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 morphological
asymmetry 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 at
the junction between the leaving organ and main plant body. We have characterized the bop1 bop2 abscission phenotype to
determine how BOP1 and BOP2 contribute to the known abscission developmental framework. Histological analysis and petal
breakstrength measurements of bop1 bop2 flowers show no differentiation of floral AZs. Furthermore, vestigial cauline leaf AZs
are 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 of
cauline leaves promoted by the INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) gene under the control of the constitutive CaMV
35S 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 and
BOP2 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 of
inflorescence lateral organs.