Overexpression of the Malus hupehensis MhTGA2 Gene, a Novel bZIP Transcription Factor for Increased Tolerance to Salt and Osmotic Stress in Transgenic Tobacco

Basic leucine zipper (bZIP) proteins form one of the largest families of transcription factors, and their individual contributions in a particular regulatory network remain difficult to assess. Here, we identified and functionally characterized the Malus hupehensis bZIP transcription factor designated MhTGA2 from M. hupehensis leaves treated with salicylic acid (SA). The MhTGA2 protein was localized in the nucleus. The expression of MhTGA2 was higher in leaves than in stems and roots. This gene was significantly induced by SA, methyl jasmonate, and 1-aminocyclopropane-1-carboxylic acid and weakly induced by abscisic acid. The gene was clearly induced by low temperature (4°C) and NaCl and weakly induced by polyethylene glycol but not induced by apple ring spot pathogens. Overexpression of the MhTGA2 gene in transgenic tobacco plants conferred enhanced resistance to NaCl at the stage of seed germination and in seedlings, and it conferred resistance to mannitol at the stage of seed germination. Furthermore, overexpression of MhTGA2 led to higher expression levels of pathogenesis-related proteins and osmotic-stress-related genes in transgenic tobacco than in wild-type plants. These results suggest that the MhTGA2 transcription factor might be active in systemic acquired resistance and might function as a possible regulator of enhanced environmental-stress tolerance. MhTGA2 is thus a potentially constructive candidate gene for engineering salt and osmotic stress tolerance in cultivated plants.