IntroductionPesticideshave been used for many decades.
Pesticides have played a vital role in themodern agricultural practices as well as in the sustenance creation. Inadvanced agricultural practices productions is increased by application ofpesticides to the fields in bulk amounts to restrict certain target organismslike insects, fungi, bacteria and other weeds that tends to grow with the maincrops of economic benefit (Liu and Xiong, 2001). But these chemical compoundsin addition to the main purpose they serve, can also effect other entities ofthe environment and can negatively impact the soil as well as surface andgroundwater (Castillo et al., 2008). Thebroad utilization of certain man-made organic chemicals in the previous decadeshas prompted various long haul environmental issues. There are two main factorsthat makes the pesticides intact in a soil for a very long period of timeincluding the physiochemical nature of pesticide and the absence of certainmicroorganisms that are responsible for the breakdown of these chemicals(Sibanda et al.
, 2011). Otherthan the health problems related to the use of pesticides, one of the mostimportant problem is that these synthetic chemicals are very resistant todegradation and remain in the environment for prolonged periods and can become partof the food chain having very harmful effects on ecological system as well ason human beings (Liu and Xiong, 2001). There is another issue with chemicalpesticides that if these pesticides are expired before their use they aretransformed to another form that is not able to be used anymore and they losetheir strength and efficiency to function properly as many of the pesticideshave a shelf life of two years and after this period it expires. When theexpired pesticides are degraded into other chemical constituents, thesechemical components in some cases are more dangerous and harmful than theinitial form (Binod and Bhupendra, 2009).
Specifically,the extensive use of pesticides in farmlands and public well-being sectorsaround the globe has brought about various incidences of sullying foodstuff,agricultural lands as well as surface and groundwater. In fact, after more thantwo decades of prohibiting or constraining the use of those pesticides that arevery resistant to degradation (e.g. Organochlorines), their remainder can befound in the environment (Chaudhry et al., 2002). Different pesticides possessdifferent attributes that decides how they will perform in a soil. Extreme andcontinued use of pesticide can cause environmental degradation. Pesticides areresponsible for the deterioration of soil, groundwater, continental and coastalwaters as well as air quality (Surekha et al.
, 2008).Ex-situ Methods for Treatment ofPesticides and their DrawbackAlthoughmany countries have adopted different regulations regarding the control onextreme release of toxic substances into the environment and numerous Europeannations have begun to take ecological issues into consideration seriously withzero resistance approach towards the polluted soil. But tremendous expensesrelated to the most of the innovative clean-up strategies have prompted flexibilityin this approach in many circumstances. The ex-situ method presents manyproblems regarding the collection of soil sample and then its transportation tothe site where it is treated with certain chemicals for the removal ofpesticides. Collectively, this method is not environmental-friendly because thesoil is disturbed by excavation and then treated with chemicals which in turnrequire proper disposal.
The ex-situ approach for the treatment of pesticidesmakes the modern and innovative clean-up methods very expensive so the in-situapproaches have gained importance (Chaudhry et al., 2005). Phytoremediation isa favorable strategy for the treatment of contaminated environment (Chaudhry etal., 2005).
Bioremediation: A Favorableapproach for Cleanup of ChemicalsBioremediationis a clean-up methodology that involves the use of microorganisms forcorrecting the contamination resulted from the use of broad range of chemicals(Singh and Walker, 2006). It is an environmental friendly and practically soundsolution 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 ofphytoremediation, that involves the utilization of plants and microorganismsrelated to those plants for the removal of toxic substances like pesticides,has been emerged as an economic and practical technique that do not pose theproblems associated with ex-situ approach such as landfilling and incineration(Smits, 2005). Bioaugmentation is a procedure for enhancing the ability of asoil or water that is contaminated with a chemical pesticide or any othercontaminant by introducing particular strains of microbes that are capable of degradingmany pollutants (Thierry et al., 2008). The process of bioremediation can beimproved through the application of soil nutrients, oxygen, trace minerals,electron acceptors or donors, it is termed as biostimulation (Scow and Hicks,2005).
Microbescan be utilized for the breakdown and removal of various dangerous syntheticcompounds that are foreign to the natural environment, e.g. pesticides. Thistechnique is found to be useful for the remediation of contaminated sites(Mervat, 2009). Method of removal of toxic substances with the help of livingorganism are more convenient as compared to the traditional methods ofremediation due to the fact that microbes breakdown various hazardoussubstances effectively without producing harmful by-products (Pieper, 2000;Furukawa, 2003).Microbesare able to react with the pesticides both chemically and physically and canbring about changes in the structure of these substances ultimately leading tothe breakdown of the concerned compounds thus making them non-hazardous (Raymondet al., 2001; Wiren-Lehr et al., 2002).
