David E. Salt

Phytoremediation: a novel strategy for the removal of toxic metals from the environment using 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) Rhizoflltration—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 hi soils. Biological mechanisms of toxic metal uptake, translocation and resistance as well as strategies for improving phytoremediation are also discussed.

Genome-wide association study of 107 phenotypes in Arabidopsis thaliana inbred lines

Although pioneered by human geneticists as a potential solution to the challenging problem of finding the genetic basis of common human diseases, genome-wide association (GWA) studies have, owing to advances in genotyping and sequencing technology, become an obvious general approach for studying the genetics of natural variation and traits of agricultural importance. They are particularly useful when inbred lines are available, because once these lines have been genotyped they can be phenotyped multiple times, making it possible (as well as extremely cost effective) to study many different traits in many different environments, while replicating the phenotypic measurements to reduce environmental noise. Here we demonstrate the power of this approach by carrying out a GWA study of 107 phenotypes in Arabidopsis thaliana, a widely distributed, predominantly self-fertilizing model plant known to harbour considerable genetic variation for many adaptively important traits. Our results are dramatically different from those of human GWA studies, in that we identify many common alleles of major effect, but they are also, in many cases, harder to interpret because confounding by complex genetics and population structure make it difficult to distinguish true associations from false. However, a-priori candidates are significantly over-represented among these associations as well, making many of them excellent candidates for follow-up experiments. Our study demonstrates the feasibility of GWA studies in A. thaliana and suggests that the approach will be appropriate for many other organisms.

Mechanisms of Cadmium Mobility and Accumulation in Indian Mustard

Indian mustard (Brassica juncea L.), a high biomass crop plant, accumulated substantial amounts of cadmium, with bioaccumulation coefficients (concentration of Cd in dry plant tissue/concentration in solution) of up to 1100 in shoots and 6700 in roots at nonphytotoxic concentrations of Cd (0.1 [mu]g/mL) in solution. This was associated with a rapid accumulation of phytochelatins in the root, where the majority of the Cd was coordinated with sulfur ligands, probably as a Cd-S4 complex, as demonstrated by x-ray absorption spectroscopy. In contrast, Cd moving in the xylem sap was coordinated predominantly with oxygen or nitrogen ligands. Cd concentrations in the xylem sap and the rate of Cd accumulation in the leaves displayed similar saturation kinetics, suggesting that the process of Cd transport from solution through the root and into the xylem is mediated by a saturable transport system(s). However, Cd translocation to the shoot appeared to be driven by transpiration, since ABA dramatically reduced Cd accumulation in leaves. Within leaves, Cd was preferentially accumulated in trichomes on the leaf surface, and this may be a possible detoxification mechanism.

Phytoremediation of metals: using plants to remove pollutants from the environment.

Phytoremediation uses plants to remove pollutants from the environment. The use of metal-accumulating plants to clean soil and water contaminated with toxic metals is the most rapidly developing component of this environmentally friendly and cost-effective technology. The recent discovery that certain chelating agents greatly facilitate metal uptake by soil-grown plants can make this technology a commercial reality in the near future.

Bioconcentration of heavy metals by plants

Abstract Certain plants can concentrate essential and non-essential heavy metals in their roots and shoots to levels far exceeding those present in the soil. Metal-accumulating plant species are invariably restricted to metalliferous soils found in different regions around the world. The mechanisms of metal accumulation, which involve extracellular and intracellular metal chelation, precipitation, compartmentalization and translocation in the vascular system, are poorly understood. Interest in these mechanisms has led to the development of phytoremediation—a new technology to use plants to clean up soil and water contaminated with heavy metals.

Engineering tolerance and hyperaccumulation of arsenic in plants by combining arsenate reductase and γ-glutamylcysteine synthetase expression

We have developed a genetics-based phytoremediation strategy for arsenic in which the oxyanion arsenate is transported aboveground, reduced to arsenite, and sequestered in thiol–peptide complexes. The Escherichia coli arsC gene encodes arsenate reductase (ArsC), which catalyzes the glutathione (GSH)-coupled electrochemical reduction of arsenate to the more toxic arsenite. Arabidopsis thaliana plants transformed with the arsC gene expressed from a light-induced soybean rubisco promoter (SRS1p) strongly express ArsC protein in leaves, but not roots, and were consequently hypersensitive to arsenate. Arabidopsis plants expressing the E. coli gene encoding γ-glutamylcysteine synthetase (γ-ECS) from a strong constitutive actin promoter (ACT2p) were moderately tolerant to arsenic compared with wild type. However, plants expressing SRS1p/ArsC and ACT2p/γ-ECS together showed substantially greater arsenic tolerance than γ-ECS or wild-type plants. When grown on arsenic, these plants accumulated 4- to 17-fold greater fresh shoot weight and accumulated 2- to 3-fold more arsenic per gram of tissue than wild type or plants expressing γ-ECS or ArsC alone. This arsenic remediation strategy should be applicable to a wide variety of plant species.

