Wound-induced apoplastic peroxidase activities: their roles in the production and detoxification of reactive oxygen species

The transcriptomic and biochemical responses of blood clams (Tegillarca granosa) to prolonged intermittent hypoxia

The blood clam (Tegillarca granosa), a marine bivalve of ecological and economic significance, often encounters intermittent hypoxia in mudflats and aquatic environments. To study the response of blood clam foot to prolonged intermittent hypoxia, the clams were exposed to intermittent hypoxia conditions (0.5 mg/L dissolved oxygen, with a 12-h interval) for 31 days. Initially, transcriptomic analysis was performed, uncovering a total of 698 differentially expressed genes (DEGs), with 236 upregulated and 462 downregulated. These genes show enrichments in signaling pathways related to glucose metabolism, sugar synthesis and responses to oxidative stress. Furthermore, the activity of the enzyme glutathione peroxidase (GPx) and the levels of gpx1 mRNA showed gradual increases, reaching their peak on the 13th day of intermittent hypoxia exposure. This observation suggests an indirect protective role of GPx against oxidative stress. The results of this study make a significantly contribute to our broader comprehensive of the physiological, biochemical responses, and molecular reactions governing the organization of foot muscle tissue in marine bivalves exposed to prolonged intermittent hypoxic conditions.

Ensifer sp. GMS14 enhances soybean salt tolerance for potential application in saline soil reclamation

Rhizosphere microbiomes play an important role in enhancing plant salt tolerance and are also commonly employed as bio-inoculants in soil remediation processes. Cultivated soybean (Glycine max) is one of the major oilseed crops with moderate salt tolerance. However, the response of rhizosphere microbes me to salt stress in soybean, as well as their potential application in saline soil reclamation, has been rarely reported. In this study, we first investigated the microbial communities of salt-treated and non-salt-treated soybean by 16S rRNA gene amplicon sequencing. Then, the potential mechanism of rhizosphere microbes in enhancing the salt tolerance of soybean was explored based on physiological analyses and transcriptomic sequencing. Our results suggested that Ensifer and Novosphingobium were biomarkers in salt-stressed soybean. One corresponding strain, Ensifer sp. GMS14, showed remarkable growth promoting characteristics. Pot experiments showed that GMS14 significantly improved the growth performance of soybean in saline soils. Strain GMS14 alleviated sodium ions (Na+) toxicity by maintaining low a Na+/K+ ratio and promoted nitrogen (N) and phosphorus (P) uptake by soybean in nutrient-deficient saline soils. Transcriptome analyses indicated that GMS14 improved plant salt tolerance mainly by ameliorating salt stress-mediated oxidative stress. Interestingly, GMS14 was evidenced to specifically suppress hydrogen peroxide (H2O2) production to maintain reactive oxygen species (ROS) homeostasis in plants under salt stress. Field experiments with GMS14 applications showed its great potential in saline soil reclamation, as evidenced by the increased biomass and nodulation capacity of GMS14-inoculated soybean. Overall, our findings provided valuable insights into the mechanisms underlying plant-microbes interactions, and highlighted the importance of microorganisms recruited by salt-stressed plant in the saline soil reclamation.

Mechanisms of Antioxidant Resistance in Different Wheat Genotypes under Salt Stress and Hypoxia

Various stressors lead to an increase in ROS and damage to plant tissues. Plants have a powerful antioxidant system (AOS), which allows them to neutralize excess ROS. We detected an intense fluorescent glow of ROS in the cells of the cap, meristem, and elongation zones in the roots of wheat Triticum aestivum (Orenburgskaya 22 variety) and Triticum durum (Zolotaya variety). An increase in ROS was accompanied by DNA breaks in the nuclei of wheat root cells, the release of cytochrome c from mitochondria into the cytoplasm, and the translocation of phosphatidylserine into the outer layer of the plasma membrane under salt stress and hypoxia. The different resistances of the two wheat varieties to different abiotic stresses were revealed. The soft wheat variety Orenburgskaya 22 showed high resistance to salt stress but sensitivity to hypoxia, and the durum wheat variety Zolotaya showed tolerance to hypoxia but high sensitivity to salt stress. Different activations of AOS components (GSH, MnSOD, Cu/ZnSOD, CAT, PX, GPX, and GST) were revealed in different wheat genotypes. The basis for the tolerance of the Zolotaya variety to hypoxia is the high content of glutathione (GSH) and the activation of glutathione-dependent enzymes. One of the mechanisms of high resistance to salt stress in the Orenburgskaya 22 variety is a decrease in the level of ROS as a result of the increased activity of the MnSOD and Cu/ZnSOD genes. Identifying the mechanisms of plant tolerance to abiotic stress is the most important task for improving breeding varieties of agricultural plants and increasing their yield.

Stimulation of the Defense Mechanisms of Potatoes to a Late Blight Causative Agent When Treated with Bacillus subtilis Bacteria and Chitosan Composites with Hydroxycinnamic Acids

Phytophthora infestans is, worldwide, one of the main causal agents of epiphytotics in potato plantings. Prevention strategies demand integrated pest management, including modeling of beneficial microbiomes of agroecosystems combining microorganisms and natural products. Chitooligosaccharides and their derivatives have great potential to be used by agrotechnology due to their ability to elicit plant immune reactions. The effect of combining Bacillus subtilis 26D and 11VM and conjugates of chitin with hydroxycinnamates on late blight pathogenesis was evaluated. Mechanisms for increasing the resistance of potato plants to Phytophthora infestans were associated with the activation of the antioxidant system of plants and an increase in the level of gene transcripts that encode PR proteins: basic protective protein (PR-1), thaumatin-like protein (PR-5), protease inhibitor (PR-6), and peroxidase (PR-9). The revealed activation of the expression of marker genes of systemic acquired resistance and induced systemic resistance under the influence of the combined treatment of plants with B. subtilis and conjugates of chitin with hydroxycinnamates indicates that, in this case, the development of protective reactions in potato plants to late blight proceeds synergistically, where B. subtilis primes protective genes, and chitosan composites act as a trigger for their expression.

Signal Mediators in the Implementation of Jasmonic Acid's Protective Effect on Plants under Abiotic Stresses

Plant cells respond to stress by activating signaling and regulatory networks that include plant hormones and numerous mediators of non-hormonal nature. These include the universal intracellular messenger calcium, reactive oxygen species (ROS), gasotransmitters, small gaseous molecules synthesized by living organisms, and signal functions such as nitrogen monoxide (NO), hydrogen sulfide (H₂S), carbon monoxide (CO), and others. This review focuses on the role of functional linkages of jasmonic acid and jasmonate signaling components with gasotransmitters and other signaling mediators, as well as some stress metabolites, in the regulation of plant adaptive responses to abiotic stressors. Data on the involvement of NO, H₂S, and CO in the regulation of jasmonic acid formation in plant cells and its signal transduction were analyzed. The possible involvement of the protein components of jasmonate signaling in stress-protective gasotransmitter effects is discussed. Emphasis is placed on the significance of the functional interaction between jasmonic acid and signaling mediators in the regulation of the antioxidant system, stomatal apparatus, and other processes important for plant adaptation to abiotic stresses.

