Modulations in growth, structure, cell viability and antioxidant enzyme of a nodule bacterium Mesorhizobium ciceri induced by pesticides

A rapid increase in the application of synthetic pesticides in agricultural practices causes a severe hazard to agro-sustainability and soil microflora. Considering these threatening problems, the present study aimed to investigate the impact of different groups of pesticides on growth, cellular morphostructure, permeability and antioxidant enzyme of a symbiotic Gram-negative bacterium recovered from chickpea nodules. Pesticide-tolerant strain BRM5 was characterized and phylogenetically (accession no. KY013481) identified as Mesorhizobium ciceri. Strain BRM5 tolerated up to the level of 1200, 1600, 1600, 2400, 2400, 1800, 2400, 2400 and 3200 μg mL−1 of glyphosate, quizalofop, atrazine, kitazin, metalaxyl, hexaconazole, fipronil, monocrotophos and imidacloprid, respectively. Among all concentrations, 3X concentrations of each pesticide had most fatal to bacterial cells. The growth pattern of strain BRM5 was decreased linearly with increasing incubation times. Scanning electron micrograph (SEM) images of pesticide-treated bacterial cells revealed the distorted, broken, irregular, misshaped and larger-sized cell as compared to untreated cells. Upon staining with acridine orange (AO)/propidium iodide (PI), active/living cells appeared as green colour, whereas dead cells appeared as red colour under confocal microscope (CLSM). This is the clear indication of loss in cell viability following the pesticide exposure. The generation of oxidative stress (catalase) by strain BRM5 induced under pesticides pressure further revealed the pesticidal toxicity. CAT activity of BRM5 was decreased with the enhancement in pesticide concentrations. For instance, CAT activity of BRM5 was decreased by 45%, 71% and 83% when cultured in liquid medium treated with 3X doses of each glyphosate, kitazin and fipronil, respectively. Overall, the present study visibly indicated that pesticides inhibited the growth, disrupted the morphology and reduced the permeability and antioxidant producing ability of bacteria. So, from this study, it is suggested that before applying the pesticides in legume production strategies, the recommended pesticides be carefully examined to avoid the toxicity problems, if any.