Page 84 - Plant-Based Remediation Processes

72 O.P. Abioye et al.

An increase in cysteine content, in response to lead toxicity, has been demonstrated
in Arabidopsis thaliana (Liu et al. 2009). Glutathione protects plants from lead
stress by quenching lead-induced ROS (Verbruggen et al. 2009; Liu et al. 2009).
Moreover, as the substrate for phytochelatin (PC) biosynthesis, the glutathione-
related proteins play an important role in heavy metal detoxification and homeosta-
sis (Liu et al. 2009). Lead treatment can induce different GSH genes, including
glutathione-synthetase, -peroxidase, and -reductase, and glutamylcysteine synthe-
tase. Glutathione can also enhance accumulation of proline in stressed plants, a role
that is associated with reducing damage to membranes and proteins (Liu et al.
2009). Gupta et al. (2010) reported the role of GSH in lead detoxification in S.
alfredii, although this was accomplished without any induction of PC. This suggests
that GSH may play an important role in detoxifying lead, under stress conditions
where PCs are absent. PCs and metallothioneins (MTs) are the best characterized
metal-binding ligands in plant cells. These ligands belong to different classes of
cysteine-rich heavy metal-binding protein molecules. PCs, the most frequently
cited metal protective proteins in plants, are low-molecular-weight, metal-binding
proteins that can form mercaptide bonds with various metals (Maestri et al. 2010)
and play an important role in their detoxification in plants (Brunet et al. 2009; Liu
et al. 2009; Gupta et al. 2010; Yadav 2010; Jiang and Liu 2010). These thiols are
biologically active compounds, whose function is to prevent oxidative stress in
plant cells (Verbruggen et al. 2009; Gupta et al. 2010). Their general structure is
(γ-glutamyl-cys) n Gly where n ¼ 2–11, and they are synthesized by the action of
γ-glutamylcysteine dipeptidyl transpeptidase (phytochelatin synthase; PCS) from
GSH (Yadav 2010). Lead is known to stimulate the production of PC and activate
PCS (Mishra et al. 2006; Clemens 2006; Andra et al. 2009; Vadas and Ahner 2009;
Singh et al. 2010). It has been proposed that in vivo, phytochelatins are involved in
the cellular detoxification and accumulation of several metals, including lead,
because of their ability to form stable metal–PC complexes (Clemens 2006;
Yadav 2010). Phytochelatin sequesters soluble lead in the cytoplasm before
transporting it to vacuoles and chloroplasts (Piechalak et al. 2002; Małecka et al.
2008; Jiang and Liu 2010), thus reducing the deleterious effect of Pb 2+ in the cells.
The mechanism regulating the passage of the lead–PC complex through the tono-
plast is, however, not yet known. Gisbert et al. (2003) reported significantly
increased uptake and tolerance to lead and Cd following the induction and
overexpression of a wheat gene encoding for phytochelatin synthase (TaPCS1)in
Nicotiana glauca.

4.5.2 Antioxidant Enzymes

To cope with the increased production of ROS and to avoid oxidative damage,
plants have a system of antioxidant enzymes that scavenge the ROS that are present
in different cell compartments (Brunet et al. 2009; Singh et al. 2010; Gupta et al.
2010). Lead-induced toxicity may inhibit the activity of these enzymes or may