Activity of Hydrolases and Their Inhibitors in Potato Plants Treated with Bacillus subtilis, Salicylic, and Jasmonic Acids and Affected by the Combined Effect of the Late Blight and the Lack of Moisture

The effect of Bacillus subtilis in combination with salicylic (SA) and jasmonic (JA) acids on the activity of amylases, cellulases, proteases, and their inhibitors in potato leaves in connection with the development of resistance to Phytophthora infestans (Mont.) de Bary in conditions of moisture deficiency have been investigated. Plants grown from microtubers were treated with Bacillus subtilis suspension (108 cells/mL) and with a mixture of bacteria with SA (10−6 M), JA (10−7 M), and SA + JA and were then infected with P. infestans (107 spores/mL) and cultivated under drought. Treatment with B. subtilis bacteria, especially in combination with signaling molecules, contributed to a decrease in the degree of pathogen infestation on plants grown with a lack of moisture. Both salicylate and jasmonate signaling pathways play an important role in the regulation of hydrolase activity and the stimulation of plant resistance. The revealed differences in the degree of hydrolase inhibitors activation under the influence of B. subtilis bacteria and signal molecules suggest different paths to the formation of resistance to P. infestans in potato under drought conditions.

Local problems need global solutions: The metabolic needs of regenerating organisms

The vast majority of species that belong to the plant or animal kingdom evolved with two main strategies to counter tissue damage-scar formation and regeneration. Whereas scar formation provides a fast and cost-effective repair to exit life-threatening conditions, complete tissue regeneration is time-consuming and requires vast resources to reinstall functionality of affected organs or structures. Local environments in wound healing are widely studied and findings have provided important biomedical applications. Less well understood are organismic physiological parameters and signalling circuits essential to maintain effective tissue repair. Here, we review accumulated evidence that positions the interplay of local and systemic changes in metabolism as essential variables modulating the injury response. We particularly emphasise the role of lipids and lipid-like molecules as significant components long overlooked.

NOX-like ROS production by glutathione reductase

In organisms from bacteria to mammals, NADPH oxidase (NOX) catalyzes the production of beneficial reactive oxygen species (ROS) such as superoxide (O₂⁻). However, our previous research implicated glutathione reductase (GR), a canonical antioxidant enzyme, as a source of extracellular superoxide in the marine diatom Thalassiosira oceanica. Here, we expressed and characterized the two GR isoforms of T. oceanica. Both coupled the oxidation of NADPH, the native electron donor, to oxygen reduction, giving rise to superoxide in the absence of glutathione disulfide, the native electron acceptor. Superoxide production by ToGR1 exhibited similar kinetics as representative NOX enzymes, and inhibition assays agreed with prior organismal studies, supporting a physiological role. ToGR is similar to GR from human, yeast, and bacteria, suggesting that NOX-like ROS production by GR could be widespread. Yet unlike NOX, GR-mediated ROS production is independent of iron, which may provide an advantageous way of making ROS under micronutrient stress.

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Glutathione reductase expressed from a microbial phototroph is found to produce ROS

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This promiscuous reaction shows similar kinetics and inhibition as NOX-derived ROS

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The GR pathway of ROS production has cellular benefits under physiological stress

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GR may function similarly to NOX in other taxa, providing metabolic versatility

The Prospective Effect of Cinnamon and Chia on Submandibular Salivary Glands After Ciprofloxacin Administration in Albino Rats (Histological, Histochemical, and Ultrastructural Study)

Ciprofloxacin (CPFX®) is potent fluoroquinolone but has severe side effects. Cinnamon (CIN) and chia seeds are potent antioxidants. The current work aimed to compare the effect of CIN extract and chia seeds on CPFX®-treated submandibular salivary glands (SMGs). Thirty-two male albino rats were divided into four groups: Group 1: received saline. Group 2: received CPFX®. Group 3: received CIN extract after 4 h of CPFX® administration. Group 4: received ground chia seeds after 4 h of CPFX® administration. After 10 days, histological, histochemical, and ultrastructural examinations were done. Different examinations illustrated normal features of SMG in Groups 1 and 3. Group 2 showed degenerative signs. Group 4 showed normal features in some areas. Statistical results illustrated that Group 2 had highest mean vacuolation area%. Highest mean of PAS optical density (OD) was for Group 2. Concerning mercuric bromophenol blue stain OD; Group 1 showed highest mean OD. CPFX® has the deteriorative effect on SMG structure and ultrastructure. It leads to increased levels of glycosaminoglycans (GAGs) and decreased levels of total proteins. CIN extract showed more ameliorative effect compared to chia seeds.

Enhanced Carbonylation of Photosynthetic and Glycolytic Proteins in Antibiotic Timentin-Treated Tobacco In Vitro Shoot Culture

Antibiotics are used in plant in vitro tissue culture to eliminate microbial contamination or for selection in genetic transformation. Antibiotic timentin has a relatively low cytotoxic effect on plant tissue culture; however, it could induce an enduring growth-inhibiting effect in tobacco in vitro shoot culture that persists after tissue transfer to a medium without antibiotic. The effect is associated with an increase in oxidative stress injury in plant tissues. In this study, we assessed changes of reactive oxygen species accumulation, protein expression, and oxidative protein modification response associated with enduring timentin treatment-induced growth suppression in tobacco (Nicotiana tabacum L.) in vitro shoot culture. The study revealed a gradual 1.7 and 1.9-fold increase in superoxide (O₂^•−) content at the later phase of the propagation cycle for treatment control (TC) and post-antibiotic treatment (PA) shoots; however, the O₂^•− accumulation pattern was different. For PA shoots, the increase in O₂^•− concentration occurred several days earlier, resulting in 1.2 to 1.4-fold higher O₂^•− concentration compared to TC during the period following the first week of cultivation. Although no protein expression differences were detectable between the TC and PA shoots by two-dimensional electrophoresis, the increase in O₂^•− concentration in PA shoots was associated with a 1.5-fold increase in protein carbonyl modification content after one week of cultivation, and protein carbonylation analysis revealed differential modification of 26 proteoforms involved in the biological processes of photosynthesis and glycolysis. The results imply that the timentin treatment-induced oxidative stress might be implicated in nontranslational cellular redox balance regulation, accelerates the development of senescence of the shoot culture, and contributes to the shoot growth-suppressing effect of antibiotic treatment.

Endosperm weakening: The gateway to a seed's new life

Seed germination is a crucial stage in a plant's life cycle, during which the embryo, surrounded by several tissues, undergoes a transition from the quiescent to a highly active state. Endosperm weakening, a key step in this transition, plays an important role in radicle protrusion. Endosperm weakening is initiated upon water uptake, followed by multiple key molecular events occurring within and outside endosperm cells. Although available transcriptomes have provided information about pivotal genes involved in this process, a complete understanding of the signaling pathways are yet to be elucidated. Much remains to be learnt about the diverse intercellular signals, such as reactive oxygen species-mediated redox signals, phytohormone crosstalk, environmental cue-dependent oxidative phosphorylation, peroxisomal-mediated pectin degradation, and storage protein mobilization during endosperm cell wall loosening. This review discusses the evidences from recent researches into the mechanism of endosperm weakening. Further, given that the endosperm has great potential for manipulation by crop breeding and biotechnology, we offer several novel insights, which will be helpful in this research field and in its application to the improvement of crop production.

ROS Homeostasis and Plant Salt Tolerance: Plant Nanobiotechnology Updates

Salinity is an issue impairing crop production across the globe. Under salinity stress, besides the osmotic stress and Na+ toxicity, ROS (reactive oxygen species) overaccumulation is a secondary stress which further impairs plant performance. Chloroplasts, mitochondria, the apoplast, and peroxisomes are the main ROS generation sites in salt-stressed plants. In this review, we summarize ROS generation, enzymatic and non-enzymatic antioxidant systems in salt-stressed plants, and the potential for plant biotechnology to maintain ROS homeostasis. Overall, this review summarizes the current understanding of ROS homeostasis of salt-stressed plants and highlights potential applications of plant nanobiotechnology to enhance plant tolerance to stresses.

