Selenium (Se) inhibits cadmium (Cd) root-to-shoot translocation and accumulation in the shoots of pak choi; however, the mechanism by which Se regulates Cd retention in roots is still poorly understood. A time-dependent hydroponic experiment was conducted to compare the effects of selenite and selenate on Cd translocation and retention in the roots. The underlying mechanisms were investigated regarding Se biotransformation and metal transportation in roots using HPLC and transcriptome analyses. Selenite showed reducing effects on Cd translocation and accumulation in shoots earlier than selenate. Selenite is mainly biotransformed into selenomethionine (80% of total Se in roots) at 72 h, while SeO₄²⁻ was the dominant species in the selenate treatments (68% in shoots). Selenite up-regulated genes involved in the biosynthesis of lignin, suberin, and phytochelatins and those involved in stress signaling, thereby helping to retain Cd in the roots, whereas essentially, selenate had opposite effects and impaired the symplastic and apoplastic retention of Cd. These results suggest that cell-wall reinforcement and Cd retention in roots may be the key processes by which Se regulates Cd accumulation, and faster biotransformation into organic seleno-compounds could lead to earlier effects.

硒 (Se) 抑制镉 (Cd) 从根到芽的易位和在小白菜芽中的积累；然而，硒调节根部镉保留的机制仍知之甚少。进行了时间依赖性水培实验，以比较亚硒酸盐和硒酸盐对镉在根中的易位和保留的影响。使用 HPLC 和转录组分析研究了根中 Se 生物转化和金属运输的潜在机制。亚硒酸盐比硒酸盐更早地表现出减少镉在芽中转运和积累的作用。72 小时，亚硒酸盐主要生物转化为硒代蛋氨酸（占根中总硒的 80%），而 SeO ₄ ²⁻是硒酸盐处理中的优势物种（芽中占 68%）。亚硒酸盐上调参与木质素、木栓质和植物螯合素生物合成以及参与应激信号传导的基因，从而有助于将镉保留在根中，而本质上，硒酸盐具有相反的作用，并损害镉的共质体和质外体保留。这些结果表明，细胞壁强化和根部镉保留可能是硒调节镉积累的关键过程，更快地生物转化为有机硒化合物可能会导致更早的效应。