Advances in gene engineering of plant for cold resistance Overexpression of a Zinc-finger Protein Gene AmZFPG from Ammopiptanthus mongolicus Confers Tolerance to Cold , Drought and Salt Stress in Transgenic Tobacco Establishment of Genetic Transformation Regeneration System in Brassica juncea Overexpression of a Zinc-finger Protein Gene AmZFPG from Ammopiptanthus mongolicus Confers Tolerance to Cold , Drought and Salt Stress in Transgenic Tobacco