Antioxidative defense under salt stress

Salt tolerance is a complex trait involving the coordinated action of many gene families that perform a variety of functions such as control of water loss through stomata, ion sequestration, metabolic adjustment, osmotic adjustment and antioxidative defense. In spite of the large number of publications on the role of antioxidative defense under salt stress, the relative importance of this process to overall plant salt tolerance is still a matter of controversy. In this article, the generation and scavenging of reactive oxygen species (ROS) under normal and salt stress conditions in relation to the type of photosynthesis is discussed. The CO₂ concentrating mechanism in C4 and CAM plants is expected to contribute to decreasing ROS generation. However, the available data supports this hypothesis in CAM but not in C4 plants. We discuss the specific roles of enzymatic and non enzymatic antioxidants in relation to the oxidative load in the context of whole plant salt tolerance. The possible preventive antioxidative mechanisms are also discussed.Key words: salt stress, generation of ROS, type of photosynthesis, scavenging of ROS, preventive antioxidative defense     

Antioxidative response mechanisms in halophytes: Their role in stress defence

Normal growth and development of plants is greatly dependent on the capacity to overcome environmental stresses. Environmental stress conditions like high salinity, drought, high incident light and low or high temperature cause major crop losses worldwide. A common denominator in all these adverse conditions is the production of reactive oxygen species (ROS) within different cellular compartments of the plant cell. Plants have developed robust mechanisms including enzymatic or nonenzymatic scavenging pathways to counter the deleterious effects of ROS production. There are a number of general reviews on oxidative stress in plants and few on the role of ROS scavengers during stress conditions. Here we review the regulation of antioxidant enzymes during salt stress in halophytes, especially mangroves. We conclude that (i) antioxidant enzymes protect halophytes from deleterious ROS production during salt stress, and (ii) genetic information from mangroves and other halophytes would be helpful in defining the roles of individual isoforms. This information would be critical in using the appropriate genes for oxidative stress defence for genetic engineering of enhanced stress tolerance in crop systems.     

Plant Growth-Promoting Rhizobacteria-Mediated Adaptive Responses of Plants Under Salinity Stress

In this recent era, several approaches have been developed to alleviate the adverse effects of salinity stress in different plants. However, some of them are not eco-friendly. In this context, evolving sustainable approaches which enhance the productivity of saline soil without harming the environment are necessary. Many recent studies showed that plant growth-promoting rhizobacteria (PGPR) are known to confer salinity tolerance to plants. Salt-stressed plants inoculated with PGPR enhance the growth and productivity of crops by reducing oxidative damage, maintaining ionic homeostasis, enhancing antioxidant machinery, and regulating gene expressions. The PGPR also regulates the photosynthetic attributes such as net photosynthetic rate, chlorophyll, and carotenoid contents and enhances the salinity tolerance to plants. Moreover, PGPR has a great role in the enhancement of phytohormones and secondary metabolites synthesis in plants under salt stress. This review summarizes the current reports of the application of PGPR in plants under salt stress and discusses the PGPR-mediated mechanisms in plants of salt tolerance. This review also discusses the potential role of PGPR in cross-talk with phytohormones and secondary metabolites to alleviate salt stress and highlights the research gaps where further research is needed.

     

Free radical scavenging and antioxidant potential of mangrove plants: a review

Free radicals derived from reactive oxygen species and reactive nitrogen species are generated in our body by normal cellular metabolism which is enhanced under stress conditions. The most vulnerable biological targets of free radicals are cell structures including proteins, lipids and nucleic acids. Since antioxidants synthesized in the body are not sufficient under oxidative stress, their exogenous supply is important to prevent the body from free radical-induced injury. Recent researches have shown that antioxidants of plant origin with free radical scavenging property could have great importance as therapeutic agents in management of oxidative stress. Mangrove plants growing in inhospitable environment of the intertidal regions of land and sea in tropics and sub-tropics are equipped with very efficient free radical scavenging system to withstand the variety of stress conditions. These mangrove plants possess variety of phytochemical and are rich in phenolic compounds such as flavonoids, isoflavones, flavones, anthocyanins, coumarins, lignans, catechins, isocatechins, etc., which served as source of antioxidants. Isolation and identification of these antioxidant compounds offer great potential for their pharmaceutical exploitations. However, no comprehensive literature is available on antioxidants’ studies in mangrove plants in particular. Hence, the present review discusses the antioxidant potential of mangrove plants with its specific role under salt stress as well as the progress made so far in evaluation of antioxidant activities of different mangrove species.     

