Overexpression of a Malus baccata (L.) Borkh WRKY Factor Gene MbWRKY33 Increased High Salinity Stress Tolerance in Arabidopsis thaliana

The WRKY transcription factor family is a significant family of plant transcription factors (TFs). Plant growth and development are often influenced by abiotic factors, such as salinity and low temperature. Numerous studies have demonstrated that WRKY TFs primarily influence plant responses to adversity. However, there are few studies on the role of WRKY genes in the stress responses of <i>Malus baccata</i> (L.) Borkh. We cloned the <i>MbWRKY33</i> gene from <i>Malus baccata</i> for this research, and its roles in salt stress tolerance were analyzed. Phylogenetic tree analysis revealed that MbWRKY33 and PbWRKY33 have the highest homology. Subcellular localization revealed that MbWRKY33 was located within the nucleus. An analysis of tissue-specific expression showed that <i>MbWRKY33</i> had relatively high expression levels in young leaves and roots. Moreover, <i>Arabidopsis thaliana</i> plants overexpressing <i>MbWRKY33</i> exhibited stronger resistance to salt stress compared with the wild type (WT) and the unloaded line empty vector (UL). Under the treatment of 200 mM NaCl, transgenic <i>Arabidopsis thaliana</i> plants exhibited significantly higher activities of antioxidant enzymes like superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) than the control. In contrast, the WT and the UL lines had elevated levels of malondialdehyde (MDA) and reactive oxygen species (ROS). In addition, <i>MbWRKY33</i> elevates transgenic plant resistance to salt stress by regulating the expression levels of <i>AtNHX1, AtSOS1</i>, <i>AtSOS3, AtNCED3, AtSnRK2,</i> and <i>AtRD29a</i>. Results indicated that <i>MbWRKY33</i> in <i>Malus</i> might be linked to high-salinity stress responses, laying a foundation for understanding WRKY TFs' reaction to such stress.