The Apple Receptor-Like Kinase MdSRLK3 Positively Regulates Resistance Against Pathogenic Fungus Valsa mali by Affecting the Ca 2+ Signaling Pathway.

is the main pathogenic fungus that causes the apple Valsa canker, a destructive disease severely threatening apple production in the world. However, the underlying key components involved in resistance against in apple trees remain largely unexplored. Here, we isolated and functionally characterized a G-type lectin S-receptor-like protein kinase MdSRLK3 from the cultivar Royal Gala derivative line GL-3. qRT-PCR showed that the relative expression of in apple branches reached its highest level at 24 h post inoculation, which was 13.42 times higher than without inoculation. Transient overexpression of enhanced apple resistance against , while transient silencing of reduced its resistance against the pathogen. More importantly, stable silencing of resulted in reduced resistance against this fungus. Furthermore, we demonstrated that positively regulated apple resistance by affecting the Ca signaling pathway, and the regulation was also related to the HO and callose signaling pathways. Overall, our data reveal that is a positive regulator of apple immunity.