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

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K. Gasic and S.S. Korban
Generally, there are two main approaches for phytoremediation: 1) developing
transgenic plants capable of phytoextraction, and 2) identifying and/or developing
plants capable of surviving on contaminated soils.
Rugh et al. (1997) expressed a bacterial mercuric reductase gene (merA) in
yellow poplar and reported higher elemental mercury release from soil at approximately
10 times the rate of that of untransformed plants. More recently, phytoremediation of
organomercurial compounds via chloroplast engineering has been reported (Ruiz et al.,
2003). Genes coding for mercuric ion reductase (mer A) and organomercurial lyase
(merB) were integrated into tobacco (Nicotiana tabacum) chloroplast genomes, and
transplastomic were found to be substantially more resistant than wild-type plants to
the highly toxic organomercurial compound phenylmercuric acetate. This novel approach
might have applications to other metals as well.
In order to study the effects of MerA and MerB genes in plants, Pilon-Smits
and Pilon (2000) developed MerA and MerB double-transgenic plants and evaluated
their tolerance to organic mercury in comparison to their wild-type and MerA/MerB
siblings. It was observed that double-transgenic plants showed the highest tolerance
to organic mercury as they converted organic mercury to elemental mercury which was
then released from the plant through volatilization.
In fact, volatilization of selenium in Brassica juncea plants was enhanced by
overexpression of cystathionine-gamma-synthase, an enzyme that catalyzes the first
step in the conversion of Se-cysteine to volatile dimethylselenide (van Huysen et al.,
2003). Transgenic Indian mustard [Brassica juncea (L.) Czern.] plants overproducing
gamma-glutamylcysteine synthase (ECS), gluthatione synthase (GS), or adenosine triphosphate
sulfurylase (APS) were evaluated for phytoremediation of metal-contaminated
mine tailings (Bennett et al., 2003). The ECS and GS transgenics accumulated significantly
more metal in their shoots than wild-type plants, and significantly removed more
metal from the soil. This was the first field study to demonstrate enhanced
phytoextraction potential of transgenic plants using polluted environmental soil.
In order to reduce heavy metals in the food chain, plants that transfer lower
amounts of heavy metals to shoots are critically important. The possibility of using a
bacterial transporter gene to reduce heavy metal content in shoots has been reported
(Lee et al., 2003). E. coli ZntA gene, which codes for a Pb (II) / Cd (II) / Zn (II) pump, has
been used to transform Arabidopsis. Transgenic plants have shown improved resistance
to Pb (II) and Cd (II), but with lowered contents of Pb and Cd in aerial parts of the
plant.
Although several advances have been made in understanding plant’s capability
to either survive on and/or accumulate heavy metals in various tissues, additional
knowledge is yet to be discovered. Pursuing further understanding of the fundamental
mechanisms involved in hyperaccumulation processes that naturally occur in metal
hyperaccumulating plants should allow for the development of plants that are more
ideally suited for phytoremediation of metal contaminated soils. Although it is proposed
that genetic hypertolerance is controlled by a small number of genes (Macnair
1993), molecular mechanisms accounting for hypertolerance remain poorly understood.