Reactive oxygen species (ROS) are produced as a normal product of plant cellular metabolism. Various environmental stresses lead to excessive production of ROS causing progressive oxidative damage and ultimately cell death. Despite their destructive activity, they are well-described second messengers in a variety of cellular processes, including conferment of tolerance to various environmental stresses. Whether ROS would serve as signaling molecules or could cause oxidative damage to the tissues depends on the delicate equilibrium between ROS production, and their scavenging. Efficient scavenging of ROS produced during various environmental stresses requires the action of several nonenzymatic as well as enzymatic antioxidants present in the tissues. In this paper, we describe the generation, sites of production and role of ROS as messenger molecules as well as inducers of oxidative damage. Further, the antioxidative defense mechanisms operating in the cells for scavenging of ROS overproduced under various stressful conditions of the environment have been discussed in detail.

https://downloads.hindawi.com/archive/2012/217037.pdf

Reactive Oxygen Species, Oxidative Damage, and Antioxidative Defense Mechanism in Plants under Stressful Conditions

Metal contamination issues are becoming increasingly common in India and elsewhere, with many documented cases of metal toxicity in mining industries, foundries, smelters, coal-burning power plants and agriculture. Heavy metals, such as cadmium, copper, lead, chromium and mercury are major environmental pollutants, particularly in areas with high anthropogenic pressure. Heavy metal accumulation in soils is of concern in agricultural production due to the adverse effects on food safety and marketability, crop growth due to phytotoxicity, and environmental health of soil organisms. The influence of plants and their metabolic activities affects the geological and biological redistribution of heavy metals through pollution of the air, water and soil. This article details the range of heavy metals, their occurrence and toxicity for plants. Metal toxicity has high impact and relevance to plants and consequently it affects the ecosystem, where the plants form an integral component. Plants growing in metal-polluted sites exhibit altered metabolism, growth reduction, lower biomass production and metal accumulation. Various physiological and biochemical processes in plants are affected by metals. The contemporary investigations into toxicity and tolerance in metal-stressed plants are prompted by the growing metal pollution in the environment. A few metals, including copper, manganese, cobalt, zinc and chromium are, however, essential to plant metabolism in trace amounts. It is only when metals are present in bioavailable forms and at excessive levels, they have the potential to become toxic to plants. This review focuses mainly on zinc, cadmium, copper, mercury, chromium, lead, arsenic, cobalt, nickel, manganese and iron.

Heavy metals, occurrence and toxicity for plants: a review

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

Response to cadmium in higher plants

Toxic metal pollution of waters and soils is a major environmental problem, and most conventional remediation approaches do not provide acceptable solutions. The use of specially selected and engineered metal-accumulating plants for environmental clean-up is an emerging technology called phytoremediation. Three subsets of this technology are applicable to toxic metal remediation: (1) Phytoextraction--the use of metal-accumulating plants to remove toxic metals from soil; (2) Rhizofiltration--the use of plant roots to remove toxic metals from polluted waters; and (3) Phytostabilization--the use of plants to eliminate the bioavailability of toxic metals in soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

Phytoremediation is emerging as a potential cost-effective solution for the remediation of contaminated soils. Because contaminants such as lead (Pb) have limited bioavailability in the soil, a means of solubilizing the Pb in the soil and facilitating its transport to the shoots of plants is vital to the success of phytoremediation. Indian mustard (Brassica juncea) was used to demonstrate the capability of plants to accumulate high tissue concentrations of Pb when grown in Pb-contaminated soil. Concentrations of 1.5% Pb in the shoots of B. juncea were obtained from soils containing 600 mg of Pb/kg amended with synthetic chelates such as EDTA. The accumulation of Pb in the tissue corresponded to the concentration of Pb in the soil and the concentration of EDTA added to the soil. The accumulation of Cd, Cu, Ni, and Zn from contaminated soil amended with EDTA and other synthetic chelators was also demonstrated. The research indicates that the accumulation of metal in the shoots of B. juncea can be enhanced t...

