Document 14681201

International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 193

ISSN 2278-7763

Rhizoremediation of pesticides: mechanism of microbial interaction in mycorrhizosphere

Kriti Kumari Dubey * and M.H. Fulekar *#

* Environmental Biotechnology Laboratory, Department of Life Sciences, University of

Mumbai, Santacruz (E), Mumbai-400 098, India

# School of Environmental science and Sustainable Development, Central University of

Gujarat, Gandhinagar, Gujarat-482030,India corresponding author: mhfulekar@yahoo.com: Tel: +91-2226528847, Fax: +91-

222652605

Abstract:

Rhizosphere bioremediation or rhizodegradation is the enhanced biodegradation of recalcitrant organic pollutants by root-associated bacteria and fungi under the influence of selected plant species. Use of selected vegetation and sound plant management practices, increase the total proportion of pollutant degraders in numbers and activity health.

IJOART treatments of soil have spurred the development of new remediation technologies. The use of plants and native microorganisms to degrade or remove pollutants has emerged as a powerful technology for in situ remediation. An understanding of the mechanisms of pollutant degradation in the rhizosphere environment is important for successful implementation of this technology. Recent studies have demonstrated that plants and rhizosphere associated microorganism produce pesticide-degrading enzymes that can mineralize different groups of pesticides and their metabolites with greater efficiency .

Thus, rhizoremediation appears a very promising technology for the removal of pesticides from polluted soil. The aim of present review is to provide improved understanding of mechanism of microbial interaction in rhizosphere, which will help to translate the results of simplified bench scale and pot experiments to the full complexity and heterogeneity of field experiments with predictable remedial success.

Introduction:

Environmental pollution has become an increasing global concern. The modern technological innovations, production and processes have generated wastes which

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ISSN 2278-7763 contain the complex inorganic and organic compounds. The treatment of the wastes has become great concern to the environmentalists. Pesticides waste generated through chemical processing in pesticide industry and their commercial, agricultural and domestic usages have enhanced the level of hazardous environmental contaminants.

Pesticides wastes find their ways in soil-water causing environmental pollution.

Pesticides contamination in soils, surface water and ground water poses major environmental problem worldwide. Environmental management of the pesticides has become a major concern to the environmentalists. There is an urgent need to develop cost- effective and sustainable technology to remove contaminants from the environment or to detoxify them. Rhizoremediation and bioremediation has attracted an increasing attention of scientists, industries and government agencies that are facing the challenge of remediation and restoration of hazardous wastes. The recent advances in remediation technology using microbial consortium and identified potential degrader have been found effective for the treatment of pesticides in soil-water environment

(Fulekar, 2005). Rhizoremediation technology uses plant roots and associated microbial IJOART natural microbial flora for the enhanced degradation of pollutants in the rhizosphere.

Bioremediation techniques can be used to remove hazardous waste pesticides which have already polluted the environment. In bioremediation microorganisms breakdown most compounds for their growth and energy needs. Bioremediation and phytoremediation are innovative technologies that have the potential to alleviate pesticide contamination. The process of bioremediation usually occurs in soil, whereby pesticides are broken down into less active/toxic compounds by fungi, bacteria, and other microorganisms that use pesticides as energy and carbon sources. It is estimated that 1 g of soil contains more than one hundred million bacteria (5000–7000 different species) and more than ten thousand fungal colonies (Dindal, 1990; Melling, 1993). The use of microbial metabolic potential for eliminating soil pollutants provides a safe and economic alternative to other commonly used physico-chemical strategies (Vidali,

2001). Indigenous microorganisms (natural attenuation) can be used for detoxification of contaminants in the environment. The application of in situ bioremediation with naturally occurring microorganism has been revealed in scientific reports



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(Bhupathiraju et al., 2002).Detoxification of pesticides by indigenous soil microorganisms and/or enzymes isolated from microbes has been well explained in this review article. Soil conditions strongly influence the effectiveness of bioremediation

(Morra, 1996; Riser-Roberts, 1998). The effects of soil moisture, temperature, aeration, pH, and organic matter content on the biodegradation of pesticides have been investigated in many studies (Bending et al., 2006; Charnay et al., 2005; Rasmussen et al., 2005). Therefore, a brief section on soil factors affecting pesticide biodegradation has also been included in this article. The aim of present review is to understand the mechanism of rhizoremediation of pesticides in rhizosphere, with emphasis on certain aspects of plant associated microbes with remediating potential of pesticides and their relevant remediation efforts.

