A protein phosphatase 2C, AP2C1 interacts with and negatively regulates the function of CIPK9 under potassium deficient conditions in Arabidopsis

Abstract

Potassium (K+) is a major macronutrient required for plant growth. In response to low- K+ condition, an adaptive mechanism entails activation of the Ca2+ signaling network consisting of calcineurin B-like proteins (CBLs) and their interacting kinases (CIPKs) in plants. The CBL-interacting protein kinase 9 (CIPK9) is previously implicated in low-K+ responses in Arabidopsis thaliana. Here, we report a protein phosphatase 2C (PP2C), AP2C1, as an interactor of CIPK9. Fluorescence resonance energy transfer (FRET), bimolecular fluorescence complementation (BiFC) and co-localization analyses revealed that CIPK9 and AP2C1 interact in the cytoplasm. AP2C1 dephosphorylates the auto-phosphorylated form of CIPK9 in vitro, presenting a regulatory mechanism for CIPK9 function. Furthermore, genetic and molecular analysis revealed that ap2c1 null mutants (ap2c1-1 and ap2c1-2) are tolerant to low-K+ conditions, retained higher K+ content and showed higher expression of K+ deficiency related genes contrary to cipk9 mutants (cipk9-1 and cipk9-2). In contrast, transgenic plants overexpressing AP2C1 were sensitive to low-K+ conditions. Thus, this study shows that AP2C1 and CIPK9 interact to regulate K+-deficiency responses in Arabidopsis. CIPK9 functions as positive regulator whereas, AP2C1 acts as a negative regulator of Arabidopsis root growth and seedling development under low-K+ conditions.