Chl b content decreased. The level of the proline amino acid associated with drought and salt stress was increased. Here APX, POD, and SOD activity were strongly detectable in roots and reduced later under recovery conditions. RT-qPCR revealed up-regulation of antioxidant genes at S1 and S3 in the root but down-regulation in recovery conditions. This study found a significant halophyte index for understanding the processes of salinity tolerance in A. littoralis. These findings may provide insight into the role of antioxidant enzymes during salt stress and the mechanism underlying the plant's response to stress.

Identification and charactering of APX genes provide new insights in abiotic stresses response in Brassica napus

Ascorbate peroxidase (APX) plays an important role in scavenging H₂O₂ and balancing ROS content in plant cells, which is of great significance for the growth and development of life and resistance to external stress. However, up to now, APXs in Brassica napus (B. napus) have not been systematically characterized. In this study, a total of 26 BnaAPX genes were identified, which were distributed on 13 chromosomes and divided into five phylogenetic branches. Gene structure analysis showed that they had a wide varied number of exons while BnaAPXs proteins contained more similar motifs in the same phylogenetic branches. qRT-PCR analysis of 26 BnaAPX gene expression patterns showed that three putative cytosol BnaAPX genes BnaAPX1, BnaAPX2, BnaAPX9, two putatice microsomal genes BnaAPX18 and BnaAPX25 were up-regulated rapidly and robustly under high salt, water shortage and high temperature stresses. In addition, the above three abiotic stresses led to a significant increase in APX activity. The results provide basic and comprehensive information for further functional characterization of APX gene family in B. napus.

Characterization of APX and APX-R gene family in Brassica juncea and B. rapa for tolerance against abiotic stresses

APX and APX-R gene families were identified and characterized in two important oilseed species of Brassica. Gene expression under abiotic stress conditions, recombinant protein expression, and analysis further divulged their drought, heat, and salt-responsive behavior. Ascorbate peroxidases (APX) are heme-dependent enzymes that rid the cells of H₂O₂ and regulate diverse biological processes. In the present study, we performed APX gene family characterization in two Brassica sp. (B. juncea and B. rapa) as these are commercially important oilseed crops and affected severely by abiotic stresses. We identified 16 BjuAPX and 9 BraAPX genes and 2 APX-R genes each in B. juncea and B. rapa genomes, respectively. Phylogenetic analysis divided the APX genes into five distinct clades, which exhibited conservation in the gene structure, motif organization, and sub-cellular location within the clade. Structural analysis of APX and APX-R proteins revealed the amino acid substitutions in conserved domains of APX-R proteins. The expression profiling of BjuAPX and BraAPX genes showed that 3 BjuAPX, 7BraAPX, and 2 BraAPX-R genes were drought and heat responsive. Notably, BjuAAPX1a, BjuAPX1d, BjuAAPX6, BraAAPX1a, BraAAPX2, and BraAAPX3b showed high expression levels in RT-qPCR. Cis-regulatory elements in APX and APX-R gene promoters supported the differential behavior of these genes. Further, two stress-responsive genes BjuAPX1d and BraAAPX2 were cloned, characterized, and their roles were validated under heat, drought, salt, and cold stress in bacterial expression system. This study for the first time reports the presence of APX activity in dimeric and LMW form of purified BraAAPX2 protein. The study may help pave way for developing abiotic stress-tolerant Brassica crops.

Visualizing Filoviral Nucleoproteins Using Nanobodies Fused to the Ascorbate Peroxidase Derivatives APEX2 and dEAPX

Fusions of single-domain antibodies (sdAbs, nanobodies) to enzymatic reporters make convenient molecular probes to detect the presence of an antigen of interest. We have previously fused the monomeric hyperactive ascorbate peroxidase derivative APEX2 to anti-Ebolavirus and anti-Marburgvirus sdAbs to generate immunoreagents useful in detecting nucleoprotein (NP) on western blots, ELISA, and within cells following transfection of NP expression plasmids or following virus infection. Here we present the methods used to overexpress and purify these sdAb-APEX2 fusion proteins, and to employ them as probes in various scenarios with colorimetric and fluorometric signal development. We also introduce a dimeric hyperactive ascorbate peroxidase derivative dEAPX that enables bivalent sdAb probes to be produced while avoiding the need to generate more complex tandem sdAbs, leveraging avidity for improved signal strength. The APEX2 and dEAPX reagents appear interchangeable with any existing detection platform and the methods described here should enable a user to study their antigen of interest by simply swapping out the sdAb for their recombinant affinity reagent of choice.

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.

Exogenous Ca2+ enhances antioxidant defense in rice to simulated acid rain by regulating ascorbate peroxidase and glutathione reductase

Exogenous calcium enhances rice tolerance to acid rain stress by regulating isozymes composition and transcriptional expression of ascorbate peroxidase and glutathione reductase. Calcium (Ca) participates in signal transduction in plants under abiotic stress, and addition of Ca²⁺ is beneficial to alleviate damage of plants caused by acid rain. To clarify the effect of exogenous Ca²⁺ on tolerance of plants to acid rain stress, we investigated regulation of Ca²⁺ (5 mM) on activities, isozymes composition and transcriptional expression of ascorbate peroxidase (APX) and glutathione reductase (GR), redox state, and H₂O₂ concentration and growth in rice leaves and roots under simulated acid rain (SAR) stress. SAR (pH 3.5/2.5) decreased the total activities of APX and GR in rice by decreasing the concentration of APX isoforms (APXII in leaves and APXIII in roots) as well as activation degree of GR isozymes and transcription level of GR1, indicating that SAR (pH 3.5/2.5) destroyed the redox state in rice cells and induced H₂O₂ excessive accumulation, and inhibited growth of rice. Exogenous Ca²⁺ alleviated SAR-induced inhibition on activities of APX and GR by regulating the concentration, activation, and transcription of their isozymes, and then maintained the redox level of cells and protected cells from oxidative damage, being beneficial to the growth of rice. Therefore, the promotion of exogenous Ca²⁺ on activities of APX and GR can be important to enhance rice tolerance to acid rain by maintaining redox state and avoiding oxidative damage.

Genetic variation in high light responses of Theobroma cacao L. accessions

Cacao (Theobroma cacao L.) is a shade-tolerant tree species, but in recent years it has increasingly been cultivated under full sun conditions in an orchard system where photoinhibition is likely. Here we investigate the extent of photoinhibition in 17 cacao accessions from a range of genetic groups, growing under high light conditions. The ability of the photosynthetic systems to respond to high light was assessed using chlorophyll fluorescence parameters (diurnal F _v /F _m and instantaneous light response curves), and differences in photosynthetic pigment content were compared using biochemical assays. Damage due to photoinhibition was assessed using electrolyte leakage, lipid peroxidation, and reactive oxygen species scavenging systems were compared using biochemical assays (for APX, CAT and SOD). There was significant variation between the 17 accessions for photosynthetic parameters, although in all cases the light saturation points were well below the midday light levels. Light acclimation of photosynthetic pigments was evident and variation in the total chlorophyll to total carotenoid ratio was significantly correlated with electrolyte leakage. Significant genetic variation was observed across the 17 accessions in the activities of CAT, APX and SOD. Across all accessions, photoprotection appeared to be restricted by the ability of leaves to generate SOD. Significant negative correlations were observed between SOD activity and both APX activity and electrolyte leakage, while significant positive correlations were observed between electrolyte leakage and both APX and CAT activity. Accessions with higher light saturation points, as well as high carotenoid and high SOD concentrations were able to tolerate the moderately high light, however, none of the accessions were clearly superior to the commonly grown Amelonado accession. The results imply that screening for SOD activity, total carotenoid content and light saturation point can aid in selection of genotypes with better tolerance to high light.

