Page 260 - Plant-Based Remediation Processes

12 Transgenic Approaches to Enhance Phytoremediation of Heavy Metal-Polluted Soils 253

In yeast S. cerevisiae, Cd is detoxified by transport of cytosolic bis
(glutathionato)cadmium complex to vacuoles by ABC-type YCF1 transporter (Li
et al. 1997). Accordingly, heterologous expression of YCF1 gene in A. thaliana
rendered transgenic plant with an enhanced tolerance to Cd and Pb (Song et al.
2003). Quite surprisingly, the YCF1 plant also efficiently translocated these metals
to shoots, which, compared to WT, accumulated 1.5 higher metal levels from
media with 70 μM Cd or 750 μM Pb. Moreover, the same phenotype was observed
in the YCF1-transformed B. juncea (Bhuiyan et al. 2011b). Transgenic lines
showed better growth and 1.5- to 2.1-fold higher Cd and Pb content in shoots
than did WT plants grown in the same hydroponic solutions containing 150 mM
Cd of 1 M Pb. Although the expression of the mammalian hMRP1 gene, encoding
a different type the ABC-type multidrug resistance-associated transporter, did not
alter Cd accumulation in the organs of N. tabacum, transgenes showed improved
Cd tolerance compared to WT controls, manifested by the continuous growth of
transgene plantlets, reduced chlorosis, and a 25 % faster root elongation on media
containing 100–240 μM Cd (Yazaki et al. 2006). Mammalian ATP-binding cas-
sette (ABC) transporters involved in the multidrug resistance of cancer cells can
efflux cytotoxic compounds that show a wide variety of chemical structures and
biological activities. Human multidrug resistance-associated protein (hMRP1) is
one of the most intensively studied ABC transporters and many substrates have
been identified, including both organic and inorganic compounds (Zhou et al.
2008). Interestingly, in mammals, members of the MRP family are found in
plasma membrane, while in N. tabacum, hMRP1 is localized in vacuolar
tonoplasts. Besides detoxification of Cd, presumably transported to vacuoles as
glutathione–Cd conjugate, hMRP1 also conferred vacuolar uptake and resistance
to model organic xenobiotic daunorubicin, an anthracycline-type DNA-
intercalating drug, suggesting that MRP transporters could be beneficial in
constructing plants for the remediation of a complex polluted environment
(Yazaki et al. 2006).
In eukaryotic cells, intracellular membrane transport involves vesicle formation
and fusion with a target membrane. This process is mediated by numerous
components. In particular, the specificity of membrane fusion is mediated by
membrane-associated proteins called SNAREs (soluble N-ethyl-maleimide-sensitive
factor attachment protein receptors) (Wickner 2010). An interesting approach to
promote sequestration of Cd in vacuoles emerged from finding that A. thaliana
SNARE proteins, SYP111 and SYP121 are involved in the transport of secretory
vesicles at the plasma membrane, and AtVAM3 (SYP22) provides target SNARE
function during the late stages of vacuolar assembly (Uemura et al. 2004). In the yeast
model, these SNARE functions were capable to direct bacterial transporter MerC
from the plasma membrane to the vacuolar tonoplast, thereby promoting vacuolar
sequestration of Hg as well as Cd (Kiyono et al. 2010, 2011). More recently, Kiyono
et al. (2012) demonstrated feasibility of this approach in A. thaliana. The transgenic
seedlings with MerC-SYP121 fusion in the tonoplast were more resistant to Cd than
WT and whole seedlings accumulated by 20 % more metal when grown hydroponi-
cally in the presence of 20 μMCd.