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5 Impact of Metal/Metalloid-Contaminated Areas on Plant Growth 95

abundant under a contamination environment, studies evaluating metal accumula-
tion by R. ulmifolius are not yet common.

5.4.3 Transgenic Plants

Biotechnology has already been successfully employed to manipulate metal
uptake and tolerance properties in selected plant species. Tolerance of trans-
genic plants to the presence of toxic levels of metals such as Cd (Kawashima
et al. 2004), Zn, Cr, Cu, Pb (Bennet et al. 2003), As (Lee et al. 2003a, b), and
Se (Berken et al. 2002) has been reported. A combination of transporter genes
has also been used in rapidly growing plant species, leading to promising
results (Lee et al. 2003a, b; Song et al. 2004). For example, in tobacco
(Nicotiana tabacum) increased metal tolerance has been obtained by expressing
the mammalian metallothionein, metal binding protein, genes. Possibly, the
most spectacular application of biotechnology for environmental restoration
has been the bioengineering of plants capable of volatilizing mercury from
soil contaminated with methyl mercury. Methyl mercury, a strong neurotoxic
agent, is biosynthesized in Hg-contaminated soils. To detoxify these substances,
transgenic plants (Arabidopsis thaliana and Nicotiana tabacum)were
engineered to express bacterial genes merBand merA. In these modified plants,
merB catalyses the protonolysis of the carbon–mercury bond with the genera-
2+
tion of Hg , a less mobile mercury species. Subsequently, MerA converts Hg
(II) to Hg (0), a less toxic, volatile element, which is released into the
atmosphere.
Overexpression of genes involved in phytochelatins (PCs) enabled the
development of useful plants for phytoremediation, e.g. under Cd and As stress
(Dhankher et al. 2002; Gasic and Korban 2007;Guo et al. 2008;Blumetal.
2010). Plants expressing SRS1p/ArsC and Act1P/γ-ECS showed two- to three-
fold elevated accumulation of As per gram of tissue in comparison to wild
plants expressing γ-ECS or Act1P alone (Dhankher et al. 2002). Simultaneous
overexpression of both AsPCS1 andYCF1intransgenic Arabidopsis thaliana
resulted in longer roots and higher Cd and As accumulation than single-gene
transgenic lines and wild plants (Guo et al. 2012). Transgenic Brassica juncea,
grown either hydroponically or in soils, shows higher uptake of Se and
enhanced Se tolerance compared to the wild species (Pilon-Smits et al.
1999). To engineer Se tolerance the seleno-cysteine methyltransferase (SMT)
gene has been transferred from the Se hyperaccumulator A. bisulcatus to Se-
non-tolerant B. juncea. SMT transgenic plants of B. juncea grownina
contaminated soil accumulate 60 % more Se than the wild type (Zhao and
McGrath 2009 and literature reported therein) (Wenzel 2009; Rascioa and
Navari-Izzo 2011).