Lettuce-derived rhizosphere polyamine-producing bacteria and their potential to reduce Cd and Pb accumulation in lettuce (Lactuca sativa L.)

Abstract Microbial immobilization is a novel and environmentally friendly technique in which microbes are used to immobilize heavy metals in the soil. Polyamine-production screening media and high-pressure liquid chromatography were used to isolate polyamine-producing bacteria from Cd- and Pb-polluted rhizosphere soil of lettuce. Enhancement of lettuce growth and reduced accumulation of Cd and Pb in lettuce by the polyamine-producing strains were investigated. Two polyamine-producing bacteria Bacillus megaterium N3 and Enterobacter bugandensis XY7 were obtained by screening. The Cd and Pb removal rates of strains N3 and XY7 ranged from 73.5% to 87.6% in solution containing 1 mg L-1 Cd2+ and 5 mg L-1 Pb2+. Strains N3 and XY7 increased the dry weight (22.2%-66.7%), improved the polyamines contents and reduced the Cd (57.3%-74.2%) and Pb (44.1%-62.9%) contents of lettuce. Moreover, the strains decreased the bioavailable Cd (27.1%-62.1%) and Pb (35.4%-73.7%) contents but increased polyamines contents and the relative abundance of aguA gene in lettuce rhizosphere soil. The results suggested that strains N3 and XY7 decreased the uptake of Cd and Pb and their translocation into lettuce shoots by i) bacterial cell adsorption, ii) chelation of polyamines to Cd and Pb, iii) decreasing available Cd and Pb in the rhizosphere soil and Cd and Pb translocation from the roots to the shoots, and iv) increasing the polyamine contents and the relative abundance of aguA gene in lettuce rhizosphere soil. Our findings may provide technical support for the remediation of heavy metal contaminated farmlands by bacteria and for safe growth of vegetables crop species.