Page 24 - Plant-Based Remediation Processes

1 Phytoremediation Protocols: An Overview 11

plant for use of phytodegradation should have (1) highly developed root system that
has the ability to secret a considerable amount of enzyme for degradation of the
xenobiotics, (2) tolerance to the xenobiotics at a concentration found in soil, (3) fast
growth, and (4) a relatively high biomass (Wang and Chen 2007). The enzymes
secreted from plant root into soil include laccases, dehalogenase, nitroreductase,
nitrilases, and peroxidases (Carreira and Wolfe 1996; Schnoor et al. 1995; Duran
and Esposito 2002; Jansen et al. 2004; Wang et al. 2004). In a field test reported by
Wolfe et al. (1993), plant-derived enzymes nitroreductases and laccases showed
significant degradation of TNT, dinitromonoaminotoluene, mononitrodiami-
notoluene and triaminotoluene. Another study reported the degradation of various
nitroaromatic compounds by nitroreductase secreted by plants (Boyajian and
Carreira 1997). In another report, laccases have been shown to be useful for the
degradation of a variety of persistent environmental pollutants including alkenes,
bisphenol A, and synthetic dyes (Mayer and Staples 2002). The presence of plant-
derived enzymes capable of degrading environmentally hazardous xenobiotics thus
can be successfully exploited for the development of future phytoremediation
strategies (Salt et al. 1998).

1.7 Phytostimulation

It is also called rhizospheric biodegradation and is based on the secretion by plants
in root exudates which support the growth and metabolic activities of diverse fungal
and bacterial communities in the rhizosphere capable of degrading varied pollutants
(Anderson et al. 1994). The secreted enzymes can transform the chemicals in the
rhizosphere; therefore, the plants do not need to take up the pollutants for detoxifi-
cation (Fig. 1.4). Plants are able to increase the abundance of soil microflora in the
rhizosphere by 1–4 orders of magnitude compared to the surrounding bulk soil and
these microflora show greater range of metabolic capabilities than the microbes in
the surrounding loose soil (Walton et al. 1994; Salt et al. 1998). Some plants such as
mulberry (Morus rubra) preferentially harbor PCB degrading microbes in the
rhizosphere (Wenzel et al. 1999). Rhizospheric microorganisms may also decon-
taminate areas by volatilizing pollutants such as polynuclear aromatic
hydrocarbons (PAH) or by increasing the production of humic substances from
organic pollutants (Cunningham et al. 1996; Dec and Bollag 1994).

1.7.1 Genetically Modified Plants for Improved Phytostimulation

The most promising approach of rhizospheric phytodegradation is the production of
transgenic plants targeted for secreting the enzymes or factors involved in phase I
and phase II detoxification process in plants (Spaczynski et al. 2012). Xenobiotics,
such as PCB, various herbicides, and explosives can be successfully degraded by