Monodehydroascorbate Reductase Plays a Role in the Tolerance of Chlamydomonas reinhardtii to Photooxidative Stress

The ascorbate-glutathione cycle coming of age

Concepts regarding the operation of the ascorbate-glutathione cycle and the associated water/water cycle in the processing of metabolically generated hydrogen peroxide and other forms of reactive oxygen species (ROS) are well established in the literature. However, our knowledge of the functions of these cycles and their component enzymes continues to grow and evolve. Recent insights include participation in the intrinsic environmental and developmental signalling pathways that regulate plant growth, development, and defence. In addition to ROS processing, the enzymes of the two cycles not only support the functions of ascorbate and glutathione, they also have 'moonlighting' functions. They are subject to post-translational modifications and have an extensive interactome, particularly with other signalling proteins. In this assessment of current knowledge, we highlight the central position of the ascorbate-glutathione cycle in the network of cellular redox systems that underpin the energy-sensitive communication within the different cellular compartments and integrate plant signalling pathways.

Reactive oxygen species: Multidimensional regulators of plant adaptation to abiotic stress and development

Reactive oxygen species (ROS) are produced as undesirable by-products of metabolism in various cellular compartments, especially in response to unfavorable environmental conditions, throughout the life cycle of plants. Stress-induced ROS production disrupts normal cellular function and leads to oxidative damage. To cope with excessive ROS, plants are equipped with a sophisticated antioxidative defense system consisting of enzymatic and non-enzymatic components that scavenge ROS or inhibit their harmful effects on biomolecules. Nonetheless, when maintained at relatively low levels, ROS act as signaling molecules that regulate plant growth, development, and adaptation to adverse conditions. Here, we provide an overview of current approaches for detecting ROS. We also discuss recent advances in understanding ROS signaling, ROS metabolism, and the roles of ROS in plant growth and responses to various abiotic stresses.

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.

Modulation in gene expression and enzyme activity suggested the roles of monodehydroascorbate reductase in development and stress response in bread wheat

Monodehydroascorbate reductase (MDHAR) is a crucial enzymatic antioxidant of the ascorbate-glutathione pathway involved in reactive oxygen species scavenging. Herein, we identified 15 TaMDHAR genes in bread wheat. Phylogenetic analysis revealed their clustering into three groups, which are also related to the subcellular localization in the peroxisome matrix, peroxisome membrane, and chloroplast. Each TaMDHAR protein consisted of two conserved domains; Pyr_redox and Pyr_redox_2 of the pyridine nucleotide disulfide oxidoreductase family. The occurrence of diverse groups of cis-regulatory elements in the promoter region and their interaction with numerous transcription factors suggest assorted functions of TaMDHARs in growth and development and in light, phytohormones, and stress responses. Expression analysis in various tissues further revealed their importance in vegetative and reproductive development. In addition, the differential gene expression and enhanced enzyme activity during drought, heat, and salt treatments exposed their role in abiotic stress response. Interaction of MDHARs with various antioxidant enzymes and biochemicals related to the ascorbate-glutathione cycle exposed their synchronized functioning. Interaction with auxin indicated the probability of cross-talk between antioxidants and auxin signaling. The miR168a, miR169, miR172 and others interaction with various TaMDHARs further directed their association with developmental processes and stress responses. The current study provides extensive information about the importance of TaMDHARs, moreover, the precise role of each gene needs to be established in future studies.

Early leaf responses of cell physiological and sensor-based signatures reflect susceptibility of wheat seedlings to infection by leaf rust

Leaf rust caused by Puccinia triticina Erikss. can have devastating effects on wheat (Triticum aestivum L.), causing severe economic losses. This comprehensive study serves to facilitate our understanding of the impact of carbohydrate and antioxidant metabolism in association with sensor-based phenotyping and leaf rust stress responses in wheat seedlings. After 24 h of inoculation (hai) very susceptible variety to leaf rust (Ficko) increased cell-wall invertase (cwInv; EC 3.2.1.26), compared to other varieties that significantly increased cwInv later. This could mean that the Ficko variety cannot defend itself from leaf rust infections once symptoms have started to develop. Also, Ficko had significantly decreased amounts of cytoplasmic invertase (cytInv; EC 3.2.1.26) at 8 hai. The downregulation of cytInv in susceptible plants may facilitate the maintenance of elevated apoplastic sucrose availability favoring the pathogen. The significant role of vacuolar invertase (vacInv; EC 3.2.1.26) in moderately resistant varieties was recorded. Also, a significant decrease of glucose-6-phosphate dehydrogenase (G6PDH; EC 1.1.1.49) and UDP-glucose pyrophosphorylase (UGPase; EC 2.7.7.9) in moderately resistant varieties might restrict normal development of leaf rust due to reduced sugar. During plant-pathogen interaction, when the invader spreads systemically throughout the plant, the main role of ascorbate peroxidase (APX; EC 1.11.1.11) activity in one moderately resistant variety (Olimpija) and catalase (CAT; EC 1.11.1.6) activity in another moderately resistant variety (Alka) is to protect the plant against oxidative damage in the early stages of infection. Non-invasive phenotyping with a sensor-based technique could be used as a rapid method for pre-symptomatic determination of wheat leaf rust resistance or susceptibility.

