Genome Wide Transcriptomic Analysis of <i>WRKY</i>Gene Family Response to Biotic Stresses in <i>Malus</i>×<i>domestica</i>

Apple (Malus × domestica Borkh.) is a perennial woody plant that often suffers from various biological stresses. Many harmful pathogens can infect apple trees and lead to reduced production. We comprehensively identified the WRKY genes in the apple genome and analyzed their expression in response to several biological stressors, including Alternaria alternata, Pythium ultimum, Botryosphaeria dothidea, Erwinia amylovora, Penicillium expansum, Gymnosporangium yamadae, and Apple replant disease. There were 113 MdWRKYs identified in the apple genome. Twenty-two MdWRKYs were differentially expressed in response to at least five pathogens. Promoter sequence analysis showed that these genes carried many defense- and stress-responsive elements, such as MeJA-response elements, salicylic acid-response elements, and W-box elements, in their promoters. Transient expression assays showed that MdWRKY40a and MdWRKY54h played negative roles in defense against B. dothidea infection. WRKY40 and WRKY60 and the MdWKRY33s might play important roles in responding to pathogens and are conserved in some plants. These differentially expressed MdWRKYs might play key roles in the apple response to multiple pathogens.