Crop Improvement and Abiotic Stress Tolerance Promoted by Moringa Leaf Extract

Moringa leaf extract (MLE) has been shown to promote beneficial outcomes in animals and plants. It is rich in amino acids, antioxidants, phytohormones, minerals, and many other bioactive compounds with nutritional and growth-promoting potential. Recent reports indicated that MLE improved abiotic stress tolerance in plants. Our understanding of the mechanisms underlying MLE-mediated abiotic stress tolerance remains limited. This review summarizes the existing literature on the role of MLE in promoting plant abiotic stress acclimation processes. MLE is applied to plants in a variety of ways, including foliar spray, rooting media, and seed priming. Exogenous application of MLE promoted crop plant growth, photosynthesis, and yield under both nonstress and abiotic stress conditions. MLE treatment reduced the severity of osmotic and oxidative stress in plants by regulating osmolyte accumulation, antioxidant synthesis, and secondary metabolites. MLE also improves mineral homeostasis in the presence of abiotic stress. Overall, this review describes the potential mechanisms underpinning MLE-mediated stress tolerance.     

Plant sugars are crucial players in the oxidative challenge during abiotic stress: extending the traditional concept

Plants suffering from abiotic stress are commonly facing an enhanced accumulation of reactive oxygen species (ROS) with damaging as well as signalling effects at organellar and cellular levels. The outcome of an environmental challenge highly depends on the delicate balance between ROS production and scavenging by both enzymatic and metabolic antioxidants. However, this traditional classification is in need of renewal and reform, as it is becoming increasingly clear that soluble sugars such as disaccharides, raffinose family oligosaccharides and fructans – next to their associated metabolic enzymes – are strongly related to stress‐induced ROS accumulation in plants. Therefore, this review aims at extending the current concept of antioxidants functioning during abiotic stress, with special focus on the emanate role of sugars as true ROS scavengers. Examples are given based on their cellular location, as different organelles seem to exploit distinct mechanisms. Moreover, the vacuole comes into the picture as important player in the ROS signalling network of plants. Elucidating the interplay between the mechanisms controlling ROS signalling during abiotic stress will facilitate the development of strategies to enhance crop tolerance to stressful environmental conditions.     

Unraveling salt stress signaling in plants

Salt stress is a major environmental factor limiting plant growth and productivity. A better understanding of the mechanisms mediating salt resistance will help researchers design ways to improve crop performance under adverse environmental conditions. Salt stress can lead to ionic stress, osmotic stress and secondary stresses, particularly oxidative stress, in plants. Therefore,to adapt to salt stress, plants rely on signals and pathways that re-establish cellular ionic, osmotic, and reactive oxygen species(ROS) homeostasis. Over the past two decades, genetic and biochemical analyses have revealed several core stress signaling pathways that participate in salt resistance. The Salt Overly Sensitive signaling pathway plays a key role in maintaining ionic homeostasis,via extruding sodium ions into the apoplast. Mitogenactivated protein kinase cascades mediate ionic, osmotic,and ROS homeostasis. SnR K2(sucrose nonfermenting1-related protein kinase 2) proteins are involved in maintaining osmotic homeostasis. In this review, we discuss recent progress in identifying the components and pathways involved in the plant's response to salt stress and their regulatory mechanisms. We also review progress in identifying sensors involved in salt-induced stress signaling in plants.     