Mechanism of Pesticide Degradation in the Rhizosphere:

“

Chemicals released by plants may enhance xenobiotic degradation, and it may therefore be beneficial to use plants in the remediation of contaminated soils. The term rhizosphere ”

IJOART higher than the non-rhizospheric soil. The rhizosphere is the zone of soil around the root in which microbes are influenced by the root system forming a dynamic root-soil interface (Kuiper et al., 2004; Pilon-Smits, 2005; Barea et al., 2005).

There are three separate, but interacting, components recognized in the rhizosphere:

1) Rhizosphere (soil): the zone of soil influenced by roots through the release of substrates that affect microbial activity.

2) Rhizoplane: the root surface, including the strongly adhering soil particles.

3) Root tissue: that some endophytic microorganisms (endophytes) are able to colonize

(Barea et al., 2005).

The differing physical, chemical, and biological properties of the root-associated soil, compared with those of the bulk soil, are responsible for changes in microbial diversity and for increased numbers and metabolic activities of microorganisms in the rhizosphere microenvironment, the phenomenon called the rhizosphere effect (Barea et al., 2005; Kuiper et al., 2004; Pilon-Smits, 2005; Salt et al., 1998).



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Densities of rhizospheric bacteria can be as much as two to four orders of magnitude greater than populations in the surrounding bulk soils and display a greater range of metabolic capabilities, including the ability to degrade a number of recalcitrant xenobiotics (Pilon-Smits, 2005; Salt et al., 1998). Therefore, to find an accelerated rate of biodegradation of organic pollutants is found in vegetated soils compared with nonvegetated soils. Rhizosphere effects on xenobiotic biotransformation have been studied for a variety of compounds, although the mechanisms by which certain plants enhance biodegradation are still poorly understood. Differences in plant tolerance to phytotoxic compounds in soils may be related to the plants’ ability to induce microorganisms that will detoxify these xenobiotics in the soil environment . Research on phytoremediation, through trial and error, has focused on densely rooted, fast growing grasses and plants, such as Brassica sp., with fine root systems. Mulberry ( Morus alba L .) and poplar

( Populus deltoides ) trees have been used successfully in the phytoremediation of chlorophenols and chlorinated solvents such as trichloroethylene (TCE) (Stomp et al.

1993). Salicylic acid, flavonoids, and monoterpenes are structurally analogous to many IJOART

Phytoremediation is also a cost-effective and innovative technology that uses plants to clean up a broad range of organic and inorganic wastes (Cunningham et al.,

1995; Licht & Isebrands, 2005; Salt et al., 1998). Plants can bioaccumulate xenobiotics in their above-ground parts, which are then harvested for removal. Plants may contribute to remediation in several ways, by reducing the leaching of contaminants, aerating soil, phytodegradation/transformation, phytovolatilization, evapotranspiration, and rhizoremediation (Amos & Younger, 2003; Chang et al., 2005;

Cunningham et al., 1995). The selection of bioremediation or phytoremediation for cleanup of a contaminated site may depend upon prevailing conditions that support the application of microbes, plants, and/or both. Without the microbial contribution, phytoremediation alone may not be a viable technology for many hydrophobic organic pollutants (Chaudhry et al., 2005). The use of rhizomicrobial populations present in the rhizosphere of plants for bioremediation is referred to as Rhizoremediation ( Kuiper et al., 2004). The term consists of both stimulation and rhizodegradation describing, thus, the importance of both the plant and the microbes in this beneficial interaction.



