Physiology and Molecular Biology of Stress ... - KHAM PHA MOI

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K. Gasic and S.S. Korban
tathione with a C-terminal linked Glu instead of Gly as well as isophytochelatins (Glu)
(iso-PC(Glu), (γGlu-Cys) n
)-Glu, n=2-3) (Meuwly et al., 1993, 1995) and desGly-PC homologues
lacking an N-terminal γ-linked Glu (Chassaigne et al., 2001) were isolated from
maize plants exposed to cadmium. Additionally, desglycine phytochelatins (des Gly-
PC, (γGlu-Cys) n
), lacking a C-terminal amino acid residue, were first discovered in maize
(Bernhard and Kägi, 1987). PC-related peptides, including isophytochelatins (Gln) (iso-
PC(Gln) (γGlu-Cys)n-Gln, n=3-4), were identified in horseradish hairy roots exposed to
cadmium (Kubota et al., 2000). Oven et al. (2002b) reported on the isolation and functional
characterization of a homo-phytochelatin synthase GmhPCS1 from Glycine max,
a plant known to accumulate homo-phytochelatins rather than phytochelatins upon
exposure to heavy metals. The catalytic properties of GmhPCS1 were compared with
those of the phytochelatin synthase AtPCS1 from A. thaliana. The incorporation of
both gluthatione and homoglutathione into homophytochelatin, n=2, was demonstrated
using GmhPCS1 and AtPCS1. These findings suggested that the presence of the GSH
substrate and its’ conforms and not the specificity of the enzyme determines the nature
of PCs synthesized in any given plant. In addition to bis(glutathionato)•metal complexes,
various other metal•thiolates were shown to contribute to the activation of
phytochelatin synthase. Previously, it has been reported that metal binding per se was
not responsible for catalytic activation of AtPCS1. Moreover, the dependence of AtPCS1
on heavy metal ions for activity in media containing GSH and other thiol peptides was
a reflection of this enzyme’s requirement for glutathione-like peptides containing blocked
thiol groups for activity (Vatamaniuk et al., 2000). However, findings of Oven et al.
(2002b) suggested that the proposed model for AtPCS1 activation could not fully explain
the catalytic mechanism of phytochelatin synthase.
Activation of the detoxicative-phytochelatin system was observed in the cytosol
of root cells of three plant species, Vicia faba, Pisum sativum, and Phaseolus
vulgaris, belonging to the Fabacea family and exposed to lead ions (Piechalak et al.,
2002). This system was composed of phytochelatins (PCs) in roots of V. faba,
homophytochelatins (hPCs) in P. vulgaris roots, and both PCs and hPCs in P. sativum
roots.
Therefore, current results on PCs and their role in heavy metal stress in plants
show that PCs are synthesized in plants in response to heavy metal stress and due to
various metals. However, analysis of naturally occurring heavy metal hyperaccumulating
plants has demonstrated involvement of some other mechanisms, yet be elucidated,
that allow these plants to survive on contaminated soils and accumulate excess heavy
metals.
3.2. Metallothioneins
Metallothioneins (MTs) belong to a family of gene-encoded low-molecular weight cysteine-rich
metal-binding proteins (Robinson et al., 1993). Although, the precise physiological
function of MTs have not yet been fully elucidated, proposed roles include the