Melatonin confers tolerance to nitrogen deficiency through regulating MdHY5 in apple plants

SUMMARY Nitrogen (N) is an essential nutrient for crop growth and development, significantly influencing both yield and quality. Melatonin (MT), a known enhancer of abiotic stress tolerance, has been extensively studied. However, its relationship with nutrient stress, particularly N deficiency, and the underlying regulatory mechanisms of MT on N absorption remain unclear. In this study, exogenous MT treatment was found to improve the tolerance of apple plants to N deficiency. Apple plants overexpressing the MT biosynthetic gene N ‐acetylserotonin methyltransferase 9 ( MdASMT9 ) were used to further investigate the effects of endogenous MT on low‐N stress. Overexpression of MdASMT9 improved the light harvesting and heat transfer capability of apple plants, thereby mitigating the detrimental effects of N deficiency on the photosynthetic system. Proteomic and physiological data analyses indicated that MdASMT9 overexpression enhanced the trichloroacetic acid cycle and positively modulated amino acid metabolism to counteract N‐deficiency stress. Additionally, both exogenous and endogenous MT promoted the transcription of MdHY5 , which in turn bound to the MdNRT2.1 and MdNRT2.4 promoters and activated their expression. Notably, MT‐mediated promotion of MdNRT2.1 and MdNRT2.4 expression through regulating MdHY5 , ultimately enhancing N absorption. Taken together, these findings shed light on the association between MdASMT9 ‐mediated MT biosynthesis and N absorption in apple plants under N‐deficiency conditions.