NH4+ Toxicity, Which Is Mainly Determined by the High NH4+/K+ Ratio, Is Alleviated by CIPK23 in Arabidopsis

Abstract

Ammonium (NH₄⁺) toxicity is always accompanied by ion imbalances, and NH₄⁺ 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-NH₄⁺-dependent under low-K⁺ condition. However, the correlation of K⁺ and NH₄⁺ 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 NH₄⁺ and K⁺ was applied. Comparative treatments showed that NH₄⁺ toxicity, which is triggered mainly by the high ratio of NH₄⁺ to K⁺ (NH₄⁺/K⁺ ≥ 10:1 for cipk23) but not by the absolute concentrations of the ions, results in leaf chlorosis. Under high NH₄⁺/K⁺ ratios, CIPK23 is upregulated abundantly in leaves and roots, which efficiently reduces the leaf chlorosis by regulating the contents of NH₄⁺ 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 NH₄⁺ toxicity. Taken all together, CIPK23 might function in different tissues to reduce stress-induced NH₄⁺ toxicity associated with high NH₄⁺/K⁺ ratios by regulating the NH₄⁺-K⁺ balance in Arabidopsis.

Keywords: CIPK; K+; NH4+ toxicity; leaf chlorosis; root development.