Chloroplast dehydroascorbate reductase and glutathione cooperatively determine the capacity for ascorbate accumulation under photooxidative stress conditions

Ascorbate is an indispensable redox buffer essential for plant growth and stress acclimation. Its oxidized form, dehydroascorbate (DHA), undergoes rapid degradation unless it is recycled back into ascorbate by glutathione (GSH)-dependent enzymatic or non-enzymatic reactions, with the enzymatic reactions catalyzed by dehydroascorbate reductases (DHARs). Our recent study utilizing an Arabidopsis quadruple mutant (∆dhar pad2), which lacks all three DHARs (∆dhar) and is deficient in GSH (pad2), has posited that these GSH-dependent reactions operate in a complementary manner, enabling a high accumulation of ascorbate under high-light stress. However, as Arabidopsis DHAR functions in the cytosol or chloroplasts, it remained unclear which isoform played a more significant role in cooperation with GSH-dependent non-enzymatic reactions. To further comprehend the intricate network of ascorbate recycling systems in plants, we generated mutant lines lacking cytosolic DHAR1/2 or chloroplastic DHAR3, or both, in another GSH-deficient background (cad2). A comprehensive comparison of ascorbate profiles in these mutants under conditions of photooxidative stress induced by various light intensities or methyl viologen unequivocally demonstrated that chloroplastic DHAR3, but not cytosolic isoforms, works in concert with GSH to accumulate ascorbate. Our findings further illustrate that imbalances between stress intensity and recycling capacity significantly impact ascorbate pool size and tolerance to photooxidative stress. Additionally, it was found that the absence of DHARs and GSH deficiency do not impede ascorbate biosynthesis, at least in terms of transcription or activity of biosynthetic enzymes. This study provides insights into the robustness of ascorbate recycling.

Clustered regularly interspaced short palindromic repeats (CRISPR) technology and genetic engineering strategies for microalgae towards carbon neutrality: A critical review

Carbon dioxide is the major greenhouse gas and regards as the critical issue of global warming and climate changes. The inconspicuous microalgae are responsible for 40% of carbon fixation among all photosynthetic plants along with a higher photosynthetic efficiency and convert the carbon into lipids, protein, pigments, and bioactive compounds. Genetic approach and metabolic engineering are applied to accelerate the growth rate and biomass of microalgae, hence achieve the mission of carbon neutrality. Meanwhile, CRISPR/Cas9 is efficiently to enhance the productivity of high-value compounds in microalgae for it is easier operation, more affordable and is able to regulate multiple genes simultaneously. The genetic engineering strategies provide the multidisciplinary concept to evolute and increase the CO₂ fixation rate through Calvin-Benson-Bassham cycle. Therefore, the technologies, bioinformatics tools, systematic engineering approaches for carbon neutrality and circular economy are summarized and leading one step closer to the decarbonization society in this review.

Protein-protein interactions in plant antioxidant defense

The regulation of reactive oxygen species (ROS) levels in plants is ensured by mechanisms preventing their over accumulation, and by diverse antioxidants, including enzymes and nonenzymatic compounds. These are affected by redox conditions, posttranslational modifications, transcriptional and posttranscriptional modifications, Ca²⁺, nitric oxide (NO) and mitogen-activated protein kinase signaling pathways. Recent knowledge about protein-protein interactions (PPIs) of antioxidant enzymes advanced during last decade. The best-known examples are interactions mediated by redox buffering proteins such as thioredoxins and glutaredoxins. This review summarizes interactions of major antioxidant enzymes with regulatory and signaling proteins and their diverse functions. Such interactions are important for stability, degradation and activation of interacting partners. Moreover, PPIs of antioxidant enzymes may connect diverse metabolic processes with ROS scavenging. Proteins like receptor for activated C kinase 1 may ensure coordination of antioxidant enzymes to ensure efficient ROS regulation. Nevertheless, PPIs in antioxidant defense are understudied, and intensive research is required to define their role in complex regulation of ROS scavenging.

