1

Expression

Fig. 11.1 Possible molecular mechanisms of phytoremediation for heavy metal-contaminated soils, in combination with signaling pathways and transcription regulation

Fig. 11.1 Possible molecular mechanisms of phytoremediation for heavy metal-contaminated soils, in combination with signaling pathways and transcription regulation

(Dugas and Bartel 2004; Shao et al. 2008), but recently their participation in stress responses has been paid more attention (Sunkar and Zhu 2004; Shao et al. 2008). The predicted targets of number of Arabidopsis thaliana microRNA families, designated as miR398, are the mRNAs coding for cytoplasmic and chloroplast Cu-Zn-superoxide dismutase (Cu,Zn-SOD:CSD1 and CSD2) and a subunit of mitochondrial cytochrome C oxidase (COX5b-1). It was shown that miR398 expression is down-regulated transcriptionally by heavy metals, light and other oxidative stresses. This down-regulation of miR398 is important for up-regulation of mRNAs coding for Cu-Zu-SOD and oxidative stress response (Sunkar et al. 2006). Further studies indicated that the same microRNA (mir398) regulated copper homeostasis and mediated this regulation by controlling the degradation of Cu-Zn-SOD mRNA when Cu was limited (Yamasaki et al. 2007). It is clear that posttranscriptional processes involving microRNAs play important roles in regulating plant heavy metal dependent genes, which is a fine performance of acclimating mechanisms of higher

DNA Modifications & DNA Damages

Direct Regulation

Chromatin Regulation

Heavy Metals

Secondary Mediators

Posttranscriptional Regulation

Responsive Genes microRNA

mRNA

Proteins

PhenotypeEe

Fig. 11.2 A framework for the gene expression and regulation when plants are exposed to heavy metals plants under the changing environment. A possible framework for the gene expression and regulation when plants are exposed to heavy metals is summarized in Fig. 11.2.

3 Important Standards for Heavy Metal Hyperaccumulator Plants

How do heavy metal hyperaccumulator plants achieve this remarkable bioaccumulation of soil heavy metals? Researchers have identified several characteristics that are important:

1. The plant must be able to tolerate high levels of the element in root and shoot cells; hypertolerance is the key property which makes hyperaccumulation possible. Such hypertolerance is believed to result from vacuolar compartmen-talization and chelation. The most direct demonstration used isolated vacuoles from protoplasts of tobacco cells which had accumulated high levels of Cd and Zn. Whether hypertolerance in the known hyperaccumulators is due to an enhancement of these mechanisms is not yet known. However, electron microprobe analysis supports vacuolar compartmentation for Zn in the leaves of the hyperaccumulator Thlaspi caerulescens.

2. A plant must have the ability to translocate an element from roots to shoots at high rates. Normally root Zn, Cd or Ni concentrations are 10 or more times higher than shoot concentrations, but in hyperaccumulators, shoot metal concentrations can exceed root levels. Researchers recently found that although the chemical forms of Ni found in extracts of leaves of Alyssum hyperac-cumulators are the chelates with malate and citrate, in the xylem exudate histidine chelates about 40% of the total Ni present; nearly all of the histi-dine in exudate is chelated with Ni. Whether Ni2+ or a mixed chelate such as Ni (histidine, malate) is pumped into the xylem by a membrane transporter remains unknown. Additions of histidine to nutrient solution increased Ni tolerance and transport to shoots by Alyssum montanum, a non-hyperaccumulator species.

3. There must be a rapid uptake rate for the element at levels which occur in soil solution. Here quite different patterns have been observed in different groups of hyperaccumulators. Studies showed that T. caerulescens accumulated Zn and Cd from nutrient solution only about as well as tomato and Silene vulgaris did, but tomato was severely injured at 30 ^M Zn, S. vulgaris at 320 ^M Zn, and T caerulescens only at 10,000 ^M Zn. Because this species can keep tolerating and accumulating Zn and Cd at high soil solution levels, it is found in nature with 1-4% Zn while surrounding plants are <0.05% Zn (Zn excluders). Further, studies have shown that Zn hypertolerant genotypes of T. caerulescens require much higher solution Zn2+ (104-fold) and leaf Zn concentrations (100300 mg kg-1 vs. 10-12 mg kg-1 in normal plants) to grow normally than do related non-hyperaccumulator species. By implication, the highly effective com-partmentalization to reduce the toxicity of Zn and Cd appears to require the plant to accumulate much more Zn to have adequate supply. In contrast, the Ni-hyperaccumulator Alyssum species accumulate remarkably higher shoot Ni levels compared to other species grown at the same Ni2+ activity in solution. The Se-hyperaccumulating species similarly accumulate higher shoot Se levels and many can volatilize Se at high rates growing beside plants with more normal levels and slow volatilization.