In silico characterization and homology modeling of cytosolic APX gene predicts novel glycine residue modulating waterlogging stress response in pigeon pea

Ascorbate peroxidase (APX) is a member of the family of heme-containing peroxidases having a similar structure with Cytochrome c peroxidase (CCP) that effectively scavenge cytosolic and chloroplastic hydrogen peroxide (H₂O₂) under various stresses. In this study, computational characterization and homology analysis of APX protein from waterlogging tolerant (ICPL 84023) and sensitive (ICP 7035) pigeon pea genotypes were carried out resulting in 100% homology with Glycine max in case of former and 99% in later genotypes respectively with 97.39% alignment coverage among each other. The model structure was further refined by various tools like PROCHECK, ProSA, and Verify3D. The planned model of the APX enzyme was then tested to dock with H₂O₂along with molecular dynamics (MD) simulation analysis. The docked complex of ICPL 84023 showed the best G-score (23.39 kcal/mol) in comparison to ICP 7035 (16.74 kcal/mol) depicting the higher production of APX for scavenging reactive oxygen species (ROS) production making this genotype more tolerant. The important binding residues in the ICPL 84023-H₂O₂complex (SER1, THR4, GLU23, and GLY13) have shown less fluctuation than the ICP 7035-H₂O₂ complex (SER1, THR4, and GLU23). Overall, our results showed that amino acid residue glycine in ICPL 84023 APX gene has a high binding affinity with H₂O₂ which could be a key factor associated with waterlogging stress tolerance in pigeon pea.

Grafting in Hylocereus (Cactaceae) as a tool for strengthening tolerance to high temperature stress

The Hylocereus species that are grown as exotic fruit crops are very often farmed under marginal agronomic conditions, which may include exposure to high temperatures. Here we present a pioneering investigation of grafting as an agro-technique to improve heat tolerance in Hylocereus. To this end, we studied the diploid species H. undatus, the tetraploid H. megalanthus and its di-haploid gamete-derived line 2719, and the interspecific-interploid tetraploid Z-10, all grafted onto H. undatus as the rootstock. Self-grafted, grafted and non-grafted plants were acclimated for one week (to obtain baseline values) and then exposed to heat stress (45/35 °C day/night) for three days, followed by a one-week recovery period under optimal temperatures (30/22 °C). A comparison of the physiological, biochemical and molecular performances of the grafted and self-grafted plants under heat stress and during the recovery period vs those of non-stressed plants (control; 30/22 °C) showed that the grafted and self-grafted plants performed better in most of the assessments: grafted and self-grafted plants recovered more rapidly from the heat stress and suffered far less stem damage. An unexpected - but important - finding that may have implications for other crop was that the self-grafted plants showed better performance than non-grafted plants throughout the trial. Our findings provide support for grafting as a strategy for coping with the stress induced by extremely high temperatures. This study thus paves the way for further investigations of grafting in Hylocereus as a valuable technique that will maintain crop productivity in the face of increasing worldwide temperatures.

Enhancing antioxidant systems by preharvest treatments with methyl jasmonate and salicylic acid leads to maintain lemon quality during cold storage

The effects of preharvest treatments with 0.1 mM methyl jasmonate (MeJA) and 0.5 mM salicylic acid (SA) on quality parameters of lemon fruit and their relationship with antioxidant systems, gene expression and bioactive compounds at harvest and during cold storage were evaluated. Results showed that total antioxidant activity, total phenolic content and the major individual phenolics (hesperidin and eriocitrin) were always higher in treated fruit than in controls. The activity of the antioxidant enzymes catalase, peroxidase and ascorbate peroxidase was also increased at harvest by SA and MeJA treatments, especially the last enzyme, for which the expression of its codifying gene was also enhanced. In addition, treated fruit had lower weight and firmness losses, respiration rate and production of ethylene than controls. Moreover, sugars and organic acids were maintained at higher concentration in flavedo and juice as a consequence of preharvest SA and MeJA treatments, showing an effect on maintaining fruit quality properties.

Expression of Ascorbate Peroxidase Derived from Cyanidioschyzon merolae in Mammalian Cells

BACKGROUND/AIM

Ascorbate peroxidase (APX) derived from Cyanidioschyzon merolae, a primitive red alga living in high temperature and acidic environments, has greater anti-oxidative capacity than similar peroxidases occurring in other plants. In the present study, we examined whether expression of Cyanidioschyzon merolae-derived APX (cAPX) in mammalian cells increases cellular anti-oxidative capacity.

MATERIALS AND METHODS

The cAPX gene was introduced into the mouse fibroblast-like cell line C3H10T1/2. Production of reactive oxygen species (ROS) and/or cell viability was assessed after heat, H₂O₂ and acid stimulation.

RESULTS

Heat and H₂O₂ stimulation resulted in ROS production. cAPX-expressing cells were more tolerant to oxidative stress induced by heat, H₂O₂ and acid stimulations than control cells lacking cAPX.

CONCLUSION

Introduction of cAPX increases the anti-oxidative capacity in mammalian cells.

Identification and expression profiling analysis of ascorbate peroxidase gene family in Actinidia chinensis (Hongyang)

Ascorbate peroxidase (APX) is one of the important antioxidant enzymes in the active oxygen metabolism pathway of plants and animals, especially it is the key enzyme to clear H₂O₂ in chloroplast and the main enzyme of vitamin C metabolism. However, knowledge about APX gene family members and their evolutionary and functional characteristics in kiwifruit is limited. In this study, we identified 13 members of the APX gene family in the kiwifruit (cultivar: Hongyang) genome according the APX proteins conserved domain of Arabidopsis thaliana. Phylogenetic analysis by maximum likelihood split these 13 genes into four groups. The APX gene family members were distributed on nine chromosomes (Nos. 4, 5, 11, 13, 20, 21, 23, 25, 28). Most of the encoded hydrophilic and lipid-soluble enzymes were predicted to be located in the cytoplasm, nucleus and chloroplast. Among them, AcAPX4, AcAPX5, AcAPX8, AcAPX12 were transmembrane proteins, and AcAPX8 and AcAPX12 had the same transmembrane domain. The gene structure analysis showed that AcAPXs were composed of 4-22 introns, except that AcAPX10 was intron-free. Multiple expectation maximization for motif elicitation program (MEME) analyzed 13 APX protein sequences of Actinidia chinensis and identified 10 conserved motifs ranging in length from 15 to 50 amino acid residues. Additionally, the predicted secondary structures of the main motifs consisted of α-helix and random coils. The gene expression of fruits in different growth stages and bagging treatment were determined by qRT-PCR. The results showed that 8 AcAPXs had the highest expression levels during the color turning period and only the gene expression of AcAPX3 was consistent with the ascorbic acid content; five AcAPXs were consistent with the ascorbic acid content after bagging. Our data provided evolutionary and functional information of AcAPX gene family members and revealed the gene expression of different members in different growth stages and bagging treatments These results may be useful for future studies of the structures and functions of AcAPX family members.