Induction of phosphatidylinositol 3-kinase-mediated endocytosis by salt stress leads to intracellular production of reactive oxygen species and salt tolerance

Salt imposes immediate problems for plant cells, such as osmotic stress, impaired ion homeostasis and sodium toxicity, followed by a secondary oxidative stress caused by generation of reactive oxygen species (ROS). Here, we analyzed the production of ROS during salt stress. We show that salt stress triggered plasma membrane internalization, resulting in the production of ROS within endosomes. The intracellular ROS were produced by NADPH oxidase in response to the ionic but not the osmotic stress. Both endocytosis and ROS production were suppressed in phosphatidylinositol (PtdIns) 3-kinase (PI3K) mutants, PI3K being a key regulator of vesicle trafficking in animals and plants, and by wortmannin, which is a specific inhibitor of PI3K and PI4K. Endocytosis and the production of ROS were rescued by supplementation of seedlings with exogenous PtdIns 3-phosphate (PtdIns3P), less with PtdIns4P, but not with PtdIns(4,5)P(2). Surprisingly, despite reduced oxidative stress, the mutants and the wortmannin-treated plants exhibited a phenotype overly sensitive to salt, as also resulted from treatment with diphenyleneiodonium, a suicide inhibitor of NADPH oxidase, suggesting a positive role for ROS in salt tolerance. In summary, our results show that salt stress responses, such as increased plasma membrane endocytosis and the intracellular production of ROS, are coordinated by phospholipid-regulated signaling pathways, and suggest that ROS act in the signal transduction of the salt tolerance response.     

Reactive oxygen species and temperature stresses: A delicate balance between signaling and destruction

Temperature stress can have a devastating effect on plant metabolism, disrupting cellular homeostasis, and uncoupling major physiological processes. A direct result of stress-induced cellular changes is the enhanced accumulation of toxic compounds in cells that include reactive oxygen species (ROS). Although a considerable amount of work has shown a direct link between ROS scavenging and plant tolerance to temperature stress, recent studies have shown that ROS could also play a key role in mediating important signal transduction events. Thus, ROS, such as superoxide (O₂^–), are produced by NADPH oxidases during abiotic stress to activate stress-response pathways and induce defense mechanisms. The rates and cellular sites of ROS production during temperature stress could play a central role in stress perception and protection. ROS levels, as well as ROS signals, are thought to be controlled by the ROS gene network of plants. It is likely that in plants this network is interlinked with the different networks that control temperature stress acclimation and tolerance. In this review paper, we attempt to summarize some of the recent studies linking ROS and temperature stress in plants and propose a model for the involvement of ROS in temperature stress sensing and defense.     

植物中活性氧的产生及清除机制

环境胁迫使植物细胞中积累大量的活性氧,从而导致蛋白质、膜脂、DNA及其它细胞组分的严重损伤。植物体内有效清除活性氧的保护机制分为酶促和非酶促两类。酶促脱毒系统包括超氧化物歧化酶(SOD)、抗坏血酸过氧化物酶(APX)、过氧化氢酶(CAT)和谷胱甘肽过氧化物酶(GPX)等。非酶类抗氧化剂包括抗坏血酸、谷胱甘肽、甘露醇和类黄酮。利用基因工程策略增加这些物质在植物体内的含量,从而获得耐逆转基因植物已取得一定的进展。     

Effects of reactive oxygen species and interplay of antioxidants during physical exercise in skeletal muscles

A large number of researches have led to a substantial growth of knowledge about exercise and oxidative stress. Initial investigations reported that physical exercise generates free radical-mediated damages to cells; however, in recent years, studies have shown that regular exercise can upregulate endogenous antioxidants and reduce oxidative damage. Yet, strenuous exercise perturbs the antioxidant system by increasing the reactive oxygen species (ROS) content. These alterations in the cellular environment seem to occur in an exercise type-dependent manner. The source of ROS generation during exercise is debatable, but now it is well established that both contracting and relaxing skeletal muscles generate reactive oxygen species and reactive nitrogen species. In particular, exercises of higher intensity and longer duration can cause oxidative damage to lipids, proteins, and nucleotides in myocytes. In this review, we summarize the ROS effects and interplay of antioxidants in skeletal muscle during physical exercise. Additionally, we discuss how ROS-mediated signaling influences physical exercise in antioxidant system.     

