Genome-wide investigation of defensin genes in apple (Malus×domestica Borkh.) and in vivo analyses show that MdDEF25 confers resistance to Fusarium solani

Apple replant disease is a complex soil syndrome that occurs when the same fields are repeatedly utilized for apple orchard cultivation. It can be caused by various pathogens, and Fusarium solani is the main pathogen. Fusarium solani disrupts the structure and function of the orchard soil ecosystem and inhibits the growth and development of apple trees, significantly impacting the quality and yield of apples. In this study, we conducted a transcriptome comparison between uninoculated apple saplings and those inoculated with F. solani. The differentially expressed genes were mainly enriched in processes such as response to symbiotic fungus. Plant defensins are antimicrobial peptides, but their roles during F. solani infection remain unclear. We performed a genome-wide identification of apple defensin genes and identified 25 genes with the conserved motif of eight cysteine residues. In wild-type apple rootstock inoculated with F. solani, the root surface cells experienced severe damage, and showed significant differences in the total root length, total root projection area, root tips, root forks, and total root surface area compared to the control group. qRT-PCR analysis revealed that MdDEF3 and MdDEF25 were triggered in response to F. solani infection in apples. Subcellular localization showed specific expression of the MdDEF3-YFP and MdDEF25-YFP proteins on the cell membrane. Overexpressing the MdDEF25-YFP fusion gene enhanced resistance against F. solani in apple, providing a new strategy for the future prevention and biological control of apple replant disease.