The Apoplast: A Key Player in Plant Survival

The apoplast comprises the intercellular space, the cell walls, and the xylem. Important functions for the plant, such as nutrient and water transport, cellulose synthesis, and the synthesis of molecules involved in plant defense against both biotic and abiotic stresses, take place in it. The most important molecules are ROS, antioxidants, proteins, and hormones. Even though only a small quantity of ROS is localized within the apoplast, apoplastic ROS have an important role in plant development and plant responses to various stress conditions. In the apoplast, like in the intracellular cell compartments, a specific set of antioxidants can be found that can detoxify the different types of ROS produced in it. These scavenging ROS components confer stress tolerance and avoid cellular damage. Moreover, the production and accumulation of proteins and peptides in the apoplast take place in response to various stresses. Hormones are also present in the apoplast where they perform important functions. In addition, the apoplast is also the space where microbe-associated molecular Patterns (MAMPs) are secreted by pathogens. In summary, the diversity of molecules found in the apoplast highlights its importance in the survival of plant cells.

Reactive oxygen species regulate auxin levels to mediate adventitious root induction in Arabidopsis hypocotyl cuttings

Adventitious root (AR) formation from leafy stem cuttings is critical for breeding of many forest and horticultural species. In addition to the plant hormone auxin, wound-induced signaling caused by the cutting excision is also essential for AR initiation. Here we found that reactive oxygen species (ROS) are rapidly generated at the excision site as a wound-induced signal and propagated throughout the hypocotyl cutting after excision of the Arabidopsis (Arabidopsis thaliana) primary root. ROS propagation was not observed in the presence of an NADPH oxidase inhibitor (diphenylene iodonium chloride) or in a knockout mutant of the NADPH oxidase gene respiratory burst oxidase homolog protein D (RBOHD). Respiratory burst oxidase homolog protein D was specifically upregulated in hypocotyl cuttings at 0.5 h post excision (hpe). Together, these data suggest that RBOHD mediates ROS propagation in hypocotyl cuttings. We also found that auxin levels increased significantly in the shoot apex at 5 hpe and at the base of the cutting at 6 hpe; these effects were blocked by treatment with ROS scavengers. Consistent with this, transcript levels of auxin biosynthesis and polar-transport genes generally increased between 1 to 6 hpe. Collectively, our results suggest that wound-induced ROS participate in AR induction through regulation of auxin biosynthesis and transport.

Modulation of Aleurone Peroxidases in Kernels of Insect-Resistant Maize (Zea mays L.; Pob84-C3R) After Mechanical and Insect Damage

Peroxidases (PODs) have many biological functions during the plant life cycle. In maize kernels, endosperm PODs have been identified as biochemical contributors to resistance against Sitophilus zeamais, but their identities have not been determined. In this study, we identified these PODs and determined whether their contributions are basal or inducible. Semi-purification and LC-MS/MS analyses showed that the protein ZmPrx35 is the predominant soluble endosperm POD from kernels of Pob84-C3R. Subsequent time-course analyses after mechanical damage showed that POD activity was regulated in a fluctuating kinetics pattern and that zmprx35 mRNA expression levels reflected this pattern. After 48 h of infestation with S. zeamais or Prostephanus truncatus, soluble endosperm POD activities were 1.38- or 0.85-fold, respectively. Under the same conditions, zmprx35 expression was induced 1.61-fold (S. zeamais infestation) and 1.17-fold (P. truncatus infestation). These findings suggest that ZmPrx35 contributes to the protective responses of aleurone cells against wounding and pest attacks, which could be enhanced/repressed by insect factors. Our data also provide evidence that the mechanisms of resistance of maize Pob84-C3R kernels toward the insect pests S. zeamais and P. truncatus are independent.

Transcriptome analysis reveals differential immune related genes expression in Ruditapes philippinarum under hypoxia stress: potential HIF and NF-κB crosstalk in immune responses in clam

Background

Hypoxia is an important environmental stressor in aquatic ecosystems, with increasingly impacts on global biodiversity. Benthic communities are the most sensitive parts of the coastal ecosystem to eutrophication and resulting hypoxia. As a filter-feeding organism living in the seafloor sediment, Ruditapes philippinarum represents an excellent “sentinel” species to assess the quality of marine environment. In order to gain insight into the molecular response and acclimatization mechanisms to hypoxia stress in marine invertebrates, we examined hypoxia-induced changes in immune-related gene expression and gene pathways involved in hypoxia regulation of R. philippinarum.

Results

We investigated the response of the Manila clam R. philippinarum to hypoxia under experimental conditions and focused on the analysis of the differential expression patterns of specific genes associated with hypoxia response by RNA-seq and time course qPCR analysis. A total of 75 genes were captured significantly differentially expressed, and were categorized into antioxidant/oxidative stress response, chaperones/heat shock proteins, immune alteration, and cell proliferation/apoptosis. Fourteen hypoxia responsive genes were validated significantly up/down regulated at different time 0, 2, 5, and 8 d in gills of R. philippinarum in hypoxia challenged group. Functional enrichment analysis revealed the HIF signaling pathway and NF-κB signaling pathway play pivotal roles in hypoxia tolerance and resistance in R. philippinarum.

Conclusion

The HIF signaling pathway and NF-κB signaling pathway play a critical role in hypoxia tolerance and resistance in Manila clam. The immune and defense related genes and pathways obtained here gain a fundamental understanding of the hypoxia stress in marine bivalves and provide important insights into the physiological acclimation, immune response and defense activity under hypoxia challenge. The reduced metabolism is a consequence of counterbalancing investments in immune defense against other physiological processes.

Comparative Genomics Screen Identifies Microbe-Associated Molecular Patterns from 'Candidatus Liberibacter' spp. That Elicit Immune Responses in Plants

Citrus huanglongbing (HLB), caused by phloem-limited 'Candidatus Liberibacter' bacteria, is a destructive disease threatening the worldwide citrus industry. The mechanisms of pathogenesis are poorly understood and no efficient strategy is available to control HLB. Here, we used a comparative genomics screen to identify candidate microbe-associated molecular patterns (MAMPs) from 'Ca. Liberibacter' spp. We identified the core genome from multiple 'Ca. Liberibacter' pathogens, and searched for core genes with signatures of positive selection. We hypothesized that genes encoding putative MAMPs would evolve to reduce recognition by the plant immune system, while retaining their essential functions. To efficiently screen candidate MAMP peptides, we established a high-throughput microtiter plate-based screening assay, particularly for citrus, that measured reactive oxygen species (ROS) production, which is a common immune response in plants. We found that two peptides could elicit ROS production in Arabidopsis and Nicotiana benthamiana. One of these peptides elicited ROS production and defense gene expression in HLB-tolerant citrus genotypes, and induced MAMP-triggered immunity against the bacterial pathogen Pseudomonas syringae. Our findings identify MAMPs that boost immunity in citrus and could help prevent or reduce HLB infection.

