Soybean CHX-type ion transport protein GmSALT3 confers leaf Na⁺ exclusion via a root derived mechanism, and Cl⁻ exclusion via a shoot derived process

Soybean (Glycine max) yields are threatened by multiple stresses including soil salinity. GmSALT3 (a cation-proton exchanger protein) confers net shoot exclusion for both Na⁺ and Cl⁻ and improves salt tolerance of soybean; however, how the ER-localized GmSALT3 achieves this is unknown. Here, GmSALT3's function was investigated in heterologous systems and near isogenic lines that contained the full-length GmSALT3 (NIL-T; salt-tolerant) or a truncated transcript Gmsalt3 (NIL-S; salt-sensitive). GmSALT3 restored growth of K⁺-uptake-defective Escherichia coli and contributed towards net influx and accumulation of Na⁺, K⁺ and Cl⁻ in Xenopus laevis oocytes, while Gmsalt3 was non-functional. Time-course analysis of NILs confirmed shoot Cl⁻ exclusion occurs distinctly from Na⁺ exclusion. Grafting showed that shoot Na⁺ exclusion occurs via a root xylem-based mechanism; in contrast, NIL-T plants exhibited significantly greater Cl⁻ content in both the stem xylem and phloem sap compared to NIL-S, indicating that shoot Cl⁻ exclusion likely depends upon novel phloem-based Cl⁻ recirculation. NIL-T shoots grafted on NIL-S roots contained low shoot Cl⁻, which confirmed that Cl⁻ recirculation is dependent on the presence of GmSALT3 in shoots. Overall, these findings provide new insights on GmSALT3's impact on salinity tolerance and reveal a novel mechanism for shoot Cl⁻ exclusion in plants.