Unravelling cadmium toxicity and tolerance in plants: Insight into regulatory mechanisms

Cadmium (Cd) toxicity has been widely studied in different plant species; however, the mechanism involved in its toxicity as well as the cell response against the metal have not been well established. In this work, using pea (Pisum sativum) plants, we studied the effect of Cd on antioxidants, reactive oxygen species (ROS), and nitric oxide (NO) metabolism of leaves using different cellular, molecular, and biochemical approaches. The growth of pea plants with 50 μm CdCl2 affected differentially the expression of superoxide dismutase (SOD) isozymes at both transcriptional and posttranscriptional levels, giving rise to a SOD activity reduction. The copper/zinc-SOD down-regulation was apparently due to the calcium (Ca) deficiency induced by the heavy metal. In these circumstances, the overproduction of the ROS hydrogen peroxide and superoxide could be observed in vivo by confocal laser microscopy, mainly associated with vascular tissue, epidermis, and mesophyll cells, and the production of superoxide radicals was prevented by exogenous Ca. On the other hand, the NO synthase-dependent NO production was strongly depressed by Cd, and treatment with Ca prevented this effect. Under these conditions, the pathogen-related proteins PrP4A and chitinase and the heat shock protein 71.2, were up-regulated, probably to protect cells against damages induced by Cd. The regulation of these proteins could be mediated by jasmonic acid and ethylene, whose contents increased by Cd treatment. A model is proposed for the cellular response to long-term Cd exposure consisting of cross talk between Ca, ROS, and NO.

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Cellular Response of Pea Plants to Cadmium Toxicity: Cross Talk between Reactive Oxygen Species, Nitric Oxide, and Calcium

When plants are exposed to stressful environmental conditions, the production of Reactive Oxygen Species (ROS) increases and can cause significant damage to the cells. Antioxidant defenses, which can detoxify ROS, are present in plants. A major hydrogen peroxide detoxifying system in plant cells is the ascorbate-glutathione cycle, in which, ascorbate peroxidase (APX) enzymes play a key role catalyzing the conversion of H2O2 into H2O, using ascorbate as a specific electron donor. Different APX isoforms are present in distinct subcellular compartments, such as chloroplasts, mitochondria, peroxisome, and cytosol. The expression of APX genes is regulated in response to biotic and abiotic stresses as well as during plant development. The APX responses are directly involved in the protection of plant cells against adverse environmental conditions. Furthermore, mutant plants APX genes showed alterations in growth, physiology and antioxidant metabolism revealing those enzymes involvement in the normal plant development.

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Plant responses to stresses: role of ascorbate peroxidase in the antioxidant protection

Pollution of the biosphere by the toxic metals is a global threat that has accelerated dramatically since the beginning of industrial revolution. The primary source of this pollution includes the industrial operations such as mining, smelting, metal forging, combustion of fossil fuels and sewage sludge application in agronomic practices. The metals released from these sources accumulate in soil and in turn, adversely affect the microbial population density and physico-chemical properties of soils, leading to the loss of soil fertility and yield of crops. The heavy metals in general cannot be biologically degraded to more or less toxic products and hence, persist in the environment. Conventional methods used for metal detoxification produce large quantities of toxic products and are cost-effective. The advent of bioremediation technology has provided an alternative to conventional methods for remediating the metal-poisoned soils. In metal-contaminated soils, the natural role of metal-tolerant plant growth promoting rhizobacteria in maintaining soil fertility is more important than in conventional agriculture, where greater use of agrochemicals minimize their significance. Besides their role in metal detoxification/removal, rhizobacteria also promote the growth of plants by other mechanisms such as production of growth promoting substances and siderophores. Phytoremediation is another emerging low-cost in situ technology employed to remove pollutants from the contaminated soils. The efficiency of phytoremediation can be enhanced by the judicious and careful application of appropriate heavy-metal tolerant, plant growth promoting rhizobacteria including symbiotic nitrogen-fixing organisms. This review presents the results of studies on the recent developments in the utilization of plant growth promoting rhizobacteria for direct application in soils contaminated with heavy metals under a wide range of agro-ecological conditions with a view to restore contaminated soils and consequently, promote crop productivity in metal-polluted soils across the globe and their significance in phytoremediation.

Role of plant growth promoting rhizobacteria in the remediation of metal contaminated soils

Selenium (Se) is an essential micronutrient for humans and animals, but lead to toxicity when taken in excessive amounts. Plants are the main source of dietary Se, but essentiality of Se for plants is still controversial. However, Se at low doses protects the plants from variety of abiotic stresses such as cold, drought, desiccation and metal stress. In animals, Se acts as an antioxidant and helps in reproduction, immune responses, thyroid hormone metabolism. Selenium is chemically similar to sulfur, hence taken up inside the plants via sulfur transporters present inside root plasma membrane, metabolized via sulfur assimilatory pathway, and volatilized into atmosphere. Selenium induced oxidative stress, distorted protein structure and function, are the main causes of Se toxicity in plants at high doses. Plants can play vital role in overcoming Se deficiency and Se toxicity in different regions of the world, hence, detailed mechanism of Se metabolism inside the plants is necessary for designing effective Se phytoremediation and biofortification strategies.

