Page 264 - Plant-Based Remediation Processes

12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 257

in threefold higher levels of Cu in leaves of transgenic plants compared to those
from WT (Thomas et al. 2003). However, CUP1 tobacco did not show improved
capacity to accumulate Cd. Both Cd tolerance and accumulation was improved in
N. tabacum transformed with a fusion gene encoding HisCUP1, the CUP1 addi-
tionally modified with an N-terminal hexahistidine (His) extension (Macek et al.
2002; Pavlikova et al. 2004). In these plants, HisCUP represented 10 % of cellular
cysteine-rich peptides involving glutathione and PCs (Kr ˇı ´z ˇkova ´ et al. 2007). More-
over, transgenic plants grown on sandy soil amended with 0.2 mg kg 1 Cd showed
by 50 % reduced retention of Cd in roots and twofold higher levels of Cd in shoots
than the control WT plants. Increased Cu accumulation was reported for roots, but
not shoots, of A. thaliana, expressing the plant MT gene PsMTA1 of pea Pisum
sativum (Evans et al. 1992). In contrast, expression of PsMTA1 in the white poplar
Populus alba rendered Cu-tolerant plants, which translocated to shoots 3 times
more Cu than did WT plants (Balestrazzi et al. 2009; Turchi et al. 2012). Some
increase in uptake of Cu and Cd in shoots, but also higher retention of the metals in
roots, was observed in A. thaliana producing CcMT1 of pigeonpea Cajanus cajan
(Sekhar et al. 2011). Shoots of CcMT1 Arabidopsis accumulated by 50 % and 30 %
higher concentrations of Cu and Cd, respectively. While most flax (Linum
usitatissimum) show Cd-tolerant phenotype, they retain 70 % of accumulated Cd
compartmentalized in roots (Bjelkova et al. 2011; Najmanova et al. 2012). The
translocation of Cd to shoots has been improved in flax, which expressed high-
affinity Cd-binding α-domain of mammalian MT1 isoform 1a (Vrbova et al. 2012).
1
When tested in soils amended with Cd at 20 and 360 mg kg , the mature
transgenic flaxes contained in stems, respectively, 3.3- and 1.9-fold higher Cd
levels than WT.
Transgenic plant N. tabacum was also constructed to demonstrate contribution
of MTs to Hg tolerance and accumulation (Ruiz et al. 2011). When grown in
hydroponic media with 15 μM Hg, transgenic tobacco producing mouse MT1
showed healthier growth and twofold Hg in leaves and stems than control WT
plants. Periplasmic protein MerP is a component of bacterial Hg resistance, which
is responsible for funneling metal ions to the uptake transporters MerT, MerC, or
MerF (Silver and Phung 2005). When produced in A. thaliana, MerP got localized
in the cell membrane and vesicles of plant cells (Hsieh et al. 2009). Unlike the WT
control, MerP plants germinated on media with 12.5 μM Hg and accumulated
5.35 μgHgg 1 of fresh seedling weight.

12.4.5 Modifications for Phytovolatilization of Mercury

+
Phytovolatilization of Hg and organomercurial compounds (R-Hg ) involves the
accumulation of metal species in plant cells and their subsequent conversion to
0
volatile metallic Hg , which can be liberated to atmosphere through leaf evapora-
tion. To this end, genetic determinants of widespread bacterial resistance to Hg and
+
R-Hg are employed, which involve merA encoding mercuric reductase, which