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

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B. Rathinasabapathi and R. Kaur
and this is responsible for depletion of glutathione (Klapheck et. al., 1995; Schneider
and Bergmann, 1995). Elevated γ-glutamylcysteine synthetase (γ-ECS) activity was
shown to correlate with Cd resistance (Chen and Goldsborough, 1994). Poplar
transformants overexpressing bacterial γ-ECS in the cytosol synthesized higher levels
of glutathione (2-fold) even when exposed to different Cd concentrations (0–1000 µg g -1
soil) for 14 d (Arisi et. al., 2000). In B. juncea transformed with E. coli gsh1 gene,
encoding a γ-glutamylcysteine synthetase in the chloroplast, there was a three-fold
increase in glutathione levels in comparison to non-transgenic control plants (Zhu et.
al., 1999a). Also, expression of an Escherichia coli glutathione synthase gene (gsh2) in
B. juncea resulted in a five-fold increase in root concentrations of glutathione, but only
following exposure to cadmium (Zhu et. al., 1999b). A bacterial glutathione reductase
(cp GR) expressed in B. juncea targeted to plastids exhibited 50 times higher activity of
glutathione reductase and enhanced Cd tolerance than wild plants (Pilon-Smits et. al.,
2000). The overexpression of a tobacco glutathione-S-transferase gene (parB) in
Arabidopsis was reported to enhance Cu, Al, and Na tolerance (Ezaki et. al., 2000).
Overexpression of enzymes γ-glutamylcysteine synthase (γ-ECS), glutathione synthetase
(GS) and phytochelatin synthetase (PCS) in Arabidopsis significantly improved
tolerance to As and Hg (Dhankher et. al., 2002). Harada et. al., (2001) also created
transgenic plants with enhanced phytochelatin levels, through overexpression of cysteine
synthase. The resulting transgenics demonstrated increased Cd tolerance, but
lower Cd accumulation. The transgenic tobacco plants over-expressing cysteine synthase
in the cytosol and chloroplasts, exhibited significantly more tolerance towards
heavy metals such as Cd, Se and Ni (Kawashima et. al., 2004). Dhankher et. al., (2003)
found that overexpression of bacterial arsenate reductase gene (arsC ) provided Cd (II)
resistance in tobacco and Arabidopsis plants.
3.2.4. Metallothioneins
Low molecular weight cysteine rich proteins called metallothioneins (MT) are synthesized
in the cells in response to toxic heavy metals and help stabilize the concentration
of the heavy metals in the cells (Hamer, 1985; Steffens, 1990). Transgenic tobacco
plants expressing a metallothionein gene were able to grow in concentrations of up to
200 ìM CdSO4 (Liu et. al., 2000). Transformants of B. oleracea expressing the yeast
metallothionein gene CUP1 tolerated 400 ìM Cd, whereas wild-type plants were unable
to grow at concentrations above 25 ìM cadmium in a hydroponic medium (Hasegawa
et. al., 1997).
3.2.5. Cation Diffusion Facilitators
Recently, a family of cation efflux transporters was discovered known as cation diffusion
facilitator (CDF) proteins or metal tolerance proteins (MTP) that might play a vital