Development of the Experimental Method to Evaluate Sodium Ion Exclusion in the Mature Root Zone of <i>Arabidopsis thaliana</i>

Sodium (Na) exclusion is vital for plants to cope with salt stress, and the plasma membrane Na+/H+ antiporter, SALT OVERLY SENSITIVE 1 (SOS1), has been known to play a central role in this process although the function of this molecule is not completely understood. In this study, we performed 22Na+ radiotracer experiments to investigate the function of SOS1 in the mature root zone. Arabidopsis thaliana seedlings were placed on the gel medium, which had air gaps to separate the medium at the mature root zone from that at the root apex zone. Then, 22Na+ was added to the root apex zone for 4 h, and the radioactivity in the mature root zone with 5 mm in length and in the mature root zone gel was measured. As a result, we found that knock-out of SOS1 caused 22Na+ accumulation in the mature root zone and reduction of 22Na+ exclusion to the gel, demonstrating that Na+ delivered via the xylem is excluded to the rhizosphere by SOS1 in the mature root zone.