The ascorbate peroxidase 1 regulates ascorbic acid metabolism in fresh-cut leaves of tea plant during postharvest storage under light/dark conditions

Postharvest storage conditions affect the ascorbic acid (AsA) levels in fresh-cut leaves of horticultural crops. However, the detailed mechanism of AsA metabolism in the fresh-cut leaves of tea plant (Camellia sinensis) during postharvest storage under light/dark conditions remains unclear. To investigate the AsA mechanism, we treated fresh-cut tea leaves with light/dark during postharvest storage. An ascorbate peroxidase 1 (CsAPX1) protein involved in AsA metabolism was identified by iTRAQ analysis. Gene expression profile of CsAPX1 encoding ascorbate peroxidase (APX) was regulated by light/dark conditions. AsA accumulation and APX activity were suppressed by light/dark conditions. SDS-PAGE analysis showed that the molecular mass of recombinant CsAPX1 protein was about 34.45 kDa. Subcellular localization indicated that CsAPX1 protein was a cytosol ascorbate peroxidase. Overexpression CsAPX1 in Arabidopsis indicated that the decrease of AsA content and APX activity in transgenic lines were less significant than that of WT during postharvest storage under light/dark conditions. These data suggested that CsAPX1 involved in regulating AsA metabolism through effecting on the changes of AsA accumulation and APX activity in fresh-cut tea leaves during postharvest storage under light/dark conditions.

Assessment of changes in growth traits, oxidative stress parameters, and enzymatic and non-enzymatic antioxidant defense mechanisms in Lepidium draba plant under osmotic stress induced by polyethylene glycol

Lepidium draba is a weed with the medicinal properties which few researches have been done on it. In this study, some traits, related to the osmotic stress, in 14-day-old L. draba sprouts that were grown 9 days in the presence of various doses of polyethylene glycol 6000 (PEG 6000) including 0, 3, 6, 9, and 12%, with different osmotic potentials (- 0.04, - 0.12, - 0.23, - 0.34, and - 0.48 MPa, respectively) were investigated. Based on our results, germination percentage besides stem and root lengths decreased with increasing the concentrations of PEG. The contents of electrolyte leakage, malondialdehyde, other aldehydes, total protein, free amino acids, total soluble carbohydrate as well as free proline increased with increasing the concentrations of PEG. Also, for the first time, our results have proven that under osmotic stress, there is an adverse relationship between hydrogen peroxide content and the activity of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase enzymes, such that hydrogen peroxide content decreased with induction of PEG up to 6% and after that increased, while the activity of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase enzymes increased up to 6% PEG and after that decreased. The expression levels of catalase, peroxidase, ascorbate peroxidase, and guaiacol peroxidase genes showed the same pattern as was seen for these enzyme activities. According to the results of this study, it can be deduced that decreasing H₂O₂ content cannot be the main reason for other oxidative stress parameters to decrease. In this study, P5CS and P5CR gene expression levels increased with increasing levels of PEG up to 12% which was completely similar to free proline content. Based on our results, L. draba can be considered as a semi-tolerant plant to osmotic stress.

PtrCDPK10 of Poncirus trifoliata functions in dehydration and drought tolerance by reducing ROS accumulation via phosphorylating PtrAPX

Calcium-dependent protein kinases (CDPKs) are important calcium signaling components that have been shown to play crucial roles in modulating plant abiotic stress responses. However, the physiological and regulatory roles of most CDPKs are still poorly understood. Here, we report the functional characterization of PtrCDPK10 from trifoliate orange (Poncirus trifoliata (L.) Raf.) in dehydration and drought stress tolerance. PtrCDPK10, categorized in the Type III subgroup of the CDPK family, was localized to the nucleus and plasma membrane. Transcript levels of PtrCDPK10 were up-regulated by dehydration, salt and ABA treatments. Transgenic trifoliate orange plants overexpressing PtrCDPK10 showed enhanced dehydration tolerance compared with the wild type (WT), whereas VIGS (virus-induced gene silencing)-mediated knockdown of PtrCDPK10 resulted in elevated susceptibility to dehydration and drought stresses. Yeast two-hybrid screening identified several proteins that interacted with PtrCDPK10, including an ascorbate peroxidase (PtrAPX). PtrCDPK10 was shown to phosphorylate PtrAPX based on an in vitro kinase assay. PtrCDPK10-overexpressing transgenic lines exhibited higher PtrAPX mRNA abundance and APX activity and accumulated dramatically less ROS in comparison with the WT, while PtrCDPK10-silenced VIGS lines showed decreased PtrAPX expression and increased ROS level. Taken together, these results demonstrate that PtrCDPK10 promotes dehydration and drought tolerance by, at least in part, phosphorylating APX to modulate ROS homeostasis.

Glycolytic profile shift and antioxidant triggering in symbiont-free and H2O2-resistant Strigomonas culicis

During their life cycle, trypanosomatids are exposed to stress conditions and adapt their energy and antioxidant metabolism to colonize their hosts. Strigomonas culicis is a monoxenous protist found in invertebrates with an endosymbiotic bacterium that completes essential biosynthetic pathways for the trypanosomatid. Our research group previously generated a wild-type H₂O₂-resistant (WTR) strain that showed improved mitochondrial metabolism and antioxidant defenses, which led to higher rates of Aedes aegypti infection. Here, we assess the biological contribution of the S. culicis endosymbiont and reactive oxygen species (ROS) resistance to oxidative and energy metabolism processes. Using high-throughput proteomics, several proteins involved in glycolysis and gluconeogenesis, the pentose phosphate pathway and glutathione metabolism were identified. The results suggest that ROS resistance decreases glucose consumption and indicate that the metabolic products from gluconeogenesis are key to supplying the protist with high-energy and reducing intermediates. Our hypothesis was confirmed by biochemical assays showing opposite profiles for glucose uptake and hexokinase and pyruvate kinase activity levels in the WTR and aposymbiotic strains, while the enzyme glucose-6P 1-dehydrogenase was more active in both strains. Regarding the antioxidant system, ascorbate peroxidase has an important role in H₂O₂ resistance and may be responsible for the high infection rates previously described for A. aegypti. In conclusion, our data indicate that the energy-related and antioxidant metabolic processes of S. culicis are modulated in response to oxidative stress conditions, providing new perspectives on the biology of the trypanosomatid-insect interaction as well as on the possible impact of resistant parasites in accidental human infection.

