| Summary: | Mulberry (<i>Morus</i> spp., Moraceae) is an important economic crop plant and is rich in flavonoids and anthocyanidins in ripe fruits. Anthocyanins are glycosides of anthocyanidins. Flavanone 3-hydroxylase (F3H) catalyzes the conversion of naringenin into dihydroflavonols and is responsible for the biosynthesis of flavonols and anthocyanidins. In this study, <i>MazsF3H</i> was cloned and characterized from <i>Morus atropurpurea</i> var. <i>Zhongshen 1.</i> Conserved motif analysis based on alignment and phylogenetic analysis indicated that <i>MazsF3H</i> belonged to 2-oxoglutarate-dependent dioxygenase and <i>MazsF3H</i> clustered with F3Hs from other plants. <i>MazsF3H</i> was located in both nucleus and cytosol. <i>MazsF3H</i> was expressed in stems, leaves, stigmas and ovaries, except buds. <i>F3H</i> expression levels showed a positive and close relationship with anthocyanin content during the anthocyanin-rich fruit ripening process, while it showed a negative correlation with anthocyanin content in <i>LvShenZi</i>, whose fruits are white and would not experience anthocyanin accumulation during fruit ripening. Significantly different <i>F3H</i> expression levels were also found in different mulberry varieties that have quite different anthocyanin contents in ripe fruits. Overexpression <i>MazsF3H</i> in tobacco showed unexpected results, including decreased anthocyanin content. Down-regulation of <i>F3H</i> expression levels resulted in co-expression of the genes involved in anthocyanin biosynthesis and a significant decrease in anthocyanin content, but the change in total flavonoid content was subtle. Our results indicated that F3H may play quite different roles in different varieties that have quite different fruit colors. In addition, possible complex regulation of flavonoid biosynthesis should be further explored in some of the featured plant species.
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