Gene expression in the smut fungus Ustilago esculenta governs swollen gall metamorphosis in Zizania latifolia

Ustilago esculenta, a smut fungus, can induce the formation of culm galls in Zizania latifolia, a vegetable consumed in many Asian countries. Specifically, the mycelia-teliospore (M-T) strain of U. esculenta induces the Jiaobai (JB) type of gall, while the teliospore (T) strain induces the Huijiao (HJ) type. The underlying molecular mechanism responsible for the formation of the two distinct types of gall remains unclear. Our results showed that most differentially expressed genes relevant to effector proteins were up-regulated in the T strain compared to those in the M-T strain during gall formation, and the expression of teliospore formation-related genes was higher in the T strain than the M-T strain. Melanin biosynthesis was also clearly induced in the T strain. The T strain exhibited stronger pathogenicity and greater teliospore production than the M-T strain. We evaluated the implications of the gene regulatory networks in the development of these two type of culm gall in Z. latifolia infected with U. esculenta and suggested potential targets for genetic manipulation to modify the gall type for this crop.

Indoleacetic Acid Levels in Wheat and Rice Seedlings under Oxygen Deficiency and Subsequent Reoxygenation

The lack of oxygen and post-anoxic reactions cause significant alterations of plant growth and metabolism. Plant hormones are active participants in these alterations. This study focuses on auxin-a phytohormone with a wide spectrum of effects on plant growth and stress tolerance. The indoleacetic acid (IAA) content in plants was measured by ELISA. The obtained data revealed anoxia-induced accumulation of IAA in wheat and rice seedlings related to their tolerance of oxygen deprivation. The highest IAA accumulation was detected in rice roots. Subsequent reoxygenation was accompanied with a fast auxin reduction to the control level. A major difference was reported for shoots: wheat seedlings contained less than one-third of normoxic level of auxin during post-anoxia, while IAA level in rice seedlings rapidly recovered to normoxic level. It is likely that the mechanisms of auxin dynamics resulted from oxygen-induced shift in auxin degradation and transport. Exogenous IAA treatment enhanced plant survival under anoxia by decreased electrolyte leakage, production of hydrogen peroxide and lipid peroxidation. The positive effect of external IAA application coincided with improvement of tolerance to oxygen deprivation in the 35S:iaaM × 35S:iaaH lines of transgene tobacco due to its IAA overproduction.

Carnosic Acid Content Increased by Silver Nanoparticle Treatment in Rosemary (Rosmarinus officinalis L.)

Biosynthesis of carnosic acid (CA), one of the most industrially valuable medicinal compounds present in rosemary (Rosmarinus officinalis L.) leaves, is affected by various plant stressors. In this study, effects of silver nanoparticle (AgNP) treatment on the secondary metabolism and CA production of rosemary plants were investigated. AgNP of 0, 25, 50, 100, and 200 ppm were utilized on hydroponically grown plants using foliar spray. Efficient absorbance and translocation of AgNPs to the plant roots were confirmed by XRF (X-ray fluorescence) analysis. The fluctuations of important antioxidant compounds such as CA content, phenolics, flavonoids, and acid ascorbic were analyzed and their correlations evaluated. Results revealed that application of 200 ppm AgNPs for 12 days increased CA level more than 11%, as compared to the control plants. Furthermore, significant positive correlations were observed between total flavonoids and CA content under AgNP treatment, suggesting that AgNP acted as an elicitor and triggered the enhancement of CA accumulation effectively. These data suggest that concentration-dependent AgNP may be used to boost antioxidant activity and phytochemical contents of other medicinal plants.

Phytotoxic and genotoxic effect of Aluminum to date palm (Phoenix dactylifera L.) in vitro cultures

BACKGROUND

Al is a common metallic element found in earth's crust and is a toxic pollutant present at high concentrations in acidic soil, thus affecting plant growth. Despite being well studied as a toxic element, the effects of Al on date palm have not been investigated. This study aimed to assess the toxic effects of different Al concentrations on the development and growth of date palm callus and evaluate the biochemical and molecular response of date palm cells under Al stress.

RESULTS

Our study revealed the phytotoxicity of Al concentrations (50, 100, 150 and 200 mg.l-1) on date palm callus. The fresh and dry weight and the number of produced embryos were significantly decreased in response to Al concentration. At 150 mg.l-1, the embryo number decreased to 1.66 compared with the 19.33 in the control treatment. At high Al concentration (200 mg.l-1), the callus failed to produce any embryo. Biochemical analysis revealed that Al exposure had negative effect on callus. Total soluble carbohydrates, total soluble protein and free amino acids were decreased in plants receiving 200 mg.l-1 Al treatment compared with those in the untreated ones. A similar decline was observed in total soluble protein and free amino acid in response to Al treatment. Significant accumulations of malondialdehyde, H2O2 and peroxidase activity accompanied the increase in Al concentration in cultured tissues, revealing the generation of toxic reactive oxygen species in affected cultures. The genotoxic effect of Al at high concentrations (150 and 200 mg.l-1) was revealed by protein patterns.

CONCLUSION

Our findings revealed for the first time the phytotoxicity of Al to date palm callus. At 200 mg.l-1, Al prevented the embryo production of date palm callus. At 50, 100, 150 and 200 mg.l-1, Al negatively affected the biochemical characteristics of date palm callus. At 150 and 200 mg.l-1, Al induced changes in protein expression. These data showed that the tissue culture technique can be used as a valuable approach in heavy metal toxicity studies.

Physiological and Transcriptomic Changes during the Early Phases of Adventitious Root Formation in Mulberry Stem Hardwood Cuttings

The initiation and induction of root primordia are of great importance for adventitious root (AR) formation in cutting propagation of horticultural and forestry crops. However, the underlying mechanisms orchestrating these early phases of AR formation remain largely unexplored. Here, we investigated the physiological and transcriptomic changes during the early AR phases in mulberry stem hardwood cuttings. The results showed that the concentrations of soluble proteins increased, whereas concentrations of soluble sugars and starch were decreased. Indole-3-acetic acid (IAA) and zeatin had a rapid transit peak at 6 h after planting (hAP) and declined thereafter. The activities of peroxidase and catalase persistently increased and indole-3-acetic acid oxidase was maintained at a higher stable level from 0 hAP, while the activities of polyphenol oxidase fluctuated with soluble phenolics and IAA levels. The comparative transcriptome identified 4276 common genes that were differentially regulated at −6, 0 and 54 hAP. They were separated into five clusters with distinct biological functions such as defense response and photosynthesis. Considerable common genes were assigned to pathways of sugar metabolism, mitogen-activated protein kinase, and circadian rhythm. The gene co-expression network analysis revealed three major co-expressed modules involved in stress responses, hormone signaling, energy metabolism, starch metabolism, and circadian rhythm. These findings demonstrate the positive effect of auxin on AR induction, and uncovered the crucial roles of stress responses, hormone signaling and circadian rhythm in coordinating the physiological changes during the early phases of AR formation in mulberry stem hardwood cuttings.

Tight Regulation of Extracellular Superoxide Points to Its Vital Role in the Physiology of the Globally Relevant Roseobacter Clade

There is a growing appreciation within animal and plant physiology that the reactive oxygen species (ROS) superoxide is not only detrimental but also essential for life. Yet, despite widespread production of extracellular superoxide by healthy bacteria and phytoplankton, this molecule remains associated with stress and death. Here, we quantify extracellular superoxide production by seven ecologically diverse bacteria within the Roseobacter clade and specifically target the link between extracellular superoxide and physiology for two species. We reveal for all species a strong inverse relationship between cell-normalized superoxide production rates and cell number. For exponentially growing cells of Ruegeria pomeroyi DSS-3 and Roseobacter sp. strain AzwK-3b, we show that superoxide levels are regulated in response to cell density through rapid modulation of gross production and not decay. Over a life cycle of batch cultures, extracellular superoxide levels are tightly regulated through a balance of both production and decay processes allowing for nearly constant levels of superoxide during active growth and minimal levels upon entering stationary phase. Further, removal of superoxide through the addition of exogenous superoxide dismutase during growth leads to significant growth inhibition. Overall, these results point to tight regulation of extracellular superoxide in representative members of the Roseobacter clade, consistent with a role for superoxide in growth regulation as widely acknowledged in fungal, animal, and plant physiology.IMPORTANCE Formation of reactive oxygen species (ROS) through partial reduction of molecular oxygen is widely associated with stress within microbial and marine systems. Nevertheless, widespread observations of the production of the ROS superoxide by healthy and actively growing marine bacteria and phytoplankton call into question the role of superoxide in the health and physiology of marine microbes. Here, we show that superoxide is produced by several marine bacteria within the widespread and abundant Roseobacter clade. Superoxide levels outside the cell are controlled via a tightly regulated balance of production and decay processes in response to cell density and life stage in batch culture. Removal of extracellular superoxide leads to substantial growth inhibition. These findings point to an essential role for superoxide in the health and growth of this ubiquitous group of microbes, and likely beyond.

