Biological responses of symbiotic Rhizobium radiobacter strain VBCK1062 to the arsenic contaminated rhizosphere soils of mung bean

•We characterized arsenic resistant Rhizobium in a metal contaminated soil.•Cell morphology, protein expression and EPS production was examined in Rhizobium.•FTIR and SEM-EDS analyses demonstrated arsenic-EPS interactions.•Arsenate uptake (biosorption) by in EPS was significantly higher compared to whole cell.•The results contribute to better understanding of cellular and metabolic changes in Rhizobium.

The rationale could be that mung bean is cultivated in areas of arsenic contamination and therefore it is worth investigating how Rhizobium is impacted by arsenic exposure. The objective(s) of the study deals with relationship between Rhizobium metal tolerance and its adaptations to metal stressed environment. The selected strain was recovered from root nodules of Vigna radiata, based on viscous EPS production and arsenic tolerant capacity, identified as R. radiobacter by 16 S rDNA sequencing. Batch studies were performed to evaluate toxic effects of heavy metal ions in decreasing order of MIC As(V) (10 mM), Cu(1.5 mM), Pb(0.18 mM), Cr(0.1 mM), Ni(0.08 mM) and Cd(0.04 mM). Scanning electron microscopy analysis of Arsenic resistant strain revealed evident changes in cell morphology. SDS-PAGE results showed altered expression of proteins in response to arsenate. One unique protein of approximately 21 kDa was highly expressed in 5 mM arsenate, but same protein was down regulated in 10 mM arsenate. The exopolysaccharide components such as total carbohydrates, proteins and uronic acids were significantly enhanced by 41%, 25% and 33% (P Value <0.05) and also produced EPS under Arsenic stressed conditions. Fourier transformed spectroscopy analysis demonstrated arsenic metal ion-EPS interactions. The results obtained from SEM–EDS analysis clearly revealed mucous nature of Rhizobial-EPS surrounding bacterial cells and confirmed the role of EPS in arsenate sequestration (10% as weight). Interestingly total arsenate uptake by strain VBCK1062 in whole-cell pellet and EPS were 0.045 mg and 0.068 mg g−1 of biomass respectively. Thus these results significantly contribute to better understanding of plant-metal-microbe interactions, cellular-metabolic changes and As-enhanced EPSs, hence can serve as potential bioremediation agent for As-contaminated agrogeoecosystems.

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