A Protein Kinase, Calcineurin B-Like Protein-Interacting Protein Kinase9, Interacts with Calcium Sensor Calcineurin B-Like Protein3 and Regulates Potassium Homeostasis under Low-Potassium Stress in Arabidopsis1[W][OA]

Potassium (K) is an essential macronutrient for plant growth and development. Previous studies have demonstrated that Calcineurin B-Like Protein1 (CBL1) or CBL9 and CBL-Interacting Protein Kinase23 (CIPK23) regulate K uptake in Arabidopsis (Arabidopsis thaliana) roots by modulating K channel Arabidopsis K Transporter1. In this study, we show that the protein kinase CIPK9 interacts with the calcium sensor CBL3 and plays crucial roles in K homeostasis under low-K stress in Arabidopsis. Arabidopsis wild-type plants showed leaf chlorotic symptoms when grown for 10 d on low-K (100 mM) medium. Here, we show that plants lacking CIPK9 displayed a tolerant phenotype to low-K stress, which still maintained green leaves when the wild-type plants showed typical K-deficient symptoms. Overexpressing lines of CIPK9 resulted in a low-K-sensitive phenotype compared with wild-type plants. Furthermore, CBL2 and CBL3 were identified as upstream regulators of CIPK9. Both CBL2and CBL3-overexpressing lines displayed similar low-K-sensitive phenotypes and K contents to CIPK9overexpressing lines. However, only cbl3 mutant plants, but not cbl2 mutant plants, showed the low-K-tolerant phenotype similar to cipk9 mutants. Taken together, these results demonstrate that CIPK9 and CBL3 work together and function in K homeostasis under low-K stress in Arabidopsis.