Conservation of the BZR-IDA/IDL Regulatory Module in Organ Abscission between Litchi and Arabidopsis

The IDA/IDL (INFLORESCENCE DEFICIENT IN ABSCISSION or like) peptides are conserved core components of the plant organ abscission machinery. However, the transcriptional networks governing IDA/IDL expression and their upstream regulators remain poorly understood. We previously identified LcIDL1 as a potential key positive regulator of litchi (Litchi chinensis Sonn.) fruitlet abscission. In the present study, yeast one-hybrid screening revealed that LcIDL1 promoter is directly bound by the transcription factor BRASSINAZOLE RESISTANT2 (LcBZR2), an essential constituent of the brassinosteroid (BR) signaling pathway functioning as a negative regulator of fruitlet abscission. Furthermore, we further demonstrated that LcBZR2 exerts trancriptional repression on LcIDL1 by specifically binding to its promoter. Consistently, proLcIDL1:GUS signals in the abscission zone (AZ) were suppressed in a 35S:LcBZR2 background. To test the functional conservation of this regulatory module, we examined Arabidopsis and found that the BR-insensitive mutant bri1 exhibits significantly accelerated floral organ abscission, elevated AtIDA expression in the floral AZ, and altered AZ cellular morphology. Moreover, AtBZR1 was also shown to suppress AtIDA transcription by specifically binding to its promoter. Collectively, these findings reveal functionally conserved BZR-IDA/IDL regulatory module in both litchi and Arabidopsis, which is critical for mediating organ abscission.