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.