Page 119 - Plant-Based Remediation Processes

108 M.d.S. Santos-Dı ´az

After treatment of roots with DES a reduction on the entry of Ni into the symplasm
was observed, reflecting that there is a coupling between the proton gradient
+
generated by H ATPase and the transport of Ni (Boominathan and Doran 2003a).
On the other hand, A. bertolonii hairy roots presented 2.4–500 times higher
levels of the antioxidant enzymes superoxide dismutase and catalase than Nicoti-
ana tabacum. However, the exposure to Ni reduced the activity of these enzymes
and increased the levels of H 2 O 2 without affecting the growth of cultures. The
authors suggest that other mechanisms for tolerating reactive oxygen species
(ROS) must be involved, as enhanced vacuolar compartmentation (Boominathan
and Doran 2002).

6.5.3 Uranium

Uranium presents both chemical and radiological hazards, the former being the
greater risk factor (Ribera et al. 1996). Hairy root cultures have been used for
several years to extract uranium from aqueous solutions. Metal removal was
performed within a short period of incubation by hairy root cultures from Brassica
juncea and Chenopodium amaranticolor (Eapen et al. 2003). At 500–5,000 μM
uranyl acetate, a near-linear uptake was observed for C. amaranticolor roots,
whereas B. juncea cultures showed saturation. For all concentrations used, 90 %
of the uranium was taken up by the root tissue within 10 h of treatment, and both
-1
cultures were able to accumulate 8,000 μgg dry weight. Hairy roots from carrot
and Armoracia rusticana also are able to remove uranium from medium (Soudek
et al. 2011; Straczek et al. 2009). Accumulation of uranium in A. rusticana was very
fast, reaching the maximum at 1–2 h, and was dependent on initial concentration
(50–500 μM), indicating that metal uptake is due to a simple or facilitated passive
diffusion more than an active transport. Roots of carrot were more sensitive to
uranium concentration presenting toxicity symptoms at 6 days at 15, 20, and
-1
30 mg L uranium (Straczek et al. 2009). Accumulation of uranium is influenced
by the phosphate concentration on culture medium. On C. amaranticolor and
B. juncea root cultures the phosphate reduces uranium uptake (Eapen et al.
2003); meanwhile in A. rusticana cultures, this ion improved 50 % metal accumu-
lation (Soudek et al. 2011). It has been described that in a pH range from 4 to 7.5,
uranium exists as a phosphate complex which can be transported to the aerial parts
(Vandenhove et al. 2007). Considering that the pH of culture media usually is
5.5–5.7, the formation of uranium–PO 4 complexes under in vitro conditions is
possible. Then the different responses observed in A. rusticana vs C. amaranticolor
and B. juncea cultures relative to phosphate content could be related to their
capacity for internalizing the uranium–PO 4 complex. A. rusticana root cultures
presented a light increase in peroxidase and glutathione-S-transferase activities in
the presence of uranium but data are not conclusive. Further work is needed to
explain the mechanism of tolerance in A. rusticana root cultures.