Rhizosphere: Zone of ExcessiveMicrobial ActivityRhizosphereis 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 photosynthesisand other processes, plant roots release a plenty of organic compounds such asroot exudates and mucilage, microorganisms take up these compounds fulfilltheir energy requirements (Brimecombe et al., 2007). Plants possess variousmechanisms for transportation and secretion of these organic compounds into therhizosphere 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 alsoaffects some physical parameters of soil around the root zone including changesin pH, water potential and oxygen availability (Dakora & Phillips, 2002). Therate of exudation is altered with the age of the plant (Haller & Stolp,1985). Presence of different mineral nutrients and certain pollutants in thesoil also effects the root exudation (Rovira et al.
, 1983).Synergistic Relationship betweenPlants and Microbes Pesticide DegradationNormally,both of the plants and soil microbes have many impediments with regard to theirindividual capacities to degrade organic compounds. But these restrictions tothe removal and degradation of organic pesticides can be dealt by the interactionbetween the plant roots and soil microbes in such a way that their effect isenhanced (Chaudhry et al., 2005). Themicroorganisms that are involved in the conversion of organic compounds tonon-hazardous substances, require energy. While converting the organiccompounds to less hazardous substances, microorganisms may face energy losses.The energy requirements of microbes are fulfilled by plant root exudates thatprovide sufficient energy to the microbial community of soil.
Plants also getadvantage as various nutrients that are held in soil becomes available to theplants through microbial activity and breakdown of many substances that arehazardous to the plants occurs (El-Shatnawi and Makhadmeh, 2001). Insteadof single type of microorganisms, there is a huge diversity of microbialcommunity in the plant rhizosphere zone that has synergistic relationships (Andersonand Coats, 1995). These interactions between plant roots and soil microfloraresult in the increased growth of plant and treatment of polluted soil.Increasedmicrobial activity in the rhizosphere zone provides the conducive environmentfor the simultaneous breakdown of many compounds resistant to degradation thatare held with the soil (Walton & Anderson, 1990; Shann, 1995).Thedegree of degradation of certain environmental pollutants through plant andmicrobial interaction differs significantly with respect to different speciesof plants and different soil types. Some plant species are more effectivedegraders of pollutants than others.
Various factors are responsible for thisvaried behavior availability of oxygen, plant roots, water fluxes and changingpH (Chaudhry et al., 2005). Mycorrhizal SymbiosisMutualisticassociation between plants roots and fungi is termed as Mycorrhizal Symbiosis. Fungigives water and nutrients to the plants that are essential for the growth ofplants.
Plants are able to uptake these nutrients and water through the fungalmycelial arrangement that covers a wide area in the rhizosphere zone. Thesefungi also secret certain organic acids and convert insoluble complex compoundsto soluble minerals which are taken up by plants. The plant partners ofmycorrhizal association also provide the fungi with essential carbohydrate forgrowth and development (Smith and Read 1997).
Some bacterial communities alsoinhabit the mycorrhizal root zone and enhance the formation of mycorrhiza(Bertaux et al. 2003).Plant Growth Promoting RhizosphericBacteriaCertainbacterial communities inhabits the rhizosphere either alone or in associationwith mycorrhizal fungi. These rhizospheric bacteria effects the growth ofplants in many ways. They can either enhance the plant growth or can havedrastic impacts on the growth of plants (Barea et al., 2005). Some soil microbes also tend to producebio-surfactants that are beneficial for the breakdown of organic pollutantsthrough their increased accessibility to plants (Lafrance and Lapointe, 1998).Nevertheless, the effect of the rhizosphere bacteria on the degradation actiongenerally differs with the genetic makeup of the microbe and the plant speciesthat are in association with the bacteria and also dependent on prevailingenvironmental state (Brimecombe et al.
2007). Pseudomonas spp. and Bacillusspp. are the most commonly known rhizosphere bacteria (Brimecombe et al.,2007). PlantGrowth Promoting Rhizobacteria are generally in association with root surfaceand enhance the development of plant through various means such as theincreased provision of mineral nutrients, production of plants hormones andthrough prevention of outbreak of certain diseases in plants (Tarkka et al.
2008). Bacterial and Fungal Degradation ofPesticidesThegenetic constitution and variety of microorganism community that areresponsible for the breakdown and removal of pesticides can differ from thosethat are present in the soil and this diversity may also vary from one crop toanother (Martin-Laurent et al., 2006). Degradation of most of the chemicalpesticides occur easily but some pesticides are very persistent and areresistant to degradation (Aislabie and LloydJones, 1995; Richins et al.
, 1997;Mulchandani et al., 1999). Phytoremediation of organic toxins isaugmented through bacterial activities. In this procedure, plants and soilmicroflora act together and furnish certain nutrients in the rhizosphere zonethat results in the enhanced microbial action for the breakdown and removal ofpesticides (Mirsal, 2004). Mycorrhizalfungi and bacteria inhabiting rhizosphere are found to enhance plant growth andbreakdown of certain contaminants in a soil with excessive amounts ofpollutants. For instance, ectomycorrhizal associations can show significanttolerance against harmful organic compounds like m-toluene (Sarand et al.,1999), petroleum constituents (Sarand et al.
