The activated oxygen role of peroxisomes in senescence

Short-term anaerobic treatment maintained the quality of Actinidia arguta by activating the antioxidant defense system

BACKGROUND

This study aimed to investigate the effect of 6, 12, and 24 h short-term anaerobic treatment on kiwiberry quality and antioxidant properties at 5 °C.

RESULTS

Short-term anaerobic treatment was found to delay ripening and softening in kiwiberries, evident from changes in ethylene release, total soluble solids, starch, protopectin, and fruit texture. The 24 h treatment group exhibited the lowest decay rate of 12% on day 49, a 38% reduction compared with the control group. Anaerobic treatment reduced flesh translucency and decay in the fruit. The 12 h and 24 h treatments enhanced the activities of superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase, and increased the level of total phenolics, flavonoids, anthocyanins, and ascorbic acid. Moreover, it lowered oxidative damage in cell membranes, evidenced by reduced malondialdehyde content and relative conductivity.

CONCLUSION

These results indicate that anaerobic treatment maintains the fruit quality by stimulating its antioxidant defense system. Therefore, short-term anaerobic treatment emerges as a promising method for kiwiberry storage. © 2024 Society of Chemical Industry.

Potentilla sericea stress-responsive spermine synthase PsSPMS enhances cadmium tolerance in Arabidopsis thaliana

Potentilla sericea is resistant and tolerates rough management. It is an excellent garden groundcover for ecological restoration and soil consolidation for slope protection. Polyamines have functions such as promoting tissue growth and physiological resistance, while spermine synthase catalyzes the production of spermine. The PsSPMS gene from Potentilla sericea was cloned and transformed into Arabidopsis thaliana to study the response of transgenic Arabidopsis thaliana to cadmium stress. The results showed that the contents of spermidine, spermine as well as glutathione were higher in PsSPMS overexpressing Arabidopsis thaliana than the control, while the contents of putrescine were less than the control. Net photosynthetic rate, stomatal conductance, chlorophyll content, water use efficiency, electron transfer rate, PSII-related parameters, proline content, superoxide dismutase, and glutathione reductase activities were higher in PsSPMS overexpressing Arabidopsis thaliana than the control, while malondialdehyde, superoxide anion, and hydrogen peroxide contents were lower than the control. Correlation analysis showed significant differences between the indicators (P < 0.05 and P < 0.01). Expression of AtSPMS, AtSPD3, AtGSH2 and AtGR in transgenic Arabidopsis thaliana was higher than that of the control. Therefore, this study provides a genetic reference for the cultivation of cadmium-tolerant plants through genetic engineering and lays the foundation for further research on cadmium-tolerant Potentilla sericea.

Salinity stress and nanoparticles: Insights into antioxidative enzymatic resistance, signaling, and defense mechanisms

Salinized land is slowly spreading across the world. Reduced crop yields and quality due to salt stress threaten the ability to feed a growing population. We discussed the mechanisms behind nano-enabled antioxidant enzyme-mediated plant tolerance, such as maintaining reactive oxygen species (ROS) homeostasis, enhancing the capacity of plants to retain K+ and eliminate Na+, increasing the production of nitric oxide, involving signaling pathways, and lowering lipoxygenase activities to lessen oxidative damage to membranes. Frequently used techniques were highlighted like protecting cells from oxidative stress and keeping balance in ionic state. Salt tolerance in plants enabled by nanotechnology is also discussed, along with the potential role of physiobiochemical and molecular mechanisms. As a whole, the goal of this review is meant to aid researchers in fields as diverse as plant science and nanoscience in better-comprehending potential with novel solutions to addressing salinity issues for sustainable agriculture.

Ellagic Acid Treatment Improves Postharvest Quality of Tomato Fruits by Enhancing the Antioxidant Defense System

Ellagic acid (EA) is a bioactive polyphenol compound with numerous biological activities, such as anti-inflammatory, antiestrogenic, antioxidant, and anticancer activities. In this research, we investigated the effect of postharvest tomato fruits with EA treatment. Our results showed that at 25°C for 20 days, compared to a control group, the weight loss rate, titratable acidity, and soluble solid concentration of tomato fruits treated with 50 μM EA were lower. The content of soluble protein in the EA-treated group was approximately 1.65 times higher than in the control group, and EA treatment greatly inhibited the changes in lycopene and vitamin C content. Moreover, EA treatment reduced malondialdehyde content, electrolyte leakage, and the production of reactive oxygen species. Furthermore, EA treatment upregulated the expression of antioxidant-related genes and induced the activities of the antioxidant enzyme. In summary, our results showed that EA could retard senescence and preserve the quality characteristics of harvested tomato fruits via enhancing the antioxidant responses.

Storage Quality Variation of Mushrooms (Flammulina velutipes) after Cold Plasma Treatment

Flammulina velutipes is susceptible to mechanical damage, water loss, microbial growth, and other factors that lead to postharvest deterioration, thereby shortening the storage period. The purpose of this study was to analyze the effects of cold plasma treatment on the physicochemical properties and antioxidant capacity of F. velutipes during storage at 4 °C for 21 days. Compared to the control group, cold plasma cold sterilization (CPCS) treatment (150 Hz, 95 kV for 150 s) effectively inhibited the growth and multiplication of microorganisms on the surface of F. velutipes, with no significant effect on the fresh weight change and the superoxide anion generation rate, but with a higher postharvest 1,1-dephenyl-2-picrylhydrzyl (DPPH) clearance rate. Moreover, CPCS increased antioxidant enzyme activities, delayed both malondialdehyde (MDA) accumulation and vitamin C loss, inhibited the browning reaction and polyphenol oxidases (PPO) activity and protected F. velutipes cell membrane from disruption. In general, CPCS not only achieved bacteriostatic effects on F. velutipes during storage, but also reduced cell damage from free radical oxidation, resulting in better postharvest quality and longer shelf life.

Functional Characterization and Phenotyping of Protoplasts on a Microfluidics-Based Flow Cytometry

A better understanding of the phenotypic heterogeneity of protoplasts requires a comprehensive analysis of the morphological and metabolic characteristics of many individual cells. In this study, we developed a microfluidic flow cytometry with fluorescence sensor for functional characterization and phenotyping of protoplasts to allow an unbiased assessment of the influence of environmental factors at the single cell level. First, based on the measurement of intracellular homeostasis of reactive oxygen species (ROS) with a DCFH-DA dye, the effects of various external stress factors such as H₂O₂, temperature, ultraviolet (UV) light, and cadmium ions on intracellular ROS accumulation in Arabidopsis mesophyll protoplasts were quantitatively investigated. Second, a faster and stronger oxidative burst was observed in Petunia protoplasts isolated from white petals than in those isolated from purple petals, demonstrating the photoprotective role of anthocyanins. Third, using mutants with different endogenous auxin, we demonstrated the beneficial effect of auxin during the process of primary cell wall regeneration. Moreover, UV-B irradiation has a similar accelerating effect by increasing the intracellular auxin level, as shown by double fluorescence channels. In summary, our work has revealed previously underappreciated phenotypic variability within a protoplast population and demonstrated the advantages of a microfluidic flow cytometry for assessing the in vivo dynamics of plant metabolic and physiological indices at the single-cell level.

