FIGURE 4.

The possible pathway of Na⁺ transport from the mesophyll to the salt gland, and the way in which Na⁺ is secreted out. Na⁺ is transported into the salt gland through the bottom penetration area, which is not covered with cuticles (1), and the plasmodesmata (2). In the salt gland, the ions can be directly transported into the intercellular space (3). The ions are wrapped in vesicles for transport from the cytosol to the plasma membrane and secreted out of the salt gland cells (4). Plasmodesmata play an important role in ion delivery among the salt gland component cells (5). Various ion transporters are widely involved in moving salt in (6) and out (7). H⁺-ATPase participated in salt secretion by establishing electric potential difference and proton motive force across plasma membrane (8). Water channels (e.g., PIP) also take part in all the processes as a medium for ion transport (9). The ions are eventually exuded from the secreting pores at the top of the salt gland due to high hydrostatic pressure (10). Ion transporters responsible for influx (dark blue) and efflux (red) are asymmetrically distributed in the plasma membrane of salt gland cells. PLAS, plasmodesmata; HKT1, high-affinity K⁺ transporter 1; CNGC, cyclic nucleotide-gated cation channel; NSCC, non-selective cationic channel; PIP, plasma membrane intrinsic protein; NHX, Na⁺/H⁺ antiporter; SOS1, Na⁺/H⁺ antiporter; CLC, chloride channel.