Med8, a subunit of mediator complex, has proved to possess crucial functions in many organisms from yeast to human. Med8 protein is a regulator that specifically binds to upstream activating sequences (UASs) of SUC2 promoter, downstream repressing sequences (DRSs) of the HXK2 gene, and to the carboxy-terminal domain of the RNA polymerase II. In the plant, the med8 mutant of Arabidopsis thaliana displayed delayed anthesis and an increased number of leaves during the vegetative period. A loss-of-function mutation in the Mediator complex subunit 8 (MED8) causes small flowers as a result of reduced cell expansion. Analysis of the med8 med25-2 double mutant reveals that MED8 acts independently of MED25 to regulate cell expansion and organ growth. The MED8 subunit of the A. thaliana Mediator regulates plant immunity to B. cinerea through interacting with the transcription factor FAMA, which was discovered to be a key component in B. cinerea resistance.