Deleterious effects of Schinus terebinthifolius Raddi seed flour on cowpea weevil, Callosobruchus maculatus (F.), larval development

Schinus terebinthifolius, Raddi, has been extensively studied due to its anti-inflammatory and antibiotic properties. S. terebinthifolius was also toxic to some insects, however little has been explored about the nature of its insecticide compounds or the toxicity of this plant to insect species. In this work, we investigate the toxicity of S. terebinthifolius seed flour against the insect C. maculatus. S. terebinthifolius seed flour interfered with the post hatch development of the C. maculatus larvae, decreasing larval survival, mass and length. Using DEAE-cellulose chromatography, five protein fractions were isolated, a non-retained fraction (NRF) and four retained fractions, eluted with 0.25, 0.5, 0.7 and 1.0 M NaCl. Proteins with varying molecular masses were observed in all fractions. The majority protein bands were identified by mass spectrometry analysis and among the main identified proteins are 11S globulins (such glycinin), lipoxygenase, chitinases, 7S globulins (vicilins, canavalin and β conglycinin), annexin, catalase and sucrose binding protein. All DEAE-protein fractions were toxic to the insect, interfering with the post hatch larval development and survival. Decreases greater than 90% were observed in the larval mass and length at 20 days after oviposition (DAO) for larvae raised on diet containing 0.5% of some fractions. Alterations in the level of proteins, glucose and in the activity of the enzymes lipases and cysteine proteases were also detected in these larvae. Our results show that seeds of S. terebinthifolius have an arsenal of toxic proteins with potential for the control of the insect C. maculatus.

Effects of three graphene-based materials on the growth and photosynthesis of Brassica napus L

The environmental safety and threats of graphene-based materials (GBMs) to the agroecosystem have attracted increasing attention in recent years. However, the mechanisms underlying the effects of GBMs on plants remain unclear. Here, we investigated the phytotoxicity of reduced graphene oxide (RGO), graphene oxide (GO) and amine-functionalized graphene (G-NH2) on Brassica napus L. The results revealed that RGO impaired photosynthesis mainly by decreasing the chlorophyll content and Rubisco activity. A further gene-level analysis suggested that this effect of RGO might be due to its toxicity on sulfate transmembrane transporter and nitrogen metabolism, which ultimately led to nutrient imbalance. However, GO directly damaged the photosystem by disrupting the chloroplast structure, and a decrease in Rubisco activity indicated that GO also inhibits carbon fixation. Further gene-level analysis demonstrated that GO has toxicity on the chloroplast membrane, photosystem, photosynthethic electron transport and F-type ATPase. In addition, G-NH2 at 10-1000 mg L-1 showed no significant toxicity. These findings shed light on the potential mechanism for the toxicity of GBMs on plants for risk assessment.

Physiological and Comparative Transcriptomic Analysis Provide Insight Into Cotton (Gossypium hirsutum L.) Root Senescence in Response

Nitrogen (N) deficiency is one of the pivotal environmental factors that induce leaf senescence. However, little is known regarding the impact of low N on root senescence in cotton. Thus, the objective of this study was to investigate the effect of low nitrogen on root senescence. In this study, the molecular mechanism of cotton root senescence in response to nitrogen deficiency was investigated by combing physiological and transcriptomic analysis when no nitrogen and normal nitrogen (138mg N·kg-1 soil). The results showed that: (1) nitrogen starvation induced the premature senescence of leaf, while delaying root senescence. (2) The increase in catalase (CAT) activity at 60, 80, and 100days after emergence (DAE), combined with decrease of malonaldehyde content at 60, 80, and 100 DAE, and the content of abscisic acid (ABA), all of these contributed to the delay of root senescence by low nitrogen treatment. (3) To study the molecular mechanisms underlying root senescence, the gene expression profiling between low nitrogen and normal nitrogen treatments were compared pairwise at 20, 40, 60, 80, and 100 DAE. A total of 14,607 genes were identified to be differentially expressed at these five points. (5) Most genes involved in glutathione (GSH) and ascorbate peroxidase (APX) synthesis were upregulated, while ABA, apoptosis, caspase, and cell cycle-related differentially expressed genes (DEGs) were downregulated. Coupled with the physiology data, these results provide new insights into the effect of nitrogen starvation on root senescence.

Luxury zinc supply acts as antiaging agent and enhances reproductive fitness in Arabidopsis thaliana

Developmental senescence in plants is an age dependent process affected by phytohormones, nutrient status, and environmental factors, while the antiaging effects of zinc are recognized in humans. This study explores the possible influence of a high, non-toxic Zn-supply (12 μM) on senescence and reproductive fitness in A. thaliana. Auxin-resistance mutant, axr1-12, and auxin overexpressing YUCCA6 mutant, yuc6-1D, and their corresponding background genotypes were grown until complete rosette senescence to quantify the fruit biomass and seed number. Gene expression of different antioxidant, auxin and senescence-associated markers were analyzed after the onset of senescence. All mutants showed delayed developmental senescence. Luxury Zn delayed senescence in wild type, but not in the mutant genotypes. Excluding axr1-12 mutants, which showed very low expression of the auxin gene marker INDOLE-3-ACETIC ACID INDUCIBLE 2 (IAA2), enhanced expression of the senescence markers SENESCENCE-ASSOCIATED GENE 12 (SAG12) and AUXIN RESPONSE FACTOR 2 (ARF2) coincided with decreased expression of IAA2. Delayed senescence and total number of seeds per plant were related to higher expression of the peroxisomal antioxidant enzymes Cu/Zn superoxide dismutase (SOD3) and catalase (CAT2). These results evidence that high Zn-induced delayed senescence and improved reproductive fitness in Arabidopsis are related to an auxin-independent mechanism that retains antioxidant activity.

Abiotic Stress and Reactive Oxygen Species: Generation, Signaling, and Defense Mechanisms

Climate change is an invisible, silent killer with calamitous effects on living organisms. As the sessile organism, plants experience a diverse array of abiotic stresses during ontogenesis. The relentless climatic changes amplify the intensity and duration of stresses, making plants dwindle to survive. Plants convert 1-2% of consumed oxygen into reactive oxygen species (ROS), in particular, singlet oxygen (1O2), superoxide radical (O2•-), hydrogen peroxide (H2O2), hydroxyl radical (•OH), etc. as a byproduct of aerobic metabolism in different cell organelles such as chloroplast, mitochondria, etc. The regulatory network comprising enzymatic and non-enzymatic antioxidant systems tends to keep the magnitude of ROS within plant cells to a non-damaging level. However, under stress conditions, the production rate of ROS increases exponentially, exceeding the potential of antioxidant scavengers instigating oxidative burst, which affects biomolecules and disturbs cellular redox homeostasis. ROS are similar to a double-edged sword; and, when present below the threshold level, mediate redox signaling pathways that actuate plant growth, development, and acclimatization against stresses. The production of ROS in plant cells displays both detrimental and beneficial effects. However, exact pathways of ROS mediated stress alleviation are yet to be fully elucidated. Therefore, the review deposits information about the status of known sites of production, signaling mechanisms/pathways, effects, and management of ROS within plant cells under stress. In addition, the role played by advancement in modern techniques such as molecular priming, systems biology, phenomics, and crop modeling in preventing oxidative stress, as well as diverting ROS into signaling pathways has been canvassed.

