Proposed working model illustrating how HLS1 SUMOylation regulates apical hook development. SIZ1 induces the SUMOylation of HLS1 in darkness, and SUMOylated HLS1 assembles into oligomers to regulate apical hook formation. During the dark-to-light transition, light induces the translocation of phyB from the cytoplasm to the nucleus, where phyB interacts with HLS1 to suppress HLS1 oligomerization. In addition, HY5 suppresses SIZ1 transcription to decrease HLS1 SUMOylation and oligomerization. Non-SUMOylated HLS1 displays a higher affinity for phyB, which also prevents HLS1 oligomerization. The decrease of oligomeric HLS1 leads to the deactivation of HLS1 and thus initiating apical hook unfolding.