Page 265 - Plant-Based Remediation Processes

258 P. Kotrba

0
converts Hg to nontoxic volatile metallic Hg , and merB coding for organomercu-
+
rial lyase, liberating Hg from R-Hg (Silver and Phung 2005). The main advantage
of phytovolatilization is the removal of Hg from a site without the need for plant
harvesting and disposal. Although there could be some skepticism regarding the
safety of such strategy, safety assessment studies on mercury phytovolatilization
have indicated that the advantage of wide dispersion and dilution in the atmosphere
and eventually to other environment components outweigh the potential risks (Lin
et al. 2000; Moreno et al. 2005). Expression of merA, merB, or a combination of
both, in A. thaliana (Bizily et al. 2003; Yang et al. 2003), N. tabacum (He et al.
2001; Ruiz et al. 2003, Haque et al. 2010), rice Oryza sativa (Heaton et al. 2003),
saltmarsh cordgrass Spartia alterniflora (Czako ´ et al. 2006), yellow poplar
Liriodendron tulipifera tulipifera (Rugh et al. 1998), and cottonwood Populus
+
deltoides (Che et al. 2003; Lyyra et al. 2007), resulted in Hg and R-Hg -tolerant
phenotypes (Table 12.2). To achieve efficient volatilization of mercury, use of
modified versions of merA optimized for plant codon preferences (merApe9 and
merA18) were shown instrumental in achieving efficient production of MerA and
pronounced mercury volatilization in A. thaliana, N. tabacum, and L. tulipifera
(Rugh et al. 1998; He et al. 2001). While cytoplasmatic MerB allowed A. thaliana
plants to grow at fivefolds higher methyl mercury concentrations compared to WT
controls, the additional expression of merApe9 further improved tolerance by a
0
factor of 10 and promoted efficient phenyl mercury removal and Hg volatilization
from a model solution (Bizily et al. 2000). More than a 10-fold higher volatilization
rate was further achieved by the targeting of MerB in the endoplasmatic reticulum
(ER) of merA/merB double transformant (Bizily et al. 2003). The likely reason was
that ER-localizing MerB exhibited more than a 20-times higher specific activity
than in MerB plants with cytoplasmic MerB.

12.5 Genetic Engineering of Plant Symbionts

Several of the plant-associated bacteria and fungi have been reported to accelerate
phytoremediation in metal-contaminated soils by promoting plant growth and
health and play a significant role in accelerating phytoremediation (Miransari
2011; Rajkumar et al. 2012). In an aspect specific to plant–metal interaction,
rhizosphere microbiota plays in its mutualistic associations with plants an important
dual role in metal homeostasis: scavenging of metal micronutrients and their supply
to the host plant; detoxification of excess essential metals and nonessential metal
species as well. In general, the plant-associated bacteria migrate from the bulk soil
to colonize the rhizosphere and roots of plants. Endophytic bacteria have been
isolated from many different plant species as those colonizing the apoplast or
symplast without causing negative effects on their host. Mycorrhizal fungi, espe-
cially species forming arbuscular mycorrhizae (e.g., an abundant Glomus spp.),
develop symbiotic association with most of terrestrial plants. An original approach
to modulate the heavy-metal accumulation capability in leguminous plants by