Variant biochemical responses: intrinsic and adaptive system for ecologically different rice varieties

India has a diverse range of agro-ecological conditions which support the cultivation of different rice varieties differing in the adaptation which is so important for sustainable development of rice crop. Specific ecotypes of rice adapted to diverse conditions have divergence in their morphology, physiology, biochemistry, molecular function, agronomy, and stress response. In the present study, 12 different rice varieties viz., PB-1, PB-1509, Pusa-RH-10, CSR-30, HKR-47, PR-126, Govind, Sharbati, ADT-37, ADT-39, ADT-45, White Ponni, were selected for the study of intrinsic biochemical behaviour and these varieties belong to different Agro-ecological zones and basmati or non-basmati rice varieties. Amongst intrinsic biochemicals activity, the differential response of radical scavenging, superoxide dismutase (SOD), catalase (CAT) and guaiacol peroxidase (POX) activities, were observed in the selected rice varieties at 14 days old seedling stage, developed under controlled growth conditions. Comparatively, North India region rice varieties displayed an enhanced intrinsic biochemical response than south India region rice varieties. Similarly, basmati rice varieties showed increased biochemical response compared to non-basmati rice varieties. Thus, the differential biochemical responses (radical scavenging, SOD, CAT, and POX activities) observed creates a significant difference between rice varieties and provides valuable information about rice ecotype-biochemical interaction for sustainable adaptive value under different ecological conditions.

Silicon in Horticultural Crops: Cross-talk, Signaling, and Tolerance Mechanism under Salinity Stress

Agricultural land is extensively affected by salinity stress either due to natural phenomena or by agricultural practices. Saline stress possesses two major threats to crop growth: osmotic stress and oxidative stress. The response of these changes is often accompanied by variety of symptoms, such as the decrease in leaf area and internode length and increase in leaf thickness and succulence, abscission of leaves, and necrosis of root and shoot. Salinity also delays the potential physiological activities, such as photosynthesis, transpiration, phytohormonal functions, metabolic pathways, and gene/protein functions. However, crops in response to salinity stress adopt counter cascade mechanisms to tackle salinity stress incursion, whilst continuous exposure to saline stress overcomes the defense mechanism system which results in cell death and compromises the function of essential organelles in crops. To overcome the salinity, a large number of studies have been conducted on silicon (Si); one of the beneficial elements in the Earth's crust. Si application has been found to mitigate salinity stress and improve plant growth and development, involving signaling transduction pathways of various organelles and other molecular mechanisms. A large number of studies have been conducted on several agricultural crops, whereas limited information is available on horticultural crops. In the present review article, we have summarized the potential role of Si in mitigating salinity stress in horticultural crops and possible mechanism of Si-associated improvements in them. The present review also scrutinizes the need of future research to evaluate the role of Si and gaps to saline stress in horticultural crops for their improvement.

Melatonin: Awakening the Defense Mechanisms during Plant Oxidative Stress

Melatonin is a multifunctional signaling molecule that is ubiquitously distributed in different parts of a plant and responsible for stimulating several physio-chemical responses to adverse environmental conditions. In this review, we show that, although plants are able to biosynthesize melatonin, the exogenous application of melatonin to various crops can improve plant growth and development in response to various abiotic and biotic stresses (e.g., drought, unfavorable temperatures, high salinity, heavy metal contamination, acid rain, and combined stresses) by regulating antioxidant machinery of plants. Current knowledge suggests that exogenously applied melatonin can enhance the stress tolerance of plants by regulating both the enzymatic and non-enzymatic antioxidant defense systems. Enzymic antioxidants upregulated by exogenous melatonin include superoxide dismutase, catalase, glutathione peroxidase, and enzymes involved in the ascorbate–glutathione cycle (ascorbate peroxidase, monodehydroascorbate reductase, dehydroascorbate reductase, and glutathione reductase), whereas levels of non-enzymatic antioxidants such as ascorbate, reduced glutathione, carotenoids, tocopherols, and phenolics are also higher under stress conditions. The enhanced antioxidant system consequently exhibits lower lipid peroxidation and greater plasma membrane integrity when under stress. However, these responses vary greatly from crop to crop and depend on the intensity and type of stress, and most studies to date have been conducted under controlled conditions. This means that a wider range of crop field trials and detailed transcriptomic analysis are required to reveal the gene regulatory networks involved in the between melatonin, antioxidants, and abiotic stress.

Multilevel Regulation of Peroxisomal Proteome by Post-Translational Modifications

Peroxisomes, which are ubiquitous organelles in all eukaryotes, are highly dynamic organelles that are essential for development and stress responses. Plant peroxisomes are involved in major metabolic pathways, such as fatty acid β-oxidation, photorespiration, ureide and polyamine metabolism, in the biosynthesis of jasmonic, indolacetic, and salicylic acid hormones, as well as in signaling molecules such as reactive oxygen and nitrogen species (ROS/RNS). Peroxisomes are involved in the perception of environmental changes, which is a complex process involving the regulation of gene expression and protein functionality by protein post-translational modifications (PTMs). Although there has been a growing interest in individual PTMs in peroxisomes over the last ten years, their role and cross-talk in the whole peroxisomal proteome remain unclear. This review provides up-to-date information on the function and crosstalk of the main peroxisomal PTMs. Analysis of whole peroxisomal proteomes shows that a very large number of peroxisomal proteins are targeted by multiple PTMs, which affect redox balance, photorespiration, the glyoxylate cycle, and lipid metabolism. This multilevel PTM regulation could boost the plasticity of peroxisomes and their capacity to regulate metabolism in response to environmental changes.

Role of Cytokinins in Senescence, Antioxidant Defence and Photosynthesis

Cytokinins modulate a number of important developmental processes, including the last phase of leaf development, known as senescence, which is associated with chlorophyll breakdown, photosynthetic apparatus disintegration and oxidative damage. There is ample evidence that cytokinins can slow down all these senescence-accompanying changes. Here, we review relationships between the various mechanisms of action of these regulatory molecules. We highlight their connection to photosynthesis, the pivotal process that generates assimilates, however may also lead to oxidative damage. Thus, we also focus on cytokinin induction of protective responses against oxidative damage. Activation of antioxidative enzymes in senescing tissues is described as well as changes in the levels of naturally occurring antioxidative compounds, such as phenolic acids and flavonoids, in plant explants. The main goal of this review is to show how the biological activities of cytokinins may be related to their chemical structure. New links between molecular aspects of natural cytokinins and their synthetic derivatives with antisenescent properties are described. Structural motifs in cytokinin molecules that may explain why these molecules play such a significant regulatory role are outlined.

