Mechanisms of phytoremediation: biochemical and ecological interactions between plants and bacteria

The use of plants to reduce contaminant levels in soil is a cost-effective method of reducing the risk to human and ecosystem health posed by contaminated soil sites. This review concentrates on plant-bacteria interactions that increase the degradation of hazardous organic compounds in soil. Plants and bacteria can form specific associations in which the plant provides the bacteria with a specific carbon source that induces the bacteria to reduce the phytotoxicity of the contaminated soil. Alternatively, plants and bacteria can form nonspecific associations in which normal plant processes stimulate the microbial community, which in the course of normal metabolic activity degrades contaminants in soil. Plants can provide carbon substrates and nutrients, as well as increase contaminant solubility. These biochemical mechanisms increase the degradative activity of bacteria associated with plant roots. In return, bacteria can augment the degradative capacity of plants or reduce the phytotoxicity of the contaminated soil. However, the specificity of the plant-bacteria interaction is dependent upon soil conditions, which can alter contaminant bioavailability, root exudate composition, and nutrient levels. In addition, the metabolic requirements for contaminant degradation may also dictate the form of the plant-bacteria interaction i.e., specific or nonspecific. No systematic framework that can predict plant-bacteria interactions in a contaminated soil has emerged, but it appears that the development of plant-bacteria associations that degrade contaminants in soil may be related to the presence of allelopathic chemicals in the rhizosphere. Therefore, investigations into plants that are resistant to or produce allelopathic chemicals is suggested as one possible method of identifying plant-bacteria associations that can degrade contaminants in soil.Key words: phytoremediation, mechanisms, rhizosphere, bacterial inoculants.NRC publication: Ye