The dual role of selenium in mitigating cadmium toxicity and inhibiting trace element uptake in rice highlights the critical trade-off for safe agricultural production.

Cadmium (Cd) accumulation in rice poses a serious threat to global food security and human health. While selenium (Se), a plant-beneficial nutrient, has demonstrated effectiveness in alleviating Cd stress in rice through mechanism that involve regulation of trace element transporters to inhibit Cd uptake and transport, however, it is unclear whether this intervention might inadvertently cause a rice grain-quality problem of essential micronutrient deficiency. In this study, we explored the changes of 17 mineral elements in rice under Se-Cd interaction based on ionomics and Cd transporter expression levels by combining laboratory and field samples. Se supplementation effectively reduced Cd levels in brown rice (48.40 %), but significantly suppressed the expression of OsNramp5 (41.26 %), OsNramp1 (15.28 %), OsHMA2 (20.67 %), OsIRT1 (17.88 %), and OsIRT2 (22.92 %) and reduced 8 trace elements concentrations in brown rice (23.45 % for Fe, 18.86 % for Mn, 19.21 % for Zn, 12.80 % for Cu, 35.24 % for Mo, 30.17 % for Ni, 31.08 % for Co and 32.18 % for Ti). The negative "L" shaped correlation between trace elements and the Se/Cd molar ratio in brown rice in the field confirmed that trace element concentrations in brown rice rapidly decreased when the Se/Cd molar ratio exceeded a certain threshold (Se: Cd > 3). Pathway analysis and ion network mapping elucidated the mechanism of Se-Cd interaction on the regulation of the rice ionome. Se supplementation led to the replacement of Cd concentration regulation, which was originally driven by trace elements, by macro-elements. These results suggest that Se acts as a double-edged sword, reducing Cd concentration in brown rice at the expense of inhibiting trace element uptake. Our study advocates for enhanced threshold-guided rational Se application to harmonize food safety and nutritional security in global rice production.