Page 18 - Plant-Based Remediation Processes

1 Phytoremediation Protocols: An Overview 5

improve or modify the metal accumulating plants: the conventional breeding and
genetic engineering. Although a number of reports exist on successful crop breed-
ing (Gleba et al. 1999; Dushenkov et al. 2002; Alkorta et al. 2004; Nehnevajova
et al. 2007) yielding improved metal accumulator plants, the major constraint in
developing such hybrid is sexual incompatibility between the taxa. Transgenic
plants have opened new avenues in phytoremediation technology by expressing
the desired gene and overcoming the limitations imposed by sexual incompatibility.

1.3.1 Transgenic Approaches to Develop Metal-Accumulating
Plants

Metallophytes have distinct biological mechanisms that enable them to tolerate
high tissue metal concentration. Recent progress in understanding the molecular
basis of metal accumulation and tolerance by metallophytes has provided a strong
scientific basis for creating transgenics that enhance phytoextraction potential.
Some of the possible areas of genetic manipulation are outlined below:
• Metallothioneins (MT) and phytochelatins (PCs) are known as metal-chelating
proteins, responsible for the detoxification and accumulation of metals
(Hirata et al. 2005). Genetic manipulation of the plants for synthesis of metal
chelators will improve the capability of plants for metal uptake by increasing the
availability of such metals (Pilon-Smits and Pilon 2002; Clemens et al. 2002;
Lee et al. 2003).
• Genes involved in metal uptake, translocation, and sequestration in plants are well
studied. Introduction or overexpression of any of these genes into candidate plants
(Table 1.1) could be a way to enhance the previously mentioned pathway in non-
hyperaccumulators (Clemens et al. 2002). Transgenic plants overexpressing the
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genes encoding the enzymes for histidine biosynthesis and ACC deaminase, Hg -
reductase, glutathione synthetase, arsenate reductase, and aldolase/aldehyde reduc-
tase, were shown to become more tolerant to the toxic levels of metals and carried
out phytoextraction with increasing potential (Stearns et al. 2005;Thomasetal.
2003; Bennett et al. 2003; Shah and Nongkynrih 2007).
• The repression of an endogenous gene expression by inserting an antisense RNA
can also result in enhanced metal uptake by plants (Shah and Nongkynrih 2007).
• The introduction of an additional metal-binding domain to the implemented
protein further enhances the metal-binding capacity (Kotrba et al. 1999).
• Another promising approach is overexpressing the enzymes catalyzing rate-
limiting steps. ATP sulfurylase (APS) is such a rate-limiting enzyme in the
selenium detoxification processes. The overexpression of APS in transgenic
Brassica juncea led to three times more uptake and accumulation of selenium
in comparison to wild plants (Pilon-Smits et al. 1999).