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

Heavy Metal Stress
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is enzymatically synthesized in two steps. In the first step, γ-glutamylcysteine synthetise
(GCS) catalyzes formation of γ-glutamyl-cysteine, and in the second, GSH synthetase
(GS) catalyzes the production of GSH through the addition of a glycine residue (Cobbett
et al., 1998). In Arabidopsis, these two reactions are encoded by GSH1 (May and
Leaver, 1995) and GSH2 (Wang and Oliver, 1996), respectively. GSH-deficient cadmiumsensitive
mutants of Arabidopsis plants have been developed; these have either diminished
capacity in producing gluthatione, cad2 (Howden et al., 1995a) or RML1 (Vernoux
et al., 2000), or have a mutation in the gene coding for phytochelatin synthase, cad1
(Howden et al., 1995b), thus synthesizing fewer phytochelatins resulting in hypersensitivity
to both Cd and Cu. The enzyme PC synthase is constitutively expressed, but its
activity is dependent on the presence of a heavy metal. Plant PC synthase genes have
been cloned from wheat (TaPCS1) and A. thaliana (AtPCS1) (Clemens et al., 1999;
Vatamaniuk et al., 1999). Later, it has been shown that PC synthase genes in wheat
(Clemens et al., 1999) and in Arabidopsis (Lee and Korban, 2002) are regulated at the
transcriptional level. However, transcriptional regulation of AtPCS1 in Arabidopsis is
observed only during early developmental stages, and it disappears as plants grow
older (Lee and Korban, 2002). Molecular characterization of phytochelatin synthase
expression in transgenic Arabidopsis has revealed its presence in leaves, roots, cotyledons,
and stems, but not in root-tips or root hairs throughout all stages of plant development
(Lee et al., 2002). Earlier, PC synthase has been reported to be present in roots
and stems, but not in leaves or fruits of tomato plants (Chen et al., 1997).
Glu+Cys
GCS
GSH1; CAD2/RML1
γGlu-Cys
GS GSH2 Glu
GSH
PCS
PC
AtPCS1, AtPCS2, TaPCS1
⊕ HM
Figure 2.
Phytochelatin biosynthesis pathway
Several attempts were made to alternate expression levels of enzymes involved
in the phytochelatin biosynthesis pathway in plants (Strohm et al., 1995; Zhu et al.,
1999a, b; Creissen et al., 1999; Arisi et al., 2000; Gisbert et al., 2003; Lee et al., 2003a, b)
with varying success. Higher Cd tolerance and accumulation were observed when E.
coli genes coding for either γ-glutamylcysteine synthetase (γ-ECS), gshI, or GSH syn-