Receptor‐like cytoplasmic kinase MdPBL34 phosphorylates melatonin biosynthetic enzyme MdSNAT5 to trigger disease resistance to apple Alternaria blotch

Summary Melatonin is known to play a pivotal role in plant immunity, but the regulation of melatonin production during pathogen infection is largely unknown. The regulatory role of receptor‐like cytoplasmic kinase MdPBL34 in regulating melatonin synthetic enzyme MdSNAT5 and its interaction with partner MdGRF3 was explored by biochemical analyses. The stable transgenic apple ( Malus domestica Borkh.) lines with overexpressed and RNA‐interfered MdSNAT5 , as well as transiently transformed apple leaves with overexpressed MdGRF3 or RNA‐interfered MdPBL34 , were generated. MdSNAT5 phosphorylation by MdPBL34 and its stabilization by MdGRF3 by reducing ubiquitination was examined. We revealed Alternaria alternata f. sp. Mali infection promoted melatonin production, which further enhanced apoplastic reactive oxygen species (ROS) burst. Overexpression of MdSNAT5 conferred transgenic apple plants enhanced Alternaria blotch resistance. The pathogen attack activates MdPBL34 kinase, which phosphorylates MdSNAT5 specifically at Ser148. Phosphorylation significantly enhances MdSNAT5 enzyme activity and facilitates its interaction with MdGRF3. MdGRF3 stabilizes MdSNAT5 by decreasing its ubiquitin‐mediated degradation for increased melatonin levels. The role of the MdPBL34‐MdSNAT5‐MdGRF3 pathway was confirmed by employing a specific phospho‐Ser148 antibody across several tolerant and susceptible apple cultivars. Our findings offer valuable insights and practical strategies for breeding disease‐resistant apple cultivars and developing melatonin‐based sustainable disease management approaches.