4 Biotechnology and Phytoremediation of Heavy Metal Contaminated Soils

Biotechnology approaches to develop phytoremediation plants have been examined. Traditional plant breeding can only use available genetic diversity within a species to combine the characteristics needed for successful phytoremediation. Researchers expected that increasing the concentrations of metal binding proteins or peptides in plant cells would increase metal binding capacity and tolerance. Although plant cell cultures expressing mammalian metallothioneins (MTs) or phytochelatins (PCs) are more tolerant of acute Cd toxicity, the transfer of mammalian metallothionein genes to higher plants appears to provide no benefit for phytoremediation. Further, when natural metal hypertolerant plants were examined, the concentration of PCs showed no difference, suggesting that hypertolerance to Cd and Zn in these plants was not due to the hyperaccumulation of PC peptides. The evidence for the role of PCs is that their presence does correlate with normal levels of metal tolerance, since mutations that abolished PC production in Arabidopsis and fission yeast resulted in hypersensitivity to Cd. Cd-sensitive (hypotolerant) single gene mutants cadi and cad2 of Arabidopsis thaliana have been identified and studied (e.g. PC synthesis). For a plant species with normal tolerance (A. thaliana), PCs were essential for the normal level of tolerance (Cunningham et al. 1995; Wu et al. 2006; Doty 2008; Shao et al. 2008).

Although these studies have allowed cloning of genes involved in acute Cd tolerance, and characterization or confirmation of metabolic pathways, the environmental relevance of findings from such acute Cd exposure has not been established. An alternative view of Cd-catalyzed PC biosynthesis is that chelation of PCs with Cd alleviates the feedback inhibition of the PC-synthase; as long as Cd activity in the cytoplasm is high, an enzyme supports more transfer to form more PCs and longer PCs. Because the level of Zn present in nearly all environments is 100 times higher than that of Cd, if an acute toxic Cd dose is provided, the plants would be killed by Zn. Even the formation of the sulfide-stabilized high molecular weight Cd-PC complex in vacuoles may result from the acute toxic Cd supply without Zn. Further, the finding that the hmt1 vacuolar membrane pump protein (which restored Cd hypertolerance to mutant fission yeast) transported both Cd-PCs and PCs without Cd, raises questions about how the pump works to induce Cd hypertolerance in vivo. Cadmium (Cd) phytotoxicity in soil is a recent anthropogenic effect, whereas Zn phytotoxicity and co-accumulation of trace levels of Cd are normal biogeochem-ical phenomena. It seems increasingly likely that the Cd hypertolerance mechanisms are incidental biochemical phenomena. Although Cd-PCs can be found at low levels in plants in the environment, they account for only a small fraction of the tissue Cd (Suzuki et al. 2001; Jonak et al. 2004).

Another goal of developing transgenic plants with increased metal binding capacity was to use these metal-binding factors to keep Cd in plant roots, thus reducing Cd movement to the food chain or into tobacco. Vacuolar compartmentation of Cd only in roots may reduce Cd translocation to shoots; expression in plants of the hmt1 vacuolar pump for Cd-PCs from fission yeast has not yet been successful, and modification of gene sequences may be required before its effectiveness can be tested (similar to the mercury reductase gene sequence changes). The expression of MT as the whole protein, the Cd binding '-domain' part of the protein, or a fusion protein with -glucuronidase, under several promoters increased Cd tolerance of tobacco and other plants, but had little effect on Cd transport to shoots(Pence et al. 2000). Recently use of the improved 35S2 promoter may have increased the ability of MT to keep Cd in roots, however, tests have not yet progressed to soil studies which must be the important measure of success. Some promising genes that are involved in phytoremediation of heavy metal-contaminated soils in plant roots are listed in Table 11.2.

Table 11.2 Some promising genes involved in phytoremediation of heavy metal-contaminated soils in plant roots

Functions of

Table 11.2 Some promising genes involved in phytoremediation of heavy metal-contaminated soils in plant roots

Functions of

Gene

gene products

Plant species

Roles of gene products

Gene regulation

Atfro2

Ferric chelate

Arabidopsis thaliana

iron reductase

Induction

Detox Diet Basics

Detox Diet Basics

Our internal organs, the colon, liver and intestines, help our bodies eliminate toxic and harmful  matter from our bloodstreams and tissues. Often, our systems become overloaded with waste. The very air we breathe, and all of its pollutants, build up in our bodies. Today’s over processed foods and environmental pollutants can easily overwhelm our delicate systems and cause toxic matter to build up in our bodies.

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