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 (137)Cs(+) and (42)K(+)in skor mutants compared to wild-type but also a different distribution of (137)Cs(+) and (42)K(+) 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 (133)Cs(+) and (85)Rb(+). 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.