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The benzenoid ester, methylbenzoate is one of the most abundant scent compounds detected in the majority of snapdragon (Antirrhinum majus) varieties. It is produced in upper and lower lobes of petals by enzymatic methylation of benzoic acid in the reaction catalyzed by S-adenosyl-L-methionine:benzoic acid carboxyl methyltransferase (BAMT). To identify the location of methylbenzoate biosynthesis, we conducted an extensive immunolocalization study by light and electron microscopy at cellular and subcellular levels using antibodies against BAMT protein. BAMT was immunolocalized predominantly in the conical cells of the inner epidermal layer and, to a much lesser extent, in the cells of the outer epidermis of snapdragon flower petal lobes. It was also located in the inner epidermis of the corolla tube with little BAMT protein detected in the outer epidermis and in the yellow hairs within the tube on the bee's way to the nectar. These results strongly suggest that scent biosynthetic genes are expressed almost exclusively in the epidermal cells of floral organs. Immunogold labeling studies reveal that BAMT is a cytosolic enzyme, suggesting cytosolic location of methylbenzoate biosynthesis. The concentration of scent production on flower surfaces that face the pollinators during landing may increase pollination efficiency and also help to minimize the biosynthetic cost of advertising for pollinators.
Flowers of many plant species attract pollinators by producing complex mixtures of volatile compounds that give each species their unique, characteristic fragrances. Among the various floral stimuli, odors play a prominent role in the location and selection of flowers by insects, particularly moths, which search and visit flowers at night (Dobson, 1994). Although little is known about how insects respond to individual components found within floral scents, it is clear that they are capable of distinguishing complex floral scent mixtures and that discriminatory visitation based upon floral scent has important implications for plant reproductive success. Therefore, floral scent can be considered to perform a vital function in the plant life cycle.
Floral fragrances belong to a broad category of secondary metabolites and are dominated by terpenoids (monoterpenes and sesquiterpenes), phenylpropanoid, and benzenoid compounds (Knudsen et al., 1993). Despite the importance of floral scent compounds in plant biology, floral scent research has, until recently, remained largely descriptive. A total of 700 floral scent compounds have been described...