CIPK23 regulates HAK5-mediated high-affinity K+ uptake in Arabidopsis roots

Abstract

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 that when the external K+ concentration is very low (<10 µM), K+ nutrition depends exclusively on the high-affinity K+ transporter HAK5. Low-K+ induced transcriptional activation of the gene encoding HAK5 has been previously reported. Here we show the posttranscriptional regulation for HAK5 transport activity by phosphorylation. Expression in a heterologous system showed that the Ca2+ sensors CBL1, CBL8, CBL9 and CBL10, together with CBL-interacting kinase CIPK23 activated HAK5 in vivo. This activation produced an increase in the affinity and the Vmax 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 in K+ deficient medium with Arabidopsis single mutants hak5, akt1 and cipk23, double mutants hak5 akt1, hak5 cipk23, akt1 cipk23, and the triple mutant hak5 akt1 cipk23 confirmed the regulatory role of CIPK23 in planta.