Oxidative stress induced by cadmium in lettuce (Lactuca sativa Linn.): Oxidative stress indicators and prediction of their genes

Environmental contamination with heavy metals is of concern as plants have the ability to absorb chemical toxicants facilitating the entry of toxic metals into the food chain. Lettuce (Lactuca sativa Linn.) was cultured in four nutrient solutions containing different concentrations of cadmium (0, 3, 6, and 9 mmol). The impact of heavy metal on the morphological features, antioxidant properties and antioxidant enzymes activity were investigated with primary focus on superoxide dismutase, ascorbate peroxidase, peroxidase and catalase enzymes. In silico methods were utilized in the study of the genes of these enzymes. Significant changes were observed in the morphological features of the plant with plants appearing stunted, more spherical and yellow in colour. A decrease in the dry mass of the plant was also detected. The Translocation factor (TF) for cadmium was significantly high in lettuce. Enhanced antioxidant enzymatic activity suggests that these enzymes are integrally involved in the defense mechanism of the plant to heavy metal stress. Also observed was an increase in total soluble protein, and total phenolic content. Total flavonoid content was not significantly affected. Fourteen genes encoding for ascorbate peroxidase and nineteen genes for superoxide dismutase were identified in lettuce. These enzymes varied from each other with regards to the number of exons and amino acids present, as well as their location within the cell. Plants exhibit various response mechanisms to combat heavy metal contamination.

Isolation, purification and characterization of an ascorbate peroxidase from celery and overexpression of the AgAPX1 gene enhanced ascorbate content and drought tolerance in Arabidopsis

BACKGROUND

Celery is a widely cultivated vegetable abundant in ascorbate (AsA), a natural plant antioxidant capable of scavenging free radicals generated by abiotic stress in plants. Ascorbate peroxidase (APX) is a plant antioxidant enzyme that is important in the synthesis of AsA and scavenging of excess hydrogen peroxide. However, the characteristics and functions of APX in celery remain unclear to date.

RESULTS

In this study, a gene encoding APX was cloned from celery and named AgAPX1. The transcription level of the AgAPX1 gene was significantly upregulated under drought stress. AgAPX1 was expressed in Escherichia coli BL21 (DE3) and purified. The predicted molecular mass of rAgAPX1 was 33.16 kDa, which was verified by SDS-PAGE assay. The optimum pH and temperature for rAgAPX1 were 7.0 and 55 °C, respectively. Transgenic Arabidopsis hosting the AgAPX1 gene showed elevated AsA content, antioxidant capacity and drought resistance. Less decrease in net photosynthetic rate, chlorophyll content, and relative water content contributed to the high survival rate of transgenic Arabidopsis lines after drought.

CONCLUSIONS

The characteristics of APX in celery were different from that in other species. The enhanced drought resistance of overexpressing AgAPX1 in Arabidopsis may be achieved by increasing the accumulation of AsA, enhancing the activities of various antioxidant enzymes, and promoting stomatal closure. Our work provides new evidence to understand APX and its response mechanisms to drought stress in celery.

Effect of alumina (Al2O3) nanoparticles and macroparticles on Trigonella foenum-graceum L. in vitro cultures: assessment of growth parameters and oxidative stress-related responses

The impact of 100 μg ml-1 alumina (Al2O3) nanoparticles (NPs) on Trigonella foenum (fenugreek) in vitro cultures was studied within 3 weeks (on days 1, 7, 14, and 21) and compared with the control and bulk (macrometer-sized particles) alumina-treated groups. Transmission electron microscopy (TEM) and dynamic light scattering were used for the characterization of NPs. The results of TEM analysis represented a nearly spherical shape for the NPs with agglomeration. The zeta potential of alumina NPs was - 25.4 ± 2.5 mV and the averaged diameter was 20 ± 5 nm. Atomic absorption and inductively coupled plasma-optical emission spectroscopy provided evidence for the release and uptake of aluminum. Treatment of cultures with NPs led to an increase in the formation of lateral roots. Treatment of fenugreek with bulk alumina caused a significant decrease in the number of leaves on day 21 (p < 0.05) and the root length on days 14 and 21 compared with the control group (p < 0.05). Alumina NP has led to a significant increase in the malondialdehyde content on days 7, 14, and 21 (p < 0.001). The glutathione content was decreased significantly in NP and bulk-treated groups on days 1 and 7 (p < 0.05). FRAP assay results showed that NPs and bulk alumina led to a decrease in the antioxidant power on days 7, 14, and 21 (p < 0.001). The increased activity of catalase (p < 0.001) and ascorbate peroxidase (p < 0.001) was observed in the bulk-treated group. Lignin content had a significant increase in response to NPs on days 14 and 21 (p < 0.001). Conclusively, alumina nano/macro particles affected agronomical and physiological properties of T. foenum; however, smaller sized particles do not necessarily imply greater toxicity, while uptake of the aluminum ions should be considered seriously.

Cloning and overexpression of the ascorbate peroxidase gene from the yam (Dioscorea alata) enhances chilling and flood tolerance in transgenic Arabidopsis

Minghuai 1 (MH1) is a yam (Dioscorea alata) cultivar with high tolerance to flooding but sensitivity to chilling. MH1 responded differently to chilling and flooding according to various physiological parameters and antioxidant enzymes. Flooding led to an increase in ascorbate peroxidase (APX) activity in both roots and leaves, while chilling did not affect APX activity. The full length DaAPX ORF sequence from MH1 (750 bp) was then cloned. Phylogenetic analysis showed that plant cytosolic APXs into four major clusters and DaAPX was closely related to Oncidium. The DaAPX gene driven by a 35S promoter was transferred into Arabidopsis. The gene expression and enzyme activity of APX in the DaAPX transgenic lines 1-3 were significantly higher than in wild type (WT) plants. Compared to WT plants, seedling growth characteristics were significantly better in all transgenic lines under chilling, flooding, and oxidative stresses, indicating that the overexpression of DaAPX in Arabidopsis enhanced tolerance to several abiotic stresses. MH1 plants supplied with H₂O₂ presented an increase in the activity of APX leading to enhanced tolerance to chilling. Functional characterization of the APX gene should improve our understanding of the chilling- and flood-response mechanism in the yam.

Adequate zinc nutrition improves the tolerance against drought and heat stresses in chickpea

Two chickpea genotypes viz. Bhakar-2011 (desi) and Noor-2013 (kabuli) were sown in soil filled pots supplied with low (0.3 mg kg^-1) and high (3 mg kg^-1 soil) zinc (Zn) under control (70% water holding capacity and 25/20 °C day/night temperature), drought (35% water holding capacity) and heat (35/30 °C day/night temperature) stresses. Drought and heat stresses reduced rate of photosynthesis, photosystem II efficiency, plant growth and Zn uptake in chickpea. Low Zn supply exacerbated adverse effects of drought and heat stresses in chickpea, and caused reduction in plant biomass, carbon assimilation, antioxidant activity, impeded Zn uptake and enhanced oxidative damage. However, adequate Zn supply ameliorated adverse effect of drought and heat stresses in both chickpea types. The improvements were more in desi than kabuli type. Adequate Zn nutrition is crucial to augment growth of chickpea plants under high temperature and arid climatic conditions.

