Introduction another issue with chemical pesticides that if

Introduction

Pesticides
have been used for many decades. Pesticides have played a vital role in the
modern agricultural practices as well as in the sustenance creation. In
advanced agricultural practices productions is increased by application of
pesticides to the fields in bulk amounts to restrict certain target organisms
like insects, fungi, bacteria and other weeds that tends to grow with the main
crops of economic benefit (Liu and Xiong, 2001). But these chemical compounds
in addition to the main purpose they serve, can also effect other entities of
the environment and can negatively impact the soil as well as surface and
groundwater (Castillo et al., 2008).  The
broad utilization of certain man-made organic chemicals in the previous decades
has prompted various long haul environmental issues. There are two main factors
that makes the pesticides intact in a soil for a very long period of time
including the physiochemical nature of pesticide and the absence of certain
microorganisms that are responsible for the breakdown of these chemicals
(Sibanda et al., 2011).

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Other
than the health problems related to the use of pesticides, one of the most
important problem is that these synthetic chemicals are very resistant to
degradation and remain in the environment for prolonged periods and can become part
of the food chain having very harmful effects on ecological system as well as
on human beings (Liu and Xiong, 2001). There is another issue with chemical
pesticides that if these pesticides are expired before their use they are
transformed to another form that is not able to be used anymore and they lose
their strength and efficiency to function properly as many of the pesticides
have a shelf life of two years and after this period it expires. When the
expired pesticides are degraded into other chemical constituents, these
chemical components in some cases are more dangerous and harmful than the
initial form (Binod and Bhupendra, 2009).

Specifically,
the extensive use of pesticides in farmlands and public well-being sectors
around the globe has brought about various incidences of sullying foodstuff,
agricultural lands as well as surface and groundwater. In fact, after more than
two decades of prohibiting or constraining the use of those pesticides that are
very resistant to degradation (e.g. Organochlorines), their remainder can be
found in the environment (Chaudhry et al., 2002). Different pesticides possess
different attributes that decides how they will perform in a soil. Extreme and
continued use of pesticide can cause environmental degradation. Pesticides are
responsible for the deterioration of soil, groundwater, continental and coastal
waters as well as air quality (Surekha et al., 2008).

Ex-situ Methods for Treatment of
Pesticides and their Drawback

Although
many countries have adopted different regulations regarding the control on
extreme release of toxic substances into the environment and numerous European
nations have begun to take ecological issues into consideration seriously with
zero resistance approach towards the polluted soil. But tremendous expenses
related to the most of the innovative clean-up strategies have prompted flexibility
in this approach in many circumstances. The ex-situ method presents many
problems regarding the collection of soil sample and then its transportation to
the site where it is treated with certain chemicals for the removal of
pesticides. Collectively, this method is not environmental-friendly because the
soil is disturbed by excavation and then treated with chemicals which in turn
require proper disposal. The ex-situ approach for the treatment of pesticides
makes the modern and innovative clean-up methods very expensive so the in-situ
approaches have gained importance (Chaudhry et al., 2005). Phytoremediation is
a favorable strategy for the treatment of contaminated environment (Chaudhry et
al., 2005).

Bioremediation: A Favorable
approach for Cleanup of Chemicals

Bioremediation
is a clean-up methodology that involves the use of microorganisms for
correcting the contamination resulted from the use of broad range of chemicals
(Singh and Walker, 2006). It is an environmental friendly and practically sound
solution for the removal of substances that are toxic to the environment
(Al-Mihanna et al., 1998). Three main approaches to bioremediation includes biostimulation,
bioaugmentation and phytoremediation (Singh, 2009). The latest approach of
phytoremediation, that involves the utilization of plants and microorganisms
related to those plants for the removal of toxic substances like pesticides,
has been emerged as an economic and practical technique that do not pose the
problems associated with ex-situ approach such as landfilling and incineration
(Smits, 2005). Bioaugmentation is a procedure for enhancing the ability of a
soil or water that is contaminated with a chemical pesticide or any other
contaminant by introducing particular strains of microbes that are capable of degrading
many pollutants (Thierry et al., 2008). The process of bioremediation can be
improved through the application of soil nutrients, oxygen, trace minerals,
electron acceptors or donors, it is termed as biostimulation (Scow and Hicks,
2005).

Microbes
can be utilized for the breakdown and removal of various dangerous synthetic
compounds that are foreign to the natural environment, e.g. pesticides. This
technique is found to be useful for the remediation of contaminated sites
(Mervat, 2009). Method of removal of toxic substances with the help of living
organism are more convenient as compared to the traditional methods of
remediation due to the fact that microbes breakdown various hazardous
substances effectively without producing harmful by-products (Pieper, 2000;
Furukawa, 2003).

Microbes
are able to react with the pesticides both chemically and physically and can
bring about changes in the structure of these substances ultimately leading to
the breakdown of the concerned compounds thus making them non-hazardous (Raymond
et al., 2001; Wiren-Lehr et al., 2002).

