Field performance and heavy metal concentrations of transgenic flue-cured tobacco expressing a mammalian metallothionein-beta-glucuronidase gene fusion

Root-selective expression of AtCAX4 and AtCAX2 results in reduced lamina cadmium in field-grown Nicotiana tabacum L

To assess the impact of enhanced root vacuole cadmium (Cd) sequestration on leaf Cd accumulation under a low Cd dose, as generally occurs in agriculture, leaf Cd accumulation was examined in field-grown tobacco plants expressing genes encoding the high-capacity-Cd, tonoplast-localized, divalent cation/H antiporters AtCAX4 and AtCAX2 (AtCAX, Arabidopsis cation exchanger). It has been shown previously that root tonoplast vesicles isolated from plants expressing these genes, directed by root-selective promoters, show enhanced Cd transport activity, and young plants show enhanced root Cd accumulation when grown in solution culture containing 0.02 microM Cd, a moderate Cd dose. In this article, we present results which show that the lower leaves of mature plants expressing AtCAX2 or AtCAX4, under the control of two different root-selective promoters, accumulate 15%-25% less lamina Cd than control plants when grown in the field (3 years, three different collection methods). Reciprocal grafting experiments of AtCAX2 shoots onto control roots (and vice versa), grown in solution culture with 0.005 microM Cd, indicated that the root controls Cd translocation and accumulation in the shoot in control and AtCAX2 and AtCAX4 tobacco plants exposed to low Cd concentration. The results are consistent with a model in which supplementation of Cd/H antiporter activity in root cell tonoplasts enhances root Cd sequestration, resulting in decreased translocation of Cd to the shoot of field-grown plants. These results suggest that human Cd intake from food and tobacco use could be reduced via the enhancement of root vacuolar sequestration of this pollutant.

Prospects of genetic engineering of plants for phytoremediation of toxic metals

Bioremediation is gaining a lot of importance in recent times as an alternate technology for removal of elemental pollutants in soil and water, which require effective methods of decontamination. Phytoremediation--the use of green plants to remove, contain or render harmless environmental pollutants--may offer an effective, environmentally nondestructive and cheap remediation method. The use of genetic engineering to modify plants for metal uptake, transport and sequestration may open up new avenues for enhancing efficiency of phytoremediation. Metal chelator, metal transporter, metallothionein (MT), and phytochelatin (PC) genes have been transferred to plants for improved metal uptake and sequestration. Transgenic plants, which detoxify/accumulate cadmium, lead, mercury, arsenic and selenium have been developed. A better understanding of the mechanisms of rhizosphere interaction, uptake, transport and sequestration of metals in hyperaccumulator plants will lead to designing novel transgenic plants with improved remediation traits. As more genes related to metal metabolism are discovered, facilitated by the genome sequencing projects, new vistas will be opened up for development of efficient transgenic plants for phytoremediation.

Vacuolar compartmentalization: a second-generation approach to engineering plants for phytoremediation

Engineering plants with greater metal tolerance and accumulation properties is the key to developing phytoremediators. A recent study by Won-Yong Song et al. has shown that overexpressing the yeast vacuolar transporter YCF1 increases Pb and Cd tolerance and consequently increases the accumulation of these metals in shoots of transgenic Arabidopsis plants even though expression levels of YCF1 were relatively low. This technology can be used to engineer advanced phytoremediators, increasing their ability to pump heavy metals into a safe compartment while requiring only a small amount of transporters rather than a large amount of chelating peptide material.

Yeast metallothionein in transgenic tobacco promotes copper uptake from contaminated soils

Metallothioneins (MTs) are metal-binding proteins that confer heavy metal tolerance and accumulation in yeast. To augment higher plant metal sequestration, the yeast metallothionein (CUP 1) was introduced into tobacco plants. The CUP 1 gene expression and copper and cadmium phytoextraction were determined. To confirm transformation, selfed and kanamycin-resistant third generation plants were subjected to DNA blot and polymerase chain reaction (PCR) analysis. A 4 mM CuSO(4) stress for 7 days resulted in a decline in CUP 1 transcripts versus nonstress conditions. Despite low mRNA levels, CUP 1 transformants accumulated up to seven times more copper in older versus younger leaves during copper stress. Pooled leaves of transgenic plants grown in soils from copper stamp-sands contained two to three times the copper content as that of the control plants. Unlike some previous reports featuring MT overexpression in plants, CUP 1 seedlings did not significantly sequester or demonstrate tolerance to CdCl(2). Using this transgenic approach, yeast CUP 1 expression under nonstressed conditions contributed to copper metal phytoextraction during a subsequent copper challenge. This strategy could be incorporated into plants designed for enhanced phytoremediation of metal contaminants.

