Abstract: The strigolactone (SL) and gibberellin (GA) signals jointly regulate various aspects of plant growth and development. However, the molecular regulatory network underlying the crosstalk between the SL and GA signaling pathways remains poorly understood. In this study, we found that ELONGATED HYPOCOTYL 5 (MdHY5) serves as an integrator of SL and GA signals in apple (Malus × domestica), and the WUSCHEL-related homeobox (WOX) transcription factor MdWOX8 enhances anthocyanin biosynthesis by directly activating MdHY5 expression. Furthermore, the MdWOX8-MdHY5 module is a crucial hub linking SL and GA signals by interacting with the SL signaling repressor SUPPRESSOR OF MORE AXILLARY GROWTH 2-LIKE 8 (MdSMXL8) and the GA signaling repressor REPRESSOR-of-ga1-3-LIKE 3a (MdRGL3a). MdSMXL8 inhibits the activation of the MdHY5 promoter by MdWOX8, and MdRGL3a promotes the function of MdWOX8 by enhancing its transcriptional activation of MdHY5 and disrupting the MdSMXL8–MdWOX8 interaction. Moreover, the E3 ubiquitin ligase SEVEN IN ABSENTIA 1 (MdSINA1) negatively regulates MdWOX8 homeostasis by targeting it for ubiquitination and degradation in response to SL and GA signals. Overall, our results reveal a precise regulatory network that integrates SL and GA at the transcriptional and post-translational levels, with the MdWOX8-MdHY5 module at its core.

Key words: anthocyanin, apple, gibberellin, phytohormone crosstalk, strigolactone, transcriptional regulation, ubiquitination modification