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Table 1. Plant species shown to facilitate microbial degradation of pesticides in the rhizosphere:

Plant rhizosphere

Pesticide Summary Reference

Sugarcane 2,4-D High population of 2,4-Ddegrading microorganisms in the rhizosphere of sugarcane

Rice Sato, 1989. Benthiocarb Eightfold increase in heterotrophic bacteria in the rhizosphere

Corn Atrazine Increase in production of Seibert et al., 1981 atrazine degradation

Kochia

Zinnia anguistifolia

Atrazine, metolachlor,

Increased mineralization compared to

Mefenoxam Pseudomonas fluorescens

Anderson et al.(1994) IJOART and Chrysobacterium

Pai et al.(2001) indologenes

Rye grass Chlorpyrifos Increased degradation in rhizo-sphere soils

Pennisetum pedicellatum

Chlorpyrifos

Cypermethrin

Selective enrichment of degraders in rhizosphere

Fenvalerate soil

Sandman and ,

Loos , 1984

Korade and Fulekar

(2010)

Dubey and Fulekar

(2011a)

Plant-microbial interactions in the rhizosphere offer very useful means for remediating environments contaminated with recalcitrant organic compounds

(Chaudhry et al., 2005). Plant roots can act as a substitute for the tilling of soil to incorporate additives (nutrients) and to improve aeration (Kuiper et al., 2004; Aprill &

Sims, 1990).Various grass varieties and leguminous plants have shown to be suitable for



ISSN 2278-7763 rhizoremediation (Kuiper et al., 2001, 2004). The mucigel secreted by root cells, lost root cap cells, the starvation of root cells, or the decay of complete roots provides nutrients in the rhizosphere (Kuiper et al., 2004; Lynch & Whipps, 1990). In addition, plants release a variety of photosynthesis derived organic compounds (Pilon-Smits,

2005; Salt et al., 1998). These root exudates contain water soluble, insoluble, and volatile compounds including sugars, alcohols, amino acids, proteins, organic acids, nucleotides, flavonones, phenolic compounds and certain enzymes (Chaudhry et al.,

2005; Pilon-Smits, 2005; Salt et al., 1998; Anderson et al., 1993).

The rate of exudation changes with the age of a plant, the availability of mineral nutrients and the presence of contaminants (Chaudhry et al., 2005). The nature and the quantity of root exudates, and the timing of exudation are crucial for a rhizoremediation process. The root exudates mediate acquisition of minerals by plants and stimulate microbial growth and activities in the rhizosphere in addition to changing some physicochemical conditions. Plants might respond to chemical stress in the soil by changing the composition of root exudates controlling, in turn, the metabolic activities IJOART

Smits, 2005; Salt et al., 1998; Anderson et al., 1993).

Cometabolism: Cometabolism is defined as the oxidation of non growth substrates during the growth of an organism on another carbon or energy source

(Kuiper et al., 2004). Some co-metabolized recalcitrant pollutants such as the pesticide lindane (organochlorine) are only transformed and not effectively mineralized by microorganisms (Paul et al., 2005). Microbes living in the rhizosphere, Rhizomicrobia, in turn, can promote plant health by stimulating root growth (regulators), enhancing water and mineral uptake, and inhibiting growth of pathogenic or other, non-pathogenic soil microbes (Pilon-Smits, 2005; Kuiper et al., 2004).

The microbial transformations of organic compounds are usually not driven by energy needs but a necessity to reduce toxicity due to which microbes may have to suffer an energy deficit (Chaudhry et al., 2005). Thus, the processes may be enhanced or driven by the abundant energy that is provided by root exudates. Such stimulation of soil microbial communities by root exudates also benefits plants through increased availability of soil-bound nutrients and degradation of phytotoxic soil contaminants



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(Chaudhry et al., 2005). This might allow the spread of roots into deeper soil layers.

