Page 22 - Plant-Based Remediation Processes

1 Phytoremediation Protocols: An Overview 9

2003), Oryza sativa (Heaton et al. 2003), yellow poplar L. tulipifera (Rugh et al.
1998), overexpressing bacterial merA and/or merB become more tolerant to Hg 2+
and R-Hg + and release 10 times higher elemental Hg as compared to
nontransformed plants. It has been reported that transgenic plants in which MerB
is targeted in the endoplasmic reticulum rather than cytoplasm, release mercury in
tenfold higher volatile form (Bizily et al. 2003).

1.4.2 Detoxification of Selenium by Plants

Two pathways dominate in the natural detoxification of selenium (Se) in plants.
In most species, selenium is most toxic after metabolization into analogues of
amino acid cysteine and methionine. Selenium hyperaccumulating plant species
have a specific enzyme, selenocysteine methyltransferase (SMT) which is respon-
sible for converting selenate into methyl selenocysteine (MetSeCys), ultimately
incorporated into the proteins and thus resulting in hyperaccumulation of selenium.
In a second detoxification mechanism, selenate can be metabolized into dimethyl-
selenide (DMSe) which is 100 times less toxic than selenate and selenite in soil and
volatilized from leaves and roots (Terry et al. 2000). Transgenic Indian mustard
(Brassica juncea L.) transformed with the SMT gene from Se-hyperaccumulator
Astragalus bisulcatus releases a higher DMSe in addition to an improved Se
accumulation and tolerance in comparison to the control plants (LeDuc et al. 2004).

1.5 Rhizofiltration

This phytoremediation method can be defined as the use of aquatic plants, either
floating or submerged to absorb, concentrate, and remove hazardous compounds
particularly heavy metals or radionuclides from aqueous environment by their roots
(January et al. 2008; Eapen et al. 2003) (Fig. 1.3). A suitable plant for rhizofiltration
should have larger root system through which toxic metals are taken up from
solution over an extended period. Such plant should be capable of producing up
2
to 1.5 kg (dry weight) of roots per month per m of water surface (Dushenkov et al.
1997). Rhizofiltration usually involves in hydroponically cultivated plants in a
stationary or moving aqueous system wherein the plant roots absorb pollutants
from the water (Salt et al. 1995). Candidate plant for rhizofiltration includes the
Indian mustard (Brassica juncea), sunflower (Helianthus annuus), and corn
(Zea mays) (Brooks and Robinson 1998). Success of rhizofiltration greatly depends
on the physicochemical characteristics of the plants, which may favor the process of
bio-adsorption (Olgu{n and Sanchez-Galvan 2012).
Dushenkov et al. (1995) reported that within 24 h, submerged roots of sunflower
plants were able to substantially reduce the levels of Cd, Cr, Cu, Mn, Ni, Pb, Sr, U,
and Zn in water bringing metal concentration close to or below the discharge limit.