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

Heavy Metal Stress
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5. HYPERACCUMULATORS
Hyperaccumulation or hypertolerance refers to the plant’s ability to grow on soils
enriched in heavy metals such as Zn, Ni, Cu, Cd, or Pb, and it is usually metal-specific
(Verkleij and Schat, 1990). A limited number of plant species, ~400 (Baker et al., 2000),
have been classified as heavy metal accumulator plants. Despite recent advances
(Assunção et al., 2001; Clemens, 2001; Macnair, 2002), mechanisms underlying metal
hyperaccumulation are still largely unknown. Analysis of 20 Brassicaceae accessions
collected from a wide geographical area has been initiated to identify potential metal
hyperaccumulating species for use in large-scale genomic efforts to uncover genes
involved in metal hyperaccumulation (Peer et al., 2003). Thus far, the first phase has
revealed that the accession T. caerulescenes Félix de Pallières shows promise as a
model hyperaccumulator. T. caerulescens is one of the relatively well-studied metal
hyperaccumulators that grows on different metalliferous soil types, such as calamine
and serpentine, as well as on nonmetalliferous soils. By comparing plant populations
growing on contrasting soil types, it has been revealed that high–level tolerances are
metal-specific and confined to those metals that are enriched at toxic levels in soils of a
particular plant population site (Assunção et al., 2003).
Among others, A. halerii ssp. halleri (accession Langelsheim) is a naturally
selected Zn- and Cd-tolerant Zn hyperaccumulator. This species differs strikingly from
its close relative A. thaliana by specifically accumulating Zn in aboveground tissues.
Recently, Becher et al. (2004) have used A. thaliana GeneChips to identify genes potentially
involved in either cellular metal uptake or detoxification in shoots of A. halerii.
Based on transcript levels, Zn tolerance in A. halleri involves high constitutive expression
of metal homeostasis genes in shoots to accommodate for higher basal levels of
Zn accumulation, and possibly to accommodate for sudden increases in Zn influx into
shoots. Identified candidate genes coded for proteins closely-related to the following
A. thaliana proteins, AtZIP6, a putative cellular Zn uptake system and a member of the
zinc-regulated transporter (ZRT)-iron regulated transporter (IRT)-like protein (ZIP)-family
of metal transporters, the putative P-type metal ATPase AtHMA3, the cation diffusion
facilitator ZAT/AtCDF1, and the nicotianamine synthase AtNAS3.
6. PHYTOREMEDIATION
Phytoremediation is a process whereby capabilities of green plants to take up and
accumulate heavy metals are exploited for purposes of environmental clean-up. Most
naturally-occurring hyperaccumulating plants have low biomass, and are restricted in
their growth to certain areas. Therefore, the tools of molecular biology are utilized to
implement all mechanisms involved in heavy metal resistance and accumulation in
order to develop an environmentally friendly clean-up system for contaminated soils.
Recently many transgenic plants with increased heavy metal resistance and
uptake of heavy metals have been developed solely for purposes of phytoremediation
(Table 2).