Assessing the effects of nickel on, e.g., Medicago sativa L. nodules using multidisciplinary approach

Industrial wastes and fertilizers can introduce excessive levels of nickel (Ni) into the environment, potentially causing threats to plants, animals, as well as human beings. However, the number of studies on the effects of Ni toxicity on nodules is fairly limited. To address this issue, the effects of increasing Ni concentration on alfalfa nodules were assessed at chemical, biochemical, and transcriptomic levels. For this purpose, plants were grown in soils supplied with Ni (control, 0 mg/kg; C1, 50 mg/kg; C2, 150 mg/kg; C3, 250 mg/kg; and C4, 500 mg/kg) for 90 days. Ni loads in leaves, roots, and nodules were monitored after the exposure period. A set of biochemical biomarkers of oxidative stress was determined in nodules including antioxidants and metal homeostasis as well as lipid peroxidation. Gene expression levels of the main targets involved in oxidative stress and metal homeostasis were assessed. Our data indicated a high concentration of Ni in leaves, roots, and nodules where values reached 25.64 ± 3.04 mg/kg, 83.23 ± 5.16 mg/kg, and 125.71 ± 4.53 mg/kg in dry weight, respectively. Moreover, a significant increase in nodule biomass was observed in plants exposed to C4 in comparison to control treatment and percentage increased by 63%. Then, lipid peroxidation increased with a rate of 95% in nodules exposed to C4. Enzymatic activities were enhanced remarkably, suggesting the occurrence of oxidative stress, with increased superoxide dismutase (SOD), glutathione reductase (GR), and ascorbate peroxidase (APX). Our results showed also a significant upregulation of SOD, GR and APX genes in nodules. Nodule homoglutathione (HGSH) levels increased with the different Ni concentrations, with a remarkable decrease of glutathione S-transferase (GST) activity and glutathione (GSH) content for the highest Ni concentration with 43% and 52% reduction, respectively. The phytochelatin (PC) and metallothionein (MT) concentrations increased in nodules, which implied the triggering of a cellular protection mechanism for coping with Ni toxicity. The results suggested that Ni promotes a drastic oxidative stress in alfalfa nodules, yet the expression of MT and PC to reduce Ni toxicity could be used as Ni stress bioindicators. Our findings provide new insights into the central role of alfalfa nodules in limiting the harmful effects of soil pollution. Therefore, nodules co-expressing antioxidant enzymes may have high phytoremediation potential.

Multi-omics approaches and genetic engineering of metabolism for improved biorefinery and wastewater treatment in microalgae

Microalgae, a group of photosynthetic microorganisms rich in diverse and novel bioactive metabolites, have been explored for the production of biofuels, high value-added compounds as food and feeds, and pharmaceutical chemicals as agents with therapeutic benefits. This article reviews the development of omics resources and genetic engineering techniques including gene transformation methodologies, mutagenesis, and genome-editing tools in microalgae biorefinery and wastewater treatment. The introduction of these enlisted techniques has simplified the understanding of complex metabolic pathways undergoing microalgal cells. The multiomics approach of the integrated omics datasets, big data analysis, and machine learning for the discovery of objective traits and genes responsible for metabolic pathways was reviewed. Recent advances and limitations of multiomics analysis and genetic bioengineering technology to facilitate the improvement of microalgae as the dual role of wastewater treatment and biorefinery feedstock production are discussed. This article is protected by copyright. All rights reserved.

Heavy metal-induced stress in eukaryotic algae-mechanisms of heavy metal toxicity and tolerance with particular emphasis on oxidative stress in exposed cells and the role of antioxidant response

Heavy metals is a collective term describing metals and metalloids with a density higher than 5 g/cm3. Some of them are essential micronutrients; others do not play a positive role in living organisms. Increased anthropogenic emissions of heavy metal ions pose a serious threat to water and land ecosystems. The mechanism of heavy metal toxicity predominantly depends on (1) their high affinity to thiol groups, (2) spatial similarity to biochemical functional groups, (3) competition with essential metal cations, (4) and induction of oxidative stress. The antioxidant response is therefore crucial for providing tolerance to heavy metal-induced stress. This review aims to summarize the knowledge of heavy metal toxicity, oxidative stress and antioxidant response in eukaryotic algae. Types of ROS, their formation sites in photosynthetic cells, and the damage they cause to the cellular components are described at the beginning. Furthermore, heavy metals are characterized in more detail, including their chemical properties, roles they play in living cells, sources of contamination, biochemical mechanisms of toxicity, and stress symptoms. The following subchapters contain the description of low-molecular-weight antioxidants and ROS-detoxifying enzymes, their properties, cellular localization, and the occurrence in algae belonging to different clades, as well as the summary of the results of the experiments concerning antioxidant response in heavy metal-treated eukaryotic algae. Other mechanisms providing tolerance to metal ions are briefly outlined at the end.

