Cultivable rhizobacteria improve castor bean seedlings root and plant growth in Pb–Zn treated soil

Abstract Culturable bacteria are widely used in the remediation of heavy metal-contaminated soil associated with different plants. In this study, a pot experiment of castor bean (Ricinus communis L.) was performed to determine the effect of culturable bacterial communities on the growth of castor bean, as well as their response to the presence of Pb and Zn during the 21-day incubation. The results indicated that the inoculation of the cultivable bacterial communities promoted plant growth, whether Pb/Zn-contaminated or not, and a well-developed root system was also detected in the bacterial community after treatment. Under heavy metal stress, the root surface area of castor bean decreased significantly, but after inoculation, it was close to the level of the normal (uncontaminated) samples. Sequence analysis at the phylum and genus taxonomic levels illustrated 13 identified phyla and 125 genera were observed in the control and Pb/Zn-contaminated soil. The input of contaminants affected the bacterial community structure, suggesting that although Proteobacteria, Bacteroidetes, and Actinobacteria were the dominant phyla in the two samples, the Ensifer and Cupriavidus genera were decreased in contaminated soil. However, Arcticibacter, a phosphate-solubilizing bacterium, was enriched by the Pb/Zn treatment. Compared with the control group, Pb and Zn exposure in the treatment group resulted in a new community structure. A microflora consisting of a number of microorganisms undergoing a long duration period of adaptation and symbiosis can possess systematic functions with high stabilities, so it has considerably high potential in intensifying the effect of phytoremediation.