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250                                                         P. Kotrba

            Table 12.1 High biomass plants genetically modified for enhanced phytoextraction of heavy
            metals
            Plant          Modification    Metal species affected  References
                                            2+
            Indian mustard  Uptake and    Cd ,Zn 2+           Xu et al. (2009),
                              translocation                     Bhuiyan et al.
                                                                (2011a),
                                                                Lang et al. (2011)
                           Vacuolar       Cd 2+               Bhuiyan et al. (2011b)
                              sequestration
                                                    2+
                                                        2+
                                            2+
                                                2+
                           PC and GSH     Cd ,Zn ,Cu ,Pb ,    Zhu et al. (1999a, b),
                                                      2
                                                2
                              synthesis     CrO 4 ,WO 4 ,       Pilon-Smits et al.
                                                2
                                            MoO 4               (2000),
                                                                Reisinger et al.
                                                                (2008)
                                            2+
                                                 3
                                                       2
                           Sulfur metabolism  Cd ,VO 4 , CrO 4 ,  Pilon-Smits et al.
                                                2     2         (1999),
                                            WO 4 , MoO 4
                                                                Wangeline et al.
                                                                (2004),
                                                                Lindblom et al.
                                                                (2006)
                                                        2+
                                                2+
                                                    2+
                                            2+
            Shrub tobacco  PC synthesis   Pb ,Cd ,Zn ,Cu ,    Gisbert et al. (2003),
                                            Ni 2+               Martı ´nez et al.
                                                                (2006)
            Aspen          PC synthesis   Pb 2+               Couselo et al. (2010)
                                            2+
            Poplar         GSH synthesis  Cd ,Zn 2+           Arisi et al. (1997),
                                                                Koprivova et al.
                                                                (2002),
                                                                Bittsa ´nszky et al.
                                                                (2005)
                                            2+
                           Metallothionein  Cu ,Zn 2+         Balestrazzi et al. (2009),
                                                                Turchi et al. (2012)
            Flax           Metallothionein  Cd 2+             Vrbova et al. (2012)
            of contaminated soils. Therefore some efforts, especially for improved
            phytostabilization and rhizofiltration, should also be directed to improve root
            sequestration by metal-complex formation and deposition in vacuoles.
            12.4.1 Plants Engineered for Improved Metal Uptake
                    and Translocation
            The transport of essential metals or alkali cations across plasma membranes by
            means of primary and secondary active transporters is of central importance in the
            metal homeostasis network in all organisms. Relatively broad substrate specificity of
            transporters makes them a promising tool to improve toxic metal uptake for
            phytoremediation. The N. tabacum plasma membrane transporter NtCBP4
            (calmodulin-binding protein), for example, is structurally similar to vertebrate and
            invertebrate K +  and to nonselective cation channels (Arazi et al. 1999).
            Overexpression of entire NtCBP4 resulted in transgenic tobacco in 20 % increased
            uptake and translocation of Pb to shoots, reflected in the higher sensitivity of
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