Biosynthesis of flavour precursors in onion and  garlic 

 
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Comparison between garlic tissue culture grown on media containing normal or low levels of sulphur. Five primers (1 - 5) were used to amplify arbitrary portions of mRNA extracted from two samples grown under each condition. The two LHS lanes in each set are from normal sulphur levels, the two RHS lanes from low sulphur.


The bands on the gel, indicating gene expression, show that expression of most genes is unchanged by growth conditions. The yellow boxes indicate genes whose expression is different in the two growth conditions.
Expression on normal rather than low sulphur (two LHS bands darker) Expression increased in low sulphur (two RHS bands darker) Expression increased in low sulphur (two RHS bands darker) Expression increased in normal sulphur (two LHS bands darker)

The major flavour precursors are S-alk(en)yl cysteine sulphoxides (CSO) and related compounds. The balance of different members of the family gives rise to the characteristic odours of onion and garlic. In onion, the major CSO is isoalliin (trans-(+)-S-(1-propenyl)-L-cysteine sulphoxide). Garlic contains a different major CSO, alliin ((+)-S-allyl-L-cysteine-sulfoxide). In both plants, glutathione derivatives of the CSOs may be important in flavour biosynthesis, storage and degradation.

We are starting to analyse the CSO biosynthetic pathway in garlic, using a combination of chemical, physiological and molecular biology approaches. We are seeking to identify chemical intermediates, proteins and genes involved in this pathway.