Heterologous expression of vacuolar H+-PPase enhances the electrochemical gradient across the vacuolar membrane and improves tobacco cell salt tolerance

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Abstract
The vacuolar H+-translocating inorganic pyrophosphatase (H+-PPase) uses pyrophosphate as substrate to generate the proton electrochemical gradient across the vacuolar membrane to acidify vacuoles in plant cells. The heterologous expression of H+-PPase genes (TsVP from Thellungiella halophila and AVP1 from Arabidopsis thaliana) improved the salt tolerance of tobacco plants. Under salt stress, the transgenic seedlings showed much better growth and greater fresh weight than wild-type plants, and their protoplasts had a normal appearance and greater vigor. The cytoplasmic and vacuolar pH in transgenic and wild-type cells were measured with a pH-sensitive fluorescence indicator. The results showed that heterologous expression of H+-PPase produced an enhanced proton electrochemical gradient across the vacuolar membrane, which accelerated the sequestration of sodium ions into the vacuole. More Na+ accumulated in the vacuoles of transgenic cells under salt (NaCl) stress, revealed by staining with the fluorescent indicator Sodium Green. It was concluded that the tonoplast-resident H+-PPase plays important roles in the maintenance of the proton gradient across the vacuolar membrane and the compartmentation of Na+ within vacuoles, and heterologous expression of this protein enhanced the electrochemical gradient across the vacuolar membrane, thereby improving the salt tolerance of tobacco cells.