Potential of a rhizobacterium on removal of heavy metals from aqueous solution and promoting plant root elongation under heavy metal toxic conditions

Abstract

Our study isolated and screened rhizobacteria for the ability to produce indole-3-acetic acid (IAA) and grow in the presence of cadmium (Cd), copper (Cu), and lead (Pb). Fifteen isolates resistant to Cd, Cu and Pb, called multi-resistant-heavy metals rhizobacteria, were found. The isolate producing the highest level of IAA was an Enterobacter cloaca, designated as CdRR1, capable of removing Cd, Cu and Pb from aqueous solutions, with the best removal efficiency found for Pb. The effects of E. cloacae CdRR1 on root elongation and translocation of Cd, Cu and Pb in Helianthus annuus L. and Sorghum bicolor L. were investigated. Under toxic conditions of Cd, Cu and Pb, E. cloacae stimulated the shoot and root elongation of H. annuus and S. bicolor, except for the shoots of S. bicolor in the presence of Pb. Interestingly, E. cloacae enhanced the growth and heavy metal accumulation in the lateral roots of H. annuus under Cd and Pb toxic conditions. In addition, increased Pb translocation from the roots to the shoots in H. annuus was found in seedlings with E. cloacae inoculation. Similar findings of increased Cu and Pb translocations in S. bicolor were seen in seedlings with E. cloacae inoculation. These results demonstrate that E. cloacae CdRR1, a potent multi-heavy metal-resistant rhizobacterium, can remove heavy metals from solution, and also promote shoot and root elongation and heavy metal translocation in both H. annuus and S. bicolor.