Isolation and engineering of plant growth promoting rhizobacteria Pseudomonas aeruginosa for enhanced cadmium bioremediation

Abstract

Although many bacteria are tolerant to heavy metals and play important roles in the immobilization of heavy metals, they cannot always be dependably reproduced under field conditions. In this work, a cadmium (Cd)-resistant bacterium was isolated from a Cd-contaminated oil field and identified as Pseudomonas aeruginosa (Pse-w). We then determined various plant growth promoting features such as the solubilization of phosphate, and the production of indole-3-acetic acid and siderophores. Lastly, we engineered the strain Pse-w-MT by targeting metallothioneins to the cell surface of Pse-w to immobilize Cd²⁺ and promote plant growth. Our data revealed that Pse-w exhibited high levels of resistance to Cd²⁺ (4 mM) and showed various plant growth promoting features. The engineered strain Pse-w-MT was found to adsorb Cd²⁺ mainly via extracellular deposition, and had an enhanced ability for immobilizing Cd²⁺ ions from the external media. Furthermore, the inoculation of Cd-polluted soil with Pse-w-MT significantly elevated the shoot and root biomass and leaf chlorophyll content. Similarly, plants inoculated with Pse-w-MT demonstrated markedly lower Cd²⁺ accumulation in the root and shoot system. It was concluded that plant growth promoting rhizobacteria with a high Cd²⁺ tolerance was an ideal candidate to be engineered for bioremediation and plant growth promotion against Cd-induced stress.