Characterization and Functional Analysis of a Small GTP-binding Protein AtRAB Interacting with H+-Pyrophosphatase AVP1 in Arabidopsis thaliana Physiological Mechanisms of Enhanced Uptake of Potassium Nutrition in EdHP1 (H+-pyrophosphatase) Transgenic Tobacco (Nicotiana tabacum) Cloning of a Vacuolar H+-pyrophosphatase Gene from the Halophyte Suaeda corniculata whose Heterologous Overexpression Improves Salt, Saline-alkali and Drought Tolerance in Arabidopsis