Rhizomicrobia may also accelerate remediation processes by increasing the humification of organic pollutants (Salt et al, 1998). In particular, the release of oxidoreductase enzymes (e.g. peroxidase) by microbes, as well as by plant roots, can catalyze the polymerization of contaminants onto the soil humic fraction and root surfaces. Usually, several bacterial populations degrade pollutants more efficiently than a single species/strain due to the presence of partners, which use the various intermediates of the degradation pathway more efficiently (joint metabolism) (Kuiper et al., 2004; Pelz et al., 1999).

During rhizoremediation, the degradation of a pollutant, in many cases, is the result of the action of a consortium of bacteria (Kuiper et al., 2004).

The colonization of different niches of plant roots by different strains has also been recognized (Kuiper et al., 2001, 2004; Dekkers et al., 2000). Interestingly, the close proximity of the different strains and the formation of mixed micro-colonies were observed only in the presence of the pollutant. However, very few studies report the directed introduction of a microbial strain or consortium for xenobiotic degradation IJOART



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Root Pieces

Roots

Serial Dilution

1 ml plate

Screw Cap Bottle

Petri Dishes

With Agar

Sterile Petri Dish

Fungi, Bacteria and Actinomycetes

Colonies Growing

Agar with Root

IJOART

Figure 1: Serial dilution of soil and plating methods for isolation of microorganisms from soil, rhizosphere soil and root surface.

Factors affecting Bioremediation of pesticides in the soil environment:

Soil Conditions:

The success of bioremediation depends on a number of soil physico-chemical factors such as moisture, redox conditions, temperature, pH, organic matter, nutrients and nature, and amount of clay that affect microbial activity and chemical diffusion in soils.

Soil water affects not only the moisture available to microorganisms, but also the redox conditions in soil that may lead to different biochemical reactions. Schroll et al. (2006) quantified the effect of soil moisture on the aerobic microbial mineralization of selected pesticides (isoproturon, benzolin-ethyl, and glyphosphate) in different soils. They found a linear correlation ( p < 0.0001) between increasing soil moisture (within a soil water potential range of − 20 and − 0.015 MPa) and increased relative pesticide mineralization.



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Optimum pesticide mineralization was obtained at a soil water potential of − 0.015 MPa.

Further increase in water reduced the pesticide mineralization because surplus water restricts oxygen diffusion and availability and can make the environment anoxic.

Temperature:

Temperature and pH are the major factors affecting the biodegradation of pesticides in soil. Temperature not only affects the rates of biochemical reactions, as all microbial activities depend on thermodynamics, but also has a direct impact on cell physiology-altering proteins and cell membrane permeability (Alberty, 2006; Guillot et al., 2000; Mastronicolis et al., 1998). The bacterial isolates were able to rapidly degrade fenamiphos and chlorpyrifos between 15 and 35 ◦ C, but their degradation ability was sharply reduced at 5 or 50 ◦ C (Singh et al., 2006). Similar results were reported by

Siddique et al. (2002), who studied biodegradation of HCH isomers in soil slurry. They observed that an incubation temperature of 30 ◦ C was optimum for effective degradation of α - and γ -HCH isomers. pH:

IJOART optimum pH between 6.5 and 7.5, which equals their intracellular pH. A Pandoraea sp. isolated from an enrichment culture (Okeke et al., 2002) degraded HCH isomers over a pH range of 4 to 9 (Siddique et al., 2002), but the optimum pH for growth and biodegradation of α - and γ -isomers of HCH in soil slurries was 9. Singh et al. (2006) also reported the similar results while studying the biodegradation of organophosphate pesticides in soil. Degradation rate was slower in lower pH soils in comparison with neutral and alkaline soils. Though soil pH has a direct effect on biochemical reactions, it may influence adsorption/desorption of pesticides on soil matrix and hence bioavailability and biodegradation. Lower soil pH can increase the adsorption of weakly acidic pesticides.

