Studies on 1-deoxynojirimycin biosynthesis in mulberry (Morus alba L.) seeds through comparative transcriptomics

Mulberry (Morus alba L.) plants are rich in 1-deoxynojirimycin (DNJ), which is a potential α-glucosidase inhibitor exhibiting various physiological activities. Compared to other tissues, Morus alba L. seeds contain the highest DNJ content, however, the DNJ biosynthesis mechanisms are unclear. In this study, we examined fruits of 27 mulberry varieties and found that variety MS02 had the highest DNJ levels (22.28 mg/g), whereas variety MS15 contained the lowest DNJ levels (0.37 mg/g). Through comparative transcriptomics, 1,719 differentially expressed genes (DEGs) were identified, 1,170 of which were upregulated, and 549 were downregulated in MS02 compared to MS15. DEGs were associated with cellular processes, metabolic processes, and catalytic activity. Specifically, nine DEGs were identified to be involved in alkaloid biosynthesis pathways, according to Kyoto Encyclopaedia of Genes and Genomes enrichment analysis, and four enzymes, i.e. polyphenol oxidase, tyrosine aminotransferase, aromatic-L-amino-acid decarboxylase, and tropinone reductase, are proposed to play important roles in DNJ biosynthesis. In conclusion, DNJ biosynthesis in mulberry seeds appears to be mediated by upregulation of polyphenol oxidase, tyrosine aminotransferase, aromatic-L-amino-acid decarboxylase, and tropinone reductase.