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Neohesperidosides are disaccharides that are present in some flavonoids and impart a bitter taste, which can significantly affect the commercial value of citrus fruits. In this study, we identified three flavonoid-7- O - di -glucosyltransferase (dGlcT) genes closely related to 1,2-rhamnosyltransferase (1,2RhaT) in citrus genomes. However, only 1,2RhaT was directly linked to the accumulation of neohesperidoside, as demonstrated by association analysis of 50 accessions and co-segregation analysis of an F1 population derived from Citrus reticulata × Poncirus trifoliata. In transgenic tobacco BY2 cells, over-expression of CitdGlcT s resulted in flavonoid-7- O -glucosides being catalysed into bitterless flavonoid-7- O - di -glucosides, whereas over-expression of Cit1,2RhaT converted the same substrate into bitter-tasting flavonoid-7- O -neohesperidoside. Unlike 1,2RhaT, during citrus fruit development the dGlcT s showed an opposite expression pattern to CHS and CHI, two genes encoding rate-limiting enzymes of flavonoid biosynthesis. An uncoupled availability of dGlcTs and substrates might result in trace accumulation of flavonoid-7- O - di -glucosides in the fruit of C. maxima (pummelo). Past human selection of the deletion and functional mutation of 1,2RhaT has led step-by-step to the evolution of the flavor-related metabolic network in citrus. Our research provides the basis for potentially improving the taste in citrus fruit through manipulation of the network by knocking-out 1,2RhaT or by enhancing the expression of dGlcT using genetic transformation. [ABSTRACT FROM AUTHOR]