Sequential protease deployment under acidic conditions degrades host defense proteins and drives Valsa mali...

Apple Valsa canker, caused by the ascomycete fungus Valsa mali, is a severe disease threatening apple (Malus domestica) production, particularly in East Asia. The pH at the infection site decreases from 6.0 to around 3.5, facilitating fungal pathogenicity. This study explores the role of the sedolisin protease (VmTPP3) in V. mali pathogenicity. VmTPP3 is a critical pathogenicity factor in V. mali, as its deletion significantly reduces fungal pathogenicity. Using yeast 2-hybrid (Y2H), bimolecular fluorescence complementation (BiFC), and co-immunoprecipitation (co-IP) assays, we demonstrated that VmTPP3 interacts with plant nonspecific lipid transfer proteins (MdnsLTP1). Furthermore, MdnsLTP1 overexpression in apple stabilizes the abscisic acid (ABA) receptor (MdPYL4), preventing its degradation by VmTPP3 and subtilase (VmSpm1), thereby enhancing disease resistance. Protease assays revealed optimal pH activities for VmTPP3 and VmSpm1 at 3.5 and 4.0, respectively, suggesting a sequential deployment of these effectors during infection. These findings provide insights into the pH-dependent strategies of V. mali and highlight potential targets for improving apple resistance against Valsa canker.