The CBL-Interacting Protein Kinase CIPK23 Regulates HAK5-Mediated High-Affinity K+ Uptake in Arabidopsis Roots

Plant growth and development requires efficient acquisition of essential elements. Potassium (K + ) is an important macronutrient present in the soil solution at a wide range of concentrations. Regulation of the K + uptake systems in the roots is essential to secure K + supply. It has been shown in Arabidopsis ( Arabidopsis thaliana ) that when the external K + concentration is very low ( + nutrition depends exclusively on the high-affinity K + transporter5 (HAK5). Low-K + -induced transcriptional activation of the gene encoding HAK5 has been previously reported. Here, we show the posttranscriptional regulation of HAK5 transport activity by phosphorylation. Expression in a heterologous system showed that the Ca 2+ sensors calcineurin B-like (CBL1), CBL8, CBL9, and CBL10, together with CBL-interacting protein kinase23 (CIPK23), activated HAK5 in vivo. This activation produced an increase in the affinity and the V max of K + transport. In vitro experiments show that the N terminus of HAK5 is phosphorylated by CIPK23. This supports the idea that phosphorylation of HAK5 induces a conformational change that increases its affinity for K + . Experiments of K + (Rb + ) uptake and growth measurements in low-K + medium with Arabidopsis single mutants hak5 , akt1 , and cipk23 , double mutants hak5 akt1 , hak5 cipk23 , and akt1 cipk23 , and the triple mutant hak5 akt1 cipk23 confirmed the regulatory role of CIPK23 in planta.