<scp>MT</scp> @ <scp> SiO ₂ </scp> Enhances <scp>MEK5</scp> ‐ <scp>MAPK6</scp> ‐ <scp>NAC32</scp> Mediated Salicylic Acid Synthesis Which Increases Resistance to <i>Glomerella</i> Leaf Spot in Apple

Glomerella leaf spot (GLS), a fungal disease caused by Colletotrichum fructicola, is a major destructive disease of apples but research on control measures is limited. Melatonin (MT) is a phytohormone-like compound that affects plant growth and stress response but is prone to light-induced degradation, resulting in low stability and efficacy. Therefore, we developed a melatonin silicon-based nanomaterial (MT@SiO₂) to enhance the stability of melatonin and increase its potential use on plants. Our results indicated that MT@SiO₂ significantly enhanced apple leaf resistance to GLS. We demonstrated that MT@SiO₂ at an optimal concentration of 50 μM significantly mitigated GLS infection in 'Gala' apples by elevating the level of salicylic acid. The core transcription factor gene MdNAC32 was identified in our transcriptome analysis and found to respond to both GLS infection and MT@SiO₂ treatment. MdNAC32 directly activates the transcription of MdPBS1/2 which promotes the synthesis of SA. Transient overexpression and silencing experiments demonstrated that MdPBS1/2 positively regulates GLS resistance. In addition, we found that the MEK5-MAPK6 module can phosphorylate MdNAC32, which regulates MdPBS1/2 expression. Overall, our results indicate that MT@SiO₂ enhances the activity of the MEK5-MAPK6-NAC32-MdPBS1/2 module by inducing SA accumulation, resulting in enhanced resistance in apples to GLS. The use of the melatonin-based nanomaterial improved the efficacy of MT and highlights the potential use of conjugated nanomaterials to modulate disease resistance in apples. Our study also provides new insights into the involvement of NAC and MAPK pathways in plant defense response to microbial pathogens.