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

TL;DR

This study shows that the SOS1 protein in plant roots is crucial for removing sodium, helping plants tolerate salty conditions.

Contribution

The study provides the first visual and quantitative evidence that SOS1 in the mature root zone excludes nearly all sodium from plants.

Findings

SOS1 excludes 90–95% of sodium from roots, mainly in the mature root zone.

SOS1 excludes sodium from both phloem and xylem sources in the mature root.

SOS1 is not active in the root apex or in sos1 mutants.

Abstract

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.

This manuscript uses radiotracer and MIFE techniques to visualize and quantify Na+ dynamics in Arabidopsis roots. We demonstrate for the first time that SOS1 Na+/H+ antiporter contributes to ~95%–98% of Na+ exclusion in the mature root zone, where phloem‐derived Na+ is excluded to the rhizosphere. The study provides visual and quantitative evidence for SOS1's dominant role in Na+ recirculation and exclusion under salt stress.

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