Molecular Mechanisms Underlying the Acclimation of Chlamydomonas reinhardtii Against Nitric Oxide Stress

The acclimation mechanism of Chlamydomonas reinhardtii to nitric oxide (NO) was studied by exposure to S-nitroso-N-acetylpenicillamine (SNAP), a NO donor. Treatment with 0.1 or 0.3 mM SNAP transiently inhibited photosynthesis within 1 h, followed by a recovery, while 1.0 mM SNAP treatment caused irreversible photosynthesis inhibition and mortality. The SNAP effects are avoided in the presence of the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (cPTIO). RNA-seq, qPCR, and biochemical analyses were conducted to decode the metabolic shifts under NO stress by exposure to 0.3 mM SNAP in the presence or absence of 0.4 mM cPTIO. These findings revealed that the acclimation to NO stress comprises a temporally orchestrated implementation of metabolic processes: (1). modulation of NADPH oxidase (respiratory burst oxidase-like 2, RBOL2) and ROS signaling pathways for downstream mechanism regulation, (2). trigger of NO scavenging elements to reduce NO level; (3). prevention of photo-oxidative risk through photosynthesis inhibition and antioxidant defense system induction; (4). acclimation to nitrogen and sulfur shortage; (5). attenuation of transcriptional and translational activity together with degradation of damaged proteins through protein trafficking machinery (ubiquitin, SNARE, and autophagy) and molecular chaperone system for dynamic regulation of protein homeostasis. In addition, the expression of the gene encoding NADPH oxidase, RBOL2, showed a transient increase while that of RBOL1 was slightly decreased after NO challenge. It reflects that NADPH oxidase, a regulator in ROS-mediated signaling pathway, may be involved in the responses of Chlamydomonas to NO stress. In conclusion, our findings provide insight into the molecular events underlying acclimation mechanisms in Chlamydomonas to NO stress.

Diverse Biosynthetic Pathways and Protective Functions against Environmental Stress of Antioxidants in Microalgae

Eukaryotic microalgae have been classified into several biological divisions and have evolutionarily acquired diverse morphologies, metabolisms, and life cycles. They are naturally exposed to environmental stresses that cause oxidative damage due to reactive oxygen species accumulation. To cope with environmental stresses, microalgae contain various antioxidants, including carotenoids, ascorbate (AsA), and glutathione (GSH). Carotenoids are hydrophobic pigments required for light harvesting, photoprotection, and phototaxis. AsA constitutes the AsA-GSH cycle together with GSH and is responsible for photooxidative stress defense. GSH contributes not only to ROS scavenging, but also to heavy metal detoxification and thiol-based redox regulation. The evolutionary diversity of microalgae influences the composition and biosynthetic pathways of these antioxidants. For example, α-carotene and its derivatives are specific to Chlorophyta, whereas diadinoxanthin and fucoxanthin are found in Heterokontophyta, Haptophyta, and Dinophyta. It has been suggested that AsA is biosynthesized via the plant pathway in Chlorophyta and Rhodophyta and via the Euglena pathway in Euglenophyta, Heterokontophyta, and Haptophyta. The GSH biosynthetic pathway is conserved in all biological kingdoms; however, Euglenophyta are able to synthesize an additional thiol antioxidant, trypanothione, using GSH as the substrate. In the present study, we reviewed and discussed the diversity of microalgal antioxidants, including recent findings.

Recent Developments in Enzymatic Antioxidant Defence Mechanism in Plants with Special Reference to Abiotic Stress

The stationary life of plants has led to the evolution of a complex gridded antioxidant defence system constituting numerous enzymatic components, playing a crucial role in overcoming various stress conditions. Mainly, these plant enzymes are superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferases (GST), ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), and dehydroascorbate reductase (DHAR), which work as part of the antioxidant defence system. These enzymes together form a complex set of mechanisms to minimise, buffer, and scavenge the reactive oxygen species (ROS) efficiently. The present review is aimed at articulating the current understanding of each of these enzymatic components, with special attention on the role of each enzyme in response to the various environmental, especially abiotic stresses, their molecular characterisation, and reaction mechanisms. The role of the enzymatic defence system for plant health and development, their significance, and cross-talk mechanisms are discussed in detail. Additionally, the application of antioxidant enzymes in developing stress-tolerant transgenic plants are also discussed.