Conserved and differential transcriptional responses of peroxisome associated pathways to drought, dehydration and ABA

Plant peroxisomes are important components of cellular antioxidant networks, dealing with ROS generated by multiple metabolic pathways. Peroxisomes respond to environmental and cellular conditions by changing their size, number, and proteomic content. To investigate the role of peroxisomes in response to drought, dehydration and ABA treatment we took an evolutionary and comparative genomics approach. Colonisation of land required evolution of dehydration tolerance in the absence of subsequent anatomical adaptations. Therefore, the model bryophyte Physcomitrella patens, the model dicot Arabidopsis thaliana and wheat (Tricitcum aestivum), a globally important cereal crop were compared. Three sets of genes namely 'PTS1 genes' (a proxy for genes encoding peroxisome targeted proteins), PEX genes (involved in peroxisome biogenesis) and genes involved in plant antioxidant networks were identified in all 3 species and their expression compared under drought (dehydration) and ABA treatment. Genes encoding enzymes of β-oxidation and gluconeogenesis, antioxidant enzymes including catalase and glutathione reductase and PEX3 and PEX11 isoforms showed conserved up-regulation, and peroxisome proliferation was induced by ABA in moss. Interestingly, expression of some of these genes differed between drought sensitive and resistant genotypes of wheat in line with measured photosynthetic and biochemical differences. These results point to an underappreciated role for peroxisomes in drought response.

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

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

A Central Role for Triacylglycerol in Membrane Lipid Breakdown, Fatty Acid β-Oxidation, and Plant Survival under Extended Darkness

Neutral lipid metabolism is a key aspect of intracellular homeostasis and energy balance and plays a vital role in cell survival under adverse conditions, including nutrient deprivation in yeast and mammals, but the role of triacylglycerol (TAG) metabolism in plant stress response remains largely unknown. By thoroughly characterizing mutants defective in SUGAR-DEPENDENT1 (SDP1) triacylglycerol lipase or PEROXISOMAL ABC TRANSPORTER 1 (PXA1), here we show that TAG is a key intermediate in the mobilization of fatty acids from membrane lipids for peroxisomal β-oxidation under prolonged dark treatment. Disruption of SDP1 increased TAG accumulation in cytosolic lipid droplets and markedly enhanced plant tolerance to extended darkness. We demonstrate that blocking TAG hydrolysis enhances plant tolerance to dark treatment via two distinct mechanisms. In pxa1 mutants, in which free fatty acids accumulated rapidly under extended darkness, SDP1 disruption resulted in a marked decrease in levels of cytotoxic lipid intermediates such as free fatty acids and phosphatidic acid, suggesting a buffer function of TAG accumulation against lipotoxicity under fatty acid overload. In the wild type, in which free fatty acids remained low and unchanged under dark treatment, disruption of SDP1 caused a decrease in reactive oxygen species production and hence the level of lipid peroxidation, indicating a role of TAG in protection against oxidative damage. Overall, our findings reveal a crucial role for TAG metabolism in membrane lipid breakdown, fatty acid turnover, and plant survival under extended darkness.

Plant Responses to Salt Stress: Adaptive Mechanisms

This review deals with the adaptive mechanisms that plants can implement to cope with the challenge of salt stress. Plants tolerant to NaCl implement a series of adaptations to acclimate to salinity, including morphological, physiological and biochemical changes. These changes include increases in the root/canopy ratio and in the chlorophyll content in addition to changes in the leaf anatomy that ultimately lead to preventing leaf ion toxicity, thus maintaining the water status in order to limit water loss and protect the photosynthesis process. Furthermore, we deal with the effect of salt stress on photosynthesis and chlorophyll fluorescence and some of the mechanisms thought to protect the photosynthetic machinery, including the xanthophyll cycle, photorespiration pathway, and water-water cycle. Finally, we also provide an updated discussion on salt-induced oxidative stress at the subcellular level and its effect on the antioxidant machinery in both salt-tolerant and salt-sensitive plants. The aim is to extend our understanding of how salinity may affect the physiological characteristics of plants.

Proteomic Identification of Differentially Expressed Proteins during Alfalfa (Medicago sativa L.) Flower Development

Flower development, pollination, and fertilization are important stages in the sexual reproduction process of plants; they are also critical steps in the control of seed formation and development. During alfalfa (Medicago sativa L.) seed production, some distinct phenomena such as a low seed setting ratio, serious flower falling, and seed abortion commonly occur. However, the causes of these phenomena are complicated and largely unknown. An understanding of the mechanisms that regulate alfalfa flowering is important in order to increase seed yield. Hence, proteomic technology was used to analyze changes in protein expression during the stages of alfalfa flower development. Flower samples were collected at pre-pollination (S1), pollination (S2), and the post-pollination senescence period (S3). Twenty-four differentially expressed proteins were successfully identified, including 17 down-regulated in pollinated flowers, one up-regulated in pollinated and senesced flowers, and six up-regulated in senesced flowers. The largest proportions of the identified proteins were involved in metabolism, signal transduction, defense response, oxidation reduction, cell death, and programmed cell death (PCD). Their expression profiles demonstrated that energy metabolism, carbohydrate metabolism, and amino acid metabolism provided the nutrient foundation for pollination in alfalfa. Furthermore, there were three proteins involved in multiple metabolic pathways: dual specificity kinase splA-like protein (kinase splALs), carbonic anhydrase, and NADPH: quinone oxidoreductase-like protein. Expression patterns of these proteins indicated that MAPK cascades regulated multiple processes, such as signal transduction, stress response, and cell death. PCD also played an important role in the alfalfa flower developmental process, and regulated both pollination and flower senescence. The current study sheds some light on protein expression profiles during alfalfa flower development and contributes to the understanding of the basic molecular mechanisms during the alfalfa flowering process. These results may offer insight into potential strategies for improving seed yield, quality, and stress tolerance in alfalfa.

Gene expression and promoter analysis of a novel tomato aldo-keto reductase in response to environmental stresses

The functional role of an uncharacterized tomato (Solanum lycopersicum) aldo-keto reductase 4B, denoted as SlAKR4B, was investigated. The gene expression of tomato SlAKR4B was detected at a high level in the senescent leaves and the ripening fruits of tomato. Although d-galacturonic acid reductase activities tended to be higher in tomato SlAKR4B-overexpressing transgenic tobacco BY-2 cell lines than those in control cell lines, SlAKR4B gene expression was not well correlated with l-ascorbic acid content among the cell lines. The analysis of the transgenic cell lines showed that tomato SlAKR4B has enzyme activities toward d-galacturonic acid as well as glyceraldehyde and glyoxal, suggesting that the SlAKR4B gene encodes a functional enzyme in tomato. Gene expression of SlAKR4B was induced by NaCl, H2O2, and plant hormones such as salicylic acid and jasmonic acid, suggesting that SlAKR4B is involved in the stress response. The transient expression assay using protoplasts showed the promoter activity of the SlAKR4B gene was as high as that of the cauliflower mosaic virus 35S promoter. Also, the promoter region of the SlAKR4B gene was suggested to contain cis-element(s) for abiotic stress-inducible expression.

