The relationship between the endocytosis, ion homeostasis, and salt tolerance in Arabidopsis thaliana was studied using mutant plants with the insertion in the ARA7/AtRabF (AT4G19640) gene, encoding a small Rab5 GTPase that represents one of the key regulators of the vesicular transport. This mutation is characterized by an increased expression of the AtARA7 protein. A 14-nucleotide deletion in the 5'-untranslated region (5'-UTR) and the loss of the first three nucleotides at the 5'-end of mRNA were revealed in the mutant allele. Though these sequence changes slightly decreased the relative level of ARA7 transcripts in leaves of mutant plants under normal conditions, an increased content of the corresponding ARA7 protein in leaves was observed. This, in turn, resulted in significant phenotypic changes in plants grown on a NaCl-containing growth media. Electron microscopy studies revealed some changes on ultrastructural level in the root cells of mutant plants. The most significant differences between the mutant and wild-type (WT) plants included the presence of electron dense deposits on the tonoplast and increase in (a) the degree of vacuolization and vesiculation of the cytosol, (b) the content of fusing microvacuoles, and (c) the content of multivesicular bodies and autophagic structures that indicated changes in the endocytosis pathway and the vacuole formation dynamics. At the whole plant level, the mutant phenotype was characterized by a positive response of mutant plants to a long-term salt stress compared to WT plants. Mutant plants also had an increased mass of roots and leaves, heightened content of K+, and decreased average content of Na+. This indicates that mutants developed a higher degree of salt tolerance when compared to WT plants. Mutation-induced changes in the cell ultrastructure probably reflect perturbations in vesicular transport and autophagy resulting in changes in ion homeostasis and salt tolerance of plants.

Flotillin membrane proteins are involved in many cellular processes and physiological functions. However, these proteins’ participation in plant response to stresses remains poorly understood. In the present report, the possible involvement of flotillin Flot1 in Na + and K + homeostasis in cells under sodium chloride salinization of the medium was studied in Arabidopsis thaliana (L.) Heynh. For this purpose, the Flot1 gene transcription was analyzed in the roots and leaves of wild-type (WT) and flot1 mutant plants with a T-DNA insertion in the promoter using the quantitative Real Time-RT PCR (qRT-PCR). Along with this, a mutant phenotype was studied (growth characteristics, content of Na + and K + ions in organs, and ultrastructure of root cells) under normal conditions and in the presence of 100 mM NaCl in the culture medium. The mutation led to the activation of AtFlot1 expression, which was more noticeable in the roots during salinization. Under these conditions, the mutants had larger organ mass, lower Na + content, and higher K + content in organs than that of WT. The study of the ultrastructure of A. thaliana root cells in mutant plants showed more intensive formation of post-Golgi vesicles and multivesicular bodies (MVB) in the cytoplasm. In both mutant and WT plants, the presence of sodium chloride in the nutrient solution stimulated the formation of MVB and microvacuoles in the cytoplasm and fusion of the latters into larger structures. It is assumed that changes in the ultrastructure of root cells that are caused by mutation and salinization reflect the stimulation, respectively, of vesicular trafficking and biogenesis of vacuoles—processes involved in maintaining Na + and K + cell homeostasis.