The Arabidopsis Receptor-like Kinase CAP1 Promotes Shoot Growth under Ammonium Stress.

High levels of ammonium (NH 4 + ) in soils inhibit plant growth and nitrogen utilization efficiency. Elucidating the underlying mechanisms of NH 4 + toxicity is essential for alleviating the growth inhibition caused by high NH 4 + . Our previous work showed that [Ca 2+ ] cyt -associated protein kinase 1 (CAP1) regulates root hair growth in response to NH 4 + in Arabidopsis thaliana , and the cap1-1 mutant produces short root hairs under NH 4 + stress conditions. However, it is unclear whether CAP1 functions in other physiological processes in response to NH 4 + . In the present study, we found that CAP1 also plays a role in attenuating NH 4 + toxicity to promote shoot growth. The cap1-1 mutant produced smaller shoots with smaller epidermal cells compared with the wild type in response to NH 4 + stress. Disruption of CAP1 enhanced the NH 4 + -mediated inhibition of the expression of cell enlargement-related genes. The cap1-1 mutant showed elevated reactive oxygen species (ROS) levels under NH 4 + stress, as well as increased expression of respiratory burst oxidase homologue genes and decreased expression of catalase genes compared with the wild type. Our data reveal that CAP1 attenuates NH 4 + -induced shoot growth inhibition by promoting cell wall extensibility and ROS homeostasis, thereby highlighting the role of CAP1 in the NH 4 + signal transduction pathway.