Xylem K⁺ loading modulates K⁺ and Cs⁺ absorption and distribution in Arabidopsis under K⁺-limited conditions

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.