Cellular and subcellular localization of S-adenosyl-L-methionine: Benzoic acid carboxyl methyltransferase, the enzyme responsible for biosynthesis ofthe volatile ester methylbenzoate in snapdragon flowers

The benzenoid ester, methylbenzoate is one of the most abundant scent compo unds 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 :benz oic acid carboxyl methyltransferase (BAMT). To identify the location of met hylbenzoate biosynthesis, we conducted an extensive immunolocalization stud y 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 exten t, 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 BA MT protein detected in the outer epidermis and in the yellow hairs within t he 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 biosynthe sis. The concentration of scent production on flower surfaces that face the pollinators during landing may increase pollination efficiency and also he lp to minimize the biosynthetic cost of advertising for pollinators.