Cesium accumulation in nodules is involved in mitigating cesium transfer to shoot

Radiocesium (¹³⁷Cs) transfer from soil to crops is largely regulated by soil potassium (K) levels owing to the chemical similarity between K and cesium (Cs). However, the mitigation of Cs translocation in soybean through soil K is lower than in other crops, highlighting the importance of clarifying soybean-specific Cs translocation mechanisms. Although root nodule symbiosis has been proposed to alter nutrient transport systems, its impact on Cs dynamics remains unclear. We hypothesized that Cs translocation mechanisms are altered under root nodule symbiosis. To elucidate these mechanisms, we conducted field experiments using three soybean genotypes with different nodulation abilities and analyzed their elemental distribution patterns. Additionally, hydroponic experiments using inoculated soybeans were conducted to investigate ¹³⁷Cs distribution. We found that Cs concentrations were consistently higher in nodules than in other organs. Radioisotope imaging also showed predominant ¹³⁷Cs accumulation in nodules. Covariance analysis revealed that Cs translocation to shoot was lower in genotypes with higher nodule formation under the same soil exchangeable K conditions. Furthermore, increased nodule formation, especially nodule number, was associated with reduced Cs translocation to shoot. These results suggest that nodules contribute to suppressing Cs translocation to shoot and provide new insights into Cs dynamics under root nodule symbiosis.