Soil salinity is a severe threat to agriculture and plant growth. Under high salinity conditions, ammonium (NH4+) is the predominant inorganic nitrogen source used by plants due to limited nitrification. However, how ammonium shapes the plant response to salt stress remains a mystery. Here, we demonstrate that the growth of Arabidopsis (Arabidopsis thaliana) seedlings is less sensitive to salt stress when provided with ammonium instead of nitrate (NO3-), a response that is mediated by ammonium transporters (AMTs). We further show that the kinase SALT OVERLY SENSITIVE2 (SOS2) physically interacts with and activates AMT1
1 by directly phosphorylating the non-conserved serine residue Ser-450 in the C-terminal region. In agreement with the involvement of SOS2, ammonium uptake was lower in sos2 mutants grown under salt stress relative to the wild type. Moreover, AMT-mediated ammonium uptake enhanced salt-induced SOS2 kinase activity. Together, our study demonstrates that SOS2 activates AMT1
1 to fine-tune and maintain ammonium uptake and optimize the plant salt stress response.