Nitric oxide-dependent regulation of sweet pepper fruit ripening

Ripening is a complex physiological process that involves changes in reactive nitrogen and oxygen species that govern the shelf-life and quality of fruits. Nitric oxide (NO)-dependent changes in the sweet pepper fruit transcriptome were determined by treating fruits at the initial breaking point stage with NO gas. Fruits were also harvested at the immature (green) and ripe (red) stages. Fruit ripening in the absence of NO resulted in changes in the abundance of 8805 transcripts whose function could be identified. Among these, functional clusters associated with reactive oxygen/nitrogen species and lipid metabolism were significantly modified. NO treatment resulted in the differential expression of 498 genes framed within these functional categories. Biochemical analysis revealed that NO treatment resulted in changes in fatty acid profiling, glutathione and proline contents, and the extent of lipid peroxidation, as well as increases in the activity of ascorbate peroxidase and lipoxygenase. These data provide supporting evidence for the crucial role of NO in the ripening of pepper fruit.

Lacking catalase, a protistan parasite draws on its photosynthetic ancestry to complete an antioxidant repertoire with ascorbate peroxidase

Background

Antioxidative enzymes contribute to a parasite’s ability to counteract the host’s intracellular killing mechanisms. The facultative intracellular oyster parasite, Perkinsus marinus, a sister taxon to dinoflagellates and apicomplexans, is responsible for mortalities of oysters along the Atlantic coast of North America. Parasite trophozoites enter molluscan hemocytes by subverting the phagocytic response while inhibiting the typical respiratory burst. Because P. marinus lacks catalase, the mechanism(s) by which the parasite evade the toxic effects of hydrogen peroxide had remained unclear. We previously found that P. marinus displays an ascorbate-dependent peroxidase (APX) activity typical of photosynthetic eukaryotes. Like other alveolates, the evolutionary history of P. marinus includes multiple endosymbiotic events. The discovery of APX in P. marinus raised the questions: From which ancestral lineage is this APX derived, and what role does it play in the parasite’s life history?

Results

Purification of P. marinus cytosolic APX activity identified a 32 kDa protein. Amplification of parasite cDNA with oligonucleotides corresponding to peptides of the purified protein revealed two putative APX-encoding genes, designated PmAPX1 and PmAPX2. The predicted proteins are 93% identical, and PmAPX2 carries a 30 amino acid N-terminal extension relative to PmAPX1. The P. marinus APX proteins are similar to predicted APX proteins of dinoflagellates, and they more closely resemble chloroplastic than cytosolic APX enzymes of plants. Immunofluorescence for PmAPX1 and PmAPX2 shows that PmAPX1 is cytoplasmic, while PmAPX2 is localized to the periphery of the central vacuole. Three-dimensional modeling of the predicted proteins shows pronounced differences in surface charge of PmAPX1 and PmAPX2 in the vicinity of the aperture that provides access to the heme and active site.

Conclusions

PmAPX1 and PmAPX2 phylogenetic analysis suggests that they are derived from a plant ancestor. Plant ancestry is further supported by the presence of ascorbate synthesis genes in the P. marinus genome that are similar to those in plants. The localizations and 3D structures of the two APX isoforms suggest that APX fulfills multiple functions in P. marinus within two compartments. The possible role of APX in free-living and parasitic stages of the life history of P. marinus is discussed.

Electronic supplementary material

The online version of this article (10.1186/s12862-019-1465-5) contains supplementary material, which is available to authorized users.

The chaperone-like protein CDC48 regulates ascorbate peroxidase in tobacco

There is increasing evidence that the chaperone-like protein CDC48 (cell division cycle 48) plays a role in plant immunity. Cytosolic ascorbate peroxidase (cAPX), which is a major regulator of the redox status of plant cells, has previously been shown to interact with CDC48. In this study, we examined the regulation of cAPX by the ATPase NtCDC48 during the cryptogein-induced immune response in tobacco cells. Our results not only confirmed the interaction between the proteins but also showed that it occurs in the cytosol. cAPX accumulation was modified in cells overexpressing NtCDC48, a process that was shown to involve post-translational modification of cAPX. In addition, cryptogein-induced increases in cAPX activity were suppressed in cells overexpressing NtCDC48 and the abundance of the cAPX dimer was below the level of detection. Furthermore, the levels of both reduced (GSH) and oxidized glutathione (GSSG) and the GSH/GSSG ratio decreased more rapidly in response to the elicitor in these cells than in controls. A decrease in cAPX activity was also observed in response to heat shock in the cells overexpressing NtCDC48, indicating that the regulation of cAPX by NtCDC48 is not specific to the immune response.

Deciphering the main determinants of O3 tolerance in Euramerican poplar genotypes

Tropospheric ozone (O₃) is the main secondary pollutant and considered to be the most damaging for growth and productivity. O₃ is well known to induce oxidative stress and Reactive Oxygen Species accumulation in leaf tissues. Several mechanisms have been suggested to enable trees to cope with such stress; however, their relative contribution to O₃ tolerance is still unclear. Here, ten Euramerican poplar genotypes (Populus deltoides × nigra) were investigated regarding their response to 120 ppb of O₃ for 3 weeks in order to determine main mechanisms and identify the key traits and strategies linked to a better tolerance to O₃-induced oxidative stress. Results showed that ascorbate peroxidase and ascorbate regeneration through monodehydroascorbate reductase are the main determinants of O₃ tolerance in Euramerican poplar, in protecting photosynthesis capacity from oxidative stress and therefore, maintaining growth and productivity. Besides, stomatal closure was harmful in sensitive genotypes, suggesting that avoiding strategy can be further deleterious under chronic ozone. Finally, O₃-induced early senescence appeared essential when up scaling leaf-level mechanistic response to whole-plant productivity, in fine-tuning resource reallocation and photosynthesis area.

Regulation of ROS through proficient modulations of antioxidative defense system maintains the structural and functional integrity of photosynthetic apparatus and confers drought tolerance in the facultative halophyte Salvadora persica L

The facultative halophyte Salvadora persica L. grow in arid, semiarid and saline areas. In present study, drought induced alterations in growth, ion homeostasis, photosynthesis, chlorophyll fluorescence, ROS regulation and antioxidative defense components were analyzed in S. persica with an aim to elucidate the drought tolerance mechanisms. In response to drought, significant reductions in growth, photosynthesis, and photosynthetic pigments were observed in S. persica. However, leaf relative water content (RWC %) did not change significantly. In S. persica seedlings, the growth, photosynthetic pigment contents and photosynthesis were resumed to control level within 7 d, when the drought treated plants were re-irrigated. However, quantum yield of PSII (ΦPSII), rate of electron transport (ETR), maximum efficiency of PSII (Fv/Fm), and photochemical quenching (qP) remained unaffected under water deficit stress. The results suggest that both non-stomatal as well as stomatal limitations can account for photosynthetic reduction. The ionomics studies revealed no significant alterations in levels of Na⁺, K⁺, Ca²⁺, B, Cu²⁺, Fe²⁺, Mo, and Zn²⁺ in leaf tissue during drought. However, there was increase in levels of Na⁺, K⁺, Ca²⁺ and Mg²⁺ in root tissue in response to drought. The activity of different enzymatic antioxidants like SOD, APX, and GR remained unaffected during drought, whereas POX activity increased and CAT activity declined under drought stress in comparison to control. This result proposes that vital ROS scavenging enzymes like SOD, APX and GR are at threshold levels to maintain the appropriate concentration of ROS. In S. persica, the ratio of AsA/DHA and GSH/GSSG (which are the indicators of redox potential of cell) remained steady or increased under drought which indicates that cellular redox level is maintained in this halophyte. Although ROS levels (H₂O₂ and O₂^•-) increased significantly under drought stress, electrolyte leakage and lipid peroxidation level remained unchanged in response to water deficit condition which indicates that minimal increase in ROS level under drought stress act in signaling for activation of ROS scavenging enzymes. Our results propose that decline in growth and photosynthesis is a vital energy conservation strategy of S. persica under drought condition. The rapid recovery of growth, photosynthesis and water relations in S. persica following drought seems to be a critical mechanism permitting this plant to withstand and survive under drought environment. In addition, our results implicate that efficient regulations of antioxidative enzymes in leaf tissue contribute in regulating the ROS level and cellular redox status, thereby protecting the plant from drought induced oxidative damage in S. persica. Consequently ion homeostasis, plant water status, and integrity of photosynthetic apparatus is maintained in S. persica subjected to drought. The results of present study propose that S. persica is a drought tolerant halophyte and it can be a potential candidate for restoration of degraded saline lands of coastal ecosystem.

