A native Zn-solubilising bacterium from mine soil promotes plant growth and facilitates phytoremediation

An important bottleneck in phytoremediation technologies, which focus on extraction of potentially toxic trace elements, is the low solubility and bioavailability of metals, a handicap that worsens over time. To overcome this barrier, we explored the possibility of using Zn-solubilising bacteria with plant growth-promoting characteristics. Zn-tolerant soil bacteria were isolated from an ancient Zn-Pb mine using cultivation media with a high ZnSO4 concentration (2.5 mM). Among the bacteria able to grow on this medium, those with plant growth-promoting (PGP) characteristics (P and Zn solubilisation and indole-3-acetic acid production) were selected. The best candidates (seven strains) were identified after 16S rDNA sequencing, and Pseudomonas fluorescens NCBI Accession No. MT218317 was selected for a microbial-assisted phytoremediation experiment with Sinapis alba plants. Fifteen days after inoculation, both shoot and root growth were higher in inoculated than in un-inoculated white mustard plants. Soil metal contents (total and exchangeable fractions) remained practically invariable. In contrast, the PGP bacterium raised the plant concentrations of some of the micronutrients, together with Cd and Pb. In addition, the bioaccumulation factors for Cd and Zn were higher in the bacterium-inoculated plants. Increased metal concentrations in plants on bacterium-inoculated soil can be due to either or both bacterium-induced enhancement of micronutrients availability and production of beneficial compounds induced by the bacterium. The artificial inoculation of P. fluorescens MT218317 promoted the biomass of S. alba plants potentiating the phytoextraction of Zn and phytostabilisation of Cd and Pb on a metal-contaminated soil. The novelty of the study is the use of metal-solubilising microorganism in combination with metal-tolerant plants for improving the restoration of soils containing potentially toxic elements.