Page 259 - Plant-Based Remediation Processes

252 P. Kotrba

than threefold increased rate of foliar Hg accumulation as compared to WT controls
(Sasaki et al. 2006). However, MerC Arabidopsis seedlings also acquired a Hg
hypersensitive phenotype. Overproduction of MerC would be still an attractive
approach, seemingly viable when further supported with some form of Hg detoxifi-
cation. Polyphosphate kinase ppk gene is in the bacterium Klebsiella aerogenes,a
key enzyme in polyphosphate (polyP) synthesis. Nagata et al. (2006b) showed that
expression of ppk in N. tabacum rendered plant more Hg tolerant, seemingly
because of binding/precipitation of intracellular Hg ions by polyP. Transgenic ppk
plants growing in soil with 10 nmol Hg g 1 were able to accumulate 6 times more
Hg than WT plants (Nagata et al. 2006a). To promote further accumulation of Hg in
N. tabacum, the ppk plant was transformed with a bacterial merT gene encoding Hg-
uptake transporter MerT (Nagata et al. 2009). When the merT/ppk plants were
grown hydroponically in the presence of 0.1–2.5 μM Hg, they had 1.3–3 times
higher foliar concentrations of Hg than tobacco expressing mere ppk. To extend the
use of this system to phytoextraction of organomercurial compounds, which could
not be transported by MerT, the merB/merT/ppk tobacco was developed that
expressed also merB gene (Nagata et al. 2010). This bacterial gene encodes organo-
mercurial lyase used to liberate Hg from organomercurials (Silver and Phung 2005).
The feasibility of this approach was demonstrated with merB/merT/ppk tobacco
callus, which was more resistant to methyl mercury and accumulated more Hg from
methyl mercury added to the culture media than merT/ppk or WT lines.

12.4.2 Plants Engineered for Improved Compartmentalization
of Metals

Subcellular sequestration of metal ions may, besides chemical complexation by
ligands in cytoplasm, involve the transport into vacuoles as the final metabolically
inactive sink. Manipulation of vacuolar exchange activity in N. tabacum by the
overproduction of the metal ion/H + antiporters CAX2 and CAX4 (calcium
exchanger 2 and 4) of A. thaliana provided transgenic plants the ability to effi-
ciently detoxify Cd, Zn, and Mn (Hirschi et al. 2000; Korenkov et al. 2007a, b). The
CAX2 or CAX4 plants showed an improved uptake of metal ions in the roots but
not in shoots, which accumulated 70–80 % less metals than the roots. However, the
net metal uptake was elevated in shoots due to the acquired metal tolerance and
markedly improved aboveground biomass yields (e.g., the Cd content of CAX2 and
CAX4 plants grown in media with 3 μM Cd accumulated 3.4 and 2.4 times higher
than that in WT). A site-directed mutagenesis approach was used to alter His338 of
an activated N-terminal truncated form of A. thaliana CAX1 to obtain CAXcd
variant with H338N substitution and high apparent Cd transport activity (Shigaki
et al. 2005). When CAXcd was constitutively expressed in petunia (Petunia
hybrida Dreams™ Red), transgenic plants treated with either 50 or 100 μMCd
showed more vigorous growth compared with controls and accumulated up to 2.5-
fold more Cd in their leaves than WT (Wu et al. 2011).