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

• Main Authors: Sujuan Shi, Fangzheng Xu, Yuqian Ge, Jingjing Mao, Lulu An, Shuaijun Deng, Zia Ullah, Xuefeng Yuan, Guanshan Liu, Haobao Liu, Qian Wang
• Format: Article
• Language: English
• Published: MDPI AG 2020-04-01
• Series: Plants
• Subjects: NH₄⁺ toxicity; <i>CIPK</i>; K⁺; leaf chlorosis; root development
• Online Access: https://www.mdpi.com/2223-7747/9/4/501

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 <i>Arabidopsis thaliana</i>, the typical leaf chlorosis phenotype in the knockout mutant of <i>calcineurin B-like interacting protein kinase 23</i> (<i>CIPK23</i>) is high-NH₄⁺-dependent under low-K⁺ condition. However, the correlation of K⁺ and NH₄⁺ in the occurrence of leaf chlorosis in the <i>cipk23</i> 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 <i>cipk23</i>) but not by the absolute concentrations of the ions, results in leaf chlorosis. Under high NH₄⁺/K⁺ ratios, <i>CIPK23</i> 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 <i>CIPK23</i> in alleviating NH₄⁺ toxicity. Taken all together, <i>CIPK23</i> might function in different tissues to reduce stress-induced NH₄⁺ toxicity associated with high NH₄⁺/K⁺ ratios by regulating the NH₄⁺–K⁺ balance in <i>Arabidopsis</i>.