Mercury detoxification using genetic engineered Nicotiana tabacum

Phytoremediation is a low cost alternative solution to soil contamination compared with
traditional removal and/or disposal techniques. One of the phytoremediation technologies is
the phytovolatilization, whereby the contaminant is not primarily accumulated in above-ground
tissues, but is instead transformed by the plant into the atmosphere. The detoxification of
highly toxic organmercurial compounds and subsequent volatilization of elemental mercury is
a unique example for the successful phytoremediation based on genetic engineering
approach. For mercury removal techniques we constructed a dicistronic construct containing
the bacterial mercury detoxification genes merA and merB under the control of the
Arabidopsis Actin2 promoter and terminator. The resulted construct was introduced to
Agrobacterium competent cells using heat shock transformation method. In the mean time,
we germinated tobacco (Nicotiana tabacum var. Gold leaf) seeds on MS media and infected
leaf discs of 6-8 weeks tobacco seedlings with Agrobacterium containing Ti plasmid harboring
merA/merB dicistronic construct. The results showed that about 90% of tobacco seedlings
were carrying the mer genes. Tobacco seeds were collected from wild type and transgenic
lines and tested for mercury resistance. The results showed that transgenic plants are
resistant to both Phenyl Mercuric Acetate (PMA) and HgCl2. The root length and dry weight of
wild and transgenic seedlings growing on both media amended with mercury compounds and
media without mercury (control) were scored. The results showed that the root lengths and
dry weight of the transgenic lines are significantly higher by 60 and 17-folds, respectively,
compared to wild type. The results showed clear evidence that the transgenic plants are
resistant to both organic and inorganic mercury compounds and can be used to clean up
mercury contaminated sites.