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JXB Advance Access published online on July 2, 2004
Journal of Experimental Botany, doi:10.1093/jxb/erh138
© 2004 by Oxford University Press
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Received January 12, 2004
Accepted February 26, 2004

Sulphur Metabolism Special Issue Article

Biosynthesis of the flavour precursors of onion and garlic

Meriel G. Jones 1*, Jill Hughes 1, Angela Tregova 1, Jonothan Milne 1, A. Brian Tomsett 1, Hamish A. Collin 1

1 School of Biological Sciences, The Biosciences Building, The University of Liverpool, Liverpool L69 7ZB, UK

* To whom correspondence should be addressed. E-mail: m.g.jones{at}liv.ac.uk.


  Abstract

Onion (Allium cepa), garlic (A. sativum) and other Alliums are important because of the culinary value of their flavours and odours. These are characteristic of each species and are created by chemical transformation of a series of volatile sulphur compounds generated by cleavage of relatively stable, odourless, S-alk(en)yl cysteine sulphoxide flavour precursors by the enzymes alliinase and lachrymatory-factor synthase. These secondary metabolites are S-methyl cysteine sulphoxide (MCSO, methiin; present in most Alliums, some Brassicaceae), S-allyl cysteine sulphoxide (ACSO, alliin; characteristic of garlic), S-trans-prop-1-enyl cysteine sulphoxide (PECSO, isoalliin; characteristic of onion), and S-propyl cysteine sulphoxide (PCSO, propiin; in onion and related species). Information from studies of the transformation of putative biosynthetic intermediates, radiolabelling, and from measurements of sulphur compounds within onion and garlic have provided information to suggest a biosynthetic pathway. This may involve alk(en)ylation of the cysteine in glutathione, followed by cleavage and oxidation to form the alk(en)yl cysteine sulphoxide flavour precursors. There is also evidence that synthesis of the flavour precursors may involve (thio)alk(en)ylation of cysteine or a precursor such as O-acetyl serine. Both routes may occur depending on the physiological state of the tissue. There are indications from the effects of environmental factors, such as the availability of sulphur, that control of the biosynthesis of each flavour precursor may be different. Cysteine and glutathione metabolism are discussed to indicate parallels with Allium flavour precursor biosynthesis. Finally, possible avenues for exploration to determine the origin in planta of the alk(en)yl groups are suggested.

Keywords: Alliaceae; alliinase; S-alk(en)yl cysteine sulphoxide; cysteine synthase; flavour precursor biosynthesis; garlic; glutathione metabolism; {gamma}-glutamyl alk(en)yl cysteine sulphoxide; onion.
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