Ascorbic acid metabolism and functions: A comparison of plants and mammals

Ascorbic acid is synthesised by eukaryotes, the known exceptions being primates and some other animal groups which have lost functional gulonolactone oxidase. Prokaryotes do not synthesise ascorbate and do not need an ascorbate supply, so the functions that are essential for mammals and plants are not required or are substituted by other compounds. The ability of ascorbate to donate electrons enables it to act as a free radical scavenger and to reduce higher oxidation states of iron to Fe²⁺. These reactions are the basis of its biological activity along with the relative stability of the resulting resonance stabilised monodehydroascorbate radical. The importance of these properties is emphasised by the evolution of at least three biosynthetic pathways and production of an ascorbate analogue, erythroascorbate, by fungi. The iron reducing activity of ascorbate maintains the reactive centre Fe²⁺ of 2-oxoglutarate-dependent dioxygenases (2-ODDs) thus preventing inactivation. These enzymes have diverse functions and, recently, the possibility that ascorbate status in mammals could influence 2-ODDs involved in histone and DNA demethylation thereby influencing stem cell differentiation and cancer has been uncovered. Ascorbate is involved in iron uptake and transport in plants and animals. While the above biochemical functions are shared between mammals and plants, ascorbate peroxidase (APX) is an enzyme family limited to plants and photosynthetic protists. It provides these organisms with increased capacity to remove H₂O₂ produced by photosynthetic electron transport and photorespiration. The Fe reducing activity of ascorbate enables hydroxyl radical production (pro-oxidant effect) and the reactivity of dehydroascorbate (DHA) and reaction of its degradation products with proteins (dehydroascorbylation and glycation) is potentially damaging. Ascorbate status influences gene expression in plants and mammals but at present there is little evidence that it acts as a specific signalling molecule. It most likely acts indirectly by influencing the redox state of thiols and 2-ODD activity. However, the possibility that dehydroascorbylation is a regulatory post-translational protein modification could be explored.

Cytosolic APX2 is a pleiotropic protein involved in H2O2 homeostasis, chloroplast protection, plant architecture and fertility maintenance

Rice cytoplasmic APX2 is a pleiotropic protein, densely distributed around chloroplasts. It plays key roles in H₂O₂ homeostasis and chloroplast protection, and is related to plant architecture and fertility regulation. Ascorbate peroxidases (APXs) catalyze the conversion of H₂O₂ into H₂O. In this report, we systematically investigated the function of cytosolic APX2 using a T-DNA knockout mutant. Loss of OsAPX2 altered rice architecture including shoot height and leaf inclination, resulting in shoot dwarfing, leaf dispersion and fertility decline. Sixty-five differentially expressed proteins were identified in flag leaves of the milk-ripe stage, mainly involved in photosynthesis, glycolysis and TCA cycle, redox homeostasis, and defense. The absence of APX2 severely impacted the stability of chloroplast proteins, and dramatically reduced their expression levels. Subcellular localization showed that APX2 was enriched around each chloroplast to form a high concentration sphere, highlighting chloroplasts as key targets protected by the protein. Accumulation of H₂O₂ was suppressed in the KO-APX2 mutant, which may benefit from increased CAT activity and functional complementation of APX family members. Unexpectedly, the accumulation of soluble sugar, especially sucrose increased significantly, suggesting that APX2 was involved in regulation of sugar metabolism. Obviously, roles of the cytosolic APX2 are very profound and complex in rice. It can be concluded that the cytosolic APX2 is a pleiotropic protein and an important regulator in ROS homeostasis, chloroplast protection, carbohydrate metabolism as well as plant architecture and fertility maintenance.

Nickel accumulation by the green algae-like Euglena gracilis

Nickel accumulation and nickel effects on cellular growth, respiration, photosynthesis, ascorbate peroxidase (APX) activity, and levels of thiols, histidine and phosphate-molecules were determined in Euglena gracilis. Cells incubated with 0.5-1mM NiCl₂ showed impairment of O₂ consumption, photosynthesis, Chl a+b content and APX activity whereas cellular integrity and viability were unaltered. Nickel accumulation was depressed by Mg²⁺ and Cu²⁺, while Ca²⁺, Co²⁺, Mn²⁺ and Zn²⁺ were innocuous. The growth half-inhibitory concentrations for Ni²⁺ in the culture medium supplemented with 2 or 0.2mM Mg²⁺ were 0.43 or 0.03mM Ni²⁺, respectively. Maximal nickel accumulation (1362mg nickel/Kg DW) was achieved in cells exposed to 1mM Ni²⁺ for 24h in the absence of Mg²⁺ and Cu²⁺; accumulated nickel was partially released after 72h. GSH polymers content increased or remained unchanged in cells exposed to 0.05-1mM Ni²⁺; however, GSH, cysteine, γ-glutamylcysteine, and phosphate-molecules all decreased after 72h. Histidine content increased in cells stressed with 0.05 and 0.5mM Ni²⁺ for 24h but not at longer times. It was concluded that E. gracilis can accumulate high nickel levels depending on the external Mg²⁺ and Cu²⁺ concentrations, in a process in which thiols, histidine and phosphate-molecules have a moderate contribution.

