Selenium-mediated rhizosphere oxidative remodelling and root internal defence synergistically mitigate arsenic accumulation in rice

Although selenium (Se) application is known to reduce arsenic (As) accumulation in rice, the mechanisms by which Se alleviates As toxicity-particularly those involving the regulation of rhizosphere reactive oxygen species and influence of Fe-(hydro)oxide-As dynamics at the root-soil interface-remain unclear. Therefore, to clarify the mechanisms underlying the inhibition of Se-mediated As uptake, this study investigated how different Se dosages (0, 50, 200, and 400 μg Na₂SeO₃ kg⁻¹ soil) affect antioxidant enzyme activities and phytochelatin (PC) biosynthesis in rice grown in As-contaminated soil and promote As transformation in the rhizosphere. The results showed that the Se-200 and Se-400 treatments significantly reduced As levels in brown rice by 52.4 % and 35.7 %, respectively, compared with the control. The Se-200 treatment significantly enhanced root superoxide dismutase and catalase activity and PC content (P < 0.05) and increased the •OH and H₂O₂ concentrations in the rhizosphere (P < 0.05). In addition, Se promoted the formation of Fe-(hydro)oxides and oxidation of As(III) to As(V), with the As(III)/As(total) ratio increasing by 18.9 %, which decreased As mobility in the rhizosphere. Microbial analysis further revealed a 91.4 % increase in the abundance of the aioA gene (encoding As(III) oxidase) under the Se-200 treatment, with Bacillus and Pseudomonas identified as the dominant genera. Collectively, the Se-200 treatment mitigated As-induced oxidative stress in rice roots, enhanced As(V) immobilisation via Fe-(hydro)oxides, and altered the rhizosphere microbial community to reduce As bioavailability. These findings provide new insights into Se-based strategies for managing As-contaminated paddy soils.