Table 1.
Gene expression, localization, and function of HKTs in rice, wheat, and barley
| Plant species | Gene | Gene expression, localization, and function | Reference |
| Oryza sativa | OsHKT1;1 | Mainly expressed in the phloem of leaves; the expression in old leaves is up-regulated by salt stress, which protects young leaves by accumulating Na+ in the old leaves | Wang et al., 2012 |
| O. sativa | OsHKT1;3 | Expressed in the root stele, phloem, and leaf vascular bundle; localized on the golgi; associated with leaf curl | Jabnoune et al., 2009 |
| O. sativa | OsHKT1;4 | Mainly expressed in the vascular tissue of shoot; Na+ was unloaded from the stem and leaf sheath during the reproductive growth stage, but this function is not significant during the vegetative growth stage | Suzuki et al., 2016 |
| O. sativa | OsHKT1;5 (SKC1) | Expressed in the plasma membrane of xylem parenchyma cells in root and leaf sheath; unloading Na+ from the xylem; reducing Na+ accumulation in leaves | Ren et al., 2005; Kobayashi et al., 2017 |
| O. sativa | OsHKT2;1 | Expressed in root epidermis and cortical cells; gene expression is reduced under salt stress; involved in root Na+ absorption | Garciadeblás et al., 2003; Kader and Lindberg, 2008 |
| O. sativa | OsHKT2;2 | Mainly expressed in root and mesophyll cells; gene expression in phloem is significantly enhanced under salt stress; K+/Na+ symporter | Kader et al., 2006; Yao et al., 2010 |
| O. sativa (cv. Nona Bokra) | No-OsHKT2;2/1 | Gene expression is induced under salt stress; increasing salt tolerance by enhancing selective absorption of K+ | Oomen et al., 2012 |
| O. sativa | OsHKT2;3 | Expressed in roots, basal stem, leaves, and sheaths, but no function; possibly pseudogenes | Garciadeblás et al., 2003 |
| O. sativa | OsHKT2;4 | Expressed in root epidermis, stems, internodes, vascular bundles in leaf sheath, and phloem; high affinity to K+, low affinity to Na+; ion affinity is inhibited by K+ | Horie et al., 2011; Sassi et al., 2012; Zhang et al., 2017 |
| Triticum monococcum | TmHKT1;4-A (Nax1) | Expressed in roots and sheaths; unloading Na+ from the vascular bundle; keeping Na+ in leaf sheath; unloading Na+ from the phloem | Lindsay et al., 2004; James et al., 2006 |
| T. monococcum | TmHKT1;5-A (Nax2) | Expressed in the plasma membrane of vascular tissues in root; unloading xylem Na+; reducing sodium accumulation in shoots | Byrt et al., 2007; Munns et al., 2012 |
| Triticum aestivum | TaHKT1;5-D (Kna1) | Expressed in the plasma membrane of stele cells in root; unloading Na+ from xylem; reducing Na+ accumulation in shoots | Byrt et al., 2014 |
| T. aestivum | TaHKT2;1 | Expressed in the plasma membrane of root cortex; Na+/K+ cotransporter under low Na+ condition; Na+ uniporter under high Na+ condition | Rubio et al., 1995; Gassmann et al., 1996 |
| Hordeum vulgare | HvHKT2;1 | Gene expression is up-regulated in the roots under low K+ condition; K+/Na+ cotransporter; when overexpressed in barley, the salt tolerance is enhanced, which may be related to the specific distribution of Na+ in leaves | Mian et al., 2011 |
| H. vulgare | HvHKT1;1 | Expressed in the plasma membrane of the root epidermis and steles; participating in the lateral transport of Na+ in the roots, and finally reducing the accumulation of Na+ in the shoots; involved in maintaining the balance of K+ and Ca2+ in the roots | Han et al., 2018 |