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

Heavy Metal Stress
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a precursor for GSH. Transgenic tobacco plants expressing a wheat OASTL gene, cys1,
show increased cysteine biosynthesis and involvement in plant responses to oxidative
stress (Youssefian et al., 2001). Domigues-Solis et al. (2001) have reported that an
Arabidopsis OASTL gene, Atcys-3A, is involved in cadmium tolerance. Transgenic
tobacco plants overexpressing cysteine synthase in both the cytosol and the chloroplast
showed significant increases in tolerance for Cd, Se, and Ni supplemented in an
agar medium (Kawashima et al., 2004). F 1
transgenic plants demonstrating higher resistance
towards these metals also showed enhanced accumulation of Cd in shoots.
Several studies proposed the involvement of either organic or amino acid
chelation in enhancing the rate of root-to-shoot transport of transition metal ions (Lee
et al., 1977; White et al., 1981; Senden and Wolterbeek, 1992; Krämer et al., 1996; Liao et
al., 2000). Correlations were observed in xylem sap concentrations between copper and
nicotianamine (Pich et al., 1994; Liao et al., 2000), and between copper and histidine
(Liao et al., 2000). As a result, it has been suggested that both amino acids, nicotianamine
and histidine, were involved in chelation of copper ions in the xylem sap.
Some amino acids, particularly histidine and proline, also play roles in the
chelation of metal ions both within plant cells and in the xylem sap (Rai, 2002). Exposure
of the hyperaccumulator Alyssum lesbiacum to nickel (Ni) is known to result in a dosedependant
increase in xylem sap concentrations of both Ni and the chelator-free histidine
(His). Recently, Kerkeb and Krämer (2003) have reported that an enhanced release
of Ni into the xylem is associated with concurrent release of His from an increased rootfree
His pool. Particularly in B. juncea roots, Ni 2+ uptake is independent of simultaneous
uptake of His.
Many plants have been reported to accumulate proline (Pro) when exposed to
heavy metals (Alia and Saradhi, 1991; Basi and Sharma 1993a,b; Costa and Morel 1994;
Talanova et al., 2000). Siripornadulsil et al. (2002) have demonstrated that increased Pro
levels provide enhanced protection against Cd in microalgae. It is interesting to note
that Pro reduces Cd stress not by sequestering Cd, but by reducing Cd-induced free
radical damage and maintaining a stringent reducing environment (higher GSH levels)
within the cell.
The chelation strategy involves extrusion of low-molecular mass secondary
amino acids (mugineic acids), known as “phytosiderophores”, that chelate the sparingly
soluble iron. In plants, metal phytosiderophore uptake is known to be important
for supplying the demands for micronutrients. Among higher plants, graminaceous
species are reported to secrete phytosiderophores, hexadentate metal chelators with
high affinity for Fe (III), to efficiently acquire iron (Fe) from alkaline soils with low Fe
solubility (Schaaf et al., 2004). The phytosiderophore system in plants serves the
acquisition of Fe and other metals as well. It has been shown that phytosiderophore
release is also triggered under conditions of Zn deficiency (Tolay et al., 2001). Although,
with high affinity for ferric iron, phytosiderophores of the mugineic acid family
also chelate other heavy metals, such as Cu, Mn, and Zn as well as other non-essential
metals (Treeby et al., 1989; Römheld and Awad, 2000).