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

Heavy Metal Stress
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following: (a) participation in maintaining the homeostasis of essential transition metals;
(b) sequestration of toxic metals, such as cadmium and mercury; and (c) protection
against intracellular oxidative damage. MTs are involved in heavy metal homeostasis
and tolerance to Cd 2+ and Zn 2+ in mammals (Masters et al., 1994) and to Cu and Cd 2+ in
yeast (Hamer et al., 1985; Yu et al., 1994). The first plant MT involved in Zn 2+ binding,
E c
(early Cys-labelled) protein, was identified in wheat (Lane et al., 1987). Since then,
more than 50 MT-like sequences have been found in plants (Rauser, 1999).
Isolation and characterization of type 1 metallothionein cDNA (mcMT1) from
a heavy metal tolerant plant, Festuca rubra cv. Merlin, was recently reported (Ma et al.,
2003). Functional complementation studies using the Saccharomyces cerevisiae cup1∆
mutant ABDE-1 (metal-sensitive) confirmed the functional nature of this mcMT1 gene
in sequestering both essential (Cu, Zn) and non-essential metals (Cd, Pb, Cr).
Expression and regulation of Arabidopsis MT genes have revealed that plant
MTs have distinct functions in metal homeostasis, especially for Cu (Murphy et al.,
1997; Garcia-Hernandez et al., 1998; Guo et al., 2003). MT1a and MT2b are involved in
the distribution of Cu via the phloem; while, MT2a and MT3 chaperone excess metals in
mesophyll cells and root tips. These functional capabilities may allow MTs to play
roles in mobilization of metal ions from senescing leaves and sequestration of excess
metal ions in trichomes. An Mt2b-like gene was identified in copper tolerant Silene
vulgaris (Moench) Garcke (van Hoof et al., 2001). When expressed in yeast, this gene,
Sv2b, restored tolerance to cadmium and copper in different hypersensitive strains.
Two distinct class I type 3 metallothionein-like genes, MT3-A and MT3-B,
were isolated from oil palm (Abdullah et al., 2002). Both MT3 genes were expressed in
the mesocarp throughout the ripening period. Moreover, MT3-B was also expressed in
roots while MT3-A was induced in senescing leaves.
Isolation of a cDNA sequence encoding a type 2 metallothionein (MT)-like
protein, htMT2, from Helianthus tuberosus L. was reported by Chang et al. (2004). This
protein was mainly present in internodes and nodes, but was not detected in roots. It
was suggested that HtMT2 might be involved in either transport or availability of Cu 2+
and Zn 2+ to some apometal enzymes or apometal proteins.
3.3. Organic Acids and Amino Acids
The presence of different concentrations of organic acids among various ecotypes of
metal-tolerant plants in their natural habitat has deemed these substances as likely
cellular chelators (Rauser, 1999). Studies have demonstrated that the primary constituents
of root exudates are low-molecular weight organic acids that play essential roles in
making sparingly soluble soil Fe, P, and other metals available to growing plants
(Römheld and Awad, 2000; Yang and Crowley, 2000).
Various plant species have developed mechanisms to enable them to grow on
acid soils where toxic levels of aluminum, Al 3+ , can limit plant growth. Some plants