Occurrence of Benzoic Acid Esters as Putative Catabolites of Prunasin in Senescent Leaves of Prunus laurocerasus

Prunus laurocerasus is an evergreen shrub containing large quantities of the cyanogenic glycoside prunasin (1) in its leaves, which decomposes to prunasin amide (2) or glucose-1-benzoate (4) when the leaves become chlorotic as a result of senescence or pseudosenescence. This study was aimed at the systematic identification of senescence-associated metabolites to contribute further insight into the catabolism of 1. LC-ESIMS profiles of senescent and green leaves were analyzed by principal component analysis. In senescent leaves, the concentrations of 36 compounds were increased significantly including several benzoic acid derivatives, of which prunasin amide-6'-benzoate (5) and prunasin acid-6'-benzoate (6) were isolated and identified. The observed metabolic changes were also induced by treatment of P. laurocerasus shrubs with exogenous ethylene. The data presented support an oxidative catabolism of 1 without release of hydrogen cyanide and the remobilization of its nitrogen in the course of senescence. The results are discussed in the context of functional diversification and drug discovery, where senescent plant material represents a widely unexplored source for the discovery of natural products.

ROS Generation in Peroxisomes and its Role in Cell Signaling

In plant cells, as in most eukaryotic organisms, peroxisomes are probably the major sites of intracellular H₂O₂ production, as a result of their essentially oxidative type of metabolism. In recent years, it has become increasingly clear that peroxisomes carry out essential functions in eukaryotic cells. The generation of the important messenger molecule hydrogen peroxide (H₂O₂) by animal and plant peroxisomes and the presence of catalase in these organelles has been known for many years, but the generation of superoxide radicals (O₂^·- ) and the occurrence of the metalloenzyme superoxide dismutase was reported for the first time in peroxisomes from plant origin. Further research showed the presence in plant peroxisomes of a complex battery of antioxidant systems apart from catalase. The evidence available of reactive oxygen species (ROS) production in peroxisomes is presented, and the different antioxidant systems characterized in these organelles and their possible functions are described. Peroxisomes appear to have a ROS-mediated role in abiotic stress situations induced by the heavy metal cadmium (Cd) and the xenobiotic 2,4-D, and also in the oxidative reactions of leaf senescence. The toxicity of Cd and 2,4-D has an effect on the ROS metabolism and speed of movement (dynamics) of peroxisomes. The regulation of ROS production in peroxisomes can take place by post-translational modifications of those proteins involved in their production and/or scavenging. In recent years, different studies have been carried out on the proteome of ROS metabolism in peroxisomes. Diverse evidence obtained indicates that peroxisomes are an important cellular source of different signaling molecules, including ROS, involved in distinct processes of high physiological importance, and might play an important role in the maintenance of cellular redox homeostasis.

Overexpression of human peroxisomal enoyl-CoA delta isomerase2 HsPECI2, an ortholog of bamboo expressed during gregarious flowering alters salinity stress responses and polar lipid content in tobacco

Peroxisomal enoyl-CoA delta isomerase2 (PECI2) is one of the key enzymes that has critical role in lipid metabolism and plant development during salt stress. Seven out of ten tobacco plants overexpressing human PECI2 (HsPECI2) with PTS1-sequence showed hypersensitivity to salt. Under salt-stress, T2 transformed plants (HsPECI2) displayed reduced primary root, delayed shoot-growth, and visibly smaller rosette leaves turning pale yellow as compared to the pKYLX71 vector control plant. Also, we found altered reactive oxygen species (ROS) levels and reduced catalase activity in 100mM sodium chloride (NaCl) treated HsPECI2 transformed plant compared with the pKYLX71 counterpart. ESI-MS/MS data showed that the polar lipids were differentially modulated upon salt treatment in HsPECI2 transformed and pKYLX71 plants as compared with the respective untreated counterpart. Notably, the levels of monogalactosyldiacylglycerol and phosphatidic acid varied significantly, whereas phosphatidylcholine, phosphatidylserine and digalactosyldiacylglycerol contents were moderately upregulated. In parallel, abscisic acid (ABA) responsiveness assay confirmed insensitivity of HsPECI2 transformed plant towards ABA. Overall our data proclaim that HsPECI2 play multifunctional role in normal development and response to salinity stress apart from its primary role in β-oxidation.

Decreased glutathione reductase2 leads to early leaf senescence in Arabidopsis

Glutathione reductase (GR) catalyzes the reduction of glutathione disulfide (GSSG) to reduced glutathione (GSH) and participates in the ascorbate-glutathione cycle, which scavenges H2 O2 . Here, we report that chloroplastic/mitochondrial GR2 is an important regulator of leaf senescence. Seed development of the homozygous gr2 knockout mutant was blocked at the globular stage. Therefore, to investigate the function of GR2 in leaf senescence, we generated transgenic Arabidopsis plants with decreased GR2 using RNAi. The GR2 RNAi plants displayed early onset of age-dependent and dark- and H2 O2 -induced leaf senescence, which was accompanied by the induction of the senescence-related marker genes SAG12 and SAG13. Furthermore, transcriptome analysis revealed that genes related to leaf senescence, oxidative stress, and phytohormone pathways were upregulated directly before senescence in RNAi plants. In addition, H2 O2 accumulated to higher levels in RNAi plants than in wild-type plants and the levels of H2 O2 peaked in RNAi plants directly before the early onset of leaf senescence. RNAi plants showed a greater decrease in GSH/GSSG levels than wild-type plants during leaf development. Our results suggest that GR2 plays an important role in leaf senescence by modulating H2 O2 and glutathione signaling in Arabidopsis.

Compartment-specific investigations of antioxidants and hydrogen peroxide in leaves of Arabidopsis thaliana during dark-induced senescence

The aim of this study was to gain insight into the compartment-specific roles of ascorbate and glutathione in leaf senescence in Arabidopsis thaliana. The subcellular distribution of ascorbate, glutathione, and hydrogen peroxide (H₂O₂) was analyzed by transmission electron microscopy and correlated with the activity of antioxidative enzymes in wildtype plants and the ascorbate- and glutathione-deficient mutants vtc2-1 and pad2-1, respectively. Both mutants showed earlier and stronger senescence than the wildtype indicating the importance of a functioning ascorbate and glutathione cycle in the induction and regulation of senescence. Glutathione levels dropped drastically and up to 93 % in all cell compartments of wildtype plants and the vtc2-1 mutant within the first day of dark-induced senescence while ascorbate contents remained unchanged until the very end. Glutathione contents in mitochondria of pad2-1 mutants decreased more slowly over the first 7 days than compared to the other plants indicating an important role of glutathione in mitochondria in this mutant during senescence. The strongest decrease (84 %) of glutathione contents in wildtype plants at this time point was found in mitochondria indicating an important role of mitochondria for the induction of senescence and cell death events. Due to the general decrease of the antioxidative capacity, a strong accumulation of H₂O₂ was observed in cell walls, plastids, and the cytosol in all plants. Activities of glutathione reductase, dehydroascorbate reductase and catalase were strongly reduced while ascorbate peroxidase and monodehydroascorbate reductase were increased. The initial rapid drop of glutathione levels seemed to be the trigger for senescence, while ascorbate appeared to be the key factor in regulating senescence through controlling H₂O₂ levels by the oxidation of reduced ascorbate to monodehydroascorbate and the subsequent reduction to ascorbate by monodehydroascorbate reductase.