Boivin et al. (2005) compared the adsorption and desorption processes of five pesticides (from very weak base to weakly acidic chemicals) in thirteen contrasting field soils and found a significant correlation between bentazone (weakly acidic pesticide) adsorption and soil pH. Sorption of the neutral form likely involves non-specific interactions along with hydrophobic interactions and the presence of hydrogen bonds.



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In the case of bentazone, the coexistence of both neutral and ionized forms could explain the increased sorption at low pH values. No significant adsorption was observed with the other weakly acidic pesticide (2, 4-D) used in the experiment due to its complete ionization at lower pH and greater repulsion between electronegative charges of soil constituents and those of the ionized molecules.

Soil organic matter:

Soil organic matter also affects biodegradation of pesticides in soil by providing nutrients for cell growth and controlling pesticide movement by adsorption/desorption processes. Perrin-Ganier et al. (2001) monitored biodegradation of isoproturon

(herbicide) by adding sewage sludge, nitrogen (N), and phosphorus (P) separately and observed that N and P had the greatest effect on isoproturon degradation. Sewage sludge did not affect isoproturon degradation significantly despite increase of organic matter with sludge addition (Perrin-Ganier et al., 2001). In soil, the main source of organic matter that provides nutrients is crop residues. Different groups of pesticides behave differently in soils. Boivin et al. (2005) correlated adsorption of non-acidic IJOART biodegradation rates. In another study, Fenlon et al. (2007) found that diazinon

(organophosphate pesticide) only appreciably mineralized in two of the organic soils when assessed in organically and conventially managed soils compared to cypermethrin

(pyrethroid), which degraded significantly in all the investigated soils.

Recent research studies implying rhizosphere bioremediation of pesticides:

 Shaw and Burns, 2004 reported that the mineralization of [U14 C]2,4dichlorophenoxyacetic acid (2,4-D) in rhizosphere soil with no history of herbicide application collected over a period of 0 to 116 days after sowing of Lolium perenne and

Trifolium pratense . The relationships between the mineralization kinetics, the number of 2,4-D degraders, and the diversity of genes encoding 2,4D/α -ketoglutarate dioxygenase ( tfdA ) were investigated. The rhizosphere effect on [ 14 C]2,4-D mineralization (50 μg g −1 ) was shown to be plant species and plant age specific. In comparison with nonplanted soil, there were significant ( P < 0.05) reductions in the lag phase and enhancements of the maximum mineralization rate for 25- and 60-day T.



ISSN 2278-7763 pratense soil but not for 116-day T. pratense rhizosphere soil or for L. perenne rhizosphere soil of any age. Numbers of 2,4-D degraders in planted and nonplanted soil were low (most probable number, <100 g −1 ) and were not related to plant species or age. Single-strand conformational polymorphism analysis showed that plant species had no impact on the diversity of α Proteobacteria tfdA -like genes, although an impact of 2,4-D application was recorded. Results indicated that enhanced mineralization in T. pratense rhizosphere soil is not due to enrichment of 2,4-D-degrading microorganisms by rhizodeposits and an alternative mechanism in which one or more components of the rhizodeposits induce the 2,4-D pathway was suggested.

 Yu et al, 2003 reported the degradative characteristics of butachlor in nonrhizosphere, wheat rhizosphere, and inoculated rhizosphere soils were measured. The rate constants for the degradation of butachlor in non-rhizosphere, rhizosphere, and inoculated rhizosphere soils were measured to be 0.0385, 0.0902, 0.1091 at 1 mg/kg,

0.0348, 0.0629, 0.2355 at 10 mg/kg, and 0.0299, 0.0386, 0.0642 at 100 mg/kg, respectively. The corresponding half-lives for butachlor in the soils were calculated to IJOART rhizosphere inoculated with the bacterial community designated HD which is capable of degrading butachlor. It was concluded that rhizosphere soil inoculated with microorganisms-degrading target herbicides is a useful pathway to achieve rapid degradation of the herbicides in soil.