Rhizosphere: Zone of Excessive
Microbial Activity

Rhizosphere
is defined as the specific zone of soil inhabited and affected by plant roots.
It is an area of increased microbial activity. As a result of photosynthesis
and other processes, plant roots release a plenty of organic compounds such as
root exudates and mucilage, microorganisms take up these compounds fulfill
their energy requirements (Brimecombe et al., 2007). Plants possess various
mechanisms for transportation and secretion of these organic compounds into the
rhizosphere including passive and active mechanisms (Badri and Vivanco 2009;
Weston et al. 2012). The mineral uptake by plants is facilitated by root exudates.
These root exudates also trigger mycelial growth in rhizosphere zone and also
affects some physical parameters of soil around the root zone including changes
in pH, water potential and oxygen availability (Dakora & Phillips, 2002). The
rate of exudation is altered with the age of the plant (Haller & Stolp,
1985). Presence of different mineral nutrients and certain pollutants in the
soil also effects the root exudation (Rovira et al., 1983).

Synergistic Relationship between
Plants and Microbes Pesticide Degradation

Normally,
both of the plants and soil microbes have many impediments with regard to their
individual capacities to degrade organic compounds. But these restrictions to
the removal and degradation of organic pesticides can be dealt by the interaction
between the plant roots and soil microbes in such a way that their effect is
enhanced (Chaudhry et al., 2005).

The
microorganisms that are involved in the conversion of organic compounds to
non-hazardous substances, require energy. While converting the organic
compounds to less hazardous substances, microorganisms may face energy losses.
The energy requirements of microbes are fulfilled by plant root exudates that
provide sufficient energy to the microbial community of soil. Plants also get
advantage as various nutrients that are held in soil becomes available to the
plants through microbial activity and breakdown of many substances that are
hazardous to the plants occurs (El-Shatnawi and Makhadmeh, 2001).

Instead
of single type of microorganisms, there is a huge diversity of microbial
community in the plant rhizosphere zone that has synergistic relationships (Anderson
and Coats, 1995). These interactions between plant roots and soil microflora
result in the increased growth of plant and treatment of polluted soil.

Increased
microbial activity in the rhizosphere zone provides the conducive environment
for the simultaneous breakdown of many compounds resistant to degradation that
are held with the soil (Walton & Anderson, 1990; Shann, 1995).

The
degree of degradation of certain environmental pollutants through plant and
microbial interaction differs significantly with respect to different species
of plants and different soil types. Some plant species are more effective
degraders of pollutants than others. Various factors are responsible for this
varied behavior availability of oxygen, plant roots, water fluxes and changing
pH (Chaudhry et al., 2005).   

Mycorrhizal Symbiosis

Mutualistic
association between plants roots and fungi is termed as Mycorrhizal Symbiosis. Fungi
gives water and nutrients to the plants that are essential for the growth of
plants. Plants are able to uptake these nutrients and water through the fungal
mycelial arrangement that covers a wide area in the rhizosphere zone. These
fungi also secret certain organic acids and convert insoluble complex compounds
to soluble minerals which are taken up by plants. The plant partners of
mycorrhizal association also provide the fungi with essential carbohydrate for
growth and development (Smith and Read 1997). Some bacterial communities also
inhabit the mycorrhizal root zone and enhance the formation of mycorrhiza
(Bertaux et al. 2003).

Plant Growth Promoting Rhizospheric
Bacteria

Certain
bacterial communities inhabits the rhizosphere either alone or in association
with mycorrhizal fungi. These rhizospheric bacteria effects the growth of
plants in many ways. They can either enhance the plant growth or can have
drastic impacts on the growth of plants (Barea et al., 2005).  Some soil microbes also tend to produce
bio-surfactants that are beneficial for the breakdown of organic pollutants
through their increased accessibility to plants (Lafrance and Lapointe, 1998).
Nevertheless, the effect of the rhizosphere bacteria on the degradation action
generally differs with the genetic makeup of the microbe and the plant species
that are in association with the bacteria and also dependent on prevailing
environmental state (Brimecombe et al. 2007). Pseudomonas spp. and Bacillus
spp. are the most commonly known rhizosphere bacteria (Brimecombe et al.,
2007).

Plant
Growth Promoting Rhizobacteria are generally in association with root surface
and enhance the development of plant through various means such as the
increased provision of mineral nutrients, production of plants hormones and
through prevention of outbreak of certain diseases in plants (Tarkka et al.
2008).

Bacterial and Fungal Degradation of
Pesticides

The
genetic constitution and variety of microorganism community that are
responsible for the breakdown and removal of pesticides can differ from those
that are present in the soil and this diversity may also vary from one crop to
another (Martin-Laurent et al., 2006). Degradation of most of the chemical
pesticides occur easily but some pesticides are very persistent and are
resistant to degradation (Aislabie and LloydJones, 1995; Richins et al., 1997;
Mulchandani et al., 1999).

 Phytoremediation of organic toxins is
augmented through bacterial activities. In this procedure, plants and soil
microflora act together and furnish certain nutrients in the rhizosphere zone
that results in the enhanced microbial action for the breakdown and removal of
pesticides (Mirsal, 2004).

