įive mutant alleles of ASYMMETRIC LEAVES1 ( AS1), a key determinant of polarity and cell fate in lateral organs, were identified through a screen for floral organ shedding mutants (Additional file 1: Table S1). HAWAIIAN SKIRT (HWS), an F-box protein, prevents the fusion of adjacent sepals and influences the timing of their abscission. The BELL-type homeodomain transcription factor, ARABIDOPSIS THALIANA HOMEOBOX GENE1 (ATH1), represses growth in regions that will become the floral organ-receptacle boundaries and is required for stamen abscission zone formation. Anatomical evidence of abscission zone differentiation cannot be detected in bop1 bop2 flowers and the sepals, petals and stamens remain strongly attached. The transcriptional activators BLADE-ON-PETIOLE1 (BOP1) and BOP2 redundantly control patterning of the proximal regions of developing leaves and floral organs. Several genes that regulate lateral organ and/or inter-organ boundaries also affect floral organ abscission. Less is known about the regulatory circuits that control abscission zone differentiation at the boundaries between the outer floral organs and underlying receptacle. Genetic analysis in Arabidopsis has been particularly productive in revealing factors that activate the cell separation phase of floral organ abscission. The process of organ abscission allows plants to detach leaves, floral organs, fruit and seeds at specific points in their life cycles or in response to environmental cues. Our results indicate that AS1 activity is critical for the proper placement of the floral organ abscission zones, and influences the timing of organ shedding. We show here that loss of BP activity in as1 flowers is sufficient to restore the positions of the sepal and petal abscission zones, the sepal-receptacle boundary of the medial sepals and the timing of their abscission. AS1 has previously been shown to restrict expression of the KNOX gene, BREVIPEDICELLUS ( BP), from the sepals. Abscission of the medial sepals is delayed in as1 flowers loss of chlorophyll in the senescing sepals contrasts with proximal zones that remain green. Movement of the petal abscission zones is also apparent. In as1 mutant flowers, the sepal abscission zones are displaced into inverted V-shaped positions, leaving behind triangular stubs of tissue when the organs abscise. Here we show a unique role for AS1 in establishing the positions of the sepal and petal abscission zones in Arabidopsis flowers. By repressing expression of a set of KNOTTED1-LIKE HOMEODOMAIN ( KNOX) genes from developing leaf primordia, AS1 and its partner ASYMMETRIC LEAVES2 allow the patterning and differentiation of leaves to proceed. The ASYMMETRIC LEAVES1 (AS1) MYB transcription factor plays a critical role in setting boundaries between newly formed leaf primordia and the shoot meristem. The sepals, petals and stamens of Arabidopsis flowers detach via abscission zones formed at their boundaries with the underlying receptacle.