Salt Overly Sensitive 1 (SOS1) is the transporter responsible for Na+ efflux from root cells and is a key molecule in salt stress tolerance. This study aimed to identify the specific tissues that require SOS1 function by employing a set of Arabidopsis mutants with root cell type-specific expression of SOS1. The Na+ exclusion ability was quantified through the live imaging that captured the dynamics of radiotracer, 22Na. Of the 22Na applied to a cotyledon and transported to the root within 24 hours, 75% was excluded from the root and only 25% remained in the wild type (WT) plant. In the sos1 mutant, on the contrary, almost all 22Na remained in the root, thus the 22Na residual rate was 100%. The expression of SOS1 in the cells either in meristematic tissue or in stele did not recover the 22Na exclusion inability of sos1. However, when SOS1 was expressed in the epidermis cells, the 22Na residual rate was improved to 50%. Salinity tolerance was evaluated by the relative root length (RRL) under 30 mM NaCl stress condition compared to that under non-stressed condition. The RRL for WT and sos1 mutants was 80% and 20%, respectively. The highest recovery of the RRL, about 70%, was achieved by SOS1 expression in epidermis cells followed by expression in stele cells, with the RRL of about 55%. In conclusion, the most significant contribution to Na+ exclusion and salinity tolerance was found to be SOS1 in the epidermis cells.