SALT OVERLY SENSITIVE 1 Na ⁺ /H ⁺ Exchanger Operates in Mature Root Zone and Is a Major Contributor to Root Na ⁺ Exclusion During Shoot‐to‐Root Na ⁺ Recirculation

The Na⁺/H⁺ antiporter SALT OVERLY SENSITIVE 1 (SOS1) is a key component of Na⁺ exclusion and plant salt tolerance. Although previous studies have suggested that SOS1 functions in both the root apex and mature root zone, their contributions remain unclear due to limited methodological resolution and originated mostly from transcriptional analysis. Here, we performed isotopic tracing techniques to visualize and quantify Na⁺ exclusion. Real-time imaging of shoot-applied ²²Na⁺ showed that ²²Na⁺ gradually disappeared from roots in wild-type (WT) plants, whereas it did not in sos1 mutants. To confirm that this reduction reflected active Na⁺ exclusion to the rhizosphere, we used the Microelectrode Ion Flux Estimation, which revealed significant Na⁺ efflux at the mature root zone of WT plants following shoot Na⁺ application, while no such efflux was observed at the root apex or in either root zone of sos1 mutants. Further quantification using a radioisotope-based method showed that approximately 90%-95% of Na⁺ derived from both the phloem and xylem was excluded from WT roots, primarily via SOS1, with the mature root zone identified as the major contributor. This study provides visual and quantitative evidence for the crucial contribution of SOS1 to Na⁺ exclusion in the mature root.