Construction of antifreeze gene CBF2 expression vector and transformation into alfalfa callus Cloning of a 4CL gene fragment and construction of a RNAi expression vector of Pennisetum purpureum Cloning and Characterization of a GDP Dissociation Inhibitor Protein Gene from Medicago sativa L. and Expression in Tobacco Transformation of 741 Poplar with Double Bt Genes and the Insect-Resistance of the Transgenic Plant