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Nickel tolerance and biosorption potential of rhizobia associated with horse gram [Macrotyloma uniflorum (Lam.) Verdc.]

Abstract The rhizobia isolated from root nodules of horse gram were screened for nickel (Ni) tolerance in vitro. The strain HGR-4 could tolerate 1000 µg g−1 of Ni. It was also observed that horse gram plants associated with HGR-4 have shown Ni stress tolerance in Ni amended soils up to a concentration of 100 µg g−1. In another experiment, the plants associated with HGR-4 have shown higher nodulation, nitrogen level, and leghaemoglobin content at 80 µg g−1 of Ni than control plants without HGR-4 inoculation. Analysis of biosorption potential of Ni in horse gram plants inoculated by the strain HGR-4 was done using atomic absorption spectroscopy revealed maximum biosorption in bacterial root nodules. Besides, there was a reduction in the content of the heavy metal in the soil samples which demonstrates a fair amount of heavy metal extraction and accumulation of Ni by rhizobia associated root nodules of the horse gram. This study demonstrates that the strain HGR-4 (GQ483457 Rhizobium sp. ATCC BAA-2335) could be a potential source for phytoextraction of Ni contaminated soils upon its association with horse gram. The study could be of use in phytoremediation of metal (Ni) contaminated soils in the future. Novelty statement: The phytoremediation of nickel (Ni) using of rhizobia associated with horse gram remains unevaluated till now. Horse gram associated with rhizobia could produce nodules and fix nitrogen even in Ni amended soils. The biosorption potential of the rhizobial strains was analyzed from both root nodules and soil. These findings imply that horse gram plants associated with these rhizobial strains could be used to remediate Ni metal in contaminated soils.