Overexpression of Malus baccata WRKY63 Enhances Cold Tolerance by Increasing the Antioxidant Level Associated with ROS Scavenging

During their natural growth, plants encounter adverse environmental conditions, such as chilling injury, freezing injury, drought, and salt damage, collectively known as abiotic stresses. Several studies have shown that WRKY proteins regulate various abiotic stress responses and plant developmental processes. However, researchers have rarely investigated WRKY genes associated with the stress response in apples. Within this research, <i>Malus baccata</i> (L.) Borkh as the experimental material. We isolated and cloned <i>MbWRKY63</i> and investigated its function in low-temperature stress tolerance. Subcellular localization analysis shows that MbWRKY63 localizes to the cell nucleus. Tissue-specific expression analysis revealed that <i>MbWRKY63</i> is relatively highly expressed in the young leaves and root tissues of apples. Under low-temperature treatment at 4 °C, <i>Arabidopsis thaliana</i> plants that overexpressed <i>MbWRKY63</i> showed greater cold stress resistance than the wild type (WT) and the empty vector (UL) control. In transgenic plants, the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) were significantly enhanced; meanwhile, the contents of proline, malondialdehyde (MDA), and chlorophyll also changed significantly. In addition, by regulating the expression levels of <i>AtKIN1</i>, <i>AtCBF1</i>, <i>AtCBF2</i>, <i>AtCBF3</i>, <i>AtCOR47</i>, and <i>AtCOR15a</i>, <i>MbWRKY63</i> enhanced the low-temperature stress tolerance in transgenic <i>Arabidopsis</i>. The results suggest that <i>MbWRKY63</i> in apples may be involved in the response to low-temperature stress, laying a foundation for understanding the role of WRKY transcription factors (TFs) in abiotic stress responses.