SsHKT1;1 is coordinated with SsSOS1 and SsNHX1 to regulate Na + homeostasis in Suaeda salsa under saline conditions

Under saline conditions, Suaeda salsa, as a typical halophyte, accumulates large amounts of Na+ in its leaves during optimal growth. Key transporters involved in Na+ accumulation in plants are HKT-type protein, the plasma membrane Na+/H+ transporter SOS1, and the tonoplast Na+/H+ antiporter NHX1. In this study, the function of SsHKT1;1 and its coordinate expression with SsSOS1 and SsNHX1 to regulate Na+ homeostasis in S. salsa was investigated. We showed, by yeast complementation assays, that SsHKT1;1 encoded a Na+-selective transporter, which located to the plasma membrane and was preferentially expressed within the stele, and was particularly abundant in xylem parenchyma and pericycle cells. When compared with a treatment of 25 mM NaCl, 150 mM NaCl greatly decreased the transcripts of SsHKT1;1, but maintained a relatively constant level of the expression of SsSOS1 in roots. Consequently, the synergistic effect of SsHKT1;1 and SsSOS1 would result in greater Na+ loading into the xylem under 150 mM NaCl than 25 mM NaCl. In leaves, 150 mM NaCl up-regulated the abundance of SsNHX1 compared with levels in 25 mM NaCl. This enabled the permanent sequestering of Na+ into leaf vacuoles. Overall, SsHKT1;1 functioned in reducing Na+ retrieval from the root xylem, and played an important role in coordinating with SsSOS1 and SsNHX1 to maintain Na+ accumulation in S. salsa under saline conditions.