Isolation and characterization of root nodule-associated bacteria from chickpea (Cicer arietinum) for evaluating plant growth-promoting activities and heavy metal tolerance in West Bengal, India

Two bacterial strains were isolated from root nodules of the chickpea plant ( Cicer arietinum ) in West Bengal and characterized to assess their potential for heavy metal (HM) tolerance and plant growth-promoting (PGP) attributes. Phylogenetic analysis based on the 16S rRNA gene identified these strains as belonging to the genera Enterobacter sp. and Labrys sp. Chromium (Cr) was selected as the heavy metal for this experiment. Endophytic bacteria, known for their PGP abilities, were assessed for their potential to sustain PGP traits under Cr (VI) stress. Bacterial strains, SMAJ_63 and SMAJ_180, were evaluated for HM tolerance, with IC ₅₀ values of 500 µM and 270 µM for Cr (VI), respectively. PGP attributes, including indole acetic acid (IAA) production and phosphate solubilization, were determined, with strain SMAJ_63 exhibiting IAA production of 23.29 µg/mL. Additionally, their IC ₅₀ and MIC values were determined for other heavy metals such as arsenic (As), cadmium (Cd), cobalt (Co), and copper (Cu), as well as their tolerance to salinity and drought stress. Among the tested metals, Cd exhibited the highest toxicity, while Cu was the least toxic. The overall toxicity order of different HMs is as follows: Cd > Cr > Co > As > Cu for both the strains. At 50 µM Co and Cu, promoted bacterial growth in strain SMAJ_180 and SMAJ_63, respectively. Furthermore, strain SMAJ_63 demonstrated the ability to produce exopolysaccharides (EPS) 0.67 g/L, which play a crucial role in stress tolerance and soil health improvement.