Strigolactones inhibit axillary bud outgrowth through transcription factor MdSPL6-mediated regulation of ABA levels in apple

Lateral branching contributes to plant architecture and agricultural yield. In apple (Malus domestica), the dormancy of axillary buds constrains early branching and fruit production. Among various phytohormones, strigolactones (SLs) play a pivotal role regulating axillary bud outgrowth. This study investigates the inhibitory effects of SLs on apple axillary bud growth and elucidates the underlying molecular mechanisms. Treatment with the SL analog racemic-GR24 (rac-GR24) markedly inhibited the outgrowth of axillary buds. We discovered that the transcription factor in apple SQUAMOSA PROMOTER BINDING PROTEIN-LIKE 6 (MdSPL6) interacts with SUPPRESSOR OF MAX2-LIKE 7 (MdSMXL7), a key component of SL signaling, to regulate branching patterns. The Mdspl6 mutant exhibited enhanced lateral branching, confirming the role of MdSPL6 as a suppressor of bud growth. Through DNA affinity purification sequencing (DAP-seq), we identified apple HOMEOBOX PROTEIN53 (MdHB53) and TEOSINTE BRANCHED1, CYCLOIDEA, PCF18 (MdTCP18) as downstream target genes of MdSPL6. Overexpression of these genes led to elevated levels of abscisic acid (ABA), implicating the function of ABA in the SL-mediated inhibition of bud outgrowth. Our results demonstrate that SLs regulate apple axillary bud growth through MdSPL6 and its downstream targets by modulating ABA levels, offering insights into the genetic control of plant architecture and identifying potential targets for breeding apple varieties with optimized branching and enhanced yield.