Floral scent is typically a complex mixture of low molecular weight volatile compounds (100-200 Da) which gives the flower its unique, characteristic fragrance. The major components in the fragrance of snapdragon flowers are the aromatic ester methylbenzoate and two monoterpene olefins, (E)-beta-ocimene and myrcene. Petunia floral scent consists almost exclusively of benzenoid/phenylpropanoid-related compounds, and is dominated by methylbenzoate, benzaldehyde, phenylacetaldehyde and benzyl benzoate. Using a functional genomic approach, a cDNA encoding a putative salicylic acid carboxyl methyltransferase (SAMT) was isolated in snapdragon and its potential involvement in the production and emission of methylbenzoate was analyzed. Using a similar approach, genes encoding enzymes potentially responsible for the formation of methylbenzoate and benzylbenzoate and phenyl ethyl benzoate in petunia were isolated and characterized. The molecular mechanisms responsible for postpollination changes in floral scent emission were investigated in snapdragon and petunia flowers using methylbenzoate, one of the major scent compounds emitted by these flowers, as an example. In both species, a pollination-induced decrease in methylbenzoate emission begins only after pollen tubes reach the ovary. Petunia and snapdragon both synthesize methylbenzoate from benzoic acid and S-adenosyl- L-methionine (SAM); however, they use different mechanisms to down-regulate its production after pollination. In petunia, expression of the gene responsible for methylbenzoate synthesis is suppressed by ethylene. In snapdragon, a decrease in S-adenosyl-L-methionine:benzoic acid carboxyl methyltransferase (BAMT) activity and a decrease in the ratio of SAM to S-adenosyl-L-homocysteine ("methylation index") after pollination are concomitant with the decrease in methylbenzoate emission, and are therefore likely involved in this post-pollination change in emission.
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