Orchestration of Cu-Zn SOD and class III peroxidase with upstream interplay between NADPH oxidase and PM H+-ATPase mediates root growth in Vigna radiata (L.) Wilczek

Post-germination plant growth depends on the regulation of reactive oxygen species (ROS) metabolism, spatiotemporal pH changes and Ca⁺² homeostasis, whose potential integration has been studied during Vigna radiata (L.) Wilczek root growth. The dissipation of proton (H⁺) gradients across plasma membrane (PM) by CCCP (protonophore) and the inhibition of PM H⁺-ATPase by sodium orthovanadate repressed SOD (superoxide dismutase; EC 1.15.1.1) activity as revealed by spectrophotometric and native PAGE assay results. Similar results derived from treatment with DPI (NADPH oxidase inhibitor) and Tiron (O₂- scavenger) denote a functional synchronization of SOD, PM H⁺-ATPase and NOX, as the latter two enzymes are substrate sources for SOD (H⁺ and O₂-, respectively) and are involved in a feed-forward loop. After SOD inactivation, a decline in apoplastic H₂O₂ content was observed in each treatment group, emerging as a possible cause of the diminution of class III peroxidase (Prx; EC 1.11.1.7), which utilizes H₂O₂ as a substrate. In agreement with the pivotal role of Ca⁺² in PM H⁺-ATPase and NOX activation, Ca⁺² homeostasis antagonists, i.e., LaCl₃ (Ca⁺² channel inhibitor), EGTA (Ca⁺² chelator) and LiCl (endosomal Ca⁺² release blocker), inhibited both SOD and Prx. Finally, a drastic reduction in apoplastic OH (hydroxyl radical) concentrations (induced by each treatment, leading to Prx inhibition) was observed via fluorometric analysis. A consequential inhibition of root growth observed under each treatment denotes the importance of the orchestrated functioning of PM H⁺-ATPase, NOX, Cu-Zn SOD and Prx during root growth. A working model demonstrating postulated enzymatic synchronization with an intervening role of Ca⁺² is proposed.

Membrane-Bound Class III Peroxidases: Unexpected Enzymes with Exciting Functions

Class III peroxidases are heme-containing proteins of the secretory pathway with a high redundance and versatile functions. Many soluble peroxidases have been characterized in great detail, whereas only a few studies exist on membrane-bound isoenzymes. Membrane localization of class III peroxidases has been demonstrated for tonoplast, plasma membrane and detergent resistant membrane fractions of different plant species. In silico analysis revealed transmembrane domains for about half of the class III peroxidases that are encoded by the maize (Zea mays) genome. Similar results have been found for other species like thale-cress (Arabidopsis thaliana), barrel medic (Medicago truncatula) and rice (Oryza sativa). Besides this, soluble peroxidases interact with tonoplast and plasma membranes by protein⁻protein interaction. The topology, spatiotemporal organization, molecular and biological functions of membrane-bound class III peroxidases are discussed. Besides a function in membrane protection and/or membrane repair, additional functions have been supported by experimental data and phylogenetics.

Differential expression of SofDIR16 and SofCAD genes in smut resistant and susceptible sugarcane cultivars in response to Sporisorium scitamineum

Proteomic profiling of the stalk of a smut resistant and a susceptible sugarcane cultivars revealed the presence of dirigent and dirigent-like proteins in abundance in the pool of high molecular mass (HMMG) and mid-molecular mass (MMMG) glycoproteins, produced as part of the defensive response to the fungal smut pathogen. Quantitative RT-PCR analysis showed that expression levels of SofDIR16 (sugarcane dirigent16) and SofCAD (sugarcane cinnamyl alcohol dehydrogenase) were higher in the smut resistant My 55-14 cultivar than in the sensitive B 42231 cultivar prior to infection. Inoculation with fungal sporidia or water decreased the level of SofCAD transcripts in My 55-14, indicating that regulation of SofCAD expression does not take part of the specific response to smut infection. In contrast, SofDIR16 expression was almost nullified in My 55-14 after inoculation with fungal sporidia, but not after water injection. It is proposed that the decreased expression of dirigent proteins induces the formation of lignans, which are involved in the defense response of the smut resistant My 55-14 cultivar.

RNA-Seq analysis of differentially expressed genes in the grand jackknife clam Solen grandis under aerial exposure

Aerial exposure tolerance has been long considered as an important trait for the life survival under acute environmental stress. In this study, we utilized RNA-seq-based transcriptomic profiling to characterize the molecular responses of grand jackknife clam in response to aerial exposure. This assembly yielded 190,856 unigenes with an average length of 1147 bp, a minimum length of 201 bp, and a maximum length of 51,869 bp, with an N50 length of 1875 bp. After differential expression analysis, a total of 1344 genes were captured significantly differentially expressed, and were categorized into antioxidant/oxidative stress response, immune alteration, and apoptosis. GO and KEGG analyses revealed that signal transduction, immune response, cellular component organization or biogenesis, and energy production processes were the most highly enriched pathways among the genes that were differentially expressed under aerial exposure stress. All these pathways could be assigned to the following biological functions in the aerial exposure tolerant Solen grandis: signaling, transporter activity, macromolecular complex, cellular component organization or biogenesis, and molecular transducer activity. This study highlighted candidate genes linked to stress response during aerial exposure and provide a useful resource for further work on gills tissue or for selection of aerial exposure tolerant phenotypes.

The Role of Hydrogen Peroxide in Mediating the Mechanical Wounding-Induced Freezing Tolerance in Wheat

Systemic wound response (SWR), a well-characterized systemic signaling response, plays crucial roles in plant defense responses. Progress in understanding of the SWR in abiotic stress has also been aided by the researchers. However, the function of SWR in freezing stress remains elusive. In this study, we showed that local mild mechanical wounding enhanced freezing tolerance in newly occurred systemic leaves of wheat plants (Triticum aestivum L.). Wounding significantly increased the maximal photochemical efficiency of photosystem II, net photosynthetic rate, and the activities of the antioxidant enzymes under freezing stress. Wounding also alleviated freezing-induced chlorophyll decomposition, electrolyte leakage, water lose, and membrane peroxidation. In addition, wounding-induced freezing stress mitigation was closely associated with the ratio between reduced glutathione (GSH) and oxidized glutathione (GSSG), and the ratio between ascorbate (AsA) and dehydroascorbate (DHA), as well as the contents of total soluble sugars and free amino acids. Importantly, pharmacological study showed that wounding-induced freezing tolerance was substantially arrested by pretreatment of wheat leaves with the scavenger of hydrogen peroxide (H₂O₂) or the inhibitor of NADPH oxidase (RBOH). These results support the hypothesis that local mechanical wounding-induced SWR in newly occurred leaves is largely attributed to RBOH-dependent H₂O₂ production, which may subsequently induce freezing tolerance in wheat plants. This mechanism may have a potential application to reduce the yield losses of wheat under late spring freezing conditions. Highlights: In our previous research, we found that local mechanical wounding could induce freezing tolerance in the upper systemic leaves of wheat plants. Surprisingly, in this paper, we further demonstrated that local mechanical wounding could also increase freezing resistance in newly occurred leaves of wheat plants. RBOH mediated H₂O₂ and ascorbate-glutathione cycle participate in this systemic wound response.

