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.