 Liao et al, 2008 reported t he degradative characteristics of simazine (SIM), microbial biomass carbon, plate counts of heterotrophic bacteria and most probably number (MPN) of SIM degraders in uninoculated non-rhizosphere soil, uninoculated rhizosphere soil, inoculated non- rhizosphere soil, and inoculated rhizosphere soil were measured. At the initial concentration of 20 mg SIM/kg soil, the half-lives of SIM in the four treated soils were measured to be 73.0, 52.9, 16.9, and 7.8 d, respectively, and corresponding kinetic data fitted first- order kinetics. The experimental results indicated that higher degradation rates of SIM were observed in rhizosphere soils, especially in inoculated rhizosphere soil. The degradative characteristics of SIM were found to be closely related to microbial process. Rhizosphere soil inoculated with



ISSN 2278-7763 microorganisms-degrading target herbicides offering a useful pathway to achieve rapid degradation of the herbicides in soil was suggested.

 Pai et al, 2001 studied the fate of the fungicide mefenoxam in a containerized rhizosphere system. The rhizosphere system used Zinnia angustifolia (Tropic Snow) in a bark/sand potting mix and was compared to bulk potting mix (no plants). Rhizosphere microbial populations were allowed to establish for 3 weeks prior to fungicide addition

(20 μg per g mix). Mefenoxam and degradation product concentrati ons were determined by High HPLC or capillary electrophoresis after extraction. Seventy eight percent of the fungicide originally applied to the rhizosphere was degraded after 21 days compared to 44% in bulk system (no plant). The primary degradation product was the free acid N-(2, 6-dimethylphenyl)-N-(methoxyacetyl)-DL-alanine, which accounted for 71% of the applied parent chemical after 30 days. N-(2,6-dimethylphenyl)acetamide was also detected, but in lesser amounts. Bacterial populations in the h. rhizosphere was found to increase during the 30-day period, which was correlated with an increase in degradation of the parent compound. Pure cultures of Pseudomonas fluorescens and degraded the ap

IJOART

 Sun et al, 2004 conducted an experiment to investigate the degradation of aldicarb, an oxime carbamate insecticide, in sterile, non-sterile and plant-grown soils, and the capability of different plant species to accumulate the pesticide. The degradation of aldicarb in soil followed first-order kinetics. Half lives (t1/2) of aldicarb in sterile and non-sterile soil were 12.0 and 2.7 days, respectively, which indicated that microorganisms played an important part in the degradation of aldicarb in soil. Aldicarb was found to disappear more quickly in the soil with the presence of plants, and t1/2 of the pesticide were 1.6, 1.4 and 1.7 days in the soil grown with corn, mung bean and cowpea, respectively. Comparison of plant-promoted degradation and plant uptake showed that the enhanced removal of aldicarb in plant-grown soil was mainly due to plant-promoted degradation in the rhizosphere.

 Drakeford et al, 2003 studied the degradation of Isoxaben{N-[3-(1-ethyl-1methylpropyl)-5-isoxazolyl]-2,6-dimethoxybenzamide} is a pre-emergence herbicide



ISSN 2278-7763 was studied in potting mix (80% bark, 20% sand) with three different regimes (sterile, bulk and rhizosphere). The rhizosphere regime contained Switch Grass ( Panicum virgatum ), and plants were allowed to grow for 14 days before adding isoxaben (10

μg/g potting mix). Isoxaben was degraded to 0.5 μg/g in 60 days giving a half -life of 7 days. Two degradation products were detected: 3-nitrophthalic acid in the rhizosphere and bulk regimes and 4-methoxyphenol in the sterile regime. Microbial population shifts were determined by fatty acid methyl ester profile analysis and were influenced by the introduction of a plant (rhizosphere regime) and by isoxaben addition.

