BRUTUS ‐ LIKE1 ( BTSL1 ) and BTSL2 repress abscisic acid responses during early seedling growth by targeting ABI5 for degradation

SUMMARY Abscisic acid (ABA) signaling plays a critical role in plant growth and responses to abiotic stresses. The transcription factor ABA INSENSITIVE 5 (ABI5) is a key regulator of ABA signaling in Arabidopsis thaliana , and its protein homeostasis is essential for controlling seed germination and early seedling development. In this study, we identified BRUTUS‐LIKE1 (BTSL1) and BTSL2, two RING‐type E3 ubiquitin ligases, as essential regulators of ABA‐inhibited cotyledon greening during early seedling development. Biochemical analyses revealed that BTSL1 and BTSL2 target ABI5 for ubiquitination and degradation. Genetic epistasis analysis demonstrated that ABI5 functions downstream of BTSL1/2, as the ABA‐sensitive phenotype of btsl1 btsl2 mutants depends on ABI5. Consistently, loss‐of‐function mutations in BTSL1 and BTSL2 resulted in enhanced ABI5 activity in planta . Collectively, our findings uncover a regulatory mechanism by which BTSL1 and BTSL2 modulate early seedling growth by controlling ABI5 protein stability.