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 in-dustry 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 beta-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.