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The Transcription Factor MdMYB2 Influences Cold Tolerance and Anthocyanin Accumulation by Activating SUMO E3 Ligase MdSIZ1 in Apple.

Conjugation of the small ubiquitin-like modifier (SUMO) peptide to target proteins is an important post-translational modification. SAP AND MIZ1 DOMAIN- CONTAINING LIGASE1 (MdSIZ1) is an apple (Malus domestica Borkh.) SUMO E3 ligase that mediates sumoylation of its targets during plant growth and development under adverse environmental conditions. However, it is unclear how MdSIZ1 senses the various environmental signals and whether sumoylation is regulated at the transcriptional level. In this study, we analyzed the MdSIZ1 promoter and found that it contained an MBS (MYB Binding Site) motif that was essential for the response of MdSIZ1 to low temperature and drought. Subsequently, we used yeast one-hybridization screening to demonstrate that a MYB transcription factor, MdMYB2, directly bound to the MBS motif in the MdSIZ1 promoter. Phenotypic characterization of MdMYB2 and MdSIZ1 suggested that the expression of both MdMYB2 and MdSIZ1 substantially improved cold tolerance in plants. MdMYB2 was induced by low temperature and further activated the expression of MdSIZ1, thereby promoting the sumoylation of MdMYB1, a key regulator of anthocyanin biosynthesis in apple. MdMYB2 promoted anthocyanin accumulation in apple fruits, apple calli, and Arabidopsis (Arabidopsis thaliana) in an MdSIZ1-dependent manner. In addition, the interaction of MdMYB2 and the MdSIZ1 promoter substantially improved plant tolerance to cold stress. Taken together, our findings reveal an important role for transcriptional regulation of sumoylation and provide insights into plant anthocyanin biosynthesis regulation mechanisms and stress response.