Abstract
Nobiletin, a polymethoxy flavone (PMF), is specific to citrus and has been reported to exhibit important health-supporting properties. Nobiletin has six methoxy groups at the 3′,4′,5,6,7,8-positions, which are catalyzed by O-methyltransferases (OMTs). To date, researches on OMTs in citrus fruit are still limited. In the present study, a novel OMT gene (CitOMT) was isolated from two citrus varieties Satsuma mandarin (Citrus unshiu Marc.) and Ponkan mandarin (Citrus reticulata Blanco), and its function was characterized in vitro. The results showed that the expression of CitOMT in the flavedo of Ponkan mandarin was much higher than that of Satsuma mandarin during maturation, which was consistent with the higher accumulation of nobiletin in Ponkan mandarin. In addition, functional analysis showed that the recombinant protein of CitOMT had methylation activity to transfer a methyl group to 3′-hydroxy group of flavones in vitro. Because methylation at the 3′-position of flavones is vital for the nobiletin biosynthesis, CitOMT may be a key gene responsible for nobiletin biosynthesis in citrus fruit. The results presented in this study will provide new strategies to enhance nobiletin accumulation and improve the nutritional qualities of citrus fruit.
Highlights
Flavonoids are a group of secondary metabolites that include more than 10,000 kinds of derivatives[1]
The major flavonoids accumulated in citrus fruit are divided into two groups, flavanone glycosides, such as naringin, hesperidin, neo-hesperidin, and polymethoxy flavone (PMF), such as, nobiletin, sinensetin, and tangeretin[35]
The accumulation of flavonoids was investigated in two citrus varieties, Satsuma mandarin and Ponkan mandarin
Summary
Flavonoids are a group of secondary metabolites that include more than 10,000 kinds of derivatives[1]. According to their structure, flavonoids are divided into several subgroups, including flavanones, flavones, flavonols, flavanols, isoflavones, and anthocyanidins[2]. Type II OMTs (caffeoyl-CoA OMT: CCoAOMT) are ion-dependent enzymes involved in the monolignol biosynthesis with the molecular weight of 23–27 kDa. CCoAOMTs act on a narrow range of substrates, which act only on 5-hydroxyferuloyl CoA and caffeoyl C oA11,12. The roles of nobiletin in human health have been investigated extensively, and it was suggested that nobiletin was beneficial to human health through its various activities, such as anti-depressant[25], anti-allergy[26], anti-pigmentation of skin[27], and inhibition of heart failure progression[28]. The development of functional foods or supplements, which are rich in nobiletin, may be of great importance for human health
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