Page 57 - Plant-Based Remediation Processes

3 Metal/Metalloid Phytoremediation: Ideas and Future 45

amounts of reactive species lead to disruption of cell equilibrium. The level of ROS
generation depends on heavy metal characteristic, speciation form, and concentra-
tion. Metals can be divided into two groups: redox active (Fe, Cu, Cr, Co) and redox
inactive (Cd, Zn, Pb, Ni, Al, etc.) (Hossain et al. 2012). Metals such as Cu or Fe are
•
known to be directly involved in the formation of O 2 and consequently H 2 O 2 and
•
also highly reactive OH via the Haber-Weiss and Fenton reactions. However,
oxidative stress in plants exposed to metals such as Cd or Pb is an effect of their
interaction with membrane lipids and proteins, antioxidative enzymes, elements of
the electron transport chain and consequently disruption of their functioning (Hall
2002; Metwally et al. 2005; Romero-Puertas et al. 2007). Increased levels of ROS
lead to lipid peroxidation, protein oxidation, disturbances in membrane permeability
and cell division, but simultaneously they may also function as signalling molecules.
Due to its characteristics, H 2 O 2 in particular may be perceived as a signalling
molecule (Dat et al. 2000). Many authors have observed an increase of H 2 O 2
concentration in response to different metals in various plants such as lupine,
tomato, A. thaliana, barley, pea and bean (Cho and Park 2000; Maksymiec and
Krupa 2006; Małecka et al. 2009). The increase of ROS induces activation of
antioxidative mechanisms at the molecular and biochemical level or may activate
the apoptosis pathway. The activated elements of defence systems differ depending
on metals, plant development, organ or tissue. Among the early activated
mechanisms, antioxidative enzymes play an important role in maintaining the cell
balance. The most studied elements of this system include superoxide dismutase
(SOD; EC 1.15.1.1), ascorbate peroxidase (APX; EC 1.11.1.11), monodehy-
droascorbate reductase (MDHAR; EC 1.6.5.4), dehydroascorbate reductase
(DHAR; EC 1.8.5.1), glutathione reductase (GR; EC 1.6.4.2), catalase (CAT; EC
1.11.1.6), glutathione peroxidase (GPX; EC 1.11.1.9), and glutathione S-transferase
(GST; EC 2.5.1.18). In particular, superoxide dismutase, catalase and peroxidase are
important to balance the production and elimination of ROS in plant cells. Also
participating in the antioxidative response are low molecular weight compounds
including ascorbate (AsA) and glutathione (GSH) which can directly quench ROS,
cooperate with antioxidative enzymes such as APX, GPX, GST or GR and also
regulate gene expression of proteins involved in the stress response.
The level of enzyme activity is dependent on metal concentration and properties,
exposure time and tolerance ability of plants. An increase in CAT activity was
observed in response to Cd, Fe and Pb in Nicotiana plumbaginifolia, Pisum sativum
and Brassica juncea (Minglin et al. 2005; Małecka et al. 2009). On the other hand,
Romero-Puertas et al. (2007) reported a decrease of CAT activity in pea plants
treated with 50 μM CdCl 2 . The author suggested that enzyme inhibition was caused
by protein oxidation due to metal presence which led to upregulating the transcrip-
tion of the corresponding gene. Also for other enzymes there is a similar lack of a
clear pattern between activity, metal ions and plants. For example, there are reports
not only indicating increase of Cu and Zn-SOD activity in tomato, pea and Indian
mustard plants treated with Cu, Cd, Pb or Fe (Pich and Scholz 1993; Lin et al. 2007;
Małecka et al. 2008) but also describing decrease of enzyme activity in tomato,
lupine and pea plants treated with different metals (Romero-Puertas et al. 2007).