The Apple Receptor-Like Kinase MdSRLK3 Positively Regulates Resistance Against Pathogenic Fungus <i>Valsa mali</i> by Affecting the Ca²⁺ Signaling Pathway

Abstract

<i>Valsa mali</i> 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 <i>V. mali</i> 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 <i>MdSRLK3</i> in apple branches reached its highest level at 24 h post <i>V. mali</i> inoculation, which was 13.42 times higher than without inoculation. Transient overexpression of <i>MdSRLK3</i> enhanced apple resistance against <i>V. mali</i>, while transient silencing of <i>MdSRLK3</i> reduced its resistance against the pathogen. More importantly, stable silencing of <i>MdSRLK3</i> resulted in reduced resistance against this fungus. Furthermore, we demonstrated that <i>MdSRLK3</i> positively regulated apple resistance by affecting the Ca²⁺ signaling pathway, and the regulation was also related to the H₂O₂ and callose signaling pathways. Overall, our data reveal that <i>MdSRLK3</i> is a positive regulator of apple immunity.