| Summary: | Nanoparticle (NP) application is most effective in decreasing metalloid toxicity. The current study aimed to evaluate the effect of <i>Bacillus subtiles</i> synthesized iron oxide nanoparticles (Fe<sub>3</sub>O<sub>4</sub> NPs) against arsenic (As) stress on rice (<i>Oryza sativa</i> L.) seedlings. Different concentrations of As (5, 10 and 15 ppm) and <i>Bacillus subtilis</i> synthesized Fe<sub>3</sub>O<sub>4</sub> NPs solution (5, 10 and 15 ppm) alone and in combination were applied to rice seedlings. The results showed that As at 15 ppm significantly decreased the growth of rice, which was increased by the low level of As. Results indicated that <i>B. subtilis</i> synthesized Fe<sub>3</sub>O<sub>4</sub> NP-treated plants showed maximum chlorophyll land protein content as compared with arsenic treatment alone. The antioxidant enzymes such as SOD, POD, CAT, MDA and APX and stress modulators (Glycine betain and proline) also showed decreased content in plants as compared with As stress. Subsequently, <i>Bacillus subtilis</i> synthesized Fe<sub>3</sub>O<sub>4</sub> NPs reduced the stress associated parameters due to limited passage of arsenic inside the plant. Furthermore, reduction in H<sub>2</sub>O<sub>2</sub> and MDA content confirmed that the addition of <i>Bacillus subtilis</i> synthesized Fe<sub>3</sub>O<sub>4</sub> NPs under As stress protected rice seedlings against arsenic toxicity, hence enhanced growth was notice and it had beneficial effects on the plant. Results highlighted that Fe<sub>3</sub>O<sub>4</sub> NPs protect rice seedlings against arsenic stress by reducing As accumulation, act as a nano adsorbent and restricting arsenic uptake in rice plants. Hence, our study confirms the significance of <i>Bacillus subtilis</i> synthesized Fe<sub>3</sub>O<sub>4</sub> NPs in alleviating As toxicity in rice plants.
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