The HD-Zip I transcription factor MdHB-7 negatively regulates Valsa canker resistance in apple (Malus domestica)

Valsa canker caused by Valsa mali, one of the major fungal diseases affecting the development of the apple industry in recent years, has led to serious reductions in the yield and quality of apples. HD-Zip transcription factors are a distinct class of transcription factors in higher plants involved in the response to biotic stresses, and they play key roles in plant immunity; however, they are understudied in apples. In this study, the expression of MdHB-7 was induced by V. mali infection. Compared with wild-type (WT) apple plants, transgenic apple plants overexpressing MdHB-7 (MdHB-7-OE) showed reduced V. mali resistance, whereas plants with the expression of MdHB-7 knocked down (MdHB-7-RNAi) showed high V. mali resistance. Upon V. mali infection, the salicylic acid (SA) content and the expression levels of SA-related genes were lower in MdHB-7-OE plants than in WT plants. The content of polyphenols and the activities of chitinase and β-1, 3 glucanase were lower in MdHB-7-OE plants than in WT plants. In addition, H2O2 accumulation was higher in MdHB-7-OE plants than in WT plants following V. mali infection. The opposite patterns were observed in MdHB-7-RNAi plants. Overall, MdHB-7 plays a negative role in regulating the defense of apple against V. mali by altering the content of SA and polyphenols, the activities of defense-related enzymes, and the content of H2O2.