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128 S. Chatterjee et al.

(Patten and Glick 1996; Gahoonia et al. 1997; Barker and Banfield 1998;
Gamalero et al. 2002; Calvaruso et al. 2006; Kidd et al. 2009).

7.10 Selection of Plants and Enhancing the Efficacy
of the Process

Improvement of biomass production is most important for the application of
phytoextraction technology that results in a higher metal extraction or total metal
yield. As for example, inoculation of rhizobacteria Pseudomonas fluorescens bio-
type F, isolated from heavy metal contaminated soil, helped to improve the growth
of sunflower plants (Helianthus annuus) and their tolerance to arsenate in soil (Raab
et al., 2005). Bacterial production of IAA and siderophores played important roles
to develop tolerance towards arsenate (Prasad 2007). Few studies suggest that
application of transgenic plants along with rhizospheric PGPR improve plant
biomass that will help in phytoextraction (Farwell et al. 2006). Pseudomonas putida
HS-2 (isolated from Ni-contaminated soil) applied to the transgenic canola (Bras-
sica napus) showed trends of higher accumulation of total Ni per plant. However,
Kuffner et al. (2008) reported that rhizobacterial strains which were found to
increase Cd/Zn uptake and accumulation and consequently growth of Salix caprea
were neither phytohormone-producing strains nor siderophore producers.
Application of bioremediation practices depend upon the detoxification of toxic
metals and xenobiotics through metabolism. It is reported that among various
molecules, proteins like cytochrome P450, phytochelatins, and metallothioneins
are very important biomolecules in this process. Augmenting the expression of
these biomolecules may help to improve the efficiency of bioremediating agent
(Choi et al. 1996; Clemens et al. 1999; Morris et al. 1999; Cobbett 2000; Cobbett and
Goldsbrough 2002; Morant et al. 2003; Gillam 2008). Genetic supplementation by
creating transgenic plants to increase remediation potential of highly toxic element
is an alternative approach in this technology. It has been shown that tobacco plants
carrying MerA gene from E. coli (encoding mercuric reductase) can mobilize
mercury 5–8 times higher than control counterpart (Ke et al. 2001; Glick 2004).
Similarly, over expressing two bacterial genes (encoding arsenate reductase (arsC)
and γ-glutamylcysteine synthetase (γ-ECS)) in the small weed Arabidopsis thaliana
significantly increased the accumulation of arsenic in leaves (Doucleff and Terry
2002). Reduction of arsenate to arsenite is catalyzed by the arsC, while γ-ECS
catalyzes the first step in the synthesis pathway of phytochelatins, increasing the
pool of thiol compounds including phytochelatins, all through the body of the plant.
After detoxification of arsenite by thiol compounds forming arsenic–protein
thiolates, may be stored and/or partitioned in the vacuole enabling arsenic to
accumulate at greater amounts in the leaves of these transgenic plants (Doucleff
and Terry 2002; Dhankher et al. 2002; Michel et al. 2007).