Mutation of the ARA7/AtRabF2b Gene, That Increases the Content of the Ara7 Protein Regulating Endocytic Trafficking Pathways, Improves Salt Tolerance of the Arabidopsis thaliana (L.) Heynh Plants

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.