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3112887 
Book/Book Chapter 
Biodegradation of Organic Xenobiotic Pollutants in the Rhizosphere 
Azaizeh, H; Castro, PML; Kidd, P 
2011 
Plant Ecophysiology 
8 
191-215 
WOS:000284721300009 
Soil contamination by xenobiotic organic compounds is a serious problem in most industrialized countries, causing acute and diffuse contamination of soil and waters on a global scale. Microbial transformation plays a major role in contaminant degradation of many persistent organic pollutants (POPs). However, microbial degradation can be limited by factors such as contaminant bioavailability, - adsorption and mass transfer, while combined plant-microbial systems can overcome these drawbacks, leading to more efficient contaminant degradation at the soil-root interface or rhizosphere. Hypotheses that support improved degradation within the rhizosphere compared to nonvegetated soils include (i) increase in microbial density, diversity and/or metabolic activity, (ii) catabolic enzyme induction. (iii) co-metabolism of contaminants with similar structures to rhizodeposits, (iv) improved contaminant bioavailability, and (v), selective increase in the number and activity of pollutant degraders. Root exudates or rhizodeposits not only provide a nutrient-rich habitat for microorganisms but can potentially enhance biodegradation of xenobiotics in different ways: they may facilitate the co-metabolic transformation of pollutants with similar structures, induce genes encoding enzymes involved in the degradation process, increase contaminant bioavailability, and/or selectively increase the number and activity of pollutant degraders in the rhizosphere. The combination of microbial bioremediation and phytoremediation in this complementary manner is known as rhizoremediation, phytostimulation or rhizosphere bioremediation. Bacteria, fungi and mycorrhizal fungi are a major component of the rhizosphere and form mutualistic associations with most plant species and their involvement in the biotransformation and biodegradation of various xenobiotic organic compounds is discussed. The diversity of bacterial and fungal genes and degradation pathways expressed in the rhizosphere is potentially huge, and the ways in which plants and associated symbionts enhance biodegradation remains much unexplored. Although a wide range of microbes able to degrade highly stable, toxic organic compounds such as polycyclic and aliphatic hydrocarbons have been discovered, the environmental pollution caused by these compounds remains an unsolved problem.