Klebsiella pneumoniae (HR1) assisted alleviation of Cd(II) toxicity in Vigna mungo: a case study of biosorption of heavy metal by an endophytic bacterium coupled with plant growth promotion

Abstract

An endophytic bacterium (HR1 isolate) previously isolated from the root nodules of Vigna mungo and identified as Klebsiella pneumoniae was screened for Cd(II) tolerance. Application of this bacterium to the V. mungo plants in the form of seed bacterization was further assessed for concurrent growth promotion under different concentrations of Cd(II). Inductively coupled plasma–optical emission spectroscopy (ICP–OES) of the bacterial isolate cultured with 100 µg/mL CdCl2 was done to reveal the mechanism of Cd(II) adsorption by the isolate. Application of this Cd(II) tolerant bacterium to the V. mungo plants in the form of seed bacterization was further assessed for growth promotion under different concentrations of Cd(II) in pots. The bacterial isolate was found to be a potential plant growth promoter with attributes such as phosphate solubilisation, indole acetic acid, hydrogen cyanide and siderophore production, and at the same time was able to withstand Cd(II) concentrations up to 100 µg/mL. ICP–OES revealed that Cd(II) adsorption took place mainly by the process of ion exchange (86.3%) rather than physical adsorption, complexation and intracellular accumulation. Also, the Cd(II) tolerant isolate was able to improve plant growth in terms of shoot and root length, biomass and germination percentage when compared to its uninoculated counterparts along with increasing the accumulation of proline and antioxidative enzymes such as peroxidase, catalase and ascorbate peroxidase. Based on our findings, it could be concluded that the Cd(II) tolerant bacterium was a potent heavy metal scavenger along with an able plant growth promoter.