MdPR4, a pathogenesis-related protein in apple, is involved in chitin recognition and resistance response to apple replant disease pathogens.

To maximize breeding and exploitation of disease resistance traits for managing apple replant disease (ARD), it is of great importance to understand the mechanisms of apple root resistance. Currently, little is known about the functions of the specific genes that confer resistance traits in apple root. In this study, molecular, biochemical, and genetic approaches allowed an in-depth understanding of the role of the MdPR4 gene in the defense response of apple root. The MdPR4 encoding gene showed upregulation following ARD pathogen inoculation in our previous transcriptome data. Subcellular localization analyses revealed that MdPR4 is localized on the plasma membrane, endoplasmic reticulum, and apoplast, which is mainly determined by its signal peptide. Molecular docking analysis between MdPR4 protein with chitin molecule and in vitro MdPR4 chitin affinity assay proved its chitin-binding ability, which provided evidence for its role in chitin-mediated immune responses. Purified MdPR4 protein and MdPR4 overexpressed apple callus inhibited spore germination and mycelial growth of ARD-related Fusarium spp. pathogens. These data support the conclusion that MdPR4 is a chitin-binding protein in apple vegetative tissues that may play an important role in defense activation in response to ARD pathogen infection.