Fig. 1.

Outline of p-menthane monoterpene metabolism in peppermint glandular trichomes. The following enzymes are involved in this pathway: indicated by “(1),” (−)-limonene synthase; indicated by “(2),” (−)-limonene 3-hydroxylase; indicated by “(3),” (−)-trans-isopiperitenol dehydrogenase; indicated by “(4),” (−)-trans-isopiperitenone reductase; indicated by “(5),” (+)-cis-isopulegone isomerase; indicated by “(6),” (+)-menthofuran synthase; indicated by “(7a),” (+)-pulegone reductase [(−)-menthone-forming activity]; indicated by “(7b),” (+)-pulegone reductase [(+)-isomenthone-forming activity]; indicated by “(8a),” (−)-menthone: (−)-menthol reductase [(−)-menthol-forming activity]; indicated by “(8b),” (−)-menthone: (−)-menthol reductase [(+)-neoisomenthol-forming activity]; indicated by “(9a),” (−)-menthone: (+)-neomenthol reductase [(+)-neomenthol-forming activity]; indicated by “(9b),” (−)-menthone: (+)-neomenthol reductase [(+)-isomenthol-forming activity]. The subcellular compartmentation of p-menthane metabolic enzymes is color-coded: Cyt (blue), cytosol; ER (orange), endoplasmic reticulum; Lpl (green), leucoplasts; Mit (red), mitochondria. The inhibition of PR by (+)-menthofuran, as demonstrated in the present manuscript, is indicated by a red arc with an orthogonal red line.