Abstract
Three genes encoding flavonoid 3'-hydroxylase (F3'H) in apple (Malus x domestica), designated MdF3'HI, MdF3'HIIa, and MdF3'HIIb, have been identified. MdF3'HIIa and MdF3'HIIb are almost identical in amino acid sequences, and they are allelic, whereas MdF3'HI has 91% nucleotide sequence identity in the coding region to both MdF3'HIIa and MdF3'HIIb. MdF3'HI and MdF3'HII genes are mapped onto linkage groups 14 and 6, respectively, of the apple genome. Throughout the development of apple fruit, transcriptional levels of MdF3'H genes along with other anthocyanin biosynthesis genes are higher in the red-skinned cv Red Delicious than that in the yellow-skinned cv Golden Delicious. Moreover, patterns of MdF3'H gene expression correspond to accumulation patterns of flavonoids in apple fruit. These findings suggest that MdF3'H genes are coordinately expressed with other genes in the anthocyanin biosynthetic pathway in apple. The functionality of these apple F3'H genes has been demonstrated via their ectopic expression in both the Arabidopsis (Arabidopsis thaliana) transparent testa7-1 (tt7) mutant and tobacco (Nicotiana tabacum). When grown under nitrogen-deficient conditions, transgenic Arabidopsis tt7 seedlings expressing apple F3'H regained red color pigmentation and significantly accumulated both 4'-hydrylated pelargonidin and 3',4'-hydrylated cyanidin. When compared with wild-type plants, flowers of transgenic tobacco lines overexpressing apple F3'H genes exhibited enhanced red color pigmentation. This suggests that the F3'H enzyme may coordinately interact with other flavonoid enzymes in the anthocyanin biosynthesis pathway.
Highlights
Flavonoids are ubiquitous secondary metabolites in higher plants, and play important roles in myriad activities, such as protecting plants from UV radiation and pathogen infection, providing flowers and seeds with pigmentation to attract pollinators and seed dispersers, and reducing the risk of oxidative damage to human health (Regan et al, 2001; Schaefer et al, 2004; Veeriah et al 2006)
We report on the isolation of a gene family encoding flavonoid 3’-hydroxylase (F3’H) in apple, and investigate the functionality of these F3’H genes via their ectopic expression in both Arabidopsis and tobacco
We have identified two apple MdF3’H gene families that have higher levels of expression in the red-skinned cv
Summary
Flavonoids are ubiquitous secondary metabolites in higher plants, and play important roles in myriad activities, such as protecting plants from UV radiation and pathogen infection, providing flowers and seeds with pigmentation to attract pollinators and seed dispersers, and reducing the risk of oxidative damage to human health (Regan et al, 2001; Schaefer et al, 2004; Veeriah et al 2006). Flavonoids consist mainly of anthocyanins, chalcone, flavone, flavonol, flavanone, and isoflavonoids. Of these flavonoid molecules, anthocyanins are broadly distributed in flowering plants, and predominantly contribute to both flower and fruit colors. Naringenin is hydroxylated, at the 3’ position of the central ring, by flavanone 3-hydroxylase (F3H) to produce dihydrokaempferol (DHK). DHK can be further hydroxylated at the 3’ or at both 3’ and 5’ positions of the B-ring to produce dihydroquercetin (DHQ) and dihydromyricetin (DHM), respectively. DHK, DHQ, and DHM generally lead to the production of the brick-red/orange pelargonidin-, red/pink cyanidin-, and blue/violet delphinidin-based pigments, respectively (Grotewold, 2006). The hydroxylation pattern plays an important role in coloration. The hydroxylation pattern is an important determinant of the flavonoid stability and antioxidant capacity (Rice-Evans et al, 1996; Croft et al, 1998)
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