Mn<sub>3</sub>O<sub>4</sub> nanozymes increased rice salt tolerance via ROS scavenging and maintenance of Na<sup>+</sup>/K<sup>+</sup> homeostasis

<bold>Objective</bold> This study aims to investigate the effects of manganese tetraoxide（Mn₃O₄）nanozymes on the regulation of reactive oxygen species（ROS）homeostasis and Na⁺/K⁺ ion homeostasis in rice under salt stress，thus providing a theoretical basis for the application of nanomaterials in crop salt tolerance. <bold>Method</bold> In this study，the indica-type three-line hybrid rice variety <italic>“Quanyou 737”</italic> was selected as the test material to perform an indoor experiment.Polyacrylic acid（PAA）-coated Mn₃O₄ nanozymes（PMO）were used as the test nanomaterial.Rice plants were treated with 200 mmol/L NaCl for salt stress.The ability of PMO in maintaining ROS homeostasis in rice plants under salt stress was comprehensively evaluated by measuring the contents of ROS such as hydrogen peroxide（H₂O₂）and superoxide anion（O₂•⁻），as well as the activities of antioxidant enzymes including SOD，CAT，and POD in rice leaves.The impacts of PMO on the regulation of ion homeostasis in rice were assessed by detecting Na⁺ and K⁺ contents and their ratio，combined with qRT-PCR analysis of the expression of related ion transporter genes. <bold>Result</bold> Rice seedlings treated with 200 mg/L PMO exhibited a better phenotype of rice seedlings than control，which have higher plant height and fresh/dry weight，indicating a significant improvement in growth status.After PMO treatment，the contents of H₂O₂ and O₂•⁻ in rice leaves decreased significantly by 20.6% and 18.4%，respectively.The activities of major antioxidant enzymes such as SOD，CAT，and POD were significantly enhanced，indicating increased ROS scavenging capacity and alleviated oxidative stress.Meanwhile，the Na⁺ content in rice leaves decreased by 16.9%，the K⁺ content increased by 20.5%，and the Na⁺/K⁺ ratio decreased significantly.The expression levels of ion transporter genes <italic>OsNHX1</italic>，<italic>OsSOS1</italic>，and <italic>OsGORK</italic> showed no significant difference compared to the control，while the expression of <italic>OsHKT</italic> and <italic>OsHAK</italic> genes was significantly upregulated，leading to a marked improvement in ion homeostasis. <bold>Conclusion</bold> Under salt stress，200 mg/L PMO significantly improved the salt tolerance of rice plants by maintaining ROS homeostasis and ion homeostasis.