Genome-wide investigation and expression profiling of APX gene family in Gossypium hirsutum provide new insights in redox homeostasis maintenance during different fiber development stages

Ascorbate peroxidase (APX) is a member of heme-containing peroxidases which catalyze the H₂O₂-dependent oxidation of a wide range of substrates in plants and animals. As is known, H₂O₂ acts as a signaling molecule in the regulation of fiber development. Our previous work reported that ascorbate peroxidase 1 (GhAPX1) was important for cotton fiber elongation. However, knowledge about APX gene family members and their evolutionary and functional characteristics in cotton is limited. Here, we report 26 GhAPX genes by genome-wide investigation of tetraploid cotton Gossypium hirsutum. Phylogenetic and gene structure analyses classified these APX members into five clades and syntenic analysis suggested two duplication events. Expression profiling of the 26 APXs revealed that ten members are expressed in cotton fibers. Notably, GhAPX10A, GhAPX10D, GhAPX12A, and GhAPX12D showed high expression levels in 30-day fiber, while GhAPX1A/D, GhAPX3A/D, and GhAPX6A/D showed very low expression levels. The enzyme activity and H₂O₂ content assays revealed that cotton fiber kept high enzyme activity and the lowest H₂O₂ level in 30-day fibers, indicating that other than GhAPX1, the newly reported APX members are responsible for the reactive oxygen species homeostasis in the cotton fiber maturation stages. Expression profiling of ten fiber-expressed APXs after phytohormone treatments revealed their regulation patterns by different stimuli, suggesting that GhAPX1, GhAPX12A, and GhAPX12D are responsible to most phytohormone treatments. Our data provided evolutionary and functional information of GhAPX gene family members and revealed that different members are responsible to redox homeostasis during different cotton fiber development stages.

Study of Hydrogen Peroxide as a Senescence-Inducing Signal

In many plant species, leaf senescence correlates with an increase in intracellular levels of reactive oxygen species (ROS) as well as differential regulation of anti-oxidative systems. Due to their reactive nature, reactive oxygen species (ROS) were considered to have only detrimental effects for long time. However, ROS turned out to be more than just toxic by-products of aerobic metabolism but rather major components in different signaling pathways. Considering its relatively long half-life, comparably low reactivity, and its ability to cross membranes, especially hydrogen peroxide, has gained attention as a signaling molecule. In this article, a set of tools to study hydrogen peroxide contents and the activity of its scavenging enzymes in correlation with leaf senescence parameters is presented.

The role of Ala134 in controlling substrate binding and reactivity in ascorbate peroxidase

Ascorbate peroxidase (APX) is a class I heme peroxidase. It has two sites for binding of substrates. One is close to the γ-heme edge and is used for oxidation of ascorbate; the other is at the δ-heme edge and is used for binding of aromatic substrates [Gumiero et al., (2010) Arch. Biochem. Biophys. 500, 13-20]. In this work, we have examined the structural factors that control binding at the δ-heme edge by replacement of Ala134 in APX with a proline residue that is more commonly found in other class II and III peroxidases. Kinetic data indicate that replacement of Ala134 by proline has only a small effect on the catalytic mechanism, or the oxidation of ascorbate or guaiacol. Chemical modification with phenylhydrazine indicates that heme accessibility close to the δ-heme edge is only minorly affected by the substitution. We conclude that the A134P mutation alone is not enough to substantially affect the reactivity of APX towards aromatic substrates bound at the δ-heme edge. The data are relevant to the recent application of APX (APEX) in cellular imaging.

Ultrasonication of in vitro potato single node explants: Activation and recovery of antioxidant defence system and growth responses

The ability to use sound or ultrasound (US) to modify plant growth in vitro, and if possible, to improve yield or productivity, would benefit horticultural scientists. In this study, potato (Solanum tuberosum L. cv. Desirée) in vitro node segments with a single leaf were exposed to US (35 kHz, 70 W, for 20 min). Morphological, physiological and biochemical parameters were measured. Treatment with US 24 h after ultrasonication temporarily accelerated shoot growth but inhibited the development and growth of roots due to a decrease in the level of AA directly after ultrasonication. At the end of the subculture period, i.e., 4 weeks after US treatment, shoot length increased 20% more than control shoots after 4 weeks, while shoot fresh weight was 24% higher than that of control shoots, representing the long-term after-effect of the US treatment. The antioxidant defence system was induced, partly by intensive plantlet growth and development from node segments, and partly by abiotic stress caused by the US treatment. Immediately (0 h) or 24 h after ultrasonication, superoxide dismutase, ascorbate peroxidase, and glutathione reductase activity increased significantly, as did the concentration of low molecular weight antioxidants (GSSG, GSH, AA, TCPa). However, there was no glutathione peroxidase activity, most likely due to the lack of selenium in the basal in vitro growth medium. Therefore, the glutathione-S-transferase path of the ascorbate-glutathione pathway was induced both by metabolic processes and by abiotic stresses and took part in the reduction of organic peroxides using glutathione. US treatment ameliorated the ratios of ascorbic acid/glutathione and reduced/oxidized glutathione, ensuring the development of plantlets with significantly improved shoot parameters, such as higher shoot length and fresh weight, by the end of the subculture period.

Biochemistry and Physiology of Reactive Oxygen Species in Euglena

Reactive oxygen species (ROS) such as superoxide and hydrogen peroxide are by-products of various metabolic processes in aerobic organisms including Euglena. Chloroplasts and mitochondria are the main sites of ROS generation by photosynthesis and respiration, respectively, through the active electron transport chain. An efficient antioxidant network is required to maintain intracellular ROS pools at optimal conditions for redox homeostasis. A comparison with the networks of plants and animals revealed that Euglena has acquired some aspects of ROS metabolic process. Euglena lacks catalase and a typical selenocysteine containing animal-type glutathione peroxidase for hydrogen peroxide scavenging, but contains enzymes involved in ascorbate-glutathione cycle solely in the cytosol. Ascorbate peroxidase in Euglena, which plays a central role in the ascorbate-glutathione cycle, forms a unique intra-molecular dimer structure that is related to the recognition of peroxides. We recently identified peroxiredoxin and NADPH-dependent thioredoxin reductase isoforms in cellular compartments including chloroplasts and mitochondria, indicating the physiological significance of the thioredoxin system in metabolism of ROS. Besides glutathione, Euglena contains the unusual thiol compound trypanothione, an unusual form of glutathione involving two molecules of glutathione joined by a spermidine linker, which has been identified in pathogenic protists such as Trypanosomatida and Schizopyrenida. Furthermore, in contrast to plants, photosynthesis by Euglena is not susceptible to hydrogen peroxide because of resistance of the Calvin cycle enzymes fructose-1,6-bisphosphatse, NADP⁺-glyceraldehyde-3-phosphatase, sedoheptulose-1,7-bisphosphatase, and phosphoribulokinase to hydrogen peroxide. Consequently, these characteristics of Euglena appear to exemplify a strategy for survival and adaptation to various environmental conditions during the evolutionary process of euglenoids.

Rice peroxisomal ascorbate peroxidase knockdown affects ROS signaling and triggers early leaf senescence

H₂O₂, which is continually produced by aerobic metabolism, is a cytotoxic molecule when in high levels. However, low levels can act as a signaling molecule able to regulate the expression of stress responses, senescence, programmed cell death, plant growth, and development. Ascorbate peroxidase (APX) enzyme plays an essential role in the control of intracellular H₂O₂ levels. Here, the function of a gene encoding a peroxisomal APX (OsAPX4) from rice (Oryza sativa L.) was studied. OsAPX4 gene expression can be detected in roots and panicles, but the highest expression level occurs in leaves. Silencing of OsAPX4 and OsAPX3 expression in RNAiOsAPX4 did not affect the growth of plants under growth chamber conditions, but aging transgenic plants interestingly displayed an early senescence phenotype. Leaf fragments from silenced plants were also more sensitive to induced senescence conditions. RNAiOsAPX4 plants did not present detectable changes in intracellular H₂O₂ levels, but biochemical analyses showed that transgenic plants displayed some decreased APX activity in the chloroplastic fraction. Also, the peroxisomal enzyme glycolate oxidase exhibited lower activity, whereas catalase activity was similar to non-transformed rice. The results imply that OsAPX4 gene has an important role in leaf senescence pathway mediated by ROS signaling.

