Changes of Ni biogeochemistry in the rhizosphere of the hyperaccumulator Thlaspi goesingense

Processes in the rhizosphere of metal hyperaccumulator species are largely unknown. We investigated root-induced changes of Ni biogeochemistry in the rhizosphere of Thlaspi goesingense Hálácsy in a rhizobox experiment and in related soil chemical and Ni uptake studies. In the rhizobox, a root monolayer was separated from rhizosphere soil by a nylon membrane. Rhizosphere soil was then sliced into 0.5 mm layers and analyzed for changes in soluble (water-extractable, Ni _S) and labile (1 M NH ₄NO ₃-extractable, Ni _L) Ni pools. Ni _L in the rhizosphere was depleted due to excessive uptake in T. goesingense. Ni _S in the rhizosphere increased in contrast to expectations based on the experimental Ni desorption isotherm. Mathematical simulations following the Tinker–Nye–Barber approach overestimated the depletion of the Ni _L and predicted a decrease of Ni _S in the rhizosphere. In a hydroponic experiment, we demonstrated that T. goesingense takes up Ni ²⁺ but excludes metal–organic complexes. The model output was then improved in later versions considering this finding. A sensitivity analysis identified I _max and K _m, derived from the Michaelis–Menten uptake kinetics experiment to be the most sensitive of the model parameters. The model was also sensitive to the accuracy of the estimate of the initial Ni concentration (C _Si) in soil solution. The formation of Ni–DOM complexes in solution could not explain the poor fit as in contrast to previous field experiments, the correlation between soluble Ni and dissolved organic carbon (DOC) was weak. Ion competition of Ni with Ca and Mg could be ruled out as explanation of enhanced Ni solubility in the rhizosphere as the molar ratio of Ni/(Ca + Mg) in solution was not affected. However, a decreased Vanselov coefficient Kv near the root plane indicated (an apparent) lower selectivity of the exchange complex for Ni, possibly due to adsorption of oxalate exuded by T. goesingense roots or associated rhizosphere microbes. This conclusion is supported by field data, showing enhanced oxalate concentrations in the rhizosphere of T. goesingense on the same experimental soil. The implications for phytoextraction and bio-available contaminant stripping (BCS) as well as for future modeling and experimental work are discussed.     

Evaluation of lead and chromium tolerance and accumulation level in Gomphrena celosoides: a novel metal accumulator from lead acid battery waste contaminated site in Nigeria

Abstract

Biology, tolerance, and metal (Pb and Cr) accumulating ability of Gomphrena celosoides were studied under hydroponic conditions. The seedlings were raised in Hoagland’s solution containing different concentrations of Pb (0, 500, 1000, 1500, 2000, 3000, 4000, and 5000?mg l^?1) and Cr (0, 50, 100, 150, 200, 300, and 400?mg l^?1). Biomass and metal accumulation in different plant parts were determined at seven (7) and fourteen (14) days after stress. Antioxidant enzyme activities, protein, and proline contents were estimated in stressed and unstressed plants. Gomphrena celosoides was able to tolerate Pb and Cr concentrations up to 4000 and 100?mg l^?1, respectively in hydroponic solution. Metal accumulation was concentration and duration dependent with the highest Pb (21,127.90 and 117,985.29?mg kg^?1) and Cr (3130.85 and 2428.90?mg kg^?1) in shoot and root, respectively found in the plants exposed to 5000?mg l^?1 Pb and 400?mg l^?1 Cr for 14?days. Proline, antioxidant enzyme activities, and protein contents were the highest in plant exposed to higher Pb and Cr concentrations for 7 and 14?days. Gomphrena celosoides could be considered as Pb and Cr accumulator with proline and increase in antioxidant enzyme activities being the tolerance mechanisms.     

The Arabidopsis metal tolerance protein AtMTP3 maintains metal homeostasis by mediating Zn exclusion from the shoot under Fe deficiency and Zn oversupply

Zinc ions are required to maintain the biological activity of numerous proteins. However, when mislocalized or accumulated in excess, Zn(2+) ions are toxic because of adventitious binding to proteins and displacement of other metal ions, among them Fe(2+), from their binding sites. Heterologous expression of a previously uncharacterized Arabidopsis thaliana metal tolerance protein, MTP3, in the zrc1 cot1 mutant of budding yeast restores tolerance to, and cellular accumulation of, zinc and cobalt. An MTP3-GFP fusion protein localizes to the vacuolar membrane when expressed in Arabidopsis. Ectopic over-expression of MTP3 increases Zn accumulation in both roots and rosette leaves of A. thaliana, and enhances Zn tolerance. Exposure of wild-type plants to high but non-toxic concentrations of Zn or Co, or Fe deficiency, strongly induce MTP3 expression specifically in epidermal and cortex cells of the root hair zone. Silencing of MTP3 by RNA interference causes Zn hypersensitivity and enhances Zn accumulation in above-ground organs of soil-grown plants and of seedlings exposed to excess Zn or to Fe deficiency. Our data indicate that, in wild-type A. thaliana, the AtMTP3 protein contributes to basic cellular Zn tolerance and controls Zn partitioning, particularly under conditions of high rates of Zn influx into the root symplasm.     