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

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

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

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

Proteomics analysis reveals that foreign cp4-epsps gene regulates the levels of shikimate and branched pathways in genetically modified soybean line H06-698

Although genetically modified (GM) glyphosate-resistant soybeans with cp4-epsps gene have been widely planted all over the world, their proteomic characteristics are not very clear. In this study, the soybean seeds of a GM soybean line H06-698 (H) with cp4-epsps gene and its non-transgenic counterpart Mengdou12 (M), which were collected from two experiment fields in two years and used as 4 sample groups, were analyzed with label-free proteomics technique. A total of 1706 proteins were identified quantitatively by label-free quantification, and a total of 293 proteins were detected as common differential abundance proteins (DAPs, FC is not less than 1.5) both in two groups or more. Functional enrichment analysis of common DAPs identified from four groups, shows that most up-regulated proteins were clustered into stress response, carbon and energy metabolism, and genetic information processing. Further documentary analysis shows that 15 proteins play important roles in shikimate pathways, reactive oxygen species (ROS) and stress response. These results indicated that the change of protein abundance in different samples were affected by various factors, but except shikimate and branched pathways related proteins, only ROS and stress-related proteins were found to be stably regulated by cp4-epsps gene, and no unexpected and safety-related proteins such as antinutritional factors, allergenic proteins, and toxic proteins were found as DAPs. The influence of foreign genes in genetically modified plants is worthy of attention and this work provides new clues for exploring the regulated proteins and pathways in GM plants.

The Multiple Roles of Ascorbate in the Abiotic Stress Response of Plants: Antioxidant, Cofactor, and Regulator

Ascorbate (ASC) plays a critical role in plant stress response. The antioxidant role of ASC has been well-studied, but there are still several confusing questions about the function of ASC in plant abiotic stress response. ASC can scavenge reactive oxygen species (ROS) and should be helpful for plant stress tolerance. But in some cases, increasing ASC content impairs plant abiotic stress tolerance, whereas, inhibiting ASC synthesis or regeneration enhances plant stress tolerance. This confusing phenomenon indicates that ASC may have multiple roles in plant abiotic stress response not just as an antioxidant, though many studies more or less ignored other roles of ASC in plant. In fact, ACS also can act as the cofactor of some enzymes, which are involved in the synthesis, metabolism, and modification of a variety of substances, which has important effects on plant stress response. In addition, ASC can monitor and effectively regulate cell redox status. Therefore, we believe that ASC has atleast triple roles in plant abiotic stress response: as the antioxidant to scavenge accumulated ROS, as the cofactor to involve in plant metabolism, or as the regulator to coordinate the actions of various signal pathways under abiotic stress. The role of ASC in plant abiotic stress response is important and complex. The detail role of ASC in plant abiotic stress response should be analyzed according to specific physiological process in specific organ. In this review, we discuss the versatile roles of ASC in the response of plants to abiotic stresses.

Ascorbate peroxidase 4 plays a role in the tolerance of Chlamydomonas reinhardtii to photo-oxidative stress

Ascorbate peroxidase (APX; EC 1.11.1.11) activity and transcript levels of CrAPX1, CrAPX2, and CrAPX4 of Chlamydomonas reinhardtii increased under 1,400 μE·m⁻²·s⁻¹ condition (HL). CrAPX4 expression was the most significant. So, CrAPX4 was downregulated using amiRNA technology to examine the role of APX for HL acclimation. The CrAPX4 knockdown amiRNA lines showed low APX activity and CrAPX4 transcript level without a change in CrAPX1 and CrAPX2 transcript levels, and monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR) activities and transcript levels. Upon exposure to HL, CrAPX4 knockdown amiRNA lines appeared a modification in the expression of genes encoding the enzymes in the ascorbate–glutathione cycle, including an increase in transcript level of CrVTC2, a key enzyme for ascorbate (AsA) biosynthesis but a decrease in MDAR and DHAR transcription and activity after 1 h, followed by increases in reactive oxygen species production and lipid peroxidation after 6 h and exhibited cell death after 9 h. Besides, AsA content and AsA/DHA (dehydroascorbate) ratio decreased in CrAPX4 knockdown amiRNA lines after prolonged HL treatment. Thus, CrAPX4 induction together with its association with the modulation of MDAR and DHAR expression for AsA regeneration is critical for Chlamydomonas to cope with photo-oxidative stress.