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An Overview of Selenium Uptake, Metabolism, and Toxicity in Plants.

Antioxidant metabolism of coffee cell suspension cultures in response to cadmium

Nickel elicits a fast antioxidant response in Coffea arabica cells

Selenium (Se) is an essential element for humans and animals that is required for key antioxidant reactions, but can be toxic at high concentrations. We have investigated the effect of Se in the form of selenite on coffee cell suspension cultures over a 12-day period. The antioxidant defence systems were induced in coffee cells grown in the presence of 0.05 and 0.5 mm sodium selenite (Na2SeO3). Lipid peroxidation and alterations in antioxidant enzymes were the main responses observed, including a severe reduction in ascorbate peroxidase activity, even at 0.05 mm sodium selenite. Ten superoxide dismutase (SOD) isoenzymes were detected and the two major Mn-SOD isoenzymes (bands V and VI) responded more to 0.05 mm selenite. SOD band V exhibited a general decrease in activity after 12 h of treatment with 0.05 mm selenite, whereas band VI exhibited the opposite behavior and increased in activity. An extra isoenzyme of glutathione reductase (GR) was induced in the presence of selenite, which confirmed our previous results obtained with Cd and Ni indicating that this GR isoenzyme may have the potential to be a marker for oxidative stress in coffee.

Selenium-induced oxidative stress in coffee cell suspension cultures

A presenca de metais pesados no ambiente e atualmente, um dos principais problemas de contaminacao ambiental, uma vez que, os metais liberados no ambiente contaminam o solo e entram na cadeia alimentar atraves das plantas, causando efeitos toxicos a curto e a longo prazo aos animais e seres humanos. No caso do metal pesado Niquel (Ni), foi constatado que a sua presenca nas plantas pode diminuir o crescimento, reduzir a taxa de fotossintese e provocar alteracoes, tanto nas atividades enzimaticas quanto metabolicas.Pouca informacao esta disponivel na literatura, com referencia a resposta antioxidante das plantas a expressao a esse metal. Neste sentido, os objetivos deste trabalho foram realizar diferentes ensaios para avaliar o efeito fitotoxico do Ni em plântulas de C. juncea. Neste estudo foram analisados parâmetros bioquimicos relativos a atividade das enzimas antioxidantes, Catalase (CAT), Superoxido Dismutase (SOD) e Glutationa Redutase (GR). Constatou-se que, nao houve alteracao da atividade GR e CAT nas raizes. Entretanto, atividades destas enzimas apresentaram aumento significativo na parte aerea. O aumento na atividade da GR, na parte aerea, pode ser explicado pelo fato de ser o ciclo Halliwell-Asada o principal mecanismo que age na desintoxicacao de Especies Ativas de Oxigenio. Quanto a atividade da SOD, quando comparados ambos tecidos, foi constatado que na parte aerea foi pouco alterada, entretanto, a atividade da SOD foi estimulada nas raizes na presenca do Ni. Para aumentar a precisao dos resultados, quantificou-se pela tecnica de Fluorescencia de Raios X, a concentracao de NiCl2 e o seu efeito na absorcao de nutrientes nas plântulas de crotalaria. Nesta analise, observou-se o acumulo do metal nas raizes e baixa translocacao para a parte aerea. Tambem foi analisada a concentracao de Malonaldeido (MDA) na parte aerea de plântulas de C. juncea, sendo constatada, a ocorrencia de peroxidacao lipidica na presenca do Ni. Finalmente, determinou-se o efeito do Ni sobre as concentracoes de aminoacidos soluveis em raizes e na parte aerea, como estrategia desta planta para superar o efeito do Ni , nao tendo sido observadas variacoes significativas nas concentracoes dos mesmos com os diferentes tratamentos com Ni. Palavra?Chave: Aminoacidos, Atividade Enzimatica, Crotalaria, Fitotoxicidade, Niquel, Poluicao Ambiental 
Abstract

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Rui A. Gomes-Junior

Papers

Antioxidant metabolism of coffee cell suspension cultures in response to cadmium

Nickel elicits a fast antioxidant response in Coffea arabica cells

Selenium-induced oxidative stress in coffee cell suspension cultures

Response of Crotalaria juncea to nickel exposure

Antioxidant stress responses of plants to cadmium.