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The present study demonstrates the metal toxicity ameliorating and growth promoting abilities of three different bacterial isolates when applied to rice as host plant. The three bacterial strains included a cadmium resistant Ochrobactrum sp., a lead resistant Bacillus sp. and an arsenic resistant Bacillus sp. designated as CdSP9, PbSP6, and AsSP9, respectively. When these isolates were used as inocula applied to metal-treated rice plants of variety Satabdi, the germination percentage, relative root elongation (RRE), amylase and protease activities were increased. The toxic effect of metal was reduced in presence of these bacteria. The overall biomass and root/shoot ratio were also enhanced by bacterial inoculation. Hydroponic studies showed that the superoxide dismutase (SOD) activity and malondialdehyde (MDA) level, which had been increased in the presence of metal stress in rice roots, were lowered by the bacterial inoculation. In addition, all three strains were 1-aminocyclopropane-1-carboxylate (ACC) deaminase and catalase positive, whereas siderophore producing ability was lacking in PbSP6. However, both PbSP6 and AsSP9 were protease positive and could hydrolyse starch. The data indicate that these bacteria have promise for bioremediation as well as for plant growth promotion.

Application of heavy metal resistant plant growth promoting rhizobacteria has an important role as they help to evade metal-induced toxicity in plants on one hand and enhance plant growth on the other. The present study is therefore focused on the characterization of a cadmium resistant bacterial strain isolated from heavy metal contaminated rhizospheric soil designated as S8. This S8 strain was selected in terms of cadmium resistance and plant growth promoting traits. Moreover, it also showed resistance to lead and arsenic to a considerable extent. The selected strain S8 was identified as Klebsiella michiganensis by modern approaches of bacterial taxonomy. The plant growth promoting traits exhibited by the strain include 1-aminocyclopropane-1-carboxylic acid deaminase activity (58.33 ng α-keto butyrate/mg protein/h), Indole-3-acetic acid production (671 μg/ml), phosphate solubilization (71.98 ppm), nitrogen fixation (3.72 μg of nitrogen fixed/h/mg protein) etc. Besides, the strain also exhibited high cadmium removal efficiency (73-97%) from the medium and intracellular accumulation as well. Its efficiency to alleviate cadmium-induced toxicity was determined against a rice cultivar in terms of morphological and biochemical changes. Enhanced growth and reduced oxidative stress were detected in presence of the bacterium. On the basis of these results, it can be concluded that K. michiganensis strain S8 is cadmium accumulating plant growth promoting rhizobacterium that can be applied in cadmium contaminated agricultural soil to achieve better productivity of rice.

A cadmium-resistant bacterium designated as CdSP9 was isolated from the slag disposal site of IISCO, Burnpur, West Bengal, India. The isolate was identified as Ochrobactrum sp. on the basis of 16S rDNA sequence-based molecular phylogenetic approach and phenotypic characteristics. It is a Gram negative, short rod (0.5-1.0 micro), aerobic bacterium, growing well in LB medium between temperatures 10-42 degrees C, pH 6.0-9.0, and between 2 and 6% NaCl. The most preferred nitrogen and carbon sources for the strain are L: -proline, L: -lysine and fructose, maltose, respectively. Superoxide toxicity minimization by increased level of SOD activity also occurs in this bacterium. The heavy metal accumulation efficiency as determined by atomic absorption spectroscopy was found to be 0.214 mg/g of the dry weight at late log phase. The accumulation efficiency was directly proportional to the optimum growth conditions.

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