K+ uptake in plant roots. The systems involved, their regulation and parallels in other organisms

Potassium (K+) is an essential macronutrient for plants. It is taken into the plant by the transport systems present in the plasma membranes of root epidermal and cortical cells. The identity of these systems and their regulation is beginning to be understood and the systems of K+ transport in the model species Arabidopsis thaliana remain far better characterized than in any other plant species. Roots can activate different K+ uptake systems to adapt to their environment, important to a sessile organism that needs to cope with a highly variable environment. The mechanisms of K+ acquisition in the model species A. thaliana are the best characterized at the molecular level so far. According to the current model, non-selective channels are probably the main pathways for K+ uptake at high concentrations (>10 mM), while at intermediate concentrations (1 mM), the inward rectifying channel AKT1 dominates K+ uptake. Under lower concentrations of external K+ (100 μM), AKT1 channels, together with the high-affinity K+ uptake system HAK5 contribute to K+ acquisition, and at extremely low concentrations (<10 μM) the only system capable of taking up K+ is HAK5. Depending on the species the high-affinity system has been named HAK5 or HAK1, but in all cases it fulfills the same functions. The activation of these systems as a function of the K+ availability is achieved by different mechanisms that include phosphorylation of AKT1 or induction of HAK5 transcription. Some of the characteristics of the systems for root K+ uptake are shared by other organisms, whilst others are specific to plants. This indicates that some crucial properties of the ancestral of K+ transport systems have been conserved through evolution while others have diverged among different kingdoms.