The MdSLR1b-MdKNOX15-MdGA20ox2/MdGA2ox7 module regulates dwarfing by mediating the gibberellin pathway in apple

Dwarf and high-density cultivation is a major trend in the apple industry, yet the regulatory mechanisms remain largely unexplored. In this study, we identified MdKNOX15, a KNOX transcription factor highly expressed in spur-type and dwarfing rootstock varieties, as a critical regulator of plant height. Overexpression of MdKNOX15 caused a gibberellin (GA)-deficient dwarf phenotype, whereas VIGS-mediated knockdown increased endogenous GA content and plant height. Exogenous GA alleviated, and uniconazole exacerbated, the dwarfism. Using DAP-seq and RNA-seq, we identified MdGA2ox7 and MdGA20ox2 as direct target genes. MdKNOX15 activated MdGA2ox7 and repressed MdGA20ox2 by binding to (T)TGAC elements in their promoters. Functional assays confirmed that MdGA2ox7 negatively, and MdGA20ox2 positively, regulates apple height. Furthermore, the DELLA homolog MdSLR1b physically interacted with MdKNOX15. Overexpression of MdSLR1b reduced plant height, while its knockdown alleviated the dwarfism induced by MdKNOX15 overexpression. MdKNOX15 enhanced MdSLR1b protein stability, especially under GA treatment, and MdSLR1b amplified the transcriptional regulation of downstream genes by MdKNOX15. Collectively, we propose a molecular model in which the MdKNOX15-MdSLR1b-MdGA2ox7/MdGA20ox2 module governs apple plant height, providing a theoretical basis for breeding dwarf apple varieties.