Valsa mali secretes an effector protein VmEP1 to target a K homology domain‐containing protein for virulence in apple

The K homology (KH) repeat is an RNA‐binding motif that exists in various proteins, some of which participate in plant growth. However, the function of KH domain‐containing proteins in plant defence is still unclear. In this study, we found that a KH domain‐containing protein in apple (Malus domestica), HEN4‐like (MdKRBP4), is involved in the plant immune response. Silencing of MdKRBP4 compromised reactive oxygen species (ROS) production and enhanced the susceptibility of apple to Valsa mali, whereas transient overexpression of MdKRBP4 stimulated ROS accumulation in apple leaves, indicating that MdKRBP4 is a positive immune regulator. Additionally, MdKRBP4 was proven to interact with the VmEP1 effector secreted by V. mali, which led to decreased accumulation of MdKRBP4. Coexpression of MdKRBP4 with VmEP1 inhibited cell death and ROS production induced by MdKRBP4 in Nicotiana benthamiana. These results indicate that MdKRBP4 functions as a novel positive regulatory factor in plant immunity in M. domestica and is a virulence target of the V. mali effector VmEP1.