**5. Rhizoremediation**

Rhizospheric microbes can degrade the majority of environmental pollutants and degradation process stops when the microbe is deprived of food. These microbes have access to the best food source available in soil, namely root exudates [61]. Researchers have described an enrichment method for the isolation of microbes [62], which combine the properties ofdegra‐ dation of a selected pollutant and excellent root colonization. They have termed this process 'rhizoremediation' instead of phytoremediation to emphasize the roles of the root exudates and the rhizosphere competent microbe. The high concentration of metals in soils and their uptake by plants harmfully influence the growth, symbiosis and consequently the yields of crops [63] by disintegrating cell organelles and disrupting the membranes, acting as genotoxic substance [64] disrupting the physiological process, such as, photosynthesis or by inactivating the respiration, protein synthesis and carbohydrate metabolism. *Pseudomonas putida* is a root colonizer of potential interest for the rhizoremediation of pollutants and the biological control of pests [65]. According to hypothesis when a suitable rhizosphere strain is inoculated together with a suitable plant (e.g., coating bacteria on plant seed), these well-equipped bacteria might settle on the root together with the normal indigenous population, thereby enhancing the bioremediation process. Pioneer work about degradation of compounds in the rhizosphere was mainly focused for herbicides and pesticides [66]. In the past two decades, a large number of publications on rhizodegradation of various organic toxicants using different plants and/or microbial inoculants have been published [37, 67-71].

degradation of pollutants [59]. The root exudates consists of water soluble, insoluble, and volatile compounds including sugars, alcohols, amino acids, proteins, organic acids,

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Normally a symbiotic relationship develops between plant and soil microbes in the rhizo‐ sphere, where plants provide nutrients necessary for the microbes to flourish, while the microbes provide a healthier soil environment where plant roots can grow. Specifically, plants loosen soil and transport oxygen and water into the rhizosphere [26]. In addition, plants exude specific phytochemicals (sugars, alcohols, carbohydrates, etc.) that are primary sources of food (carbon) for the specific soil organisms that aid in providing the healthier soil environment [79]. Alternatively, the exuded phytochemical may be an allelopathic agent meant to suppress other plants from growing in the same soil [26]. In return for exporting these phytochemicals, plants are protected from competition, soil pathogens, toxins and other chemicals that are naturally present or would otherwise be growing in the soil environment [26]. Microbial populations can be several orders of magnitude higher in a vegetated soil compared to an unvegetated soil. Rhizodegradation, sometimes called phytostimulation, rhizosphere biodegra‐ dation or plant assisted bioremediation/degradation, is the enhanced breakdown of a contaminant by increasing the bioactivity using the plant rhizosphere environment to stimu‐

Several factors that influence the rate of rhizoremediation of PAHs in soil e.g. soil type, texture, particle size, nutrients and organic matter content which can limit the bioavailability of pollutants [67]. The conditions that increase the possibility of degradation include the presence of low molecular weight PAHs, relatively recent PAHs emission or deposition, moderate soil pH, the presence of appropriate PAHs degrading bacteria and plants to facilitate decomposi‐ tion by virtue of large root surface area or uptake affinity [80]. Root microbe interactions are considered the primary process of PAHs phytoremediation [81]. Natural attenuation in vegetated settings is thought to degrade one, two and three chain PAHs in periods ranging from16 to 126 days [82]. By-products of degradation are thought to be less toxic and may serve as an energy source for other soil organisms. Research suggests that PAHs with fewer benzene rings are more easily digested by soil microbes. Johnson *et al*. [32] suggests that microbial degradation of PAHs and other hydrophobic substrates is believed to be limited by the amounts dissolved in the water phase, with sorbed, crystalline and non-aqueous phase liquid

The main problem for soil bioremediation is the bioavailability of the pollutant. Most of organic pollutants are highly hydrophobic compounds that dissolve poorly in water and many of them can form complexes with soil particle, this lack of bioavailability often lowers removal efficiencies [26]. Bioavailability is a dynamic process, determined by the rate of substrate mass

nucleotides, flavonones, phenolic compounds and certain enzymes [68].

late the microbial populations [26].

**6. Factor influencing PAH degradation**

dissolved PAHs being unavailable to PAH degrading organisms.

transfer to microbial cells relative to their intrinsic catabolic activity [26].

Field contaminated soils that have undergone prolonged periods of ageing generally appear to be much less responsive to rhizodegradation than freshly spiked soil [72-74]. Wenzel [67] concluded that low bioavailability is a main cause of failure of rhizodegradation in field contaminated and aged spiked soils. This has important implications for the applicability of rhizodegradation as well as for the evaluation of data obtained on freshly or only shortly aged, spiked soil material. Other strategies to enhance rhizodegradation (e.g. inoculation of degrader strains) are likely to fail where low bioavailability is the main constraint.

Interestingly, microbial treatments appeared to be successful at the laboratory experi‐ ments [75] but failed when applied to long term contaminated soil on field experiments [76]. This indicates again the importance of the experimental scale and of bioavailability. In view of the still disappointing and controversial results of traditional inoculation [77], enhanced rhizodegradation requires more sophisticated approaches. Enhanced degrada‐ tion capabilities of inoculated microorganisms may be obtained by induction of a nutrition‐ al bias towards the inoculated strains. Only recently, Narasimhan *et al*. [78] identified root exudate compounds (phenylpropanoids) that created a nutritional bias in favour of enhanced PCB degradation. A successful rhizoremediation process could depend on the highly branched root system of the plant species where a large number of bacteria harbor, establishment of primary and secondary metabolism, survival and ecological interactions with other organisms [48]. Plant roots can act as a substitute for the tilling of soil to incorporate additives (nutrients) and to improve aeration in soil [48]. Plants also release a variety of photosynthesis derived organic compounds (root exudates), which might help in degradation of pollutants [59]. The root exudates consists of water soluble, insoluble, and volatile compounds including sugars, alcohols, amino acids, proteins, organic acids, nucleotides, flavonones, phenolic compounds and certain enzymes [68].

Normally a symbiotic relationship develops between plant and soil microbes in the rhizo‐ sphere, where plants provide nutrients necessary for the microbes to flourish, while the microbes provide a healthier soil environment where plant roots can grow. Specifically, plants loosen soil and transport oxygen and water into the rhizosphere [26]. In addition, plants exude specific phytochemicals (sugars, alcohols, carbohydrates, etc.) that are primary sources of food (carbon) for the specific soil organisms that aid in providing the healthier soil environment [79]. Alternatively, the exuded phytochemical may be an allelopathic agent meant to suppress other plants from growing in the same soil [26]. In return for exporting these phytochemicals, plants are protected from competition, soil pathogens, toxins and other chemicals that are naturally present or would otherwise be growing in the soil environment [26]. Microbial populations can be several orders of magnitude higher in a vegetated soil compared to an unvegetated soil. Rhizodegradation, sometimes called phytostimulation, rhizosphere biodegra‐ dation or plant assisted bioremediation/degradation, is the enhanced breakdown of a contaminant by increasing the bioactivity using the plant rhizosphere environment to stimu‐ late the microbial populations [26].
