Page 118 - Plant-Based Remediation Processes

6 Metal Remediation via In Vitro Root Cultures 107

Interaction of Cd with organic acids in hairy root cultures has also been
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described, but plays a minor role. For example, the treatment with 20 μgg Cd
causes 13 % of the metal to be associated with organic acids in T. caerulescens roots
(Boominathan and Doran 2003a). Distribution of Cd can be affected by treatment of
+
roots with diethylstilbestrol (DES), an inhibitor of plasma membrane H ATPase
that collapses the proton gradient. DES significantly altered the uptake and distri-
bution of Cd in T. caerulescens cultures promoting a shift of Cd away from the cell
walls into the symplasm, and a substantial increase in Cd concentration inside the
cells. This result shows that Cd tolerance and hyperaccumulation are not affected
by disruption of the transmembrane proton gradient (Boominathan and Doran
2003a). Other effects observed during Cd 2+ exposure in hairy roots include changes
in protein and PC content. Hairy roots from Adenophora lobophylla (species with
low resistance to environmental stress) and A. potaninii (species widely distributed
and vigorous growth) cultivated in 10–200 μM Cd increased 1.7- to 2-fold the
protein content compared to control cultures. In addition, A. lobophylla
accumulated more Cd, PCs, glutathione (GSH), and cysteine than A. potaninii.
The shift of protein pattern and the lower accumulation of Cd in A. potaninii
suggested a possible Cd exclusion system (Wu et al. 2001).

6.5.2 Nickel

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Nickel occurs in soil with typical concentrations of 1–200 μgg . It has phytotoxic
effects on plant growth, photosynthesis, and membrane function (Pandolfini et al.
1992). Ni recovery from phytomining processes is important since the metal has
relatively high commercial value (Robinson et al. 1997). To investigate Ni uptake,
Nedelkoska and Doran (2001) compared Ni tolerance and Ni accumulation in hairy
roots from Alyssum bertolonii, A. tenium, and A. troodii. In short-term experiments
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(9 h) the highest Ni content was 17,500 μgg in A. bertolonii, and 1,100 μgg in
A. tenium and A. troodii. Growth of hairy roots from A. bertolonii was unaffected
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by the presence of 20 μgg Ni, while growth of A. tenium was inhibited. The
addition of EDTA to the medium improved growth of hairy roots and reduced the
Ni content in root biomass, suggesting a protective effect of EDTA in chelating Ni
ions in solution (Nedelkoska and Doran 2001). In hyperaccumulator plants, Ni is
complexed with organic acids (malic, citric, malonic) or amino acids (histidine,
glutamine). This mechanism of tolerance is also present in hairy roots but is not the
main response. It has been reported that A. bertolonii hairy roots contained
high constitutive levels of citric, malic, and malonic acids. After treatment with
25 μgg -1 Ni about 28 % of the total Ni was associated with organic acids and
85–95 % with the symplasm (Boominathan and Doran 2003a). In A. bertolonii
hairy roots the Ni was distributed along length of roots including root tips
(Boominathan and Doran 2003a), but virtually all metal is located in the symplasm.