, 1998), or polycyclic aromatichydrocarbons (Leyval and Binet 1998; Wenzel 2009).Mycorrhizalfungi and bacteria can tolerate extreme soil states and enhance the treatmentof polluted soils through direct consumption of constituents of pesticidepollutants as nutrients and through enhancing plant growth (Schützendübel andPolle 2002; Fomina et al. 2005; Baum et al. 2006; Zimmer et al.
2009; Wenzel2009).Mycorrhizalfungi and bacteria can act together in the rhizosphere zone at different stagesof cellular integration, extending from relatively simple interactions likesurface linkages to more complex, close and essential association. Thisinteraction serves to enhance the plant growth and development as well as it isalso important for maintaining the interaction of organisms among themselvesand with their surroundings in the rhizosphere zone (Perotto and Bonfante1997). Plantsand microbes undergo a sequence of chemical reactions that can cause breakdownof a number of chemical pesticides that are discharged into the environment.Due to long lasting nature some pesticides, the performance of certain microbesand plants to breakdown these chemicals is significantly limited.
As forexample, organochlorine compounds cannot be easily degrade by plants and marinephytoplankton (Shimabukuro et al., 1982). Aboutsixty-six bacterial strains were being studied that were found to beresponsible for the breakdown of atrazine when they used atrazine as a sourceof nitrogen and citrate as a source of carbon. These strains were beingseparated from unplanted and maize rhizosphere to which atrazine pesticide hadbeen applied. These degrader bacterial communities were the members of Actinobacteria, Bacteroidetes andProteobacteria (Martin-Laurent et al., 2006).
Manybacteria and fungi that inhabits the soil has the ability to breakdown ormineralize many different pesticides. The rate of adsorption, movement andbreakdown of chemical pesticides can be affected by the application of organicmatter and mineral nutrients (Briceño et al., 2007).Mostcommonly bacteria are thought of the most important degraders of chemicalpesticides but filamentous fungi have certain attributes that make them more favorablein different types of environments thus making fungi more successful degradersthan bacteria. Use of fungi is considered to be the most favorable techniquefor biological breakdown of certain pollutants that are resistant todegradation (Glaser and Lamar, 1995). Despite the fact that fungi are notcapable of movement, they adapt themselves to the changing environment (Read,2007). Fungi do not undergo a series of chemical reactions to degradecontaminants instead they secret certain enzymes outside the cell that act onthe pollutant for its degradation (Nawaz et al.
, 2011).Ifa specific enzyme is not present in a soil, it can cause the pesticide toreside in a soil for a very long period of time. Similarly, if particular amicroorganism that degrade a particular pesticide is not present in the rhizospherezone of soil or if the microbial community responsible for degradation ofpesticides is declined due to toxic effects of pesticides, then these microbesare introduced into the soil to augment the action of the microbial communitythat is already present (Singh, 2008). ConclusionLongterm application of chemical pesticides to agricultural fields and other sitescan lead to the environmental damage and can affect the natural productivity ofthe soil.
The toxic effects associated with the use of pesticides cansignificantly affect the microbial status of the soil. Some chemical pesticidesare persistent and extremely harmful to the environment as they can leach downthrough the soil and can contaminate the groundwater as well as thesepesticides can also pollute surface water. These pesticides also pose threat tomany wildlife species and ultimately to human beings when they are transferredfrom one organism to another through food chain. Decontamination of pesticidesis very important and crucial to maintain the productivity of the soil. Thetraditional practices for the remediation of soil or water contaminated withpesticides are very expensive and not ecofriendly. So, bioremediation isemerged as a cost effective and environmental friendly technique that involvesthe utilization of certain microbes with potential degrading capacity for thedegradation of pesticides.
Fungi and bacteria are considered as potentialbiodegraders. These microbes can interact with plant roots in the rhizospherezone to enhance the process of degradation of pollutants. Plants have complexinteractions with microbes in the rhizosphere zone. Microbial activity in therhizosphere zone is very important for the treatment of disturbed soils and canincrease the fertility of the plants. Fungi are more favorable degradersbecause they can tolerate harsh environmental condition which many bacterialstrains are unable to tolerate.
Fungi secrets specific extracellular enzymesthat degrades various pollutants. In favorable environmental conditions thatare conducive for the growth of microbial strains responsible for degradation,this technique is considered as an effective and ecofriendly approach forcleanup of environment polluted with toxic pollutants. In addition to thisapproach, biological control instead of synthetic chemical pesticide is aneffective approach and it can control the problem at source.