Cadmium (Cd) is a toxic heavy metal and an abiotic stressor to plants; however, inoculation of endophytic bacteria can raise resistance in plants against Cd, as well as improve plant growth. In the present study, two endophytic bacterial strains were isolated from Solanum nigrum, identified as Serratia sp. IU01 and Enterobacter sp. IU02 by 16S DNA sequencing. Both IU01 and IU02 were tolerant up to 9.0 mM of Cd in culture broth and successive increase in Cd concentration from 0 mM to 9.0 mM, led to an increase in the SOD enzyme activity of the isolates. Both strains were capable of indole-3-acetic acid (IAA) synthesis and phosphate solubilization, detected through gas spectrometry-mass chromatography (GC-MS) and Pikovskaya agar medium respectively. Brassica juncea plants stressed with 0-25 mg/kg Cd showed retardation in all growth attributes, however, inoculation of strain IU01 and IU02 significantly promoted the plant growth attributes as compared to control. Moreover, antioxidant enzymes and metabolites against reactive oxygen species (ROS) including polyphenol oxidase (PPO), peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), alcohol dehydrogenase (ADH), reduced glutathione (GSH), malondialdehyde (MDA), flavonoid and polyphenolic contents were also significantly relieved by inoculation of IU01 and IU02 in plant exposed to different concentration of Cd stress as compared to control plants. Phytohormone production, phosphate solubilization, and/or antioxidative support of IU01 and IU02 might be responsible for growth promotion and Cd resistance in the plant.
Keywords: Antioxidants; Cadmium accumulation; Endophytic bacteria; Indole-3-acetic acid; Stress response gene.
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