Rhizoremediation of pesticides: mechanism of microbial … · 2013. 8. 4. · Rhizosphere bioremediation or rhizodegradation is the enhanced biodegradation of recalcitrant organic

International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 193 ISSN 2278-7763

Copyright © 2013 SciResPub. IJOART

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: [email protected]: 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

in the rhizosphere, leading to enhanced rhizodegradation of recalcitrant pesticides.

Pesticides are capable of persisting in the environment and causing concerns for human

health. The increasing costs and limited efficiency of traditional Physico-chemical

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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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

consortium to degrade environmental pollutants/toxins from soil with an aim of

restoring area sites to a condition useable for intended purpose. Rhizoremediation

takes advantage of plant roots natural symbiosis with mycorrhiza and root associated

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” describes the soil volume around plant roots, which is influenced by the

activities of the living roots. Rhizosphere is a complex environment that supports a huge

number of metabolically active microbial populations, several orders of magnitude

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 non-

vegetated 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

anthropogenic compounds in that they are small, mobile chemicals that are amenable to

cellular uptake and may interact through signal transduction pathways to induce the

production of specific degradative enzymes.

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-D-

degrading microorganisms

in the rhizosphere of

sugarcane

Sandman and ,

Loos,1984

Rice Benthiocarb Eightfold increase in

heterotrophic bacteria in the

rhizosphere

Sato, 1989.

Corn Atrazine Increase in production of

atrazine degradation

Seibert et al., 1981

Kochia Atrazine,

metolachlor,

and

trifluralin

Increased mineralization

compared to

nonrhizosphere soils

Anderson et

al.(1994)

Zinnia

anguistifolia

Mefenoxam Pseudomonas fluorescens

and Chrysobacterium

indologenes

Pai et al.(2001)

Rye grass Chlorpyrifos Increased degradation in

rhizo-sphere soils

Korade and Fulekar

(2010)

Pennisetum

pedicellatum

Chlorpyrifos

Cypermethrin

Fenvalerate

Selective enrichment of

degraders in rhizosphere

soil

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

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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

of rhizosphere microorganisms (Chaudhry et al., 2005). Some organic compounds in

root exudates may serve as carbon and nitrogen sources for the growth and long-term

survival of microorganisms that are capable of degrading organic pollutants (Pilon-

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

activities (bioaugmented rhizoremediation), which is able to efficiently colonize the

root (Korade and Fulekar, 2009, Kuiper et al., 2001; 2004).

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Roots

Root Pieces

Screw Cap Bottle

Serial Dilution

1 ml plate

Petri Dishes With Agar

Fungi, Bacteria and Actinomycetes

Sterile Petri Dish

Agar with Root

Colonies Growing 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:

The biodegradation of a compound is dependent on specific enzymes secreted by

microorganisms. These enzymes are largely pH-dependent and bacteria tend to have

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

pesticides (atrazine, isoproturon, and trifluralin) to soil organic matter content. Singh et

al. (2006) studied the biodegradation of organophosphate pesticides (fenamifos and

chlorpyrifos) and did not find any impact of soil organic matter on the pesticide

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 [U-14C]2,4-

dichlorophenoxyacetic 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 2,4-D/α-ketoglutarate

dioxygenase (tfdA) were investigated. The rhizosphere effect on [14C]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.

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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 non-

rhizosphere, 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

be 18.0, 7.7, 6.3 days at 1 mg/kg, 19.9, 11.0, 2.9 days at 10 mg/kg, and 23.2, 18.0, 10.8

days at 100 mg/kg, respectively. The experimental results show that the degradation of

butachlor can be enhanced greatly in wheat rhizosphere, and especially in the

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 the 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

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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 concentrations 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

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 Chrysobacterium indologenes isolated from the rhizosphere system

degraded the applied fungicide (10 μg/ml) almost completely to the free acid within 54

h.

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-1-

methylpropyl)-5-isoxazolyl]-2,6-dimethoxybenzamide} is a pre-emergence herbicide

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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

ryegrass rhizosphere, and the microorganisms surviving in the rhizospheric soil spiked

at highest concentration (100 mg/kg) was assessed and used for isolation of

chlorpyrifos degrading microorganisms. The potential degrader identified by 16s rDNA

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 × 106 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

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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

finding shows that Chlorpyrifos was more toxic than Cypermethrin and Fenvalerate at

higher concentrations (75 and 100mg/kg) for the germination, survival and subsequent

growth of Cenchrus setigerus, and Pennisetum pedicellatum. The heterotrophic microbial

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

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rhizosphere. Further understanding of the critical factors influencing the plant-microbe-

toxicant interaction in soils will permit more rapid realization of this new approach to

in situ bioremediation.

References:

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