Influence of the rhizosphere on the biodegradation of organic xenobiotics - A case study with 2,4-dichlorophenoxyacetic acid

The rhizosphere is the unique environment at the plant-soil interface and a zone of intense soil-plant-microbe interaction. One recorded consequence of this complex interaction is the enhanced microbial biodegradation of organic xenobiotics. Mechanisms suggested to explain rhizosphere enhanced biodegradation range from the direct role of rhizodeposits acting as xenobiotic structural analogs to more indirect possibilities such as increased bioavailability in the rhizosphere. Our aim was to learn more about the mechanisms of enhanced biodegradation in the rhizosphere using the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) as a model xenobiotic. We report that 2,4-D mineralization is enhanced by root exudates and root debris of legume species. This rhizosphere enhancement may be due to the action of a rhizodeposit component in inducing the 2,4-D pathway. We discuss the possibility that the stimulatory component belongs to the chemical class flavonoids.