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3 Metal/Metalloid Phytoremediation: Ideas and Future 47

formed soluble complexes with heavy metals and in that form may be transported
for example to vacuoles. Metal presence also induces production of specific ligands
such as metallothioneins (MTs) and phytochelatins (PCs). A large number of recent
studies in plants involving sensitive, tolerant, mutant, transgenic, and hyper-
accumulator plants concentrate on glutathione and phytochelatins as key elements
in their tolerance and accumulation strategies. Phytochelatins (PCs) were first
isolated by Grill et al. (1985) from a cell-suspension culture of Rauvolfia
serpentina. They have the structure [(γ-Glu–Cys)n–Gly], where n is the number
of replications of the (γ-Glu–Cys) units, which is generally in the range 2–11. The
enzyme responsible for their synthesis is c-glutamyl cysteine dipeptidyl
transpeptidase (phytochelatin synthase: PCS), the substrate of which is glutathione
(Grill et al. 1989). The enzyme is also expressed constitutionally but is primarily
activated by the presence of heavy metals. First phytochelatins bind metals and
form low-molecular-weight (LMW) complexes, then form high-molecular-weight
(HMW) complexes with acid-labile sulphur, which are more stable. The HMW
metal–PC complexes are then transported to the vacuole, where under acidic pH the
metals form complexes with organic acids (citrate, oxalic acid and malate) and
probably with amino acids, while the phytochelatins are either decomposed by
hydrolases or return to the cytosol (Sanita ` di Toppi and Gabbrielli 1999). The
literature on the relationship between heavy-metal tolerance and phytochelatin
synthesis contains many contradictions (Arisi et al. 2000;Pa ´l et al. 2006;
Barałkiewicz et al. 2009). Wo ´jcik and Tukiendorf (2011) found that tolerant
maize accumulated far less Cd than the more sensitive rice or wheat. The use of
more sensitive techniques allowed observation of both oxidised and reduced form
of glutathione and phytochelatins in plants treated with cadmium and lead
(Barałkiewicz et al. 2009). The authors showed that only Cd was chelated by
phytochelatins while Pb bound with proteins and phytochelatins probably play a
role as antioxidants. This may suggest that phytochelatins are important
components in the detoxification of heavy metals, but they are unlikely to be
responsible for metal tolerance, avoidance or hyperaccumulation.
In conclusion, numerous important processes are affected by metal presence and
have consequences for plant condition and growth. Some of these mechanisms are
specifically involved in defence against heavy metals and their impacts on plant
functioning, such as phytochelatins, antioxidants and proline. An important field for
further research may be the tolerance mechanism of plants exhibiting hyperaccu-
mulation abilities.

3.4 Genetic Background of Plant Adaptation
to and Hyperaccumulation of Metal(loid)s

Two adaptation strategies are observed in plants growing on metalliferous soils: the
common “excluder” strategy which restricts accumulation of toxic metal(loid)s to
the root, and the more advanced hyperaccumulator strategy of translocation to the
shoot. Toxic metal uptake, translocation and accumulation interacts with uptake