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

The involvement of endocytosis in the Na⁺ ion uptake from the external medium by the cells of suspension culture derived from A. thaliana (Col-0) leaves was investigated. Na⁺ ion 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 membranous structures was recorded using fluorescence microscopy by colocalization of FM4-64, a marker of endocytosis structures, and Asante NaTRIUM Green-2 TMA⁺ salt (ANG-2 TMA), a membrane impermeable probe for sodium ions, that enabled the detection of Na⁺ absorbed by the cells via endocytosis but not through ion channels or transporters of the plasma membrane. Following a 1.5-h incubation of the cells in the presence of NaCl, FM4-64 and ANG-2 TMA, fluorescence of the probes was colocalized in structures with sizes ranging from 800 to 3000 nm. It was shown by electron microscopy that NaCl added to the cell incubation medium stimulated vesiculation and vacuolization of the cytoplasm, formation of plasma membrane invaginations, as well as fusion of microvacuoles with each other. The size of the structures, in which the colocalization of the two probes was detected by fluorescent microscopy, matched the size of the microvacuoles revealed by the electron microscopy. The obtained results indicate the capture of sodium ions contained in the apoplast by endocytosis invaginations, their subsequent internalization by the cells, and transfer into microvacuoles.