Redox proteomics: basic principles and future perspectives for the detection of protein oxidation in plants

The production and scavenging of chemically reactive species, such as ROS/RNS, are central to a broad range of biotic and abiotic stress and physiological responses in plants. Among the techniques developed for the identification of oxidative stress-induced modifications on proteins, the so-called 'redox proteome', proteomics appears to be the best-suited approach. Oxidative or nitrosative stress leaves different footprints in the cell in the form of different oxidatively modified components and, using the redox proteome, it will be possible to decipher the potential roles played by ROS/RNS-induced modifications in stressed cells. The purpose of this review is to present an overview of the latest research endeavours in the field of plant redox proteomics to identify the role of post-translational modifications of proteins in developmental cell stress. All the strategies set up to analyse the different oxidized/nitrosated amino acids, as well as the different reactivities of ROS and RNS for different amino acids are revised and discussed. A growing body of evidence indicates that ROS/RNS-induced protein modifications may be of physiological significance, and that in some cellular stresses they may act causatively and not arise as a secondary consequence of cell damage. Thus, although previously the oxidative modification of proteins was thought to represent a detrimental process in which the modified proteins were irreversibly inactivated, it is now clear that, in plants, oxidatively/nitrosatively modified proteins can be specific and reversible, playing a key role in normal cell physiology. In this sense, redox proteomics will have a central role in the definition of redox molecular mechanisms associated with cellular stresses.     

Oxidative stress as a mediator of life history trade-offs: mechanisms, measurements and interpretation

The concept of trade-offs is central to our understanding of life-history evolution. The underlying mechanisms, however, have been little studied. Oxidative stress results from a mismatch between the production of damaging reactive oxygen species (ROS) and the organism's capacity to mitigate their damaging effects. Managing oxidative stress is likely to be a major determinant of life histories, as virtually all activities generate ROS. There is a recent burgeoning of interest in how oxidative stress is related to different components of animal performance. The emphasis to date has been on immediate or short-term effects, but there is an increasing realization that oxidative stress will influence life histories over longer time scales. The concept of oxidative stress is currently used somewhat loosely by many ecologists, and the erroneous assumption often made that dietary antioxidants are necessarily the major line of defence against ROS-induced damage. We summarize current knowledge on how oxidative stress occurs and the different methods for measuring it, and highlight where ecologists can be too simplistic in their approach. We critically review the potential role of oxidative stress in mediating life-history trade-offs, and present a framework for formulating appropriate hypotheses and guiding experimental design. We indicate throughout potentially fruitful areas for further research.     

Plant responses to water stress: Role of reactive oxygen species

Terrestrial plants most often encounter drought stress because of erratic rainfall which has become compounded due to present climatic changes.Responses of plants to water stress may be assigned as either injurious change or tolerance index. One of the primary and cardinal changes in response to drought stress is the generation of reactive oxygen species (ROS), which is being considered as the cause of cellular damage. However, recently a signaling role of such ROS in triggering the ROS scavenging system that may confer protection or tolerance against stress is emerging. Such scavenging system consists of antioxidant enzymes like SOD, catalase and peroxidases, and antioxidant compounds like ascorbate, reduced glutathione; a balance between ROS generation and scavenging ultimately determines the oxidative load. As revealed in case of defence against pathogen, signaling via ROS is initiated by NADPH oxidase-catalyzed superoxide generation in the apoplastic space (cell wall) followed by conversion to hydrogen peroxide by the activity of cell wall-localized SOD. Wall peroxidase may also play role in ROS generation for signaling. Hydrogen peroxide may use Ca2+ and MAPK pathway as downstream signaling cascade. Plant hormones associated with stress responses like ABA and ethylene play their role possibly via a cross talk with ROS towards stress tolerance, thus projecting a dual role of ROS under drought stress.     