 Korade and Fulekar, 2009 reported the potential of ryegrass for rhizosphere bioremediation of chlorpyrifos in mycorrhizal soil by the green house pot culture experiments. The pot cultured soil amended at initial chlorpyrifos concentration of 10 mg/kg was observed to be degraded completely within 7 days where the rest amended concentrations (25–100 mg/kg) decreased rapidly under the influence of ryegrass mycorrhizosphere as the incubation progressed till 28 days. This bioremediation of chlorpyrifos in soil is attributed to the microorganisms associated with the roots in the IJOART analysis using BLAST technique was Pseudomonas nitroreducens PS-2. Further, bioaugmentation for the enhanced chlorpyrifos biodegradation was performed using

PS-2 as an inoculum in the experimental set up similar to the earlier. The heterotrophic bacteria and fungi were also enumerated from the inoculated and non-inoculated rhizospheric soils. In bioaugmentation experiments, the percentage dissipation of chlorpyrifos was 100% in the inoculated rhizospheric soil as compared to 76.24, 90.36 and 90.80% in the non-inoculated soil for initial concentrations of 25, 50 and 100 mg/kg at the 14th, 21st and 28th day intervals respectively.

 Abhilash et al, 2011 reported the combined rhizoremediation potential of

Staphylococcus cohnii subspecies urealyticus in the presence of tolerant plant Withania somnifera grown in lindane spiked soil. Withania was grown in garden soil spiked with

20 mg kg−1 of lindane and inoculated with 100 ml of microbial culture (8 .1 × 10 6 CFU).

Effect of microbial inoculation on plant growth, lindane uptake, microbial biomass carbon, dehydrogenase activity, residual lindane concentration and lindane dissipation percentage were analyzed. The microbial inoculation significantly enhances the growth



ISSN 2278-7763 and lindane uptake potential of test plant (p < 0.05). Furthermore, there was an enhanced dissipation of lindane observed in microbial inoculated soil than the dissipation rate in non-inoculated soil (p < 0.01) and the dissipation rate was positively correlated with the soil dehydrogenase activity and microbial biomass carbon (p <

0.05). Study concluded that the integrated use of tolerant plant species and rhizospheric microbial inoculation can enhance the dissipation of lindane, and have practical application for the in situ remediation of contaminated soils.

 Dubey and Fulekar (2011a and b) reported the experiment carried out to evaluate the potential use of grass species Pennisetum pedicellatum for the rhizospheric bioremediation of pesticides Chlorpyrifos. The effect of the three pesticides on the germination of grass seeds was investigated using pesticide spiked soil at the concentrations 10, 25, 50, 75 and 100 mg/kg, while unspiked soil has been taken as control. The heterotrophic microbial numbers were also enumerated in the developing rhizospheric zone and inthe bulk soil in order to assess developing microbial associations for biodegeradation of pesticides in mycorrhizosphere. The research growth of

IJOART populations were found to be higher in the mycorrhizosphere soil of co-cropping system of Cenchrus setigerus and Pennisetum pedicellatum as compared to individual mycorrhizospheres of Cenchrus setigerus and Pennisetum pedicellatum , for all the three pesticides at each concentration ranging from 10 mg/kg to 100mg/kg. This study will help in selection plants for further investigation of the rhizospheric bioremediation of

Chlorpyrifos, Cypermethrin and Fenvalerate contaminated soil. Enhanced chlorpyrifos degradation was reported in Pennisetum rhizosphere.

Conclusions:

The potential role of plants and associated rhizomicrobial population in facilitating microbial degradation for in situ bioremediation of surface soils contaminated with hazardous organic compounds is substantial. Support for this concept comes from the fundamental microbial ecology of the rhizosphere, documented acceleration of microbial degradation of agricultural chemicals in the root zone, and recent research addressing degradation of agricultural and nonagricultural hazardous pesticides in the



ISSN 2278-7763 rhizosphere. Further understanding of the critical factors influencing the plant-microbetoxicant interaction in soils will permit more rapid realization of this new approach to in situ bioremediation.

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