Role of novel bacterial Raoultella sp. strain X13 in plant growth promotion and cadmium bioremediation in soil

Heavy metal pollution in agricultural soils has become a widespread serious problem with the rapid industrialization and urbanization in the past two decades. Cadmium (Cd²⁺) is of the most concern in soils due to its high toxicity. It is necessary to develop remediation strategies to remove or neutralize its toxic effects in Cd-contaminated soil. Microbial bioremediation is a promising technology to treat heavy metal-contaminated soils. In this study, Cd-resistant bacterium, isolated from heavy metal-polluted soil in Southern China, was characterized as Raoultella sp. strain X13 on the basis of its biochemical profile and 16S rRNA. We investigated the characterization of Cd²⁺ distribution in different cellular compartments after Cd²⁺ uptake. Cd²⁺ uptake by strain X13 was mainly by ion exchange and chelation binding tightly to the cell wall. In addition, X13 plant growth-promoting characteristics suggested that X13 could solubilize phosphate and produce indole acetic acid. Pot experiments for the remediation of Cd-contaminated soil in situ by X13 inoculation demonstrated that X13 application to Cd-contaminated soils significantly promoted pak choi growth and improved production. We also found that X13 substantially reduced the Cd²⁺ bioavailability for pak choi. Therefore, strain X13 is an effective treatment for potential application in Cd²⁺ remediation as well as for sustainable agronomic production programs in Cd-contaminated soils.