Cytospora canker, caused by Cytospora mali, is the most destructive disease in production of apples (Malus domestica). Adding potassium (K) to apple trees can effectively control this disease. However, the underlying mechanisms of apple resistance to C. mali under high-K (HK) status remain unknown. Here, we found that HK (9.30 g/kg) apple tissues exhibited high disease resistance. The resistance was impeded when blocking K channels, leading to susceptibility even under HK conditions. We detected a suite of resistance events in HK apple tissues, including upregulation of resistance genes, callose deposition, and formation of ligno-suberized tissues. Further multiomics revealed that the phenylpropanoid pathway was reprogrammed by increasing K content from low-K (LK, 4.30 g/kg) status, leading to increases of 18 antifungal chemicals. Among them, the physiological concentration of coumarin (1,2-benzopyrone) became sufficient to inhibit C. mali growth in HK tissues, and exogenous application could improve the C. mali resistance of LK apple branches. Transgenic apple calli overexpressing beta-glucosidase 40 (MdBGLU40), which encodes the enzyme for coumarin synthesis, contained higher levels of coumarin and exhibited high resistance to C. mali even under LK conditions. Conversely, the suppression of MdBGLU40 through RNAi reduced coumarin content and resistance in HK apple calli, supporting the importance of coumarin accumulation in vivo for apple resistance. Moreover, we found that the upregulation of transcription factor MdMYB1r1 directly activated MdBGLU40 and the binding affinity of MdMYB1r1 to the MdBGLU40 promoter increased in HK apple tissue, leading to high levels of coumarin and resistance in HK apple. Overall, we found that the accumulation of defensive metabolites strengthened resistance in apple when raising K from insufficient to optimal status, and these results highlight the optimization of K content in fertilization practices as a disease management strategy.

Potassium (K) fertilization is crucial for plant resistance to pathogens, but the underlying mechanisms remain unclear. Here, we investigate the molecular mechanism by which the addition of K promotes resistance in Nicotiana benthamiana to Phytophthora parasitica. We found that N. benthamiana with high K content (HK, 52.3 g/kg) produced more ethylene in response to P. parasitica infection, compared to N. benthamiana with low-K content (LK, 22.4 g/kg). An exogenous ethylene application effectively increased resistance in LK N. benthamiana to the level under HK status, demonstrating the involvement of ethylene in the HK-associated resistance in N. benthamiana. Further, transcriptome analysis showed that NbSAMS1, encoding ethylene biosynthesis, was induced to upregulate P. parasitica about five times higher in HK than in LK N. benthamiana. NbSAMS1 overexpression enhanced resistance in LK plants, whereas NbSAMS1 silencing reduced resistance in HK plants, confirming its importance in conferring resistance. Furthermore, we identified a calcium-binding protein, NbCBP1, which interacted with NbSAMS1, promoting its expression in HK N. benthamiana. Silencing NbCBP1 compromised resistance in HK N. benthamiana, whereas its overexpression improved resistance in LK N. benthamiana. Notably, NbCBP1 protected NbSAMS1 from degradation by the 26S proteasome, thereby sustaining ethylene accumulation in HK N. benthamiana in response to P. parasitica infection. Thus, our research elucidated some mechanisms of the NbCBP1-NbSAMS1 module associated with disease resistance in HK N. benthamiana.

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.