Rice Na⁺ absorption mediated by OsHKT2;1 affected Cs⁺ translocation from root to shoot under low K⁺ environments

¹³⁷Cs diffused into the environment due to a nuclear power plant accident has caused serious problems for safe crop production. In plants, Cs⁺ is similar in its ionic form to K⁺. Cs⁺ is absorbed and transported mainly by the K⁺ transport mechanism. However, the full picture of the genes contributing to Cs⁺ transport and the transport mechanism of rice is still unclear. This study focused on OsHKT2;1, a candidate Cs⁺ transporter under low K⁺ conditions. To verify the ability of OsHKT2;1 to transport Cs⁺, the OsHKT2;1 mutant (hkt2;1) was grown in a ¹³⁷Cs-contaminated paddy field in Fukushima. The ¹³⁷Cs concentration in hkt2;1 aboveground was higher than in the wild type (WT), and the K concentration in these samples did not change between WT and hkt2;1, whereas the Na concentration was lower in hkt2;1. Uptake experiments with radioactive tracers (²²Na⁺, ⁴³K⁺, and ¹³⁷Cs⁺) in hydroponic systems with different elemental compositions showed a negative correlation between Na⁺ and Cs⁺ accumulation in rice shoot cultivated under low K⁺ conditions. These results indicated that OsHKT2;1 does not directly contribute to Cs⁺ uptake but is an important factor in regulating Cs⁺ translocation by controlling Na⁺ accumulation. This indicates the possibility of controlling rice Cs content by regulating the Na⁺ environment during cultivation.