Specificity versus redundancy in the RAP2.4 transcription factor family of Arabidopsis thaliana: transcriptional regulation of genes for chloroplast peroxidases

BACKGROUND

The Arabidopsis ERFIb / RAP2.4 transcription factor family consists of eight members with highly conserved DNA binding domains. Selected members have been characterized individually, but a systematic comparison is pending. The redox-sensitive transcription factor RAP2.4a mediates chloroplast-to-nucleus redox signaling and controls induction of the three most prominent chloroplast peroxidases, namely 2-Cys peroxiredoxin A (2CPA) and thylakoid- and stromal ascorbate peroxidase (tAPx and sAPx). To test the specificity and redundancy of RAP2.4 transcription factors in the regulation of genes for chloroplast peroxidases, we compared the DNA-binding sites of the transcription factors in tertiary structure models, analyzed transcription factor and target gene regulation by qRT-PCR in RAP2.4, 2-Cys peroxiredoxin and ascorbate peroxidase T-DNA insertion lines and RAP2.4 overexpressing lines of Arabidopsis thaliana and performed promoter binding studies.

RESULTS

All RAP2.4 proteins bound the tAPx promoter, but only the four RAP2.4 proteins with identical DNA contact sites, namely RAP2.4a, RAP2.4b, RAP2.4d and RAP2.4h, interacted stably with the redox-sensitive part of the 2CPA promoter. Gene expression analysis in RAP2.4 knockout lines revealed that RAP2.4a is the only one supporting 2CPA and chloroplast APx expression. Rap2.4h binds to the same promoter region as Rap2.4a and antagonizes 2CPA expression. Like the other six RAP2.4 proteins, Rap2.4 h promotes APx mRNA accumulation. Chloroplast ROS signals induced RAP2.4b and RAP2.4d expression, but these two transcription factor genes are (in contrast to RAP2.4a) insensitive to low 2CP availability, and their expression decreased in APx knockout lines. RAP2.4e and RAP2.4f gradually responded to chloroplast APx availability and activated specifically APx expression. These transcription factors bound, like RAP2.4c and RAP2.4g, the tAPx promoter, but hardly the 2CPA promoter.

CONCLUSIONS

The RAP2.4 transcription factors form an environmentally and developmentally regulated transcription factor network, in which the various members affect the expression intensity of the others. Within the transcription factor family, RAP2.4a has a unique function as a general transcriptional activator of chloroplast peroxidase activity. The other RAP2.4 proteins mediate the fine-control and adjust the relative availability of 2CPA, sAPx and tAPx.

Association between radionuclides (210Po and 210Pb) and antioxidant enzymes in oak (Quercus coccifera) and mastic tree (Pistacia lentiscus)

The activity levels of naturally occurring radionuclides Polonium-210 and lead-210 in different subjects including plant species have direct or indirect impact on human beings. High levels of ionising radiation cause oxidative stress and the interaction between antioxidative defense and radionuclides is not well established in plant systems. In this study, we aimed to understand the impact of oxidative stress caused by ²¹⁰Po and ²¹⁰Pb in two Mediterranean plants; Quercus coccifera and Pistacia lentiscus. We analysed the constitutive and seasonal levels of ²¹⁰Po, ²¹⁰Pb, lipid peroxidation levels, superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities in the field-collected samples. The highest activity concentrations of ²¹⁰Po and ²¹⁰Pb were detected in both plants in summer and Q. coccifera had higher levels than that of P. lentiscus. SOD and APX activity trends were different between oak and mastic; as compared to P. lentiscus, Q. coccifera efficiently used the two major components of antioxidative defense. Lipid peroxidation levels were low in both plants in all seasons except that of spring which were in good agreement with high antioxidant enzyme activities. In conclusion, we found that high ²¹⁰Po and ²¹⁰Pb activity concentrations in oak and mastic did not interfere with their growth and life cycles. The ability of both plants for survival and adaptation to Mediterranean environmental constraints provided an additional advantage for coping radionuclide induced oxidative stress as well.

Arsenic-induced stress activates sulfur metabolism in different organs of garlic (Allium sativum L.) plants accompanied by a general decline of the NADPH-generating systems in roots

Arsenic (As) contamination is a major environmental problem which affects most living organisms from plants to animals. This metalloid poses a health risk for humans through its accumulation in crops and water. Using garlic (Allium sativum L.) plants as model crop exposed to 200μM arsenate, a comparative study among their main organs (roots and shoots) was made. The analysis of arsenic, glutathione (GSH), phytochelatins (PCs) and lipid peroxidation contents with the activities of antioxidant enzymes (catalase, superoxide dismutase, ascorbate-glutathione cycle), and the main components of the NADPH-generating system, including glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), NADP-malic enzyme (NADP-ME) and NADP-isocitrate dehydrogenase (NADP-ICDH) was carried out. Data showed a correlation among arsenic accumulation in the different organs, PCs content and the antioxidative response, with a general decline of the NADPH-generating systems in roots. Overall, our results demonstrate that there are clear connections between arsenic uptake, increase of their As-chelating capacity in roots and a decline of antioxidative enzyme activities (catalase and the ascorbate peroxidase) whose alteration provoked As-induced oxidative stress. Thus, the data suggest that roots act as barrier of arsenic mediated by a prominent sulfur metabolism which is characterized by the biosynthesis of high amount of PCs.

Overexpression of the ascorbate peroxidase gene from eggplant and sponge gourd enhances flood tolerance in transgenic Arabidopsis

Previously, we found that the flood resistance of eggplant (Solanum melongena) and sponge gourd (Luffa cylindrica) enhanced ascorbate peroxidase (APX) activity under flooding, and consequently, both the SmAPX and LcAPX genes were cloned. In this study, the SmAPX and LcAPX genes were transferred under a ubiquitin promoter to Arabidopsis (At) via Agrobacterium tumefaciens. The expression and amount of APX and APX activities of the SmAPX and LcAPX transgenic lines were significantly higher than those of non-transgenic (NT) plants under a waterlogged condition. Furthermore, the SmAPX, LcAPX, At-sucrose synthases (SUS)-1, phosphoenolpyruvate carboxylase (PEPC), and lactate dehydrogenase (LDH) genes were overexpressed in all transgenic Arabidopsis lines after flooding treatment. Compared to NT plants, the malondialdehyde (MDA) contents and H₂O₂ accumulation were significantly lower, but germination rates were significantly higher in all transgenic lines with higher APX activity, indicating that the overexpression of SmAPX and LcAPX in Arabidopsis could enhance flood tolerance by eliminating H₂O₂. Moreover, Arabidopsis seedlings overexpressing SmAPX and LcAPX also displayed greater resistance to flooding and less oxidative injury than NT plants subjected to flooding condition.