Mycorrhizal
fungi and bacteria inhabiting rhizosphere are found to enhance plant growth and
breakdown of certain contaminants in a soil with excessive amounts of
pollutants. For instance, ectomycorrhizal associations can show significant
tolerance against harmful organic compounds like m-toluene (Sarand et al.,
1999), petroleum constituents (Sarand et al., 1998), or polycyclic aromatic
hydrocarbons (Leyval and Binet 1998; Wenzel 2009).

Mycorrhizal
fungi and bacteria can tolerate extreme soil states and enhance the treatment
of polluted soils through direct consumption of constituents of pesticide
pollutants as nutrients and through enhancing plant growth (Schützendübel and
Polle 2002; Fomina et al. 2005; Baum et al. 2006; Zimmer et al. 2009; Wenzel
2009).

Mycorrhizal
fungi and bacteria can act together in the rhizosphere zone at different stages
of cellular integration, extending from relatively simple interactions like
surface linkages to more complex, close and essential association. This
interaction serves to enhance the plant growth and development as well as it is
also important for maintaining the interaction of organisms among themselves
and with their surroundings in the rhizosphere zone (Perotto and Bonfante
1997).

Plants
and microbes undergo a sequence of chemical reactions that can cause breakdown
of a number of chemical pesticides that are discharged into the environment.
Due to long lasting nature some pesticides, the performance of certain microbes
and plants to breakdown these chemicals is significantly limited. As for
example, organochlorine compounds cannot be easily degrade by plants and marine
phytoplankton (Shimabukuro et al., 1982).

About
sixty-six bacterial strains were being studied that were found to be
responsible for the breakdown of atrazine when they used atrazine as a source
of nitrogen and citrate as a source of carbon. These strains were being
separated from unplanted and maize rhizosphere to which atrazine pesticide had
been applied. These degrader bacterial communities were the members of Actinobacteria, Bacteroidetes and
Proteobacteria (Martin-Laurent et al., 2006).

Many
bacteria and fungi that inhabits the soil has the ability to breakdown or
mineralize many different pesticides. The rate of adsorption, movement and
breakdown of chemical pesticides can be affected by the application of organic
matter and mineral nutrients (Briceño et al., 2007).

Most
commonly bacteria are thought of the most important degraders of chemical
pesticides but filamentous fungi have certain attributes that make them more favorable
in different types of environments thus making fungi more successful degraders
than bacteria. Use of fungi is considered to be the most favorable technique
for biological breakdown of certain pollutants that are resistant to
degradation (Glaser and Lamar, 1995). Despite the fact that fungi are not
capable of movement, they adapt themselves to the changing environment (Read,
2007). Fungi do not undergo a series of chemical reactions to degrade
contaminants instead they secret certain enzymes outside the cell that act on
the pollutant for its degradation (Nawaz et al., 2011).

If
a specific enzyme is not present in a soil, it can cause the pesticide to
reside in a soil for a very long period of time. Similarly, if particular a
microorganism that degrade a particular pesticide is not present in the rhizosphere
zone of soil or if the microbial community responsible for degradation of
pesticides is declined due to toxic effects of pesticides, then these microbes
are introduced into the soil to augment the action of the microbial community
that is already present (Singh, 2008).

 Conclusion

Long
term application of chemical pesticides to agricultural fields and other sites
can lead to the environmental damage and can affect the natural productivity of
the soil. The toxic effects associated with the use of pesticides can
significantly affect the microbial status of the soil. Some chemical pesticides
are persistent and extremely harmful to the environment as they can leach down
through the soil and can contaminate the groundwater as well as these
pesticides can also pollute surface water. These pesticides also pose threat to
many wildlife species and ultimately to human beings when they are transferred
from one organism to another through food chain. Decontamination of pesticides
is very important and crucial to maintain the productivity of the soil. The
traditional practices for the remediation of soil or water contaminated with
pesticides are very expensive and not ecofriendly. So, bioremediation is
emerged as a cost effective and environmental friendly technique that involves
the utilization of certain microbes with potential degrading capacity for the
degradation of pesticides. Fungi and bacteria are considered as potential
biodegraders. These microbes can interact with plant roots in the rhizosphere
zone to enhance the process of degradation of pollutants. Plants have complex
interactions with microbes in the rhizosphere zone. Microbial activity in the
rhizosphere zone is very important for the treatment of disturbed soils and can
increase the fertility of the plants. Fungi are more favorable degraders
because they can tolerate harsh environmental condition which many bacterial
strains are unable to tolerate. Fungi secrets specific extracellular enzymes
that degrades various pollutants. In favorable environmental conditions that
are conducive for the growth of microbial strains responsible for degradation,
this technique is considered as an effective and ecofriendly approach for
cleanup of environment polluted with toxic pollutants. In addition to this
approach, biological control instead of synthetic chemical pesticide is an
effective approach and it can control the problem at source.