Ectopic Expression of WRINKLED1 Affects Fatty Acid Homeostasis in Brachypodium distachyon Vegetative Tissues

Triacylglycerol (TAG) is a storage lipid used for food purposes and as a renewable feedstock for biodiesel production. WRINKLED1 (WRI1) is a transcription factor that governs fatty acid (FA) synthesis and, indirectly, TAG accumulation in oil-storing plant tissues, and its ectopic expression has led to TAG accumulation in vegetative tissues of different dicotyledonous plants. The ectopic expression of BdWRI1 in the grass Brachypodium distachyon induced the transcription of predicted genes involved in glycolysis and FA biosynthesis, and TAG content was increased up to 32.5-fold in 8-week-old leaf blades. However, the ectopic expression of BdWRI1 also caused cell death in leaves, which has not been observed previously in dicotyledonous plants such as Arabidopsis (Arabidopsis thaliana). Lipid analysis indicated that the free FA content was 2-fold elevated in BdWRI1-expressing leaf blades of B. distachyon. The transcription of predicted genes involved in β-oxidation was induced. In addition, linoleic FA treatment caused cell death in B. distachyon leaf blades, an effect that was reversed by the addition of the FA biosynthesis inhibitor cerulenin. Taken together, ectopic expression of BdWRI1 in B. distachyon enhances FA biosynthesis and TAG accumulation in leaves, as expected, but also leads to increased free FA content, which has cytotoxic effects leading to cell death. Thus, while WRI appears to ubiquitously affect FA biosynthesis and TAG accumulation in diverse plants, its ectopic expression can lead to undesired side effects depending on the context of the specific lipid metabolism of the respective plant species.

Peroxisomes sense and respond to environmental cues by regulating ROS and RNS signalling networks

BACKGROUND

Peroxisomes are highly dynamic, metabolically active organelles that used to be regarded as a sink for H2O2 generated in different organelles. However, peroxisomes are now considered to have a more complex function, containing different metabolic pathways, and they are an important source of reactive oxygen species (ROS), nitric oxide (NO) and reactive nitrogen species (RNS). Over-accumulation of ROS and RNS can give rise oxidative and nitrosative stress, but when produced at low concentrations they can act as signalling molecules.

SCOPE

This review focuses on the production of ROS and RNS in peroxisomes and their regulation by antioxidants. ROS production is associated with metabolic pathways such as photorespiration and fatty acid β-oxidation, and disturbances in any of these processes can be perceived by the cell as an alarm that triggers defence responses. Genetic and pharmacological studies have shown that photorespiratory H2O2 can affect nuclear gene expression, regulating the response to pathogen infection and light intensity. Proteomic studies have shown that peroxisomal proteins are targets for oxidative modification, S-nitrosylation and nitration and have highlighted the importance of these modifications in regulating peroxisomal metabolism and signalling networks. The morphology, size, number and speed of movement of peroxisomes can also change in response to oxidative stress, meaning that an ROS/redox receptor is required. Information available on the production and detection of NO/RNS in peroxisomes is more limited. Peroxisomal homeostasis is critical for maintaining the cellular redox balance and is regulated by ROS, peroxisomal proteases and autophagic processes.

CONCLUSIONS

Peroxisomes play a key role in many aspects of plant development and acclimation to stress conditions. These organelles can sense ROS/redox changes in the cell and thus trigger rapid and specific responses to environmental cues involving changes in peroxisomal dynamics as well as ROS- and NO-dependent signalling networks, although the mechanisms involved have not yet been established. Peroxisomes can therefore be regarded as a highly important decision-making platform in the cell, where ROS and RNS play a determining role.

Changes in production of reactive oxygen species in illuminated thylakoids isolated during development and senescence of barley

This paper presents a detailed analysis of thylakoids isolated from secondary barley leaves harvested 18, 22, 25, 29, 32, 35 and 39 days after sowing (DAS). Goal of the analysis was to investigate the production of different reactive oxygen species (ROS) during development and senescence of barley. Generation of superoxide anion (O2-•) and hydrogen peroxide (H2O2) increases during development of barley reaching the highest value right after the onset of senescence (between 25 and 29 DAS), thereafter the levels of both ROS start to decrease until 35 DAS when production of H2O2 increases again. In comparison with O2-• and H2O2, generation of singlet oxygen ((1)O2) showed continuous production of low amounts thought the duration of experiment. Oxidative damage to the thylakoid membrane was assessed by measuring lipid peroxidation. Results showed gradual increase in lipid peroxidation with progress of plant development with highest increase occurring at the late stages of senescence. A possible factor contributing to the elevation in the production of ROS could be an increase in membrane fluidity observed in our previous study. Fluidization of the membrane, allows for better penetration of oxygen inside the membrane, which can lead to an increase in the production of ROS. Indeed, the production of ROS started to increase together with observed fluidization of the membrane from 22 to 29 DAS. Thereafter, production of ROS started to decline till 35th DAS. On the last day of the measurement, chl is at 25% of its initial value, lipid peroxidation reaches the highest value and H2O2 increases again.

Senescence, Stress, and Reactive Oxygen Species

Generation of reactive oxygen species (ROS) is one of the earliest responses of plant cells to various biotic and abiotic stresses. ROS are capable of inducing cellular damage by oxidation of proteins, inactivation of enzymes, alterations in the gene expression, and decomposition of biomembranes. On the other hand, they also have a signaling role and changes in production of ROS can act as signals that change the transcription of genes that favor the acclimation of plants to abiotic stresses. Among the ROS, it is believed that H₂O₂ causes the largest changes in the levels of gene expression in plants. A wide range of plant responses has been found to be triggered by H₂O₂ such as acclimation to drought, photooxidative stress, and induction of senescence. Our knowledge on signaling roles of singlet oxygen (¹O₂) has been limited by its short lifetime, but recent experiments with a flu mutant demonstrated that singlet oxygen does not act primarily as a toxin but rather as a signal that activates several stress-response pathways. In this review we summarize the latest progress on the signaling roles of ROS during senescence and abiotic stresses and we give a short overview of the methods that can be used for their assessment.

Generation of reactive oxygen species in thylakoids from senescing flag leaves of the barley varieties Lomerit and Carina

During senescence, production of reactive oxygen species increased in thylakoids. In two barley varieties, no difference in superoxide production was observed while singlet oxygen production increased only in one variety. During senescence, chlorophyll content decreased and photosynthetic electron transport was inhibited as shown for flag leaves collected from barley varieties Lomerit and Carina grown in the field. Spin trapping electron paramagnetic resonance (EPR) was used to investigate the production of reactive oxygen species in thylakoid membranes during senescence. EPR measurements were performed with specific spin traps to discriminate between singlet oxygen on one hand and reactive oxygen intermediates on the other hand. The results show that the generation of reactive oxygen intermediates increases in both varieties during senescence. Singlet oxygen increased only in the variety cv. Lomerit while it remained constant at a low level in the variety cv. Carina. Measurements in the presence of inhibitors of photosystem II and of the cytochrome b6f complex revealed that in senescing leaves reduction of oxygen at the acceptor side of photosystem I was the major, but not the only source of superoxide anions. This study shows that during senescence the production of individual reactive oxygen species varies in different barley varieties.

