To study transcriptional regulators, an in vivo functional assay is indispensable. Here we report a new convenient vector set for a transactivation assay in plants. The system consists of a luciferase reporter controlled by a synthetic promoter with GAL4-binding sites and an effector to express any fusion protein with the GAL4 DNA binding domain. Co-transfection of the two plasmids causes transactivation of the reporter by the expressed GAL4-effector fusion if the effector exhibits transcriptional activation activity. A multicloning site was introduced into the effector vector which should facilitate the construction of GAL4-effector fusions. In tobacco transient assay by microprojectile bombardment, a 27-fold transactivation for the GAL4-B17 fusion as an effector was demonstrated with this system.