NH(4)(+) Toxicity, Which Is Mainly Determined by the High NH(4)(+)/K(+) Ratio, Is Alleviated by CIPK23 in Arabidopsis

Ammonium (NH(4)(+)) toxicity is always accompanied by ion imbalances, and NH(4)(+) and potassium (K(+)) exhibit a competitive correlation in their uptake and transport processes. In Arabidopsis thaliana, the typical leaf chlorosis phenotype in the knockout mutant of calcineurin B-like interacting protein kinase 23 (CIPK23) is high-NH(4)(+)-dependent under low-K(+) condition. However, the correlation of K(+) and NH(4)(+) in the occurrence of leaf chlorosis in the cipk23 mutant has not been deeply elucidated. Here, a modified hydroponic experimental system with different gradients of NH(4)(+) and K(+) was applied. Comparative treatments showed that NH(4)(+) toxicity, which is triggered mainly by the high ratio of NH(4)(+) to K(+) (NH(4)(+)/K(+) ≥ 10:1 for cipk23) but not by the absolute concentrations of the ions, results in leaf chlorosis. Under high NH(4)(+)/K(+) ratios, CIPK23 is upregulated abundantly in leaves and roots, which efficiently reduces the leaf chlorosis by regulating the contents of NH(4)(+) and K(+) in plant shoots, while promoting the elongation of primary and lateral roots. Physiological data were obtained to further confirm the role CIPK23 in alleviating NH(4)(+) toxicity. Taken all together, CIPK23 might function in different tissues to reduce stress-induced NH(4)(+) toxicity associated with high NH(4)(+)/K(+) ratios by regulating the NH(4)(+)–K(+) balance in Arabidopsis.