Real-time monitoring of superoxide anion radical generation in response to wounding: electrochemical study

Background

The growth and development of plants is deleteriously affected by various biotic and abiotic stress factors. Wounding in plants is caused by exposure to environmental stress, mechanical stress, and via herbivory. Typically, oxidative burst in response to wounding is associated with the formation of reactive oxygen species, such as the superoxide anion radical (O₂^•−), hydrogen peroxide (H₂O₂) and singlet oxygen; however, few experimental studies have provided direct evidence of their detection in plants. Detection of O₂^•− formation in plant tissues have been performed using various techniques including electron paramagnetic resonance spin-trap spectroscopy, epinephrine-adrenochrome acceptor methods, staining with dyes such as tetrazolium dye and nitro blue tetrazolium (NBT); however, kinetic measurements have not been performed. In the current study, we provide evidence of O₂^•− generation and its kinetics in the leaves of spinach (Spinacia oleracea) subjected to wounding.

Methods

Real-time monitoring of O₂^•− generation was performed using catalytic amperometry. Changes in oxidation current for O₂^•− was monitored using polymeric iron-porphyrin-based modified carbon electrodes (φ = 1 mm) as working electrode with Ag/AgCl as the reference electrode.

Result

The results obtained show continuous generation of O₂^•− for minutes after wounding, followed by a decline. The exogenous addition of superoxide dismutase, which is known to dismutate O₂^•− to H₂O₂, significantly suppressed the oxidation current.

Conclusion

Catalytic amperometric measurements were performed using polymeric iron-porphyrin based modified carbon electrode. We claim it to be a useful tool and a direct method for real-time monitoring and precise detection of O₂^•− in biological samples, with the potential for wide application in plant research for specific and sensitive detection of O₂^•−.

Nitric Oxide and Hydrogen Peroxide Mediate Wounding-Induced Freezing Tolerance through Modifications in Photosystem and Antioxidant System in Wheat

Mechanical wounding is a common stress caused by herbivores or manual and natural manipulations, whereas its roles in acclimation response to a wide spectrum of abiotic stresses remain unclear. The present work showed that local mechanical wounding enhanced freezing tolerance in untreated systemic leaves of wheat plants (Triticum aestivum L.), and meanwhile the signal molecules hydrogen peroxide (H₂O₂) and nitric oxide (NO) were accumulated systemically. Pharmacological study showed that wounding-induced NO synthesis was substantially arrested by pretreatment with scavengers of reactive oxygen species and an inhibitor of NADPH oxidase (respiratory burst oxidase homolog, RBOH). On the contrary, wounding-induced H₂O₂ accumulation was not sensitive to NO synthetic inhibitors or scavenger, indicating that H₂O₂ acts upstream of NO in wounding signal transduction pathways. Cytochemical and vascular tissues localizations approved that RBOH-dependent H₂O₂ acts as long-distance signal in wounding response. Transcriptome analysis revealed that 279 genes were up-regulated in plants treated with wounding and freezing, but not in plants treated with freezing alone. Importantly, freezing- and wounding-induced genes were significantly enriched in the categories of "photosynthesis" and "signaling." These results strongly supported that primary mechanical wounding can induce freezing tolerance in wheat through the systemic accumulation of NO and H₂O₂, and further modifications in photosystem and antioxidant system.

Methods to Detect Nitric Oxide in Plants: Are DAFs Really Measuring NO?

Nitric oxide, a gaseous radical molecule, appears involved in many reactions in all living organisms. Fluorescent dyes like DAF-2 and related compounds are still widely used to monitor NO production inside or outside cells, although doubts about their specificity have recently been raised. We present evidence that DAF dyes do not only react with nitric oxide but also with peroxidase enzyme and hydrogen peroxide. Both are secreted in the case of elicitation of tobacco suspension cells with cryptogein, with a fluorescence increase mimicking NO release from cells. However, HPLC separation shows that fluorescence outside cells does not at all originate from DAF-2T, the product of DAF-2 and NO, but from other yet unidentified compounds. Inside cells, other DAF molecules are formed but only a minor part is DAF-2T. The chemical nature of the novel DAF derivatives still needs to be determined.

Species-specific control of external superoxide levels by the coral holobiont during a natural bleaching event

The reactive oxygen species superoxide (O₂^·−) is both beneficial and detrimental to life. Within corals, superoxide may contribute to pathogen resistance but also bleaching, the loss of essential algal symbionts. Yet, the role of superoxide in coral health and physiology is not completely understood owing to a lack of direct in situ observations. By conducting field measurements of superoxide produced by corals during a bleaching event, we show substantial species-specific variation in external superoxide levels, which reflect the balance of production and degradation processes. Extracellular superoxide concentrations are independent of light, algal symbiont abundance and bleaching status, but depend on coral species and bacterial community composition. Furthermore, coral-derived superoxide concentrations ranged from levels below bulk seawater up to ∼120 nM, some of the highest superoxide concentrations observed in marine systems. Overall, these results unveil the ability of corals and/or their microbiomes to regulate superoxide in their immediate surroundings, which suggests species-specific roles of superoxide in coral health and physiology.

Corals may vary in their ability to regulate reactive oxygen species (ROS) that can influence coral health. Diaz and colleagues conduct in vivo measurements of the ROS superoxide at the surface of corals and find substantial species-level variation in superoxide regulation that is independent of bleaching status.

Wound signaling of regenerative cell reprogramming

Plants are sessile organisms that must deal with various threats resulting in tissue damage, such as herbivore feeding, and physical wounding by wind, snow or crushing by animals. During wound healing, phytohormone crosstalk orchestrates cellular regeneration through the establishment of tissue-specific asymmetries. In turn, hormone-regulated transcription factors and their downstream targets coordinate cellular responses, including dedifferentiation, cell cycle reactivation and vascular regeneration. By comparing different examples of wound-induced tissue regeneration in the model plant Arabidopsis thaliana, a number of key regulators of developmental plasticity of plant cells have been identified. We present the relevance of these findings and of the dynamic establishment of differential auxin gradients for cell reprogramming after wounding.

Production of superoxide from photosystem II-light harvesting complex II supercomplex in STN8 kinase knock-out rice mutants under photoinhibitory illumination

When phosphorylation of Photosystem (PS) II core proteins is blocked in STN8 knock-out mutants of rice (Oryza sativa) under photoinhibitory illumination, the mobilization of PSII supercomplex is prevented. We have previously proposed that more superoxide (O2(-)) is produced from PSII in the mutant (Nath et al., 2013, Plant J. 76, 675-686). Here, we clarify the type and site for the generation of reactive oxygen species (ROS). Using both histochemical and fluorescence probes, we observed that, compared with wild-type (WT) leaves, levels of ROS, including O2(-) and hydrogen peroxide (H2O2), were increased when leaves from mutant plants were illuminated with excess light. However, singlet oxygen production was not enhanced under such conditions. When superoxide dismutase was inhibited, O2(-) production was increased, indicating that it is the initial event prior to H2O2 production. In thylakoids isolated from WT leaves, kinase was active in the presence of ATP, and spectrophotometric analysis of nitrobluetetrazolium absorbance for O2(-) confirmed that PSII-driven superoxide production was greater in the mutant thylakoids than in the WT. This contrast in levels of PSII-driven superoxide production between the mutants and the WT plants was confirmed by conducting protein oxidation assays of PSII particles from osstn8 leaves under strong illumination. Those assays also demonstrated that PSII-LHCII supercomplex proteins were oxidized more in the mutant, thereby implying that PSII particles incur greater damage even though D1 degradation during PSII-supercomplex mobilization is partially blocked in the mutant. These results suggest that O2(-) is the major form of ROS produced in the mutant, and that the damaged PSII in the supercomplex is the primary source of O2(-).

