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Developmental boundaries play an important role in coordinating the growth and patterning of lateral organs. In plants, specification of dorsiventrality is critical to leaf morphogenesis. Despite its central importance, the mechanism by which leaf primordia acquire adaxial versus abaxial cell fates to establish dorsiventrality remains a topic of much debate. Here, by combining time-lapse confocal imaging, cell lineage tracing and molecular genetic analyses, we demonstrate that a stable boundary between adaxial and abaxial cell fates is specified several plastochrons before primordium emergence when high auxin levels accumulate on a meristem prepattern formed by the AS2 and KAN1 transcription factors. This occurrence triggers a transient induction of ARF3 and an auxin transcriptional response in AS2-marked progenitors that distinguishes adaxial from abaxial identity. As the primordium emerges, dynamic shifts …