Page 20 - Plant-Based Remediation Processes

1 Phytoremediation Protocols: An Overview 7

• Another strategy for increasing the efficiency of phytoextraction involves
increase in the metal translocation to shoots by increasing plant transpiration
(Gleba et al. 1998).
• According to Raskin (1996), transgenic plants could be developed to secrete
metal selective ligands (phytosiderophores or chelating agents) into the rhizo-
sphere, which could specifically solubilize the toxic elements (Ma and Nomoto
1996).

1.3.2 Phytoextraction with Endophytic Microbes

Researchers carried out several experiments on the application of endophytic bacteria
and mycorrhizal fungi in the phytoextraction of pollutants (Doty 2008). Endophytes
are the symbiotic microbes inhabiting in the internal plant tissue and are able to
facilitate plant growth and increase resistance of plants against pathogen and drought
(Taghavi et al. 2010). It has been recently reported that the endophytic symbiotic
bacteria Methylbacterium populum that lives within poplar can mineralize 1,3,5-
trinitro-1,3,5-triazacyclohexane (RDX) and octahydro-1,3,5,7-tetranitro-1,3,5,7-
tetrazocine (HMX) (VanAken 2009). However, the success rate of phytoextraction
of heavy metals using endophytic bacteria remains slow because of the lack of proper
strains with heavy metal resistance and detoxification capacities (Luo et al. 2011).
Besides endophytes, the arbuscular mycorrhizal (AM) fungi are also known to be
involved in the uptake of elements into plants (Doty 2008) and are reported to be
present in mutualistic association in the roots of plants growing on markedly
contaminated soil (Khade and Adholeya 2009;Javaid 2011; Miransari 2011). There-
fore, mycorrhizal fungi can be applied for significant phytoextraction by improving
several attributes like increased metal tolerance, increased biomass production, and
greater metal concentration in plant tissue (Vamerali et al. 2010). In brief, the goal of
phytoextraction is to reduce the presence of trace elements in soils through their
uptake and accumulation by plants; in contrast, phytostabilization aims to minimize
the mobile and bioavailable fraction of metals by combining the use of metal-tolerant
plants and soil amendments and thus reduces leaching through soil. In both processes
the “mobility and bioavailability of trace elements in the soil—particularly in the
rhizosphere where root uptake and exclusion takes place—is a critical factor affect-
ing their outcome and success” (Kidd et al. 2009).

1.4 Phytovolatilization

A variant of phytoextraction is phytovolatilization, where the contaminant is not
primarily concentrated in aboveground tissues, but instead transformed by the plant
into evaporable and less toxic form before releasing into the atmosphere (Kramer
2005). It is not a direct clean up method rather a dispersal technology of the