Characterization of a Caffeic Acid 8--Methyltransferase from Citrus and Its Function in Nobiletin Biosynthesis.

Nobiletin (3',4',5,6,7,8-hexamethoxyflavone) is a polymethoxylated flavonoid specifically accumulated in citrus fruit with numerous beneficial effects to human health. In this study, a novel -methyltransferase () was isolated from three citrus varieties, Ponkan mandarin ( Blanco), Nou 6 ("King mandarin" × "Mukaku-kishu"), and Satsuma mandarin ( Marc.), and its functions were characterized in vitro. The gene expression results showed that was highly expressed in the two nobiletin abundant varieties of Ponkan mandarin and Nou 6. However, the expression level of was low in the flavedo of Satsuma mandarin, in which only a small amount of nobiletin was accumulated. Functional analysis suggested that CitOMT2 was a caffeic acid 8--methyltransferase, and it catalyzed the O-methylation of 7,8-dihydroxyflavone at 8-OH. As the methylation of flavone at 8-OH was required for nobiletin biosynthesis, the results presented in this study suggested that was a key gene regulating nobiletin accumulation in citrus fruit.