Rhizoremediation of pesticides: mechanism of microbial interaction in mycorrhizosphere

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 IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 194 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART 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 costeffective 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 IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 195 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART (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). IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 196 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART 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 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. IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 197 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART 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 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 IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 198 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART 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 (PilonSmits, 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 IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 199 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART (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). IJOART International Journal of Advancements in Research & Technology, Volume 2, Issue 7, July-2013 200 ISSN 2278-7763 Copyright © 2013 SciResPub. IJOART Roots Root Pieces Screw Cap Bottle Serial Dilution