An Unconventionally Secreted Effector, VmUSP1, Targets Apple Heat Shock Protein 70 to Promote Infection

Apple Valsa canker, caused by <i>Valsa mali</i>/<i>Cytospora mali</i>, is a highly destructive disease that leads to significant economic losses annually. Pathogens manipulate host immunity to facilitate colonization and infection through the secretion of effector proteins, which are typically identified based on the presence of signal peptides. However, unconventional secretory effector proteins have been neglected, and little is known about their protogenetic roles in virulence. In this study, we demonstrate that an unconventional secreted protein 1 (VmUSP1) not only inhibits BAX- and INF1-induced cell death but also plays a crucial role in the complete virulence of <i>V. mali</i>. Furthermore, VmUSP1 lacks a typical signal peptide and exhibits characteristics of unconventional secretion. Through yeast two-hybrid, bimolecular fluorescence, and co-immunoprecipitation assays, we confirmed that VmUSP1 targets an apple (<i>Malus × domestica</i>) heat shock protein 70 (MdHSP70). MdHSP70 induces the accumulation of reactive oxygen species and callose while significantly enhancing plant resistance against pathogens. Additionally, VmUSP1 greatly compromises the MdHSP70-mediated resistance of apple against <i>V. mali</i>. Overall, these findings elucidate a mechanism by which an unconventionally secreted effector from <i>V. mali</i> suppresses host resistance by interfering with MdHSP70-mediated immune responses.