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Background  Flavonoid 3′,5′-hydroxylase (F3′5′H), an important branch point enzyme in tea plant flavan-3-ol synthesis, belongs to the CYP75A subfamily and catalyzes the conversion of flavones, flavanones, dihydroflavonols and flavonols into 3′,4′,5′-hydroxylated derivatives. However, whether B-ring hydroxylation occurs at the level of flavanones and/or dihydroflavonols,  in vivo  remains unknown.    Results  The  Camellia sinensis F3′5′H  ( CsF3′5′H ) gene was isolated from tea cDNA library. Expression pattern analysis revealed that  CsF3′5′H  expression was tissue specific, very high in the buds and extremely low in the roots.  CsF3′5′H  expression was enhanced by light and sucrose. Over-expression of  CsF3′5′H  produced new-delphinidin derivatives, and increased the cyanidin derivative content of corollas of transgenic tobacco plants, resulting in the deeper transgenic plant flower color. Heterologous expressions of CsF3′5′H in yeast were carried out to demonstrate the function of CsF3′5′H enzyme  in vitro . Heterologous expression of the modified  CsF3′5′H  ( CsF3′5′H  gene fused with  Vitis vinifera  signal peptide ,  FSI) revealed that 4′-hydroxylated flavanone (naringenin, N) is the optimum substrate for CsF3′5′H, and was efficiently converted into both 3′4′- and 3′4′5′-forms. The ratio of 3′4′5′- to 3′4′-hydroxylated products in  FSI  transgenic cells was significantly higher than  VvF3′5′H  cells.    Conclusions  CsF3′5′H is a key controller of tri-hydroxyl flavan-3-ol synthesis in tea plants, which can effectively convert 4′-hydroxylated flavanone into 3′4′5′- and/or 3′4′-hydroxylated products. These findings provide animportant basis for further studies of flavonoid biosynthesis in tea plants. Such studies would help accelerate flavonoid metabolic engineering in order to increase B-ring tri-hydroxyl product yields.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-014-0347-7) contains supplementary material, which is available to authorized users.

Functional analysis of Flavonoid 3′,5′-hydroxylase from Tea plant (Camellia sinensis): critical role in the accumulation of catechins