Potassium (K +) is an essential macronutrient for plant growth. The transcriptional regulation of K + transporter genes is one of the key mechanisms by which plants respond to K + deficiency. Among the HAK/KUP/KT transporter family, HAK5, a high-affinity K + transporter, is essential for root K + uptake under low external K + conditions. HAK5 expression in the root is highly induced by low external K + concentration. While the molecular mechanisms of HAK5 regulation have been extensively studied, it remains unclear how plants sense and coordinates K + uptake and translocation in response to changing environmental conditions. Using skor mutants, which have a defect in root-to-shoot K + translocation, we have been able to determine how the internal K + status affects the expression of HAK5. In skor mutant roots, under K + deficiency, HAK5 expression was lower than in wild-type although the K + concentration in roots was not significantly different. These results reveal that HAK5 is not only regulated by external K + conditions but it is also regulated by internal K + levels, which is in agreement with recent findings. Additionally, HAK5 plays a major role in the uptake of Cs + in roots. Therefore, studying Cs + in roots and having more detailed information about its uptake and translocation in the plant would be valuable. Radioactive tracing experiments revealed not only a reduction in the uptake of 137Cs + and 42K +in skor mutants compared to wild-type but also a different distribution of 137Cs + and 42K + in tissues. In order to gain insight into the translocation, accumulation, and repartitioning of both K + and Cs + in plants, long-term treatment and split root experiments were conducted with the stable isotopes 133Cs + and 85Rb +. Finally, our findings show that the K + distribution in plant tissues regulates root uptake of K + and Cs + similarly, depending on HAK5; however, the translocation and accumulation of the two elements are different.