POTASSIUM is the most abundant cation in higher plants and is crucial for plant nutrition, growth, tropisms, enzyme homeostasis and osmoregulation(1-4). K+ accumulation can be rate-limiting for agricultural production(2-4). K+ uptake from soils into roots is largely mediated by high-affinity K+ uptake (K-m approximate to 10-40 mu M) (refs 1, 2, 5-7). But although K+ channels allow low-affinity K+ uptake(8-10), both the transport mechanism and structure of the high-affinity K+ nutrition pathway remain unknown. Here we use expression cloning to isolate a complementary DNA encoding a membrane protein (HKT1) from wheat roots which confers the ability to take up K+. The substrate affinity, saturation and cation selectivity of HKT1 correspond to hallmark properties of classical high-affinity K+ uptake in plants(1,2,11). The transport mechanism of HKT1 uses K+-H+ co-uptake. Expression of HKT1 is localized to specific root and leaf regions which represent primary sites for K+ uptake in plants(2,3). HKT1 is important for plant nutrition and could possibly contribute to environmental alkali metal toxicities(11-13).