The BLADE-ON-PETIOLE genes are essential for abscission zone formation in Arabidopsis

The Arabidopsis BLADE-ON-PETIOLE 1 (BOP1) and BOP2 genes encode redundant transcription factors that promote morphologicalasymmetry during leaf and floral development. Loss-of-function bop1 bop2 mutants display a range of developmental defects,including a loss of floral organ abscission. Abscission occurs along specialised cell files, called abscission zones (AZs) that develop atthe junction between the leaving organ and main plant body. We have characterized the bop1 bop2 abscission phenotype todetermine how BOP1 and BOP2 contribute to the known abscission developmental framework. Histological analysis and petalbreakstrength measurements of bop1 bop2 flowers show no differentiation of floral AZs. Furthermore, vestigial cauline leaf AZsare also undifferentiated in bop1 bop2 mutants, suggesting that BOP proteins are essential to establish AZ cells in different tissues.In support of this hypothesis, BOP1/BOP2 activity is required for both premature floral organ abscission and the ectopic abscission ofcauline leaves promoted by the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene under the control of the constitutive CaMV35S promoter. Expression of several abscission-related marker genes, including IDA, is relatively unperturbed in bop1 bop2 mutants,indicating that these AZ genes respond to positional cues that are independent of BOP1/BOP2 activity. We also show that BOP1 andBOP2 promote growth of nectary glands, which normally develop at the receptacle adjacent to developing AZs. Taken together,these data suggest that BOP1/BOP2 activity is required for multiple cell differentiation events in the proximal regions ofinflorescence lateral organs