Reactive oxygen species, essential molecules, during plant-pathogen interactions

Reactive oxygen species (ROS) are continually generated as a consequence of the normal metabolism in aerobic organisms. Accumulation and release of ROS into cell take place in response to a wide variety of adverse environmental conditions including salt, temperature, cold stresses and pathogen attack, among others. In plants, peroxidases class III, NADPH oxidase (NOX) locates in cell wall and plasma membrane, respectively, may be mainly enzymatic systems involving ROS generation. It is well documented that ROS play a dual role into cells, acting as important signal transduction molecules and as toxic molecules with strong oxidant power, however some aspects related to its function during plant-pathogen interactions remain unclear. This review focuses on the principal enzymatic systems involving ROS generation addressing the role of ROS as signal molecules during plant-pathogen interactions. We described how the chloroplasts, mitochondria and peroxisomes perceive the external stimuli as pathogen invasion, and trigger resistance response using ROS as signal molecule.

2,4-D resistance in wild radish: reduced herbicide translocation via inhibition of cellular transport

Resistance to auxinic herbicides is increasing in a range of dicotyledonous weed species, but in most cases the biochemical mechanism of resistance is unknown. Using (14)C-labelled herbicide, the mechanism of resistance to 2,4-dichlorophenoxyacetic acid (2,4-D) in two wild radish (Raphanus raphanistrum L.) populations was identified as an inability to translocate 2,4-D out of the treated leaf. Although 2,4-D was metabolized in wild radish, and in a different manner to the well-characterized crop species wheat and bean, there was no difference in metabolism between the susceptible and resistant populations. Reduced translocation of 2,4-D in the latter was also not due to sequestration of the herbicide, or to reduced uptake by the leaf epidermis or mesophyll cells. Application of auxin efflux or ABCB transporter inhibitors to 2,4-D-susceptible plants caused a mimicking of the reduced-translocation resistance phenotype, suggesting that 2,4-D resistance in the populations under investigation could be due to an alteration in the activity of a plasma membrane ABCB-type auxin transporter responsible for facilitating long-distance transport of 2,4-D.

Bioinspired Immobilization of Glycerol Dehydrogenase by Metal Ion-Chelated Polyethyleneimines as Artificial Polypeptides

In this study, a novel, simple and generally applicable strategy for multimeric oxidoreductase immobilization with multi-levels interactions was developed and involved activity and stability enhancements. Linear polyethyleneimines (PEIs) are flexible cationic polymers with molecular weights that span a wide range and are suitable biomimic polypeptides for biocompatible frameworks for enzyme immobilization. Metal ion-chelated linear PEIs were applied as a heterofunctional framework for glycerol dehydrogenase (GDH) immobilization by hydrogen bonds, electrostatic forces and coordination bonds interactions. Nanoparticles with diameters from 250-650 nm were prepared that exhibited a 1.4-fold enhancement catalytic efficiency. Importantly, the half-life of the immobilized GDH was enhanced by 5.6-folds in aqueous phase at 85 °C. A mechanistic illustration of the formation of multi-level interactions in the PEI-metal-GDH complex was proposed based on morphological and functional studies of the immobilized enzyme. This generally applicable strategy offers a potential technique for multimeric enzyme immobilization with the advantages of low cost, easy operation, high activity reservation and high stability.

Activity of Redox Enzymes in the Thallus of Anthoceros natalensis

Anthocerotophyta (hornworts) belong to a group of ancient nonvascular plants and originate from a common ancestor with contemporary vascular plants. Hornworts represent a unique model for investigating mechanisms of formation of stress resistance in higher plants due to their high tolerance to the action of adverse environmental factors. In this work, we demonstrate that the thallus of Anthoceros natalensis exhibits high redox activity changing under stress. Dehydration of the thallus is accompanied by the decrease in activities of intracellular peroxidases, DOPA-peroxidases, and tyrosinases, while catalase activity increases. Subsequent rehydration results in the increase in peroxidase and catalase activities. Kinetic features of peroxidases and tyrosinases were characterized as well as the peroxidase isoenzyme composition of different fractions of the hornwort cell wall proteins. It was shown that the hornwort peroxidases are functionally similar to peroxidases of higher vascular plants including their ability to form superoxide anion-radical. The biochemical mechanism was elucidated, supporting the possible participation of peroxidases in the formation of reactive oxygen species (ROS) via substrate-substrate interactions in the hornwort thallus. It has been suggested that the ROS formation by peroxidases is an evolutionarily ancient process that emerged as a protective mechanism for enhancing adaptive responses of higher land plants and their adaptation to changing environmental conditions and successful colonization of various ecological niches.

Detection of hydrogen peroxide in Photosystem II (PSII) using catalytic amperometric biosensor

Hydrogen peroxide (H2O2) is known to be generated in Photosystem II (PSII) via enzymatic and non-enzymatic pathways. Detection of H2O2 by different spectroscopic techniques has been explored, however its sensitive detection has always been a challenge in photosynthetic research. During the recent past, fluorescence probes such as Amplex Red (AR) has been used but is known to either lack specificity or limitation with respect to the minimum detection limit of H2O2. We have employed an electrochemical biosensor for real time monitoring of H2O2 generation at the level of sub-cellular organelles. The electrochemical biosensor comprises of counter electrode and working electrodes. The counter electrode is a platinum plate, while the working electrode is a mediator based catalytic amperometric biosensor device developed by the coating of a carbon electrode with osmium-horseradish peroxidase which acts as H2O2 detection sensor. In the current study, generation and kinetic behavior of H2O2 in PSII membranes have been studied under light illumination. Electrochemical detection of H2O2 using the catalytic amperometric biosensor device is claimed to serve as a promising technique for detection of H2O2 in photosynthetic cells and subcellular structures including PSII or thylakoid membranes. It can also provide a precise information on qualitative determination of H2O2 and thus can be widely used in photosynthetic research.

Mechano-stimulated modifications in the chloroplast antioxidant system and proteome changes are associated with cold response in wheat

BACKGROUND

Mechanical wounding can cause morphological and developmental changes in plants, which may affect the responses to abiotic stresses. However, the mechano-stimulation triggered regulation network remains elusive. Here, the mechano-stimulation was applied at two different times during the growth period of wheat before exposing the plants to cold stress (5.6 °C lower temperature than the ambient temperature, viz., 5.0 °C) at the jointing stage.

RESULTS

Results showed that mechano-stimulation at the Zadoks growth stage 26 activated the antioxidant system, and substantially, maintained the homeostasis of reactive oxygen species. In turn, the stimulation improved the electron transport and photosynthetic rate of wheat plants exposed to cold stress at the jointing stage. Proteomic and transcriptional analyses revealed that the oxidative stress defense, ATP synthesis, and photosynthesis-related proteins and genes were similarly modulated by mechano-stimulation and the cold stress.

