FIGURE 2.

Main pathways for root K⁺ uptake and their regulatory mechanisms. HAK5-type transporters are high-affinity K⁺ transporters involved in K⁺ uptake at very low concentrations. When the external K⁺ concentration increases, the inward-rectifier K⁺ channel AKT1 together with HAK5, contributes to K⁺ uptake. At K⁺ concentrations above 200 μM, HAK5 is not present and AKT1 is the main system for low-affinity K⁺ uptake. At very high concentrations, other unknown systems can secure K⁺ supply if AKT1 is not functional. The HAK5 and AKT1 uptake systems are subjected to finely tuned regulation. At low K⁺ concentrations, a hyperpolarization of the plasma membrane induces HAK5 transcription. The signal cascade of HAK5 regulation is dependent on ethylene and ROS production. In addition, low external K⁺ likely produces a specific cytoplasmic Ca²⁺ signal that is registered by the Ca²⁺ sensor CBL1, which induces CIPK23 recruitment to the plasma membrane resulting in the phosphorylation and subsequent activation of HAK5 and AKT1. The channel activity is downregulated through dephosphorylation by the AIP1 phosphatase, and interaction with CBL10 and vitamin B6. Other subunits such as KC1 and the SNARE protein SYP121 also cooperate in AKT1 regulation. It can be concluded that whereas HAK5 is subjected to transcriptional and post-transcriptional regulation, K⁺ uptake through AKT1 is mainly regulated post-transcriptionally. The CIPK23/CBL1 complex emerges as a key regulator of K⁺ nutrition.