UHPLC-MS/MS, transcriptomic, and genomic analyses unravel the molecular mechanism on melatonin biosynthesis in...

Melatonin, a neurohormone and an antioxidant agent, has been identified in plant-derived foods, such as vegetables, fruits, medicinal herbs, and wine. However, there is no more report about melatonin and its biosynthesis in plant sprouts. This study determined melatonin contents in Tartary buckwheat (TB, Fagopyrum tartaricum L.) sprouts via ultra-high-performance liquid with tandem mass spectrometry (UHPLC-MS/MS) analysis, and the mechanisms underlying melatonin biosynthesis were investigated by integrating genome re-sequencing and transcriptome analyses. Melatonin contents in the sprouts of 16 TB varieties ranged from 0 to 0.187 ng g^-1 fresh weight (FW). Specifically, the TB varieties Heifeng No.1 (HF1), Chiku (CK), and Zhaoku No. 1 (ZK1) had relatively high melatonin contents, reaching 0.187, 0.183, and 0.125 ng g^-1 FW, respectively, whereas no melatonin was detected in Yunqiao No. 1 (YQ1), YQ2, and Xiqiao No. 3 (XQ3). 330 differentially expressed genes (DEGs) were identified by comparative transcriptome analysis between the high- and non-melatonin varieties. Ten putative melatonin biosynthetic genes were identified, among which FtCYP96A15, FtCYP81D1, and FtCYP82C4 may be the key regulatory genes. Twelve transcription factors such as FtABR1, FtWRKY41, and FtMYB80 may regulate melatonin synthesis. Furthermore, genes involved in auxin, abscisic acid, and cytokinin signaling pathways exhibited more active expression in high-melatonin varieties. Additionally, genome re-sequencing analysis identified 58 genotype-specific DEGs with sequence variations, including RLKs, FtCYP71A26, FtUGT76B1, FtFAAH, and FtSEOC. This work provides valuable insights into the mechanisms underlying melatonin biosynthesis in TB sprouts and proposes candidate genes for breeding high-melatonin TB varieties.