Physiological and molecular level studies on the toxicity of silver nanoparticles in germinating seedlings of mung bean (Vigna radiata L.)

In the present study, the toxic effect of silver nanoparticles was studied in seedlings of economically important food crop mung bean (Vigna radiata L.). The plants were grown in 1/2 MS agar medium containing 0, 5, 10, 20, and 50 mg/L of silver nanoparticles for 21 days. The toxic effects were studied using different parameters such as growth, chlorophyll, and proline contents, reactive oxygen species (ROS) generation, lipid peroxidation, changes in mitochondrial membrane potential (Delta Psi m), and oxidative stress responsive gene expression. The shoot length and weight was significantly reduced upon exposure to 50 mg/L of silver nanoparticles. Significant reduction in root elongation and weight was observed upon exposure to 20 and 50 mg/L of silver nanoparticles. The total chlorophyll content significantly reduced after exposure to 50 mg/L of silver nanoparticles. However, proline content was increased significantly upon exposure to 20 and 50 mg/L of silver nanoparticles. The hydrogen peroxide level and lipid peroxidation levels increased significantly in roots after exposure to 20 and 50 mg/L of silver nanoparticles. Histochemical staining with nitroblue tetrazolium showed a concentration-dependant increase in superoxide formation in roots. Roots treated with 2', 7'-dichlorodihydro-fluorescein diacetate and 3, 3'-diaminobenzidine showed a concentration-dependant increase in ROS generation. Exposure to increasing concentrations of silver nanoparticles has resulted in changes in Delta Psi m in roots as revealed by increased Rhodamine 123 fluorescence. Gene expression analysis using real-time polymerase chain reaction showed changes in the relative mRNA expression of CuZn-SOD, CAT, and APX genes indicating the plants' antioxidative defense responses under silver nanoparticle stress.