Increased Glutathione Biosynthesis Plays a Role in Nickel Tolerance in Thlaspi Nickel Hyperaccumulators

Worldwide more than 400 plant species are now known that hyperaccumulate various trace metals (Cd, Co, Cu, Mn, Ni, and Zn), metalloids (As) and nonmetals (Se) in their shoots. Of these, almost one-quarter are Brassicaceae family members, including numerous Thlaspi species that hyperaccumulate Ni up to 3% of there shoot dry weight. We observed that concentrations of glutathione, Cys, and O-acetyl-l-serine (OAS), in shoot tissue, are strongly correlated with the ability to hyperaccumulate Ni in various Thlaspi hyperaccumulators collected from serpentine soils, including Thlaspi goesingense, T. oxyceras, and T. rosulare, and nonaccumulator relatives, including T. perfoliatum, T. arvense, and Arabidopsis thaliana. Further analysis of the Austrian Ni hyperaccumulator T. goesingense revealed that the high concentrations of OAS, Cys, and GSH observed in this hyperaccumulator coincide with constitutively high activity of both serine acetyltransferase (SAT) and glutathione reductase. SAT catalyzes the acetylation of l-Ser to produce OAS, which acts as both a key positive regulator of sulfur assimilation and forms the carbon skeleton for Cys biosynthesis. These changes in Cys and GSH metabolism also coincide with the ability of T. goesingense to both hyperaccumulate Ni and resist its damaging oxidative effects. Overproduction of T. goesingense SAT in the nonaccumulator Brassicaceae family member Arabidopsis was found to cause accumulation of OAS, Cys, and glutathione, mimicking the biochemical changes observed in the Ni hyperaccumulators. In these transgenic Arabidopsis, glutathione concentrations strongly correlate with increased resistance to both the growth inhibitory and oxidative stress induced effects of Ni. Taken together, such evidence supports our conclusion that elevated GSH concentrations, driven by constitutively elevated SAT activity, are involved in conferring tolerance to Ni-induced oxidative stress in Thlaspi Ni hyperaccumulators.

MgATP-Dependent Transport of Phytochelatins Across the Tonoplast of Oat Roots.

In Cd-exposed oat (Avena sativa) roots Cd was found to be associated primarily with the phytochelatin ([gamma]-glutamylcysteinyl)3-glutamic acid [([gamma]EC)3G], with a peptide to Cd ratio of 1:3 (cysteine to Cd ratio of 1:1), even though both ([gamma]EC)2G and ([gamma]EC)3G were present in the roots. Phytochelatins are known to accumulate in the vacuoles of plant cells on exposure to Cd, but the mechanism is not clear. Here we present evidence for the transport of the phytochelatins ([gamma]EC)3G and ([gamma]EC)2G as well as the Cd complex Cd-([gamma]EC)3G across the tonoplast of oat roots. Transport of ([gamma]EC)3G had a Km, for MgATP of 0.18 mM and a Vmax of 0.7 to 1 nmol mg-1 protein min-1. Transport of ([gamma]EC)3G was also energized by MgGTP and to a lesser extent MgUTP and was highly sensitive to orthovanadate, with a 50%-inhibitory concentration of 0.9 [mu]M. The Cd complex Cd-([gamma]EC)3G and ([gamma]EC)2G were also transported in a MgATP-dependent, vanadate-sensitive manner. Therefore, this process is a candidate for the transport of both phytochelatins, and Cd as its peptide complex, from the cytoplasm into the vacuole.

Selenium uptake, translocation, assimilation and metabolic fate in plants

The chemical and physical resemblance between selenium (Se) and sulfur (S) establishes that both these elements share common metabolic pathways in plants. The presence of isologous Se and S compounds indicates that these elements compete in biochemical processes that affect uptake, translocation and assimilation throughout plant development. Yet, minor but crucial differences in reactivity and other metabolic interactions infer that some biochemical processes involving Se may be excluded from those relating to S. This review examines the current understanding of physiological and biochemical relationships between S and Se metabolism by highlighting their similarities and differences in relation to uptake, transport and assimilation pathways as observed in Se hyperaccumulator and non-accumulator plant species. The exploitation of genetic resources used in bioengineering strategies of plants is illuminating the function of sulfate transporters and key enzymes of the S assimilatory pathway in relation to Se accumulation and final metabolic fate. These strategies are providing the basic framework by which to resolve questions relating to the essentiality of Se in plants and the mechanisms utilized by Se hyperaccumulators to circumvent toxicity. In addition, such approaches may assist in the future application of genetically engineered Se accumulating plants for environmental renewal and human health objectives.

Plants, selenium and human health

Selenium is an essential nutrient for animals, microorganisms and some other eukaryotes. Although selenium has not been demonstrated to be essential in vascular plants, the ability of some plants to accumulate and transform selenium into bioactive compounds has important implications for human nutrition and health, and for the environment. Selenium-accumulating plants provide unique tools to help us understand selenium metabolism. They are also a source of genetic material that can be used to alter selenium metabolism and tolerance to help develop food crops that have enhanced levels of anticarcinogenic selenium compounds, as well as plants that are ideally suited for the phytoremediation of selenium-contaminated soils.