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JXB Advance Access originally published online on May 17, 2008
Journal of Experimental Botany 2008 59(10):2611-2625; doi:10.1093/jxb/ern117
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© 2008 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This paper is available online free of all access charges (see http://jxb.oxfordjournals.org/open_access.html for further details)

RESEARCH PAPER

Multi-substrate flavonol O-glucosyltransferases from strawberry (Fragariaxananassa) achene and receptacle

Markus Griesser1 *, Florian Vitzthum1, Barbara Fink1, Mari Luz Bellido2, Constanze Raasch1, Juan Munoz-Blanco2 and Wilfried Schwab1,{dagger}

1Biomolecular Food Technology, Technical University Munich, Lise-Meitner-Str. 34, 85354 Freising, Germany
2Departamento de Bioquímica y Biología Molecular, Edificio Severo Ochoa (C-6), Campus Universitario de Rabanales, Universidad de Córdoba, 14071 Córdoba, Spain

{dagger} To whom correspondence should be addressed. E-mail: schwab{at}wzw.tum.de

In an effort to characterize fruit ripening-related genes functionally, two glucosyltransferases, FaGT6 and FaGT7, were cloned from a strawberry (Fragariaxananassa) cDNA library and the full-length open reading frames were amplified by rapid amplification of cDNA ends. FaGT6 and FaGT7 were expressed heterologously as fusion proteins in Escherichia coli and target protein was purified using affinity chromatography. Both recombinant enzymes exhibited a broad substrate tolerance in vitro, accepting numerous flavonoids, hydroxycoumarins, and naphthols. FaGT6 formed 3-O-glucosides and minor amounts of 7-O-, 4'-O-, and 3'-O-monoglucosides and one diglucoside from flavonols such as quercetin. FaGT7 converted quercetin to the 3-O-glucoside and 4'-O-glucoside and minor levels of the 7- and 3'-isomers but formed no diglucoside. Gene expression studies showed that both genes are strongly expressed in achenes of small-sized green fruits, while the expression levels were generally lower in the receptacle. Significant levels of quercetin 3-O-, 7-O-, and 4'-O-glucosides, kaempferol 3-O- and 7-O-glucosides, as well as isorhamnetin 7-O-glucoside, were identified in achenes and the receptacle. In the receptacle, the expression of both genes is negatively controlled by auxin which correlates with the ripening-related gene expression in this tissue. Salicylic acid, a known signal molecule in plant defence, induces the expression of both genes. Thus, it appears that FaGT6 and FaGT7 are involved in the glucosylation of flavonols and may also participate in xenobiotic metabolism. The latter function is supported by the proven ability of strawberries to glucosylate selected unnatural substrates injected in ripe fruits. This report presents the first biochemical characterization of enzymes mainly expressed in strawberry achenes and provides the foundation of flavonoid metabolism in the seeds.

Key words: Achenes, detoxification, flavonols, Fragariaxananassa, multisubstrate glucosyltransferase, plant defence, salicylic acid, strawberry, xenobiotics

* Present address: Department of Pharmacology, Vanderbilt University, Robinson Research Building 572, 23rd Ave. S. at Pierce, Nashville, TN 37232, USA.

GenBankTM accession numbers: FaGT1, AY663784 [GenBank] ; FaGT2, AY663785 [GenBank] ; FaGT3, AY663786 [GenBank] ; FaGT4, AY663787 [GenBank] ; FaGT5, DQ289586 [GenBank] ; FaGT6, DQ289587 [GenBank] ; FaGT7, DQ289588.

Received 17 January 2008; Accepted 2 April 2008


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