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

Heavy Metal Stress
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of prokaryotes. Arabidopsis mutants for each of these transporters have been identified
and characterized, and their roles in metal homeostasis have been determined.
While individual mutants exhibited no apparent phenotype, hma2 hma4 double mutants
had a nutritional deficiency phenotype that could be compensated for by increasing
the level of Zn in the growth medium,but not that of either Cu or Co. Levels of Zn,
but not of other essential elements decreased in shoots of an hma2 hma4 double mutant
and to a lesser extent of an hma4 single mutant when compared to wild type.
Together, these observations indicate primary roles for HMA2 and HMA4 in essential
Zn homeostasis. HMA2promoter- and HMA4promoter-reporter gene constructs revealed
that HMA2 and HMA4 are predominantly expressed in vascular tissues of roots,
stems, and leaves, and HMA2 appears to be localized to the plasma membrane. These
observations are consistent with the roles proposed for HMA2 and HMA4 in Zn translocation.
Both hma2 and hma4 mutations confer increased sensitivity to Cd in a
phytochelatin-deficient mutant background, thus suggesting that they may also influence
Cd detoxification.
The multidrug-resistance-associated protein (MRP) is a large subfamily of
ABC transporters. Otherwise known as GS-X pumps, glutathione-conjugates, or
multispecific organic anion Mg 2+ -ATPases, MRPs are reported to participate in the
transport of both exogenous and endogenous amphipathic anions as well as
glutathionated compounds from the cytosol into the vacuole (Rea et al., 1998). AtMRPs,
subfamily of Arabidopsis ABC transporters, are plant homologues of multi-drug-associated
proteins. Their involvement in the transport of a wide variety of substances into
the vacuoles of plant cells in higher plants has been proposed (Rea et al., 1998; Rea,
1999). When the role of AtMRPs in cadmium vacuolar sequestration has been investigated,
expressed putative sequences coding for AtMRPs have been observed in roots
and shoots of plants at different levels (Bovet et al., 2003). In 4-week-old Arabidopsis
seedlings, transcript levels of four AtMRPs are up-regulated following Cd treatment,
and these are exclusively observed in the roots. Interestingly, increases in transcript
levels of AtMRP3 are most predominant. In young plantlets, a higher portion of Cd 2+ is
translocated to aerial parts compared with adult plants. Consequently, AtMRP3 transcript
levels increase in both roots and shoots of young plants suggesting that 7-dayold
seedlings do not exhibit such a strict root-to-shoot barrier as 4-week-old plants.
Expression analysis for glutathione and phytochelatin synthesis in mutant lines, as
well as their subjection to compounds producing oxidative stress have indicated that
induction of AtMRP3 is likely due to the heavy metal itself.
Sancenón et al. (2004) first reported on the physiological function of copper
transport in A. thaliana. Studies of expression pattern of COPT1 in transgenic
Arabidopsis plants overexpressing a reporter gene under the control of the COPT1
promoter revealed its presence in embryos, trichomes, stomata, pollen, and root-tips.
Moreover, investigating the involvement of COPT1 in copper acquisition in
CaMV35S::COPT1 antisense transgenic plants confirmed its role in Cu uptake. Interestingly,
COPT1 antisense plants also exhibited both increased root length, which was