Peppermint oil gland secretory cell

Figure 9.2 Outline of a functional genomics approach to evaluate ESTs from a peppermint oil gland secretory cell cDNA library.

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

Figure 9.4 Monoterpene biosynthesis in peppermint oil gland secretory cells. The enzymes involved in this pathway are (1) 1-deoxy-D-xylulose 5-phosphate synthase, (2) 2-C-methyl-D-erythritol 4-phosphate reductoisomerase, (3) 2-C-methyl-D-erythritol 4-phosphate cytidyltransferase, (4) 4-(cytidine 5 -diphospho)-2-C-methyl-D-erythritol kinase, (5) 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase, (6) isopentenyl diphosphate isomerase, (7) geranyl diphosphate synthase, (8)...

Table 9.2 Incorporation rate of [2-14C]-pyruvate into monoterpenes of isolated peppermint oil gland secretory cells in the presence of fosmidomycin, a specific inhibitor of 1-deoxy-D-xylulose 5-phosphate reductoisomerase, an enzyme of the mevalonate-independent pathway of isoprenoid biosynthesis.

The peppermint oil gland secretory cell cDNA library has proven to provide a highly enriched source of candidate genes involved in essential oil biosynthesis. A functional genomics approach has successfully been employed to clone genes involved in the mevalonate-independent pathway of isoprenoid biosynthesis and in the peppermint-specific steps producing (-)-menthol and (-)-menthone. The optimization of LC-MS technology to profile phosphoiylated carbohydrates and... 

LANGE, B.M., KETCHUM, R.E.B., CROTEAU, R., Isoprenoid biosynthesis metabolite profiling of peppermint oil gland secretory cells and application to herbicide target analysis, Plant Physiol., 2001,127, 305-314. 

Turner, G., Gershenzon, J., Nielson, E.E., Eroehlich, J.E. and Croteau, R. (1999) Limonene s)mthase, the enzyme responsible for monoterpene biosynthesis in peppermint, is localized to leucoplasts of oil gland secretory cells. Plant Physiol., 120, 879-86. 

Importantly, these genomic libraries can be enriched for the appropriate gene transcripts if the tissue or cell type from which the genetic material is harvested conesponds to the site of natural product production. For example, 25% of the clones in cDNA obtained exclusively from the oil gland secretory cell of peppermint Mentha x piperita) appear to be involved in oil metabolism [28]. In another example, mRNA for construction of a C. roseus cDNA Ubraiy obtained from epidermal cells resulted in a collection enriched in genes involved in alkaloid biosynthesis [21]. 

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