CONCLUSIONS

It was concluded that mechano-stimulated modifications of the chloroplast antioxidant system and proteome changes are related to cold tolerance in wheat. The findings might provide deeper insights into roles of reactive oxygen species in mechano-stimulated cold tolerance of photosynthetic apparatus, and be helpful to explore novel approaches to mitigate the impacts of low temperature occurring at critical developmental stages.

Membrane water permeability of maize root cells under two levels of oxidative stress

Changes in the total water permeability of two cell membranes (plasmalemma and tonoplast), estimated by the effective diffusion coefficient of water (D ef), were controlled using the NMR method. The time dynamics of D ef in maize (Zea mays L.) root cells was studied in response to (i) root excision from seedling and the following 6-h incubation in the growth medium (wound stress) and (ii) the superposition of wound stress plus paraquat, which induces the excess of reactive oxygen species (ROS). The dynamics of lipid peroxidation, oxygen consumption, and heat production was studied to estimate general levels of oxidative stress in two variants of experiments. Under wound stress (the weak oxidative stress), the reversible by dithiothreitol increase in cell membrane water permeability was observed. The applicability of mercury test to aquaporin activity in our experiments was verified. The results of wound stress effect, obtained using this test, are discussed in terms of oxidative upregulation of aquaporin activity by ROS. The increase of oxidative stress in cells (wound-paraquat stress), contrary to wound stress, was accompanied by downregulation of membrane water permeability. In this case, ROS is supposed to affect the aquaporins not directly but via such processes as peroxidation of lipids, inactivation of some intracellular proteins, and relocalization of aquaporins in cells.

Extracellular haem peroxidases mediate Mn(II) oxidation in a marine Roseobacter bacterium via superoxide production

Manganese (Mn) oxides are among the strongest sorbents and oxidants in environmental systems. A number of biotic and abiotic pathways induce the oxidation of Mn(II) to Mn oxides. Here, we use a combination of proteomic analyses and activity assays, to identify the enzyme(s) responsible for extracellular superoxide-mediated Mn oxide formation by a bacterium within the ubiquitous Roseobacter clade. We show that animal haem peroxidases (AHPs) located on the outer membrane and within the secretome are responsible for Mn(II) oxidation. These novel peroxidases have previously been implicated in direct Mn(II) oxidation by phylogenetically diverse bacteria. Yet, we show that in this Roseobacter species, AHPs mediate Mn(II) oxidation not through a direct reaction but by producing superoxide and likely also by degrading hydrogen peroxide. These findings point to a eukaryotic-like oscillatory oxidative-peroxidative enzymatic cycle by these AHPs that leads to Mn oxide formation by this organism. AHP expression appears unaffected by Mn(II), yet the large energetic investment required to produce and secrete these enzymes points to an as yet unknown physiological function. These findings are further evidence that bacterial peroxidases and secreted enzymes, in general, are unappreciated controls on the cycling of metals and reactive oxygen species (ROS), and by extension carbon, in natural systems.

Nitration of plant apoplastic proteins from cell suspension cultures

Nitric oxide causes numerous protein modifications including nitration of tyrosine residues. This modification, though one of the greatest biological importance, is poorly recognized in plants and is usually associated with stress conditions. In this study we analyzed nitrotyrosines from suspension cultures of Arabidopsis thaliana and Nicotiana tabacum, treated with NO modulators and exposed to osmotic stress, as well as of BY2 cells long-term adapted to osmotic stress conditions. Using confocal microscopy, we showed that the cell wall area is one of the compartments most enriched in nitrotyrosines within a plant cell. Subsequently, we analyzed nitration of ionically-bound cell-wall proteins and identified selected proteins with MALDI-TOF spectrometry. Proteomic analysis indicated that there was no significant increase in the amount of nitrated proteins under the influence of NO modulators, among them 3-morpholinosydnonimine (SIN-1), considered a donor of nitrating agent, peroxynitrite. Moreover, osmotic stress conditions did not increase the level of nitration in cell wall proteins isolated from suspension cells, and in cultures long-term adapted to stress conditions; that level was even reduced in comparison with control samples. Among identified nitrotyrosine-containing proteins dominated the ones associated with carbon circulation as well as the numerous proteins responding to stress conditions, mainly peroxidases.

BIOLOGICAL SIGNIFICANCE

High concentrations of nitric oxide found in the cell wall and the ability to produce large amounts of ROS make the apoplast a site highly enriched in nitrotyrosines, as presented in this paper. Analysis of ionically bound fraction of the cell wall proteins indicating generally unchanged amounts of nitrotyrosines under influence of NO modulators and osmotic stress, is noticeably different from literature data concerning, however, the total plant proteins analysis. This observation is supplemented by further nitroproteome analysis, for cells long-term adapted to stressful conditions, and results showing that such conditions did not always cause an increase in nitrotyrosine content. These findings may be interpreted as characteristic features of apoplastic protein nitration.

Roles of apoplastic peroxidases in plant response to wounding

Apoplastic class III peroxidases (EC 1.11.1.7) play key roles in the response of plants to pathogen infection and abiotic stresses, including wounding. Wounding is a common stress for plants that can be caused by insect or animal grazing or trampling, or result from agricultural practices. Typically, mechanical damage to a plant immediately induces a rapid release and activation of apoplastic peroxidases, and an oxidative burst of reactive oxygen species (ROS), followed by the upregulation of peroxidase genes. We discuss how plants control the expression of peroxidases genes upon wounding, and also the sparse information on peroxidase-mediated signal transduction pathways. Evidence reviewed here suggests that in many plants production of the ROS that comprise the initial oxidative burst results from a complex interplay of peroxidases with other apoplastic enzymes. Later responses following wounding include various forms of tissue healing, for example through peroxidase-dependent suberinization, or cell death. Limited data suggest that ROS-mediated death signalling during the wound response may involve the peroxidase network, together with other redox molecules. In conclusion, the ability of peroxidases to both generate and scavenge ROS plays a key role in the involvement of these enigmatic enzymes in plant stress tolerance.

A proposed interplay between peroxidase, amine oxidase and lipoxygenase in the wounding-induced oxidative burst in Pisum sativum seedlings

Plant surfaces form the barrier between a plant and its environment. Upon damage, the wound healing process begins immediately and is accompanied by a rapid production of extracellular reactive oxygen species (ROS), essential in deterring pathogens, signalling responses and cell wall restructuring. Although many enzymes produce extracellular ROS, it is unclear if ROS-producing enzymes act synergistically. We characterised the oxidative burst of superoxide (O2(·-)) and hydrogen peroxide (H2O2) that follows wounding in pea (Pisum sativum L.) seedlings. Rates of ROS production were manipulated by exogenous application of enzyme substrates and inhibitors. The results indicate significant roles for di-amine oxidases (DAO) and peroxidases (Prx) rather than NADPH oxidase. The burst of O2(·-) was strongly dependent on the presence of H2O2 produced by DAO. Potential substrates released from wounded seedlings included linoleic acid that, upon exogenous application, strongly stimulated catalase-sensitive O2(·-) production. Moreover, a 65kD plasma membrane (PM) guaiacol Prx was found in the secretome of wounded seedlings and showed dependence on linoleic acid for O2(·-) production. Lipoxygenases are suggested to modulate O2(·-) production by consuming polyunsaturated fatty acids in the apoplast. Overall, a O2(·-)-producing mechanism involving H2O2-derived from DAO, linoleic acid and a PM-associated Prx is proposed.