The key enzyme for transcription of protein-encoding genes in eukaryotes is RNA polymerase II (RNAPII). The recruitment of this enzyme during transcription initiation and its passage along the template during transcription elongation is regulated through the association and dissociation of several complexes. Elongator is a histone acetyl transferase complex, consisting of six subunits (ELP1–ELP6), that copurifies with the elongating RNAPII in yeast and humans. We demonstrate that point mutations in three Arabidopsis thaliana genes, encoding homologs of the yeast Elongator subunits ELP1 , ELP3 (histone acetyl transferase), and ELP4 are responsible for the phenotypes of the elongata2 ( elo2 ), elo3 , and elo1 mutants, respectively. The elo mutants are characterized by narrow leaves and reduced root growth that results from a decreased cell division rate. Morphological and molecular phenotypes show that the ELONGATA ( ELO ) genes function in the same biological process and the epistatic interactions between the ELO genes can be explained by the model of complex formation in yeast. Furthermore, the plant Elongator complex is genetically positioned in the process of RNAPII-mediated transcription downstream of Mediator. Our data indicate that the Elongator complex is evolutionarily conserved in structure and function but reveal that the mechanism by which it stimulates cell proliferation is different in yeast and plants.