Phytoremediation: a technology using green plants to remove contaminants from polluted areas

Phytoremediation is an emerging cost-effective, non-intrusive, esthetically pleasing, and low cost technology using the remarkable ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. Phytoremediation technology is applicable to a broad range of contaminants, including metals and radionuclides, as well as organic compounds like chlorinated solvents, polychlorobiphenyls, polycyclic aromatic hydrocarbons, pesticides/insecticides, explosives, and surfactants. The use of plants to transport and concentrate metals from the soil into the harvestable parts of roots and above-ground shoots, usually called 'phytoextraction', has appeared on the scene as a valid alternative to traditional physicochemical remediation methods that do not provide acceptable solutions for the removal of metals from soils. Positive results are becoming available regarding the ability of plants to degrade certain organic compounds. Nonetheless, despite the firm establishment of phytoremediation technology in the literature and in extensive research study and in small-scale demonstrations, full-scale applications are currently limited to a small number of projects. At present, the phytoremediation of metal pollutants from the environment could be approaching commercialization.

Differential expression and characterization of three metallothionein-like genes in Cavendish banana (Musa acuminata)

Metallothioneins (MTs) are cysteine-rich polypeptides that are involved in metal detoxification and homeostasis in both prokaryotes and eukaryotes. In this study, we report the isolation and characterization of three members (MT2A, MT2B and MT3) of the MT-like gene family from ripening banana fruit and their differential expression in various banana organs and during fruit development and ripening. All members of the MT-like gene encode small cysteine-rich polypeptides of 65-79 amino acid residues. MT2A shared a high sequence similarity (54-77%) with several type-2 MTs in plants, while MT3 was highly homologous (51-61%) with type-3 MTs. The three members expressed differentially in various organs but transcripts were generally more abundant in reproductive than vegetative organs. During fruit development, the MT2A transcript was barely detectable in ovary but increased to a high level in young fruit at 20 days after shooting (DAS) and declined gradually thereafter as fruit developed. In contrast, both MT2B and MT3 expressed poorly in young fruits (20-60 DAS) and transcripts were detected only in fruits at later stages of development. As ripening progressed, expression of MT2A decreased but that of MT3 increased. Expression of MT members during ripening appeared to be differentially regulated by ethylene, whose levels were low in FG and TY fruit but surged climacteristically in MG and declined sharply as ripening advanced further. Exogenous application of ethylene at 5 ppm or higher concentrations down-regulated MT2A expression and the inhibitory effect of ethylene could be partially suppressed by the presence of norbornadiene, an inhibitor of ethylene action. Ethylene had no effect on transcript accumulation of MT2B and MT3. However, MT3 expression was greatly enhanced in response to metals such as CdSO4, CuSO4 and ZnSO4. These results suggest that increased MT3 expression may be associated with excess metal ions present in ripening fruit tissues. This study also provided evidence, for the first time, that ethylene and metals play a regulatory role in expression of MT-like genes in banana.

Genetic engineering in the improvement of plants for phytoremediation of metal polluted soils

Metal concentrations in soils are locally quite high, and are still increasing due to many human activities, leading to elevated risk for health and the environment. Phytoremediation may offer a viable solution to this problem, and the approach is gaining increasing interest. Improvement of plants by genetic engineering, i.e. by modifying characteristics like metal uptake, transport and accumulation as well as metal tolerance, opens up new possibilities for phytoremediation. So far, only a few cases have been reported where one or more of these characteristics have been successfully altered; e.g. mercuric ion reduction causing improved resistance and phytoextraction, and metallothionein causing enhanced cadmium tolerance. These, together with other approaches and potentially promising genes for transformation of target plants are discussed.

Phytoremediation of soil metals

The phytoremediation of metal-contaminated soils offers a low-cost method for soil remediation and some extracted metals may be recycled for value. Both the phytoextraction of metals and the phytovolatilization of Se or Hg by plants offer great promise for commercial development. Natural metal hyperaccumulator phenotype is much more important than high-yield ability when using plants to remove metals from contaminated soils. The hypertolerance of metals is the key plant characteristic required for hyperaccumulation; vacuolar compartmentalization appears to be the source of hypertolerance of natural hyperaccumulator plants. Alternatively, soil Pb and Cr6+ may be inactivated in the soil by plants and soil amendments (phytostabilization). Little molecular understanding of plant activities critical to phytoremediation has been achieved, but recent progress in characterizing Fe, Cd and Zn uptake by Arabidopsis and yeast mutants indicates strategies for developing transgenic improved phytoremediation cultivars for commercial use.