GmSALT3 confers shoot Na+ and Cl- exclusion in soybean via two distinct processes.

Soybean (Glycine max) yields are threatened by multiple stresses including soil salinity. GmSALT3 confers net shoot exclusion for both Na+ and Cl- to soybean and improved salt tolerance; however, how the ER-localised GmSALT3 achieves this is unknown. Here, GmSALT3 function was investigated in heterologous systems, and in near isogenic lines that contained the full-length GmSALT3 gene (NIL-T; salt-tolerant) or a truncated transcript (NIL-S; salt-sensitive). A time course analysis of the NILs confirmed shoot Cl- exclusion breaks down days prior to Na+ exclusion. Shoot Na+ exclusion occurs via a root xylem-based mechanism in NIL-T lines, as is common amongst plants, with Na+ content significantly greater in NIL-S stem xylem sap. 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 a novel phloem-based Cl- recirculation back to NIL-T roots. In heterologous expression systems, GmSALT3 restored growth of K+-uptake-defective E. coli, whilst in Xenopus laevis oocytes, GmSALT3 contributed to net influx and accumulation of Na+, K+, and Cl-. Overall, these findings provide new insights on the impact of ER-localised GmSALT3 on plasma membrane ion fluxes and reveals a novel mechanism for shoot Cl- exclusion.