Full Journal - Journal of Cell and Molecular Biology - Haliç Üniversitesi

intracellular availability of essential metals within
certain limits. They may as well serve to reduce the
availability of nonessential metals. The capacity of
these systems can be varied by de novo synthesis of
the chelating compounds. The heavy metal-PC
complexes may also be relevant for animal and
human nutritional studies since these complexes
reflect the state of heavy metal chelation within these
plants (Gekeler et al.,1989). The "phytochelatin
response" or synthesis of Cd-binding polypeptides, is
one of the few examples in plant stress biology in
which it can be readily demonstrated that the stress
response (PC synthesis) is truly an adaptive stress
response (Steffens, 1990).
It has been shown that the energy necessary for PC
production is considerable: since PCs derive from
GSH, Cd-stressed cells have to restore the GSH used
to form them, by activating the enzymes catalyzing
GSH biosynthesis (Sanità di Toppi and Gabbrielli,
1999). However as discussed by Ow (1996), other
factors can be more important than the PC level for
efficient Cd detoxification, such as a PC reductase
enzyme which seems to be essential to guarantee
sufficient reducing power to prevent the oxidation of
Cd-induced PCs and ineffective Cd-binding. Also in
higher plants high levels of PCs may not be sufficient
for complete Cd detoxification. The rapid formation
of HMW complex, highly stabilized by S 2- groups,
seems to particularly decisive in Cd detoxification
(Speiser et al., 1992; Zenk, 1996).
Because of their ability to bind heavy metal ions,
PCs are considered to play a role in cellular metal
homeostasis and metal detoxification (Grill et al.,
1988; Rauser, 1990; Steffens, 1990). Moreover, it is
suggested that PCs are involved in differential heavy
metal tolerance, i.e. naturally or artificially selected
intraspesific heritable differences in the ability to
tolerate high levels of metal exposure. Increased
metal tolerance could involve increased or
accelerated production of PCs or the formation of
more stable metal-PC complexes due to either an
increase in the PC chain length or an increase in the
incorporation of labile sulfide into the complex
(Rauser, 1995).
It is possible that the plants can cope effectively
with Cd stress by means of the mechanisms of
avoidance, detoxification and repair which are
provided by PCs and vacuolar compartmentalizationi.e.
amount of proteins, rapidity in HMW formation,
number of γ-Glu-Cys units, high incorporation of
Phytochelatin and cadmium 53
S 2- , level of reduction of PCs. If Cd levels are low in
the soil or culture medium but the exposure time is
long, a general cellular homeostatic process may be
the plant management to chronic Cd stress (Sanità di
Toppi and Gabbrielli, 1999). If Cd levels are high or
very high and the exposure time is short, plant can
manage this acute Cd stress by detoxification
(constitutive response) and repair. Cd tolerance
(adaptive response), which is mostly observed in high
concentrations of Cd with a long exposure time, in
higher plants should be defined as the natural or
artificially given capacity regulated by interacting
genetic and environmental factors, thus the
development of tolerance should be a long-term
process (Sanità di Toppi and Gabbrielli, 1999). The
mechanisms required to adapt to highly contaminated
environments may involve just one of these processes
(Meharg, 1994).
Molecular genetic approaches have brought
important advances in our understanding of PC
biosynthesis (Clemens, 2001). The identification of
PC-synthase genes from plants and other organisms
and also the possibility to utilize the plant genes in
mammalian cells to enhance the resistance to Cd
toxicity by PC activity, is a significant breakthrough
that will lead to a better understanding of the
regulation of a critical step in PC biosynthesis.
Nonetheless, we must keep in mind the numerous
other aspects of PC biosynthesis and function, and the
ways in which they, too, are regulated at a cellular and
physiological level in response to heavy-metal
exposure. These include aspects of sulfur
assimilation, GSH and sulphide biosynthesis, PC
compartmentalization and the signal pathways
through which metal toxicity leads to gene regulation
(Cobbett, 2000, 2001).
Increasing pollution of the environment caused by
heavy metals is becoming a significant problem in the
modern world. The use of plants and the concept of
phytoremediation of contaminated soils has been
increasingly supported by research in recent years
(Salt et al., 1998; Cobbett, 2000; Rennenberg and
Will, 2000). To investigate the heavy-metal
detoxification processes will allow us to explore the
mechanisms by which some species are capable of
hyper-accumulation of metals such as Cd and how
they may be best used for phytoremediation.
Understanding the genetic and molecular basis of
such mechanisms is an important goal in developing
plants for the phytoremediation of contaminated
environments.