Comparative transcriptome analysis of mulberry reveals anthocyanin biosynthesis mechanisms in black (Morus atropurpurea Roxb.) and white (Morus alba L.) fruit genotypes

BACKGROUND: To gain a better understanding of anthocyanin biosynthesis in mulberry fruit, we analyzed the transcriptome of the mulberry varieties Da 10 (Morus atropurpurea Roxb., black fruit) and Baisang (Morus alba L., white fruit). RESULTS: We found that whereas Da 10 had high levels of cyanidin 3-O-glucoside (Cy), and pelargonidin 3-O-glucoside (Pg), Baisang contained only Cy, at low levels. Based on a comparative transcriptome analysis, we annotated more than 27,085 genes (including 1735 new genes). Genes that were differentially expressed between Da 10 and Baisang were detected at three stages of fruit development: S1 [4256 genes, 10 days post-anthesis (DPA)], S2 (5612 genes, 19 DPA), and S3 (5226 genes, 28 DPA). Anthocyanin biosynthesis was found to be associated with the expression of 15 core genes and 5 transcription factors. Relative to Baisang, Da 10 showed a significant upregulation of genes involved in the early stages (production of the intermediate compounds chalcone and dihydroflavonol) and late stages (production of Cy and Pg) of anthocyanin biosynthesis. Baisang showed a significant downregulation of the genes involved in the early stages of anthocyanin biosynthesis and overexpression of flavanone 3-hydroxylase (FLS), resulting in the generation of quercetin and/or myricetin but not anthocyanins. CONCLUSIONS: The biosynthesis of anthocyanin in mulberry fruit is initiated from the precursor, phenylalanine, and mediated by the upregulation of dihydroflavonol 4-reductase, anthocyanidin synthase, anthocyanidin 3-O-glucosyltransferase, and cyanidin-3-O-glucoside 2-O-glucuronosyltransferase, and downregulation of FLS to produce Cy and Pg.