Reactive oxygen species (ROS) were initially recognized as toxic by-products of aerobic metabolism. In recent years, it has become apparent that ROS plays an important signaling role in plants, controlling processes such as growth, development and especially response to biotic and abiotic environmental stimuli. The major members of the ROS family include free radicals like O2● −, OH● and non-radicals like H2O2 and 1O2. The ROS production in plants is mainly localized in the chloroplast, mitochondria and peroxisomes. There are secondary sites as well like the endoplasmic reticulum, cell membrane, cell wall and the apoplast. The role of the ROS family is that of a double edged sword; while they act as secondary messengers in various key physiological phenomena, they also induce oxidative damages under several environmental stress conditions like salinity, drought, cold, heavy metals, UV irradiation etc., when the delicate balance between ROS production and elimination, necessary for normal cellular homeostasis, is disturbed. The cellular damages are manifested in the form of degradation of biomolecules like pigments, proteins, lipids, carbohydrates and DNA, which ultimately amalgamate in plant cellular death. To ensure survival, plants have developed efficient antioxidant machinery having two arms, (i) enzymatic components like superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), guaiacol peroxidase (GPX), glutathione reductase (GR), monodehydroascorbate reductase (MDHAR) and dehydroascorbate reductase (DHAR); (ii) non-enzymatic antioxidants like ascorbic acid (AA), reduced glutathione (GSH), α-tocopherol, carotenoids, flavonoids and the osmolyte proline. These two components work hand in hand to scavenge ROS. In this review, we emphasize on the different types of ROS, their cellular production sites, their targets, and their scavenging mechanism mediated by both the branches of the antioxidant systems, highlighting the potential role of antioxidant

Reactive oxygen species (ROS) and response of antioxidants as ROS-scavengers during environmental stress in plants

Preface.- Contributors.- List of Abbreviations.- Section 1: Basic Principles: Introduction.-Sources of Heavy Metals and Metalloids in Soils.- Chemistry of Heavy Metals and Metalloids in Soils.- Methods for the Determination of Heavy Metals and Metalloids in Soils.- Effects of Heavy Metals and Metalloids on Soil Organisms.- Soil-Plant Relationships of Heavy Metals and Metalloids.- Heavy Metals and Metalloids as Micronutrients for Plants and Animals.-Critical Loads of Heavy Metals for Soils.- Section 2: Key Heavy Metals And Metalloids: Arsenic.- Cadmium.- Chromium and Nickel.- Cobalt and Manganese.- Copper.-Lead.- Mercury.- Selenium.- Zinc.- Section 3: Other Heavy Metals And Metalloids Of Potential Environmental Significance: Antimony.- Barium.- Gold.- Molybdenum.- Silver.- Thallium.- Tin.- Tungsten.- Uranium.- Vanadium.- Glossary of Specialized Terms.- Index.

Heavy metals in soils : trace metals and metalloids in soils and their bioavailability

The contamination of soils and water with metals has created a major environmental problem, leading to considerable losses in plant productivity and hazardous health effects. Exposure to toxic metals can intensify the production of reactive oxygen species (ROS), which are continuously produced in both unstressed and stressed plants cells. Some of the ROS species are highly toxic and must be detoxified by cellular stress responses, if the plant is to survive and grow. The aim of this review is to assess the mode of action and role of antioxidants in protecting plants from stress caused by the presence of heavy metals in the environment.

Making the life of heavy metal-stressed plants a little easier

Effect of exogenous salicylic acid under changing environment: a review

Unprecedented bioaccumulation and biomagnification of heavy metals (HMs) in the environment have become a dilemma for all living organisms including plants. HMs at toxic levels have the capability to interact with several vital cellular biomolecules such as nuclear proteins and DNA, leading to excessive augmentation of reactive oxygen species (ROS). This would inflict serious morphological, metabolic, and physiological anomalies in plants ranging from chlorosis of shoot to lipid peroxidation and protein degradation. In response, plants are equipped with a repertoire of mechanisms to counteract heavy metal (HM) toxicity. The key elements of these are chelating metals by forming phytochelatins (PCs) or metallothioneins (MTs) metal complex at the intra- and intercellular level, which is followed by the removal of HM ions from sensitive sites or vacuolar sequestration of ligand-metal complex. Nonenzymatically synthesized compounds such as proline (Pro) are able to strengthen metal-detoxification capacity of intracellular antioxidant enzymes. Another important additive component of plant defense system is symbiotic association with arbuscular mycorrhizal (AM) fungi. AM can effectively immobilize HMs and reduce their uptake by host plants via binding metal ions to hyphal cell wall and excreting several extracellular biomolecules. Additionally, AM fungi can enhance activities of antioxidant defense machinery of plants.

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Heavy Metal Stress and Some Mechanisms of Plant Defense Response

