Potassium transporter KUP9 participates in K⁺ distribution in roots and leaves under low K⁺ stress

Potassium (K) is a major essential element in plant cells, and KUP/HAK/KT-type K⁺ transporters participate in the absorption of K⁺ into roots and in the long-distance transport to above-ground parts. In Arabidopsis thaliana, KUP9 is involved in the transport of K⁺ and Cs⁺ in roots. In this study, we investigated KUP9 function in relation to the K⁺ status of the plant. The expression of KUP9 was upregulated in older leaves on K⁺-depleted medium, compared to the expression of the other 12 KUP genes in the KUP/HAK/KT family in Arabidopsis. When grown on low K⁺ medium, the kup9 mutant had reduced chlorophyll content in seedlings and chlorosis in older rosette leaves. Tissue-specific expression of KUP9 determined by KUP9 promoter:GUS assay depended on the K⁺ status of the plants: In K⁺ sufficient medium, KUP9 was expressed in the leaf blade towards the leaf tip, whereas in K⁺ depleted medium expression was mainly found in the petioles. In accordance with this, K⁺ accumulated in the roots of kup9 plants. The short-term ⁴³K⁺ tracer measurement showed that ⁴³K was transferred at a lower rate in roots and shoots of kup9, compared to the wild type. These data show that KUP9 participates in the distribution of K⁺ in leaves and K⁺ absorption in roots under low K⁺ conditions.