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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

Redirection of Flavonoid Biosynthesis through the Down-Regulation of an Anthocyanidin Glucosyltransferase in Ripening Strawberry Fruit^1,[W],[OA]

Biomolecular Food Technology, Technical University Munich, 85354 Freising, Germany (M.G., T.H., B.F., R.K., W.S.); Departamento de Bioquímica y Biología Molecular, Campus Universitario de Rabanales, Universidad de Córdoba, 14071 Cordoba, Spain (M.L.B., J.M.-B.); ENEA Centro Richerche Trisaia, Department of Genetics and Genomics, I–75026 Rotondella, Italy (C.R.); and Plant Research International, Business Units Cell Cybernetics and Genetics and Breeding, 6700 AA Wageningen, The Netherlands (A.A.)

Strawberry (Fragaria x ananassa) fruit contains several anthocyanins that give the ripe fruits their attractive red color. The enzyme that catalyzes the formation of the first stable intermediate in the anthocyanin pathway is anthocyanidin-3-O-glucosyltransferase. A putative glycosyltransferase sequence (FaGT1) was cloned from a strawberry fruit cDNA library and the recombinant FaGT1 transferred UDP-glucose to anthocyanidins and, to a lesser extent, flavonols, generating the respective 3-O-glucosides. Quantitative polymerase chain reaction revealed that transcripts of FaGT1 were almost undetectable in green fruits, but gene expression increased dramatically in both turning and ripe red fruit, corresponding closely to the accumulation of anthocyanins during fruit ripening. The expression of FaGT1 is fruit associated and negatively regulated by auxin. To elucidate the in planta function of FaGT1, Agrobacterium tumefaciens cells harboring an intron-hairpin construct of a partial FaGT1 sequence were injected into midsized ripening fruits. In about one-third of the injected fruits, this led to significant down-regulation of FaGT1 transcript levels that corresponded to reduced concentrations of anthocyanin pigments in ripe strawberry fruits. In contrast, significant levels of epiafzelechin—formed by anthocyanidin reductase (ANR) from pelargonidin—were identified in FaGT1-silenced fruits, indicating competition of FaGT1 and FaANR for the common anthocyanidin substrate. Thus, FaGT1 represents an important branching-point enzyme because it is channeling the flavonoid pathway to anthocyanins. These results demonstrate a method to redirect the anthocyanin biosynthesis into flavan-3-ol production to increase the levels of bioactive natural products or modify pigments in plant tissues.

² Present address: Department of Pharmacology, Vanderbilt University, Nashville, TN 37232.

³ Present address: Department of Plant Sciences, Weizmann Institute of Science, 76100 Rehovot, Israel.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Wilfried Schwab (schwab{at}wzw.tum.de).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.107.114280

^* Corresponding author; e-mail schwab{at}wzw.tum.de.

Received November 29, 2007; accepted January 21, 2008; published February 7, 2008.