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4 Remediation Mechanisms of Tropical Plants for Lead-Contaminated Environment 71

concentration in peas (Pisum sativum L. cv Sparkle) was 11,000 mg kg 1 compared
to corn, which accumulated 3,500 mg kg 1 in soils receiving equivalent amounts of
EDTA (Malone et al. 1974). Although there are some advantages associated with
the use of synthetic chelates, environmental concerns governing their impact on
these contaminated sites are in need of research. The major concern associated with
using chelates to enhance phytoremediation and increase the bioavailability of the
toxic metals is the fear of lead leaching or running off into the ground or surface
water. By making the metals more soluble in the soil matrix, leaching is more
probable, threatening the contamination of nearby water sources (Reuther 1998).

4.5.1.2 Passive Mechanisms

Even when small amounts of lead penetrate root cell membranes, it interacts with
cellular components and increases the thickness of cell walls (Krzesłowska et al.
2009, 2010). Pectin is a component of plant cell walls. Lead complexation with
pectin carboxyl groups is regarded as the most important interaction by which plant
cells can resist lead toxicity (Meyers et al. 2008; Jiang and Liu 2010). Krzesłowska
et al. (2009) observed that binding of lead to JIM5-P (within the cell wall and its
resultant thickening) acted as a physical barrier that restricted lead access to the
plasma membrane in F. hygrometrica protonemata. However, later, these authors
stated that lead bound to JIM5-P within the cell can be taken up or remobilized by
endocytosis, together with this pectin epitope (Krzesłowska et al. 2010).

4.5.1.3 Inducible Mechanisms

Recently, several authors have reported the presence of transporter proteins among
plant cells that play an important role in metal detoxification, by allowing the
excretion of metal ions into extracellular spaces (Meyers et al. 2008; Vadas and
Ahner 2009; Maestri et al. 2010). The human divalent metal transporter 1 (DMT1),
expressed in yeast, has been shown to transport lead via a pH-dependent process in
plants (Bressler et al. 2004). Simultaneously, several ATP-binding cassette (ABC)
carriers, such as AtATM3 or AtADPR12 at ATP-binding sites in Arabidopsis, were
involved in resistance to lead (Kim et al. 2006; Cao et al. 2008). Although suspected
to act against lead, this detoxification mechanism has not yet been clearly con-
firmed. Transcriptome analysis has shown that the gene expression of these carriers
is stimulated by lead (Liu et al. 2009). Cellular sequestration is considered to be an
important aspect of plant metal homeostasis and plant detoxification of heavy
metals (Maestri et al. 2010). The lead, which could be bound by certain organic
molecules (Piechalak et al. 2002; Vadas and Ahner 2009), is sequestered in several
plant cell compartments: vacuoles (Małecka et al. 2008; Meyers et al. 2008),
dictyosome vesicles (Malone et al. 1974), endoplasmic reticulum vesicles
(Wierzbicka et al. 2007), or plasma tubules (Wierzbicka 1998). Cysteine and
glutathione (GSH) are known to be nonenzymatic antioxidants in plants.