The expression of constitutively active CPK3 impairs potassium uptake and transport in Arabidopsis under low K+ stress

Potassium (K+) is a vital cation and is involved in multiple physiological functions in plants. K+ uptake from outer medium by roots is a tightly regulated process and is mainly carried out by two high affinity K+ transport proteins AKT1 and HAK5. It has been shown that calcium (Ca2+) signaling plays important roles in the regulation of K+ transport in plants. Ca2+-dependent protein kinases (CPKs) are involved in regulation of multiple K+ channels in different tissues. However, it remains to be studied whether CPKs are involved in the regulation of AKT1 and, thereby, K+ transport. Here, we have shown that constitutively active version of CPK3 (CPK3CA) is involved in K+ transport in Arabidopsis via regulating AKT1 under low K+ conditions. The constitutively active version of CPK3 (CPK3CA), as well as CPK21 (CPK21CA), inhibited K+ currents of AKT1 in Xenopus oocytes. CPK3CA inhibited only channel conductance but had no effect on channel open probability. Further, CPK3 in vivo interacted with AKT1. Under low K+ conditions, cpk3 knock-out mutants had no distinct phenotype, while the seedlings of 35S-CPK3CA overexpressing lines died even at normal K+ concentration. Further, the transgenic lines expressing CPK3CA under AKT1 promoter (ProAKT1-CPK3CA) exhibited the same phenotype as akt1 mutant with a defective root growth and leaf chlorosis. Moreover, ProAKT1-CPK3CA transgenic lines had lower root and shoot K+ contents than Col. Overall, the data reported here demonstrate that the expression of constitutively active of CPK3 impairs potassium uptake and transports in Arabidopsis under low K+ stress by inhibiting the activity of AKT1.