AtHAK5-mediated root high-affinity K+ uptake is regulated by the protein kinases AtCIPK1 and AtCIPK9.

The high-affinity K+ transporter AtHAK5 is the major contributor to root K+ uptake from diluted solutions in K+-starved Arabidopsis plants. Its functionality is tightly regulated and its activity is enhanced under K+ starvation by the transcriptional induction of the AtHAK5 gene, and by the activation of the transporter via the AtCBL1-AtCIPK23 complex. In the present study, the 26 members of the Arabidopsis CIPK protein kinase family were screened in yeast for their capacity to active AtHAK5-mediated K+ uptake. Among them, AtCIPK1 was the most efficient activator of AtHAK5. In addition, AtCIPK9, previously reported to participate in K+ homeostasis, also activated the transporter. The genes encoding AtCIPK1 and AtCIPK9 were induced in roots by K+ deprivation and atcipk1 and atcipk9 Arabidopsis KO mutants showed a reduced AtHAK5-mediated Rb+ uptake. Activation of AtHAK5 by AtCIPK1 did not occur under hyperosmotic stress conditions, where AtCIPK1 function has been shown to be required to maintain plant growth. The presented data contribute to the identification of the complex regulatory networks that control the high-affinity K+ transporter AtHAK5 and root K+ uptake.