Ultrastructural, metabolic and proteomic changes in leaves of upland cotton in response to cadmium stress

Present study explores physiological, biochemical and proteomic changes in leaves of upland cotton (ZMS-49) using 500 μM cadmium (Cd) along with control. Leaves' biomass and chlorophyll pigments decreased at 500 μM Cd. Cd contents in roots were higher than leaves. Levels of ROS ( [Formula: see text] and H2O2) both in vivo and in vitro and MDA contents were significantly increased. Chlorophyll parameters (F0, Fm, Fm(') and Fv/Fm), total soluble protein contents and APX showed a decline at 500 μM Cd. SOD, CAT and POD and GR activities significantly enhanced. Less ultrastructural alterations in leaves under Cd stress could be observed. Scanning micrographs at 500 μM Cd possessed less number of stomata as well as near absence of closed stomata. Cd could be located in cell wall, vacuoles and intracellular spaces. Important upregulated proteins were methionine synthase, ribulose 1,5-bisphosphate carboxylase, apoplastic anionic guaiacol peroxidase, glyceraldehydes-3-phosphate dehydrogenase (chloroplastic isoform) and ATP synthase D chain, (mitochondrial). Important downregulated proteins were seed storage proteins (vicilin and legumin), molecular chaperones (hsp70, chaperonin-60 alpha subunit; putative protein disulfide isomerase), ATP-dependent Clp protease, ribulose-1,5-bisphophate carboxylase/oxygenase large subunit. Increase in the activities of ROS-scavenging enzymes, less ultrastructural modification, Cd-deposition in dead parts of cells as well as active regulation of different proteins showed Cd-resistant nature of ZMS-49.

Senescence-Associated Vacuoles, a Specific Lytic Compartment for Degradation of Chloroplast Proteins?

Degradation of chloroplasts and chloroplast components is a distinctive feature of leaf senescence. In spite of its importance in the nutrient economy of plants, knowledge about the mechanism(s) involved in the breakdown of chloroplast proteins is incomplete. A novel class of vacuoles, “senescence-associated vacuoles” (SAVs), characterized by intense proteolytic activity appear during senescence in chloroplast-containing cells of leaves. Since SAVs contain some chloroplast proteins, they are candidate organelles to participate in chloroplast breakdown. In this review we discuss the characteristics of SAVs, and their possible involvement in the degradation of Rubisco, the most abundant chloroplast protein. Finally, SAVs are compared with other extra-plastidial protein degradation pathways operating in senescing leaves.

Evidences for reduced metal-uptake and membrane injury upon application of nitric oxide donor in cadmium stressed rice seedlings

Heavy metal Cadmium (Cd) contaminates the environment through various anthropogenic sources. Cadmium-induced productions of free radicals lead to oxidative stress and H2O2 formation in plants. Endogenous Nitric oxide (NO) acts as signal molecules in plant stress response and play a significant role in key regulatory pathways of plant development. This study investigates the effect of 50 μM exogenous sodium nitroprusside (SNP, NO donor), on roots and shoots of rice cv. HUR 3022 grown under 50 μM Cd-stress at 7 days of growth. Plants treated with Cd alone showed stunted growth, decreased length and weight, lower cell viability and less chlorophyll. An elevated lipid peroxidation complemented with more electrolyte leakage was noted. Levels of hydrogen peroxide and superoxide anion increased in Cd-exposed plants with corresponding increase in activity of antioxidant enzymes catalase and superoxide dismutase. Lower chlorophyll levels paralleled with more uptake of cadmium in Cd-treatments as compared to controls. Application of equimolar amount of SNP to cadmium-stressed rice in the growth medium inhibited Cd-uptake and reversed the Cd-induced toxic effects by restoring membrane integrity. The levels of H2O2 and O2(-) were considerably recovered due to SNP treatment. The results indicate that exogenous NO diminishes the deleterious effects of Cd in rice plants.

Specificity in ROS signaling and transcript signatures

SIGNIFICANCE

Reactive oxygen species (ROS), important signaling molecules in plants, are involved in developmental control and stress adaptation. ROS production can trigger broad transcriptional changes; however, it is not clear how specificity in transcriptional regulation is achieved.

RECENT ADVANCES

A large collection of public transcriptome data from the model plant Arabidopsis thaliana is available for analysis. These data can be used for the analysis of biological processes that are associated with ROS signaling and for the identification of suitable transcriptional indicators. Several online tools, such as Genevestigator and Expression Angler, have simplified the task to analyze, interpret, and visualize this wealth of data.

CRITICAL ISSUES

The analysis of the exact transcriptional responses to ROS requires the production of specific ROS in distinct subcellular compartments with precise timing, which is experimentally difficult. Analyses are further complicated by the effect of ROS production in one subcellular location on the ROS accumulation in other compartments. In addition, even subtle differences in the method of ROS production or treatment can lead to significantly different outcomes when various stimuli are compared.

FUTURE DIRECTIONS

Due to the difficulty of inducing ROS production specifically with regard to ROS type, subcellular localization, and timing, we propose that the concept of a "ROS marker gene" should be re-evaluated. We suggest guidelines for the analysis of transcriptional data in ROS signaling. The use of "ROS signatures," which consist of a set of genes that together can show characteristic and indicative responses, should be preferred over the use of individual marker genes.

Increased expression of native cytosolic Cu/Zn superoxide dismutase and ascorbate peroxidase improves tolerance to oxidative and chilling stresses in cassava (Manihot esculenta Crantz)

BACKGROUND

Cassava (Manihot esculenta Crantz) is a tropical root crop, and is therefore, extremely sensitive to low temperature; its antioxidative response is pivotal for its survival under stress. Timely turnover of reactive oxygen species (ROS) in plant cells generated by chilling-induced oxidative damages, and scavenging can be achieved by non-enzymatic and enzymatic reactions in order to maintain ROS homeostasis.

RESULTS

Transgenic cassava plants that co-express cytosolic superoxide dismutase (SOD), MeCu/ZnSOD, and ascorbate peroxidase (APX), MeAPX2, were produced and tested for tolerance against oxidative and chilling stresses. The up-regulation of MeCu/ZnSOD and MeAPX2 expression was confirmed by the quantitative reverse transcriptase-polymerase chain reaction, and enzymatic activity analyses in the leaves of transgenic cassava plant lines with a single-transgene integration site. Upon exposure to ROS-generating agents, 100 μM ROS-generating reagent methyl viologen and 0.5 M H₂O₂, higher levels of enzymatic activities of SOD and APX were detected in transgenic plants than the wild type. Consequently, the oxidative stress parameters, such as lipid peroxidation, chlorophyll degradation and H₂O₂ synthesis, were lower in the transgenic lines than the wild type. Tolerance to chilling stress at 4°C for 2 d was greater in transgenic cassava, as observed by the higher levels of SOD, catalase, and ascorbate-glutathione cycle enzymes (e.g., APX, monodehydroascorbate reductase, dehydroascorbate reducatase and glutathione reductase) and lower levels of malondialdehyde content.

