VmSpm1: a secretory protein from Valsa mali that targets apple's abscisic acid receptor MdPYL4 to suppress jasmonic acid signaling and enhance infection

Pathogenic fungi such as Valsa mali secrete effector proteins to manipulate host defenses and facilitate infection. Subtilases are identified as potential virulence factors, yet their specific roles in fruit tree pathogens, such as those affecting apple trees, are poorly understood. Our research shows VmSpm1 as a virulence factor in V. mali. Knocking it out decreased virulence, whereas its heterologous expression in apple led to reduced disease resistance. Using Y2H, BiFC, SLC, and Co-IP techniques, we demonstrated an interaction between VmSpm1 and MdPYL4. MdPYL4 levels increased during V. mali infection. The stable transgenic apple lines inoculation experiment showed that MdPYL4 correlates with enhanced resistance to Apple Valsa canker when overexpressed in apples. Furthermore, through in vitro and in vivo assays, we showed the degradative role of VmSpm1 on MdPYL4. MdPYL4 promotes the synthesis of jasmonic acid (JA) in apples in an abscisic acid-dependent manner. The degradation of MdPYL4 leads to a reduction in JA content in apples during V. mali infection, thereby impairing JA signal transduction and decreasing disease resistance in apple plants. In summary, this study reveals how V. mali utilizes VmSpm1 to subvert JA signaling, shedding light on fungal manipulation of plant hormones to disrupt immunity.