The <scp>MdGLK1</scp> ‐ <scp>MdBZR1</scp> Module Integrates Ethylene and Strigolactone Signals to Regulate Cold Tolerance via a <scp>CBF</scp> ‐Dependent Pathway in Apple

Although ethylene and strigolactone (SL) are key regulators of cold tolerance in plants, the molecular crosstalk between their signalling pathways is poorly understood. Here, we identified the transcription factor GOLDEN2-LIKE1 (MdGLK1) as a central integrator of ethylene and SL signalling during the apple (Malus × domestica) cold stress response. MdGLK1 enhanced cold tolerance by recruiting BRASSINAZOLE-RESISTANT1 (MdBZR1), a core component of brassinosteroid signalling, thereby promoting MdBZR1-mediated transcriptional activation of the cold-responsive genes C-REPEAT BINDING FACTOR1 (MdCBF1) and MdCBF2. Ethylene signalling modulated this process through ETHYLENE INSENSITIVE3-BINDING F-BOX PROTEIN1 (MdEBF1), which targets MdGLK1 for proteasomal degradation, thereby attenuating MdGLK1-enhanced cold tolerance. In addition, SUPPRESSOR OF MORE AXILLARY GROWTH2-LIKE8 (MdSMXL8), a repressor of SL signalling, interfered with the formation of the MdGLK1-MdBZR1 complex, further suppressing MdGLK1-mediated cold tolerance. Collectively, the MdGLK1-MdBZR1-MdCBF1/2 regulatory module operates as a signalling hub that integrates the ethylene and SL pathways during cold stress adaptation in apple. These findings elucidate the molecular mechanism by which MdGLK1 coordinates ethylene and SL signals to enhance cold tolerance, thereby offering potential targets for optimising the trade-off between growth and stress tolerance in plants.