Page 117 - Plant-Based Remediation Processes

106 M.d.S. Santos-Dı ´az

Table 6.1 Metal removal by hairy roots cultures
Metal pollutant Species Reference
Cadmium Calystegia sepium Metzger et al. (1992)
Solanum nigrum Macek et al. (1994)
Thlaspi caerulescens Nedelkoska and Doran (2000a)
Adenophora lobophylla Wu et al. (2001)
Adenophora potaninii
Thlaspi caerulescens Boominathan and Doran (2003a, b)
Nicotiana tabacum
Copper Hyptis capitata Nedelkoska and Doran (2000b)
Nicotiana tabacum
Polycarpaea longiflora
Euphorbia hirta
Nickel Alyssum bertolonii Nedelkoska and Doran (2001),
Alyssum tenium Boominathan and Doran (2002, 2003b)
Nicotiana tabacum
Uranium Brassica juncea Eapen et al. (2003)
Chenopodium amaranticolor
Armoracia rusticana Soudek et al. (2011)
Daucus carota Straczek et al. (2009)
Zinc Solanum niger Subroto et al. (2007)

6.5.1 Cadmium

Cd has been ranked as one of the major heavy metal hazards because it is mobile in
soils, penetrates easily into the food chain, and presents adverse effects for
human health (McLaughlin and Singh 1999). Among the first hairy roots tolerant
to Cd are those obtained from Calystegia sepium (Metzger et al. 1992) and Solanum
nigrum (Macek et al. 1994). These cultures accumulate about 1,100 μgg -1 and
24,455 μgg -1 (dry weight basis), respectively. However, hairy roots of Thlaspi
caerulescens greatly surpass these concentrations, reaching 62,800 μgg -1 dry
weight, accumulation which corresponds to 6.3 % dry weight. T. caerulescens
roots localized the metal in the cell wall fraction during 7–10 days before allowing
passage into the symplasm. This delay represents an important defensive strategy
against Cd poisoning allowing time for activation of intracellular mechanism for
heavy metal detoxification (Nedelkoska and Doran 2000a, 2003a). It has been
found that T. caerulescens roots present high endogenous activities of catalase
and superoxide dismutase, and high concentration of glutathione. In addition, the
levels of H 2 O 2 were maintained at nontoxic levels in the presence of metal. These
results show that antioxidative defenses, specifically the induction of catalase
activity, play an important role in the mechanism of tolerance to Cd in
T. caerulescens roots (Boominathan and Doran 2003a). An increase in the activity
of the enzymes superoxide dismutase (SOD) and peroxidase (POD) was also
observed in Cucumis sativus L. hairy roots in the presence of low concentration
of Cd (Zhang et al. 2009).