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12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 249

yeast cells (Persans et al. 2001). Later study suggested that MTP1 of T. goesingense
localizes in plasma membrane (Kim et al. 2004), where it could mediate transport of
metals to apoplast. In B. juncea, the CDF transporters functionally related to MTP1
of T. goesingense are CET2, CET3, and CET4 (Xu et al. 2009; Lang et al. 2011).
They can transport Zn, Cd, and Co in yeast model, and are in planta upregulated
under Zn and Cd stress. When overexpressed, they enhance tolerance to and
accumulation in shoots of Zn and Cd.
Since heavy metal ions are often subjects to vacuolar sequestration immediately
upon their entry to root cells, there is a need for their remobilization before
translocation to aboveground tissues. This can be accomplished by vacuolar ZIP
transporters or transporter of the natural resistance-associated macrophage protein
(NRAMP) family. The primary biological function of A. thaliana NRAMPs appears
to be in Fe homeostasis (Kra ¨mer et al. 2007). In A. halleri or N. caerulescens
NRAMPs show differential tissue-specific expression, thereby suggesting they may
be involved in hyperaccumulation trait of these species (Weber et al. 2004;
Hammond et al. 2006; Talke et al. 2006; van de Mortel et al. 2006). A specific
phenomenon associated with hyperaccumulation is that substantial portion of heavy
metal ions in root cells escape vacuolar sequestration. This is because metal form in
the cytoplasm complexes with nicotianamine (NA) or histidine (His). Synthesis of
NA from three S-adenosyl-methionine (SAM) molecules is present in all plants and
the role of NA seems to be in the movement of micronutrients such as Zn, Cu, or Fe
throughout the plant (Schuler and Bauer 2011). In hyperaccumulators, both NA and
His are elevated and strongly inhibit the retention of metals, particularly Ni, in
vacuoles, rendering the complex ready to mobilization into the aboveground tissues
(Ingle et al. 2005; Callahan et al. 2006, 2007; Mari et al. 2006; Richau et al. 2009).

12.4 Genetically Engineered Plants

The above paragraphs define the targets for genetic modifications of plants directed
towards the improved phytoextraction of metals from soils and sediments. These
lay in such pathways as (1) mobilization and uptake of metal from the soil,
(2) competence of metal translocation to shoots via symplast or xylem (apoplast),
including efficiency of xylem loading, (3) distribution to aboveground organs and
tissues, (4) sequestration within tissue cells, (5) expulsion of accumulated metal to
less metabolically active cells. Removal of Hg from contaminated soil by
phytovolatilization could be achieved on implementation of enzyme activities
promoting plants, (6) capacity to convert metals to volatile species for phytovola-
tilization. Research papers describing successful genetic engineering of high bio-
mass (crop) species particularly suitable for phytoextraction of metals are listed in
Table 12.1. Substantial body of information was also obtained with transgenic
model species A. thaliana and Nicotiana tabacum. Although deposition of heavy
metals in roots is not desirable in phytoextraction strategy, improved
metallotolerance in such organ could be of importance during phytostabilization