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

50 Gülriz Bayçu
PC production was found to be the main response
mechanism to Cd stress in the roots of higher plants
such as: Avena sativa, Brassica juncea, Cucumis
sativus, Glycine max, Hordeum vulgare, Lactuca
sativa, Lupinus luteus, Lycopersicon esculentum,
Oryza sativa, Phasealous vulgaris, Pisum sativum,
Raphanus sativus, Sesamum indicum, Silene vulgaris,
Zea mays (Klapheck, 1988; Leita et al.,1991; De
Knecht et al., 1994; Inouhe et al., 1994; Klapheck et
al., 1995; Salt et al., 1995; Chen et al., 1997; Sanità
di Toppi and Gabbrielli, 1999).
The fact that root tissue contains a much higher
concentration of heavy metals as well as of PCs than
the leaf tissue points to the fact that metals are
obviously immobilized to a far greater extent at the
site of metal uptake. Binding of Cd to PCs in Betula
pendula roots has been suggested as an explanation
for tolerance to Cd toxicity in this tree. It is assumed
that PCs participate in protecting the root against Cd
interferences with growth, possibly by restricting
Cd-induced changes in the nutrient composition of
the plant (Gussarsson et al., 1996).
PC is demonstrated as the major intracellular Cd
chelator in the microalga, Chlamydomonas
reinhardtii. From Cd challenged algal cells high
molecular weight (HMW) and low molecular weight
(LMW) complexes were purified and characterized
and these complexes differed in accumulation
kinetics, PC profile, acid labile sulfide content, and in
vivo turnover rate. According to this investigation, the
accumulation of LMW complex appeared to be an
early sign of metal stress and HMW complex
contributed to stable Cd sequestration. (Hu et al.,
2001). In another investigation with B. juncea, it is
suggested that the high levels of sulfur uptake by this
plant may play an important role, because Cd
exposure and the resulting burst of PC synthesis have
been reported to deplete GSH faster than biosynthesis
can replenish it (Tukendorf and Rauser, 1990).
Therefore, the production of PCs with their
advantages in stability over the LMW complex, could
contribute to higher metal tolerance by more effective
sequestration (Speiser et al., 1992).
In Ailanthus altissima roots, 200 mM Cd supply
for 1 week induced three strongly bound slow
migrating (HMW) PCs and five fast migrating
(LMW) PCs which include a very fast migrating and
strongly bound PC-Cd complex. Binding of purified
PCs from the root extract to 109 Cd 2+ were observed
through non-denaturing gel electrophoresis and direct
autoradiography. The Mr of the Cd-binding proteins
were found with the following pattern: 10.5, 18, 22,
23.5, 26, 50, 65, 75 kDa. The heterogeneity of PC-
Cds was probably due to the difference in the number
of PC-Cd chains, chain lengths, molecular weights,
S 2- amounts and number of Cd 2+ ion bounds.
Tentatively, they were assumed to represent PCs of
the type (γ-Glu-Cys)n-Gly where n=2-6 and may play
a role in Cd-detoxification and in Cd-tolerance
mechanism in this pollution resistant tree (Bayçu,
1998).
A number of observations indicate that synthesis
of PCs in response to Cd exposure is essential for
expression of tolerance to this metal. Mutants of
S. pombe that are unable to synthesize PCs have
increased sensitivity to Cd. Therefore by sequestering
the metal in a less toxic form, PCs appear to play a
critical role in the mechanism that protects cellular
metabolism from damage caused by Cd. Increased Cd
tolerance is associated with higher concentrations of
PCs and accumulation of HMW PCs (Gupta and
Goldsbrough, 1991).
Regulation of PC biosynthesis
1. PC Synthase
PC synthesis is regulated at a number of levels, most
importantly through the activation of PC synthase by
various heavy metal ions. PC synthase is a
cytoplasmic constitutive enzyme and its activity is the
expected major determinant of the rate of PC
synthesis (Grill et al., 1989; Chen et al., 1997;
Cobbett, 2000). Kinetic studies using plant cell
cultures exposed to Cd demonstrated that PC
biosynthesis occurs within minutes of exposure to the
heavy metal and is independent of de novo protein
synthesis (Rauser, 1999). Likewise, in in vitro studies
of PC synthase expressed in E. coli or in S. cerevisiae,
the enzyme was activated to varying extents by Cd,
Cu, Ag, Hg, Zn and Pb ions (Clemens et al., 1999; Ha
et al., 1999; Vatamaniuk et al., 2000). The mechanism
by which PC synthase is activated appears to be
relatively non-specific with respect to the activating
metal ion, although some metals are more effective
than the others. It is suggested that the conserved
amino-terminal domain confers the catalytic activity