/pdf/enhanced-accumulation-of-pb-in-indian-mustard-by-soil-11mg86jnpj.pdf

Enhanced Accumulation of Pb in Indian Mustard by Soil-Applied Chelating Agents

The glycoside hydrolase family 5 endocellulase, E1 (Cel5A), from Acidothermus cellulolyticus was transformed into both Nicotiana tabacum and Zea mays with expression targeted to the cell wall under a constitutive promoter. Here we explore the possibility that in planta expression of endocellulases will allow these enzymes to access their substrates during cell wall construction, rendering cellulose more amenable to pretreatment and enzyme digestion. Tobacco and maize plants were healthy and developed normally compared with the wild type (WT). After thermochemical pretreatment and enzyme digestion, transformed plants were clearly more digestible than WT, requiring lower pretreatment severity to achieve comparable conversion levels. Furthermore, the decreased recalcitrance was not due to post-pretreatment residual E1 activity and could not be reproduced by the addition of exogenous E1 to the biomass prior to pretreatment, indicating that the expression of E1 during cell wall construction altered the inherent recalcitrance of the cell wall.

/pdf/in-planta-expression-of-a-cellulolyticus-cel5a-endocellulase-266dv9ptog.pdf

In planta expression of A. cellulolyticus Cel5A endocellulase reduces cell wall recalcitrance in tobacco and maize.

Uranium phytoextraction, the use of plants to extract U from contaminated soils, is an emerging technology. We report on the development of this technology for the cleanup of U-contaminated soils. In this research, we investigated the effects of various soil amendments on U desorption from soil to soil solution, studied the physi ological characteristics of U uptake and accumulation in plants, and developed techniques to trigger U hyperac cumulation in plants. A key to the success of U phytoextraction is to increase soil U availability to plants. We have found that some organic acids can be added to soils to increase U desorption from soil to soil solution and to trigger a rapid U accumulation in plants. Of the organic acids (acetic acid, citric acid, and malic acid) tested, citric acid was the most effective in enhancing U accumulation in plants. Shoot U concentrations of Brassica juncea and Brassica chinensis grown in a U-contaminated soil (total soil U, 750 mg kg-1) increased from less than 5 mg kg-1 t...

Phytoremediation of Uranium-Contaminated Soils: Role of Organic Acids in Triggering Uranium Hyperaccumulation in Plants

Previous studies have shown that EDTA is necessary to solubilize soil Pb and facilitate its transport from the soil to the above ground plant tissues These studies have also suggested that Pb is accumulated in the plant tissue with transpiration as the driving force We conducted further studies to evaluate the relationship between EDTA soil treatment, plant transpiration, and plant accumulation of Pb and EDTA Indian mustard (Brassica juncea) plants were grown in soils containing Pb at three different concentrations (15, 30 and 48 mmol/kg) for 5 weeks before being treated with EDTA concentrations ranging from 0 to 10 mmol/kg Plant shoots and xylem sap were collected and analyzed for Pb and EDTA content using ICP and HPLC, respectively Water loss was measured for 7 days following EDTA application Transpiration was not affected at <5 mmol/kg EDTA but, at 10 mmol/kg EDTA transpiration decreased by 80%, whereas accumulation of Pb and EDTA increased In the Sassafras soil, Pb and EDTA accumulation in the plant shoots continued to increase as the applied EDTA concentration increased, except at the highest level (10 mmol/kg) In soil amended with 48 mmol/kg Pb and 10 mmol/kg EDTA, the concentrations of EDTA and Pb in shoots decreased and visible signs of phytotoxicity were observed The results presented herein support recent studies in hydroponic systems showing that EDTA and Pb are taken up by the plant and suggest that Pb is translocated in the plant as the Pb-EDTA complex The results also show that the maximum Pb accumulation by plants occurs by maximizing the concentration of the Pb-EDTA complex based on the EDTA extractable soil Pb

Michael J. Blaylock

Papers

Phytoremediation: A Novel Strategy for the Removal of Toxic Metals from the Environment Using Plants

Enhanced Accumulation of Pb in Indian Mustard by Soil-Applied Chelating Agents

In planta expression of A. cellulolyticus Cel5A endocellulase reduces cell wall recalcitrance in tobacco and maize.

Phytoremediation of Uranium-Contaminated Soils: Role of Organic Acids in Triggering Uranium Hyperaccumulation in Plants

EDTA and Pb-EDTA accumulation in Brassica juncea grown in Pb-amended soil