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

Area of Science:

• Plant Pathology
• Molecular Plant-Microbe Interactions
• Biochemistry

Background:

• Apple Valsa canker, caused by Valsa mali, is a destructive disease leading to significant economic losses.
• Pathogens secrete effector proteins to manipulate host immunity, but unconventional secretory effectors remain understudied.
• Understanding these effectors is crucial for developing novel disease control strategies.

Purpose of the Study:

• To investigate the role of unconventional secreted protein 1 (VmUSP1) from Valsa mali in apple canker virulence.
• To elucidate the mechanism by which VmUSP1 suppresses host plant immunity.
• To identify the host target of VmUSP1.

Main Methods:

• Characterization of VmUSP1 secretion pathway (lacks signal peptide, exhibits unconventional secretion).
• Functional assays to assess VmUSP1's role in inhibiting cell death and promoting virulence.
• Yeast two-hybrid, bimolecular fluorescence, and co-immunoprecipitation assays to identify VmUSP1's host target.

Main Results:

• VmUSP1 inhibits BAX- and INF1-induced cell death and is essential for Valsa mali virulence.
• VmUSP1 targets apple heat shock protein 70 (MdHSP70), a key component of plant defense.
• VmUSP1 disrupts MdHSP70-mediated resistance, which involves reactive oxygen species and callose accumulation.

Conclusions:

• Unconventionally secreted effectors, like VmUSP1, represent a significant virulence strategy for plant pathogens.
• VmUSP1 suppresses apple immunity by interfering with MdHSP70-mediated defense responses.
• These findings reveal a novel mechanism of pathogen virulence and offer potential targets for disease management.