Soil Bacteria Confer Plant Salt Tolerance by Tissue-Specific Regulation of the Sodium Transporter <italic>HKT1</italic>

Elevated sodium (Na) decreases plant growth and, thereby, agricultural productivity. The ion transporter high-affinity K transporter (HKT)1 controls Na import in roots, yet dysfunction or overexpression of HKT1 fails to increase salt tolerance, raising questions as to HKT1’s role in regulating Na homeostasis. Here, we report that tissuespecific regulation of HKT1 by the soil bacterium Bacillus subtilis GB03 confers salt tolerance in Arabidopsis thaliana. Under salt stress (100 mM NaCl), GB03 concurrently downand upregulates HKT1 expression in roots and shoots, respectively, resulting in lower Na accumulation throughout the plant compared with controls. Consistent with HKT1 participation in GB03-induced salt tolerance, GB03 fails to rescue salt-stressed athkt1 mutants from stunted foliar growth and elevated total Na whereas saltstressed Na export mutants sos3 show GB03-induced salt tolerance with enhanced shoot and root growth as well as reduced total Na. These results demonstrate that tissuespecific regulation of HKT1 is critical for managing Na homeostasis in salt-stressed plants, as well as underscore the breadth and sophistication of plant–microbe interactions.