The single RRM domain-containing protein SARP1 is required for establishment of the separation zone in Arabidopsis

Abscission is the shedding of plant organs in response to developmental and environmental cues. Abscission involves cell separation between two neighboring cell types, residuum cells (RECs) and secession cells (SECs) in the floral abscission zone (AZ) in Arabidopsis thaliana. However, the regulatory mechanisms behind the spatial determination that governs cell separation are largely unknown. The class I KNOTTED-like homeobox (KNOX) transcription factor BREVIPEDICELLUS (BP) negatively regulates AZ cell size and number in Arabidopsis. To identify new players participating in abscission, we performed a genetic screen by activation tagging a weak complementation line of bp-3. We identified the mutant ebp1 (enhancer of BP1) displaying delayed floral organ abscission. The ebp1 mutant showed a concaved surface in SECs and abnormally stacked cells on the top of RECs, in contrast to the precisely separated surface in the wild-type. Molecular and histological analyses revealed that the transcriptional programming during cell differentiation in the AZ is compromised in ebp1. The SECs of ebp1 have acquired REC-like properties, including cuticle formation and superoxide production. We show that SEPARATION AFFECTING RNA-BINDING PROTEIN1 (SARP1) is upregulated in ebp1 and plays a role in the establishment of the cell separation layer during floral organ abscission in Arabidopsis.