The onset of cell cycle in mammalian systems is primarily controlled by E2F-like transcription factors. Recent evidence shows that plant E2F homologs and their associated proteins likely play similar roles in higher plant development. We studied the function of plant E2F in gene regulation and morphogenesis using transgenic Arabidopsis plants over-expressing AtE2Fa. Examination of rosettes showed that AtE2Fa over-expression resulted in increased expression of both cell cycle promoters and cell cycle inhibitors. The positive factors up-regulated by AtE2Fa emcompassed genes for G1/S transition, DNA synthesis and mitosis, and the negative factors up-regulated by AtE2Fa included RB1 , encoding the E2F binding protein, as well as KRP3 and KRP5 , encoding the plant CDK inhibitors. Moreover, AtE2Fa over-expression in rosettes led to elevated expression of ATPK19 , the homolog of the highly conserved S6 kinase that is known to enhance cell growth. The transgenic plants exhibited narrower rosette leaves when compared to wild-type control. Consistent with elevated expression of cell cycle inhibitors and ATPK19 , the mature rosette leaves displayed reduced cell number but increased cell size. These results demonstrate that AtE2Fa controls cell division and plant development by assuming a bimodal function in balancing the expression of both positive and negative regulators involved in cell division and growth.