Apple WRKY transcription factor MdWRKY56 positively modulates drought stress tolerance

Drought seriously threatens the growth, development, and yield of crops. WRKY transcription factors (TFs) play important roles in abiotic stress responses. However, the study of WRKY TFs in woody plant apple (Malus domestica) in terms of drought tolerance regulation is limited. This study aimed to functionally characterize an apple group IIc WRKY gene MdWRKY56. The transcription abundance of MdWRKY56 was significantly upregulated under drought stress. The subcellular localization analysis revealed that the MdWRKY56 protein localized to the nucleus. Transcriptional activation experiments proved that MdWRKY56 had transcriptional activity in yeast, and its N-terminal played a decisive role in this activity. The functional analysis using MdWRKY56 transgenic Arabidopsis, apple calli, and apple plants indicated that the MdWRKY56 TF functioned positively in the modulation of osmotic and drought stress tolerance, with the MdWRKY56-overexpressed transgenic materials showing lower electrolyte leakage, malondialdehyde (MDA) content, and reactive oxygen species (ROS) accumulation, and higher fresh weight, proline content, and antioxidant enzyme activities. In conclusion, our findings indicated that MdWRKY56 served as an important positive regulator of drought stress tolerance in apple.