The antioxidant system in Suaeda salsa under salt stress

ABSTRACTSuaeda salsaL. is a typical euhalophyte and is widely distributed throughout the world. Suaeda plants are important halophyte resources, and the physiological and biochemical characteristics of their various organsand their response to salt stress have been intensively studied. Leaf succulence, intracellular ion localization, increased osmotic regulation and enhanced antioxidant capacities are important responses for Suaeda plants to adapt to salt stress. Among these responses, scavenging of reactive oxygen species (ROS) is an important mechanism for plants to withstand oxidative stress and improve salt tolerance. The generation and scavenging pathways of ROS, as well as the expression of scavenging enzymes change under salt stress. This article reviews the antioxidant system constitute of S. salsa, and the mechanisms by which S. salsaantioxidant capacity is improved for salt tolerance. In addition, the differences between types of antioxidant mechanisms in S. salsaare reviewed, thereby revealing the adaptation mechanisms of Suaeda to different habitats. The review provides important clues for the comprehensive understanding of the salt tolerance mechanisms of halophytes.KEYWORDS: Suaeda salsa, halophyte, salt-tolerance mechanism, oxidative stress, antioxidant system     

Reactive Oxygen Species and Antioxidants in Plants: An Overview

Plants exposed to biotic and abiotic stresses generate more reactive oxygen species (ROS) than their capacity to scavenge them. Biological molecules are susceptible to attack by ROS, including several proteins, polyunsaturated fatty acids and nucleic acids. The cellular arsenal for scavenging ROS and toxic organic radicals include ascorbate, glutathione, tocopherol, carotenoids, polyphenols, alkaloids and other compounds. Enzymatic antioxidants including superoxide dismutase, peroxidase, catalase and glutathione reductase detoxify either by quenching toxic compounds or regenerating antioxidants involving reducing power. Various aspects relating to sensors for ROS and signaling role of ROS in plants, improvement of antioxidant systems in transgenic plants and functional genomics approaches used to unravel the reactive oxygen gene network has been discussed.     

Comparative Analysis of Physio-Biochemical Responses to Cold Stress in Tetraploid and Hexaploid Wheat

The cellular changes induced by cold stress (CS) include responses that lead to oxidative stress and limits plant growth, metabolism, and productivity. In this study, responses of physio-biochemical to CS phases were comparatively studied in three genotypes of bread and durum wheats differing in sensitivity, two of them (Norstar, bread wheat and Gerdish, durum wheat) were tolerant to CS and the other one, SRN (durum wheat) was sensitive to CS. 14-day-old seedlings were subjected to CS (12 and 24 h) with or without cold acclimation (CA) phase. During CS, the elevated levels of electrolyte leakage index, contents of hydrogen peroxide (H₂O₂), and malondialdehyde in Norstar and Gerdish were lower than that of SRN plants. Positive correlation and co-regulation of reactive oxygen species (ROS) scavenging systems, superoxide dismutase, catalase, ascorbate peroxidase, guaiacol peroxidase, and proline especially after CA phase suggested crucial role for holding back toxic ROS levels in CS phase. However, different activities of this system resulted in different intensities of oxidative stress in acclimated and non-acclimated plants. Our results showed that a CA phase induced oxidative stress tolerance by modulating antioxidative systems. These responses confirmed the existence of wide range of genetic capacity in durum wheat to increase cold tolerance particularly in Gerdish so that the sensitivity of SRN plants to CS was linearly correlated with the more decrease of antioxidant systems. These general responses may be a sign for associating other metabolites or enzymes activities to create partly tolerance against cold-induced oxidative stress. Eventually, assessing the dynamic of cell responses in short intervals after CS without CA phases profitably could be a novel path in plant stress response investigations in a short period of time.     

Transgenic plants: An insight into oxidative stress tolerance mechanisms

Environmental stresses considerably limit plant productivity. At the molecular level the negative effect of stress is often mediated by reactive oxygen species-initiated oxidative damage. Hence, it was hypothesised that increased tolerance to several environmental constraints could be achieved through enhanced tolerance to oxidative stress. In recent years much effort has been undertaken to improve oxidative stress tolerance by transforming plants with native or bacterial genes coding either for reactive oxygen species-scavenging enzymes or for enzymes modulating the cellular antioxidant capacity. This review deals with data on transgenic plants with altered antioxidant capacity and focuses on the new insight into the antioxidant defence mechanism given by this type of experimental model.