Potassium translocation into the root xylem

Potassium is the most abundant cation in cells of higher plants and plays vital roles in plant growth and development. Since the soil is the only source of potassium, plant roots are well adapted to exploit the soil for potassium and supply it to the leaves. Transport across the root can be divided into three stages: uptake into the root symplast, transport across the symplast and release into the xylem, Uptake kinetics of potassium have been studied extensively in the past and suggested the presence of high and low affinity systems. Molecular and electrophysiological techniques have now confirmed the existence of discrete transporters encoded by a number of genes. Surprisingly, detailed characterisation of the transporters using reverse genetics and heterologous expression shows that a number of the transporters (AKT and AtKUP family) function both in the low (mu M) and high (mM) K+ range. Electrophysiological studies indicate that K+ uptake by roots is coupled to H+, to drive uptake from micromolar K+. However, thus far only Na+ coupled K+ transport has been demonstrated (HKT1). Ion channels play a major role in the exchange of potassium between the symplast and the xylem. An outward rectifying channel (KORC) mediates potassium release. Cloning of the gene encoding this channel (SKOR) shows that it belongs to the Shaker super-family. Both electrophysiological and genetic studies demonstrate that K+ release through this channel is controlled by the stress hormone abscisic acid. Interestingly, xylem parenchyma cells of young barley roots also contain a number of inward rectifying K+ channels that are controlled by C-proteins. The involvement of G-proteins emphasises once more that potassium transport at the symplast/xylem boundary is under hormonal control. The role of the electrical potential difference across the symplast/xylem boundary in controlling potassium release is discussed.