Page 253 - Plant-Based Remediation Processes

246 P. Kotrba

consistent with the idea that HMA4 is responsible for root detoxification by
translocating the metal ions to aboveground tissues. Indeed, RNAi-mediated silenc-
ing of HMA4 gene in A. halleri rendered plants which accumulated less Cd and Zn
in shoots and were more metal sensitive (Hanikenne et al. 2008). A member of the
monovalent group of P 1B -ATPases from A. thaliana, HMA5, has been reported to
be crucial for vascular translocation of Cu (Andres-Colas et al. 2006; Burkhead
et al. 2009). Several lines of evidence also suggest that efficient Cu transport by
HMA5 requires cytosolic ATX1 copper-binding chaperone, to which a role in
funneling of the metal ion to the membrane transporter is being attributed
(Puig and Thiele 2002; Shin et al. 2012). Other important transport proteins
implicated in the heavy metal translocation are yellow-stripe 1-like (YSL)
transporters of oligopeptide transporter (OPT) family; hence they transport metal
chelates rather than free-hydrated cations. Members of this family are abundant in
A. thaliana and other non-hyperaccumulating species where they respond to Fe
availability (Kra ¨mer et al. 2007; Migeon et al. 2010). There is striking evidence for
a role of at least two YSL transporters in the Zn and Ni hyperaccumulation of
N. caerulescens, in which YSL3 was shown to transport Ni-nicotianamine
complexes (Gendre et al. 2007; Haydon and Cobbett 2007). In plants, metal
chelates (Fe-citrate) are also transported by FRD3 transporter of multidrug-
resistance transporter family and high expression levels of FRD3 genes in A. halleri
and N. caerulescens compared to those in A. thaliana may suggest a role of FRD3
also in Zn translocation (van de Mortel et al. 2006; Talke et al. 2006).
The metal ions translocated from roots are in shoots subjected to redistribution
through both apoplast and symplast. This is achieved by transporters of the same
families, which are involved in the metal uptake and its radial passage. Thus xylem
unloading or uptake from general apoplast to symplast is by ZIP transporters and
the distribution to intercellular apoplast is by HMA/FRD/YSL transporters (Kra ¨mer
et al. 2007; Migeon et al. 2010). Several lines of evidence suggest that for xylem
transport free-hydrated metal ions are used, rather than complexes (Salt et al. 1999;
Lu et al. 2008; Ueno et al. 2008). The possible translocation of metal complexes
with peptide ligands (e.g., phytochelatins; see Sect. 12.4.2) via tissue symplast,
initiated by their export via transporters of ATP-binding cassette (ABC) family, has
been suggested by Bovet et al. (2005). High phytochelatin content and four times
higher Cd levels in the phloem sap, compared to xylem, in the metallophyte
rapeseed Brassica napus (Mendoza-Co ´zatl et al. 2008) provide some support to
this idea. The role of ABC transporters is well established in vacuolar sequestration
of the metal complexes (see Sect. 12.4.2), but their role in the metal-chelate
translocation remains to be elucidated.

12.3.2 Intracellular Sequestration and Detoxification
of Heavy Metals

A common feature underlying the interactions of heavy metal with the components
of a biological system is relatively high reactivity of metal ions, mostly due to their
ability to form coordination and covalent complexes. Upon the entry into the root