CONCLUSIONS

These results suggest that the expression of native cytosolic SOD and APX simultaneously activated the antioxidative defense mechanisms via cyclic ROS scavenging, thereby improving its tolerance to cold stress.

Integrated analysis of transcriptome and metabolome of Arabidopsis albino or pale green mutants with disrupted nuclear-encoded chloroplast proteins

We used four mutants having albino or pale green phenotypes with disrupted nuclear-encoded chloroplast proteins to analyze the regulatory system of metabolites in chloroplast. We performed an integrated analyses of transcriptomes and metabolomes of the four mutants. Transcriptome analysis was carried out using the Agilent Arabidopsis 2 Oligo Microarray, and metabolome analysis with two mass spectrometers; a direct-infusion Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR/MS) and a gas chromatograph-time of flight mass spectrometer. Among approximately 200 known metabolites detected by the FT-ICR/MS, 71 metabolites showed significant changes in the mutants when compared with controls (Ds donor plants). Significant accumulation of several amino acids (glutamine, glutamate and asparagine) was observed in the albino and pale green mutants. Transcriptome analysis revealed altered expressions of genes in several metabolic pathways. For example, genes involved in the tricarboxylic acid cycle, the oxidative pentose phosphate pathway, and the de novo purine nucleotide biosynthetic pathway were up-regulated. These results suggest that nitrogen assimilation is constitutively promoted in the albino and pale green mutants. The accumulation of ammonium ions in the albino and pale green mutants was consistently higher than in Ds donor lines. Furthermore, genes related to pyridoxin accumulation and the de novo purine nucleotide biosynthetic pathway were up-regulated, which may have occurred as a result of the accumulation of glutamine in the albino and pale green mutants. The difference in metabolic profiles seems to be correlated with the disruption of chloroplast internal membrane structures in the mutants. In albino mutants, the alteration of metabolites accumulation and genes expression is stronger than pale green mutants.

Early senescence of the oldest leaves of Fe-deficient barley plants may contribute to phytosiderophore release from the roots

Barley (Hordeum vulgare), which tolerates iron (Fe) deficiency, secretes a large amount of phytosiderophores from its roots. However, how barley is able to allocate resources for phytosiderophore synthesis when the carbon assimilation rate is reduced by Fe deficiency is unknown. We previously suggested that the acceleration of senescence in older leaves triggered by Fe deficiency may allow the recycling of assimilates to contribute to phytosiderophore synthesis. In this work, we show the relationship between an increase in the C/N ratio in older leaves and Fe-deficiency tolerance among three barley cultivars. The increase in the C/N ratio suggests an enhanced capacity for the retranslocation of carbohydrates or amino acids from older leaves to the sink organs. An increase in the sucrose concentration in Fe-deficient barley also suggests active redistribution of assimilates. This metabolic modulation may be supported by accelerated senescence of older leaves, as Fe deficiency increased the expression of senescence-associated genes. The older leaves of Fe-deficient barley maintained CO2 assimilation under Fe deficiency. Barley that had been Fe-deficient for 3 days preferentially allocated newly assimilated (13) C to the roots and nutrient solution. Interestingly, the oldest leaf of Fe-deficient barley released more (13) C into the nutrient solution than the second oldest leaf. Thus, the balance between anabolism and catabolism in older leaves, supported by highly regulated senescence, plays a key role in metabolic adaptation in Fe-deficient barley.

Over-expression of the peroxisomal ascorbate peroxidase (SbpAPX) gene cloned from halophyte Salicornia brachiata confers salt and drought stress tolerance in transgenic tobacco

Salicornia brachiata Roxb., an extreme halophyte, is a naturally adapted higher plant model for additional gene resources to engineer salt tolerance in plants. Ascorbate peroxidase (APX) plays a key role in protecting plants against oxidative stress and thus confers abiotic stress tolerance. A full-length SbpAPX cDNA, encoding peroxisomal ascorbate peroxidase, was cloned from S. brachiata. The open reading frame encodes for a polypeptide of 287 amino acid residues (31.3-kDa protein). The deduced amino acid sequence of the SbpAPX gene showed characteristic peroxisomal targeting sequences (RKRAI) and a C-terminal hydrophobic region of 39 amino acid residues containing a transmembrane domain (TMD) of 23 amino acid residues. Northern blot analysis showed elevated SbpAPX transcript in response to salt, cold, abscisic acid and salicylic acid stress treatments. The SbpAPX gene was transformed to tobacco for their functional validation under stresses. Transgenic plants over-expressing SbpAPX gene showed enhanced salt and drought stress tolerance compared to wild-type plants. Transgenic plants showed enhanced vegetative growth and germination rate both under normal and stressed conditions. Present study revealed that the SbpAPX gene is a potential candidate, which not only confers abiotic stress tolerance to plants but also seems to be involved in plant growth.

Autophagy as a possible mechanism for micronutrient remobilization from leaves to seeds

Seed formation is an important step of plant development which depends on nutrient allocation. Uptake from soil is an obvious source of nutrients which mainly occurs during vegetative stage. Because seed filling and leaf senescence are synchronized, subsequent mobilization of nutrients from vegetative organs also play an essential role in nutrient use efficiency, providing source-sink relationships. However, nutrient accumulation during the formation of seeds may be limited by their availability in source tissues. While several mechanisms contributing to make leaf macronutrients available were already described, little is known regarding micronutrients such as metals. Autophagy, which is involved in nutrient recycling, was already shown to play a critical role in nitrogen remobilization to seeds during leaf senescence. Because it is a non-specific mechanism, it could also control remobilization of metals. This article reviews actors and processes involved in metal remobilization with emphasis on autophagy and methodology to study metal fluxes inside the plant. A better understanding of metal remobilization is needed to improve metal use efficiency in the context of biofortification.

Cloning and Expression Analysis of a Gene Encoding for Ascorbate Peroxidase and Responsive to Salt Stress in Beet (Beta vulgaris)

BvpAPX is a full-length cDNA-encoding peroxisomal ascorbate peroxidase isolated from leaves of salt-stressed beet (Beta vulgaris) plants. A high level of identity has been reported between the deduced amino acid sequence of BvpAPX and other known ascorbate peroxidases. The genomic sequence of BvpAPX revealed a gene composed of 5 exons and 4 introns. Several sequence motifs revealed in the 5'UTR region of the gene confer to BvpAPX a putative responsiveness to various abiotic stresses. We determined the effect of salt stress on BvpAPX expression in leaves of the cultivated beet varieties, Huzar and Janosik, and their wild salt-tolerant relative B. vulgaris ssp. maritima. Plants were subjected to salt stress during a 32-day culture period (long-term salt treatment). An alternative salinization protocol consisted of an 18-h incubation of detached beet leaves in media supplemented with toxic salt concentrations (short-term salt treatment). RT-Q-PCR analysis revealed that BvpAPX expression markedly increased in leaves of plants subjected to conditions of long-term treatment with salinity, whereas BvpAPX transcript levels remained unaffected in detached leaves during short-term salt treatment. In addition, several leaf redox system parameters, such as ascorbate peroxidase activity or ascorbic acid, hydrogen peroxide, and lipid hydroperoxide concentration, were determined in the leaves of beet plants subjected to salt stress conditions.