Characterization of heavy metal-resistant rhizobia and non-rhizobia isolated from root nodules of Trifolium sp. in a lead and zinc mining area

Abstract

In this study, we evaluated the ability of rhizobacteria isolated from Trifolium sp. nodules growing on a Pb/Zn mine site to produce plant growth-promoting substances, such as siderophores, extracellular enzymes, antifungal substances, their capacity to solubilize phosphate, and also their tolerance to heavy metals and salinity. Overall, the results demonstrated that bacterial isolates showed an ability to produce multiple important plant growth-promoting traits, with remarkable ability to grow up to 20% salt concentration and resist to high levels of heavy metals up to 1300 mg/L Pb++, 1200 mg/L Zn++, 1000 mg/L Ni++, 1000 mg/L Cd++, 500 mg/L Cu++, 400 mg/L Co++, and 50 mg/L CrVI+. The order of resistance of isolates to heavy metals was reduced as follows: Pb++ > Zn++ > Ni++ > Cd++ > Cu++ > Co++ > CrVI+. All isolates had multiple metal-resistant abilities; however, the existence of the pbrA, czcD, and nccA genes responsible for resistance to Pb++, Zn++, Cd++, Co++, and Ni++, respectively, was determined by PCR and were detected only on Cupriavidus paucula RSCup01. Our results also showed that the plant growth-promoting rhizobacteria strains screened in the present study could be used as a potential inoculant for the improvement of phytoremediation in heavy metal-polluted soils.