Participation of Endocytosis in Sodium Uptake by Cells of the Suspension Culture of Arabidopsis thaliana (L.) Heynh.

The involvement of endocytosis in the Na+ uptake from the external medium by cells of suspension culture derived from A. thaliana leaves was investigated. Na+ uptake by endocytic structures occurred following the addition of NaCl at the final concentration of 100 mM to the incubation medium. The presence of Na+ in membrane structures was detected by fluorescent microscopy in the colocalization experiments employing endocytosis marker FM4-64 and a membrane-impermeable Na+ probe Asante Natrium Green-2 TMA(+) salt (ANG-2 TMA) that allows the detection of Na+ ions taken in by the cells via endocytosis but not through the plasma membrane transporters or ion channels. Following the 1.5-h incubation of the cells in the presence of NaCl, FM4-64, and ANG-2 TMA, the fluorescence of the dyes was colocalized in structures with sizes ranging from 800 to 3000 nm. Electron microscopy showed that NaCl added to the cell incubation medium stimulates vesiculation and vacuolization of the cytoplasm, occurrence of plasma membrane invaginations, as well as the fusion of the microvacuoles into larger ones. As a whole, the sizes of the structures in which the colocalization of the dyes was observed by fluorescent microscopy, matched the sizes of the microvacuoles revealed by the electron microscopy. The data obtained indicate the entrapment of Na+ ions by endocytic invaginations followed by Na+ internalization by cells and transfer to microvacuoles.