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260 P. Kotrba

engineering root-associated rhizobia was employed by Ike et al. (2007). Rhizobia
establish a symbiotic relationship with leguminous plants and forms nitrogen
8
fixing-nodule that contains more than 10 bacterial progenies. When PCS gene
AtPCS1 from A. thaliana along with a genetic fusion of four mammalian
MT-coding sequences were expressed in Mesorhizobium huakuii subsp. rengei
(strain B3), the natural capability of the bacterium to accumulate Cd from media
containing 30 μM Cd increased by 25-fold. The colonization of Chinese milkvetch
(Astragalus sinicum) with the B3 strain in rice-paddy soil containing 1 mg kg 1 Cd
promoted uptake of the metal in roots, but not in nodules, by three times. Although
the enhanced Cd accumulation phenotype of the roots was not accompanied by an
increased metal translocation to the shoots, such a strategy would be useful in the
rhizofiltration or transient phytostabilization of heavy metals in soil. The heavy
metal-tolerant endophytes have been described from many hyperaccumulating
plants (Rajkumar et al. 2012). In an attempt to investigate whether or not the
introduction of endophytes engineered for the metal resistance would enhance
phytoextraction of Ni, nickel tolerance ncc-nre genes were integrated into
chromosomes of endophytic strains Burkholderia cepacia and Herbaspirillum
seropedicae (Lodewyckx et al. 2001). Contrary to expectation, when modified
strains were inoculated into host yellow lupin Lupinus luteus and ryegrass Lolium
perenne, they apparently did not influence the growth of plants or cause an
increased translocation of Ni in planta.

12.6 Conclusion and Future Prospect

Three different approaches are currently employed to develop transgenic plants
suitable for phytoremediation. These include (1) increasing the number of metal
transporters along with modulation of the specificity of the metal uptake system
(2) enhancing intracellular ligand production and the efficiency of metal targeting
into vacuoles to keep accumulated metal in a safe form without disturbing cellular
processes and (3) biochemical transformation of metal volatile forms. A substantial
experience has been gained, which helped to prove the suitability of heterologous
and/or promoted intrinsic gene expression for the development of plants useful in
phytoremediation. It is generally accepted that understanding of metal hyperaccu-
mulation physiology and molecular basis underlying metal homeostasis and adap-
tation in hyperaccumulating species can greatly contribute to development of high
biomass phytoremediation plants. Specifically, phytoremediation plants should be
modified for effective long-distance metal translocation and repressed metal depo-
sition in the roots and creation of artificial metal sinks in shoots. To this end,
overproduction of highly mobile metal ligands such as nicotianamine by engineered
plants or endophytes, manipulations to reduce transport into root vacuoles, and the
shoot-specific expression of engineered cell-wall proteins with high-affinity bind-
ing sites for metal deposition in the apoplast of aboveground tissues could be
instrumental. Successful phytoremediation of metal pollution may further involve