Bud dormancy is a key adaptive strategy in perennial plants, enabling them to survive adverse environmental conditions. However, it poses challenges in crop cultivation, especially in fruit crops such as apple, in which synchronized bud break is crucial for consistent growth and yield. The synthetic cytokinin 6-benzylaminopurine (6-BA) promotes dormancy release; however, its molecular and metabolic mechanisms remain largely unclear. This study investigates dormancy release in nursery-grown Xianheng 01 apple rootstocks through integrated transcriptomic, metabolomic, and hormonal analyses. Dormant buds were treated with 6-BA, with morphological, biochemical, and molecular profiling performed over 30 d. 6-BA treatment increased plant height and leaf emergence by increasing the levels of cytokinins (DHZR, IPA) and decreasing the level of abscisic acid (ABA). Transcriptomics analysis identified 7,009 differentially expressed genes (DEGs) in response to 6-BA treatment. The cytokinin-responsive gene A-ARR8 exhibited a distinct expression pattern, being upregulated at 1, 3, and 6 d post-treatment but downregulated at 11 d. In contrast, ABA-related genes SnRK2a/b, PP2C, and ABF3, were consistently downregulated throughout the treatment period. Metabolomic analysis identified 2,053 metabolites, showing early-phase dominance of phenylpropanoids, and flavonoids, followed by a shift towards ABC transporter-mediated nutrient mobilization. Conjoint analysis highlighted the coordinated activation of secondary metabolite biosynthesis and cytokinin signaling. These results demonstrate that 6-BA induces dormancy release through cytokinin–ABA antagonism and phased metabolic reprogramming from stress defense to growth promotion. Our findings provide a comprehensive framework for optimizing dormancy management in apple cultivation and highlight 6-BA as an effective agrochemical for improving temperate fruit production.

Brassinosteroid is an important hormone that interacts with auxin and gibberellin to regulate plant growth and development. However, there is currently a relative lack of information regarding brassinosteroid in apple. Previous study confirmed that exogenous brassinosteroid treatments increased growth and endogenous brassinosteroid, auxin, as well as gibberellin levels of apple nursery trees. Here we succeeded to find that exogenous brassinosteroid treatments upregulated the transcript expression of auxin biosynthesis, transport, and positive signal transduction genes as well as gibberellin biosynthesis genes. In contrast, the application of exogenous brassinosteroid downregulated the expression of genes encoding negative regulators of auxin and gibberellin signal transductions. Rapid responses of several brassinosteroid-, auxin-, and gibberellin- related genes to the brassinosteroid and brassinazole treatments inferred the crosstalk of BR and IAA or GA. Furthermore, the expression levels of cell growth-related genes were also enhanced by exogenous brassinosteroid. Auxin and gibberellin treatments also influenced growth of apple tree and enhanced the expression of brassinosteroid- and cell growth-related genes. These results indicate that the growth of apple tree is regulated by the interaction between brassinosteroid, auxin, and gibberellin. Our work opened new avenues for deep portfolio of apple tree growth and laid the foundation for future studies aimed at elucidating the mechanisms regulating hormone interactions-mediated growth in apple trees.