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12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 247

symplast, virtually all heavy metal ions are thus sequestered by specialized ligands
or metallochaperones, which are together intimately involved in management of the
storage metal pool. As many metallic species exert their toxic effect by induction of
reactive oxygen species (ROS) and other free radicals, their elimination is another
challenge faced by the cell (Foyer and Noctor 2005). Some heavy metal ions are
essential for specific metabolic process, but may impair biological equilibrium
when over accumulated. For the heavy metal (hyper)accumulation to be highly
effective, the plant should be thus capable to maintain metal homeostasis and create
appropriate metal sinks. The main detoxification mechanisms on subcellular level
involve sequestration of the metal ion by ligands in cytoplasm eventually followed
by transport of the metal (complex) into vacuoles. Other aspect relevant to (hyper)
accumulation of heavy metals in aboveground tissues is their availability for
translocation that also implies limited sequestration in or efficient mobilization
from vacuoles of the root cells (Fig. 12.1).
The cysteine-rich metallothioneins (MTs) are intracellular ligands capable of
tight coordination of heavy metal ions via cysteine residues shared along the
peptide sequence in Cys–X–Cys or Cys–Cys motifs (X represents any amino
acid). Peptides of MT family have been identified in plants, animals, eukaryotic
microorganisms, and certain prokaryotes. Most of plant MTs consist of about 63–85
amino acids with two terminal cysteine-rich domains separated by a central region
without any Cys residues and they cluster into four types (Freisinger 2008). Like
animals and fungi, plants have several MT genes and MTs play a major role in the
homeostasis of essential heavy metals and the transcription of their genes is
controlled by signals instrumental during germination, organ development, and
senescence (Clemens 2006). Animal and certain fungal MTs are, besides their
function in homeostasis, known for their essential role in detoxification of toxic
heavy metal ions (Coyle et al. 2002; Bellion et al. 2006; Vas ˇa ´k and Meloni 2011). In
plants, MTs seem to be contribute to Cu tolerance in various Arabidopsis ecotypes
and in Silene vulgaris (Murphy and Taiz 1995; Jack et al. 2007) and may be
intricately involved in phloem Cu transport in A. thaliana (Guo et al. 2008b). In
N. caerulescens the role of MTs is also attributed to Cu homeostasis and expression
of type 3 MT was found particularly strong in Cd-hyperaccumulating populations
(Roosens et al. 2004, 2005; Hassinen et al. 2007).
Intracellular detoxification of most heavy metal ions in plants and in certain
yeasts relies on phytochelatins (PCs). These peptides of general structure
(γ-Glu–Cys) n X (PCn; n ¼ 2–11; X represents Gly, Ser, β-Ala, Glu, Gln, or no
residue) tightly sequester multiple metal ions in metal–thiolate complexes, rendering
them inactive in cellular processes (Clemens 2006). The metal–PC complexes
formed in cytosol could further be deposited in vacuoles, which serve as cellular
sink for toxic metal species (Fig. 12.1). In the yeast Schizosaccharomyces pombe,it
involves the transport of the complex via ABC transporter Hmt1 (Clemens and Simm
2003) and its functional homologue Abc2 from A. thaliana has been characterized
recently (Mendoza-Co ´zatl et al. 2010). Inside the vacuole, the metal–PC can accom-
modate inorganic sulfide and CdS crystallites are formed (Clemens and Simm 2003;
Clemens 2006). Alternatively, the complex dissociates and released metal cations