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

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K. Gasic and S.S. Korban
thetase, gshII, were overexpressed in Indian mustard (Zhu et al., 1999a, b).
Overexpression of the wheat PCS gene TaPCS1 in Nicotiana glauca (shrub tobacco)
also increased tolerance to metals such as Pb and Cd. Seedlings of transgenic plants
grown in mining soils containing high levels of Pb accumulated twice the level of this
heavy metal than wild type plants (Gisbert et al., 2003). Whereas, overexpression of
gshI increased γ-ECS activity and accumulation of foliar glutathione in transformed
poplar resulting in higher Cd accumulation in tissues, but had only a marginal effect on
Cd tolerance in poplar (Arisi et al., 2000). On the other hand, transgenic tobacco plants
overexpressing a chloroplast-targeted gshI exhibited a three-fold increase in foliar levels
of GSH and enhanced oxidative stress (Creissen et al., 1999). Oxidative stress
caused by gluthatione depletion, attributed to heavy metal induced phytochelatin synthesis,
was reported in a number of plant species such as Silene cucubalus (De Vos et
al., 1992) and Holcus lanatus L. (Hartley-Whitaker et al., 2001a). In a recent report, Lee
et al. (2003a) observed that overexpression of AtPCS1 in Arabidopsis lead to hypersensitivity
to Cd stress, which was attributed to the toxicity of phytochelatins at supraoptimal
levels. Concurrently, transgenic Indian mustard plants overexpressing AtPCS1
showed an increase in Cd tolerance and accumulation up to a certain level of heavy
metal in the medium (100 µM CdCl 2
), but as the level of Cd concentration increased (150-
200 µM CdCl 2
) severe inhibition in plant growth was observed (Gasic and Korban
unpublished data). In contrast, when the AtPCS1 was expressed in Escherichia coli,
bacterial cells placed in the presence of heavy metals such as Cd or As showed 20- and
50-fold increase, respectively, in metal content (Sauge-Merle et al., 2003). Similarly,
overexpression of a plant PC synthase gene in yeast resulted in enhanced resistance to
arsenite as well as arsenate (Vatamaniuk et al., 1999).
In an attempt to understand the structural/functional organization of PC synthase,
Rutolo et al. (2004) conducted a limited proteolysis analysis of the Arabidopsis
AtPCS1 enzyme followed by functional characterization of the resulting polypeptide
fragments. Their results suggested that AtPCS1 is composed of a protease resistant,
presumably highly-structured, N-terminal domain, and flanked by an intrinsically unstable
C-terminal region. The most upstream part of such a region, positions 284-372,
appeared to be important for enzyme stabilization; while, its most terminal part, positions
373-485, was likely involved in determining enzyme responsiveness to a broad
range of heavy metals.
The PC-based arsenic sequestration is essential for both normal and enhanced
arsenate tolerance. Arsenate tolerance in Holcus lanatus requires both adaptive suppression
of a high-affinity phosphate uptake system and constitutive phytochelatin
production (Hartley-Whitaker et al., 2001a,b). Conversely, arsenate hypersensitivity in
Cytisus striatus and grasses is based on reduced uptake through suppression of phosphate
transporter activity (Bleeker et al., 2003).
Isolation of a PCS cDNA clone from Brassica juncea L. cv. Vitasso, a candidate
plant species for phytoremediation, revealed a close relationship between BjPCS1
and PCS proteins from both A. thaliana and T. caerulescens (Heiss et al., 2003). When