Root K+ Acquisition in Plants: The Arabidopsis thaliana Model

K+ is an essential macronutrient required by plants to complete their life cycle. It fulfills important functions and it is widely used as a fertilizer to increase crop production. Thus, the identification of the systems involved in K+ acquisition by plants has always been a research goal as it may eventually produce molecular tools to enhance crop productivity further. This review is focused on the recent findings on the systems involved in K+ acquisition. From Epstein's pioneering work > 40 years ago, K+ uptake was considered to consist of a high- and a low-affinity component. The subsequent molecular approaches identified genes encoding K+ transport systems which could be involved in the first step of K+ uptake at the plant root. Insights into the regulation of these genes and the proteins that they encode have also been gained in recent studies. A demonstration of the role of the two main K+ uptake systems at the root, AtHKA5 and AKT1, has been possible with the study of Arabidopsis thaliana T-DNA insertion lines that knock out these genes. AtHAK5 was revealed as the only uptake system at external concentrations < 10 mu M. Between 10 and 200 mu M both AtHAK5 and AKT1 contribute to K+ acquisition. At external concentrations > 500 mu M, AtHAK5 is not relevant and AKT1's contribution to K+ uptake becomes more important. At 10 mM K+, unidentified systems may provide sufficient K+ uptake for plant growth.