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RAC protein, a member of Rho superfamily, exists in the signal transduction pathway of cell and plays an important role in many filamentous fungi.Trichoderma harzianum, which shows strong inhibition upon phytopathogen in many ways,is a typical fungus that has an effective function of biocontrol..Since thesignal transduction genes play a crucial role in the pathway of biocontrol fungi when they inhibit the phytopathogen, researches of the factors in the signal transduction pathway are contributed to reveal the biocontrol mechanism.This study aimed to analyze the function in the process of biocontrol in Trichoderma harzianum T88 by the method ofoverexpressing rac gene.To construct the overexpression plasmid, rac gene and expression vector pCAMBIA1301 were both digested by Nco Ⅰ and BstE Ⅱ, and then connected by T4 DNA ligase to build pCAMBIA1301:rac, which was subsequently transported into Agrobactrium tumefaciens strains GV3101 by electrotransformation.LB medium containing 50μg/ml kanamycin was used to screen the positive cloning.Through the colony PCR, A.tumefaciens transformants were proved to be successfully constructed.Then the overexpression plasmid was transformed into the wild type T88 by Agrobactrium tumefaciens-mediated transformation of T.harzianum, and the transformants were screened using PDA media containing Hyg 50μg/ml.T.harzianum transformant with the rac gene overexpression wassuccessfully obtained after screening and molecular identification.In further research, optical microscope and AFM will be used to observe the morphological differences between the transformation and the wild type T88.Besides, activities ofenzymes such as cellulosic hydrolysis enzyme and glucanase will be measured to reveal the influence ofenzyme secretion in the condition of rac gene overexpression.