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12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 243

essential metals (Kra ¨mer et al. 2007). Unless detoxified, nonessential metal ions
may exert their toxic effect at virtually any tissue and cellular concentration. The
property of hyperaccumulators to concentrate in their tissues heavy metal ions in
large quantities is probably a consequence of their adaptation (Verbruggen et al.
2009). However, the selective factors causing the evolution of hyperaccumulation,
which required complex alterations in the plant metal homeostasis network, are
unknown and difficult to identify retrospectively. It has been suggested that
accumulated metals execute defense function, poisoning plant tissues for
herbivores and pathogens (Boyd 2007; Noret et al. 2007).

12.3.1 Heavy Metal Uptake and Translocation

The actual bioavailability of metal ions in soil is limited, because of their presence
in mineral form, formation of hydrous oxides at pH >5, and strong binding to soil
components like humic and fulvic acids. The soil microflora can modulate the
bioavailability of metals by several mechanisms (Gadd 2007, 2010). Metabolic
activities of some microorganisms may result in immobilization of metallic species
in soil by such mechanisms as organic precipitation with oxalates, inorganic
precipitation with carbonates, phosphates, or hydroxides, redox immobilization,
sorption at cell walls and associated polymeric substances, and bioaccumulation.
+
Some microorganism may, in turn, mobilize metals through excretion of H and
carboxylic (e.g., citrate) ligands and redox conversion to mobile forms. Also plants
can solubilize metals for uptake by decreasing pH within the rhizosphere or by
various organic chelators (root exudates; Fig. 12.1), such as carboxylates or
phytosiderophores from the mugineic acid family (Chaney et al. 2007; Nair et al.
2007). Although the concept of developing transgenic plants with enhanced secre-
tion of such ligands is plausible, there is no definite answer to the question of
whether, and how, would such modification promote the metal uptake.
Following mobilization, the initial contact of the metal ion with root cell
involves its biosorption at the cell wall via ion-exchange and chelation at cellulose,
hemicellulose, pectin, and some minor polymers. The transport of most divalent
heavy metals into root cells (Fig. 12.1) seems to be driven by members of the zinc-
regulated transporter, iron-regulated transporter (ZIP) family (Kra ¨mer et al. 2007;
Migeon et al. 2010). It is of particular interest to note that, unlike non-
hyperaccumulating species, hyperaccumulator Noccaea caerulescens (alpine
pennygrass; previously named Thlaspi caerulescens) constitutively overexpress in
its roots ZIP1 gene, whose products mediate high-affinity Zn transport as well as
low-affinity Cd uptake (Pence et al. 2000; Hammond et al. 2006; van de Mortel et al.
2006; Milner et al. 2012). Gene expression analyses in N. caerulescens and in
another Zn-, Cd-hyperaccumulator A. halleri have further highlighted
overexpression of more ZIP members and physiological studies provided strong
evidence that multiple uptake system are involved in the root uptake of Cd and Zn,
which show differential preference for these metal ions (Lin et al. 2009; Verbruggen