Abiotic stresses like heavy metals, drought, salt, low temperature, etc are the major factors that limit crop productivity and yield These stresses are associated with production of certain deleterious chemical entities called reactive oxygen species (ROS), which include hydrogen peroxide (H₂O₂), superoxide radical (O₂(-)), hydroxyl radical (OH(-)), etc ROS are capable of inducing cellular damage by degradation of proteins, inactivation of enzymes, alterations in the gene and interfere in various pathways of metabolic importance Our understanding on ROS in response to abiotic stress is revolutionized with the advancements in plant molecular biology, where the basic understanding on chemical behavior of ROS is better understood Understanding the molecular mechanisms involved in ROS generation and its potential role during abiotic stress is important to identify means by which plant growth and metabolism can be regulated under acute stress conditions ROS mediated oxidative stress, which is the key to understand stress related toxicity have been widely studied in many plants and the results in those studies clearly revealed that oxidative stress is the main symptom of toxicity Plants have their own antioxidant defense mechanisms to encounter ROS that is of enzymic and non-enzymic nature  Coordinated activities of these antioxidants regulate ROS detoxification and reduces oxidative load in plants Though ROS are always regarded to impart negative impact on plants, some reports consider them to be important in regulating key cellular functions; however, such reports in plant are limited Molecular approaches to understand ROS metabolism and signaling have opened new avenues to comprehend its critical role in abiotic stress ROS also acts as secondary messenger that signals key cellular functions like cell proliferation, apoptosis and necrosis In higher eukaryotes, ROS signaling is not fully understood In this review we summarize our understanding on ROS and its signaling behavior in plants under abiotic stress

https://www.tandfonline.com/doi/pdf/10.4161/psb.23681

Reactive oxygen species signaling in plants under abiotic stress

The article presents an overview of the mechanism of chromium stress in plants. Chromium is known to be a toxic metal that can cause severe damage to plants and animals. Chromium-induced oxidative stress involves induction of lipid peroxidation in plants that causes severe damage to cell membranes. Oxidative stress induced by chromium initiates the degradation of photosynthetic pigments causing decline in growth. High chromium concentration can disturb the chloroplast ultrastructure thereby disturbing the photosynthetic process. Like copper and iron, chromium is also a redox metal and its redox behaviour exceeds that of other metals like Co, Fe, Zn, Ni, etc. The redox behaviour can thus be attributed to the direct involvement of chromium in inducing oxidative stress in plants. Chromium can affect antioxidant metabolism in plants. Antioxidant enzymes like SOD, CAT, POX and GR are found to be susceptible to chromium resulting in a decline in their catalytic activities. This decline in antioxidant efficiency is an important factor in generating oxidative stress in plants under chromium stress. However, both metallothioneins and organic acids are important in plants as components of tolerance mechanisms and are also involved in detoxification of this toxic metal.

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Chromium stress in plants

The effect of short-term Pb and Cr (0, 100 and 1000μM) stress in moss Taxithelium nepalense (Schwaegr.) Broth., the possible generation of oxidative stress, antioxidant metabolism and changes in the chloroplast and cell membrane ultrastructure were investigated. In moss cells, treatment of Pb and Cr for 12 and 24 h decreased the dry mass and total chlorophyll content with marked inhibition under Pb. Both Pb and Cr accumulated after 24 h of their treatment where highest accumulation of Pb was visible than that of Cr. The ultrastructural studies at 1000 μM of Pb and Cr showed distortion of the thylakoid, distortion of chloroplast membrane and changes in the chloroplast structure. Chloroplast distortion was highly visible under Pb than that of Cr. The distortion in the cell membrane was evident at high concentration of Pb, while under Cr, minor changes were visible as compared to controls. Both Pb and Cr significantly increased the production of ROS like H2O2 and O2− radical with marked production after 24 h under Pb than that of Cr. The alteration in metabolism of activated oxygen in moss cells was evidenced by the increase in the lipid peroxidation in moss cells, with pronounced effect after 24 h than that of 12 h after Pb and Cr treatment. The SOD activity showed an increasing trend followed by decrease in CAT, POX and GR activity after 12 and 24 h. Both ascorbate and glutathione showed higher accumulation under Pb followed by Cr. The results showed that at high concentration of metals, oxidative stress could be induced in moss cells characterized by the generation of ROS and initiation lipid peroxidation that inhibited the major antioxidant metabolism. Both physiological and ultrastructural studies suggested the possible induction of oxidative stress in Taxithelium nepalense (Schwaegr.) Broth. under Pb and Cr toxicity.

Toxic effects, oxidative stress and ultrastructural changes in moss Taxithelium nepalense (Schwaegr.) broth. under chromium and lead phytotoxicity

Summary. In the present investigation we studied the role of salicylic acid in cadmium induced physiological and biochemical changes and the possible induction of oxidative stress in developing rice roots. 24h cadmium treatment resulted in the inhibition of root dry biomass, root elongation and increased cadmium accumulation in roots. Salicylic acid presoaking for 16hrs resulted in protection against cadmium, minor changes in root dry biomass, root elongation and minimal accumulation of cadmium. Cadmium applied to salicylic acid free plants increased the level of lipid peroxidation, hydrogen peroxide content and superoxide radical production. Salicylic acid treatment decreased the toxic level of cadmium manifested by lower lipid peroxidation, lesser production of hydrogen peroxide and reduction in the generation of superoxide radical. The extent of membrane integrity loss was higher in salicylic acid free plants compared to salicylic acid treated ones, under cadmium application. To test the antioxidant efficiency of various enzymes, the activity of superoxide dismutase, catalase, peroxidase and glutathione reductase were measured. Higher increase in the activity of these antioxidant enzymes was observed in salicylic acid free plants than in the salicylic treated ones. Elevated level of ascorbate and glutathione was observed in salicylic acid free plants. Glutathione, however, decreased at higher concentration. Data show the potentiating effect of salicylic acid in regulating cadmium induced oxidative stress in Oryza sativa L. roots.

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Shuvasish Choudhury

Papers

Reactive oxygen species signaling in plants under abiotic stress

Chromium stress in plants

Toxic effects, oxidative stress and ultrastructural changes in moss Taxithelium nepalense (Schwaegr.) broth. under chromium and lead phytotoxicity

Role of salicylic acid in regulating cadmium induced oxidative stress in oryza sativa l. roots

Arsenic stress in rice: redox consequences and regulation by iron.