AtKC1 and CIPK23 Synergistically Modulate AKT1-Mediated Low-Potassium Stress Responses in Arabidopsis

In Arabidopsis (Arabidopsis thaliana), the Shaker K+ channel AKT1 conducts K+ uptake in root cells, and its activity is regulated by CBL1/9-CIPK23 complexes as well as by the AtKC1 channel subunit. CIPK23 and AtKC1 are both involved in the AKT1-mediated low-K+ (LK) response; however, the relationship between them remains unclear. In this study, we screened suppressors of low-K+ sensitive [lks1 (cipk23)] and isolated the suppressor of lks1 (sls1) mutant, which suppressed the leaf chlorosis phenotype of lks1 under LK conditions. Map-based cloning revealed a point mutation in AtKC1 of sls1 that led to an amino acid substitution (G322D) in the S6 region of AtKC1. The G322D substitution generated a gain-of-function mutation, AtKC1(D), that enhanced K+ uptake capacity and LK tolerance in Arabidopsis. Structural prediction suggested that glycine-322 is highly conserved in K+ channels and may function as the gating hinge of plant Shaker K+ channels. Electrophysiological analyses revealed that, compared with wild-type AtKC1, AtKC1(D) showed enhanced inhibition of AKT1 activity and strongly reduced K+ leakage through AKT1 under LK conditions. In addition, phenotype analysis revealed distinct phenotypes of lks1 and atkc1 mutants in different LK assays, but the lks1 atkc1 double mutant always showed a LK-sensitive phenotype similar to that of akt1. This study revealed a link between CIPK-mediated activation and AtKC1-mediated modification in AKT1 regulation. CIPK23 and AtKC1 exhibit distinct effects; however, they act synergistically and balance K+ uptake/leakage to modulate AKT1-mediated LK responses in Arabidopsis.