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

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B. Rathinasabapathi and R. Kaur
drogenase (MDH) displayed higher synthesis of citric acid and Al tolerance (Tesfaye et
al., 2001).
Transgenic Arabidopsis plants expressing MerA gene showed significantly
enhanced tolerance to Hg (II) and volatilized elemental mercury (Rugh et. al., 1996).
Also, transgenic MerB Arabidopsis plants were significantly more tolerant to methylmercury
and other organomercurials (Bizily et. al., 1999). Transgenic tobacco and yellow
poplar expressing both MerA and MerB genes also showed enhanced mercury tolerance
(Rugh et. al., 2000).
Transgenic tobacco overexpressing NtCBP4 (Nicotiana tabacum calmodulinbinding
protein) demonstrated enhanced tolerance to Ni 2+ and hypersensitivity to Pb 2+
due to reduced Ni 2+ accumulation and Pb 2+ accumulation respectively (Arazi et. al.,
1999). Furthermore, expression of a truncated tobacco NtCBP4 channel in transgenic
plants and disruption of the homologous Arabidopsis CNGC1 gene conferred improved
tolerance to Pb 2+ (Sunkar et. al., 2000). In an approach to enhance Se assimilation by
plants, the plastidic Arabidopsis APS1 cDNA encoding ATP sulfurylase, was expressed
in B. juncea under the control of a 35S promoter. Transgenic plants displayed a little
increase in Se tolerance, and accumulated about 2-fold more Se in their shoots (Pilon-
Smits et. al., 1999). Grichko et. al. (2000) found that the overexpression of 1-
aminocyclopropane-1-carboxylic acid (ACC) deaminase led to an enhanced accumulation
of a variety of metals, as well as higher metal tolerance.
3.2.7. Arsenic Hyperaccumulation
Arsenic is a toxic carcinogen. In many areas of the world, the soil and ground water are
contaminated with arsenic both due to natural and human activities. Current methods
used to remediate arsenic contaminated water and soil are expensive. Recent research
has uncovered the extraordinary ability of the Chinese brake fern (Pteris vittata) to
hyperaccumulate arsenic (Ma et al. 2001), leading to phytoremediation technology to
remediate arsenic contaminated environment.
Arsenic exists most commonly in two forms, As V and As III, the latter being
more reactive and toxic to cells. To understand how the Chinese brake fern
hyperaccumulates arsenic, the roots were exposed to As V in a nutrient solution medium
and the total arsenic and the species of As were analyzed in the plants’ fronds and
roots. About 90% of the arsenic accumulated in the fern occurred in the form of As III
and the accumulation was more in the shoots than in the roots (Tu et al., 2002; Figure 6).
This shows that the fern has an efficient arsenic uptake system in its roots, and the
fren’s tissue reduces As V to As III and arsenic hyperaccumulation occurs in the
fronds.
The mechanisms involved in arsenic hyperaccumulation in this fern are actively
researched in many laboratories in the world including that of the authors. P.vittata
genes involved in arsenic uptake, arsenate reduction, translocation, vacuolar transport