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<p>In this study, we used silicon (Si, in the form of K₂SiO₃, 2 mM) to alleviate the toxicity of aluminum oxide (Al₂O₃) nanoparticles (NPs) in barley (<em>Hordeum vulgare</em> L.). Using Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) analyses, we showed that the Al₂O₃ NPs were taken up by barley plants. Barley growth was negatively affected by the addition of 3 g l^-1 nano-Al₂O₃, whereas the diminishing effect of NPs on barley growth was not obvious when 1 g l^-1 nano-Al₂O₃ was applied, indicating that the nano-Al₂O₃ action is dependent on nano-Al₂O₃ dose. Si pretreatment ameliorated toxic effects of high nano-Al₂O₃ on root growth. Si pretreatment did not decrease nano-Al₂O₃ entry into roots but reduced nano-Al₂O₃ accumulation in the shoot. The restriction of the root-to-shoot translocation of nano-Al₂O₃was one of the important mechanisms for Si to mitigate high nano-Al₂O₃ toxicity. The occurrence of oxidative stress was found under 3 g l¹ nano-Al₂O₃treatment, as evaluated by the accumulation of malondialdehyde (MDA). Exogenous addition of Si could alleviate toxicity symptoms induced by Al₂O₃ nanoparticles by reducing lipid peroxidation via enhancing antioxidant activity of catalase as well as by limiting the root-to-shoot translocation of nano-Al₂O₃. These data provide the first direct evidence that the Si pretreatment ameliorates nano Al₂O₃ phytotoxicity in plants.</p>