MdKIM1-MdMYB306 enhances apple resistance to Cytospora mali via brassinosteroid biosynthesis under sufficient potassium status.

Potassium (K) supplementation enhances apple (Malus ×domestica) resistance to Cytospora mali, thereby reducing apple Cytospora canker occurrence in the field. Nevertheless, the molecular mechanisms underlying this K-enhanced resistance remain largely unelucidated. Here, we found that brassinosteroid (BR) biosynthesis was active in apple tissues under sufficient K (SK) conditions, resulting in an elevation of brassinolide (BL), the bioactive BR. Exogenous BL application under low K (LK) conditions restored apple resistance in both detached branches and field-grown trees, while the BR inhibitor brassinazole (BRZ) attenuated SK-dependent resistance. Notably, BR biosynthetic gene MdBR6ox2 was upregulated under SK conditions. Silencing of MdBR6ox2 reduced BL content and consequently compromised resistance to C. mali in SK apple calli. Furthermore, we identified that MdMYB306 was responsible for transactivating MdBR6ox2 expression via binding to the MBS cis-element in its promoter. Gene function analysis confirmed the positive role of MdMYB306 in regulating BR biosynthesis. Importantly, K supplementation facilitated the expression of MdKIM1, which interacted with MdMYB306, induced conformational changes in MdMYB306, and enhanced its transactivation capacity for MdBR6ox2. Genetic evidence confirmed the importance of the MdKIM1-MdMYB306 module in initiating BR biosynthesis and increasing resistance in apple under SK status. Collectively, our findings uncovered a MdKIM1-MdMYB306-MdBR6ox2 module that promotes BR accumulation under SK conditions, providing a promising strategy for managing Cytospora canker via combined application of K and BR.