The balance between cellular proliferation and differentiation is a key aspect of development in multicellular organisms. Using high-resolution expression data from the Arabidopsis root, we identified a transcription factor, UPBEAT1 (UPB1), that regulates this balance. Genomewide expression profiling coupled with ChIP-chip analysis revealed that UPB1 directly regulates the expression of a set of peroxidases that modulate the balance of reactive oxygen species (ROS) between the zones of cell proliferation and the zone of cell elongation where differentiation begins. Disruption of UPB1 activity alters this ROS balance, leading to a delay in the onset of differentiation. Modulation of either ROS balance or peroxidase activity through chemical reagents affects the onset of differentiation in a manner consistent with the postulated UPB1 function. This pathway functions independently of auxin and cytokinin plant hormonal signaling. Comparison to ROS- regulated growth control in animals suggests that a similar mechanism is used in plants and animals.