Evolution and function of phytochelatin synthases

Both essential and non-essential transition metal ions can easily be toxic to cells. The physiological range for essential metals between deficiency and toxicity is therefore extremely narrow and a tightly controlled metal homeostasis network to adjust to fluctuations in micronutrient availability is a necessity for all organisms. One protective strategy against metal excess is the expression of high-affinity binding sites to suppress uncontrolled binding of metal ions to physiologically important functional groups. The synthesis of phytochelatins, glutathione-derived metal binding peptides, represents the major detoxification mechanism for cadmium and arsenic in plants and an unknown range of other organisms. A few years ago genes encoding phytochelatin synthases (PCS) were cloned from plants, fungi and nematodes. Since then it has become apparent that PCS genes are far more widespread than ever anticipated. Searches in sequence databases indicate PCS expression in representatives of all eukaryotic kingdoms and the presence of PCS-like proteins in several prokaryotes. The almost ubiquitous presence in the plant kingdom and beyond as well as the constitutive expression of PCS genes and PCS activity in all major plant tissues are still mysterious. It is unclear, how the extremely rare need to cope with an excess of cadmium or arsenic ions could explain the evolution and distribution of PCS genes. Possible answers to this question are discussed. Also, the molecular characterization of phytochelatin synthases and our current knowledge about the enzymology of phytochelatin synthesis are reviewed.     

A role for nickel in osmotic adjustment in drought-stressed plants of the nickel hyperaccumulator <Emphasis Type="Italic">Stackhousia tryonii</Emphasis> Bailey

The hypothesis that hyperaccumulation of certain metals in plants may play a role in osmotic adjustment under water stress (drought) was tested in the context of nickel hyperaccumulator Stackhousia tryonii. Field-collected mature plants of S. tryonii, grown in native ultramafic soil, were pruned to soil level and the re-growth exposed to five levels of water stress (20, 40, 60, 80 and 100% field capacity; FC) for 20 weeks. Water stress had significant (P<0.05) influence on growth (biomass), water potential and shoot Ni concentrations, with progressively more impact as water stress was increased from 80 to 40% FC. Shoot Ni concentration increased significantly from 3,400 μg g⁻¹ dry weight (at 100% FC) to 9,400 μg g⁻¹ dry weight (at 20% FC). Assuming that Ni is uniformly distributed through the shoot tissue, the Ni concentration could account for 100% at the 80 and 60% FC conditions, and 50% at the 40 and 20% FC conditions of plant osmotic regulation. The results are consistent with a role of Ni in osmotic adjustment and protection of S. tryonii plants against drought.     

Variation in Heavy Metal Accumulation and Genetic Diversity at a Regional Scale Among Metallicolous and Non-Metallicolous Populations of the Facultative Metallophyte Biscutella laevigata subsp. laevigata

Biscutella laevigata is a facultative metallophyte, with populations on non-metalliferous and metalliferous soils. Some of its metallicolous populations have been shown to hyperaccumulate thallium or lead in nature. Only Tl hyperaccumulation has been experimentally confirmed. We aimed to compare the patterns of metal (hyper)accumulation and genetic diversity among populations of B. laevigata subsp. laevigata in NE Italy.

None of the populations exhibited foliar hyperaccumulation of Cu, Zn, or Pb. The root-to-shoot accumulation rates for these metals were unchanged or decreased rather than enhanced in the metallicolous populations, in comparison with the non-metallicolous ones. Hyperaccumulation of Tl was confined to the population of the Cave del Predil mine. This population was genetically very distinct from the others, as demonstrated by AFLP-based cluster analysis. The two other mine populations did not surpass the threshold for Tl hyperaccumulation, but showed enhanced foliar Tl concentrations and root-to-shoot translocation rates, in comparison with the non-metallicolous populations. Genetic analysis suggested that adaptation to metalliferous soil must have been independently evolved in the metallicolous populations.     

Nickel distribution in the hyperaccumulator Alyssum serpyllifolium Desf. spp. from the Iberian Peninsula

Abstract

The relative concentration and distribution of nickel (Ni) in vegetative tissues (leaves, stems and trichomes) and reproductive organs (seeds) was studied using energy-dispersive X-ray microanalysis (EDXS) and scanning electron microscopy (SEM) in two previously studied Ni-hyperaccumulator subspecies of Alyssum serpyllifolium Desf. growing naturally in ultramafic soils of the Iberian Peninsula: A. serpyllifolium ssp. lusitanicum Dudley & P. Silva and A. serpyllifolium ssp. malacitanum Rivas Goday ex G. López. Both taxa showed that Ni accumulates preferentially in the leaves, exhibiting a homogeneous distribution on both epidermis surfaces. The highest Ni concentrations were found inside the epidermal cells and at the base of trichome stalks. Ni accumulation in seeds was lower than in the vegetative organs. The location of Ni in these plants allows us to postulate that its accumulation is a protection mechanism against external stress.     

花卉植物应用于污染土壤修复的可行性研究

植物修复是解决污染土壤问题的有效途径之一,而已报道的超积累植物的种类非常有限.如果能从物种繁多的花卉植物中筛选出修复植物,不但能够弥补这一不足,而且还能在美化环境的同时,产生一定的经济效益.从植物修复的重要性和修复植物的筛选出发,概括了修复植物的判断标准及基本特征.通过描述花卉植物资源及其在环境保护中的作用,列举花卉植物与其它植物相比的优势,分析花卉植物的耐性、积累性和修复类型,探讨花卉植物应用于污染土壤修复实践的可行性.从花卉中筛选超积累植物,将为污染土壤的修复工作提供的生物材料.     

Phytoremediation—An Overview

The use of plants (directly or indirectly) to remediate contaminated soil or water is known as phytoremediation. This technology has emerged as a more cost effective, noninvasive, and publicly acceptable way to address the removal of environmental contaminants. Plants can be used to accumulate inorganic and organic contaminants, metabolize organic contaminants, and encourage microbial degradation of organic contaminants in the root zone. Widespread utilization of phytoremediation can be limited by the small habitat range or size of plants expressing remediation potential, and insufficient abilities of native plants to tolerate, detoxify, and accumulate contaminants. A better understanding and appreciation of the potential mechanisms for removing contaminants from the root zone and the interaction between plants, microorganisms, and contaminants will be useful in extending the application of phytoremediation to additional contaminated sites.