Page 252 - Plant-Based Remediation Processes

12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 245

et al. 2009;Kra ¨mer 2010). Although ZIP transporters, such as ZIP2 of Arabidopsis
thaliana, may be involved also in import of Cu (Yamasaki et al. 2009), the high
affinity uptake of this is being attributed to proteins of the Cu-transporter protein
(COPT) family (Puig et al. 2007; Burkhead et al. 2009). Members of COPT (syn.
CTR) family are well characterized from many eukaryotes as essential for Cu
uptake and homeostasis (Puig and Thiele 2002). Moreover, the human plasma
membrane hCTR1 has been demonstrated recently to execute also the high-affinity
+
Ag uptake (Bertinato et al. 2010). In A. thaliana COPT1 was the first to be
identified as the root Cu uptake transporter essential for healthy plant growth
(Sancenon et al. 2004). Moreover, overexpression COPT1 or of the otherwise
uncharacterized COPT3 genes resulted in overaccumulation of Cu and sensitivity
to Cu excess (Andres-Colas et al. 2010).
Efficient translocation of metal ions to aboveground organs requires radial
passage from root symplast to xylem apoplast (Fig. 12.1). Here we refer to the
symplast–apoplast concept, which considers that all the cells of higher plants are
connected forming symplast. Continuous semipermeable membrane then separates
the symplast from the apoplast, the nonliving parts of the plant tissue (cell walls,
xylem, and intercellular space). The passage of metal ions from root cells into
apoplast (xylem) occurs via specific membrane transporters and is generally tightly
regulated. Among them, P 1B -type ATPases, in plants referred to as HMA (heavy
metal ATPase) transporter, were characterized from both non-hyperaccumulators
and hyperaccumulating species. Transporters of this type play an important role in
transporting heavy metal ions against their gradient in virtually all kingdoms of life
(Argu ¨ello et al. 2007; Migeon et al. 2010) on the expense of ATP (hence the name
ATPases). They constitute two functional groups: those transporting monovalent
metal ions (Ag, Cu) and those transporting divalent metal ions (Zn, Cu, Co, Cd, Pb).
In bacteria and certain fungi, P 1B -ATPases are among the main players in the metal
tolerance (Silver and Phung 2005; von Rozycki and Nies 2009). Since the charac-
terization of the AtHMA4 of the divalent group from A. thaliana (Mills et al. 2003,
2005), the role of its homologues in hyperaccumulating species is being well
established. The key difference that seems to greatly contribute to hyperaccu-
mulation is triplication and quadruplication of HMA4 genes in genomes of
´
A. halleri and N. caerulescens, respectively (Hanikenne et al. 2008;O Lochlainn
et al. 2011). In hyperaccumulating A. halleri as well as in N. caerulescens, HMA4
gene is more expressed in both roots and shoots compared with their
non-hyperaccumulating Zn- and Cd-sensitive relatives (Verbruggen et al. 2009),

ä
Fig. 12.1 (continued) ATPase family; YSL yellow-stripe 1-like transporters of OPT (oligopeptide
transporter) family; FRD Fe-citrate transporter “ferric reductase defective”; ABC ATP-binding
cassette family; MTP metal transporter proteins of CDF (cation diffusion facilitator) family (MTP
in B. juncea also referred to as cation efflux transporters [CAT]); CAX Ca/cation exchanger family;
NRAMP natural resistance-associated macrophage protein family. Me metal; L general ligand (e.g.
citric acid, nicotianamine; extracellular [phyto]siderophores); MT metallothionein; GSH glutathi-
one; PC phytochelatin