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Vacuolar H + -ATPase (V-H + -ATPase) is highly conserved, multisubunit proton pumps that play key roles in adaptation to abiotic stress in plants. Function of apple V-ATPase subunit B1 ( MdVHA-B1 ) has been identified in drought tolerance, however, it remains unclear whether MdVHA-B1 is involved in salt tolerance. In this work, we characterized the function of MdVHA-B1 gene in salt stress response. GUS and Western blotting assays showed that MdVHA-B1 was highly induced by salt stress. Yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays demonstrated that MdVHA-B1 interacts with MdSOS2L1, a Arabidopsis SOS2-like protein kinase, in apple. Compared with wild-type (WT) apple calli, V-ATPase activities in the transgenic calli were significantly enhanced under salt stress condition. Furthermore, Na + and K + were accumulated much higher in transgenic tomato plants than the WT control. These results suggest that MdVHA-B1 may confer salt tolerance through salt overly sensitive (SOS) signal pathway in apple to regulate the ion balance. [ABSTRACT FROM AUTHOR]